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

Piaggio, Paola, McMorn, Paul, Murphy, Damien, Bethell, D., Page, P. C. B., Hancock, F. E., Sly, C., Kerton, Owain J. and Hutchings, Graham John 2000. Enantioselective epoxidation of (Z)-stilbene using a chiral Mn(III)-salen complex: effect of immobilisation on MCM-41 on product selectivity. Journal of the Chemical Society - Perkins Transactions 2 2000 (10) , pp. 2008-2015. 10.1039/B005752P

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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. The enantioselection of the immobilised chiral Mn–salen complex is shown to decrease with reaction time at −10 °C, but the cis∶trans epoxide ratio remains unchanged; whereas for the non-immobilised complex in solution the enantioselection is independent of reaction time. Iodobenzene, a decomposition product formed from iodosylbenzene, is found to act as a poison for the immobilised catalyst, leading to a slower reaction and lower enantioselection.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Cardiff Catalysis Institute (CCI)
Subjects: Q Science > QD Chemistry
Publisher: Royal Society of Chemistry
ISSN: 1364-5471
Last Modified: 12 Mar 2020 14:57

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