Metallacyclopentanes, Metallacyclopentenes, and Metallacyclopentadienes

There have been many reports on the preparation and applications of five-membered zirconacycles, namely, zirconacyclopentanes, zirconacyclopentenes and zirconacyclopentadienes [1-5]. 

For the preparation of 5-membered zirconacycles, although it is possible to use di-Grignard or dilithium reagents for the preparation of zirconacyclopentanes [ 10], the reductive coupling of unsaturated organic substrates, either intramolecular or intermolecular, on a zirconocene(II) species is the most common way. 

Compared with symmetrical metallacyclopentadienes, unsymmetrical met-allacyclopentadienes from two different alkynes are not always easily prepared. Several methods have been reported for preparation of unsymmetrical zir-conacyclopentadienes in which the key step is the addition of a second alkyne to zirconocene-alkyne complexes generated in situ [20]. Formation of homocoupling products from the same alkyne is often the biggest problem (Eq. 21). 

Cyclic gem-metallozirconocene compounds such as a-lithiated zirconacyclopentadienes, a-stannyl or gallio-zirconacyclopentenes, and a,o -bis(trimethylstannyl) zirconacyclopentadienes have been prepared (Fig. 3) [21-25]. These multifunctionalized zirconacycles can be expected to have useful applications in organic synthesis. 

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