ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Iron-Catalyzed Unexpected Easy Access to
Stereodefined Trimethylsilyl Vinyl Ketenes
Guobi Chai, Chunling Fu, and Shengming Ma*
Laboratory of Molecular Recognition and Synthesis, Department of Chemistry,
Zhejiang University, Hangzhou 310027, Zhejiang, P. R. China
Received June 12, 2012
ABSTRACT
Stereodefined trimethylsilyl vinyl ketenes with polysubstitution have been synthesized highly regio- and stereoselectively via an iron-catalyzed
reaction of 2-trimethylsilyl-2,3-allenoates with Grignard reagents in good to excellent yields. The reaction was believed to proceed via a conjugate
addition and elimination mechanism. Applications of the products for the synthesis of stereodefined R-silyl-β,γ-unsaturated enones, a
stereodefined triene, and polysubstituted phenols have been carefully demonstrated.
The utility of ketenes in organic synthesis has been well
demonstrated in the past decades.1 For example, vinyl
ketenes could serve as both two- and four-carbon building
blocks for the assembly of a variety of carbocyclic
systems.2 However, they are highly unstable and often
generated as intermediates for in situ trapping, which
greatly restrict their application. In 1980, Danheiser et al.
reported the synthesis of trialkylsilyl vinyl ketenes by
the dehydrohalogenation of R-silyl-R,β-unsaturated acid
chlorides (Scheme 1, route a).3 The presence of the silyl
group could stabilize vinyl ketenes4 and allow them to
serve as enophiles in [4 þ 2] cycloadditions and reactive
carbonyl compounds;5 in 1998, Danheiser et al. reported
the photochemical Wolff rearrangement of R0-silyl-R0-
diazo-R,β-unsaturated ketones to afford trialkylsilyl vinyl
ketenes (Scheme 1, route b);6a in the same paper,6a thistype
of vinyl ketenes was also prepared by heating the R-silyl
cyclobutenone (Scheme 1, route c).6 Moreover, treatment
of the Fischer-type chromium carbene complexes with
silyl-substituted alkynes could also afford such vinyl
ketenes (Scheme 1, route d).7 Herein, we present our recent
unexpected observation for the synthesis of stereodefined
trimethylsilyl vinyl ketenes via an iron-catalyzed reaction
of 2-trimethylsilyl-2,3-allenoates with Grignard reagents
(Scheme 1, route e).
(1) Tidwell, T. T. Ketenes II; Wiley: Hoboken, NJ, 2006.
(2) For some representative examples, see: (a) Day, A. C.;
McDonald, A. N.; Anderson, B. F.; Bartczak, T. J.; R. Hodder,
O. J. J. Chem. Soc., Chem. Commun. 1973, 247. (b) Danheiser, R. L.;
Martinez-Davila, C.; Sard, H. Tetrahedron 1981, 37, 3943. (c) Danheiser,
R. L.; Gee, S. K.; Sard, H. J. Am. Chem. Soc. 1982, 104, 7670. (d) Berge,
J. M.; Rey, M.; Dreiding, A. S. Helv. Chim. Acta 1982, 65, 2230. (e)
Jackson, D. A.; Rey, M.; Dreiding, A. S. Tetrahedron Lett. 1983, 24, 4817.
(f) Danheiser, R. L.; Gee, S. K. J. Org. Chem. 1984, 49, 1672. (g)
Danheiser, R. L.; Brisbois, R. G.; Kowalczyk, J. J.; Miller, R. F. J. Am.
Chem. Soc. 1990, 112, 3093. (h) Collomb, D.; Doutheau, A. Tetrahedron
Lett. 1997, 38, 1397.
(3) Danheiser, R. L.; Sard, H. J. Org. Chem. 1980, 45, 4810.
(4) For reviews of the chemistry of silylketenes, see ref 1 and (a)
Pommier, A.; Kocienski, P.; Pons, J.-M. J. Chem. Soc., Perkin Trans. 1
1998, 2105. (b) Pons, J.-M.; Kocienski, P. J. In Science of Synthesis;
Fleming, I., Ed.; Thieme: Stuttgart, 2001; Vol. 4, p 657. (c) George, D. M.;
Danheiser, R. L. In Science of Synthesis; Danheiser, R. L., Ed.; Thieme:
Stuttgart, 2006; Vol. 23, p 53.
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Tetrahedron Lett. 2008, 64, 915.
(6) (a) Loebach, J. L.; Bennett, D. M.; Danheiser, R. L. J. Org. Chem.
1998, 63, 8380. (b) Benda, K.; Knoth, T.; Danheiser, R. L.; Schaumann,
E. Tetrahedron Lett. 2011, 52, 46.
€
(7) (a) Dotz, K. H. Angew. Chem., Int. Ed. Engl. 1979, 18, 954. (b)
€
€
€
Dotz, K. H.; Fugen-Koster, B. Chem. Ber. 1980, 113, 1449. (c) Wulff,
W. D.; Tang, P.-C. J. Am. Chem. Soc. 1984, 106, 1132. (d) Wulff, W. D.;
Xu, Y.-C. J. Org. Chem. 1987, 52, 3263. (e) Moser, W. H.; Sun, L.;
Huffman, J. C. Org. Lett. 2001, 3, 3389. (f) Li, Z.; Moser, W. H.; Zhang,
W.; Hua, C.; Sun, L. J. Organomet. Chem. 2008, 693, 361.
(8) (a) Tsuji, J.; Sugiura, T.; Minami, I. Tetrahedron Lett. 1986, 27,
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10.1021/ol3016176
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