Substituent, temperature and solvent effects on keto-enol equilibrium in symmetrical pentane-1,3,5-triones. Nuclear magnetic resonance and theoretical studies

Keto-enol tautomeric equilibrium in several beta -triketones has been inves tigated using NMR spectroscopy and theoretical methods. The equilibrium inv olves two slow and two fast enolization processes in solution. In solvents of low polarity and at room temperature, the predominant tautomeric species is the dienol form. However, the equilibrium is significantly shifted to t he more polar triketo form on raising the temperature up to 140 degreesC an d by using solvents of higher polarity. Structures and stabilities of both long and short-lived tautomeric forms, as well as transition-state structur es and barrier heights of enolization processes, were calculated using semi empirical and density functional quantum chemical methods.