PM3-CL CALCULATION OF DIELS-ALDER TRANSITION STRUCTURES OF HETERO DIENOPHILE ADDITION TO BUTADIENE - COMPARISON WITH PM3 AND AB-INITIO GENERATED TRANSITION STRUCTURES

Transition structures for Diels-Alder addition of formaldehyde, formal dimine, diazene, and nitrosyl hydride to butadiene were studied with a b initio and PM3-CI methods. Activation energies were evaluated and co mpared 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 ster eoselectivity of the reactions. It was concluded that the ab initio ca lculations tend to generate transition structures with a very small de gree of asynchronicity, whereas, PM3-CI on the other hand, tends to ge nerate highly asynchronous transition structures. That is specially pr onounced in the case when one of the two new forming bonds in the tran sition structures is C-N. All of the studied methods predict high endo vs. exo N-H addition of hetero dienophiles to butadiene thus suggesti ng the usefulness of these reactions in stereoselective organic synthe ses.