Dehydrogenative\nPhotocyclization\nof 3‑Styryl\nIndoles to Fused Indole Systems

The synthesis of\n5-aryl- or 5,6-diaryl-11<i>H</i>-benzo[<i>a</i>]carbazoles has been achieved from 3-styryl indoles through\ncatalyst-free photoinitiated dehydrogenative cyclization transformation,\nproviding a range of structurally diverse products in excellent yields\nunder mild conditions. The protocol is also applicable to furan and\nthiophene samples. This Mallory-type reaction takes place in an argon\natmosphere without external oxidants. Finally, detailed mechanistic\nstudies were performed, and kinetic isotope effect experiments indicate\nthat dehydrogenative annulation reaction involves photoinduced 6π-electrocyclic\nring-closing and hydrogen evolution cascade processes.