NITROGEN INVERSION BARRIER OF 2-METHYL-2-AZABICYCLO[2.2.1]HEPTANE - THE ROLE OF TORSIONAL STRAIN IN PYRAMIDAL INVERSION

Low-temperature C-13 NMR measurements indicate that the endo isomer of 2-methyl-2-azabicyclo-[2.2.1]heptane is about 0.3 kcal mol(-1) more s table than the exo isomer. Rate constants for inversion from the endo to exo isomer were determined by NMR line shape analysis. The inversio n barrier, Delta G(double dagger), of 7.2 kcal mol(-1) is lower than t hat in model acyclic amines, despite an internal CNC bond angle that i s less than the tetrahedral angle of 109.47 degrees. Comparison with 7 -methyl-7-azabicyclo-[2.2.1]heptane that has a small internal CNC angl e and an unusually high barrier, as well as other cyclic and bicyclic amines, leads to the conclusion that torsional (eclipsing) strain play s a significant role along with angle strain in determining inversion barriers. Molecular mechanics calculations of the change in energy bet ween pyramidal ground state and planar transition state account reason ably well for the observed barriers. New measurements of inversion bar riers and their dependence on solvent are also reported for 2-methyl-2 -azabicyclo[2.2.2]octane and 1-methyl-4-piperidone.