Contrastive photoreduction pathways of benzophenones governed by regiospecific deprotonation of imidazoline radical cations and additive effects

[GRAPHICS] In the photoreaction of benzophenones with 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI), benzhydrols were major products. Addition of H2O accelerated the reaction with no change in the product distribution, while AcOH, PhOH, and metal salts such as LiClO4 and Mg(ClOL4)(2) were effective additives to produce benzpinacols. In contrast, benzpinacols were exclusively formed regardless of the solvent and the additive in the reactions with 2-(o-hydroxyphenyl)-1,3-dimethylbenzimidazoline (o-HPDMBI). These observations are consistent with the hypothesis that DMPBI center dot+ donates a proton at the C-2 position to the benzophenone ketyl radicals while o-HPDMBI center dot+ donates a phenol proton.