Oxidation by hydroperoxides

Metal Deactivators. The abiUty of metal ions to catalyse oxidation can be inhibited by metal deactivators (19). These additives chelate metal ions and increase the potential difference between the oxidised and reduced states of the metal ions. This decreases the abiUty of the metal to produce radicals from hydroperoxides by oxidation and reduction (eqs. 15 and 16). Complexation of the metal by the metal deactivator also blocks its abiUty to associate with a hydroperoxide, a requirement for catalysis (20). 

SCHEME 11. Preparation of alkyl hydroperoxides by oxidation of zinc-organometalUcs... 

The formation of some organic hydroperoxides by oxidation with molecular oxygen is catalytically promoted by metals like silver or copper 171). A dissociative chemisorption of oxygen cannot be active in these processes they probably proceed via the chemisorption of O7 ions (or O2 molecules forming a covalent bond resonating with an ionic bond). 

Roginskii, S. Z., Berlin, A. A., Kutseva, L. N., Aseeva, R. M., Cherkashina, L. G., Sherle, A. I., Matseeva, N. G. Catalytic Properties of Organic Polymers with a Conjugated Bond System. The Formation of Hydroperoxides by Oxidation of Alkylaromatic Hydrocarbons and Cychlohexane. Dokl. SSSR, Chemistry Section (English Transl.) 148,35 (1963) (1963),... 

RadAk et al. 2000). Cardiac muscle of ferf-butyl hydroperoxide-medicated trained animals accumulated significantly less carbonylated proteins than that of untrained controls medicated with tert-butyl hydroperoxide only (P <0.05). ferf-Butyl hydroperoxide by oxidation modified myocardial proteins less in trained than in untrained rats (P <0.05). 

Section 16 7 Dialkyl ethers are useful solvents for organic reactions but must be used cautiously due to their tendency to form explosive hydroperoxides by air oxidation in opened bottles... 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Sales demand for acetophenone is largely satisfied through distikative by-product recovery from residues produced in the Hock process for phenol (qv) manufacture. Acetophenone is produced in the Hock process by decomposition of cumene hydroperoxide. A more selective synthesis of acetophenone, by cleavage of cumene hydroperoxide over a cupric catalyst, has been patented (341). Acetophenone can also be produced by oxidizing the methylphenylcarbinol intermediate which is formed in styrene (qv) production processes using ethylbenzene oxidation, such as the ARCO and Halcon process and older technologies (342,343). 

The avermectins also possess a number of aUyflc positions that are susceptible to oxidative modification. In particular the 8a-methylene group, which is both aUyflc and alpha to an ether oxygen, is susceptible to radical oxidation. The primary product is the 8a-hydroperoxide, which has been isolated occasionally as an impurity of an avermectin B reaction (such as the catalytic hydrogenation of avermectin B with Wilkinson s rhodium chloride-triphenylphosphine catalyst to obtain ivermectin). An 8a-hydroxy derivative can also be detected occasionally as a metaboUte (42) or as an impurity arising presumably by air oxidation. An 8a-oxo-derivative can be obtained by oxidizing 5-0-protected avermectins with pyridinium dichromate (43). This also can arise by treating the 8a-hydroperoxide with base. 

The process can be modified to give predominandy or solely /-butyl alcohol. Thus, /-butyl hydroperoxide (and /-butyl alcohol) produced by oxidation of isobutane in the first step of the process can be decomposed under controlled, catalytic conditions to give gasoline grade /-butyl alcohol (GTBA) in high selectivity (19—22). 

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

In this process (Figure 10-6), cumene is oxidized in the liquid phase. The oxidation product is concentrated to 80% cumene hydroperoxide by... 

A non-kinetic study of the oxidation of cumyl hydroperoxide by Pb(lV) to acetophenone and dimethylphenylcarbinol gives useful complementary data. 

The deoxygenation of peroxycarbonates (53) with phosphines and phosphites has been examined. Reaction with phosphites favours pyrocarbonate formation (Path A) whilst phosphines favour carbonate formation (Path B). Secondary phosphine oxides are oxidized to phosphinic acids by perbenzoic acid. The kinetics of the deoxygenation of hydroperoxides by triphenylphosphine have been examined and the reaction shown to be catalysed by strong acids. ... 

Rgure 2.3 The antioxidant activity of butyiated hydroxytoluene in the presence of exogenous iipid hydroperoxides. The oxidation of LDL was monitored by measuring the increase in absorbance at 234 nm as described in Fig. 2.2 and the lag phase (time before the phase of maximum rate of oxidation) estimated as described by Esterbauer et at. (1989). Samples of LDL were supplemented with the cortcentrations of 13-hydroperoxyoctadecanoic acid (13-HPODE) indicated and in the presence of 3 fM BHT. The lag phase in the absence of BHT for this preparation of LDL was 48 min. 

The elimination is promoted by oxidation of the addition product to the selenoxide by f-butyl hydroperoxide. The regioselectivity in this reaction is such that the hydroxy group becomes bound at the more-substituted end of the carbon-carbon double bond. The regioselectivity of the addition step follows Markovnikov s rule with PhSe+ acting as the electrophile. The elimination step specifically proceeds away from the oxygen functionality. 

Basically, three reactions were evoked to support the occurrence of 5a-C-centered radicals 10 in tocopherol chemistry. The first one is the formation of 5a-substituted derivatives (8) in the reaction of a-tocopherol (1) with radicals and radical initiators. The most prominent example here is the reaction of 1 with dibenzoyl peroxide leading to 5a-a-tocopheryl benzoate (11) in fair yields,12 so that a typical radical recombination mechanism was postulated (Fig. 6.6). Similarly, low yields of 5a-alkoxy-a-tocopherols were obtained by oxidation of a-tocopherol with tert-butyl hydroperoxide or other peroxides in inert solvents containing various alcohols,23 24 although the involvement of 5 a-C-centered radicals in the formation mechanism was not evoked for explanation in these cases. 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

Aqueous cyanide effluent containing a little methanol in a 2 m3 open tank was being treated to destroy cyanide by oxidation to cyanate with hydrogen peroxide in the presence of copper sulfate as catalyst. The tank was located in a booth with doors. Addition of copper sulfate (1 g/1) was followed by the peroxide solution (27 1 of 35 wt%), and after the addition was complete an explosion blew off the doors of the booth. This was attributed to formation of a methanol vapour-oxygen mixture above the liquid surface, followed by spontaneous ignition. It seems remotely possible that unstable methyl hydroperoxide may have been involved in the ignition process. 

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. 

---