Pressure dependence of the stereoselectivity in the photodenitrogenation of diazabicyclo[2.2.1]hept-2-ene in supercritical fluids: evidence for the diazenyl diradical

The present study shows that the preference for the stereoselective inversion upon the photolytic nitrogen extrusion from the stereolabeled DBH derivative exo-d2-diazabicyclo[2.2.1]heptene (1) in supercritical media (sc-CO2, sc-C2H6) depends on pressure. The increase of pressure up to 200 bar leads to ca. 2.3-fold decrease of the stereoselectivity (kinv/kret ratio) in sc-CO2 and sc-C2H6. Analysis of the observed pressure dependence in terms of collisional (self-diffusion coefficient) and frictional (viscosity) effects discloses that frictional impositions by the fluid media account best for the experimental observations reported herein. This pressure-dependent viscosity effect is consistent with a stepwise denitrogenation mechanism for the photolysis of DBH in the supercritical phase, which proceeds through an unsymmetrical, nitrogen-containing transient, namely the singlet diazenyl diradical.