Alkyl hydroperoxides isomerization

The importance of alkylperoxy radicals as intermediates had long been realized (see Sect. 2) and their subsequent reaction to yield the alkyl-hydroperoxide or decomposition products such as aldehydes and alcohols had been reasonably successful in describing the mechanism of the autocatalytic oxidation of alkanes. However, even though 0-heterocycles (which cannot be derived from intermediate aldehydes) had been found in the products of the oxidation of n-pentane as early as 1935 [66], the true extent of alkylperoxy radical isomerization reactions has been recognized only recently. Bailey and Norrish [67] first formulated the production of O-heterocycles in terms of alkylperoxy radical isomerization and subsequent cyclization in order to explain the formation of 2,5-dimethyl-tetrahydrofuran during the cool-flame oxidation of n-hexane. Their mechanism was a one-step process which involved direct elimination of OH. However, it is now generally formulated as shown in reactions (147) and(I67)... 

Example 5.5. Oxidation of paraffins to secondary alcohols (see also Section 10.6.2). Alcohols can be produced by oxidation of paraffins with air or oxygen at moderate temperatures (typically 120 to 180° C) in the presence of borate esters or boroxines [16-19]. These intercept the alkyl hydroperoxide, the first oxidation product, preventing it from generating radicals that would cause further degradation including scission of carbon-carbon bonds and produce aldehydes, ketones, and acids. The peroxy borates so formed then are hydrolyzed to yield the alcohol. The carbon atoms at the chain ends are largely immune to oxidation, so the product consists predominantly of isomeric secondary alcohols. 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

During PP oxidation, hydroxyl groups are formed by the intramolecular isomerization of alkyl radicals. Since PP oxidizes through an intense intramolecular chain transfer, many of the alkyl radicals containing hydroperoxy groups in the 0-position to an available bond can undergo this reaction. An isomerization reaction has also been demonstrated for the liquid-phase oxidation of 2,4-dimethylpentane [89], Oxidation products contain, in addition to hydroperoxides, oxide or diol. 

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

Alkyl substituted cyclopropanols and cyclopropanone hemiacetals 115,116a) aiso undergo oxidative cleavage when exposed to air or peroxides the initial products are hydroperoxides such as 148. In the case of l-methoxy-2,2-dimethylcyclopropanol, the reaction can be followed by observing the emission peaks in the NMR spectrum, and these CIDNP effects have enabled identification of radical intermediates.1154) With di-f-butylperoxylate (TBPO), the isomeric radicals 143 and 144 are formed and these may undergo a diverse number of further reactions as indicated by the complex product mixture given in Table 20. 

As indicated in Scheme VII/32, cyclononanone (VII/165) is transformed into hydroperoxide hemiacetal, VII/167, which is isolated as a mixture of stereoisomers. The addition of Fe(II)S04 to a solution of VII/167 in methanol saturated with Cu(OAc)2 gave ( )-recifeiolide (VII/171) in quantitative yield. No isomeric olefins were detected. In the first step of the proposed mechanism, an electron from Fe2+ is transferred to the peroxide to form the oxy radical VII/168. The central C,C-bond is weakened by antiperiplanar overlap with the lone pair on the ether oxygen. Cleavage of this bond leads to the secondary carbon radical VII/169, which yields, by an oxidative coupling with Cu(OAc)2, the alkyl copper intermediate VII/170. If we assume that the alkyl copper intermediate, VII/170, exists (a) as a (Z)-ester, stabilized by n (ether O) —> <7 (C=0) overlap (anomeric effect), and (b) is internally coordinated by the ester to form a pseudo-six-membered ring, then only one of the four -hydrogens is available for a syn-//-elimination. [111]. This reaction principle has been used in other macrolide syntheses, too [112] [113]. 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

However, production of xylenols from isomeric xylene mixtures or individual isomers via propylene alkylation has not been attempted so far, neither established commercially nor even been tried in a laboratory or pilot plant. As in benzene and toluene alkylation processes it has been reported that Mitsubishi Gas Chemical Co., Japan obtained 3,5 xylenol by oxidation of 3,5-dimethyl cumene by alkylation of m-xylene with propylene to 3,5-dimethyl cumene hydroperoxide and thereafter its cleavage to 3,5-xylenol. Economics of the process did not justify its commercialization [1,38]. 

However, from economic and environmental point of view both USA and Japan use the propylene alkylation route, as this method of manufacture is more amenable to continuous operations with recycle stream. The alkylation with propylene and isomerization are carried out upto 240° C with traditional solid phosphoric acid (SPA) catalyst and more recently with anhydrous AICI3 catalyst. Final catalytic oxidation at 90-110°C gives the hydroperoxide, as in cumene and cymene processes, which on cleavage with dilute sulfuric acid gives 2-naphthol in high overall yield. [53]... 

Peroxy radicals ROO are key species in the mechanisms of oxidative and combustion systems. At the same time they have been among the most difficult radicals to study experimentally. There are only limited or no thermochemical information available for unsaturated alkylperoxy and hydroperoxide species. An explanation for this paucity of data could be the fact that these species are unstable and short-lived, and therefore difficult to study and characterise by experimental methods. The difficulty arises in part from the lability of these radicals towards reversible unimolecular dissociation into R + O2 and then to reversible isomerization into hydroperoxy alkyl radicals ROOH, both of these reactions occurring at comparable rates at temperatures below 450 K [2]. 

On the other hand, hydrogen at the allylic carbon-8 or carbon-11 of OA can be abstracted under the presence of free radicals and other initiators to produce two kinds of delocalized three-carbon alkyl radicals (Porter et al., 1995). And then, oxygen attack at the end-carbon positions of these intermediates prodnces a mixture of four kinds of isomeric OA mono-hydroperoxides (Frankel, 1998a). When the reactivity of OA and SFA snch as PA is compared, SFA shonld be less reactive to oxidation than OA becanse of no double bond in its molecnle. However, results obtained in the comparative study on the oxidative stability of vegetable oil TAG were more complicated. 

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