Benzoquinones Wacker process

In 1960, Moiseev and coworkers reported that benzoquinone (BQ) serves as an effective stoichiometric oxidant in the Pd-catalyzed acetoxylation of ethylene (Eq. 2) [19,20]. This result coincided with the independent development of the Wacker process (Eq. 1, Scheme 1) [Ij. Subsequently, BQ was found to be effective in a wide range of Pd-catalyzed oxidation reactions. Eor example, BQ was used to achieve Wacker-type oxidation of terminal alkenes to methyl ketones in aqueous DMF (Eq. 3 [21]), dehydrogenation of cyclohexanone (Eq. 4 [22]), and alcohol oxidation (Eq. 5 [23]). In the final example, 1,4-naphthoquinone (NQ) was used as the stoichiometric oxidant. 

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

The industrial Wacker process is carried out in aqueous hydrochloric acid using PdClj/CuCh as the catalyst under oxygen pressure. The oxidation of higher terminal alkenes under the same conditions is slow and sometimes accompanied by undesired by-products formed by the chlorination of carbonyl com-poimds by CuCh, and isomerization of double bonds. Earlier examples of oxidation of various alkenes, mainly in aqueous solutions, have been tabulated.The pseudo-first-order rate constants for oxidation of various alkenes, relative to the value for cycloheptene, with PdCb in the presence of benzoquinone in aqueous solution have been rqwrted. An accelerating effect of surfactants such as sodium lauryl sulfate on the stoichiometric oxidation of higher alkenes in an aqueous solution has been reported. 

The Wacker process is carried out in aqueous HCl solution and low-boiling acetaldehyde is removed continuously by distillation. However, the oxidation of higher alkenes is carried out in organic solvents which can mix both alkenes and water. DMF is widely used as a solvent for this purpose. The oxidation is a useful synthetic method of producing ketones from alkenes and is used extensively [19]. Some organic compounds are used as stoichiometric oxidants. Benzoquinone is most widely used. 

In the last example, benzoquinone is used as a stoichiometric oxidant instead of Cu that is used as a catalyst in the Wacker process. With multiple catalysis inspired from biological processes, Backwal succeeded in using oxygen from air to ace-toxylate cyclohexene by acetic acid. The redox catalysts are iron phthalocyanine (FePc) and dihydroquinone coupled with PdCl2 ... 

Asymmetric induction has also been achieved in the cyclization of aliphatic alcohol substrates where the catalyst derived from a spirocyclic ligand differentiates enantiotopic alcohols and alkenes (Equation (114)).416 The catalyst system derived from Pd(TFA)2 and (—)-sparteine has recently been reported for a similar cyclization process (Equation (115)).417 In contrast to the previous cases, molecular oxygen was used as the stoichiometric oxidant, thereby eliminating the reliance on other co-oxidants such as GuCl or/>-benzoquinone. Additional aerobic Wacker-type cyclizations have also been reported employing a Pd(n) system supported by A-heterocyclic carbene (NHC) ligands.401,418... 

When 1,3-dienes containing a tethered alcohol are subjected to Wacker-type reactions, the initial intramolecular oxypalladation event creates a 7r-allylpalladium species, which can then undergo an additional bond-forming process to effect an overall 1,4-difunctionalization of the diene with either cis- or // -stereochemistry (Scheme 18).399 An array of substrate types has been shown to participate in this reaction to generate both five- and six-membered fused or ro-oxacycles.435-437 Employing chiral benzoquinone ligands, progress toward the development of an asymmetric variant of this reaction has also been recorded, albeit with only modest levels of enantioselectivity (up to 55% ee).438... 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

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. 

An alternate synthesis of enantiopure (—)-diversonol was recently reported by Tietze et al. using an enantioselective transition-metal-catalyzed domino process (Scheme 14.48). Key step in the synthesis is the formation of a chromane with concomitant introduction of the quaternary stereocenter at C-4a with 96% ee using a domino Wacker/carbonylation/methoxylation reaction in the presence of (S,S)-Bn-BOXAX (310) as ligand [73]. In this reaction phenol 308, containing an alkene moiety, was treated with catalytic amounts of Pd(tfa)2 and 310 in MeOH in the presence of CO and p-benzoquinone the latter is necessary to reoxidize Pd to Pd". One can assume that the first step is an enantiofacial coordination of the Pd" species, which is attacked by the phenolic hydroxyl group to give intermediate... 

A powerful piece of methodology has been developed involving nucleophilic attack on both an r and an T]p-complex in an adaptation of the Wacker reaction. Treatment of cyclohexadiene 9.268 with palladium acetate in acetic acid gives an -complex 9.281 (Scheme 9.76). If the reaction is done in the presence of sodium acetate, this salt will act as a nucleophile to give an t -complex 9.283 via an initial t -complex 9.282. Reductive elimination to form a bond between a ring carbon and the acetate ligand from palladium then gives the diacetate product 9.280. As the first acetate attacks trans to Pd, and the second acetate comes from Pd, the product is the trans isomer. The palladium is now in its zero oxidation state, but inclusion of benzoquinone reoxidizes it to palladium (II) and makes the entire process catalytic. There is an occasional... 

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