Correlations Between Carbon-13 NMR and X-Ray Crystallography for Pyrroles Containing Electron Withdrawing Groups
Paine, J.B., David Dolphin, Trotter, J., and Greenhough, T.J.
Can. J. Chem.
The 13C NMR chemical shifts of benzyl 4-ethyl-3,5-dimethylpyrrole-2-carboxylate (I), Me E-3-(3-ethyl-4,5-dimethylpyrrol-2-yl)-2-cyanopropenoate (II), and Me E-3-(5-benzyloxy)carbonyl-3-ethyl-4-methylpyrrol-2-yl)-2-cyanopropenoate (III) were compared to their x-ray crystallographic structures. The 13C NMR chemical shifts were determined by direct comparison of a series of closely related homologs. I is monoclinic, space group P21/c, with a 14.934(2), b 6.674(2), c 15.269(2)Å and β 101.96(1)+; Z = 4. II is monoclinic space group P21/n, with a 7.3030(3), β 13.478(1), c 12.985(1) Å, and β 97.48(1)+; Z = 4. III is monoclinic, space group P21/c, values of a 11.157(2), b 13.109(2), c 14.068(1) Å, and β 115.47(1)+; Z = 4. The structures were determined by direct methods and refined by full-matrix least squares to R values of 0.052, 0.040, 0.038 for 862, 2032, 1483 reflections. Atomic coordinates are given. The molecules are approx. planar, except for deviations of the Ph rings and of the terminal C atoms of the C3-Et 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 substituents; exocyclic bond lengths also exhibit differences from normal values. The competitive effects of the various electron-withdrawing groups were correlated to change in bond lengths, 13C chemical shifts, and the chem. reaction of the pyrrolic nucleus.