A critical evaluation of the s-cis-trans isomerism of 2-acetylpyrrole and its N-methyl derivative through infrared and NMR spectroscopies and theoretical calculations.

Literature data are controversial regarding the conformational equilibria of 2-acetylpyrrole (AP) and its N-methyl derivative (AMP). Now, a detailed study through infrared spectroscopy and theoretical calculations has shown that previous data were erroneously interpreted, since only a N,O-cis conformer is present in solution and that it is the stable conformer in the isolated state (ΔE(trans-cis) = 5.05 kcal mol(-1), for AP; ΔE(trans-cis) = 7.14 kcal mol(-1), for AMP). Carbonyl and NH absorption data in different solvents, supported by theoretical results taking into account the solvent effects [at IEFPCM-B3LYP/6-311++G(3df,3p) level of theory] clearly demonstrated that only the N,O-cis conformer is present in solution. However, a doublet was observed for AP, in CCl4, which can be attributed to this conformer and the lowest wavenumber component to the cis dimer form, stabilized through intermolecular hydrogen bonds (NH · · · OC). The overall preference for the N,O-cis conformer, in AP and AMP, as interpreted by the NBO analysis, indicated that the hyperconjugative effect is the main contribution to stabilize this rotamer, overcoming the possible steric repulsion. (13)C NMR experiments at low temperature in two different solvents (CS2/CDCl2 and acetone-d6) confirmed the occurrence of a single conformer since no separated signals were observed.