Preparation and structure of an unexpected dehydrogenation product from 2,6-diphenylcyclohexanone oxime

The synthesis, spectroscopic studies, computational analysis, and crystal structure of (6S)-2,6-diphenylcyclohex-2-enone oxime are described. The oxime crystallizes in the monoclinic space group P2 l /c with a = 11.5269(11) A, b = 6.6724(6) A, c = 19.0105(18) A, β = 100.863(3)°, V = 1435.9(2) A3, and Z = 4. Semi-empirical (AM1), ab initio (MP2/6-31G*), and density functional theory (B3LYP/6-31G*) calculations suggest that there are two low-energy conformations available for the oxime. Significant differences were observed between the dihedral angles in the conformers. Good agreement was found between the crystal data and one of the conformers from the calculations. The absence of certain splitting patterns in the 1H NMR spectrum of the title compound is in contrast to what would be expected on the basis of the dihedral angles and suggests that rapid interconversion is possible in solution. Analysis of the crystal packing suggests that one conformer is favored in the solid state, stabilized by packing interactions, and interconversion is prohibited due to a blocking effect.