Ultrafast radical clock reactions

In addition to their use for the calibration of rates for radical reactions, radical clocks can be employed to distinguish between ionic and radical pathways. In the simplest embodiment of this idea, a suitable clock reaction that undergoes a known fast rearrangement with easily identifiable products is incorporated into the reaction system to be studied. This approach has been exploited in the pioneering work of Newcomb and co-workers in studies of the mechanism of cytochrome P450 oxidation reactions [13]. Newcomb has developed a range of ultrafast radical clocks able to detect radical species with lifetimes of 80-200 fs. 

Despite the accumulated evidence for the oxygen rebound mechanism, recent results using more sophisticated and ultrafast radical clocks have challenged the oxygen rebound mechanism and revived the concerted insertion mechanism. Atkinson and Ingold used several radical clocks including phenyl-substituted cyclopropanes in P450-catalyzed hydroxylation reactions. 

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