Kinetic Resolution of Chiral ∝-Olefins Using Enantiopure Ziegler-Natta Polymerization Catalysts

Citation

Min, Endy Yeo-Jung (2006) Kinetic Resolution of Chiral ∝-Olefins Using Enantiopure Ziegler-Natta Polymerization Catalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZD0X-9Y24. https://resolver.caltech.edu/CaltechETD:etd-11162005-125732

Abstract

Towards the goal of kinetic resolution of chiral olefins, a series of enantiopure C1 symmetric metallocenes has been synthesized for use in the polymerization of chiral olefins. The new precatalysts were based on the parent precatalyst {(SiMe2)2[[eta]5-C5H(CHMe2)2][[eta]5-C5H2((S)-CHMeCMe3)]}ZrCl2, (S)-2, which has a doubly, silylene-linked ligand framework. The new precatalysts include {(SiMe2)2[[eta]5-C5H(CHEt2)2][[eta]5-C5H2((S)-CHMeCMe3)]}ZrCl2, (S)-3, {(SiMe2)2[[eta]5-C5H(CHCy2)2][[eta]5-C5H2((S)-CHMeCMe3)]}ZrCl2, (S)-4 (Cy = cyclohexyl), {(SiMe2)2[[eta]5-C5H(CHTMS2)2][[eta]5-C5H2((S)-CHMeCMe3)]}ZrCl2, (S)-5 (TMS = trimethylsilyl), and {(SiMe2)2[[eta]5-C5H(CHMe2)2][[eta]5-C5H2((S)-CHEtCMe3)]}ZrCl2, (S)-6. The zirconocene dichlorides (S)-2, (S)-3, (S)-4, and (S)-5 have an enantiopure 3,3-dimethyl-2-butyl ("methylneopentyl") substituent on the "upper" cyclopentadienyl ligand. The zirconocene dichloride (S)-6 has an enantiopure 2,2-dimethyl-3-pentyl ("ethylneopentyl") substituent on the "upper" cyclopentadienyl ligand. When activated with methylaluminoxane (MAO), these metallocenes show unprecedented activity for the polymerization of racemic monomers bearing substitution at the 3- and/or 4-positions. In addition, due to the optically pure nature of these single site catalysts, polymerization of racemic monomers serves as a transition metal mediated kinetic resolution strategy. The polymeric product is enriched with the faster reacting enantiomer, while the recovered monomer is enriched with the slower reacting enantiomer. The two components are easily separated, thus affecting the resolution. A modest kinetic resolution was achieved (s = kfaster/kslower = ca. 2) with most olefins surveyed. In the case of 3,4-dimethyl-1-pentene and 3,4,4-trimethyl-1-pentene, high levels of separation were obtained (s > 12). X ray crystal structure determinations for (S)-2, (S)-3, and (S)-4 have been used to examine the prevailing steric interactions expected in the diastereomeric transition states for propagation during polymerization. In comparison to (S)-2, slight improvements in the selectivity of 3-methyl-1-hexene and 3,5,5-trimethyl-1-hexene were observed with polymerizations using (S)-3. Likewise, the polymerizations of 3-methyl-1-pentene and 3,5,5-trimethyl-1-hexene using (S)-6 showed a modest increase in selectivity, relative to (S)-2. The kinetic resolution of chiral olefins containing a polar functionality also has been attempted with (S)-2. Although the selectivity of these polymerization experiments is yet to be determined, preliminary work indicates that NMR can be used to analyze the (S)-Mosher esters of the olefins to obtain the enantiomeric excess.

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