Solvent-induced E/Z isomerization of 2-(furylmethylidene)-1-hydrazinecarbothioamide: The N–H⋅⋅⋅O intramolecular hydrogen bond as promoting factor

Abstract

It is proved by the 1H and 13C data that the thiosemicarbazone with the furyl substituent is synthesized as the E isomer. However, this thiosemicarbazone undergoes the solvent-induced E/Z isomerization when standing in chloroform, despite the fact that the other thiosemicarbazones with a heteroaryl or aryl substituent do not exhibit such phenomenon. It is established that the N–H⋅⋅⋅O intramolecular hydrogen bond in the Z isomer of furyl thiosemicarbazone promotes the E/Z isomerization. As it is shown by calculations, the energy difference between the E and Z isomers is negligible in this case, although the E isomer is significantly preferable in energy (by 2.5–5.2 kcal mol–1) in the other thiosemicarbazones. At the same time, no E/Z isomerization is observed when the furyl thiosemicarbazone stands in the DMSO‑d6 solution. The formation of a strong hydrogen-bonded complex of furyl thiosemicarbazone with the DMSO molecule via the N–H⋅⋅⋅O intermolecular hydrogen bond leads to the fact that the E→Z conversion becomes an energy unfavorable process. A comparison of the calculated 1H and 13C chemical shifts in the different conformations with measured values indicates that the syn(O,N)-conformation of the Z isomer of furyl thiosemicarbazone is stabilized by the N–H⋅⋅⋅O intramolecular hydrogen bond. However, a conformational equilibrium between the syn(O,N)- and anti(O,N)-forms is of the case for the E isomer. Thus, furyl thiosemicarbazone is an example of molecular switch controlled by interaction with the environment, rather than using hard UV radiation.