Chelate Restrained Boron Cations for Intermolecular Electrophilic Arene Borylation

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 degrees C. The [CatB](+) moiety was shown to be strongly Lewis acidic on the basis of H-1 and P-31{H-1} 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 H-2 the only byproduct. Successful catalysis was dependent on the robust [CbBr6](-) anion and the use of electrophile-resistant borane sources,