SCIPCM study of the solvent effect on the conformations and keto-enol tautomerism of dihydroresorcino

M. I. SANCHO; A.H. JUBERT; S.E. BLANCO; F.H. FERRETTI; E.A. CASTRO

Cordoba (Argentina)

Conferencia; 9 Conferencia Latinoamericana de Fisicoquímica Orgánica; 2007

Asociación Argentina de Investigación Fisicoquímica - Sociedad Argentina de Investigación en química Orgánica. Universidad Nacional de Rio Cuarto

The solvent effect on the conformations and keto-enol tautomerism of dihydroresorcinol were studied by means of a B3LYP/6-31G(d) (Becke hybrid three-parameter nonlocal exchange functional combined with the Lee-Yang-Parr dynamic correlation functional) method that makes uses of the SCIPCM model. (self-consistent isodensity polarizable continuum model) The studied compound presents two non planar conformers, and their thermodynamic stabilities differ in 1.245 (EtOH) 1.225 (MeOH) and 1.189 (Water) Kcal mol-1. This means that solvents with greater basitiy disfavour the conformational reaction Conf 1<==> Conf 2. When the equilibrium is reached at 298 K, there is a 14% of Conf 1 and a 86% of Conf 2.Cyclohexane-1,3-dione presents the characteristic Boat and Chair conformations, been the Chair form the most thermodynamically stable in the used hydroxylic solvents. The energy difference between both conformations decreases when the solvent hydrogen bond donating (HBD) capability increases. The characteristics equilibrium constants of the conformational reaction Boat <==> Chair were related to solvents semiempirical solvation parameters, and the following linear equations were obtained: (a) Ln KC = 0.0392 alfa + 2.1764 (r>0.99); (b) Ln KC = 0.0426 beta + 2.2424 (r>0.98). These equations indicate that solvents with lower hydrogen bond acceptor capability favour the formation of the Chair conformer. When the equilibrium is reached, there is a 9.8% of the Boat form and a 90.2% of the Chair form. The obtained activation energy values in ethanol methanol and water were 5.219, 5.213 y 5.201 kcal mol-1, respectively.The tautomer 3-hydroxy-cyclohexane-1-one posses the oxygenated groups in the same plane, approximately. This plane forms a dihedral angle of 30º approximately with the plain that contains the C atom in position 5. For the tautomerization reaction cyclohexane-1,3-dione (Chair)<==>3-hydroxy-cyclohexane-1-one the following theoretical  empirical correlations were obtained (c) Ln KT = 0.3463 alpha 8.6826 (r>0.99); (d) Ln KT = ?0.3799 beta 8.0971 (r>0.99). This means that solvents with greater HDB increase the thermodynamic stability of the enolic form. The conclusion arrived is that solvents perform a more intense effect on the tautomeric equilibriums than the conformational equilibriums.