Tautomeric and conformational properties of dipivaloylmethane

Abstract The tautomeric and structural properties of 5-hydroxy-2,2,6,6-tetramethyl-3-heptanone, (dipivaloylmethane, C(CH 3 ) 3 C(O)CH 2 C(O)C(CH 3 ) 3 ) have been studied by means of gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ). Both, quantum chemistry and GED analyses resulted in the presence of 100(5)% enol tautomer at 296(3)K. Quantum chemical calculations predict the existence of two enol conformers in about equal amounts. In both conformers the enol ring possesses C s symmetry and they possess different torsional orientations of the two tert-butyl groups. The experimental data refinement results in an enol tautomer, in which the tert-butyl group adjacent to the carbonyl group possesses an intermediate orientation between those in “enol1” and “enol 2” forms (torsional angle is about 30°), and the tert-butyl group adjacent to the hydroxyl group slightly deviates from orientation in the theoretical conformers (by about 10°). The enol ring possesses C S symmetry with a strongly asymmetric hydrogen bond. The experimental geometric parameters are reproduced very closely by the B3LYP/aug-cc-pVTZ method.