The radical cation of anti-tricyclooctadiene and its rearrangement products

The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0(25)]octa-3,7-diene, TOD) is subjected to ionization by gamma-irradiation in Freon matrices, pu lse radiolysis in hydrocarbon matrices, and photoinduced electron transfer in solution. The resulting species are probed by optical and ESR spectrosco py (solid phase) as well as by CIDNP spectroscopy (solution). Thereby it is found that ionization of anti-TOD invariably leads to spontaneous decay to two products, that is bicyclo[4.2.0]octa-2,4,7-triene (BOT) and 1,4-dihydr opentalene (1,4-DHP), whose relative yield strongly depends on the conditio ns of the experiment. Exploration of the C8H8.+ potential energy surface by the B3LYP/6-31G* density functional method leads to a mechanistic hypothes is for the observed rearrangements which involves a bifurcation between a p athway leading to the simple valence isomer, BOT.+, and another one leading to an unprecedented other valence isomer, the anti form of the bicyclo[3.3 .0]octa-2,6-diene-4,8-diyl radical cation (anti-BOD.+). The latter product undergoes a very facile H-shift to yield the radical cation of 1,3a-dihydro pentalene (1,3a-DHP.+) which ultimately rearranges by a further I-I-shift t o the observed product, 1,4-DHP.+.