A simple and versatile method for the synthesis of acetals from aldehydes and ketones using bismuth triflate

Article abstract of DOI:10.1021/jo0258249

Acetals are obtained in good yields by treatment of aldehydes and ketones with trialkyl orthoformate and the corresponding alcohol in the presence of 0.1 mol % Bi(OTf)₃·4H₂O. A simple procedure for the formation of acetals of diaryl ketones has also been developed. The conversion of carbonyl compounds to the corresponding 1,3-dioxolane using ethylene glycol is also catalyzed by Bi(OTf)₃· 4H₂O (1 mol %). Two methods, both of which avoid the use of benzene, have been developed.

Full text of DOI:10.1021/jo0258249

A Sim p le a n d Ver sa tile Meth od for th e Syn th esis of Aceta ls fr om
Ald eh yd es a n d Keton es Usin g Bism u th Tr ifla te
Nicholas M. Leonard,† Matthew C. Oswald,† Derek A. Freiberg, Bryce A. Nattier,
Russell C. Smith, and Ram S. Mohan*
Department of Chemistry, Illinois Wesleyan University, Bloomington, Illinois 61701
rmohan@titan.iwu.edu
Received April 15, 2002
Acetals are obtained in good yields by treatment of aldehydes and ketones with trialkyl orthoformate
and the corresponding alcohol in the presence of 0.1 mol % Bi(OTf)3‚4H2O. A simple procedure for
the formation of acetals of diaryl ketones has also been developed. The conversion of carbonyl
compounds to the corresponding 1,3-dioxolane using ethylene glycol is also catalyzed by Bi(OTf)3‚
4H2O (1 mol %). Two methods, both of which avoid the use of benzene, have been developed.
In tr od u ction
in the presence of an acid catalyst.14 Several other
methods that do not require protic acid catalysis have
also been developed.15 In addition, mild methods for the
formation of acetals under almost neutral conditions have
also been developed.16 The synthesis of dialkyl acetals
from diaryl ketones is more difficult, and the standard
acetalization conditions generally do not work with diaryl
ketones. Triflic acid (20 mol %) has been shown to be a
useful catalyst for synthesis of diaryl ketones.17 Many of
the existing methods for acetalization suffer from certain
drawbacks and often use reagents that are toxic and
corrosive. For example, TiCl4, BF3.Et2O, CF3SO3H, and
TMSOTF are rather corrosive. In addition, few of these
methods have broad applicability.
Due to our continued interest in the use of bismuth
compounds as environmentally friendly reagents for
organic synthesis, we undertook a study of the utility of
bismuth triflate as a catalyst for acetal formation. Herein
we report that bismuth triflate (0.1-1.0 mol %) is a
versatile and robust catalyst for acetal formation. The
highly catalytic nature of this reagent and its wide
applicability should make this procedure an attractive
alternative to existing methods for acetal formation.
Aldehydes and ketones are frequently protected as
acetals in the course of a total synthesis.1a-c In addition,
acetals can be converted to a variety of other useful
functional groups and hence serve as useful intermedi-
ates in synthesis.1d,e Thus, conversions of carbonyl com-
pounds to the corresponding acyclic and cyclic acetals
(1,3-dioxolanes) are important synthetic transformations
that have received much attention. Acetalization is
typically carried out by treatment of the carbonyl com-
pound with the alcohol and/or the corresponding ortho-
formate in the presence of an acid catalyst. Some methods
for the formation of dimethyl acetals include dry HCl in
CH3OH,2 LaCl3/CH3OH/(CH3O)3CH,3 DCC-SnCl4,4 (CH3)
3SiOCH3/Me3SiOTf,5 (CH3O)3CH/p-TsOH,6 CH3OH/
(CH3O)4Si/dry HCl,7 and CH3OH/PhSO2NHOH.8 Fewer
methods are known for the synthesis of diethyl acetals.
These include (CH3CH2O)3CH/HCl,9 (CH3CH2O)3CH/
FeCl3 in refluxing ethanol,10 (CH3CH2O)3CH/Amberlyst,11
DDQ,12 and ZrCl4.13 The formation of 1,3-dioxolanes
(cyclic acetals) is usually carried out using ethylene glycol
† Both students contributed equally to the project.
(1) (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 3rd ed.; J ohn Wiley and Sons: New York, 1999. (b) Hanson,
J . R. Protecting Groups in Organic Synthesis, 1st ed.; Blackwell Science,
Inc: Malden, MA, 1999. (c) Kocienski, P. J . Protecting Groups, 1st ed.;
Georg Thieme Verlag: Stuttgart, 1994. (d) Bergstrom, R. G. In The
Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and Their
Sulphur Analogues, Supplement E; Patai, S., Ed.; Wiley: New York,
1980; Part 2, Chapter 20. (e) Schnitz, E.; Eichorn, I. In The Chemistry
of the Ether Linkage; Patai, S., Ed.; Wiley: New York, 1967; Chapter
7.
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Wilson, B. M. J . Chem. Soc. 1953, 3864.
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1357.
(6) Wenkert, E., Goodwin, T. E. Synth. Commun. 1977, 7, 409.
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(8) Hassner, A.; Wiederkehr, R.; Kascheres, A. J . J . Org. Chem.
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Am. Chem. Soc. 1958, 80, 5837. (c) Fieser, L. F.; Stevenson, R. J . Am.
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(15) (a) Ethylene glycol/TMSCl: Chan, T. H.; Brook, M. A.; Chaly,
T. Synthesis 1983, 203. (b) AlPO4: Bautista, F. M.; Campelo, J . M.;
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336, 620. (c) CdI2: Laskar, D. D.; Prajapati, D.; Sandhu, J . S. Chem.
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6151. (b) BF3‚Et2O: Torok, D. S.; Figueroa, J . J .; Scott, W. J . J . Org.
Chem. 1993, 58, 7274. (c) Ti(OR)4/TiCl4: Mahrwald, R. J . Prakt. Chem.
1994, 336, 361. (d) Envirocats: Beregszaszi, T.; Molnar, A. Synth.
Commun. 1997, 27, 3705. (e) Pt-Mo/ZrO2: Reddy, B. M.; Reddy, V. R.;
Giridhar, D. Synth. Commun. 2001, 31, 1819. (f) Microwave irradia-
tion: Perio, B.; Dozias, M.-J .; J acquault, P.; Hamelin, J . Tetrahedron
Lett. 1997, 38, 7867. (g) TiCl4/Et3N: Clerici, A.; Pastori, N.; Porta, O.
Tetrahedron 1998, 54, 15679. (h) N-Bromosuccinimide: Karimi, B.;
Seradj, H.; Ebrahimian, G.-R. Synlett 1999, 9, 1456.
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10.1021/jo0258249 CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/18/2002
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J . Org. Chem. 2002, 67, 5202-5207

Synthesis of Acetals from Aldehydes and Ketones
SCHEME 1
trialkylorthoformate (1.5 equiv) and the corresponding
alcohol (8 equiv) in the presence of 0.1 mol % Bi(OTf)3‚
4H2O. Smooth reaction was observed at room tempera-
ture but, not surprisingly, at a considerably slower rate.
Unlike triflic acid or other air-sensitive Lewis acids such
as BF3‚Et2O or TiCl4, bismuth triflate is a relatively
stable, easy to handle solid that is insensitive to small
amounts of air and moisture. While acetal formation was
observed in the presence of the catalyst and trialky-
lorthoformate alone, the addition of the alcohol acceler-
ated the rate of the reaction. When the reaction was
attempted using the catalyst and alcohol alone (no
trialkylorthoformate), significant acetal formation was
not observed. The addition of molecular sieves or a drying
agent such as anhydrous magnesium sulfate did not
accelerate the reaction significantly. Aromatic aldehydes
with both activating and deactivating groups (entries
1-4), conjugated aldehydes (entries 5 and 6), and ali-
phatic aldehydes (entries 7 and 8) underwent smooth
transformation to the corresponding acetal in good yield.
In almost all cases, the crude product was found to be
>98% pure (determined by GC and 1H and 13C NMR).
Commercial citral (entry 6), available as a mixture of E/ Z
isomers, was subjected to the reaction conditions and, it
was found that the double bond in citral did not undergo
isomerization during acetal formation. This indicates that
the reaction conditions are relatively mild and not
sufficiently acidic to cause isomerization of the double
bond. The conversion of ketones (entries 9-11) to the
corresponding acetal was also successfully carried out.
While a large number of methods exist for synthesis of
dimethyl acetals, fewer methods are available for syn-
thesis of diethyl acetals. One reported method that is
catalytic in nature uses DDQ as the catalyst.12 DDQ,
however, is rather toxic and corrosive, and hence it is
desirable to have safer alternatives. Using bismuth
triflate as a catalyst (0.1 mol %), we were able to
synthesize a variety of diethyl acetals (entries 12-16) in
good yields. The best yields were obtained using 2.4 equiv
of triethylorthoformate and 6.2 equiv of ethanol.
Diaryl ketones are quite resistant to the standard
conditions for acetalization, and hence indirect routes to
their preparation have been developed. For example, the
dimethyl acetals of 4,4′-dichlorobenzophenone and 4,4′-
dimethoxybenzophenone have been prepared by alkoxy-
halogen exchange from the corresponding dihalodiaryl-
methanes.28 To study the versatility of bismuth triflate,
we extended our methodology to a series of benzophe-
nones (Table 2). Gratifyingly, the corresponding acetals
were obtained in excellent yields with only 1.0 mol %
bismuth triflate. When the substituted benzophenones
were insoluble in methanol/trimethyl orthoformate, ni-
tromethane was used as a cosolvent. A wide variety of
substituted benzophenones underwent smooth reaction
to give both dimethyl and diethyl acetals. However,
fluorenone (entry 7) failed to give the corresponding
acetal. It has been reported that even with 20 mol %
triflic acid as a catalyst, the acetalization of fluorenone
was not successful.17
Bismuth compounds have attracted recent attention due
to their low toxicity, low cost, and good shelf life.18,19 We
recently reported the use of bismuth triflate as a catalyst
for rearrangement of epoxides to aldehydes and ketones,20
formation of acylals,21 and deprotection of acetals.22
Bismuth triflate has also been used as a catalyst for
Friedel-Crafts acylations,23 sulfonylations,24 Diels-Alder
reactions,25 and aza-Diels-Alder reactions.26
Resu lts a n d Discu ssion
Herein we report a mild and general procedure for the
conversion of aldehydes and ketones to the corresponding
acetals in good yields using bismuth triflate (0.1-1.0 mol
%) as a catalyst (Scheme 1). The catalyst is very versatile
and works well for the synthesis of dimethyl acetals,
diethyl acetals as well as 1,3-dioxolanes from a variety
of aldehydes and ketones.
Although bismuth triflate is not commercially avail-
able, it can be easily synthesized in large quantities at a
relatively low cost.27 A wide variety of aldehydes and
ketones underwent smooth reaction to give the corre-
sponding acetal in good yield (Table 1). The experimental
procedure for synthesis of acetals is straightforward and
involves heating the aldehyde or ketone as a solution in
(18) (a) Reglinski, J . In Chemistry of Arsenic, Antimony and
Bismuth; Norman, N. C., Ed.; Blackie Academic and Professional: New
York, 1998; pp 403-440. (b) Marshall, J . A. Chemtracts 1997, 1064-
1075. (c) Suzuki, H.; Ikegami, T.; Matano, Y. Synthesis 1997, 249.
(19) Organobismuth Chemistry; Suzuki, H.; Matano. Y., Eds.;
Elsevier: Amsterdam, 2001.
(20) Bhatia, K. A.; Eash, K. J .; Leonard, N. M.; Oswald, M. C.;
Mohan, R. S. Tetrahedron Lett. 2001, 42, 8129.
(21) Carrigan, M. D.; Eash, K. J .; Oswald, M. C.; Mohan, R. S.
Tetrahedron Lett. 2001, 42, 8133.
(22) Wieland, L. C.; Carrigan, M. C.; Sarapa, D.; Smith, R. S.;
Mohan, R. S. J . Org. Chem. 2002, 67, 1027.
(23) (a) Labrouille`re, M.; Le Roux, C.; Gaspard, H.; Laporterie, A.;
Dubac, J . Tetrahedron Lett. 1997, 38, 8871. (b) Re´pichet, S.; Le Roux,
C.; Dubac, J .; Desmurs, J . R. Eur. J . Org. Chem. 1998, 2743.
(24) Re´pichet, S.; Le Roux, C.; Hernandez, P.; Dubac, J . J . Org.
Chem. 1999, 64, 6479.
(25) (a) Garrigues, B.; Gonzaga, F.; Robert, H.; Dubac, J . J . Org.
Chem. 1997, 62, 4880. (b) Robert, H.; Garrigues, B.; Dubac, J .
Tetrahedron Lett. 1998, 39, 1161.
(26) Laurent-Robert, H.; Garrigues, B.; Dubac, J . Synlett 2000, 1160.
(27) Labrouille`re, M.; Le Roux, C.; Gaspard, H.; Laporterie, A.;
Dubac, J .; Desmurs, J . R. Tetrahedron Lett. 1999, 40, 285. Bismuth
triflate synthesized by this procedure is reported to be mainly the
tetrahydrate. Recently, another procedure for the synthesis of bismuth
triflate has been reported. Repichet, S.; Zwick, A.; Vendier, L.; Le Roux,
C.; Dubac, J . Tetrahedron Lett. 2002, 43. 993.
The standard method for synthesis of 1,3-dioxolanes
consists of heating a mixture of the carbonyl compound
(28) Hardie, W. R.; Hidalgo, J .; Halverstadt.; Allen, R. E. J . Med.
Chem. 1966, 9, 127.
J . Org. Chem, Vol. 67, No. 15, 2002 5203

Leonard et al.
TABLE 1. F or m a tion of Acyclic Aceta ls fr om Ald eh yd es a n d Keton es Usin g Bi(OTf)3‚4H2O
a
b
Superscripted numbers accompanying entry numbers refer to literature references for the product. All reactions were carried out at
reflux temperatures unless otherwise mentioned. c Refers to the yield of the isolated product. The crude product was estimated to be
g98% pure unless otherwise mentioned. Product is commercially available. e Purified by flash chromatography. f Contains 5% SM.
Contains 2% triethyl orthoformate. Contains 5% triethyl orthoformate. Purified by Kugelrohr distillation.
d
g
h
i
5204 J . Org. Chem., Vol. 67, No. 15, 2002

Synthesis of Acetals from Aldehydes and Ketones
TABLE 2. F or m a tion of Acyclic Aceta ls fr om Dia r yl Keton es Usin g Bi(OTf)3‚4H2O
a
b
Superscripted numbers against entry numbers refer to literature references for the product. All reactions were carried out at reflux
temperature. c Refers to the yield of crude product unless otherwise mentioned. Purity of the crude product was estimated to be g98% by
1H and 13C NMR spectroscopy. Nitromethane was used a cosolvent. e Purified by flash chromatography.
d
in ethylene glycol and removing the water as an azeo-
trope with benzene. This method is not particularly
suitable for small-scale synthesis and also uses the
carcinogenic solvent benzene. We have developed two
practical methods for the synthesis of 1,3-dioxolanes
(Table 3). In one method (method A), toluene is used as
a solvent and 1.0 mol % Bi(OTf)3‚4H2O serves as the
catalyst. Under these conditions, at 110 °C, the reactions
reached >95% conversion and the pure dioxolane was
then isolated by flash chromatography. The advantage
of this method is that no Dean-Stark trap is needed,
making the procedure suitable for small-scale reactions.
A search for a substitute for the highly toxic solvent
benzene led to fluorobenzene (which, unlike benzene, is
not classified as a carcinogen) as the solvent of choice
for removal of the water formed as an azeotrope (method
B). Again the reaction was catalyzed by as little as 1.0
mol % Bi(OTf)3‚4H2O. As in the synthesis of dialkyl
acetals, the addition of molecular sieves did not increase
the percent conversion or yield.
dimethyl and diethyl acetals as well as 1,3-dioxolanes
from a wide variety of aldehydes and ketones. The
procedure also works very well with diaryl ketones, which
are often resistant to the standard acetalization condi-
tions. The highly catalytic nature of this reagent coupled
with its low cost, noncorrosive nature, ease of handling,
and wide applicability should make this method for
synthesis of acetals particularly attractive.
Exp er im en ta l Section
Methanol was dried over magnesium metal.29 Fluoroben-
zene, trimethylorthoformate, and triethylorthoformate were
(29) (a) Lund, H.; Bjerrum, J . Berichte 1931, 64, 210. (b) Lund, H.
Berichte 1934, 37, 936. (c) Lund, H. J . Am. Chem. Soc. 1952, 74, 3188.
(30) Luche, J .-L.; Gemal, A. L. J . Chem. Soc. Chem. Commun. 1978,
976.
(31) Heywood, D. L.; Phillips, B. J . Org. Chem. 1960, 25, 1699.
(32) Yamato, M.; Hashigaki, K.; Kokubu, N.; Tashiro, T.; Tsuruo,
T. J . Med. Chem. 1986, 29, 1202.
(33) Reddy, B. M.; Reddy, V. R.; Giridhar, D. Synth. Commun. 2001,
31, 1819.
(34) Suzuki, T.; Oriyama, T. Synth. Commun. 1999, 29, 1263.
(35) Wang, W.-B.; Shi, L.-L.; Huang, Y.-Z. Tetrahedron Lett. 1990,
46, 3315.
Con clu sion s
(36) Carbera, A.; Va´zquez, D.; Velasco, L.; Salmo´n, M.; Arias, J . L.
J . Mol. Catal. 1992, 75, 101.
(37) Gihani, M. E.; Heaney, H. Synlett 1993, 583.
Bismuth triflate (0.1-1.0 mol %) has been shown to
be a versatile and robust catalyst for the synthesis of
J . Org. Chem, Vol. 67, No. 15, 2002 5205

Leonard et al.
TABLE 3. F or m a tion of 1,3-Dioxola n es fr om Ald eh yd es a n d Keton es
a
b
Superscripted numbers against entry numbers refer to literature references for the product. Determined by GC analysis of the
reaction mixture. c Refers to the yield of pure (g98%), isolated product after purification by flash chromatography. Relative to the aldehyde,
d
2 equiv of 2,2-dimethoxypropane was added as a water scavenger. e Product was found to be g98% pure by 1H and 13C NMR. GC analysis,
however, showed the product to be 90% pure. f Product is commercially available.
used as received. Ethylene glycol was dried over 4 Å sieves
before use. Commercially available 200 proof ethanol was dried
over anhydrous magnesium sulfate prior to use. Reactions
were heated at the indicated temperature using a temperature-
controlled oil bath.
Rep r esen ta tive P r oced u r e for Syn th esis of Dim eth yl
a n d Dieth yl Aceta ls. A solution of p-chlorobenzaldehyde
(3.00 g, 21.3 mmol) and trimethylorthoformate (3.39 g, 31.95
mmol) in methanol (7.0 mL) was stirred as bismuth triflate
(14.0 mg, 0.0213 mmol, 0.1 mol %) was added. The resulting
mixture was heated at reflux for 3 h and then cooled to room
temperature. Saturated aqueous NaHCO3 (20 mL) was added,
and the mixture was stirred well and extracted with ether (2
× 20 mL). The combined organic layers were washed with
water (3 × 15 mL) and saturated aqueous NaCl (15 mL). The
organic layer was dried (Na2SO4), and the solvents were
removed on a rotary evaporator to yield 3.56 g (89%) of a
colorless oil (>99% pure by GC and 1H and 13C NMR) that
was identified as the corresponding dimethyl acetal by 1H and
13C NMR spectroscopy.
Rep r esen ta tive P r oced u r e for Syn th esis of Aceta ls
fr om Dia r yl Keton es. A mixture of 4,4′-difluorobenzophenone
(1.00 g, 4.58 mmol) in methanol (3 mL) and trimethylortho-
formate (3.16 g, 3.3 mL, 29.8 mmol) was stirred at room
temperature as bismuth triflate (30 mg, 0.0458 mmol, 1.0 mol
%) was added. As the mixture was heated, a solution resulted
(38) Fadel, A.; Yefsah, R.; Salaun, J . Synthesis 1987, 37.
5206 J . Org. Chem., Vol. 67, No. 15, 2002

Synthesis of Acetals from Aldehydes and Ketones
that was further heated at reflux for 24 h and allowed to cool
to room temperature. The solution was then poured onto 10%
aqueous NaHCO3 (15 mL), which resulted in precipitation of
a solid. The mixture was cooled in ice, and the solid formed
was collected by suction filtration and dried in a vacuum
desiccator to afford 1.14 g (94%) of white crystals. The product
has been reported in the literature, but 13C NMR data has not
been previously reported: 13C NMR (67.5 MHz, CDCl3) δ 49.2,
102.2, 114.8 (d, J CF ) 21.1 Hz), 128.7 (d, J CF ) 8.3 Hz), 138.19
(d, J CF ) 3.1 Hz), 162.2 (d, J CF ) 244.9 Hz).
Dim eth yl Aceta l of 4-Nitr oben zop h en on e. A mixture of
4-nitrobenzophenone (2.00 g, 8.80 mmol) in methanol (4.5 g,
5.7 mL, 0.141 mol) and trimethylorthoformate (6.07 g, 6.3 mL,
57.2 mmol) was stirred at room temperature as bismuth
triflate (57.8 mg, 0.0880 mmol, 1.0 mol %) was added. The
resulting mixture was heated at reflux, and nitromethane (5
mL) was added until a solution resulted. The solution was
refluxed for 24 h and allowed to cool to room temperature. It
was then poured onto 10% aqueous NaHCO3 solution (15 mL),
and the mixture was extracted with CH2Cl2 (2 × 20 mL). The
combined organic layers were washed with H2O (2 × 15 mL)
and saturated NaCl (15 mL), dried (Na2SO4), and concentrated
on a rotary evaporator to give 2.00 g (83%) of off-white
temperature as Bi(OTf)3‚4H2O (0.140 g, 0.213 mmol, 1.0 mol
%) was added. The mixture was heated under N2 at 110 °C in
an oil bath. After 4.5 h, the mixture was poured onto 10%
aqueous NaHCO3 (25 mL) and extracted with ether (45 mL).
The organic layer was then washed with H2O (4 × 30 mL),
saturated NaCl (30 mL), dried (Na2SO4), and concentrated on
a rotary evaporator to yield 3.72 g of an oil that was purified
by flash chromatography on 300 g of silica to yield 2.83 g (72%)
of a clear liquid that was consistent with the expected
dioxolane (>98% pure as confirmed by GC analysis and 1H
and 13C NMR spectroscopy).
Syn th esis of 1,3-Dioxola n es (Meth od B). A round-bottom
flask was charged with p-chlorobenzaldehyde (3.00 g, 21.34
mmol), fluorobenzene (10 mL), ethylene glycol (10.60 g, 0.171
mol), and bismuth triflate (0.140 g, 0.213 mmol, 1.0 mol %).
The flask was equipped with a Dean-Stark trap, and the well-
stirred mixture was heated at 110 °C using an oil bath. After
5 h, the mixture was poured on to 10% aqueous NaHCO3
solution (30 mL) and extracted with ether (3 × 40 mL). The
organic layer was then washed with H2O (4 × 30 mL) and
saturated NaCl (30 mL), dried, and concentrated on a rotary
evaporator to yield 3.00 g (76%) of a colorless oil that was
consistent with the expected dioxolane (>98% pure as con-
firmed by GC and 1H and 13C NMR spectroscopy).
crystals: IR (KBr) 2967, 2834, 1559, 1343, 1212, 1090 cm-1
;
1H NMR (270 MHz, CDCl3) δ 3.13 (s, 6 H), 7.24 (m, 3 H), 7.45
(m, 2 H), 7.67 (m, 2 H), 8.4 (m, 2 H); 13C NMR δ 49.3, 102.1,
123.2, 126.6, 127.8, 127.9, 128.2, 140.8, 147.1, 149.5; mp 61-
63 °C. Anal. Calcd. for C15H15NO2: C, 65.92; H, 5.53; N, 5.13.
Found: C, 65.84; H, 5.55; N, 5.17.
Ack n ow led gm en t. We thank the National Science
Foundation for an RUI grant. R.M. also thanks The
Camille and Henry Dreyfus Foundation for a teacher-
scholar award.
Syn th esis of 1,3-Dioxola n es (Meth od A). A mixture of
p-chlorobenzaldehyde (3.00 g, 21.34 mmol), ethylene glycol
(5.30 g, 85.4 mmol), and toluene (6 mL) was stirred at room
J O0258249
J . Org. Chem, Vol. 67, No. 15, 2002 5207