Diels-Alder reactions of 2-azabutadienes with aldehydes: Ab initio and density functional theoretical study of the reaction mechanism, regioselectivity, acid catalysis, and stereoselectivity

The Diels-Alder reaction of 2-azabutadiene with aldehydes has been studied using high level ab initio molecular orbital and density functional methods. Multiconfigurational calculations were carried out on the concerted and stepwise mechanisms. At the CASPT2F/6-31G*//CASSCF/6-31G* level of theory, the [pi 4s + pi 2s]-cycloaddition of 2-azabutadiene with formaldehyde is predicted to be a concerted reaction, in good agreement with the experimental evidence. The regio- and stereoselectivity of the reaction was studied at the HF/6-31G*, MP2/6-31G*, and Becke3LYP/6-31G* levels of theory. The density functional calculations appears to give a good description of the basic features of the reaction. The decisive role played by the Lewis acid catalyst in reducing the reaction barrier and increasing the stereoselectivity was evaluated. It is shown that the Lewis acid coordination to the dienophile significantly changes the geometrical and electronic character of the transition structure. The electrostatic interaction between the Lewis acid and the nitrogen lone pair of the 2-azabutadiene appears to be important in the preference for the exo-coordination of the catalyst in the transition structure.