Alkyl radicals from diacyl peroxides

Treatment of alkyl isonicotinates or 4-cyanopyridine with alkyl free radicals (generated from diacyl peroxides) afforded 2-alkyl-substituted molecules. ... 

Knowledge of the cyclization rate of the 5-hexenyl radical has also been used to study cage reactions of alkyl radicals generated in solution from diacyl peroxides or to distinguish between intra- and intermolecular pathways in the Wittig rearrangement. It has also been used to study the mechanism of the deshalogenation of 1,4-, 1,5-, and 2,3-dihaloalkanes by alkali... 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

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). 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

In general, aliphatic diacyl peroxide initiators should be considered as sources of alkyl, rather than of aeyloxy radicals. With few exceptions, aliphatic acyloxy radicals have a transient existence at best. For certain diacyl peroxides (36) where R is a secondary or tertiary alkyl group there is controversy as to whether loss of carbon dioxide occurs in concert with 0-0 bond cleavage. Thus, ester end groups observed in polymers prepared with aliphatic diaeyl peroxides are unlikely to arise directly from initiation, but rather from transfer to initiator (see 3.3,2.1.4),... 

Diacyl peroxides (16. R1 = R2 = alkyl or aryl) also undergo radical induced decomposition either by direct radical displacement on the oxygen-oxygen bond, or by radical addition to, or abstraction from, the hydrocarbyl group adjacent to die peroxide. 

Oxidation reactions of hydrocarbons have a typical course. From the low rates, the reaction accelerates successively due to the consecutive formation of another source of free radicals which increases the rate of the primary initiation reaction. The amplification of the number of reactive free radicals is caused mainly by the decomposition of alkyl hydroperoxides, dialkyl and diacyl peroxides and peracids which are formed as intermediates in the oxidation reaction. 

The alkyl groups of two identical carboxylic acids can be coupled to symmetrical dimers in the presence of a fair number of functional groups (equation 1). Since free radicals are the reactive intermediates, polar substituents need not be protected. This saves the steps for protection and deprotection that are necessary in such cases when electrophilic or nucleophilic C—C bond-forming reactions are involved. Furthermore, carboxylic acids are available in a wide variety from natural or petrochemical sources, or can be readily prepared from a large variety of precursors. Compared to chemicd methods for the construction of symmetrical compounds, such as nucleophilic substitution or addition, decomposition of azo compounds or of diacyl peroxides, these advantages make the Kolbe electrolysis the method of choice for the synthesis of symmetrical target molecules. No other chemical method is available that allows the decarboxylative dimerization of carboxylic acids. 

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