Mechanisms aerobic oxidation

Identical kinetics are found for the uranyl ion-catalysed aerobic oxidation of ascorbic acid and a similar mechanism has been put forward These results and others afford a sequence of catalytic activity for the aerobic oxidation of ascorbic acid ... 

The oxidation of alcohols to the corresponding carbonyl compounds is one of the key reactions in organic synthesis and nnmerous methods have been developed over the years to accomplish this transformation [16], A general mechanism for Pd-catalysed aerobic oxidation is shown below (Scheme 10.5). 

Scheme 10.6 Mechanism of aerobic oxidation catalysed by complex 13 [23] Table 10.2 Oxidative kinetic resolution of alcohols using (-)-sparteine [25]...

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. 

Figure 32. Proposed mechanism for the aerobic oxidation of benzyl alcohol by Complex E. [Adapted from (212).]...

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

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. 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

Somewhat related to the chemistry observed with the peptidylglycine-a-hydroxylating monoxygenase (PHM), the aerobic oxidation of cyclohexane has been studied by several groups. Murahashi et al. have reported that with CuCl2 and acetaldehyde and in the presence of 18-crown-6, cyclohexanone was obtained as the major product (cyclohexanol was formed as the major by-product) under relativelymild conditions (70 °C) [109,110]. Turnover numbers of up to 1600 were achieved with 61% yield of cyclohexanone at 1 atm of O2, but the precise mechanism has not been elucidated (Eq. 7). 

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. 

The catalytic mechanism for BQ-coupled Pd-catalyzed oxidation reactions is formally identical to that of the aerobic oxidation reactions (Scheme 2). Benzoquinone replaces O2 as the oxidant for Pd and hydroquinone is formed as a by-product. This similarity suggests that it might be possible to convert... 

A general simplified mechanism for palladium-catalyzed aerobic oxidation reactions and the different intermediates is given in Scheme 13. 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

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... 

Working with a mutated bacterial strain, Isbister et al. (62) demonstrated a novel mechanism of aerobic oxidation of dibenzothiophene which involved the specific excision of the sulfur atom from the molecule (Figure 11). Studies with -labeled dibenzothiophene showed the release of the radioactivity into the aqueous phase and ion chromatography showed the appearence of sulfate. There was no radioactive carbon dioxide released when this microorganism was incubated with 14C-labeled dibenzothiophene. GC-MS analysis showed that the oxidation product was 2,2 -dihydroxybiphenyl. Kargi and Robinson (52) also report the release of sulfate from dibenzothiophene. This OSC served as the sole carbon and sulfur source in their cultures of the aerobic thermophile Sulfolobus acidocaldarius. 

Recently, N-hydroxyphthalimide (NHPI) catalysis has been applied to a variety of aerobic oxidations of organic compounds [12], We have reported how NHPI, in the presence of Co(II) salts, is able to generate the phthalimido N-oxyl (PINO) radical, which rapidly abstracts hydrogen from aromatic and aliphatic aldehydes, in a free-radical chain mechanism under aerobic conditions (Scheme 14.2) [13]. The role of oxygen is to oxidize Co(II) to Co(III), which is also involved in the oxidation of the intermediate (Equation 14.8). 

The mechanism of the aerobic oxidation of alcohols depends on the particular catalyst used. Two general mechanisms can be considered (1) the direct oxygenation of alcohols by 02 through a free-radical chain process initiated by the catalyst, and (2) the direct oxidation of the alcohol by the catalyst, which is then regenerated by 02. Both mechanisms are well illustrated [6] by the aerobic oxidations catalyzed by the persistent tetramethylpiperidine-N-oxyl (TEMPO) radical 1 and the nonpersis-tent phthalimide-N-oxyl (PINO) radical 2. 

Pd(II) complexes have also been used for aerobic oxidation of alcohols to the corresponding carbonyl compounds - the mechanism likely involves the reduction of Pd(II) to Pd(0) by the alcohol. In this reaction also the role of 02 is the regeneration of Pd(II) through a Pd-peroxo intermediate [8] (Scheme 3). 

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