C−H Bond Activation by Hyperconjugation with Al−C Bonds and by Chelating Coordination of the Hydride Ion

On treating di(tert-butyl)butadiyne with dimethylaluminum hydride under different reaction
conditions two unprecedented organoelement compounds, containing cationic carbon atoms stable in
solution at room temperature, were obtained. A vinyl cation (2) in which the cationic carbon atom is part of
a CC double bond was produced from 3 equiv of the hydride, whereas a large excess of the hydride
yielded an aliphatic carbocation (3) by complete hydroalumination of all C−C multiple bonds. Each compound
is zwitterionic with the hydride counterion effectively coordinated in a chelating manner by two strongly
Lewis acidic aluminum atoms. In agreement with quantum-chemical calculations the C−H bond activation
and the stabilization of the cationic species are further supported by a strong hyperconjugation with Al−C
single bonds. This considerably diminishes the effective positive charge at the respective cationic carbon
atoms.