Trinuclear Pd₃O₂ Intermediate in Aerobic Oxidation Catalysis

Abstract

The activation of O2 is a key step in selective catalytic aerobic oxidation reactions mediated by transition metals. The bridging trinuclear palladium species, [(LPdII)3(μ3‐O)2]2+ (L=2,9‐dimethylphenanthroline), was identified during the [LPd(OAc)]2(OTf)2‐catalyzed aerobic oxidation of 1,2‐propanediol. Independent synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and is a competent intermediate in the catalytic aerobic oxidation of alcohols. The formation and catalytic activity of the trinuclear Pd3O2 species illuminates a multinuclear pathway for aerobic oxidation reactions catalyzed by Pd complexes.

Mechanism of Catalytic Oxidation of Styrenes with Hydrogen Peroxide in the Presence of Cationic Palladium(II) Complexes

“…Analysis of catalytic reactions by ESI-MS techniques coupled with other mechanistic tools and kinetic studies can provide valuable insights into complex catalytic mechanisms. 48,55,[58][59][60][61] We have recently used ESI-MS to investigate the mechanisms of aerobic Pd(II) catalysis for alcohol oxidation 56,62 and H2O2 disproportionation. 57 Oxidations of 0.2 M styrene in ambient air with 1 M H2O2 in the presence of 2 mM A in MeCN/H2O were investigated by taking aliquots and diluting them directly before analyzing the reaction mixture by ESI-MS (Scheme 3).…”

“…This ion is formulated as the trimeric [PBO3Pd3O2] 2+ , an ion analogous to that observed and characterized in O2 or H2O2 mediated oxidations with cationic Pd neocuproine complexes. [56][57] After one hour of reaction, the major observed species is an ion at 421.0151 m/z, corresponding to the formula [PBOPd-CH2COPh] + , [C] + which we assign as a Pd-enolate. 63 This ion remains the dominant species throughout the reaction.…”

“…42 While the use of aqueous H2O2 for Wacker-type oxidations dates to the 1960s, 43 considerably less is known regarding the mechanism of alkene oxidations mediated by H2O2 [44][45] relative to those employing O2, [46][47][48][49] or alkyl hydroperoxides. [50][51][52][53][54] Herein we describe kinetic investigations, isotope labeling studies and insitu high-resolution mass spectrometry [55][56][57] of Pd(II)/H2O2 catalyzed oxidation of styrenes. These studies provide clear evidence for a mechanism involving the initial formation of a Pd hydroperoxide intermediate 50,54 which reacts with styrene to generate a Pd-alkylperoxide intermediate that liberates acetophenone by at least two competitive processes, one of which involves an unusual palladium enolate intermediate (Scheme 1).…”

Detection of Palladium(I) in Aerobic Oxidation Catalysis

“…[ 6] The majority of Pd-catalyzed aerobic oxidations proceed through a catalytic cycle wherein Pd II mediates substrate oxidation and Pd 0 is reoxidized by O 2 (Scheme 1). [7] The Pd 0 intermediate in this cycle is metastable and susceptible to decomposition via aggregation into Pd nanoparticles and/or bulk metallic Pd. Use of ancillary ligands can help to prevent catalyst decomposition and/or promote reoxidation of Pd 0 by O 2 , thereby avoiding a requirement for undesirable oxidants, such as stoichiometric Cu II , Ag I , or benzoquinone (BQ).…”

Significant and Synergistic Intensification of Aerobic Oxidation of Activated Alcohols in Water at Ambient Condition by Adding Perfluoro‐Surfactant

“…In view of environmental impact and sustainable development, utilization of molecular oxygen as an oxidant is highly desired . Since the activation of O 2 is a key step in selective catalytic aerobic oxidation reaction that involves two‐electron oxidation, noble metals (including gold, palladium, platinum, ruthenium, etc) in the form of complexes or nanoparticles have been employed to activate O 2 during the selective oxidation of alcohols. Nevertheless, considering the high cost of noble metals, it would be highly desirable to develop low‐cost base metal catalysts .…”