• azoalkanes;
  • denitrogenation mechanism;
  • deuterium-labeling;
  • dynamics effects;
  • substituent effect

A stereochemical deuterium-labeling study of the thermal and photochemical denitrogenation of the exo-selectively dideuterated azoalkane 7,7-diethoxy-2,3-diazabicyclo[2.2.1]hept-2-ene (AZb-d2) was performed in order to obtain information about the denitrogenation mechanism of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives. A double-inversion product 5,5-diethoxybicyclo[2.1.0]pentane (CPb), that is, inv-CPb, was selectively formed over the retention product ret-CPb in a ratio of inv-CPb/ret-CPb = 73/27 under flash vacuum pyrolysis (FVP) conditions (365 °C/~1.0 × 10−2 mmHg). The formation of inv-CPb was slightly higher than that of ret-CPb in the direct photolysis of AZb-d2 at 70 °C. An approximately 1:1 mixture of the two products was observed after the direct photolysis at 7 °C. A stereoselective formation of 1,5-diethoxycyclopent-1-ene (MG-d2) was found in the denitrogenation of AZb-d2 under the FVP conditions. Computational studies at the broken-symmetry (BS)-(U)CCSD/6-31G(d) level of theory revealed that the stepwise C–N bond cleavage is the energetically favored process of the denitrogenation mechanism. The SH2 trajectory of the nitrogen-extrusion process through an equatorial conformation of the intermediary diazenyl diradical DZb-eq was found for the formation of inv-CPb. An alternative route for inv-CPb was found through an axial conformation of DZb-ax, which was suggested by the trajectory calculations using a Born–Oppenheimer molecular dynamics at the UB3LYP/6-31G(d). Copyright © 2011 John Wiley & Sons, Ltd.