BF3·2CF3CH2OH (BF 3·2TFE), an efficient superacidic catalyst for some organic synthetic transformations

Article abstract of DOI:10.1021/jo0604181

BF₃ · 2CF₃CH₂OH complex was found to be a very effective superacidic catalyst comparable in acid strength to at least that of 100% anhydrous sulfuric acid for various acid-catalyzed organic transformations such as isomerizations, rearrangements, ionic hydrogenation of various ketones, and aromatics with triethylsilane and nitration of aromatics with metal nitrate. Studies of the pivalaldehyde-methyl isopropyl ketone rearrangement and the benzopinacol to phenanthrene transformation suggest that the complex has an acidity comparable to that of 100% anhydrous sulfuric acid. The structure and properties of the 1:2 boron trifluoride-trifluoroethanol complex have been further studied using NMR (¹H, ¹³C, ¹⁹F, ¹¹B) and DFT calculations at the B3LYP/6- 311++G**//B3LYP/6-31G* level.

Full text of DOI:10.1021/jo0604181

BF ‚2CF CH OH (BF ‚2TFE), an Efficient Superacidic Catalyst for
3
3
2
3
Some Organic Synthetic Transformations
,
†
†
†
‡
G. K. Surya Prakash,* Thomas Mathew, Eric R. Marinez, Pierre M. Esteves,
†
,†
Golam Rasul, and George A. Olah*
Donald P. and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry,
UniVersity of Southern California, 837 Bloom Walk, Los Angeles, California 90089-1661, and Instituto de
Qu ´ı mica, UniVersidade Federal do Rio de Janeiro, Cidade UniVersit a´ ria CT Bloco A,
2
1949-900 Rio de Janeiro, Brazil
gprakash@usc.edu
ReceiVed February 27, 2006
3 3 2
BF ‚2CF CH OH complex was found to be a very effective superacidic catalyst comparable in acid strength
to at least that of 100% anhydrous sulfuric acid for various acid-catalyzed organic transformations such
as isomerizations, rearrangements, ionic hydrogenation of various ketones, and aromatics with triethylsilane
and nitration of aromatics with metal nitrate. Studies of the pivalaldehyde-methyl isopropyl ketone
rearrangement and the benzopinacol to phenanthrene transformation suggest that the complex has an
acidity comparable to that of 100% anhydrous sulfuric acid. The structure and properties of the 1:2
1
13
19
11
boron trifluoride-trifluoroethanol complex have been further studied using NMR ( H, C, F, B) and
DFT calculations at the B3LYP/6-311++G**//B3LYP/6-31G* level.
2
-4
Introduction
to boric acid and hydrofluoric acid. Meerwein and collabora-
5
,6
tors found that BF3 combines with water to form both a
monohydrate and a dihydrate.
With organic compounds containing oxygen atoms as
n-donors (Lewis bases), boron trifluoride in general forms
coordination compounds of 1:1 or 1:2 compositions. With
ethers and esters, boron trifluoride forms 1:1 coordination
BF3 has received wide application as a Lewis acid catalyst
in a variety of acid-catalyzed reactions. Starting with Meerwein’s
pioneering work, its remarkable propensity to form coordination
compounds with a variety of inorganic and organic bases were
1
1
also explored. Unlike other boron trihalides, boron trifluoride
does not undergo hydrolysis with water under usual conditions
(2) Rochester, C. Acidity Functions, Organic Chemistry Series; Academic
Press: New York, 1970.
(3) Vinnik, M. I.; Manelis, G. B.; Chirkov, N. M. Russ. J. Inorg. Chem.
1957, 2, 306.
†
University of Southern California.
Universidade Federal do Rio de Janeiro.
‡
(
1) Topchiev, A. V.; Zavgorodnii, S. V.; Paushkin, Y. M. Boron
(4) Farcasiu, D.; Ghenciu, A. J. Catal. 1992, 134, 126.
(5) Meerwein, H. Ber. Dtsch. Chem. Ges. 1933, 66, 411.
(6) Meerwein, H.; Pannwitz, W. J. Prakt. Chem. 1934, 141, 123.
Trifluoride and its Compounds as Catalysts in Organic Chemistry:
Pergamon Press: New York, 1959.
10.1021/jo0604181 CCC: $33.50 © 2006 American Chemical Society
3
952
J. Org. Chem. 2006, 71, 3952-3958
Published on Web 04/21/2006

BF
3
3 2 3
‚2CF CH OH (BF ‚2TFE), an Efficient Superacidic Catalyst
complexes. With simple aliphatic ethers, the complexes are
stable liquids, which even can be distilled without decomposi-
tion. For example, the coordination complexes of BF3 with
dimethyl and diethyl ethers have boiling points of 126.6 and
SCHEME 1
125.7 °C, respectively, at atmospheric pressure. However, their
electrical conductivity is significantly lower than the conductiv-
ity of the coordination compounds of boron trifluoride with
compounds containing hydroxyl groups.
1
6-18
of alkenes or alcohols to carboxylic acids,
and in the Ritter
1
9
reaction of alkenes to formamides. Recently, we found that
N-halosuccinimides in boron trifluoride monohydrate can act
as efficient halogenating systems for deactivated aromatics.20
Boron trifluoride forms stable adducts of two types with
7
alcohols: BF3‚ROH and BF3‚2ROH. All of the adducts exhibit
strong acidity and show good electrical conductivity but are
weaker than boron trifluoride hydrates.1 Similar to boron
trifluoride monohydrate, complexes of the type BF3‚ROH are
less stable and cannot be distilled without dehydration or
decomposition. However, the complexes BF3‚2ROH are stable
and can be distilled under vacuum.
We found that the BF ‚2CF CH OH complex could be easily
3
3
2
prepared by passing BF3 into ice-cooled 2,2,2-trifluoroethanol.
Since the formation of the complex is highly exothermic,
continuous cooling is required. Preparation of the complex can
also be carried out by cooling the container in an acetone/dry
ice bath. However, precautions have to be taken to avoid
freezing (when freezing starts, the frozen complex was taken
out from the cooling bath and slowly warmed to ensure melting).
The introduction of BF3 was continued until no further weight
increase was observed. The complex has the stoichiometry of
BF3‚2CF3CH2OH and is a colorless, highly hygroscopic liquid
with a density of 1.62 g/mL (since it loses some BF3 above
room temperature, its atmospheric boiling point could not be
determined). Similar to other BF3-alcohol complexes, the ionic
Since fluorine is the most electronegative element, by
replacing hydrogen with fluorine the acidity of alcohols can be
increased. Due to the strong electron-withdrawing inductive
effect of a trifluoromethyl group, the pKa of 2,2,2-trifluoroet-
8a
hanol (CF3CH2OH) is 12.4, thus considerably more acidic than
8
b,c
both methanol (pKa ) 15.2) and ethanol (pKa ) 15.9).
Therefore, we anticipated that the coordination complex of BF3
with trifluoroethanol would exhibit both increased acidity and
higher catalytic activity than the BF3 complexes of water,
alcohols, and others. Herein, we report the preparation and
properties of 1:2 complex of BF3 and 2,2,2-trifluoroethanol and
its successful use as a strong acid catalyst in a wide variety of
organic transformations.
+
-
form is suggested to be (CF3CH2OH2) (BF3OCH2CF3) though
the structure is not as yet as clearly determined as other BF3
complexes. It shows high acidity and catalytic activity indica-
tive of a new strong acid system. The OH proton in the proton
NMR of the complex is shifted to downfield by 5 ppm showing
effective BF3 complexation making it more acidic. We report
the results of a variety of organic transformations using this
complex as an effective and a convenient catalyst.
BF3‚2CF3CH2OH-Catalyzed Synthetic Transformations.
a) Isomerization of Pivalaldehyde to Methyl Isopropyl
Ketone. The BF3‚2CF3CH2OH complex has been investigated
as an acid catalyst in a variety of acid-catalyzed organic
transformations. Acid-catalyzed isomerization of epoxides to
9
Results and Discussion
Comparison of Boron Trifluoride-Trifluoroethanol Com-
plex with Boron Trifluoride Hydrates. Physical as well as
chemical properties of boron trifluoride monohydrate and
dihydrate have been extensively studied. Boron trifluoride
dihydrate, BF3‚2H2O, is a colorless liquid that melts at 5.9-
(
6
1
.0 °C with a boiling point of 58.5-60 °C and has a density of
2
1,22
aldehydes and ketones has been well studied.
When the
1
,9
.65 g/mL. It does not etch glass or shows indication of
isomerization of styrene oxide to phenyl acetaldehyde was
attempted with the BF3‚2CF3CH2OH complex, the reaction was
found to be highly exothermic and generated a mixture of
products due to aldol condensation. However, during our studies
of various methods for the synthesis of methyl isopropyl ketone
formation of free HF. However, BF3‚H2O is a less stable
colorless liquid that melts at 6.2 °C with a density of 1.80 g/mL.
The monohydrate fumes strongly in air and loses boron
trifluoride readily when warmed above room temperature.
Neither complexes are ionized as solids, but they are highly
1
(Scheme 1), we found that the pivalaldehyde-methyl iso-
propyl ketone rearrangement proceeded quantitatively with
9
ionized in the liquid state. The dihydrate exists in the liquid
+
state predominantly in the hydronium ionic form, [H3O] , and
has a Hammett acidity of Ho ) -6.85 which is similar to that
of 100% nitric acid.2,3 The monohydrate has a Hammett acidity
(12) Takematsu, A.; Sugito, K.; Nakane, R. Bull. Chem. Soc. Jpn. 1978,
5
1, 2082.
13) (a) Yoneda, N.; Hasegawa, E.; Yoshida, H.; Aomura, K.; Ohtsuka,
H. Mem. Tac. Eng., Hokkaido UniV. 1973, 13, 227; (b) Chem. Abstr. 1973,
9, 146068f.
14) (a) Haruko, T.; Okumura, Y.; Imai, C. Jpn. Patent 6357537, 1988;
b) Chem. Abstr. 1989, 110, 195118q.
15) (a) Kolesnikov, I. M.; Miragaleev, I. G.; Kruglyak, A. A. Zh. Prikl.
Khim. 1971, 44, 625; (b) Chem. Abstr. 1971, 74, 125002m.
16) Roland, J. R.; Wilson, J. D. C.; Hanford, W. E. J. Am. Chem. Soc.
of Ho ) -11.4 close to that of 100% anhydrous sulfuric acid
(
(
Ho ≈ -12.0). Boron trifluoride monohydrate was reported to
7
be an efficient acid catalyst in a variety of organic transforma-
tions. Some of the well-known examples are oligomerization
(
(
10
of 1-alkenes for the manufacture of lubricating oils, alkylation
(
of aromatics with alkenes,1
1-15
the Koch-Haaf carbonylation
(
1
950, 72, 2122.
(
(
7) Bowlus, H.; Nieuwland, J. A. J. Am. Chem. Soc. 1931, 53, 3835.
8) (a) Ballinger P.; Long, F. A. J. Am. Chem. Soc. 1959, 81, 1050. (b)
(17) Koch, H. Brennstoff-Chem. 1955, 36, 321.
(18) Bahrman, H.; New Synthesis with Carbon Monoxide; Falbe, J., Ed.;
Ballinger P.; Long, F. A. J. Am. Chem. Soc. 1960, 82, 795. (c) Olmstead,
W. N.; Margolin, Z.; Bordwell, G. F. J. Org. Chem. 1979, 45, 3295.
Springer: Berlin, 1980; p 372.
(19) Sasaki, I.; Nikizaki, S. U.S. Patent 3574760, 1971.
(20) Prakash, G. K. S.; Mathew, T.; Hoole, D.; Wang, Q.; Esteves, P.
M.; Rasul, G.; Olah, G. A. J. Am. Chem. Soc. 2004, 123, 11556.
(21) Parker, R. E.; Isaacs, N. S. Chem. ReV. 1959, 59, 737.
(22) Prakash, G. K. S.; Mathew, T.; Krishnaraj, S.; Marinez, E. R.; Olah,
G. A. Appl. Catal. A: Gen. 1999, 181, 283.
(
9) Greenwood, N. N.; Martin, R. L. J. Chem. Soc. 1951, 1915.
(10) Madgavakar, A. M.; Swift, H. E. Ind. Eng. Chem. Prod. Res. DeV.
1
983, 22, 675.
(11) Oyama, T.; Hamano, T.; Nagumo, K.; Nakane, R. Bull. Chem. Soc.
Jpn. 1978, 51, 1441.
J. Org. Chem, Vol. 71, No. 10, 2006 3953

Prakash et al.
SCHEME 2
SCHEME 4
BF3‚2CF3CH2OH is partially soluble in dichloromethane but
remains biphasic in benzene. A possible explanation for the
formation of benzopinacolone 4 in dichloromethane is that the
catalyst is highly diluted due to its partial dissolution, preventing
the formation of dicationic intermediate 6 in dichloromethane
and thus the rearrangement to benzopinacolone is preferred.
Since the catalyst is immiscible with benzene, benzopinacol is
highly ionized in the strongly acidic phase resulting in the
formation of a highly energetic dicationic intermediate 6 which
produces 9,10-diphenylphenanthrene. These results further
indicate superacidic nature of the catalyst. On the other hand,
when the Fries rearrangement of phenyl acetate was attempted
with BF3‚2CF3CH2OH the major product was found to be
phenol (ester hydrolysis product) with hydroxyacetophenone
only as a minor product (<5%). The system is thus not acidic
enough to effect the Fries rearrangement at room temperature.
SCHEME 3. Mechanism for Phenanthrene Formation
(c) Friedel-Crafts Acylation. Friedel-Crafts acylation
reactions were also explored using the BF3‚2CF3CH2OH
catalyst. Treatment of 2-phenylmethylbenzoic acid (R-phenyl-
o-toluic acid) gave anthrone (7) in quantitative yield through
intramolecular Friedel-Crafts acylation (Scheme 4). However,
BF3‚2CF3CH2OH complex. Since the quantitative rearrangement
of pivalaldehyde 2 to methyl isopropyl ketone 1 requires high
acidity (optimized at Ho ) -11.3),2
3,24
this indicates that the
2
-(2-phenylethyl)benzoic acid did not give the expected product,
acidity of the complex is comparably high.
dibenzocycloheptadienone (10,11-dihydro-5H-dibenzo[a,d]cy-
clohepten-5-one);, instead, the corresponding ester of trifluo-
roethanol, trifluoroethyl-2-(2-phenylethyl)benzoate, was ob-
tained. This shows that in the latter the rate of Friedel-Crafts
acylation is very slow compared to the esterification by
trifluoroethanol. Formation of anthrone (7) from 2-phenylm-
ethylbenzoic acid indicates that intramolecular acylation is
kinetically more favorable for the formation of the six-membered
ring than esterification.
(b) Pinacol-Pinacolone Rearrangement. Pinacol rearrange-
ment was also studied in the presence of BF3‚2CF3CH2OH. It
was found that the products were highly dependent upon the
solvent used in the reaction. In dichloromethane, benzopinacol
(3) afforded benzopinacolone (4) in 95% yield. However, in
benzene, the major product was 9,10-diphenylphenanthrene (5,
6
2
5%) along with 2% benzopinacolone (4) as shown in Scheme
.
Recently Olah, Klumpp, et al. have described the preparation
Attempted intermolecular acylation reactions did not give the
desired acylated products. During attempted acetylation of
benzene with acetic anhydride or acetyl chloride at 0 °C, no
acetophenone was observed. Similarly, no methylbenzophenones
were formed during attempted benzoylation of toluene with
benzoyl chloride. Instead, the product obtained was 2,2,2-
trifluoroethylbenzoate in quantitative yield. Based on these
observations, it is possible that during the Fries rearrangement
of phenyl acetate the intermediate acetyl cation esterified
trifluoroethanol to 2,2,2-trifluoroethyl acetate instead of acety-
lating phenol.
of substituted phenanthrenes from aryl pinacols catalyzed by
25
trifluoromethanesulfonic acid in excellent yields. A mechanism
was suggested for the formation of phenanthrenes 5 involving
the formation of a protosolvated dicationic intermediate 6 from
pinacol 3 followed by electrophilic cyclization (Scheme 3). It
is known that with weaker sulfuric acid, the rearrangement of
benzopinacol 3 produces quantitatively only benzopinacolone
4
, which suggests that superacidity is required for phenanthrene
formation. Involvement of possible superelectrophilic activation
of electrophiles by protosolvation in superacid-catalyzed reac-
tions was suggested.26 This could explain the mechanistic
pathways for the observed formation of varied products in a
wide variety of superacid catalyzed reactions recently stud-
(d) Ionic Hydrogenation of Ketones and Polycyclic Aro-
matics. Larsen and Chang have described the deuteration of
activated aromatics and polycyclic aromatics via a deuteration-
27,28
29
ied.
deprotonation sequence using BF ‚D O. They also found that
3
2
BF3‚H2O is an efficient acid catalyst in the ionic hydrogenation
of polycyclic atromatics using triethylsilane as the hydride
source. Studies by Eckert-Maksic et al. showed that BF3‚H2O/
Et3SiH system can be used for ionic hydrogenation of aromatic
(
23) Olah, G. A.; Mathew, T.; Marinez, E. R.; Esteves, P. M.; Etzkorn,
M.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 2001, 123, 11556.
24) Olah, G. A.; Prakash, G. K. S.; Mathew, T.; Marinez, E. R. Angew.
Chem., Int. Ed. 2000, 2647.
25) Olah, G. A.; Klumpp, D. A.; Neyer, G.; Wang, Q. Synthesis 1996,
21.
30
(
(
3
(28) (a) Koltunov, K. Y.; Repinskaya, I. B. Russ. J. Org. Chem. 2002,
38, 437. (b) Koltunov, K. Y.; Prakash, G. K. S.; Rasul, G.; Olah, G. A.
Tetrahedron 2002, 58, 5423. (c) Koltunov, K. Y.; Prakash, G. K. S.; Rasul,
G.; Olah, G. A. J. Org. Chem. 2002, 67, 4330.
(29) Larsen, J. W.; Chang, L. W. J. Org. Chem. 1978, 43, 3602.
(30) Larsen, J. W.; Chang, L. W. J. Org. Chem. 1979, 44, 1168.
(
(
26) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767.
27) (a) Olah, G. A.; Rasul, G.; York, C.; Prakash, G. K. S. J. Am. Chem.
Soc. 1995, 117, 11211. (b) Klumpp, D. A.; Beak, D. N.; Prakash, G. K. S.;
Olah, G. A. J. Org. Chem. 1997, 62, 6666. (c) Hwang, J. P.; Prakash, G.
K. S.; Olah, G. A. Tetrahedron 2000, 56, 7199.
3954 J. Org. Chem., Vol. 71, No. 10, 2006

BF
3
3 2 3
‚2CF CH OH (BF ‚2TFE), an Efficient Superacidic Catalyst
TABLE 1. Reduction of Ketones with Triethylsilane/BF3‚2TFE
dibenz[a,h]anthracene, respectively. Earlier studies by Harvey
32
et al. showed that reduction of anthracene, naphthacene, benz-
a]anthracene and dibenz[a,h]anthracene with lithium in liquid
[
ammonia under usual conditions proceeded exclusively in the
meso region which was in agreement with Streitwieser’s
prediction based on molecular orbital calculations of the
3
3
positions of highest electron density (Table 2). Phenanthrene
did not undergo reduction under these conditions.
(e) Nitration of Aromatics. A variety of nitrating agents are
known for the nitration of aromatics. Nitric acid or its derivatives
and metal nitrates with monodendate and bidendate nitrate
ligands have been extensively studied as nitrating agents.34
Though there are several reports on the use of inorganic nitrates
in various acids such as trifluoroacetic acid, polyphosophoric
acid, and sulfuric acid, the yields of the nitrated products were
found to be low with deactivated aromatic systems.35 Olah and
co-workers reported a homogeneous nitration system using silver
3
6
nitrate/boron trifluoride in acetonitrile solution. Relatively
electron-rich aromatics are nitrated by this system. Later, it has
been found that nitration of aromatics with potassium nitrate
or nitric acid catalyzed by boron trifluoride monohydrate
proceeded with good to excellent yields even for some of the
3
7
deactivated aromatic systems as well. Since the boron trif-
luoride-trifluoroethanol complex exhibits the properties of a
strong acid as evident from the reactions discussed, we have
carried out the nitration of aromatics with potassium nitrate
catalyzed by BF3‚2TFE. In all cases, the catalyst was found to
be very efficient and the reaction proceeded in good to excellent
yields (Table 3).
organosulfur compounds and disubstituted naphthalenes.31
Reduction of aldehydes and ketones, a crucial step encountered
frequently in organic synthesis, has also been carried out by
triethylsilane. Aryl ketones and aldehydes with electron-donating
ring substituents give the completely deoxygenated products in
high yields. However, with aliphatic aldehydes and ketones, the
reduction generally stops after the transfer of one equivalent of
hydride and a variety of products, including alcohols, esters,
silyl ethers, and olefins were obtained depending upon the
substrate and the reaction conditions. Due to the high catalytic
activity of BF3‚2CF3CH2OH, we studied the ionic hydrogenation
with triethylsilane for the reduction of ketones with this catalyst.
The reductions proceeded well giving the reduced products in
good yields (Table 1). In the case of acenaphthenone (entry
vii), not only the carbonyl function but also the aromatic ring
closer to the carbonyl function underwent hydrogenation.
The BF3‚2TFE catalyst is not only effective for the reduction
of aliphatic and aromatic ketones but also for some studied
arenes. The yield of the products is significantly higher in
comparison with the yield reported for reactions carried out in
the presence of BF3‚H2O. Reduction of 9,10-dimethylanthracene
gave both cis- and trans-9,10-dimethyl-9,10-dihydroanthracenes
DFT Calculations. Density functional theory (DFT) calcula-
tions were performed on the BF3-TFE system in order to get
a better understanding of its structure and properties. The
structure of a cluster formed by three units of the complex [CF3-
CH2OH]2‚BF3 was considered in these calculations in order to
get an idea of the main interactions that occur within the liquid.
A geometry optimization procedure for this system was
performed at the B3LYP/6-31G* level. The vibrational analysis
showed no imaginary frequencies. NMR chemical shifts cal-
culations were performed at this geometry at the GIAO-B3LYP/
6
-311++G** level. Energy differences correspond to enthalpies
at 298 K and 1 atm, computed at the B3LYP/6-311++G**//
B3LYP/6-31G* level. All calculations were performed with the
Gaussian 98 package.
3
8
(59:41) in 83% yield. Both isomers were separated by fractional
(32) Harvey, R. G.; Arzadon, L.; Grant, J.; Urberg, K. J. Am. Chem.
Soc. 1969, 91, 4535 and references cited therein.
crystallization. The less soluble cis isomer precipitates out first
from the solution and was recrystallized from a mixture of
hexane and CH2Cl2 (9:1). The trans isomer could be separated
(33) Streitwieser, A., Jr. Molecular Orbital Theory for Organic Chemists;
John Wiley & Sons: New York, 1961; p 425.
(34) (a) Olah, G. A.; Malhotra, R.; Narang, S. C. Nitration, Methods
and Mechanism; VCH Publishers: New York, 1989. (b) Topchiev, A. V.
Nitration of Hydrocarbons and Other Organic Compounds; Pergamon
Press: New York, 1959.
(with some amount of cis isomer) from the mother liquor by
evaporation of the solvent.
Reduction of benz[a]anthracene and dibenz[a,h]anthracene
gave mainly 7,12-dihydrobenz[a]anthracene and 7,14-dihydro-
(35) (a) Crivello, J. V. J. Org. Chem. 1981, 46, 3056. (b) Sastry, S.;
Kudav, N. A. Ind. J. Chem. 1979, 18b, 198. (c) Majumdar, M. P.; Kudav,
N. A. Ind. J. Chem. 1976, 14b, 1012.
(
36) Olah, G. A.; Fung, A. P.; Narang, S. C.; Olah, J. A. J. Org. Chem.
(31) (a) Eckert-Maksic, M.; Margetic, D. Energy Fuels 1991, 5, 327.
1981, 46, 3533.
(b) Eckert-Maksic, M.; Margetic, D. Energy Fuels 1993, 7, 315.
(37) Olah, G. A.; Wang, Q.; Li, Xi.; Bucsi, I. Synthesis 1992, 1085.
J. Org. Chem, Vol. 71, No. 10, 2006 3955

Prakash et al.
TABLE 2. Reduction of Arenes with Triethylsilane/BF3‚2TFE
TABLE 3. BF3‚2TFE-Catalyzed Nitration of Aromatics with
KNO3
DFT calculations indicate the complex formation involving
bonding of BF3 to the oxygen atom of CF3CH2OH and the
prevalence of strong hydrogen bond between this complex with
a second trifluorethanol (TFE) molecule. In a limiting case this
could lead to complete transfer of the proton to trifluoroethanol
in accordance with the equilibrium
TABLE 4. Experimental and Calculated (at B3LYP/6-311++G**//
B3LYP/6-31G* level) Gas-Phase Proton Affinities for Selected
Acids39
gas-phase proton affinity (kcal/mol)
The acidic species can be considered to be the protonated
acid
exptl
calcd
+
TFE species (CF3CH2OH2 ) or, more probably, the solvated
H2SO4
FSO3H
167.2
167.5
153.4
164.6
111.9
134.4
137.1
164.7
179.4
189.1
+
CF3CH2OH2 . Experimental gas-phase proton affinity for its
conjugated base, TFE, is 167.4 kcal/mol (obtained from the
CF3SO3H
167.2
116
133.1
139.6
167.4
NIST database).39 The DFT calculations indicate that the proton
HF
HCl
HBr
affinity for TFE is 164.7 kcal/mol, in agreement with the
experimental value. This value is an indication of the acidity
CF3CH2OH (TFE)
(CF3CH2OH)2‚BF3
(CF3CH2OH)2‚BF3]3
+
of the [CF3CH2OH2] species in the gas phase. The higher the
[
proton affinity, the lower the ability of the protonated species
(
38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(conjugate acid) to transfer the proton for another basic species.
For comparison, we have tabulated the proton affinities (PA)
of TFE and of other acids in Table 4.
Comparing these results, one can see that the acidity of the
protonated TFE in the gas phase is comparable to that of triflic
acid or sulfuric acid. Since acidity also depends on solvation
of the species, we compared some solvation effects. Thermo-
dynamic properties for solvation of the protonated TFE by a
second molecule was measured in the gas phase (Scheme 5).
Experimental results indicate that the first solvating molecule
(39) Available on the Internet: webbook.nist.gov/chemistry.
3956 J. Org. Chem., Vol. 71, No. 10, 2006

BF
3
3 2 3
‚2CF CH OH (BF ‚2TFE), an Efficient Superacidic Catalyst
FIGURE 1. Geometries and NMR chemical shifts obtained for complexes BF
TFE) , obtained after energy minimization procedures at the B3LYP/6-31G* level. Indicated chemical shifts refer to the GIAO-B3LYP/6-311++G**/
B3LYP/6-31G* level (averaged values, when applicable). References for NMR chemical shifts calculations are BF O (boron), SiMe (carbon
3
‚TFE, BF
3
‚2TFE and its dimer, (BF
3
‚2TFE)
2 3
, and trimer, (BF ‚
2
3
3
‚Et
2
4
3
and hydrogen), and CFCl (fluorine).
SCHEME 5
give a better understanding of solvation effects, resulted in the
structures shown in Figure 1. The structure that shows NMR
chemical shifts in best agreement with the experimental values
is the trimer. It is noteworthy that there is O- -H- -O and F- -
H- -O hydrogen bonding involved in the structure. The hydrogen-
fluorine bonding in the complex is especially interesting,
suggesting that compounds that are more basic than coordinated
BF3 would potentially be protonated and undergo typical acid-
catalyzed reactions as the ones presented in this paper. The
calculated proton affinities for the monomer and trimer form
are, respectively, 179.4 and 189.1 kcal/mol. These values, higher
than the isolated TFE molecule, are compatible with the higher
solvation experienced by the excess proton into these more
complex systems. Calculated NMR chemical shifts (shown in
stabilizes the system by 31.8 kcal/mol. Results of our DFT
calculations for the same reaction give the ∆H value, -26.5
kcal/mol.
As expected, the deprotonation enthalpy for this solvated
species is higher (∆H ) 187.6 kcal/mol), if compared with the
isolated protonated molecule (∆H ) 164.7 kcal/mol).
The calculations on the 1:1 (BF3‚TFE) and 1:2 (BF3‚2TFE)
complexes, considering the latter as a monomer (BF3‚2TFE), a
dimer [(BF3‚2TFE)2], and a trimer [(BF3‚2TFE)3], aiming to
J. Org. Chem, Vol. 71, No. 10, 2006 3957

Prakash et al.
Figure 1) agree reasonably with the experimental chemical shifts
Preparation of BF
trifluoroethanol (100 g, 1.0 mol) was taken in a dry Nalgene bottle
(500 mL), closed, and weighed. The cap of the nalgene bottle was
3
‚2CF
3
CH
2
OH. Freshly distilled 2,2,2-
obtained for the liquid systems. Gas-phase proton affinity data
for the CF3CH2OH2+ (the conjugated acid of the TFE) and the
replaced with a cap containing an inlet for the introduction of BF
and a safety outlet for the release of excess pressure. The bottle
was cooled to 0 °C using an ice bath, and BF was slowly bubbled
into 2,2,2-trifluoroethanol with stirring and continuous cooling until
the solution was saturated with BF (no further weight increase
was observed) and the excess BF was released through the safety
outlet. The bottle was closed and weighed again. The solution was
found to be a 1:2 complex of BF and 2,2,2-trifluoroethanol with
35 g (0.52 mol) of BF incorporated. The complex was stored in
the refrigerator for further use. H NMR (300 MHz, CDCl
used as the standard; NMR tube containing the complex was fitted
with a capillary containing CDCl ): δ 4.28 (q, 2H, J ) 8.12
3
calculations on the structure of the trimer suggest the acidity
of BF3‚2CF3CH2OH is in the range between H2SO4 and triflic
acid, thus being a moderate superacid. This is in agreement with
its behavior in acid-catalyzed reactions presented here, which
indicate that its acidity is about or higher than acid systems
with Ho -11.3.
3
3
3
3
Conclusion
3
1
3
was
In conclusion, we have found that a 1:2 complex, BF3‚2CF3-
CH2OH (BF3‚2TFE) formed from BF3 and CF3CH2OH can act
as an efficient superacidic catalyst for many organic synthetic
transformations. Gas-phase proton affinity data for the CF3CH2-
3
H-F(â)
Hz); 8.84 (s, 1H). 13C NMR (75 MH
(q, CH , JC-C-F ) 38.15 Hz), 122.37 (q, CF
, CDCl
3
as standard): δ 61.19
, JC-F ) 276.50 Hz).
Z
2
3
1
9
+
F NMR (282.2 MHz, CFCl as standard): δ -152.76 (BF ),
OH2 (the conjugated acid of the TFE) and the calculations on
3
3
-
78.05 (t, CF
3
, JF-H(â) ) 9.16 Hz. 11B NMR (96 MHz, BF
3
-diethyl
the structure of the trimer (BF3‚2TFE)3 suggest the acidity of
BF3‚2TFE is in the range between H2SO4 and triflic acid, thus
being a moderate superacid. This is in agreement with its
behavior in acid-catalyzed reactions such as pivalaldehyde-
methyl isopropyl ketone rearrangement and the benzopinacol
to phenanthrene transformation now studied. We expect that
BF3‚2TFE can serve as an efficient nonoxidizing strong Brønsted
acid catalyst for many electrophilic reactions.
ether as standard): δ -0.495. Density: 1.62 g/mL.
Acknowledgment. This paper is dedicated to the loving
memory of Professor C. P. Joshua, University of Kerala, India.
Support of our work by the Loker Hydrocarbon Research
Institute is gratefully acknowledged. P.M.E. acknowledges
CNPq and FAPERJ (Brazil) for financial support.
Supporting Information Available: 1H NMR, 13C NMR, 19
F
NMR, and 11B NMR of BF
‚2TFE complex, H NMR of products
1
3
1
Experimental Section
entries i-vii in Table 1, H NMR of products entries i, ii, and iv-
vi in Table 2, 13C spectrum of entry iii in Table 2, H NMR of
products entries i-vii in Table 3, and Cartesian coordinates for
1
3 3
Caution! BF and BF complexes are highly irritating and toxic
and can cause severe damages to eyes, skin, and lungs. Precautions
have to be taken (chemical safety goggles and rubber gloves) while
handling them. Gas masks approved for acid gases or those with
an independent supply of oxygen or air should be readily available
in convenient locations in the event of an emergency.
structures of TFE‚BF
3
, (TFE)
2
‚BF
3
, [(TFE)
2 3 2 2
‚BF ] , and [(TFE) ‚
BF optimized at the B3LYP/6-31G* level. This material is
3 3
]
available free of charge via the Internet at http://pubs.acs.org.
JO0604181
3958 J. Org. Chem., Vol. 71, No. 10, 2006