Highly Efficient and Selective Epoxidation of Alkenes by Photochemical Oxygenation Sensitized by a Ruthenium(II) Porphyrin with Water as Both Electron and Oxygen Donor

Abstract

Visible light irradiation of a reaction mixture of carbonyl-coordinated tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(II) (Ru(II)TMP(CO)) as a photosensitizer, hexachloroplatinate(IV) as an electron acceptor, and an alkene in alkaline aqueous acetonitrile induces selective epoxidation of the alkene with high quantum yield (Phi = 0.6, selectivity = 94.4% for cyclohexene and Phi = 0.4, selectivity = 99.7% for norbornene) under degassed conditions. The oxygen atom of the epoxide was confirmed to come from a water molecule by an experiment with H(2)(18)O. cis-Stilbene was converted into its epoxide, cis-stilbeneoxide, without forming trans-stilbeneoxide. trans-Stilbene, however, did not exhibit any reactivity. Under neutral conditions, an efficient buildup of the cation radical of Ru(II)TMP(CO) was observed at the early stage of the photoreaction, while an addition of hydroxide ion caused a rapid reaction with the cation radical to promote the reaction with reversion to the starting Ru(II)TMP(CO). A possible involvement of a higher oxidized state of Ru such as Ru(IV), Ru(V), Ru(VI) through a dismutation of the Ru(III) species was excluded by an experiment with Ru(VI)TMP(O)(2). Decarbonylation of the Ru complex was also proven to be invalid. A reaction mechanism involving an electron transfer from the excited triplet state of Ru(II)TMP(CO) to hexachloroplatinate(IV) and subsequent formation of OH(-)-coordinated Ru(III) species, leading to an oxo-ruthenium complex as the key intermediate of the photochemical epoxidation, was postulated.

The Water Oxidation Bottleneck in Artificial Photosynthesis: How Can We Get Through It? An Alternative Route Involving a Two‐Electron Process

“…60 %) was induced under deaerated conditions. [35] The net chemistry indicates that the water molecule serves as both an efficient electron and oxygen atom donor in the reaction (Scheme 4). A reaction mechanism involving the formation of the oxo-complex of the ruthenium porphyrin was postulated.…”

“…A reaction mechanism involving the formation of the oxo-complex of the ruthenium porphyrin was postulated. [35] Elucidation of the molecular mechanism would surely lead to a breakthrough in understanding how the relatively inert molecule, water, can be chemically activated upon irradiation with visible light. Preliminary experimental results have already excluded possible mechanisms involving processes such as: (1) direct electron transfer from the alkene to the excited Ru-porphyrin; (2) the formation of higher oxidation states of Ru, such as Ru IV and Ru VI species; and (3) the de-carbonylation of Ru II TMP(CO).…”

“…The cation radical of Ru II TMP(CO) was revealed to exert a key role in the photochemical epoxidation. [35] The experimental results, together with theoretical (DFT) calculations, have convinced us that the mechanism involves two reaction intermediates: the hydroxyl-coordinated one-electron-oxidized Ru-complex [Intermediate (I)] and its deprotonated form, an oxo-type complex of the Ru-porphyrin [Intermediate (II)]. [36] Water activation is accomplished at the metal center of the complex through the ligation of a hydroxide ion to the cation radical of the Ru-porphyrin, followed by subsequent two-electron conversion of the metal center ( Figure 3).…”

Homogeneous and Heterogeneous Free-Based Porphyrins Incorporated to Silica Gel as Fluorescent Materials and Visible Light Catalysts Mimic Monooxygenases

“…Peculiar spectral properties and ability of the porphyrins to undergo photoinduced multielectron transfer without changing their structure has opened new fields of their application not only for light-driven biocatalysis but also for application in solar energy conversion, photodegradation of a wide range of organic and inorganic chemicals in air and water, elimination of bacteria, viruses, cancer cells and splitting of water as well as construction of sensing materials to mimic the human nose (electronic nose) and photosynthetic systems (Paolesse et al, 2002). There are a lot of contributions of porphyrin model studies to the photooxidation catalysis in homogeneous systems (Weber et al, 1994;Quici et al, 1993;Maldotti et al, 1996;Funyu et al, 2003). Simple photoexcited Fe III porphyrins were used to induce hydrocarbon oxygenation to the corresponding ketones under aerobic conditions.…”

Three‐Membered Heterocycles. Structure and Reactivity

“…Some interesting ruthenium porphyrins have also been reported, including a dioxoruthenium(VI) species [459] and the ruthenium(II) catalyst 418b, which functions as a photosensitizer capable of effecting the selective epoxidation of alkenes (e.g., 422) using water as an oxygen source, although the method suffers from the limitation of requiring hexachloroplatinate as an electron acceptor [460]. Metalloporphyrins of all stripes have been appended to solid supports, and the reader is directed to a recent and effective review on the topic for further information [461].…”