Properties of Aminopyrazines

2-Aminopyrazine is a yellowish crystalline solid with m.p. 118-120 (420). It is a weak base with reported pAj, values of 3.14 (123) and 2.96 (821) (others are recorded in Chapter X). The similarity in the ultraviolet spectra (in water) of the neutral molecules of 2-amino-, 2-methylamino-, and 2-dimethylaminopyrazines, and of their monocations, and their similar basic strengths (2.96,3.42,3.27) (821) supports the conclusion that in aqueous solution 2-aminopyrazine exists mainly in the amino form (20) (821). The ultraviolet spectrum of 2-aminopyrazine cation differs from that of the neutral molecule of pyrazine, thus indicating that protonation does not take place at the extranuclear nitrogen atom (821) 2-aminopyrazine in fact protonates at Ni (see below). 

The ultraviolet spectra in aqueous solution (821) of the methiodides isolated from the methylation of 2-amino- and 2-dimethylaminopyrazines with methyl iodide in methanol (821) differed from those obtained by protonation of 2-amino-and 2-dimethylaminopyrazine, respectively. This methiodide of 2-aminopyrazine was rapidly decomposed by aqueous alkali but did not form l-methyl-2-oxo(or imino)-l, 2-dihydropyrazine or 2-methylaminopyrazine (821). These and other observations were consistent with protonation of 2-amino(2-methylamino or 2-dimethylamino)pyrazine at N, and with the methiodides isolated involving quater-nization at N4 (821). It has been claimed from studies of ultraviolet and infrared spectra and from reactions with cyanoguanidine that in 2-aminopyrazine p-toluenesulfonate, the amino group is protonated (1189). 

Spin couplings between an aromatic N nucleus and ring protons in the p.m.r. spectrum of 3-amino-l-methylpyrazinium iodide have been examined (704). 

Proton magnetic resonance measurements in dimethyl sulfoxide indicate that 2-aminopyrazine and its 3-methyl derivative exist predominantly in the amino form (979, 1086), in agreement with theoretical and experimental electronic spectra (1083). Proton magnetic resonance analysis of the methylation of 2-aminopyrazine with methyl iodide in dimethyl sulfoxide at room temperature showed it to be methylated 8.8 times as fast as pyrazine to give both 3-amino-l-methyl- and 2-amino-l -methylpyrazinium iodides in the ratio 2.9 I (666). Proton magnetic resonance spectra of ionized 2-aminopyrazine in liquid ammonia have also been reported (665). 

Polarographic reduction of 2-aminopyrazine and 2-amino-3-carboxypyrazine to 1,4-dihydro derivatives that give 1,2-dihydro derivatives by acid catalysis has been studied (1190). 

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This section covers primary, secondary, tertiary, and quaternary aminopyrazines (both nuclear and extranuclear) but not (functionally substituted amino)pyrazines such as hydrazino-, hydroxyamino-, or azidopyrazines. General discussions have appeared on the spectra of 2-pyrazinamine,255 257 991 the proton-sponge properties of 2,3,5,6-tetra(pyridin-2-yl)pyrazine in relation to its fine structure,925 the fluorescene properties of 3,6-diamino-2,5-pyrazinedicarboxylic acid derivatives in relation to their fine structures,1646,1659 the basic properties of aminopyrazines and other such azines in relation to their electronic structures,412,928 and the fine structures of 3-amino-2-pyrazinecarboxylic acid1340 and l,4-diacetyl-2,3-diphenylpiperazine.559... 

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