Photocatalyzed Intramolecular C–S Bond Formation: Green Synthesis of 2-Substituted Benzothiazoles

Visible-light-promoted oxidative cyclization reaction of thiobenzanilides for the intramolecular C-S bond construction has been developed. By employing acridinium salt as the photosensitizer, catalytic amount of tetramethylpiperidine-1-oxyl (TEMPO) as the hydrogen atom transfer (HAT) agent, and molecular oxygen from air as the terminal oxidant, 2-substituted benzothiazoles are efficiently synthesized under bases and transition metal-free conditions. A key feature of this transformation is that TEMPO as the HAT agent could be regenerated in the presence of molecular oxygen from air; this approach could tolerate various functional groups, offering a sustainable, environmentally benign photochemical pathway for the synthesis of benzothiazoles.