Semiconductor photocatalysis. Part 4. Hydrogen evolution and photoredox reactions of cyclic ethers catalysed by zinc sulphide

Abstract

ZnS-catalysed photoredox reactions of aqueous solutions of cyclic ethers, i.e. tetrahydrofuran, dihydrofurans, 2,5-dimethyltetrahydrofuran, 3-methyloxetane, 1,4-dioxane, and tetrahydropyran are described. A freshly prepared ZnS suspension displays efficient and selective photocatalysis especially for furan derivatives under band-gap irradiation (> 290 nm); reduction of water by the conduction band electrons generates H₂ and one-hole oxidation of the substrates leads to carbon–carbon bond formation through coupling of intermediary radicals. In the photolysis of tetrahydrofuran, oligomerization from the dimeric product (1) to octamer was observed with efficient H₂ evolution (φ_½H2= 0.13–0.62). Further oxidation of intermediary radicals and photoreduction of the resulting oxidation products were also confirmed; e.g. photolysis of 2,5-dimethyltetrahydrofuran gave exclusively the ring-opened product (9) with its photoreduced product, hexane-2,5-diol (10).