Enantioselective acylation using a second-generation P-aryl-2-phosphabicyclo[3.3.0]octane catalyst

Area of Science:

• Organic Chemistry
• Asymmetric Catalysis

Background:

• Phosphabicycloalkanes are valuable chiral ligands in asymmetric catalysis.
• Developing stable and easily handled precursors for these ligands is crucial for broader application.
• Air-stable phosphine salts offer advantages over sensitive free phosphines.

Purpose of the Study:

• To describe the synthesis of novel P-aryl-2-phosphabicyclo[3.3.0]octane tetrafluoroborate salts.
• To evaluate the utility of these salts as precursors for phosphine ligands in kinetic resolution.
• To assess the cost-effectiveness and convenience of the synthetic route.

Main Methods:

• Synthesis of P-aryl-2-phosphabicyclo[3.3.0]octane tetrafluoroborate salts (3a and 3c).
• In situ deprotonation of the salts using triethylamine to generate active phosphine ligands (1a and 1c).
• Application of the generated phosphines in the kinetic resolution of secondary alcohols.

Main Results:

• Successful synthesis of air-stable phosphabicycloalkane tetrafluoroborate salts.
• Demonstration of in situ generation of active phosphine ligands from the salts.
• Achieved high enantioselectivities in the kinetic resolution of secondary alcohols, comparable to free phosphines for certain substrates (s ≈ 40 or lower).
• Incorporation of a specific aryl group (3,5-di-tert-butyl-4-methoxyphenyl) enabled the use of a more economical starting material.

Conclusions:

• The synthesized P-aryl-2-phosphabicyclo[3.3.0]octane tetrafluoroborate salts are convenient and stable precursors for chiral phosphine ligands.
• The in situ generation method is effective for small-scale kinetic resolution experiments.
• This approach provides a practical alternative to using free phosphines, especially when cost and handling are considerations.