Nitrogen-Centered Radical Clocks

Nitrogen-centered radicals have been applied in synthetic conversions infrequently, in part because convenient sources of nitrogen-centered radicals have not been readily available until recently [49, 50]. Nitrogen-centered radical conversions are 

Cyclization of the aminyl radical 22 is slow and reversible [53], and the kinetics of related cyclizations and ff-fragmentations were measured directly by LFP [54]. Protonation of the dialkylaminyl radicals gives dialkylaminium cation radicals such as 24 that react much more rapidly [55], and Lewis acid complexes of aminyl radicals such as 23 are intermediate in reactivity [56]. Clocks such as 23 and 24 are in equilibrium with the neutral aminyl radicals, and the concentrations of the proto-nated or complexed forms are necessary if one is to use these clocks equilibrium constants for protonations and Lewis acid complexations in some solvents were determined in the initial kinetic calibration studies. 

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Figure 8. Nitrogen-centered radical clocks and their rate constants for rearrangements at 20 °C...

When the probe reaction being calibrated is a unimolecular process, one measures the rate constant of a radical clock directly for the initial absolute kinetic values, and, thus, the method is inverted in approach from that used for alkyl radical kinetics. LFP studies of unimolecular process give more precise data than those of bimolecular processes, and the approach typically starts with inherently good kinetic data. The synthetic efforts necessary for production of appropriate radical precursors are a drawback to this method, but it is, nonetheless, useful for establishing absolute kinetics for some classes of radicals where little kinetic information was available, such as nitrogen-centered radicals discussed later. 

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