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 °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.