In-Line Proximity Effects in Extended 7-Azanorbornanes. 2. A Major Reduction of N-Inversion Barriers in Symmetrically Flanked Systems

Through-space compression effects from a pair of flanking (sentinel) groups X are shown to cause a flattening of the bridging nitrogen in symmetrically fused 7-azanorbornanes which is evaluated by a substantial reduction of the nitrogen inversion barrier as measured by low-temperature (1)H NMR spectroscopy. This effect is attributed to destabilization of the ground state of the amine.