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 η 5 -Cp*-coordinated boron cation could serve as a powerful Lewis acid for organic catalytic transformations. The boron cation) stabilized in a boronium-like electronic structure binds to Et 3 PO readily and displays an acceptor number exceeding that of B(C 6 F 5 ) 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.

A tricoordinate borenium ion has received considerable attention in recent years for its applications in Lewis acid catalysis. Over the years, asymmetric catalysis mediated by a chiral borenium ion has also been developed. To stabilize the electrondeficient boron atom, a series of chloroborane masked borenium ions featuring the symmetrical [B−Cl−B] + linkage are prepared and utilized as the catalyst for the enantioselective Diels−Alder cycloaddition of cyclopentadiene and 2,2,2-trifluoroethyl acrylate. The presence of a Cp* ligand is critical in realizing the cyclic diboron compounds, and the stability of the resulting [B−Cl−B] + cation is dependent on the steric bulkiness of the oxazolidinone moiety. The stereoselectivity of the Diels−Alder cycloaddition is controlled by the substituents of the chiral oxazolidinone ligand and could be further improved via the coordination of SnCl 4 at the bridging chloride of the [B−Cl−B] + cation.