Please cite this article in press as: R.A. Sheldon, Recent advances in green catalytic oxidations of alcohols in aqueous media, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.024 ARTICLE IN PRESS G Model CATTOD-9241; No. of Pages 10 Catalysis Today xxx (2014) xxx–xxx Contents lists available at ScienceDirect Catalysis Today j our na l ho me page: www.elsevier.com/locate/cattod Recent advances in green catalytic oxidations of alcohols in aqueous media Roger A. Sheldon ∗ Biocatalysis and Organic Chemistry, Delft University of Technology, Julianalaan 136, 2628BL Delft, The Netherlands a r t i c l e i n f o Article history: Received 2 July 2014 Accepted 1 August 2014 Available online xxx Keywords: Catalytic oxidation Alcohol oxidations in water Dioxygen and hydrogen peroxide as oxidants N-oxy radical catalysts Enzymatic oxidations Laccase a b s t r a c t Catalytic oxidations of alcohols, with dioxygen or hydrogen peroxide as the primary oxidant, in aque- ous reaction media are reviewed. Selective alcohol oxidations with hydrogen peroxide generally involve early transition elements, mostly tungsten, molybdenum and vanadium, in high oxidation states and peroxometal complexes as the active oxidants. Aerobic oxidations, in contrast, involve oxidative dehy- drogenation, usually catalyzed by late transition elements, e.g. water soluble palladium(II)-diamine complexes, or supported nanoparticles of Pd or Au as hybrid species at the interface of homogeneous and heterogeneous catalysis. Alternatively, water soluble organocatalysts, exemplified by stable N-oxy radi- cals such as TEMPO and derivatives thereof, in conjunction with copper catalysts, are efficient catalysts for the aerobic oxidation of alcohols. Metal-free variants of these systems, e.g. employing nitrite or nitric acid as a cocatalyst, are also effective catalysts for aerobic alcohol oxidations. Finally, enzymatic aerobic oxidations of alcohols employing oxidases as catalysts are described. In particular, the laccase/TEMPO system is receiving much attention because of possible applications in the selective oxidations of diols and carbohydrates derived from renewable resources. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Selective oxidations of primary and secondary alcohols to the corresponding aldehydes (or carboxylic acids) and ketones, respectively, are pivotal reactions in organic synthesis. Traditional methods involve stoichiometric inorganic or organic oxidants such as hexavalent chromium, manganese dioxide or the Swern or Dess- Martin reagents, respectively [1]. Although such methods have broad scope they are very atom inefficient, involve the use of toxic and/or hazardous reagents and generate copious amounts of inorganic or organic waste. Consequently, there is an on-going quest for sustainable catalytic technologies, with broad substrate scope, using oxygen or hydrogen peroxide as the primary oxidants [2,3]. However, hypochlorite is widely favored as an oxidant in the pharmaceutical and fine chemical industries, rather than oxygen or hydrogen peroxide, because of the potential explosion hazards associated with the use of the latter oxidants. We note, however, that the use of water as an inert, non-inflammable solvent allevi- ates this problem. Nonetheless, hypochlorite is inexpensive and the ∗ Tel.: +31 15 2782675; fax: +31 15 2781415. E-mail addresses: r.a.sheldon@tudelft.nl, r.sheldon@clea.nl relatively low tonnages involved in pharma and fine chemicals mean that the generation of one or more equivalents of sodium chloride waste is not really an issue. Similarly, the use of environmentally unfriendly organic sol- vents as reaction media should, where possible, be avoided. In this context, water has several advantages: it is abundantly available, inexpensive, odorless, non-toxic and non-inflammable. Indeed, a variety of commercially important catalytic processes, such as hydrogenation, carbonylation, hydroformylation, olefin metathe- sis, polymerization and telomerization, is already performed in an aqueous medium at industrial scale [4]. These processes gener- ally involve catalysis by low-valent transition metal complexes of phosphine ligands, and organometallic compounds as key interme- diates. Replacement of the hydrophobic phosphine ligands used in organic media with highly water soluble hydrophilic equiv- alents, e.g. sulfonated triarylphosphines, allows such reactions to be conducted in aqueous/organic biphasic media. In contrast, catalytic oxidations generally involve transition metals in high oxi- dation states, coordinated to relatively simple hard nitrogen and/or oxygen-containing ligands, and coordination complexes as reac- tive intermediates. For example, in heme-dependent oxygenase and peroxidase enzymes active high-valent oxoiron intermediates are stabilized by coordination to a macrocyclic porphyrin ligand http://dx.doi.org/10.1016/j.cattod.2014.08.024 0920-5861/© 2014 Elsevier B.V. All rights reserved.