Mechanism, hydroperoxides

It seems very probable that the epoxidation reaction proceeds through a two-stage mechanism. Hydroperoxide oxidizes the catalyst to peroxo complex and the this complex performs epoxidation of olefins. 

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

Suitable catalysts are /-butylphenylmethyl peracetate and phenylacetjdperoxide or redox catalyst systems consisting of an organic hydroperoxide and an oxidizable sulfoxy compound. One such redox initiator is cumene—hydroperoxide, sulfur dioxide, and a nucleophilic compound, such as water. Sulfoxy compounds are preferred because they incorporate dyeable end groups in the polymer by a chain-transfer mechanism. Common thermally activated initiators, such as BPO and AIBN, are too slow for use in this process. 

New radicals come exclusively from the decomposition of the intermediate hydroperoxide (eq. 4), provided no other radical sources, eg, peroxidic impurities, are present. Hydroperoxides have varying degrees of stabiUty, depending on their stmcture. They decompose by a variety of mechanisms and are not necessarily efficient generators of new radicals via thermolysis (19,20). 

The simple hydroperoxide mechanism so far discussed is incomplete for representing reactions with significant products other than hydroperoxides. It can be adequate for oxidations of certain unsaturates, aldehydes, and alkylaromatics where the yield of the corresponding hydroperoxide can exceed 90%. 

Earlier reports have indicated that esters can form before significant amounts of acids accumulate (16). The Bayer-ViUiger oxidations of ketones with intermediate hydroperoxides and/or peracids have been suggested as ester forming mechanisms (34,56). However, the reactions of simple aUphatic ketones with peracetic acid are probably too slow to support this mechanism (57,58). Very early proposals for ester formation, although imaginative, appear improbable (59). 

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). 

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

The decomposition products show that the initially formed radicals react by several mechanisms. These hydroperoxides are decomposed by bases, apparentiy by an ionic pathway, and are surprisingly stable in acids (33). 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

Homogeneous Oxidation Catalysts. Cobalt(II) carboxylates, such as the oleate, acetate, and naphthenate, are used in the Hquid-phase oxidations of -xylene to terephthaUc acid, cyclohexane to adipic acid, acetaldehyde (qv) to acetic acid, and cumene (qv) to cumene hydroperoxide. These reactions each involve a free-radical mechanism that for the cyclohexane oxidation can be written as... 

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

Transition metal-catalyzed epoxidations, by peracids or peroxides, are complex and diverse in their reaction mechanisms (Section 5.05.4.2.2) (77MI50300). However, most advantageous conversions are possible using metal complexes. The use of t-butyl hydroperoxide with titanium tetraisopropoxide in the presence of tartrates gave asymmetric epoxides of 90-95% optical purity (80JA5974). 

Whilst the R radieal again reacts as in equation (7.2) the hydroperoxide may decompose by the following mechanisms... 

Materials that promote the decomposition of organic hydroperoxide to form stable products rather than chain-initiating free radicals are known as peroxide decomposers. Amongst the materials that function in this way may be included a number of mercaptans, sulphonic acids, zinc dialkylthiophosphate and zinc dimethyldithiocarbamate. There is also evidence that some of the phenol and aryl amine chain-breaking antioxidants may function in addition by this mechanism. In saturated hydrocarbon polymers diauryl thiodipropionate has achieved a preeminent position as a peroxide decomposer. 

A. Preparation of hydroperoxide. In a 2-1. three-necked round-bottomed flask, equipped with a mechanical stirrer (Note 1), a dropping funnel, and a thermometer, is placed 800 g. of 30% hydrogen peroxide (Note 2). The flask is surrounded by an ice bath and rapid stirring is started. In the meantime, 800 g. of 95-96% sulfuric acid is added to 310 g. of cracked ice and the solution is cooled to 10°. When the temperature of the hydrogen peroxide reaches 5-10°, the cold sulfuric acid is added slowly from the dropping funnel during a period of about 20 minutes (Note 3). The temperature of the solution should not exceed 20° during the... 

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. ... 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

Henbest and Jackson have rationalized these remote directive effects on the basis of the well-supported mechanism of alkaline epoxidations. The initial step in the reaction is the reversible addition of the hydroperoxide ion... 

The Dakin reaction proceeds by a mechanism analogous to that of the Baeyer-Villiger reaction. An aromatic aldehyde or ketone that is activated by a hydroxy group in the ortho or para position, e.g. salicylic aldehyde 12 (2-hydroxybenzaldehyde), reacts with hydroperoxides or alkaline hydrogen peroxide. Upon hydrolysis of the rearrangement product 13 a dihydroxybenzene, e.g. catechol 14, is obtained ... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

The basis to the chain breaking donor (CB—D) mechanism, which was the first antioxidant mechanism to be investigated, was laid down by the late 1940s [10-12]. Many reducing agents, e.g., hindered phenols and aromatic amines, which reduce the ROO to hydroperoxide in a CB—D step have already been empirically selected and used for rubbers and by this time also for the newer plastics industry (e.g., Table la, AO 1-8 and 9-12). The major mechanistic landmarks of the antioxi-... 

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. 

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. 

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