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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, Pr i , Bu t , CH=CH 2 , C≡CH, Ph, PhSO 2 , and 4 O 2 NC 6 H 4 ) 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. , 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).Electrophilic substitution is the most important class of reactions of five membered heterocycles with one het eroatom, which allows one to prepare compounds with various substituents. The present paper summarizes the results of theoretical studies of the features of the sub strate and positional selectivities in electrophilic substitu tion reactions in pyrrole, furan, thiophene, and seleno phene derivatives and the corresponding benzoannelated systems that have been rationalized only recently.It is well known that the effect of the heteroatom is manifested in the higher reactivities of the α positions. This is generally interpreted as the result of higher stabil ity of the corresponding σ complex (A) due to favorable charge delocalization compared to its isomer (B) result ing from the attack at the β position (Scheme 1).The reactivities and positional selectivities of pyrrole, furan, and thiophene and the reactivity of selenophene in electrophilic substitution reactions were studied quantita tively. 1,2 There are tremendous differences in the reactiv ity, which drops by approximately ...

Positional selectivity (α : β ratio) of electrophilic substitution in pyrrole, N-methylpyrrole, and N-tert-butylpyrrole was analyzed by ab initio , MP2/6-31G(d)//RHF/6-31G(d)] and DFT [B3LYP/6-31G(d)] calculations of some σ-complexes derived from the substrates. The results of calculations with the use as model electrophilic species of trimethylsilyl cation [MP2/6-31G(d)//RHF/6-31G(d) and B3LYP/6-31G(d)] and SO 3 molecule [B3LYP/6-31G(d)] instead of proton are fairly consistent with the experimental data, according to which trimethylsilylation of pyrrole and its N-substituted derivatives with trimethylsilyl trifluoromethanesulfonate, as well as sulfonation with pyridine-sulfur trioxide complex, gives the corresponding β-substituted products.We previously performed a quantum-chemical study on positional selectivity in electrophilic substitution in derivatives of pyrrole, furan, thiophene, selenophene, and their benzo-fused analogs [1]. Our results allowed us to rationalize the absence of correlation between the reactivity (substrate selectivity) and positional selectivity (ratio of the α-and β-substituted products). Very strong differences in the reactivity (in the series pyrrole >> furan > selenophene > thiophene the reactivity decreases by about 10 orders of magnitude) may be interpreted in terms of different conditions for electron density delocalization over the ring atoms in intermediate σ-complexes (primarily in more thermodynamically favorable ionic species like A), which involves overlap of carbon π-orbitals with n-orbitals of heteroatoms belonging to different groups and periods of the Periodic Table (Scheme 1) [2,3]. However, the selectivity (α : β) conformed to a different series: furan > selenophene > thiophene > pyrrole, which was not rationalized so far. The results of our ab initio [RHF/6-31G(d), MP2/6-31G(d)//RHF/6-31G(d)] and DFT [density functional theory; B3LYP/6-31G(d)] calculations of pyrrole, furan, thiophene, and selenophene molecules, as well as of the corresponding benzo-fused systems and hetarenium ions formed upon protonation (E = H) [1], were consistent with our previous hypothesis [4, 5] implying predominant contribution of heteroatoms to the stabilization of σ-com-

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