Calibration of Intramolecular Radical Cation Probes Based on Arylalkene Cycloadditions1

Abstract

The radical cations of three probe molecules (1−3) that undergo intramolecular cyclobutanation or Diels−Alder reactions have been generated by both photoionization and photosensitized electron transfer. In each case, flash photolysis experiments indicate formation of an initial radical cation with λmax at ∼600 nm that is localized on the arylalkene moiety. The radical cations of 1 and 3 have decay rate constants of 1.2 × 109 and 3.0 × 108 s-1 and can only be observed by picosecond spectroscopy. These cyclization rate constants agree reasonably well with previous estimates based on product and Stern−Volmer quenching studies using a triarylaminium salt as the electron transfer catalyst. The transient experiments are consistent with cyclization of 1•+ to give a cyclobutane radical cation that has a lifetime of ∼100 ns and decays by either reduction to cyclobutane or rearrangement to a hexatriene radical cation, λmax at 500 nm (10), but does not revert to 1•+. Consistent with this, product studies under photo...