Ketones Tebbe reaction

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. 

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. 

Titanacyclobutenes, prepared readily from Tebbe reagent and alkynes, react with aldehydes and ketones to form insertion products which undergo facile retro-Diels-Alder reaction to afford substituted 1,3-dienes (equation 107)185. 

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. 

Another interesting feature of Tebbe s reagent is its capability for converting ketones to terminal olefins. For example, reaction with cyclo-... 

The Wittig reaction is carried out under strongly basic conditions, and is not possible with the five-membered keto ester 195, because an enolate of the ketone is formed. The Tebbe reagent, as described later, reacts with both esters and ketones to give a mixture of products. Selective reaction of the ketone in 195 without attacking... 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

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. 

McMurry coupling, Tebbe reagent, Petasis reagent and Takeda reagent) and catalytic alke-nation of aldehydes and ketones plus the related reactions in each case. 

The Tebbe reagent (4.85) converts aldehydes and ketones to alkenes. The reaction of the Tebbe reagent (4.85) with cyclohexanones in toluene produces the corresponding methylenecyclohexanes in >65% yields. For example (4-methylenecyclohex-l-yl)benzene (4.88) was prepared jfrom 4-phenylcyclohexanone (4.87) in 96% crude yield on reaction with the Tebbe reagent (4.85). ... 

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