Wacker cycle

It was found, however, that in the RhCl3-catalyzed reaction a complementary Wacker cycle operates as well.292,531,540 The 73 oxorhodium(III) complex formed... 

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 ... 

It is clear that the Wacker cycle in a CuPdY zeolite incorporates the traditional features of the homogeneous catalysis combined with typical effects of a zeolite (303, 310). It also follows that whereas other cation exchangers in principle will show Wacker activity after cation exchange with Cu/Pd ions, the cage and pore architecture will probably be less suitable for Wacker chemistry than those of the faujasite structure. This is the case for fluoro-tetrasilicic mica, a synthetic layer silicate that swells under reaction conditions and allows access to the interlayer space (311). 

The findings of Sen and Halpern (36, see Reaction 7) suggest alternative pathways for the Rh-catalyzed co-oxidation, in which the phosphine is oxidized by liberated peroxide and the ketone is formed during the reduction of Rh(III) to Rh(I) (a Wacker cycle). Read and Walker (87) ruled out Wacker chemistry, since addition of water in their system (OH is needed for the reduction, Reactions 7 and 8) decreased ketone yield somewhat. More convincing evidence for the absence of Wacker type chemistry comes from isotope studies using H2018, work just published by Tang et al. (89). 

Figure 12.3. Rhodium-catalyzed conversion of alkenes into methyl ketones with molecular oxygen involving both a peroxymetalation and a Wacker cycle.

Holland and Milner [474] re-examined this reaction recently in some detail. It was found that reaction of [(C8Hi4)2RhQ]2 with dioxygen in benzene at 74 gave equimolar amounts of 2-cyclooctene-l-one, cyclooctanone and water, equation (279). Reaction proceeds via a pathway which is independent of radical chains and does not involve a Wacker cycle. By contrast, cyclooctene was autoxidized by cobalt naphthenate by a radical pathway to give mainly cyclooctene oxide and this reaction was completely suppressed by the radical inhibitor 2,6-di-f-butyl-p-cresol.. This inhibitor had no effect on reaction (279). Labehng experiments with added H20 showed that water did not take part in this reaction, ruling out a Wacker process. 

These reactions constitute the cycle their sum gives the stoichiometry of the Wacker oxidation ... 

The free HCl and Cl generated in the catalytic cycle produce environmentally harmful chlorinated by-products to the extent that more than 3 kg of HCl need to be added to the reactor per tonne of acetaldehyde produced to keep the catalytic cycle going. Modified catalysts such as ones based on palladium/ phosphomolybdovanadates have been suggested as a way of reducing byproduct formation to less than 1% of that of the conventional Wacker process. These catalysts have yet to make an impact on commercial acetic production, however. 

In contrast to the usual Wacker-conditions, optimum rates and catalyst stability in the Pd/batophenanthroHne-catalyzed olefin oxidations was observed in the presence of NaOAc (pH s 11.5). Under such conditions, the catalyst-containing aqueous phase could be recycled with about 2-3 % loss of activity in each cycle. In the absence of NaOAc precipitation ofPd-black was observed after the second and third cycles. Nevertheless, kinetic data refer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallic palladium. 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

Although the oxidation of ethylene to acetaldehyde was known for a number of years,506 its utility depended on the catalytic regeneration of Pd(0) in situ with cop-per(II) chloride discovered by Smidt and coworkers.507 508 Air oxidation of Cu(I) to Cu(n) makes a complete catalytic cycle. This coupled three-step transformation is known as the Wacker process [Eqs. (9.97)-(9.99)]. The overall reaction [Eq. (9.100)] is the indirect oxidation with oxygen of alkenes to carbonyl compounds ... 

Gerdes, J., Lemke, H, Baisch, H, Wacker, H -H, Schwab, U, and Stem, H. (1984) Cell-cycle analysis of a proliferation-associated human nuclear antigen defined by the MAb Ki-67. J Immunol 133, 1710-1715... 

The cycle approach for oxidation has been adopted at an industrial level for the Wacker-Chemie process for acetaldehyde production, in which ethylene is first put in contact with the oxidized catalyst solution, containing palladium chloride, and in the second step the solution containing the reduced catalyst is sent to a regeneration reactor containing cupric chloride and inside which also air is fed. The regenerated catalyst solution is returned to the first oxidation stage. Another industrial application is the Lummus process for the anaerobic ammoxidation of o-xylene to o-phthaloni-trile [68]. Du Pont has developed the oxidation of n-butane to maleic anhydride catalyzed by V/P/O, in a CFBR reactor, and built a demonstration unit in Spain [69] however, a few years ago the plant was shut down, due to the bad economics. 

Another example is the palladium-catalyzed oxidation of ethylene to acetaldehyde in the presence of oxygen and cupric salts, the so-called Wacker reaction. This catalytic cycle combines two stoichiometric processes, which involve first the reduction of Pd11 to Pd°, followed by reoxidation with Cu11. The understanding of the first step of this process came from the earlier work of Kharasch et al., who showed that the stoichiometric dinuclear complex shown in Figure 2.14 decomposed in the presence of water to acetaldehyde (ethanal), Pd° and HC1 [38]. 

Figure 3.13 a Nucleophilic attack of water on coordinated ethene in the Wacker oxidation cycle b attack ofethoxide on coordinated... 

Figure 3.30 a The three stoichiometric redox reactions and b the net reaction of the Wacker oxidation system c a simplified representation of the Pd and the Cu catalytic cycles (the reverse reaction arrows are omitted, for clarity). 

Figure 1. Schematic catalytic cycle of the Wacker-Hoechst ethene oxidation.

The Wacker catalytic cycle is known in detail due to the detailed work of several authors (298-301) and consists of linked elementary reactions known from organometallic chemistry (302). Even in a chloride medium there is firm evidence for the identity of the rate-determining step (303). The cycle, however, takes into account kinetic, spectroscopic, isotopic, and stereospecific evidence (Scheme 2). 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

Fig Proposed mechanism for the oxidation of ethylene to acetaldehyde in the Wacker process. Chloride ligands have been omitted. The oxidation number ofpalladium is + 2 at all stages of this cycle except the upper left where eductive elimination of acetaldehyde gives Pd (0), which is oxidised by Cu (II). The complete cycle for the reoxidation of Cu (I) is not shown. 

Figure 8.2 Catalytic cycle for the Wacker oxidation of ethylene to acetaldehyde.

Figure 28 Representation of the mechanistic cycle involved in the Wacker reaction the conversion of ethylene into acetaldehyde.

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