Dehydrogenative Photocyclization of 3-Styryl Indoles to Fused Indole Systems

The synthesis of 5-aryl- or 5,6-diaryl-11<i>H</i>-benzo[<i>a</i>]carbazoles has been achieved from 3-styryl indoles through catalyst-free photoinitiated dehydrogenative cyclization transformation, providing a range of structurally diverse products in excellent yields under mild conditions. The protocol is also applicable to furan and thiophene samples. This Mallory-type reaction takes place in an argon atmosphere without external oxidants. Finally, detailed mechanistic studies were performed, and kinetic isotope effect experiments indicate that dehydrogenative annulation reaction involves photoinduced 6π-electrocyclic ring-closing and hydrogen evolution cascade processes.