No abstract

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.