Semiconductor Photocatalysis: Reaction Mechanisms for the Photoreductive cis–trans Isomerization of Electron-Deficient Alkenes Catalyzed by CdS Powder

Highly pure commercially available CdS powder (99.999%) catalyzes the effective cis–trans photoisomerization of electron-deficient alkenes under visible light irradiation using triethylamine (TEA) as an electron donor, accompanying the formation of the dihydro compound as a two-electron reduction product. The photoisomerization does not occur at all in the absence of TEA. Donor effect, solvent effect, deuterium incorporation experiments for photocatalysis, and MOPAC molecular orbital calculation (MNDO/PM3) of the intermediary radical anions from the alkenes were investigated in order to elucidate the mechanism of this photoisomerization. These results reveal that the CdS-catalyzed cis–trans photoisomerization should proceed through two pathways involving the photoreduction of alkenes: one through the back electron transfer from the radical anion of the alkene (alkene−•) towards the radical cation of TEA (TEA+•), both formed by photoexcited conduction band electrons and holes of CdS, respectively. The other is the reoxidation of a radical intermediate (alkyl•), formed by protonation of alkene−•, by TEA+•.