We have studied the mechanism of the reaction between the most stable and the most represented in literature tautomers of metformin and methylglyoxal by density functional theory calculations. Designed models included 16 explicit water solvent, which forms hydrogen bond networks around the reactants and intermediates molecules, facilitating intramolecular proton transfer in some steps of the reaction mechanism. The reaction takes place in five steps, namely: (1) formation of a dimethylguanide–guanylhydrazone–acetylcarbinol adduct, (2) formation of a zwitterionic triazepine derivative by ring closure, (3) dehydration of this intermediate, (4) an imine–enamine tautomerism, (5) an enol–keto tautomerism. The first step was found as the rate-determining step for the reaction, having the reaction an overall activation energy value of 9.7 kcal mol−1.
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