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Structure of a 4-Nitroso-5-aminopyrazole and Its Salts:  Tautomerism, Protonation, and E/Z Isomerism

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journal contribution
posted on 14.11.2003, 00:00 by Marcus H. Holschbach, Dionisia Sanz, Rosa M. Claramunt, Lourdes Infantes, Sam Motherwell, Paul R. Raithby, María Luisa Jimeno, David Herrero, Ibon Alkorta, Nadine Jagerovic, José Elguero
The structures of 1-benzyl-4-nitroso-5-aminopyrazole (1) and its hydrochloride (1H+) have been determined in the solid state and in solution in DMSO, methanol, and ethanol. The free base exists in solution as a mixture of amino/nitroso tautomers 2a and 2b rather than in the imino/oxime tautomers 3. The conjugated cation 1H+ results from the protonation of the nitroso group. X-ray crystallography showed that both amino hydrogen atoms of 2a form NH···ON hydrogen bonds:  one is intramolecular, the other links adjacent molecules in an infinite chain.

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