| Abstract
| - A ruthenium complex of meso-tetrakis(2,6-dichlorophenyl)porphyrin, [RuII(TDCPP)(CO)(EtOH)],is immobilized into mesoporous MCM-41 molecular sieves; the supported Ru catalyst can effecthighly selective heterogeneous alkene epoxidations using 2,6-dichloropyridine N-oxide as terminaloxidant. Aromatic and aliphatic alkenes can be efficiently converted to their epoxides in good yieldsand selectivities, and cis-alkenes such as cis-stilbene, cis-β-methylstyrene, and cis-β-deuteriostyreneare epoxidized stereospecifically. Oxidation of cycloalkenes, e.g., norbornene and cyclooctene, canbe carried out effectively using the heterogeneous Ru-catalyzed reaction while these alkenes areunreactive in the zeolite-based titanium silicate (TS-1)-catalyzed conditions (Murugavel, R.; Roesky,H. W. Angew. Chem., Int. Ed. Engl.1997, 36, 477). On the other hand, the Ru/M-41(m) catalystdisplays size selectivity in the (+)-limonene oxidation where the terminal CC bond (vs internaltrisubstituted CC bond) becomes more readily oxidized. Bulky 3,4,6-tri-O-benzyl-d-glucal hasfailed to react under the heterogeneous Ru-catalyzed conditions, whereas the smaller acetylderivative is converted to a 3:1 mixture of α- and β-glycal epoxides. The Ru/M-41(m) catalyst canbe used repeatedly, and 67% of its initial activity is retained after 11 691 turnovers (three runs).The loss of activity is attributed to catalyst leaching and/or deactivation. On the basis of Hammettcorrelation (ρ+ = −0.72, R = 0.997) and product studies (cyclohexene and cis-alkenes as thesubstrates), a reactive dioxoruthenium(VI) porphyrin intermediate is not favored. An oxoruthenium(V) complex or oxoruthenium(IV) porphyrin cation radical could be the key intermediate for thishighly selective epoxidation reaction.
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