Preparation of silyl enol ethers using (bistrimethylsilyl)acetamide in ionic liquids

Article abstract of DOI:10.1021/ol015602h

(matrix presented) Ionic liquids have been used for the preparation of silyl enol ethers from aldehydes and ketones with (bistrimethylsilyl)acetamide (BSA) in good yields.

Full text of DOI:10.1021/ol015602h

ORGANIC
LETTERS
2001
Vol. 3, No. 7
1037-1039
Preparation of Silyl Enol Ethers Using
(Bistrimethylsilyl)acetamide in Ionic
Liquids
Michael Smietana and Charles Mioskowski*
Laboratoire de Synthe`se Bio-organique, CNRS and UniVersite´ Louis Pasteur,
Faculte´ de Pharmacie, 74 Route du Rhin, 67401 Illkirch, France
mioskow@aspirine.u-strasbg.fr
Received January 23, 2001
ABSTRACT
Ionic liquids have been used for the preparation of silyl enol ethers from aldehydes and ketones with (bistrimethylsilyl)acetamide (BSA) in
good yields.
Synthetic utility and importance of silyl enol ethers has been
well established, and their preparations have been extensively
studied1 and reviewed.2 The most common methods use silyl
chlorides or silyl triflates/base combinations and need careful
attention during workup of the reaction and isolation of the
enol ether.
Because of their solvent properties, ionic liquids are
attracting increasing attention.5 They afford significant
environmental benefits and can contribute to green chemistry.
They are low melting, nonvolatile, and exhibit good physi-
cochemical properties and can be recovered by careful
washing in ether or pentane.
Silylations with silazane-type reagents such as (bistri-
methylsilyl)acetamide (BSA) are generally mild and nearly
neutral. The use of a co-base to capture the acid proton is
not needed and is operationally more convenient due to the
ease of preparation and handling. However, the reactivity
of these silicon-based reagents often remains low and
unsatisfactory. To our knowledge, the only example of the
silylation of enolizable aldehydes and ketones with BSA uses
hexamethylphosphoramide (HMPA) as the solvent in the
presence of small quantities of sodium metal.3,4 According
to the authors, this reaction does not work with any other
solvent and the high toxicity of HMPA requires special
precautions.
Recently ionic liquids have been reported as alternative
solvents for polymerization,6 hydrogenation,7 regioselective
alkylation,8 Friedel-Crafts reactions,9 dimerization of al-
kenes,10 Diels-Alder reactions,11 and cross-coupling reac-
tions.12 In this Letter we report an efficient, fairly concen-
(5) (a) Welton, T. Chem. ReV. 1999, 99, 2077. (b) Wasserscheid, P.;
Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772.
(6) Abdul-sada, A. A. K.; Ambler, P. W.; Hodgson, P. K. G.; Seddon,
K. R.; Stewart, N. J. World Pat. WO 9521871, 1995.
(7) (a) Chauvin, Y.; Mussman, L.; Olivier, H. Angew. Chem., Int. Ed.
Engl. 1995, 38, 3097. (b) Fisher, T.; Sethi, A.; Welton, T.; Woolf, J.
Tetrahedron Lett. 1999, 40, 793. (c) Adams, C. J.; Earle, M. J.; Seddon, K.
R. Chem. Commun. 1999, 25. (d) Dyson, P. J.; Ellis, D. J.; Parker, D. G.;
Welton, T. Chem. Commun. 1999, 1043. (e) Monteiro, A. L.; Zinn, F. K.;
de Souza, R. F.; Dupont, J. Tetrahedron: Asymmetry 1997, 8, 177. (f)
Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; de Souza, R. F.; Dupont, J.
Polyhedron 1996, 15, 1217.
(1) For recent papers about the formation of enoxysilanes, see: (a)
Hydrio, J.; Van de Weghe, P.; Collin, J. Synthesis 1997, 68. (b) Ishino, Y.;
Kita, Y.; Maekawa, H.; Ohno, T.; Yamasaki, Y.; Miyata, T.; Nishiguchi, I.
Tetrahedron Lett. 1999, 40, 1349.
(8) (a) Badri, M.; Brunet, J. J.; Perron, R. Tetrahedron Lett. 1992, 33,
4435. (b) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Chem. Commun.
1998, 2245.
(2) Brownbridge, P. Synthesis 1983, 1. Brownbridge, P. Synthesis 1983,
89.
(3) Dedier, J.; Gerval, P.; Frainnet, E. J. Organomet. Chem. 1980, 185,
183.
(9) (a) Boon, J. A.; Levisky, J. A.; Pflug, J. L.; Wilkes, J. S. J. Org.
Chem. 1986, 54, 480. (b) Luer, G. D.; Bartak, D. E. J. Org. Chem. 1982,
47, 1238. (c) Adams, C. J.; Earle, M. J.; Roberts, G.; Seddon, K. R. Chem.
Commun. 1998, 2097.
(4) El Gihani, M. T.; Heaney, H. Synthesis 1998, 357.
(10) Ellis, B.; Keim, W.; Wasserscheid, P. Chem. Commun. 1999, 337.
10.1021/ol015602h CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/07/2001

are outlined in Table 1. The reactions are completed in 4 h
and produce the enoxysilanes in high yields. In the case of
carbonyl compounds that can lead to two regioisomeres, the
thermodynamic one is formed preferentially (entries 2, 3, 4,
and 6). It is noteworthy that the product distribution changes
with R-heteroatom-substitued ketones (entries 10 and 11).
Moreover, when chlorotrimethylsilane is used as reagent
under the same conditions, only the crotonization products
are formed. With other silazane-type reagents, such as bis-
(trimethylsilyl urea) or trimethylsilyl imidazole, no reaction
is observed.
Table 1. Synthesis of Enoxysilanes from Aldehydes and
Ketones with BSA
To examine the behavior of other molten salts, we have
considered the reactivity of cyclohexanone in the presence
of BSA. The results are reported in Table 2. The best yields
Table 2. Synthesis of 1-(Trimethylsilyloxy)cyclohexene in
Different Molten Salts
entry
solvent
temp (°C)
yield (%)a
1
2
3
4
5
6
7
[PBu4]Br
[PBu4]Cl
[NBu4]Br
[NBu4]Cl
[BuPy]Br
[bmim]Cl
[bmim][PF6]
105
85
105
85
90
90
87
80
90
45
15
0
90
0
a Isolated yields.
were obtained with phosphonium and ammonium salts
(entries 1-4). In the case of phosphonium salts, bromide
and chloride led to similar yields (entries 1 and 2). On the
other hand, the ammonium salts gave different results.
Whereas the bromide seems to be a very good medium (entry
3), the yield decreased significantly with the chloride (entry
4). Longer reaction times and amounts of BSA were required
to increase the yield. With N-butylpyridinium bromide14
([BuPy][Br]), very small amounts of enoxysilane were
formed but no starting ketone was recovered (entry 5). We
also examined the behavior of room-temperature ionic liquids
at various temperatures. Surprisingly, the generation of silyl
enol ethers was not observed with 1-butyl-3-methylimid-
azolium chloride ([bmim][Cl]) nor with 1-butyl-3-methyl-
imidazolium hexafluorophosphate ([bmim][PF6]),15 even at
90 °C (entries 6 and 7). In both cases, the starting ketone
was entirely recovered.
1
a Selectivity was determined by H NMR. b Isolated yield (%).
trated, and environmentally friendly preparation of enoxysi-
lanes from aldehydes and ketones by treatment with BSA
in different ionic liquids.
Typically, the enoxysilanes were obtained in tetrabutyl-
ammonium bromide at 105 °C and isolated directly by
stripping the product from the crude mixture.13 The results
In conclusion, these results demonstrate the potential of
molten salts, e.g., tetrabutylammonium bromide, for the
generation of enoxysilanes with BSA in neutral conditions
without any base addition. The nature of the salt and
(13) Typical procedure for the preparation of enoxysilanes: A few
degrees above the melting temperature of tetrabutylammonium bromide (5
mmol; ca. 105 °C), the carbonyl compound was added (2 mmol), followed
by addition of BSA (2.4 mmol). The mixture was stirred for 4 h at this
temperature and directly distilled under reduced pressure whithout any
workup to give pure silyl enol ether.
(14) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J.
Mater. Chem. 1998, 8, 2627.
(15) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.;
Rogers, R. D. Chem. Commun. 1998, 1765.
(11) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green Chem. 1999,
1, 23.
(12) (a) Kaufmann, D. E.; Nouroozian, M.; Henze, H. Synlett 1996, 1091.
(b) Bo¨hm, V. P. W.; Herrmann, W. A. Chem. Eur. J. 2000, 6, 1017. (c)
Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.; Seddon,
K. R. Org. Lett. 1999, 1, 997. (d) Zim, D.; de Souza, R. F.; Dupont, J.;
Monteiro, A. L. Tetrahedron Lett. 1998, 39, 7071. (e) Song, C. E.; Roh, E.
J. Chem. Commun. 2000, 837. Siriex, J.; Ossberger, M.; Betzemeier, B.;
Knochel, P. Synlett 2000, 1613.
1038
Org. Lett., Vol. 3, No. 7, 2001

especially the nature of the cation proved to be important
parameters. These new reaction conditions open an important
alternative to the use of toxic solvents. Studies are underway
to develop other significant applications to reduce toxic waste
by using ionic liquids.
nologie is gratefully acknowledged. We thank Stephan
Eberhard for his careful reading of the final manuscript.
Supporting Information Available: Spectroscopy data
and analytical data of compounds prepared in Table 1. This
material is available free of charge via the Internet at
http//pubs.acs.org.
Acknowledgment. Financial support from the Ministe`re
de l’Education Nationale de la Recherche et de la Tech-
OL015602H
Org. Lett., Vol. 3, No. 7, 2001
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