Lewis-Acidic Properties and Catalytic Behavior of Hydroboration- and Diolysis-Derived NHC-Stabilized Borenium Cations

In this work, we report the synthesis and characterization of a series of thirteen N-heterocyclic carbene-stabilized borenium cations featuring varied steric and electronic properties of the supporting NHC ligands (IMe₂, IMes, SIMes). The series comprises borenium species bearing distinct substituents, including hydrides, carbon-based groups (sp³- and sp²-hybridized), and diol-derived moieties forming five- to seven-membered rings. The Lewis acidity was quantified in terms of Gutmann's Acceptor Number (AN), revealing that most of the prepared boreniums exhibit higher acidity than the benchmark Lewis acid B(C₆F₅)₃. These borenium species were subsequently evaluated as catalysts in two model transformations-homocoupling of diazo derivatives and hydrosilylation of acetophenones. The markedly different catalytic behavior observed across the series highlights the decisive influence of the structural and electronic factors on the global Lewis acidity of the boron center and, consequently, on its catalytic performance.