Copper-catalysed oxidative reactions

Copper-catalysed Oxidative Reactions under Molecular Oj gen... 

Caeruloplasmin (Cp) is the major copper-containing protein of human serum and is an acute phase reactant, being synthesised in increased amounts by the liver in inflammation [27]. Several groups have suggested that the pool of non-Cp-bound copper, capable of catalysing oxidative reactions, was elevated in rheumatoid extracellular fluids [28,29], However, these reports are likely to be artifactual, since Cp readily loses its copper during storage-induced oxidation/proteolysis [30,31]. 

Finally, copper is not entirely inert during wort boiling and appears to catalyse oxidation reactions involving polyphenols, giving rise to greater colour in boiled worts than is the case with stainless steel. While this may not be desirable, the oxidation of some compounds with sulphydryl groupings in copper vessels is said to reduce undesirable sulphurous aromas from the final beer. 

In 2012, Christmann et al. developed an enantioselective tandem oxidation-Diels-Alder reaction of a trienol which constituted the key step in the synthesis of the key decalin subunit of UCS1025A. The first step of this one-pot process was a copper-catalysed oxidation of this trienol into... 

Phase-transfer catalysed oxidation of sulphoxides to sulphones using copper(II) permanganate or a mixture of potassium permanganate and copper(II) sulphate is also possible156. In this case hexane is used as the solvent for the organic phase and the reaction is carried out under reflux for 24 hours. Sulphones are prepared by this method in quantitative yields and the mechanism proposed is given in equation (51). 

The palladium catalysed substitution reaction of allylic systems has also been utilised in the formation of five membered rings. Intramolecular nucleophilic attack of the amide nitrogen atom on the allylpalladium complex formed in the oxidative addition of the allyl acetate moiety on the catalyst led to the formation of the five membered ring (3.63.). In the presence of a copper(II) salt the intermediate pyrroline derivative oxidized to pyrrole.80... 

Asymmetric microbial oxidation afforded the (-)-epoxide which has been explored as a building block ring opening reactions with organometallic nucleophiles, and via Friedel-Crafts reactions have been reported. [226,227]. A non-biotransformative approach to this epoxide has also been described [228]. Copper(II)-catalysed oxidative hydrolysis (Eq. 72) affords a lactic acid analogue in high enantiomeric purity [229]. 

The efficiency of a new chiral non-racemic and C2-symmetric 2,2-bipyridyl ligand (6) in copper(I)-catalysed asymmetric allylic oxidation reactions of the cyclic alkenes with f-butyl peroxybenzoate has been evaluated. On performing the reaction of cyclopentene, cyclohexene, and cycloheptene in acetonitrile the corresponding product, (lS)-cycloalk-2-enyl benzoate, was isolated in up to 69% yield and in 91% ee 29... 

Corey chose a Wittig-style (HWE) reaction to control the aldol process and copper-catalysed addition of vinyl Grignard for the conjugate addition. Oxidation with NaI04 and catalytic OSO4 gave the keto-aldehyde 35 which cyclised cleanly under equilibrating conditions. 

Iron and copper catalyse the formation of oxyradicals. Three reactions are relevant in this context (1) Autoxidation of metal complexes may yield the superoxide radical which by itself is not very reactive, but is a precursor of more reactive radical species. (2) The one-electron reduction of hydrogen peroxide -the Fenton reaction - results in hydroxyl radicals via a higher oxidation state of iron [2]. (3) A similar reaction with organic peroxides leads to alkoxyl radicals, although a recent report alleges that hydroxyl radicals are also formed [3]. There is a fourth radical, the formation of which does not require mediation by a metal complex. This is the alkyldioxyl radical, ROO , which is formed at a... 

Toluene has been oxidized by the silver ion catalysed reaction with peroxy-disulfate. The reaction produces a mixture of bibenzyl, benzaldehyde and benzoic acids.299 Russian workers have described the conversion of 4-methoxy-toluene to the benzaldehyde by oxidation with peroxydisulfate in the presence of silver or copper ions and oxalic acid.300 The presence of copper salts in iron or copper catalysed peroxydisulfate oxidation is believed to suppress side-reactions.301 Phillips have patented a palladium(II)/tin(IV)/persulfate system for the oxidation of toluene derivatives.302 The reactions are carried out in carboxylic acid solvents (Figure 3.78). 

A copper catalysed click (azide-alkyne cycloaddition) reaction has been used to prepare a fluorous-tagged TEMPO catalyst (Figure 7.20). TEMPO is a stable organic free radical that can be used in a range of processes. In this case, its use in metal-free catalytic oxidation of primary alcohols to aldehydes using bleach as the terminal oxidant was demonstrated. The modified TEMPO can be sequestered at the end of the reaction on silica gel 60 and then released using ethyl acetate for reuse in further reactions in this way the TEMPO was used four times with no loss in activity. 

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. 

Early work showed that the rate of the silver ion-catalysed oxidation of oxalate is much faster than the oxidations of other substrates under similar conditions King ). Allen showed that with solutions of very low copper concentration, but not de-aerated, the rate is only slightly faster compared with other substrates. However, Kalb and Allen found that oxygen is a powerful inhibitor of the silver ion-catalysed oxidation, and that in the absence of oxygen low concentrations of copper have no effect on the rate. They studied the silver ion-catalysed reaction in the absence of oxygen. With peroxodisulphate concentrations greater than 0.004 M the rate equation is... 

In contrast to silver-catalysed cumene oxidation, the evidence concerning the mechanism of copper-catalysed reactions favours radical initiation via surface hydroperoxide decomposition. Gorokhovatsky has shown that the rate of ethyl benzene oxidation responds to changes in the amount of copper(ii) oxide catalyst used, in a manner which is characteristic of this mechanism. Allara and Roberts have studied the oxidation of hexadecane over copper catalysts treated in various ways to produce different surface oxide species, Depending on the catalyst surface area and surface oxide species present, a certain critical hydroperoxide concentration was necessary in order to produce a catalytic reaction. At lower hydroperoxide levels, the reaction was inhibited by the oxidized copper surface. XPS surface analysis of the copper catalysts showed a... 

The number of S- and N-arylations with arylbismuth reagents is relatively limited. The reaction of thiols with pentavalent organobismuth reagents leads mostly to oxidation products and, in some instances, to 5-phenylation. In the case of the nitrogen derivatives, the most efficient arylation by arylbismuth reagents are reactions which are copper-catalysed, and these will be reviewed in a later part of this chapter (see section 6.7). 

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