Wacker oxidation reaction conditions

A direct route for acetone from propylene was developed using a homogeneous catalyst similar to Wacker system (PdCl2/CuCl2). The reaction conditions are similar to those used for ethylene oxidation to acetaldehyde. ... 

PdCOTfj CIPr) generated in situ from [Pd(p,-Cl)(Cl)(IPr)]j and AgOTf was reported to catalyse the copper-free Wacker-type oxidation of styrene derivatives using ferf-butyl hydroperoxide (TBHP) as the oxidant (Table 10.7) [41]. Reaction conditions minimised oxidative cleavage of styrene, which is a common side-reaction in Wacker-type oxidations. However, when franx-stilbene was used as a substrate, a significant amount of oxidative cleavage occurred. 

Under the conditions of the Wacker oxidation, 4-trimethylsilyl-3-alkyn-l-ols give 7 -lactones. Similarly, A-carbamoyl or A-acetyl 4-trimethylsilyl-3-alkynamines cyclize to y -lactams. Formulate a mechanism for these reactions. (Hint In DzO,... 

In comparable reaction conditions as Pd +Cu +Y, Pd + and Cu2+ exchanged pentasil and ferrierite zeolites show a different type of activity [31]. The main products formed by propylene oxidation on these catalysts are acrolein and propionaldehyde below 120°C and 2-propanol above 120 C. Above 150°C consecutive oxidation of 2-propano1 to acetone is observed. The catalytic role of Pd and Cu in the 2-propanol synthesis is proposed to follow the Wacker concept. It is striking that when Pd + and Cu2+ are exchanged in 10-membered ring zeolites, oxidation of a primary carbon atoms seems possible, as acrolein and propionaldehyde are obtained from propylene. 

The well-known Wacker oxidation of terminal alkenes to methylketones has been used for many years on a large scale. It requires a catalytic amount of Pd(II) together with stoichiometric CuCl2 under aerobic conditions. But it is hmited by palladiiun decomposition and chlorinated byproducts. Therefore, a lot of research has been devoted to modifying the reaction, but most of the time copper cocatalysts were necessary. Another problem is the often observed cleavage of the double bond and the production of aldehydes. 

It is well known that the properties of supercritical fluids are sensitive to pressure, and thus pressure may drastically influence the catalytic activity or the product selectivity when a reaction takes place in supercritical conditions. The favorable pressure for the Wacker oxidation of styrene is around 16 MPa of total pressure including 3 MPa 02, at which the selectivity toward acetophenone reaches 92 %, while under a total pressure of 9 MPa, the selectivity for acetophenone is lower (86 %). However, C02 with a higher pressure of over 20 MPa might retard the interaction between the substrate and the catalyst, and might cause a low concentration of substrate in the vicinity of the catalyst, thus resulting in a relatively low yield [38]. 

The Pd-catalyzed conversion of terminal alkenes to methyl ketones is a reaction that has found widespread use in organic chemistry [87,88]. These reactions, as well as the industrial Wacker process, typically employ CuCh as a co-catalyst or a stoichiometric oxidant. Recently Cu-free reaction conditions were identified for the Wacker-type oxidation of styrenes using fBuOOH as the oxidant. An NHC-coordinated Pd complex, in-situ-generated (I Pr)Pd(OTf)2, served as the catalyst (Table 5) [101]. These conditions min-... 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

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. 

The antiviral marine natural product, (-)-hennoxazole A, was synthesized in the laboratory of F. Yokokawa." The highly functionalized tetrahydropyranyl ring moiety was prepared by the sequence of a Mukaiyama aldol reaction, cheiation-controiied 1,3-syn reduction, Wacker oxidation, and an acid catalyzed intramolecular ketalization. The terminal olefin functionality was oxidized by the modified Wacker oxidation, which utilized Cu(OAc)2 as a co-oxidant. Interestingly, a similar terminal alkene substrate, which had an oxazole moiety, failed to undergo oxidation to the corresponding methyl ketone under a variety of conditions. 

Heteropolyacids can be very useful in oxidation reactions. In contrast with metal chelate catalysts that usually become oxidized and deactivated eventually, the heteropolyacids are extremely stable to oxidation. Acetaldehyde is produced commercially from ethylene by the Wacker reaction with a palladium(II) chloride catalyst, copper(II) chloride, oxygen, and water. The corrosive conditions are a disadvantage of the process. Catalytica Inc. has devised a process (6.55) that uses only 1% as much palladium and chloride as the usual process. It uses a small amount of palladium (II) chloride with a partial sodium salt of phosphomolybdovanadic acid.287... 

Under reaction conditions essentially those for the Wacker oxidation, electron-deficient 1-alkenes (e.g., acrylic esters) are trimerized to jarovides 1,3,5-trisubstituted benzenes. 

Feringa from The Netherlands was the first to report an abnormal Wacker oxidation in 1986.21 Dec-1 -ene was oxidized in the presence of a tertiary alcohol to a mixture of aldehyde 8 and methyl ketone 9. Surprisingly, aldehyde 8 was isolated as the major product (8/9 = 70/30). Even more strikingly, styrene was transformed exclusively to its corresponding phenylacetaldehyde under the same conditions. Feringa proposed that the aldehyde formation involved an oxygen transfer reaction via initial cycloaddition of the nitro-palladium complex, followed by p-hydrogen elimination. 

Under standard catalytic conditions (10% PdCh, 1 eq. CuCI, O2, in DMF/H2O), the Wacker oxidation of 4-methoxystyrene proceeded to give a mixture of the two possible products. As expected, the Markovnikov product, methylketone 31 was predominate and only a small amount of aldehyde 30 was isolated (31 30 = 8.4 1). However, Spencer and coworkers performed the reaction in the absence of the reoxidant CuCI and observed a reversal of the usual regioselectivity.26 Thus, reaction of 4-methoxystyrene with 2 equivalents of PdCb gave aldehyde 30 as the major product. The authors explained the regioselectivity by the involvement of a possible 4-palladium-styrene complex 29. 

TON = 4.4). Under optimized conditions compound 166 can be directly converted to euchrestifoUne (159) by reaction with 0.1 equiv. of paUadium(ll) acetate and 0.1 equiv. of copper(Il) acetate for 2 days. The turnover number for the one-pot triple C-H bond activation (TON of 4.0) is in the same order of magnitude as for the individual reactions (Wacker oxidation and oxidative cyclization). Euchrestifoline (159) was further transformed into girinimbine (168) by reduction of the ketone using lithium aluminum hydride followed by elimination under acidic conditions. 

The cyclopentenone 19 can be prepared by an intramolecular aldol reaction from the diketone 18. This reaction is best achieved with a base such as KOH in MeOH and heat. The diketone 18 can be prepared by Wacker oxidation of the alkene 17. Standard conditions for the Wacker oxidation are 10 mol% PdCla, CuCl, O2, DMF, H2O (see Scheme 5.115). The alkene 17 is prepared by allylation of the enamine of cyclohexanone. See J. Tsuji, I. Shimizu and K. Yamamoto, Tetrahedron Lett. (1976), 2975. 

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