Modern Oxidation of Alcohols using Environmentally Benign Oxidants

The major drawbacks of this method are the use of NaOCl as the oxidant, the need for the addition of bromine ions and the necessity to use chlorinated solvents. Recently a great deal of effort has been devoted towards a greener oxoammonium-based method, by, for example, replacing TEMPO by heterogeneous variations or the replacement of NaOCl by a combination of metal as the co-catalyst and molecular oxygen as the oxidant. Examples of heterogeneous variants of TEMPO are anchoring 

Noble metal salts, e.g., of Pd or Pt , undergo reduction by primary and secondary alcohols in homogeneous solution. Indeed, the ability of alcohols to reduce Pd has already been described in 1828 by Berzelius who showed that K2PdCl4 was reduced to palladium metal in an aqueous ethanolic solution [22]. The reaction involves a P hydride elimination from an alkoxymetal intermediate and is a commonly used method for the preparation of noble metal hydrides [Eq. (1)]. In the presence of dioxygen this leads to catalytic oxidative dehydrogenation of the alcohol, e.g., with palladium salts [23-27]. 

Peroxometal pathways are typically observed with early transition metal ions with a d° configuration, e.g., Mo, W, Ti and Re , which are relatively weak oxidants. Oxometal pathways are characteristic of late transition elements and first row transition elements, e.g., Cr, Mn Os , Ru and Ru, which are strong oxidants in high oxidation states. Some metals can operate via both pathways depending, inter alia, on the substrate, e.g., vanadium(V) operates via a peroxometal pathway in olefin epoxidations but an oxometal pathway is involved in alcohol oxidations [28]. 

Ruthenium compounds are widely used as catalysts in organic synthesis [29, 30] and have been studied extensively as catalysts for the aerobic oxidation of alcohols [31], In 1978, Mares and coworkers [32] reported that RuCh nH20 catalyzes the aerobic oxidation of secondary alcohols into the corresponding ketones, albeit in modest 

Modern Oxidation of Alcohols using Environmentally Benign Oxidants 

Oxoammonium based Oxidation of Alcohols - TEMPO as Catalyst 

MCM-41, [15] orbyentrapping TEMPO in sol-gel [16]. Alternatively, in our group we developed an oligomeric TEMPO (Eigure 5.3) derived from Chimassorb 944 [17]. 

TEMPO (0.60mmol/g ether linker) [15] ( ) SI02 TEMPO (0.87mmol/g, amine linker) [13] ( ) SiOj TEMPO (0.40mmol/g, amide linker) [14]  

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I. W. C. E. Arends and R. A. Sheldon, Modern Oxidation of Alcohols Using Environmentally Benign Oxidants, in Modem Oxidation Methods, Wiley-VCH Verlag GmbH Co. KGaA, 2005, pp. 83-118. 

S8 I 5 Modern Oxidation of Alcohols using Environmentally Benign Oxidants... 

In this multi-authored book selected authors in the field of oxidation provide the reader with an up to date of a number of important fields of modern oxidation methodology. Chapter 1 summarizes recent advances on the use of green oxidants such as H2O2 and O2 in the osmium-catalyzed dihydroxylation of olefins. Immobilization of osmium is also discussed and with these recent achievements industrial applications seem to be near. Another important transformation of olefins is epoxidation. In Chapter 2 transition metal-catalyzed epoxidations are reviewed and in Chapter 3 recent advances in organocatalytic ketone-catalyzed epoxidations are covered. Catalytic oxidations of alcohols with the use of environmentally benign oxidants have developed tremendously during the last decade and in Chapter 4 this area is reviewed. Aerobic oxidations catalyzed by N-hydroxyphtahmides (NHPI) are reviewed in Chapter 5. In particular oxidation of hydrocarbons via C-H activation are treated but also oxidations of aUcenes and alcohols are covered. 

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