Palladium Oxidation in Catalytic Systems Biomimetic Approach J' Piera, J'A' Bckvall: Angew' Chem' In

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Palladium Oxidationin Catalytic Systems-
Biomimetic ApproachJ. Piera, J.-A. Bäckvall
Angew. Chem. Int. Ed. 2008, 47, 3506-3523
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Molecular Oxygen and Hydrogen Peroxideas
Stoichiometric Oxidants
Both environmentally friendly, both show high
efficiency per weight, both give rise to
non-toxic side-products and no waste O2 -
inexpensive (air can often be used) ? - rigorous
safety handling is required for large-scale
preparations ? H2O2 - liquid miscible with
water, easy to handle ? - can undergo
radical-induced decomposition to water and oxygen
?
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Molecular Oxygen and Hydrogen Peroxideas
Stoichiometric Oxidants
BUT - the direct oxidation of palladium by
either O2 or H2O2 is rare and only few examples
are known oxidation of alcohols to ketones,
alkenes to carbonyl compounds and intramolecular
heterocyclizations of alkenes - this is due to
the faster decomposition of the reduced metal in
comparison to the electron transfer rate between
Mn and the oxidant - Result Pd-black
precipitation from soluble palladium species
(Pd-H, Pd0)
- Solution - use of Pd0-stabilizing ligands or -
employment of electron-transfer mediators (ETM)
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Palladium-Catalyzed Aerobic Oxidation with ETMs
Idea - squeeze in an electron-transfer
mediator (ETM) between the substrate-selective
redox catalyst and O2 or H2O2 - this ETM would
then carry the electrons from Mn to the oxidant
along a low-energy pathway, which would compete
kinetically with side reactions of the reduced
metal (precipitation, decomposition)
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Palladium-Catalyzed Aerobic Oxidation with ETMs
Wacker oxidation an illustrative example of the
principle using an ETM

• - the direct reoxidation of Pd0 back to Pd2 by
  O2 in water is difficult, since the energy
  barrier is higher than that for catalyst
  decomposition (Pd black precipitation),
• - the facile transfer of electrons from Pd0 to
  CuCl2 may be due to a close interaction between
  the palladium and copper,
• - and CuCl in known to be readily oxidized by air
  to CuCl2 in aqueous solutions

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Palladium-Catalyzed Aerobic Oxidation with ETMs
Problems associated with the Wacker process -
presence of Cl- ions (to stabilize the system)
has a negative effect on the rate (inverse square
dependence on the chloride anion concentration),
- diminished chemoselectivity, formation of
chlorinated by-products, - reactor corrosion due
to the HCl presence Solution chloride-free
Wacker oxidation
Iron phthalocyanine Fe(Pc) (oxygen carrier)
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Palladium-Catalyzed Aerobic Oxidation with ETMs
Aerobic 1,4-oxidation of 1,3-dienes (metal
macrocycles)
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Palladium-Catalyzed Aerobic Oxidation with ETMs

• Why ETMs work
• - processes are based on a stepwise low-energy
  electron transfer that has lower redox potentials
  analogy to the respiratory chain in biological
  systems,
• - there are 4 oxidants with decreasing oxidation
  potentials (O2, (MLm)ox, BQ, and Pd2) and 4
  components that can be oxidized (MLm, HQ, Pd0,
  and diene),
• - there is a coordination between those redox
  couples that interact O2 coordinates to MLm, HQ
  binds to the metal macrocycle, Pd0 is known to
  coordinate to BQ, and finally Pd2 forms a
  complex with diene

Redox potentials 02/H2O 1.229 eV BQ/HQ
0.6992 eV PdCl42-/Pd0 0.561 eV Pd(OH)2/Pd0
0.07 eV
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Palladium-Catalyzed Aerobic Oxidation with ETMs
Aerobic oxidation of methane to methyl
trifluroacetate (nitrous oxide)
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Palladium-Catalyzed Aerobic Oxidation with ETMs
Aerobic allylic oxidation of olefins
(hetero-polyacids)