Ab Initio/IGLO/GIAO-MP2 Studies of Fluorocarbocations: Experimental and Theoretical Investigation of the Cleavage Reaction of Trifluoroacetic Acid in Superacids

Article abstract of DOI:10.1021/jo9611327

The structures of a number of fluorocarbocations were calculated at the correlated MP2/6-31G* level. 13C and 19F NMR chemical shifts of fluorocarbocations were calculated for the first time using IGLO and GIAO-MP2 methods.The data showed good correlation of calculated 19F and 13C NMR chemical shifts with the experimental shifts of fluorocarbocations.The correlation for GIAO-MP2-calculated 19F NMR chemical shifts with the experimental data is excellent.Using theoretical calculations as guidance, the protolytic cleavage of trifluoroacetic acid (CF3COOH) in superacid forming CF4 was also investigated experimentally and by ab initio calculations.This reaction is suggested to involve the gitonic CF3C(OH)(OH)²⁺ dication as an intermediate.

Full text of DOI:10.1021/jo9611327

J . Org. Chem. 1996, 61, 9253-9258
9253
Ab In itio/IGLO/GIAO-MP 2 Stu d ies of F lu or oca r boca tion s:
Exp er im en ta l a n d Th eor etica l In vestiga tion of th e Clea va ge
Rea ction of Tr iflu or oa cetic Acid in Su p er a cid s1a
G. K. Surya Prakash,* Golam Rasul, Arwed Burrichter, Kenneth K. Laali, and
George A. Olah*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California,
University Park, Los Angeles, California 90089-1661
Received J une 14, 1996X
The structures of a number of fluorocarbocations were calculated at the correlated MP2/6-31G*
1
3
19
level. C and F NMR chemical shifts of fluorocarbocations were calculated for the first time
1
9
13
using IGLO and GIAO-MP2 methods. The data showed good correlation of calculated F and
C
NMR chemical shifts with the experimental chemical shifts of fluorocarbocations. The correlation
for GIAO-MP2-calculated 19F NMR chemical shifts with the experimental data is excellent. Using
theoretical calculations as guidance, the protolytic cleavage of trifluoroacetic acid (CF
3
COOH) in
superacids forming CF was also investigated experimentally and by ab initio calculations. This
4
2
+
reaction is suggested to involve the gitonic CF
3
C(OH)(OH
2
)
dication as an intermediate.
In tr od u ction
studies of a series of fluorocarbocations which were
earlier characterized by 19F NMR spectroscopy under
long-lived stable ion conditions. This permits comparison
of calculated data with experimentally observed results.
Such calculations have become increasingly useful to
study electron deficient intermediates but were so far not
reported for fluorocarbocations. We have also calculated
Fluorocarbocations play an important role as interme-
diates in the electrophilic addition to fluoroolefins. The
study of fluorocarbocations is of considerable interest
because of the dualistic effect of a fluorine as a substitu-
ent. Due to its high electronegativity, a fluorine atom
adjacent to a carbocationic center is inductively desta-
bilizing. On the other hand, the nonbonded electron pairs
on the fluorine atom can stabilize the positive charge
through back-donation (n-p interaction). Numerous
fluorocarbocations were observed as stable, long-lived
ions by 13C and F NMR spectroscopy since the pioneer-
the structures and 13C and F NMR chemical shifts for
19
+
still elusive perfluorinated cations, such as CF
3
.
In
addition, using theoretical calculations as a guide, we
report the experimental and theoretical study of the
protolytic cleavage of trifluoroacetic acid, CF
strong superacids giving CF . Based on ab initio calcula-
tions, it is suggested that the reaction involves the CF C-
3
COOH, in
19
4
2
ing work of Olah, Chambers, and Comisarow, although
in some cases difficulties arose because of rapid fluoride
ion exchange between superacid and fluorocarbocations.
3
2
+
(OH)(OH
2
)
dication as an intermediate.
+
2
+ 3a
Attempts to directly observe CH
3
CHF
and CF
3
by
NMR spectroscopy under superacid stable ion conditions
Resu lts a n d Discu ssion
+
were unsuccessful. The intermediacy of CF
3
was
suggested,3b however, in the ionization and subsequent
Ab initio calculations were carried out by using the
6
GAUSSIAN-94 package of programs. Optimized geom-
etries were obtained at the MP2/6-31G* level, and
selected parameters of the ions are given in Figures 1
and 2. Vibrational frequencies at the HF/6-31G*//HF/6-
decarbonylation of trifluoroacetyl fluoride (CF COF) with
3
+
+
5 3 3
SbF at low temperature. Whereas no CF CO and CF
1
9
cations could be observed directly, F NMR spectroscopy
showed already at relatively low temperature (-50 °C)
the formation of tetrafluoromethane (CF ). It was pro-
4
posed that the great strength of the C-F bond in CF
3
ca. 140 kcal/mol) leads to rapid quenching of CF to CF
3
1G* level were used to characterize stationary points
as minima. IGLO calculations were performed according
4
4
+
to the reported method at IGLO II level using MP2/6-
(
4
7
3
1G* geometries. Huzinaga Gaussian lobes were used
even in low-nucleophilicity fluorinated superacid media.
as follows: Basis II, C, O, or F, 9s 5p 1d contracted to
[
[
51111, 2111, 1]; d exponent, 1.0; H, 5s 1p contracted to
311, 1]; p exponent, 0.70. GIAO-SCF and GIAO-MP2
(4) Kutzelnigg, W. Isr. J . Chem. 1980, 19, 193. Schindler, M.;
Kutzelnigg, W. J . Chem. Phys. 1982, 76, 1919. Schindler, M. J . Am.
Chem. Soc. 1987, 109, 1020. Kutzelnigg, W.; Fleischer, U.; Schindler,
M. NMR Basic Principles Prog. 1991, 91, 651.
(5) Gauss, J . J . Chem. Phys. Lett. 1992, 191, 614. Gauss, J . J . Chem.
Phys. 1993, 99, 3629.
4
We wish to report now our ab initio/IGLO /GIAO-MP25
(
6) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
X
Abstract published in Advance ACS Abstracts, December 15, 1996.
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T. A.; Peterson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; Defrees, D. J .; Baker, J .;
Stewart, J . J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian-
94 (Revision A.1); Gaussian, Inc.: Pittsburgh, PA, 1995.
(
1) (a) Considered Stable Carbocations 301. For part 300, see:
Prakash, G. K. S.; Rasul, G.; Liang, G.; Olah, G. A. J . Chem. Phys.
996, 100, 15805. (b) For a review of fluorocarbocations, see: Olah, G.
A.; Mo, Y. K. Adv. Fluorine Chem. 1973, 7, 69.
2) Olah, G. A.; Chamber, R. D.; Comisarow, M. B. J . Am. Chem.
Soc. 1967, 89, 1268.
3) (a) Olah, G. A.; Heiliger, L.; Prakash, G. K. S. J . Am. Chem. Soc.
989, 111, 8020. (b) Olah, G. A.; Germain, A.; Lin, H. C. J . Am. Chem.
Soc. 1975, 97, 5481.
1
(
(
1
(7) Huzinaga, S. Approximate Atomic Wave Function; University of
Alberta: Edmonton, Alberta, 1971.
S0022-3263(96)01132-2 CCC: $12.00 © 1996 American Chemical Society

9
254 J . Org. Chem., Vol. 61, No. 26, 1996
Prakash et al.
F igu r e 1. Selected MP2/6-31G*-optimized parameters of 1-11.
Ta ble 1. 9F NMR a n d Selected 13C NMR Ch em ica l
1
Sh ifts of F lu or oca r boca tion sa
IGLO GIAO- GIAO-
II//MP2/ SCF/ MP2/
no.
cations
[CF3]+
atom 6-31G* tzp/dz tzp/dz
expt
1
F
C
F
C
F
C
F
C
F
C
F
C
F
C(F)
C(H)
F
C(F)
C(H)
F
58.6
162.1
215.0
277.5
126.9
216.8
183.7
295.7
162.5
293.0
159.1
311.4
-56.5
168.2
160.7
-51.7
154.0
146.7
-50.6
138.3
73.3
81.3
51.7
+
+
+
+
+
+
167.1 169.2
230.8 258.8
279.0 279.7
143.8 135.5
222.0 219.0
2
3
4
5
6
7
CH3[CHF]+
CH3[CFF]+
96.4b
(CH3)2[CF]+
C2H5[CFCH3]+
200.3 219.2 185.0
296.3 295.9 282.8
c
c
c
c
c
c
183.5
283.6
149.4
294.0
F igu r e 2. Selected MP2(FU)/6-31G*-optimized parameters of
2 and 13.
1
[c-C5H8F]+
5
calculations using tzp/dz basis set have been performed
with the ACES II program.8 Chemical shifts are listed
in Table 1.
[c-C H F]+
-46.8 -50.2 -67.0d
170.3 181.2
160.9 163.7
-42.1 -49.5
156.0 163.3
3
2
8
9
[c-C3HF2]+
So far the appplication of IGLO and GIAO methods to
fluorocarbocations has not been explored. The IGLO
147.0 147.9
9
19
[c-C3F3]+
-42.0 -55.4 -63.1e
140.5 145.2
method has been applied only to calculate the F NMR
chemical shifts of a number of small neutral molecules.
With the use of the large basis set TZP (triple-ú plus
polarization functions), satisfactory agreement between
C
F
1
1
0
1
[t-CHFdOCH3]+
90.4
72.8
49.9f
C(F)
C(H)
F
189.9
75.0
68.1
192.2 186.5
76.5
84.5
83.1
69.1
gas phase 19F NMR chemical shifts with calculated 19
F
[c-CHFdOCH3]+
41.8f
NMR chemical shifts has been found. IGLO calculations
C(F)
C(H)
184.3
74.7
188.0 180.9
76.1 82.0
with DZ (double-ú) level were shown to be unreliable for
1
9
9
F NMR chemical shifts. The application of the GIAO-
a
Both experimental and theoretical 19F and 13C NMR Chemical
b
shifts are referenced to CFCl3, TMS, respectively. Reference 15.
c
Reference 18. d Reference 19. e Reference 20. f Reference 21.
(8) Stanton, J . F.; Gauss, J .; Watts, J . D.; Lauderdale, W.; Bartlett,
R. J . ACES II, an ab initio program system; University of Florida:
Gainesville, FL, 1991.
MP2 method, which includes dynamic electron correla-
tion in chemical shift calculations, to the chemical shifts
(9) Fleischer, U.; Schindler, M. Chem. Phys. 1988, 120, 103.

Ab Initio/IGLO/GIAO-MP2 Studies of Fluorocarbocations
J . Org. Chem., Vol. 61, No. 26, 1996 9255
of 13C, 17O, 15N, 19F, etc., show, in some cases, significant
5
improvements over the chemical shifts results computed
at the SCF level.
+
CF
3
, 1. In the gas phase, trifluoromethyl cation (1),
1
0
is stable and has been observed as an abundant species.
We have recently reported the calculated energy and
structure of 1.11 The MP2/6-31G*-optimized structure of
1
is a planar D3h with a shorter C-F bond length of 1.246
. Since
Å compared to the C-F bond length (1.33 Å) of CF
fluorine possesses 2p nonbonded electron pairs, the back-
4
donation 2p-2p overlap is maximum in ion 1, resulting
1
2
in the shorter C-F bond length. However, Reynolds
+
calculated the relative stability of CF
3
compared to other
trihalomethyl cations (CX
3
+, X ) Cl and Br). The
calculated order of stability was found to be Cl > Br >>
F. Therefore, the electron-withdrawing power of the
+
three fluorine atoms in CF
3
surpasses their π-donating
ability. The IGLO II- and GIAO-MP2-calculated 13
C
NMR chemical shifts of 1 are 162.1 and 169.2 ppm,
respectively, close to the predicted value of δ 140.0
obtained from comparison with other known trihalo-
methyl carbocations.3 The GIAO-MP2-calculated
19
F
NMR chemical shift of ion 1 is δ 51.7.
+
CH
3
CHF , 2. So far 2 has also not been observed in
2
solution under stable ion conditions. Structure 2 was
found to be the global minimum on the potential energy
surface. The hydrogen-bridged structure 2a is not a
minimum at the MP2/6-31G* level and converged into 2
upon optimization at the MP2/6-31G* level. C-H hy-
perconjugation is responsible for the shorter C-C bond
(
1.433 Å) in 2. Structure 2 has previously been calculated
1
3
by Reynolds at MP2/6-31G**, and the results were
similar to those obtained in the present study.
Compared to 1, the GIAO-MP2-calculated 13C and 19
F
NMR chemical shifts of 2 of δ 279.7 and 258.8, respec-
tively, are much more deshielded. Correlation has little
effect on 13C NMR chemical shift calculations. Accord-
ingly, both IGLO II (δ 277.5)- and GIAO-SCF (δ 279.0)-
calculated 13C NMR chemical shifts of 2 are very close to
the corresponding GIAO-MP2-calculated value of δ 279.7.
+
CH
3
C(F )F , 3. Methyldifluorocarbenium ion (3) was
observed14 as a long-lived ion by ionizing CH
SO ClF solution at -80 °C and characterized by H and
F NMR spectroscopy. The longer C-C bond (1.452 Å)
3
3 5
CF in SbF /
1
2
1
9
of 3 compared to that (1.433 Å) of 2 can be accounted for
on the basis that both fluorine atoms in ion 3 are capable
of back-donation and thus stabilize the ion by resonance.
The GIAO-MP2-calculated averaged 19F NMR chemical
shift of 3 is δ 135.5, which deviates from the experimental
value (δ 96.4) by 39.1 ppm. However, the overall cor-
relation of the GIAO-MP2-calculated 19F NMR chemical
shifts with the experimental chemical shifts of the
(
(
10) Murdoch, H. D.; Weiss, E. Helv. Chim. Acta 1962, 45, 1927.
11) Olah, G. A.; Rasul, G.; Heiliger, L.; Prakash, G. K. S. J . Am.
F igu r e 3. Plot of calculated vs experimental 19F NMR
chemical shifts of fluorocarbenium ions: (a) GIAO-MP2/tzp/
dz vs experimental, (b) GIAO-SCF/tzp/dz vs experimental, and
Chem. Soc. 1996, in press. Olah, G. A.; Rasul, G.; Yudin, A. K.;
Burrichter, A.; Prakash, G. K. S.; Chistyakov, A. L.; Stankevich, I. V.;
Akhrem, I. S.; Gambaryan, N. P.; Vol’pin, M. E. J . Am. Chem. Soc.
(c) IGLO II vs experimental.
1
996, 118, 1446.
(
(
(
12) Reynolds, C. H. J . Chem. Soc., Chem. Commun. 1991, 975.
13) Reynolds, C. H. J . Am. Chem. Soc. 1992, 114, 8676.
14) Olah, G. A.; Comisarow, M. B. J . Am. Chem. Soc. 1969, 91, 2955.
fluorocarbocations is excellent as shown in Figure 3. The
GIAO-MP2-calculated 13C NMR chemical shift of 13 (δ

9
256 J . Org. Chem., Vol. 61, No. 26, 1996
Prakash et al.
2
19.0) indicates a moderate shielding effect as compared
-63.1, which is deshielded by 57.8 ppm when compared
to neutral perfluorocyclopropene (δ -120.9). The struc-
tural feature and calculated and experimental chemical
shifts of this trifluoro-substituted cyclopropenium ion 9
are very similar to those of mono- and difluoro substi-
tuted cyclopropenium ions 7 and 8, respectively (Figure
1).
to 2 (δ 279.7). However, no experimental 13C NMR
chemical shift of 3 is available to make comparisons with
the theoretical results.
+
(
CH
3
)
2
CF , 4. The C
2
+
structure 4 is the most stable
conformer of (CH
calculations. A similar C
be the most stable conformer for the 2-propyl cation.
3
)
2
CF at the MP2/6-31G* level of
2
structure was also shown to
1
5
+, 10 a n d 11. When R,R-
-SO
tr a n s- a n d cis-CHF dOCH
difluoromethyl methyl ether was treated with SbF
3
1
3
Whereas the GIAO-MP2-calculated C NMR chemical
5
2
shift of δ 295.9 agrees well with the experimental
at -40 °C, two isomeric methoxyfluorocarbenium ions
chemical shift of δ 282.8, the GIAO-MP2-calculated 19
F
20
were obtained, of which 70% is 10 and 30% is 11 as
1
NMR chemical shift of δ 219.2 again deviated by 34 ppm
measured by integration of the H NMR signals.
from the experimental value of δ 185.0.
+
CH
3
CH
2
C F CH
3
, 5. The optimized structure 5 shows
a long C(CH
3
)-C(CH ) bond (1.578 Å) aligned parallel
2
+
with the p-orbital of C thus permitting maximum C-C
hyperconjugation. This type of long bond has also been
found in tertiary alkyl carbocations.16 Ion 5 has also been
At the MP2/6-31G*//MP2/6-31G* level the ion 11 is
only 3.2 kcal/mol more stable than the ion 10. At the
MP4(SDTQ)//6-31G*//MP2/6-31G* level the ion 11 is still
observed in superacid solution under stable ion condi-
tions.17 Because of the size of the molecule, we were not
3
.2 kcal/mol more stable than the ion 10. This is,
able to calculate its chemical shifts at the GIAO-MP2
level. Experimental and IGLO-calculated chemical shifts
however, not in agreement with the experimental results
where 10 was formed predominantly. The calculated
structures of ions 10 and 11 show that the ions are
predominantly carboxonium ions rather than carbenium
are shown in Table 1.
+
c-C
5
H
8
F , 6. The twisted C
2
structure 6 is found to
be the global minimum of 1-fluoro-1-cyclopentyl cation.
ions as indicated from the calculated C(CH )-O bond
2
The ion was previously prepared17 by Olah et al. by
distances (1.24 Å) of 10 and 11, which are close to the
CdO distance in carbonyl compounds. The calculated
chemical shifts are summarized in Table 1.
treating 1,1-difluorocyclopentane with a SbF
5
2
-SO ClF
solution at -78 °C. The 19F NMR spectrum of ion 6
contains a deshielded quintet centered at δ 149.4 and can
Ch em ica l Sh ift Cor r ela tion . The GIAO-MP2-cal-
culated 19F NMR chemical shifts are in excellent agree-
ment with the experimental data (Figure 3a) and are
clearly superior to the GIAO-SCF (Figure 3b)- and IGLO
be compared to the IGLO-calculated value of δ 159.1.
+
c-C
3
H
2
F , 7. The monofluorocyclopropenium ion (7)
is the simplest substituted three-membered-ring H u¨ ckel
aromatic system. The ion was studied by NMR and
vibrational spectroscopy.18 The two calculated C-C force
constants obtained from vibrational spectroscopy of
1
9
II (Figure 3c)-calculated F NMR chemical shifts. In the
+
+
+
3 3 2 3 2
series CF , CH C F , and (CH ) C F, both the calculated
and experimental δ values indicate an increase in the
deshielding effect at the carbocationic center with de-
creasing fluorine substitution. Presumably, this is due
to an increase in fluorine back-donation into the carboca-
0 2
monofluorocyclopropenyl-d and -d cations correspond
to a weaker C-C bond opposite to the fluorine-substi-
tuted carbon and a stronger C-C bond adjacent to the
fluorine-substituted carbon. Our calculated structure 7
also shows that the longer (1.385 Å) C-C bond is opposite
to the fluorine-substituted carbon and the shorter (1.367
Å) C-C bond is adjacent to the fluorine-substituted
carbon. The interaction of the fluorine atom with the
cyclopropenyl ring is substantial as shown by the shorter
C-F bond (1.274 Å), which is even shorter than that of
ions 4-6. Both IGLO- and GIAO-calculated 19F NMR
chemical shifts of 6 agree well with the experimental data
1
9
tionic center. Both calculated and experimental F NMR
chemical shifts indicate that this effect is more pro-
nounced in the monofluorinated derivatives.
P r otolytic Clea va ge of Tr iflu or oa cetic Acid . We
3
have previously attempted to observe the trifluoromethyl
cation (1) by protolytic cleavage of trifluoroacetic acid
(
CF
only protonated trifluoroacetic acid was observed, and
attempts to dehydrate it to trifluoroacetyl cation (CF
3 3 5
COOH) or its esters with FSO H:SbF . However,
3
-
(
Table 1).
c-C HF
known experimentally. The calculated C-F bond length
1.272 Å) of 8 is very close to the C-F bond length of 7.
+
CO ) and subsequently via decarbonylation to cation 1
were unsuccessful.3 In fact, even at 60 °C in neat “Magic
Acid” no cleavage of protonated trifluoroacetic acid was
+, 8. Difluorocyclopropenium ion (8) is not
2
3
(
observed by 13C and F NMR spectroscopy.
19
3b
This is
The two shorter C-C bonds (1.382 and 1.373 Å) indicate
that the ion 8 also has substantial aromatic character.
The calculated chemical shifts are also very close to those
surprising, since protonated carboxylic acids in general
readily dehydrate to yield the corresponding acylium
cations.21 CF
4
was not observed, but because of its
of monofluorocyclopropenium ion (7) (Table 1).
volatility, small quantities of CF (bp ) -128 °C) are
4
+
c-C
3
F
3
, 9. Trifluorocyclopropenium ion (9) is ob-
difficult to observe in solution by 19F NMR spectroscopy.
We have now reinvestigated the ionization of trifluoro-
acetic acid with Magic Acid by employing more sensitive
served1 experimentally by treating perfluorocyclopro-
pene with an excess of SbF at 0 °C. 9 is characterized
by a single peak in the fluorine NMR spectrum at δ
9
5
4
gas IR spectroscopy to detect CF possibly formed during
the reaction. Indeed, FT-IR analysis of gas samples
taken after reacting trifluoroacetic acid with excess
FSO H:SbF (50 mol % SbF ) at room temperature for
3 5 5
(
15) Schleyer, P. v. R.; Koch, W.; Liu, B.; Fleischer, U. J . Chem. Soc.,
Chem. Commun. 1989, 1098.
16) Schleyer, P. v. R.; Carneiro, J . W. M.; Koch, W.; Forsyth, D. J .
Am. Chem. Soc. 1991, 113, 3990.
(
3
0 min in an autoclave showed a strong absorption band
(
17) Olah, G. A.; Liang, G.; Mo, Y. K. J . Org. Chem. 1974, 39, 2394.
(
18) Craig, N. R.; Lai, R. K.; Matus, L. G.; Miller, H.; Palfrey, S. L.
J . Am. Chem. Soc. 1980, 102, 38.
19) Sargeant, P. B.; Krespan, C. G. J . Am. Chem. Soc. 1969, 91,
15.
(20) Olah, G. A.; Bollinger, J . M. J . Am. Chem. Soc. 1967, 89, 2993.
(21) Olah, G. A.; White, A. M.; O’Brien, D. H. Chem. Rev. 1970, 70,
561.
(
4

Ab Initio/IGLO/GIAO-MP2 Studies of Fluorocarbocations
J . Org. Chem., Vol. 61, No. 26, 1996 9257
at 1277.3 cm- indicative of CF
1
. The assignment of the
4
Sch em e 1
peak to tetrafluoromethane was confirmed by adding
pure CF gas to the sample. The formation of CF
4
4
indicates protolytic cleavage of protonated trifluoroacetic
+
acid to CF
3
CO and subsequent decarbonylation to the
+
CF
SbF
3
cation which is then quenched by fluoride ion (from
or the acid system) to form CF .
4
-
6
Interestingly, no protolytic cleavage occurred when
trifluoroacetic acid was reacted with superacid systems
weaker than 1:1 M Magic Acid. For example, no forma-
tion of CF
containing only 1, 2, 5, and 10 mol %, respectively, of
SbF were used. There are two possible reasons for the
lack of CF formation in the latter acid systems: (a) The
4 3 5
was detected when mixtures of FSO H:SbF
5
4
protolytic cleavage of trifluoroacetic acid is dependent on
the acidity of the superacid employed, or (b) the fluoride
concentration in these acid systems is not sufficient for
+
the quenching of CF
3
4
cations to CF . We have also
attempted to react trifluoroacetic acid with a mixture of
FSO H and KF (ca. 1:1 M). This acid system generates
3
HF in situ as a fluoride ion source for the quenching of
+
CF
3
4 4
cations to CF . However, no CF formation was
detected under these conditions, indicating that the
protolytic cleavage of trifluoroacetic acid is mainly de-
pendent on the acidity of the superacid system. This
acidity dependence can be rationalized by further pro-
tolytic (i.e., superelectrophilic) activation of protonated
trifluoroacetic acid, involving a diprotonation equilibrium
with a reactive gitonic dication (mechanism I) rather than
direct cleavage of the monoprotonated trifluoroacetic acid
(mechanism II).
Protolytic activation of protonated trifluoroacetic acid
1
2 (mechanism I) leads to a highly electron deficient,
superelectrophilic, gitonic dication (13), which is sub-
stantially more reactive than its parent monocation 12.22
Its subsequent dehydration and decarbonylation leads to
+
the CF
3
cation (1) which is then readily quenched by
-
fluoride ion (from SbF
6
) to form CF
4
. Similar diproto-
nation equilibria have previously23 been suggested in the
ionization reactions of formic acid and acetic acid,
respectively, in excess superacid. On the other hand, the
direct cleavage of monoprotonated trifluoroacetic acid 12
+
protonation of water to H O (∆H ) -167.0 kcal/mol at
3
the MP2(fu)/6-31G* + ZPE level) in the superacid media
+
is considered. Finally, fluoride abstraction by CF (1)
3
-
from SbF6 leads to the formation of CF4.
(
mechanism II) would involve tautomerization to form
Tautomerization of 12 to form cation 15 (mechanism
II), on the other hand, is endothermic by 15.9 kcal/mol
at the MP2(fu)/6-31G* + ZPE level. Subsequent dehy-
cation 15, which subsequently would dehydrate and
decarbonylate to yield the CF
+
3
cation (1).
In order to investigate the two mechanistic possibili-
ties, we have carried out ab initio calculations. Dipro-
tonated trifluoroacetic acid 13 (mechanism I) was found
to be a stable minimum structure at the MP2/6-31G*
level and can be viewed as a donor-acceptor complex of
H
dration of cation 15 to form trifluoroacetyl cation (CF
3
-
+
CO , 16) was calculated to be endothermic by 26.8 kcal/
mol at the same level. Decarbonylation of 16 to form
+
CF
3
(1) is also endothermic by 19.6 kcal/mol; the latter
two reactions become exothermic, however, if the subse-
+
2
O and protonated trifluoroacetyl dication (CF
3
COH2
,
+
quent protonation of H
2
O and CO to H
3
O (∆H ) -167.0
1
4) with a long C-C bond of 1.599 Å (Figure 2).
+
kcal/mol) and COH (∆H ) -97.6 kcal/mol), respectively,
Protonation of 12 to form dication 13 was calculated to
be slightly endothermic by only 2.6 kcal/mol at the MP2-
is considered. As in mechanism I, fluoride abstraction
+
-
by CF
Based on the calculated data, it is suggested that the
protolytic cleavage of CF COOH in superacid media
3
(1) from SbF
6
4
leads to the formation of CF .
(
fu)/6-31G* + ZPE level. Attempts to find a stable
minimum for protiotrifluoroacetyl dication (14) failed
+
3
because of its spontaneous dissociation into CF
protonated carbon monoxide COH . Simultaneous de-
hydration and deprotonation of dication 13 into CF
COH , and H
kcal/mol at the MP2(fu)/6-31G* + ZPE level. This
reaction is even more exothermic if the subsequent
3
(1) and
+
involves the cleavage of protio(trifluoromethylcarboxo-
+
+
nium) dication (CF
3
C(OH)(OH
2
)
2
, 13) (mechanism I)
3
(1),
+
rather than the direct cleavage of monoprotonated tri-
fluoroacetic acid 12 (mechanism II). This is in accord
with the observed acidity dependence of the reaction. In
addition, theoretical calculations have shown that dipro-
tonation of trifluoroacetic acid is energetically feasible,
and dication 13 was found to be a stable minimum
structure at the MP2(fu)/6-31G* level.
2
O was calculated to be exothermic by 37.0
(
(
22) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767.
23) Hartz, N.; Rasul, G.; Olah, G. A. J . Am. Chem. Soc. 1993, 115,
1
277.

9
258 J . Org. Chem., Vol. 61, No. 26, 1996
Prakash et al.
Con clu sion s
on a Nicolet 800 FT-IR spectrometer using a gas IR cell
equipped with NaCl windows.
The structures of a series of fluorocarbocations were
calculated at the correlated MP2/6-31G* level. C and
F NMR chemical shifts of these structures were calcu-
lated using IGLO and GIAO-MP2 methods. The data
3
P r otolytic Clea va ge of CF COOH w ith Ma gic Acid .
Trifluoroacetic acid (1 mL) was placed into a bomb (stainless
steel) equipped with a magnetic stirrer and cooled to -78 °C.
3 5
After the addition of 4 mL of FSO H/SbF (1:1 molar ratio)
1
3
1
9
1
9
13
showed good correlation of calculated F and C NMR
chemical shifts with the experimental chemical shifts of
the fluorocarbocations. The correlation for GIAO-MP2-
the reactor was closed and allowed to warm up to room
temperature under continuous stirring. A gas sample was
taken from the reactor after about 2 h. FT-IR analysis showed
-
1
calculated 19F NMR chemical shifts with the experimen-
tal data is excellent. The protolytic cleavage of trifluo-
a strong absorption band at 1278 cm . The assignment of
this peak to CF was confirmed by adding pure CF gas to the
sample. The same experimental procedure was used for the
4
4
3 4
roacetic acid (CF COOH) in superacids giving CF was
also investigated, and the reaction mechanism is sug-
gested to involve the intermediacy of the reactive gitonic
attempted protolytic cleavage of CF
acid systems (see text).
3
COOH in various other
2
+
CF
3
C(OH)(OH
2
)
dication (13).
Ack n ow led gm en t . Support of our studies by the
Loker Hydrocarbon Research Institute and the National
Science Foundation is gratefully acknowledged. A.B.
wishes to thank the Konrad-Adenauer Foundation for
a scholarship.
Exp er im en ta l Section
COOH, KF, CF , and H SO are commercially available
CF
3
4
2
4
products (Aldrich) and were used as received. Antimony
pentafluoride (Allied-Chemical) and fluorosulfonic acid (3 M)
were double distilled prior to use. IR spectra were obtained
J O9611327