Wacker oxidation solvents

The method is basically an application of the Wacker oxidation except that the catalyst used is palladium acetate ( Pd(AcO)2 or Pd(02CCH3)2). the solvent is acetic acid or tert-butyl alcohol and the oxygen source is the previously suggested hydrogen peroxide (H202)[17]. 

Wacker oxidation of l-alkenes. The Wacker oxygenation of 1-alkenes to methyl ketones involves air oxidation catalyzed by PdCl2 and CuCU, which is necessary for reoxidation of Pd(0) to Pd(II).1 This oxygenation is fairly sluggish and can result in chlorinated by-products. A new system is comprised of catalytic amounts of Pd(OAc)2, hydroquinone, and 1, used as the oxygen activator.2 The solvent is aqueous DMF, and a trace of HClOj is added to prevent precipitation of Pd(0). Oxygenation using this system of three catalysts effects Wacker oxidation of 1-alkenes in 2-8 hours and in 67-85% yield. 

The impressive activity achieved by Teles catalyst was improved some years later by the use of CO as an additive [92]. In this study, Hayashi and Tanaka reported a TOF of 15600h 1, at least two orders of magnitude higher than [as-PtCl2(tppts)2], for the hydration of alkynes, providing an alternative synthetic route to the Wacker oxidation. Although several solvents were tested, the best results were obtained with aqueous methanol, and sulfuric acid or HTfO as acidic promoters. Unlike Utimoto s observation, in this case terminal propargylic alcohols partially (17-20%) delivered anti-Markovnikov product, in addition to the Markovnikov species. Some years before, Wakatsuki et al. had already reported the anti-Markovnikov hydration of terminal alkynes catalyzed by ruthenium(II) [93]. 

Wacker oxidation. Tsuji et al.s have developed two procedures for oxidation of 1-alkenes to methyl ketones with oxygen that are catalyzed by PdCl2 (7, 278 9, 327). The solvent in both cases is aqueous DMF. One method uses PdCl2-CuCl (molar ratio 1 10) the other uses PdCl2 and p-benzoquinone (molar ratio 1 100). Both procedures are about equivalent for oxidation of simple l-alkenes to methyl ketones, but the former method is usually more effective for oxidation of more complex 1-alkenes. 

Textbook chemistry (297,298) teaches that palladium is the preferred catalyst for aerobic oxidation of olefins. When water is the solvent, nucleophilic water addition to coordinated olefins is the key step in the so-called Wacker cycle. Wacker oxidation occurs regiospecifically because a carbonyl group is formed at that carbon atom of the double bond where the nucleophile in a Markovnikov-like addition would enter. The Wacker reaction thus yields methylketones from primary alkenes ... 

However, all these systems suffer from high concentrations of chloride ion, so that substantial amounts of chlorinated by-products are formed. For these reasons there is a definite need for chloride- and copper-free systems for Wacker oxidations. One such system has been recently described, viz., the aerobic oxidation of terminal olefins in an aqueous biphasic system (no additional solvent)... 

Moreover, it was disclosed that PdCl2 in combination with N,N-dimethylaceta-mide (DMA) solvent could offer a simple and efficient catalyst system for acid-and Cu-free Wacker oxidation [102]. The reaction is illustrated in Fig. 4.37. A wide range of terminal olefins could be oxidized to form the corresponding methyl ketones in high yields, reaching a TOF up to 17 h-1. The Pd-DMA catalyst layer could be recycled. Furthermore this system is also capable of per-... 

For example, PEG-200 and PEG-400 (the number refers to the average molecular weight) were used as solvents for the aerobic oxidation of benzylic alcohols catalyzed by the polyoxometalate, H5PV2Mo10O40 [8]. Combination of the same polyoxometalate with Pd(II) was used to catalyze the Wacker oxidation of propyl-... 

The point has been made that the conditions of p-chloroethanol formation are not the same as used for the Wacker oxidation. Cu Pd chlorine-bridged dimers are likely reactants under higher [Cl ] reaction conditions, which may lead to a different reaction mechanism. However, a second stereochemical study also obtained results consistent with trans hydroxypaUadation. When cfr-l,2-dideuteroethene is oxidized in water with PdCl2 under a CO atmosphere, the product is tran5 -2,3-dideutero-jS-propiolactone (Scheme 37). The reaction conditions were, once again, not identical with standard Wacker process conditions, since the solvent was acetonitrile water, the temperature was —25°C, the bis-ethene PdCl2 complex was used, and there was no excess Cl present. Nevertheless, it is clear that, under many reaction conditions, a trans addition of water onto ethene coordinated to Pd is the favored reaction stereochemistry. 

When water is replaced by alcohols as the reaction solvent in Wacker oxidation, the primary products of ethene oxidation are acetals. If base is present, vinyl ethers and acetals... 

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). 

Monflier, E., Tilloy, S., Blouet, E., Barbaux, Y., Mortreux, A. Wacker oxidation of various olefins in the presence of per(2,6-di-0-methyl)-P-cyclodextrin mechanistic investigations of a multistep catalysis in a solvent-free two-phase system. J. Mol. Catal. A Chemical 1996,109, 27-35. 

The Waclcer oxidation (pronounced vocker ) is used industrially to convert ethylene and O2 into acetaldehyde. The Wacker oxidation is catalyzed by PdCl2 and CUCI2 and requires H2O as solvent. The O atom in the product comes from the water, not the O2. 

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]. 

Hou et al. [22] compared reaction selectivity and conversion for the Wacker oxidation of 1-hexene in four different reaction systems (without solvent, or using SCCO2, IL or CO2-IL as solvent). The conversions in all reaction systems were similar. The selectivity in the CO2- based expanded liquid was significantly higher, and was found to increase with pressure. The higher selectivity of this system can be explained by partial dissolution of a reactant in the CO2 gas phase, which leaves less reactant in contact with catalyst in the liquid phase, decreasing isomerization. An enhanced mass transfer in the C02-expanded liquid may also lead to reduced isomerization. Recycling experiments were performed for supercritical CO2 and CO2 -t IL. The catalyst was stable in both systems, but more stable in the latter. The conclusion that can be drawn is that not only is selectivity enhanced, but also catalyst stability. 

Initial experiments were done in water and resulted in low cyclohexene conversions, low product selectivities, and extensive palladium deactivation by Pd black formation. The low cyclohexanone yield originated from overoxidation of cyclohexanone to 2-cyclohexenone, which undergoes further oxidation to a plethora of by-products. The low cyclohexene conversion can be attributed to the aforementioned low reactivity of the internal double bond as well as the low solubility of cyclohexene in water. Several reaction media have been described in which higher alkenes are oxidized to ketones in organic solvent-based systems. Some typical examples are DMF [4], water mixtures with chlorobenzene, dodecane, sulfolane [5], 3-methylsulfolane andM-methylpyrrolidone [6], or alcohols [7]. These solvent systems indeed lead to increased cyclohexene conversions but still suffer from overoxidation and catalyst deactivation by Pd black formation. Hence, the goal of our research was to find a variation to the Wacker oxidation without over-oxidation of the product and deactivation of the palladium catalyst. 

Wacker oxidation of 1-hexene has been carried out in scC02-[BMIM][PFg] at 40 °C and 12.5 MPa total pressure (2.1 MPa O2) [47]. The main products for this reaction were 2-hexanone and 3-hexanone, with 2-hexanone being the desired product. The conversion approached 100% after 17 h in aU the solvent systems studied (SCCO2 only, [BMIM][PFg] only, scC02-[BMlM][PFg]) as well as in a solventless reaction. Very similar rates were observed in scC02and in scC02-[BMlM][PFgj. 

A similar acac-type palladium complex catalyzes the Wacker oxidation of terminal alkenes, leading to methyl ketones in a binary solvent system comprising benzene and bromoperfluorooctane using t-BuOOH as an oxidant. A nickel hex-afluoroacetylacetonate catalyst was employed in a propylene dimerization, using a binary solvent system comprising toluene and perfluorodecalin. ... 

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