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

Abstract

AbstractThe benzophenone‐sensitized laser jet photolysis of 2,3‐diazabicyclo[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°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‐diyldiracidal (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.