Self-association, tautomerism and E-Z isomerization of isatin-phenylsemicarbazones - spectral study and theoretical calculations

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 CHCl3 and benzene, phenylsemicarbazone concentrations above 1x10-5 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 stableZ-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 degrees C. Moreover, decrease in phenylsemicarbazone concentration below 1x10-4 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 (c) 2013 John Wiley & Sons, Ltd.