Radical clock reactions, table

Table 10.2 A Selection of common radical clock reactions.

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... 

The kinetic data for these reactions are numerous, as shown in Table VI. Most of values were obtained by radical clock methods. The ring expansion of radical 7 has been employed as the clock in a study that provided much of the data in Table VI.74 Cyclizations of 5-hexenyl-type radicals also have been used as clocks,75-77 and other competition reactions have been used.78 Hydrogen atom abstraction from n-Bu3GeH by primary alkyl radicals containing a trimethylsilyl group in the a-, >8-, or y-position were obtained by the indirect method in competition with alkyl radical recombi-... 

Table VIII contains rate constants for reactions of tin hydrides with carbon-centered radicals. A striking feature of Table VIII in comparison to other tables in this work is the high percentage of reactions for which Arrhenius parameters were determined by direct LFP or the LFP-clock method. These results are expected to be among the most accurate listed in this work. Scores of radical clocks have been studied with Bu3SnH, but the objectives of those studies were to determine rate constants for the clocks using tin hydride trapping as the calibrated basis reaction.

Cyclizations of amidyl radicals have been studied both synthetically and kinetically. A detailed study on the rates of a variety of amidyl radical reactions was determined by both LFP and indirect competition methods (Table l) In addition, the rate constants for reactions with BusSnH and PhSH were also reported (thus giving a range of simple amidyl radical clocks). The results obtained will be useful in synthetic sequenceplanning involving amidyl radicals. 

The free-radical clock methodology has been also applied to calibrate unim-olecular radical reactions based on the A h values of Table 3.2 and, when avail-... 

In a second time, the reduction of an unsaturated halide was an interesting goal because such a reaction could be an easy access to cyclic molecules via a radical cyclisation in competition with the hydrogen transfer. 6-bromohex-l-ene is one of the compound whose free radical reduction was the most extensively studied, according to the radical clock status of the 6-hex-1-enyl radical11. Therefore, it appeared of interest to study it with the above system. The results obtained are reported in table 2. 

Limited examples of substituted alkyl radical clocks are available. Fortunately, some calibrated clocks that are available have rate constants in the middle ranges for radical reactions and should be useful in a number of applications. Examples of clocks based on the 5-exo cyclization of the 5-hexenyl radical are shown in Table 2. The data for the series of radicals 2-1 and 2-2 [17, 32, 34, 35] are from indirect studies, whereas the data for radicals 2-3 and 2-4 [3, 35-38] are from direct LFP studies. The striking feature in these values is the apparent absence of electronic effects on the kinetics as deduced from the consistent values found for secondary radicals in the series 2-1 and 2-3. The dramatic reduction in rate constants for the tertiary radical counterparts that contain the conjugating ester, amide and nitrile groups must, therefore, be due to steric effects. It is likely that these groups enforce planarity at the radical center, and the radicals suffer a considerable energy penalty for pyramidalization that would relieve steric compression in the transition states for cyclization. 

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. 

In order to study the lifetimes of various radicals in new reactions, one requires several radical clocks with varying lifetimes. Incorporation of these clocks into the molecules under study is used both to show that radical intermediates do or do not exist, and if they do, their lifetimes relative to the clock. Several free radical clocks with their rate constants for rearrangement are shown in Table 8.7. Such a collection has been termed an horlogerie, after a French term for a small shop that sells clocks. Seven orders of magnitude can be spanned by choosing the correct clocks. 

The 1,2-vinyl shift shown in Eq. 11.72 proceeds via a familiar structure, the cyclopro-pylcarbinyl radical we introduced in the context of free radical clocks (Table 8.7). In this case, the two species involved, the allylcarbinyl and cyclopropylcarbinyl radicals, are both discrete chemical entities that have been thoroughly characterized by EPR spectroscopy (Eq. 11.74). The equilibrium very strongly favors the ring-opened form, making the clock reaction, the opening of cyclopropylcarbinyl, essentially irreversible. 

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