Acylperoxy radical

Actually, it is much more likely that two acylperoxy radicals combine with liberation of oxygen and C02 i to form two isopropyl radicals which would then react with oxygen to yield isopropylperoxy radicals (reaction (ID) ... 

A second mechanism involving as intermediate step a stable hydroxylamine ether (isopropyl I-ether) is also a possibility (reaction (15)). In a second step the ether would undergo cleavage by the acylperoxy radical with formation of isobutyric acid and acetone and liberation of the nitroxide (reaction (16)) ... 

In the case of DiPK, we were likewise able to show in additional experiments that in all probability it is the isobutyryl peroxy radical c and not the isopropyl peroxy radical d that is responsible for oxidation of the amine II to the nitroxide I. When namely the two oxygen-centered radicals are produced independently of one another in accordance with reactions (19) and (20) only in the case of the acylperoxy radical the formation of the nitroxide can be observed ... 

Acenaphthene oxide is known to be unstable under acidic conditions and is therefore isolated in low yields when prepared by common methods. The highly strained acenaphthene was smoothly epoxidized in quantitative yield at —30°C in CH3CN by using 51 . C -stilbene was epoxidized with 51 at —35 °C in CH3CN to a mixture of the trans-(70%) and c/3-epoxides (30%) (equation 79). This result shows that the peroxysulfonyl intermediate must be a radical like the acylperoxy radical ArC(0)00 ° and the phenyl nitroso oxide radical PhNOO . ... 

The stereoselective nature of the reaction supports the suggestion that epoxidation in this case does not occur by acylperoxy radicals but rather by peracids generated from autoxidation of aldehydes. 

The acylperoxy radical was found to epoxidize olefins much faster than peracids also formed under reaction conditions. The result ruled out the role of the latter.267 The addition of RCO3 was observed to occur 105 faster than that of ROO. The relative reactivity of alkenes suggests a strongly electrophilic radical forming the polar transition state 30 ... 

Sawaki Y, Ogata Y (1984) Reactivities of acylperoxy radicals in the photoreaction of a-diketones and oxygen. J Org Chem 49 3344-3349... 

Examination of Table III reveals that reactivities of peroxy radicals are strongly dependent on their structure. Reactivities are influenced by both steric and polar effects,26,30-32 and, in general, increase as the electron-withdrawing capacity of the a substituent increases. Acylperoxy radicals, which possess a strong electron-withdrawing substituent, are considerably more reactive than other alkylperoxy radicals. For example, the benzoylperoxy radical is 4 X 104 times more reactive than the ferf-butylperoxy radical. 

Autoxidation of aldehydes is analogous to that of hydrocarbons. Acylperoxy radicals are involved as principal chain carriers and peracids are the primary products in the following manner ... 

An example of the utilization of active oxygen is illustrated by the cooxidation of aldehydes and olefins,51 in which both the acylperoxy radical and the peracid are used for epoxidation of the olefin ... 

This reaction affords much higher yields of epoxide than those obtained from the autoxidation of the olefin alone since acylperoxy radicals are more selective than alkylperoxy radicals in favoring addition relative to abstraction. 

Pozzi has also shown that perfluorocarbons are excellent replacements for chlorinated solvents in the epoxidation of alkenes with sacrificial aldehydes in the absence of metal catalysts [77]. It is believed that the acylperoxy radical, which is formed by the reaction of oxygen with an aldehyde, is responsible for the epoxidation (Eqn. (6)). 

Irradiation of an alkene in the presence of molecular oxygen and an a-diketone furnishes the core-sponding oxirane in high yields. The reaction proceeds in the complete absence of nucleophiles, and thus can avoid formation of by-products arising from the reaction of nuclec hiles with sensitive oxiranes. The photoepoxidation proceeds via addition of an acylperoxy radical to the alkene. Photochemical epoxidation of cholesteryl acetate (176) has been carried out (equation 64a) the major epoxidation product is the sp,6 -epoxide (177a). In MCPBA epoxidation of (176) the major product is (177b). 

The activation energy of reaction (37) [136] is 10.8 kcal. mole while that of reaction (34) will be close to zero, hence reaction (37) will be favoured at higher temperatures. The acylperoxy radical formed in reaction (34) may also decompose or it may abstract hydrogen to form peracetic acid, viz. 

Photooxidation suitable for the epoxidation of aromatic olefins also occurs with a-diketones (benzil, biacetyl), benzophenone, benzoin, and a-ketoacids, Isotopic mechanistic studies point to a reaction via a biradical. Photooxidation with an a-diketone or a-ketoacid recently has been interpreted in terms of a photochemical a-cleavage leading to an acylperoxy radical, which can effectively transfer an oxygen atom to olefins. Vinylallenes have similarly been photooxidized in the presence of biacetyl. ... 

Oxiranes may also be prepared by the cooxidation of aldehydes and olefins. There are two assumptions as regards the mechanism the oxidation occurs via either an acylperoxy radical or a peracid. The peracid oxidation is stereospecific. Experiments carried out with a view to assessing the radical versus nonradical mechanism indicate that the extent of the radical epoxidation depends on the structure of the olefin and the olefin/aldehyde ratio. Cooxidation in the presence of oxygen was achieved by irradiating the aldehyde and carrying out the reaction with the alkene after a suitable quantity of peracid had been obtained. Enantioselective epoxidation has been described in the reaction of (1-phenyl-alkylidene)malonitriles 63 catalyzed by optically active tertiary amines. ... 

Aldehydes and ketones are the major immediate precursors of acetic acid in paraffin oxidation. The reported mechanisms for the oxidations of these intermediates are somewhat involved and are not of primary concern for the present purpose. A point of interest, however, is that acylperoxy radicals, intermediates in aldehyde oxidations, are much stronger hydrogen abstractors than alkylperoxy radicals [8]. Aldehyde oxidations have been more extensively covered elsewhere [10, 18, 39, 41-43]. Acetic acid is quite resistant to further oxidation [10] and tends to be a terminal product. 

The reduction of acylperoxy radicals by Mn , by the analog of eq. (20), tends to reduce the concentration of acylperoxy radicals in the system ... 

There is an interesting exception to this observation. As noted above, aldehyde oxidations tend to be very fast and to have relatively long kinetic chain lengths. Most chain terminations occur via bimolecular reactions of acylperoxy radicals (eq. (7a)) these reactions result in carbon dioxide generation and are inefficient. If one adds manganese catalyst, some of the acylperoxy radicals will be reduced to peroxy acid and Mn " will be produced. Mn can carry the chain via an analog of reaction (18), but does not participate in chain termination reactions. As a result, kinetic chain lengths and rates tend to increase. 

To some degree, this rate increase reduces the extent to which the concentration of acylperoxy radicals can be suppressed and limits the efficiency increase that can be obtained. It can also lead to oxygen starvation. However, if one adds a cupric-ion promoter, acyl radicals can be intercepted in an analog of eq. (32) ... 

POM supported on mesoporous MCM-41, and amino-modified MCM-41 acylperoxy radical intermediates oxidize substrate... 

Aerobic epoxidation of different alkenes, including a number of natural terpenes, efficiently occurs under mild reaction conditions in the presence of isobutyraldehyde as a reductant and MNaY and MNaZSM-5 type zeolites (M=Co(II), Cu(II), Ni(II) and Fe(III)) as catalysts. Yields of the epoxidation products vary from 80 up to 99% depending on the olefin and catalyst. The reaction proceeds via chain radical mechanism, acylperoxy radicals being the main epoxidizing species. 

The mechanism of metal phthalocyanine catalysed oxidation by molecular oxygen -isobutyraldehyde system is not established at this stage. The iron[14], manganese[15] and cobalt tetrasulphonato-[16] phthalocyanines are known to form superoxo complexes with dioxygen and are known to catalyse autoxidation reactions[13]- The acyl radical formation thus can be initiated by interaction of metal phthalocyanine-dioxygen superoxo complex with isobutyraldehyde. The acyl radical in presence of oxygen can yield acylperoxy radical or peracid as the oxidising speceis[17]. 

HAS also react directly with acylperoxy radicals (Eq. 20). They play an important role only in the later stages of PO oxidation and react with NH faster than ROO. ... 

---