| Résumé | 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 photosensitized electron transfer conditions demonstrate that both substituted cyclobutane (5) and dihydronaphthalene (8) products are formed, with the relative amounts depending on the concentration of 1. Radical cation 2•⁺ is relatively long-lived with an apparent decay rate constant of 3 × 10⁶ s⁻¹; transient studies again provide evidence for rearrangement to a hexatriene radical cation. The observed decay rate is much lower than previous estimates and may indicate that the initial cyclobutane radical cation undergoes rapid cleavage (as observed for the all-trans-1,2-dianisyl-3,4-dimethylcyclobutane radical cation) in competition with rearrangement. The results illustrate some of the limitations that may be encountered in the development and calibration of intramolecular radical cation probes based on arylalkene cycloaddition chemistry. |
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