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Positional selectivity in reactions of pyrrole and its N-substituted derivatives with electrophiles

“…5 It was shown that, in contrast to the known sequence of relative reactivity (substrate selectivity) in electrophilic substitution reactions (pyrrole >> furan > selenophene > thiophene 1,2 ), which may be explained by the different conditions for delocalization of electron density on ring atoms with the participation of heteroatoms belonging to a different group and period of the Periodic System, the positional selectivity reduces in the sequence: furan>> selenophene ≥ thiophene > pyrrole, corresponding to that for the relative stability of the onium state of the elements (O + < Se…”

“…5,19,20 The orientation of electrophilic substitution in the pyrrole series depends also on the nature of the electrophile. Electrophilic hydrogen−deuterium exchange was investigated 21 .…”

“…The geometries of the neutral molecules of N-isopropylpyrrole (1d) and N-(t-butyl)pyrrole (1e) are evidence of steric strain. The bonds N (1) −С (2) and N (1) −С (5) in 1d differ markedly from one another in length, while in 1e the distances between the centers of the hydrogen atom in position 2 and the nearest H atoms of two methyl groups (2.39 Å) are lower or equal to the sum of Van der Waals radii. Also, the distance from the center of H atom in the 5 position to the center of the nearest H atom of the methyl group (2.33 Å) is lower than the sum of Van der Waals atoms that is equal to 2.4 Å.…”

“…5 The present communication is devoted to the orientation of electrophilic substitution reactions of N-substituted pyrroles, for which a considerable effect of a substituent at the nitrogen atom on the α:β-ratio should be expected, taking into account the foregoing. It is important to mention that, according to numerous data of quantum chemical calculations carried out by semi-empirical methods 6 (see also the references cited in the review 7 devoted to Nvinylpyrrolium ions), σ-complexes modeling the substitution at the α-position of Nunsubstituted pyrroles are thermodynamically preferable than their β-isomers, while kinetic factors (higher negative charge) favor β-substitution.…”

“…On going from the neutral molecule to the cation in the case of 2H-pyrrolium ions, the bonds N (1) −С (2) , С (2) −С (3) , С (4) −С (5) are lengthened; on the other hand, the bonds N (1) −С (5) , С (3) −С (4) are shortened. In the case of the 3H-pyrrolium ions, the bonds N (1) −С (2) and С (4) −С (5) are shortened but the bonds N (1) −С (5) , С (2) −С (3) , and С (3) −С (4) are lengthened. This clearly reflects the changes in their multiplicity on going from a neutral molecule to a σ-complex.…”