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Investigation of silylated onium ions

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Investigation of silylated onium ions

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. U M I films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy subm itted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UM I a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. ProQuest Information and Learning 300 North Zeeb Road, Ann Arbor, M l 48106-1346 USA 800-521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INVESTIGATION OF SILYLATED ONIUM IONS Copyright 2002 by Chulsung Bae A Dissertation Presented to THE FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA hi Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMISTRY) May 2002 Chulsung Bae Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 3073742 __ ___ f t p UMI UM I Microform 3073742 Copyright 2003 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, M l 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA The Graduate School University Park L O S ANGELES, CALIFORNIA 90089-1695 This d issertation , w ritten b y _____ Chulsung Bae_______________________ U nder th e direction o f hi&... D issertation C om m ittee, a n d approved b y a il its m em bers, has been p resen ted to an d a ccep ted b y The G raduate School, in p a rtia l fu lfillm en t o f requirem ents fo r th e degree o f DOCTOR OF PHILOSOPHY D ate Novenber 16, 2001 DISSER TA T^N C jO M M flTE E Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DEDICATION To Mom, Dad, and my wife Amie ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS I would like to express my sincere thanks to Professor G. K. Surya Prakash for his wonderful guidance over the past four years. His immeasurable support is greatly appreciated. I also would like to thank Professor George A. Olah for providing a most challenging and fun atmosphere to work in during my study at the Loker Hydrocarbon Research Institute. I would like to thank Professor Golam Rasul for many helpful discussion and contribution in theoretical studies of my work. Dr. Manfred Kroll who started project of Chapter 1 with me is acknowledged for teaching me new techniques and experimental skills. To all my colleagues at the Loker Hydrocarbon Research Institute, I appreciate the friendship and your support. I will keep the fond memories forever. Special thanks to Mihirbaran Mandal, Jinbo Hu, Giovanni Bemasconi, and Xin Yao in the Loker Hydrocarbon Research Institute, and Jooho Kim in physical chemistry. Last, I would like to express my love and thanks to my father, mother, brother Chulmin, sister Minyoung, and wife Amie. I could have never reached here without their unimaginable support. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS INVESTIGATION OF SILYLATED ONIUM IONS Dedication............................................................................................................... ii Acknowledgements.................................................................................................iii List of tables.......................................................................................................... vii List of figures....................................................................................................... viii List of schemes........................................................................................................xi Abstract................................................................................................................. xii Chapter 1. Approaches towards Trfalkylsilyl Cation 1.1. Introduction and Historical Background..........................................1 1.2. Results and Discussion....................................................................11 1.2.1. Attempted preparation of tris( I -adamantyl)silyl cation via 1-adamantyl anion................................................................11 1.2.2. Attempted preparation of tris( I -adamantyl)silyl cation via 1,3-dehydroadamantane 17 1.2.3. Further application of 1,3-dehydroadamantane: Synthesis of 1,3-bis(N,N,-difluoroamino)adamantane..........................22 1.3. Conclusions....................................................................................25 1.4. Experimental procedure................................................................. 26 1.5. References...................................................................................... 54 Chapter 2. Silylated Sulfonium Ions 2.1. Introduction.................................................................................... 60 iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.3. Conclusions................................................................................ 137 4.4. Experimental Section.................................................................. 137 4.5. References.................................................................................. 149 Bibliography....................................................................................................... 152 vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Table 2.1. 1 3 C and 2 9 Si NMR Chemical Shifts of Silylsulfonium Ions and their precursors................................................................................ 63 Table 3.1 l3 C and 2 9 Si NMR chemical shifts of silylated ketones 2a, 3a and their precursors................................................................................ 93 Table 3.2 1 3 C and 2 9 Si NMR chemical shifts of silylated dimethylcarbonate 4a, N,N-dimethylacetamide Sa, N,N-tetramethylurea 6a, and their precursors........................................................................................ 94 Table 3.3. 2 9 Si NMR chemical shifts for attempted silylation of weak bases (CS2 , COS, C02 , CO, N2 0, CH3 CN)............................................... 99 Table 4.1. ,3C NMR chemical shifts of ketenes RiR2 Cp=Co=0........................129 Table 4.2. Total energies (-au), ZPE, and relative energies of silylated ketenes............................................................................................133 Table 4.3. I3 C and 2 9 Si NMR chemical shifts of diphenylketene, protonated diphenylketene, and silylated diphenylketene............................... 134 vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Figure 1.1. lH NMR spectrum of 1,3-dibromoadamantane.................................37 Figure 1.2. 1 3 C NMR spectrum of 1,3-dibromoadamantane................................38 Figure 1.3. lH NMR spectrum of 1,3-dehydroadamantane..................................39 Figure 1.4. ,3C NMR spectrum of 1,3-dehydroadamantane................................ 40 Figure 1.5. lH NMR spectrum of Ad2Sil2 ...........................................................41 Figure 1.6. l3 C NMR spectrum of Ad2Sil2 ..........................................................42 Figure 1.7. *H NMR spectrum of Ad2SiH2 ......................................................... 43 Figure 1.8. l3C NMR spectrum of Ad2SiH2 ........................................................ 44 Figure 1.9. Mass spectrum of Ad2SiH2 ...............................................................45 Figure 1.10. lH NMR spectrum of Ph3C+ I^C&FsV..............................................46 Figure 1.11. l3 C NMR spectrum of Ph3C+ BCC^s)/.............................................47 Figure 1.12. 1 9 F NMR spectrum of PhsC* B(C 6Fs)4 * ............................................. 48 Figure 1.13. 1 H NMR spectrum of 1,3-bis(N,N-difluoroamino)adamantane.........49 Figure 1.14. 1 3 C NMR spectrum of l,3-bis(N,N-difluoroamino)adamantane........50 Figure 1.15. l9 F NMR spectrum of 1,3-bis(N,N-difluoroamino)adamantane........ 51 Figure 1.16. Mass spectrum of 1,3-bis(N,N-difluoroamino)adamantane...............52 Figure 1.17. FT-IR spectrum of 1,3-bis(N,N-difluoroamino)adamantane............. 53 Figure 2.1. lH NMR spectrum of tris(trimethylsilyl)sulfonium ion at -78 °C. .. 73 Figure 2.2. lH NMR spectrum of tris(trimethylsilyl)sulfonium ion at variable temperatures.....................................................................................74 Figure 2.3. 1 3 C NMR spectrum of tris(trimethylsilyl)sulfonium ion at — 78 °C. .........................................................................................................75 Figure 2.4. l3C NMR spectrum of tris(trimethylsilyl)sulfonium ion at room temperature...................................................................................... 76 Figure 2.5 2 9 Si NMR spectrum of tris(trimethylsilyl)sulfonium ion at -78 °C. ........................................................................................................ 77 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.6. 2 9 Si NMR spectrum of tris(trimethylsilyl)sulfonium ion at room temperature......................................................................................78 Figure 2.7. lH NMR spectrum of methylbis(trimethylsilyl)sulfonium ion...........79 Figure 2.8. I3 C NMR spectrum of methyIbis(trimethyIsilyl)sulfonium ion..........80 Figure 2.9. 2 9 Si NMR spectrum of methylbis(trimethylsilyl)sulfonium ion.........81 Figure 2.10. DFT B3LYP/6-3IG* optimized structures of silylsulfonium ions 1,2, and 5...................................................................................... 82 Figure 3.1. l3C NMR spectrum of triethylsilylated acetophenone ion.............. 107 Figure 3.2. Expanded l3C NMR spectrum of triethylsilylated acetophenone ion. 108 Figure 3.3. 2 9 Si NMR spectrum of triethylsilylated acetophenone ion 109 Figure 3.4. 1 3 C NMR spectrum of triethylsilylated 2-cyclohexen-1-one ion. . 110 Figure 3.5. Expanded l3C NMR spectrum of triethylsilylated 2-cyclohexen-1 -one ion............................................................................................... I ll Figure 3.6. 2 9 Si NMR spectrum of triethylsilylated 2-cyclohexen-1-one ion. 112 Figure 3.7. lH NMR spectrum of triethylsilylated dimethylcarbonate ion 113 Figure 3.8. l3C NMR spectrum of triethylsilylated dimethylcarbonate ion 114 Figure 3.9. 2 9 Si NMR spectrum of triethylsilylated dimethylcarbonate ion. ... 115 Figure 3.10. lH NMR spectrum of triethylsilylated N,N-dimethylacetamide ion. 116 Figure 3.11. l3C NMR spectrum of triethylsilylated N,N-dimethylacetamide ion. .................................................................................................... 117 Figure 3.12. 2 9 Si NMR spectrum of triethylsilylated N,N-dimethylacetamide ion. 118 Figure 3.13. lH NMR spectrum of triethylsilylated tetramethylurea ion 119 Figure 3.14. l3C NMR spectrum of triethylsilylated tetramethylurea ion 120 Figure 3.15. 2 9 Si NMR spectrum of triethylsilylated tetramethylurea ion 121 ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.16. DFT B3LYP/6-31G* calculated structures of triethylsilylated carboxonium ions 2 a'- 6a'.......................................................... 122 Figure 4.1. HOMO and LUMO molecular orbitals of ketene CH2=C=0......... 129 Figure 4.2. DFT B3LYP/6-31 l+G* calculated structures for trimethylsilylated kemes 3 - 4 .................................................................................. 140 Figure 4.3. 'H NMR spectrum of diphenylketene............................................ 142 Figure 4.4. l3C NMR spectrum of diphenylketene........................................... 143 Figure 4.5. Mass spectrum (70 eV) of diphenylketene..................................... 144 Figure 4.6. lH NMR spectrum of triethylsilylated diphenylketene ion 145 Figure 4.7. l3C NMR spectrum of triethylsilylated diphenylketene ion 146 Figure 4.8. Expanded > 3 C NMR spectrum of triethylsilylated diphenylketene ion. .................................................................................................... 147 Figure 4.9. 2 9 Si NMR spectrum of triethylsilylated diphenylketene ion 148 X Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF SCHEMES Scheme 1.1. Literature claims about silyl cations in the condensed phase.............3 Scheme 1.2. The allyl leaving group approach for the preparation of trimesitylsilyl cation..........................................................................7 Scheme 1.3. Lambert’s procedure for the generation of trimesitylsilyl cation using Et3Si ClfcC^Plfc as an electrophile........................................... 9 Scheme 1.4. Preparation of allyltris( 1 -adamantyl)silane and its conversion to tri(l-adamantyl)silyl cation.............................................................. 10 Scheme 1.5. The preparation of trityl tetrakis(pentafluorophenyl)borate (Ph3 C+TPFPB-)..................................................................... 10 Scheme 1.6. Reported methods for preparation and reactions of l-adamantyllithium.......................................................................... 14 Scheme 1.7. Attempted preparation and reactions of l-adamantyllithium............16 Scheme 1.8. Attempted synthesis of allyltris(l-adamantyl)silane via 1,3-dehydroadamantane 21 xi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4 encompasses theoretical and experimental study of electrophilic silylation of ketenes. The in situ generated silyl cation can react with Cp of ketenes (C-silylation) or react with oxygen (O-silylation). The question of C- vs. O-silylation of various ketenes is investigated by DFT calculations. The structure and enthalpies of C- as well as O-silylated ketene cations are computed. In the case of silylated diphenylketene, experimental result of l3C and 2 9 Si NMR spectroscopy is compared with calculated NMR chemical shifts of IGLO method. Surprisingly, in solution C- silylated product of diphenylketene is found to be formed exclusively, although it is energetically less stable than O-silylated product by 5.4 kcal/mol by DFT calculations in the idealized gas phase. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. observed in solution or in solid state, their expected z 9 Si NMR chemical shifts are computed using theoretical calculations.4 Based on the correlation of 2 9 Si to > 3 C NMR chemical shifts in analogous compounds Olah estimated the 2 9 Si chemical shift of (CH3> 3 Si+ to be in the range of 225 to 275 ppm.5 With the use of advanced calculations (ab initio/IGLO) this value has been corrected to be around 355 ppm.6 The geometry of trivalent trialkylsilyl cations are expected to be planar with C-Si-C angle close to 120 °. Until recently dozens of claims have been made about the preparation and observation of free and stable trivalent silyl cations, mostly by J. B. Lambert.7 Since the 1980s Lambert and coworkers, based on conductance and cryoscopy results, claimed several times that they managed to prepare free silyl cations using perchlorate as a counteranion.7 b 'h In 1993 when independent claims of Lambert7 1 and Reed8 * about trialkylsilyl cations were made, the topic became the most controversial subject in organosilicon chemistry. Later both claims were refuted independently by several investigators.9 Now it is accepted that both silicon species prepared by Lambert and by Reed are silylated arenium and onium ions in which the positive charge is substantially delocalized into the aromatic ring and to bromine, respectively.1 0 The claims about silyl cations in the condensed phase are summarized in Scheme 1.1. Great efforts by many leading physical organic chemists7 ,8 ,1 1 have been made to prepare and observe long lived trialkylsilyl cations in solution. When all these 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. studies are taken into account, there are four factors critical for successful preparation of the silyl cations. CH2C |2 Ph3CH + (AC3H7S)3Si+CI04* (APrS)3SiH + Ph3C CIO4 ----------- - * * (/-C3H7S)3Si-CI0 4 covalent CH3CN P h 3SiH + P h 3C CIO4' ------- P h 3CH + P h 3Si CIO4* P h 3Si-CI0 4 covalent R2MeSiH + Ph3C* CI04' ^ h3— - ph3CH + R2MeSi* CIO4* R2MeSi-CIQ4 covalentj R3SiH + Ph3C* B(C6F5)4- where R = Me, Et, APr, Me3Si C gO e or Toluene Ph3CH + R3S f B(C6F5)4* B(C6F5)4 Silyl aremum ion R3SiH + Ph3C* (Brs-CBuHg)* Where R3 = Et3, APr3, ABu3, ABu2Me Toluene Ph3CH + R3Si* (Br6-CBnH6)‘ R3Si— Br ^BuCHgBrg)* Silyl bromonium ion Scheme l.l. Literature claims about silyl cations in the condensed phase. Actual species are in rectangular forms. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1. The use of low coordinating solvents. 2. The use of anions with very low nucleophilicity (lacking halogen and oxygen nucleophiles). 3. The use of bulky substituents on Si atom to prevent it from external nucleophilic attack (kinetic protection). 4. A special methodology for the initiation of the cation formation. Until recently, most investigators followed “Corey‘s hyride transfer”1 2 reaction that uses triphenylmethyl (trityl) salts as a hydride abstracting agent from silanes (eqn. 1). The reaction, typically carried out in hydrocarbon solvents such as benzene or toluene, generates trivalent silyl cations in situ as well as formation of triphenylmethane because it trades the weaker Si-H bond for the stronger C-H bond. Although silicon generally makes stronger bond than carbon to electronegative elements such as oxygen and halogens, the reverse is true for its bond with hydrogen. Where Ar = C6H5 CF3 c f3 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The saturated C-H bond dissociation energy generally exceeds the Si-H energy by ca. 10 kcal/raol (CH 4 105 and SilLj 91.6 kcal/mol; (CH3 )3 CH 96.4 and (CH3 )3 SiH 90.3 kcal/mol).1 3 Due to kinetic instability of the silyl cation, the in situ generated silyl cation reacts rapidly with any nucleophiles in the system, even with low nucleophilicity solvents. Thus the extreme reactivity of the in situ formed silyl cation prevents it from being observed or isolated as long-lived ion. The in situ generation of trialkylsilyl cations by Corey’s hydride transfer method has been used for the preparation of silylated onium ions.1 4 These types ions will be discussed in subsequent chapters (silylated sulfonium ions in chapter 2; silylated carboxonium ions in chapter 3; silylated acylium ions in chapter 4). Although tetraphenylborate anion is less reactive toward some electrophilic cationic compounds than other widely used counterions (i.e., CKV, BF4 \ PF 6*, SbF6~ etc.), it still suffers from facile degradation and a tendency to n-coordinate through one of its phenyl groups. To solve these problems Kira et a/.1 4 a and Boudjouk et a/.l4 c independently introduced terakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB)1 5 as a low nucleophilicity counteranion of trityl salt. TFPB is indeed a low nucleophilicity and much more stable anion than tetraphenylborate, and soluble in many organic solvents. In spite of low nucleophilicity of TFPB when trityl TFPB was used for the preparation of trialkylsilyl cations by Corey’s hydride transfer reaction, it gave fluoro- and chlorosilanes in weakly coordinating solvents (i.e., CD2CI2) and silylnitrilium ions in strongly coordinating solvent (i.e., butyronitrile). Lambert and coworkers,^ almost at the same time, also reported that use of tetrakis- 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (pentafluorophenyl)borate (TPFPB)1 6 a ,l6 d as a counteranion of trityl salt resulted in no interaction between observed silyl cationic species and the anion. They reasoned that multiple substitution with fluorine atoms rendered the anion much more stable and less nucleophilic than other known anions. In search of low nucleophilicity counteranions Reed et al.% have also examined hexabrominated c/oso-carborane, CBuBr6H6* and hexachlorinated c/oso-carborane, C B n O ^ ' anions. Although there have been numerous claims about preparation of trialkylsilyl cations based on Corey's hydride transfer method7 ,8 , all of them turned out to be erroneous.9 It is safe to state that so far no long-lived trialkylsilyl cations have been observed in solution or solid state. Low nucleophilicity and chemical inertness are also important for the choice of solvents. Schleyer1 7 found out that even rare gases, such as argon and the C-H bonds of alkanes, coordinate with tricoordinated silicon center. This observation seemingly doomed the possibility of generation of free silyl cations in condensed phase because any solvent or anion would necessarily possess some nucleophilicity and consequently would bind with silicon to some extent. Therefore silicon atom has to. be protected sterically to prevent attack of external nucleophile (for example use of mesityl1 8 * '* 1 or duryl1 8 * groups by Lambert et al). However, in such case it is difficult for trityl cation to remove hydride from the silane because the silicon center in the starting material is sterically shielded by the bulky mesityl (or duryl) substituents. To circumvent this problem Lambert and coworkers exchanged hydride for allyl as a reactive site, which is outside the steric sphere of influence of the bulky substituents 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (so called “allyl leaving group approach”). Reaction of the positively charged electrophile E+ with allyl double bond would result in kinetically unstable (3-silyl carbocation which decomposes by expulsion of trimesitylsilyl (or tridurylsilyl) cations. The driving force of this step is the relief of steric strain, i.e., on loss of allyl, the three bulky groups snap into a cage that prevent access of external nucleophiles to central silicon atom. Using this approach Lambert and coworkers successfully managed to prepare long lived trimesitylsilyl cation1 8 a ,l8 d (a first long lived 3 e q .DurBr L Na/_HSiCI3> > Dur3SjC| 2. PCI5 3 eq. MesLi + SiCI4 ► Mes3SiCI M e s a S r^ N ^ (or Dur3Si) MesaSi— (or Dur3Si) Where Mes3Si+ TPFPB (or Dur3Si*) Scheme 1.2. The allyl leaving group approach for the preparation of trimesitylsilyl and tridurylsilyl cations. 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. trialrylsilyl cation) and tridurylsilyl cation1 8 6 in solution as shown in Scheme 1.2. The trimesitylsilyl cation is now generally accepted as the first example of free trivalent silyl cation,1 9 albeit stabilized by good electron donating mesityl (aromatic) substituents. While investigating preparation of trimesitylsilyl cation Lambert also probed various electrophiles for initiation reaction with allyl group and found that trityl cation is relatively inefficient as an electrophile. He achieved better results when he used triethylsilylbezenium ion EtjSi-CCeDe)^ or (5-silylcarbenium ion Et3SiCH2C+ Ph2 .2 0 By using the latter approach a single resonance S2 9 Si 225.5 for the trimesitylsilyl cation was observed. The reaction was carried out at room temperature under nitrogen and the resulting cation was stable for several weeks. The reaction solution always consists of two immiscible layers (Scheme 1.3). The lower layer contains ionic species and the upper layer contains deuterated solvent and apolar byproducts. The lower ionic layer can be considered as a liquid clathrate, which is defined as a “nonstoichiometric liquid inclusion compound” formed upon the interaction of aromatic molecules with salts.2 1 The upper layer containing triphenylmethane was syringed off, and the lower layer containing triethylsilylbezenium ion was used as the actual reagent for subsequent reaction with 1,1-diphenylethylene. Once trimesitylsilyl cation is formed, aromatic solvents are not able to move into the coordination sphere of trimesitylsilyl cation because of the steric constraints. Stronger and smaller nucleophiles such as CD3CN, however, may be able to penetrate the coordination sphere. 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. We embarked on a project to achieve preparation of the first trialkylsilyl cation in solution, tris(l-adamantyl)silyl cation, following the “Allyl Leaving Group Approach” (Scheme 1.4). The electrophile with low nucleophilic counteranion trityl TPFPB is synthesized as a stable yellow solid according to a literature procedure1 6 as shown in Scheme 1.5. removed colorless light brown Ph Ph' 1.3 eq. ph CH2^^ Ph Ph 1.1 eq. ^ ^ \^ S iM e s 3 ■ P h Ph ^ deep green Where Mes = Scheme 1.3. Lambert’s procedure for the generation of trimesitylsilyl cation using EtsSi CH2CEI 1 2 as an electrophile. 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ad3Si- Ad3S r E TPFPB" TPFPB Where Ad E*= Ph3C* Et3 Si~ 7 v 5 ) D ---- Et3s r CPh2 Ad3Si*TPFPB~ Scheme 1.4. Preparation of allyltris(l-adamantyl)silane and its conversion to tri(l-adamantyl)silyl cation. F F 1) nBuLi r 2) BCIa C6F5L i • A Scheme 1.5. The preparation of trityl tetrakis(pentafluorophenyl)borate (Ph3 C+TPFPB). 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.2. Results and Discussion 1.2.1. Attempted preparation of tris(l-adamantyl)silyl cation via 1-adamantyl anion Organosilicon compounds are often prepared by the reaction of organometallic reagents with chloro- or fluorosilanes. Therefore, , we decided to use reaction of 1- adamantyl anion with chloro- or fluorosilanes to prepare tris(l-adamantyl)silyl derivatives. However, soon it became apparent that preparation of silyladamantane derivatives by the adamantly anion approach is difficult and often not reproducible since organometallic derivatives of adamantane are extremely difficult to prepare. There are three possible methods to prepare 1-adamantyl anion according to available literature reports and examples of these methods are illustrated in Scheme 1.6. Method 1. Preparation of l-adamantyllithium (AdLi) by treating 1-adamantyl iodide (Ad-I) and 2 equivalent of f-BuLi at low temperature (-60 ~ -70 °C),2 2 so called metal-halogen exchange method. Since excess of t-BuLi is used to complete metal-halogen exchange, products generally consists of mixture of AdLi adduct and r-BuLi adduct.2 2 * ’ 2 2 0 Method 2. Preparation of AdLi from reaction of l-adamantyl halide (Ad-X, where X = Cl, Br) with Li metal.2 3 This method sometimes uses broken glass in the reaction mixture to enhance the surface of Li metal. li Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Method 3. Barbier reation of Ad-X (X = Cl, Br): One step condensation between Ad-X and a reagent in the presence of a metal (Li or Mg).2 4 Although it is generally accepted that Barbier reaction proceeds via an organometallic compound generated in situ, in some cases such as 1-adamantyl system radical pathway is known to compete with the organometallic pathway.2 4 a ,2 4 b All of our attempts to prepare 1-adamantyl anion according to the literature methods and react it further with nucleophiles failed (see Scheme 1.7). When Ad-I and /-BuLi were used to prepare AdLi (method 1 in Scheme 1.7), they did not yield any coupling reaction with silyl compounds. The major product was adamantane (Ad-H). When we used Li metal to prepare AdLi (method 2 in Scheme 1.7), with or without broken glass, we recovered only the starting material Ad-Br after treating with allyltrichlorosilane. Therefore to verify existence of AdLi in solution in both method 1 and method 2, we trapped it with carbonyl compounds as shown in Scheme 1.7. All reactions of AdLi, prepared in either method 1 or method 2, with carbonyl compounds furnished major product of adamatane and low yield of alcohol adducts. Attempted Barbier reaction of Ad-Br and ketones (i.e., PlfeCO or (CHshCO) in the presence of Li metal (method 3 in Scheme 1.7) also gave their corresponding alcohol adducts in low yields. In conclusion, preparation of silyladamantane derivatives could not be achieved by using 1-adamantyl anion approach because preparation of this bridgehead anion is complicated by problems related to reproducibility and low yield. Kraus et al.2 4 d and Xiao2 5 independently pointed these problems in recent reports. 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Several investigators2 6 independently have claimed that silyladamantanes could be obtained by Wurtz reaction of Ad-Cl or Ad-Br. For example, Roberts2 6 * reported that he synthesized tetrakis(l-adamantyl)silane in low yield by Wurtz coupling reaction of Ad-Cl with tetrachlorosilane using sodium, although he could not prepare trimethylsilyladamantane by the same method (eqn. 2). Our repetition of Robert’s procedure of tetrakis(l-adamantyl)silane gave only recovery of l-adamantyl chloride after reflux in cyclohexane for 12 ~ 18 h. Even when 1-adamantyl iodide was used instead of 1 -adamantyl chloride in the reaction, only the starting material was recovered. Ci + SiCU Na Cyclohexane reflux + (CH3)3SiCI N a / » Cl Cyclohexane reflux 18% (2) Ref. 26(a) We also considered the use of commercialized Rieke® 1-adamantylzinc bromide2 7 * as a source of 1-adamantyl anion. But this method could not be used because of unavailability of the commercial reagent at the time. Other reported Rieke® highly reactive metals2 7 b ,c such as Ca, Mn are also candidates for the source of 1 -adamantyl anion. 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Method 1 2 eq. f-BuLi Ad-I L i + f-Bul AdLi I PhCHO OH OH Ph Ad Ph f-Bu 30 - 40 % 30 . 4 0 % f-BuLi , ^ j | ^ (.C4Hg + Li, Ref. 22(a) .OH 2 eq. t-BuLi Ad-I --------------► [AdLi Et20 , -60 °C ~ ~ j r Rrt22«» 75 % Ad-Ad 2 eq. f-BuLi ^ lAdUI ek^ — * " Y ) - Re,- 22(c) I I I 15% R N X 0 E t R = Ad 14 ° /( OEt R = f-Bu 43 ^ , 2.5 eq. f-BuLi MesSiCI3 A dW W J PhH^redux* Me!^ Jf'Cl! Ref-22(d) „ isolated as white solid 25 % 70 C -> reflux but not characterized Scheme 1.6. Reported methods for preparation and reactions of 1 -adamantyllithium. 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Method 2 Ad-X — -tL---- ► [AdLi] ° 2° . Ad'D Ref. 23(a) x-C I.B r S g f , non-enolizable n u 9 H ? H L i ketone r - r - V I 1 M '° n-CsH12 lA ‘1 L ' 1 Et20 /n -C 5H1 2 J Z ^ / \ d Ph^T ^Ph f-B u 'T N -B u J d2 0 4- w Ad Ad Ad.D 34% 47% 30% 80 % Ref. 23(b) Method 3 9 L j f-Bur-Bu Ad-X [AdLi] ► No Reaction Ei20 , -20 C 7g% Et20, -20 °C Ref. 24(a), (b) c» n OH i *j w i ■ ^ O blzU r I ♦ Ad-Ad + Ad-H ^ Ad -X + L i + y J. f-Bu^N-Bu -2 0 °C f-Bu / f-Bu 19% 35% X = C I, Br Ad 40% n EtO ° 0H ? H Ad-Br + L i -25°C->rt A d ^ A d A d ^ A d A d ^ A d 20o F ‘ OH Et20 ^ J L ' i . Ad' 'Ad ~ ~ ' \*d 50 % 24 % Ref. 24(C) OH O THF A d-B r + Li + A d ' "O C H 3 O Inp I Y X A OCH3 5 0 ^ 1 . Ad^ k Ad * AdA Ad * Adi > Ad 1 6 % ^ / 0 2 7 % /— \ Et2O orTH F / ---\ /OH Ad-Br + Li + ( V = 0 - / \ ___/ Sonication \ / X 2 8 % Ref. 24(d) 7 3 % Scheme 1.6. (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Method 1 Ad-H + Ad-Ad + Ad-f-Bu ( SiCl3 >70% minor 2 eq. t-BuLi A<1'' n-C5H12/B 2o ‘ [AdU1 Ad-H ♦ Ad-Ad * Ad-I-Bu -70 - -60 °C ^ >60% minor HSiCI3 SiF Only Ad-H recovered Ad-H + Ad-Ad + *-BuAdSiF2 34% 12% 23% Ad-I 2 eq. t-BuLi n-CcHio -70 - -60 °C PhCHO Ad-H (75%) + Ph(Ad)CHOH (-20%) [AdLi] (CHafeCO Ad-H (39%) + Ad(CH3)2COH (27%) + Ad-Ad (5%) + Ad-f-Bu (7%) Method 2 ^ v^ /S i C l 3 0nly A d 'B r r e c o v e r e d . ^ « Li Ph2CO Ad-Br— —* [AdLi] ----------- * Ph2AdCOH n-C5H12 . |(CH3)2C O Ad-H Ad-Ad + (CH3)2AdCOH 70 % 9 % 21 % 2 % Method 3 OH Ad-H minor n coarse glass Ad -Br + Li + V -------- » PtT^Ph n * C s H l 2 Ph' I 'Ph Ad 11 % n coarse glass 0H Ad -Br + Li + V --------------------► C H a^ C H a n" CsHl2 CHT/ ^CH3 65% Ad 35% Scheme 1.7. Attempted preparation and reactions of 1-adamantyllithium. Ad-H Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.2.2. Attempted preparation of tris(l-adamantyl)silyl cation via 1,3-dehydroadamantane Hydrosilylation reactions are frequently used to prepare organosilicon compounds. The addition of Si-H bond of silanes across the strained C-C single bond of 1,3-dehydroadamantane (tetracyclo[3.3.3.33, 7 .0l,3]dacane or 1,3-DHA) is a promising method for preparation of silyladamantane derivatives, even though only a few examples have been reported. 2 8 1,3-DHA is one of the most easily accessible highly strained hydrocarbons and possesses an unique cyclopropyl group which readily undergoes addition reaction to form a variety of mono- and disubstituted adamantanes. 2 9 Dehalogenation of 1,3-dibromoadamantane, 3 0 which was prepared in good yields by bromination of adamantane using BBr3, Br2 , and catalytic amounts of AlBr3, with Na/K alloy in dry ether affords 1,3-DHA. Filtration of Na/K alloy under argon atmosphere followed by sublimation at 60 °C in order to avoid decomposition of the product furnished 1,3-DHA as a white solid in 77% yield with ca. 85% purity (the rest 15% is pure adamantane) (see Scheme 1.8). The most unique feature of the structure of 1,3-DHA is that it contains “inverted carbon atoms” 3 1 in a cyclopropyl moiety. The nature of the central bond (C1-C3) is <5 overlap of p orbitals. 2 9 0 J H NMR spectrum of 1,3-DHA (Fig. 1.3) reveals the unique properties of the unusual bond. The chemical shift difference of the geminal C4 hydrogen is quite large due to their proximity to the central bond. Since H u> is close 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to the plane of anisotropic cyclopropyl ring, it is in the deshielded zone with respect to the p-orbital extending out of the ring, while H 4 a is in a zone above the plane and therefore it is less affected.2 9 ” Introduction of an especially strained cyclopropane ring into a homoadamantane ring system also resulted in chemical shifts to lower field for carbon atoms situated across from the cyclopropyl group. Although cyclopropane carbon chemical shifts are usually at higher field than in corresponding ring opened compounds,3 2 the cyclopropyl Ci and C2 of 1,3-DHA are deshielded by 8.8 and 11.6 ppm, respectively, compared to those of adamantane.3 3 More remarkable are the remote effect of the 1,3-bonding in 1,3-DHA; the bridgehead Cs is deshielded by 25.9 ppm compared to the corresponding carbon of adamantane. It seems there is a through space interaction of n-like orbitals of the cyclopropyl group with the carbon atom situated directly across from the ring (Cs). Similar low field 1 3 C NMR chemical shifts of long range effect was reported in 2,4-dehydroadamantane.3 4 These low field chemical shifts may reflect the increased contribution of p-character in these highly strained ring systems. 6 1,3-DHA 10 NC CN-derivative of 1,3-DHA 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A single crystal structure of cyano-derivative of 1,3-DHA (1- cyanotetracyclo[3.3.1.13 ,7 .03 ,7 ]dacane) provided details of the unusual geometry at the “inverted” bridgehead carbon atoms.3 5 The central bond length (C3-C7, 1.643 A) agrees well with theoretically calculated value for tricyclo[l,l,l,01 ,3 ]pentane (1.60 A), but the side bonds (C3-C1 0, 1.493 A; C7-C1 0, 1.476 A) appear to be somewhat shorter than the theoretical value (1.53 A). The side bonds lengths correspond closely with the value expected for C sp 2-C sp 3 bonds, 1.515 A, giving some support for the assumption of sp2 hybridization at the bridgehead, with pa overlap for the central bond. 1,3-DHA is known to be reasonably thermally stable provided moisture and oxygen are excluded.2 9 ,,b The thermal stability and reactivity of 1,3-DHA could be explained by the following; The most strained bond in the three-membered ring is the central bond. The cleavage of this bond would lead to relief of ring strain. However, because of the rigid structure, the bridgehead orbitals cannot move far from each other, and will therefore always have significant overlap. Only the attack of free radicals or ions from out side of the ring could change the situation and make the cleavage of the central bond an easy process. Because of the extreme strain of the three-membered ring and the nature of the two inverted carbon atoms, 1,3-DHA is very reactive to many reagents via both free radical and ionic mechanisms. It can also undergo hydrosilylation under chloroplatinic acid catalyst.2 8 8 Radical reaction of 1,3-DHA will be discussed in detail in next section, synthesis of l,3-bis(N,N- difluoroamino)adamantane. 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Upon addition of diiodosilane to cooled (0 °C) pentane solution of 1,3-DHA an exothermic reaction ensued forming light sensitive di(l-adamantyl)diiodosilane (Scheme 1.8). Reduction of di(l-adamantyl)diiodosilane with UAIH4 afforded di(l- adamantyl)silane as a white air stable solid. Unfortunately, attempted hydrosilylation of di(l-adamantyl)silane with another 1,3-DHA did not lead to the formation of tri(l- adamantyl)silane, but gave recovery of di(l-adamantyl)silane. Even under various reaction conditions, including a sealed tube reaction for extended reaction time and presence of different catalysts, only decomposed products of 1,3-DHA and di(l- adamantyl)silane were observed by GC-MS spectrometry. Attempted adamatylation of di(l-adamantyl)diiodosilane with AdLi-Et20, prepared by Method I in Scheme 1.6., also failed. Only decomposed products of Ad2Sil2 and adamantane were observed as analyzed by GC-MS spectrometry. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Br2 (excess) B B r3 [AlBrJ 1.5h, reflux 1 , - p *-v 2. AllyiLi ■ / 84% 54% CH2 Na/K Et2 0 7 7 % 0.5 6 Q. H 2 Sil 2 pentane 0*C Reflux LiAIH T H F I - / 76% J AdLi-Et20 \ of Method 1 in Schem e 6 Scheme 1.8. Attempted synthesis of allyltris(l-adamantyl)silane via 1,3-dehydroadamantane. 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.2.3. Further application of 1,3-dehydroadamantane: Synthesis of l,3-bis(N,N,-difluoroamino)adamantane Propellanes containing three membered rings have been of considerable interest because of their different reactivity from that of other cyclopropane derivatives.3 1 Besides they show unusually large interatomic distances between the two “inverted carbons”, their electronic environments seem to be remarkably different from those of normal cyclopropanes. It has been reported that the hybridization at the bridgehead carbon of propellanes is close to sp2 and central bond is formed by a overlap of p orbitals.3 6 1 AcOH X2 OAc (3) X = I. COCH3 l,3-dehydroadamantane2 9 a ’ b 1 and 1,1,1-propellane3 7 2 are one of the most easily accessible highly strained hydrocarbons among reported propellanes and they serve 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. as versatile precursors for the synthesis of 1 -substituted and 1,3-disubstituted derivatives as shown in eqn 3. The propellane 2 has been prepared by reacting 1,1- dibromo-2 ,2 -bis(chloromethyl)cyclopropane and methyllithium,3 7 c and has been known to be relatively thermally stable undergoing a variety of reactions, such as free radical addition3 8 and polymerization. 3 9 Similarly one step synthesis from 1,3- dibromoadamantane and Na-K. alloy in ether furnishes 1 with moderate thermal stability. Although it was described in Pincock’s original report2 9 * that the weak 1,3- bond of 1 readily undergoes both free radical and electrophilic attack, reactions of 1 with free radicals have been relatively unexplored in relation to 2 . Tetrafluorohydrazine N2F4 has been widely used for the synthesis of both organic and inorganic compounds containing the difluoroamino group. 4 0 Due to its low N-N bond strength, ca. 20 kcal/mol, N2F4 actually exist in facile equilibrium between difluoroamino radical. 4 1 Therefore, most of the reported reactions of tetrafluorohydrazine are typical free radical reactions of the difluoroamino radical. For example addition of the NF2 radical to various hydrocarbons such as olefins, 4 2 acetylenes, 4 3 anthracene4 4 has yielded their corresponding 1,2- or 1,4- bis(difluoroamino) compounds. To our best knowledge, however, direct synthesis of 1,3-disubstituted difluoroamino compound is unprecedented. Here we report synthesis of l,3-bis(N,N-difluoroamino)adamantane by radical reaction of 1,3- dehydroadamantane and tetrafluorohydrazine. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NF- 3 Na-K alloy 1 Initially, when we treated 1 with tetrafluorohydrazine in pentane solution either at 0 °C or at room temperature under argon, we observed formation of l,3-bis(N,N- difluoroamino)adamantane 3 and other complex mixtures. We reasoned that high reactivity of I toward tetrafluorohydrazine under the reaction conditions induced less selectivity, so we decided to decrease the reactivity of 1 by employing lower temperature. Thus, when tetrafluorohydrazine was bubbled through pentane solution of 1,3-dehydroadamanetane at -78 °C under argon atmosphere, l,3-bis(N,N- diflouroamino)adamantane 3 was formed exclusively in 83 % yield (eqn. 4). After concentration in vacuo the product 3 was isolated by flash column chromatography on neutral alumina (n-hexane as eluent) and characterized by GC-MS, FT-IR, and nuclear magnetic resonance spectroscopy. The 1 9 F NMR spectrum of 3 (Fig. 1.15) showed a single resonance at 22.2 ppm, indicative of the tertiary difluoroamino group. The FT-IR spectrum of 3 (Fig. 1.16) also exhibited strong N-F absorption at 938, 872, 823 cm'1 . The *H NMR spectrum of 3 (Fig. 1.13) consisted of peaks at 8 2.47, 2.10, 1.89, and 1.68 representing hydrogens on C2, C5, C4 , and C6, respectively. The relative area ratio was 1:1:4:1 as required for the structure of 3. I3 C NMR 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. spectrum (Fig. 1.14) clearly support structure of 3; triplet at 70.6 ppm for both Ci and C3 (Jc-f = 7.5 Hz), singlet at 34.8 ppm for C6, triplet at 34.6 ppm for C4 (Jc-f - 6.7 Hz), pentet at 33.1 ppm for C2 (Jc-f = 8.5 Hz), and singlet at 28.4 ppm for Cs- Unfortunately, attempted reaction of tetrafluorohydrazine with 2 in ether to synthesize its 1,3-difluoroamino derivative of 1,1,1-propellane resulted in violent explosion. In summary, first example of 1,3-disubstituted difluoroamino compound 3 is prepared by radical reaction of tetrafluorohydrazine and 1,3-dehydroadamantane. All spectroscopic data support the structure 3. 1.3. Conclusions Preparation of first example of trialkylsilyl cation tris(l-adamantyl)silyl cation via 1-adamantyl anion approach could not be achieved because preparation of this bridgehead anion is complicated by problems related to reproducibility and low yield. Preparation of tris(l-adamantyl)silyl derivatives via 1,3-dehydroadamantane resulted in partial success giving synthesis of di(l-adamantyl)silyl derivatives. Attempted incorporation of third adamantly group failed. As an extension of study on 1,3-dehydroadamantane, reaction with tetrafluorohydrazine was investigated. The addition of difluoroamino radical of tetrafluorohydrazine to the strained bond in the 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. three-membered ring of 1,3-dehydroadamantane produced first example of 1,3- disubstitutued difluoroamino compound l,3-bis(N,N-difluoroamino)adamantane. 1.4. Experimental procedure General Instruments: The 'H NMR spectra were recorded on a 300 MHz Varian Unity 300 NMR spectrometer equipped with a variable temperature probe. Ail chemical shifts are reported relative to internal TMS at 8 0.0 or to the signal of a residual protonated solvent: CDCU 5 7.27 or C6D6 8 7.15. The l3C NMR spectra were recorded at 59.6 MHz, and chemical shifts were reported relative to internal TMS at 5 0.0 or to the l3 C signal of solvent: CDCI3 8 77.2 or 8 128.0. Signals in NMR spectra are described as: s, singlet; d, doublet; t, triplet; q, quartet; p, pentet; m, multiplet; br, broad. Coupling constants (J) are given in Hertz (Hz). Infrared spectra (IR) were recorded on a Perkin Elmer FT-IR spectrometer 2000. Melting points (m.p.) were determinded on Mel-Temp II capillary melting point apparatus and were uncorrected. GC-MS spectra were obtained on Hewlett Packard 5890 Gas Chromatograph connected to Hewlett Packard 5971 Mass Selective Detector at 70 eV. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of 1-adamantyl bromide (1.075 g, 5 mmol) was added to the flask using a syringe and the solution was stirred for 90 min at 35 °C. This AdLi solution was later used for the subsequent reactions in Method 2 of Scheme 6, but no desired product was obtained in any of the cases. 1 Attempted Barbier reaction2 4 6 : Pieces of broken glass, pentane (10 mL), and lithium powder (20 mmol) were placed in a dry 100 mL Schlenk flask. The flask was then cooled to -30 °C. Afler vigorous stirring of the reaction mixture for 10 min, pentane solution (10 mL) of 1-adamantyl bromide (5 mmol) and ketone (acetone or benzophenone) (5 mmol) were added to the reaction mixture using a syringe over 20 min, while the medium continued to be vigorously stirred. After 15 min the solution was poured into ice water (100 mL) and extracted with Et20. The organic phase was neutralized, dried with sodium sulfate, and subjected to GC-MS spectrometric analysis to determine the percentage of the various components. Attempted Wurtz reaction2 6 * : Cyclohexane (5 mL) and sodium (2.3 g, 100 mmol) were placed in a dry 100 mL Schlenk flask. Cyclohexane solution (30 mL) of 1-adamantyl chloride (7.68 g, 45 mmol) and SiCL (2.55 g, 15 mmol) were introduced by a syringe through a rubber septum. The rubber septum was replaced to a reflux condenser and the solution was refluxed at 75 °C for 18 h with vigorous stirring. The reaction mixture was Altered and the filtrate was subjected to GC-MS 28 permission of the copyright owner. Further reproduction prohibited without permission. spectrometric analysis. GC-MS analysis indicated the presence of only starting material, 1-adamantyl chloride. Synthesis of 1-adamantyl iodide (Ad-I)4 5 : 1-Adamantanol (5 g, 33 mmol) and 47% HI solution (75 mL) were placed in a 200 mL round-bottomed flask equipped with a reflux condenser. The solution was refluxed at 90 °C for 1 h in a closed system, poured into water (150 mL), and extracted with EtaO (2 x 200 mL). The organic phase was washed with aq. NaHCOa and water, and dried over MgS0 4 . Filtration and concentration in vacuo afforded a crude product. Recrystallization from ethanol (100 mL) gave pure 1-adamantyl iodide as white solid. Yield 50 %. m.p. 73-74 °C. Synthesis of 1,3-dibromoadamantane3 0 : To a dry 250 mL three-necked round- bottomed flask equipped with a reflux condenser were added Br2 (50 mL) and BBr3 (2.5 mL, 25 mmol) at 0 °C using external ice bath under argon atmosphere. A few milligrams of AlBr3 was then added to the flask at 0 °C under argon. Adamantane (13.6g, 100 mmol) was added portion wise to the flask under blanket of argon at 0 °C. (Caution: HBr evolves immediately!) The reaction medium was gradually warmed up to 55 °C and refluxed for 90 min. CCLt (100 mL) was added and excess Br2 was decomposed by adding NaHSCh. (Caution: ice need to be added intermittently to cool the solution!) The aqueous layer was extracted with CCL (2 x 100 mL) and combined organic CCL layers were washed with aq. Na2C0 3 . 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Evaporation of CCU and recrystallization from /i-hexane (200 mL) afforded the 1,3- dibromoadamantane as white crystals (24.56 g). Yield: 84 %. m.p. 108-110 °C (sealed tube). Lit3 0 112 °C. 'H NMR (CDC13 ) (Fig. l.l) 5 1.70 (s, 2H), 2.26 (s, 2H), 2.30 (s, 2H), 2.87 (s, 2H); ,3C NMR(CDC13 ) (Fig. 1.2) 8 33.5, 35.0, 46.9, 58.9, 62.1. Synthesis of l,3-dehydroadamantane2 5 ,2 9 b : To a dry 500 mL three-necked round-bottomed flask were placed a magnetic stirring bar and dry Et20 (400 mL) under argon. Cut sodium (1.25 g) and potassium (6.25 g) fine pieces in degassed hexane were introduced into the flask under argon. Et20 solution (100 mL) of 1,3- dibromoadamantane (15 g, 5 mmol) was added to the flask and the solution was vigorously stirred for 12 h under argon at room temperature. The left over Na-K alloy was filtered over celite in a glass frit under argon. Evaporation of Et20 from the filtrate under vacuum gave crude product as off-white solid. Sublimation at 60 °C (1 mm) furnished 1,3-dehydroadamantane as white solid with ca. 85 % purity (5.86 g, 44 mmol). Yield; 77 %. *H NMR (CgDg) (Fig. 1.3) 8 1.14-1.17 (d, J = 10.5 Hz, 4H, H 4a), 1.66 (s, 2H, H2 ), 1.89-1.93 (d, J = 9.6 Hz, 4H, &,„), 2.05 (s, 2H, H«), 2.73 (s, 2H, Hs); l3C NMR(C6D6 ) (Fig. 1.4) 8 37.2, 37.5,45.9,49.5, 54.4. Synthesis of di(l-adamantyl)diiodosilane (Ad2 Sil2)2 5 ,2 8 b : hi a argon filled glove box, 1,3-dehydroadamantane (5.3 g, 85 % purity, 31 mmol) was loaded into a dry 100 mL Schlenk flask. The flask was brought out from the glove box and cooled 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in an ice bath. Pentane (SO mL) and diiodosilane (4.25 g, 15 mmol) were added through a rubber septum by syringe. With the rubber septum replaced by a reflux condenser equipped with an inlet tube for an argon atmosphere, the reaction mixture was refluxed for 2 h. During the reaction time, the flask was covered with aluminum foil to avoid photo-decomposition of the product. The white solid product was isolated by filtration using glass frit under argon atmosphere and was washed in pentane and dried under vacuum, giving 6.303 g of white solid. This crude product was used for next reaction without further purification. The product should be kept in cool, dry, dark place. Yield; 76 %. lH NMR (CgDe) (Fig. 1.5) 8 1.47-1.58 (q, 12H, CH2 in Ad), 1.73 (s, 6 H, CH in Ad), 2.02 (d, 12H, CH2 in Ad); ,3C NMR (CfiDg) (Fig. 1.6) 5 22.7,28.1,37.0,38.6. Synthesis of di(l-adamantyl)silane (Ad2SiH2 )2 5 ,2 8 b : In a argon filled glove box, di(l-adamantyl)diiodosilane (1.49 g, 2.71 mmol) and LiAlftt (1.0 g, 26mmol) were loaded into a dry 100 mL Schlenk flask. The flask was brought out from the glove box and THF (40 mL) was added through a rubber septum. The reaction mixture was stirred for 2.5 h at room temperature and excess LiAlH* was destroyed with ice water. Extraction with n-hexane (3 x 50 mL), dry over MgS04 , and evaporation of the solvent gave white solid of di(l-adamantyl)silane. Recrystallization from n- hexane gave pure Ad2SiH2 (0.43 g). Yield; 54%. m.p. 135 °C. lH NMR (CDCI3) (Fig. 1.7) 5 1.73 (s, 12H, CH2 in Ad), 1.81 (s, 6 H, CH in Ad), 1.89 (s, 12H, CH2 in 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. for 10 min at -78 °C before it was gradually warmed to room temperature (1 h). The precipitate was allowed to settle (1 h) and the pentane solution of tris(pentafluorophenyl)boron was siphoned out of the reaction flask by means of double tripped needle under argon pressure. Removal of solvent under reduced pressure gave B(C6Fs) 3 as sticky off-white solid (8.55 g, 16.7 mmol). Yield; 84 %. This sticky crude product was used for next reaction without further purification or characterization. Synthesis of lithium tetrakis(pentafluorophenyl)borate (Li* B(C«Fs)4* )l6b,l6c: Bromopentafluorobenzene (4.94 g, 20 mmol) was added to a dry 500 mL Schlenk flask. The starting material was diluted with Et20 (100 mL) and the resulting solution was cooled to -78 °C. Over 20 min, n-BuLi was added as a 1.6 M solution in hexanes (12.5 mL, 20 mmol). A white solid formed quickly, and the reaction mixture was allowed to stir for an additional 10 min. B(C6Fs) 3 (8.55 g, 16.7 mmol) in pentane (150 mL) was added to the flask at -78 °C and the resulting off-white suspension was slowly wanned to room temperature (1 h). The precipitate was allowed to settle (1 h) and the majority of liquid solution was siphoned out the reaction flask by means of double tipped needle. Filtration of the precipitate over glass frit and drying in vacuo gave Li* B(C6Fs)4 * as off-white solid (3.14 g, 4.58 mmol). Yield; 27 %. (This reaction gave lower yield than that reported in the literaturel6 b because starting material B(C6F$)3 was not pure. Considering two steps the overall yield was similar to that of literature 84 % x 27 % = 23 %; litl6 b 50 % x 33 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vigorous stirring. The reaction mixture was maintained at -78 °C for 15 min and then stirred at -5 °C for 1 h. The volatile liquid was transferred to another 50 mL Schlenk flask by static vacuum distillation. Based on iodine consumption of this solution3 7 9 , the yield of 1 ,1 , 1 -propellane is between 73 and 91 %. This pentane/ether solution of 1 , 1 , 1 -propellane was used for subsequent reaction with tetrafluorohydrazine. Synthesis of l,3-bis(N,N-difluoroamuio)adainantane: 1,3-Dehydroadamantane (78 mg, 85 % purity, 5.0 mmol) and pentane (25 mL) were added to a dry 100 mL Schlenk flask under argon. The solution was cooled to -78 °C using dry ice / acetone bath and tetrafluorohydrazine ( 2 0 mmol) was gently bubbled through the solution for 15 min under argon atmosphere. The reaction mixture was slowly warmed to room temperature and argon was bubbled for 1 0 min to remove unreacted tetrafluorohydrazine in the solution. Concentration of the solution in vacuo gave crude product as white slurry. Purification by flash column chromatography using n- haxane as eluent on neutral alumina afforded l,3-bis(N,N,difluoroamino)adamantane as a white solid (98 mg, 83 %). m.p. 76-78 °C; lH NMR (CDCI3 , TMS) (Fig. 1.13) 8 1 . 6 8 (t, 2H), 1.89 (d, 8 H), 2.10 (s, 2H), 2.47 (br. s, 2H); I3 C NMR (CDCI3 , TMS) (Fig. 1.14) 8 28.44 (d), 33.06 (p, JC -f 8.5 Hz, a), 34.61 (t, JC -f 6.7 Hz, c), 34.76 (e), 70.63 (t, Jc-f 7.5 Hz, b); I9 F NMR (CDC13 , CFCI3) (Fig. 1.15) 8 22.1; MS (70 eV) (Fig. 1.16) m I z (relative intensity) 238 (M+, 7), 186 (M*-NF2 , 100), 135 (Ad+ , 67); FT-IR (KBr) (Fig. 1.17) 823, 872, 938, 1013,1357, 1461,2932 cm*1 . 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1 .1 . ' H N M R sp ectru m o f 1,3-dibromoadamantane. 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1 .2 , l3 C N M R sp ectru m o f 1,3-dibromoadamantane. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. n CO * n < D CM m 4 0 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1 .4 . ,JC N M R sp ectru m o f 1,3-dehydroadamantane. 5 Q . a > * 'C Q . E n C M a o “ e g O d * i e g o '-8 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1 .6 . ,3 C N M R sp ectru m o f Ad2 Sil2. 75 (A © CM m CM C O 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1.7. 'H N M R spectrum o f Ad2SiH2. m CM 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1 .8 . ,3 C N M R sp ectru m o f AdzSiHj, Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Abundance 1 5 0 0 0 0 0 - Scan 1209 (16.661 m in): CB0713A1.D 1400000 1300000 1200000 1 1 0 0 0 0 0 - 300 1000000 9 0 0 0 0 0 - 800000 700000 - 600000 500000 400000 - 255 107 55 164 m 100000 1?9 187 1 9 9 0 5 219227 244 iiH-ni m m nriTi Timi r iVl’ i I i i i i t n 149 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 M/Z -> Figure 1.9. Mass spectrum of AdzSif^. • . u » U S' + u .c CL L . o E E 8 S- x © E 3) IE 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CO 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1.11. ,3C N M R sp ectru m o f PhjC+ B(C6 F 5) 4'. li. I L . U. u. © 00 CO Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1.12.1 9 F N M R spectrum o f PiiiC* B(C6Fs)4 *99'I m ’ © o ’ w lA 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1.13. ‘H N M R sp ectru m o f l,3-bis(N,N-difluoroamino)adamantane. a .CM Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1.14, l3 C N M R sp ectru m o f l,3-bis(N,N-difluoroamino)adamantane. u T * tM M 5 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1.15. ,9F N M R sp ectru m o f l,3-bis(N,N-difluoroamino)adamantane. u. o rt O o e m o o m a s i n m o u > o o o o m m C 5 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1.16. M ass sp ectru m o f l,3-bis(N,N-difluoroamino)adamantane. 2 0 5 J A O O <n O « in O m O a g o 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 1.17. F T -IR sp ectru m o f l,3-bis(N,N-difluoroamino)adamantane. 1.5. References 1. For reviews about silyl cations, see a) Corriu, R. J. P.; Henner, M. J. Organomet. Chem. 1974, 74, 1. b) Lambert, J. B.; Kania, L.; Zhang, S. Chem. Rev. 1995,95, 1191. 2. a) Weber, W. P.; Felix, R. A.; Willard, A. K. Tetrahedrom Lett. 1970, 907. b) Shim, S. K.; Beauchamp, J. L. J. Am. Chem. Soc. 1989, 111, 900. c) Cecchi, P.; Crestoni, M. E.; Grandinetti, F.; Vinciguerra, V. Angew. Chem. Int. Ed. Engl. 1996, 35,2522. 3. For a review, see Stable Carbocation Chemistry, G. K. S. Prakash and G. A. Olah Eds.; John Wiley & Sons, New York, 1996. 4. For theoretical calculations about Si+ cations, see a) Apeloig, Y.; Schleyer, P. v. R. Tetrahedron Lett. 1977, 4647. b) Apeloig, Y. Godleski, S. A.; Heathcock, D. J.; McKelvey, J. M. Tetrahedron Lett. 1981, 3297. c) Godleski, S. A.; Heathcock, D. J.; McKelvey, J. M. Tetrahedron Lett. 1982, 4453. d) Truong, T.; Gordon, M. S.; Boudjouk, P. Organometallics 1984, 3, 484. e) Olsson, L.; Ottosson, C. -H.; Cremer, D. J. Am. Chem. Soc. 1995, 117,7460. f) Arshadi, M.; Johnels, D.; Edlund, U.; Ottosson, C. -H.; Cremer, D. J. Am. Chem. Soc. 1996, 118, 5120. g) Ottosson, C. -H.; Cremer, D. Organometallics 1996, 15, 5309. h)Ottosson, C. -H.; Cremer, D. Organometallics 1996, 15, 5495. i) Kraka, E.; Sosa, C. P.; Grafenstein, J.; Cremer, D. Chem. Phys. Lett. 1997, 279, 9. j) Ottosson, C. -H.; Szabo, K. J.; Cremer, D. Organometallics 1997, 16,2377. 5. Olah, G, A. Organometallics 1982, 1, 1485. 6. Olah, G. A.; Rasul, G.; Heiliger, L.; Bausch, J.; Prakash, G. K. S. J. Am. Chem. Soc. 1992,114,7737. 7. a) Lambert, J. B.; Zhang, S.; Stem, C. L.; Huffman J. C. Science 1993, 260, 1917. b) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun. 1993, 383. c) Lambert, J. B.; Kania, L.; Schilf, W.; McConnell J. A. Organometallics 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1991, 10, 2578. d) Lambert, J. B.; Schulz Jr., W. J.; McConnell, J. A.; Schilf, W. J. Am. Chem. Soc. 1988, 110, 2201. e) Lambert, J. B.; Schilf, W. J. Am. Chem. Soc. 1988, 110, 6364. f) Lambert, J. B.; McConnell, J. A.; Schilf, W.; Schulz Jr., W. J. J. Chem. Soc. Chem. Commun. 1988, 455. g) Lambert, J. B.; McConnell, J. A.; Schulz Jr., W. J. J. Am. Chem. Soc. 1986, 108, 2482. h) Lambert, J. B.; Schulz Jr., W. J. J. Am. Chem. Soc. 1983, 105, 1671. i) Lambert, J. B.; Sun, H. -N. J. Am. Chem. Soc. 1976, 98, 5611. 8. a) Xie, Z.; Manning, J.; Reed, R. W.; Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am. Chem. Soc. 1996, 118, 2922. b) Xie, Z.; Bau, R.; Benesi, A.; Reed, C. A. Organometallics 1995, 14, 3933. c) Xie, Z.; Bau, R.; Reed, C. A. J. Chem. Soc. Chem. Commun. 1994, 2519. d) Xie, Z.; Liston, D. J.; Jelinek, T.; Mitro, V.; Bau, R.; Reed, C. A. J. Chem. Soc. Chem. Commun. 1993, 384. e) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402. f) Reeds, C. A. Acc. Chem. Res. 1998, 31,325. 9. a) Pauling, L. Science 1994, 263, 983. b) Olah, G. A.; Rasul, G.; Li, X. -Y.; Buchholz, H. A; Sandford, G.; Prakash, G. K. S. Science 1994, 263, 983. c) Schleyer, P. v. R.; Buzek, P.; Muller, T.; Apeloig, Y.; Siehl, H. -U. Angew. Chem. Int. Ed. Engl. 1993, 32 ,1471. d) Olsson, L.; Cremer, D. Chem. Phys. Lett. 1993,215,433. 10. Olah, G. A.; Rasul, G.; Prakash, G. K. S. J. Organomet. Chem. 1996, 521, 271. 11. a) Olah, G. A.; Mo, Y. K. J. Am. Chem. Soc. 1971, 93, 4942. b) Prakash, G. K. S.; Keyaniyan, S.; Aniszfeld, R.; Heiliger, L.; Olah, G. A.; Stevens, R. C.; Choi, H. -K.; Bau, R. J. Am. Chem. Soc. 1987, 109, 5123. c) Olah, G. A.; Heiliger, L; Li, X. -Y.; Prakash, G. K. S. J. Am. Chem. Soc. 1990, 112, 5991. d) Olah, G. A.; Rasul, G.; Heiliger, L.; Bausch, J.; Prakash, G. K. S. J. Am. Chem. Soc. 1992,114, 7737. 12. a) Corey, J. Y. J. Am. Chem. Soc. 1975, 97, 3237. b) Corey, J. Y.; West, R. J. Am. Chem. Soc. 1963, 85, 2430. 13. a) Walsh, R. In The Chemistry o f Organic Silicon Compounds; Patai, S. Rappoport, Z. Eds.; John Wiley & Sons; Chichester, 1989; Part I, pp 371- 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 391. b) Brinkman, E. A.; Salomon, K.; Tumas, W.; Brauman, J. I. J. Am. Chem. Soc. 1995, 117,4905. 14. a) Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114, 6697. b) Kira, M.; Hino, T,; Sakurai, H. Chem. Lett. 1993, 153. c) Bahr, S. R.; Boudjouk. P. J. J. Am. Chem. Soc. 1993, 115, 4514. d) Olah, G. A.; Li, X-Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1995,117, 8692. 15. Nishida, H.; Takada, N.; Yoshimura, M.; Sonoda, T.; Kobayashi, H. Bull. Chem. Soc. Jpn. 1984, 57, 2600. 16. a) Chien, J. C. W.; Tsai, W. -M.; Ransch, M. D. J. Am. Chem. Soc. 1991, 113, 8570. b) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245. c) Stehling, U. M.; Stein, K. M.; Kesti, M. R.; Waymouth, R. M. Macromolecules 1998, 31, 2019. d) Toskas, G.; Moreau, M.; Masure, M.; Sigwalt, P. Macromolecules 2001, 34,4730. 17. C. Maerker, J. Kapp, P. v. R. Schleyer hi Organosilicon Chemistry II; N. Auner, J. Weiss., Eds.; VCH, Weinheim Germany, 1996, pp 329. 18. a) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B. J. Am. Chem. Soc. 1999, 121, 5001. b) Lambert, J. B.; Stem, C. L.; Zhao, Y.; Tse, W. C.; Shawl, C. E.; Lentz, K. T.; Kania, L. J. Organomet. Chem. 1998, 568, 21. c) Muller, T.; Zhao, Y.; Lambert, J. B. Organometallics 1998, 17, 278. d) Lambert, J. B.; Zhao, Y. Angew. Chem. Int. Ed. Engl. 1997, 36, 400. e) Lambert, J. B.; Lin, L. J. Org. Chem. in press. 19. a) Schleyer, P. v. R. Science 1997, 275, 39. b) Belzner, J. Angew. Chem. Int. Ed. Engl. 1997, 36,1277. 20. Lambert, J. B.; Zhao, Y. J. Am. Chem. Soc. 1996, 118, 7867. 21. J.L. Atwood In Coordination Chemistry o f Aluminum; G. H. Robinson Ed.; VCH, New York, 1993, pp 197. 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22. a) Lansbury, P. T.; Sidler, J. D. Chem. Commun. 1965, 373. b) Wieringa, J. H.; Strafing, J.; Wynberg, H. Synth. Commun. 1972, 2, 191. c) Groth, U. Huhn, T.; Porsch, B.; Schmeck, C.; Schollokopf, U. Liebigs Ann. Chem. 1993, 715. d) Shepherd, B. D.; Powell, D. R.; West, R. Organometallics 1989, 8, 2664 23. a) Molle, G; Dubois, J. E.; Bauer, P. Synth. Commun. 1978, 8, 39. b) Molle, G.; Dubois, J. E.; Bauer, P. Tetrahedron Lett. 1978, 3177. c) Molle, G.; Bauer, P.; Dubois, J. E. J. Org. Chem. 1982, 47, 4120. d) Molle, G.; Bauer, P.; Dubois, J. E. J. Org. Chem. 1983, 48, 2975. 24. a) Bauer, P.; Molle, G. Tetrahedron Lett. 1978, 4853. b) Molle, G.; Bauer, P. J. Am. Chem. Soc. 1982, 104, 3481. c) Lomas, J. S. Nouveau Journal De Chimie 1984, 8, 365. d) Kraus, G. A.; Siclovan, T. M. J. Org. Chem. 1994, 59,922. 25. Manchao Xiao, Ph. D. thesis, Washington University, 1988, pp 68. 26. a) Roberts, R. M. G. J. Organomet. Chem. 1973, 63, 159. b) Sakurai, H.; Kondo, F. J. Organomet. Chem. 1975, 92, C46. 27. a) Hanson, M. V.; Brown, J. D.; Rieke, R. D.; Niu, Q. J. Tetrahedron Lett. 1994, 7205. b) Guijarro, A.; Rosenberg, D. M.; Rieke, R. D. J. Am. Chem. Soc. 1999, 121, 4155. c) Kim, S. -H.; Hanson, M. V.; Rieke, R. D. Tetrahedron Lett. 1996, 2197. d) Wu, T. -C.; Xiong, H.; Rieke, R. D. J. Org. Chem. 1990, 55, 5045. e) Rieke, R. D.; Hanson, M. V. Tetrahedron 1997, 53, 1925. f) Rieke, R. D. Aldrichimica Acta 2000, 33, No. 2,52. 28. a) No, B. I.; Son, V. V.; Belyakova, T. V.; Ushchenko, V. P.; Kulikova, N. I. J. Gen. Chem. U.S.S.R. 1982, 52, 1904.; Zhumal Obshchei Khimii 1982, 52, 2138. b) Pae, D. H.; Xiao, M.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc. 1991, 113,1281. 29. a) Pincock, R. E.; Torupka, E. J. J. Am. Chem. Soc. 1969, 91, 4593. b) Pincock, R. E.; Schmidt, J.; Scott, W. B.; Torupka, E. J. Can. J. Chem. 1972, 50, 3958. c) Wiberg, K. B.; connon, H. A.; Pratt, W. E. J. Am. Chem. Soc. 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1979, 101, 6970. d) Kogay, B. E.; Sokolenko, W. A. Tetrahedron Lett. 1983, 613. e) Fokin, A. A.; Gunchenko, P. A.; Yaroshinsky, A. I.; Yurchenko, A. G.; Krasutsky, P. A. Tetrahedron Lett. 1995,4479. 30. Baughman, G. L. J. Org. Chem. 1964, 29, 238. 31. For review, see Wiberg, K. B. Acc. Chem. Res. 1984, 17, 379. 32. Levy, G. C.; Nelson, G. L. Carbon-13 Nuclear Magnetic Resonance for Organic Chemists, Wiley-Interscience, New York, 1972, pp 24,44, 57. 33. Pincock, R. E.; Fung, F. -N. Tetrahedron Lett. 1980, 19. 34. Geluk, H. W.; de Boer, Th. J. Tetrahedron 1972,28, 3351. 35. Gibbons, C. S.; Trotter, J. Can. J. Chem. 1973, 51, 87. 36. a) Stohrer, W. D.; Hoffmann, R. J. Am. Chem. Soc. 1972, 94, 779. b) Newton, M. D.; Schulman, J. M. J. Am. Chem. Soc. 1972, 94, 773,4391. 37. a) Wiberg, K. B.; Walker, F. H. J. Am. Chem. Soc. 1982, 104, 5239. b) Semmler, K.; Szeimies, G.; Belzner, J. J. Am. Chem. Soc. 1985, 107,6410. c) Lynch, K. M.; Dailey, W. P. Org. Synth. Coll. Vol. 75, 98. 38. a) Siberg, K. B.; Waddell, S. T.; Laidig, K. Tetrahedron Lett. 1986, 27, 1553. b) Wiberg, K. B.; Waddell, S. T. J. Am. Chem. Soc. 1990,112, 2194. 39. a) Kaszynski, P.; Michl, J. J. Am. Chem. Soc. 1988, 110, 5225. b) Schlttter, A. -D. Angew. Chem. Int. Ed. Engl. 1988, 27,296. 40. a) Petty, R. C.; Freeman, J. P. J. Am. Chem. Soc. 1961, 83, 3912. b) Ruff, J. K. Chem. Rev. 1967, 67, 665. 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41. Johnson, F. A.; Colburn, C. B. J. Am. Chem. Soc. 1961, 83, 3043. 42. Petry, R. C.; Freeman, J. P. J. Org. Chem. 1967,32,4034. 43. a) Petry, R. C.; Parker, C. O.; Johnson, F. A.; Stevens, T. E.; Freeman, J. P. J. Org. Chem. 1967, 32, 1534. b) Sausen, G. N.; Logothetis, A. L. J. Org. Chem. 1967, 32,2261. 44. Logothetis, A. L. J. Org. Chem. 1966, 31, 3686. 45. Schleyer, P. v. R.; Nicholas, R. D. J. Am. Chem. Soc. 1961, 83,2700. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 2. Silylated Sulfonium Ions 2.1. Introduction Sulfonium salts containing a tricoordinated sulfur atom attached to three carbon atoms have been well known since the early 1900. Trialkylsulfonium ions are more stable than the corresponding trialkyloxonium ions and can be easily prepared by the alkylation of saturated dialkyl sulfides under mild conditions because of sufficient nucleophilicity of the sulfur atom.1 Sulfonium salts and the ylides derived from them have become increasingly useful in organic synthesis2 and more recently sulfonium ions have found specific industrial applications as cationic initiators for polymerization.3 Although there has been increasing interest in the chemistry of heterosulfonium salts in which one or more of the ligand is a heteroatom (halogen, sulfur, oxygen, or nitrogen), silyl-substituted sulfonium ions are rare and their structures have not been confirmed.4 Hydride transfer of organohydrosilane to trityl cations, known as Corey hydride transfer,5 has been well adopted for the production of silyl cation6 and synthesis of silylated onium ions.7 Olah and Prakash previously reported the preparation and NMR spectroscopic characterization of long lived trisilyloxonium ions.7 c Now we would like to report the preparation and NMR spectroscopic (lH, 1 3 C, 2 9 Si) characterization of long-lived trisilylsulfonium and methyldisilylsulfonium ions. We 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. have also calculated the structure and geometry of the sulfonium ions using density functional theory (DFT) method. l3C and 2 9 Si NMR chemical shifts of the ions were also computed using the IGLO method and the results were compared with the experimental data. 2.2. Results and Discussion 2.2.1. Synthesis of Trisilylsulfonium Ion and NMR studies In contrast to studies on trisilyloxonium ion reported by Olah and Prakash,7 c when the hydride abstraction from trimethylsilane by trityl TPFPB was carried out in the presence 3-5 equiv. of hexamethyldisilathiane 3 at -78 °C, trisilylsulfonium was not formed. In a separate experiment, it was shown that hexamethyldisilathiane undergoes fast exchange with trityl TPFPB in CD2CI2 even at -78 °C as shown by lH, 1 3 C, and 2 9 Si NMR. To avoid this exchange, hexamethyldisilathiane and trityl TPFPB were used in equimolar amount, so that all hexamethyldisilathiane could react with the in situ generated trimethylsilyl cation. Thus when trityl TPFPB in CD2CI2 solution was added to a mixture of trimethylsilane and 1 equiv. of hexamethyldisilathiane at -78 °C under argon in a NMR tube, successful formation of tris(trimethylsilyl)sulfonium ion 1 was observed (eqn. 1). 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Me3Si—S— R + Me3SiH + Ph3C* BfCgFs)^ C D 2C I2 3 R = SiMe3 4 R = CH3 -78°C 4 R = CH + (Me3Si)2~S-R + Ph3CH (1) B(C8F5)4* 1 R = SiMe3 2 R = CH3 lH NMR spectrum recorded at -78 °C (Fig. 2.1) shows a peak at 8 0.75 corresponding to methyl groups of tris(trimethylsilyl)sulfonium ion. l3C NMR (Fig. 2.3) and 2 9 Si NMR (Fig. 2.5) resonances for the trisilylsulfonium ion 1 appeared at 2.9 and 38.8 ppm, respectively (see Table 2.1). The 2 9 Si NMR shift of 1 is deshielded by 26 ppm compared to that of progenitor, hexamethyldisilathiane (S2 9 Si 13). Even though the chemical shifts cannot be directly related to positive charge density, the results qualitatively indicate that the positive charge of the trisilylsulfonium ion is delocalized both on S and Si atoms. To examine the stability of 1 in the solution, we further recorded the NMR spectra of the trisilylsulfonium ion at different temperatures. The NMR spectra of the trisilylsulfonium ion reaction mixture did not change below -60 °C. However, the signals of 1 and small amount of remaining hexamethyldisilathiane coalesced on raising temperature above -60 °C (Figure 2.2). This exchange was clearly indicated by the observed averaged peaks in lH, l3C (Fig. 2.4), and 2 9 Si NMR (Fig. 2.6) at room temperature; the resonances were 0.68, 3.4, and 8.7 ppm, respectively. It is interesting to note that this exchange process is 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thermally reversible and a sharp singlet resonance at 8 38.8 was obtained in the 2 9 Si NMR spectrum upon cooling the solution back to -78 °C. We treated trimethylsilyl trifluoromethanesulfonate (TMSOTf) with hexamethyldisilathiane in CD2CI2 to study the effect other low nucleophilicity counter anions. However, we observed only the starting materials by 'H, 1 3 C, and 2 9 Si NMR with no evidence for the formation of trisilylsulfonium ion. This result can be explained by the strength of the stronger Si-O bond in TMSOTf compared to Si-S bond in 1. Table 2.1. NMR Chemical Shifts of Some Silylsulfonium Ions and their Precursors0 Compounds 5lH (SiCH3 ) exp. 8l3C (SiCH3 ) exp. calcd. 82 9 Si exp. calcd. Me3 SiH 0. 11 -2.6 -2.2 -16.3 -16.3 (MeaSi^S 3 0.33 4.1 3.7 12.9 15.3 (Me3 Si)3 S+ 1 0.75 2.9 3.0 38.8 42.9 Me3 SiSCH3 4 0.30 0.3 0.3 15.9 16.0 (Me3 Si)2 S+ CH3 2 0.68 -0.2 -0.3 40.8 46.8 0 Experimental and calculated chemical shifts are referenced to TMS Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.2.2. Synthesis of Methyldisilylsulfonium Ion and NMR Studies Similarly, methylthiotrimethylsilane 4 and trimethylsilane in the presence of I equiv. of trityl TPFPB formed the corresponding methylbis(trimethylsilyl)sulfonium ion 2, whose NMR data are also shown in Table 2.1. Solution of ion 2 was found to be more stable than that of ion 1 and 'H, i3C, and 2 9 Si NMR chemical shifts of 2 did not show any temperature dependence up to room temperature. The *H NMR chemical shifts of 2 (Fig. 2.7) at 8 0.68 (Si-CH3> and 8 2.37 (S-CH3) were found deshielded compared to the progenitor, methylthiotrimethylsilane at 8 0.30 (Si-CH3) and 8 2.02 (S-CH3). The 1 3 C NMR resonances at -0.2 (Si-CH3) and 10.8 ppm (S- CH3) also confirm the formation of 2 (Fig. 2.8). 2 9 Si NMR chemical shift of 2 was found be at 40.8 ppm (Fig. 2.9). Attempted preparation of 2 either by reacting methylthiotrimethylsilane and TMSOTf or by reacting hexamethyldisilathiane and methyl iodide did not take place; only starting materials were observed in both cases. Independent reaction of hexamethyldisilathiane and methyl triflouromethanesulfonate (MeOTF) in either CD2CI2 or CD3CN resulted in trimethylsulfonium triflate and TMSOTF quantitatively (eqn. 2). The products were identified by lH, l3C, and 2 9 Si NMR spectroscopy. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c h 3-o s o 2c f 3 CD2CI2 or + Me3S i-0 S 0 2CF3 + (CH3>3S*CF3S 0 3* (2) Me3Si—S -R 3 R = SiMe3 4 R= CH3 c d3 c n 1H 5 = 0.49 ppm 2.79 ppm 13C 8 = 0.3 ppm 27.4 ppm 29Si 8= 45 ppm 2.2.3. Attempted Synthesis of Dimethylsilylsulfonium Ion When dimethylsulfide was allowed to react with trimethylsilane in the presence of 1 equiv. of trityl TPFPB in CD2 C 12 at -78 °C, we observed a white precipitate that was characterized as trimethylsulfonium ion and triphenylmethane by NMR spectroscopy. Even under a variety of conditions no evidence for the formation of dimethylsilylsulfonium ion was found. In CD3 CN solvent, in the above experiment at -40 °C, the NMR data showed only the formation of trimethylsulfonium ion (lH 5 2.79, l3C 8 27.4 ppm) and triphenylmethane as well as few unidentified silyl compounds. Attempts to prepare dimethylsilylsulfonium ion by reacting thiomethyltrimethyl- silane 4 and MeOTF in either CD2 Cl2 or CD3CN also resulted in TMSOTf and trimethylsulfonium ion. The formation of trimethylsulfonium ion instead of dimethylsilylsulfonium ion is presumably due to displacement of Me3 Si group in 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. initially formed dimethylsilylsulfonium ion to produce more stable trimethylsulfonium ion. 8 Since the sulfonium ions are known to disproportionate, the dimethylsilylsulfonium ion appears to desilylate and react with another molecule of MeOTf giving trimethylsulfonium ion in all cases. Abel et al. have claimed that a mixture of /i-butylthiotrimethylsilane and methyl iodide upon standing in the dark for several days resulted in w-butylmethyl- (trimethylsilyl)sulfonium iodide as a fine white precipitate. 9 The evidence for the formation of silylsulfonium ion was based on poor elemental analysis data. We have repeated the work of Abel et al. by reacting methylthiotrimethylsilane and methyl iodide in either CD2CI2 or CD3CN. The precipitation did occur in both solvents after two days, however, no evidence for the formation of dimethylsilylsulfonium ion was obtained by t3C, and 2 9 Si NMR spectroscopy. The white precipitate was found to be only the trimethylsulfonium ion ('H 5 2.73, I3 C 8 27.4). Recently Franek and El- Sayed reported the first preparation of dialkylsilylsulfonium ions by reacting ethyl- or propylthiotrimethylsilane and Meerwein's trimethyloxonium salt. The claim for the purported silylsulfonium ions was based on the 'H NMR data. 1 0 Repetition of this work by treating ethylthiotrimethylsilane and trimethyloxonium tetrafluoroborate in CD3NO2 at -10 °C gave almost identical reported lH NMR resonances ( 8 0.22, 1.50, 2.91, 3.34) with a product yield of 50 %.u However, lH NMR integration and l3C NMR data (50 b s 8.9, 24.7, 38.9) suggest that the observed species is actually ethyldimethylsulfonium tetrafluoroborate (eqn. 3). The presence 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of trimethylfluorosilane1 2 was also confirmed by 2 9 Si and l9 F NMR spectra (lit.1 3 2 9 Si, 5 32.9 in acetone-d^ relative to internal TMS, doublet, J = 274 Hz; 1 9 F, 8 -156 relative to internal CFCh). Trimethylfluorosilane is produced by the reaction of initially formed ethylmethyl(trimethylsilyl)sulfonium ion with the F ions of BF4* moiety. Since ethylthiotrimethylsilane requires 2 equiv. of (CH3> 3 0 + BF4' to form the ethyldimethylsulfonium ion (eqn. 3), when both reagents were mixed in 1:1 ratio, only half of the sulfide reacted. This result is consistent with the Franek and El- Sayed's work wherein roughly 56 % yield of the product was observed. When a similar silylonium ion, trimethylsilylnitrilium tetrafluoroborate (Me3SiNCCH3+ BF4*) was reported by Wenkert et al.lA Bassindale refuted their claim,1 3 concluding that the reported preparation of silylnitrilium tetrafluoroborate actually gave trimethylfluorosilane and boron trifluoride coordinated acetonitrile complex. Me3 Si—S-CH2CH3 + (CH 3)3 0+ BF4* . bf4- Me3 Si—S-CH2CH3 CH3 (3) BF4 * h3 c - s - ch2 ch3 ch3 (CH3)3 0 + bf4- H3 C-S-CH2 CH3 + Me3SiF 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.2.4. Density Functional Theory (DFT)/ IGLO studies To substantiate the observed experimental results we have calculated the structures and l3C and 2 9 Si NMR chemical shifts of tris(trimethylsily)sulfonium 1 and methylbis(trimethylsilyl)sulfonium 2 as well as dimethyl(trimethylsilyl)- sulfonium 5 ions. The structures were fully optimized at the density functional theory (DFT) B3LYP/6-31G* level. The minimum energy structure of tris(trimethylsilyl)sulfonium ion was found to be of C3 symmetry, as shown in Figure 2.10, with Si-S bond distance of 2.291 A. The sulfur atom in 1 is modestly pyramidal. The pyramidalization level is about 16 degrees, expressed as the out-of plane bending angle of the central sulfur atom relative to the plane defined by its three bonding partners. Optimized structures of methylbis(trimethylsilyl)sulfonium and dimethyl(trimethylsilyl)- sulfonium ions were found to be of Q symmetry and Cs symmetry, respectively, with pyramidalization levels of 19 and 21 degrees (Figure 2.10). The relative stability of trisilylsulfonium and trisilyloxonium ions were compared by using the following isodesmic reaction (eqn. 4). Trimethylsilyl group transfer from silylated sulfonium ion (Me3Si)3S+ 1 to (Me3Si) 2 0 giving silylated oxonium ion (Me3Si)3 0 * and (Me3Si)2S was computed to be endothennic by 11.8 kcal/mol. This again indicates that trialkylsulfonium ions are more stable than corresponding trialkyloxonium ions, hi comparison, for methyl group transfer from methylated 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sulfonium ion (CH3 )3 S+ to (CH3 )20 giving methylated oxonium (CH3 )3 0 + and (CH3 )2 S was also calculated to be endothermic by 16.4 kcal/mol (eqn. 5). (Me3Si)3S* + (Me3Si)20 ► (Me3Si)30 + + ( M e ^ S (4) (CH3)3S* + (CHgfeO ► (CH3)30 + + (CHafeS (5) We have also computed the 1 3 C and 2 9 Si NMR chemical shifts of 1 and 2 at the IGLO II// B3LYP/6-31G* level with reasonable degree of accuracy (Table 2.1). The calculated S 2 9 Si of 1 is 42.9 ppm, agrees well with the experimental value of 38.8 ppm. The calculated 6 U C of 1 is 3.0 ppm, also very close to the experimental value of 2.9 ppm. The calculated average 5 2 9 Si of 2 is 46.8 ppm, 6.0 ppm more deshielded than the experimental value of 40.8 ppm. On the other hand, the calculated average 5 1 3 C of Si-CH3 in 2 is -0.3 ppm, close to the experimental value of -0.2 ppm. The 8 2 9 Si of 5 was computed to be 54.4 ppm. 2.3. Conclusions The first silylsulfonium ions, tris(trimethylsilyl)sulfonium and methylbis- (trimethylsilyl)sulfonium ions, were prepared as long-lived species and unequivocally characterized by jH, 1 3 C, and 2 9 Si NMR spectroscopy. The strong 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. tendency for formation of more stable trimethylsulfonium ion in solution has prevented successful synthesis of dimethylsilylsulfonium ion. The structural parameters and chemical shifts of the ions were also computed by DFT/IGLO methods. The calculated results agree reasonably well with the experimental data. 2.4. Experimental Section General Instruments: The lH NMR spectra were recorded on a 300 MHz Varian Unity 300 NMR spectrometer equipped with a variable temperature probe. All chemical shifts are reported relative to external TMS at 5 0.0 or to the signal of a residual protonated solvent: CD2CI2 at 8 5.37. The 1 3 C NMR spectra were recorded at 59.6 MHz, and chemical shifts were reported relative to external TMS at 5 0.0 or to the 1 3 C signal of the solvent: CD2CI2 at 8 53.8. The 2 9 Si NMR spectra were recorded at 59.6 MHz, and chemical shifts were reported relative to external TMS at 8 0.0. Signals in NMR spectra are described as in experimental section of chapter 1. Materials: Trimethylsilane (Gelest), hexamethyldisilathiane, methylthiotri- methylsilane, CD2CI2 (Aldrich) are commercially available and were used as received. Trityl tetrakis(pentafluorophenyl)borate (Ph3C+ TPFPB) was prepared according to a literature method.1 5 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Tris(trimethylsilyl)sulfonium Ion. Trimethylsilane (0.1 mL, ca. 0.8 mmol) was condensed in a 5mm J. Young NMR tube and cooled to -78 °C in a dry ice/acetone bath. Hexamethyldisilathiane (0.24 mmol) and trityl TPFPB (0.23 mmol) in CD2CI2 (0.75 mL) were added subsequently to the NMR tube. The NMR tube sealed off at - 78 °C and the NMR spectra of the sample were recorded at -78 °C (Fig. 2.1. !H NMR spectrum at -78 °C; Fig. 2.2. 'H NMR spectrum at variable temperatures; Fig. 2.3. 1 3 C NMR spectrum at -78 °C; Fig. 2.4. ,3C NMR spectrum at room temperature; Fig. 2.5. 2 9 Si NMR spectrum at -78 °C; Fig. 2.6. 2 9 Si NMR spectrum at room temperature). Methylbis(trimethylsilyl)sulfonium Ion. By using a similar procedure as described above, the reaction of trimethylsilane (0.1 mL) and methylthiotrimethylsilane (0.17 mmol) in the presence of 1 equiv. of trityl TPFPB (0.17 mmol) in CD2CI2 at -78 °C gave a solution of methylbis(trimethylsilyl)sulfonium TPFPB with triphenylmethane (Fig. 2.7. !H NMR spectrum at -78 °C; Fig. 2.8. 1 3 C NMR spectrum at -78 °C; Fig. 2.9. 2 9 Si spectrum at -78 °C NMR). Calculation Methods, Basis Set, and Geometry. Calculations were carried out with the Gaussian 98 program system.1 6 The geometry optimizations were performed using the DFT1 7 method at the B3LYP1 8 /6-31G* levels.1 9 Vibrational frequencies at the B3LYP/6-31G*//B3LYP/6-3lG* level were used to characterize stationary 71 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. points as minima and to evaluate zero point vibrational energies (ZPE) which were scaled by a factor of 0.96. Isodesmic energies were calculated at the B3LYP/6- 31G*//B3LYP/6-31G* + ZPE level. UC and 2 9 Si NMR calculations were performed according to the reported method using IGLO programs2 0 at the IGLO 1 1 ’ levels using B3LYP/6-31G* geometries. Huzinaga2 1 Gaussian lobes were used as follows; Basis II” : Si, 11s 7p 2d contracted to [5111111, 211111, 11], d exponent = 1.4 and 0.35; C, 0 :9 s 5p Id contracted to [51111,2111, I], d exponent: 1.0, H, 3s contracted to [21]. The I3 C and 2 9 Si NMR chemical shifts were referenced to TMS (calculated absolute shift i.e 8(Si) = 380.6 and 8(C) = 196.4) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. m a a . 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.1. * H N M R sp ectru m o f tris(trimethylsilyl)sulfonium io n a t -7 8 °C. -78 °C •I IN -40 °C JU L -20 °C 15 °C Figure 2.2.'H NMR spectrum of tris(trimethylsilyl)suIfonium ion at variable temperature. 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ph3CH ( O 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2.3.1 3 C N M R sp ectru m o f tris(trimethylsilyl)sulfonium io n a t -7 8 °C, C/J 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 2.5.2 9 S i N M R sp ectru m o f tris(trimethylsilyl)sulfonium io n a t -7 8 °C. C O ■}! H I* 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 2.7. ' H N M R sp ectru m o f methylbis(trimethylsilyl)sulfonium ion. 80 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (CH3)3 SiS+ (CH3 )25 (Cs) Figure 2.10. (continued) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.5. References 1. Olah G. A.; Laali, K. K.; Wang, Q.; Prakash, G. K. S. Onium Ions, John Wiley & Sons, New York, 1998. 2. Stirling, C. J. M. Ed. The Chemistry o f Sulphonium Group, in The Chemistry o f Functional Groups, Patai, S. Series Ed., Wiley, New York, 1981, Part 1 and 2. 3. (a) Sundell, P.-E.; Jonsson, S.; Jult, A. J. Polym. Sci. Polym. Chem. 1991, 29, 1535. (b) Hamazu, F; Akashi, S.; Koizumi, T.; Takata, T; Endo, T. J. Polym. Sci. Polym. Chem. 1991, 29, 1845. (c) Decker C.;Le Xuam, H.; Nguyen Thi Viet, T. J. Polym. Sci. Polym. Chem. 1995, 33, 2759. (d) Crivello, J. V. Adv. Polym. Sci., 1984,62,1. 4. Although there have been two reports on the preparation of silylsulfonium, ions, their claims are not true. See ref. 9 and 10. 5. (a) Corey, J. Y. J. Am. Chem. Soc. 1975, 97, 3237. (b) Corey, J. Y.; West R. J. Am. Chem. Soc. 1963, 85, 2430. 6. Lambert, J. B.; Kania, L.; Zhang, S. Chem. Rev. 1995,95, 1191. 7. (a) Olah, G. A.; Rasul, G.; Prakash, G. K. S. J. Organomet. Chem. 1996, 521, 271. (b) Prakash, G. K. S.; Wang, Q.; Rasul, G.; Olah, G. A. J. Organomet. Chem. 1998, 550,119. (c) Olah, G. A.; Li, X. -Y.; Wang, Q.; Rasul, G; Prakash, G. K. S. J. Am. Chem. Soc. 1995, 117, 8962. (d) Kira, M; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992,114, 6697. 8. Ray, F. E.; Levine, I. J. Org. Chem. 1937, 2,267. 9. Abel, E. W.; Aimitage, D. A.; Bush, R. P. J. Chem. Soc. 1964,2455. 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10. Franek, W.; El-Sayed, I. Sulfur Lett. 1996, 19, 181. 11. Reported 'H NMR resonances (CDCI3) are 8 0.18, 1.44, 2.85, 3.30. Yield 56%. 12. Observed Me3 SiF has 'H 8 0.22 (d, J = 7.8 Hz), ,3C 8 -0.1 (J = 15.1 Hz), 2 9 Si 8 34.3 (J = 273 Hz), ,9F 8 = -156 ppm. 13. Bassindale, A. R.; Stout, T. Tetrahedron Lett. 1984, 25, 1631. 14. Caputo, R.; Ferreri, C.; Palumbo, G.; Wenkert, E. Tetraheron Lett. 1984, 25, 577. 15. Chien, J. C. W.; Tsai, W. -M.; Ransch, M. D. J. Am. Chem. Soc. 1991, 113, 8570. 16. Gaussian 98 (Revision A.5), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, R. E.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; M. Head-Gordon, M.; Pople, J. A., Gaussian, Inc., Pittsburgh PA, 1998. 17. Ziegler, T. Chem. Rev. 1991, 91, 651. 18. Becke's Three Parameter Hybrid Method Using the LYP Correlation Functional: Becke, A. D. J. Chem. Phys. 1993, 98,5648. 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C hapter 3. Silylated Carboxooium Ions 3.1. Introduction Carboxonium la and oxonium lb ions are important intermediates in many acid catalyzed reactions.1 However, unlike saturated oxonium ions positive charge of carboxonium ions can delocalize into the neighboring carbon atom through resonance interaction. Therefore, ions la exhibit both carboxonium and carbenium ion character and they are sometimes called oxocarbenium ions. Long-lived protonated carboxonium (la, R=H) ions can be readily obtained by protonation with strong acids and characterized by various physical methods. In some cases, even crystal structures have been determined. Among all the physical methods, NMR spectroscopy has been used most extensively to study the structures of the carboxonium ions. Alkylated carboxonium ions (la, R-alkyl) are typically prepared by alkylation of carbonyl compounds using alkyl triflates, fluorosulfates, or fluoroalkane/SbFs. \ © © R la R = H, alkyl, silyl lb 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Silylated carboxonium ions (la, R=silyl) have not been well studied because of paucity of electrophilic silylation methods on carbonyl compounds under appropriate conditions. The first purported preparation of trialkylsilyl carboxonium ion has been achieved by reaction of trimethylsilyl trifluoromethanesulfonate with p,p'- bis(dimethylamino)-benzophenone. 2 The gas-phase protonation of 2-(trimethyl- siloxy)propene using ion cyclotron resonance spectroscopy has been reported by Hendewerk and coworkers. 3 They suggested protonation of the silyl enol ether occurs at carbon and the C-protonated ion is structurally identical with the long-lived complex formed in collisions of trimethylsilyl cation with acetone. Only recently, a more detailed study of silylcarboxonium ions has been examined by NMR spectroscopy. Kira and coworkers described characterization, based on NMR study, of trimethylsiloxydiphenylcarbenium ion that was prepared using hydride transfer method by reacting trimethylsilane and trityl cation in the presence of benzophenone. 4 This method of in situ generation of trialkylsilyl cation in solution by hydride transfer of organohydrosilane to trityl cation (Corey hydride transfer5 ), has also been successfully used to silylate a number of organic compounds and to prepare silylated onium ions. 6 hi a previous study, 7 Olah and Prakash have reported 1 3 C and 2 9 Si NMR study of silylcarboxonium ions prepared as long-lived ions by the reaction of the corresponding esters with trimethylsilane and triphenylmethyl tetrakis- (pentafluorophenyl)borate (trityl TPFPB, Ph3C+ BCCgFs)^. Now as a part of my thesis work we felt it of interest to extend these studies by undertaking a systematic 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NMR study of a variety of silylated carbonyl compounds such as ketones, enones, carbonates, amides, and urea. To rationalize the experimental results, we have also calculated the structure of model trimethylsilylated carboxonium ions using density functional theory (DFT) method. 1 3 C and 2 9 Si NMR chemical shifts of the ions were also computed using the IGLO method and the results were compared with the experimental data. The silylated carboxonium ions 2a - 6 a were prepared as stable ions by silylation of the corresponding carbonyl compounds with triethylsilane and trityl TPFPB at low temperature in dichloromethane solution as illustrated in eqn. 1 . X \ PhjCB^eFsV \ © ,C = 0 , C = O X B / CD2C1 2 / -78 °C 2a, 3a, 4a, 5a, 6 a Et3 SiH ® © :C = O X B(C6F5 ) 4 + Ph3 CH (1) Y © :OX 5 6 CHa 2a X = SiEt3 2b X = H 3a X = SiEt3 3bX = H c h 3o © :OX CH3 © (CH3)2N Ch3° 4aX = SiEt3 OX (CH3)2N 5aX = SiEt3 5b X = H (CH3)2N 6 a X = SiEt3 4b X = H 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2. Results and Discussion 3.2.1. Silylated Ketones The 1 3 C and 2 9 Si NMR spectra of silylated ions 2a and 3a generated at -78 °C are summarized in Table 3.1. For comparison ,3C NMR data of related protonated ketones 2b and 3b as well as their precursors are grouped in Table 3.1. Upon triethylsilylation ,3C NMR chemical shift of carbonyl carbon of 2a (8 ,3C 226.0) was found to be deshielded by 28 ppm from that of acetophenone (Fig. 3.1). The corresponding carbonyl carbon chemical shift of protonated acetophenone 2 b was reported at 8 l3C 219.6.8 1 * The carbon chemical shift of CH3 in 2a, however, is only slightly deshielded. The adjacent phenyl group also helps to stabilize the carbocation A 14 center. Similar behavior was also observed for protonated ion 2b. Although C NMR chemical shifts cannot be directly related to positive charge density, they do reflect the charge density at carbons of similar hybridization and substitution. 9 The 2 9 Si NMR signal of 2a (Fig. 3.3) was located at S2 9 Si 59.3, deshielded by 59.1 ppm compared to that of triethylsilane. Both 2 9 Si and carbonyl carbon 1 3 C NMR resonances of 2 a are considerably more deshielded than those of reported trimethylsiloxydiphenylcarbenium ion (8 2 9 Si 52.3 and SI3C 208.3),4 prepared by trimethylsilylation of benzophenone. This is easily rationalized by the presence of two phenyl groups that delocalize the positive charge from the neighboring 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. carbenium ion center in silylated benzophenone. Although two distinct isomers of 2a were expected due to the restricted rotation around C-0 bond, only one isomer was observed from our 2 9 Si and 1 3 C NMR studies. Protonation of acetophenone also leads to observation of only one isomer. 8 Protonated a,3-unsaturated ketone 3b has been prepared as a stable cation in Magic Acid and studied by ‘H and ,3C NMR spectroscopy. 1 0 ,1 1 Olah and coworkers reported, 10 based on NMR studies, that the site of protonation of 2-cyclohexen-l-one was carbonyl oxygen atom and the resulting positive charge of the ion was distributed between the carbonyl oxygen atom and the allylic carbons, i.e. Ci and C3. Forsyth and coworkers also prepared 3b as stable ion and even identified syn and anti isomers of the ion based on ,3C NMR spectroscopy. 11 In our study 1 3 C NMR spectrum of 3a (Fig. 3.4) indicates that silylation of the a,(3-unsaturated ketone results in similar charge distribution to that of 3b (see Table 3.1). 1 3 C NMR spectrum of 3a shows that both Ci and C3 are significantly deshielded compared with those of the parent compound. The deshielding caused by silylation is larger for C3 (A5 1 3 C 34.5) than Ci (A51 3 C 17.7). Difference in 1 3 C NMR resonance of C2 is 3.2 ppm and as anticipated it bears very little positive charge. The 2 9 Si NMR signal of 3a (S2 9 Si 50.5) (Fig. 3.6) is very close to that of silylated benzophenone. 4 These results clearly show the extensive charge delocalization of 3a among Ct, C3, and Si atoms. In previous investigations of protonated a , 3 -unsaturated ketones, only an average structure (due to rapid proton exchange) of 3b was detected in FSO3H/SO2CIF,1 1 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. while both syn and anti isomers of 3b were observed in stronger acid medium FSO3H-SbF5(l:l)/SO 2ClF.l0,n From our NMR study only one isomer of 3a was indicated under the reaction conditions. Our attempts to silylate benzaldehyde as well as cyclohexanone under similar conditions were not successful leading to a number of triethylsilyated products. 3.2.2. Silylated Dimethyl Carbonate Silylation of dimethyl carbonate in CD2CI2 at -78 °C afforded 4a, whose lH NMR spectrum showed two peaks at 6!H 4.38 and 4.31 representing the two non equivalent methoxy groups (Fig. 3.7). The non-equivalence of the two methoxy groups at low temperature is due to restricted rotation about CH3O-C bond of 4a on the NMR time scale. This type of restricted rotation was also found in protonated dimethyl carbonate 4b (8lH 4.52 and 4.40 at -80 °C).1 2 The 1 3 C NMR chemical shift of carbonyl carbon of 4a measured at -78 °C (Fig. 3.8) was located at 81 3 C 160.5 (see Table 3.2). This low field of carbon chemical shift of carbonyl group of 4a is in good agreement with those of related ions, CH30C(0H)2+ (8l3C 161.8) and C(OH)3+ (5 I3 C 164.5).1 2 A single l3C NMR peak for two non-equivalent methoxy groups of 4a (8l3C 63.2) appears to be fortuitous. 2 9 Si NMR spectrum of 4a (Fig. 3.9) displayed a single resonance at S2 9 Si 54.7. 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Table 3.1. ,3C and 2 9 Si NMR chemical shifts of silylated ketones and their precursors0 ,6 Cmpd ,3r '-^catbonyl *-CH3 C»rom 2,Si 2a 226.0 (230.4/ 29.3 (25.6/ 144.9,128.7,128.3,125.9 59.3 (56.5/ 2br f 219.6 27.0 147.1,132.6,132.6,130.6 0 • PhCCH3 198.0 26.5 137.1,133.1,128.4,128.2 I3f> l) p D p D p D p D p ^ Cl C2 C3 C4 C5 ^ C6 3a 31/ o 50.5 217.4 126.9 185.2 28.3 21.2 36.4 (57.0/ (232.4/ (124.9/ (205.5/ (28.2/ (22.1/ (36.8/ 217.2 124.7 183.1 27.4 20.3 33.2 199.7 130.1 150.7 25.8 22.8 38.2 " l3 C and 2 9 Si NMR chemical shifts are in ppm relative to external tetramethylsilane. b In CD2CI2 at -78 °C, unless otherwise indicated. c Calculated values of trimetl Nishimura, J. J. Am. Chem. Soc. 1974, 96, 3548. e DeMember, J. R. J. Am. Chem. Soc. 1985,107,818. otherwise indicated. c Calculated values of trimethylsilylated ion in parenthesis. d Olah, G. A. Westerman, P. W . Nishimura, J. J. Am. Chem. Soc. 1974, 96, 3548. e In CDCI3 at room temperature.f Forsyth, D. A. Osterman, V. M. S O u > Table 3 .2 .1 3 C and 2 9 Si NMR chemical shifts of 4a, 5a, 6a, and their precursors"’ 6 Compounds ,3 Cc a r t > o n y i I3 Co- c h j o r n - chi 1 3 Cc- c u j 2 9 Si 4a 160.5 (172.4)* 63.3(63.3)* 54.7(66.0)* Dimethyl carbonate*1 156.6 54.9 5a major isomer 5a minor isomer 174.7(190.9)* 173.5 37.5,39.7 (37.8,38.4)* 40.0,41.8 19.7 (21.3)* 20.5 40.7 (49.7)* N,N,-Dimethylacetamider f 171.0 35.1,38.0 21.5 6a 164.6(177.0)* 39.5 (39.1)* 39.8 (44.9)* Tetramethylurear f 165.3 38.6 " ,3 C and 2 9 Si NMR chemical shifts are in ppm relative to external tetramethylsilane. b In CD2CI2 at -78 °C, unless otherwise indicated.* Calculated values of trimethylsilylated ion in parenthesis. d In CDCI3 at room temperature.* Average calculated value. 3.2.3. Silylated N,N-Dimethylacetamide Two methyl groups on nitrogen of N,N-dimethyIacetamide have different environments because of hindered rotation about the OC-NMe2 bond, due to its partial double bond character. Thus, ‘H and 1 3 C NMR spectra of pure N,N- dimethylacetamide show two distinct peaks of equal area (8lH 3.03, 2.94 and S,3C 38.0, 35.1) for N(CHj) 2 in addition to a peak at higher field of OC-CHj (S'H 2.08 and 81 3 C 21.5). Although monoprotonation of N,N-dimethylformamide, either in 100% sulfuric acid1 3 or in fluorosulphuric acid,1 4 has been known to give two non equivalent signals of N(CH3 ) 2 in 'H NMR at room temperature, monoprotonated N,N-dimethylacetamide under the above conditions apparently gave a single peak for N(CH3 ) 2 group (S'H 3.45 ppm) even at -80 °C, and this was attributed to a very small chemical shift difference between the two methyl groups.1 3 Silylation of N,N-dimethylacetamide at -78 °C produced two distinct isomers of 5a, one major and one minor based on relative peak heights, as indicated by 'H and l3C NMR spectroscopy. The lH NMR spectrum (Fig. 3.10) of the major isomer recorded at -78 °C consists of a peak at 8*H 2.25 representing OC-CHj and two peaks at S'H 3.07, 3.13 representing two non-equivalent methyl groups of N(CH3 )2 - The corresponding peaks for minor isomer are S’H 2.46, 3.32, and 3.28, respectively. The l3 C NMR spectrum (Fig. 3.11) of each isomers of 5a consists of four peaks. The peaks at Sl3C 174.7 (major) and 173.5 (minor) are assigned to carbonyl carbon. The 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. peaks at 5l3 C 39.5, 37.1 (major) and 5l3 C 41.4, 39.5 (minor) are assigned for two distinct non-equivalent methyl groups on nitrogen, due to restricted rotation about OC-N(CH3 ) 2 bond. Methyl group of CH3-CO appeared at 8 1 3 C 19.3 (major) and 20.1 (minor). Interestingly, it is found that 5a exhibits only one discernible signal in 2 9 Si NMR (S2 9 Si 40.7) (Fig. 3.12). We reasoned that extremely low receptivity of 2 9 Si resulted in observation cf silicon atom of only major isomer. 3.2.4. Silylated Tetramethylurea The lH NMR spectra of tetramethylurea shows one signal peak at 5*H 2.81 for the two methyl groups of N(CHs)2 . Two ,3C NMR signals at 51 3 C 165.8 and 38.6 are representing carbonyl carbon and N(CH3)2, respectively. Reaction of tetramethylurea with triethylsilane and trityl TPFPB in CD2CI2 at -78 °C provided the evidence for the formation of 6a, whose 2 9 Si NMR chemical shift was found to be at S2 9 Si 39.8 (Fig. 3.15). Interestingly, the 2 9 Si NMR signal assigned to 6a did not change with variation of temperature, up to even room temperature. NMR resonance of N(CH3 ) 2 of 6a recorded at -78 °C (Fig. 3.13) appeared as two signals at 5lH 3.01 and 3.06. However, the 1 3 C NMR spectrum (Fig. 3.14) showed only one averaged signal ofN(CH 3 ) 2 at 5l3C 39.5. Carbonyl carbon of 6a showed little change on l3 C NMR (Sl3C 164.6) compared with that of the precursor. 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.5. Attempted silylation of other weak bases via triethylsilyltoluenium ion Electrophilic activation of carbon dioxide is rare. Protonation of carbon dioxide in superacid media indicated a limited equilibrium1 3 and alkylation of carbon dioxide was not successful. Only recently, silylation of carbon dioxide mediated by SiF3* in the gas phase has been reported1 6 (eqn. 2). Protonation of carbon monoxide occurs on carbon to form formyl cation, which was observed in HF/SbFs under high CO pressure by 1 3 C NMR by Gladysz and Horvath1 7 (eqn. 3). O-Protonation of carbon monoxide is energetically less favored, although O-Protonated carbon monoxide product, the isoformyl cation, has been detected in the interstellar space. SiF3+ F3Si--------C 0 2 C-protonation ^ HCO* CO (3) O-protonation COI-T S = C = = X = O .S R F /S b F 5 S 0 2 R = CH3, C2H5 =X*R (4) 97 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carbon disulfide and carbonyl sulfide can be readily alkylated with alkylfluorides in the presence of SbFs. In the case of carbonyl sulfide, alkylation occurs exclusively on sulfur.1 8 (eqn. 4). As a continuing effort on electrophilic silylation of weak bases, we decided to silylate CO2, CO, COS, CS2, N2O. In the cases of CO and COS, we expected that O- silylated products could be obtained due to high oxophilicity of silicon. When Corey hydride transfer reaction5 was carried out in CD2CI2 in the presence of CS2 or COS, only chlorosilanes were formed (eqn. 5). The products were identified by !H, l3C, and 2 9 Si NMR spectroscopy. On our second strategy, we prepared triethyl-toluenium TPFP (Et3Si-CeD5+ -CD3 B(C6Fs)4") according to Lambert’s procedure,1 9 then attempted to replace toluene to weak bases, such as CS2, COS etc (eqn. 6). The driving force of the ligand exchange is recovery of aromaticity in toluene. The results of 2 9 Si NMR chemical shifts are summarized in Table 3.3. For comparison calculated 52 9 Si of complexes, formed by trimethylsilyl cation and the corresponding bases, are also shown in parenthesis. Even though 2 9 Si NMR chemical shift of triethylsilyl tolueneum ion (S2 9 Si 81.9) changed after reacting with bases, the observed 52 9 Si values deviate substantially from the corresponding calculated values. Except for reaction with CH3CN, all of the ligand exchange reactions of triethylsilyl toluenum ion with weak bases were energetically unfavorable (i.e., endothermic) based on DFT calculations. It is not possible to analyze the silyl species obtained by the ligand exchange. Thus, no definite conclusion could be made at this moment. 98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD2CI2 M e3 SiH + Ph3C * B (C 6F5 ) 4 * + CS2 CD2CI2 Et3SiH + Ph3C* B(C6F5)4‘ + COS ------------ Toluene-d0 Et3SiH + Ph3C+ B(C0F 5) 4’ ► Et3Si room temp I * < 6 1 Et3Si-X+ B(C6F5)4* X = CS2, COS, C 0 2, CO, N02. c h3cn Table 3.3.2 9 Si NMR chemical shifts for attempted silylation of weak bases according to eqn. 6. C 0 3 B(C6F5)4* Me3SiCI 1 H 805, 0.51 ^C 5 ^ 3.8 29Si8ob,32.2 (5) Et3SiCI 1H Sob, 0 .8 5 ,1-02 13C Sob, 6.57. 7 45 MSi Sob, 37.1 EtsSi-X* Si S o b s (S cale) x = c s 2 x = c o s x = c o 2 x = c o x = n 2 o x = c h 3 c n 48.3 (107.4) 59.8 (129.6) 57.2 (160.2) 60.0 59.9 37.8 (51.5) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.6. Density Functional Theory (DFT) / IGLO Study To further rationalize the experimental results we have carried out density functional theory (DFT)2 4 and IGLO2713C and 29Si NMR chemical shift calculations on model trimethylsilylated carbonyl compounds, 2a' - 6a*. We have fully optimized the structure of the ions at the B3LYP/6-31G* level (for simplification we calculated only the most stable conformation, other less stable conformers are expected to give similar results but are not important for the topic discussed in this paper). Optimized geometries are depicted in Figure 3.16. In each of the trimethylsilylated ions 2a' - 6a' the bond Si-0 lies almost in the same carbocationic plane. The C-O bond lengths of the ions are in the range of 1.276 - 1.307 A. This indicates that although the silylated carbonyl compounds are resonance hybrids of the oxocarbenium ion and carboxonium ion, the latter is the predominant contributor of the overall structure. Whreas the structures of trimethylsilylated dimethyl carbonate 4a' and trimethylsilylated N,N-dimethylacetamide 5a' are planar or almost planar, the structure of trimethylsilylated tetramethyl urea 6a' is considerably twisted due to steric repulsion by the two bulky dimethylamino groups. 13 29 We also reproduced the C and Si NMR chemical shifts of 2a' - 6a' at the IGLO Wl B3LYP/6-31G* level with reasonable accuracy (Table 3.1 and Table 3.2). The calculated 82 9 Si of 2a', 3a', 4a', 5a' and 6a' are 56.5, 57.0, 66.0, 49.7 and 44.9, respectively, and these values agree reasonably well with the experimental values of to o Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59.3, 50.5, 54.7,40.7 and 39.8, respectively. The calculated 5*3 C of carbonyl carbon of 2a’ - 6a' are 230.4, 232.4, 172.4, 190.9 and 177.0, about 5-15 ppm more deshielded than the corresponding experimental values. We previously reported2 0 that the calculated S> 3 C of carbocationic center of various cycloalkylcarboxonium ions at the IGLO 1 1 7 / B3LYP/6-31G* level are also 11-15 ppm more deshielded than the experimentally observed results. 3.3. Conclusions A series of silylated carbonyl compounds were prepared as long-lived ions and characterized by *H , l3C, and 2 9 Si NMR spectroscopy. The NMR study indicates that silylated carbonyl compounds are resonance hybrids of the oxocarbenium and carboxonium ions, latter are the predominant contributors of the overall structures. Comparison of the NMR data with those of corresponding protonated carboxonium ions demonstrated that silylation on oxygen atom of carbonyl group resulted in significant influence on charge distribution within the molecules. The structures and ,3C and 2 9 Si NMR chemical shifts of the silylated carboxonium ions were also calculated by DFT/IGLO method. The calculated 1 3 C and 2 9 Si NMR chemical shifts of the model trimethylsilated carbonyl compounds agree reasonably well with the experimental data. Attempted silylation of weak bases (CS2, COS, CO2, CO, N2O) 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. showed deshielded in silicon NMR spectra, but no clear conclusion can be drawn from the results. 3.4. Experimental Section General Instruments: The 'H NMR spectra were recorded on a 300 MHz Varian Unity 300 NMR spectrometer equipped with a variable temperature probe. All chemical shifts are reported relative to external TMS at 5 0.0 or to the signal of a residual protonated solvent: CD2CI2 at 8 5.37. The l3C NMR spectra were recorded at 59.6 MHz, and chemical shifts were reported relative to external TMS at 5 0.0 or to the I3 C signal of the solvent: CD2CI2 at 5 53.8. The 2 9 Si NMR spectra were recorded at 59.6 MHz, and chemical shifts were reported relative to external TMS at 5 0.0. Signals in NMR spectra are described as in experimental section of chapter 1. Materials: Triphenylmethyl tetrakis(pentafluorophenyl)borate (Ph3C+ BfCeFsV) was prepared according to a literature method2 1 (see experimental section of chapter 1). Triethylsilane, acetophenone, CD2CI2 , and N,N-dimethylacetamide were commercially available and dried according to general procedures2 2 before use. 2- Cyclohexen-1 -one, dimethylcarbonate, and tetramethylurea were purchased from Aldrich and used as received. 1 0 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Preparation of 2a: Ph3 C+ B(C6F5)4 * (102 mg, 0.11 mmol) and dry CD2CI2 (0.5 mL) were placed in 5 mm NMR tube under argon atmosphere in a glove box. The NMR tube was taken out from the glove box and then cooled using dry ice / acetone to -78 °C still under argon. Acetophenone (0.12 mg, 0.1 mmol) and triethylsilane (58 mg, 0.5 mmol) in dry CD2CI2 (0.25 mL), prepared in the glove box and cooled to - 78 °C, were added to the NMR tube under vortex stirring. The NMR tube was sealed at -78 °C and the NMR spectra of the samples were recorded at -78 °C (Fig. 3.1.I3 C NMR spectrum at -78 °C; Fig. 3.3.2 9 Si NMR spectrum at -78 °C). Preparation of 3a: By using a similar procedure as described above, the reaction of triethylsilane (18 mg, 0.16 mmol) and 2-cyclohexen-1 -one (16 mg, 0.17 mmol) in the presence of PhsC'1 ’ B(C6FsV (101 mg, 0.11 mmol) in CD2CI2 (0.75 mL) at -78 °C gave a solution of 3a with triphenylmethane (Fig. 3.4. l3C NMR spectrum at -78 °C; Fig. 3.6.2 9 Si NMR spectrum at -78 °C). Preparation of 4a: By using a similar procedure as described above, the reaction of triethylsilane (17 mg, 0.15 mmol) and dimethylcarbonate (12 mg, 0.13 mmol) in the presence of Ph3C+ B(C6Fs)4 ~ (100 mg, 0.11 mmol) in CD2CI2 (0.75 mL) at -78 °C gave a solution of 4a with triphenylmethane (Fig. 3.7. lH NMR spectrum at -78 °C; Fig. 3.8. l3C NMR spectrum at -78 °C; Fig. 3.9.2 9 Si NMR spectrum at -78 °C). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Preparation of 5a: By using a similar procedure as described above, the reaction of triethylsilane (17 mg, 0.15 mmol) and N,N-dimethylacetamide (11 mg, 0.12 mmol) in the presence of Ph3C+ BfCeFs)/ (110 mg, 0.12 mmol) in CD2CI2 (0.75 mL) at -78 °C gave a solution of 5a with triphenylmethane (Fig. 3.10. 'H NMR spectrum at -78 °C; Fig. 3.11. I3 C NMR spectrum at -78 °C; Fig. 3.12.2 9 Si NMR spectrum at -78 °C). Preparation of 6a: By using a similar procedure as described above, the reaction of triethylsilane (22 mg, 0.19 mmol) and tetramethylurea (17 mg, 0.15 mmol) in the presence of Ph3C+ BtC^FsV (122 mg, 0.13 mmol) in CD2CI2 (0.75 mL) at -78 °C gave a solution of 6a with triphenylmethane (Fig. 3.13. lH NMR spectrum at -78 °C; Fig. 3.14. ,3C NMR spectrum at -78 °C; Fig. 3.15.2 9 Si NMR spectrum at -78 °C). Attempted trimethylsilylation of CS2 by Corey hydride transfer reaction: Trimethylsilane (0.1 mL), ca. 0.8 mmol) was condensed in a 5 mm J. Young NMR tube and cooled to -78 °C in a dry ice/acetone bath. CS2 (18 mg, 0.24 mmol) and Ph3C+ B(C6F5 )4 ‘ (228 mg, 0.25 mmol) in CD2CI2 (0.75 mL) were added subsequently to the NMR tube. The NMR tube was sealed off at -78 °C and shaken on Vortex Stirrer, and the NMR spectra of the sample were initially recorded at -78 °C. However, due to high viscosity of the sample at -78 °C the sample had to be warmed to -20 °C under argon atmosphere at which it became less viscous red solution. Then the NMR spectra of the sample recorded at -20 °C. Based on NMR spectra, the 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. formed silyl species was identified as MesSiCl. lH NMR (CD2CI2) 8 0.51; ,3C NMR (CD2CI2) 5 3.8; 2 9 Si NMR 5 32.2. Attempte triethylsilylation of COS by Corey hydride transfer reaction: COS (ca. 0.1 mL) was condensed in a 5 mm J. Young NMR tube and cooled to -78 °C in a dry ice/acetone bath. Triethylsilane (70 mg, 0.60 mmol) in CD2CI2 (0.15 mL) and Ph3C+ B(C6F5)4 * (108 mg, 0.12 mmol) in CD2CI2 (0 . 6 mL), each solution prepared in a glove box and cooled to -78 °C, were added subsequently to the NMR tube. The NMR tube was sealed off at -78 °C and shaken on Vortex Stirrer, and the NMR spectra of the sample were initially recorded at -78 °C. However, due to high viscosity of the sample at -78 °C the sample had to be warmed to room temperature under argon atmosphere at which it became less viscous red solution. Then the NMR spectra of the sample recorded at room temperature. Based on NMR spectra, the formed silyl species was identified as EtsSiCl. *H NMR (CD2CI2) 8 0.85, 1.02; I3 C NMR (CD2CI2) 8 6.57, 7.45; 2 9 Si NMR 8 37.1. Preparation of EtjSi-CjDs^CDs B(C6F5 ) 4 ' ; 19 In a argon filled glove box, Ph3C+ B (C < > F 5)4 * (225 mg, 0.24 mmol) was dissolved in dry Toluene-dg (1 mL) in a 5 mL J. Young NMR tube. Addition of triethylsilane (80 mg, 0.69 mmol) produced two layers, the lower one consisting of a light brown oil. The colorless top phase was 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. taken off with a syringe to remove the triphenylmethene by product and excess of triethylsilane. Attempted silylation of CS2 with triethylsilyltoluenium ion: Weak base was either added (for CS2 127 mg, 1.67 mmol; for CH3CN 78 mg, 1.9 mmol) or gently bubbled (COS; CO2; CO; N2O) for 30 min. to the above solution of Et3 Si-C <sD 5+ -CD3 B(CgFs)4 * . The observed 2 9 Si NMR chemical shifts are summarized in Table 3.3. Calculation Methods, Basis Set, and Geometry. Calculations were carried out with the Gaussian 98 program system. 2 3 The geometry optimizations were performed using the DFT2 4 method at the B3LYP2 5 /6-31G* level. 2 6 Vibrational frequencies at the B3LYP/6-31G*//B3LYP/6-31G* level were used to characterize stationary points as minima and to evaluate zero point vibrational energies (ZPE) which were scaled by a factor of 0.98. Energies were calculated at the B3LYP/6- 31 G*//B3LYP/6-31G* + ZPE level. I3 C and 2 9 Si NMR calculations were performed according to the reported method using IGLO programs2 7 at the IGLO II levels using B3LYP/6-311+G* geometries. Huzinaga2 8 Gaussian lobes were used as follows; Basis 1 1 " : Si, 11s 7p 2d contracted to [5111111, 211111, 11], d exponent - 1.4 and 0.35; C, O: 9s 5p Id contracted to [51111, 2111, 1], d exponent: 1.0, H: 5s lp 13 29 contracted to [311, 1], p exponent: 0.70. The C and Si NMR chemical shifts were referenced to TMS (calculated absolute shift i.e 5(Si) = 379.3 and 8 (C) = 196.8) 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. © o m „ c o ©O O J N 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 3.1. ,3 C N M R sp ectru m o f triethylsilylated acetophenone ion. L L J CO ©O 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 3.2, Expanded ,3C N M R sp ectru m o f triethylsilylated acetophenone ion. < o Ui 5 5 ©O c JO u e o e u X < D ¥ JS *s « % 'S j : U X o E 5 8 Q . ( A 22 3 00 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 1 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. „ CO ©O 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 3.7. ' H N M R sp ectru m o f triethylsilylated dimethylcarbonate ion. UJ ©8 ©O e 0 1 s o •e C Q o ■ > » .3 u s ■ 3 * o u c « % ’< 7 5 .C V •c < — o E E «* S- 0 0 r * S s 3 00 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. o J N 5 5 n OJ L U ^ 5 5 ©O A x o 9, X 115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3 .9 .29S i N M R spectrum o f triethylsilyiated dimethylcarbonate ion. ©o ©o 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 3,11. ,3 C N M R sp ectru m o f triethylsilylated N,N-dimethylacetamide ion. CO • S ’ Ui < • » U i „ CO © o X s X 2 u . a s CO < n UJ X o 'a a . a 1 1 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3.12,2 9 Si N M R spectrum o f triethylsilylated N,N-dimethylacetamide ion. Q , a . ® o Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3f 13. ' H N M R spectrum o f triethylsilylated tetramethylurea ion. UJ CO ©O 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 3,14, ,3 C N M R sp ectru m o f triethylsilylated tetramethylurea ion. V) < n UJ Ui _ 5 5 ©O A < 2 x o X a a > 'C u 0 E E 1 & C /3 rn 2 3 o o lb 121 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. M 74 W ) a U a £ 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 3,16. continued. 3.5. References 1 . For a review of carboxonium ions see: Olah, G. A.; Laali, K. K.; Wang. Q.; Prakash, G. K. S. “Onium Ions”; John Wiley & Sons; New York, 1998; pp 269-345. 2. Emde, H.; Gdtz, A.; Hofmann, K.; Simchen, G. Liebigs Ann. Chem. 1981, 1643. 3. Hendewerk, M. L.; Weil, D. A.; Stone, T. L.; Ellenberger, M. R.; Fameth, W. E.; Dixon, D. A. J. Am. Chem. Soc. 1982,104,1794. 4. Kira, M.; Hino, T.; Sakurai, H. Chem. Lett. 1992, 555. 5. a) Corey, J. Y. J. Am. Chem. Soc. 1975,97, 3237. b) Corey, J. Y.; West, R. J. Am. Chem. Soc. 1963,85,2430. 6. a) Prakash, G. K. S.; Bae, C.; Wang, Q.; Rasul, G.; Olah, G. A. J. Org. Chem. 2000, 65, 7646. b) Kira, M.; Hino, T.; Sakurai, H. J. Am. Chem. Soc. 1992, 114,6697. c) Kira, M.; Hino, T.; Sakurai, H. Chem. Lett. 1993, 153. d) Bahr, S. R.; Boudjouk, P. J. Am. Chem. Soc. 1993,115, 4514. e) Olah, G. A.; Li, X. -Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1995, 117, 8962. f) Lambert, J. B.; Zhang, S.; Stem, C. L.; Huffman, J. C. Science 1993, 260, 1917. 7. Prakash, G. K. S.; Wang, Q.; Rasul, G.; Olah, G. A. J. Organometallic Chem. 1998, 550,119. 8. a) Olah, G. A.; Westerman, P. W.; Forsyth, D. A. J. Am. Chem. Soc. 1975, 97, 3419. b) Krishnamurthy, V. V.; Prakash, G. K. S.; Iyer, P. S.; Olah, G. A. J. Am. Chem. Soc. 1984,106, 7068. 124 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9. a) Lauterbur, P. C. J. Am. Chem. Soc. 1961, 83, 1838. b) Spiesecke, H.; Schneider, W. G.; Tetrahedron lett. 1961, 468. c) LaLancette, E. A.; Benson, R. E.; J. Am. Chem. Soc. 1965, 87, 1941. d) Olah, G. A.; Bollinger, J. M.; White, A. M. J. Am. Chem. Soc. 1969, 91, 3667. e) Olah, G. A.; Mateescu, G. D. J. Am. Chem. Soc. 1970,92, 1430. 10. Olah, G. A.; Halpem, Y.; Mo, Y. K.; Liang, G. J. Am. Chem. Soc. 1972, 94, 3554. 11. Forsyth, D. A.; Osterman, V. M.; DeMember, J. R. J. Am. Chem. Soc. 1985, 107, 818. 12. Olah, G. A.; White, A. M. J. Am. Chem. Soc. 1968, 90, 1884. 13. Fraenkel, G.; Franconi, C. J. Am. Chem. Soc. 1960, 82,4478. 14. Gillespie, R. J.; Birchall, T. Can. J. Chem. 1963,41,148. 15. Olah, G. A.; Shen, J. J. Am. Chem. Soc. 1973, 95, 3582. 16. Cecchi, P.; Crestoni, M. E.; Grandinetti, F.; Vinciguerra, V. Angew. Chem. Int. Ed. Engl. 1996, 35,2522. 17. deRege, P. J. F.; Gladysz, J. A.; Horvath, I. Science 1997, 276, 776. 18. Olah, G. A.; Bruce, M. R.; Clouet, F. L. J. Org. Chem. 1981,46,438. 19. a) Lambert, J. B.; Zhao, Y.; Wu, H. J. Org. Chem. 1999, 64, 2729. b) Lamdert, J. B.; Zhao, Y.; Wu, H.; Tse, W.; Kuhlmann, B. J. Am. Chem. Soc. 1999,121,5001. 125 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20. a) Head, N. J.; Rasul, G.; Mitra, A.; Heshemi, A. B.; Prakash, G. K. S.; Olah, G. A. J. Am. Chem. Soc. 1995, 117, 12107. b) Prakash, G. K. S.; Rasul, G.; Liang, G.; Olah, G. A. J. Phys. Chem. 1996, 100, 15805. 21. Chien, J. C. W.; Tsai, W. -M.; Ransch, M. D. J. Am. Chem. Soc. 1991, 113, 8570. 22. Perrin, D. D.; Armarego, W. L. F. Purification o f Laboratory o f Chemicals, Third ed., Pergamon Press, Oxford, 1988. 23. Gaussian 98 (Revision A.5), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, R. E.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al- Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; M. Head-Gordon, M.; Pople, J. A., Gaussian, Inc., Pittsburgh PA, 1998. 24. Ziegler, T. Chem. Rev., 1991, 91,651. 25. Becke's Three Parameter Hybrid Method Using the LYP Correlation Functional: Becke, A. D. J. Chem. Phys. 1993, 98, 5648. 26. Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. “Ab Initio Molecular Orbital Theory”; Wiley-Interscience; New York, 1986. 27. a) Kutzelnigg, W.; Fleischer, U.; Schindler, M. NMR Basic Principles and Progress, 1991, 91, 651. b) Kutzelnigg, W. Isr. J. Chem. 1980, 27, 789. c) Fleischer, U., Schindler, M., Kutzelnigg, W. J. Chem. Phys. 1987, 86, 6337. d) Schindler, M. J. Am. Chem. Soc. 1987,109, 5950. 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28. Huzinaga, S. “Approximate Atomic Wave Function”; University of Alberta; Edmonton, Alberta, Canada, 1971. 127 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4. Silylation of Ketenes 4.1. Introduction Ketenes are unusual alkenes as well as carbonyl compounds that are reactive towards both electrophiles and nucleophiles. Because of unique nature of HOMO and LUMO molecular orbitals (Fig. 4.1) ketenes are highly polarized molecules with Cp and oxygen atoms bearing substantial negative charge and Ca bearing the positive charge, as indicated on their 1 3 C NMR chemical shifts.1 ,2 The > 3 C NMR chemical shifts of ketenes are expected to provide an indication of charge distribution on the carbons, and therefore a clue as to potential reactivity. Some values are given in Table 4.1. The low field values of Ca are typical for carbonyl groups, but the values for are very high fields compared to ordinary alkenes, which are typically in the range 100-150 ppm. The chemical behaviors of ketenes have been studied extensively and ketenes have played major role in organic synthesis.1 Electrophiles attack at Cp or oxygen centers from above or below the plane of the ketenes, and nucleophiles attack at Ca in plane. Protonation of ketenes has been the subject of numerous experimental and theoretical studies. Theoretical studies3 ' 8 show that the protonation at Cp of ketene la (to form acylium ion 2a in eqn. 1) is more favorable than protonation at oxygen (to form O-protonated ion) by 43 kcal/mol. hi solution the site of proton addition to 128 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ketenes has been mostly determined from kinetic studies of acid-catalyzed hydration.9 - * 1 3 LUMO HOMO Figure 4.1. HOMO and LUMO molecular orbitals of ketene CH2=C=0 (ref. 1). Table 4.1. l3C NMR chemical shifts of ketenes Ri R.2Cp=Ca=0.a '6 Ri R2 Ca c P H H 194.0 2.5 Me H 200.0 10.9 Me Me 204.9 24.2 Ph Me 205.6 33.8 Ph Ph 201.2 47.6 Et3 Si H 179.2 -4.9 Me3 Si Me3 Si 166.8 1.7 a Chemical shifts are referenced to TMS. b from ref. 1. 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. We have previously shown that protonation of diphenylketene and of di-tert- butylketene in Magic Acid solution at low temperature resulted in the formation of corresponding acylium ions 2b and 2c, as indicated by 1 3 C NMR signals for CO+ at 8 ,3C 154.7 and 154.1, respectively (eqn. I) . 14 M ° H* Rs © 'c=c=o ---- ► x c-c»o (1 ) R' r' H 1 a R = H 2 a R = H 1 b R = P h 2 b R = Ph 1 c R = f-Bu 2 c R = t-Bu P\ H2PtCI6 P \ / / H ; c=c=0 + R , S iH c = c R O SiR ’3 R R = Et, Ph, n-Pr, n-Bu R. .CN R. M e3Si-CN \ / r.=r.=n __________^ _ / R /C=c=o ------------- ; c = c : R = H, Ph, C F 3 R O SiM e3 (2) Trialkylsilyl cations are still elusive in the condensed phase due to their kinetic instability. The in situ generation of trialkylsilyl cations in solution, by hydride transfer/abstraction from organohydrosilanes by the triphenylmethyl (trityl) cation (so called “Corey hydride transfer” 1 5 see chapter 1), has been adopted for preparation of a number of silylated onium ions. 1 6 ~ 2 1 However, electrophilic silylation of ketenes is not yet reported. Thus far, silylation of ketenes employing silicon compounds has been known to produce only silyl enol ethers, formed by nucleophilic addition of 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. leaving group, such as CN' or N3* , at Ca followed by O-silylation (eqn. 2) . 2 2 We would like to disclose a theoretical and experimental study of electrophilic silylation of ketenes in solution in this chapter. 4.2. Results and Discussion 4.2.1. Density Functional Theory (DFT) / IGLO Study In principle, the incipient silyl cation may react with Cp of ketene to give ( 3 silylacylium ion, or react with oxygen to give silylcarboxonium ion due to strong Si O bond formation (eqn. 3). Rt "Me3Si+" R. © R1 ©,SiMe3 ,C=C=0 ,C -C -0 or ;C=C=0 (3) R2 R2SiMe3 R 2 1 a R 1 = R 2 = H 3 a R^ = R 2 s H 4 a R 1 = R2 = H 1b R 1 = R 2 = Ph 3 b R t = R2 = Ph 4 b R i = R 2 = Ph 1d R , = H, R2 = CH3 3 d R, = H, R2 = CH3 4 d R, = H, R2 = CH3 1 a R 1 = R2 — CH3 3 a R^ = R2 ~ CH3 4 a R^ — R2 — CH3 1f Rt = CH3l R2 = Ph 3 f R t = CH3i R2 = Ph 4 f R t = CH3, R2 = Ph Initially, we carried out density functional theory (DFT) calculations to ascertain the relative stability and structural information of Cp- and O-silylated isomers of each of l a and l b . We have fully optimized the structures at the B3LYP/6-311+G* 131 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. level. Optimized geometries are depicted in Figure 4.2. Two possible structures of trimethylsiiylated ketene 3a and 4a were calculated. Both were found to be stable minima on the potential energy surface (PES) as indicated by the frequency calculations at the B3LYP/6-31GV/B3LYP/6-31 G* level. It was found that C- silylated 3a was more stable than O-silylated 4a by 8.2 kcal/mol at the B3LYP/6- 311 +G*//B3LYP/6-311+G* + ZPE level (Table 4.2). We have also calculated Cp- and O-trimethylsilyated diphenylketene 3b and 4b, respectively, at the same level of theory. However, unlike silylation of ketene la, C-silylated 3b is in fact 5.4 kcal/mol less stable than O-silylated 4b as shown in Table 4.2. Interestingly, the cation 3b is characterized with relatively long Si-Cp bond distance of 2.121 A (Figure 4.2), which indicate the involvement of substantial Si-C hyperconjugation. In contrast, in the case of protonated diphenylketene, the same level of calculations showed that oxygen protonated form is 31.6 kcal/mol less stable than Cp-protonated form. In order to look into the effect of substituent on the relative stability of Cp- and O- silylated isomers of ketenes, we calculated both isomers with different substituents (3c - 3e and 4c - 4e in eqn. 3). At the same level of DFT calculation 3d is more stable than 4d by 4.1 kcal/mol. However, when ketenes have both substituents bulkier than H, O-silylated isomers were found to be more stable product. Thus, 4e is more stable than 3e by 0.3 kcal/mol and 4f is more stable than 3f by 1.9 kcal/mol (Table 4.2). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4.3. l3C and 2 9 Si NMR chemical shifts of diphenylketene, protonated diphenylketene, and silylated diphenylketene. ,3C (Ca) ,3C (Cp) 1 3 C (Ph) 2 9 Si lb- 201.2 47.0 126.6,127.7,129.3,130.8 2b* 154.7 48.9 127.8,128.7, 129.5, 139.8 3bc 174.3 34.2 44.5 4bc 174.9 69.5 94.7 3b,a 187.1 59.3 128.4, 130.1, 130.2, 133.2 49.6 a Observed chemical shifts in CD2CI2 at -78 °C in reference to tetramethylsilane. * from reference 14.c calculated chemical shifts. 4.2.2. Triethylsilylation of Diphenylketene Diphenylketene is relatively stable and easily prepared by various methods. It was originally prepared by Zinc reduction of a-chlorodiphenylacetyl chloride. 2 3 Other preparative methods are 0 oxidation of benzil monobydrazone with yellow mercury (II) oxide followed by thermal rearrangement of a-diazoketone2 4 ii) dehydrohalogenation of diphenylacetyl chloride with triethylamine2 5 iii) debromination of a-bromodiphenylacetyl bromide by the reaction of 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. charge delocalization in 3b' than in 2b due to {3-silyl hyperconjugation. 2 9 While ion 2 b is primarily stabilized by delocalization involving oxygen (oxonium ion character), ion 3b' is seemingly stabilized involving oxygen atom as well as Si-C hyperconjugation (both oxonium ion and ketene-like character, the latter results in deshielding of the acylium ion carbon) as shown in eqn. 5. The 2 9 Si NMR spectrum of 3b* showed only a single resonance at 5 49.6 ppm, substantially deshielded from that of Et3 SiH (S2 9 Si 0.2 in CD2CI2). This value also suggests some transfer of positive charge to silicon through hyperconjugation. Et3SiH PK Ph3C* B(C6F5)4- Phs 0 © C =C =0 r n r , C— O O B(C6Fs)4 (4) < CD2p'2 Ph iiC f. PK 1b -78 V SiEt3 3b' © © Ph2C -C = 0 Ph2C -C = 0 —— Ph2C =C =0 = Ph2C ^C=0 (5) SiEt3 SiEt3 ®SiEt3 SiEt3 Oxonium form Oxocarbenium form Ketane-like form 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. recorded at 59.6 MHz, and chemical shifts were reported relative to external TMS at 8 0.0. Signals in NMR spectra are described as in experimental section of chapter 1. Materials: Triphenylmethyl tetrakis(pentafluorophenyl)borate (Ph3C+ B(C6Fs)4*) was prepared according to a literature method2 7 (see chapter 1). Triethylsilane, triethylamine, CD2CI2, and diethyl ether were commercially available and dried according to general procedures3 0 before use. Diphenylacetyl chloride (90 % purity) was purchased from Aldrich and used as received. Diphenylketene (lb):2 S In a dry 500 mL Schlenk flask equipped with a magnetic stirring bar and a dropping funnel was placed diphenylacetyl chloride (90 % purity, 25.57 g, 110 mmol) and dry ether (200 mL). The flask was cooled in an ice bath and triethylamine (10.1 g, 100 mmol) was added dropwise under argon for 30 minutes to the stirred solution. White solid (triethylamine hydrochloride) precipitated * immediately, and the ether solution became viscous with bright yellow in color. After addition of the triethylamine, the solution was stirred at 0 °C for 1 h. The Schlenk flask was tightly stoppered under argon and stored in refrigerator (-20 °C) overnight. The triethylamine hydrochloride was filtered under argon using glass frit and washed with dry ether until the washings are colorless, c.a. 100 mL. The ether is removed under reduced pressure and the residual orange-color oil was transferred to 50 mL round-bottomed flask. Vacuum distillation twice afford 9.59 g of pure diphenylketene as an orange oil. b.p. 96 °C / 0.5 mm (lit.2 5 118 ~ 120 °C / 1 mm). 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The literature pointed out that the critical step in the procedure appeared to be distillation. Diphenylketene can be stored at 0 °C in a tightly stopped bottle for several weeks without decomposition. Yield 70 %. 'H NMR (CDCI3) (Fig. 4.3) 8 7.2 - 7.4 (m); 1 3 C NMR (CDCI3) (Fig. 4.4) 8 47.0 (Ph2C), 126.2 (para), 127.7 (ortho), 129.3 (meta), 130.8 (ipso), 201.2 (=0=0); MS (70 eV) (Fig. 4.5) m/z (relative intensity) 207 (M*, I), 194 (40), 165 (100). Silylation of diphenylketene (3b'): Diphenylketene (25 mg, 0.13 mmol) in 5 mL J. Young NMR tube, trityl TPFPB (101 mg, 0.11 mmol) and CD2CI2 (0.5 mL) in a 50 mL Schlenk flask, and triethylsilane (25 mg, 0.22 mmol) and CD2CI2 (0.25 mL) in another 50 mL Schlenk flask were prepared in argon filled glove box. To the NMR tube, cooled to -78 °C in a dry ice / acetone bath, was added CD2CI2 solution of triethylsilane followed by CD2CI2 solution of trityl TPFPB, which had been cooled to -78 °C, using syringe under argon. The NMR tube containing dark red solution was sealed at -78 °C under argon and NMR spectroscopy were obtained at - 78 °C. ‘H NMR (CD2CI2) (Fig. 4.6) 8 0.94, 1.03, 7.13-7.59 (m); l3C NMR (CD2C 12 ) (Fig. 4.7) 8 4.3, 5.9, 59.3, 128.4, 130.1, 130.2, 133.2, 187.1; 2 9 Si NMR (CD2C 12 ) (Fig. 4.9) 8 49.6. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. nri 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4,2. continued. CM 142 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 4.3, * H N M R sp ectru m o f diphenylketene. CM 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F igure 4.4, ,3C N M R sp ectru m o f diphenylketene. Scan 1053 (1 2 .0 7 6 m in ): CB100BA3.D 650000 ■ 600000- 550000 5 0 0 0 0 0 - 450000 400000 350000 300000 66 250000 200000 150000 100000 139 86 50000 - 91 69 126 40 207 100 110 120 130 140 150 160 170 180 190 200 210 »/Z Figure 4.5. Mass spectrum (70 eV) of diphenylketene. O Ph3 C H (J £ © O ■ m s>i o -w £ £ • I B _ o s « u " > » B U j : a. T 8 J3 ’S ■ > » £ « o E 2 u c. M 05 s z x V O 2 3 0 0 145 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. * ©o X o £ C L LL ,O-C0 o = tt ■ a. 146 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4 .7 .1 3 C N M R spectrum o f iriethyIsilylaled diphenylketene ion. O Ph3 C H # Ph3 C‘ ♦ B(C6F s)4 ©o V \ i .0 -0 ) £ £ 147 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.8. Expanded l3C N M R spectrum of triethylsilylated diphenylketene ion. 4.5. References 1 . Tidwell, T.T. Ketenes; John Wiley and Sons; New York, 1995. 2. For a review see: Seikaly, H. R.; Tidwell, T.T. Tetrahedron 1986,42,2587. 3. Leung-Toung, R.; Peterson, M. R.; Tidwell, T. T; Gsizmadia, I. G.; J. Mol. Structure (Theochem) 1989,183, 319. 4. Nobes, R. H.; Bouma, W. J.; Radom, L. J. Am. Chem. Soc. 1983,105,309. 5. Armitage, M. A.; Higgins, M. J.; Lewars, E. G.; March, R. E. J. Am. Chem. Soc. 1980,102, 5064. 6. Vogt. J.; Williamson, A. D.; Beauchamp, J. L. J. Am Chem. Soc. 1978, 100, 3478. 7. Traeger, J. C.; McLoughlin, R. G.; Nicholson, A. J. C. J. Am. Chem. Soc. 1982, 104, 5318. 8. Hopkinson, A. C. J. Chem. Soc. Perkin Trans. II. 1973, 795. 9. Allen, A. D.; Tidwell, T. T. Tetrahedron Lett. 1991, 32, 847. 10. Allen, A. D.; Stevenson, A.; Tidwell, T. T. J. Org. Chem. 1989, 54, 2843. 11. Allen, A. D.; Tidwell, T. T. J. Am. Chem. Soc. 1987, 109, 2774. 12. Allen. A.D.; Kresge, A. J.; Schepp, N. P.; Tidwell, T.T. Can. J. Chem. 1987, 65,1719. 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Kabir, S. H.; Seikaly, H. R.; Tidwell, T. T. J. Am. Chem. Soc. 1979, 101, 1059. Olah, G. A.; Alemayehu, M.; Wu, A.; Faroog, O.; Prakash, G. K. S. J. Am. Chem. Soc. 1992, 114,8042. a) Corey, J. Y. J. Am. Chem. Soc. 1975,97, 3237. b) Corey, J. Y.; West, R. J. Am. Chem. Soc. 1963, 85,2430. Lambert, J. B.; Zhang, S.; Stem, C. L.; Huffman, J. C. Science 1993, 260, 1917. Prakash, G. K. S.; Bae, C.; Wang, Q.; Rasul, G.; Olah, G. A. J. Org. Chem. 2000,65,7646. Steinberger, H. -U.; Muller, T.; Auner, N.; Maerker, C.; Schleyer, P. von R. Angew. Chem. Int. Ed. Engl. 1997, 36, 626. a) Kira, M.; Hino, T.; Sakurai, H. Chem. Lett. 1993, 153. b) Bahr, S. R.; Boudjouk, P. J. Am. Chem. Soc. 1993,115,4514. Olah, G. A.; Li, X. -Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1995,117, 8962. Prakash, G. K. S.; Wang, Q.; Rasul, G.; Olah, G. A. J. Organomet. Chem. 1998, 550,119. a) Frainnet, E.; Causse, J. Bull. Soc. Chim. Fr. 1968,3034. b) Hertenstein, U.; Hiinig, S.; Reichelt, H.; Schaller, R. Chem. Ber. 1982,115,261. Staudinger, H.; Ber. 1905, 38,1735. 150 of the copyright owner. Further reproduction prohibited without permission. 24. Smith, L. I.; Hoehn, H. H. Org. Syn. Coll. Vol. 3 ,1955, 356. 25. Taylor, E. C.; McKillop, A.; Hawks, G. H. Org. Syn. 1973, 52, 36. 26. Darling, S. D.; Kidwell, R. L. J. Org. Chem. 1968, 33, 3974. 27. a) Chien, J. C. W.; Tsai, W. -M.; Ransch, M. D. J. Am. Chem. Soc. 1991, 113, 8570. b) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245. c) Stehling, U. M.; Stein, K. M.; Kesti, M. R.; Waymouth, R. M. Macromolecules 1998, 31, 2019. d) Toskas, G.; Moreau, M.; Masure, M.; Sigwalt, P. Macromolecules 2001, 34,4730. 28. Olah, G. A.; White, A. M. J. Am. Chem. Soc. 1969, 91, 5801. 29. For examples of stable (J-silyl carbocation see a) Siehl, H. -U.; Kaufmann, F. -P.; Apeloig, Y.; Braude, V.; Danovich, D.; Bemdt, A.; Stamatis, N. Angew. Chem. Int. Ed. Engl. 1991, 30, 1479. b) Siehl, H, -U.; Kaufmann, F. -P. J. Am. Chem. Soc. 1992, 114, 4937. c) Prakash, G. K. S.; Reddy. V. P.; Rasul, G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1992, 114, 3076. d) Lambert, J. B.; Zhao, Y. J. Am. Chem. Soc. 1996, 118, 7867. e) Lambert, J. B.; Zhao, Y.; Wu. H. J. Org. Chem. 1999, 64, 2729. f) Muller, T.; Meyer, R.; Lennartz, D.; Siehl, H. -U. Angew. Chem. Int. Ed. 2000, 39, 3074. 30. Perrin, D. D.; Annarego, W. L. F. Purification o f Laboratory o f Chemicals, Third ed., Pergamon Press, Oxford, 1988. 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Creator Bae, Chulsung (author)

Core Title Investigation of silylated onium ions

School Graduate School

Degree Doctor of Philosophy

Degree Program Chemistry

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemistry, organic,OAI-PMH Harvest

Language English

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Advisor Prakash, G.K. Surya (committee chair), Olah, George A. (committee member), Petruska, John (committee member)

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