Organo-functionalized activated carbons as supports for the covalent attachment of a chiral manganese(III) salen complex

Abstract

A modified Jacobsen-type catalyst, possessing an hydroxyl group on the aldehyde fragment of the salen ligand, was anchored through cyanuric chloride onto: (i) an air oxidized activated carbon functionalized with 3-aminotriethoxysilane and (ii) a nitric acid activated carbon treated with thionyl chloride and functionalized with 1,8-diaminooctane. The new heterogeneous catalysts, as well as the precedent materials, were characterized by nitrogen elemental analysis, Mn ICP-AES, nitrogen adsorption at 77 K, TG-IR and XPS. The results showed that the amine derivatives, cyanuric chloride and the manganese(III) salen complex were sequentially anchored onto the two activated carbons (AC). These Mn based materials acted as active and enantioselective heterogeneous catalysts in the epoxidation of α-methylstyrene, using NaOCl as oxidant. The heterogenized complex showed higher catalytic activity than the homogeneous catalyst. The increase of distance of the manganese(III) salen complex to the carbon surface did not have a significant effect on the asymmetric inductions, which were lower than the homogeneous catalyst. Nevertheless, the catalyst bearing the shorter amine alkyl chain showed higher catalytic activity due to unique steric effects that prevent hindrances to diffusion of the substrates to the active catalytic sites.