Pyrroles from ketoximes and acetylene - III. Conjugation in 2-alkyl- and 2,3-dialkyl-1-vinylpyrroles and their ¹³C NMR spectra

The ¹³C NMR spectra of eight 2-alkyl- and 2,3-dialkyl-1-vinylpyrroles were studied. The ¹³C chemical shifts of all of the carbon atoms of the ring and the vinyl group depend substantially on the position and structure of the alkyl substituent. As the branched character of the alkyl group in the 2 position increases, the signal of the β-carbon atom of the vinyl group is shifted to weak field due to weakening of the p-π conjugation in the N-vinyl group because of disruption of its coplanarity with the pyrrole ring. The conjugation between the double bond and the pyrrole π system involves competition for possession of the p electrons of the nitrogen atom.