Oxidation with Peroxygen Compounds

The oxidation of benzylic alcohols was quantitative within hours and selective to the corresponding benzaldehydes, but the oxidation of allylic alcohols was less selective. The oxidation of aliphatic alcohols was slower but selective. In mechanistic studies considering oxidation of benzylic alcohols, similar to the oxidation of alkylarenes, a polyoxometalate-sulfoxide complex appears to be the active oxidant. Further isotope-labeling experiments, kinetic isotope effects, and especially Hammett plots showed that oxidation occurs by oxygen transfer from the activated sulfoxide and elimination of water from the alcohol. However, the exact nature of the reaction pathway is dependent on the identity of substituents on the phenyl ring. 

Summarizing the information disclosed in the section above, one notes that polyoxometalates appear to be versatile oxidation catalysts capable of activating various mono-oxygen donors such as iodosobenzene, periodate, ozone, nitrous oxide, and sulfoxides. Some of these reactions are completely new from both a synthetic and mechanistic point of view. The various reaction pathways expressed are also rather unusual and point to the many options and reaction pathways available for oxidation catalyzed by polyoxometalates. 

From a mechanistic point of view it is important to realize that polyoxometalates may interact with peroxygen oxidants in several different ways depending on the composition, structure, and redox potential of the polyoxometalate compounds. On the one hand, one may expect reaction pathways typical for any oxotungstate or 

Others have been also been intrigued by this research. In a computational study, it was concluded that the catalyst precursor is better formulated as having four hydroxy terminal ligands rather than two aqua and two 0x0 ligands [60]. Further computational studies by the same team suggested that the reactivity of 

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Title Modification of Nanotubes by Oxidation with Peroxygen Compounds... 

In the present section we have highlighted research that has been carried out using polyoxometalates as catalysts for oxidation with peroxygen compounds. Not all of the synthetic applications have been noted, but those missing have been previously reviewed [2]. It is important to stress that from a synthetic point of view various substrates with varying functional groups can be effectively transformed to desired products. In addition, interesting reaction selectivity can be obtained in certain cases. 

In the present section we have highHghted research that has been carried out using polyoxometalates as catalysts for oxidation with peroxygen compounds. Not all of the synthetic applications have been noted, but those missing have been reviewed... 

A range of metal catalysts can be employed with peroxygen species for the effective oxidation of sulfur compounds. For example, branched-chain high molecular weight mercaptans are difficult to oxidize with hydrogen peroxide. However, this difficulty is overcome if the reaction is conducted with hydrogen peroxide in the presence of a copper(II) salt.395 The formation of a copper(I) mercaptide followed by its oxidation are believed to be the key steps. 

Dioxiranes. These organic peroxygen compounds are relative newcomers to synthetic chemistry, but in the last ten years it has been shown that they are among the most powerful and versatile non-metal oxidants available to the organic chemist, with the ability to oxidise amines to nitro-compounds, to epoxidise very unreactive double bonds, and to hydroxy late alkanes and aromatic side-chains [26]. 

Metal oxene compounds are formed from the interaction of a metal centre with a peroxygen and this reaction is classed as a two-electron transfer oxidation (Figure 2.16). 

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