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

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-1 H-pyrrole-3,4-dicarbonitrile. Although the conclusions to the two problems have already been published elsewhere by one of the present authors (DM), 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. However, the most important thing about the present work is to establish that 1 H-pyrrole is a peculiar azole, while the pyrrolide anion behaves like a classic azole.