Homogeneous catalytic oxidation of thymol with dinuclear Cu(II)-2-phenyl propionate-bipyridine complex

A carboxylate bridged dimeric metal–organic framework involving a monocarboxylic acid (phenylpropionic acid) and N-donor ligand (2,2-bipyridine), Cu2(μ2-η2:η1(ppa)2(μ2-η1:η1 (ppa)bpy)2]·ClO4·H2O, structurally characterized by X-Ray diffraction. The Cu(II)-complex exhibits high activity and selectivity as a catalyst for the oxidation of thymol to thymoquinone without additive in a acetonitrile with TBHP. Under optimised conditions, the maximum conversion of thymol (100 %) was obtained with 100 % selectivity to thymoquinone after 35 min at the thymol to catalyst mole ratio of 267 (TOF = 449 h−1) in acetonitrile at 55 °C. The second reported example of Cu(II)-mediated homogeneous oxidation of thymol in an organic solvent, with the catalyst recycled four times without loss of activity, was also described. [Display omitted] A carboxylate-bridged dinuclear copper(II) complex, [Cu2(μ2-η2:η1(ppa)2(μ2-η1:η1 (ppa)bpy)2]·ClO4·H2O (ppa (C9H9OO) = 2-phenyl propionate, bpy = 2,2′-bipyridine), was prepared and its structure was determined by X-ray diffraction analysis. The octacoordinated Cu(II) dimer complex exhibits an asymmetric conformation, with the metal centers linked by two distinct modes of the carboxylate group. These include an asymmetric chelating bridging bidentate involving two ppa ligands and a syn-syn bidentate bridging mode involving one ppa ligand. The complex was employed as a catalyst for the oxidation of thymol, demonstrating a high degree of quantitative conversion and selectivity with TBHP across a range of solvents at moderate temperatures. The homogeneous catalytic system, comprising Cu(II)-2-phenyl propionate-bipyridine, tert-butylhydroperoxide (TBHP) as oxidant, and a polar aprotic solvent (acetonitrile or acetone), demonstrated high turnover numbers (TOF) in the absence of any additives. The impact of temperature, solvent, the ratio of thymol to TBHP, different oxidants and the quantity of catalyst on the catalytic activity and product selectivity was examined. Under optimized conditions, the maximum conversion of thymol (100 %) was obtained with 100 % selectivity to thymoquinone with TBHP after 35 min at the thymol to catalyst mole ratio of 267 (TOF = 449 h−1) in acetonitrile (MeCN) at 55 °C. Based on the available evidence, this activity and selectivity to thymoquinone represent the highest reported level of performance achieved by a homogeneously copper-catalysed process.