Mechanistic insights into a Zn-assisted ring-chain tautomerism process involving a 1,2,3,4-tetrahydroquinazoline and a Schiff base

Density functional theory calculations and spectroscopic monitoring of the reactions of the chain tautomer N-{2-[(8-hydroxyquinolin-2-yl)methyleneamino]benzyl}-4-methylbenzene-sulfonamide (H2Lchain) and the ring tautomer 2-(3-tosyl-1,2,3,4-tetrahydroquinazolin-2-yl)quinolin-8-ol (H2Lring) against Zn(OAc)2•2H2O have been performed in order to clarify the role of the metal ion in the ring-opening/closing reaction under investigation. The results obtained show that, among the processes leading to zinc complexes that could occur, the path leading to Zn(HLring)2 is the energetically more favorable one. Thus, this path would be preferred in the presence of sufficient free ligand in solution, regardless of its tautomeric form. The latter is due to the formation of energetically similar key intermediate complexes Zn(OAc)(HLchain) and Zn(OAc)(HLring). The process to afford Zn2(Lchain)2 by dimerization of the intermediate complex Zn(OAc)(HLchain) is energetically unfavourable and is only obtained in good yield by decreasing the stoichiometry of the ligand and at high reaction temperature.