Reaction of nitroanilines with aldehydes. Refinement of the Doebner–Miller reaction mechanism

Due to intramolecular hydrogen bonding between the amino and nitro groups, o-nitroaniline is incapable of forming Schiff bases in the reactions with acetaldehyde and crotonaldehyde but is converted to quinoline derivative under Doebner–Miller reaction conditions via addition to the C=C double bond of the α,β-unsaturated aldehyde. Under analogous conditions, p-nitroaniline possessing a free amino group gives rise to the product of Doebner–Miller quinoline synthesis through intermediate formation of Schiff base dimer. The reaction of p-nitroaniline with benzaldehyde also yields the corresponding Schiff base, whereas o-nitroaniline is converted to N-benzyl derivative.