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Indexed by:期刊论文

Date of Publication:2012-06-01

Journal:CHEMICAL SCIENCE

Included Journals:SCIE、EI、Scopus

Volume:3

Issue:6

Page Number:2094-2102

ISSN No.:2041-6520

Abstract:The kinetic resolution of racemic terminal epoxides with CO2 as reagent via enantioselective coupling represents an attractive method for affording enantiopure epoxides and optically active organic carbonates. A multichiral cobalt(III) complex in conjunction with an ammonium salt is an efficient catalyst system for the asymmetric coupling reaction of CO2 and racemic epoxides, and up to 97.1% ee for cyclic carbonate product and the highest k(rel) (kinetic resolution coefficient) to date of 75.8 was obtained. The variation of nucleophilic co-catalyst and its relative loading dramatically changes the product selectivity and enantioselectivity. Both the anion and cation of ammonium salts have significant effects on the catalytic kinetic resolution process. An ammonium salt consisting of an anion with poor leaving ability and a bulky cation benefits for improving the enantioselectivity. The excess co-catalyst loading favors selective production of cyclic carbonate via the intramolecular cyclic elimination of the formed linear carbonate. A higher k(rel) was observed in the excess co-catalyst loading, in comparison with one equivalent co-catalyst loading, predominantly resulting in polymer formation at the same temperature. It was also found that the excess co-catalyst loading led to significant increases in the regioselective ring-opening at the methylene carbon of various terminal epoxides, including styrene oxide with an electron-withdrawing group. The present study also offers a detailed mechanistic explanation of the role of the nucleophilic co-catalyst in the asymmetric coupling of racemic epoxides with CO2 using multichiral Co(III) complexes.