A debrominative oxygenation protocol has been developed for the conversion of α-bromo-α,α-dialkyl-substituted carbonyl compounds to their corresponding α-hydroxy analogues. For example, stirring a solution of α-bromoisobutyrophenone and 2-aryl-1,3-dimethylbenzimidazoline (BIH-Ar) at room temperature under an air atmosphere leads to the efficient formation of α-hydroperoxyisobutyrophenone, which can be converted to α-hydroxyisobutyrophenone using Me2S reduction. In contrast, reaction of α-bromoacetophenone under the same conditions produces the α-hydrogenated product acetophenone. α-Keto-alkyl and benzimidazolyl radicals (BI•-Ar), generated via dissociative electron transfer from BIH-Ar to α-bromoketone substrates, serve as key intermediates in the oxidation and reduction processes. The dramatic switch from hydrogenation to oxygenation is attributed to a steric effect of α-alkyl substituents, which causes hydrogen atom abstraction from sterically crowded BIH-Ar to α-keto-alkyl radicals to be slow and enable preferential reaction with molecular oxygen. Generation of the α-keto-alkyl radical and BI•-Ar intermediates in these process and their sterically governed hydrogen atom transfer reactions are supported by results arising from DFT calculations. Moreover, an electron spin resonance study showed that visible light irradiation of phenyl benzimidazoline (BIH-Ph) in the presence of molecular oxygen produces the benzimidazolyl radical (BI•-Ph). The addition of thiophenol into the reaction of α-bromoisobutyrophenone and BIH-Ph predominantly produced α-phenylthiolated isobutyrophenone even if a high concentration of molecular oxygen exists. Furthermore, the developed protocol was applied to other α-bromo-α,α-dialkylated carbonyl compounds.

Desulfonylation reactions of α-sulfonylketones promoted by photoinduced electron transfer with 2-hydroxyarylbenzimidazolines (BIH-ArOH) were investigated. Under aerobic conditions, photoexcited 2hydroxynaphthylbenzimidazoline (BIH-NapOH) promotes competitive reduction (forming alkylketones) and oxidation (producing α-hydroxyketones) of sulfonylketones through pathways involving the intermediacy of α-ketoalkyl radicals. The results of an examination of the effects of solvents, radical trapping reagents, substituents of sulfonylketones, and a variety of hydroxyaryl-and aryl-benzimidazolines (BIH-ArOH and BIH-Ar) suggest that the oxidation products are produced by dissociation of α-ketoalkyl radicals from the initially formed solvent-caged radical ion pairs followed by reaction with molecular oxygen. In addition, the observations indicate that the reduction products are generated by proton or hydrogen atom transfer in solvent-caged radical ion pairs derived from benzimidazolines and sulfonylketones. The results also suggest that arylsulfinate anions arising by carbon-sulfur bond cleavage of sulfonylketone radical anions act as reductants in the oxidation pathway to convert initially formed α-hydroperoxyketones to α-hydroxyketones. Finally, density functional theory calculations were performed to explore the structures and properties of radical ions of sulfonylketones as well as BIH-NapOH.