Polymer-Supported (Salen)Mn Catalysts for Asymmetric Epoxidation: A Comparison between Soluble and Insoluble Matrices

Abstract

This paper describes the synthesis of both soluble and insoluble polymer-supported chiral (salen)Mn complexes and their use in asymmetric epoxidation reactions. These studies were undertaken to establish whether high enantioselectivities could be achieved with a polymer-bound catalyst and if the complexes could be recovered and reused for multiple cycles. Poly(ethylene glycol) monomethyl ether (MeO-PEG) and non-cross-linked polystyrene (NCPS) were used as soluble supports while JandaJel and Merrifield resins served as insoluble supports. Each polymer was linked to the salen catalyst through a glutarate spacer. The soluble catalysts were recovered by precipitation with a suitable solvent while the insoluble catalysts were simply filtered from the reaction mixture. Three olefins were utilized as epoxidation substrates: styrene, cis-β-methylstyrene, and dihydronaphthalene. Best results were obtained with cis-β-methylstyrene as the enantioselectivity obtained with each polymer-bound catalyst (86−90%) was equivalent to that achieved with the analogous commercially available, solution-phase (salen)Mn catalyst 1 (88%). The soluble polymer-supported catalysts 5 and 6 could be used twice before a decline in yield and enantioselectivity was observed and the JandaJel attached catalyst 7 could be used for three cycles in some cases. The Merrifield-bound catalyst 8 was found to lose activity with each use. This work presents the most effective (salen)Mn catalyst that has been attached to gel-type resin to date and may have practical applications in high-throughput organic chemistry.