Acyl Radical Clocks

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

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Figure 7. Acyl radical clocks and their rate constants for reactions at 20 °C...

Radical clock competition kinetic studies of reactions of Bu3SnH with acyl radicals have been reported. Relative rate constants for reactions of... 

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