Mechanisms of Diels-Alder reactions between pyridines and dienophiles: A DFT investigation

Mechanisms of the Diels-Alder (D-A) reactions between N-alkyl and aryl-1,2-dihydropyridines and dienophiles have been investigated by M06-2X-D3/6-31 + G(d,p) basis set, and SMD model was employed to simulate the solvent effect. The computational results show that the productive process of endo-product is favorable with lower energy barrier. The calculations of substituent effect indicate that there are some relations between the energy barrier and the productivity. The lower energy barrier means the higher productivity, and the barrier of retro D-A reaction could also have influence on the productivity. Finally, natural bond orbital Fukui function (NOFF) and frontier molecular orbital theory (FMOT) are employed to analyze the structures and reveal the substances of D-A reactions.