Divalent <i>ansa</i>-Zirconocenes:  Stereoselective Synthesis and High Activity for Propylene Polymerization

The reduction of ZrCl₄(PR₃)₂ with Li powder, in the presence of a stoichiometric amount of <i>trans</i>-1,4-diphenyl-1,3-butadiene, affords the Zr(II) diene complexes (<b>1</b>) in 90−93% yields. This reaction consists of a rate-limiting step for the formation of the chloride-bridged Zr(III) dimer (<b>2</b>) and a fast diene-driven disproportionation of <b>2</b> to <b>1</b> and ZrCl₄(PR₃)₂ that re-enters the reduction cycle. The reaction of <b>1</b> with Li₂{Me₂Si(2-Me-4-Ph-Ind)₂} in toluene produces quantitatively the desired racemic, divalent <i>ansa</i>-zirconocene (<b>3</b>) that is a highly active isospecific propylene polymerization catalyst upon activation with common activators.