Zirconocene-mediated and/or catalyzed unprecedented coupling reactions of alkoxymethyl-substituted styrene derivatives

Reactions of o-(alkoxymethyl)styrene derivatives with a stoichiometric amount of zirconocene-butene complex (zirconocene equivalent, "Cp(2)Zr") brought about an insertion of the zirconocene species into a benzylic carbon-oxygen bond. The oxidative insertion of Cp(2)Zr to the benzylic carbon-oxygen bond is a result of sequential reactions: (i) formation of zirconacyclopropane by the ligand exchange with o-(alkoxymethyl)styrene, (ii) elimination of the alkoxy group through an aromatic conjugate system giving metalated o-quinodimethane species, and (iii) transfer of zirconium metal to the benzylic position. Through use of a catalytic amount of "Cp(2)Zr", however, unprecedented homo-coupling reactions (dimerization) of o-(alkoxymethyl)styrene derivatives occurred to give a tetracyclic compound. On the other hand, reactions of o-(1-alkoxyisopropyl)styrene derivatives gave rise to the analogous tetracyclic compounds regardless of the amount of "Cp(2)Zr" (stoichiometric or catalytic). Heterocoupling product between o-(1-alkoxyisopropyl)styrene and styrene congeners was obtained in high cis stereo- and regioselectivity by treating o-(1-alkoxyisopropyl)styrene derivatives with "Cp(2)Zr" in the presence of an excess amount of styrene derivatives.