Cycloadditions, radical cation probes

These results provide the first detailed calibration for a series of intramolecular radical cation probes based on cycloaddition chemistry. The cyclization rate constants cover several orders of magnitude in timescale, an ideal case for using 1—3 as probes for radical cations of different lifetimes. However, the time-resolved experiments demonstrate that the application of radical cation probes, at least those based on aryl alkene cycloaddition chemistry, may be considerably less straightforward than similar experiments with free radical probes or clocks. Some of the problems that need to be addressed include the variation of products with the reaction conditions and method of radical cation generation, and the possibility of reversibility of the initial adduct formation. Furthermore, at least some radical cation reactions are quite sensitive to solvent and this may mean that calibrations for radical cation cycloadditions will have to be done in a variety of solvents. 

The following three sections discuss recent time-resolved experiments on inter- and intramolecular cycloadditions of aryl-alkene radical cations. These studies address some of the mechanistic issues raised by the earlier studies and also provide kinetic data for the cycloadditions of a number of aryl and diaryl-alkene radical cations. Such kinetic data are essential for the development of this chemistry as a useful synthetic strategy and as a mechanistic probe for radical cation chemistry. 

Intramolecular radical cation cycloadditions of three P-alkyl-4-methoxystyrene-containing substrates have been used recently as mechanistic probes to test for the intermediacy of radical... 

The data on cycloadditions of alkene radical cations indicate that dimerization will usually compete efficiently with cross additions and demonstrate the necessity for obtaining detailed kinetic data in order to design appropriate synthetic methods based on radical cation chemistry. The mechanistic data obtained from both time-resolved and steady-state experiments demonstrate the complexity of cycloaddition chemistry. This may be a particular limitation in the use of cycloaddition reactions in the design of mechanistic probes for assessing whether a particular reaction involves radical cation intermediates. The results also highlight the importance of using both product studies and the kinetic and mechanistic data obtained from time-resolved methods to develop a detailed understanding of the reactions of radical cations. 

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