Electron transfer catalyzed [2+2] cycloreversion of benzene dimers

The catalysis of the [2 + 2] cycloreversion of the anti-o,o'-benzene dimer 1 and the syn-o,o'-naphthalene-benzene dimer 2 through thermal and photoind uced electron transfer is studied using experimental and computational meth ods. The reaction of the radical cations formed by electron transfer is at least 10(5) times faster than the thermal background reaction. It is demons trated that the photoinduced electron transfer catalyzed reaction proceeds via an electron transfer sensitized pathway and that the observed inverse s econdary deuterium isotope effect of 0.91 +/- 0.02 on the reaction is due t o the equilibrium isotope effect on the electron transfer step. The relevan ce of these findings on the mechanism of the electron transfer catalyzed [2 + 2] cycloreversion of the biologically important cis,syn-cyclobutane-thym ine dimer is also discussed.