Correlations between carbon-13 nuclear magnetic resonance and X-ray crystallography for pyrroles containing electron-withdrawing groups

The 13 C nmr chemical shifts of benzyl 4-ethyl-3,5-dimethylpyrrole-2-carboxylate ( 1 ), methyl E-3-(3-ethyl-4,5-dimethylpyrrol-2-yl)-2-cyanopropenoate ( 2 ), and methyl E-3-(5-(benzyloxy)carbonyl-3-ethyl-4-methylpyrrol-2-yl)-2-cyanopropenoate ( 3 ) have been compared to their X-ray crystallographic structures. The 13 C nmr chemical shifts were determined by direct comparison of a series of closely related homologs.Crystal data for 1 , 2 , and 3 are as follows: 1 , monoclinic, P2 1 /c, a = 14.934(2), b = 6.674(2), c = 15.269(2) Å, β = 101.96(1)°, Z = 4; 2 , monoclinic, P2 1 /n, a = 7.3030(3), b = 13.478(1), c = 12.985(1) Å, β = 97.48(1)°, Z = 4; 3 , monoclinic, P2 1 /c, a = 11.157(2), b = 13.109(2), c = 14.068(1) Å, β = 115.47(1)°, Z = 4. The structures were determined with diffractometer data by direct methods, and refined by full-matrix least-squares techniques to R = 0.052, 0.040, 0.038 for 862, 2032, 1483 reflexions, respectively. The molecules are approximately planar, except for deviations of the phenyl rings and of the terminal carbon atoms of the C 3 -ethyl groups from the molecular planes. The bond lengths in the pyrrole rings differ from those in pyrrole itself, as a result of the presence of the strongly electron-withdrawing sustituents; exocyclic bond lengths also exhibit differences from normal values.The competitive effects of the various electron withdrawing groups have been correlated to change in bond lengths, 13 C chemical shifts, and the chemical reactions of the pyrrolic nucleus.