Wacker process

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The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) originally referred to the oxidation of ethylene to acetaldehyde by oxygen in water in the presence of a palladium tetrachloride catalyst [1]. The same basic reaction is currently used to produce aldehydes and ketones from a number of alkenes. This chemical reaction, a German invention, was the first organometallic and organopalladium reaction applied on an industrial scale. The Wacker process is similar to hydroformylation, which is also an industrial process and also leads to aldehyde compounds. The differences are that hydroformylation promotes chain extension, and uses a rhodium-based catalyst system. The Wacker process is an example of homogeneous catalysis. The palladium complex with ethylene is reminiscent of Zeise's salt, K[PtCl3(C2H4)] which is a heterogeneous catalyst.

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[edit] Reaction mechanism

The modern understanding of the reaction mechanism for the Wacker process (olefin oxidation via palladium(II) chloride) is described below:

The catalytic cycle can be described as follows:

\mathrm{[PdCl_4]^{2-} + C_2H_4 + H_2O \rightarrow CH_3CHO + Pd + 2HCl +2Cl^-}
\mathrm{Pd + 2CuCl_2 + 2Cl^- \rightarrow [PdCl_4]^{2-} + 2CuCl}
\mathrm{2CuCl + 1/2O_2 + 2HCl \rightarrow 2CuCl_2 + H_2O}

Note that all catalysts are regenerated and only the alkene and oxygen are consumed. Without copper(II) chloride and hydrochloric acid as oxidizing agents, the palladium would precipitate out and the reaction would come to a halt (the stoichiometric reaction without catalyst regeneration was discovered in 1894). Air, pure oxygen, or a number of other oxidizers can then oxidise the resultant CuCl back to CuCl2, allowing the cycle to repeat.

The initial stoichiometric reaction was first reported by Phillips [2] [3] and the The Wacker reaction was first reported by Smidt et al. [4] [5] [6].

[edit] Mechanism Summary

Substantial mechanistic investigation on the olefin oxidation cycle has elucidated much of the oxidation process, though some questions remain.[7] Several interesting key points were found:

(1) there is no H/D exchange seen in reactions with fully-deuterated ethylene, C2D4, thus ruling out keto-enol tautomerization.

(2) the lack of a kinetic isotope effect in C2D4 shows that hydride transfer is not a rate-determining step.

(3) a significant competitive isotope effect with C2H2D2 shows the rate determining step should be prior to oxidized product formation.

For these reasons, modern understanding of this process has the rate-determining step occurring before a series of hydride rearrangements.

The bulk of mechanistic studies on the Wacker Process debated whether nucleophilic attack occurred via an external (anti-addition) pathway or via an internal (syn-addition) pathway. Stereochemical studies by Patrick M. Henry[8] confirmed that both pathways occur at different chloride concentrations. Syn-addition occurs under low-chloride reaction concentrations (< 1M, industrial process conditions), while anti-addition occurs under high-chloride (> 3M) reaction concentrations.

Another key step in the Wacker process is the migration of the hydrogen from oxygen to chlorine and formation of the C-O double bond. This step is generally regarded to proceed through a so-called β-hydride elimination with a four-membered cyclic transition state:

Wacker hydride elimination

One in silico study [9] argues that the transition state for this reaction step is unfavorable (activation energy 36.6 kcal/mole) and proposes an alternative reductive elimination reaction mechanism in which the proton directly attaches itself to chlorine with an activation energy of 18.8 kcal/mol. The proposed reaction step gets assistance from a water molecule acting as a catalyst.

Wacker process alternative transition state

[edit] Wacker-Tsuji oxidation

The so-called Wacker-Tsuji oxidation is the laboratory scale version of the above reaction, for example the conversion of 1-decene to decanone with palladium(II) chloride and copper(II) chloride in a water / dimethylformamide solvent mixture in the presence of air [10].

[edit] References

  1. ^ Translated in part from de:Wacker-Verfahren.
  2. ^ F.C. Phillips, Am. Chem. J., 1894, 16, 255-277.
  3. ^ F.C. Phillips, Z. Anorg. Chem., 1894, 6, 213-228.
  4. ^ J. Smidt, W. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Rüttinger, and H. Kojer, Angew. Chem., 1959, 71, 176-182.
  5. ^ W. Hafner, R. Jira, J. Sedlmeier, and J. Smidt, Chem. Ber., 1962, 95, 1575-1581.
  6. ^ J. Smidt, W. Hafner, R. Jira, R. Sieber, J. Sedlmeier, and A. Sabel, Angew. Chem., Int. Ed. Engl., 1962, 1, 80-88.
  7. ^ vida infra - specific citations needed Review Henry, Patrick M. In Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E., Ed.; Wiley & Sons: New York, 2002; p 2119;
  8. ^ citation needed
  9. ^ Inaccessibility of -Hydride Elimination from -OH Functional Groups in Wacker-Type Oxidation John A. Keith, Jonas Oxgaard, and William A. Goddard, III J. Am. Chem. Soc.; 2006; 128(10) pp 3132 - 3133; Abstract
  10. ^ Jiro Tsuji, Hideo Nagashima, and Hisao Nemoto Organic Syntheses, Coll. Vol. 7, p.137 (1990); Vol. 62, p.9 (1984) Link
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