Tandem ring-contraction/decarbonylation of 2,4-diphenyl-3H-1-benzazepine to 2,4-diphenylquinoline

Attempted free-radical bromination of 2,4-diphenyl-3H-1-benzazepine (3) with NBS led to an unusual ring-contraction reaction, giving rise to 2,4-diphenylquinoline (5) in high yield. This is a convenient path for the synthesis of a quinoline in one step from the easily accessible 1-benzazepine. We have elucidated the mechanism of ring-contraction reaction using 13C-labeled and deuterated benzazepines, and DFT calculations. There is strong experimental evidence that the departing carbon atom is initially part of a reactive intermediate dibromomethyl cation, which leads to methyl formate upon reaction with methanol. In the absence of methanol, water can complete the ring-contraction. In this case, the departing carbon atom is in the form of carbon monoxide.