Visible-Light-Mediated TiO2‑Catalyzed Aerobic Dehydrogenation of N‑Heterocycles in Batch and Flow

We report a simple and environmentally friendly method for synthesizing N-containing heterocycles via a visible-light-mediated aerobic dehydrogenation reaction. Using a nontoxic, stable, and inexpensive titanium dioxide catalyst, a variety of substituted quinoline, indole, quinoxaline, and 3,4-dihydroisoquinoline derivatives could be synthesized using the green oxidant molecular oxygen. Improved reactivity and scalability of this reaction were demonstrated by adapting the photochemical multiphasic reaction to a continuous flow system. To gain insight into the mechanism, we also conducted several mechanistic studies, including absorption analysis, light on–off testing, and NMR analysis. Especially, oxygen is reduced to hydrogen peroxide, and dimethyl sulfoxide is a critical scavenger of the oxidant byproduct for ensuring high yields.