Synthesis and Characterization of a Polymer-Supported Salen Ligand for Enantioselective Epoxidation

We report the synthesis of a covalently linked salen complex onto the surface of a 98 vol % styrene/2 vol % divinylbenzene polymeric support. The solid-phase method allows manipulation of the substituents on the salen complex in a more versatile manner than can be done for the homogeneous ligand and the production of a support for catalysis that guarantees accessibility to the supported active sites. The ligand was constructed in a stagewise manner that allows the possibility for combinatorial syntheses. Immobilized salen complex 1 was formed by the sequential treatment of the resin with 2,4,6-trihydroxybenzaldehyde, a chiral trans -1,2-diaminocyclohexane, and 3,5-di- tert -butylsalicylaldehyde. After loading the immobilized ligand with Mn, the support effected the enantioselective epoxidation of 1,2-dihydronaphthalene, cis -β-methylstyrene, and styrene as test olefins with enantiomeric excesses of 46%, 79% ( cis -epoxide), and 9%, respectively, using aqueous sodium hypochlorite as the oxidant. The heterogeneous catalyst also showed a selectivity toward the formation of the trans-epoxide in the case of cis -β-methylstyrene. The heterogeneous system allows ease of separation and recycle, albeit with diminished enantioselectivity in reuse.