Titanium carbene complex, Tebbe methylenation

Exchange of the oxygen atom of a carbonyl function for the methylene group of the proposed titanium carbene complex (the Tebbe reagent) to yield terminal alkenes ... 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

A few years later, Tebbe and co-workers found that the methylene-bridged metallacycle 3, which has become known as the Tebbe reagent, is useful for the methylenation of ketones and aldehydes [5]. Titanocene-methylidene 4, the active species of this olefination, also transforms carboxylic acid derivatives into heteroatom-substituted olefins. Because the carbene complex 4 is much less basic than conventional olefination reagents such as phosphorus ylides, it can be employed for the olefination of carbonyl compounds possessing highly acidic a-protons or of highly hindered ketones, and has become an indispensable tool in organic synthesis. Various methods for the preparation of titaniumcarbonyl olefination. This chapter focuses on the use of metal-carbene complexes and some related species in carbonyl olefination (Scheme 4.2). 

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