A-Hydroperoxides

The ff-oxidation of carbonyl compounds may be performed by addition of molecular oxygen to enolate anions and subsequent reduction of the hydroperoxy group, e.g. with triethyl phosphite (E.J. Bailey, 1962 J.N. Gardner, 1968 A,B). If the initially formed a-hydroperoxide possesses another enolizable a-proton, dehydration to the 1,2-dione occurs spontaneously, and further oxidation to complex product mitctures is usually observed. 

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... 

From cumene Almost all the phenol produced in the United States is prepared by this method Oxi dation of cumene takes place at the benzylic posi tion to give a hydroperoxide On treatment with dilute sulfuric acid this hydroperoxide is converted to phenol and acetone... 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

An important descriptor of a chain reaction is the kinetic chain length, ie, the number of cycles of the propagation steps (eqs. 2 and 3) for each new radical introduced into the system. The chain length for a hydroperoxide reaction is given by equation (10) where HPE = efficiency to hydroperoxide, %, and 2/ = number of effective radicals generated per mol of hydroperoxide decomposed. For 100% radical generation efficiency, / = 1. For 90% efficiency to hydroperoxide, the minimum chain length (/ = 1) is 14. 

Higher valence-state metal ions can abstract hydrogen from a hydroperoxide (25) (eq. 35) or from a substrate (eq. 36). 

The reaction of a hydroperoxide with 2-methylaziridine [75-55-8] has been described (94). The reaction of ethyleneknine with phenols (95) and carboxyHc acids (96,97) produces ethylamine ethers and esters, respectively. However, these reactions frequentiy yield product mixtures which contain polyaminoalkylated oxygen nucleophiles and polymers, in addition to the desked products (1). The selectivity of the reaction can often be improved by using less than the stoichiometric amount of the aziridine component (98,99). 

Substantial decomposition of phenoHc resins begins above 300°C. In the presence of oxygen, the methylene bridging group is converted to a hydroperoxide which in turn yields alcohols and ketones on decomposition. 

The second process involves reaction of propylene with peroxides, as in the Oxirane process (97), in which either isobutane or ethylbenzene is oxidized to form a hydroperoxide. 

Styrene. Commercial manufacture of this commodity monomer depends on ethylbenzene, which is converted by several means to a low purity styrene, subsequendy distilled to the pure form. A small percentage of styrene is made from the oxidative process, whereby ethylbenzene is oxidized to a hydroperoxide or alcohol and then dehydrated to styrene. A popular commercial route has been the alkylation of benzene to ethylbenzene, with ethylene, after which the cmde ethylbenzene is distilled to give high purity ethylbenzene. The ethylbenzene is direcdy dehydrogenated to styrene monomer in the vapor phase with steam and appropriate catalysts. Most styrene is manufactured by variations of this process. A variety of catalyst systems are used, based on ferric oxide with other components, including potassium salts, which improve the catalytic activity (10). 

Limonene (15) can be isomerized to terpiaolene (39) usiag Hquid SO2 and a hydroperoxide catalyst (/-butyl hydroperoxide (TBHP)) (76). Another method uses a specially prepared orthotitanic acid catalyst with a buffer such as sodium acetate (77). A selectivity of about 70% is claimed at about 50% conversion when mn at 150°C for four hours. 

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). 

The synergistic effect of a hydroperoxide decomposer, eg, dilauryl thiodipropionate [123-28-4] (34), and a radical scavenger, eg, tetrakis[methylene(3,5-di-/ f2 butyl-4-hydroxyhydrocinnamate)]methane (9), ia protecting polypropylene duting an oxygen-uptake test at 140°C is shown ia Table 3. 

Imidazole rings also survive most oxidation conditions, but photosensitized oxidation of imidazoles can give diarylbenzamidines through a hydroperoxide (136) (70AHC(12)103). 

Another approach is to use an easily oxidized substance such as acetaldehyde or methylethyl ketone, which, under the reaction conditions, forms a hydroperoxide. These will accelerate the oxidation of the second methyl group. The DMT process encompasses four major processing steps oxidation, esterification, distillation, and crystallization. Figure 10-16 shows a typical p-xylene oxidation process to produce terephthalic acid or dimethyl terephthalate. The main use of TPA and DMT is to produce polyesters for synthetic fiber and film. 

Q Protonation of a hydroperoxide oxygen by an acid HA makes the neighboring oxygen electrophilic and allows the aromatic ring to react, giving a carbocalion intermediate. 

Strong evidence has been provided that the photoxygenation proceeds via a hydroperoxide intermediate which is transformed into an epoxide. The epoxide is then opened by the nucleophilic solvent. 

In the postulated bioluminescence mechanism, firefly luciferin is adenylated in the presence of luciferase, ATP and Mg2+. Luciferyl adenylate in the active site of luciferase is quickly oxygenated at its tertiary carbon (position 4), forming a hydroperoxide intermediate (A). 

The moiery transferred will most often be a hydrogen atom, for example, when the transfer agent is a thiol (e.g. n-butancthiol - Scheme 6.6, Section 6.2.2,1), a hydroperoxide (Section 3,3.2.5), the solvent (6.2.2.5), etc. 

The oxidation of sulphoxides with peracid resins occurs rapidly and quantitatively at 20°C using dioxane as solvent150. Vanadium(V), generated on a polymer support by treatment of vanadium(IV) with a hydroperoxide (equation 49) may be used to oxidize successfully sulphoxides to sulphones in very good yields151. 

How does oxygen insert in between the B—bonds Let s take a closer look at the reagents— a hydroxide ion can deprotonate hydrogen peroxide to form a hydroperoxide anion ... 

Alkane oxidation via a hydroperoxide was suggested many years ago, and seems to be operative in Acinetobacter sp. strain M-1 that has, in addition, a rather unusual range of substrates that include both n-alkanes and -alkenes. The purified enzyme contains FAD and requires copper for activity (Maeng et al. 1996). 

If molybdenum(VI) is generated by treatment of a polymer-supported complex containing molybdenum(V) with a hydroperoxide, then this polymer-supported oxidant may also be used to prepare sulphones from sulphoxides. In this case the yield is not good unless the sulphoxide is repeatedly passed through a column containing the oxidant or the reaction is performed by stirring the polymer and the sulphoxide together at 56 °C for 16 hours . ... 

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