Zirconacycles skeletal rearrangements

Zirconacyclopentanes can readily undergo skeletal rearrangements, exemplified by those shown in Scheme 41. Although the process involving Zr reveals only the final zirconacycle, examination by NMR spectroscopy of the corresponding Hf reaction shows the formation and decay of the 2,5- and 2,4-dimethylhafnacyclopentanes. All of the three hafnacycles as well as the zirconacyclic product are >98% dl.229... 

Scheme 48 Skeletal rearrangement of zirconacycles via dipolar zirconates.

A few other interesting and potentially important consequences of the reversible formation of five-membered zirconacycles include stereo- and regioselective skeletal rearrangement, as exemplified by Scheme 1.57 [197], and 1,3-C=C bond and Zr migration (Scheme 1.58) [191,192], supporting the associative mechanism for alkene displacement (Generalization 22 ). 

The reaction of zirconacyclobutene-silacyclobutene fused compound 46 formed from Si-tethered diyne 45 with Cp2Zr(II) species has been used in the synthesis of fused aromatic heterocycles 47 (Scheme 11.18) [20]. When the zirconacycles 46 reacted with nitriles, the corresponding pyrrolo[3,2-c]pyridines 47 were formed. This indicates the unexpectedly strong effects of the alkynylsilyl groups on this unusual skeletal rearrangement of zirconacycles. 

Except the well-known olefin-metathesis protocol, generally, there is lack of preparative methods for four-membered metallacycles. Takahashi and coworkers reported formation of zirconacyclobutene-silacyclobutene fused ring compound via a novel skeletal rearrangement of zirconacycles (Eq. 14) [14]. The structure of this compound has been characterized by X-ray analysis [14b]. 

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