Oxidation with palladium complexes

Kinetics and Mechanism of the Oxidation with Palladium Complexes... 

Oxidative addition involving carbon-to-oxygen bonds is of relevance to the catalysis with palladium complexes. The most reactive carbon-oxygen bond is that between allylic fragments and carboxylates. The reaction starts with a palladium zero complex and the product is a ir-allylic palladium(II) carboxylate Figure 2.16. 

Olefin Activation. Mechanistic studies on the 1,4-oxidation of 1,3-dienes led to the discovery of a new palladium catalyst [Pd(DA)2], which was readily prepared from the reaction of Pd2(dba)3 and the Diels-Alder adduct derived from 1,3-cyclohexadiene and/>-benzoquinone. With p-benzoquinone as the stoichiometric oxidant, this palladium complex proves more reactive and selective than the Pd(II) carboxylate, typically used in... 

The rate-determining process is considered to involve the conversion of the initial intermediate complex into a c-bonded 1,3-oxypalladium adduct which undergoes further rapid reactions possibly involving two specific, consecutive 1,2-hydride shifts. In the reaction with cyclohexene and other unsaturated hydrocarbons of this type under acidic conditions (I.5M-HCIO4), a fast oxidation with palladium acetate is observed, the products being cyclohexanone and other acetate-containing species. A slower oxidation is reported, however, in the presence of oxygen. 

Reaction with Palladium Complexes. 7r-Allylpalladium complexes undergo oxidative cleavage with copper(II) chloride to form allyl chlorides with the concomitant release of PdCl2 (eq 26). ... 

Oxidation of the enynes shown in Scheme 83 with palladium complexes gives indenone derivatives in moderate yield. 

Oxidative addition—Reaction of the carbon electrophile with palladium-(0) complex 5 to give a palladium-(II) complex 6. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

In 2003, Sigman et al. reported the use of a chiral carbene ligand in conjunction with the chiral base (-)-sparteine in the palladium(II) catalyzed oxidative kinetic resolution of secondary alcohols [26]. The dimeric palladium complexes 51a-b used in this reaction were obtained in two steps from N,N -diaryl chiral imidazolinium salts derived from (S, S) or (R,R) diphenylethane diamine (Scheme 28). The carbenes were generated by deprotonation of the salts with t-BuOK in THF and reacted in situ with dimeric palladium al-lyl chloride. The intermediate NHC - Pd(allyl)Cl complexes 52 are air-stable and were isolated in 92-95% yield after silica gel chromatography. Two diaster corners in a ratio of approximately 2 1 are present in solution (CDCI3). 

Following this pnblication, the anthors tested a series of Pd-NHC complexes (33-36) for the oxidative carbonylation of amino compounds (Scheme 9.8) [44,45]. These complexes catalysed the oxidative carbonylation of amino compounds selectively to the nreas with good conversion and very high TOFs. Unlike the Cu-NHC catalyst 38-X, the palladium complexes catalysed the oxidative carbonylation of a variety of aromatic amines. For example, 35 converted d-Me-C H -NH, d-Cl-C H -NH, 2,4-Me3-C H3-NH3, 2,6-Me3-C H3-NH3, and 4-Ac-C H3-NH3 to the corresponding nreas with very high TOFs (>6000) in 1 h at 150°C, in 99%, 87%, 85%, 72%, and 60% isolated yields, respectively (Pco,o2 = 3.2/0.8 MPa). 

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