Electrophile affinity and positional selectivity in electrophilic substitution reactions of N-substituted pyrroles |
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Authors: | L I Belen??kii N D Chuvylkin I D Nesterov |
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Institution: | 1. N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991, Moscow, Russian Federation
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Abstract: | In terms of the density functional theory (DFT) using B3LYP/6-31G(d) method with full geometry optimization, gas-phase quantum chemical calculations were performed for ??-complexes formed upon the attack of electrophiles E (E = H+, Me+, Me3Si+, Br+, NO2 +, MeCO+, SO3) on ??- and ??-positions of furan, thiophene, selenophene, pyrrole, and N-substituted pyrroles (NR-pyrroles, R = Me, But, SiMe3, SiPri 3, C6H4NO2-p, SO2Ph, CHO, COOMe) and for respective ??- and ??-substituted electrophilic substitution products. The energy differences between the ??- and ??-isomers of the ??-complexes characterize the preferred direction of the electrophilic attack, while the energy differences between the isomeric products make it possible to estimate the energy preference of a particular product. An analysis of the results obtained demonstrates the effect of the structure of heterocycle, the nature of electrophile, and thermodynamic and steric factors on the positional selectivity (??/?? ratio) in electrophilic substitution reactions of ??-electron rich five-membered heteroaromatics. |
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