Aerobic Visible-Light Photoredox Radical C–H Functionalization: Catalytic Synthesis of 2-Substituted Benzothiazoles

Abstract

An aerobic visible-light driven photoredox catalytic formation of 2-substituted benzothiazoles through radical cyclization of thioanilides has been accomplished. The reaction features C-H functionalization and C-S bond formation with no direct metal involvement except the sensitizer. The reaction highlights the following: (1) visible-light is the reaction driving force; (2) molecular oxygen is the terminal oxidant, and (3) water is the only byproduct.

A new era for homolytic aromatic substitution: replacing Bu3SnH with efficient light-induced chain reactions

“…Some authors use inert atmospheres, while others purposely add air or oxygen in order to facilitate the rearomatization. 81,83 Further the catalyst itself can undergo competitive HAS (at the aromatic ligands), which ultimately leads to its deactivation. 111 However recent reports of visible-light mediated syntheses of heterocycles demonstrate good yields can be achieved in the absence of an external catalyst including, the coupling of electron-deficient arenes onto indoles through electron donor-acceptor complexes, 112 and synthesis of polycyclic indolizines which themselves can act as photocatalysts.…”

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

“…74 Oxygen is proposed to act as an oxidative quencher of *Ru(bpy) 3 2+ , accepting an electron from the photocatalyst to give superoxide and the strongly oxidizing Ru(bpy) 3 3+ (Scheme 25). While no reaction of thiobenzanilide 104 occurs in the absence of base, added 1,8-diazabicycloundec-7-ene (DBU) deprotonates 104 to give the more readily oxidized imidothiolate anion 105 .…”

Dual Catalysis Strategies in Photochemical Synthesis

“…Photooxidation and deprotonation 308 of 265 affords thioamidyl radical 267 , which undergo cyclization to afford 268 . 309 The Co III catalyst is then proposed to be sequentially reduced by Ru I and by radical 268 . Proton transfer from the pentadienyl cation to the Co I complex results in the formation of rearomatized benzothiazole 266 and regenerates the Co III catalyst with evolution of hydrogen gas.…”