Carbon-13 NMR Studies on some 5-Substituted Quinoxalines

Article abstract of DOI:10.1002/mrc.1270140417

Eleven 5-substituted quinoxalines (NO2, NH2, COOH, OCH3, CH3, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods.Their 13C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities.The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines.The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl3, CD3COCD3, CD3CN, CD3OD, C6D6 and CD3COOD.In all solvents an abnormally low ²J(CF) (ca. 12 Hz) was found for C-4a and no C-F spin-spin splitting could be detected for the three-bond coupling of C-8a.Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide.There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6.A linear relationship also exists between the chemical shift of C-8 ('para' position) and the Hammett ?_p parameter of the substituent.