KINETICS OF CYCLOPROPYLCARBINYL RADICAL RING OPENINGS MEASURED DIRECTLY BY LASER FLASH-PHOTOLYSIS

The kinetics of ring openings of cyclopropylcarbinyl radicals containi ng reporter groups were measured directly by laser flash photolysis (L FP) methods. The reporter groups are aryl-substituted cyclopropanes at the position of the radical center in the incipient product, and the initial ring-opening reaction gives an aryl-substituted cyclopropylcar binyl radical that opens ''instantly'' on the nanosecond time scale to give benzylic and diphenylalkyl radical products that are detected by UV spectroscopy. Three reporter group de signs have been studied. The most useful design is that in substituted ans-2-(2,2-diphenylcyclopro pyl)-cyclopropyl)methyl radicals (7), The important synthetic intermed iate for production of this series is l-1R-cyclopropyl)-1S*,2R*-cyclo propanecarboxamide (15a) which has been prepared diastereomerically pu re. The syntheses of 15a and a series of PTOC ester radical precursors derived from 15a are described. Laser photolysis (355 nm) of PTOC eat ers gave acyloxyl radicals that decarboxylated to give cyclopropylcarb inyl radicals that were studied. The substitutions at the cyclopropylc arbinyl position of radicals 7 were H, H (7a); H, Me (7b); Me, Me (7c) ; H, CO2Et (7d); Me, CO2Et (7e); and H, Ph (7f). The kinetics of ring openings of radicals 7 were measured in THF and in four cases in CH3CN in the temperature range -41 to +50 degrees C. Alkyl radicals 7a and 7b displayed no kinetic solvent effect, whereas ester-substituted radi cals 7d and 7e did. The ester-substituted radicals react about as fast as the alkyl-substituted radicals due to favorable polarization in th e transition states for ring opening. The phenyl-substituted radical 7 e ring opens about 3 orders of magnitude less rapidly than the parent radical 7a. The structures and energies of the ground states, rotation al transition states, and ring-opening transition states for a series of substituted cyclopropylcarbinyl radicals corresponding to 7a-e were computed by the hybrid density functional theory B3LYP/6-31G, and th e ring-opening kinetics were compared to those predicted from the comp utational barriers. The precision in the Arrhenius functions for ring openings of radicals 7 permits meaningful comparisons of the log A ter ms to those predicted from thermochemical considerations.