A metal‐free synthesis of biologically important benzazepines is achieved through a single synthetic operation involving an oxidative C–H bond functionalization and ring expansion with diazomethanes as key reagent. This represents a new, strong methodology for the straightforward construction of the seven‐ring N‐heterocyclic structures under mild conditions using a 2,2,6,6‐tetramethylpiperidine 1‐oxyl (TEMPO) oxoammonium salt as oxidant. Moderate to good yields are achieved from simple, readily available tetrahydroisoquinolines, and this methodology has been further successfully applied for the synthesis of the 3‐benzazepine drug Lorcaserin. A possible mechanistic pathway for the ring expansion step, comprising the extrusion of nitrogen in a concerted asynchronic process, is proposed based on both mechanistic proof and density function theory (DFT) calculations.
