[η5‑Cp*B-Mes]+: A Masked Potent Boron Lewis Acid

The chemistry of the boron cation has been revitalized in the past decade due to its newfound application in stoichiometric and catalytic organic reactions. To extend the frontier of boron cation catalysis, we came to discover that a mesityl-substituted η5-Cp*-coordinated boron cation could serve as a powerful Lewis acid for organic catalytic transformations. The boron cation [Cp*B-Mes]­[B­(C6F5)4] ([1]­[B­(C6F5)4]) stabilized in a boronium-like electronic structure binds to Et3PO readily and displays an acceptor number exceeding that of B­(C6F5)3 on the Gutmann–Beckett acidity scale. The steric and electronic stabilization exerted by the electron-donating Cp* renders the highly Lewis acidic boron cation an easy-to-handle catalyst for hydrodeoxygenation of aryl ketones at ambient temperature. The exceptional catalytic performance of [1]+ implies that the incorporation of a coordinatively flexible substituent at boron is critical in bringing catalytic activity and stability to boron cation catalysts.