Stereoselective formation of inverted housane in the denitrogenation of the diazenyl diradical photolytically derived from a stereolabeled diazabicyclo[2,2,1]hept-2-ene with bridgehead substituents as a function of solvent- and temperature-varied viscosity

It has been experimentally demonstrated that viscosity differentiates the e xtent to which inverted and retained housanes are formed in the liquid-phas e denitrogenation of a substituted diazabicyclo[2,2,1]hept-2-enes (DBH)-typ e azoalkane, namely syn-2,3-diaza-4,7-dimethyl-1-ethyl bicyclo[2,2,1]hepten e. The fact that the viscosity profiles coincide for the inversion/retentio n product ratio, assessed for a set of diverse solvents at constant tempera ture and through the temperature variation in a single solvent (n-butanol), shows that the temperature dependence of the stereoselectivity is controll ed by bulk viscosity effects. The simple free-volume model rationalizes the viscosity behavior of the inversion/ retention ratio. Since the viscous me dium differentiates between the inversion and retention reaction channels, we conclude that an unsymmetrical, nitrogen-containing intermediate, presum ably the singlet diazenyl diradical, intervenes in the liquid-phase deazeta tion process. (C) 2001 Elsevier Science B.V. All rights reserved.