Air-Metal catalyst transition-metal catalyzed reactions using molecular oxygen

Mechanism + Description

There may be a variety of mechanisms operation depending on the metal, phase (homo/hetero) and substrate. Most operate via coordination of the alcohol to the metal, dehydrogenation to generate the ketone/aldehyde and a metal hydride. This is oxidized by O2 back to the active higher oxidation state species. Some systems may generate H2.

General comments

Potentially one of the most atom efficient methods for oxidation of alcohols to aldehydes, ketones and carboxylic acids, this is tempered by the hazards of using air / O2 in the presence of flammable solvents. There is currently a focus on the use of flow/ continuous processing to minimize/negate this hazard. A number of metals have been reported as showing activity in this transformation – Cu, Pd, Au, and Ir – examples are given in the references below.

Key references

JACS, 2003, 125, (8), 2195-2199 Zeolite-Confined Nano-RuO2:  A Green, Selective, and Efficient Catalyst for Aerobic Alcohol Oxidation
Advances in Inorganic Chemistry Volume 56, 2004, Pages 211–240 – aerobic oxidation of alcohols review
Chemistry--A European Journal (2009), 15(12), 2915-2922 - Mechanism of Pd(OAc)2/pyridine catalyst reoxidation by O2
Green Chemistry (2010), 12(7), 1180-1186 – scaled Pd catalysed aerobic oxdn using 8% O2 in N2
Advances in Inorganic Chemistry (2006), 58, 235-279 –  Review of Cu complexes and enzymes including aerobic oxidation
Chemical Society Reviews (2012), 41(8), 3381-3430  - Reviews of recent advances in transition metal catalysed oxidations
Green Chemistry (2012), 14(3), 547-564  Transition metal based catalysts in the aerobic oxidation of alcohols
Tetrahedron (2007), 63(35), 8430-8434 - Aerobic oxidation of alcohol in aqueous solution catalyzed by gold
Chemical Reviews (2011), 111(3), 1825-1845 - Organic Synthesis Involving Iridium-Catalyzed Oxidation
Green Chem, 2006, (8), 735-741 Ruthenium hydroxide on magnetite as a magnetically separable heterogeneous catalyst for liquid-phase oxidation and reduction
Chem Asian J. 2008, 3, 196-214 Recent Advances in Immobilized Metal Catalysts for Environmentally Benign Oxidation of Alcohols

Relevant scale up example

No scale up examples identified

Green Review

  1. Atom efficiency (by-products Mwt)
    The exact atom efficiency cannot be determined given the range of metal-air conditions, however in general molecular oxygen will be one of the greenest conditions available if coupled with a catalytic metal.
  2. Safety Concerns
    The use of high temperatures and flammable solvents in oxygen containing atmospheres should be given due consideration.
  3. Toxicity and environmental/aquatic impact
    Varying metals have different human toxicity and ecotoxicity issues. In all cases the use of catalytic quantities should be beneficial and recovery (and zero release) of precious metals is advised for both environmental and financial reasons.
  4. Cost, availability & sustainable feedstocks
    Many of the metals outlined are simple complexes with good commercial availability on scale, however some of them are more novel (nano-clusters, complexes etc) that are in their infancy.
  5. Sustainable implications
    Potentially a very green oxidation technology if the hazards are managed. If possible, base metals should be used as catalysts. Precious metals have a high LCA to produce, and several like Ir and Pd are on the medium to high risk level for depletion.
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