A Borenium-Borane Composite for Exhaustive Reduction of Oxo-Chemicals

Borenium ions have attracted significant attention in organic transformations due to their strong Lewis acidity. The reported borenium ions are often stabilized by sterically demanding substituents and strong coordination bonds. Herein, we have synthesized a small steric borenium-equivalent NH3BH2OTf and subjected it to the exhaustive reduction of a carboxylic functional group to a methyl group, which shows broad functional group tolerance. This system can also undergo a reductive deoxygenation reaction of alcohols, ethers, and other oxo-chemicals (>100 examples). The mechanistic studies revealed that the in situ-generated NH3BH2OTf/[NH3BH2(sol)]OTf, rendering the borenium-like properties, plays a crucial role in these transformations by interacting with the O atom of substrates to activate the carbonyl group and facilitating the cleavage of the C–O bond. This work has not only offered a system for the exhaustive reduction of oxo-chemicals but is also of great significance for providing insight into the application of the borenium ions in various reactions.