PM3 STUDY OF THE STEREOCHEMISTRY OF HETERODIENOPHILE CYCLOADDITIONS TO PYRROLE - ENDO LONE-PAIR EFFECT

A semiempirical PM3 theoretical study of heterodienophile addition to pyrrole, a heterocyclic aromatic diene is presented. The results are c ompared with the ethylene addition to pyrrole. Three approaches were u sed to determine the reactivity of the dienophiles: frontier orbital e nergy correlation, comparison of the calculated activation barriers, a nd comparison of the structural features of the transition state struc tures, all generated by PM3 calculations. On the basis of the frontier molecular orbital theory it was concluded that the reaction is LUMO d ienophile controlled and that all studied heterodienophiles are more r eactive than ethylene. Because pyrrole has a higher HOMO energy than c yclopentadiene, it was concluded that pyrrole is more reactive than cy clopentadiene as the diene in Diels-Alder reactions with normal electr on demand. On the other hand, reaction energy barriers predict that al most all heterodienophiles except oxygen are less reactive than ethyle ne. Discrepancies in the frontier orbital and activation energy predic tions are discussed in the light of repulsion interactions between the heteroatom endo lone pair and the si-system of pyrrole. In this way t he stereoselectivity of the heterodienophile cycloadditions to pyrrole , as well as the geometries of the transition structures, can be expla ined.