Synthetic and mechanistic studies into the kinetic resolution of alpha-olefins using C1- and C2- symmetric zirconocene polymerization catalysts

摘要

Mechanistic and synthetic investigations into the kinetic resolution of racemic alpha-olefins by polymerization catalysis using C1- and C2-symmetric zirconocenes are reported. The importance of chain end control as a stereocontrol element was probed with ethylene and propylene copolymerizations catalyzed by the C1-symmetric catalyst, {1,2-(SiMe2)2(eta-5-3,5-C5H1(CHMe2)2)(eta-5-4-C5H2((S)-CHMeCMe3)]}ZrCl2/MAO. Selectivity factors and pentad analysis of racemic alpha-olefin and propylene polymerizations catalyzed by a similar C1-symmetric catalyst, {1,2-(SiMe2)2(eta-5-3,5-C5H1(CHMe2)2)(eta-5-4-C5H2((S)-CHEtCMe3)]}ZrCl2/MAO, indicate that site epimerization does not limit selectivity during kinetic resolution.To avoid some of the issues involved with the C1-symmetric catalysts, a route to enantiopure C2-symmetric zirconocenes was pursued. With the aid of the chiral auxiliary, (R)-N2,N2’-di-p-tolyl-1,1’-binaphtyl-2,2’-diamine, enantiospecefic synthesis of (S,S)-{C2H4-1,2-(1-indene)2}ZrCl2 was accomplished and its use for kinetic resolution was investigated. Although synthetically useful selectivities were not observed, it was determined that the C2-symmetric catalyst does not racemize during polymerization, which substantiates a more thorough investigation of catalysts based on {C2H4-1,2-(1-indene)2}ZrCl2.