Selective alcohols aerobic oxidation

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

The application of ionic liquids as a reaction medium for the copper-catalyzed aerobic oxidation of primary alcohols was reported recently by various groups, in attempts to recycle the relatively expensive oxidant TEMPO [150,151]. A TEMPO/CuCl-based system was employed using [bmim]PF6 (bmim = l-butyl-3-methylimodazolium) as the ionic liquid. At 65 °C a variety of allylic, benzylic, aliphatic primary and secondary alcohols were converted to the respective aldehydes or ketones, with good selectiv-ities [150]. A three-component catalytic system comprised of Cu(C104)2, dimethylaminopyridine (DMAP) and acetamido-TEMPO in the ionic liquid [bmpy]Pp6 (bmpy = l-butyl-4-methylpyridinium) was also applied for the oxidation of benzylic and allylic alcohols as well as selected primary alcohols. Possible recycling of the catalyst system for up to five runs was demonstrated, albeit with significant loss of activity and yields. No reactivity was observed with 1-phenylethanol and cyclohexanol [151]. 

Similarly, a water-soluble palladium complex of a sulfonated phenanthroline ligand catalyzed the highly selective aerobic oxidation of primary and secondary alcohols in an aqueous biphasic system in the absence of any organic solvent (Figure 1.8) [40]. The liquid product could be recovered by simple phase separation, and the aqueous phase, containing the catalyst, used with a fresh batch of alcohol substrate, affording a truly green method for the oxidation of alcohols. 

Gold NPs deposited on carbons are active and selective for mild oxidations in liquid phase although they exhibit almost no catalytic activity in the gas phase. Examples are aerobic oxidation of mono-alcohols, diols, glycerol, glucose, alkenes and alkanes. 

The partial oxidation of alcohols, to afford carbonyl or carboxylic compounds, is another synthetic route of high industrial interest For this, scC02 was investigated as a reaction medium for the aerobic oxidation of aliphatic, unsaturated, aromatic and benzylic acids with different catalytic systems, mainly based on the use of noble metals, both in batch [58-64] and in continuous fixed-bed reactors [65-70]. In this context, very promising results have been obtained when studying the catalytic activity of supported palladium and gold nanoparticles in the oxidation of benzyl alcohol to benzaldehyde these allowed conversions and selectivities in excess of 90% to be achieved [71-73]. 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

In this context, Leitner et al. have developed a novel catalytic system for selective aerobic oxidation of alcohols based on highly dispersed Pd nanoparticles in a PEG matrix with scC02 as the substrate and product phase (Scheme 3.2) [13]. 

The nitroxyl-based systems are the most important and widely investigated homogeneous catalysts for the aerobic and non-aerobic oxidation of alcohols [9]. The different mechanisms with persistent (Scheme 1) and nonpersistent (Scheme 2) nitroxyl radicals is reflected in the selectivity of primary alcohol oxidation. Several... 

Copper-Catalysed Aerobic Oxidation of Selected Primary Alcohols... 

These conditions were next applied to the aerobic oxidation of a variety of primary alcohols. A selection of pertinent examples is displayed in Table IX. 

A combination of RuCl2(Ph3P)3 and the stable nitroxyl radical, 2,2, 6,6 -tetra-methylpiperidine-N-oxyl (TEMPO) is a remarkably effective catalyst for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding aldehydes and ketones, respectively, with above 99% selectivity [87]. The best results were obtained using 1 mol % of RuCl,(Ph3P)3 and 3 mol % of TEMPO (Eq. 21). 

Recently two heterogeneous TPAP catalysts were developed which could be recycled successfully and displayed no leaching In the first example the tetraalkylammonium perruthenate was tethered to the internal surface of mesoporous silica (MCM-41) and was shown [ 101] to catalyse the selective aerobic oxidation of primary and secondary allylic and benzylic alcohols (Fig. 17). Surprisingly, both cyclohexanol and cyclohexenol were unreactive although these substrates can easily be accommodated in the pores of MCM-41. No mechanistic interpretation for this surprising observation was offered by the authors. 

Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. Masutani et al. [107] described (nitrosyl)Ru(salen) complexes, which can be activated by illumination to release the NO ligand. These complexes demonstrated selectivity for oxidation of the alcoholic group versus epoxidation, which was regarded as evidence for the intermediacy of Ru-oxo moieties. Their excellent alcohol coordination properties led to a good enantiomer differentation in the aerobic oxidation of racemic secondary alcohols (Fig. 19) and to a selective oxidation of primary alcohols in the presence of secondary alcohols [108]. 

Another improvement is the use of a Ru/TEMPO catalyst combination for the selective aerobic oxidations of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively (Fig. 1.22) [72]. The method is effective (>99% selectivity) with a broad range of primary and secondary aliphatic, allylic and benzylic alcohols. The overoxidation of aldehydes to the corresponding carboxylic acids is suppressed by the TEMPO which acts as a radical scavenger in preventing autoxidation. 

Recently, an alternative to the catalytic system described above was reported [204]. The new catalytic procedure for the selective aerobic oxidation of primary alcohols to aldehydes was based on a CunBr2(Bpy)-TEMPO system (Bpy=2,2 -bipyridine). The reactions were carried out under air at room temperature and were catalyzed by a [copper11 (bipyridine ligand)] complex and TEMPO and base (KOtBu) as co-catalysts (Fig. 4.70). 

Dijksman, A., Marino-Gonzalez, A., Payeras, A.M., Arends, W., and Sheldon, R.A. 2001. Efficient and selective aerobic oxidation of alcohols into aldehydes and ketones using Ruthenium/TEMPO as the catalytic system. Journal of the American Chemical Society, 123 6826-33. 

A number of systems consist of a palladium salt, typically PdCb or Pd(OAc)2, with abase. For example, PdCb-NaOAc catalyzes the aerobic oxidation of secondary alcohols in ethylene carbonate under nuld conditions. Sheldon has carried out mechanistic investigations on a number of related Pd systems and shown that water-soluble complexes of Pd(II) with phenanthrohnes are stable, recyclable catalysts for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic liquid liquid system. The active catalyst is a dihydroxy-bridged palladium dimer. 

PS has also been used in the copper catalysed aerobic oxidation of primary alcohols (Scheme 9.3). The selective oxidation of primary alcohols into aldehydes can be complicated by overoxidation to carboxylic acids or even decomposition products. These side reactions were not observed in PS, and a high turnover frequency (>31 h ) was achieved. The product could be easily isolated by extraction into -pentane and the PS catalyst-containing phase could be recycled three times. 

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