Synthesis, structural characterization and reactivity of new trisubstituted N1-acylamidrazones: solid state and solution studies

Abstract

A series of new linear trisubstituted N¹-acylamidrazones have been investigated using a variety of analytical techniques and theoretical calculations to check the influence of the type of N¹-acyl substituent on the resonance forms and conformational behavior in the solid state and in solution. The 1D- and 2D-NMR experiments, supported by computational studies, revealed that in solution all amidrazones exhibit conformational syn/anti isomerism that results from the hindered rotation around the amide bond. In the case of the propenoic acid derivative, the ROESY, HSQC, and HMBC experiments proved a third equilibrium structure that corresponds to the planar ylide form. The SC XRD data confirmed that the compounds tend to exist as Z-anti conformers of their hydrazone-amide tautomeric species in the solid state. The carboxyl-amide heterosynthon is favoured provided that the carboxyl group is not involved in the intramolecular hydrogen bonding. Interestingly, after replacement of the cyclic C¹-substituent by the smaller propanoic acid unit, which results in lower steric hindrance, both syn and anti conformers can exist. Furthermore, in the former case both the neutral molecules and zwitterions are present in the single crystal. The relative stability of the distinct crystal forms was examined using TG-DSC methods supplemented with cohesive energy calculations (CRYSTAL). The results indicate that the unsolvated forms are stable in a wide range of temperatures; however, inclusion of solvent molecules makes them prone to cyclization at higher temperatures. This paper reports the first observation of temperature-induced cyclization of N¹-acylamidrazone to its cyclic triazole derivative in the solid state.