Competitive Desulfonylative Reduction and Oxidation of α‐Sulfonylketones Promoted by Photoinduced Electron Transfer with 2‐Hydroxyaryl-1,3-dimethylbenzimidazolines under Air

Eietsu Hasegawa, Shyota Nakamura, Kazuki Oomori, Tsukasa Tanaka, Hajime Iwamoto, Kan Wakamatsu

Desulfonylation reactions of α-sulfonylketones promoted by photoi nduced electron transfer with 2- hydroxyarylbenzimidazolines (B IH-ArOH) were investigated. Under aerobic condit ions, photoexcited 2-hydroxynaphthylbenzimidazoline (BIH-NapOH) promotes competitive reduction (forming alkylk etones) and oxidation (producing α-hydroxyketones) of sulfonylketones through pathways involving the intermediacy of α-ketoalkyl radicals. The results of an examin ation 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 mol. 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 benzimid azolines 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, d. functional theory calcul ations were performed to explore the structures and properties of radical ions of sulfonylketones as well as BIH-NapOH.

J. Org. Chem.

86

3

2556

2569

10.1021/acs.joc.0c02666