Methyl radicals, oxidation source

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

Radical nucleophile oxidation based on one-electron oxidation, known as the Minisci reaction, is employed for the functionalization of /V-heterocycles with acidic hydrogen peroxide in the presence of iron(II) salts (Figure 3.112).472 A range of A-heterocycles (pyridines, pyrazines, quinolines, etc.) which are activated towards attack by nucleophilic radicals when protonated are suited to this chemistry. The Minisci reaction is suitable for the preparation of carboxylic amides (from formamide), carboxylic esters (from pyruvic esters via a hydroxyhydroperoxide), aldehydes (from 1,3,5-trioxane) and alkylated pyridines (either from carboxylic acids or from alkyl iodides in dimethyl sulfoxide).473 The latter reaction uses dimethyl sulfoxide as the source of methyl radical (Figure 3.112). 

Thus, acetaldehyde (ethanal) may be expected as a prominent molecular intermediate, which also readily undergoes further oxidation. An additional source of CH3CHO is from the oxidation of propene, formed as an intermediate by OH radicals (see Chapter 1). The oxidation of acetaldehyde is particularly well documented over a wide temperature range [152-156], an important feature being the generation of methyl radicals and carbon monoxide at temperatures in excess of 650 K, in the sequence of... 

The oxidative dehydrogenation reactions over these catalysts are similar to the gas phase result of shock tube experiments determined by Skinner et al. (ref. 6). This observation supports the fact that the recombination reactions of methyl radicals in the gaseous phase are an important source of ethane and that the ethene is a secondary product derived from ethane. This secondary reaction proceeds in the gaseous phase as well as the catalyst surface. The major role of the MgO surface is to produce the methyl radical efficiently. The active sites for cleaving the H-CH3 bond should be moderated by Li to enforce C2 selectivity. In addition to gas phase oxidation, the direct surface oxidation of the hydrocarbon adsorbate is very significant especially for acidic materials. 

Show the mechanism of one-electron transposition initiation with chemical equations based on a free-radical oxidation by diphenyliodonium hexafluorophosphate. (Let the source of the free-radical be photodecomposition of methyl benzoin ether.)... 

An elegant model had been developed to describe the transfer of a methyl cation from the cobalt-corrinoid Fe-S protein to a proximal nickel center [116], Methyl transfer from a methyl-cobalt species to a Ni(I) complex yields a stable Ni(II)-Me species concomitant with oxidation of a second equivalent of the Ni(I) complex (Eq. 12.9). In this system, the methyl cobalt species is equivalent to a methyl cation source, although mechanistic studies revealed the involvement of methyl radicals formed via homolysis of an initially generated Co(II)-methyl species [117]. 

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

Local overheating and ignition occurred when solid benzoyl peroxide was put into a beaker which had been rinsed out with methyl methacrylate. Contact between the peroxide, a powerful oxidant and radical source, and oxidisable or polymerisable materials should only be under controlled conditions. 

The oxidative decarboxylation of carboxylic acids is the most convenient source for the alkylation of protonated heteroaromatic bases owing to their easy availability and the high versatility of the reaction, which permits methyl, primary, secondary, and tertiary alkyl radicals to be obtained under very simple experimental conditions. The following methods have been utilized. 

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