Steric structure of alkyl 2-aryl(hetaryl)hydrazono-3-fluoroalkyl-3-oxopropionates

Steric structure of fluorinated 2-arylhydrazono-3-oxo esters was studied by H-1, F-19, and C-13 NMR spectroscopy and X-ray analysis. It was found that these compounds in the crystalline state and in solutions in acetone-d (6), DMSO-d (6), and CDCl3 exist as Z isomers with the ester fragment involved in intramolecular hydrogen bond with the hydrazone NH proton. Exceptions are alkyl 2-arylhydrazono-4,4-difluoro-3-oxobutanoates which exist in acetone-d (6) as mixtures of Z and E isomers, the former prevailing. Unlike fluorinated analogs, ethyl 2-(4-methylphenyl)hydrazono-3-oxobutanoate in crystal has the structure of E isomer in which intramolecular hydrogen bond is formed between the NH proton and acetyl carbonyl group. The same compound in acetone-d (6), DMSO-d (6), and CDCl3 gives rise to a mixture of Z and E isomers, the latter prevailing.