Synthesis of N-heteroaryl(trifluoromethyl)hydroxyalkanoic acid esters by highly efficient solid acid-catalyzed hydroxyalkylation of indoles and pyrroles with activated trifluoromethyl ketones

Article abstract of DOI:10.1002/adsc.200505117

The synthesis of N-heteroaryl(trifluoromethyl)hydroxyalkanoic acid esters by solid acid-catalyzed Friedel-Crafts hydroxyalkylation of indoles and pyrroles with ethyl 3,3,3-trifluoropyruvate and ethyl 4,4,4-trifluoroacetoacetate is described. The inexpensi

Full text of DOI:10.1002/adsc.200505117

FULL PAPERS
DOI: 10.1002/adsc.200505117
Synthesis of N-Heteroaryl(trifluoromethyl)hydroxyalkanoic Acid
Esters by Highly Efficient Solid Acid-Catalyzed
Hydroxyalkylation of Indoles and Pyrroles with Activated
Trifluoromethyl Ketones
´
Mohammed Abid, Bela Tçrçk*
Department of Chemistry, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
Fax: (þ1)-906-487-2061
Current address: Department of Chemistry, University of Massachusetts Boston, 100 Morrisey Blvd., Boston, MA
02125-3393, USA
E-mail: bela.torok@umb.edu
Received: March 18, 2005; Accepted: May 9, 2005; Published online: September 26, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The synthesis of N-heteroaryl(trifluoro- yield (up to 98%) and 100% selectivity under mild ex-
methyl)hydroxyalkanoic acid esters by solid acid-cat- perimental conditions, during very short reaction
alyzed Friedel–Crafts hydroxyalkylation of indoles times. Beyond these, the ease of product isolation,
and pyrroles with ethyl 3,3,3-trifluoropyruvate and catalyst stability and handling make this process an
ethyl 4,4,4-trifluoroacetoacetate is described. The in- attractive, environmentally benign alternative for
expensive and readily available K-10 montmorillonite the synthesis of the target compounds.
is found to be an efficient catalyst for the synthesis of
a wide variety of trifluoromethylated indol-3-yl- and
pyrrol-2-yl-hydroxypropionic and -butanoic acid es- Keywords: ethyl trifluoroacetoacetate; ethyl trifluoro-
ters. Using a series of substituted indoles and pyrroles pyruvate; Friedel–Crafts hydroxyalkylation; heteroge-
the corresponding products were isolated in excellent neous catalysis; K-10; solid acid catalysis
Introduction
methods include but are not limited to the superacid cat-
alytic hydroxyalkylations with ethyl 3,3,3-trifluoropyru-
Since the first publication on biologically active organo- vate,[3c] application of Grignard reagents[5b, c] or organo-
fluorine compounds,[1a] the synthesis of these materials cuprates.[5d] Despite the several available processes[6] the
has received significant attention in the pharmaceutical development of new environmentally benign methods
and materials sciences.[1] Among these compounds the for the synthesis of trifluoromethylated compounds is
trifluoromethyl group-containing molecules are espe- still in great demand.
cially important due to their unique physical and biolog-
Friedel–Crafts chemistry is one of the most funda-
ical properties.[2] Some of the most well-known drugs are mental C C bond forming reactions in organic chemis-
Prozacꢀ (anti-depressant), Diflucanꢀ (anti-fungal try.[7] Two major applications of the aromatic electro-
agent), Casodexꢀ (anti-cancer agent) and Desflurane philic substitution reaction such as acylation and alkyla-
(inhalation anesthetic).[2] Mosherꢁs acid and its deriva- tion, provide a wide range of highly valuable products.[8]
tives are another important class of CF3-containing com- The conventional methods for carrying out the Friedel–
pounds, which are widely used as chiral NMR resolution Crafts reaction involve the use of traditional Lewis or
À
agents.[3]
Brønsted acid catalysts.[1,2] Although these acids are ex-
Indolyl-alkanecarboxylic acids including trifluorome- cellent catalysts, many of them cannot meet the recent
thylated derivatives have also attracted considerable at- environmental and safety standards for fine chemical
tention in recent years due to their excellent biological production. As a result, the development of environ-
activity.[4] Their synthesis can be achieved by enantiose- mentally benign catalysts attracted significant atten-
lective Friedel–Crafts reactions of trifluoromethyl pyru- tion.[9] However, the search for active and selective het-
vate with aromatic and heteroaromatic compounds cat- erogeneous acid catalytic systems is still highly desira-
alyzed by chiral copper(II) complexes.[5a] Alternative ble.
Adv. Synth. Catal. 2005, 347, 1797 – 1803
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1797

´
Mohammed Abid, Bela Tçrçk
FULL PAPERS
Continuing our efforts in developing environmentally tively. Some weak solid acids (e.g., Amberlyst, Dowex)
friendly, heterogeneous catalytic synthetic methods, could not activate 2b, while the superacidic Nafion-
herein, we report the synthesis of trifluoromethylated H[9c] showed slow reaction rates, probably due to its lim-
hydroxyl(N-heteroaryl)alkanoic acid esters by solid ited surface area (~ 10 m2 gÀ1). Finally, K-10 montmor-
acid-catalyzed hydroxyalkylations. This method is based illonite was found to be the catalyst of choice for these
on the application of K-10 montmorillonite and trifluoro- transformations. It is a well-known strong solid acid
methylated a- and considerably less reactive b-keto es- for a variety of acid-catalyzed organic transforma-
ters. Beyond the new procedure, several new products tions[9b] and also used for bifunctional catalysis.[10] K-10
have been synthesized and this process provided an op- is prepared from natural montmorillonite by high tem-
portunity to explore their further chemical and biomed- perature mineral acid treatment. According to solid
ical potential.
state 29Si NMR studies its main constituent is a quartz-
like material, in addition to kaolinite and montmorillon-
ite.[11] The acid strength of K-10 highly surpasses the
acidity of the other solids (SiO2, Al2O3, TiO2 etc.), how-
ever, it is a significantly weaker acid than the superacidic
Results and Discussion
Due to the extended interest in trifluoromethylated in- Nafion-H.[9,12] Its Hammett acidity constant is H0 ¼ À8,
dolyl-carboxylic acid derivatives[4,5] we intended to de- this value is as high as the acidity of concentrated nitric
velop a new, economic and easily adaptable solid acid- acid.[9b,12] Besides, its BET surface exceeds 250 m2 gÀ1
,
catalyzed process for the synthesis of these compounds. which is significantly higher than that of other solid acids
The general scheme of the reaction is summarized in (~10–20 m2 gÀ1) surface area. Based on the preliminary
Scheme 1.
investigations further reactions have been carried out
As a first step, several solid acid catalysts have been with K-10 montmorillonite as a catalyst.
tested in the reaction of indole with 2a and 2b, respec-
As a next step, we have selected the hydroxyalkylation
of indole with reactants 2a and 2b, respectively, as a
probe reaction to determine optimum experimental
conditions. The results are summarized in Table 1.
As shown, 2a reacts with indole with a very slow rate,
no product formation has been observed with 2b even
after extended reaction time (entries 1, 5). We observed
that the reaction time can drastically be reduced by us-
ing K-10 montmorillonite as catalyst at room tempera-
ture in the case of indole 2a reaction (entry 2). However,
K-10 was not able to activate 2b under identical condi-
tions. Using elevated temperatures (60–708C) and mi-
crowave irradiation, respectively, we observed excellent
reaction rates. Using microwave irradiation, however,
the isolated yields are only moderate to good, probably
due to a limited reagent loss from the open reaction ves-
sel. Conventional heating combined with a closed pres-
sure tube reactor excluded reagent loss and gave the
maximum yields (Table 1entries 4 and 8). As such,
this system was applied in further reactions.
Scheme 1. K-10 montmorillonite-catalyzed hydroxyalkyla-
tion of indoles (1) and pyrroles (3) with ethyl 3,3,3-trifluoro-
pyruvate (2a) and ethyl 4,4,4-trifluoroacetoacetate (2b).
Table 1. Hydroxyalkylation of indole with ethyl 3,3,3-trifluoropyruvate (2a) and ethyl 4,4,4-trifluoroacetoacetate (2b) under
various experimental conditions.
Entry
Substrate
Reactant
Catalyst
Solvent
Temp. [ 8C]
Time [h]
Yield [%]
1Indole
2a
2a
2a
2a
2b
2b
2b
2b
–
Ether
Ether
–
Toluene
Ether
Ether
–
25
24
1
89
85
80
97
0
0
75
98
2
3
4
5
6
7
8
Indole
Indole
Indole
Indole
Indole
Indole
Indole
K-10
K-10
K-10
–
K-10
K-10
K-10
25
mw[a]
60
0.2
0.066
24
24
0.5
1
25
25
mw
70
Toluene
[a] mw¼microwave irradiation (1200 W power).
1798
asc.wiley-vch.de
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2005, 347, 1797 – 1803

Synthesis of N-Heteroaryl(trifluoromethyl)hydroxyalkanoic Acid Esters
FULL PAPERS
Table 2. K-10 montmorillonite-catalyzed hydroxyalkylation of substituted indoles with ethyl 3,3,3-trifluoropyruvate (2a) in to-
luene at 608C in a closed pressure tube.
Entry
1
Substrate
Time [min]
4
Product
Yield [%][a]
98
2
3
4
5
6
9
5
6
6
5
98
97
95
98
97
7
8
9
4
6
4
98
96
95
10
4
98
[a]
Isolated yield.
Adv. Synth. Catal. 2005, 347, 1797 – 1803
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1799

´
Mohammed Abid, Bela Tçrçk
FULL PAPERS
Table 3. K-10 montmorillonite-catalyzed hydroxyalkylation of substituted indoles with ethyl 4,4,4-trifluoroacetoacetate (2b) in
toluene at 708C in a closed pressure tube.
Entry
1
Substrate
Time [h]
1
Product
Yield [%][a]
98[b]
2
3
4
5
6
2
80[b]
98[b]
85[b]
92[b]
89[b]
0.75
1
1
1.5
7
8
9
1
95[b]
90[b]
88[b]
91[b]
1.5
1.5
1
10
[a]
Isolated yield.
New compounds.
[b]
The first set of synthetic studies involved substituted
As Table 2 clearly shows, K-10 is an excellent catalyst
indoles and 2a. The results of K-10-catalyzed hydroxyal- for the hydroxyalkylation reaction. Using a wide variety
kylations are tabulated in Table 2.
of substituted indoles, excellent yields have been ob-
served. The substituents did not affect significantly the
1800
asc.wiley-vch.de
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2005, 347, 1797 – 1803

Synthesis of N-Heteroaryl(trifluoromethyl)hydroxyalkanoic Acid Esters
FULL PAPERS
Table 4. K-10 montmorillonite-catalyzed hydroxyalkylation of substituted pyrrole derivatives with ethyl 3,3,3-trifluoropyru-
vate (2a, n¼0) and ethyl 4,4,4-trifluoroactoacetate (2b, n¼1) in a closed pressure tube.
Entry
1
Substrate
Reactant
Temp. [ 8C]
Time [min]
6
Product
Yield [%][a]
98
2a
60
2
3
2a
2a
60
60
5
5
97
95
4
5
2a
2b
60
70
6
93
30
98[b]
6
7
2b
2b
2b
70
70
70
30
30
30
94[b]
89[b]
86[b]
8
[a]
Isolated yield.
New compounds.
[b]
reactivity of indoles, reaction times varied from 4 to 9 the reactions. The yields obtained were excellent and
minutes. The reactions proceeded smoothly and most in the 88–98% range.
of them were completed in an average time of 5 minutes.
Pyrroles also readily underwent hydroxyalkylation at
Despite the short reaction times, the isolated yields indi- their a-position under the same reaction conditions. In
cated practically quantitative product formation. No by- this case, any excess of the hydroxyalkylating agents re-
product formation was observed.
sulted in the formation of 2,5-disubstituted pyrroles. In
Based on the excellent result shown in Table 2, a sim- order to avoid the formation of such by-products, equi-
ilar sequence of reactions was carried out with substitut- molar amounts of these reactants have been used. Re-
ed indoles and 2b. The results are provided in Table 3.
According to the optimization (Table 1), a slight in-
sults are summarized in Table 4.
Table 4 shows that, using 1:1 pyrrole/reactant ratio,
crease in reaction temperature (708C) was necessary the expected mono-hydroxyalkylated products have
in this case. As 2b is a significantly weaker electrophile been formed in excellent yields (86–98%). The products
than 2a, the reactions usually require longer contact have easily been isolated, no significant by-product for-
times to achieve yields similar to those listed in Table 1. mation has been observed.
However, the longest reaction still required only 2 hours
The key step in the proposed mechanism of the reac-
and the yields obtained were excellent. Although elec- tion is the adsorption of the trifluoro-keto ester on the
tron-donating substituents slightly increased the reac- surface of the catalyst. We propose that the carbonyl
tivity of indoles, the electronic or steric nature of the oxygen interacts with a surface Lewis acid center. This
substituents did not significantly affect the outcome of adsorption/complex formation anchors the volatile re-
Adv. Synth. Catal. 2005, 347, 1797 – 1803
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1801

´
Mohammed Abid, Bela Tçrçk
FULL PAPERS
product was separated from catalyst by filtration. The solvent
and excess of TFP were removed under vacuum. The products
have been isolated as crystals or oils and purified by flash chro-
matography when needed.
actant and initiates strong electrophilic character on the
carbonyl carbon, resulting in a surface-bound inter-
mediate of carbocationic nature. Similar carbonyl-an-
choring effect has been observed in the hydrogenation
of cinnamaldehyde on Pt/K-10 catalyst.[10] The forma-
tion of such intermediate is clearly supported by experi-
ments carried out with microwave irradiation in an open
reaction vessel (Table 1, entries 3 and 7). As ethyl tri-
fluoropyruvate has a low boiling point (428C) without
such chemical adsorption a significant amount of 2a
would have evaporated and have been lost from the re-
action mixture. The high yields unambiguously indicate
that such extensive reactant loss does not occur even
from the open vessel. After the adsorption the reaction
follows the conventional Friedel–Crafts mechanism.[3]
General Procedure for the Synthesis of 3-Hydroxy-3-
(indol-3-yl)-4,4,4-trifluorobutanoic Acid Ethyl Esters
(Table 3, Entries 1–10)
Indole (0.75 mmol) and ethyl 4,4,4-trifluoroacetoacetate
(1.125 mmol) were dissolved in 3 mL of toluene in a Teflon
screw-cap pressure tube and 500 mg of K-10 were added. The
reaction mixture was then immersed into an oil bath preheated
to 708C. The reaction mixture was stirred by magnetic stirrer
and the progress of reaction was monitored by TLC (CH2Cl2
eluent). After satisfactory conversion, the product was separat-
ed from catalyst by filtration. The solvent was removed under
vacuum. The oily residue was purified by flash chromatogra-
phy to obtain the pure product.
Conclusion
In conclusion, a new solid acid-catalyzed, effective, eco-
nomic and clean Friedel–Crafts hydroxyalkylation reac-
tion has been developed to incorporate hydroxyl(tri-
fluoromethyl)alkanoic acid moiety into indoles and pyr-
roles. Our method was able to provide the products in
excellent yields and selectivities, under mild conditions,
in short reaction times. In addition, it made possible the
selective synthesis of a wide range of 4,4,4-trifluorome-
thyl(N-heteroaryl)hydroxybutanoic acid esters for the
first time. This simple and effective process provides a
novel way for the synthesis of the target compounds
that might open new possibilities in their chemical and
biomedical applications.
General Procedure for the Synthesis of 2-Hydroxy-2-
(pyrrol-2-yl)-3,3,3-trifluoropropionic Acid Ethyl
Esters (Table 4, Entries 1–4)
Pyrrole (0.75 mmol) and ethyl 3,3,3-trifluoropyruvate
(0.75 mmol) were dissolved in 3 mL of toluene in a Teflon
screw-cap pressure tube and 500 mg of K-10 were added. The
reaction mixture was then immersed into an oil bath preheated
to 608C. The reaction mixture was stirred by magnetic stirrer
and the progress of reaction was monitored by TLC (CH2Cl2
eluent). After satisfactory conversion, the product was separat-
ed from catalyst by filtration. The solvent and excess of TFP
were removed under vacuum. The products have been isolated
as oils.
Experimental Section
General Procedure for the Synthesis of 3-Hydroxy-3-
(pyrrol-2-yl)-4,4,4-trifluorobutanoic Acid Ethyl Esters
(Table 4, Entries 5–8)
Materials
All indoles, pyrroles, ethyl 3,3,3-trifluoropyruvate and ethyl
4,4,4-trifluoroacetoacetate were purchased from Aldrich and
used without further purification. K-10 was a Fluka product.
The CDCl3 used as solvent for NMR studies and the CFCl3
used as reference compound in 19F NMR measurements were
obtained form Aldrich. Other solvents used in synthesis were
Fisher products with a minimum purity of 99.5%.
Indole (0.75 mmol) and ethyl 4,4,4-trifluoroacetoacetate
(0.075 mmol) were dissolved in 3 mL of toluene in a Teflon
screw-cap pressure tube and 500 mg of K-10 were added. The
reaction mixture was then immersed into an oil bath preheated
to 708C. The reaction mixture was stirred by magnetic stirrer
and the progress of reaction was monitored by TLC (CH2Cl2
eluent). After satisfactory conversion, the product was separat-
ed from catalyst by filtration. The solvent was removed under
vacuu,. The oily residue was purified by flash chromatography
to obtain the pure product.
General Procedure for the Synthesis of 2-Hydroxy-2-
(indol-3-yl)-3,3,3-trifluoropropionic Acid Ethyl Esters
(Table 2, Entries 1–10)
Indole (0.75 mmol) and ethyl 3,3,3-trifluoropyruvate
(1.125 mmol) were dissolved in 3 mL of toluene in a Teflon
screw-cap pressure tube and 500 mg of K-10 montmorillonite
were added. The reaction mixture was immersed into an oil
bath preheated to 608C. The reaction mixture was stirred by
magnetic stirrer and the progress of reaction was monitored
by TLC (CH2Cl2 eluent). After satisfactory conversion, the
Product Identification and Analysis
The products were characterized by NMR (1H, 13C, 19F) spec-
troscopy and mass spectrometry. The complete NMR and
MS characterization of all products is provided in the Support-
ing Information
1802
asc.wiley-vch.de
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2005, 347, 1797 – 1803

Synthesis of N-Heteroaryl(trifluoromethyl)hydroxyalkanoic Acid Esters
FULL PAPERS
[6] G. K. S. Prakash, A. Yudin, Chem. Rev. 1997, 97, 757; P.
Lin, J. Jiang, Tetrahedron, 2000, 56, 3635; G. K. S. Pra-
kash, M. Mandal, J. Fluorine Chem. 2001, 112, 123.
[7] a) G. A. Olah, Friedel–Crafts and Related Reactions, Wi-
ley, New York, 1965; b) G. A. Olah, R. Krisnamurthy,
G. K. S. Prakash, Friedel-Crafts Alkylation, in: Compre-
hensive Organic Synthesis, (Eds.: B. M. Trost, I. Fleming),
Pergamon Press, Oxford, 1991, Vol. 3, p. 293; R. M. Rob-
erts, A. A. Khalaf, Friedel–Crafts Alkylation Chemistry,
Acknowledgement
Financial support provided by Michigan Technological Univer-
sity and National Institute of Health (R-15 AG025777-01, B. T.)
is gratefully acknowledged.
References
´
´
[1] a) J. Fried, E. T. Sabo, J. Am. Chem. Soc. 1954, 76, 1455;
b) I. Ojima, J. R. McCarthy, J. T. Welch, Biomedical
Frontiers of Fluorine Chemistry, American Chemical So-
ciety, Washington, DC, 1996; c) P. V. Ramachandran,
Asymmetric Fluoroorganic Chemistry, ACS Symp. Series,
ACS, Washington, DC, 2000; d) T. Himaya, Organofluor-
ine Compounds, Springer-Verlag, Berlin, Heidelberg,
2001; e) B. Tçrçk, G. K. S. Prakash, Adv. Synth. Catal.
2003, 345, 165–168; P. Kirsch, Modern Fluoroorganic
Chemistry: Synthesis, Reactivity, Applications, Wiley-
VCH, New York, Heidelberg, 2004.
[2] R. Filler, in: Asymmetric Fluoroorganic Chemistry, (Ed.:
P. V. Ramachandran), ACS Symp. Series, ACS, Washing-
ton, DC, 2000, Chap. 1, p.1.
[3] a) J. A. Dale, H. S. Mosher, J. Am. Chem. Soc. 1968, 90,
3732; b) J. A. Dale, H. S. Mosher, J. Am. Chem. Soc.
1973, 95, 512; c) P. Yan, B. Tçrçk, G. K. S. Prakash,
G. A. Olah, Synlett 2003, 4, 527.
[4] M. Pappolla, P. Bozner, C. Soto, H. Shao, N. K. Robakis,
M. Zagorski, B. Frangiones, J. Ghiso, J. Biol. Chem. 1998,
273, 7185; Y.-J. Chyan, B. Poeggeller, R. A. Omar, D. G.
Chain, B. Frangione, J. Chiso, M. A. Pappolla, J. Biol.
Chem. 1999, 274, 21937; B. Poeggeller, L. Miravalle,
M. G. Zagorski, T. Wisniewski, Y.-J. Chyan, Y. Zhang,
H. Shao, T. Bryant-Thomas, R. Vidal, B. Frangione, J.
Ghiso, M. A. Pappolla, Biochemistry 2001, 40, 14995;
P. E. Bendheim, B. Poeggeler, E. Neria, V. Ziv, M. A.
Pappola, D. G. Chain, J. Mol. Neurosci. 2002, 19, 213;
K. Kato, S. Fujii, Y. F. Gong, S. Tanaka, M. Katayama,
H. Kimoto, J. Fluorine Chem. 1999, 99, 5; M. Tçrçk,
M. Abid, B. Tçrçk, J. Biol. Chem. in preparation.
[5] a) W. Zhuang, N. Gathergood, R. G. Hazell, K. A. Jør-
gensen, J. Org. Chem. 2001, 66, 1009; N. Gathergood,
W. Zhuang, K. A. Jørgensen, J. Am. Chem. Soc. 2000,
122, 12517; M. P. A. Lyle, N. D. Draper, P. D. Wilson,
Org. Lett. 2005, 7, 901; b) K. Aikawa, S. Kainuma, M.
Hatano, K. Mikami, Tetrahedron Lett. 2004, 45, 183;
c) G. Blay, I. Fernandez, A. Marco-Aleixandre, B. Mon-
je, J. R. Pedro, R. Ruiz, Tetrahedron 2002, 58, 8565; d) B.
Tçrçk, M. Abid, G. London, J. Esquibel, M. Tçrçk, S. C.
Mhadgut, P. Yan, G. K. S. Prakash, Angew. Chem. Int.
Ed. 2005, in press.
Marcel Dekker, New York, 1984; A. Molnar, in: Ency-
clopedia of Catalysis, (Ed.: I. T. Horvath), Wiley, New
York, 2003, Vol. 1, p. 40.
[8] G. A. Olah, A. Molnar, Hydrocarbon Chemistry, 2nd edn.,
Wiley, New York, 2003.
´
´
[9] a) B. C. Gates, in: Encyclopedia of Catalysis, (Ed.: I. T.
Horvath), Wiley, New York, 2003, Vol. 2, p. 104;
b) H. A. Benesi, B. H. C. Winquest, Adv. Catal. 1978,
27, 97; R. S. Varma, K. P. Naicker, D. Kumar, R. Dahiya,
P. J. Liesen, J. Microwave Power Electromagn. Energy
1999, 34, 113; K. V. N. S. Srinivas, B. Das, J. Org. Chem.
2003, 68, 1165; M. Balogh, P. Laszlo, Organic Chemistry
Using Clays, Springer-Verlag, Berlin, 1993; R. S. Varma,
R. Dahiya, S. Kumar, Tetrahedron Lett. 1997, 38, 2039;
c) G. A. Olah, P. S. Iyer, G. K. S. Prakash, Synthesis
1986, 513; G. A. Olah, G. K. S. Prakash, P. S. Iyer, M. Ta-
shiro, T. Yamamoto, J. Org. Chem. 1987, 52, 1881; T. Ya-
mamoto, C. Hideshima, G. K. S. Prakash, G. A. Olah J.
Org. Chem. 1991, 56, 3955; G. A. Olah, B. Tçrçk, T.
Shamma, M. Tçrçk, G. K. S. Prakash, Catal. Lett. 1996,
´
´
42, 5; B. Tçrçk, I. Kiricsi, A. Molnar, G. A. Olah, J. Catal.
´
´
´
2000, 193, 132; d) T. Beregszaszi, B. Tçrçk, A. Molnar,
G. A. Olah, G. K. S. Prakash, Catal. Lett. 1997, 48, 83;
I. V. Kozhevnikov Chem. Rev. 1998, 98, 171; M. Misono,
in: Encyclopedia of Catalysis, (Ed.: I. T. Horvath), Wiley,
New York, 2003, Vol. 3, p. 433.
´
[10] G. Szçllçsi, B. Tçrçk, L. Baranyi, M. Bartok, J. Catal.
´
1998, 179, 619; K. Balazsik, B. Tçrçk, G. Szakonyi, M.
´
Bartok, Applied Catal. A 1999, 182, 53; B. Tçrçk, K. Ba-
´
´
lazsik, I. Kun, G. Szçllçsi, G. Szakonyi, M. Bartok, Stud.
Surf. Sci. Catal. 1999, 125, 555; B. Tçrçk, G. London, M.
´
Bartok, Synlett, 2000, 631.
´ ´
´
[11] T. Cseri, S. Bekassy, F. Figueras, E. Cseke, E. de Menorv-
al, R. Dutartre, Appl. Catal. A 1995, 132, 141; C. Cativie-
la, F. Figueras, J. M. Fraile, J. I. Garcia, J. A. Mayoral, E.
´
de Menorval, E. Pires, Appl. Catal. A 1993, 101, 253; B.
´
´
´
Tçrçk, G. Szçllçsi, M. Rozsa-Tarjani, M. Bartok, Mol
Cryst Liquid Cryst 1998, 311, 289.
[12] G. A. Olah, J. Sommer, G. K. S. Prakash, Superacids, Wi-
ley, New York, 1989.
Adv. Synth. Catal. 2005, 347, 1797 – 1803
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1803