Temperature and Viscosity Dependence in the Stereoselective Formation of the Inverted Housane for the Photochemical Nitrogen Loss from the Deuterium-Stereolabeled Parent Diazabicyclo[2.2.1]hept-2-ene

• Published in: Journal of the American Chemical Society 2001-07, Vol.123 (29), p.7109-7112
• Main Authors: Adam, Waldemar, Grüne, Matthias, Diedering, Manfred, Trofimov, Alexei V
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja005887d

For the liquid-phase photolytic denitrogenation of the stereolabeled DBH derivative exo-d 2-diazabicyclo[2.2.1]heptene (exo-d 2-1), the k inv/k ret ratio of the inverted [2(inv)] and retained [2(ret)] housanes (bicyclo[2.1.0]pentanes) depends on the viscosity of the medium. For this purpose, the viscosity was varied by changing the solvent (various alcohols and diols, n-hexane, and acetonitrile) at constant temperature and by changing the temperature (−50 to +100 °C) in one single solvent, namely n-butanol. This viscosity effect is consistent with a stepwise denitrogenation mechanism in the liquid-phase photolysis of DBH, which proceeds through an unsymmetrical, nitrogen-containing transient, namely the singlet diazenyl diradical. The simple free-volume model adequately accounts for the observed viscosity behavior of the k inv/k ret ratio in terms of frictional effects. The temperature dependence discloses a small but measurable difference in the internal activation energies for the inversion and retention processes of the proposed diazenyl diradical.