Mixed Chloro-/Fluoroaryl Boranes as Lewis Acid Catalysts for C–O Bond Cleavage/Reduction

A simplified one-pot procedure for the synthesis of a useful class of mixed chloro-/fluoroaryl boranes following the formula B(C₆Cl₅)_n(C₆F₅)_3–n is reported. Standard organolithium and Grignard reagents are utilized, and intermediate arylzinc or arylcopper reagents are not required. With regard to their applications to C–O cleavage reactions, the increased steric bulk of having ortho-chlorine enhances the catalyst tolerance of water, allowing for C–O deoxygenations under atmospheric conditions. As the degree of chlorination increases, the reactivity of the borane catalyst decreases with B(C₆Cl₅)(C₆F₅)₂ being the most active of the boranes studied. This mono-C₆Cl₅ borane was able to cleave or reduce the C–O bond of ethers, carbonyls, and sugars under benchtop (open air) conditions. B(C₆Cl₅)(C₆F₅)₂ thus demonstrates a favorable balance of water tolerance and preserved reactivity relative to that of BCF itself.