Metal-free visible light-induced cross-dehydrogenative coupling of benzocyclic imines with water/P(O)H compounds: efficient access to functionalized benzazepines/ones

Visible light-induced cross-dehydrogenative coupling is a general and highly efficient protocol for the synthesis of functionalized benzazepines/ones in a one-pot fashion, and is reported with good to excellent yields and high chemoselectivity. Control and electron paramagnetic resonance experiments reveal radical–radical cross-coupling and oxidative dehydrogenation processes via single-electron chemistry in this reaction. The reaction is free of metal catalysts and external oxidants, and has mild and redox-neutral reaction conditions. A coupling reaction rather than the addition of active benzocyclic imines and nucleophiles is first established. More importantly, the functionalized benzazepines/ones can be synthesized on the gram scale using a continuous-flow technique. The product benzo[d]thiazol-2-yldiphenylphosphine oxide presents superior flame retardancy in epoxy resin, in which only a 0.46 wt% ultra-low phosphorus loading is needed to reach the highest flame-retardant rating of the polymer material.