25 / 2023-07-21 22:36:09

Bifunctional phosphine/phosphoric acids chiral ligands for enantioselective catalysis via Tethered Counterion-Directed Catalysis

enantioselective catalysis,bifunctional phosphorus,gold catalysis

Phosphorus in Synthetic Chemistry > 2) Phosphorus compounds as ligands

The access to enantioenriched molecules via selective catalytic methods is of utmost importance. The ligand-based approach (Fig 1a) to enantioselective catalysis often relies on the use of either functionalized or very bulky ligands to trigger geometrical constraints and steric repulsions. The alternative Asymmetric Counteranion-Directed Catalysis (ACDC) approach^1a relies on the use of chiral counteranions as the source of stereochemical information during the key bond-forming step (Fig 1b). The strong pairing between the cationic metal-substrate complex and the chiral anion is hence central to the transfer of the chiral information. This concept,^1a pioneered in Au(I)^1c and Pd(0)^1a catalysis, was rapidly applied to other transition metals. However, 15 years later, the field did not bloom as much as expected and examples remain sporadic. Why ACDC has met little success probably results from the flexible and poorly defined spatial arrangements of the chiral phosphate-cation pairs (Fig 1b). In this context, we have established (Fig1c) that tethering the chiral anion to the metal center could be a suitable strategy to increase geometrical constraints and molecular organization in the intermediates involved in catalytic processes and therefore generate higher enantioselectivities. This strategy, named Tethered Counterion-Directed Catalysis (TCDC) relies on the use of CPA-Phos chiral ligands that are phosphine/phosphoric acid new chiral bifunctional ligands.^2a The corresponding Au(I) complexes have been prepared and used successfully as catalysts in a number of enantioselective reactions. The synthesis, coordination and catalytic applications of these chiral ligands will be discussed.²