The conformational preferences of N-alkylpyrrole mono- and di-esters: an infrared and nuclear magnetic resonance study
Abstract
The conformational preferences of 19 N-alkylpyrrole 2- and 3-mono-, and 2,3-, 2,4-, and 2,5-di-esters have been investigated by i.r. and (in two cases) n.m.r. spectrometry. The 2-mono-esters are thought to exist in solution in only one rotameric form, probably syn-s-trans, whereas two rotational isomers may be present in the 3-mono-ester case. In the di-esters, the doublet i.r. carbonyl absorptions exhibited by the 2,5-compounds can best be rationalised in terms of a mixture of syn-syn and anti-syn conformers; 2,4-di-esters display asymmetric singlet i.r. carbonyl bands, ascribed to an equilibrium between syn-cisoid and syn-transoid forms; in the 2,3-di-ester case, the wellseparated i.r. carbonyl doublet is thought to arise from anti-cisoid and anti-transoid isomers, although the possibility of interactions between adjacent substituents makes the interpretation more problematic.
2,5-Dimethoxycarbonyl-N-t-butyl- and -N-adamantyl-pyrrole have been examined by variable-temperature proton and carbon-13 n.m.r. spectrometry for evidence of rotational isomerism due to restricted rotation about either the N-alkyl group bond or the ring-carbonyl group bonds.