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

Baar, Cliff R. and Levy, Christopher J. and Min, Endy Y.-J. and Henling, Lawrence M. and Day, Michael W. and Bercaw, John E. (2004) Kinetic Resolution of Chiral α-Olefins Using Optically Active ansa-Zirconocene Polymerization Catalysts. Journal of the American Chemical Society, 126 (26). pp. 8216-8231. ISSN 0002-7863. https://resolver.caltech.edu/CaltechAUTHORS:20170420-063814924

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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.


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URLURL TypeDescription
http://dx.doi.org/10.1021/ja040021jDOIArticle
http://pubs.acs.org/doi/abs/10.1021/ja040021jPublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/ja040021jPublisherSupporting Information
Additional Information:© 2004 American Chemical Society. Received 16 January 2004. Published online 9 June 2004. Published in print 1 July 2004. This work was been supported by the NSF (Grant No. CHE-0131180). We would like to thank Richard E. Marsh for his assistance in solving the structure of (S)-2 and Professor Malcolm Green for suggesting the use of thiolate ligands. C.R.B. and C.J.L. would like to thank NSERC (Canada) for postdoctoral fellowships.
Funders:
Funding AgencyGrant Number
NSFCHE-0131180
Natural Sciences and Engineering Research Council of Canada (NSERC)UNSPECIFIED
Issue or Number:26
Record Number:CaltechAUTHORS:20170420-063814924
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170420-063814924
Official Citation:Kinetic Resolution of Chiral α-Olefins Using Optically Active ansa-Zirconocene Polymerization Catalysts Cliff R. Baar, Christopher J. Levy, Endy Y.-J. Min, Lawrence M. Henling, Michael W. Day, and John E. Bercaw Journal of the American Chemical Society 2004 126 (26), 8216-8231 DOI: 10.1021/ja040021j
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76743
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:20 Apr 2017 16:24
Last Modified:03 Oct 2019 17:49

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