Titanocene complexes, Tebbe methylenation

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... 

Titanocene methylene-Zinc iodide complex, CpjTiCHj ZnI, (1). The reagent is prepared by reaction of CpjTiCE with CHjfZnl), in THF. It is related to the Tebbe reagent (8, 83-84 10, 87-88), but is easier to prepare. 

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). 

The readion of titanocene dichloride with the methylene di-Grignard reagent affords the organotitanium species 14, which behaves like the Tebbe reagent 14 reacts with olefins to produce various titanacydobutanes 11 [47]. The interesting spiro metallacycle 15, which is formed from two equivalents of 14, reacts with benzophenone to produce 1,1-diphenylethylene (80% yield) and forms the titanocene-methylidene complex 16 on treatment with trimethyl phosphine (Scheme 4.14) [48]. 

Grubbs reagents, titanacycles 50 and 51 were prepared by the reaction of Tebbe reagent with a terminal alkene in the presence of a Lewis base. When these complexes are heated, reactive titanocene methylidene 5 is regenerated and will methylenate carbonyl group (49 to 53). ... 

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