Olefin complexes Wacker process

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. 

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... 

Two selective processes are important in the oxidation of ethylene the production of ethylene oxide and acetaldehyde. The first process is specifically catalyzed by silver, the second one by palladium-based catalysts. Silver catalysts are unique and selective for the oxidation of ethylene. No similar situation exists for higher olefins. The effect of palladium catalysts shows a resemblance to the liquid phase oxidation of ethylene in the Wacker process, in which Pd—C2H4 coordination complexes are involved. The high selectivity of the liquid phase process (95%), however, is not matched by the gas phase route at present. 

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. 

Historically the homolytic type of catalysis has been known and studied for a long time. The heterolytic catalysts represent a relatively recent innovation but, nevertheless, include important developments such as the Wacker process for the oxidation of olefins. Regardless of the mechanism involved, the most important characteristics of metal catalysts for effecting oxidation are the accessibility of several oxidation states as well as the accommodation of various coordination numbers, both of which are properties of transition metal complexes. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

The Wacker process, of course, gives highly selective oxidation of olefins to aldehydes or ketones (42) the function of the 02 is to reoxidize the catalyst, and again any formation of a dioxygen complex is incidental, although such a species could be involved in the reoxidation step. Reoxidation of Cu(I) to Cu(II)/Cu(III) by 02 appears to be involved in certain Cu-containing oxidase systems, for example, ascorbic-acid oxidase (43, 44). 

The electrophilic activation of a C—C multiple bond as a result of coordination to an electron-deficient metal ion is fundamental to much of organometallic chemistry, both conceptually and in synthetic applications (11). The Wacker process, a classic example of an efficient catalytic oxidation, is an important industrial reaction, used for the conversion of ethylene into acetaldehyde. The catalytic reaction begins with the coordination of ethylene to a Pd(ll) center, leading to activation of the ethylene moiety. The key step is the reaction of the metal-olefin complex with a nucleophile to give substituted metal-alkyl species (12). The integration of this reaction into a productive catalytic cycle requires the eventual cleavage of the newly generated M—C bond. 

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

William Christopher Zeise (1789-1847) was a Danish apothecary and professor in Copenhagen, Denmark. He synthesized the first metal-olefin complex by serendipity (this term is explained in Chapter 4), when he treated platinum(IV) chloride with ethanol and potassium chloride K[PtCl3( -C2H4)], sal kalico-platinicus inflammabilis , cf [73], TT-Complexation of olefins at transition metals nowadays comprises a key feature of homogeneous catalysis in terms of olefin activation, with the Wacker-Hoechst process being a prominent example (cf. Section 2.4.1). 

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 ). 

The palladium chloride-coppeifll) chloride couple (28, 29) used industrially in the Wacker process oxidizes olefins to carbonyl compounds. Experimental kinetic and isotope effect data (30) seem to indicate that a TT-olefin complex is initially formed in a series of preequilibrium steps. The rate-determining step is postulated to be a rearrangement of the TT-olefin complex to a cr-complex followed by the final breakdown of the cr-complex to products. Figure 13 depicts the widely accepted Henry mechanism (31). 

Homogeneous catalysis with defined soluble transition metal complexes as catalysts has become one of the most effective means of transforming simple olefins into more valuable materials. The technically important hydroformylation of olefins to aldehydes or alcohols the Wacker process the dimerization of propylene to linear hexenes the oligomerization of ethylene to linear a-olefins are only a few examples. A feature common to all these processes is the insertion of a substrate olefin molecule, which is coordinatively bonded to the transition metal center M, into a metal-carbon or metal-hydrogen bond present at the same center ... 

Complexes [PdCl3(olefin)] are also unstable. They are also formed as unstable intermediate compounds during catalytic oxidation of olefins, particularly during oxidation of ethylene to acetaldehyde in the Wacker process. Stability constants for [PdCl3(olefin)] complexes are given in Table 6.11. 

In a variant of the Wacker process, Stille and co-workers coupled the anti-addition of HO-Pd-X to an olefin with a lactonization process to confirm the stereochemistry of the hydroxypalladation of olefins (Scheme 31).[" 3],[44] -pjjg j-gsuit supports the notion that the nucleophile attacks the olefin-palladium complex in an anti fashion. Further evidence for this came from the reaction of the di-dideuterated ethylene with CO and water, which led to a lactone with the two deuterium atoms tmns to each other. 

The organometallic chemistry of pahadium(II) is similar to that of platinum(II) except that the palladium compounds are less stable. This lability permits a wide variety of useful catalytic reactions (e.g., palladium olefin complexes in the Wacker process). Prominent examples are the formation and reaction of r-allyl complexes. The r-allyl complexes can be formed from an olefin bound to palla-dium(II) on heating or by the reaction of an allyl halide... 

In recent years it has become evident that many metal-catalyzed reactions proceed via a substrate metal 7r-complex intermediate. Commercially, the most significant of these include the polymerization of ethylene, the hydro-formylation of olefins yielding aldehydes (oxo process), and the air oxidation of ethylene-producing acetaldehyde (Wacker process). 

In fact, even a simple olefin is activated by 7r-complexation, the attack by nucleophiles being then possible under smooth conditions this principle is the basis of the Wacker process. Therefore, jr-complexation of olefins or the formation of Jt-allyl metal complexes offers a versatile and efficient alternative to the organic chemist for decreasing the electron density on unsaturated systems. 

Alike olefins, allenes also undergo palladium mediated addition in the presence of N-H or O-H bonds. Although these reactions show some similarity to Wacker-type processes, from the mechanistic point of view they are quite different. Allenes, such as the cr-aminoallene in 3.69., usually undergo addition with palladium complexes (e.g. carbopalladation in 3.69. and 3.70., or hydropalladation in 3.71.), which leads to the formation of a functionalized allylpalladium complex. Subsequent intramolecular nucleophilic attack by the amino group leads to the closure of the pyrroline ring.87... 

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