Novel substituent effects on the mechanism of the thermal denitrogenation of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives, stepwise versus concerted

The substituent effect on the thermal denitrogenation mechanism of 7,7-disubstituted 2,3-diazabicyclo2.2.1]hept-2-enes, concerted versus stepwise, has been investigated in detail. Unrestricted DFT calculations at the B3LYP/6-31G(d) level of theory suggest that azoalkanes that possess electron-withdrawing substituents at C(7) prefer to expel the nitrogen molecule in a stepwise manner. The activation energy is calculated to be ca. 36 kcal/mol for the dihydroxy-substituted azoalkane. In contrast, the preferred mechanism of the concerted denitrogenation is predicted for azoalkanes that possess electron-donating substituents at C(7). The activation energy is computed to be ca. 28 kcal/mol for the silyl-substituted azoalkane. The theoretical prediction of the substituent effects on the mechanistic change is supported by analyzing the activation parameters of the azoalkane decompositions. The activation enthalpy for the decomposition of the 7,7-diethoxy-substituted azoalkane is determined to be 39.1 kcal/mol, which is 13.1 kcal/mol higher in energy for the denitrogenation of the 7-silyl-substituted azoalkane. These dramatic substituent effects can be reasonably explained by the preferred electronic configuration of the lowest singlet state of the cyclopentane-1,3-diyls produced during the denitrogenation of the azoalkanes.