The effect of ortho and para substituents on the formation of the E and Z isomers of the arylhydrazones obtained from diazonium coupling with methyl 3-aminocrotonate and 3-aminocrotononitrile

Reaction of arene diazonium salts with 3-aminocrotononitrile or methyl 3-aminocrotonate affords the 2-aryl-hydrazono-3-oxobutanenitrile (1 or 3 and 3′) or the methyl 2-arylhydrazono-3-oxobutanoate (2 and 2′ or 4 and 4′). A series of these hydrazones has been prepared with a range of electron-withdrawing and -donating substituents in the ortho or para position of the aryl moiety. The hydrazones have been characterized by spectroscopic methods, with emphasis on the 1 H NMR spectra, which have been used to determine the configuration of the hydrazones as E or Z or a mixture of the two. The para-substituted hydrazononitriles (1) are formed as a single species, namely the Z isomer, whereas the ortho isomers are formed as a mixture of E and Z configurations (3 and 3′). The hydrazonobutanoates (2 and 2′ or 4 and 4′) are formed as E/Z mixtures regardless of the position of the substituent in the aryl moiety. Complete assignments of all signals in the 1 H NMR spectra have been made on the basis of the ability of the various substituents to participate in intramolecular hydrogen bonding and a mechanism is proposed to account for the variations in the proportions of E and Z isomers and the effect of the nature of the substituent on this ratio. 13 C NMR spectra of selected hydrazones have been recorded as an aid to structure assignment. Key words: hydrazone, diazonium, NMR spectroscopy, E/Z isomers, crotonic acid derivatives.