Radical clock reactions

LFP-Clock Method. In this method, rate constants for the radical clock reactions are measured directly by LFP, and the clocks are used in conventional competition kinetic studies for the determination of second-order rate constants. The advantages are that the clock can be calibrated with good accuracy and precision in the solvent of interest, and light-absorbing reagents can be studied in the competition reactions. The method is especially useful when limited kinetic information is available for a class of radicals. 

The first-order ft-scission of the ferf-butoxy radical is one of the oldest radical clock reactions and has been used for over 50 years for the measurement of the relative rates of hydrogen abstraction from organic compounds (AH) in solution (Scheme 10.14). At low conversions, when the concentration of AH has not appreciably changed, the ratio of the rate constants for hydrogen atom abstraction, kAH> and /3-scission, kp, can be determined simply by analysis for acetone and ferf-butyl alcohol formation in the reaction. This is most conveniently achieved by gas chromatography ... 

Table 10.2 A Selection of common radical clock reactions.

Radical clock reaction Rate constant, /ctemp (s 1)... 

There are numerous other examples of radical clock reactions in the literature used both for simple rate determinations to facilitate the quest for selectivity in synthesis and for more detailed probing of mechanistic pathways. 

The rearrangement of a-cyclopropyl radicals (162) to the ring-opened alkyl radical (163) has been recognized as an extremely rapid (ca. 10 s" ) and useful radical clock reaction (equation 30), with equilibrium greatly favoring the unstrained acyclic partner. In recent years this process has been used in enzymatic redox reactions to probe the nature of... 

For greatest accuracy, the rate constant of the radical clock reaction should be similar to the rate constant of the competing reaction. For this reason, investigators have developed a number of radical clocks with varying rate constants for reaction, as shown in Table 5.3. In addition to applications in the study of chemical reactions, radical clock reactions have been used extensively in studies of biological processes, such as the peroxidation of... 

TABLE 5.3 Room Temperature Rate Constants for Radical Clock Reactions... 

Lusztyk, J., MaiUard, B., Deyard, S., Lindsay, D. A., and Ingold, K. U., Kinetics for the reaction of a secondary alkyl radical with tri-n-butylgermanium hydride and calibration of a secondary alkyl radical clock reaction, /. Org. Chem., 52, 3509,1987. 

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