Palladium olefin oxidation

The palladium chloride process for oxidizing olefins to aldehydes in aqueous solution (Wacker process) apparendy involves an intermediate anionic complex such as dichloro(ethylene)hydroxopalladate(II) or else a neutral aqua complex PdCl2 (CH2=CH2)(H2 0). The coordinated PdCl2 is reduced to Pd during the olefin oxidation and is reoxidized by the cupric—cuprous chloride couple, which in turn is reoxidized by oxygen, and the net reaction for any olefin (RCH=CH2) is then... 

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. ... 

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... 

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. 

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. 

The proposed mechanism is illustrated in Figure 8-5.60a Oxidative addition of the phenyl triflate to the palladium(0)-BINAP species A gives phenylpalla-dium triflate B. Cleavage of the triflate and coordination of 2,3-dihydrofuran on B yields cationic phenyl palladium olefin species C. This species C bears a 16-electron square-planar structure that is ready for the subsequent enantio-selective olefin insertion to complete the catalytic cycle (via D, E, F, and G). The base and catalyst precursor have profound effects on the regioselectivity and enantioselectivity. 

In contrast to the usual Wacker-conditions, optimum rates and catalyst stability in the Pd/batophenanthroHne-catalyzed olefin oxidations was observed in the presence of NaOAc (pH s 11.5). Under such conditions, the catalyst-containing aqueous phase could be recycled with about 2-3 % loss of activity in each cycle. In the absence of NaOAc precipitation ofPd-black was observed after the second and third cycles. Nevertheless, kinetic data refer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallic palladium. 

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... 

Metal-Halogen Counpounds. One of the few examples of an olefin insertion into a metal-halogen compound has been reported by Tsuji. The reaction, which also supports the idea that sigma-bonded metal-carbon compounds are intermediates in the palladium chloride-olefin oxidation reaction, was the addition of carbon monoxide to the ethylene palladium chloride 7r-complex in nonaqueous solvents to produce a moderate yield of 3-chloropropionyl chloride (96). 

Palladium chloride or the chloropalladite ion catalyze the oxidation of olefins to aldehydes or ketones, presumably by forming unstable palladium-olefin complex intermediates 196). A reaction of great industrial importance is the palladium chloride/cupric chloride catalyzed oxidation of ethylene to acetaldehyde 195). The first stage is presumably the oxidative hydrolysis of ethylene,... 

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. 

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. 

Finally a kinetic study of the oxidation of ethylene by palladium (II) acetate gave a rate-[NaOAc] profile similar to Figure 1 which could also be interpreted as conversion of less reactive trimer to more reactive dimer. However at [NaOAc] > 0.2M the decrease in rate with increase in [NaOAc] is much greater than that shown in Figure 1 and corresponds to a 1/[NaOAc] term in the rate expression for reaction of dimer. This difference in rate expression between exchange and olefin oxidation could have very interesting mechanistic implications. For instance, the added acetate inhibition term could result from the need for a vacant coordination site on the Pd (II) before hydride elimination can occur. The scheme is shown in Equations 31 and 32. 

Insertion Reactions of Compounds of Metals and Metalloids Involving Unsaturated Substrates M. F. Lappert and B. Prokai Olefin Oxidation with Palladium (II) Catalyst in Solution A. Aguil6... 

Smidt, J. and Sieber, R. (1959) Reactions of palladium dichloride with olefinic double bonds. Angew. Chem., 71, 626. Moiseev, I.I., Levanda, O.G. and Vargaftik, M.N. (1974) Kinetics of olefin oxidation by tetrachloropalladate in aqueous solution. /. Am. Chem. Soc., 96, 1003. 

Tsuji, J. Nagashima, H. Palladium-catalyzed oxidative coupling of aromatic compounds with olefins using tert-butyl perbenzoate as hydrogen acceptor. Tetrahedron 1984, 40, 2699-2702. 

Scheme 1. Palladium-mediated oxidative coupling reactions of olefins and water.

Wacker reactions, which is why we refer to them here [12, 15-17]. Finally, the general concept of a Wacker reaction could be regarded as the palladium-catalyzed oxidative coupling of heteronucleophiles and olefins, and this can obviously be extended to nitrogen nucleophiles and others [18] conversely, the principle of the Cu(I)/Cu(II)/02 reoxidation system for Pd(0) can be applied to other oxidation reactions (for example that of CO to C02), but the present overview is limited to sp2-C-H activation in olefins. 

A study of the olefin oxidation catalyst system, palladium acetate-MOAc (M = Li or Na), has shown that in the absence of acetate ion, Pd acetate-acetic acid exists as the trimeric species [Pd3(OAc)6].32 Reaction with MOAc is not instantaneous, and u.v.-visible spectra indicate an initial equilibrium involving trimer - dimer (9). When M = Na conversion into dimer is complete at 0.2M-NaOAc. Further addition of... 

J. Tsuji, Synthetic Applications of the Palladium-Catalysed Oxidation of Olefins to Ketones, Synthesis 1984, 369. 

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

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