Peroxy radicals epoxidation

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Diphenylphosphinic chloride reacts with the superoxide anion radical (Oi ) in CH3CN under mild conditions to form the diphenylphosphinic peroxy radical intermediate 77, which shows strong oxidizing abilities in the epoxidation of alkenes, oxidation of sulfoxides to sulfones, desulfurization of thioamides to amides and oxidation of triphenylphos-phines to triphenylphosphine oxide in good to excellent yields (equation 105). ... 

W. F. Brill For olefins producing high yields of epoxide in the liquid phase, much of the epoxide may be formed directly from the radical formed by the addition of peroxy radical to olefin without involving polyperoxide chains. While there are no indications that the addition is reversible in the liqud phase, it may be expected to become so at sufficiently high temperatures. The behavior of cis-trans isomers in the gas phase may clarify this matter. 

Direct oxidation of alkenes with molecular oxygen11,255,256 initiated by free radicals to yield epoxides occurs through addition of peroxy radicals to produce the more stable P-peroxy alkyl radicals (28) 257... 

In cooxidation of alkenes and aldehydes, and in photosensitized epoxidations acyl peroxy radicals are the epoxidizing agents. The mechanism of the former oxidation, on the basis of kinetic measurements and the nonstereospecificity of the reaction, involves alkyl peroxy radicals formed through the cr-bonded radical 29 as the intermediate ... 

The formation of propene oxide as a side product of the acrolein formation or dimerization reactions is reported by many authors. Daniel et al. [95,96] demonstrated that propene oxide is formed by surface-initiated homogeneous reactions which may involve peroxy radical intermediates. The epoxidation is increased by a large void fraction in the catalyst bed or a large postcatalytic volume. In view of these results, the findings of Centola et al. [84] are understandable, as the wall of the empty reactor may have been sufficiently active to initiate the reaction. 

Viewed in this way, reactions yielding cyclic ethers should be thought of as two-step processes, epoxides probably arising from peroxy radical additions to olefins followed by 1,3-displacements, and larger rings arising via intramolecular hydrogen abstraction by peroxy radicals followed by 1,4- or 1,5-displacements. In all such reactions it is probably the first step which is slow and which determines the yield of product observed. 

Caps and coworkers studied the solvent effect in the epoxidation of stilbene by varying solvents and the supports [200], In methylcyclohexane (MCH), the activated radical species proposed were MCH peroxy radicals, which were formed by the radical transfer from TBHP and reaction with molecular oxygen. Except for MCH, the solvent effect is not fully understood however, the choice of solvent and supports that can trap or stabilize the radical species affected the catalytic performance of Au. 

Aldehydes are easily oxidized (19) and are well-known sensitizer compounds benzaldehyde enhances oxidation of linear alkenes producing the corresponding epoxides (15). An interesting feature of radical epoxidation is the generation of the alkoxy radical RO, which is more reactive and less selective than the peroxy radical (16)... 

Of special attention is the single-stage method of olefm oxide production by their conjugated oxidation with another, more easily oxidizing compound (aldehyde, ketone, etc.) [134,139], For the epoxidation of an olefm, active oxygen of peroxy radicals is used in such a system ... 

The principal possibility of olefm epoxidation by peroxy radicals is indicated. The simplest representative of this class can be obtained from H202. 

In terms of metal-promoted reactions in the presence of oxygen and aldehydes, a detailed mechanistic study of the conversion of alkenes to oxiranes suggested that the primary oxidant in these reactions is also an acyl peroxy radical derived from the aldehyde <1996IC1045>. A second investigation of the Mukaiyama epoxidation (oxygen, Ni(acac)2, and an aldehyde) suggested that reactions proceed via peroxyacyl radicals and a peracid <2004JOC3453>. 

Fig. 3 Proposed reaction pathway of P-carotene prooxidant activity. P-C, P-carotene ROO% peroxyl radical ROO-P-C-, P-carotene radical R00-P-C-00% P-carotene peroxy radical P-CO, P-carotene epoxide RO% alkoxy radical. (Adapted in part from Ref. l)...

In some cases, an important contribution ofhomogeneous radical reactions to PO formation is postulated [32f] in practice, an allyl radical is initially formed at the catalyst surface, tvhich then interacts tvith O2 to form peroxy radicals, leading to the formation of peroxodimers or hydroperoxides. The hydroperoxides epoxidize propylene to PO and the peroxodimers may decompose to form PO. 

Golub (16) found, using Ir and NMR, that functional groups of epoxide and alcohol are mainly formed In the oxidation of poly-butadlene in the temperature range of 90-180°C. These two products suggest the addition of the peroxy radical to the double bond to form an alkoxy radical and an epoxide group ... 

Atmospheric oxygen could add to the alkyl radical before the formation of epoxide, resulting In a dlalkyl peroxide and a peroxy radical. 

Addition reactions of peroxy radicals with olefins (Equation 4.63) have often been described (Mayo, 1958 Hamberg and Gotthammar, 1973). Among the stable products are epoxides, possibly formed by elimination of alkoxy radicals. The structural constraints on epoxide formation are quite stringent and the overall rate constants for their formation can vary by three or more orders of magnitude. 

Inhaled ozone is known to initiate free-radical autooxidation of unsaturated fatty acids in animal pulmonary lipids (Pryor et al., 1981). These reactions lead to the formation of such typical autooxidation products as conjugated dienes and short-chain alkanes like ethane and pentane. Whether these reactions also occur in water treatment is uncertain. Glaze et al. (1988) showed that 9-hexadecenoic acid (83) reacted readily in aqueous solution to form the expected C, and C, aldehydes and acids. Linoleic acid (84) was converted to a mixture of aldehydes and acids (Carlson and Caple, 1977) notably, 3-nonenal (85) was among the products. Isolation of an unsaturated aldehyde is significant because of the high reported toxicity of these compounds. Carlson and Caple (1977) also implied that the epoxide of stearic acid was formed when an aqueous solution of oleic acid was ozonized the product probably derives from an indirect attack on the double bond by peracids or peroxy radicals (Equation 5.39). Nevertheless, it is conceivable that similar reactions could occur in natural waters. 

Attack of NO3 on isoprene apparently proceeds in much the same manner, but there is considerable controversy about the precise reaction pathway because of the variety of peroxy radicals that can be formed. The products, such as 4-nitroxy-2-methyl-l-butan-3-one and methacrolein, are consistent with the initial addition of NO3 to the terminal carbon atoms to form nitro-oxy-peroxy radicals in the presence of oxygen apparently the NO3 adds preferentially to position 1 (Fig. 12). 3-methyl-4-nitroxy-2-butenal was found as the main product in these experiments. The nitro-oxy-peroxy radicals can react with NO2, in the presence of O2, to yield thermally unstable nitroxy-peroxynitrate compounds. One particularly important feature of the addition of NO3 is the extent to which the initial adduct, which might eliminate NO2 to form an epoxide, is actually converted to the nitro-oxy-peroxy radicals in the atmosphere. 

The primary interaction of NO3 with radical species X at ambient temperatures yields rovibrationally excited products XONO2 which in the gas phase immediately decompose to yield XO and NO2. This way X = H, CH3, HO, NO, HO2, CH3O2, Cl and CIO are oxidised by NO3 with high efficiency. This mechanism has been verified for X = H, HO and CH3 by ab initio calculations on MP2-level of theory. Interaction of NO3 with unsaturated organics starts through an electrophilic radical attack at the double bond forming a rovibrationally excited adduct radical with free rotation at the attacked bond. The adduct radical either spontaneously eliminates NO2 to form a stable epoxide or becomes stabilised in air by formation of a nitroalkyl-peroxy radical. 

Another kind of metalloporphyrin catalyzed olefin epoxidation utilizes aldehydes, which generate peroxy radicals and subsequently peroxy acids [95,98,258]. The latter are complexed by the metalloporphyrin and serve as sources of 0-atoms for epoxidation either as the peroxy acid complex or as the oxidized ir-cation radical (oxometal species) [96,97]. The catalysis of propene epoxidation by... 

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