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

P. Piaggio, P. McMorn, D. Murphy, D. Bethell, P. C. B. Bulman Page, F. E. Hancock, C. Sly, O. J. Kerton, G. J. Hutchings

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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-diamin e] 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 degrees 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.
Original languageEnglish
Pages (from-to)2008-2015
Number of pages8
JournalJournal of the Chemical Society-Perkin Transactions 2
Issue number10
Publication statusPublished - 2000

Keywords

  • ELECTRON-SPIN-ECHO
  • CIS-OLEFINS
  • ASYMMETRIC EPOXIDATION
  • ALKENES
  • TRANS-EPOXIDES
  • (SALEN)MANGANESE(III) COMPLEXES
  • MOLECULAR-SIEVES
  • ZEOLITE
  • HETEROGENEOUS AZIRIDINATION
  • CATALYST

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