Oxygen-Centered Radical Clocks

Rate constants for some alkoxyl radical fragmentations are shown in Fig. 9. The tcrt-butoxyl radical (26) fragmentation to acetone and the methyl radical has been studied for years, but the rate constant shown below is from a very recent work that employed time-resolved ESR methods [2]. The cumyloxyl radical (27) fragmentation was studied directly by LFP methods, taking advantage of the IR and UV absorbances of this radical [57]. The rate constants for the reversible ring opening of the cyclopentyloxyl radical (28) were determined by competition kinetics [58], and one should note that the kinetic values are at 80 °C. 

Alkoxyl radical cleavages have log A terms of about 13 as expected for fragmentation reactions. Increasing alkyl substitution, such that the product alkyl radicals are increasingly more stable, results in considerable kinetic accelerations [59]. One should note that the kinetics of alkoxyl radical fragmentations arc sensitive to 

This brief overview was intended to introduce the concepts of the radical clock method with a relatively limited number of examples. Extensive tables of radical kinetics exist, and many reactions can be used as clocks. In regard to the kinetic values available, however, one should appreciate that determinations of radical kinetics tend to involve a series of increasingly precise and accurate approximations. For that reason, more recently determined rate constants usually were selected for this overview. When using radical clocks, one is well advised to search a reference in the forward direction for improved kinetic values, especially those involving recalibrations of absolute rate constants. 

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On the other hand, participation of free radicals should be considered in most cases of oxygenations. Both of carbon and oxygen centered radicals can take part in the oxygenation processes. Apart from the apparent autoxidation process, it is not easy to differentiate explicitly metal-based mechanisms from free radical mechanisms. Different types of the radical-clock reagents have been developed for detection of radicals of different lifetime, especially in the discussions on the radical-rebound mechanisms. 

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