Photochemical epoxidation of cyclohexene sensitized by tetraphenylporphyrinatoantimony(V) in the presence of water acting both as an electron and an oxygen donor

Abstract

Visible light irradiation of a reaction mixture involving tetraphenylporphyrinatoantimony(V)(Sb^VTPP){[Sb^VTPP(OMe)Br]Br 1 or [Sb^VTPP(OH)₂]OH 2}, methylviologen (MV²⁺), hydroxide ion and cyclohexene 6 in acetonitrile–water (95:5) under degassed conditions induced formation of MV˙⁺(one-electron reduced product of MV²⁺), and oxidation of cyclohexene (with cyclohexene oxide 7 and cyclohex-2-enol 8 as major products and cyclohexane-1, 2-diol 9 in small amounts in the case of 1, and cyclohexene oxide 7 as the sole product in the case of 2). One third of the obtained diol was cis isomer. The oxygen atom of the water molecule in the reaction system was confirmed as being incorporated in the oxidized products of cyclohexene by an experiment using H₂¹⁸O. The overall quantum yield of the photochemical epoxidation was in the order of 10^–3, while that of MV˙⁺ production were in the order of 10^–1. Hydroxide ion was shown to have both a negative effect on radical anion formation by reductive quenching of the excited triplet state of Sb^VTPP, and also a positive effect by acting as an oxygen source itself for the epoxidation. A key intermediate for the epoxidation was presumed to be an antimony(VI) oxo-type porphyrin complex generated by an electron transfer to MV²⁺ from a singly and doubly deprotonated excited triplet Sb^VTPP. The different product distributions obtained for 1 and 2 are discussed in relation to the geometrical structures of the presumed key intermediate oxo-complexes.