Substrate and positional selectivity in electrophilic substitution reactions in pyrrole, furan, thiophene, and selenophene derivatives and related benzoannelated systems |
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Authors: | L. I. Belen'kii T. G. Kim I. A. Suslov N. D. Chuvylkin |
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Affiliation: | (1) N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow, Russian Federation;(2) Higher Chemical College, Russian Academy of Sciences, 9 Miusskaya pl., 125820 Moscow, Russian Federation |
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Abstract: | Data on the relative reactivities (substrate selectivity) of five-membered heterocycles in electrophilic substitution reactions and positional selectivity (α : β ratio) in these reactions were analyzed. Unlike the substrate selectivity (pyrrole ≫ furan > selenophene > thiophene) determined by the position of heteroatoms in the Periodic Table, the positional selectivity decreases in the order corresponding to the change in the relative stability of the onium states of the elements (O+ < Se+ ≤ S+ < N+) and reflects the predominant role of heteroatoms in the stabilization of σ complexes formed upon β-substitution. These differences in the positional selectivity of the parent heterocycles have a substantial effect on the orientation in electrophilic substitution reactions in their derivatives and the corresponding benzoannelated systems. This interpretation was confirmed by ab initio quantum chemical calculations (RHF/6–31G(d) and MP2/6– 31G(d)//RHF/6–31G(d)) and density functional theory calculations (B3LYP/6–31G(d)). Quantum chemical calculations were performed by the above-mentioned methods for model N-R-pyrroles (R = Me, Et, Pri, But, CH=CH2, C≡CH, Ph, PhSO2, and 4-O2NC6H4) and their α- and β-protonated σ complexes. The results of these calculations demonstrated that it is the steric factors and charges on the β-C, α-C, and N atoms and the substituents at the N atom (the kinetic control), as well as the nature of the electrophile, rather than the difference in the relative stabilities of the onium states of N+ (which depends on the nature of the substituent at the N atom and reflects the role of the heteroatom in stabilization of σ complexes formed via β-substitution; the thermodynamic control) that are responsible for the type of orientation (α or β) that prevails. Dedicated to Academician V. I. Minkin on the occasion of his 70th birthday. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 837–846, April, 2005. |
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Keywords: | pyrrole furan thiophene selenophene indole benzo[b]furan benzo[b]thiophene benzo[b]selenophene electrophilic substitution substrate selectivity positional selectivity cationic σ complexes relative stability of onium states of chalcogens quantum chemical calculations ab initio methods (RHF/6– 31G(d) MP2/6– 31G(d)) DFT B3LYP/6– 31G(d) |
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