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

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 epoxidati on of (Z)-stilbene using iodosylbenzene as oxygen donor, with particular in terest in the effect of reaction conditions on the cis:trans ratio of the e poxide 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 n on-immobilised complex under the same conditions. Increasing the level of M n-exchange in the Al-MCM-41 increases the amount of trans-epoxide, whereas increasing the iodosylbenzene:substrate ratio increases the amount of cis p roduct formed. Increasing the reaction temperature also increases the amoun t of trans-epoxide for the homogeneous Mn-complex under the same conditions . A series of experiments is described in which the external ion-exchange s ites on Al-MCM-41 are preferentially silanised, which enables the cis/trans selectivity for external and internal sites to be determined. Mn-salen imm obilised on the external surface of Al-MCM-41 gives the same cis:trans rati o as that observed with the non-immobilised Mn-salen complex in solution, w hereas Mn-salen immobilised within the pores gives the cis-epoxide preferen tially. The enantioselection of the immobilised chiral Mn-salen complex is shown to decrease with reaction time at -10 degrees C, but the cis:trans epoxide ra tio remains unchanged; whereas for the non-immobilised complex in solution the enantioselection is independent of reaction time. Iodobenzene, a decomp osition product formed from iodosylbenzene, is found to act as a poison for the immobilised catalyst, leading to a slower reaction and lower enantiose lection.