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Trifluoromethanesulfonates (triflates) for organic syntheses
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Trifluoromethanesulfonates (triflates) for organic syntheses
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TRIFLUOROMETHANESULFONATES (TRIFLATES)
FOR ORGANIC SYNTHESES
by
Andreas Archut
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 1994
Copyright 1994 Andreas Archut
UNIVERSITY O F SO U T H E R N CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELE6, CALI FO RNIA 9 0 0 0 7
ft
This thesisi written by
Andreas Archut
under the direction o f hj.P. Thesis C om m ittee,
and approved by all its m embers, has been p re
sented to and accepted by the D ean of The
Graduate School, in partial fu lfillm en t o f the
requirements fo r the degree of
Master o f Science
Dean
THESISfCOMMT
CHairman
M einer Familie
“The great tragedy of Science -
the slaying of a beautiful hypothesis by an ugly fact.”
Thomas Henry Huxley, 1825 - 1895
iii
Acknowledgements
First of all I would like to express my gratitude to my academic advisor,
Professor George A. Olah for his support during my work at the Loker
Hydrocarbon Research Institute. Your excitement for chemistry has been infecting
and your words of support kept me going when the molecules did not seem to do
what I wanted. Undoutably, you are a noble teacher, although the spelling of
“noble” has been changed last October...
I would like to thank Professor G.K.Surya Prakash for his advice and
patience with all the little problems I bothered him with.
Not only when it came to mastering the shoals of the English language, my
roommate Dr. Robert W. Reed helped me to overcome difficulties and problems on
the way.
Many thanks to Dr. Klaus Weber for lending me his patience and expertise
during endless coffee breaks on the “Deutsche Bank.”
My thanks for the friendly reception to all the members of the Olah and
Prakash groups, but especially Dr. Herwig Buchholz, Dr. Denis Deffieux, Dr. Jorg-
Stephan Brunck, Dr. Patrice Batamack, Dr. Golam Rasul and Dr. Robert Anisfeld
and the graduate students Tatyana Shamma, Mike Heagy, Arwed Burrichter, Andrei
Yudin, Chentao York and Jay Struckhoff.
For support on the computational battlefield I would like to thank Harold
Holcomb, Andre Schreiber and Mark Tapsak. Further thanks for advice and help to
Terri Drought, Reiko Choy, Michelle Dea, Paul Langford and Jim Merrit. A special
“Dankeschon" to "Hauswirt" John Devon.
Thanks to friends in the “Old World” go to Oliver Braun, Klaus Veiling and
Holger Henke (“der Abgesandte des Ostens").
Support by the Konrad-Adenauer-Stiftung, Germany, the Graduate School
and a Harold Molton Fellowship granted by the Loker Hydrocarbon Research
Institute is gratefully acknowledged.
I thank all the others I forgot to mention for their patience and pardon,
v
Abstract
The convenient synthesis of organic and inorganic trifluoromethanesulfonate
(triflate) reagents with potenital use in organic syntheses was investigated.
Methylene bistriflate was successfully prepared in a metathesis reaction of
diiodomethane and silver triflate, however, attempts to isolate the compound failed.
Attempts to generate the parent vinyl triflate by the addition o f triflic acid to
trimethylsilyl-substituted acetylenes were unsuccessful.
Trichloromethyl triflate, possibly a powerful trichloromethylation reagent
and as such a carboxylate synthon was prepared from bromotrichloromethane and
silver triflate.
A number of metal triflates were prepared, by an alternative method to the
literature procedures, via the metathesis reactions of silver triflate and the
corresponding metal chlorides. The use of a triflate ion exchange resin for the
generation o f organic triflates was investigated.
Table of Contents
Trifluoromethanesulfonates (Triflates) for Organic Syntheses
C hapter 1: General Introduction 1
1.1 Triflic Acid - a Versatile Member of the Superacid Family 1
1.2 Practical Use of Triftc Acid and Its Derivatives 3
1.3 The Triflate Ion and Its Role in Chemistry 4
1.4 Triflic Acid Esters in Organic Syntheses 5
1.5 References 6
C hapter 2: Methylene Bistriflate 8
2.1 General Introduction 8
2.2 Methylene Bistriflate - First Formation 9
2.3 Preparation o f Triflate Esters 9
2.4 Conversion of 1,3,5-Trioxane with Triflic Anhydride 10
2.5 Metathesis of Silver Triflate with Dihalomethanes 13
2.6 Conclusion 19
2.7 References 20
C hapter 3: Vinyl Triflates 21
3.1 Application of Vinyl Triflates in Chemistry 21
3.1.1 Studies of Reactive Intermediates 21
3.1.2 Vinylic Carbon-Carbon Bond Formations 22
3.1.3 Vinylic Carbon-Heteroatom Bond Formations 22
3.2 Generation of Vinyl Triflates 23
3.3 The Target Compounds of this Thesis 25
3.4 Possible Approaches to the Synthesis of Vinyl Triflate 26
3.5 Attempted Preparation of Vinyl Triflate 29
3.5.1 Addition of Triflic Acid to Acetylenes 29
3.5.2 Reaction of Triflic Anhydride with Acetaldehyde 30
3.6 Conclusion and Outlook 32
3.7 References 34
Chapter 4: Trichloromethyl Triflate 37
4.1 Introduction 37
4.2 Nucleophilic Trichloromethylation Reactions 37
4.3 Electrophilic Trichloromethylations 38
4.4 Trichloromethyl and Trifluoromethyl Triflate 39
4.5 The Reaction of Trichloromethyl Triflate with Benzene 41
4.6 Attempted Synthesis o f Trichloromethyl Triflate 42
4.6.1 Metathesis with Silver Triflate 42
4.6.2 Yields and Reaction Conditions 43
4.6.3 Side Product Formation 45
4.7 Conclusion and Outlook 48
viii
4.8 References 49
Chapter 5: M etal Triflates 50
5.1 Binary Element-Triflate Compounds 50
5.2 Element-Triflate Compounds in Synthesis 50
5.2.1 Non Metal Triflates 51
5.2.2 Metal Triflates 51
5.2.3 Metal Triflates in Hydration Studies 51
5.2.4 Metal Triflates as Friedel-Crafts Catalysts 51
5.3 General Approaches in the Formation of Metal Triflates 52
5.4 Preparation o f Metal Triflates by Silver Triflate Metathesis 55
5.5 Potential Catalytic Activity of Nickel Triflate 56
5.6 Triflate Ion Exchange Resin 57
5.7 References 60
Chapter 6: Experimental 62
6.1 General Features 62
6.1.1 Chemicals Used 62
6.1.2 Reaction Conditions 62
6.1.3 Hazards and Precautions 62
6.1.4 Spectroscopy 63
6.1.5 Ultrasound 63
6.2 Methylene Bistriflate Experiments 64
ix
6.2.1 Reactions o f 1,3,5-Trioxane with Triflic Anhydride 64
6.2.2 Metathesis of Diiodomethane with Silver Triflate 64
6.2.3 Reaction of Dibromomethane with Silver Triflate 65
6.3 Vinyl Triflates 65
6.3.1 Reaction of Acetylenes with Triflic Acid 65
6.3.2 Reaction of Acetaldehyde with Triflic Anhydride 66
6.4 Trichloromethyl Triflate 67
6.4.1 Attempted Trichloromethylation o f Benzene 67
6.4.2 Formation of CCljOTf at Elevated Temperature 67
6.4.3 Formation of CCbOTf at Ambient Temperature 68
6.4.4 Control Experiments: Sonication o f CCl3 Br and CCL 68
6.5 Metal Triflates 69
6.5.1 Preparation of Nickel Triflate 69
6.5.2 Cobalt Triflate 69
6.5.3 Iron (III) Triflate 69
6.5.4 Preparation o f Mercury (II) Triflate 70
6.5.5 Attempted Preparation of Ruthenium (III) Triflate 70
6.6 Gattermann-Koch Formylation of Toluene 71
6.7 Reaction o f Triflic Acids with Metals 71
6.8 Triflate Ion Exhchange Resin 72
6.8.1 Preparation o f the Ion Exchange Resin 72
x
6.8.2 Preparation of Methyl Triflate by ton Exchange 72
6.8.3 Formation of Cyclohexyl Triflate 73
6.8.4 Attempted Formation ofMethylene Bistriflate 73
6.8.5 Attempted Formation o f Trichloromethyl Triflate 73
6.9 References 74
Chapter 7: Conclusion 75
xi
List of Schemes
Scheme l.l Triflic Acid and the Target Compounds o f This Thesis. 5
Scheme 2.1 Potential Applications ofMethylene Bistriflate. 8
Scheme 2,2 Common Procedures for the Formation o f Triflate Esters. 10
Scheme 2.3 Suggested Mechanism for the Formation of Vinyl Triflate. 11
Scheme 2.4 Bistriflates form Hindered Ketones. 11
Scheme 2.5 Attempted Synthesis of Methylene Bistriflate. 12
Scheme 2.6 Triflic Anhydride Catalyzed Polymerization of Trioxane. 12
Scheme 2.7 Estimated Energy Profile ofMethylene Bistriflate
Preparation and Decomposition. 13
Scheme 2.8 GCMS Investigation of Methylene Bistriflate. 16
Scheme 3.1 Vinyl Triflates as Precursors for Vinyl Cations and
Alkylidene Carbenes. 21
Scheme 3.2 The Suzuki Reaction. 22
Scheme 3.3 Alkanes and Alkenes from a Carbonyl Function
via Vinyl Triflate. 23
Scheme 3.4 Vinyl Triflate Formations. 24
Scheme 3.5 Triflating Reagents. 25
Scheme 3.6 Preparation o f Vinyl Triflate via an Iodonium Triflate. 25
Scheme 3.7 Projected Synthesis of Vinyl Triflate. 28
Scheme 3.8 Potential Further Approaches.
32
Scheme 4.1 Trichloromethylation with Sodium Trichloroacetate. 38
Scheme 4.2 Decomposition of Trichloromethyl Triflate. 39
xii
Scheme 4.3 The Two Possible Bond Breaking Mechanisms in
Trihalomethyl Triflate. 40
Scheme 4.4 Metathesis of Bromotrichloromethane with Silver Trifate. 42
Scheme 4.5 Radical Processes o f Bromotrichloromethane. 46
Scheme 5.1 Friedel-Crafts Acylation with Scandium Triflate. 52
Scheme 5,2 Gattermann-Koch Type Formylation of Benzene with
Nickel Triflate. 57
Scheme 5.3 Triflate Formation via Ion Exchange. 58
xiii
List of Tables
Table 1.1 Hammett Acidities of Protic Superacids.
2
Table 3.1 Addition of Triflic Acid to Substituted Acetylenes. 27
Table 4.1 Conversion of Halotrichloromethanes with
Triflating Reagents. 43
Table 5.1 Metal Triflates and Their Preparation Procedures. 53
1. General Introduction
The focus of this thesis are derivatives of trifluoromethanesulfonic acid,
CF3SO3H. Its trade name “triflic” acid has been introduced in 1968 by Streitwieser1
and has become of common use in chemical publications. Derivatives are called
“triflates” accordingly. The trifluoromethanesulfonates have found a wide
application in organic, inorganic and organometallic chemistry and related fields.2,3
The unique properties of triflic acid and its derivatives have led to an ever increasing
number of publications since the acid was first reported in 1954 by Haszeldine and
Kidd.4
1.1 Triflic Acid - a Versatile Member of the Superacid Family
Since Hull and Conant first used the term “superacid”s in 1927,6 these acids
have found an important role in modem chemistry. By definition Br0nsted acids
stronger then 100% sulfuric acid are called superacids and some are up to billions of
times stronger than HjSC>4. Superacids have been found to stabilize carbo- and other
cations and thus to make these highly reactive intermediates accessible to standard
spectroscopy methods (especially nuclear magnetic resonance, NMR). They catalyze
a great variety of synthetic reactions in organic chemistry (e.g. isomerization of
hydrocarbon fuels,5 Friedel-Crafts reactions,7 e tc.).
1
Describing the acidity of superacids in terms of pH values as done with
aqueous solutions of organic and mineral acids 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 “leveling effect"8 In non-aqueous
systems an acid’s strength can be determined by its ability to protonate weak bases,
A quantitative measurement of acidities has been established in form of the Hammett
Acidity Function (Ho). The logarithmic Ho value is calculated by the formula
H0= pKB H + - log BH7B
where pKbh+ is the dissociation constant and BH7B is the ionization ratio between a
weak base B and its corresponding acid BH+ .9 Pure sulfuric acid is situated at -12.6
on the logarithmic scale, whereas some of the strongest, complex superacids reach
values of -25 (“magic acids”, fluoroantimonic acid and related systems).1 0
Trifluoromethanesulfonic acid has been found to be one of the strongest protic
superacids with an Hoof -14.1.5 ,1 1
Acid Ho value Reference
h 2 s o 4 - 12.6 11
CFjSOjH -14.6 5
C2F5SO3H -14.0 5, 12
C4F9SO3H -13.2 5 ,9
FSO3H -15.6 12
Table 1.1: Hammett Acidities of Protic Superacids.
2
Triflic acid is slightly lower in acidity than fluorosulfonic acid, one of the
strongest known protic acids (table 1.1).3 Unlike the latter, triflic acid is a relatively
redox stable and hence a non-oxidizing acid. It does not give off fluoride ions, even
at elevated temperatures and in the presence of strong nucleophiles.3 Its wide liquid
range (melting point < -35°C; boiling point 162°C) and its good solubility in a
variety of solvents make triflic acid the superacid catalyst of choice in many organic
syntheses.
1 .2 Practical Use of Triflic Acid and Its Derivatives
Triflic acid is produced on an industrial scale by electrochemical fluorination
of methanesulfenyl halides.1 3 It has found practical use in acid-catalyzed refining
processes of hydrocarbons. For example, the acid can be used in the alkylation of
low molecular weight alkanes with alkenes, especially in the production of high
octane gasoline from butanes and isobutylene.1 4
The metal salts of triflic acid have attracted commercial attention as electro
lytes for non-aqueous electrochemical cells and are applied as catalyst in cationic
polymerizations.1 5
3
1 .3 The Triflate Ion and Its Role in Chemistry
With triflic acids being one of the strongest protic acids known, its corre
sponding Br0nsted base (the triflate anion CF3SO3) is a very weak base. The anion
has only weakly basic sites due to good charge delocalization, and it is therefore of
low nucleophilicity. Organic chemists appreciate the triflate as a “super” leaving
group, 104 to 106 times more efficient than methylsulfonates and para-
toluenesulfonates {vide infra)} Triflates have therefore played a prominent role in
solvolysis studies e.g. of vinyl species and in forming of vinyl cations1 6 ,1 7 * 1 8 {vide
infra) where conventional “good” leaving groups would not be sufficient
Triflates are of substantial interest in organometallic and inorganic chemistry
as weakly coordinating anions (gegenions) for positively charged complexes.19,20
The triflate ion has low symmetry compared to tetrahedral or octahedral anions such
as perchlorate CIO4', tetrafluoroborate BET and hexafluorophosphate PF5 ‘, resulting
in stronger interactions with solvent molecules.2 1 Triflate has also been suggested
as a replacement for perchlorate in the laboratory because it is far more stable and
does not cause any explosion hazard.2 0
1 .4 Triflic Acid Esters in Organic Syntheses
Triflate esters have found many applications in synthetic organic chemistry.
Stang et al,2 2 have reviewed this topic in great detail. The esters are excellent alkyl
4
group transfer agents, as the triflate group can be replaced by soft nucleophiles
under very mild conditions. The high reactivity of triflate esters, however, imposes
difficulties in the preparation and isolation of certain desirable triflate species.
My thesis deals with the attempt to find facile and convenient methods to
generate potentially useful triflate reagents. Some of these compounds were already
known species but have not been applied to syntheses because they were not readily
accessible. Specifically, methylene bistriflate, the parent vinyl triflate and the
trichloromethyl triflate ester were studied in my work (scheme 1.1). A detailed
description of these compounds, as well as the chemistry involved in their formation
and application are discussed in the following chapters.
O -C H jr-O ,
TfO-H O F
1 2
F Q
C l
3 4
Scheme 1.1: Triflic Acid (1) and the Target Compounds of this Thesis: Methylene Bistriflate
(2), Vinyl Triflate (3) and Trichloromethyl Triflate (4).
1.5 References
Streitwieser, A. Jr.; Wilkins, C.L.; Kiehlmann E. J. Am. Chem. Soc. 1968, 90,
1598
2 Senning, A. Chem. Rev. 1965,65, 385
3 Howells, R.D.; Me Crown, J.D. Chem. Rev. 1977,77, 69
4 Haszeldine, R.N.; Kidd J.M. J. Chem. Soc. 1954,4228
5 Olah, G.A.; Prakash, G.K.S.; Sommer J. Superacids, Wiley-Interscience, New
York, 1991, and references therein
6 Hull, N.F.; Conant, J.B. J. Am. Chem. Soc. 1927,49, 3047
7 a) Olah, G.A. Friedel-Crqfts Chemistry, Wiley-Interscience, New York, 1973;
b) Friedel-Crqfts and Related Reactions, Vols. 1 - 4 (Ed.; G.A. Olah), Wiley-
Interscience, New York, 1963-65
8 Lowry, T.H.; Richardson, K.S. Mechanism and Theory in Organic Chemistry,
3rd ed., Harper Collins, New York, 1987
9 Olah, G.A.; Meidar, D. Kirk-Othmer Encyclopedia o f Chemical Technology,
3rd. ed., vol. 11, pp 269-300, Wiley-Interscience, New York, 1980 and
references therein
1 0 Olah, G.A. Angew. Chem. Intl. Ed. Engl. 1993,32,767 and references therein
1 1 Olah, G.A.; Prakash,G.K.S.; Sommer, J. Science 1979,206, 13
1 2 Grondin, J.; Sagnes, R.; Commeyras, A. Bull. Soc. Chim. Fr. 1976,1779
1 3 Gramstad, T., Haszeldine, R.N. J. Chem. Soc. 1956, 173
1 4 Shultz, A. Kirk-Othmer Encyclopedia o f Chemical Technology, 3rd. ed., vol.
22, p51, Wiley-Interscience, New York, 1983 and references therein
1 5 Guenthner, R.A. Kirk-Othmer Encyclopedia o f Chemical Technology, 3rd ed.
vol. 10, pp 952-955, Wiley-Interscience, New York, 1980 and references
therein
6
16 Summerville, R.H.; Senker, C.A.; Schleyer, P.v.R.; Dueber, T.E.; Stang, P.J.
J. Am. Chem. Soc. 1974, 96, 1100
17 Summerville, R.H.; Schleyer, P.v.R. J. Am. Chem. Soc. 1974, 96, 1110
18 Stang, P.J.; Rappoport, Z.; Hanack M.; Subramanian, L.R.; Vinyl Cations,
Academic Press, New York, 1979
19 Strauss, S.H. Chem. Rev. 1993, 93, 927
20 Lawrance, G.A. Chem. Rev. 1986,86, 17
21 Bergstrom, P. A.; Lindgren, J. J. Mol. Struc. 1990,239,103
22 Stang, P.J.; Hanack, M.; Subramanian, L.R. Synthesis, 1982, 85
7
2. Methylene Bistriflate
2.1 General Introduction
The emphasis of my work was to find a convenient synthesis of methylene
bistriflate. Once prepared, a methane derivative with two excellent leaving groups
should be a versatile reagent for organic syntheses. Methyl triflate1 and alkyl
triflates are known to be powerful alkylating agents.2,3 Considering it as a Ci-
building block it should be useful for linking nucleophilic sites within a complex or
two different molecules and thus for forming an intra- or inter-molecular methylene
a)
n n ;
2 TfO®
b)
OH OH
Q
O
Scheme 2.1: Potential Applications ofMethylene Bistriflate: a) with N-nucleophiles and
b) with O-nucleophiles.
bridge. This could be used for ring closures between oxygen, nitrogen or C-H-acidic
carbon atoms as well as other nucleophilic species.
8
2.2 Methylene Bistriflate - First Formation
The formation of methylene bistriflate has been reported by DesMarteau and
Katsuhara in 1980,4 It was obtained in 45% yield by reacting trifluoromethane-
sulfonic acid hypochloride with dichloromethane at -195°C in a specially designed
glass reactor and trapping the products in cooling traps. Considering the required
experimental difficulties and the hazardous triflic hypochloride this method seems
not to be suitable for the use in most synthetic organic laboratories. It was therefore
a goal to find an alternative, safer and more convenient access to the desired
compound.
F3C-|-0-CH2-0-jj-CF3
Methylene Bistriflate
2.3 Preparation of Triflate Esters
The preparation of triflate esters has been accomplished by a variety of
different routes.5 The reaction of alcohols with triflic anhydride, preferably in the
presence of base, has been widely used. Alcohols were also reported to react with
triflic acid under elimination of water to yield triflate esters. In certain cases the
elimination of dinitrogen from diazocompounds and reaction with triflic acid has
been used. Further methods involve the addition of triflic acid to carbon-carbon
9
multiple bonds as well as enolate formation from carbonyl compounds with base and
triflic anhydride {vide infra). Finally, a method of generating triflates is the
metathesis of alkyl halides with silver triflate.
CH3 CH2 OH + (CF3 S 0 2 ) 2 0 -------- ► CF3 SO3 CH2 CH3 + TfOH
CH3 CH2 OH + CF3 SO3 H ---------► CF3 SO3 CH2 CH3 + H2 O
RX + AgCF3 S0 3 -------- ► CF3 SO3 R + AgX
Scheme 2.2: Common Procedures T or the Formation of Triflate Esters.
Although these methods were mostly used to form mono triflates, it should
be possible to generate geminal bistriflates as well. Two procedures promised to be
most applicable to this synthetic problem: the reaction of the corresponding carbonyl
compound with triflic anhydride or the metathesis of a methylene dihalide and silver
triflate. A third approach was the use of an ion exchange resin with diiodomethane.
This (unsuccessful) attempt will be described in chapter 5.
2.4 Conversions of 1,3,5-Trioxane with Triflic Anhydride
Carbonyl compounds are known to react with triflic anhydride to form vinyl
triflates.6 Stang suggested that this reaction proceeds via a bistriflate intermediate.7
Whether this intermediate is a covalent geminal bistriflate or an ion pair species is
10
not yet clear. In the case of enolizable carbonyl compounds the intermediate reacts
further by proton elimination, usually supported by a base.
O
A
Tf20
TfO. „OTf TfO. -
X o r X
OTf
base
OTf
■ TfOH
Scheme 23: Suggested Mechanism for the Formation of Vinyl Triflates.
An exception are carbonyl compounds where enol formation is hindered,
such as the norbornen-5-one and the 7-norbomanone systems.8 In these cases the
geminal bistriflate is the final product and can be isolated and characterized.
T T p O
T f O ^ T f
Scheme 2.4: Bistriflates from Hindered Ketones: 5-Norbomenone and Norbornan-7-one.
To obtain the desired compound formaldehyde was reacted with triflic
anhydride. Due to the volatility of monomeric formaldehyde (b.p. -21°C) no
reaction ensued. Thus, its trimer 1,3,5-trioxane was used in the experiments.
11
? ? 1^ 2*. 3 TfO-CH2-OTf
Scheme 2.5: Attempted Synthesis of Methylene Bistriflate from Trioxane and Triflic
Anhydride.
The addition of trioxane to triflic anhydride at various temperatures,
however, did not result in detectable product formation. Temperatures above 0°C
and a too fast addition resulted in polymerization, presumably by the mechanism
indicated in scheme 2.6. Even when polymerization did not appear, analysis by
NMR only revealed starting materials.
TfO0
r ' i + T f° T f — - r \
Ck CL J X
'T f
r ' i
n o ^ o
T f-^ O C H ,)® TfCP — — *• Tf-(-OCH2) ® +(.TfCf3
Scheme 2.6: Triflic Anhydride Catalyzed Polimerization of Trioxane.
A facile decomposition pathway of methylene bistriflate is the formation of
formaldehyde and elimination of triflic anhydride, which DesMarteau has observed
12
for methylene bis-fluorosulfate compounds9 and predicted for bistriflates.4 Later on
in my study this has been confirmed by experimental results (vide infra). Hence
trioxane and triflic anhydride are probably situated at the lower end of the energy
profile of this decomposition reaction, and due to the polymerization side-reactions
it is not likely an option to force the reaction to run backwards.
Energy
trioxane + T£>0
Reaction Coordinate
Scheme 2.7: Estimated Energy Profile of Methylene Bistriflate Preparation and
Decomposition.
2.5 Metathesis of Silver Triflate with Dihalomethane
Compounds similar to the desired methylene bistriflate are methylene
bistoluenesulfonate (“tosylate”) and methylene bismethanesulfonate (“mesylate").
13
These have been prepared from diiodomethane and the corresponding silver
sulfonate salt. 10 However, unlike methylene bistriflate, the bistosylate and bis-
mesylate are fairly stable solids at room temperature. Although considered good
leaving groups, mesylates and tosylates are far less effective than triflates. 11 This is
shown convincingly by the fact, that methylene bistosylate and bismesylate could be
recrystallized from absolute ethanol!
A preparation of a,o>bistriflates has been reported by Chapman et at.1 2 from
a,oo-dibromo alkanes. Chapman reported that attempts to prepare methylene bis
triflate from dibromomethane failed. He explained this by the deactivating nature of
the bromine substituents making a cationic intermediate in a nucleophilic
substitution too unstable for reaction to occur. Bullock, Hembre et a lP reported
the synthesis of 1, 1-ditriflatoethane (“ethylidene bistriflate”) from the corresponding
diiodioethane in 87% yield. The latter result was encouraging that the metathesis of
diiodomethane with silver triflate would result in bistriflate formation.
The experiments were carried out in dried solvents and under inert gas
atmosphere at varying temperatures. The best results were obtained when an excess
of silver triflate was sonicated in pentane for 10 hours. Sonication is known to
promote reactions under heterogenous conditions. 14 Without sonication reactions in
pentane or dichloromethane at room temperature or in the refluxing solvent
provided lower overall conversion and a dominant amount of mono-substituted
product rather than the bistriflate. In addition, the heterogenous reactions were
difficult to control, since surface properties of the solid and insoluable silver triflate
could not be predicted. This is indicated by greatly varying yields making single
experiments difficult to reproduce. When acetonitrile, in which silver triflate is well
soluable, was used as solvent, no formation of either mono- or disubstituted product
was observed and it is likely that the silver triflate reacted with the solvent itself as
has been reported by Chapman et al.n for a similar reaction.
The methylene bistriflate formed in the sonication experiment could be
identified using gas chromatography/ mass spectrometry. Three distinct fractions
were obtained upon separation by gas chromatography, of which the bistriflate
appeared with the shortest retention time (approximately 3.00 minutes). Diiodo
methane was observed at a retention time of about 5.00 minutes, whereas the iodo-
triflatomethane appeared in between at about 4.20 minutes. This may be caused by
the bulky triflate substituents. Unlike iodide, triflate groups are less polarizable and
therefore less attracted by the polymer material in the chromatography column.
However, even when a low ionization energy was applied, the bistriflate molecular
peak could not be observed, except for characteristic fragments. Considering the
distinct gas chromatographic separation of the three compounds and the fact, that
unlike in the mass spectra of iodotriflatomethane and diiodomethane, no fragments
containing iodine could be found in the mass spectrum of the first fraction, this
seems to be sufficient evidence for the formation of the desired product. Integration
gave a yield of approximately 60 % (non-isolated product).
Schem e 2.8: GCM S Investigation o f M ethylene Bistriflate.
A bundance
4 0 0 0 0 0 -i
3 5 0 0 0 0 -
3 0 0 0 0 0 *
2 5 0 0 0 0
200000
1 5 0 0 0 0
100000
5 0 0 0 0
rim e ->
Abundance
1 4 0 0 0 0
120000 -
1 0 0 0 0 0 -
aoooo
6 0 0 0 0
99
163
4 0 0 0 0 -
2 0 0 0 0 *
133
" 0
90____100 110 120 130 140 150 160 170 M/Z - > 70 80 60
Above: Gas chromatography spectrum of a mixture of methylene bistriflate (3.20 minutes),
iodo triflato methane (4.20 minutes) and diiodomethane (5.00 minutes). The signal at
3.45 minutes is an impurity (bromo iodo methane).
Below: Mass spectrum of diiodomethane; m/z = 163 CF3 SO3CH2*; m/z = 133 CF3S C > 2+ ; m/z
99 CF3 CH20 + ; m/z = 69 CF3+ .
16
Scheme 2.8 (continued)
ftbundance
3 0 0 0 -
2 5 0 0 -
2000
1500
163
1 0 0 0
500
127 141
99
m.
SO 70 90 100 110 120 130 1 4 0 1 5 0 160 170
A bundance
2 5 0 0 0 0
200000
1 5 0000
141
1 0 0 0 0 0
127
5 0 0 0 0
254
134
26 >
140 160 180 200 220 2 4 0 260
Above: Mass spectrum of iodo triflato methane; m/z = 163 CFaSOaCHj*; m/z = 141 ICHi ;
m/z = 133 CFaStV; m/z = 1271*; m/z = 99 CFaCHiO+; m/z = 69 CF3*.
Below: Mass spectrum of diiodomethane; m/z = 268 M **; m/z = 254 = h ; m/z = m/z = 141
ICHj+; m/z = 1271*.
17
The instability and high reactivity of methylene bistriflate made it difficult to
prepare a sample for NMR. After removal of silver iodide and excess silver triflate
by filtration and evaporation of pentane in vacuo, the residue was taken up in
deuterated benzene. When the 13C{ *H} and ’H spectra were recorded the bistriflate
had apparently decomposed into 1,3,5-trioxane and triflic anhydride, as shown by
comparison with commercially available trioxane and triflic anhydride. The identifi
cation of trioxane is another proof for the actual formation of the desired product
since trioxane is not likely to be formed by the other reactants involved in the
reaction. Trace moisture in the deuterated solvent may have catalysed the decompo
sition reaction by hydrolyzing one of the two triflate ligands. It could not be distin
guished if formaldehyde was formed besides trioxane, because the former would
have evaporated under the given reaction conditions and thus could not have been
observed in the NMR. No higher aggregated oligomers of formaldehyde have been
formed, however, since only one distinct signal for trioxane was observed in the
spectra and no precipitation occured.
18
2.6 Conclusion
The ultrasound assisted metathesis of diodomethane and silver triflate is a
facile procedure for the preparation of methylene bistriflate and can be carried out
with relatively low preparative difficulty. Further improvements will have to be
made to give the synthesis preparative value. The isolation of the reaction product
could be achieved by low temperature trap to trap distillation. It may also be feasible
to prepare the bistriflate and react it in situ without isolation.
19
2.7 References
1 Beard, C.D.; Baum, K.; Grakauskas, V. J. Org. Chem, 1973,38, 3673
2 Stang, P.J.; Hanack, M.; Subramanian, L.R. Synthesis 1982, 85
3 Stang, P.J.; White, M.R. Aldrichimica Acta 1983,16, 15
4 Katsuhara, Y.; Des Marteau, D.D. J. Fluorine Chem. 1980,16, 257
5 Howell, R.D.; McCrown, J.D. Chem. Rev. 1977, 77, 69 and references
therein
6 Ritter, K, Synthesis 1993,735 and references therein
7 Stang, P J.; Treptow, W. Synthesis 1980,283
8 Martinez, A.G.;Rios, I.E.; Vilar, E.T. Synthesis 1979,382
9 DesMarteau, D.D. Inorg. Chem. 1968, 7,434
10 Emmons, W.D.; Ferris, A.F. J. Am. Chem. Soc. 1953, 75,2257
11 Streitwieser, A.Jr.; Wilkins, C.L.; Kiehlmann, E.J. J. Am. Chem. Soc. 1968,
90, 1598
12 Chapman, R.D.; Andreshak, J.L.; Herrlinger, S.P. J. Org. Chem. 1986,51,
3792
13 Bullock, R.M.; Hembre, R.T.; Norton, J.R. J. Am. Chem. Soc. 1988,110,
7868
14 Einhom, C.; Einhom, J.; Luche, J.L. Synthesis 1989, 787
20
3. Vinvl Triflates
Vinyl triflates have found their place as an important class of triflic acid
derivatives. This section of my thesis deals with the attempt to prepare the parent
vinyl triflate itself by applying the general procedures for the formation of this type
of compounds.
3.1 Applications of Vinyl Triflates in Chemistry
3.1.1 Studies of Reactive Intermediates
The generation and application of vinyl triflates has recently been reviewed
in detail.' Trifluoromethanesulfonates (triflates) have been precursors for the
generation of highly reactive intermediates such as vinyl cations2 and alkylidene
carbenes.3 A
Scheme 3.1: Vinyl Triflates as Precursors for Vinyl Cations and Alkylidene Carbenes.
r : .OTf R’
21
3.1.2 Vinylic Carbon-Carbon Bond Formations
In synthetic organic chemistry vinyl triflates as well as aryl triflates are used
as reagents in transition metal catalyzed cross coupling reactions with
organometallic compounds such as organocuprates, -stannanes, -boranes, Grignard
reagents and others. A recent example of this now widely applied type of reaction is
the Suzuki reaction, which creates biaryls and vinyl-substituted aromatics by
coupling arylboronic acids with aryl or vinyl triflates under Pd°-catalysis.5 '6
Pd(0), b ase
R’ -O T f + R—B{OH)2 — --------► R'— R
R = alkyl, alkenyl, aryl
R’ = alkenyl, aryl
Scheme 3.2: The Suzuki Reaction.
The cross coupling reaction of alkenes with aryl or vinyl halides under
palladium (II) catalysis is known as the Heck reaction.7 In the same fashion vinyl
and aryl triflates can be coupled with alkenes in the presence of a base that binds the
formed triflic acid.
3.1.3 Vinylic Carbon-Heteroatom Bond Formations
Besides vinyl carbon-carbon bond formations a number of vinyl carbon-
heteroatom bond formations have also been reported. 1 A summary of these reactions
would by far exceed the scope of my thesis. A few examples illustrate the versatility
of this type of reactions. Vinyl triflates react with thiols to give vinylic thioethers.8
22
The triflate group can also be replaced by hydrogen to give the corresponding
alkene or alkane (Scheme 3.3): hydrogenation with hydrogen gas and a transition
metal catalyst gives the corresponding alkane,9 reaction with tributyltin hydride and
lithium chloride in the presence of palladium (0 ) results in the corresponding
alkene. 10 Overall, these reactions allow the removal of a carbonyl function and the
simultaneous formation of an saturated or unsaturated hydrocarbon.
O
Tf20
base
H2/ P t0 2/ EtOH v OTf
HSnBu3/ U CI
Pd(PPh3)4 ~
Scheme 3.3: Alkanes or Alkenes from a Carbonyl Function via a Vinyl Triflate.
3.2 Preparation of Vinyl Triflates
Vinyl triflates have first been prepared by Jones and Maness11 and by Stang
and Summerville. 12 In 1969 Jones and Maness obtained their vinyl triflates in the
reaction of a vinyl acyltriazene with triflic acid. Stang and Summerville synthesized
mono- and disubstituted vinyl triflates via the addition of triflic acid to carbon-
carbon triple bonds. 12,13,14 Although the latter synthesis is often very facile and
23
convenient its conditions are often not suitable for (acid) sensitive substrates. In
1970 Stang et al. offered a different approach by reporting the conversion of
carbonyl compounds with triflic anhydride to give vinyl triflates in moderate yields
(18-50%).1 5 This reaction works best in the presence of a base which binds the
triflic acid that is eliminated in the reaction. It has became a standard method1 6
although yields are often lower and the reaction takes longer than in the addition of
triflic acid.
CH.
l = N N A r
-Ft'
TfOH ^ Arv /OTf
A r = = V\r
TfOH / O T f
n OTf
T ?2Q.basV
Scheme 3.4: Vinyl Triflate Formations, a) from Vinyl Acyltriazene; b) Addition of Triflic
Acid to an Acetylene; c) Reaction of Triflic Anhydride with a Ketone in the Presence of a
Base.
A number of refinements to this preparation have been reported. When a
hindered, non-nucleophilic base is used instead of pyridine yields increase
significantly.1 7 N-phenyltrifluoromethanesulfonimine was used as inflating reagent
24
with regiospecifically generated metal enolates to give regio-defined products.1 8
Similar inflating agents, where the N-phenyl entity is replaced by N-pyridyl, show
additional chelation abilities (scheme 3.5).1 9
Scheme 3.5: Triflating Reagents - a) N-Phenyltriflimide; b) N-(2-pydridyl)trifiimide,
3.3 The Target Compounds
Although numerous vinyl triflates have been synthesized during the 25 years
since the first vinyl triflate studies, it was not until 1991 that Stang and Ullmann2 0
reported the synthesis of the parent vinyl triflate (scheme 3,6). This procedure
followed an unusual route via an iodonium ion. Tri-n-butyl(vinyl)tin was reacted
with cyano(phenyl)iodonium triflate to give ethenyl(phenyl)iodonium triflate in 75%
yield. The addition of silver triflate results in 15 to 20% vinyl triflate.
25
©
J — Ph
SnBu3 + Ph!{CN)OTf
TfOe
AgOTf>
.OTf
-Agl
Scheme 3.6: Preparation of Vinyl Triflate rut an lodonium Triflate (Stang 1991).
This part of this thesis focuses on a more simple and convenient preparation
of vinyl triflate. If the compound was more easily accessable it could be a useful
reagent in the synthetic reactions mentioned above. The classical syntheses of vinyl
triflates, the addition of triflic acid to acetylene and the conversion of acetaldehyde
with triflic anhydride in the presence of a hindered base, are potential methods for its
preparation.
3.4 Possible Approaches to the Preparation of Vinyl Triflate
The addition of triflic acid to acetylenes is known to result in higher
conversion than the reaction of carbonyl compounds with triflic anhydride (vide
supra), and was therefore investigated first. However, this reaction would involve
the use of acetylene gas which is neither convenient to handle nor was it expected to
be very reactive. The latter can be estimated from the yield in the addition of triflic
acid to substituted acetylenes in table 3 .1. The results indicate, that with decreasing
substitution of the acetylene with electron-donor groups the formation of the
26
addition product becomes less and less favorable. Whereas 2-butyne and propyne
give moderate yields (68% and 67% respectively)2 1 the conversion drops to 17 %
product formation in the case of 1 -bromo-2-propyne.2 2 The experiment would
therefore have to show whether acetylene itself was reactive enough to give the
desired product.
Acetylene Product Yield (Ref.)
h3c c h 3
6 8 % (22)
— — c h 3
67%
“ * * c 3h 7
“ C C3 H 7
55 %(22)
— CH2Br
OH
CH2B r
17 96(23)
Table 3.1: Addition of Triflic Acid to Substituted Acetylenes.
To avoid acetylene gas as reagent, trimethylsilyl (TMS) substituted
acetylenes, TMS-acetylene and bis(TMS)acetylene, were chosen as substrates.
Scheme 3.7 shows the feasible pathways the reaction could take. If the sensitive
TMS-substitutents survived the treatment with triflic acid, TMS-substituted vinyl
triflates would be obtained (pathway A in scheme 3.7). The potential products
would be interesting reagents by themselves and could be easily converted into the
27
parent vinyl triflate by desilylation reactions e.g, with fluoride reagents (B).23,24
Abstraction of the trimethylsilyl groups by triflic acid (C) would finally form
acetylene. This reaction is known as proto-desilylation and it is driven by the energy
gained in the silicon-oxygen bond formation. In fact, comparable reaction
condititons are applied in the synthesis of defined silyl triflates especially from triflic
acid and phenyl trialkylsilylates.25,26 The acetylene formed in this way could then
react with triflic acid to give the desired product (D).
TMS = -Si(CH3)3
Scheme 3.7: Projected Synthesis of Vinyl Triflate. Addition of Triflic Acid to TMS-Acetylenes
(A) followed by Desilylation with Fluoride (B). A Possible Side Reaction is Protodesilylation
(C) which would result in Acetylene that may undergo Addition of Triflic Acid itself (D).
TfOH
OTf
TMS
A
C
a
TMSOTl V
B
O T f
TfOH
H 2C —
H - TMSOTf
H
D
F® B
TMS TMS
TfOH
A
28
Trimethylsilyl vinyl triflates can also be prepared from silyl ketones, which
react similar to other carbonyl compounds. In this manner, cx-silylvinyl triflate has
been prepared by Schiavelli et air1 in 52 % yield. Bis(trimethylsilyl) vinyl triflate is
still unknown. The required silyl ketones can be obtained by the oxidation of silyl
alcohols,2 8 or in the hydrolysis of cyclic silyl dithioacetals.2 9
3.5 Attempted Preparation of Vinyl Triflate
3.5.1 Addition of Triflic Acid to Acetylenes
In a number of experiments triflic acid was added to solutions of
trimethylsilyl-acetylene or bis(trimethylsilyl)acetylene in a variety of different
solvents and at various temperatures. In no case could formation of the desired
products be observed by NMR or mass spectroscopy. However, traces of
trimethylsilyl triflate could be observed by gas chromatography/ mass spectroscopy.
Increasing the mole ratio of triflic acid in order to provide for the desilylation of and
the addition to the acetylene did not afford vinyl triflate species, even when triflic
acid was used as the solvent.
These observations suggest that the triflic acid reacted by abstracting TMS-
groups rather than adding to the carbon-carbon triple bond. The remaining
acetylene, that is formed from TMS- and bis(TMS)acetylene, was not found to react
with triflic acid in any way. It just bubbled out of the solution.
29
Hanack and coworkers30 reported in 1973 similar observations in the
unsuccessful attempt to prepare vinyl triflate by reacting triflic acid with acetylene.
They argued that the volatility and polymerization tendency of the product caused
the preparative attempt to fail. Nevertheless, when they carried out the reaction with
nonafluorobutanesulfonic acid, the “vinyl nonaflate” was obtained (but no yield was
reported). Since the volatility of the product should have played a minor role (its
reported boiling point is 80°C)2 1 and polymerization could not be observed in the
investigated reactions, the reason for the unsuccessful preparation must be the low
reactivity of acetylene. The electron density of its triple bond (i.e. its Lewis basicity)
may not be sufficient for protonation by triflic acid to occur. This assumption is
supported by the reported results for the addition of triflic acid to alkyl substituted
acetylenes {vide infra). The collected data clearly indicates that the yield of vinyl
triflate depends directly on the degree of substitution of the acetylene with electron
donor groups. Even if an equilibrium between free and protonated acetylene had
been established, the addition of the barely nucleophilic triflate anion would be
another barrier towards the formation of the desired vinyl triflates.
3.5.2 Reaction of Triflic Anhydride with Acetaldehyde
As an alternative route to vinyl triflate the reaction of acetaldehyde with
triflic anhydride in the presence of base was investigated. As a non-nucleophilic base
1,8 -bis(dimethylamino)naphthalene (“proton sponge”1 1 1 ) was selected.
30
■ C H ;
l,8-Bis(dimethylamino)napbUiaIene
Since acetaldehyde has a low boiling point (16°C at atmospheric pressure)
the reaction had to be performed at an appropriately low temperature. Triflic
anhydride was added slowly to a solution of acetaldehyde in pentane at about -20°C.
The reaction mixture immediately turned orange, then deeply red and finally dark
brown. The volatile products were distilled into a cooling trap under vacuum.
Investigation by NMR spectroscopy did not reveal the desired product. When a
longer reaction time or an initial reaction temperature of 0°C was chosen, a tarry
black precipitate formed instantly.
Unlike similar reactions with less volatile aldehydes and ketones the reaction
temperature had to be low enough to avoid evaporation of the aldehyde. This might
have prevented the reaction from occuring. Although acetaldehyde and triflic
anhydride seem to have low reactivity towards vinyl triflate formation,
polymerization occurs readily. Triflic anhydride possibly promoted the
polymerization in a similar fashion to that observed for trioxane (chapter 2 ).
Further investigations are needed to finally determine whether suitable
reaction conditions will lead to the desired product.
31
3.6 Conclusion and Outlook
The attempt to apply the general procedures for the preparation of vinyl
triflate to the preparation of the parent compound proved to be unsuccessful.
However, it may be possible to achieve the addition reaction of triflic acid if the
acetylene can be further activated e.g. by transition metal catalysis. Acetylene is
known to be activated by forming complexes with metals complexes of e.g.
L— F
L
L
h— OTf
R
M = Rh, Ru
H OCOR"
R- -R'
R'COOH
Scheme 3.8: Potential Further Approaches. Acetylene complexes of transition metals (above
left); Rhodium - triflate complexes (above right); Mercury catalysed addition of carboxylic
acids to acetylenes (below).
rhodium31 and ruthenium.32 Triflic acid undergoes oxidative addition with Rh(I)
complexes,33 Insertion reactions of carboxylic acids and acetylenes under transition
metal catalysis are well-known processes.34,35 Although the nucleophilicity of the
triflate anion is far lower than of carboxylic acids, the activation resulting from the
32
coordination of the acetylene to the metal cation may be sufficient to promote the
reaction. Mercury (II) salts are reagents for the acetoxylation of alkynes.36,37 In a
similar fashion vinyl nonaflate was prepared from acetylene and nonafluorobutane-
sulfonic acid with mercury oxide as catalyst.30 This seems to be a possible pathway
for further investigations.
33
3.7 References
Ritter, K. Synthesis, 1993,735 and references therein
2 Stang, P.J.; Rappoport, Z.; Hanack, M.; Subramanian, L.R. Vinyl Cations,
Academic Press, New York, 1979
3 Stang, P.J.; Magnum, M.G.; Fox, D.P.; Haak, P ../. Am. Chem. Soc. 1974, 96,
4562
4 Stang, P.J. Acc. Chem. Res. 1978, 11, 107
5 Oh-e, T.; Miyaura, N.; Suzuki, A. J. Org. Chem. 1993,58, 2201
6 Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett, 1992, 207
7 March, J. Advanced Organic Chemistry, 4th ed., Wiley-
Interscience.NewYork, !992, pp 717-718
8 Philips, D.; O’Neill, B.T. Tetrahedron Lett. 1990,31, 3291
9 Martinez, A.G.; Alvarez, R.M.; Cassado, M.M.; Subramanian, L.R.; Hanack,
M, Tetrahedron 1987,43, 275
10 Scott, W J.; Stille, J.K. J. Am. Chem. Soc. 1986,108, 3033
1 1 Jones, W.M.; Maness, D.D. J. Am. Chem. Soc. 1969, 9 1 ,4314
12 Stang, P.J.; Summerville, R.H. J. Am. Chem. Soc. 1969, 9 1 ,4600
13 Summerville, R.H.; Schleyer, P.v.R. J. Am. Chem. Soc. 1972, 94, 3629
14 Summerville, R.H.; Schleyer, P.v.R. J. Am. Chem. Soc. 1974, 96, 1110
15 Dueber, T.E.; Stang, P.J.; Pfeifer, W.D.; Summerville R.H.; Imhoff, M.A.;
Schleyer, P.v.R.; Hummel, K.; Bocher, S.; Harding, C.E.; Hanack, M. Angew.
Chem. Intl. Ed. Engl. 1970, 82, 521
16 Stang, P.J.; Dueber, T.E. Org. Synth., 1974,5 4 ,79
34
Stang, P.J.; Treptow, W. Synthesis, 1980, 283
McMurry, J.E.; Scott, W.J. Tetrahedron Lett. 1983,24, 979
Comins, D.L.; Dehghani, A. Tetrahedron Lett. 1992,33, 6299
Stang, P.J.; Ullmann, J. Angew. Chem. Intl. Ed. Engl. 1991,103, 1469
Summerville, R.H.; Senker, C.A.; Schleyer, P.v.R.; Dueber, T.E.; Stang, P.J.
./. Am. Chem. Soc. 1974, 96, 1100
Crisp, G.T.; Meyer, A.G. Synthesis, 1994, 667
Recent examples of this method: Chyall, L.J.; Brickhouse, M.D.; Schnute,
M.E.; Souires, R.R. J. Am. Chem. Soc. 1994,116,6 8 1
Maeda, Y.; Shirai, N.; Sato, Y. J. Chem. Soc. Perkin Trans. 1 1994, 393
Hiibich, D.; Effenberger, F. Synthesis, 1978,755
Uhlig, W. Organometallics 1994,13, 2843
Schiavelli, M.D.; Jung, D.M.; Morrison, D.S. J. Org. Chem. 1981,46, 92
Brook, A.G.; Duff, M.; Jones P.F.; Davis, N.R. J. Am. Chem. Soc. 1967, 59,
431
Corey, E.J.; Seebach, D.; Freedman, R. J. Am. Chem. Soc. 1967, 59,434
Eckes, L.; Subramanian, L.R.; Hanack, M. Tetrahedron Lett. 1973,22, 1967
Bianchini, G ; Meli, A.; Peruzzini, M.; Zanobini, F. Organometallics 1990, 9,
1155
Ruppin, C ; Dixneuf, P.H. Tetrahedron Lett. 1986,27, 6323
Bianchini, C.; Masi, D.; Meli, A.; Peruzzini, M.; Zanobini, F. J. Am. Chem.
Soc. 1988,110, 6411
Rotem, M.; Shvo, Y. Organometallics 1983,2, 1689
35 Mitsudo, T.; Hori, Y.; Yamakawa. Y.; Watanabe, Y. J. Org. Chem. 1987, 52,
2230
36 Larock, R.C.; Oertle, K.; Beatty, K..M. /. Am. Chem. Soc. 1980,102, 1966
37 Bach, R.D.; Woodard, R.A.; Anderson, T.J.; Glick, M.D. J. Org. Chem. 1982,
4 7 ,3707
36
4. Trichloromethvl Triflate
4.1 Introduction
As a potential reagent of substantial preparative interest, trichloromethyl
triflate has found special attention in the course of my studies. With triflate as an
excellent leaving group the compound can be expected to be a source of the
trichloromethyl cation which would readily serve as a trichloromethylation agent
F C - ^ * ° Cl
F a C v4-c l
C l
The introduction of the trichloromethyl group itself can be of synthetic use where
this type of substituent is needed on an aromatic system. Furthermore the CCI3-
group can be hydrolysed to give a carboxylic acid -COOH. From this perspective
CChOTf can be considered a carbon dioxide equivalent
4.2 Nucleophilic Trichloromethylation Reactions
Trichloromethylation can be achieved by a number of routes. Most
commonly, nucleophilic trichloromethylation via an anionic trichloromethyl species
as intermediate has been used. One synthetic approach is the conversion of an
electrophilic compound with the sodium salt of trichloroacetic acid, which looses
37
carbon dioxide to form the trichloromethyl anion, which then can undergo
nucleophilic attack. 1
4.3 Electrophilic Trichloromethylation
A reaction proceeding via a carbocationic species (i.e. undergoing
electrophilic aromatic substitution) can be accomplished by treating
halotrichloromethane XCCI3 with a Lewis acid such as aluminum trichloride or
boron trichloride.2 In this case, however, a mixture of products, involving mono-,
di-, tri- and even tetrasubstituted substrate will be obtained depending on the
halogene X and the strength of the used Lewis acid. The trichloromethyl cation itself
has been prepared by Olah and coworkers under superacidic conditions from carbon
tetrachloride and SbFs.3
More controlable conditions should be obtained when the substitutent X is
significantly more reactive than the three chlorine substituents. This may be true for
X = iodine, but in this case the reaction will have to face different problems in redox
CCI3
Scheme 4.1: Anionic Trichloromethylation with Sodium Trichloroacetate.
38
stability of the iodinated compound. Triflate seems to be a useful substituent.
Instead of stoichiometric amounts of Lewis acid, catalytic amounts of triflic acid
may suffice and even this may not be necessary. In addition, the triflate group is
stable towards redox processes.
Trichloromethyl triflate has been prepared in 1969 by Schmeisser et a l4 in a
metathesis of mercury triflate and bromotrichloromethane. It was obtained in 78 %
when the two components were brought to reaction at 0°C for 10 hours. The ester
is thermally unstable and it partially decomposes to give off phosgene upon
distillation (scheme 4.2).
4.4 Trichloromethyl and Trifluoromethyl Triflate
Closely related to the topic of this investigation is trifluoromethyl triflate.5
Interestingly, this compound shows different chemical properties from
trichloromethyl triflate: Whereas the latter trichloromethylates aromatic rings, the
latter does not not show similar behaviour..
In principle, there are two different cleavages possible in the trihalomethyl
triflate system: cleavage of the C -0 bond results in the formation of the
+ CISO2 CF3
Scheme 4.2: Decomposition of Trichloromethyl Triflate.
39
trihalomethyl cation, while breaking of the O-S bond creates a trihalomethoxy anion
and the trifluoromethanesulfonyl cation. Trifluoromethyl triflate has been reported
to react in the latter way.6 When the triflate ester was added to pyridine, no
trifluoromethyl pyridinium triflate could be detected. In contrast, trifluoromethyl
triflate gives trifluoromethanesulfonylation (“triflylation”) when it is reacted with
enamines.7
? 6
X3C H f - c f 3
b O a)
Scheme 4.3: The Two Possible Bond Breaking Mechanisms in Trihalomethyl
Triflate.
TfO"
By which route the trihalomethyl triflate reacts depends on the relative stability of
the cleavage products. The formation of the trifluoromethyl cation is highly
unfavorable. Unlike the trichloromethyl cation, it has not yet been observed even in
superacidic media.4 Cleavage of the sulfur-oxygen bond leads to the formation of
trifluoromethanolate which readily decomposes to fluoride and fluorophosgene.
Studies have shown that this decomposition also occurs with trichloromethyl triflate
(vide infra), but only at elevated temperatures.6
40
4.5 The Reaction of Trichloromethyl Triflate with Benzene
To investigate the synthetic value of trichloromethyl triflate its reactivity
towards benzene was investigated. First, the ester was prepared by the metathesis of
bromotrichloromethane and silver triflate in dichloromethane. Then an excess of
benzene was added. After two hours only starting material was observed by NMR.
After three days the mixture was hydrolysed with ice water, neutralized and
extracted with diethyl ether. GCMS analysis revealed unhydrolyzed trichloromethyl
benzene and traces of benzophenone. Although these results were qualitative, they
indicate that the triflate ester gives the desired trichloromethylation reaction. The
reaction may be catalysed by triflic acid that is freed in the process. Interestingly,
arylation on CCU did not only occur once (by replacement of the triflate group) but
twice to give dichlorodiphenylmethane which then hydrolysed to benzophenone.
The in situ formed trichloromethyl triflate could have acted as the
trichloromethylation reagent. It is also possible, that the transient trichlormethyl
cation, a strong electrophile, underwent electrophilic addition to benzene
immediately. Considering the low nucleophilicity of the triflate anion, the formation
of the triflate ester and electrophilic addition to benzene by the trichloromethyl
cation can be assumed to be highly competitive reaction pathways.
41
4.6 Attempted Synthesis of Trichloromethyl Triflate
4.6.1 M etathesis with Silver Triflate
The high yield Schmeisser and coworkers (vide supra) reported for the
reaction of mercury (II) triflate and bromotrichloromethane encouraged the use of a
slightly different yet similar system for the preparation of the target compound.
Unlike mercury (II) triflate, silver triflate is easily accessable.
Cl3 CBr + AgOTf -------- ► Cl3 COTf + AgBi
Scheme 4.4: Metathesis of Bromotrichloromethane with Silver Triflate.
In addition, the silver bromide formed in the reaction is almost insoluble in
all solvents that could be used as the reaction medium. It should hence provide a
powerful driving force to the metathesis reaction.
The metathesis reaction was carried out under various conditions: the
reactants were combined neat and in different solvents, the reaction temperature was
varied between room temperature and refluxing solvent and-in analogy to the good
results for methylene bistriflate (see chapter 2 ) the application of ultrasound was
investigated. Table 4.1 summarizes the experimental results.
42
Substrate
CCI,X
Triflating
reagent
Solvent Reaction
Conditions
Reaction
Time
Products
CCUBr AgOTf pentane 25°C 18 hours < 10% CCljOTf
traces CiCU
CCUBr AgOTf
CH jC Ij 25aC 8 hours traces CCUOTf
traces C2C U
CCUBr AgOTf CD2CI2 25°C, sonic.* 2 hours no reaction
CCUBr AgOTf benzene 25°C 7 days no reaction
CCl3 Br AgOTf acetonitrile reflux 8 hours no CCUOTf
CCljBr triflate resin” pentane 25°C 24 hours no reaction
CCIjBr AgOTf none 25°C, sonic. 1 hours traces CCUOTf
major C2 C U
C C L , AgOTf
C C L , reflux 20 hours' 7% CCUOTf
traces CiCU
e c u AgOTf e c u 25°C, sonic. 1.5 hours no reaction
CCUBr AgOTf e c u 25°C 30 hours 1% conversion
CCUOTf+
CiCU
CCl3 Br AgOTf e c u 25°C 48 hours ca. 6% CCUOTf
ca. 0.5% C,CU
CCl3 Br AgOTf C C L , 25°C 96 hours 10% CCUOTf
3% CiCU
none none e c u 25°C, sonic. 2.5 hours no reaction
CCUBr AgOTf
e c u reflux 24 hours 2.6% CCUOTf
traces C2 C U
CCljBr none e c u 25°C, sonic. 2 hours traces C2 C U
Table 4.1: Conversion of Halotrichlorometbane with Triflating Reagents. “ sonication
conditions (see chapter 6 for details); b see chapter 5 ;c Procedure reported by Chapman and
coworkers.
43
4.6.2 Yields and Reaction Conditions
In most of the reactions the desired trichloromethyl ester was obtained in,
however, overall low yield. The formation of the ester was always accompanied by
the formation of side products (vide infra). These side products, namely
tetrachloroethylene and hexachloroethane, even became the major product when the
reaction was carried out without a solvent as in the procedure by Schmeisser with
mercury triflate and halotrichloromethanes. When the reaction was done in benzene
or acetonitrile, the only solvents used in which silver triflate is soluble, no desired
product was observed. In the case of acetonitrile either silver triflate or eventually
formed trichloromethyl triflate may have reacted with the solvent, which is a
reasonably strong N-nucleophile.
To obtain detectable products under the heterogenous reaction conditions in
non-polar solvents as pentane dichloromethane and carbon tetrachloride, the
reaction time had to be increased to days. The attempt to speed up the reaction by
means of ultrasound application (similar to the experiments leading to methylene
bistriflate; see chapter 2 ) resulted in significant formation of side products, which
became the mayor products under these conditions.
In an attempt to obtain more controlable reaction conditions, bromo
trichloromethane was replaced by carbon tetrachloride. Chapman and coworkers
had observed qualitatively the formation of the trichloroester when they carried out
prolonged metathesis reactions with silver triflate in boiling carbon tetrachloride. In
44
the repetition of the reported procedure, trichloromethyl triflate was found to be the
major product (7 %, GC-MS but not isolated yield), however, accompanied by
significant amounts of hexachloroethane.
4.6.3 Side Product Formation
The major side product found in the reactions was hexachloroethane. Its
presence among the reaction products indicates the behaviour of
bromotrichloromethane. Under the given reaction conditions the halogenated
methane undergoes radical reactions rather than metathesis with silver triflate. After
all, BrCClj is known as a mild brominating reagent.8 It has been used in the radical
side chain bromination of aromatics.9 These reaction usually involve the homolytic
cleavage of the carbon-bromine bond as the initiation step. The resulting
trichloromethyl and bromine radicals then start the radical chain reaction by
abstracting a hydrogen atom from the alkyl side chain of the aromatic. In the
BrCClj/ AgOTf system the the formation of hexachloroethane (initiated by the
homolytic cleavage of the carbon-bromine bond in BrCClj) occurs without
irradiation, but an increased formation of side products is observed at elevated
temperatures and when sonication is applied.
The silver cation seems to catalyse the formation of trichloromethyl radicals,
presumably by a single electron transfer process, which would explain the frequent
45
observation of a metallic-gray silver precipitation besides yellow silver bromide. In
two control experiments neat carbon tetrachloride and CCljBr in CCU were
sonicated for 2 to 2,5 hours. Whereas CCL* did not show any reactions, with CCljBr
trace amounts of hexachloroethane were formed. In the presence of silver triflate the
overall quantity of hexachlroethane is much larger.
Once formed, the trichloromethyl radical can react in two different fashions:
Two radicals can combine to give hexachloroethane, or it can cleave again into
dichlorocarbene and a chlorine radical.
BrCCl3 *-
CCI3 + Br'
CCI3 ♦ :c c i2 + a
Scheme 4.5: Radical Processes of Bromotrichloromethane.
Hexachloroethane appeared to be the major side product in all reactions. It
could be identified by mass spectroscopy as the C2G s+ cation (m/z = 199). The non-
occurance of the molecular peak is a common observation for halogenated
compounds. Formation of dichlorocarbene seemed to have played only a secondary
role. Nevertheless, the generation of dichlorocarbene from the CClj radical appeared
under relatively mild conditions. CClj is usually obtained from the trichloromethyl
anion, as in the Reimer-Tiemann formylation of phenols with chloroform and
base, 10’11 or in the pyrolysis of chloroform or CCL* at temperatures above 1000°C. 12
The C2CI4 was detectable in the mass spectra of the product mixtures, however
trapping of the carbene with tetramethylethylene did not result in detectable
quantities of 1,1-dichlorotetramethylcyclopropane. Hydrogen abstraction and thus
formation of a bromoalkane is not possible when carbon tetrachloride is used as
solvent and therefore significant amounts of CiCU and C2CI4 are observed. The
overall amount of these side products is much lower in reactions carried out in
pentane and dichloromethane, because here hydride abstraction and brominatton of
the solvent are feasible.
47
4.7 Conclusion and Outlook
The reaction of bromotrichloromethane with silver triflate has proven to be
more complex than just a simple metathesis reaction. Under any reaction condition
tried significant amounts of perhalogenated side products were so that the overall
yield in trichloromethyl triflate was unsatisfyingly low. Best results were obtained
with CCUBr in CCI4 after four days at room temperature. With a yield of <10 %,
however, the reaction does not seem to have synthetic value. For the generation of
trichlormethyl triflate the method suggested by Schmeisser is therefore still the
method of choice. Mercury triflate can be prepared from silver triflate and mercury
(II) chloride in excellent yields (see chapter 5). Besides metathesis reactions, other
approaches may give better results. The chlorination of methyl triflate, however, a
considerable approach to the desired compound, does not appear to be a suitable
pathway: treatment of methyl triflate with a number of chlorinating reagents or
chlorine (with either a radical starter or irradiation) has been reported to give only
trifiuoromethanesulfonyl chloride, hydrogen chloride and formaldehyde. 13
48
4.8 References
for a recent example see Grignon-Dubois, M.; Diaba, F.; Grellier-Marly, M.C.
Synthesis 1994, 800
2 Olah, G.A.; Heiliger, L.; Prakash, G.K.S. J. Am. Chem. Soc. 1989, H i , 8020
3 Olah, G.A. Friedel-Crafts Chemistry, Wiley-Interscience, New York, 1973
4 Schmeisser, M.; Satori, P.; Lippsmeier, B. Chem. Ber. 1969,102,2150
5 Burdon, J.; McLoughlin V.C.R. Tetrahedron 1965,27,1
6 Taylor, S.L.; Martin, J.C. J. Org. Chem. 1987.5 2 ,4147
7 Kobayashi, Y.; Yoshida, T.; Kumadaski, L. Tetrahedron Lett. 1979,40, 3865
8 March, J. Advanced Organic Chemistry, 4th ed., Wiley-Interscience, New
York, 1992, p 691 and references therein
9 Huyser, E.S. J. Chem. Soc. 1960, 82,391 + 394
10 March, J. Advanced Organic Chemistry, 4th ed., Wiley-Interscience, New
York, 1992, pp 544 - 545 andr references therein
1 1 Sykes, P. Reaktionsmechanismen der Organischen Chemie (in German), 9th
ed., Verlag Chemie, Weinheim, 1988, p 341
12 Skell, P.S.; Cholod, M.S. J. Am. Chem. Soc. 1969, 91,6035
13 Bruce, M.R. Ph.D. Dissertation, University of Southern California 1980, p 107
49
5. Metal Triflates
5.1 Binary Element-Triflates
As all strong acids, triflic acid forms binary salts with a great number main
group elements as well as transition metals (vide infra). Most of these metal salts
are very stable compounds with ionic character, high melting points and a are
typically hygroscopic.
The binary main group element-triflate compounds which have been reported
I A ^
(boron, aluminum, gallium), show the typical behaviour of predominantly covalent
bonded species: they have low melting points, are highly hygroscopic and are strong
Lewis acids. An increasing number of triflate salts and related binary main group
element - triflate compounds have been reported in the literature (vide infra),J
5.2 Binary Element-Triflates in Organic Syntheses
5.2.1 Non Metal Triflates
The non-metal-triflates are known for their high Lewis acidity. Especially
boron tristriflate has aroused much attention as Lewis acid reagent in Friedel-Crafts
type reactions.2 In combination with triflic acid, it forms the strongly superacidic
system H2S0 3 CF3 + [BfCFsSOj) ^ ' . 1
50
5.2.2 Metal Triflates
Silver triflate has probably found the most widespread application in organic
chemistry as a triflating agent with alkyl halide,4 ,s ‘ 6 since the formation of insoluble
silver halides in the metathesis reaction provides a strong driving force for the
reaction .and thus leads to high overall yields. Magnesium and zinc triflate have been
found to catalyze the formation of thioketals from ketones and dithiols in excellent
yields.7
5.2.3 Metal Triflates in Hydration Studies
Some triflate salts such as nickel (II) and lanthanum (EH) triflate have been
the subject of hydration studies of the triflate ion.8 These studies show that the
triflate ion undergoes strong hydrogen bonding to solvent water (in an aqeous
solution) and hydrate water (in the solid state) at the oxygen end whereas no
hydrogen bonding was found for the CF3-group.
5.2.4 Metal Triflates as Friedel-Crafts Catalysts
Transition metal triflates have recently attracted attention as novel Friedel-
Crafts catalysts. Lanthanide and scandium triflates were reported to be reusable
catalysts in the Friedel-Crafts acylation of aromatic compounds.9,10 The commonly
51
used aluminum trichloride is required in stoichiometric quantity since it coordinates
with the acylation product and is hydrolyzed in the aqueous work-up procedure.
M eO
(M eC0)2O
0.2 eq. Sc(TfO)3
‘ COMe
Scheme S.l: Friedel-Crafts Acylation with Scandium Triflate.1 0
Unlike AICI3, lanthanide and scandium triflates can be used in catalytic amounts, and
because they are water-tolerant (i.e. they are not hydrolyzed by water), they can be
recovered and recycled after a simple drying procedure. If solvent complexed triflate
salts (hydrates and others) are desired to be desolvated it is usually sufficient to heat
the compound in vacuo for several hours.
5.3 General Approaches in the Formation of Metal Triflates
A number of methods for the preparation of metal triflates have been
reported in the literature. Table 5.1 compiles a number of triflate salts and their
preparation. Dixon et al. have published a survey of the most often used
procedures. 11 Metal chlorides, and especially alkali metal chlorides, react with triflic
acid to give the corresponding metal triflate and HCI gas. This procedure was
discovered accidentally during the attempt to record the infrared spectrum of triflic
52
acid using sodium chloride windows. 15 Silver chloride, however, is inert towards
triflic acid and can be used in infrared spectroscopy samples instead.
Table 5.1: Metal triflates and their preparation procedure.
Triflates from Halides Preparation Conditions Ref.
Aluminum Tristriflate A ids + c f 3s o 3h -78°C, freon 113 2
Boron Tristriflate b c i 3 + c f 3s o 3 h
BBr3 + AgCF3S 0 3
-78°C, freon 113
heptane soln.
1 , 2
12
Gallium Tristriflate a i q 3 + c f 3s o 3 h
-78°C, freon 113
2
Ruthenium (HI) Triflate RuCI3 + AgCF3S 0 3 reflux in MeOH 13
Triflates via C arbonates Preparation Conditions Ref.
Cobalt (H) Triflate C0 CO3 + c f 3s o 3 h in H2 0 8
Magnesium (II) Triflate M gC03 + CF3S 0 3 H reflux in MeOH 7
Silver (I) Triflate Ag2C 0 3 + CF3S 0 3 H in H20 14
Zinc (II) Triflate ZnC 0 3 + CF3S 0 3 H reflux in MeOH 7
Triflates from Oxides Preparation Conditions Ref.
Lanthanide (HI) Triflates Ln20 3 + CF3S 0 3 H
100°C, in H2 0 9
Lanthanum (HI) Triflate La20 3 + CF3S 0 3 H neat addition 15
Scandium (ID) Triflate Sc20 3 + CF3S 0 3 H in H20 10
Mercury (II) Triflate 1. CS2 + HgF2
2. Hg(CF3S)2 + H20 2
pressure, 250 °C
100 - 105 °C
16
continued
53
Table 5,1 continued
Special M ethods Preparation Conditions Ref.
Aluminum (D3) Triflate AI4C3 + c f 3s o 3 h 180-200°C 2
Cobalt (II) Triflate Co(PhC02) 2 + CF3S 0 3 H neat addition 9, 17
Copper (I) Triflate Cu(CF3S 0 3) 2 + Cu° benzene soln. 18
Nickel (II) Triflate Ba(CF3S 0 3) 2 + N iS0 4 in H20 16
Sodium Triflate Ba(CF3S 0 3) 2 + Na2S 0 4 in H20 19
The procedure using metal halides has not only been applied in the
preparation of binary metal triflate salts or positively charged metal complexes. One
of the first preparations of triflate salts takes advantage of the insolubility of barium
sulfate in aqueous solutions.20 When solutions of metal sulfates are combined with
an aqueous barium triflate solution, barium sulfate precipitates quantitatively. Filtra
tion and evaporation of water yields the desired triflate salt. The precipitation of
silver chloride from a combined solution of silver triflate and a metal halide is a
similar process (vide infra).
Finally, a more specific but limited procedure is the anion exchange of triflate
against the base of a weaker acid. Copper (II) m-bromobenzoate was converted into
the corresponding triflate in this fashion. Certain metal triflates have also been
prepared with an excess triflic acid and a metal oxide or carbonate. In this case the
triflate salt and water or carbon dioxide are formed respectively.
54
5.4 Preparation of Metal Triflates by Silver Triflate Metathesis
In the course of this thesis a number of metal salts have been synthesized
using the metathesis with silver triflate. Nickel and cobalt (II) triflate were prepared
by the addition of stoichiometric amounts of silver triflate dissolved in methanol to
concentrated methanolic solutions of the hydrated nickel and cobalt chloride salts.
Silver chloride precipitated immediately and prolonged stirring ensured complete
conversion. The resulting solutions (intense green for nickel and deep violet for
cobalt) were then evaporated to dryness, redissolved in acetone to faciliate removal
of residual solvent, and dried in vacuo at I56°C for at least 6 hours. The drying
process could be followed easily by monitoring the color change of the salts: green
nickel (II) triflate hydrate turns light yellow upon dehydration, and the deep purple
cobalt triflate hydrate takes up a light lavender color. Both substances are very
stable (melting points > 400 °C) and can be stored in a desiccator supplied with a
drying agent for months without change in color or decomposition. Iron (III)
triflate21 was prepared for the first time by using this method. It is a dark brown
solid with a melting point above 400 °C. All three substances are fairly hygroscopic
and are slowly converted to the hydrate salt when exposed to moisture. The silver
triflate metathesis, however, was found to be unsuccessful in an attempt to prepare
rhodium (III) triflate by this method. The black anhydrous rhodium trichloride does
not dissolve in a solution of silver triflate in methanol, and no white silver chloride
55
precipitation was observed, even when the heterogenous mixture is refluxed for
several days.
Whether triflic acid was capable of reacting with elemental metals was also
investigated. The chosen metals (cobalt, copper, chromium, zinc and aluminum) did
neither dissolve nor show (hydrogen) gas development after several hours. This may
be due to the fact that triflic acid, unlike sulfuric acid, does not exhibit any oxidizing
properties or due to the formation of an inert oxide surface on the metals. Alkali
metals may instead react violently with the acid as they do with aqueous mineral
acid solutions, but these reactions would be too violent to be controlled.
5.5 Potential Catalytic Activity of Nickel Triflate
The catalytic properties of the majority of triflate salts have not yet been
investigated to any extent. Nickel (II) triflate was chosen as the subject of the
investigation of its catalytic potential in the well known Gattermann-Koch formy-
lation22,2J ,24 of benzene. In the original experiment, Gattermann and Koch25 used
hydrochloric acid as protonating agent, aluminum chloride as Lewis acid and copper
(I) chloride as carrier (i.e. as complexation agent to bind the CO gas) at room tem
perature and atmospheric pressure. Nickel (II) chloride can be used in place of the
CuCl, however it is less effective.25 It has been shown, that no carrier is needed
56
when triflic acid is used as protonating agent at higher temperature in an autoclave
under high pressure.26
It was therefore investigated, if triflic acid supported by nickel (II) triflate
would formylate benzene at ambient temperature and atmospheric pressure. Carbon
monoxide was passed through a benzene solution of triflic acid and nickel triflate for
12 hours. The reaction mixture was then neutralized and extracted and analyzed by
gas chromatography/ mass spectroscopy. No benzaldehyde was found.
Apparently, the affinity of carbon monoxide towards nickel triflate was not
high enough to allow a reaction to occur. Nickel (II) is rather electron rich with
eight electrons remaining in its outer shell and may have been further deactivated by
the formation of 7t-complexes with the solvent benzene. Lower transition metals
show much higher Lewis acidity; scandium (III), as described above, has no outer
shell electrons and in therefore a much stronger Lewis acid.
5.6 Triflate Ion Exchange Resin
Hassner and Stem27 reported in 1986 a new method for the preparation of
azides under very mild conditions. The motivation in this case was the occasionally
CO/Ni(TfO)2/TfOH
■CHO
Scheme 5.2: Attempted Formylation of Benzene with Nickel Triflate.
57
hazardous and (due to low solubility) difficult manipulation of inorganic azide
reagents. They managed to charge a commercially available anion exchange resin
(Amberlite IRA 400 (Cl)) with a concentrated sodium azide solution. The thus
prepared azide resin reacted smoothly at ambient temperature with alkyl halides
(iodides better than bromides better than chlorides) to give the corresponding alkyl
azides. The procedure was so effective, that even dichloromethane, which served as
the solvent in some of the reported reactions, reacted with the resin after two weeks
to give the highly unstable diazidomethane.
Although the triflate ion is far less nucleophilic than the azide ion, it was
explored to obtain a similar triflating resin. Since chloride would have a much higher
affinity for the quaternary ammonium sites in the resin, an equilibrium had to be
established in which a large excess of triflate ions could displace the chloride present
in the resin.
A 20 % sodium triflate solution was prepared by the addition of triflic acid
to a concentrated sodium carbonate solution. The molarity given by Hassner and
Stem for the azide resin (2.55 mmol azide per gramm resin) was adopted and a
fourfold excess of sodium triflate solution was added. After stirring for two weeks
to allow the establishment of the equilibrium the resin was filtered off and dried in
vacuo for several days.
As a control experiment methyl iodide was added to a suspension of the
resin in dry acetonitrile. After stirring for 16 hours at room temperature a sample
58
was taken and analyzed by gas chromatography/mass spectroscopy. Traces of
methyl triflate was observed along with unreacted methyl iodide. Whether methyl
chloride was formed as well under the reaction conditions from residual chloride in
the resin could not be determined, because it would have evaporated under the
work-up conditions.
©
NR3 ------------------- ► ROTi
TfO0
Scheme S3: Triflate Formation via Ion Exchange.
The experiment was repeated with cyclohexyl iodide as a less volatile alkyl
halide. In this case only starting material was recovered. Similar reactions were
carried out with diiodomethane and bromotrichloromethane but no methylene bis-
triflate and trichloromethyl triflate were observed.
The extremely low nucleophilicity of the triflate ion seems to limit the appli
cability of the ion exchange resin. The use of the resin system in the preparation of
inflates was therefore not further pursued.
59
5.7 References
Engelbrecht, A.; Tschager, E. Z. anorg. allg. Chem. 1977,433, 19
2 Olah, G.A.; Farooq, O.; Morteza, F.F.; Olah, J.A. J. Am. Chem. Soc. 1988,
110, 2560
3 Lawrance, G.A. Chem. Rev. 1986, 86, 17
4 Howell, R.D.; Me Crown, J.D. Chem. Rev. 1977, 77,69
5 Stang, P.J.; Hanack, M.; Subramanian, L.R. Synthesis, 1982, 85
6 Stang, P.J.; White, M.R, Atdrichimica Acta, 1983,1 6 ,15
7 Corey, E.J.; Shimoji, K. Tetrahedron Lett. 1983,24, 169
8 Jansky, M.T.; Yoke, J.T, J. inorg. nucl. Chem. 1979,41, 1707
9 Kobayashi, S.; Hachiya, I. J. Org. Chem. 1994,59,2233
10 Kawada, A.; Mitamura, S.; Kobayashi, S. Synlett 1994, 545
11 Dixon, N.E.; Lawrance, G.A.; Lay, P.A.; Sargeson, A.M.; Taube, H.
Inorganic Syntheses, 1986,2 4 ,243
12 Farooq, O. Ph.D. Dissertation, University of Southern California 1984, p42
13 Rawle, .S.; Sewell, T.J.; Cooper, S.R. Inorg. Chem. 1987,2 6 ,3769
14 Haszeldine, R.N.; Kidd, J.M. J. Chem. Soc. 1954,4228
15 Bergstrom, P.A.; Lindgem, J. J. Mol. Struc. 1990,2 3 9 ,103
16 Schmeisser, M.; Satori, P.; Lippsmeier, B. Chem. Ber. 1970,103, 8 6 8
17 Arduini, A.L.; Garnett, M.; Thompson, R.C. Can. J. Chem., 1975,5 3 ,3812
18 Dines, M.B. Sep. Sci. 1973,8, 661
19 Miles, M.G.; Doyle, G.; Cooney, R.P.; Tobias, R.S. Spectrochimica Acta,
1969,25A, 1515
60
Kupferschmidt, W.C.; Jordan, R.B. Inorg. Chem. 1982,21, 2089
Haynes, J.S.; Sams, J.R.; Thompson, R.C. Can. J. Chem. 1981,59, 669
Houben-Weyl, Methoden der Organischen Chemie, Thieme, Stuttgart,
vol.7/1, p 16-20,1954
Crounse, N.N. Org. React. 1949, J , 290
Truce, W.E. Org. React., 1957, 9, 37
Gattermann, L.; Koch, J.A. Chem. Ber. 1897,30, 1622
Olah, G.A.; Laali, K.; Farooq, O. J. Org. Chem. 1985,5 0 ,1483
Hassner, A.; Stem, M. Angew. Chem. Intl. Ed. Engl. 1986, 98, ???
6. Experimental
6.1.1 Chemicals Used
Chemicals, if not otherwise indicated, were purchased from Aldrich
Chemical Co. and used without further purification if not described otherwise.
Trifluoromethanesulfonic Acid (triflic acid) was obtained from the 3 M Co. and was
distilled twice prior to use.
6.1.2 Reaction Conditions
Because trifluoromethanesulfonic acid and most of its derivatives are
extremely moisture sensitive precautions had to be taken to avoid contamination of
the reagents. Most reagents were manipulated in Schlenk type reaction vessels under
an inert gas atmosphere which was either nitrogen or argon, unless otherwise
described in the text. 1 Solvents were usually dried and distilled before use following
standard literature procedures.2 ‘ 3
6.1.3 Hazards and Precautions
Triflic acid is known to be one of the strongest protic acids accessible. Its
esters are extremely reactive alkylation agents. Thus triflic acid and triflates should
be handled with utmost care. Tests show, that eye contact with the acid can result in
62
permanent eye demage,4 The acid and its derivatives have also acute inhalation
toxicity and should therefore only be manipulated in a well ventilated hood. Skin
contact has to be avoided. The treatment of triflic acid bums is the same as for other
strong acids.5,6
Triflic acid is corrosive towards rubber, cork, cellulose and common grease
for glass joints. Hence, the reaction apparati should be connected and protected
from air and moisture proof with teflon tape or sleeves. Storage of triflic acid in
glass bottles with glass or teflon stoppers is highly recommended.
6.1.4 Spectroscopy
Spectroscopic data was gathered with the Varian Unity-300 and Varian
VXR-200 superconducting spectrometers (NMR), the Hewlett-Packard gas
chromatograph/ mass spectrometer model 5890/5971A (GCMS) and the Nicolet
800 SX FTIR infrared spectrometer (IR).
6.1.5 Ultrasound
Ultrasound assisted experiments were done in a Sonicator® Ultrasonic Liquid
Processor W-385.
63
6.2 Methylene Bis triflate Experiments
6.2.1 Reactions of 1,3,5-Trioxane with Triflic Anhydride
0.41 g of 1,3,5-trioxane were dissolved in 2.5 ml dry dichloromethane. 0.5
ml triflic anhydride in 2 ml dry dichloromethane were placed under nitrogen
atmosphere in a Schlenk flask equipped with a rubber septum. The trioxane solution
was added dropwise with a hypodermic syringe. After stirring for one hour at room
temperature a milky white precipitation occurred that was not further investigated.
In the same manner 0.16 g trioxane in 2.5 ml CH2G 2 were added slowly to
0.30 ml triflic anhydride in 3.0 ml CH2CI2 under cooling by means of a dry
ice/acetone bath. Addition was completed after two hours and the mixture was then
allowed to warm up to room temperature under continuous stirring. Upon standing
for 1 0 hours the solution turned slightly yellow and no precipitation was observed.
The i3C{‘H}-NMR spectrum showed a quartet at 118 ppm (CFj) and singlets at
205.95 ppm, 93.43 ppm and 53.57 ppm (CH2Q 2).
6.2.2 M etathesis of Diiodomethane with Stiver Triflate
In a dry Schlenk flask equipped with a reflux condenser under argon
atmosphere 1.29 g silver triflate were placed and covered with 10 ml dry pentane.
The pentane was heated to a full boil 0.2 ml of diiodomethane were added in two
64
portions 30 minutes apart. The mixture was then heated under reflux for two more
hours and was allowed to cool to room temperature and stirred for four more hours.
The GCMS spectra indicated the formation of the mono-substituted product
(m/z=290).
6.2.3 Reaction of Dibromomethane with Silver Triflate
A solution of 2.00 g silver triflate in 10 ml acetonitrile were heated under
reflux in a Schlenk flask with reflux condenser under argon atmosphere. 0.27 ml
dibromomethane were added at once. Only after seven days a little yellow
precipitate occurred. The mixture was separated from the dissolved silver triflate by
bulb-to-bulb distillation. GCMS spectroscopy only showed starting material CHzBr;.
6.3 Vinyl Triflates
6.3.1 Reactions o f TM S-Acetylene and Bis(TMS)Acety!ene with Triflic Acid
0.8 ml (trimethylsilyl)acetylene in 5.0 ml dry pentane were placed in Schlenk
flask under argon atmosphere and cooled to -40°C by an acetonitrile/dry ice bath.
1 .0 ml triflic acid were added slowly over a small dropping funnel over a period of
three minutes. While constantly stirring the mixture was allowed to warm up
gradually to room temperature within four hours.
65
2.0g bis{trimethylsilyl)acetylene (11.7 mmol) in 10 ml dry pentane were
placed in a Schlenk flask under argon atmosphere. 0.51 ml triflic acid were added
dropwise at -78°C (dry ice/acetone bath). The cooling bath was removed after the
addition was completed and the mixture stirred for 24 h at room temperature.
3 ml triflic acid were placed in a Schlenk flask with a rubber septum under
argon atmosphere. The reaction vessel was cooled to -23°C with an ice/salt bath and
then 1.0 ml (trimethylsilyl)acetylene was added slowly. The solution turned dark
brown immediately and gas development was observed during the addition by
turning off the inert gas flow and watching the bubbler at the outlet of the inert
gas/vacuum line. The mixture was diluted with dichloromethane and neutralized
with pyridine until no further precipitation of pyridinium triflate was observed.
6.3.2 Reaction of Acetaldehyde with Triflic Anhydride
In a roundbottom Schlenk flask 4.98 g l,8 -bis(dimethylamino)naphthalene
was dissolved in 100 ml dry pentane. The solution was cooled by means of an ice/
salt bath to -23°C. Then 3.0 ml of acetaldehyde were added. Under vigorous
stirring, 1.0 ml of triflic anhydride was added slowly. After the addition was
complete, the ice/ salt bath was replaced by an ice bath and the mixture was allowed
to warm up to room temperature within four hours. The solvent and residual
acetaldehyde were removed under reduced pressure while the reaction flask was
66
held at 0°C by an ice bath. The ice bath was removed and any volatile residue was
distilled into a cooling trap cooled by liquid nitrogen. Deuterated benzene was
added. l3C{ *H} and 'H NMR did not show the desired product.
6.4 Trichloromethyl Triflate
6.4.1 Attempted in situ Trichloromethylation of Benzene
2.0 g silver triflate (7.784 mmol) were covered with 5 ml dry
dichloromethane in a 25 ml Schlenk flask under argon atmosphere. 0.85 ml
bromotrichloromethane (8.62 mmol) were added an the mixture was stirred at room
temperature for 18 h. 13C-NMR investigation showed unreacted C G 3Br (5 = 67.48
ppm) and product CCI3 ( 8 = 108.8 ppm) and CF3 ( 8 - 117.82 ppm, Jch 321.4 Hz).
All chemical shifts are in reference to CDCI3.
Then 2.0 ml dry benzene were added and the mixture was left at room
temperature for three days. The mixture was then hydrolyzed, the aqueous layer
neutralized and extracted with ether. GCMS investigation showed the presence of
Ph-CCl3 and benzophenone Ph-CO-Ph.
6.4.2 Formation of CCI3OTf at Elevated Temperature
In a Schlenk flask equipped with a reflux condenser 0.38 ml CCl3Br, 1.0 g
silver triflate and 9.0 ml dry tetrachloromethane were placed under argon
67
atmosphere. The mixture was heated to reflux for 24 h. After cooling to room
temperature a sample was investigated by GCMS. (Yields: <5% CCljOTf; <1%
C2CIs)
6.4.3 Formation of CCbO Tf at Ambient Temperature
In a flame-dried Schlenk flask equipped with a rubber septum 1,0 g silver
triflate and 9.5 ml tetrachloromethane were placed under argon atmosphere. While
the mixture was stirred at room temperature 0.4 ml CCl3Br were added with a
hypodermic syringe. Samples were taken after 30, 48 and 96 hours and investigated
by GCMS. (Yields: < 1% CCl3 OTf + C2C1 6 (30 h); <6 % CCl3OTf; <0.5% C2Q 6 (48
h); <10% CCIjOTf; <3% C2Cl6 (96 h)).
6.4.4 Control Experiments: Sonication o f CCbBr and CCI4
In a Schlenk flask 0.5 ml of bromotrichloromethane in 5.0 ml of carbon
tetrachloride were sonicated under inert gas atmosphere for 2 hours. GCMS
investigation showed besides a large excess of starting material traces of
hexachloroethane.
In the same way 5.0 ml of carbon tetrachloride were sonicated under inert
gas atmosphere for 2.5 hours. GCMS spectroscopy only showed starting material.
68
6.5 Metal Triflates
6.5.1 Preparation of Nickel Triflate
2.78 g nickel (II) chloride hexahydrate (11.7 mmol) and 6.0 g Silver Triflate
(23.4 mmol) were dissolved separately in a small volume of methanol. Upon
combination of the two solutions (total 35 ml) a white precipitation occurred. After
stirring overnight the precipitate was filtered off and the methanol rotary-evaporated
under vacuum. The viscous green residue became an amorphous solid upon cooling.
It was redissolved in acetone. Evaporation of the acetone and storage in the vacuum
oven (30 in. Hg vac.) at 160°C for five hours gave a light yellow salt. Yield: 3.68 g
(10.3mmol), 88.03 % of the theory.
6.5.2 Cobalt Triflate
As described for nickel triflate 2.78 g C0 CI2 ' 6 HjO and 6.0 g silver triflate
were brought to reaction and resulted in 3.34 g (9.4 mmol) of a lavender salt, 80.34
% of the theory.
6.5.3 Iron (IQ) Triflate
1.01 g iron (III) chloride hexahydrate and 2.88 g silver triflate were dissolved
separately in methanol to result in a 20 ml solution. To complete precipitation of
69
silver chloride it was heated under reflux conditions overnight. The methanoi was
evaporated, the residue dissolved in acetone and dried at 150 in the vacuum oven
for six hours. The reaction yielded 1.28 g of a dark brown solid (2.6 mmol), 34% of
the theory,
6.5.4 Preparation of Mercury (II) Triflate
2.64 g mercury (U) chloride (9.72 mmol) and 5.0 g silver triflate (19.45
mmol) were each dissolved in about 10 ml methanol. Upon combining the two
solutions a milky white precipitation occurred immediately. To assure complete
precipitation, it was stirred overnight. The silver chloride was then filtered of and
washed with methanol. The filtrate was rotary-evaporated and the residue further
dried under vacuum at 110 to 115 °C over 30 hours. The reaction yielded 4.8 g of a
white powdery solid (9.64 mmol, 99 % of theory) which was used without further
analysis.
6.5.5 Attempted Preparation of Rhodium (HI) Triflate
Rhodium (III) chloride (0.1 g), 0,37 g silver triflate and 6 ml methanol were
combined in a 10 ml Schlenk flask and heated under reflux conditions overnight
The dark ruthenium chloride did not dissolve in methanol and no white precipitate of
silver chloride was observed after refluxing for several days.
70
6.6 Gattermann-Koch Formylation of Toluene with Nickel Triflate
In a three-necked round bottom flask equipped with a gas inlet (needle
through rubber septum), magnetic stirrer and a gas outlet connected to a bubbler a
mixture of 1,0 g nickel triflate (2.8 mmol), o.25 ml triflic acid (2,8 mmol) and 3.0 ml
dry, freshly distilled toluene was placed. Then a moderate stream of CO was
bubbled through the mixture under constant stirring at room temperature. After five
hours the mixture was hydrolyzed with ice water, washed with saturated sodium
bicarbonate solution and extracted with dichloromethane. The organic layer was
separated, dried over magnesium sulfate and the solvent removed by rotary-
evaporation. Only toluene was observed in the 'H-NMR spectrum.
6.7 Reaction of Triflic Acid with Metals
Small amounts of cobalt, copper, chromium, zinc and aluminum were placed
in test tubes. To each metal 0.25 ml triflic acid were added with a glass syringe. No
reaction (dissolving of metal, gas development) was observed.
71
6.8 Triflate Ion Exchange Resin
6.8.1 Preparation o f the Ion Exchange Resin
To a saturated aqueous solution of 10 g sodium carbonate cooled to 0°C in
an ice bath 16.68 ml of triflic acid were added slowly through a dropping funnel.
Since the reaction proceeded violently the addition took more then 1.5 hours. The
solution was then stirred at about 60°C to remove all COi . Since a large excess of
sodium triflate was required in the second step and not a distinct amount the
reaction was assumed to be quantitative (hence resulting in a solution of 16.2 g
sodium triflate in 100 ml H2O). To the triflate solution was then added a 10 g
amount of ion exchange resin Amberlite IRA 400 (Cl) . It was stirred at room
temperature for 6 days. The resin was filtered and washed with small amounts of
water and methanol. It was then dried under vacuum at room temperature for 12
hours.
6.8.2 Preparation of Methyl Triflate by Ion Exchange
1 0 g of the prepared triflate-charged ion exchange resin were covered with
10 ml dry acetonitrile. Under stirring at room temperature 0.2 ml methyl iodide (3.2
mmol) were added. After 16 h the resin was filtered off and the filtrate was
investigated by GCMS. The GCMS spectrum showed one major peak indicating the
72
formation of the triflate (m/z = 164, [CHjOTf]; m/z = 134, M - CH20 =
[H S0 2CF3 ]+ ; m/z = 118, M - CH20 2 = [HSOCF3 ]+ .
6.8.3 Attempted Formation of Cyclohexyl Triflate by Ion Exchange
In the same fashion as described for methyl iodide 1.1 ml cyclohexyl iodide
(8.24 mmol) were added to a suspension of 4.12 g triflate ion exchange resin in 10
ml methylene chloride. GCMS investigation showed only starting material.
6.8.4 Attempted Formation of Methylene Ditriflate by Ion Exchange
In the same fashion as described for methyl iodide 0.32 ml diiodomethane
were added to a suspension of 3.96 g triflate ion exchange resin in 10 ml methylene
chloride. GCMS investigation showed only starting material.
6.8.5 Attempted Formation of Trichloromethyl Triflate by Ion Exchange
In the same fashion as described for methyl iodide 0.5 ml CCfjBr were added
to a suspension of 4 g triflate ion exchange resin in 15 ml pentane. GCMS
investigation showed only unreacted starting material.
73
6.9 References
1 Gill, G.B.; Whiting, D. A. Aldrichimica Acta, 1986,19, 31
2 Purification o f Laboratory Chemicals, Perrin, D.D.; Armarego, W.L.F. Eds.;
Pergamon Press, New York, 3rd ed., 1992
J Loewenthal, Zass, E. Der clevere Organiker, Barth, Ed., Verlag der
Wissenschaften, Leipzig; Berlin; Heidelberg, 1993
4 Guenthner, R.A. Kirk-Othmer Encyclopedia o f Chemical Technology, 3rd ed.,
Wiley-Interscience, New York, 1980, pp 952*955
s Le Fevre, M.J. First Aid Manual fo r Chemical Accident, 2nd ed., Von
Nostrand Reinhold, New York, 1989
6 Schmitz, A. personal communication
74
7. Conclusion
Trifluoromethanesulfonic acid and its derivatives, organic triflates as well as
metal triflates, have become important reagents in modem chemistry. The
extraordinary properties of triflate as a leaving group have been applied in
numberous organic syntheses. The facile preparation of the triflate compounds that
were the target of my thesis promise to use this property and open new approaches
to preparative problems.
Methylene bistriflate has been shown to be more easily accessable. Although
it could not yet be isolated in pure form it may now find application in the formation
of methylene bridged species.
The parent vinyl triflate has been found not to be accessable by the common
preparations for vinyl triflates, presumably due to the low reactivity of the starting
materials (acetylene or acetaldehyde, respectively).
Trichloromethyl triflate is a potentially very useful synthetic organic reagent.
Although not readily available by the metathesis with silver triflate, it can be
prepared from mercury (II) triflate and bromotrichloromethane. It was
demonstrated, however, that trichloromethyl triflate unlike trifluoromethyl triflate
tends to cleave at the carbon-oxygen bond, hence providing the potential to be used
in electrophilic trichloromethylation reactions.
75
Mercury (II) triflate as well as other metal triflates were conveniently
prepared by metathesis of mercury (II) chloride and silver triflate. The possible use
of these metal triflates as catalysts in Friedel-Crafts type reactions is to be explored.
76
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Asset Metadata
Creator
Archut, Andreas
(author)
Core Title
Trifluoromethanesulfonates (triflates) for organic syntheses
School
Graduate School
Degree
Master of Science
Degree Program
Chemistry
Degree Conferral Date
1994-12
Publisher
University of Southern California
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University of Southern California. Libraries
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Tag
chemistry, organic,OAI-PMH Harvest
Language
English
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Olah, George A. (
committee chair
), Bau, Robert (
committee member
), Surya, G.K. Surya (
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