Intermolecular radical clock study

The pioneering work on the calibration of intramolecular cy-clization of the 5-hexenyl radical by Ingold and co-workers provided the basis for the development of a large number of radical clocks." These are now used both for the calibration of rate constants for intermolecular radical reactions and as mechanistic probes to test for the intermediacy of radical intermediates in a variety of processes. Furthermore, the ready availability of bimolecular rate constants from competitive product studies using free radical clocks without the use of time-resolved experiments has greatly enhanced the synthetic utility of free radical chemistry. The same concept has recently been extended to radical ion chemistry. For example, rate constants for carbon—carbon bond cleavage reactions of a variety of radical cations and anions derived from substituted diarylethanes have been measured by direct time-resolved techniques. " ... 

Further evidence to exclude the triplet radical pathway includes the use of cyclopropyl substrates, which serve as a radical clock. In all cases, the reaction proceeds with no indication of ring fragmentation. The nature of the transition state of the C—H insertion step has been analyzed, via a Hammett study of the intermo-lecular C—H amination with p-substituted benzenes. A negative q value of 0.73 is obtained for the intermolecular reaction with trichloroethylsulfamate [71]. Such data indicate that there is a small, but significant, preference for electron-rich substrates, thus the resonance does contribute to the stabilization of a partial positive charge at the insertion carbon in the transition state. A kinetic isotope value of 1.9 is observed for competitive intramolecular C—H amination with a deuterated substrate (Eq. (5.21)). 

Section 8.2.3 centers on benzoyloxyl and related radicals these studies compare the reactivity of benzoyloxyl in intermolecular reactions with its own cleavage to phenyl and CO2, which is effectively used as a clock . Typical substrates have included numerous vinyl monomers. 

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