Dimethyl-1,4-Dithiines From 2-Chloroprop-2-Enylisothiouronium Salt in KOH/Hydrazine Hydrate Medium: Experiment and Quantum Chemical Study

Abstract

We report on the synthesis of 2,6-dimethyl-1,4-dithiine from 2-chloroprop-2-enylisothiouronium salt in the KOH/N₂H₄·H₂O medium. The yield of the target product amounted to 33% at 50°C, under the addition of a tenfold excess of potassium hydroxide. The structure of the product was characterized by ¹H and ¹³C NMR spectroscopy. The proposed mechanism of the formation of 2,6-dimethyl-1,4-dithiine and possible side reactions were explored by means of quantum chemistry methods. The complete reaction profile of the formation has been constructed and analyzed, and kinetics modeling of all the stages has been carried out. Our major finding is identifying the key reaction intermediate—methylidenethiirane—formed following the alkaline hydrolysis of the isothiouronium salt, which is responsible for the 1,2-migration of the sulfur atom.