Kinetics of cyclopropylcarbinyl radical ring openings measured directly by laser flash photolysis

The kinetics of ring openings of cyclopropylcarbinyl radicals containing reporter groups were measured directly by laser flash photolysis (LFP) 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 cyclopropylcarbinyl 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 (trans-2-(2,2-diphenylcyclopropyl)-cyclopropyl)methyl radicals (7), The important synthetic intermediate for production of this series is N-methoxy-N-methyl-trans-2-(2,2-diphenpl-1R*-cyclopropyl)-1S*,2R*-cyclopropanecarboxamide (15a) which has been prepared diastereomerically pure. The syntheses of 15a and a series of PTOC ester radical precursors derived from 15a are described. Laser photolysis (355 nm) of PTOC esters gave acyloxyl radicals that decarboxylated to give cyclopropylcarbinyl radicals that were studied. The substitutions at the cyclopropylcarbinyl 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 radicals 7d and 7e did. The ester-substituted radicals react about as fast as the alkyl-substituted radicals due to favorable polarization in the transition states for ring opening. The phenyl-substituted radical 7e ring opens about 3 orders of magnitude less rapidly than the parent radical 7a. The structures and energies of the ground states, rotational 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 the ring-opening kinetics were compared to those predicted from the computational barriers. The precision in the Arrhenius functions for ring openings of radicals 7 permits meaningful comparisons of the log A terms to those predicted from thermochemical considerations.