An infrared study of the conformations and association of pyrrole-2-carbaldehydes and pyrrole-2-carboxylates

Abstract

The NH and CO bands of solutions of pyrrole-2-carboxylates, pyrrole-2-carbaldehydes, and their N–Me analogues have been studied. Knowledge gained from the spectra of the aldehydes, which are known to adopt the N,O-syn-conformation, was applied in deducing the conformational preferences of the esters from their i.r. absorptions. The N-Me esters exist predominantly or exclusively in the N, carbonyl O-syn-arrangement. With the N–H esters both syn- and anti-rotamers are present, the former predominating, and in the case of t-butyl pyrrole-2-carboxylate the syn-form was shown to be thermochemically more stable than the anti-form by ca. 4.8 kJ mol^–1. The intermolecular associations of pyrrole-2-carbaldehyde and of t-butyl pyrrole-2-carboxylate give species containing more than two molecules of the substrates and involve similar enthalpy changes (ca. 22 kJ mol^–1).