Synthesis and intramolecular cyclization of bisthiadiazinylmethane derivatives

3,4-Disubstituted derivatives of 1,2,5-thiadiazole 1,1-dioxide. Ethanol addition reactions and electroreduction of 3-methyl-4-phenyl and 3,4-dimethyl derivatives in acetonitrile and ethanol solvents

3-Methyl-4-phenyl-1,2,5-thiadiazole 1,1-dioxide (TMP), as well as 3,4-dimethyl-1,2,5-thiadiazole 1,1-dioxide (TMM), react with ethanol (EtOH), which adds to one of their C = N double bonds. The equilibrium constants for the addition reaction are measured in mixed acetonitrile (ACN) – EtOH solvents by means of UV spectroscopy in the case of TMP, and by 13C NMR spectroscopy in the case of TMM, since TMM presents only terminal UV absorption. Both equilibrium constants are also estimated through cyclic voltammetry (CV) experiments. In the case of TMP, the ethanol molecule adds to the C = N bond located on the methyl-substituted side of the substrate, according to 13C NMR spectroscopy and CV results. The electroreduction characteristics of the substrates and their ethanol addition products are studied using CV techniques in ACN, EtOH, and CAN–EtOH solvent mixtures. The radical anion formed by the first electron transfer to TMM is unstable and decomposes rapidly while that corresponding to TMP undergoes a relatively slow homogeneous second-order reaction with the substrate (k = 3 × 102 M−1 s−1). The equilibrium constant for EtOH addition and the voltammetric properties of the substrates are compared with those of the previously studied 3,4-diphenyl derivative (TPP). Key words: electrochemistry, thiadiazoles, structure–reactivity relations, kinetics.