Enantioselective epoxidation of (Z )-stilbene using a chiral Mn(III)–salen complex: effect of immobilisation on MCM-41 on product selectivity

Abstract

Manganese-exchanged Al-MCM-41 modified by the chiral salen ligand [(R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] has been investigated as a heterogeneous catalyst for the enantioselective epoxidation of (Z)-stilbene using iodosylbenzene as oxygen donor, with particular interest in the effect of reaction conditions on the cis∶trans ratio of the epoxide product. Immobilisation of the chiral Mn–salen complex in Al-MCM-41 increases the cis∶trans ratio of the epoxide product when compared to the non-immobilised complex under the same conditions. Increasing the level of Mn-exchange in the Al-MCM-41 increases the amount of trans-epoxide, whereas increasing the iodosylbenzene∶substrate ratio increases the amount of cis product formed. Increasing the reaction temperature also increases the amount of trans-epoxide for the homogeneous Mn-complex under the same conditions. A series of experiments is described in which the external ion-exchange sites on Al-MCM-41 are preferentially silanised, which enables the cis/trans selectivity for external and internal sites to be determined. Mn–salen immobilised on the external surface of Al-MCM-41 gives the same cis∶trans ratio as that observed with the non-immobilised Mn–salen complex in solution, whereas Mn–salen immobilised within the pores gives the cis-epoxide preferentially.