Acylperoxy radicals oxidations

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 . ... 

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. 

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. 

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

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. ... 

Acylperoxy radicals arising from the oxidation of aldehydes apparently terminate via a tetroxide which then cleaves to form, first, primary carbon radicals and second, primary alkylperoxy radicals which terminate rapidly [130,131]. Termination rate coefficients for aliphatic aldehydes have values ranging from 0.7 X 107 to 10 X 1071 mole-1 s-1 [10]. 

In very general terms, the Co-Br-catalyzed oxidation is a particular case of the free radical chain oxidation, common for all liquid phase oxidations of hydrocarbons [8-10]. The free radical chain oxidation occurs with four types of free radicals alkyl, alkoxy, alkylperoxy, and acylperoxy radicals [11, 12]. Other key active intermediates are hydroperoxides and peracids [11,12]. The nomenclature and structures are displayed in Figure 4.2. 

Some aerobic oxidation reactions progress effectively in SCCO2 without a catalyst. Aerobic oxidation of olefins (e.g. cw-cyclooctene and (/f)-(-F)-limonene) in the presence of aldehydes (e.g. 2-methyl-propionaldehyde) in SCCO2 (d = 0.75 g/mL) gave the corresponding epoxides without a catalyst. It is speculated that the stainless steel from autoclave walls triggered the formation of acylperoxy radicals from the aldehyde and oxygen, and the reaction proceeded via a non-catalytic radical... 

The metal ion catalyzed oxidation of aldehydes to carboxylic acids has been extensively studied and a free-radical chain mechanism is now firmly established. The acylperoxy radical intermediate has been utilized for converting olefins to epoxides. 

Cobalt(II)-porphyrins a-d (Figure 35) are versatile catalysts promoting the oxidation of the organic substrates listed in (Figure 36) by a combination of molecular oxygen and 2-methylpropanal under ambient conditions . Typically 10 mmol ofhydrocarbon and 20 mmol ofthe aldehyde are stirred in an autoclave in 15 mL acetonitrile for 12-15 hours under O2 at room temperature. Although not stated explicitly by the authors, acyl free radicals are obviously the key intermediates, converted by O2 to acylperoxy radicals responsible for the oxidation. 

An important reaction of the acylperoxy radical is with NO2 to form an acylperoxy nitrate. In the example shown, the oxidation of acetaldehyde gives acetyl peroxy radicals which can react with NO2 to form peroxyacetyl nitrate, CH3C(0)02N02, generally known as PAN ... 

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