Racemization during radical reactions

Carboxylic acids with a chiral center at the a-carbon give racemic Kolbe dimers [399,400]. Addition products of radicals derived from such carboxylic acids to olefins are also racemic [401]. Racemization is also observed in anodic methoxylation of carboxylic acids [402], These facts may suggest that Kolbe rdicals interact so weakly with the anode surface that their configuration cannot be stabilized during the reaction. However, it can also be stated that the radicals interact sterically with the anode so that the geometry of the dimers is influenced by the anode material (see Sec. IV). 

Transformations involving trigonalization of asymmetric sp carbons (such as forming radicals and cabenium ions, etc.) typically lead to racemic products when no other asymmetric centers exist in the molecule. However, there are circumstances that this generalization does not hold. One such circumstance involves the formation of reaction intermediates that are conformationaUy chiral. These conformers are relatively stable and lead to the formation of nonracemic products even though the only asymmetric sp carbon is literally destroyed during the reaction. 

Due to the very limited amount of experimental data available, a choice between the two mechanisms is even more difficult for reaction III than for reaction II. Orban et investigated the photochemical rearrangement, in pentane, of an aliphatic, optically active ketone with a single asymmetric carbon atom in the y position. Their results demonstrate a partial retention of configuration during photochemical cyclobutanol formation, which can be explained neither by a biradical mechanism alone, if this involves a long-lived radical, nor by a concerted mechanism alone. The results are reconcilable with the competitive participation of both mechanisms, but they are just as compatible with the assumption of the production of a short-lived biradical whose rates of racemization and of cyclization are comparable. 

The cobaloxime system has been well studied, and stereochemical tests show complete racemization of the alkyl group during insertion. Rearrangements expected for a radical mechanism [reaction (g)] are also observed . A reaction between O2 an organic free radical has been suggested as a key step for most of the above reactions ... 

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