Wacker type oxidation of olefins

Catalytic oxidation of olefins to ketones and aldehydes 3.2.1. Wacker type oxidation of olefins 

The most important and extensively studied Pd(II) catalyzed olefin oxidation is the conversion of ethylene to acetaldehyde, which is best known as the Wacker Process [129,130]. The field has been reviewed several times [131-136] and a comprehensive treatise by Henry is also available [137]. 

Although the detailed treatment of Wacker chemistry is beyond the scope of this book, it seems appropriate to briefly summarize the main features of Pd(II) catalyzed olefin oxidation on the example of ethylene oxidation, which can be given by the equation  

The oxidation is made catalytic via reoxidation of Pd(0) by Cu(II), and of the resulting Cu(I) hy 0 or other reoxidation reagents, like e.g. 

The homogeneous Wacker Process is normally carried out in aqueous HCl solution, in a two-step process. Early procedures, in which ethylene oxidation and catalyst regeneration were performed in one reactor, ran a serious risk of explosions. In the two-step process ethylene oxidation and catalyst regeneration occur separately. Typical conditions are 7-14 atm ethylene at 100-110 °C, affording 99% product. Palladium reoxidation is done at the same temperature and 7-14 atm O2. 

---

The Pd(II) hinge in cage 2 can also participate in a chemical transformation. The catalytic Wacker-type oxidation of olefins took place when 8-nonen-l -ol (32) was heated for 5 h at 80 °C in the presence of cage 2 (5 mol%), giving 9-hydroxynonan-2-one (33)... 

The Wacker-type oxidation of olefins is one of the oldest homogeneous transition metal-catalyzed reactions [1], The most prominent example of this type of reaction is the oxidation of ethylene to acetaldehyde by a PdCl2/CuCl2/02 system (Wacker-Hoechst process). In this industrial process, oxidation of ethylene by Pd(ll) leads to Pd(0), which is reoxidized to Pd(ll) via reduction of Cu(ll) to Cu(l). To complete the oxidation-reduction catalytic cycle, Cu(l) is classically reoxidized to Cu(ll) by O2 [2, 3], The use of bidentate ligands [4], bicomponent systems constituted of benzoquinone and iron(ll) phfhalocyanine [5] or chlorine-free oxidants such as ferric sulfate [6], heteropoly acid [7], and benzoquinone [8], make it possible to increase the selectivity reaction by avoiding the formation of chlorinated products. 

Although the Wacker-type oxidation of olefins has been applied since the early 1980s, processes involving higher olefins are stiU the subject of investigations due to their poor solubility in water. Particularly interesting in this context is the inverse phase-transfer catalysis using water-soluble host molectdes. Indeed, upon a careful choice of the substituent, these receptor molecules avoid the isomerization into internal olefins or make it possible to perform substrate selective oxidations that cannot be achieved a biphasic medium with conventional transition metal catalysts. 

The Pd(Quinox)Cl2, (Quinox = 2-(2-quinolyl)-4,5-dihydrooxazole) catalysed Wacker-type oxidation of olefins bearing homoallylic alcohols by TBHP led to the corresponding /3-hydroxy ketones in good yields. Since the oxidation was catalyst controlled, it was significantly faster than the substrate-controlled Tsuji-Wacker oxidation. The bis- and fra-homoallylic alcohols were oxidized to cyclic peroxyketals, presumably via nucleophilic attack of the methyl ketone. Kinetics of the Wacker-type oxidation of olefins by TBHP in the presence of Quinox (ligand), and (54) as the catalyst reveal first-order dependence on ligand and olefin, and rate saturation in TBHP, as expected of the proposed mechanism (Scheme 9)... 

Intermediates that control the selectivity of the Wacker-type oxidations of olefins. 

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... 

PdS04/H9PV6Mo604o/CuS04 in the presence of chemically modified P-cyclodextrins were also used as CPTC systems in the Wacker-type oxidation of higher a-olefins (Cg-Cje) to the corresponding 2-ketones (Equation 18) with high yields (90-98%) in an aqueous/organic two phase system.545,571... 

It is surprising that the Wacker-type oxidation of 1-octene to 2-octanone is faster with the Co-salophen/zeolite catalyst than with the free complex. However, it is known that the Pd(II)-catalyzed oxidation of terminal olefins to ketones is accelerated by the presence of a catalytic amount of strong acid [1,2]. An explanation of the fester rate of the zeolite-encapsulated Co-salophen in this case is therefore that the acidic sites in the zeolite accelerate the reaction. 

Lambert, A., Derouane, E. G., Kozhevnikov, I. V. Kinetics of One-Stage Wacker-Type Oxidation of C2-C4 Olefins Catalysed by an Aqueous PdCl2-Heteropoly-Anion System. J. Catai. 2002, 211, 445-450. 

The Wacker-type oxidation of the olefins is one of the oldest homogeneous transition metal-catalyzed reactions. The mechanism of the oxidation of ethylene to acetaldehyde by a PdCl2/CuCl2/02 system is shown in Figure 23. Interestingly, the selectivity of the oxidation of olefins with longer alkyl chains is dependent on their solubility in water. Furthermore, the production of chlorinated side-products and isomerized olefins has also occurred for olefins with low water solubility. In order to avoid the solubility issues, co-solvents such as DMSO, acetone, THF, dioxane, acetonitrile, DMF, and ethanol were used and DMF seemed to be the best. ... 

Mitsudome, T., Mizumoto, K., Mizugaki, T., et al. (2010). Wacker-Type Oxidation of Internal Olefins Using a PdCl2/N,N-Dimethylacetamide Catalyst System under Copper-Free Reaction Conditions, Angew. Chem. Int. Ed., 49, pp. 1238-1240. 

Wacker cyclization has proved to be one of the most versatile methods for functionalization of olefins.58,59 However, asymmetric oxidative reaction with palladium(II) species has received only scant attention. Using chiral ligand 1,1 -binaphthyl-2,2 -bis(oxazoline)-coordinated Pd(II) as the catalyst, high enantioselectivity (up to 97% ee) has been attained in the Wacker-type cyclization of o-alkylphenols (66a-f) (Scheme 8-24). 

Palladium-catalyzed, Wacker-type oxidative cycHzation of alkenes represents an attractive strategy for the synthesis of heterocycles [139]. Early examples of these reactions typically employed stoichiometric Pd and, later, cocat-alytic palladium/copper [140-142]. In the late 1970s, Hegedus and coworkers demonstrated that Pd-catalyzed methods could be used to prepare nitrogen heterocyles from unprotected 2-allylanilines and tosyl-protected amino olefins with BQ as the terminal oxidant (Eqs. 23-24) [143,144]. Concurrently, Hosokawa and Murahashi reported that the cyclization of allylphenol substrates can be accomplished by using a palladium catalyst with dioxygen as the sole stoichiometric reoxidant (Eq. 25) [145]. 

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. 

Among the several types of homogeneously catalyzed reactions, oxidation is perhaps the most relevant and applicable to chemical industry. The well-known Wacker oxidation of ethylene to ethylene oxide is the classic example, although this is not a true catalytic process since the palladium (II) ion becomes reduced to metallic palladium unless an oxygen carrier is present. Related to this is the commercial reaction of ethylene and acetic acid to form vinyl acetate, although the mechanism of this reaction does not seem to have yet been discussed publicly. Attempts to achieve selective oxidation of olefins or hydrocarbons heterogeneously do not seem very successful. 

Wacker-type oxidative reactions of olefins with nucleophiles, reactions of zr-allyl-palladium complexes with nucleophiles, reactions based on chelation, and trans-metallation of organomercury compounds. 

To explain the observed behavior, it has been suggested [163,164,169,171] that the oxidation of terminal olefins to methyl ketones takes place via two complementary reactions occurring in a coupled mode (Scheme 12). These reactions are the activation of dioxygen (path A) and a Wacker type oxidation (path B). In path A, the cationic rhodium(II) complex 17, formed from RhCl, olefin, EtOH and... 

Wacker-type oxidations have not only been used to prepare commodity chemicals, but have been used in complex molecule synthesis. Examples of the use of three classes of the oxidative functionalizations of olefins in natural products synthesis are shown in Schemes 16.27-16.30. 

Olefins, which are capable of undergoing Wacker-type oxidation, were also studied under the conditions of high and low chloride. As expected, oxidation, whose kinetics obeyed Eq. 1, occurred at low [Cl ] while exchange, whose kinetics obeyed Eq. 2, occurred at high [Cl ]. The reaction schemes for allyl alcohol, 3, and the trisubstituted... 

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]. 

In fact, Wacker type oxidations (largely applied for aldehyde synthesis, acetoxylation reactions) can be considered as an intra or, more probably according to the recent literature, as an out-of-sphere nucleophilic attack on a palladium-olefin 7r-complex. 

Wacker-type Oxidation. The palladium-catalyzed oxidation of terminal olefins to ketones (Wacker oxidation) is an important chemical process both in the laboratory and in industrial settings. Pd(dba)2 has shown useful activity in this area, for example. 

PdCl2(CH3CN)2-catalyzed dialkoxylation of internal olefins of styrene derivatives containing an o-phenol unit has been achieved because the o-phenol prevents 8-hydride elimination of the a-alkylpalladium(II) species (eq 7).4 Wacker-type oxidation products are obtained when o-anisole-derived substrates are used instead of o-phenols. Under similar reaction conditions, simple styrene derivatives afford the corresponding acetals or their hydrolysis products. The enantioselective variant of this dialkoxylation process has been subsequently developed. ... 

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]. 

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. 

---