Conflicting findings in pyrrolo[1,2-d]tetrazoles: a computational study

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Two structural problems concerning the prototropic and ring-chain tautomerism of pyrrolo[1,2-d]tetrazoles were clarified by means of DFT calculations. The first one concerns the carboxyethyl ester structure of the compound obtained by treating 7-methylpyrrolotetrazolide with ethyl chloroformate and the second one the structure of a precursor of a dye that was not a tetrazole as reported but 2-azido-5-chloro-1H-pyrrole-3,4-dicarbonitrile. Although the conclusions to the two problems have already been published elsewhere by one of the present authors (D.M.), the new NMR data of the aforementioned compound together with GIAO/B3LYP/6-311++G(d,p) calculations establish those conclusions on a solid basis. But the most important thing about the present work is to establish that 1H-pyrrole is a peculiar azole while the pyrrolide anion behaves like a classic azole.