Chiral Carborane Acids Decorated with Binol-Based Phosphonates: Synthesis, Characterization, and Application.

The syntheses of hexabrominated closo -carborates decorated with different chiral Binol-derived phosphonates and their conjugate acids are described. X-ray diffraction analysis reveals a polymeric structure for the sodium salt with the anionic units connected by [B-Br-Na-O═P] ⁺ linkages. For the acid, coordination of the proton to the phosphonate's P═O oxygen atom is assumed. The p K _a value was estimated by combining experiments and computations. Application of these Brønsted acids as chiral catalysts in an imino-ene and a Mukaiyama-Mannich reaction was moderately successful.