Kinetics of reactions of cyclopropylcarbinyl radicals and alkoxycarbonyl radicals containing stabilizing substituents: Implications for their use as radical clocks

Article abstract of DOI:10.1021/ja00086a003

The rate constants for rearrangement of α-substituted cyclopropylcarbinyl radicals have been measured by nitroxide radical-trapping (NPT). Those bearing methyl, dimethyl, or cyclopropyl substituents undergo ring opening 2-3 times more slowly than does cyclopropylmethyl radical, but the reaction is essentially irreversible under the conditions used. Phenyl and tert-butoxycarbonyl α-substituents retard the rate of ring opening more strongly and enhance the rate of ring closure of the corresponding substituted but-3-enyl radicals. Thus for c-C₃H₅CHPh at 60°C, k_{ring open} = 5.4 × 10⁵ s^-1, k_{ring close} = 1.5 × 10⁷ s^-1, and the equilibrium favors the ring closed form (K_equil = 0.04). The implications of the possible reversibility of the ring opening of substituted cyclopropylcarbinyl radicals for cyclopropane probe studies of metal hydride reduction and other chemical/biochemical reactions are assessed. Most of the cyclopropylcarbinyl radicals were generated from tert-butyl peroxyglyoxalates [ROC(O)CO₃Bu^t) via alkoxycarbonyl radicals (ROC?O). This method allowed the determination of the rate constants for decarboxylation of ROC?O when R is t-Bu, PhCH_2, c-C₃H₅CMe₂, c-C₃H₅CHMe, (c-C₃H₅)₂CH, or c-C₃H₅CHC₆H₅.