Synthesis of resins with pendently-bound chiral manganese–salen complexes and use as heterogeneous asymmetric alkene epoxidation catalysts

Abstract

Gel-type and macroporous poly(styrene–divinylbenzene) resins have been prepared by suspension polymerisation employing 4-(4-vinylbenzyloxy)salicylaldehyde, a functional comonomer. Reaction with optically active 1,2-diaminocyclohexane and functional salicylaldehydes has yielded polymer-supported chiral salen ligands site isolation allowing attachment to the matrix largely in a pendent fashion. Loadings of salen of ∼0.65 mmol g −1 can be achieved with ∼75% of groups pendent. The resin supported chiral salen ligands have been converted to the corresponding Mn(III) complexes, and the supported complexes used as catalysts in the asymmetric epoxidation of dihydronaphthalene and indene using m-chlorobenzoic acid as the oxidant, and 4-methylmorpholine N-oxide as the activator under typical Jacobsen conditions. The gel-type resin catalyst proved to be rather inactive probably as a result of formation of the oxo-bridged dimer known to be problematical in these catalyses. In contrast, a macroporous resin catalyst displayed high chemical activity, but with only modest enantioselectivity (∼20% ee). The results are discussed in terms of the accepted mechanism of action of Jacobsen's catalyst and in the context of recent results from other groups who have been immobilising these catalytic species.