Tentative mechanisms of 1,2-dihydropyridine and 4H-pyran formation via the reaction of 3-trimethylsilyl-2-propyn-1-al with 2-aminopyridine and water: experimental and quantum chemistry studies

The reaction of N-(2-pyridyl)-2-(trimethylsilylethynyl)-1,2-dihydropyridine-3,5-dicarbaldehyde self-assembling from three molecules of 3-trimethylsilyl-2-propyn-1-al and one molecule of 2-aminopyridine was monitored by 1H NMR spectroscopy. It was shown that the aldehyde and amine ratio affects the concurrent trimerization of 3-trimethylsilyl-2-propyn-1-al into 4-(trimethylsilylethynyl)-4H-pyran-3,5-dicarbaldehyde involving one molecule of water as a nucleophile. Two plausible mechanisms of 1,2-dihydropyridine and 4H-pyran cascade assembling are proposed and analyzed by quantum chemical methods (MP2 and B3LYP). The transition states were found and the activation barriers were calculated for the key steps of the reactions. The potential energy surface (PES) analysis performed do suggests an alternative radical mechanism leading to 1,2-dihydropyridine and 4H-pyran with significantly lower (by 10 kcal/mol) activation barriers at the limiting stages.