C-H Activation in Aqueous Medium. The Diverse Roles of Platinum(II) and Metallic Platinum in the Catalytic and Stoichiometric Oxidative Functionalization of Organic Substrates Including Alkanes

The oxidation of ethers, alcohols, esters, and light alkanes (ethane, propane) by K₂PtCl₄ and Pt/O₂ in aqueous medium has been studied. Results appear to indicate that unactivated C-H bonds were attacked and oxidized by Pt(II) whereas C-H bonds a to an oxygen were activated and catalytically oxidized by metallic Pt in the presence of O₂. For example, Pt(II) was found to oxidize ethane selectively to the alcohols, ethanol, and ethylene glycol. In the presence of metallic Pt and O₂, further oxidation of the alcohol functionality occurred to generate the corresponding carboxylic acids. Thus, with proper choice of the C-H activating system, it was possible to oxidize substrates with a fairly high degree of selectivity with respect to the oxidation level, as well as the particular C-H bond that was functionalized. With respect to the latter, because of the “chelate effect”, the selective activation and oxidation by Pt(II) of C-H bonds suitably distant from a coordinating oxygen was achieved. For ethers, the order of reactivity was α-C-H < β-C-H < γ-C-H.