Substrate and positional selectivity in electrophilic substitution reactions of pyrrole, furan, thiophene, and selenophene derivatives

摘要

The relative reactivity (substrate selectivity) of five-membered heterocycles on electrophilic substitution (pyrrole furan > selenophene > thiophene) and their positional selectivity (furan > selenophene ≥ thiophene > pyrrole) are not consistent. The indicated differences in positional selectivity (α: β ratios) of the parent heterocycles show up essentially in orientation on electrophilic substitution reactions of their derivatives and the corresponding benzannelated systems. It was shown that the positional selectivity was reduced in a sequence corresponding to the change in relative stability of the onium states of the elements (O~+ < Se~+ ≤ S~+ < N~+) and reflects the predominant role of the heteroatoms in stabilizing the σ-complexes formed on β-substitution, in which the positive charge is distributed between the heteroatom and one α-carbon atom (in α-isomers four atoms of the ring participate in delocalization of the charge). This interpretation was confirmed by quantum-chemical calculations carried out by the RHF/6-31G(d), MP2/6-31G(d)//RHF(6)-31G(d), and B3LYP/6-31G(d) ab initio methods.