Carbon-13 nuclear magnetic resonance spectra of some substituted pyrroles

Abstract

The ¹³C n.m.r. spectra of 55 pyrroles have been recorded and assigned. The pyrrole ring carbon chemical shifts can be predicted, to ca. 0·5 p.p.m., on the basis of additive substituent effects together with contributions due to adjacent substituents (steric compression shifts) and in the case of carbonyl functions in the α-positions, an additional ‘conjugation effect’. The substituent chemical shift parameters obtained are analogous to those for substituted thiophens, and are compared with those obtained from ethylenes and benzene derivatives. The C(2) substituents affect mainly C(3) and C(5)(apart from the directly bonded carbon) and the C(3) substituents mainly C(2), and these directive effects may be rationalised by accepted resonance considerations. The steric compression effects parallel those found for other aromatic and olefinic systems; a buttressing effect of the next-but-one substituent was also identified. Novel conjugation effects were manifest for two α-carbonyl substituents at all the ring carbons.