Two Photocatalytic Pathways for Reductive Dearomatizations of N‐Arylformylindoles by a Visible‐Light Triplet Organocatalyst

A visible‐light triplet photocatalyst 5‐acetyl‐2‐mercapto‐benzoic acid methyl ester (AcBSH) was developed through introducing acetyl into benzenethiolate to improve the efficiency of intersystem crossing (ISC) and using o ‐methoxyformyl to increase the molar extinction coefficient. In the photocatalytic system of AcBSH, the reductive dearomatization of N ‐arylformylindoles is achieved to afford the desired indolines in moderate to excellent yields, including both indoles without ( 1 ) and with 2‐substituent ( 2 ). The 2‐substituent is a steric hindrance for the conjugation between benzoyl and indole moiety, and makes indole 2 and indole 1 exhibit different reduction potentials ( E red ) and different triplet energy levels ( ΔE T ). Thereby, two kinds of substrates undergo primarily two photocatalytic pathways: i) successive SET/protonation pathway, which merging photosensitization via triplet‐triplet energy transfer (EnT) and photoinduced SET for substrates 1 , and ii) single electron transfer (SET) from the triplet photocatalyst and a concerted hydrogen atom transfer (HAT) process for substrates 2 .