Ligand-Modulated Palladium Oxidation Catalysis:  Mechanistic Insights into Aerobic Alcohol Oxidation with the Pd(OAc)₂/Pyridine Catalyst System

Abstract

Pd(OAc)(2):pyridine (1:4) is an efficient catalyst system for the oxidation of alcohols with molecular oxygen. A mechanistic study of this reaction reveals that pyridine promotes the aerobic oxidation of palladium(0) but inhibits the oxidation of alcohol by palladium(II). Kinetic results reveal that turnover-limiting substrate oxidation consists of (i) formation of a palladium(II)-alkoxide, (ii) pyridine dissociation, and (iii) beta-hydride elimination. These results provide a framework for the design and/or screening of more effective aerobic oxidation catalysts.

Selective oxidation of allylic alcohols over highly ordered Pd/meso-Al2O3 catalysts

“…Detailed kinetic investigations, alongside in situ [8], [9] and [10] and operando studies [11], [12], [13] and [14] have provided conclusive evidence that allylic alcohol oxidation occurs via a redox mechanism, catalysed by electron deficient, surface Pd(II) species present as PdO. Kumar et al [15], Scott et al [16] and Hara et al [17] have likewise reported a heterogeneous Pd(II) active species responsible for aerobic alcohol selox, while homogeneous Pd(II) complexes are well known to catalyse such alcohol oxidations [18], [19] and [20]. Interestingly, surface PtO2 has also been recently reported as the active phase in the analogous Pt catalysed aerobic selox of allylic alcohols [21].…”

Formation and growth of palladium nanoparticles inside porous poly(4-vinyl-pyridine) monitored by operando techniques: The role of different reducing agents

“…As discussed in the introduction, alcohols are often used as solvents in reactions catalyzed by Pdbased catalysts [33], or they may be used even as reactants [34][35][36][37][38][39][40][41]. Therefore, it is important to clarify if they are innocent toward the catalyst, or if they may react with it and eventually lead to its final deactivation.…”

“…In other cases, alcohols are used as reactants. For example, alcohol hydrogenation, oxidation and cross-coupling reactions are also catalyzed by palladium-based catalysts, either homogeneous or heterogeneous [34][35][36][37][38][39][40][41]. In this respect, the homogeneous Pd(OAc)2/pyridine system is by far one of the most efficient and selective catalyst for oxidation of alcohols to aldehydes.…”

“…In this respect, the homogeneous Pd(OAc)2/pyridine system is by far one of the most efficient and selective catalyst for oxidation of alcohols to aldehydes. The reaction mechanism has been proposed by Stahl and coworkers [38][39][40] as an oxidase-style mechanism in which palladium(II)-mediated substrate oxidation (Stage I in Scheme 1) and aerobic oxidation of the catalyst (Stage II in Scheme 1) occur in two independent, sequential stages. Formally, in Stage I Pd(II) species transform into Pd 0 ones, while in the Stage II oxidation by oxygen restores the Pd(II) original complex.…”

“…Scheme 1. Catalytic cycle for alcohol aerobic oxidation catalyzed by homogeneous Pd(OAc)2/Pyridine catalyst as proposed by Stahl et al [38][39][40]. Stage I (black arrows) involves alcohol oxidation into aldehyde/ketone by eliminating acetate ions as acetic acid; Stage II (grey arrows) involves the active species regeneration, by converting acetic acid and O2 into acetate ions and water.…”