Palladium -catalyzed oxidation of olefins

Use of DMF as a solvent for the oxidation of l-o1efins has been reported by Clement and Selwitz. The method requires only a catalytic amount of PdCl2 and gives satisfactory yields under mild conditions. A small amount of olefin migration product is the only noticeable contaminant in the cases reported. The procedure can be applied satisfactorily to various 1-olefins with other functional groups. This useful synthetic method for the preparation of methyl ketones has been applied extensively in the syntheses of natural products such as steroids,macrolides, dihydrojasmone, and muscone. " A comprehensive review article on the palladium-catalyzed oxidation of olefins has... 

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other palladium-catalyzed oxidations of olefins in various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

Review "Synthetic Applications of the Palladium Catalyzed Oxidation of Olefins to Ketones"... 

Tsuji, J. 1984. Synthetic applications of the palladium-catalyzed oxidation of olefins to ketones. Synthesis 369-383. 

A number of metal macrocycles were tested in the aerobic palladium-catalyzed oxidation of olefins and alcohols [3,4],... 

Intermolecular Additions of Alcohois and Carboxylates The intermolecular oxidations of olefins with alcohols as nucleophile typically generate ketals, whereas the palladium-catalyzed oxidations of olefins with carboxylic acids as nucleophile generates vinylic or allylic carboxylates. As a result, many of the oxidations with alcohols have been conducted with diols to generate stable cyclic acetal products. Both types of oxidations have been conducted on large industrial scale, and vinyl acetate is produced from the oxidative reaction of ethylene with acetic acid in the gas phase over a supported palladium catalyst. ... 

Compared with these methods, the palladium-catalyzed oxidation of 1-olefins described here is more convenient and practical. The industrial method of ethylene oxidation to acetaldehyde using PdCl2-CuCl 2-O2 original reaction of this type. The oxidation of various olefins has been carried out. ... 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Poly(ethylene oxide) polymers and poly(ethylene oxide/propylene oxide) copolymers with iminodipropionitrile (139) or iminodiacetonitrile end groups were used as ligands in the palladium-catalyzed oxidation of higher olefins (1-octene to 1-hexadecene) at 50-70 °C with atmospheric air or 1-3 bar O2. In an ethanol/water mixture 88 % yield of 2-hexanone and 92 % yield of 2-hexadecanone was obtained in 4 and 2 h, respectively, with a... 

SCHEME 136. Palladium-catalyzed oxidation of terminal olefins to methyl ketones by TBHP or H2O2... 

The most common oxidation state of palladium is H-2 which corresponds toa electronic configuration. Compounds have square planar geometry. Other important oxidation states and electronic configurations include 0 ( °), which can have coordination numbers ranging from two to four and is important in catalytic chemistry, and +4 (eft), which is octahedral and much more strongly oxidizing than platinum (IV). The chemistry of palladium is similar to that of platinum, but palladium is between 103 to 5 x 10s more labile (192). A primary industrial application is palladium-catalyzed oxidation of ethylene (see Olefin polymers) to acetaldehyde (qv). Palladium-catalyzed carbon—carbon bond formation is an important organic reaction. 

The key step in this category involves the oxidation of a coordinated substrate by a metal ion or an oxometal species (see later). Examples include the palladium(II)-catalyzed oxidation of olefins (Wacker process) and the oxidative dehydrogenation of alcohols, where the key steps are reactions (5) and (6), respectively. 

In the metallic palladium-catalyzed carbonylation of olefins, some hydrogen sources are essential hydrogen halide and molecular hydrogen were found to be the most eflFective. The following sequence of reactions was proposed for the reaction mechanism of the ester and aldehyde formation catalyzed by palladium (23). The first step of the metallic palladium-catalyzed carbonylation seems to be the formation of a palladium-hydrogen bond by the oxidative addition of hydrogen chloride... 

Aerobic Palladium-Catalyzed Oxidation of Other Types of Olefins... 

The palladium-catalyzed reaction of olefins with aryl or vinyl halides or pseudohalides in the presence of base (the Heck reaction) follows a different course from the other crosscoupling reactions after the oxidative addition step. As shown in Scheme 19.5, the olefin coordinates to the palladium after oxidative addition of the aryl or vinyl halide. Tliis coordination of olefin may occur by associative displacement of a monodentate ligand from the palladium, or it may occur by replacement of halide by the olefin to generate a cationic olefin complex. In some cases, these reactions are conducted with aryl or vinyl triflates. In this case, the olefin readily displaces the triflate to generate a cationic palladium-olefin... 

Another of the standard palladium-catalyzed C—C bond formations is the Mizoroki-Heck reaction. This reaction is based on the palladium-catalyzed coupling of olefins with aryl or vinyl halides under basic conditions [21]. The catalytic cycle that is typically proposed for this reaction is outlined in Scheme 7.7. The first step of the reaction postulated in the mechanism is an oxidative addition of R -X to a Pd(0) complex. The next step is the insertion of the alkene to the Pd complex II. In order for this to be possible, an uncharged ligand has to break away giving a neutral Pd(II) complex that will be coordinated by the olefin. The insertion of the alkene into the Pd— bond results in the C C bond-forming step to give BO. Rotation around the C—C bond and 3-hydride elimination yields the new substituted olefin and intermediate 131. Regeneration of the active catalytic species occurs by the addition of a base. 

Roussel, M. and Mimoun, H., (1980). Palladium-Catalyzed Oxidation of Terminal Olefins to Methyl Ketones by Hydrogen Peroxide, J. Org. Chem., 45, pp. 5387-5390. 

Wacker-type Oxidation. The palladium-catalyzed oxidation of terminal olefins to ketones (Wacker oxidation) is an important chemical process both in the laboratory and in industrial settings. Pd(dba)2 has shown useful activity in this area, for example. 

Oxidative Carbonylation of Ethylene—Elimination of Alcohol from p-Alkoxypropionates. Spectacular progress in the 1970s led to the rapid development of organotransition-metal chemistry, particularly to catalyze olefin reactions (93,94). A number of patents have been issued (28,95—97) for the oxidative carbonylation of ethylene to provide acryUc acid and esters. The procedure is based on the palladium catalyzed carbonylation of ethylene in the Hquid phase at temperatures of 50—200°C. Esters are formed when alcohols are included. Anhydrous conditions are desirable to minimize the formation of by-products including acetaldehyde and carbon dioxide (see Acetaldehyde). 

Wacker olefin oxidation, which is depicted in its simplest form in Eq. (6.33), contains palladium( 11)-catalyzed hydration of olefin in its important step (Eq. 6.34) and is discussed extensively [62]. In this review article we introduce two asymmetric Wacker type reactions. 

Under microwave heating, the Heck olefinations were achieved in 30-60 min, as opposed to 10-40 h by conventional heating. The recyclable heterogeneous LDH-Pd(0) catalytic system circumvents the need to use expensive and air-sensitive basic phosphines as ligands in the palladium-catalyzed coupling of chloroarenes. This novel Mg-Al layered double-hydroxide (LDH) support in the catalytic system stabilizes the nanopalladium particles and also supplies adequate electron density to the anchored palladium(O) species and facilitates the oxidative addition of the deactivated electron-rich chloroarenes. 

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