Structure Determination of the N-Methyl Isomers of 5-Amino-3,4-dicyanopyrazole and Certain Related Pyrazolo[3,4-d]pyrimidines

The position of N substitution of certain substituted 4-aminopyrazolo[3,4-d]pyrimidine derivatives has been studied by chemical and spectroscopic techniques and has resulted in the assignment of structures to the pyrazole precursors of these compounds. The more abundant pyrazole resulting from treatment of tetracyanoethylene with methylhydrazine [identical with the single pyrazole isomer originally isolated by the same condensation procedure, C. L. Dickinson, J. K. Williams, and B. C. McKusick, J. Org. Chem., 29, 1915 (1964), which was not characterized definitively with respect to the position of the N substituent] has thus been assigned as 3-amino-4,5-dicyano-1-methylpyrazole on the basis of its conversion to a pyrazolo[3,4-d]pyrimidine identical with authentic 4-amino-2-methylpyrazolo[3,4-d]pyrmidine, rather than with the authentic 1-methyl isomer. The assigned structure has been verified by X-ray crystallographic determination of 3-amino-4,5-dicyano-1-methylpyrazole. Because 3-amino-4,5-dicyano-1-methylpyrazole would not be expected to be the more abundant pyrazole on the basis of previous work, a mechanism is proposed which accounts for its formation. Also studied was the position of tautomeric equilibrium in 3-amino-4,5-dicyanopyrazole. A consideration of the ¹³C NMR spectrum of 3-amino-4,5-dicyanopyrazole, relative to those of 5-amino-3,4-dicyano-1-methylpyrazole and 3-amino-4,5-dicyano-l-methylpyrazole, as well as the N-acetyl derivatives of all three, indicated that the major tautomer was 5-amino-3,4-dicyano-1H-pyrazole. A comparison of the ultraviolet spectrum of this pyrazole with those of the two methylated isomers led to the same conclusion.