Treasures from the Free Radical Renaissance Period – Miscellaneous hexenyl radical kinetic data

Rate constant data and Arrhenius parameters have been determined for a series of substituted hexenyl radicals of differing electronic and steric demand. Electron-withdrawing groups (CF₃, CO₂Et) directly attached to the radical centre slighly accelerate 5-exo ring-closure (k_cis + k_trans ∼ 2.1 × 10⁵ s⁻¹ at 25°) relative to donating groups (OMe; 1.6 × 10⁵ s⁻¹ at 25°). Sterically demanding groups (tert-Bu), as expected, slow the cyclization process (1 × 10⁵ s⁻¹). These observations are consistent with subtle changes in activation energy for 5-exo ring-closure. Interestingly, the nature of the solvent would appear to have a significant influence on this chemistry with the cis/trans stereoselectivity sometimes improved as the solvent polarity is increased. Except for the system containing the CF₃ (electron-withdrawing) group which displays an increase in the cyclization/capture rate constant (k_c/k_H), a general decrease in the k_c/k_H ratio as solvent polarity is increased is noted; these changes have been speculated to arise mainly from changes in k_H in the various solvents employed.