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Carbon‐13 NMR studies on some 5‐substituted quinoxalines
Author(s) -
Hollstein Ulrich,
Krisov Galen E.
Publication year - 1980
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270140417
Subject(s) - substituent , chemical shift , chemistry , benzene , naphthalene , carbon 13 nmr , nitrogen , nmr spectra database , carbon fibers , spectral line , medicinal chemistry , crystallography , stereochemistry , organic chemistry , materials science , physics , astronomy , composite number , composite material
Eleven 5‐substituted quinoxalines (NO 2 , NH 2 , COOH, OCH 3 , CH 3 , OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their 13 C NMR spectra have been measured in DMSO‐ d 6 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 CDCl 3 , CD 3 COCD 3 , CD 3 CN, CD 3 OD, C 6 D 6 and CD 3 COOD. In all solvents an abnormally low 2 J (CF) (∼ 12 Hz) was found for C‐4a and no CF 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 σ p parameter of the substituent.