Ketones Tebbe reagent

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. 

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

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... 

Methyl ketones. The Tebbe reagent (1) reacts with acyl chlorides to form methyl ketones in 35-65% yield (equation I). That an intermediate titanium enolate may be involved is indicated because addition of methyl iodide before aqueous work-up results in formation of an ethyl ketone. 

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). ... 

The Tebbe reagent (4.85) gives better yield of alkenes with sterically hindered ketones than the Wittig reagent. ... 

In compounds containing both ketone and ester groups, the ketone selectively reacts in the presence of 1 equiv. of the Tebbe reagent (4.85), but with excess amount of the Tebbe reagent (4.85) both carbonyl groups are alkenated. 

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. 

In Section 10-2-2, we discussed the synthesis of Tebbe s reagent, a bridged Ti methylidene complex. This reagent converts a C=0 group to an alkene in a manner analogous to that of a phosphorus ylide (Wittig reagent).56 Conversion of a carbonyl to a terminal alkene may seem to be of limited utility until one realizes that, unlike phosphorus ylides that react only with aldehydes and ketones, Tebbe s... 

The Petasis reagent reacts with aldehydes, ketones, and esters to afford terminal olefins. In addition, tliis reagent enjoys greater air stability than the Tebbe reagent. 

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