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    Enantioselective polymerization of epoxides using biaryl-linked bimetallic cobalt catalysts: A mechanistic study

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    Type
    Article
    Authors
    Ahmed, Syud M.
    Poater, Albert cc
    Childers, M. Ian
    Widger, Peter C B
    Lapointe, Anne M.
    Lobkovsky, Emil B.
    Coates, Geoffrey W.
    Cavallo, Luigi cc
    KAUST Department
    Biological and Environmental Sciences and Engineering (BESE) Division
    Chemical Science Program
    KAUST Catalysis Center (KCC)
    Physical Science and Engineering (PSE) Division
    KAUST Grant Number
    KUS-C1-018-02
    Date
    2013-11-07
    Online Publication Date
    2013-12-03
    Print Publication Date
    2013-12-18
    Embargo End Date
    2014-11-07
    Permanent link to this record
    http://hdl.handle.net/10754/563152
    
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    Abstract
    The enantioselective polymerization of propylene oxide (PO) using biaryl-linked bimetallic salen Co catalysts was investigated experimentally and theoretically. Five key aspects of this catalytic system were examined: (1) the structural features of the catalyst, (2) the regio- and stereoselectivity of the chain-growth step, (3) the probable oxidation and electronic state of Co during the polymerization, (4) the role of the cocatalyst, and (5) the mechanism of monomer enchainment. Several important insights were revealed. First, density functional theory (DFT) calculations provided detailed structural information regarding the regio- and stereoselective chain-growth step. Specifically, the absolute stereochemistry of the binaphthol linker determines the enantiomer preference in the polymerization, and the interaction between the salen ligand and the growing polymer chain is a fundamental aspect of enantioselectivity. Second, a new bimetallic catalyst with a conformationally flexible biphenol linker was synthesized and found to enantioselectively polymerize PO, though with lower enantioselectivity than the binaphthol linked catalysts. Third, DFT calculations revealed that the active form of the catalyst has two active exo anionic ligands (chloride or carboxylate) and an endo polymer alkoxide which can ring-open an adjacent cobalt-coordinated epoxide. Fourth, calculations showed that initiation is favored by an endo chloride ligand, while propagation is favored by the presence of two exo carboxylate ligands. © 2013 American Chemical Society.
    Citation
    Ahmed, S. M., Poater, A., Childers, M. I., Widger, P. C. B., LaPointe, A. M., Lobkovsky, E. B., … Cavallo, L. (2013). Enantioselective Polymerization of Epoxides Using Biaryl-Linked Bimetallic Cobalt Catalysts: A Mechanistic Study. Journal of the American Chemical Society, 135(50), 18901–18911. doi:10.1021/ja409521z
    Sponsors
    We thank the NSF (CHE-1136607 and CHE-1112278) and the King Abdullah University of Science and Technology (KAUST; Award KUS-C1-018-02) for financial support. We also thank Dr. Ivan Keresztes and Mr. Anthony Condo for assistance with NMR experiments to confirm the structure of 8. A.P. thanks the Spanish MINECO for a Ramon y Cajal contract (RYG-2009-5226), European Commission for a Career Integration Grant (CIG09-GA-2011-293900), and Generalitat de Catalunya (2012BE100824).
    Publisher
    American Chemical Society (ACS)
    Journal
    Journal of the American Chemical Society
    DOI
    10.1021/ja409521z
    Additional Links
    https://doi.org/10.1021/ja409521z
    Relations
    Is Supplemented By:
    • [Dataset]
      Ahmed, S. M., Poater, A., Childers, M. I., Widger, P. C. B., LaPointe, A. M., Lobkovsky, E. B., … Cavallo, L. (2014). CCDC 1001988: Experimental Crystal Structure Determination [Data set]. Cambridge Crystallographic Data Centre. https://doi.org/10.5517/cc12mn6k. DOI: 10.5517/cc12mn6k HANDLE: 10754/624318
    • [Dataset]
      Ahmed, S. M., Poater, A., Childers, M. I., Widger, P. C. B., LaPointe, A. M., Lobkovsky, E. B., … Cavallo, L. (2014). CCDC 1001989: Experimental Crystal Structure Determination [Data set]. Cambridge Crystallographic Data Centre. https://doi.org/10.5517/cc12mn7l. DOI: 10.5517/cc12mn7l HANDLE: 10754/624319
    ae974a485f413a2113503eed53cd6c53
    10.1021/ja409521z
    Scopus Count
    Collections
    Articles; Biological and Environmental Science and Engineering (BESE) Division; Physical Science and Engineering (PSE) Division; Chemical Science Program; KAUST Catalysis Center (KCC)

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