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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 SiCbBr 6 (CbBr 6 = closo -1-H-CB 11 H 5 Br 6 −). A transient CatBCbBr 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 SiCbBr 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.
Grosso et al. (Tue,) studied this question.