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.