Aryl-substituted dimethylbenzimidazolines as effective reductants of photoinduced electron transfer reactions

• DOI: 10.1016/j.tet.2015.06.071
• Publication Date: 2015-06-24T20:37:46Z
• Authors: Eietsu Hasegawa; Taku Ohta; Shiori Tsuji; Kazuma Mori; Ken Uchida; Tomoaki Miura; Tadaaki Ikoma; Eiji Tayama; Hajime Iwamoto; Shin-ya Takizawa; Shigeru Murata
• Keywords: Photoinduced electron transfer; Photoexcitation; Radical ion; Aryl radical; Proton-Coupled Electron Transfer; Chromophore

Abstract Photoinduced electron transfer (PET) reactions promoted by 2-aryl substituted 1,3-dimethylbenzimidazolines (Ar-DMBIH) were investigated. Excited states of Ar-DMBIH, formed by irradiation using light above 360 nm, initiate PET reductions of various organic substrates, including transformations of epoxy ketones to aldols, free radical rearrangements such as the Dowd-Beckwith ring-expansion and 5- exo hexenyl cyclization, deprotection of N -sulfonyl-indols, and allylation of acyl formates. In these processes, Ar-DMBIH possessing 1-naphthyl, 2-naphthyl, 1-pyrenyl and 9-anthryl substituents formally act as two electron and one proton donors while the hydroxynaphthyl substituted derivative serves as a two electron and two proton donor. On the basis of the results of absorption spectroscopy studies, cyclic voltammetry and DFT calculation, a mechanistic sequence for these reduction reactions is proposed that involves initial photoexcitation of the aryl chromophore of the Ar-DMBIH followed by single electron transfer (SET) to the organic substrate to generate the radical cation of benzimidazoline and the radical anion of the substrate.