POSITIONAL SELECTIVITY IN ELECTROPHILIC SUBSTITUTION REACTIONS OF π-EXCESSIVE HETEROCYCLES

Existing experimental data on positional selectivity in electrophilic substitution reactions of π-excessive heterocycles are classified. These data are discussed basing on the results of the authors' quantum-chemical calculations [RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d)] of the σ-complexes formed during attack of electrophiles such as H + , Me + , Me 3 Si + , Br + , NO 2 + , MeCO + , and SO 3 at the α- and β-positions of furan, thiophene, selenophene, pyrrole and its N-substituted derivatives, N-R-pyrroles (R = Me, t -Bu, SiMe 3 , Si( i -Pr) 3 , C 6 H 4 (p-NO 2 ), SO 2 Ph, CHO, CO 2 Me), and the corresponding α- and β-substituted electrophilic substitution products. The differences in energies of the α-and β-isomers of the σ-complexes characterize the preferred direction of electrophilic attack, while the differences in the energies of the isomeric products make it possible to assess the energy preference of one of them. Analysis of the obtained data demonstrates the effects of the studied heterocycles' structure, the nature of the electrophile, and the thermal and steric factors on the positional selectivity (α/β ratio) in electrophilic substitution reactions of π-excessive five-membered heteroaromatic compounds. Authors: L. I. Belen'kii, N. D. Chuvylkin, and I. D. Nesterov. English translation in Chemistry of Heterocyclic Compounds , 2012, 48 (2), pp 241-257 http://link.springer.com/article/10.1007/s10593-012-0985-3