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OXIDATION OF OLEFINS

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OXIDATION OF OLEFINS

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OXIDATION OF OLEFINS

  1. 1. OXIDATION OF OLEFINS 1
  2. 2. Content:- 1- Ethylene Oxidation To Acetaldehyde: wacker process 2- Hydroxylation By metal-Oxo Complexes 3- Phase-transfer Catalysis in Oxidation Chemistry 2
  3. 3. 1-Ethylene Oxidation To Acetaldehyde: Wacker Process The Wacker process (1953) presently leads to the production of 4 million tons of acetaldehyde per year: The stoichiometric ethylene oxidation reaction has been discovered by F.C. Phillips in 1894: 3
  4. 4. In aqueous medium, PdCl2 is actually in the form [PdCl4]2–. The detailed mechanism has only been proposed in 1979 by Bäckwall and Stille the catalytic cycle below). The isomerization of the hydroxyethyl ligand by β-elimination and re-insertion before decomposition to acetaldehyde has been demonstrated by the fact that addition of D2O, known to deuterate an enol, does not lead to the incorporation of deuterium in acetaldehyde. The rate law is: It can be deduced that the rate-limiting step involves, in its transition state, a palladium complex containing an ethylene molecule and having lost two chloride ligands and a proton. In this stoichiometric reaction, the palladium metal precipitates. In the presence of oxygen, the thermodynamics is favorable to the re-oxidation of Pd0 to PdII. The structural transformation required for Pd oxidation slows down this re-oxidation, however. The Pd0 colloid formation is thus faster and the kinetics is unfavorable for catalysis. It is the introduction of CuCl2 as a cocatalyst that allowed to make this process catalytic. Indeed, CuCl2 can rapidly re-oxidize Pd0 to PdCl2 because of fast inner-sphere Cl transfer via bridging Cu and Pd. CuCl formed can be oxidized by O2. The redox CuI/CuII system works as a redox catalyst in a way very similar to that of biological systems. The coupling between coordination catalysis and redox catalysis is thus a biomimetic concept. The overall catalytic cycle follows
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  6. 6. or, in summary: The Wacker process has also been applied to the ketonization of terminal olefins. Although these applications are complicated by the isomerization of the olefins, good selectivities are now obtained in particular if DMF is added. The reaction is currently used in organic synthesis. 6
  7. 7. 2-Hydroxylation By Metal-Oxo Complexes 2.1Metal-Oxo Complexes In Oxidation Catalysis The complexes with M=O bonds have very different reactivities depending on the nature of the transition metal M. The early transition metals are very oxophilic and form M=O bonds that are not very reactive. These compounds are called oxides. On the other hand, late transition metals form labile M=O bonds because of the repulsion between the filled d metal orbitals and p oxygen orbitals. They are called metal-oxo complexes. The metal-oxo complexes can form and regenerate by transfer of an oxygen atom onto a transition metal using an oxygen atom donor such as H2O2 or from O2 by double oxidative addition giving a metal-dioxo complex. They play an essential role in oxidation catalysis. They can also, as oxidants, remove one electron from an oxidizable substrate (for instance, in the case of [MnO4]– for alkylated aromatics). There are many binary mono- and polymetallic complexes, i.e. containing only one type of metal and the oxo ligands. There are also many compounds containing one or several oxo ligands in addition to other ligands. There are many oxidation reactions that are catalyzed by metal-oxo complexes as illustrated by the non-exhaustive following table. 7
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  9. 9. 2.2.Alkene Dihydroxylation The dihydroxylation of olefins is catalyzed by OsO4. Sharpless has proposed olefin coordination on osmium followed by formation of a metallocycle analogous to metallacyclobutanes in metathesis, then generation of a 5-membered ring. Another mechanistic possibility is direct formation of the 5-membered ring metallocycle without prior coordination of the olefin on osmium. In the presence of a chiral amine such as quinine, Sharpless has demonstrated asymmetric catalysis for this dihydroxylation reaction that is also accelerated by this type of ligand. The oxidizing agent (oxygen donor) is then amine oxide. 9
  10. 10. 3-PHASE-TRANSFER CATALYSIS IN OXIDATION CHEMISTRY Although transition-metal oxide anions are used in stoichiometric amounts in phasetransfer catalysis, this technique should be noted, because it is practical. The insolubility in water of substrates combined with the insolubility in common organic solvents of sodium and potassium salts of transition-metal oxide anions led to low oxidation yields. Under these conditions, it was necessary to utilize large quantities of oxidant, well superior to stoichiometric amounts. The discovery of phase-transfer catalysis allows to use these metal oxide anions in nonpolar solvents such as toluene and methylene chloride. Good selectivities are obtained under mild conditions with this technique. It consists in involving two phases, the aqueous one and the organic one and a phase-transfer reagent, most often in catalytic amounts (which justifies the catalytic nomenclature). This latter reagent is a tetraalkylammonium- or tetraalkylphosphonium salt containing long alkyl chains or a crown ether. The principle consists in obtaining a large cation that allows: *to transport the inorganic anion into the organic phase in which it is solubilized due to the lipophilicity of the counter cation; to render this anion very reactive in the organic phase. This is *due to the decreased strength of electrostatic binding between the anion and the counter cation resulting from the large size of the latter. 10
  11. 11. Currently used anion oxides as their sodium or potassium salts include MnO4 - ,CrO4 - ,Cr2O 7 2- ,ClO-, IO4- and FeO42- The most classic examples of application are Meunier’s epoxidation (also using a transition-metal catalyst), Sharpless’ dihydroxylation and oxidative cleavage of double C=C bonds (which requires catalysis by RuO2). 11
  12. 12. For the latter reaction, a mechanism involving a thermally allowed (2+4) cycloaddition followed by electron transfer (oxidation of MnV to MnVI) and also thermally allowed chelotropic (2+2+2) elimination has been suggested: In fact, the interaction of olefins with transition-metal oxides gives five-membered metallocycles. This can result from (3+2) cycloaddition as shown above (inorganic mechanism) or (2+2) olefin addition on the metal followed by insertion of the two oxygen atoms (organometallic mechanism) as proposed by Sharpless in the case of OsO4. Indeed, MnO4 –, RuO4 and OsO4 can be viewed as 16-electron complexes with a vacant site available on the metal center for the attack of the olefin. On the other hand, in the presence of another ligand such as pyridine, the metal center is electronically saturated, and the (3+2) cycloaddition occurs. 12
  13. 13. Summary Of Oxidation Of Olefins 1-Oxidation of ethylene to acetaldehyde (Wacker process) 2-The various metal-oxo complexes catalyze numerous reactions: allylic oxidation (SeO2), olefin metathesis (MoO3), aromatic oxidation (MnO4 –), water oxidation (RuO4), alkene dihydroxylation (OsO4), oxidation of sulfides to sulfoxides ([VO(acac)2]), epoxidation of alkenes (WO2ClL2 or ReO3Me) and cyclization of 1-pentene-4-ol to THF and THP ([MoO2(acac)2]). These oxidation reactions are considerably improved by using phase-transfer catalysis (PTC) when the catalyst is an anionic oxo complex. PTC enhances the reactivity of transition-metal oxide anions by the introduction of a large organic cation such as R4N+ (with long alkyl arms for R). This organic cation, the phase-transfer catalyst, carries the oxo-anion from the aqueous phase into the organic phase and renders it very reactive by decreasing the electrostatic binding within the ion pair due to its large size. 13
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