Reactivity of substituted 4-oxothiazolidine derivatives in electron transfer reactions: A spectroelectrochemical study and mechanistic aspects

Highlights: → Spectroelectrochemical study on (5-R-4-oxothiazolidin)-N-phenylethanamide in DMSO. → R=ethoxycarbonylethylidene (2), R=ethoxycarbonylmethyl (1). → Proposed mechanism: common intermediate-dianion. → Bielectronic reduction 2+2e-, or 1-2H+ in basic DMSO. → E/Z isomerization, tautomerization, push-pull effect. - Abstract: The electrochemical reduction of (5-ethoxycarbonylmethylidene-4-oxothiazolidin-2-ylidene)-N-phenylethanamide (designated as compound 2) as a mixture of the (2E,5Z) and (2Z,5Z) isomers in DMSO to (Z)-(5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-N-phenylethanamide (compound 1) was investigated by electrochemical (cyclic voltammetry with a stationary and rotating disc electrode) and spectroelectrochemical (electron paramagnetic resonance and UV-vis absorption) in situ techniques. A reduction mechanism, which consisted of an ECE-Disp sequence and was followed by the protonation of the strong electrogenerated base dianion, was proposed. The results indicate the presence of the same intermediate species, the dianion, when starting from either compound 2 by electrochemical reduction, or from compound 1 in presence of TBOH in DMSO. The suggested mechanism, which is based on the experimental results, corroborated with the gas phase and the solvent-dependent semiempirical PM3-MO calculations, was rationalized in terms of the electronic structure and the reactivity of the intermediate species involved.