University of Southern California Dissertations and Theses

Fluorinated carbocations and carboxonium ions

(USC Thesis Other)

Fluorinated carbocations and carboxonium ions

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content FLUORINATED CARBOCATIONS
AND CARBOXONIUM IONS
by
Arwed A. Burrichter
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(Chemistry)
December 1996
Copyright 1996 Arwed A. Burrichter
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UNIVERSITY O F SO U TH ERN CALIFORNIA
THE GRADUATE SCHOOL.
UNIVERSITY PARK
LOS ANGELES, CALIFORNIA S 0 0 0 7
This thesis, written by
Arved A. Burrichter
under the direction of hhft— Thesis Committee,
and approved by all its members, has been pre
sented to and accepted by the Dean of The
Graduate School, in partial fulfillment of the
requirements for the degree of
Master of Science
O m s
Date £?ce~ be£ 1 7 , 1996
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In loving memory of my grandfather
Heinrich Burrichter (1911-1991)
ii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Acknowledgements
I am indebted to my academic advisor, Professor George A. Olah, whom I have
come to know not only as an ingenious scientist, but also as a very personable individual
His school of science as well as his strong personality have greatly influenced me and
will continue to be a role model for me in the future. I am also greatly indebted to
Professor G. K. Surya Prakash, with whom I worked together very closely on all o f my
projects and whose friendship I cherish. Many thanks go to Professor William P. Weber
for his continuous encouragement and for being on my thesis committee. My friends and
colleagues at the Loker Hydrocarbon Research Institute have made my stay at USC most
enjoyable. Of the many people who have contributed to the success of this work, I would
like to particularly acknowledge my coworkers Dr. Golam Rasul, Dr. Andrei Yudin, and
Dr. Nikolai Hartz. Further thanks go to Mrs. Reiko Choy and Dr. Robert Aniszfeld for
their help and support. I am grateful for the financial support of the Konrad-Adenauer-
Foundation which made it possible for me to come to USC. I would like to sincerely
thank Ms. Tamika Henry and her family who with their love and encouragement gave me
the much needed emotional support. Finally and above all, I thank my parents who over
the years have always lent me their compassion and caring support.
iii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table of Contents
Preface Dedication
Acknowledgements
List o f Tables
List of Figures
Abstract
u
iii
vi
vii
viii
Chapter 1 Introduction
1.1 Introduction 2
1.2 Observation of Stable Carbocations and Related Ions 5
Chapter 2
1.3 Superacid Systems 8
1.4 References
16
Fluorocarbocations and Protolytic Cleavage of Trifluoroacetic Acid
2.1 Introduction 19
2.2 Results and Discussion 20
2.2.1 Fluorocarbocations 20
2.2.1.1 c f 3 + 25
2.2.1.2 c h 3 c h f + 25
2.2.1.3 CH3C(F)F+ 26
2.2.1.4 (CH3 )2 CF+ 27
2.2.1.5 c h 3c h 2c + f c h 3 27
2.2.1.6 c-C5 H8 F+ 28
2.2.1.7 c-C3 H2 F+ 28
2.2.1.8 c-C3 HF2+ 29
2.2.1.9 c-C3 F3 + 29
2.2.1.10 trans- and czs-CHF=OCH3 + 29
2.2.1.11 NMR Chemical Shift Correlation 30
2.2.2 Protolytic Cleavage o f Trifluoroacetic Acid 34
2.3 Conclusions 39
2.4 Experimental Part 39
2.5 References 41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3 T rifluoromethyl Substituted Carboxonium Ions
3.1 Introduction
3.2 Results and Discussion
3.2.1 Protonated T rifluoroacetone
3.2.2 Methylated Trifluoroacetone
3.2.3 Protonated Hexafluoroacetone
3.2.4 Methylated Hexafluoroacetone
3.2.5 Protonated Methyl Trifluoroacetate
3.2.6 Methylated Methyl Trifluoroacetate
3.2.7 Perfluorotrialkoxymethyl Cations
3.3 Conclusions
3.4 Experimental Part
3.5 References
44
46
55
57
58
59
60
62
63
64
65
67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
List of Tables
Chapter 1
Table 1.1
Table 1.2
Table 1.3
Chapter 2
Table 2.1
Chapter 3
Table 3.1
Table 3.2
Table 3.3
Hq Values of Some Bronsted Acids 9
Physical Properties of Some Lewis Superacids 11
Hammett Acidity Function Hq of Some Conjugate 14
Bronsted/Lewis Superacids
1 9 F NMR and Selected I3 C NMR Chemical Shifts 22
of Fluorocarbocations
Total Energies (-au), ZPE (kcal/mol), and Relative Energies 47
(kcal/mol) of Protonated and Methylated Trifluoroacetone,
Hexafluoroacetone, and Methyl Trifluoroacetate
Experimental and Calculated 1 3 C NMR Shifts at the 49
IGLO E // MP2(fu)/6-31G* Level
Experimental lH NMR Chemical Shifts of Carboxonium Ions 50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
List of Figures
Chapter 1
Figure 1.1 Classification of Carbocations
Chapter 2
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Selected MP2/6-31G* Optimized Parameters of 1-11 23
Selected MP2(fu)/6-31G* Optimized Parameters of 12 and 13 24
Plot of Calculated vs Experimental l9F NMR Chemical Shifts 31
of Fluorocarbenium Ions: (a) GIAO-MP2 vs Experimental
(b) GIAO-SCF vs Experimental (c) IGLOII vs Experimental
Proposed Reaction Mechanisms for the Protolytic Cleavage of 36
Trifluoroacetic Acid
Chapter 3
Figure 3.1 MP2(fu)/6-31G* Optimized Geometries
51
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Abstract
The structures and properties o f a number of fluorinated carbocations and
carboxonium ions and dications were investigated by experimental and theoretical
methods. Cations studied include the fluorocarbocations CF3 +, CH3 CHF+ , CH3C(F)F"\
(CH3 )2C F \ CH3 CH2C+ FCH3j c-C5 H8 F+ , c-C3 H2 F+, c-C3 HF2 + , c-C3 F3 + , and CHF=OCH3",
as well as the carboxonium ions CF3 C(OH)CH3 +, CF3 C(OCH3 )CH3 \ CF3 C(OH)OCH3 +,
CF3 C(OCH3 )2 \ CF3 C(OH)CF3 +, CF3 C(OCH3 )CF3 \ C(OC2 Fs)3 +, and C(OC4 F9 )3 +. The
protolytic cleavage of trifluoroacetic acid in superacids forming CF4 was investigated and
its reaction mechanism is suggested to involve the gitonic CF3 C(OH)(OH2 )2 + dication, a
prototype superelectrophile, as an intermediate.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 1
Introduction
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.1 INTRODUCTION
Carbocations, i.e the positive ions o f carbon compounds, and related ions are an
important class of intermediates in organic chemistry and occur in a variety of acid-
catalyzed reactions, including the dehydration of alcohols, cleavage of ethers, many
additions to alkenes, nucleophilic substitutions, and a variety of rearrangements. George
A. Olah’s1 pioneering work in the 1960’s lead to the discovery and first observation of
carbocations in superacid medium and resulted in what is now generally referred to as
superacidic, stable-ion chemistry and the chemistry of stable carbocations. Superacids2
are acid systems up to billions of times stronger than conventional mineral acids such as
100 % H2 S 04. Thousands of publications from around the world have contributed and
are continuing to contribute to this very active field.
Based on the greatly enhanced reactivity of a number of cations under superacid
conditions, Olah recently3 suggested the superelectrophilic activation of these ions in a
number of acid catalyzed reactions. Superelectrophiles are electron deficient
intermediates that are further activated by protonation (protosolvation) or Lewis acid
coordination. The resulting dications are substantially more reactive than their parent
monocations. Hydrogen/deuterium exchange experiments under superacid conditions as
well as theoretical calculations give strong support to this concept.3
2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The work presented in my thesis encompasses the investigation of
fluorosubstituted carbocations and onium ions. The study of these ions is of particular
interest in the light of the increasing importance o f fluorinated organic compounds as
precursors in the development of pharmaceutical4 and agricultural5 chemicals due to their
unique properties such as oxidative, hydrolytic, and thermal stability. In addition, the
study of fluorocarbocations6 is of interest because of the dualistic effect o f fluorine as a
substituent. Due to its high electronegativity (4.0 on the Pauling electronegativity scale),
a fluorine atom adjacent to an electron-deficient carbocationic center is inductively
destabilizing. On the other hand, the nonbonded electron pairs on the fluorine atom can
stabilize the positive charge through back-donation (n-p interaction). This dualistic
behavior of fluorine as a substituent leads to interesting effects on the structure and
reactivity of many fluorocarbocations. In Chapter 2 of my thesis the effect of
fluorosubstitution on the structure of a number of important carbocations is investigated.
Trifluoromethyl substituted cations are a particularly challenging class of cationic
intermediates in view of their highly electron deficient nature. The destabilizing effect of
a trifluoromethyl group (CF3 ) adjacent to a cationic center is well documented both by
7 8 +
theory and experiment. ’ As reflected by its Hammett substituent constant (6p (p-CF3 ) =
0.61), the CF3 group is one of the strongest electron withdrawing groups and thus has a
strong inductive destabilizing effect on carbocationic centers. In addition and unlike
fluorine as a substituent, the trifluoromethyl group lacks the ability to stabilize positive
3
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
charge through n-n back-donation. Thus, trifluoromethyl-substituted cations are
generally less stable than their fluoro-substituted analogs. Chapter 3 of my thesis
explores the possibilities and limits of preparing novel, highly electron-deficient
trifluoromethyl substituted carboxonium ions.
The combination of experimental and theoretical methods provides a powerful
tool for the elucidation of complex structures and reaction mechanisms. The recent
development o f methods and algorithms for the calculation of electronic structures made
it possible to study reactive intermediates and even transition states theoretically,
allowing predictions to be made on structural and energetic details that are difficult or
impossible to achieve experimentally.9 , I0 , 1 1 In my work I have used experimental
methods such as nuclear magnetic resonance spectroscopy to characterize various
fluorinated organic ions under superacid stable ion conditions. In addition, high level ab
initio molecular orbital calculations were carried out to gain better insight into the
structural parameters and energetics of these ions.
The following two sections provide a brief historical overview of the development
of carbocation chemistry, as well as a summary of superacid systems most commonly
used for the generation and observation of carbocations and related ions.
4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.2 OBSERVATION OF STABLE CARBOCATIONS AND RELATED IONS
Carbocations are generally divided into two groups: Trivalent carbenium ions, of
which CH3 + is the parent, and pentavalent carbonium ions, o f which CH5 + is the parent
(Figure l.l) .1 2
Carbocations
Carbenium Ions Carbonium Ions
trivalent pentavalent
H
Hx ,H
hV c " ‘' ' h
H H
H H
Figure 1.1 Classification of Carbocations
Trivalent carbgnium ions contain an sp2 -hybridized electron-deficient carbon
atom, which tends to be planar in the absence of constraining skeletal rigidity or steric
interference. The carbenium ion center contains six valence electrons and is thus highly
electron deficient and its structure can be adequately described by using two-electron,
two-center bonds (Lewis valence bond structures). Trivalent carbenium ions are the key
intermediates in reactions of unsaturated 7i-donor hydrocarbons (e.g. alkenes, alkynes,
aromatics) with electrophiles, e.g. Friedel-Crafts type reactions. Pentacoordinated
carbanium ions contain five coordinated carbon atoms. They cannot be described by
5
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
two-electron, three-center bonding. The carbocation center is always surrounded by eight
electrons, but overall the carbonium ions are electron-deficient because two electrons are
shared between three atoms. Thus the bonding principles in carbonium ions are much
alike those found in boron hydrides (2e-3c bonding), e.g. diborane B2 H6. Carbonium ions
are the key intermediates in reactions of saturated cr-donor hydrocarbons (e.g. alkanes)
with electrophiles. These reactions are industrially important in the conversion of low-
boiling hydrocarbons into high-octane gasoline.1 3
The history of carbocations goes back to the turn of the 20th century, when
Norris1 4 3 and Kehrmann and Wentzel1 4 b independently discovered that colorless
triphenylmethyl alcohol gave deep yellow solutions in concentrated sulfuric acid. The
ionic character of trityl salts was first recognized by von Baeyer in 1902.l4c In 1922,
Meerwein and van Emster1 4 6 proposed the involvement o f carbocationic intermediates in
the Wagner rearrangement of camphene hydrochloride to isobronyl chloride. Similarly,
based on kinetic and stereochemical studies, Ingold and Hughes1 5 proposed the
intermediacy of carbocations in certain substitution and elimination reactions. In the
1930’s, Whitmore1 6 generalized these concepts to include many other organic reactions.
Due to their electon-deficient nature, carbocations are generally very reactive and are too
short-lived to be observed under normal reaction conditions, and even in strong acids
such as H2S04. Their transient nature arises from their extreme reactivity with
nucleophiles to form elimination or substitution products. The use of superacid systems
6
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(e.g. HF-SbF5 or FS03 H-SbF5 ) in combination with low-nucleophilicity solvents (e.g.
S 0 2 or S 02 C1F) to provide stable conditions for the direct observation of carbocations
was pioneered by Olah in the 1960’s. The first alkyl cations were generated by reacting
alkyl fluorides with antimony pentafluoride.1
h3 cx ch3
C— F + SbF5 c + SbF6‘
H3C"V s
h3 c h3c x c h 3
Other suitable precursors for the generation of carbocations in superacids include
alcohols and alkenes.1 7 Even alkanes, long known for their chemical inertness, were
found to be protonated and readily eliminate H2 under superacidic conditions.1 8
H3C 9 H3
\ + FS03H-SbFs
l_l p , , ; C — H + H --------- - --------- U £ + + H 2
h 3c V / \
h3c h 3c c h3
Hundreds of stable carbenium ions have been observed under superacid
conditions and their structures have been elucidated by methods such as NMR, ESC A,
IR, UV, and Raman spectroscopy and even X-ray crystallography.1 9
7
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.3 SUPERACID SYSTEMS
The name “superacid” was first used in 1927 by Conant2 0 to denote acids such as
perchloric acid, which he found stronger than conventional mineral acids and capable of
protonating even such weak bases as carbonyl compounds. It was, however, not until the
late 1950’s that the field of superacids started to undergo rapid development, involving
the discovery of new systems and an understanding of their nature as well as their
* • 21 22
chemistry. Gillespie proposed an arbitrary but since widely accepted definition of
superacids, defining them as any acid system that is stronger than 100% sulfuric acid.
Describing the acidity of superacids in terms of pH values as done with aqueous
acid solutions is not possible. In aqueous solution, the acidity of any system cannot be
higher than that of conjugated acid of water, i.e. the hydronium ion H3 0 + ; this is known
as the “levelling effect”. In non-aqueous systems, the strength of a strong acid can be
determined by its ability to protonated weak organic bases, such as nitroanilines. A
quantitative measurement o f acidities has been established in form of the Hammett acidity
function H0P The logartihmic H0 value is calculated by the formula:
H 0 = Pka(ln) + log — —
C|nH+
8
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
where pBQ (In) is the dissociation constant o f the indicator and InH+ /In is the ionization
ratio between a weak indicator base In and its conjugate acid InH+. 100 % sulfuric acid
has a Hq value of -11.9 on the logarithmic scale, whereas some of the strongest, complex
superacids reach values of -25 (“magic acid”, fluoroantimonic acid systems, see Table
1.3). Perchloric acid HC104, chlorosulfuric acid C1S03 H, triflic acid CF3 S03 H, and
fluorosulfuric acid F S 03 H are examples of simple Bronsted acids that exceed the acidity
o f sulfuric acid with H0 values smaller than -12 (Table l.l).2
Acid Formula
-H0
Glacial Acetic Acid CH3 COOH 2
Phosphoric Acid
h 3 p o 4
5
Nitric Acid h n o 3 6
Sulfuric Acid (100%) h 2 s o 4 11.9
Perchloric Acid h c io 4 13.0
Chlorosulfuric Acid c is o 3 h 13.8
Triflic Acid c f 3 s o 3 h 14.1
Fluorosulfuric Acid f s o 3 h 15.1
Table 1.1 H0 values o f some Bronsted Acids
Fluorosulfuric acid (FS03 H) is one o f the strongest protic acids known (H0 =
- 15.6).2 2 It is a mobile colorless liquid that fumes in moist air and has a sharp odor. It is
readily available, relatively inexpensive, easily purified by distillation, and it does not
9
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
etch glass if pure [i.e. free from HF). Its other advantages include a conveniently wide
liquid range (-89 0 to +163 °C) and a viscosity lower than that of sulfuric acid systems.
Structurally, fluorosulfonic acid can be regarded as a mixed anhydride of sulfuric and
hydrofluoric acid. Fluorosulfuric acid is commercially prepared by reaction of S 0 3 and
HF.2
S 0 3 + HF (g) FS03H
Fluorosulfuric acid is employed as a catalyst and chemical reagent in various
chemical processes including alkylation, acylation, polymerization, sulfonation,
isomerization, and production of organic fluorosulfates.2 4 As one of the strongest
Bronsted acids known, it forms a number of stable salts whose solubility is comparable to
fluoroborate (B F ^ and perchlorate (CIO^O salts. The low freezing point (-89 °C) has
proven to be advantageous in the study o f carbocations.
Lewis superacids are arbitrarily defined as those stronger than anhydrous
aluminum trichloride, the most commonly used Friedel-Crafts catalyst.2 This definition is
only arbitrary because there is no absolute scale in rating the strength of Lewis acids.
However, it was found that certain Lewis acid halides like SbF5 , AsF5 , TaFs, NbF5 , etc.
show exceptional reactivity far exceeding those of A1C13 , BF3 , and other conventional
Lewis acid halides in applications such as ionizing alkyl or cycloalkyl halides to their
corresponding carbocations and catalytic activity. Moreover, these acid fluorides also
10
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
show remarkable coordinating ability to Bronsted acids such as HF, FS03 H, CF3 S 0 3 H,
etc., resulting in vastly enhanced acidity o f the resulting conjugate acids.
Acid Formula mp(°C) bp (°C)
Antimony Pentafluoride SbFs 7 143
Arsenic Pentafluoride AsF5 -80 -53
Tantalum Pentafluoride TaFs 97 229
Niobium Pentafluoride NbFs 72 236
Boron Tris(triflate) B(OTf)3 44 68-83
Table 1.2 Physical Properties of some Lewis Superacids
Antimony pentafluoride (SbF5 ) is a colorless, highly viscous liquid at room
temperature that boils at 143 °C and solidifies at 7 °C. Its viscosity is 460 cp at 20 °C,
which is close to that of glycerol. The pure liquid can be handled and distilled in glass if
moisture is excluded. As a powerful oxidizing and a moderate fluorinating agent it
spontaneously inflames phosphorous and sodium. It reacts with water to form SbF5 2
H2 0 , an unusually stable solid hydrate that reacts violently with excess water to form a
clear solution. In solution, SbF5 is not monomeric but has a polymeric structure in which
each antimony atom is surrounded by six fluorine atoms in an octahedral arrangement
(m-fluorine bridging).2 5
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
F
I
F
In the gas phase, SbFs exists as trimers (SbF5 )3 (150 °C) and dimers (SbF5 )2 (250 °C).
On cooling, SbF5 gives a nonionic solid composed of trigonal bipyramidal molecules.
Industrially, antimony pentafluoride is prepared by direct fluorination of antimony metal
or antimony trifluoride SbF3.2
The exceptional ability of SbF5 to complex and subsequently ionize nonbonded electron-
pair donors (such as halides, alcohols, ethers, amines, etc.) to carbocations, recognized
first by Olah in the early 1960’s, has made it one of the most widely used Lewis halides
in the study of cationic intermediates and catalytic reactions. Antimony pentafluoride is
one of the strongest Lewis acids known and has been used in a number of synthetic
applications.2 6
2 Sb + 5 F2 2 SbF5
SbF3 + F2 SbF5
12
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Mixtures of strong Bronsted acids with certain Lewis acids, such as S 03 , BF3 ,
A sF5, or SbF5 are often much more acidic than the Bronsted acid itself. These superacid
systems are termed Conjugate Bronsted/Lewis Superacids (see Table 1.3). As an
example, the acidity function of FS03 H increases from - 15 to - 21 upon addition of 25
mole percent SbF5. This phenomenon can be explained by a shift in the autoprotolysis
equilibrium of FS03 H:
FS03H + FSO3H — — f s o 3h2+ + f s o 3 ‘
Addition of SbF5 (strong Lewis acid) to the system leads to complexation of the
fluorosulfate anion FS03 ' (Lewis base)
F
F I ..oF
^ Sb' ^
F I
s ° 3F
thus shifting the autoprotolysis equilibrium to the right and increasing the concentration
of highly acidic FS03 H2 * cations. The resulting acid, a mixture of FS03 H and SbF5
(“Magic Acid”) is significantly more acidic than neat fluorosulfuric acid. A number of
conjugate Bronsted/Lewis acid systems of various acidities (Table 1.3) have found
practical use in synthetic and mechanistic chemistry.
13
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Acid System Formula
-H0
Oleum H2 S 0 4:S03 12 to 15
Tetrafluoroboric Acid HF:BF3 18 to 20
Triflatoboric Acid HOTf:B(OTf)3 18 to 20
Triflic Acid / SbF5 HOTf:SbF5 20 to 22
Magic Acid FS03 H:SbF5 20 to 25
Fluoroantimonic Acid HF:SbF5 24 to 30
Table 1.3. Hammett acidity ranges (Hq) of some conjugate Bronsted/Lewis superacids
Magic Acid, a mixture o f fluorosulfuric acid FS03 H and antimony pentafluoride
27
SbF5 , is probably the best known superacid system. The fluorosulfuric acid-antimony
pentafluoride system was developed in the early 1960’s by Olah for the study of stable
carbocations. The acidity o f the Magic Acid system increases significantly with
28
increasing SbF5 content has an estimated value of H0 = -26.5 for the 90% SbF5 content.
As already mentioned, this increase in acidity is due to complexation of the fluorosulfate
anion FS03 ‘ by SbF5 which shifts the autoprotolysis equilibrium of FS 03 H to the right
(vide infra). The major reason for the wide application of Magic Acid systems compared
with others (besides its high acidity) is probably the large temperature range in which it
can be used (ca. -160° to 80 °C). Another advantage is that unlike HF containing systems
it can be handled in glass.2
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Fluoroantimonic Acid (HF:SbF5 ) is considered the strongest liquid superacid and
also the one that has the widest acidity range. Due to the excellent solvent properties of
hydrogen fluoride, HF:SbF5 is used advantageously for a variety of catalytic and
synthetic applications. 2 9 The acidity of HF was initially estimated at H 0 = 1 1 and has now
been revised to Hq of - 15.1 for highly purified anhydrous HF. 2 A dramatic increase in
acidity (H0 = - 20.5) is observed when 1 mol% SbF5 is added to anhydrous HF. 3 0 A 1:1
molar mixture of HF:SbF5 is estimated to have an Hq value of -30.3 0 The acidity may
increase still further for higher SbFs concentrations. The ionization of HF:SbFs can be
described as follows:
2 HF + SbF5 H 2F+ + SbF6'
In dilute solutions o f SbF5 in HF (< 20 % SbF5 ), however, the H3F2 + ion was found to be
the predominant cationic species, and the autoprotolysis equilibrium can be written as
follows:
3 HF + SbF5 H 3F2+ + SbF6'
Furthermore, higher homo logs of the hexafluoroantimonate anion such Sb2 Fn ' and
Sb3 FI6 - are often present as gegenions, especially when the SbF5 content is increased. 3 1
Special caution and protective clothing is required when handling HF:SbF5 .
15
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.4 REFERENCES
1) Olah, G. A. Angew. Chem. Int. Ed. Engl. 1995, 3 4 ,1393.
2) For a comprehensive monograph see: Olah, G. A.; Prakash, G. K. S.; Sommer, J.
Superacids\ Wiley-Interscience: New York, 1985.
3) Olah, G. A. Angew. Chem. Int. Ed. Engl. 1993, 32, 767.
4) Filler, R.; Kobayashi, Y.; Yagupolskii, L. M. Organofluorine Compounds in
Medicinal chemistry and Biomedical Applications', Elsevier: Amsterdam, 1993.
5) Yoshioka, H.; Nakayama, C.; Matsuo, N. J. Synth. Org. Chem., Jpn. 1984,42,
809.
6 ) For a review see: Olah, G. A.; Mo, Y. K. Advances in Fluorine Chemistry 1973,
7,69.
7) For a review see: Gassman, P. G.; Tidwell, T. T. Acc. Chem. Res. 1983,16, 279.
8 ) For a review see: Creary, X. Chem. Rev. 1991, 9 1 ,1625.
9) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular
Orbital Theory, Wiley-Interscience: New York, 1986.
10) Foresman, J. B.; Frisch, A. E. Exploring Chemistry with Electronic Structure
Methods: A Guide to Using GAUSSIAN, Gaussian, Inc.: Pittsburg, PA, 1993.
11) Lammertsma, K.; Schleyer, P. v. R.; Schwarz, H. Angew. Chem. Int. Ed. Engl.
1989,2 8 ,1321.
12) For a review see: Olah, G. A. Angew. Chem. 1973,8 5 ,183.
13) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry', Wiley-Interscience: New York,
1995.
14) (a) Norris, J. F. Am. Chem. J. 1901, 2 5 ,117 (b) Kehrmann, F.; Wentzel, F. Ber.
Dtsch. Chem. Ges. 1901, 34, 3815 (c) Baeyer, A.; Villiger, V. ibid. 1902,35,
1189 (d) Meerwein, H.; van Emster, K. Ber. Dtsch. Chem. Ges. 1922,55,2500.
16
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15) Ingold, C. K. Structure and Mechanism in Organic Chemistry; Cornell University
Press: Ithaca, NY, 1953.
16) Whitmore, F. C. J. Am. Chem. Soc. 1932, 54,3274.
17) Olah, G. A.; Halpem, Y. J. Org. Chem. 1971,36, 2354.
18) Olah, G. A.; Lukas, J. J. Am. Chem. Soc. 1967,89,2227.
19) Vogel, P. Carbocation Chemistry, Elsevier: Amsterdam, 1985.
20) Hall, N. F.; Conant, J. B. J. Am. Chem. Soc. 1927,49, 3047.
21) Gillespie, R. J.; Peel, T. E. Adv. Phys. Org. Chem. 1972, 9,1.
22) Gillespie, R. J.; Peel, T. E. J. Am. Chem. Soc. 1973,95, 5173.
23) Hammett, L. P.; Deyrup, A. J. J. Am. Chem. Soc. 1932,54, 2721.
24) Olah, G. A.; Prakash, G. K. S.; Wang, Q.; Li, X.-Y. in: Encyclopedia o f Reagents
in Organic Synthesis, Paquette, L., Ed.; Wiley: New York, 1995; Vol. 4, pp 2567.
25) Hoffinan, G. J.; Holder, B. F.; Jolly, W. L. J. Phys. Chem. 1958, 62, 364.
26) Olah, G. A.; Prakash, G. K. S.; Wang, Q.; Li, X.-Y. in: Encyclopedia o f Reagents
in Organic Synthesis, Paquette, L., Ed.; Wiley: New York, 1995; Vol. 1, pp 209.
27) Olah, G. A.; Prakash, G. K. S.; Wang, Q.; Li, X.-L. in: Encyclopedia o f Reagents
in Organic Synthesis, Paquette, L., Ed.; Wiley: New York, 1995; Vol. 4, pp 2569.
28) Gold, V.; Laali, K.; Morris, K. P.; Zduneck, L. Z. Chem. Commun. 1981, 769.
29) Olah, G. A.; Prakash, G. K. S.; Wang, Q.; Li, X.-Y. in: Encyclopedia o f Reagents
in Organic Synthesis, Paquette, L., Ed.; Wiley: New York, 1995; Vol. 4, pp 2715.
30) Sommer, J.; Canivet, P.; Schwarts, S.; Rimmelin, P. Now. J. Chim. 1981,5 ,45.
31) Bacon, J.; Dean, P. A. W.; Gillespie, R. J. Can. J. Chem. 1970, 48, 3413.
17
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 2
Fluorocarbocations and Protolytic Cleavage of
Trifluoroacetic Acid
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.1 INTRODUCTION
Fluorocarbocations1 play an important role as intermediates in the electrophilic
addition to fluoroolefins. The study of fluorocarbocations is of considerable interest
because o f the dualistic effect of fluorine as a substituent. Due to its high
electronegativity, a fluorine atom adjacent to a carbocationic center is inductively
destabilizing. On the other hand, the nonbonded electron pairs on the fluorine atom can
stabilize the positive charge through back-donation (n-p interaction). Numerous
fluorocarbocations were observed as stable, long-lived ions by l3C and I9 F NMR
spectroscopy since the pioneering work of Olah, Chambers, and Comisarow2, although in
some cases difficulties arose because of rapid fluoride ion exchange between superacid
and fluorocarbocations. Attempts to directly observe CH3 CHF+ 2 and CF3 + 3 a by NMR
spectroscopy under superacid stable ion conditions were unsuccessful. The intermediacy
of CF3 + was suggested, 315 however, in the ionization and subsequent decarbonylation of
trifluoroacetyl fluoride CF3 COF with SbF5 at low temperature. Whereas no CF3 CO+ and
CF3 + cations could be observed directly, l9F NMR spectroscopy showed already at
relatively low temperature (-50 °C) the formation o f tetrafluoromethane CF4. It was
proposed that the great strength of the C-F bond in CF4 (ca. 140 kcal/mol) leads to rapid
quenching of CF3 + to CF4 even in low nucleophilicity fluorinated superacid media.
19
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
c f 3c of
SbFs
— ►
-78 °C
-50 °C
[CFgCO*]
F
► [CF34 ]
SbF6-
*-► c f4
-CO
In this study, ab initio / IGLO4 / GIAO-MP25 calculations were carried out on a
series of fluorocarbocations which have earlier been characterized by l9F NMR
spectroscopy under long lived stable ion conditions. This permits comparison of
calculated data with experimentally observed results. Such calculations have become
increasingly useful to study electron deficient intermediates, but were so far not reported
for fluorocarbocations. We have also calculated the structures and 1 3 C and l9F NMR
chemical shifts for still elusive perfluorinated cations, such as CF3 +. In addition, the
protolytic cleavage of trifluoroacetic acid in strong superacids giving CF4 is investigated
by experimental and theoretical methods. Based on ab initio calculations it is suggested
that the reaction involves the CF3 C(OH)(OH2 )2 + dication as an intermediate.
2.2 RESULTS AND DISCUSSION
2.2.1 Fluorocarbocations
Ab initio calculations were carried out by using the GAUSSIAN-946 package of
programs. Optimized geometries were obtained at the MP2/6-31G* level and selected
parameters of the ions are given in Figure 2.1 and Figure 2.2. Vibrational frequencies at
20
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the HF/6-3 lG*//HF/6-31G* level were used to characterize stationary points as m inim a
IGLO calculations were performed according to the reported method4 at IGLO II level
using MP2/6-31G* geometries. Huzinaga/Gaussian lobes were used as follows; Basis H:
C, O or F: 9s 5p Id contracted to [51111, 2111, 1], d exponent: 1.0, H: 5s lp contracted
to [ 311, 1], p exponent: 0.70. GIAO-SCF and GIAO-MP2 calculations using the tzp/dz
basis set5 have been performed with the ACES II program . 8 Chemical shifts are listed in
Table 2.1.
So far the appplication of IGLO and GIAO methods to fluorocarbocations has not
been explored. IGLO method has been applied9 only to calculate the l9F NMR chemical
shifts of a number of small neutral molecules. With the use of large basis sets TZP (triple
zeta plus polarization functions) satisfactory agreement between gas phase I9F NMR
chemical shifts with calculated 1 9 F NMR chemical shifts has been found. IGLO
calculations with DZ (double zeta) level were shown to be unreliable for l9F NMR
chemical shifts. 9 The application of GIAO-MP2 method, which includes dynamic
electron correlation in chemical shift calculations, to the chemical shifts of I 3 C, I7 0 , l5 N,
l9 F, etc.,5 show, in some cases, significant improvements over the chemical shifts results
computed at the SCF level.
21
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19 13
Table 2.1. F and selected C NMR chemical shifts of fluorocarbocations
No. Cation Atom IGLO n//
MP2/6-3IG*
GIAO-SCF GIAO-MP2 Expt
1. [CF3r F 58.6 81.3 51.7
C+ 162.1 167.1 169.2
2. CH3[CHF] + F 215.0 230.8 258.8
C* 277.5 279.0 279.7
3. CH3[CFF] + F 126.9 143.8 135.5 96.4
c t 216.8 222.0 219.0
4. (CH3)2[CF] + F 183.7 200.3 219.2 185.0
C+ 295.7 296.3 295.9 282.8
5. C2H5[CFCH3] + F 162.5 183.5
C+ 293.0 283.6
6. [c-C5H8F] + F 159.1 149.4
C+ 311.4 294.0
7. [c -C3H2F] + F -56.5 -46.8 -50.2 -67.0
C(F) 168.2 170.3 181.2
C(H) 160.7 160.9 163.7
8. [c -C3HF2] + F -51.7 -42.1 -49.5
C(F) 154.0 156.0 163.3
C(H) 146.7 147.0 147.9
9. [c-C3F3] + F -50.6 -42.0 -55.4 -63.1
C 138.3 140.5 145.2
10. [r-CHF=OCH3] + F 73.3 90.4 72.8 49.9
C(F) 189.9 192.2 186.5
C(H) 75.0 76.5 83.1
11. [c-CHF=OCH3] + F 68.1 84.5 69.1 41.8
C(F) 184.3 188.0 180.9
C(H) 74.7 76.1 82.0
22
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2.1 Selected MP2/6-31G* optimized parameters of 1-11
1 (D3h) 2(Cs) 3(Cs)
o o
4(C2) 5 (Cl)
0 0
6(C2)
1.385
7 (C2v)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2.1 (cont) Selected MP2/6-31G* optimized parameters of 1-11
1.373
8 (C2v)
1.389
9(D3h)
10 (Cs)
1 1 (Cs)
Figure 2.2. Selected MP2(FU)/6-31G* optimized parameters of 1 2 and 13
12 (Cs)
13 (Cs)
24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2.1.1 C F3 + , 1
In the gas phase, trifluoromethyl cation 1, is stable and has been observed as an
abundant species. 1 0 We have recently reported the calculated energy and structure of 1. 1 1
The MP2/6-3IG* optimized structure o f 1 is a planar D 3 h with a shorter C-F bond length
of 1.246 A compared to C-F bond length of (1.33 A) of CF4. Since fluorine possesses 2 p
nonbonded electron pairs, the back donation 2 p-2 p overlap is maximum in ion 1 ,
resulting in the shorter C-F bond length. However, Reynolds1 2 calculated the relative
stability of CF3 + compared to other trihalomethyl cations (CX3 + , X=C1 and Br). The
calculated order o f stability was found to be Cl > Br » F. Therefore, the electron-
withdrawing power of the three fluorine atoms in CF3 + surpasses their Tt-donating ability.
IGLO II and GIAO-MP2 calculated I3C chemical shift of 1 are 162.1 and 169.2 ppm,
respectively, are close to the predicted value of S1 3 C 140.0 obtained from comparison
with other known trihalomethyl carbocations. 3 GIAO-MP2 calculated 1 9 F chemical shifts
of ion 1 is 5i9F 51.7.
2.2.1.2 CH 3 C H F \ 2
So far 2 has also not been observed in solution under stable ion conditions. 2
Structure 2 was found to be the global minimum on the potential energy surface. The
hydrogen bridged structure 2a is not a minimum at the MP2/6-31G* level and converged
into 2 upon optimization at the MP2/6-31G* level. C-H hyperconjugation is responsible
for the shorter C-C bond (1.433 A) in 2. Structure 2 has previously been calculated by
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reynolds1 3 at MP2/6-31G** and the results were similar to those obtained in the present
study.
Compared to 1, the GIAO-MP2 calculated l3C and l9 F chemical shifts of 2 of
8 I3C 279.7 and 8 1 9 F 258.8, respectively, are much more deshielded. Electron correlation
has little effect on 1 3 C chemical shift calculations. Accordingly, both IGLO H (8 1 3 C
277.5) and GIAO-SCF (8 l3C 279.0) calculated 1 3 C chemical shifts of 2 are very close to
the corresponding GIAO-MP2 calculated value of 8 I3C 279.7.
2.2.1.3 CH 3 C(F)F+ , 3
Methyldifluorocarbenium ion 3 was observed1 4 as a long-lived ion by ionizing
CH3 CF3 in SbF5 /S 0 2 ClF solution at -80 °C and characterized by 'H and l9F NMR
spectroscopy. The longer C-C bond (1.452 A) of 3 compared to that (1.433 A) of 2 can be
accounted for on the basis that both fluorine atoms in ion 3 are capable of back-donation
and thus stabilize the ion by resonance. The GIAO-MP2 calculated averaged 1 9 F chemical
shift of 3 is SI9F 135.5, which deviates from the experimental value (SI9 F 96.4) by 39.1
ppm. However, the overall correlation of the GIAO-MP2 calculated 1 9 F chemical shifts
26
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
with the experimental chemical shifts of the fluorocarbocations is excellent as shown in
Figure 2.3. The GIAO-MP2 calculated l3C chemical shift o f 13 (8 l3C 219.0) indicates a
moderate shielding effect as compared to 2 (8 1 3 C 279.7). However, no experimental 1 3 C
chemical shift of 3 is available to make comparisons with the theoretical results.
2.2.1.4 (CH 3 )2 CF+ , 4
The C 2 structure 4 is the most stable conformer of (CH3 )2 CF+ at the MP2/6-31G*
level of calculations. A similar C 2 structure was also shown to be the most stable
conformer for the 2-propyl cation. 1 5 Whereas the GLAO-MP2 calculated 1 3 C NMR
chemical shift of Sl3C 295.9 agrees well with the experimental chemical shift of 8 I3 C
282.8, the GIAO-MP2 calculated 1 9 F NMR chemical shift of S,9F 219.2 again deviated
by 34 ppm from the experimental value o f 8 l9F 185.0.
2.2.1.5 CH 3 CH 2 C+ FCH3 , 5
The optimized structure 5 shows a long C(CH3 )-C(CH2 ) bond (1.578 A) aligned
parallel with the p-orbital of C+ thus permitting maximum C-C hyperconjugation. This
type of long bond has also been found in tertiary alkyl carbocations. 1 6 Ion 5 has also been
1 7
observed m superacid solution under stable ion conditions. Because of the size of the
molecule, we were not able to calculate its chemical shifts at GIAO-MP2 level.
Experimental and IGLO calculated chemical shifts are shown in Table 2.1.
27
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2.1.6 c-C 5 H 8 F+ (6 )
The twisted C2 structure 6 is found to be the global m inim um of l-fluoro-l-
cyclopentyl cation. The ion was previously prepared1 7 by Olah et al. by treating 1,1-
difluorocyclopentane with a SbF5 -S 02ClF solution at -78 °C. The 1 9 F NMR spectrum of
ion 6 contains a deshielded quintet centered at 5 I9 F 149.4 and can be compared to the
IGLO calculated value of 8 1 9 F 159.1.
2.2.1.7 c-C 3 H 2 F+ , 7
The monofluorocyclopropenium ion 7 is the simplest substituted three-membered-
ring Huckel aromatic system. The ion was studied by NMR and vibrational
spectroscopy. 1 8 The two calculated C-C force constants obtained from vibrational
spectroscopy of monofluorocyclopropenyl-c/ 0 and -d2 cations correspond to a weaker C-C
bond opposite to the fluorine substituted carbon and a stronger C-C bond adjacent to the
fluorine substituted carbon. Our calculated structure 7 also shows that the longer (1.385
A) C-C bond is opposite to the fluorine substituted carbon and the shorter (1.367 A) C-C
bond is adjacent to the fluorine substituted carbon. The interaction of the fluorine atom
with the cyclopropenyl ring is substantial as shown by the shorter C-F bond (1.274 A),
which is even shorter than that of ion 4, 5 and 6 . Both IGLO and GIAO calculated 1 9 F
NMR chemical shifts of 6 agree well with the experimental data (Table 2.1).
28
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2.1.8 c-C 3 HF2 + , 8
Difluorocyclopropenium ion 8 is not known experimentally. The calculated C-F
bond length (1.272 A) of 8 is very close to the C-F bond length of 7. The two shorter C-C
bonds (1.382 and 1.373 A) indicate that the ion 8 also has substantial aromatic character.
The calculated chemical shifts are also very close to those of monofluorocyclopropenium
ion 7 (Table 2.1).
2.2.1.9 c-C 3 F3 + , 9
Trifluorocyclopropenium ion 9 is observed1 9 experimentally by treating
perfluorocyclopropene with excess o f SbF5 at 0 °C. 9 is characterized by a single peak in
the fluorine NMR spectrum at 8 1 9 F -63.1, which is deshielded by 57.8 ppm when
compared to neutral perfluorocyclopropene (51 9 F -120.9). The structural feature and
calculated and experimental chemical shifts of this trifluorosubstituted cyclopropenium
ion 9 are very similar to those of mono- and difluoro substituted cyclopropenium ions 7
and 8 , respectively (Figure 2.1).
2.2.1.10 trans- and c/s-CHF=OCH3 + , 10 and 11
When a,a-difluoromethyl methyl ether was treated with SbF5 -S 0 2 at -40 °C, two
isomeric methoxyfluorocarbenium ions were obtained2 0 of which 70% is 10 and 30% is
11 as measured by integration of the *H NMR signals.
29
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SbF5-S 0 2
CH3OCHF2 ------------► 10 + 11
-40 °C
At MP2/6-3lG*//MP2/6-31G* level the ion 11 is only 3.2 kcal/mol more stable than the
ion 1 0 . At MP4(SDTQ)//6-31G*//MP2/6-31G* level the ion 1 1 is still 3.2 kcal/mol more
stable than the ion 10. This is, however, not in agreement with the experimental results
where 10 was formed predominantly. The calculated structures o f ions 10 and 11 shows
that the ions are predominantly carboxonium ions rather than carbenium ions as indicated
from the calculated C(CH2 )-0 bond distances (1.24 A) of 1 0 and 1 1 , which are close to
the C=0 distance in carbonyl compounds. The calculated chemical shifts are
summarized in Table 2.1.
2.2.1.11 Chemical Shift Correlation
The GIAO-MP2 calculated l9F NMR chemical shifts are in excellent agreement
with the experimental data (Figure 2.3a) and are clearly superior to the GIAO-SCF
(Figure 2.3b) and IGLO II (Figure 2.3c) calculated l9F NMR chemical shifts.. In the
series CF3 + , CH3 C+ F2 , and (CH3 )2 C+ F both the calculated and experimental 8 l3C values
indicate an increase in the deshielding effect at the carbocationic center with decreasing
fluorine substitution. Presumably, this is due to an increase in fluorine back donation into
the carbocationic center. Both calculated and experimental I9 F NMR chemical shifts
indicate that this effect is more pronounced in the monofluorinated derivatives.
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2.3 Plot of calculated vs. experimental NMR chemical shifts of fluoro-
carbenium ions
200
100
O
£
*
J Z
o
O N
I
U-
y= -18.410 + 0.90399X RA 2 = 0.997
-100
-100 0 100 200 300
GIAO-MP2/tzp/dz calculated F-19 chemical shifts
(a) GIAO-MP2/tzp/dz vs. experimental
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2.3 (cont) Plot of calculated vs. experimental NMR chemical shifts of
fluorocarbenium ions
200
5 1 0 0 -
o
£
J Z
o
o s
I
u.
T J
1 *
>
V I
.O
O
y = - 28.438 + 0.96147x RA 2 = 0.981
100
100 200 -100 300 0
GIAO-SCF/tzp/dz calculated F-19 chemical shifts
(b) GIAO-SCF/tzp/dz vs. experimental
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2.3 (cont) Plot of calculated vs. experimental NMR chemical shifts of
fluorocarbenium ions
200
-16.050 + 1,0567x RA 2 = 0.975
S 100 -
I
*
I
Li.
-100
-100 0 100 200
IGLO I I calculated F-19 chemical shifts
(c) IGLO II vs. experimental
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2.2 Protolytic Cleavage of Trifluoroacetic Acid
We have previously3 8 ,1 * attempted to observe the trifluoromethyl cation 1 ion by
protolytic cleavage of trifluoroacetic acid CF3 COOH or its esters with FS0 3 H:SbF5 .
However, only protonated trifluoroacetic acid was observed and attempts to dehydrate it
to trifluoroacetyl cation (CF3 CO+ ) and subsequently via decarbonylation to cation 1 were
unsuccessful.3 8 ,1 * In fact, even at 60 °C in neat "Magic Acid" no cleavage of protonated
trifluoroacetic acid was observed by 1 3 C and 1 9F-NMR spectroscopy.3 1 * This is surprising,
since protonated carboxylic acids in general readily dehydrate to yield the corresponding
acylium cations. 2 1 CF4 was not observed, but because of its volatility small quantities of
CF4 (bp = -128 °C) are difficult to observe in solution by 1 9 F NMR spectroscopy. We
have now reinvestigated the ionization of trifluoroacetic acid with Magic Acid by
employing more sensitive gas-IR spectroscopy to detect CF4 possibly formed during the
reaction. Indeed, FT-IR analysis of gas samples taken after reacting trifluoroacetic acid
with excess F S 0 3 H:SbF5 (50 mol% SbF5 ) at room temperature for 30 minutes in an
autoclave, showed a strong absorption band at 1277.3 cm ' 1 indicative of CF4. The
assignment of the peak to tetrafluoromethane was confirmed by adding pure CF4 gas to
the sample. The formation of CF4 indicates protolytic cleavage of protonated
trifluoroacetic acid to CF3 CO+ and subsequent decarbonylation to the CF3 + cation which
is then quenched by fluoride ion (from SbF6 * or the acid system) to form CF4.
34
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Interestingly, no protolytic cleavage occurred when trifluoroacetic acid was
reacted with superacid systems weaker than 1:1 molar Magic Acid. For example, no
formation of CF4 was detected when mixtures of FS0 3 H:SbF5 containing only 1 , 2, 5,
and 10 mol%, respectively, o f SbF5 were used. There are two possible reasons for the
lack of CF4 formation in the latter acid systems: (a) the protolytic cleavage of
trifluoroacetic acid is dependent on the acidity of the superacid employed, or, (b) the
fluoride concentration in these acid systems is not sufficient for the quenching of CF3 +
cations to CF4. We have also attempted to react trifluoroacetic acid with a mixture of
FS03 H and KF (ca. 1:1 molar). This acid system generates HF in situ as a fluoride ion
source for the quenching of CF3 + cations to CF4. However, no CF4 formation was
detected under these conditions, indicating that the protolytic cleavage of trifluoroacetic
acid is mainly dependent on the acidity o f the superacid system. This acidity dependence
can be rationalized by further protolytic (i.e. superelectrophilic) activation o f protonated
trifluoroacetic acid, involving a diprotonation equilibrium with a reactive gitonic dication
(mechanism I) rather than direct cleavage of the monoprotonated trifluoroacetic acid
(mechanism II) as depicted in Figure 2.4.
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2.4 Reaction mechanisms for the protolytic cleavage o f trifluoroacetic acid.
cP
CF3 — Cn
OH
H*
Mechanism I
H
OH
CF3 — C v N +
OH
12
Mechanism
*0H
c f 3 — c n +
OH 2
13
gitonic dication
H20
c f 3 — c n +
OH 2
15
H20
+
+ -OH
CF3 — c '
14
c f 3c o +
16
CO
+ SbF 6
c f 3*
1
+ SbF 6
-SbF 5
CF4
36
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Protolytic activation of protonated trifluoroacetic acid 12 (mechanism I) leads to a
highly electron-deficient, superelectrophilic, gitonic dication 13, which is substantially
more reactive than its parent monocation 12.2 2 (Figure 2.4) Its subsequent dehydration
and decarbonylation leads to the CF3 + cation 1 which is then readily quenched by fluoride
ion (from SbF6 * ) to form CF4. Similar diprotonation equilibria have previously2 3 been
suggested in the ionization reactions of formic acid and acetic acid, respectively, in
excess superacid. On the other hand, the direct cleavage of monoprotonated
trifluoroacetic acid 12 (mechanism II) would involve tautomerization to form cation 15
which subsequently would dehydrate and decarbonylate to yield the CF3 + cation 1.
In order to investigate the two mechanistic possibilities we have carried out ab
initio calculations. Diprotonated trifluoroacetic acid 13 (mechanism I) was found to be a
stable minimum structure at MP2/6-31G* level and can be viewed as a donor-acceptor
complex of H2 0 and protonated trifluoroacetyl dication CF3 COH2 + 14 with a long C-C
bond of 1.599 A (Figure 2.2). Protonation of 12 to form dication 13 was calculated to be
slightly endothermic by only 2.6 kcal/mol at MP2(fu)/6-31G* + ZPE level. Attempts to
find a stable minimum for protio-trifluoroacetyl dication 14 failed because of its
spontaneous dissociation into CF3 + 1 and protonated carbon monoxide COH+.
Simultaneous dehydration and deprotonation of dication 13 into CF3 + 1, COH+, and H2 0
was calculated to be exothermic by 37.0 kcal/mol at MP2(fu)/6-31G* + ZPE level. This
reaction is even more exothermic if the subsequent protonation o f water to H3 0 + (AH =
37
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
-167.0 kcal/mol at MP2(fu)/6-31G* + ZPE level) in the superacid media is considered.
Finally, fluoride abstraction by CF3 * 1 from SbF6' leads to the formation o f CF4.
Tautomerization of 12 to form cation 15 (mechanism II) on the other hand, is
endothermic by 15.9 kcal/mol at MP2(fu)/6-31G* + ZPE level. Subsequent dehydration
of cation 15 to form trifluoroacetyl cation CF3 CO+ 16 was calculated to be endothermic
by 26.8 kcal/mol at the same level. Decarbonylation o f 16 to form CF3 + 1 is also
endothermic by 19.6 kcal/mol; the latter two reactions become exothermic, however, if
the subsequent protonation of H2 0 and CO to H 3 0 + (DH=-167.0 kcal/mol) and COH"
(AH = -97.6 kcal/mol), respectively, are considered. As in mechanism I, fluoride
abstraction by CF3 + 1 from SbF6 ‘ leads to the formation of CF4.
Based on the calculated data it is suggested that the protolytic cleavage of
CF3 COOH in superacid media involves the cleavage of protio-(trifluoromethyl
carboxonium) dication CF3 C(OH)(OH2 )2 + 13 (mechanism I) rather than the direct
cleavage of monoprotonated trifluoroacetic acid 12 (mechanism II). This is in accord
with the observed acidity dependence of the reaction. In addition, theoretical calculations
have shown that diprotonation o f trifluoroacetic acid is energetically feasible and dication
13 was found to be a stable minimum structure at MP2(fu)/6-31G* level.
38
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.3 CONCLUSIONS
The structures of a series of fluorocarbocations were calculated at the correlated
MP2/6-31G* level. I3 C and I9 F NMR chemical shifts o f these structures were calculated
using IGLO and GIAO-MP2 methods. The data showed good correlation of calculated
l9F and I3C chemical shifts with the experimental chemical shifts of the
fluorocarbocations. The correlation for GIAO-MP2 calculated 1 9 F chemical shifts with
the experimental data is excellent. The protolytic cleavage o f trifluoroacetic acid
CF3 COOH in superacids giving CF4 was also investigated and the reaction mechanism is
suggested to involve the intermediacy of the reactive gitonic CF3 C(OH)(OH2 )2 + dication
13.
2.4 EXPERIMENTAL SECTION
CF3 COOH, KF, CF4 , and H2 S0 4 are commercially available products (Aldrich)
and were used as received. Antimony pentafluoride (Allied-Chemical) and fluorosulfonic
acid (3M) were doubly distilled prior to use. IR spectra were obtained on a Nicolet 800
FT-ER spectrometer using a gas IR cell equipped with NaCl windows.
39
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Protolytic cleavage of CF3COOH with Magic Acid: 1 mL of trifluoroacetic
acid was placed into a bomb (stainless steel) equipped with a magnetic stirrer and cooled
to -78° C. After the addition o f 4 mL of FS0 3 H/SbF5 (1:1 molar ratio) the reactor was
closed and allowed to warm up to room temperature under continuos stirring. A gas
sample was taken from the reactor after about 2h. FT-ER. analysis showed a strong
absorption band at 1278 c m 'l The assignment of this peak to CF4 was confirmed by
adding pure CF4 gas to the sample. The same experimental procedure was used for the
attempted protolytic cleavage o f CF3 COOH in various other acid systems (see text).
40
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.5 REFERENCES
1. For a review o f Fluorocarbocations see: Olah, G. A.; Mo, Y. K. in: Advances
in Fluorine Chemistry, 1973, 7, 69.
2. Olah. G. A.; Chamber, R. D.; Comisarow, M. B. J. Am. Chem. Soc. 1967,89,
1268.
3. (a) Olah. G. A.; Heiliger, L.; Prakash, G. K. S. J. Am. Chem. Soc. 1989, III,
8020 (b) Olah, G. A.; Germain, A.; Lin, H. C. J. Am. Chem. Soc. 1975,97,
5481.
4. Kutzelnigg, W.; Isr. J. Chem., 1980,19, 193.; Schindler, M.; Kutzelnigg, W.;
J. Chem. Phys., 1982, 7 6 ,1919.; Schindler, M.; J. Am. Chem. Soc. 1987, 109,
1020.; W. Kutzelnigg, U. Fleischer and M. Schindler, NMR Basic Principles and
Progress, 91 (1991), 651.
5. Gauss, J.; J. Chem. Phys. Lett.;. 1992,191, 614.; Gauss, J.; J. Chem. Phys.;.
1993, 99, 3629.
6 . Gaussian 94 (Revision A.1), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill,
P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.;
Peterson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B.
B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R ; Martin, R. L.; Fox,
D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Head-Gordon,
M.; Gonzalez, C.; Pople, J. A., Gaussian, Inc., Pittsburgh PA, 1995.
7. Huzinaga, S. Approximate Atomic Wave Function. University of Alberta,
Edmonton, Alberta, 1971.
8 . Stanton, J.F.; Gauss, J.; Watts, J.D.; Lauderdale, W.; Bartlett, R.J.; ACES II, an
ab initio program system; University of Florida: Gainesville, FL, 1991.
9. Fleischer,U.; Schindler, M. Chem. Phys. 1988,120, 103.
10. Murdoch, H. D.; Weiss, E.; Helv. Chim. Acta.; 1962, 4 5 ,1927.
11. Olah, G. A.; Rasul, G.; Heiliger, L.; Prakash, G. K. S.; J. Am. Chem. Soc., 1996,
in press; Olah, G. A.; Rasul, G.; Yudin, A. K.; Burrichter; A.; Prakash, G.
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
K. S.; Chistyakov, A. L.; Stankevich, I. V.; Akhrem, I. S.; Gambaryan, N. P.;
Vol'pin, M. E.; J. Am. Chem. Soc., 1996,118,1446.
12. Reynolds, C. Yl.'J. Chem. Soc. Chem. Commun.,\99\, 975.
13. Reynolds, C. H.^7. Am. Chem. Soc. ,1992,114, 8676.
14. Olah, G. A.; Comisarow, M. B., J. Am. Chem. Soc. 1969, 91, 2955.
15. Schleyer, P.v. R.; Koch, W.; Liu, B.; Fleischer, U.; J. Chem. Soc. Chem.
Commun:, 1989, 1098.
16. Schleyer, P. v. R.; Cameiro, J. W. M.; Koch, W.; Forsyth, D.; J. Am. Chem.
Soc., 1991,113, 3990.
17. Olah, G. A.; Liang, G.; Mo, Y. K., J. Org. Chem. 1974,39,2394.
18. Craig, N. R.; Lai, R. K.; Matus, L. G.; Miller, H.; Palfrey, S. L.., J. Am. Chem.
Soc. 1980,102, 38.
19. Sargeant, P. B.; Krespan, C. G., J. Am. Chem. Soc. 1969, 91, 415.
20. Olah, G. A.; Bollinger, J. M., J. Am. Chem. Soc. 1967, 89, 2993.
21. Olah, G. A.; White, A. M.; O'Brien, D. H. Chem. Rev. 1970, 70, 561.
22. Olah, G. A. Angew. Chem. Int. Ed. Engl. 1993, 32, 767.
23. Hartz, N.; Rasul, G.; Olah, G. A. J. Am. Chem. Soc. 1993,115,1277.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 3
Trifluoromethyl Substituted Carboxonium Ions
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.1 INTRODUCTION
Carboxonium ions such as A and fi (R, = alkyl; R2 > 3 = H or alkyl) are important
intermediates in many acid-catalyzed organic reactions. Considerable interest has
centered on the elucidation of their structure and electronic properties. 1
R
A
R
c = o r 3
c — OR 3
II
B R.
/ * 2
\ ) R ,
R,
/O R 2
c x ;
o r 3
R,— C
/
O R,
O R ,
m IV
Carboxonium ions were first studied by Meerwein2 ' and their chemical behavior
reflects both their oxonium and carbenium ion nature. Ions A can be obtained by
protonation or methylation of ketones. They can be visualized as hybrids of resonance
structures (I) and (II) (Rl i 2 = alkyl; R3 = H or alkyl).2 d Carboxonium ions B result from
the protonation or alkylation of esters. They can be depicted by three resonance forms
(III), (IV), and (V) (R, 3 = alkyl; R2 = H or alkyl).2 d A number of carboxonium ions
have been isolated as stable salts with various counterions (such as BF4\ SbF6\ SbCl6 ',
A sF 6') and have found use as effective alkylating reagents in organic synthesis.2 a ,c
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The destabilizing effect o f a trifluoromethyl group (CF3 ) adjacent to a carbenium
ion center is well documented both by theory and experiment. 2 As reflected by its
Hammett3 substituent constant (8 p+ (p-CF3 ) = 0.61), the CF3 group is one o f the strongest
electron withdrawing groups and thus inductively destabilizes carbocationic centers. In
addition, the trifluoromethyl group lacks the ability to stabilize positive charge through
k-7i back-donation. Thus, trifluoromethyl-substituted carbocations are generally less
stable than their fluoro-substituted analogs. A number of trifluoromethyl-substituted
carbocations have been observed as stable, long-lived species in superacid solution 3 a,b ,
but no study of the related trifluoromethyl substituted carboxonium ions has been
reported. An investigation o f trifluoromethyl-substituted carboxonium ions is of interest
in view o f their highly electron deficient nature and because of the increasing role of
trifluoromethylated organic compounds in the development of pharmaceutical4 and
agricultural chemicals. 5 Further, as high level ab initio calculated geometries together
with IGLO6 (individual gauge fo r localized orbitals) calculated l3C NMR chemical shifts
have become increasingly useful in predicting accurate molecular structures o f such
electron deficient carbocations7 , such studies were also carried out.
Subject of this study is the preparation and characterization of the trifluoromethyl
substituted carboxonium ions CF3 C(OH)CH3 + 2, CF3 C(OCH3 )CH3 + 3,
CF3 C(OH)OCH3 + 8 , and CF3 C(OCH3 )2 + 9 by lH and 1 3 C-NMR spectroscopy under
stable ion conditions.
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.2 RESULTS AND DISCUSSION
Substitution o f one or more alkyl groups (R) in A or B by trifluoromethyl groups
is expected to significantly decrease the stability o f the corresponding carboxonium ion
due to the trifluoromethyl group’s strong electron withdrawing effect. In order to assess
the effect of trifluoromethyl substituents on the stabilities of carboxonium ions, we
attempted to prepare and observe ions containing one (2, 3, 8a-d, 9a-c), two (5, 6), and
three (10) perfluoroalkyl groups under superacidic stable ion conditions. The
experimental 'H and I3C NMR chemical shifts of the observed cations and their parent
compounds are summarized in Table 3.2 and Table 3.3.
To gain better insight into the structural parameters of trifluoromethyl-substituted
carboxonium ions, high level ab initio molecular orbital calculations were carried out on
trifluoroacetone 1, hexafluoroacetone 4, methyl trifluoroacetate 7a,b and their protonated
and methylated forms. Geometry optimizations o f all isomers were performed up to
MP2(fu)/6-31G* level and are summarized in Figure 3.1. Total energies (- a.u.), ZPE
(kcal/mol), and relative energies (kcal/mol) of the ions are given in Table 3.1. IGLO I3 C
NMR chemical shift calculations were carried out for the most stable isomers using the
MP2(fu)/6-31G* geometries and are summarized in Table 3.2.
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.1. Total energies (-a.u.), ZPE (kcal/mol) and relative energies (kcal/mol) of
protonated and methylated trifluoroacetone, hexafluoroacetone, and methyl trifluoroacetate.
Structure HF/6-31G*// ZPE' MP2(fu)/6-3lGV/ ReL Energy1
HF/6-31G* MP2(fu)/6-3IG*
Trifluoroacetone
I. (Cs) 488.53463 3733 489.62318
Protonated Trifluoroacetone
2. (Cs) 488.82419 44.68 489.90444
Methylated Trifluoroacetone
3. (Cl) 527.86221 61.42 529.07648
Hexafluoroacetone
4. (C2) 785.09484 2433 786.69468
Protonated Hexafluoroacetone
5. (Cl) 785.34595 31.62 786.93838
Methylated Hexafluoroacetone
6. (Ct) 824.39194 48.17 826.11820
Methyl Trifluoroacetate
7a. (Cs)
7b. (Cl)
563.40283
56338757
41.09
40.98
564.66942
564.65631
0.00
8.34
Protonated Methyl Trifluoroacetate
8a. (Cs) 563.69995 4830 564.95826
8b. (Cl) 563.69549 48.32 564.95404
8c. (Cl) 563.69176 48.33 564.95174
8d. (Cs) 563.69029 48.10 564.94766
0.00
2.62
4.07
6.82
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.1. (cont.) Total energies (-a.u.), ZPE (kcal/mol) and relative energies (kcal/mol) of
protonated and methylated trifluoroacetone, hexafluoroacetone, and methyl trifluoroacetate.
Structure HF/6-31G*//
HF/6-31G*
ZPE' MP2(fu)/6-31G*//
MP2(fu)/6-3lG*
ReL Energy2
Methylated Methyl Trifluoroacetate
9a. (Cl) 602 3 6 138 65.13 604.12801 0.00
9b. (Cl)
602.72360 65.13 604.11631 7.32
9c. (Cs) 602.72437 64.90 604.11379 9.16
zero-point vibrational energies at the HF/6-3lG*//HF/6-31G* level scaled by a factor of 0.89
relative energy in kcal/mol at MP2(fu)/6-3 lG*//MP2(fu)/6-3lG* + ZPE level
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.2. Experimental and Calculated I3 C NMR Shifts at IGLO II // MP2(fu)/6-3 IG* level.
Structure c=o CF, CH, OCH,
1. (Cs)
expt.
calc.
Trifluoroacetone
187.40
197.34
115.60
106.83
23.10
18.87
2. (Cs)
expt.
calc.
Protonated Trifluoroacetone
225.45
243.24
114.12
104.59
26.64
21.04
3. (Cl)
expt.
calc.
Methylated Trifluoroacetone
222.32
239.91
116.78
104.36
24.03
25.00
75.97
75.61
4. (C2)
expt.
calc.
Hexafluoroacetone
172.22
183.03
114.36
104.89
5. (Cl)
calc.
Protonated Hexafluoroacetone
224.18 104.00
6. (Cl)
calc.
7a. (Cs)
expt.
calc.
Methylated Hexafluoroacetone
Methyl Trifluoroacetate
217.79
157.98
166.68
103.71
104.57
114.48
104.89
83.37
54.36
50.51
8a. (Cs)
expt.
calc.
Protonated Methyl Trifluoroacetate
172.49
186.21
112.24
103.00
68.01
69.38
9a. (Cl)
expt.
calc.
Methylated Methyl Trifluoroacetate
171.66
184.91
114.48
103.60
68.21
72.44
65.23
69.41
49
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.3 Experimental lH-NMR Chemical Shifts of Carboxonium Ions
No. Molecule -OH -OCHj -CHj Reference
Acetone
(CHjhCO - - 2.09
t
(CH3)2COH* 14.93 - 3.45
2
(CH3)2COCH3 + - 5.23 3.40
J
Trifluoroacetone
1 CH3 (CF3)CO - - 2.42
I
2 CH3(CF3)COET 12.84 - 2.94 this study
3 CH3 (CF3)COCH3 + - 5.10 3.24 this study
Methyl Acetate
c h 3 c o 2c h 3 - 3.67 2.01
i
CH3 C(OCH3 )(OH)+ 12.72 4.53 2.83
4
CH3 C(OCH3 )2* 4.62
4.36
2.76
s
Methyl Trifluoroacetate
7a CF3 C 02 CH3 - 3.98 -
1
8a CF3 C(OCH3 )(OH)+ 12.10 6.86 - this study
9a CF3 C(OCH3 )2 + 5.15
4.61
this study
1 Pouchert. C . J.; Behnke, J . "The Aldrich Library o flsC and 'H FT NMR Spectra “ 1st Ed., Aldrich Chemical
Co., Milwaukee. 1993. 2 Olah, G . A.; Calin. M .; O’Brien. D. H . J. Am. Chem. Soc. 1967, 89, 3586. 3 O lah,
G. A.; Parker. D . G .; Yoneda, N. J. Org. Chem. 1977, 42, 32. 4 Olah, G . A.; O'Brien, D. H .; W hite, A. M .
J. Am. Chem. Soc. 1967, 89,5694. 4 Dusseau, C . H . V.; Schaafsma, S. E .: Steinberg. H .; de Boer, T. J .
Tetrahedron Letters 1969, 6, 467.
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.1. MP2(fu)/6-31G* optimized geometries
1.220
125.60
119.24
112.30
l.(C s)
1*^0998
112.63 ^ 9
117.69
F ^ 108.27 f
1.265
^ 120.77
' 111.69 H
2. (Cs)
H
105.87
126.28
119.08
120.72
109.91
dihedral H! C2 0 3 C4 = -85.38
3. (CO
1.213
1.535
r 2
0 6
II
122.05
C ^ n ^ / s
’ \ ^
C^**F
I
dihedral F5 C3 C i0 6 = -19.36
dihedral F 4 C2 Ci Og = -19.36
4. (CO
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.1. (cont) MP2(fu)/6-31G* optimized geometries
1.004 h
112.98
1.251.
p 117_53 /"I 120‘48 P
V 5
FflftSr£ 1 _ 5 4 2 C3 \ ,
dihedral F4 C2 Ci 0 6 =-19.81
dihedral F5 C3 Ci 0 6 = -8.81
5. (CO
H7
1 _ 5 6 1
° 6 . J 126.25
1-245
123.34
116.54I - p
' C i 1 107J8XS
1-540 S W
.2 1-544- C 3 ^ F
I
Fs
dihedral F4 C -> Ct 0 6 = - 3.30
dihedral F5 C3 Ct 0 6 = -172.77
dihedral H7 C8 0 6 Ci = -154.07
6. (Q)
123.83
F 110.47 ("
1.213
127.13
1531 1336 O
7a. (Cs)
114.06
104.89
O 7
119.86
11056
r,
1.211
122.01
r ' 1540 1.340 o <
F ^ 2 123.2X . •
F
3 2X . I 104.12
1 4 4 6 r ^ H *
H
dihedral F3 C, Ci 0 7 = -5.13
dihedral C5 0 4 Ct 0 7 = 175.56
dihedral H 6 C5 0 4 Ct = 167.23
7b. (CO
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.1. (cont.) MP2(fii)/6-31G* optimized geometries
121.35
121.94
106.90
120.30
1.542 1.258 O 103.14
8a. (Cs)
/ " H fJ
^ 6 J 11X67
114.99 1 2 3 2
F7. H O -38 r ( O 120.66
V / \
,...C 2 -544 1.256>,
F < /
F
124.47
1-5031 H,
102.60
5
H,
dihedral F7 Co Ct 0 6 = 6.55
dihedral H5 C4 0 3 0 != -162.1
dihedral H 9 C4 O3 Ci = - 42.9
H o
12.29 V.
119-51
108.03 (
1.283
^ 116.56
p « p r “ « M
S U 4 - V „ \ ~ - H<
H 9
dihedral F7 C2 C! 0 5 = - 5.37
dihedral Hs C4 0 3 Cx = 161.44
dihedral H 9 C4 0 3 C! = 80.52
8b. (CL )
116.49
1.284
128.32
122.34
1.544
1.265
1.486
103.45
109.40
F
8 c. (CO 8d. (Cs)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 3.1. (cont.) MP2(fu)/6-31G* optimized geometries
H. H 9
n m - c ' ) «“■«
\ L.495
123.93 ^ 9 s
1.274
123.84^
120.92
\ 119.56,
.H
/ V T . 4 ^ 4\
^ ^ C - ^ - 542 l- 2 7 0 V ^ w \
/
103.34
lH
He
110.46
/>
102.07
H,„ > >
\M 9 l
I
128.45 V s .
0 6
1.271
124.94, | U449
llM l ^ 1.274
1^49 -0 ,
V ° :5 h
126.05
1.489
H'"“
•c;
h5
dihedral F7 C, Ct 0 6 = - • -11.15
dihedral H5 C4 0 3 C! = 160.60
dihedral 0 3 C[ Og C8 = 177.56
dihedral H9 C8 Og Q = 173.45
9b. (Ct)
H
128.64
dihedral F7 C2 Cv 0 6 = -174.38
dihedral H 9 C8 0 6 Cj = -163.37
9c. (Cs)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.2.1 Protonated trifluoroacetone (2)
Trifluoroacetone 1 is readily protonated in FS0 3 H:SbF5 solution at -20 °C with
S 0 2 C1F as solvent and the resulting carboxonium ion 2 could be observed by lH and 1 3 C
NMR spectroscopy.
H3 C.
F,C
I
/
c= o
Magic Acid
-20 °C
HjC\ - / H
/ C ^ o +
F,C
anti
H3 C\ j L .
f 3 c h
syn
In theory, protonation of 1 can yield two different isomers of 2, syn and anti.
NMR spectroscopy, however, indicated the presence of only one isomer, which was
characterized by a deshielding of 38 ppm at carbonyl carbon in 2 ( 8 1 3 Ce x p C=0 225.5)
with respect to 1 (5 I 3 Ce x p C =0 187.4). In order to rationalize the experimental results,
ab initio calculations were carried out on the two possible isomers of 2. At MP2(fu)/6-
31G* level, only the syn isomer was found to be a stable minimum structure. The
experimentally observed chemical shifts of protonated trifluoroacetone (see Table 3.2
and Table 3.3) were therefore assigned to the syn isomer of 2. The IGLO II calculated
,3C NMR chemical shifts are in reasonable agreement with the experimental data. IGLO
II predicts a deshielding of 45.9 ppm for the carbonyl carbon in 2 ( 8 I 3 Cc a Ic C=0 243.2)
as compared to parent trifluoroacetone 1 ( 8 l3 Cc a |C C=0 197.3). The deshielding (38
55
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ppm) at carbonyl carbon in protonated trifluoroacetone 2 is smaller than that observed in
protonated acetone. Protonated acetone8 (5 I3 Ce x p C=0 248.7) shows a deshielding of
44 ppm as compared to acetone ( 8 l3 Ce x p C=0 205.1). This difference can be rationalized
by the strong inductive electron withdrawing effect of the trifluoromethyl group in 2
which leads to a more distinct oxonium (I) ion nature in protonated trifluoroacetone 2 as
compared to protonated acetone. Protonated trifluoroacetone 2 has also been observed as
a stable species in the gas phase by ion cyclotron resonance spectrometry. 9
MO-calculations at MP2(fu)/6-31G* level show that the eclipsed-eclipsed, e-e,
conformation with cs symmetry is preferred in protonated trifluoroacetone 2. Similar
results have previously1 0 been reported for protonated acetone. 2 is characterized by a
C= 0 bond elongation of ~ 3.6 % (0.045 A) and a shortening o f the neighboring C-
C(CH3 ) bond by ~ 3 % with respect to 1. The increase of the C=0 bond length is
substantially smaller than that found for protonated acetone (4.1 %) by Krivdin et a l n* at
a similar level of theory and can readily be explained by a stronger contribution of the
oxonium ion (I) resonance structure in protonated trifluoroacetone 2 as compared to
protonated acetone.
56
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.2.2 Methylated Trifluoroacetone (3)
Trifluoroacetone 1 is readily methylated in CH3 F:SbF5 :S0 2 solution at -60 °C and
a deshielding at the carbonyl carbon by 35 ppm is observed in the methylated species 3 (5
I 3 Cex p 222.3) as compared to the parent 1 ( 8 1 3Ce x p 187.4):
CH.ESbF, h 3 \ _ L /C H , h 3 c x
-60 °C ^ +
f 3c f 3 c c h 3
anti syn
Again, two different isomers, syn and anti, of 3 are possible. Under the experimental
conditions only one isomer was observed by NMR. In agreement with the experimental
results, ab initio calculations at the MP2(fu)/6-31G* level resulted in the syn isomer as
the only stable minimum structure for 3. The experimentally observed chemical shifts of
methylated trifluoroacetone (see Tables 3.2, 3.3) were therefore assigned to the syn
isomer of 3. IGLO II slightly overestimates the deshielding at carbonyl carbon in 3 and
predicts a deshielding of 42.6 ppm in 3 (5 l3 Cc a ]c C=0 239.9) as compared to parent
trifluoroacetone 1 ( 6 l3 Cc a |C C =0 197.3). Again, the magnitude of the deshielding at
carbonyl carbon is greater for acetone (40 ppm) than for trifluoroacetone (35 ppm).
Methylated acetone1 1 ( 8 1 3 Cex p C =0 245.5) shows a deshielding of 40 ppm with respect
57
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
to parent acetone ( 8 l3 Ce x p C=0 205.1). As in the case of protonated trifluoroacetone 2,
this can be explained by a stronger contribution o f the oxonium ion resonance structure
(I) in 3 than in methylated acetone due to the strong electron withdrawing trifluoromethyl
group. Similar to protonated trifluoroacetone 2, the calculated C=0 bond in methylated
trifluoroacetone 3 is longer by 0.038 A (3 %) compared to 1 , whereas the C-C (CH3 )
bond is shorter by 0.036 A (2.4 %), indicating charge delocalization among the three
atoms.
3.2.3 Protonated Hexafluoroacetone (5)
Protonated hexafluoroacetone 5 has been observed as an abundant species in the
gas phase by ion cyclotron resonance spectroscopy1 0 . Attempts to observe long lived
protonated hexafluoroacetone in the condensed phase in FS0 3 H:SbF5 :S0 2 solution at -60
°C, however, were unsuccessful.
F,C
F,C
\
i
c = o
Magic Acid F3C +
— x / \
-60 °C
F,C
C— OH
58
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
MO-Calculations at MP2(fu)/6-31G* level indicate a C=0 bond elongation by 0.038 A ( 3
%) in 5 with respect to hexafluoroacetone 4. The increase in C=0 bond length upon
protonation is smaller than those calculated for protonated trifluoroacetone 2 (3 . 6 %) and
protonated acetone (4.1 %). This can be rationalized by a strong contribution o f the
oxonium ion (I) resonance structure in protonated hexafluoroacetone 5. There is almost
no change in the C-C(CF3 ) bond length between 4 and 5, indicating very little charge
delocalization in the protonated species. IGLO II calculations predict the carbonyl carbon
in 5 ( 8 l3 Cc a ic C=0 224.2) to be deshielded by 41.2 ppm with respect to 4 ( 8 1 3 Cc a Ic C=0
183.0). It is, of course, possible that an extremely limited protonation equilibrium exists
in the superacid solution, but it cannot be detected by NMR spectroscopy.
3.2.4 Methylated Hexafluoroacetone (6 )
Attempts to methylate hexafluoroacetone 4 with CH3 F:SbF5 in S0 2 solution at -60
°C to a persistent carboxonium ion were also unsuccessful.
4
CH3F:SbF5
-60 °C
6
59
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
MO-calculations at MP2(fu)/6-31G* level predict a C=0 bond elongation of 0.032 A (2.6
%) for 6 with respect to the neutral parent 4. This value is similar to that calculated for
protonated hexafluoroacetone 5 (3 %). IGLO II calculations predict the carbonyl carbon
in 6 (5 1 3Cc a lc C=0 217.8) to be deshielded by 34.8 ppm with respect to parent 4 ( 6 1 3Cc a lc
C=0 183.0). Again, the NMR spectroscopic studies cannot answer the question whether
an extremely limited methylation equilibrium could be present.
3.2.5 Protonated Methyl Trifluoroacetate (8a-d)
NMR spectroscopy reveals that methyl trifluoroacetate 7 is readily protonated at
the acyl oxygen in FS0 3 H:SbF5 :S0 2 ClF solution at -50 °C.
Z 1 Magic Acid
F3 C~ C\
o c h3
7
H
>
F j C _
f3 c - < ' ° >
0
/
h3 c
H
>
F= C“ V
h3 c
F3C— C ' +
0"CH
8a 8b
8c 8d
60
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Four different isomers, 8a, 8b, 8c, and 8d, are possible for protonated methyl
trifluoroacetate. Under the experimental conditions only one isomer was observed,
which was characterized by a slight deshielding of 14.5 ppm at the carboxylic carbon in
protonated methyl trifluoroacetate ( 8 1 3Ce x p C=0 172.5) as compared to 7 (5 1 3 Ce x p C=0
158.0). As expected, protonation at the alkyl oxygen was not observed under the above
conditions. Similar to previously1 2 reported studies on protonated methyl acetate, four
stable minimum structures 8a-d were found for protonated methyl trifluoroacetate at the
MP2(fu)/6-31G* level. Structure 8a was found to be the global minimum, differing only
by a few kcal/mol from the other isomers 8b-d (see Table 3.1). Accordingly, the
experimentally observed chemical shifts (see Tables 3.2, 3.3) were assigned to the most
stable isomer 8a, accordingly. The IGLO II calculated i3 C-NMR chemical shifts of
protonated methyl trifluoroacetate 8a are in good agreement with the experimental results
(Table 3.2).
Due to the strong electron withdrawing effect of the trifluoromethyl group, the
contribution of the carbenium ion structure IV (Ri=CF3 ; R 2 =CH3; R3=H) is greatly
reduced in protonated methyl trifluoroacetate 8a-d. This explains why protonated methyl
acetate9 ( 8 1 3 Ce x p C=0 192.8) shows a considerably higher deshielding effect (22 ppm) at
the carboxylic carbon with respect to the parent methyl acetate ( 8 I 3 Ce x p C=0 170.7).
61
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8 a is characterized by a C= 0 bond elongation of 0.073 A ( 6 %) and a shortening
of the C-0 (CH3 ) bond by 0.078 A (5.8 %) as compared to methyl trifluoroacetate 7 a.
3.2.6 M ethylated Methyl Trifluoroacetate (9a-c)
Methyl trifluoroacetate is readily methylated at the acyl oxygen in
CH3 F:SbF5 :S0 2 solution at -60 °C and the methylated species could be observed as a
long lived stable ion by lH and l3C NMR spectroscopy.
F,C— C .
\
O
OCH,
C H 3F:SbFs
-60 °C
h 3 c x
v ' ' 0
f 3c — c . +
+
V CH3
h 3c x
f 3c — q x +
>
h 3c
+
/ ? '
F 3 C— C ' +
V
o -
'CH-i
-CH,
9a
9b 9c
Three isomers, 9a, 9b, and 9c, are possible for methylated methyl
trifluoroacetate. Under the stable ion conditions, however, only one isomer was observed
62
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
by NMR spectroscopy, which was exhibited by a deshielding of 13.7 ppm at the
carboxylic carbon (5 I 3 Ce x p C=0 171.7) compared to its precursor 7 ( 8 1 3 Cc x p C=0
158.0). Similar to previously1 3 a ,b reported studies on methylated methyl acetate, there are
three minimum structures were found for methylated methyl trifluoroacetate 9 a-c.
Structure 9a was found to be the global minimum and is calculated to be 9.16 kcal/mol
more stable than structure 9c at MP2/6-31G* + ZPE level. The experimental chemical
shifts of methylated methyl trifluoroacetate were assigned to structure 9a. The calculated
deshielding of 18.2 ppm (IGLO II) at the carbonyl carbon in 9a compares well with the
observed deshielding of 13.7 ppm. The magnitude of the deshielding is similar to that of
in protonated methyl trifluoroacetate 8 a-d and reflects a strong contribution of oxonium
resonance forms III and V (R,=CF3; R 2 < 3 =CH3 ). When compared to 9a-c, methylated
methyl acetate1 3 * ( 8 I 3 Ce x p C=0 192.8) shows a considerably higher deshielding ( 2 2
ppm) at the carboxylic carbon with respect to methyl acetate (5 l3 Cex p C=0 170.7).
3.2.7 Perfluorotrialkoxymethyl Cations (10)
Attempts to generate carboxonium ions with three perfluoroalkoxy groups were
unsuccessful. Perfluoroethyl orthoformate (10, RF = C2 F5 ) and perfluorobutyl
orthoformate (10, RF = C4 F9 ) did not ionize with SbF5 in Freon 113 solution at -40 °C to
63
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
give the corresponding perfluoro-triethoxymethyl (11, RF = C2 Fs) and perfluoro-
tributoxymethyl (11, RF = C4 F9 ) cations, respectively:
F—
/ O R f
c — ORf
ORF
SbF5
- X -
RpO—
ORp
ORp
SbFf i
10 11
3.3 CONCLUSIONS
We have prepared and studied by NMR spectroscopy the highly electron deficient
trifluoromethyl-substituted carboxonium ions CF3 C(OH)CH3 + 2, CF3 C(OCH3 )CH3 + 3,
CF3 C(OH)OCH3 + 8 , and CF3 C(OCH3 )2 + 9. lH and l3C NMR studies were carried out at
low temperature. The IGLO II calculated I3C chemical shifts o f these ions are in good
agreement with the experimental data, as shown in Table 3.2. Preparation of
carboxonium ions containing two or three perfluoroalkoxy groups such as
CF 3 C(OH)CF3 + 5, CF3 C(OCH3 )CF3 + 6 , and C(ORF )3 + 10) were unsuccessful under
similar conditions.
64
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.4 EXPERIMENTAL PART
Trifluoroacetone and methyl trifluoroacetate are commercially available (Aldrich)
and were distilled prior to use. Hexafluoroacetone (Aldrich) was used without further
purification. Doubly distilled FS0 3 H, SbF5 i and S 0 2 C1F were used for the preparation of
the ions. CH3 F and S 0 2 were purchased from Matheson Gas Co. and used as received.
Perfluorotriethyl orthoformate and perfluorotributyl orthoformate were prepared in Prof.
R. J. Lagow’s laboratory by direct fluorination o f the corresponding orthoformates in
Freon 113 as solvent. 1 3
lH and 1 3 C NMR spectra were obtained on a spectrometer equipped with a
variable temperature probe at 300 MHz and 75.4 MHz, respectively. lH and I3 C NMR
spectra were obtained with respect to TMS by using an acetone-d6 capillary as external
standard.
Ab initio molecular orbital calculations were carried out by using the GAUSSIAN
941 4 package of programs. Restricted Hartree-Fock calculations were performed
throughout. All geometries were fully optimized at the MP2(fu)/6-31G* level. Chemical
shifts have been evaluated using the direct IGLO7 method employing the II basis set.
MP2(fu)/6-31G* optimized geometries were used for the chemical shift calculations. The
calculated 1 3 C chemical shifts (5 values) are referenced to TMS.
65
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Protonation Experiments The appropriate precursor (~ 30 mg) was dissolved in
approximately 0.5 mL of S 0 2 or S 0 2 C1F (see text) in a 5 mm NMR tube and cooled to -
78 °C in a dry ice/acetone bath. Approximately 1.5 mL of 50% v/v solution of
FS0 3H:SbF5 (1:1 molar solution) in S 0 2 or S 0 2 C1F (see text) was added to the solution
at -78 °C. The ensuing mixture was vigorously stirred (Vortex stirrer) under periodic
cooling prior to transfer to a precooled NMR instrument.
Methylation Experiments The appropriate precursor (~ 30 mg) was dissolved in
approximately 0.5 mL of S 0 2 in a 5 mm NMR tube and cooled to -78 °C in a dry
ice/acetone bath. A solution of CH3 F:SbF5 :S0 2 was prepared by bubbling CH3 F through
a complex of SbF5 in S 0 2 (ratio of 1:5 by volume) for approximately one to two minutes
at -78 °C and subsequently added in excess to the reaction mixture. The mixture was
vigorously stirred under periodic cooling until clear (Vortex stirrer) and then transferred
to a precooled NMR instrument.
66
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.5 REFERENCES
1) (a) Perst, H. "Oxonium Ions in Organic C h e m is tr y Verlag Chemie: Weinheim,
Germany, 1971. (b) Perst, H. "Carbonium Ions"', Olah, G. A.; Schleyer, P. v. R.
Eds.; Wiley-Interscience: New York, 1976; Vol. 5, pp. 1961-2047 (c) Meerwein,
H. "Methoden der Organischen Chemie (Houben-Weyl)", 4th Edition, E. Muller,
Ed., Thieme, Stuttgart, 1965; Vol. VI/3: Sauerstoffverbindungen, p. 329. (d)
Olah, G. A.; White, A. M.; O'Brien, D. H. Chem Rev. 1970, 70, 561-591 and
references therein, (e) Hartz, N.; Rasul, G.; Olah, G. A. J. Am. Chem. Soc. 1993,
115, 1277-1285 and references therein.
2) For reviews see: (a) Gassman, P. G.; Tidwell, T. T. Acc. Chem. Res. 1983, 16,
279. (b) Creary, X. Chem. Rev. 1991,91,1625.
3) (a) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96. (b) Johnson, K. F. "The
Hammett Equation"-, Cambridge University Press, New York, 1973.
4) Filler, R.; Kobayashi, Y.; Yagupolskii, L. M.; Eds. “ Organojluorine Compounds
in Medicinal Chemistry and Biomedical Applications" Elsevier, Amsterdam,
1993.
5) Yoshioka, H.; Nakayama, C.; Matsuo, N. J. Synth. Org. Chem., Jpn. 1984, 42,
809.
6 ) (a) Kutzelnigg, W. Isr. J. Chem. 1980, 19, 193. (b) Schindler, M.; Kutzelnigg,
W. J. Chem. Phys. 1982, 76, 1919. (c) Review: Kutzelnigg, W.; Fleischer, U.;
Schindler, M. in: “NMR, Basic Principles and Progress"', Springer Verlag:
Berlin, Heidelberg, 1990; Vol. 23, p 165.
7) (a) Hehre, W.J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. "Ab Initio Molecular
Orbital Theory", John Wiley & Sons, New York: 1986 (b) Lammertsma, K.;
Schleyer, P. v. R.; Schwarz, H. Angew. Chem. Int. Ed. Engl. 1989, 28, 1321. (c)
Lammertsma, K. Rev. Chem. Interm. 1988, 9,141.
8 ) Olah, G. A.; White, A. M. J. Am. Chem. Soc. 1969, 91, 5801.
9) Drummond, D. F.; McMahon, T. B. Int. J. Mass Spectrom. Ion Phys. 1977, 25,
27.
10) (a) Krivdin, L. B.; Zinchenko, S. V.; Kalabin, G. A.; Facelli, J. C.; Tufro, M. F.;
Contreras, R. H.; Denisov, A. Yu.; Gavrilyuk, O. A.; Mamatyuk, V. I. J. Chem.
67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Soc. Faraday Trans. 1992, 8 8 , 2459-2463. (b) Wiberg, K. B.; Marquez, M.;
Castejon, H. J. Org. Chem. 1994, 59,6817.
11) Olah, G. A.; Parker, D. G.; Yoneda, N.; Pelizza, F. J. Am. Chem. Soc. 1976, 98,
2245.
12) (a) Olah, G. A.; Hartz, N.; Rasul, G.; Burrichter, A.; Prakash, G. K. S. J. Am.
Chem. Soc. 1995, 117, 6421. (b) Wiberg, K. B.; Waldron, R. F. J. Am. Chem.
Soc. 1991, 113, 7705.
13) Mlsna, T. E.; Lin, W.-H.; Hovsepian, M. M.; Lagow, R. J. Eur. J. Solid State
Inorg. Chem. 1992, 29, 907.
14) Gaussian 94 (Revision A.1), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill,
P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Peterson,
G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V.
G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B.; Nanayakkara, A.;
Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J.
L.; Replogle, E. S.; Gomperts, R ; Martin, R. L.; Fox, D. J.; Binkley, J. S.;
Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Head-Gordon, M.; Gonzalez, C.; Pople,
J. A., Gaussian, Inc., Pittsburgh PA, 1995.
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI
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 o f computer printer.
The quality of this reproduction is dependent upon the quality of the
copy submitted. Broken or indistinct print, colored or poor quality
illustrations and photographs, print bleedthrough, substandard margins,
and improper alignment can adversely afreet reproduction.
In the unlikely event that the author did not send UMI 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. Each
original is also photographed in one exposure and is included in reduced
form at the back o f the book.
Photographs included in the original manuscript have been reproduced
xerographically in this copy. Higher quality 6” x 9” black and white
photographic prints are available for any photographs or illustrations
appearing in this copy for an additional charge. Contact UMI directly to
order.
UMI
A Bell & Howell Information Company
300 North Zeeb Road, Ann Arbor MI 48106-1346 USA
313/761-4700 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.
UMI Number: 1383524
Copyright 1996 by
Burrichter, Arwed A.
All rights reserved.
UMI Microform 1383524
Copyright 1997, by UMI Company. All rights reserved.
This microform edition is protected against unauthorized
copying under Title 17, United States Code.
UMI
300 North Zeeb Road
Ann Arbor, MI 48103
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

ipso substitutions of aryl boronic acids and aryltrifluoroborates

PDF

Investigation of silylated onium ions

PDF

Trifluoromethanesulfonates (triflates) for organic syntheses

PDF

Activation and functionalization of carbon-hydrogen bonds catalysed by oxygen ligated iridium metal complexes

PDF

Development of high-performance polymer electrolyte membranes for direct methanol fuel cells

PDF

Design and synthesis of novel lipid mediators

PDF

Design and synthesis of novel heterocycles and peptidomimetics from organoboronic acids, amines and carbonyl compounds

PDF

Investigations in selective fluorinations: Novel synthetic methodologies and material syntheses

PDF

Electrochemical studies of membranes, catalysts, and fuels for direct oxidation fuel cells

PDF

Infrared spectroscopy and ab initio studies of carbon dioxide van der Waals complexes

PDF

Carbocations and electrophilic reaction intermediates.

PDF

Development of a 20-femtosecond tunable ultraviolet laser source towards study of the photochemistry of liquid water

PDF

Atropisomerism In Sterically Hindered Carbocations

PDF

I. Metal nanoparticles on high surface area latex supports: Preparation and applications, and, II. Modified semiconductor quantum dots as luminescent label for in situ hybridizations

PDF

Superelectrophilic Reactions Under Superacidic Catalysis

PDF

Novel nucleophilic and electrophilic fluoroalkylation reactions and related chemistry

PDF

Crosslinked siloxanes: Preparation and properties

PDF

Design and synthesis of foscarnet -peptide conjugates, halogenated bisphosphonates and phosphonocarboxylates and biochemical sensing using film -bulk -acoustic -resonators

PDF

Femtosecond laser studies of biological systems

PDF

Association of perfluorocarbon containing water soluble polymers

Asset Metadata

Creator Burrichter, Arwed A. (author)

Core Title Fluorinated carbocations and carboxonium ions

School Graduate School

Degree Master of Science

Degree Program Chemistry

Degree Conferral Date 1996-12

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

Tag chemistry (physical chemistry),chemistry, organic,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Olah, George A. (committee chair), Prakash, G.K. Surya (committee member), Weber, William P. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-7663

Unique identifier UC11341325

Identifier 1383524.pdf (filename),usctheses-c16-7663 (legacy record id)

Legacy Identifier 1383524.pdf

Dmrecord 7663

Document Type Thesis

Rights Burrichter, Arwed A.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA