A plausible mechanism for the nucleophile-mediated ring expansion of 4-chloromethyl-1,2,3,4-tetrahydropyrimidin-2-ones into 2,3,6,7-tetrahydro-1H-1,3-diazepin-2-ones based on DFT calculations at B3LYP/6-31+G(d,p) level is discussed. This mechanism involves the following subsequent steps: N₍₁₎H deprotonation under the action of nucleophile, intramolecular nucleophilic substitution of the chlorine atom to give cyclopropane bicyclic intermediate, nucleophile-promoted cyclopropane ring opening leading to 2,5-dihydro-1H-1,3-diazepin-2-one, and addition of nucleophiles to the C=N bond to afford the final diazepinones.