Chemoselective Aerobic Diol Oxidation by Palladium(II)-Pyridine Catalysis

Neutral and cationic palladium complexes that bear pyridine ligands [i.e., pyridine (Py), 4-ethylpyridine (4-EtPy) and 2,4,6-trimethylpyridine (2,4,6-Me(3)Py)] have been isolated and characterized in solution by (1)H and (13)C{(1)H} NMR spectroscopy, cyclic voltammetry (CV) and in the solid state by elemental analysis and single-crystal structure analysis. All palladium compounds have been scrutinized as a precursor to catalyze the aerobic oxidation of diols either in the presence or in the absence of an external base (i.e., K(2)CO(3)). As a result, the chemoselective production of the corresponding hydroxy ketones has been achieved. The bis-cationic palladium complex of the formula [Pd(4-EtPy)(4)](OTs)(2) (OTs = p-toluenesulfonate) [5b(OTs)(2)] emerged as the most promising precursor; it outperformed the neutral precursor that consisted of trans-[Pd(OAc)(2)(4-EtPy)(2)] (OAc = acetate) and 4-EtPy [3b/2(4-EtPy)] (2 mol-equiv.). An operando high-pressure (HPNMR) spectroscopic study with the precursor 5b(OTs)(2) combined with the results obtained from catalytic reactions has provided insight into the catalytic mechanism that is operative in 5b(OTs)(2)-catalyzed aerobic diol oxidation reactions.