Experimental and Theoretical Studies of a One‐Flask Synthesis of 3H‐1‐Benzazepines from 2‐Haloanilines and α,β‐Unsaturated Ketones

3H-1-Benzazepines are prepared in one step from the reaction of 2-fluoro- or 2-chloroaniline and several aryl vinyl ketones. Enones 9a-c gave benzazepines 11a-c as expected, showing that alkyl substitution at C4 and C5 in the benzazepines is possible. However, enone 9d underwent decomposition due to conjugate addition of the 2-fluoroaniline, while enone 10 gave unsaturated imine 12 as the only product able to be isolated. The failure of unsaturated imine 12 to undergo cyclization may indicate that placement of an alkyl group at C3 of the benzazepines may not be possible. Isolation of by-products buttressed by a computational study verifies a postulated multi-step process for benzazepine formation: a [1,5] sigmatropic shift in fluoroimine 3, addition elimination to give carbocation 15, formation of the unstable 5H-1-benzazepine 7, and acid-catalyzed isomerization to give the stable 3H-1-benzazepine 2. The calculations also indicate why unsaturated imine 12 fails to cyclize. NMR spectroscopic experiments show that the 3H-benzazepines undergo fast conformational exchange on the NMR time scale at room temperature except when there is an alkyl substituent at C5.