Alkene epoxidation with iodosylbenzene catalysed by polyionic manganese porphyrins electrostatically bound to counter-charged supports

Manganese(III) complexes of tetra-anionic and tetra-cationic porphyrins hav e been immobilised on counter-charged, surface-modified silica supports and on organic ion-exchange resins. The reactions of these supported manganese (III) porphyrin systems and analogous uncharged homogeneous systems have be en examined using cyclooctene and (E)- and (Z)-4- methylpent-2-ene epoxidat ions, with iodosylbenzene (PhIO) as the oxygen donor. Comparisons using the manganese porphyrin systems as catalysts for the epox idation of cyclooctene in acetonitrile reveal that, in low turnover reactio ns (maximum 136 turnovers), they all give an essentially quantitative yield of epoxide although the heterogeneous reactions are significantly slower t han the homogeneous analogues. In large scale repeat-use experiments, howev er, the supported catalysts are clearly superior, giving markedly better yi elds. The epoxidations of (E)- and (Z)-4- methylpent-2-ene with all the catalysts show a very high stereoretention, with the (Z)-alkene reacting faster than the (E)-isomer. The sterically hindered manganese(III) 5,10,15,20-tetrakis (2,6-dichloro-3-sulfonatophenyl)porphyrin (MnTDCSPP) shows the highest sele ctivity for the (Z)-isomer; by contrast the supported manganese(III) 5,10,1 5,20-tetrakis[2,3,5,6-tetrafluoro-4-(trimethylammonio)phenyl]porphyrin on D owex (MnTF(4)TMAPP-Dowex) reacts with the two alkenes at effectively the sa me rate. The mechanism of the epoxidations and the influence of the porphyrin ligand and support on the substrate selectivity are discussed.