Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System
Hiromu Fuse, Yu Irie, Masaaki Fuki, Yasuhiro Kobori, Kosaku Kato, Akira Yamakata, Masahiro Higashi, Harunobu Mitsunuma, Motomu Kanai
Abstract
We developed organocatalyst systems to promote the cleavage of stable C–H bonds, such as formyl, α-hydroxy, and benzylic C–H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7’-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π–π interactions to form electron donor–acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7’-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7’-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon–hydrogen (C–H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.