Chelate Restrained Boron Cations for Intermolecular Electrophilic Arene Borylation

Abstract

Highly electrophilic boron species that borylate arenes are generated by halide abstraction from CatBX (Cat = catecholato, C 6 H 4 O 2 2−, X = Cl or Br) by [Et 3 Si][CbBr 6 ] (CbBr 6 = [ closo -1-H-CB 11 H 5 Br 6 ] − ). A transient [CatB][CbBr 6 ] 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 1 H and 31 P{ 1 H} NMR spectroscopy of the crotonaldehyde and triethylphosphine oxide adducts, respectively. Catalytic quantities of [Et 3 Si][CbBr 6 ] 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 [CbBr 6 ] − anion and the use of electrophile-resistant borane sources.