Alternative Non-Ionic Pathway for Uncatalyzed Prins Cyclization: DFT Approach

Density functional theory calculations (ωB97X-D/6-311++G(d,p)) are employed to investigate an alternative pathway for Prins-like cyclization. Although strong acids usually catalyze this reaction, 4-amino-1,3-dioxanes are rapidly obtained in high yields without catalyst when benzenamines and acetaldehyde react at low temperatures, in aqueous medium. Considering these conditions, we applied a supermolecule model with explicit water molecules to compute the mechanism for 4-amino-1,3-dioxanes formation from the reactants. The structure of the primary solvation shell was determined by Metropolis Monte Carlo method. In the Prins-cyclization step, we found an unpredicted pathway with non-ionic transition structures or intermediates. Explicit water molecules establish a net of hydrogen bonds allowing prototropism, maintaining the electrical neutrality in the system while two protons transfer occurs. To provide data to further experimental confirmation of this hypothesis, we estimated the kinetic isotope effect for the reaction. We also investigated the use of aliphatic amines, which indicates that the reaction may be of a broader application than first observed experimentally.