Aryl and Vinyl Radical Clocks

The availability of radical clocks for -hybridized carbon systems has been limited by the high reactivity of phenyl and vinyl radicals and by the lack of appropriate methods for preparation of these radicals for direct kinetic studies. Competition kinetic studies have given relative rate constants for some radical clocks in this group, but absolute rate constants for the radical-trapping reactions used in the competitions are not generally available. In that regard, one should note that reported rate constants for reactions of BuySnH with the phenyl and 2,2-dimethylvinyl radicals [29] were later vitiated when it was found that these radicals had not been produced cleanly. 

A more recent determination of the rate constant for reaction of an aryl radical with Bu3SnH at ambient temperature is available, viz. ki = 7.8 x 10 M s [42], If one assumes that this kinetic value can be used for any aryl radical reacting with tin hydride, then the rate constants for cyclization of the aryl radical clocks shown below can be calculated from the reported relative Arrhenius functions [29]. Specifically, radicals 13 and 14 cyclize with rate constants of 5 x 10 s and 

8 X 10 s respectively, at 25 °C. For the vinyl radical clock 15, the ratio of rate 

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