Theoretical study of the possible mechanisms for the synthesis of dialkyl thiourea from dithiocarbamate

In this study, the different mechanistic ways for the conversion of dithiocarbamate to dialkyl thiourea have been investigated using the high-accurate DFT calculations. The energy details of all mechanisms were investigated in the gas phase, simple and solvent-assisted solvation models. Two general mechanisms and several different pathways have been considered to evaluate these mechanisms. The first mechanism (A) involved with the preparation of alkyl isothiocyanate, addition of alkyl amine to it and the final proton transfer. The second mechanism (B) is consisted of the addition of alkyl amine, proton transfer, and elimination of thiol. In the gas phase and solvent-assisted models, the mechanism A is preferred; while in PCM model, the mechanism B is preferred. In both mechanisms, both solvation models have enhancing effects on the thermodynamics of the reactions and stabilize the product versus the reactant (comparing with the gas phase). In mechanism A, the amine should have at least 1 proton and the dithiocarbamate should also have 1 proton in its structure. Both proton transfer steps of these mechanisms have been facilitated by the proton transfer of amine molecule and both solvation models (PCM and explicit presence of water) increased the barriers and reduced the rate of this mechanism. However, in mechanism B, the PCM solvation model reduces the barriers and accelerates the reaction but the solvent-assisted model increases the barriers and reduces the reaction's rate. The water molecule could not assist efficiently by making the proton bridge in these cases.