Alkene titanacyclobutane

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. 

Reaction of the Tebbe reagent with 3,3-dimethylcyclopropeneand4-(dimethylamino)pyridine (DMAP) affords the titanacycle 1 in 33% isolated yield. Alternatively, 1 can be prepared by alkene exchange with other titanacyclobutanes. Acidolysis of 1 with hydrochloric acid gives a 89% yield of 1,1,2-trimethylcyclopropane. ... 

Titanacyclobutanes 7 can be synthesized from Tebbe s reagent 6 with alkenes. These too give cylopropanes 8 in near quantitative yields upon treatment of the complex 7 with alkynes under photochemical conditions or thermally in the presence of oxidants (e.g. iodine, quinones, etc.), cyclopentadienones or allylpalladium complexes. ... 

Complex 17 shows versatile reactivity in both catalytic and stoichiometric reactions. It is a catalyst for olefin metathesis, in which a metallacyclobutane is proposed to be a key intermediate [52]. Grubbs et al. successfully isolated titanacyclobutanes 18 by treatment of 17 with alkenes in the presence of a Lewis base such as dimethylaminopy-ridine [56,57]. When cyclic olefins such as norbornene are treated with a catalytic amount of 18, ring opening metathesis polymerization occurs [58]. 

The four membered titanacyclobutane ring is nearly planar. Studies of Tebbe s compound are also of interest in connection with Ziegler-Natta polymerization of alkenes. Commercial catalysts incorporate TiCl and aluminium alkyls (p. 371). 

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