Titanium catalysts alkylidenation

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

The reduction of Cp2TiCl2 with Mg in the presence of P(OEt)3 affords the titanium(n) complex Cp2Ti[P(OEt)3]2 which was used as a catalyst for the carbonyl olefination of thioacetals through a titanium-alkylidene intermediate (see Section 4.05.4.2.4 alkylidene complexes).1148... 

It was found that titanacyclobutanes could be prepared by removing the aluminum from 3 with a strong Lewis base in the presence of an olefin, as shown in Eq. 9. These metallacycles were stable with respect to rearrangement to an olefin via a 3 hydride process, and showed a tendency to generate intermediate alkylidene complexes in situ. They ultimately proved to be useful as catalysts for the ring opening metathesis polymerization of a variety of strained olefins. [47,48] It should be noted that the masked titanium methylene complex 3 (Eq. 

The method has been extended to a general synthesis of a-alkylidene- and a-alkyl-ketones. The same group describes the reaction of allylic and benzylic halides with silyl enol ethers in the presence of catalytic quantities of zinc(ii) bromide. Other workers also report the use of secondary benzylic halides, allylic halides, and diethyl thioacetals as alkylating agents in the presence of Lewis acids. t-Alkyl halides also alkylate these systems with titanium(iv) chloride as catalyst. ... 

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