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1- Ethylene Oxidation To Acetaldehyde: wacker process
2- Hydroxylation By metal-Oxo Complexes
3- Phase-transfer Catalysis in Oxidation Chemistry
1-Ethylene Oxidation To Acetaldehyde: Wacker Process
The Wacker process (1953) presently leads to the production of 4 million tons of acetaldehyde
The stoichiometric ethylene oxidation reaction has been discovered by F.C. Phillips
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
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.
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.
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
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.
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
*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.
Currently used anion oxides as their sodium or potassium salts include MnO4
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).
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.
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
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.