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Zirconocene-Mediated and/or Catalyzed Unprecedented Coupling Reactions of Alkoxymethyl-Substituted Styrene Derivatives

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posted on 27.05.2005, 00:00 by Yutaka Ikeuchi, Takeo Taguchi, Yuji Hanzawa
Reactions of o-(alkoxymethyl)styrene derivatives with a stoichiometric amount of zirconocene−butene complex (zirconocene equivalent, “Cp2Zr”) brought about an insertion of the zirconocene species into a benzylic carbon−oxygen bond. The oxidative insertion of Cp2Zr 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 “Cp2Zr”, 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 “Cp2Zr” (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 “Cp2Zr” in the presence of an excess amount of styrene derivatives.