Synthetic applications of organozirconocenes
Alkenyl- and alkylzirconocenes are readily available by hydrozirconation of alkynes and alkenes, respectively. Due to the relatively low nucleophilicity of the carbon-zirconium bond in these organometallics, transmetalation schemes have been developed for carboncarbon bond formations. Especially, in siru transmetalation to copper(1) and zinc(II) species can be used for selective alkylation reactions with organic electrophiles. Alternatively, conversion of organozirconocenes to the corresponding cationic complexes enhances their reactivity considerably. Synthetic applications include protocols for orthoester formations and Diels-Alder catalysis as well as natural products (Curacin A, Manumycin) synthesis. Since the preparation of the first organochlorobis(cyclopentadienyl)zirconium(W) complex by Wailes, Weigold and Bell 25 years ago (ref. l), the application of these organometallics for carbon-carbon and carbon-heteroatom bond formations has become an important aspect of synthetic strategy and tactics. Clearly, this is due to the relative ease of preparation of alkenyl- and alkylzirconocenes by hydrozirconation of alkynes and alkenes with Cp2Zr(H)C1 (Schwartz reagent) (ref. 2). In spite of a polarization of the carbon- zirconium bond similar to Grignard reagents, the bulky cyclopentadienyl groups sterically prevent the attack of many organic electrophiles. The resulting functional group tolerance of organozirconocenes is useful for the preparation of the organometallic species, but this low reactivity must be overcome for subsequent selective carbon-carbon bond formations. Much of the development of the chemistry of organozirconocenes has therefore focused on indirect reaction pathways involving transmetalation or activation by ligand abstraction.