Aminoquinoxalines, tautomerism

Aminoquinoxaline exists predominantly as such rather than in the tautomeric imino form. This is indicated by a comparison of the basic strength of the 2-amino compound (pKo 3.90) and those of its fixed methylated tautomers, 2-dimethylaminoquinoxaline pKa 3.72) and l-methylquinoxalin-2-oneimine (pfCn 8.70). The ultraviolet spectrum of the neutral molecule of 2-dimethylaminoquinoxaline shows the expected bathochromic shifts compared to that of 2-aminoquinoxaline these spectra differ from the ultraviolet spectrum of the neutral molecule of l-methylquinoxalin-2-oneimine (Fig. 1). The mono-cations (68) and (69) derived from 2-aminoquinoxaline and l-methylquinoxalin-2-oneimine have a similar chromophoric system and show almost identical ultraviolet absorption (Fig. 2). 

It should be noted that, as all carbon positions in pyrazine are identical, the locant 2- in a monosubstituted derivative is unnecessary. All possible reduced derivatives of pyrazine 1, and several of those of its benzo analogues quinoxaline 2 and phenazine 3, are known. There are four dihydropyrazines, the 1,2-, 2,3-, 1,4-, and 2,5-isomers, two tetrahydropyrazines, the 1,2,3,4- and 1,2,3,6-, and hexahydropyrazine or piperazine, the last of which is omitted in this chapter. The reduced quinoxalines are the 1,2- and 1,4-dihydro compounds and 1,2,3,4-tetrahydroquinoxaline. The only known reduced phenazine is 1,4-dihydrophenazine. Hydroxypyrazine 4 and hydroxyquinoxaline 6 have been shown to exist in the tautomeric amide form by spectral studies, and therefore they are formulated as 2(1//)-pyrazinone 5 and 2(l//)-quinoxalinone, respectively. In contrast, aminopyrazine and aminoquinoxaline exist as described in the amino rather than the imino forms (Figure 1). 

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