New Views on the Reaction of Primary Amine and Aldehyde from DFT Study

A general theoretical investigation on the reaction of primary amine with aldehyde was carried out by density functional theory. The calculation systems involve three kinds of primary amines (methylamine, vinylamine, and phenylamine) and three kinds of aldehydes (formaldehyde, acetaldehyde, and acrylaldehyde). The steric and electronic inductive effects on the reaction mechanism were studied. Results reveal that the nucleophilic attack of primary amine on aldehyde under neutral conditions leads to carbinolamines, rather than Schiff bases. The nucleophilic attack on the protonated aldehyde produces the protonated Schiff base. The steric hindrance of the aldehyde slows down the nucleophilic attack but allows enough time to abstract a H; consequently, the formation of the protonated Schiff base is preferred. During the carbinolamine protonation, the H⁺ preferably locates on the amine nitrogen and then is abstracted by the hydroxyl oxygen over an energy barrier, leaving protonated Schiff base after timely water liberation. The formation of a prereaction potential energy well obviously softens the steric and electronic inductive effects on the active barrier for different reactants.