Hydroperoxides reaction + metal ions

This fact illustrates the point where the functions of metal salt catalysts become apparent. If oxidation to the alcohol, ketone or carboxylic acid (i.e. beyond the hydroperoxide stage) is the objective, metal catalysts should be used to promote decomposition of the hydroperoxide. The metal ion (complex) catalyzed decomposition of hydroperoxides is responsible for the sustained and rapid formation of radicals participating in a chain reaction. The most effective are metals with at least two accessible oxidation states. Both components of a redox couple may be capable of reacting with alkyl hydroperoxides ... 

The resulting alkoxyl and peroxyl radicals increase the autoxidation reaction rates of initiation and propagation phases, since the rate of cleavage of hydroperoxides by metal ions is much faster than the formation of radicals ab inicio. Metal ions and non-ionised salts may react in this way (Figure 3.65). Of the metals bound in complexes, some are effective, but some are ineffective. Metals may also become less effective in the presence of fats if micelles are formed. The catalyst for the oxidation of Kpids may be bound iron complexes. Iron bound in haem pigments has the same catalytic activity as the ions Fe + a Fe +, in aqueous solutions it is even more active, as it catalyses the cleavage of hydroperoxides as follows ... 

This reaction is one example of several possible radical transition-metal ion interactions. The significance of this and similar reactions is that radicals are destroyed and are no longer available for initiation of useful radical reactions. Consequentiy, the optimum use levels of transition metals are very low. Although the hydroperoxide decomposes quickly when excess transition metal is employed, the efficiency of radical generation is poor. 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Alkyl hydroperoxides are among the most thermally stable organic peroxides. However, hydroperoxides are sensitive to chain decomposition reactions initiated by radicals and/or transition-metal ions. Such decompositions, if not controlled, can be auto accelerating and sometimes can lead to violent decompositions when neat hydroperoxides or concentrated solutions of hydroperoxides are involved. 

The radicals are destroyed and are not available to take part in the desired radical reactions, eg, polymerizations. Thus, transition-metal ion concentrations of metal—hydroperoxide initiating systems are optimized to maximize radical generation. 

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). 

Common components of many redox systems are a peroxide and a transition metal ion or complex. The redox reactions of peroxides are covered in the sections on those compounds. Discussion on specific redox systems can be found in sections on diacyl peroxides (3,3.2.1.5), hydroperoxides (3,3.2.5) persulfate (3.3.2.6.1) and hydrogen peroxide (3.3.2.6,2). 

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). 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

Lophine emits yellow CL upon oxidation by molecular oxygen in alkaline solution. The oxidation is believed to produce a free radical [3], which is further oxidized to a hydroperoxide, which is the light-emitting species [4-6], A number of chemiluminescent derivatives of lophine have been synthesized and have been shown to exhibit varying efficiencies of CL. Lophine has been used in the analysis of metal ions such as Co2+ that catalyze the chemiluminescent reaction between it and hydrogen peroxide [7], It has also been used as a chemiluminescent indicator in titrimetry [8],... 

Luminol derivatives produce emission of light by oxidation with oxygen and hydrogen peroxide under alkaline conditions. By utilizing this reaction, peroxides such as hydrogen peroxide and lipid hydroperoxides can be determined after HPLC separation. Metal ions [e.g., iron(II), cobalt(II), etc.] catalyzing the luminol CL reaction can also be determined. 

As in the case with catalytic decomposition of hydrogen peroxide, radical generation by the reaction of metal ions with hydroperoxide consists of several steps. In an aqueous solution, first ROOH is substituted in the internal coordination sphere of the ion followed by the transfer of an electron from the ion to ROOH accompanied by the subsequent cleavage of hydroperoxide to RO and OH, for example,... 

Rate Constants of the Reaction of Transition Metal Ions and Complexes with Hydroperoxides... 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

In addition to the decay of hydroperoxides, metal complexes accelerate the reaction of phosphite with peroxyl radicals [90,91]. Phosphite forms a complex with the metal ion, and the formed complex terminates the chains more rapidly than phosphite does alone. For example, triphenyl phosphite terminates the chains in oxidized styrene with fkq = 2 x 103L mol-1 s-1 at T 393 K and the complex of this phosphite with CuCI does it with fkq = 2 x 105L mol-1 s 1, i.e., two orders of magnitude more rapidly. The values of / and kq for three phosphites and six complexes (phosphite with CuCI) are given in Table 17.10. 

The actual schemes of these reactions are very complicated the radicals involved may also react with the metal ions in the system, the hydroperoxide decomposition may also be catalysed by the metal complexes, which adds to the complexity of the autoxidation reactions. Some reactions, such as the cobalt catalysed oxidation of benzaldehyde have been found to be oscillating reactions under certain conditions [48],... 

Such a species cannot be ruled out in reactions of iron-EDTA complexes with hydroperoxides recently described by Bruice and coworkers (27). On the other hand, a hydroperoxide complex that reacts with the substrate such that bond formation fiom O to substrate is concerted with 0-0 bond breaking, as proposed by Klinman for dopamine P-monooxygenase (18), could provide compensation for the cost of 0-0 bond cleavage in the transition state. In fact, it is interesting to speculate that for each of these enzymes, the mechanism by which the substrate is oxidized may be dependent on the reactivity of the substrate. One could envision certain substrates that would react with the metal-bound hydroperoxide ligand prior to or concerted with 0-0 bond cleavage. This possibility is difficult to assess because of our lack of information concerning the reactivity of HQ2" when complexed to different metal ions. 

A further use of the system is to mediate the reaction of adamantane with carbon monoxide and oxygen to form 1-adamantanecarboxylic acid . When long-wavelength light (>300 nm) is used, hydroperoxides efficiently generate hydroxyl radicals without the use of metal ions and would be an extremely useful source of hydroxyl radicals, particularly in the design of DNA-cleaving molecules . ... 

In the development of effective catalytic oxidation systems, there is a qualitative correlation between the desirability of the net or terminal oxidant, (OX in equation 1 and DO in equation 2) and the complexity of its chemistry and the difficulty of its use. The desirability of an oxidant is inversely proportional to its cost and directly proportional to the selectivity, rate, and stability of the associated oxidation reaction. The weight % of active oxygen, ease of deployment, and environmental friendliness of the oxidant are also key issues. Pertinent data for representative oxidants are summarized in Table I (4). The most desirable oxidant, in principle, but the one with the most complex chemistry, is O2. The radical chain or autoxidation chemistry inherent in 02-based organic oxidations, whether it is mediated by redox active transition metal ions, nonmetal species, metal oxide surfaces, or other species, is fascinatingly complex and represents nearly a field unto itself (7,75). Although initiation, termination, hydroperoxide breakdown, concentration dependent inhibition... 

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