Radical anions of quinoxalines (an in situ electron paramagnetic resonance spectroelectrochemical and theoretical study)

The cathodic reduction of 10-ethyl-7 H ,10 H -pyrido[2,3- f ]quinoxalin-7-one (1a) and ethyl 10-ethyl-7-oxo-7 H ,10 H -pyrido[2,3- f ]quinoxaline-8-carboxylate (2a), as well as their 2,3- R,R -substituted derivatives ( R = CH 3 or C 6 H 5 ), represents a reversible one-electron process coupled with the generation of corresponding radical monoanions as evidenced by in situ electron paramagnetic resonance (EPR)/UV–vis-near-infrared (NIR) cyclovoltammetric experiments in N , N -dimethylformamide. The detected radical monoanions are characterized by electronic absorption bands in the vis-NIR regions, and their EPR spectra show a dominant interaction of the unpaired electron with the nitrogen and hydrogen nuclei of the pyrazine ring. The reduction behavior of 10-ethyl-7-oxo-7 H ,10 H -pyrido[2,3- f ]quinoxaline-8-carboxylic acid (3a) and its 2,3-dimethyl or 2,3-diphenyl derivatives is more complex, revealing an irreversible first reduction peak, followed by a second reversible reduction step. Based on the results of the in situ EPR/UV–vis-NIR spectroelectrochemical measurements for quinoxaline carboxylic acids, the first irreversible reduction peak was assigned to a reduction process on the pyridone ring, followed by a second reversible reduction process on the π-electron deficient pyrazine moiety, generating the radical monoanions detected by EPR spectroscopy.