Insertion of Molecular Oxygen into Palladium and Platinum-sp³ Carbon Bonds

Abstract

Synthesis of methanol from methane and terminal alcohols from linear alkanes using molecular oxygen as an oxidant under mild conditions is an attractive and environmentally-friendly transformation reaction. Insertion of molecular oxygen into a transition metal-carbon (sp³) bond is a key step to achieve the transition metal complex-catalyzed transformation. In this review, two mechanistically-different insertion reactions of oxygen into palladium and platinum-carbon (sp³) bonds are introduced. The former insertion is proposed to proceed by a radical chain mechanism, which is similar to that for the auto oxidation of organic compounds. A palladium(III) 17e^- intermediate, [Pd^IIIMe₂(OOMe)(bipy)] (bipy = 2,2'-bipyridine), which would be formed by the attack of a methylperoxy radical to [PdMe₂(bipy)], is regarded as a characteristic species in the proposed mechanism. On the other hand, the latter insertion involves loosely aggregated Pd(II) or Pt(II) methyl complexes, [Me(L)M^II…M^II(L)Me] (L = 6,6''-diamino-2,2':6',2''-terpyridine), which would be excited to triplet complexes, ³[Me(L)M^III–M^III(L)Me]*, upon exposure to light. Subsequent coordination of triplet oxygen and further conversions including reductive elimination of a carbon (sp³)- oxygen bond at the final step afford [M^II(L)OOMe].