Kinetic Resolution of Chiral α-Olefins Using Optically Active ansa-Zirconocene Polymerization Catalysts

Abstract

A series of enantiopure C_1-symmetric metallocenes, {(SiMe_2)_2[η^5-C_5H(CHMe_2)_2][η^5-C_5H_2((S)-CHMeCMe_3)]}ZrCl_2, (S)-2, {(SiMe_2)_2[η^5-C_5H(CHEt_2)_2][η^5-C_5H_2((S)-CHMeCMe_3)]}ZrCl_2, (S)-6, and {(SiMe_2)_2[η^5-C_5HCy_2][η^5-C_5H_2((S)-CHMeCMe_3)]}ZrCl_2, (S)-7 (Cy = cyclohexyl), zirconocene dichlorides that have an enantiopure methylneopentyl substituent on the “upper” cyclopentadienyl ligand, and diastereomerically pure precatalysts, {(SiMe_2)_2[η^5-C_5H((S)-CHMeCy)(CHMe_2)][η^5-C_5H_3]}ZrCl_2, (S)-8a and (S)-8b, which have an enantiopure, 1-cyclohexylethyl substituent on the “lower” cyclopentadienyl ligand, has been synthesized for use in the polymerization of chiral α-olefins. When activated with methylaluminoxane, these metallocenes show unprecedented activity for the polymerization of bulky racemic monomers bearing substitution at the 3- and/or 4-positions. Due to the optically pure nature of these single site catalysts, they effect kinetic resolution of racemic monomers:  the polymeric product is enriched with the faster reacting enantiomer, while recovered monomer is enriched with the slower reacting enantiomer. The two components are easily separated. For most olefins surveyed, a partial kinetic resolution was achieved (s = k_(faster)/k_(slower) ≈ 2), but, in one case, the polymerization of 3,4-dimethyl-1-pentene, high levels of separation were obtained (s > 15). ^(13)C NMR spectroscopy of poly(3-methyl-1-pentene) produced with (S)-2 indicates that the polymers are highly isotactic materials. X-ray crystal structure determinations for (S)-2, {(SiMe_2)_2[η^5-C_5H(CHMe_2)_2][η^5-C_5H_2((S)-CHMeCMe_3)]}Zr(SC_6H_5)_2, (S)-6, and (S)-7 have been used in combination with molecular mechanics calculations to examine the prevailing steric interactions expected in the diastereomeric transition states for propagation during polymerization. Precatalysts (S)-8a and (S)-8b are less selective polymerization catalysts for the kinetic resolution of 3-methyl-1-pentene than are (S)-2, (S)-6, and (S)-7.