Bond Metathesis of Zirconacycles

Cp2ZrCI2+ Et3AI Cp2Zr x AIEt2— Cp2Zr( IEt2 

Cp2ZrCI2+ 3 EtsAI A AIEt2 Cp2Zr-Y + A ZrCp2 Cp2Zr-v 

In many other reactions of zirconacydes catalyzed by transition metal complexes containing Cu, Ni, Pd, etc., a-bond metathesis (transmetallation) must undoubtedly be involved, but such products have not generally been identified. Partly for this reason, they are not discussed here. Readers are referred to the chapter by T. Takahashi. 

Cp2ZrCI2+ Et3AI Cp2Zr x AIEfe— Cp2Zr( AIEfe 

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Although o-bond metathesis of five-membered zirconacycles with EtMgBr [51] (Scheme 1.4) and H2ZrCp2 (Scheme 1.68) has been implicated, there are as yet very few well-established examples. The reaction of zirconacyclopentanes with alkyllithiums is interesting since it involves (i) the displacement of one of the two Cp groups, and (ii) the generation of a bimetallic species, the NMR spectroscopic data of which are consistent only with a fluxional structure as shown in Scheme 1.71 [66]. 

In these early studies, however, the concept of c-bond metathesis most probably did not exist, and the results were presented just as observed facts. Mainly in the 1990s, a wide variety of c-bond metathesis reactions of both three- and five-membered zirconacycles were reported. In Scheme 1.4, the reaction of the five-membered zirconacycle with EtMgBr via c-bond metathesis followed by another c-bond metathesis (p-H abstraction) produces the ethylmagnesation product along with ethylene-zirconocene [51], Some representative examples of c-bond metathesis reactions of three-membered zirconacycles are shown in Scheme 1.69. These are examples of stoichiometric c-bond metathesis reactions from which the products have been identified. 

Scheme 1.69. Various examples of a-bond metathesis reactions of three-membered zirconacycles.

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