Poly(styrene)-supported co-salen complexes as efficient recyclable catalysts for the hydrolytic kinetic resolution of epichlorohydrin |
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Authors: | Zheng Xiaolai Jones Christopher W Weck Marcus |
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Affiliation: | School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. |
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Abstract: | Here we describe an unprecedented synthetic approach to poly(styrene)-supported chiral salen ligands by the free radical polymerization of an unsymmetrical styryl-substituted salen monomer (H2salen = bis(salicylidene)ethylenediamine). The new method allows for the attachment of salen moieties to the polymer main chain in a flexible, pendant fashion, avoiding grafting reactions that often introduce ill-defined species on the polymers. Moreover, the loading of the salen is controlled by the copolymerization of the styryl-substituted salen monomer with styrene in different ratios. The polymeric salen ligands are metallated with cobalt(II) acetate to afford the corresponding supported Co-salen complexes, which are used in the hydrolytic kinetic resolution of racemic epichlorohydrin, exhibiting high reactivity and enantioselectivity. Remarkably, the copolymer-supported Co-salen complexes showed a better catalytic performance (>99 % ee, 54 % conversion, one hour) in comparison to the homopolymeric analogues and the small molecule Co-salen complex. The soluble poly(styrene)-supported catalysts were recovered by precipitation after the catalytic reactions and were recycled three times to afford almost identical enantiomeric excesses as the first run, with slightly reduced reaction rates. |
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