| Abstract
| - Visible light irradiation of a reaction mixture of carbonyl-coordinated tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(II) (RuIITMP(CO)) as a photosensitizer, hexachloroplatinate(IV) as an electronacceptor, and an alkene in alkaline aqueous acetonitrile induces selective epoxidation of the alkene withhigh quantum yield (Φ = 0.6, selectivity = 94.4% for cyclohexene and Φ = 0.4, selectivity = 99.7% fornorbornene) under degassed conditions. The oxygen atom of the epoxide was confirmed to come from awater molecule by an experiment with H218O. cis-Stilbene was converted into its epoxide, cis-stilbeneoxide,without forming trans-stilbeneoxide. trans-Stilbene, however, did not exhibit any reactivity. Under neutralconditions, an efficient buildup of the cation radical of RuIITMP(CO) was observed at the early stage of thephotoreaction, while an addition of hydroxide ion caused a rapid reaction with the cation radical to promotethe reaction with reversion to the starting RuIITMP(CO). A possible involvement of a higher oxidized stateof Ru such as RuIV, RuV, RuVI through a dismutation of the RuIII species was excluded by an experimentwith RuVITMP(O)2. Decarbonylation of the Ru complex was also proven to be invalid. A reaction mechanisminvolving an electron transfer from the excited triplet state of RuIITMP(CO) to hexachloroplatinate(IV) andsubsequent formation of OH--coordinated RuIII species, leading to an oxo-ruthenium complex as the keyintermediate of the photochemical epoxidation, was postulated.
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