SUBSTRATE AND POSITIONAL SELECTIVITY IN ELECTROPHILIC SUBSTITUTION REACTIONS OF PYRROLE, FURAN, THIOPHENE, AND SELENOPHENE DERIVATIVES
Keywords:
benzo[b]selenophene, benzo[b]thiophene, benzo[b]furan, indole, pyrrole, selenophene, thiophene, furan, cationic σ-complexes, quantum-chemical calculations, MP2/6-31G(d) and B3LYP/6-31G(d) methods, relative stability of the onium states of chalcogens, positional selectivity, substrate selectivity, electrophilic substitutionAbstract
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.
How to Cite
Belen'kii, L. I.; Suslov, I. A.; Chuvylkin, N. D. Chem. Heterocycl. Compd. 2003, 39, 36. [Khim. Geterotsikl. Soedin. 2003, 38.]
For this article in the English edition, see DOI https://doi.org/10.1023/A:1023060406534
