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

Experimental data on the positional selectivity (alpha:beta-ratios) in reactions of N-substituted pyrroles with electrophiles have been considered. Based on the results of quantum chemical calculations of model NR-pyrroles (R=H, Me, Et, i-Pr, t-Bu, CH=CH2, Cequivalent toCH, Ph, PhSO2, 4-O2NC6H4) and their alpha- or beta-protonated sigma-complexes, carried out using ab initio methods (RHF/6-31G(d), MP2/6-31G(d)//RHF/631G(d)), and within the framework of density functional theory (B3LYP/6-31G(d)), it has been shown that the predominant alpha- or beta-orientation is determined by steric factors and charges on the atoms beta-C, alpha-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 sigma-complexes formed by beta-substitution.