Computational study of iminium ion formation: effects of amine structure

Density functional calculations are used to explore the formation of iminium ions from secondary amines and acrolein and the subsequent reactivity of the resulting iminium ions. After establishing a feasible profile for this reaction in simulated experimental conditions, we focus on the effect of variation in amine structure on calculated barriers. This analysis shows that incorporation of a heteroatom (N or O) in the alpha-position to the reactive amine results in significantly reduced energy barriers, as does an electron-withdrawing group (carbonyl or thiocarbonyl) in the beta-position. Electron density analysis is used to monitor reactions at a detailed level, and to identify important intermolecular interactions at both minima and transition states. Barriers to reaction are linked to calculated proton affinities of secondary amines, suggesting that the relative ease of protonation-deprotonation of the amine is a key property of effective catalysts. Moreover, barriers for subsequent Diels-Alder reaction of iminium ions with cyclopentadiene are lower than for their formation, suggesting that formation may be the rate determining step in the catalytic cycle.