Lite Version|Standard version

To gain access to this content please
Log in via your home Institution.
Log in with your member or subscriber username and password.
Download

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 (kcis + ktrans ∼ 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 (kc/kH), a general decrease in the kc/kH ratio as solvent polarity is increased is noted; these changes have been speculated to arise mainly from changes in kH in the various solvents employed.

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

Page: ^ Top