Self‐association, tautomerism and <i>E–Z</i>isomerization of isatin–phenylsemicarbazones – spectral study and theoretical calculations

Abstract

The self‐association and tautomerism of ( E )‐isatin‐3‐4‐phenyl(semicarbazone) Ia and ( E )‐ N ‐methylisatin‐3‐4‐phenyl(semicarbazone) IIa were investigated in solvents of various polarity. In weakly interacting non‐polar solvents, such as CHCl 3 and benzene, phenylsemicarbazone concentrations above 1×10 −5 mol dm −3 result in the formation of dimers or higher aggregates of E ‐isomers Ia and IIa . This aggregate formation prevents room temperature E–Z isomerization of Ia and IIa to more stable Z ‐isomers. In contrast to the situation in non‐polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone Ia and IIa E ‐isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10 −4 mol dm −3 in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results. Copyright © 2013 John Wiley &amp; Sons, Ltd.