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
W. Adam et al., 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, CHEM P LETT, 340(1-2), 2001, pp. 26-32
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.