CALIBRATION OF INTRAMOLECULAR RADICAL-CATION PROBES BASED ON ARYLALKENE CYCLOADDITIONS

The radical cations of three probe molecules (1-3) that undergo intram olecular 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 r adical cation with lambda(max) at similar to 600 nm that is localized on the arylalkene moiety. The radical cations of 1 and 3 have decay ra te constants of 1.2 x 10(9) and 3.0 x 10(8) s(-1) and can only be obse rved by picosecond spectroscopy. These cyclization rate constants agre e reasonably well with previous estimates based on product and Stem-Vo lmer quenching studies using a triarylaminium salt as the electron tra nsfer catalyst., The transient experiments are consistent with cycliza tion of 1(.+) to give a cyclobutane radical cation that has a lifetime of similar to 100 ns and decays by either reduction to cyclobutane or rearrangement to a hexatriene radical cation, lambda(max) at 500 nm ( 10), but does not revert to 1(.+). Consistent with this, product studi es 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 o f 1. Radical cation 2(.+) is relatively long-lived with an apparent de cay rate constant of 3 x 10(6) s(-1); transient studies again provide evidence for rearrangement to a hexatriene radical cation. The observe d decay rate is much lower than previous estimates and may indicate th at the initial cyclobutane radical cation undergoes rapid cleavage (as observed for the all-trans-1,2-dianisyl-3,4-dimethylcyclobutane radic al cation) in competition with rearrangement. The results illustrate s ome of the limitations that may be encountered in the development and calibration of intramolecular radical cation probes based on arylalken e cycloaddition chemistry.