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

Area of Science:

• Organometallic Chemistry
• Catalysis
• Supramolecular Chemistry

Background:

• Development of novel chiral catalysts is crucial for asymmetric synthesis.
• Carboranes offer unique structural and electronic properties for functionalization.
• Chiral phosphonates are valuable ligands and catalytic moieties.

Purpose of the Study:

• To synthesize novel hexabrominated closo-carborates functionalized with chiral Binol-derived phosphonates.
• To characterize the structure and properties of these carborate derivatives and their conjugate acids.
• To evaluate their potential as chiral Brønsted acid catalysts in asymmetric reactions.

Main Methods:

• Synthesis of hexabrominated closo-carborates.
• Decoration with chiral Binol-derived phosphonates.
• X-ray diffraction analysis for structural determination.
• Estimation of pKa values using combined experimental and computational methods.
• Application as catalysts in imino-ene and Mukaiyama-Mannich reactions.

Main Results:

• Successful synthesis of chiral phosphonate-decorated hexabrominated closo-carborates and their conjugate acids.
• X-ray diffraction revealed a polymeric structure for the sodium salt linked by [B-Br-Na-O═P]+ interactions.
• Proton coordination to the phosphonate P═O oxygen was inferred for the acidic form.
• pKa values were estimated through a combination of experimental and computational approaches.
• Moderate catalytic success was observed in imino-ene and Mukaiyama-Mannich reactions.

Conclusions:

• Novel chiral phosphonate-functionalized carboranes were successfully synthesized and structurally characterized.
• These compounds exhibit Brønsted acidity and can be evaluated as chiral catalysts.
• The developed carboranes show promise for applications in asymmetric catalysis, particularly in C-C bond formation.