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

ORGANOMETALLICS(2024)

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Abstract
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 eta(2)-coordination fashion. Such a bonding interaction significantly reduces the positive charge on the boron atom [5](+), preventing the decomposition of the critical intermediate.
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