Effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical study

The proton transfer between interconversion tautomers is of importance in synthetic chemistry, such as: keto-enol, imine-enamine, oxime-nitroso [1, 2]…. The keto-enol tautomerization, especially in the -diketone compounds is a common one. Acetyl acetone, one of the -diketone compound, was studied experimentally early and thoroughly [1, 2]. This compound usually exists an equilibrium mixture of enol and keto tautomers with equilibrium
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Effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical studyJournal of Chemistry, Vol. 44 (2), P. 249 - 254, 2006 effect of solvents on tautomeric equilibrium of acetyl acetone: A Theoretical stUdy Received 22 June 2005 Nguyen Thanh Cuong, Le Kim Long and Dang Ung Van Center for computational chemistry, Faculty of Chemistry College of Natural Sciences, Vietnam National University, Hanoi summary The tautomeric equilibrium of acetyl acetone in gas and different solvents were studied by electronic structure calculations. The mechanism of reaction was proposed. In gaseous phase, the results of B3LYP level are more agreement with experiments than that of HF level. The cis-enolic form was found to be the most thermodynamic stable structure. The solvent effects on tautomeric equilibrium are estimated by performing self-consistent reaction field (SCRF)-Onsager and PCM models at B3LYP level. The latter is shown of a better model for solvation. The solvents are shown to be not effecting on the thermodynamic stabilities of enol or keto form, but have more influence on the transition state. In solvents, the activation energy decreases ca. 20 kcal.mol-1. The enol/keto concentration ratio in different solvents were calculated and compared with experimental data. In more polar solvent, the more content of keto form was found. I - Introduction constant Ke ( K e = [enol ] ). The proton transfer between interconversion [keto]tautomers is of importance in synthetic Enol form of acetyl acetone exists mainly inchemistry, such as: keto-enol, imine-enamine, gaseous phase, however, the enol/keto ratiooxime-nitroso [1, 2]…. The keto-enol depends on strongly the polarization of solvents.tautomerization, especially in the -diketone The equilibrium constant measured by 1H-NMRcompounds is a common one. Acetyl acetone, spectroscopy indicates a higher enol content inone of the -diketone compound, was studied apolar aprotic than in dipolar protic or dipolarexperimentally early and thoroughly [1, 2]. This aprotic solvents [2]. On the other hand, enolcompound usually exists an equilibrium mixture form of acetyl acetone exists two confor-of enol and keto tautomers with equilibrium mations: cis–enolic form and trans-enolic form. H O CH3 TS1 O O TS2 O O C C C C C C CH3 CH OH CH3 CH2 CH3 CH3 CH CH3 trans-enolic tautomer Keto tautomer cis-enolic tautomer The purpose of the present study is to re- of acetyl acetone by electronic structureexamine the keto - enol tautomeric equilibrium calculations in gas phase and the effects of 249solvent on tautomeric equilibrium. solute is placed in a uniform electric field of solvent with a dielectric constant . The solute is II - Calculation Methods assumed to occupy a spherical cavity of radius a0 in the medium. A dipole in the molecule is All calculations were carried out using induced by a dipole of the medium and viceGaussian 98. A3 [4]. All geometries of versa. The electric field applied to the solute bystructures were fully optimized and frequencies the solvent dipole in turn interacts with thewere calculated at the RHF/ 6-31+G(d) and molecular dipole to lead to net stabilization.B3LYP/6-31+G(d,p) level of theory. Zero-point This model has a major drawback that thevibrational and thermal corrections were molecule is a sphere that is usually far awaycalculated at the same level and scaled by 0.9 at from the realistic picture. In the second modelthe HF level [3, 4] and 0.98 at the B3LYP level (polarized continuum models-PCM), solvent is[3, 4] to account for the overestimation of assumed to be a continuous medium with avibrational frequencies at these levels. The dielectric constant that surrounds a cavityscaled ZPE corrections were included in the ...