The cationic Pd complex [(neocuproine)Pd(�-OAc)]2[OTf]2(neocuproine = 2,9-dimethyl-1,10-phenanthroline) is a versatile catalyst for the selective aerobic oxidation of primary and secondary alcohols, vicinal diols, and polyols. In competition experiments, primary alcohols are oxidized more rapidly than secondary alcohols, whereas for 1,2-diols the chemoselectivity is reversed to generate the hydroxyketone. Further, 1,2-diols are oxidized at a significantly faster rate than isolated mono-ols. High selectivity is observed even in complex polyols such as threitol and methyl glycosides. This Pd complex can also mediate the oxidative cyclization of a,?-diols to generate novel monomers for ring-opening polymerization (ROP); lactones are formed in the presence of oxygen while carbonates are formed in the presence of CO and dichloroisocyanuric acid.Pd loadings of approx. 10 mol% are required in aerobic oxidation reactions to reach high conversion due to oxidative degradation of the neocuproine ligand. Mechanistic and kinetic studies have illuminated some of the key reaction steps responsible for the high chemoselectivity of this catalyst and the species responsible for ligand degradation. Monitoring experiments by Electrospray Ionization Mass Spectrometry (ESI-MS) and Desorption Electrospray Ionization Mass Spectrometry (DESI-MS) enabled the identification of several reaction intermediates, including the novel trinuclear species [(LPd)3(�3-O)2]2+. Mechanistic insights have provided new strategies for mitigating catalyst deactivation, resulting in reaction protocols that achieve turnover numbers greater than 200 per Pd atom at room temperature.

Chemoselective aerobic Pd oxidation catalysis

248th National Meeting (2014)

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