Hydroperoxides oxidation, metal-catalyzed

Because two or more radicals, R, are produced by the reaction of one initial polymer molecule, PH, the reaction speeds up as the reaction proceeds, as shown in Figure 15.8. (Note PH = A in this figure.) Autocatalytic oxidation kinetics are often prevalent with hydroperoxide and metal catalyzed oxidation of polymers. 

Cyclohexene oxidations in the presence of a variety of acetylacetonates [442] were found to be free radical chain reactions having the same homogeneous propagation steps and yielding as the principle primary product, cyclohexenyl hydroperoxide. The metal catalyzed decomposition of the primary product appeared to give rise to varying amounts of the principle stable monomeric products of oxidation 2-cyclohexene-l-one, 2-cyclohexene-l-ol and cyclohexene oxide. 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

Transition Metal-Catalyzed Epoxidation of Alkenes. Other transition metal oxidants can convert alkenes to epoxides. The most useful procedures involve f-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium or... 

Similarly, the metal-catalyzed oxidation of aryl alkyl sulfides by t-butyl hydroperoxide carried out in a chiral alcohol gives rise to optically active sulfoxides of low optical purity (e.e., 0.6-9.8%) (57). 

Several different organonitrogen compounds readily undergo metal-catalyzed oxidation using alkyl hydroperoxide or hydrogen peroxide as oxygen transfer agents. In 1961,... 

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

The intermediate formation of alkyl peroxide complexes has been postulated, and in several cases observed with spectroscopic and spectrometric techniques in several selective procedures based on metal catalyzed oxidation with hydroperoxides, Ti and V ions being among the transition metals most widely used for this purpose. However, to date the few examples of alkyl peroxide complexes isolated and characterized in the solid state refer to (dipic)V0(00Bu-f)(H20) 8, synthesized by Mimoun and coworkers in 1983, and to a dimeric Ti complex [((/7 -OOBu-f)titanatrane)2(CH2Cl2)3] 9, synthesized by Boche and coworkers. ... 

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. 

Oxidations are catalyzed by acids, bases, pH values that are higher than the optimum, polyvalent metal ions, peroxides, hydroperoxides, and exposure to oxygen and ultraviolet (UV) illumination. These reactions may necessitate the use of antioxidant chemicals, inert atmospheres, and opaque packaging. Chelating agents... 

It is clear from a recent review of the mechanisms of metal-catalyzed oxidations of hydrocarbons (89) that by far the most extensive studies have been on the oxidation of alkenes and aromatic compounds relatively little work on alkane oxidation is to be found. The studies on these reactions show that, if the reactivity is enhanced by a hard metal, it is often because the metal becomes involved in the free-radical reactions and generates further free radicals by the chain decomposition of hydroperoxides (39) ... 

This paper presents the results of an investigation of the oxidation of substituted olefins in the presence of hydrocarbon-soluble transition metal complexes. Results indicate that the initial interaction of oxygen with the olefin probably does not occur within the coordination sphere of the metal. The best interpretation appears to be autoxidation of the olefin, initiated either by the metal or by metal catalyzed decomposition of peroxidic impurities. The initial product of an olefin having allylic hydrogens is an allylic hydroperoxide species this is usually the case in radical initiated autoxidations. Nonetheless, with some metal complexes the product profile differs markedly from that observed when radical initiators are used. In the presence of several complexes, oxidation is... 

Thus, depending on the metal complex used, cyclohexene oxidation can occur via one or more of at least three major pathways, as shown in Reaction 20 path A, radical initiated decomposition of cyclohexenyl hydroperoxide path B, metal catalyzed epoxidation of the olefin and path C, metal catalyzed epoxidation of an allylic alcohol. Ugo found that path B becomes more pronounced when molybdenum complexes are used to modify the oxidation of cyclohexene in the presence of group... 

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