An unexpected synthesis of azepinone derivatives through a metal-free photochemical cascade reaction

Funding Information: L.S. and X.T. are grateful to the CSC (China Scholarship Council) for a PhD fellowship. We gratefully acknowledge the generous allocation of computing time from the Australian National Computational Infrastructure and University of Tasmania, and the Australian Research Council (grant number DP180100904) for financial support. We thank Petra Krämer for her UV-vis measurements. Publisher Copyright: © 2023, The Author(s). Azepinone derivatives are privileged in organic synthesis and pharmaceuticals. Synthetic approaches to these frameworks are limited to complex substrates, strong bases, high power UV light or noble metal catalysis. We herein report a mild synthesis of azepinone derivatives by a photochemical generation of 2-aryloxyaryl nitrene, [2 + 1] annulation, ring expansion/water addition cascade reaction without using any metal catalyst. Among the different nitrene precursors tested, 2-aryloxyaryl azides performed best under blue light irradiation and Brønsted acid catalysis. The reaction scope is broad and the obtained products underwent divergent transformations to afford other related compounds. A computational study suggests a pathway involving a step-wise aziridine formation, followed by a ring-expansion to the seven-membered heterocycle. Finally, water is added in a regio-selective manner, this is accelerated by the added TsOH.