Transition-State-like Planar Structures for Amine Inversion with Ultralong C-C Bonds in Diamino--carborane and Diamino--dodecahedron.

Umbrella-like inversion of pyramidalized amines proceed through a planar transition state (TS). Stabilization of the TS through N(lone-pair) → σ*(C-C) "negative hyperconjugation" in diamino--carborane () causes rapid inversion in the amine, which results in the observation of a planarized -NH from the X-ray crystal structure. This proceeds through quantum mechanical tunneling across the small and narrow barrier (low pyramidalization). Tuning this secondary orbital (donor-acceptor) interaction for various derivatives of and diamino--dodecahedron () provides a rational approach toward increasing to as large as 2.001, 2.011, and 1.807 Å for (amino oxide--carborane), (di--dimethylamino--carborane), and (di--diisopropylamino-o-dodecahedron), respectively.