DYNAMIC-MODELS FOR THE THERMAL DEAZETIZATION OF 2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE

Kinetic studies on the thermal nitrogen extrusion from 2,3-diazabicyclo[2.2.1]hept-2-ene-exo,exo-5,6-d2 are reported. The ratio of rate constants for formation of the label-isomeric products (bicyclo[2.1.0]pentane-exo,exo-2,3-d2 and -endo,endo-2,3-d2) is found to exhibit no statistically significant temperature dependence. A comparison of gas-phase and solution-phase results is presented. The results are interpreted in terms of two complementary dynamic models. In the first,classical trajectory calculations are run on a three-dimensional projection (two geometric coordinates) of the potential energy hypersurface. These calculations correctly identify the major product, and reproduce the near temperature-independence of the rate-constant ratio, but do not match the ratio quantitatively. Modification of the trajectory calculations to simulate the effect of collisions with solvent molecules also qualitatively matches the observed difference between gas-phase and solution-phase behavior. In the second model, the vector of atomic displacements corresponding to the reaction coordinate at the transition state for nitrogen loss is identified by both semiempirical and ab initio calculations. The components of this vector pointing along the paths to the post-transition-state minima are computed and are shown to lead to a prediction of the product ratio that is in good (if partly fortuitous) agreement with the experimental result. The vector model is used to predict isotope effects on the product ratio, which are then investigated experimentally with 2,3-diazabicyclo[2.2.1]hept-2-ene-endo,endo-1,4,5,6,7,7-d6 and -endo-7-d.