Pyrroles tautomeric forms

In many cases, substituents linked to a pyrrole, furan or thiophene ring show similar reactivity to those linked to a benzenoid nucleus. This generalization is not true for amino or hydroxyl groups. Hydroxy compounds exist largely, or entirely, in an alternative nonaromatic tautomeric form. Derivatives of this type show little resemblance in their reactions to anilines or phenols. Thienyl- and especially pyrryl- and furyl-methyl halides show enhanced reactivity compared with benzyl halides because the halogen is made more labile by electron release of the type shown below. Hydroxymethyl and aminomethyl groups on heteroaromatic nuclei are activated to nucleophilic attack by a similar effect. 

For the NH azoles (Table 3), the two tautomeric forms are usually rapidly equilibrating on the NMR timescale (except for triazole in HMPT). The iV-methyl azoles (Table 4) are fixed chemical shifts are shifted downfield by adjacent nitrogen atoms, but more by a pyridine-like nitrogen than by a pyrrole-like iV-methyl group. 

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. 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

Upon reaction with nitrous acid, indole produces a complex mixture of products. In addition to 3-oximino-3H -indole (16), which is the stable tautomeric form of 3-nitrosoindole (17), dimeric products of the type (18) and (19) are also formed. In contrast, (16) appears to be the sole product of the nitrosation of indole with amyl nitrite and sodium ethoxide (72HC(25-2)537). Studies of the nitrosation of pyrrole are somewhat indecisive. The mononitrosopyrrole, obtained from the reaction of pyrrole with nitrous acid, has not been fully characterized, but there is some evidence that nitrosation of pyrrole with amyl nitrite and sodium ethoxide leads to the sodium salt of the 3-nitroso derivative. However, upon the addition of acid, the product rearranges to give the oxime of 3-formylisoxazole (20) (B-77MI30502). 

Comparing the [l,5]-sigmatropic hydrogen shifts in pyrrole and phosphole, at the MP2/6-31G level, Bachrach has shown that the more stable tautomeric form is, respectively, IH -pyrrole and 2//-phosphole2d. The activation enthalpies were computed to be 186 kJmoG1 for 1H-pyrrole to 2H-pyrrole transformation and only 67 kJmoG1... 

Azoloazoles represent interesting objects for study. Most of these structures are very unstable and can be regarded only as intermediates <1998JPR687>. Nonetheless, some, for instance pyrrolotetrazole and its derivatives, are relatively stable and can exist as the tautomeric forms 17 and 18 (Equation 2). In these structures only one of the heterocycles remains aromatic, for example, it is the tetrazole ring in the 5//-tautomer 18 and the pyrrole ring in the 1/7-tautomer 17. 

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 . 

Color test for pyrroles. One drop of an aqueous, ethereal, or alkaline test solution is mixed on a spot plate with one or two drops of a 5% solution of p-dimethylamino-benzaldehyde. If pyrrole or a substance capable of forming pyrrole is present, a violet color will appear. The aldehyde condenses with a tautomeric form of pyrrole (3) to give (4), which is converted by hydrogen ion into the colored quinonoid ion... 

Compound (93), written in its tautomeric form (96) (similar to a Schiff base), and 5-(D-am6i m-tetrahydroxybutyl)pyrrole-2-carboxylic acid (94) have also been considered, by Gottschalk, to be intermediates of this reaction. The final product of the reaction (95, X = CIO4) has been isolated... 

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