[89121-60-8]  · C6H11ClOSi  · Chloro(trimethylsilylmethyl)ketene  · (MW 162.67)

(intermediate capable of cycloadding to activated alkenes1 for direct conversion to a-methylenecyclobutanones and -cyclopentanones2)

Solubility: sol nonpolar organic solvents usually employed for [2 + 2] cycloadditions.

Form Supplied in: generated in situ by dehydrochlorination of the acid chloride precursor.

Chloro(trimethylsilylmethyl)ketene2 presently stands as the preferred synthetic equivalent to the very unstable methyleneketene (CH2=C=C=O)3 in [2 + 2] cycloaddition reactions. Generation of the title ketene is achieved by conversion of b-(trimethylsilyl)propionic acid4 to its acyl chloride, subsequent a-chlorination with N-Chlorosuccinimide in thionyl chloride as solvent at 70 °C,5 and ultimate dehydrochlorination with Triethylamine (eq 1).2 If an adequately reactive acceptor alkene is not present, the ketene is captured by the acid chloride to produce the ester shown.

With cyclopentadiene, vinyl ethers, and silyl enol ethers, smooth [2 + 2] cycloaddition takes place in both a regio- and stereocontrolled fashion. In a manner paralleling that observed with alkyl-substituted ketenes,6 chloro(trimethylsilylmethyl)ketene is captured with the Me3SiCH2 group projected into the more sterically crowded endo environment; the chlorine atom finds itself exo oriented (eq 2).2 Although the silicon atom is sterically congested, elimination in the presence of fluoride ion from Tetra-n-butylammonium Fluoride proceeds rapidly in DMSO solution at 20 °C to give the annelated 2-methylenecyclobutanone derivative. Treatment of the initial adduct with ethereal Diazomethane results in ring expansion via 1,2-shift of the nonhalogenated carbon atom.7 Comparable treatment of this product with F- leads to the methylenecyclopentanone.

The same chemical behavior is exhibited by vinyl ethers, as illustrated for the dihydropyran adduct in (eq 3).2 As seen before, the high level of a-substitution does not deter attack by diazomethane. Attempts to accomplish analogous Baeyer-Villiger ring expansions have failed.

When silyl ethers are the co-reactants, more highly functionalized, angularly hydroxylated products are obtained following mild acidic hydrolysis (eq 4).

Although methyl(phenylthiomethyl)ketene enters satisfactorily into analogous [2 + 2] cycloaddition, the conditions required for reductive elimination appear too vigorous for implementation with polyfunctional molecules.8,9

Related Reagents.

Dichloroketene; Diphenylketene.

1. Patai, S. The Chemistry of Ketenes, Allenes, and Related Compounds; Wiley: New York, 1980; Vol. 1, pp 757-778.
2. Paquette, L. A.; Valpey, R. S.; Annis, G. D. JOC 1984, 49, 1317.
3. Brown, R. F. C.; Eastwood, F. W.; McMullen, G. L. JACS 1976, 98, 7421.
4. Fleming, I.; Goldhill, J. JCS(P1) 1980, 1493.
5. Harpp, D. N.; Bao, L. Q.; Black, C. J.; Gleason, J. G.; Smith, R. A. JOC 1975, 40, 3420.
6. (a) Rey, M.; Roberts, S.; Dieffenbacher, A.; Dreiding, A. S. HCA 1970, 53, 417. (b) Brady, W. T.; Roe, R. JACS 1970, 92, 4618. (c) Brook, P. R.; Harrison, J. M.; Duke, A. J. CC 1970, 589.
7. (a) Greene, A. E.; Deprés, J.-P. JACS 1979, 101, 4003. (b) Deprés, J.-P.; Greene, A. E. JOC 1980, 45, 2036. (c) Annis, G. D.; Paquette, L. A. JACS 1982, 104, 4504. (d) Au-Yeung, B.-W.; Fleming, I. CC 1977, 81.
8. (a) Hassner, A.; Pinnick, H. W.; Ansell, J. M. JOC 1978, 43, 1774. (b) Mubarik Ali, S.; Roberts, S. M. CC 1975, 887.
9. Minami, T.; Ishida, M.; Agawa, T. CC 1978, 12; JOC 1979, 44, 2067.

Leo A. Paquette

The Ohio State University, Columbus, OH, USA

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