2,5-Dimethylpyrazine 1-oxide salts

Dimethylpyrazine 1-oxide (52) does not appear to react at the exocyclic oxygen with methyl iodide or benzyl chloride but instead reacts at N-4 to form the quaternary salt 53. ... 

The strong base-weakening effect of an A-oxide substituent upon a para-situated sp nitrogen atom is exemplified by a comparison of the of pyrazine [0.65 (122)] with that of pyrazine A-oxide [0.05 (745)]. The pKg of 3-methylpyrazine 1-oxide is 0.46 (745). Pyrazine A-oxides form salts thus 2,5-dimethylpyrazine A-oxide forms 1 1 addition products with hydrogen chloride, methyl iodide, and benzyl chloride (625). Thermodynamic parameters for the second protonation of tetramethylpyrazine 1,4-dioxide have been determined from measurements at 25, 40,60,80, and 90° (746). 

Quaternization of the four dimethylpyrazine A(-oxides and 2,3-diphenylpyrazine 1-oxide with methyl iodide in a sealed tube at 80 gave the 4-methylpyrazinium 1-oxide iodides, but 3-phenyl-, 2,5-diphenyl- and 3,5-diphenylpyrazine 1 -oxides and dimethylpyrazine 1,4-dioxides could not be quaternized. The quaternary salts were reduced by sodium borohydride to A -hydroxypiperazines (766). 

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