Photoinduced insertion of molecular oxygen into metal–hydrogen bonds

Insertion of Molecular Oxygen into a Gold(III)-Hydride Bond

The use of molecular oxygen as an oxidant in chemical synthesis has significant environmental and economic benefits, and it is widely used as such in large-scale industrial processes. However, its adoption in highly selective homogeneous catalytic transformations, particularly to produce oxygenated organics, has been hindered by our limited understanding of the mechanisms by which O2 reacts with transition metals. Of particular relevance are the mechanisms of the reactions of oxygen with late transition metal hydrides as these metal centers are better poised to release oxygenated products. Homogeneous catalysis with gold complexes has markedly increased, and herein we report the synthesis and full characterization of a rare AuIII-H, supported by a diphosphine pincer ligand (tBuPCP = 2,6-bis(di-tert-butylphosphinomethyl)benzene). [(tBuPCP)AuIII-H]+ was found to cleanly react with molecular oxygen to yield a stable AuIII-OOH complex that was also fully characterized. Extensive kinetic studies on the reaction via variable temperature NMR spectroscopy have been completed, and the results are consistent with an autoaccelerating radical chain mechanism. The observed kinetic behavior exhibits similarities to that of previously reported PdII-H and PtIV-H reactions with O2 but is not fully consistent with any known O2 insertion mechanism. As such, this study contributes to the nascent fundamental understanding of the mechanisms of aerobic oxidation of late metal hydrides.

Mechanistic examination of aerobic Pt oxidation: insertion of molecular oxygen into Pt–H bonds through a radical chain mechanism

DFT calculations were performed in an effort to evaluate the mechanism of O₂ insertion into the Pt–H bond of Tp^Me2Pt^IVMe₂H catalyzed by AIBN or light.

Insertion of Molecular Oxygen into a Palladium−Hydride Bond: Computational Evidence for Two Nearly Isoenergetic Pathways

The reaction of a palladiumII-hydride species with molecular oxygen to form palladiumII-hydroperoxide has been proposed as a key step in Pd-catalyzed aerobic oxidation reactions. We recently reported one of the first experimental precedents for such a step (Angew. Chem., Int. Ed. 2006, 45, 2904-2907). DFT calculations have been used to probe the mechanism for this reaction, which consists of formal insertion of O2 into the palladium-hydride bond of trans-(NHC)2Pd(H)OAc (NHC = N-heterocyclic carbene). Four different pathways were considered: (1) hydrogen atom abstraction (HAA) of the Pd-H bond by molecular oxygen, (2) reductive elimination of HX followed by oxygenation of Pd0 and protonolysis of the (eta2-peroxo)-PdII species, (3) oxygenation of palladiumII-hydride with subsequent reductive elimination of the O-H bond from an eta2-peroxo-PdIV center, and (4) formation of a cis-superoxide adduct of the palladium-hydride species followed by O-H bond formation via hydrogen atom migration. The calculations reveal that pathways 1 and 2 are preferred energetically, and both pathways exhibit very similar kinetic barriers. This result suggests that more than one pathway is possible for catalyst reoxidation in Pd-catalyzed aerobic oxidation reactions.