PICOSECOND RADICAL KINETICS - RING OPENINGS OF PHENYL SUBSTITUTED CYCLOPROPYLCARBINYL RADICALS

Rate constants for ring openings of the trans-(2-phenylcyclopropyl)carbinyl radical (1a), the cis-(2-phenylcyclopropyl)carbinyl radical (1b), and the (2,2-diphenylcyclopropyl)carbinyl radical (1c) were studied by competition kinetics using PTOC esters as radical precursors and hydrogen atom transfer trapping from benzeneselenol as the basis reaction. Radical 1a was studied in two solvents, toluene and THF; the experimental Arrhenius function for ring opening of 1a was log (k(r).s) = 13.9 - 3.3/2.3RT (R in kcal/mol). It is possible that the immediate precursor to 1a, acyloxy radical 3a, suffers a concomitant decarboxylation-ring opening process that competes with simple decarboxylation leading to 1a. The experimental rate constant for ring opening of 1a at 25-degrees-C is 3 x 10(11) s-1. Preliminary kinetic studies of radicals 1b and 1c gave Arrhenius functions of log (k(r).s) = 13.9 - 3.1/2.3RT and log (k(r).s) = 13.1 - 2.0/2.3RT, respectively, and the respective rate constants for ring openings at 25-degrees-C are 4 and 5 x 10(11) s-1. Rate constants for ring openings of substituted cyclopropylcarbinyl radicals were estimated by Marcus theory using the known rate constants and equilibrium constant for the parent system and expected DELTAG-degrees values for the substituted systems. From these results, the estimated rate constants at 25-degrees-C for ring opening of 1a and 1b were 1 x 10(11) s-1 and that for 1c was 4 x 10(11) s-1. Precursors to radicals 1, such as the corresponding hydrocarbons, represent hypersensitive radical probes that, in principle, can provide unequivocal conclusions regarding the intermediacy of a radical in a reaction.