ON THE MECHANISM OF THE BENZOPHENONE-SENSITIZED PHOTOLYSIS OF 2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE IN THE LASER JET - EVIDENCE FOR INTERMOLECULAR TRIPLET DIRADICAL REACTIONS

The benzophenone-sensitized laser jet photolysis of 2,3-diaza-bicyclo[2.2.1]hept-2-ene (1) affords, besides the previously reported cyclopentene and housane (2), also cyclopentane, cyclopentadiene, and the dimers bicyclopent-2-en-1-yl (7), 3-cyclopentylcyclopent-1-ene (8), and 1,1'-bicyclopentyl (9). As a model reaction, the pyrolysis of dimer 8 at 600-degrees-C/20 Torr leads to the other dimers 7 and 9 together with cyclopentadiene, cyclopentene, and traces of cyclopentane. Control experiments showed that H abstraction by the cyclopentane-1,3-diyl diracidal (3) from cyclohexene (as model substrate for cyclopentene) and addition to housane (2) with formation of diradical 6 are unlikely pathways. Instead, the product data available can be best explained in terms of an intermolecular disproportionation of two diradicals 3 to give the cyclopent-2-en-1-yl (4) and cyclopentyl (5) radical pair, which is subsequently converted to the observed products by in-cage and out-of-cage coupling and H transfer reactions. Such intermolecular diradical chemistry becomes feasible due to the high steady-state concentrations (ca. micromolar) generated in the laser jet. Two-photon processes take place, but are of subordinate importance.