PM3-CI calculation of Diels-Alder transition structures of hetero dienophile addition to butadiene: comparison with PM3 and ab initio generated transition structures

Abstract

Transition structures for Diels-Alder addition of formaldehyde, formaldimine, diazene, and nitrosyl hydride to butadiene were studied with ab initio and PM3-CI methods. Activation energies were evaluated and compared with the different methods. The order of the reactivity of the hetero dienophiles was predicted on the basis of the frontier orbital energies, and ab initio and semiempirical calculations of the reaction barriers were performed. These energies were used to predict the stereoselectivity of the reactions. It was concluded that the ab initio calculations tend to generate transition structures with a very small degree of asynchronicity, whereas, PM3-CI on the other hand, tends to generate highly asynchronous transition structures. That is specially pronounced in the case when one of the two new forming bonds in the transition structures is CN. All of the studied methods predict high endo vs. exo NH addition of hetero dienophiles to butadiene thus suggesting the usefulness of these reactions in stereoselective organic syntheses.