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Metals and Air

Mechanism + Description

There may be a variety of mechanisms operating depending on the metal, phase (homo/hetero) and substrate. Most operate via the coordination of the alcohol to the metal, and the dehydrogenation to generate the aldehyde and a metal hydride. This is oxidized by O2 back to the active higher oxidation state species. Some systems may generate H2 and need hydrogen acceptors. The aldehyde is then oxidized to the acid; this could occur via a number of mechanisms, including radicals.

General comments

Though potentially one of the most atom efficient methods for the oxidation of alcohols to aldehydes, ketones and carboxylic acids, its success 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: V, Au, Pd, and Pt. Metal catalysts can be homo and heterogeneous. A recent advancement in this area is the use of water as an oxidant in the homogeneous catalyzed oxidation of primary alcohols to carboxylic acids.

Key references

Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Catalytic Transformation of Alcohols to Carboxylic Acid Salts and H2 Using Water as the Oxygen Atom Source. Nat. Chem. 2013,  5, 122–125.

Parmeggiani, C.; Cardona, F. Transition Metal Based Catalysts in the Aerobic Oxidation of Alcohols. Green Chem. 2012, 14, 547-564.

Marui, K.; Higashiura, Y.; Kodama, S.; Hashidate, S.; Nomoto, A.; Yano, S.; Ueshima, M.; Ogawa, A. Vanadium-Catalyzed Green Oxidation of Benzylic Alcohols in Water Under Air Atmosphere. Tetrahedron. 2014, 70 (14), 2431-2438.

Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Recent Advances in Transition-Metal Catalyzed Reactions Using Molecular Oxygen as the Oxidant. Chem. Soc. Rev. 2012, 41, 3381-3430.

Matsumoto, T.; Ueno, M.; Wang, N.; Kobayashi, S. Recent Advances in Immobilized Metal Catalysts for Environmentally Benign Oxidation of Alcohols. Chem Asian J. 2008, 3 (2), 196-214.

Technology Solutions for Oxidation with O2/Air

Ye, X.; Johnson, M. D.; Diao, T.; Yates, M. H.; Stahl, S. S. Development of Safe and Scalable Continuous-Flow Methods for Palladium-Catalyzed Aerobic Oxidation Reactions. Green Chem. 2010, 12, 1180-1186.

Protasova, L. N.; Bulut, M.; Ormerod, D.; Buekenhoudt, A.; Berton, J.; Stevens, C. V. Latest Highlights in Liquid-Phase Reactions for Organic Synthesis in Microreactors. Org. Process Res. Dev. 2013, 17 (5), 760−791.

Greene, J. F.; Preger, Y.; Stahl, S. S.; Root, T. W. PTFE-Membrane Flow Reactor for Aerobic Oxidation Reactions and Its Application to Alcohol Oxidation. Org. Process Res. Dev. 2015, 19 (7),  858–864.

Ciriminna, R.; Pandarus, V.; Béland, F.; Xu, Y.; Pagliaro, M. Heterogeneously Catalyzed Alcohol Oxidation for the Fine Chemical Industry. Org. Process Res. Dev. 2015, 19 (11), 1554-1558.

Relevant Scale-Up Examples with Scheme

Org. Process Res. Dev. 2003, 7 (2), 198-201.
16 g scale

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.) and in their infancy.
  5. Sustainable implications
    This can potentially be a very green oxidation technology if the hazards are managed. If possible, base metals should be used as catalysts. Precious metals have a high LCI to produce and several, like Ir and Pd, are on the medium- to high-risk level for depletion.