Olefin complexes Wacker oxidation

Terminal olefins may be oxidatively cleaved by hydrogen peroxide, catalyzed by a palladium ) complex, to give methyl ketones in almost quantitative yields (equation 38)167. This methodology is an alternative to the well established Wacker protocol using palladium ) complexes. 

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. 

The reaction according to eq. (4) seems to proceed via a mechanism which is common for the homogeneous Pd-catalyzed reactions that are often referred to as Wacker oxidations (cf. Section 2.4.1, [4, 8, 9]). In fact, there are several liquid-phase olefin oxidations that are catalyzed by Pd complexes, and the nature of the reaction products depends on the solvent used (Scheme 3). 

Similar, fluorous palladium /i-dikctonatc complexes (27) have been employed for Wacker oxidation of olefins to the corresponding ketones in a biphasic system [27] (Scheme 3.10). 

Wacker oxidation of olefins to ketones catalyzed by palladium complexes is a well-known process which has been applied to numerous olefins [120]. However, selective oxidation of Cg-Cig a-olefins remains a challenge. Recently, Mortreux et al. have developed a new catalytic system for the quantitative and selective oxidation of higher a-olefins in an aqueous medium [121-123]. For example, 1-decene was oxidized to 2-decanone in 98% yield using PdS04/ H9PV6M06O40/CUSO4 as the catalyst in the presence of per(2,6-di-0-methyl)-j9-cyclodextrin, which probably played the role of a reverse phase transfer reagent [Eq. (22)]. 

Long-chain aliphatic olefins give only insufficient conversion to the acids due to low solubility and isomerization side reactions. In order to overcome these problems the effect of co-solvents and chemically modified /i-cyclodextrins as additives was investigated for the hydrocarboxylation of 1-decene [23], Without such a promoter, conversion and acid selectivity are low, 10% and 20% respectively. Addition of co-solvents significantly increases conversion, but does not reduce the isomerization. In contrast, the addition of dimethyl-/i-cyclodextrin increased conversion and induced 90% selectivity toward the acids. This effect is rationalized by a host/ guest complex of the cyclic carbohydrate and the olefin which prevents isomerization of the double bond. This pronounced chemoselectivity effect of cyclodextrins is also observed in the hydroformylation and the Wacker oxidation of water-insoluble olefins [24, 25]. More recent studies of the biphasic hydrocarboxylation include the reaction of vinyl aromatic compounds to the isomeric arylpropanoic acids [29, 30], and of small, sparingly water-soluble alkenes such as propene [31]. 

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). 

Cuprous chloride tends to form water-soluble complexes with lower olefins and acts as an IPTC catalyst, e.g., in the two-phase hydrolysis of alkyl chlorides to alcohols with sodium carboxylate solution [10,151] and in the Prins reactions between 1-alkenes and aqueous formaldehyde in the presence of HCl to form 1,3-glycols [10]. Similarly, water-soluble rhodium-based catalysts (4-diphenylphosphinobenzoic acid and tri-Cs-io-alkylmethylam-monium chlorides) were used as IPTC catalysts for the hydroformylation of hexene, dodecene, and hexadecene to produce aldehydes for the fine chemicals market [152]. Palladium diphenyl(potassium sulfonatobenzyl)phosphine and its oxide complexes catalyzed the IPTC dehalogenation reactions of allyl and benzyl halides [153]. Allylic substrates such as cinnamyl ethyl carbonate and nucleophiles such as ethyl acetoactate and acetyl acetone catalyzed by a water-soluble bis(dibenzylideneacetone)palladium or palladium complex of sulfonated triphenylphosphine gave regio- and stereo-specific alkylation products in quantitative yields [154]. Ito et al. used a self-assembled nanocage as an IPTC catalyst for the Wacker oxidation of styrene catalyzed by (en)Pd(N03) [155]. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

Wacker oxidation of an alkene in a two-phase system A reaction mixture, containing olefin (2 mmol), the corresponding amount of a metal complex (0.05 mmol), the cocatalyst (0.2 mmol) and water (2 mL) was introduced into an autoclave with a volume of 15 mL. The autoclave was purged with oxygen (0.5-1.5 MPa). The solution was vigorously stirred at 65 °C for 4h. The autoclave was than cooled, pentane (2 mL) was added and the aqueous layer with catalyst was separated from the solution of the product in pentane. 

The mechanism of the Wacker oxidation has been the subject of many mechanistic studies and much discussion for nearly 50 years. At this point, the identity of the elementary steps of this process appears to depend on the reaction conditions. The majority of the mechanistic discussion has focused on whether the C-O bond is formed by nucleophilic attack of water on a coordinated olefin or by insertion of an olefin into a metal-hydroxo complex. These elementary reactions were discussed in Qiapters 11 and 9, respectively. It appears that the mechanism involving nucleophilic attack occurs under conditions of high chloride concentration, and the mechanism involving olefin insertion occurs imder conditions of low chloride concentration. ... 

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. 

Water is a moderately reactive nucleophile involved in several well-known catalytic cycles, such as hydroxycarbonylation and Wacker oxidation of olefins. Besides these, palladium, as many other late transition metals, is reactive in the water gas shift reaction (WGS reaction) (Scheme 3), which is a source of metal hydride complexes. Further transformations triggered by the WGS reaction are versatile. 

Wacker oxidation of olefins by palladium complexes involves water as the nucleophilic reagent. Depending on conditions, the oxidation of olefins by Pd(II) salts in the presence of water gives ketones or chlorohydrins. The enantioselective procedure leading to the latter involves chiral bidentate phosphines, either sulfonated or nonsulfonated (Scheme 83, L = sulfonated (f )-TolBINAP).f ... 

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, " Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

Water is so extensively used in catalytic oxidation reactions that usually this fact is regarded as a natural feature and remains unnoticed. Wacker oxidation of olefins by palladium complexes involves water as a nucleophilic reagent, and thus the whole Wacker-type chemistry, which has developed into a powerful and versatile method of organic synthesis, is derived from aqueous catalysis [178]. The role of the nature of the co-oxidant and the mechanism of deactivation of the palladium catalyst due to aggregation and growth of inactive metal particles were recently investigated, and such study may have relevance for other processes catalyzed by phosphine-less palladium catalysts [179]. 

Optically active, vicinal chlorohydrins can serve as building blocks in much the same capacity as epoxides, azido alcohols, or diols. Enantioselec-tive access to chlorohydrins such as 165 was made possible through Henry s discoveiy of a Pd-catalyzed interrupted Wacker oxidation of olefins (Equation 28) [136, 137]. The process employs tetrasulfonated BINAP 166 as a chiral ligand embedded within the bimetallic triketone complex 164 [136]. 

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... 

Hegedus et al. have thoroughly studied the homogeneous hydroamination of olefins in the presence of transition metal complexes. However, most of these reactions are either promoted or assisted, i.e. are stoichiometric reactions of an amine with a coordinated alkene [98-101] or, if catalytic, give rise to the oxidative hydroamination products, as for example in the cyclization of o-allylanilines to 2-alkylindoles [102, 103], i.e. are relevant to Wacker-type chemistry [104]. 

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