Mechanism of Thermal Deazetization of 2,3-Diazabicyclo[2.2.1]hept-2-ene and Its Reaction Dynamics in Supercritical Fluids

The mechanism of thermal deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) is examined both theoretically and experimentally in this paper. The theoretical work involves CASPT2 ab initio MO calculations using the 6-31G(d) basis set. These calculations are shown to give relative enthalpies for the stationary points on the potential energy surface that are within ± 3 kcal/mol of the experimental values. Surprisingly, the transition structure for the rate-limiting step is found to correspond to synchronous C−N cleavage, leading to the formation of the cyclopentane-1,3-diyl biradical. The alternative mechanism, involving stepwise C−N bond scission, is found to have an activation enthalpy that is at least 7 kcal/mol higher. The synchronous mechanism is in apparent conflict with earlier calculations and experiments, all of which seemed to favor the stepwise pathway. A resolution of this disagreement is proposed. The well-known preference for inversion of stereochemistry in the deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene- exo,exo -5,6 -d 2 (DBH- d 2 ) is proposed to involve nonstatistical dynamical effects. This mechanism is similar to, but not identical with, one first proposed by Allred and Smith over 30 years ago. The deazetization of DBH- d 2 in supercritical propane is studied experimentally. The pressure dependence of the reaction stereochemistry is shown to be qualitatively and quantitatively consistent with the dynamic model, but not to be consistent with any mechanisms that would require separate reaction paths across the potential energy surface. In supercritical carbon dioxide, the pressure dependence is found not to be fit by any simple model. Calculations suggest that this may be because CO 2 forms weak complexes (binding energies of approximately 2 kcal/mol for each CO 2 ) with the cyclopentane-1,3-diyl intermediate. The calculations also suggest that similar complexes can be expected between CO 2 and simple monoradicals.