Synthesis of 2-arylhydrazono-4-chloro-3-oxobutanoates existing as (E)/(Z)-hydrazone isomers in solution: correlation between equilibrium constants and substituent constants

Various 2-arylhydrazono-4-chloro-3-oxobutanoates 10a-i synthesized by the reaction of ethyl 4-chloro-3-oxobutanoate with the 4-substituted benzenediazonium chlorides were found to exist as two isomers between the (E)- and (Z)-hydrazone forms in DMSO‑d6 solution. The equilibrium constants {K = [(Z)-form]/[(E)-form]: (8.0–3.4) at 30 °C} were clarified to correlate with the substituent constants of R (σp: 0.78 to −0.27) in the aryl group of compounds 10a-i, wherein the correlation coefficient was 0.99. The preponderance of the (Z)-hydrazone isomers in compounds 10a-i was assumed to be directed by the 4-chloro substituent, while the ratios of less stable (E)-hydrazone isomers gradually increased with decrease in σp values and rise in temperature.