Publisher Summary The liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p -xylene, acetaldehyde and vinyl acetate from ethylene, and propylene oxide from propylene. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. This chapter describes the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework. A better understanding of the catalytic action of metal complexes is essential from the point of view of increasing selectivity and efficiency. Metal-catalyzed oxidations may be conveniently divided into two types—homolytic and heterolytic. The homolytic catalysis involves soluble transition metal salts (homogeneous), such as the acetates or naphthenates of Co, Mn, and Cu, or the metal oxides (heterogeneous). Heterolytic catalysis involves reactions of organic substrates coordinated to transition metals.

Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach

We present a concise review on the most pertinent investigations that illuminate the complicated and elusive mechanism for the Wacker process, homogeneous olefin oxidation by palladium(II) catalysts. For more than four decades, multitudes of creative and elegant studies detailing the nucleophilic addition and other steps of the Wacker process have appeared contradictory, while in fact modern perspective has shown an intricate and colorful picture of the "textbook" organometallic reaction. A summary and critical analysis of previous studies is of great importance to explain resolved and highlight unresolved questions about this frequently misunderstood reaction.

The mechanism of the Wacker reaction: a tale of two hydroxypalladations.

57 – Organopalladium Compounds in Organic Synthesis and in Catalysis

Over the past eight years there has been tremendous progress in the development of Pd-catalyzed ligand-directed C H oxidation reactions to form C O, C N, C S, C halogen, and C C bonds. In marked contrast, analogous C H oxidation reactions of substrates that do not contain directing groups remain challenging. The lack of a directing group typically renders these transformations (as exemplified by C H oxygenation) kinetically slow, particularly with electron-deficient arene substrates. The palladium-catalyzed C H oxygenation of simple arenes is also plagued by low turnover numbers and competing biaryl formation, which often leads to catalyst decomposition through precipitation of palladium black. Furthermore, with substituted aromatic substrates, the site selectivity is typically low and difficult to control. As part of a program aimed at developing efficient, selective, and robust catalysts for nondirected C H functionalization reactions, we sought to identify supporting ligands that would address these limitations and promote the Pdcatalyzed C H acetoxylation of arenes. The vast majority of palladium-catalyzed arene C H oxygenations utilize simple palladium salts (e.g., Pd(OAc)2 or PdCl2), [3–5] and literature studies have provided conflicting data about the influence of added ligands on these reactions. Several reports have shown that most common ligands (e.g., 2,2’-bipyridine, 1,10-phenanthroline, pyridine, triphenylphosphine oxide, etc.) inhibit the palladium-catalyzed C H acetoxylation of arenes. In contrast, in a few related systems bidentate sp N-donor ligands (e.g., 2,2’-bipyridine and/or 1,10-phenanthroline) were shown to provide modest enhancement of catalytic activity. However, these latter reactions exhibited low turnover numbers (typically < 10); furthermore, the origin of the observed effects was not explored in detail. Herein we describe the use of careful mechanistic analysis to identify new, efficient, and general palladium catalysts for the C H acetoxylation of benzene derivatives. Remarkably, these catalysts can be formed in situ from Pd(OAc)2 and the simple ligand pyridine (pyr). Furthermore, their catalytic activities and site selectivities can be dramatically modulated through variation of the palladium/ pyridine ratio. Inspired by recent reports of Pd-catalyzed C H functionalizations, our initial explorations focused on pyridine as a ligand for the Pd-catalyzed C H acetoxylation of benzene with [PhI(OAc)2]. As shown in Figure 1, the Pd(OAc)2-catalyzed transformation proceeded to completion after 24 hours at 100 8C. However, in marked contrast, [(pyr)2Pd(OAc)2] (generated in situ from 1 equiv of Pd(OAc)2 and 2 equiv of pyr) performed very poorly, providing less than 20% yield under the same reaction conditions (Figure 1).

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Remarkably High Reactivity of Pd(OAc)2/Pyridine Catalysts: Nondirected CH Oxygenation of Arenes†

The use of stoichiometric Ag(I)-salts as additives in Pd-catalyzed C–H functionalization reactions is widespread. It is commonly proposed that this additive acts as an oxidant or as a halide scavenger promoting Pd-catalyst turnover. We demonstrate that, contrary to current proposals, phosphine ligated Ag(I)-carboxylates can efficiently carry out C–H activation on electron-deficient arenes. We show through a combination of stoichiometric and kinetic studies that a (PPh3)Ag-carboxylate is responsible for the C–H activation step in the Pd-catalyzed arylation of Cr(CO)3-complexed fluorobenzene. Furthermore, the reaction rate is controlled by the rate of Ag(I)-C–H activation, leading to an order zero on the Pd-catalyst. H/D scrambling studies indicate that this Ag(I) complex can carry out C–H activation on a variety of aromatic compounds traditionally used in Pd/Ag-mediated C–H functionalization methodologies.

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Ag(I)-Catalyzed C–H Activation: The Role of the Ag(I) Salt in Pd/Ag-Mediated C–H Arylation of Electron-Deficient Arenes

Palladium(II) acetate in acetic acid in the absence of acetate ion exists as the trimeric species Pd3(OAc)6. The reaction of the trimer with NaOAc or LiOAc at 25° was not instantaneous. Spectral st...

Interaction of Palladium(II) Acetate with Sodium and Lithium Acetate in Acetic Acid

π-Complex equilibria between dimeric Pd(II) acetate and various olefins (ethylene, styrene, 3,3-dimethyl-1-butene, vinylic and allylic esters) in acetic acid solvent have been investigated by spect...

Equilibria between Palladium(II) Acetate and Olefins in Acetic Acid containing Sodium Acetate

The kinetics of the palladium(II) acetate catalyzed exchange of vinyl propionate with acetic acid solvent to give vinyl acetate has been studied in the sodium acetate concentration range from 0 to 1 M. The exchange rate first sharply increases as [NaOAc] increases, reaches a maximum at about 0.2 M and then gradually decreases as the sodium acetate concentration is in-creased to 1.0 M. Using previous results on the equilibrium between palladium(II) acetate and sodium acetate in acetic acid it can be shown that the rate expression for exchange is: rate = (ko + kt[Pd3(OAc)6] + kd[Na2Pd2(OAc)6]) [CH2=CHO2CC2H5] where ko = 2 × 10−4 s−1, kt = 0.045 M−1 s−1, and kd = 0.089 M−1 s−1. A monomeric palladium(II) species, Na2Pd(OAc)4, formed at high [NaOAc] is unreactive. Since the rate expression does not contain a term in [NaOAc], the sodium acetate serves only to convert one palladium(II) species to another. The lack of a [NaOAc] term in the rate expression for the Na2Pd2(OAc)6 catalyzed reaction is believed to res...

Palladium(II) Catalyzed Exchange Reactions. XI. Vinyl Propionate Exchange with Acetic Acid Catalyzed by Palladium(II) Acetate

The interaction of aqueous PdCl42− with ethylene to form π-complexes has been reinvestigated using a reactor with high efficiency gas-liquid mixing. This reactor permitted study of the equilibria a...

Raj N. Pandey

Papers

Interaction of Palladium(II) Acetate with Sodium and Lithium Acetate in Acetic Acid

Equilibria between Palladium(II) Acetate and Olefins in Acetic Acid containing Sodium Acetate

Palladium(II) Catalyzed Exchange Reactions. XI. Vinyl Propionate Exchange with Acetic Acid Catalyzed by Palladium(II) Acetate

π-Complex equilibria between ethylene and PdCl42− in aqueous solution

Equilibria between palladium(ii) acetate and olefins in acetic acid containing sodium acetate