A Tamed Intermediate: The Pivotal Role of Cp* in Hypercoordinate Boron Cation Catalysis

Compared to borenium
ions, the synthetic application of its dicoordinate
variant, borinium ion, remains much less explored due to the facile
decomposition of borinium ions. Previously, we showed that [Cp*B-Mes]+ ([1]+) can be viewed as a masked
borinium ion with great potential in catalysis. In this study, we
further demonstrate that [1]+ serves as a
precatalyst in catalytic cyanosilylation of ketones. When treated
with TMSCN, [1]+ transforms into a TMSCN coordinated
boronium ion [Cp*BMes(TMSCN)2]+ ([4]+), which is highly efficient in catalyzing cyanosilylation.
However, such reagent-coordinated boronium ion cannot be accomplished
in the absence of Cp* ligand. Experimental and computational investigations
revealed that the Cp* ligand could mitigate the Lewis acidity of the
putative borenium ion, [Cp*BMes(TMSCN)]+ ([5]+), via the coordination of Cp* to boron in a η2-coordination fashion. Such a bonding interaction significantly
reduces the positive charge on the boron atom [5]+, preventing the decomposition of the critical intermediate.