Ketones Tebbe methylenation

CPv. / Ti Al Tebbe reagent methylenation of aldehydes, ketones, esters, thioesters, amides, carbonates can also induce olefin metathesis in situ preparation [711-713]... 

Eisch and Piotrowski reported the preparation of gem-dizinc compounds from diiodomethane and zinc powder in the titanocene chloride mediated methylenation of ketones in 1983 (equation 5)7. In this case, it was not mentioned that they had used pyrometallurgy zinc. The Tebbe-type reagent 2 was shown as an intermediary species. Before addition of titanocene chloride, the amount of methane was measured after hydrolysis of the reaction mixture to determine the formation of gem-dizinc species7. 

Methylenation.1 When heated in THF at 60-65°, this reagent effects methy-lenation of ketones, even readily enolizable ones, in 60-90% yield and of aldehydes (45-60% yield). It also converts esters and lactones into enol ethers, but this reaction is generally slower. It is thus an attractive alternative to the Tebbe and Grubbs reagent. 

Methylenation of ketones. The Tebbe reagent (1) or the unstable 3,pdisubstituted titanacyclobutane 2 methylenate even easily enolizable ketones rapidly at room temperature. Yields are generally high except in reactions with a very hindered ketone. 

The methylenation of ketones and aldehydes by the Wittig reaction is a well-established and selective methodology. Unlike addition-elimination methods of alkene formation, the Wittig proceeds in a defined sense, producing an alkene at the original site of the carbonyl. The Wittig reaction is not considered here, but is used as the standard by which the methods discussed are measured. The topics covered in the methylenation sections include the Peterson alkenation, the Johnson sulfoximine approach, the Tebbe reaction and the Oshima-Takai titanium-dihalomethane method. 

In 1973, Johnson reported the use of (N-methylphenylsulfonimidoyl)methyllithium (26) for addition to carbonyls, followed reductive elimination to produce the methylene derivative (28 Scheme 7). As with the Tebbe and Oshima procedures discussed in Sections 3. l.S and 3.1.6, this method can be applied to enones, ketones and, with comparatively diminished efficiency, aldehydes. The anion appears to be more nucleophillic than methylenetriphenylphosphorane, and there are several examples, detailed below, in which the Wittig reaction failed but the Johnson procedure succeeded. The addition and reductive cleavage can be combined into a single operation without isolation of the p-hydroxysulfoximine. ... 

A reaction that appears to be mechanistically similar to the Tebbe reaction was developed by Oshima in 1978. Diiodomethane or dibromomethane in the presence of zinc is treated with a Lewis acid to form, presumably, a divalent complex (72), which reacts with aldehydes and ketones to produce the corresponding methylene derivative (73 Scheme 18). This reagent complements the reactivity of the Tebbe reagent, in that the zinc methylenation is not reactive towards esters or lactones. Because it is an electrophilic reagent, it is suitable for the methylenation of enolizable ketones and aldehydes. 

R = OMe) was methylenated with the Tebbe reagent to give the same ketone... 

Hydroxytetrahydrofurans. Triisobutylaluminum acts both as a Lewis acid to catalyze the isomerization of 4-methylene-1,3-dioxolanes to 2-oxotetrahydrofurans (via C-O bond cleavage and C-C bond formation) and as a reducing agent for the ketone group of the intermediate. The 4-methyIene-1,3-dioxolanes are available by a Tebbe olefination. 

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