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Journal of Structural Chemistry

2009 year, number 6

Structural tautomerism of 4-acylpyrazolone Schiff bases and crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one

A.S. Amarasekara, O.S. Owereh, K.A. Lyssenko, T.V. Timofeeva
Keywords: 4-acylpyrazolone, Schiff-base, tautomerism
Pages: 1213-1218


The Schiff base derivatives prepared from 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one and alkyl amines are shown to remain exclusively in the amine-one(I) tautomeric form in chloroform solutions at room temperature using a combination of 1H, 13C, APT, COSY, HMQC, and HMBC NMR spectroscopic methods. The crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one showed that this
4-acylpyrazolone Schiff base stays in the amine-one(I) form in the solid state as well, and the solid state structure supports the fact that strong hydrogen bonding between amine hydrogen and the pyrazolone C3 carbonyl oxygen helps to stabilize the amine-one(I) tautomer.