Pyrrolenines, tautomeric forms

Annular tautomers are prototropic tautomers in which the migrating proton is restricted to ring atoms. For these five-membered heterocycles, annular tautomerism can only occur with pyrrole 2 and its polycyclic derivatives. There is no authenticated case of the monocyclic pyrrolenine tautomeric forms 211 and 212 predominating, presumably due to the required loss of resonance energy in these nonaromatic tautomers . 

The tautomeric forms 11 and 12, called pyrrolenines, have often been postulated for pyrrole (10), but there is no conclusive evidence for their existence. The report that two isomers exist as the pyrro-lenine forms 13 and 14 (Ar = 3,4-dimethoxyphenyl) must be regarded with considerable doubt. 

As with the alkyl and aryl derivatives of the pyrrolenines and indolenines, a tautomeric equilibrium has also been noted between the pentachloro-2//- and -3H- pyrroles, such that when the 2//-pyrrole, produced by chlorination of 2,3,4,5-tetrachloropyrrole or of 3,4-dichloromaleimide, is allowed to react with dienophiles, the adducts are those formed by cycloaddition with the 3H-pyrrole tautomer (Scheme 84) (80JOC435, 80JA7862, 81JOC3036). Cycloaddition with cyclopentadiene occurs on the 2H-pyrrole, which behaves as the dienophile. 

The substitution reaction may occur on a prototropic nonaromatic form. Both types of tautomerism, i.e., that prevailing in the parent heterocycles (pyrrole-pyrrolenine, indole-indolenine) and that typical of the hydroxy and amino derivatives, must be considered. 

Progress has been made in the interpretation of the ESR spectrum of 2,4,5-triphenylimidazolyl radicals by examining the spectra of the partially and completely deuterated species and by the application of simple molecular orbital (MO) calculations. It is evident that the radicals are not planar. Some ambiguity is still present since INDO calculations indicate that the imidazyl radical should be close to a (z-radical, while ESR shows that the triphenylimidazyl radical is of the n type. The results of ESR studies of imidazole anion-radicals have been thought to demonstrate that they exist in the tautomeric a-pyrrolenine form (84) rather than 85. However, a... 

The structures of the products other than the simple 1- or 2-alkylpyrroles formed in these reactions are not proved in details. They are formulated as pyrrolenines by analogy with reactions in the indole series. The reactions have been used as an argument that pyrroles react as resonance hybrids and not as the tautomeric pyrrolenines. 

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