Mixed Chloro-/Fluoroaryl\nBoranes as Lewis Acid Catalysts\nfor C–O Bond Cleavage/Reduction

A simplified one-pot procedure for\nthe synthesis of a\nuseful class\nof mixed chloro-/fluoroaryl boranes following the formula B(C<sub>6</sub>Cl<sub>5</sub>)<sub><i>n</i></sub>(C<sub>6</sub>F<sub>5</sub>)<sub>3–<i>n</i></sub> is reported.\nStandard organolithium and Grignard reagents are utilized, and intermediate arylzinc or arylcopper\nreagents are not required. With regard to their applications to C–O\ncleavage reactions, the increased steric bulk of having ortho-chlorine\nenhances the catalyst tolerance of water, allowing for C–O\ndeoxygenations under atmospheric conditions. As the degree of chlorination\nincreases, the reactivity of the borane catalyst decreases with B(C<sub>6</sub>Cl<sub>5</sub>)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> being\nthe most active of the boranes studied. This mono-C<sub>6</sub>Cl<sub>5</sub> borane was able to cleave or reduce the C–O bond of\nethers, carbonyls, and sugars under benchtop (open air) conditions.\nB(C<sub>6</sub>Cl<sub>5</sub>)(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> thus demonstrates a favorable balance of water tolerance and preserved\nreactivity relative to that of BCF itself.