Catalytic Role of Multinuclear Palladium–Oxygen Intermediates in Aerobic Oxidation Followed by Hydrogen Peroxide Disproportionation

Abstract

Aerobic oxidation of alcohols are catalyzed by the Pd-acetate compound [LPd(OAc)]2(OTf)2 (L = neocuproine = 2,9-dimethyl-1,10-phenanthroline) to form ketones and the release of hydrogen peroxide, but the latter rapidly undergoes disproportionation. We employ a series of kinetic and isotope labeling studies made largely possible by electrospray ionization mass spectrometry to determine the role of intermediates in causing this complex chemical transformation. The data suggested that multiple catalytic paths for H2O2 disproportionation occur, which involve formation and consumption of multinuclear Pd species. We find that the trinuclear compound [(LPd)3(μ(3)-O)2](2+), which we have identified in a previous study, is a product of dioxygen activation that is formed during aerobic oxidations of alcohols catalyzed by [LPd(OAc)]2(OTf)2. It is also a product of hydrogen peroxide activation during disproportionation reactions catalyzed by [LPd(OAc)]2(OTf)2. The results suggest that this trinuclear Pd compound is involved in one of the simultaneous mechanisms for the reduction of oxygen and/or the disproportionation of hydrogen peroxide during oxidation catalysis. Electrospray ionization mass spectrometry of hydrogen peroxide disproportionation reactions suggested the presence of other multinuclear Pd-O2 species in solution. Theoretical calculations of these compounds yield some insight into their structure and potential chemistry.

Synthesis, Characterization and Aerobic Alcohol Oxidation Catalysis of Palladium(II) Complexes with a Bis(imidazolyl)borate Ligand

“…Another mechanistic possibility entails reductive elimination of HX from the Pd(H)(X) species I to yield the corresponding Pd 0 compound and subsequent reaction with O 2 giving palladium(II)‐peroxo species III . In the former mechanism, the oxidation state of the palladium center remains +2 throughout the catalyst turnover process , , . Our –1‐charged L Ph ligand might favor palladium(II) rather than palladium(0), as suggested by the absence of peaks during the initial reductive scan within –1.1 V (v.s.…”

“…During this reaction the color of the reaction mixture turned from pale yellow to brown during the initial stages, and the brown color darkened with further progress of the reaction. These observations suggest that the catalytically active species formed from 2 might react with H 2 O 2 generated during catalytic turnover . Such a reaction with H 2 O 2 may lead to a less basic and effectively inert species, thereby serving as a means of catalyst quenching.…”

Phosphinous Acid Platinum Complex as Robust Catalyst for Oxidation: Comparison with Palladium and Mechanistic Investigations

“…Since the oxidation of 1,2‐ or 1,3‐ diols into α‐ or β‐hydroxy ketones respectively is particularly challenging owing to potential metal chelation or competitive reaction,, we focused our efforts on this last challenge. Although some efficient and selective catalytic systems have been proposed to this end,, , it should be noted that most of them are based on the use of palladium in combination with nitrogen ligands,, , rather than oxidation‐sensitive phosphine‐type ligand. To our delight, our method allowed the totally selective oxidation of such substrates in good to excellent yields as evidenced by the compounds 3i (89 %) and 3j (68 %).…”

Secondary Phosphine Oxides as Multitalented Preligands En Route to the Chemoselective Palladium‐Catalyzed Oxidation of Alcohols

“…Completion of the catalytic cycle then requires either reoxidation of a Pd 0 species or direct reaction of HPdX with oxygen to regenerate the active catalyst (Scheme , pathway a) . Despite the clear environmental benefit of using oxygen and great advances achieved thereof, many serious issues such as intolerance of sensitive functional groups and the nature of the ligands may emerge in maintaining catalytic activity and/or regio‐ and chemoselectivity . Alternatively, oxidizing alcohols in the absence of any formal reoxidant necessarily implies metal‐mediated hydrogen transfers from the substrate to a hydrogen acceptor such as electron‐poor alkenes .…”