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 stu died using high level ab initio molecular orbital and density function al 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 ag reement with the experimental evidence. The regio- and stereoselectivi ty of the reaction was studied at the HF/6-31G, MP2/6-31G*, and Becke 3LYP/6-31G levels of theory. The density functional calculations appe ars to give a good description of the basic features of the reaction. The decisive role played by the Lewis acid catalyst in reducing the re action barrier and increasing the stereoselectivity was evaluated. It is shown that the Lewis acid coordination to the dienophile significan tly changes the geometrical and electronic character of the transition structure. The electrostatic interaction between the Lewis acid and t he nitrogen lone pair of the 2-azabutadiene appears to be important in the preference for the exo-coordination of the catalyst in the transi tion structure.