Hydroperoxides, alkyl mechanism

In the oxidation of a hydrocarbon the substitution product is an alkyl hydroperoxide. The mechanism is rendered more complex by the fact that the oxygen molecule is diatomic and oxygen atom bivalent. 

Fig. 7. Hydroperoxide Decomposition Mechanism for Hydroxylamines. R mixture of long chain alkyl groups C16H33, C18H37, C20H42, and C22H45.

Reactions 33 and 35 constitute the two principal reactions of alkyl hydroperoxides with metal complexes and are the most common pathway for catalysis of LPOs (2). Both manganese and cobalt are especially effective in these reactions. There is extensive evidence that the oxidation of intermediate ketones is enhanced by a manganese catalyst, probably through an enol mechanism (34,96,183—185). 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Sulphur compounds, e.g., thiopropionate esters and metal dithiolates (Table la, AO 16 and 17), decompose hydroperoxides catalytically, i.e., one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids [19,20]. Scheme 6 shows a simplified scheme for the antioxidant mechanism of simple alkyl sulphides. 

The mechanism for such a process was explained in terms of a structure as depicted in Figure 6.5. The allylic alcohol and the alkyl hydroperoxide are incorporated into the vanadium coordination sphere and the oxygen transfer from the peroxide to the olefin takes place in an intramolecular fashion (as described above for titanium tartrate catalyst) [30, 32]. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

The formed hydroxyperoxide decomposes into free radicals much more rapidly than alkyl hydroperoxide [128]. So, the equilibrium addition of the hydroperoxide to the ketone changes the rate of formation of the radicals. This effect was first observed for cyclohexanone and 1,1-dimethylethyl hydroperoxide [128]. In this system, the rate of radical formation increases with an increase in the ketone concentration. The mechanism of radical formation is described by the following scheme ... 

Different chain mechanisms of hydroperoxide decomposition are known with the participation of alkyl, alkoxyl, and peroxyl radicals [9]. 

Oxidation of organic compounds occurs by the chain mechanism via alternating reactions of alkyl and peroxyl radicals (see Chapter 2). The accumulated hydroperoxide decomposes into radicals, thereby increasing the rate of oxidation (see Chapter 4). The oxidation of an organic compound may be retarded by one of the following three ways ... 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

We emphasize that the above mechanism is strictly valid only for H202 and alkyl hydroperoxide epoxidations of alkenes catalyzed by TS-1 and Ti-MCM-41. In view of the observation of similar titanium oxo species when H2 + 02 are brought in contact with TS-1 or Ti-MCM-41 (54), similar conclusions may be drawn for that system as well. A radical mechanism involving the Ti=0 groups had been proposed earlier by Khouw et al. (221) for the hydroxylation of alkanes. No spectroscopic investigation of the TS-l/H202/alkane has yet been reported. 

The transition metal based catalytic species derived from hydrogen peroxide or alkyl hydroperoxides are currently regarded as the most active oxidants for the majority of inorganic and organic substrates " An understanding of the mechanism of these processes is therefore a crucial point in the chemistry of catalytic oxidations. This knowledge allows one to predict not only the nature of the products in a given process, but also the stereochemical outcome in asymmetric reactions. 

In Studying asymmetric oxidation of methyl p-tolyl sulfide, employing Ti(OPr-/)4 as catalyst and optically active alkyl hydroperoxides as oxidants, Adam and coworkers collected experimental evidence on the occurrence of the coordination of the sulfoxide to the metal center. Therefore, also in this case the incursion of the nucleophilic oxygen transfer as a mechanism can be invoked. The authors also used thianthrene 5-oxide as a mechanistic probe to prove the nucleophilic character of the oxidant. 

These complexes are effective catalysts in epoxidation reactions with H2O2 and alkyl hydroperoxides. Several detailed mechanistic studies have been carried out in particular, it has been shown that, when the alkyl chain contains a double bond, no autoepoxidation is observed both in the solid state and in solution. Nevertheless, if f-BuOOH is added, the epoxidation of the olefinic moiety immediately takes place. Therefore, it has been suggested that these complexes are not the active species in the oxygen transfer step to the substrate, but they behave as catalysts for the primary peroxidic oxidant. On the basis of kinetic, spectroscopic and theoretical studies, the authors provided a mechanism, whose key steps are sketched in Scheme 12. In this context a major role appears to be played by the fluxionality of the particular ligands used . ... 

Alkyl hydroperoxides are known to be oxidized to alkylperoxy radicals by cupric, cobaltic, and manganic salts. Although CeIV has not yet been reported to oxidize hydroperoxides and although mechanisms for ionic oxidation of alkoxy derivatives have not been put forward, Reaction 1 is a possible first step. 

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