Ab initio investigation of the 3-aza-Cope reaction

The energies of reaction and activation for three archetypal aliphatic 3-aza-Cope reactions have been investigated by MP4/6-31G*//MP2/6-31G* calculations. The activation energies of these rearrangements (3-aza-1,5-hexadiene (1 to 2, eq 1), Delta E+/- = 34.6 kcal/mol; 3-azonia-1,5-hexadiene (3 to 4, eq 2), Delta E++ = 21.4 kcal/mol; 3-aza-1,2,5-hexatriene (5 to 6, eq 3), Delta E++ = 17.7 kcal/mol) were found to be consistent with the reported relative facility of substituted versions of these processes. Inclusion of electron correlation in the theoretical model was necessary to reproduce the experimentally observed ordering and relative magnitudes of the activation energies. Comparisons of the transition-structure bond orders for these three reactions show that the N-allyl ketenimine rearrangement features the least amount of bond breaking and bond making of any of these processes. Finally, a comparison of the atomic charges on nitrogen (as calculated by the methods of Mulliken, NBO analysis, and CHELPG methods using the MP2 densities) in these three 3-aza-Cope rearrangements provides evidence that charge delocalization in the transition state is an important factor only in the charged 3-aza-Cope reaction. The relative facility of the 3-aza-1,2,5-hexatriene rearrangement is probably due to the favorable reduction of steric congestion in the transition structure which is offered by the heterocumulene structure.