13C nuclear magnetic resonance spectra of quinoxaline derivatives

Abstract

The ¹³C n.m.r. spectra of a series of 5-, 6-, and 2-substituted quinoxalines have been analysed by consideration of their ¹H-coupled spectra. Typical values of the coupling constants are: C(2,3),¹J_CH 181.9,²J_CH 11.4; C(5,8), ¹J_CH 162.6, ³J_CH 6.5, C(6,7), ¹J_CH 159.4, ³J_CH 9.1; C(9), ³J_CH(2)=³J_CH(7)= 10.0, ³J_CH(5) 5.4; C(10), ³J_CH(3)=³J_CH(6)= 10.0, ³J_CH(8) 5.4 Hz. The magnitudes of the coupling constants in the benzenoid ring [C(5)–C(10)] are similar to these for the corresponding positions in naphthalene, but application of naphthalene chemical shift substitutions effects leads in some cases to the wrong peak sequence in the related quinoxalines. Within the quinoxaline series itself, however, acceptable additivity of substituent effects is found (±0.8 p.p.m.), provided that the reference compounds are carefully chosen. Analysis of mixtures of quinoxalines from substituted o-phenylenediamines and α-oxo-aldehydes is possible by consideration of the multiplicity of the ring-junction quaternary carbon signals in the fully coupled spectra.