Wacker system

A direct route for acetone from propylene was developed using a homogeneous catalyst similar to Wacker system (PdCl2/CuCl2). The reaction conditions are similar to those used for ethylene oxidation to acetaldehyde. ... 

Recently, researchers at Catalytica proposed a new technology for ethylene oxidation (368). Typical compositions are aqueous ca. O.l mM Pd2+, 5-25 mM Cl , and ca. 0.30 M NavH(3+ -v)PVxMoi2 04o (preferably x = 2-3). The Pd2+ and chloride concentrations are only 1/100 those in the oridinary Wacker system. The solutions at pHO-l result in high reaction rates and stability of Pd2+, as shown in Fig. 65. The stability of Pd2+ is further improved by the presence of chloride ion in a concentration of about 0.01 M. In this system, the phosphomolybdate serves two functions in the Pd° reoxidation (l) It solubilizes high concentrations of Vs + in aqueous solution and (2) it accelerates the reoxidation of V4+ by dioxygen. Kinetics (the reaction is first-order in Pd2+ and in ethylene concentrations and zero-order in Vs + concentration) shows that the oxidation of ethylene to produce acetaldehyde is rate-determining. 

As shown, osmium tetroxide bearing the chiral ligand interacts with the olefin to give an Os(VI) ester, which upon hydrolysis releases the chiral diol. The actual oxidant is the metal itself that reduces from Os(VIII) to Os(VI). This reaction was known since the 1930 s and in this respeet it resembles the Wacker system where ethylene is oxidized to acetaldehyde with reduction of Pd(II) to... 

In this regard, the Catalytica catalyst behaves like a self-assembled system as it can be readily formed by mixing Pt black or indissolvable Pt salt with bypm in fumed H2SO4. Noteworthily, forming Pt black or Pt salt is just the formidable problem in the Wacker system [82] as well as in the Shilov system [76]. 

One might presume that Cu(II) acts as a one-electron oxidant, which would require a two-step oxidation sequence (the reaction of Cuta with [Pt Clg] appears from kinetics to be stepwise, although the results were not completely conclusive [25]), making this finding even more remarkable. Conceivably, the actual oxidant involves a cluster of Cu(II) centers, as has been suggested for other oxidations of Pt(II) by Cu(ff), where the intermediacy of Pt(III) may appear unattractive [26] similar consideradmis may apply to the reoxidation of Pd(0) in the Wacker system (a step that is not very well characterized mechanistically). Mixed Pt(II)-Cu(II) clusters are also known [27] and might play a part here. However, other one-electron oxidants such as [Ir Cl6] and Ce(IV), where such clustering seems most unlikely, also oxidize [(CH3)Pt°Cl3] at rates competitive with protonolysis [16]. 

In a second example, we focus on an industrially relevant Wacker reaction system. In the homogeneous Wacker system, Pd + is the active intermediate that generates atomic oxygen from H2O. Cu, however, is necessary to act as a redox couple in order to reoxidize the Pd° that forms with air ... 

The biochemical oxomolybdooxidase enzymes operates similarly to the Pd " " system with the important difference that reoxidation now proceeds electrochemically. As in the Wacker system, the selective oxygen atom is generated by the oxidation of water. A possible catalytic cycle for the sulfite oxidase system l is shown in Fig. 7.14. 

Formation of acetaldehyde and metallic Pd by passing ethylene into an aqueous solution of PdCl2 was reported by Phillips in 1894 15] and used for the quantitative analysis of Pd(II)[16], The reaction was highlighted after the industrial process for acetaldehyde production from ethylene based on this reaetion had been developed[l,17,18]. The Wacker process (or reaction) involves the three unit reactions shown. The unique feature in the Wacker process is the invention of the in situ redox system of PdCl2-CuCl2. 

The direct oxidation of ethylene is used to produce acetaldehyde (qv) ia the Wacker-Hoechst process. The catalyst system is an aqueous solution of palladium chloride and cupric chloride. Under appropriate conditions an olefin can be oxidized to form an unsaturated aldehyde such as the production of acroleia [107-02-8] from propjiene (see Acrolein and derivatives). 

A variation of the Pd/Cu Wacker-Hoechst process, termed OK Technology, has been proposed by Catalytica Associates (40—46). This process avoids the use of chlorides and uses a Pd/Cu catalyst system which incorporates a polyoxoanion and a nitrile ligand. 

Acetaldehyde [75-07-0] can be obtained by the Wacker process, ia which a homogeneous CuC —PdCl system is used for the oxidation. 

In 1975 Wacker-Chemie introduced silicones under the name of m-polymers. These are also room temperature curing liquid polymers which give rubbery materials on cross-linking and are available both as one- and two-component systems. Their particular feature is that they contain dispersions of copolymers such as those of styrene and n-butyl acrylate in the shape of rods or rice grains in the fluid silicone polymer. A small amount of the organic copolymer is also grafted onto the silicone backbone. 

The Wacker process uses an aqueous solution of palladium(II) chloride, copper(II) chloride catalyst system. 

Isobutylene glycol may also be produced by a direct catalyzed liquid phase oxidation of isobutylene with oxygen in presence of water. The catalyst is similar to the Wacker-catalyst system used for the oxidation... 

Betzemeier et al. (1998) have used f-BuOOH, in the presence of a Pd(II) catalyst bearing perfluorinated ligands using a biphasic system of benzene and bromo perfluoro octane to convert a variety of olefins, such as styrene, p-substituted styrenes, vinyl naphthalene, 1-decene etc. to the corresponding ketone via a Wacker type process. Xia and Fell (1997) have used the Li salt of triphenylphosphine monosulphonic acid, which can be solubilized with methanol. A hydroformylation reaction is conducted and catalyst recovery is facilitated by removal of methanol when filtration or extraction with water can be practised. The aqueous solution can be evaporated and the solid salt can be dissolved in methanol and recycled. 

Both the regiochemistry and stereochemistry of Wacker oxidation can be influenced by substituents that engage in chelation with Pd. Whereas a single y-alkoxy function leads to a mixture of aldehyde and ketone, more highly oxygenated systems such as the acetonide or carbonate of the diol 1 lead to dominant aldehyde formation.107 The diol itself gives only ketone, which perhaps indicates that steric factors are also important. 

Early mechanistic studies have indicated that the oxypalladation step in the Wacker process proceeds through an <37z/z-pathway,399 although recent deuterium-labeling experiments have shown the viability of a yy/z-mechanism involving insertion of a metal-coordinated oxygen into the alkene.400,401 For example, with excess chloride ion present, the Wacker-type cyclization of a deuterated phenol system occurred in a primarily //-pathway, whereas the oxypalladation step favored a yy/z-mode in the absence of excess chloride ion (Scheme 16). Thus, either mechanism may be operative under a given set of experimental conditions. 

Asymmetric induction has also been achieved in the cyclization of aliphatic alcohol substrates where the catalyst derived from a spirocyclic ligand differentiates enantiotopic alcohols and alkenes (Equation (114)).416 The catalyst system derived from Pd(TFA)2 and (—)-sparteine has recently been reported for a similar cyclization process (Equation (115)).417 In contrast to the previous cases, molecular oxygen was used as the stoichiometric oxidant, thereby eliminating the reliance on other co-oxidants such as GuCl or/>-benzoquinone. Additional aerobic Wacker-type cyclizations have also been reported employing a Pd(n) system supported by A-heterocyclic carbene (NHC) ligands.401,418... 

Wacker oxidation of l-alkenes. The Wacker oxygenation of 1-alkenes to methyl ketones involves air oxidation catalyzed by PdCl2 and CuCU, which is necessary for reoxidation of Pd(0) to Pd(II).1 This oxygenation is fairly sluggish and can result in chlorinated by-products. A new system is comprised of catalytic amounts of Pd(OAc)2, hydroquinone, and 1, used as the oxygen activator.2 The solvent is aqueous DMF, and a trace of HClOj is added to prevent precipitation of Pd(0). Oxygenation using this system of three catalysts effects Wacker oxidation of 1-alkenes in 2-8 hours and in 67-85% yield. 

The system consisting of [PMo12 BVBO40](3+, ) and Pd2+ salts can catalyze the Wacker-type oxidation. What is the role of [PMo12 V O40]( 3+, ) ... 

Wacker oxidation of styrene has also been performed in [bmim][BF4] and [bmim][PF6], at 60 °C with H2O2 and PdCF as a catalyst [19]. This system gave yields of acetophenone as high as 92 % after 3 h. Hydrogen peroxide may also be used under phase transfer conditions for alkene bond cleavage, to produce adipic acid (an intermediate in the synthesis of nylon-6) from cyclohexene (Scheme 9.9). 

Acetaldehyde is the product of the Wacker process. At the end of the fifties oxidation of ethene to ethanal replaced the addition of water to acetylene, because the acetylene/coal-based chemistry became obsolete, and the ethene/petrochemistry entered the commercial organic chemicals scene. The acetylene route involved one of the oldest organometallics-mediated catalytic routes started up in the 1920s the catalyst system comprised mercury in sulfuric acid. Coordination of acetylene to mercury(II) activates it toward nucleophilic attack of water, but the reaction is slow and large reactor volumes of this toxic catalyst were needed. An equally slow related catalytic process, the zinc catalysed addition of carboxylic acids to acetylene, is still in use in paint manufacture. 

The Wacker-Hoechst process has been studied in great detail and in all textbooks it occurs as the example of a homogeneous catalyst system illustrating nucleophilic addition to alkenes. Divalent palladium is the oxidising agent and water is the oxygen donor according to the equation ... 

The nucleophilic attack of the water or hydroxide species takes place in an anti fashion i.e. the oxygen attacks from outside the palladium complex and the reaction is not an insertion of ethene into the palladium oxygen bond. This has been demonstrated in a model reaction by Backvall [4], The reaction studied was the Wacker reaction of dideuterio-ethene (cis and trans) in the presence of excess of LiCl, which is needed to form 2-chloroethanol as the product instead of ethanal. The latter product would not reveal the stereochemistry of the attack Note that all of the mechanistic work has been carried out, necessarily, on systems deviating in one aspect or another from the real catalytic one. The outcome depends strongly on the concentration of chloride ions [5],... 

Once the functionalized proline (116) was in place, it could be converted to the desired indolizidine ring system using several steps, which included a Wacker oxidation, to generate a methyl ketone... 

The electrochemical Wacker-type oxidation of terminal olefins (111) by using palladium chloride or palladium acetate in the presence of a suitable oxidant leading to 2-alkanones (112) has been intensively studied. As recyclable double-mediatory systems (Scheme 43), quinone, ferric chloride, copper acetate, and triphenylamine have been used as co-oxidizing agents for regeneration of the Pd(II) catalyst [151]. The palladium-catalyzed anodic oxidation of... 

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