Positional selectivity in reactions of pyrrole and its N-substitutedderivatives with electrophiles

Abstract

Experimental data on the positional selectivity (α:β-ratios) in reactions of N-substituted pyrroles with electrophiles have been considered. Based on the results of quantum chemical calculations of model N-R-pyrroles (R=H, Me, Et, i-Pr, t-Bu, CH=CH2, C≡CH, Ph, PhSO2, 4-O2NC6H4) and their α- or β- protonated σ-complexes, carried out using ab initio methods (RHF/6-31G(d), MP2/6-31G(d)//RHF/6-31G(d)), and within the framework of density functional theory (B3LYP/6-31G(d)), it has been shown that the predominant α- or β-orientation is determined by steric factors and charges on the atoms β-C, α-C, N, and on the substituents at the N atom. We conclude that it is not determined by differences in the relative stabilities of the onium state N+ depending on the nature of a substituent at the N atom, or reflecting the role of the heteroatom in the stabilization of σ-complexes formed by β-substitution.