Does Metal Ion Complexation Make Radical Clocks Run Fast? An Experimental Perspective.

The rate constant for the β-scission of the cumyloxyl radical (kβ) was measured in the presence of various added electrolytes in acetonitrile and DMSO solvent. The results show that in CH3CN, kβ increases in the presence of added electrolyte, roughly paralleling the size of the cation: Li+ > Mg2+ ≈ Na+ > nBu4N+ > no added electrolyte. As suggested by Bietti et al. earlier, this effect is attributable to stabilizing ion-dipole interactions in the transition state of the developing carbonyl group, a conclusion further amplified by MO calculations (gas phase) reported herein. Compared to the gas phase predictions, however, this effect is seriously attenuated in solution because complexation of the cation to the electrophilic alkoxyl radical (relative to the solvent, CH3CN) is very weak. Because the interaction of Li+ and Na+ is much stronger with DMSO than with CH3CN, addition of these ions has no effect on the rate of β-scission.