Picosecond radical kinetics. Rate constants for reaction of benzeneselenol with primary alkyl radicals and calibration of the 6-cyano-5-hexenyl radical cyclization

Article abstract of DOI:10.1021/ja00047a026

The cyclopropylcarbinyl radical ring opening was used as a radical clock to determine rate constants for benzeneselenol trapping in THF and in toluene. Hydrogen atom transfer trapping from PhSeH appeared to be partially diffusion controlled. An operational Arrhenius function for trapping in THF is log (k_T·M s) = 11.03 - 2.21/2.3RT. The recommended function for PhSeH trapping in other low-viscosity organic solvents is log (k_T·M s) = 10.87 - 2.10/2.3RT. The rate constant for trapping at 25°C is 2.1 × 10⁹ M^-1 s^-1. The kinetic values are expected to apply for PhSeH trapping of simple primary alkyl radicals. As a check on this assumption, cyclization of the 6-cyano-5-hexenyl radical (9), produced from the corresponding PTOC ester radical precursor, was calibrated with PhSH and PhSeH trapping. The two trapping agents gave essentially equivalent results. The cyclizations of both (E)- and (Z)-9 are described by log (k_r·s) = 11.0 - 3.8/2.3RT. This fast rearrangement (k_r = 1.6 × 10⁸ s^-1 at 25°C) could prove to be useful as a radical clock for timing fast second-order processes.