Pivaloyl radicals, decarbonylation

The high reactivity of protonated heteroaromatic bases towards acyl radicals is shown by the success of the reaction with the pivaloyl radical, which usually undergoes rapid decarbonylation [Eq. (33)]. 

Protonated heteroaromatic bases are therefore more reactive than simple olefins toward acyl radicals. The radical addition of pivalaldehyde to olefins is, in fact, characterized by a radical chain, whose propagation is determined by decarbonylation of the pivaloyl radical and addition of <-butyl radical to the olefin. The synthetic interest is great in the case of substrates with only one reactive position, such as benzothiazole, ... 

Decarbonylation of the pivaloyl radical, to afford the stable terf-butyl radical is rapid. 

The decarbonylation of pivaloyl radicals in aqueous solution, k - 2.5 X 10 s 147) is somewhat slower than in most non-aqueous solvents 148), but sufficiently rapid to compete with the reaction with O2 in Eq. (21) under most conditions. In the absence of O2, most of the acyl radicals produced in Eq. (19) will dissociate CO, Eq. (20), and react with CraqOO Eq. (21). A small fraction may disappear in bimolecular radical self-reactions. 

The high decarbonylation efficiency of di-t-butyl ketone 8 impHes that the acyl and alkyl fragments of the primary pair can separate and remain separated as free radicals long enough for the pivaloyl radical to lose CO with a time constant of -1.2 to 1.5 ps." " In contrast, the decarbonylation efficiency of the cyclic ketone analogs in Scheme 5 is remarkably reduced by the average proximity of the two radical termini, which facilitates faster radical-radical reactions. 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

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