Aerobic oxidation of alcohols

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

In situ generated Ni-IPr complexes were also active in this oxidation reaction, however higher catalyst loadings (5 mol%) and temperatures (60°C) were required to enable the reaction. A proposed mechanism for the aerobic oxidation of alcohols in presented in Scheme 10.8. 

Kim, S.S. St Rajagopal, G. (2004) Efficient Aerobic Oxidation of Alcohols to Carbonyl Compounds with NHPI/CAN Catalytic System. Synthetic Communications, 33, 2237-2243. 

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... 

Figure 13 Aerobic oxidation of alcohols catalyzed by water-soluble PhenS Pd(OAc)2 R = alkyl, aryl.

Gold-platinum bimetallic clusters for aerobic oxidation of alcohols under ambient conditions Miyamura H, Matsubara R, Kobayashi S (2008) Chem Commun 2031-2033. 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

The principle of hydrogen transfer reactions has been applied to a variety of oxidative transformations of alcohols with Ru11 catalysts.72 Among them, one interesting application is the aerobic oxidation of alcohols developed by Backvall,153-157 which can be performed with a catalytic... 

R. Ciriminna and M. Pagliaro, Tailoring the Catalytic Performance of Sol-Gel-Encapsulated Tetra-n-propylammonium Perruthenate (TPAP) in Aerobic Oxidation of Alcohols, Chem. Eur. J., 2003, 9, 5067. 

Other important successes have been achieved in developing clean, green, methods to oxidize alcohols, for example, the Ru/TEMPO (tetramethylpiperidiny-loxyl radical) catalysis, shown in Figure 9.4, for the aerobic oxidation of alcohols. ... 

A prime advantage of such biphasic systems is that the catalyst resides in one phase and the starting materials and products are in the second phase, thus providing for easy recovery and recycling of the catalyst by simple phase separation. A pertinent example is the aerobic oxidation of alcohols catalyzed by a water-soluble Pd-bathophenanthroline complex (Figure 9.5). The only solvent used is water, the oxidant is air, and the catalyst is recycled by phase separation. 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

Fig. 2 Schematic representation of continuous-flow aerobic oxidation of alcohols using Pd nanoparticles in a PEG/SCCO2 biphasic system...

Gabrielsson et al. reported the aerobic oxidation of alcohols catalyzed by a cationic Cp Ir complexes bearing diamine ligands such as bipyrimidine 10 (Scheme 5.8) [35], the mechanism of which is closely related to the Oppenauer-type oxidation mentioned above. In this reaction, the deprotonation of Ir hydrido species to afford Ir species, and the reoxidation of Ir to Ir by O2, are crucial. 

Aerobic Oxidation of Alcohols Istvdn E. Marko, Paul R. Giles, Masao Tsukazaki, Isabelle Chelle-Regnaut,... 

R. Cirimiima, P. Hesemaim, J. J. E. Moreau, M. Carraro, S. Campestrini, M. Pagliaro, Aerobic Oxidation of Alcohols in Carbon Dioxide with Silica- Supported lonie Liquids Doped with Perruthenate, Chem. Eur. J. 12 ( 2006) 5220-5224. 

In another example, RuCl2(PPh3)3 in [BMIMjCl was shown to be a much less active catalyst than this complex in the ionic liquids [R4N]C1 or [R4N]OH for the aerobic oxidation of alcohol (199). The weaker association of the bulky tetraalkyl-ammonium cation than the planar [BMIM] cation with the anion appears to be the principal reason for the difference. 

The effect of cations and anions of several ionic liquids was investigated for the performance of several catalyst precursors, including RuCh and RuCl2(PPh3)3, for aerobic oxidation of alcohols 199).The catalysts in the neutral ionic liquids... 

An inner-sphere hydrogen atom abstraction from the alcohol by a peroxo metal complex, thus forming a coordinated ketyl radical [(CH3)2—C —O—V(0)(00H)]" , has been proposed for the aerobic oxidation of alcohols catalyzed by peroxidic molybdenum and vanadium derivatives (Scheme 16). While in the case of Mo-catalyzed reaction the H2O2 produced is quantitatively converted to products (ketone and H2O), in the vanadium mediated process, hydrogen peroxide accumulates . In this latter case, the direct involvement of a vanadium monoperoxo species has been substantiated by ESI-MS data. 

A copper-centered mechanism for the Cu-TEMPO-catalyzed aerobic oxidation of alcohols was proposed by Sheldon and co-workers, wherein the active catalytic Cu" species is generated by oxidation of a Cu species with TEMPO, in the presence of alcohol, with formation of TEMPOH (Scheme 3) [146]. The resulting Cu" species is then capable of oxidizing the alcoholate to the aldehyde or ketone species. Regeneration of the TEMPO radical species was achieved by rapid oxidation of TEMPOH with O2. 

We wish to report here on a new and highly efficient catalyst composition for the aerobic oxidation of alcohols to carbonyl derivatives (Scheme 1). The catalyst system is based on 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), Mg(N03)2 (MNT) and N-Bromosuccinimide (NBS), utilizes ecologically friendly solvents and does not require any transition metal co-catalyst. It has been shown, that the described process represents a highly effective catalytic oxidation protocol that can easily and safely be scaled up and transferred to technical scale. 

A recent contribution reported by Aoshima and Tsukuda showed the aerobic oxidation of alcohols such as benzyl alcohol catalyzed by gold nanoclusters. These stable and durable clusters of less than 4 nm were prepared using thermosensitive vinyl ether star polymers previously obtained by living cationic polymerization. 

Figure 1.8 Aqueous biphasic aerobic oxidation of alcohols.

The aerobic oxidation of alcohols mediated by TEMPO, used in combination with other catalysts, such as CuBr Me2S,31 RuCl2(PPh3)332 or the enzyme laccase,33 must also be mentioned. 

The aerobic oxidation of alcohol under neutral or acidic conditions to produce the corresponding adds, which can avoid the neutralization of the carboxylate salts, is also an important R D issue. In Au-catalyzed alcohol oxidation in methanol, the corresponding methyl esters are obtained with high seledivity instead of carboxylic acids by using metal oxide supported Au NPs [157, 160], In this case, base is not necessary, or only a catalytic amount of base is required to promote the readion. However, in water, it was demonstrated that alcohols were not oxidized under acidic conditions [161] and only aldehydes were oxidized to carboxylic adds [162]. Even under solvent-free conditions or in organic solvents, alcohols were converted into aldehydes without base however, the alcohols were not fully oxidized to carboxylic acid under acidic conditions [163-166]. 

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... 

A new catalytic system consisting of a persistent macrocyclic aminoxyl radical and the couple Mn(N03)2-Co(NC>3)2 for the aerobic oxidation of alcohols to carbonyl compounds has been developed. The rate-determining step has been identified by studying the effect of substituents on the oxidation of benzyl alcohol. The chemistry of aminoxyl, amidoxyl, and imidoxyl radicals has been discussed.265... 

A theoretical study of the aerobic oxidation of alcohol using a Pd(OAc)2-DMSO catalyst system has brought out the essential role of DMSO in the oxidation process. 

Various mechanisms for the aerobic oxidation of alcohols catalysed by (NHC)Pd (carboxylate)2(H20) complexes [NHC = l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] were investigated using DFT combined with a solvent model. Of these, reductive j3-hydride elimination, in which the -hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provided the lowest-energy route and explained the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pKa.26S... 

The kinetics of the aerobic oxidation of alcohols catalysed by Pd(OAc)2-triethylamine have been studied experimentally and computationally. Measurement of various kinetic isotope effects and the activation parameters and also rate law derivation support a rate-limiting deprotonation of the palladium-coordinated alcohol, contrary to the previously proposed rate-limiting /3-hydride elimination.234 The catalytic efficiency of Pd(OAc)2-triethylamine and palladium alkoxides in the aerobic oxidation of alcohols has been evaluated. A new catalyst, Pd(IiPr)(OPiv)2, is found to operate efficiently at room temperature.235... 

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