Photoinitiated Olefin Epoxidation with Molecular Oxygen, Sensitized by Free Base Porphyrins and Promoted by Hexacarbonylmolybdenum in Homogeneous Solution

The photooxidation of various olefins in homogeneous solution under an oxygen atmosphere, using visible light, a dye sensitizer, and an oxygen-transfer catalyst has been investigated. The oxygen transfer from molecular oxygen to olefin involves the following steps: i) photoinduced singlet-oxygen formation, ii) alkylhydroperoxide formation through ene-reaction, iii) intermediacy of a reactive molybdenum-peroxide, and iv) olefin epoxidation of the remaining substrate or of a second olefin. Among the various sensitizers and catalysts tested, electron deficient free base porphyrin 5,10,15,20-tetrakis(2’,6’-dichlorophenyl)-b-octabromoporphyrin and molybdenum hexacarbonyl showed the best performances in terms of robustness and activity. Under proper conditions, a complete olefin conversion may be obtained, adopting molar ratios of sensitizer/catalyst/substrate=1/50/2000, with the formation of the corresponding epoxide up to 38% yield, which corresponds to 77% of the theoretical maximum. Quite interestingly, olefins reluctant to undergo ene-reaction may be epoxidized in the presence of a second sacrificial olefin yielding the corresponding epoxides with up to 80% total selectivity.