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

Abstract

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 (Ered) and different triplet energy levels (ΔET). 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.