Olefins Wacker-type

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. 

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. 

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). 

Hegedus proposed the probable course of the cyclization reaction, which follows a Wacker-type reaction mechanism. Coordination of the olefin to Pd(II) results in precipitate 110, which upon treatment with Et3N undergoes intramolecular amination to afford intermediate 111. As expected, the nitrogen atom attack occurs in a 5-exo-trig fashion to afford 112. Hydride... 

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... 

Scheme 1. Reaction scheme for Wacker type olefin oxidation on Pd +Cu +Y.

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]. 

Not only acetylene derivatives do undergo palladium catalysed intarmolecular carbon-nitrogen bond formation with amines. The similar reaction of olefins in a Wacker-type process also leads to ring closure. (0-Aminopentenes bearing a suitable leaving group in the 4-position were converted to pyrroles in a cyclization-isomerisation-elimination sequence (3.65.),82... 

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... 

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... 

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)... 

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. 

For a long time, electrocatalysis (cf. Section 3.2.8) has not been considered important in organometallie chemistry although numerous known processes depend on redox reactions (e. g., Wacker-type catalysis, oxidative olefin carbonylation. 

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. 

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. 

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 application of ethylene in Heck reactions often shows different activities from other olefins, because of Wacker-type side reactions. It was found, however, that iodo- and acceptor-substituted bromoarenes are cleanly converted in aqueous media to the corresponding styrenes utilizing a palladiium-TPPMS complex [13]. Furthermore, high-purity 0- and p-vinyltoluenes were prepared in a dimethyl-formamide/water mixture with palladium tri(o-tolyl)phosphine complexes [14]. Here, the role of water may be the dissolution of the inorganic base (potassium carbonate) in the organic media. 

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. ... 

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

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... 

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. 

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. 

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