Highly electrophilic boron species that borylate arenes are generated by halide abstraction from CatBX (Cat = catecholato, C₆H₄O₂²⁻, X = Cl or Br) by [Et₃Si][CbBr₆] (CbBr₆ = [closo-1-H-CB₁₁H₅Br₆]⁻). A transient [CatB][CbBr₆] related species reacts as a synthetic equivalent of [CatB]⁺ in intermolecular electrophilic borylation, with reactions proceeding rapidly at 25 °C. The [CatB]⁺ moiety was shown to be strongly Lewis acidic on the basis of ¹H and ³¹P{¹H} NMR spectroscopy of the crotonaldehyde and triethylphosphine oxide adducts, respectively. Catalytic quantities of [Et₃Si][CbBr₆] and CatBX were effective for the high-yielding borylation of arenes by CatBH in a highly atom efficient cycle with H₂ the only byproduct. Successful catalysis was dependent on the robust [CbBr₆]⁻ anion and the use of electrophile-resistant borane sources.