KINETICS OF REACTIONS OF CYCLOPROPYLCARBINYL RADICALS AND ALKOXYCARBONYL RADICALS CONTAINING STABILIZING SUBSTITUENTS - IMPLICATIONS FOR THEIR USE AS RADICAL CLOCKS

The rate constants for rearrangement of alpha-substituted cyclopropylc arbinyl 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 radica l, but the reaction is essentially irreversible under the conditions u sed. Phenyl and tert-butoxycarbonyl alpha-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-C(3)H(5 )CHPh at 60 degrees C, k(ring open) = 5.4 x 10(5) s(-1), k(ring close) = 1.5 x 10(7) s(-1), and the equilibrium favors the ring closed form (K-equil = 0.04). The implications of the possible reversibility of th e ring opening of substituted cyclopropylcarbinyl radicals for cyclopr opane probe studies of metal hydride reduction and other chemical/bioc hemical reactions are assessed. Most of the cyclopropylcarbinyl radica ls were generated from tert-butyl peroxyglyoxaletes (ROC(O)CO(3)Bu(t)) via alkoxycarbonyl radicals (ROCO). This method allowed the determina tion of the rate constants for decarboxylation of ROCO when R is t-Bu, PhCH(2), c-C(3)H(5)CMe(2), c-C(3)H(5)CHMe, (c-C3H5)(2)CH, or c-C3H5CH C6H(5).