Wacker process intermediates

Ab initio calculations on the trans effect at platinum(II) and rhodium(I) rationalize its operation in terms of the trigonal-bipyramidal reaction intermediates. Theoretical studies on some palladium(II) ethene compounds (all of them Wacker process intermediates) suggested a trans influence series OH >C1" >OH2 at this metal. ... 

Extensive studies on the Wacker process have been carried out in industrial laboratories. Also, many papers on mechanistic and kinetic studies have been published[17-22]. Several interesting observations have been made in the oxidation of ethylene. Most important, it has been established that no incorporation of deuterium takes place by the reaction carried out in D2O, indicating that the hydride shift takes place and vinyl alcohol is not an intermediate[l,17]. The reaction is explained by oxypailadation of ethylene, / -elimination to give the vinyl alcohol 6, which complexes to H-PdCl, reinsertion of the coordinated vinyl alcohol with opposite regiochemistry to give 7, and aldehyde formation by the elimination of Pd—H. 

Alternatively, the intermediate acetaldehyde (qv) for this process was obtained from ethylene by the Wacker process (9). A small amount of -butyl alcohol is produced in the United States by the Ziegler-Natta chain growth reaction from ethylene [74-85-1] (10). 

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... 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

In addition to the cr-v equilibrium, an exchange between the n complex (45) and free acetaldehyde has been demonstrated using, 4C-labeled acetaldehyde (46, 48). After 45 hours at room temperature, 0.46% exchange was observed. While this appears quite small, one must remember that the free vinyl alcohol/acetaldehyde ratio has been estimated to have an upper limit of only 10-7 and that in the Wacker Process the equilibrium between 7r-coordinated and free vinyl alcohol would be shifted considerably in favor of free vinyl alcohol by the overpressure of ethylene. Thus, the behavior of the ir-vinyl alcohol complexes (45) and (48) seem to support the importance of such complexes as intermediates in the Wacker and similar olefin oxidation processes. 

One of the mechanistic steps most often encountered and inferred from kinetic data is ligand dissociation, which leads to the generation of a catalytically active intermediate. If ligand is added to such a catalytic system, the rate of the reaction decreases. Examples of this in homogeneous catalytic reactions are many CO dissociation in cobalt-catalyzed hydroformylation, phosphine dissociation in RhCl(PPh3)3-catalyzed hydrogenation, Cl dissociation in the Wacker process, etc. The actual rate expressions of most of these processes are described in subsequent chapters. 

Catalytic oxidations with dioxygen can also proceed via heterolytic pathways which do not involve free radicals as intermediates. They generally involve a two-electron oxidation of a (coordinated) substrate by a metal ion. The oxidized form of the metal is subsequently regenerated by reaction of the reduced form with dioxygen. Typical examples are the palladium(II)-catalyzed oxidation of al-kenes (Wacker process) and oxidative dehydrogenation of alcohols (Fig. 4.6). 

The synthesis of acetaldehyde by oxidation of ethylene, generally known as the Wacker process, was a major landmark in the application of homogeneous catalysis to industrial organic chemistry. It was also a major step in the displacement of acetylene (made from calcium carbide) as the feedstock for the manufacture of organic chemicals. Acetylene-based acetaldehyde was a major intermediate for production of acetic acid and butyraldehyde. However the cost was high because a large energy input is required to produce acetylene. The acetylene process still survives in a few East European countries and in Switzerland, where low cost acetylene is available. 

Several important nomadical catalytic oxidations go via organometalhc mechanisms. The commercially useful Wacker process converts ethylene to acetaldehyde with air as oxidant, using Pd(II) and Cu(II) catalysts. The Pd(II) binds to the ethylene to give an organometalhc intermediate, the alkene complex. This complex subsequently uses water as the O source to oxidize the ethylene to acetaldehyde, the Pd being reduced in the process. The resulting Pd(0) is reoxidized to Pd(II) with two equivalents of Cu(n) and the Cu(I) so formed is then reoxidized by air to close the cycle. 

Over the last 50 years numerous reactions of organic compounds catalyzed by transition metal complexes have been developed (e. g., olefin oxidation -Wacker Process, hydroformylation, carbonylation, hydrogenation, metathesis, Ziegler-Natta polymerization and oligomerization of olefins) in which the reactivity of metal-carbon bonds in the active intermediate (organometallics) is crucial. 

Acetoxylations (oxyacylations) have to be seen in context with olefin oxidation to carbonyl compounds (Wacker process, Section 2.4.1). With the lowest olefin, ethylene, acetaldehyde is formed. In water-free acetic acid no reaction takes place. Only in the presence of alkali acetates - the acetate ion shows higher nu-cleophilicity than acetic acid - ethylene reacts with palladium salts (eq. (1)) to give vinyl acetate, the expected product, as first reported by Moiseev et al. [1]. Stem and Spector [2] independently used [HP04] as base in a mixture of isooctane and acetic acid. This reaction could be exploited for a commercial process to produce vinyl acetate and closed the last gap replacing acetylene by the cheaper ethylene, a petrochemical feed material, for the production of large-tonnage chemical intermediates. 

The application of organometallic compounds in medicine, pharmacy, agriculture and industry requires the accurate determination of these metals as part of their application. Most % complexes characterised by direct carbon-to-carbon metal bonding may be classified as organometallic and the nature and characteristics of the n ligands are similar to those in the coordination metal-ligand complexes. The -complex metals are the least satisfactorily described by crystal field theory (CFT) or valence bond theory (VBT). They are better treated by molecular orbital theory (MOT) and ligand field theory (LFT). There are several uses of metal 7i-complexes and metal catalysed reactions that proceed via substrate metal rc-complex intermediate. Examples of these are the polymerisation of ethylene and the hydration of olefins to form aldehydes as in the Wacker process of air oxidation of ethylene to produce acetaldehyde. 

The impetus for research in this filed stems from the industrial importance of metal-olefin complexes as intermediates and catalysts in a wide range of reactions, especially in the petrochemical industry. Major uses include the Wacker Process (oxidation of ethylene to acetaldehyde in the presence of PdCl2), the OXO process (hydroformalation of olefins), the specific hydrogenation of double bonds and the isomerisation of olefins (e.g. but-l-ene to but-2-ene in the presence of [ (C2H4 )2 RhCl ]2 ). 

Spencer and gaunt developed a palladium-catalyzed amidation reaction.57 When enone 72 was treated with carbamate Cbz-NH2, in the presence of a catalytic amount of Pd(II), the Wacker addition intermediate underwent a protonolysis rather than the conventional P-hydride elimination process. As a consequence, amidation adduct 73 was obtained as the sole product. 

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