Metal-mediated oxidation of alcohol

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. 

For the sake of completeness we also note that oxygen transfer processes can be mediated by organic catalysts which can be categorized on the same basis as metal catalysts. For example, ketones catalyze a variety of oxidations with mono-peroxysulfate (KHS05) [14]. The active oxidant is the corresponding dkmrane and, hence, the reaction can be construed as involving a peroxometal pathway. Similarly, TEMPO-catalyzed oxidations of alcohols with hypochlorite [15, 16] involve an oxoammonium salt as the active oxidant, i.e. an oxometal pathway. 

The epoxidation using optically active hydroperoxide both as chiral source and oxidant has also been studied as an alternative approach to metal-mediated AE of allylic alcohols, but there is still much room for improvement in this approach [13]. 

Metal Free Transition metal catalysts are highly effective for C—H bond activation. However, transition metal complexes are not only expensive, but also difficult to remove from the reaction products, resulting in toxicity concerns. DDQ is a well-known oxidant in organic chemistry [33]. For many years, it has been used for the oxidation of alcohols to ketones and aromatization. The first intermolecular C—C bond formation was realized by DDQ-mediated Mukaiyama-type aldol reactions [34], The reactions of electron-rich benzyl ethers and silyl enol ethers afforded 3-alkoxy-3-phenylpropionyl derivatives at ambient temperature with moderate to excellent yields (Equation 11.12). 

One method of bypassing the stoichiometry problem involves the deliberate addition of a sacrificial reagents, which accept one of the 0-atom equivalents. Examples are metal-catalyzed oxidations of alkenes with O2 in the presence of alcohols, aldehydes etc. as co-reductants. While this approach is acceptable on a small-scale [32], it is impracticable for the production of bulk chemicals. More practicable co-reductants are molecular hydrogen and carbon monoxide, provided that Reactions 13 and 14 are appropriately mediated by a catalyst. 

After precomplexation with ji-CD, a variety of alcohols, including aromatic alcohols, were oxidized to their corresponding carbonyl compounds in good yields with NaOCl-KBr in aqueous solution. A substrate-selective and transition metal-free oxidation of benzoic and allylic alcohols with NaOCl oxidant mediated by j8-CD in water was developed. In the presence of one molar equivalent of jS-CD, benzyl alcohol, 4-methoxybenzyl alcohol and some primary aromatic alcohols were oxidated to form benzaldehyde, 4-methoxybenzaldehyde and aromatic aldehydes, respectively, at 50 °C for 1-4 h. When 20% of acetone was added to the reaction system, the yield of aldehyde was dramatically decreased. 

Metal Ion Mediators A variety of metal salts and complexes have often been used as soluble mediators for the indirect electrochemical oxidation of alcohols. In particular, there have been many studies on the oxidation of henzyl alcohols to benzaldehydes. 

Miscellaneous Reactions. DTBP has been used as a hydrosi-lylation catalyst, even though catalysis by transition metal complexes have largely replaced the radical methods. DTBP has also been used as an oxidant for silanes. Other applications of DTBP include its use as an initiator for radical mediated deoxygenation of alcohols via the corresponding chloroformate or acetate ester (eq 20). It has also been used as an initiator for the reduction of lactones and esters to ethers using trichlorosilane. In a rare example of a nonradical reaction, DTBP has been used in conjunction with titanium(IV) chloride for the formation of chlorohydrin from alkenes (eq 21). ... 

Usually the stable nitroxyl radicals alone cannot directly catalyze the oxidation of alcohols with dioxygen or peroxide, so they rely on the assistance of various cocatalysts that play an important role in activating the oxidation agent. The most used cocatalysts are first row transition-metal complexes where Cu compounds with various N-donor ligands account for the prime ones. In many instances this combination serves as some kind of model to compare catalytic properties of copper compounds. For example, the performances of two asymmetric tetranuclear (with the Cu4(p—0)2(p — 0)2 404 core) and dinuclear (with the Cu2(p-0)2N202 core) copper(II) complexes were compared in the catalytic TEMPO-mediated aerobic oxidation ofbenzylic alcohols. In spite of their similarity, the complexes perform differently the tetranuclear copper(II) (R) complex is highly active leading to yields up to 99% and TONs up to 770, while the (S,R)-2 dinuclear complex is not so efficient under the same conditions. However, no solid explanation of the activity differences was proposed. 

Zhang G, Proni G, Zhao S, et al. Chiral tetranuclear and dinuclear copper(II) complexes for TEMPO-mediated aerobic oxidation of alcohols are font metal centres better than two Dalton Trans. 2014 43 12313-12320. 

Shen L, Liang S, Wu W, Liang R, Wu L. Multifimctional NH2-mediated zirconium metal-organic framework as an efficient visible-hght-driven photocatalyst for selective oxidation of alcohols and reduction of aqueous Cr(VI). Dalton Trans. 2013 42 13649-13657. 

PaUadium(ll) is also capable of mediating the oxidation of alcohols via the hydrido-metal pathway shown in Scheme 4.5. Blackburn and Schwarz first reported [73] the PdCl2-NaOAc-catalyzed aerobic oxidation of alcohols in 1977. However, activities were very low, with turnover frequencies of the order of 1 h . Subsequently, much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For example, the giant palladium cluster, Pd56ipheii6o(OAc)i8o [74], was shown to catalyze the aerobic oxidation of primary aUyhc alcohols to the corresponding a,(3-unsaturated aldehydes [Eq. (13)] [75]. 

Synthesis of optically pure compounds via transition metal mediated chiral catalysis is very useful from an industrial point of view. We can produce large amounts of chiral compounds with the use of very small quantities of a chiral source. The advantage of transition metal catalysed asymmetric transformation is that there is a possibility of improving the catalyst by modification of the ligands. Recently, olefinic compounds have been transformed into the corresponding optically active alcohols (ee 94-97%) by a catalytic hydroxylation-oxidation procedure. 

The metal-catalyzed asymmetric epoxidation of allylic alcohols with various enan-tiomerically pure hydroperoxides has been studied by several groups. This approach has been employed in the Ti- and V-mediated epoxidation of this class of substrates, in the presence of different achiral additives with modest enantioselectivities (ee ee < 46% ), which turned satisfactory (ee 72%) in the presence of the TADDOL-derived hydroperoxide TADOOH 73 . This oxidant has been recently employed in the oxovanadium sandwich-type POM [ZnW(V0)2(ZnW9034)2] catalyzed epoxidation of various allylic alcohols with very high catalytic efficiency (42000 turnovers) and enantiomeric ratios up to 95 5 98. 

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