Theoretical study on the solvent-free self-catalyzed coupling mechanism of a chiral vicinal diamine and isobutyraldehyde mediated by the outgrowth H2O

The self-catalyzed addition of a chiral vicinal diamine with two isobutyraldehyde molecules and the subsequent diaza-Cope rearrangement between two diimines were investigated systematically by M06-2X method in conjunction with the 6-311+G(d) basis set. Our theoretical model shows that, 1) the solvent-free coupling of the vicinal diamine and isobutyraldehyde molecules produces a relative stable compounds with an imidazolidine-dihydro-1,3-oxazine fused ring instead of imine intermediates; 2) the assistance of a yielded water molecule can significantly decrease the free-energy barriers of the ring-recombination and ring-opening processes by borrowing a hydrogen atom from it; 3) the two addition processes with the free-energy barriers of 150.0 and 159.0 kJ/mol, respectively, and the successive formation of the five-membered ring with a barrier of 164.1 kJ/mol control the whole reaction system.