OH-Pd(0) interaction as a stabilizing factor in palladium-catalyzed allylic alkylations

Area of Science:

• Organometallic Chemistry
• Catalysis
• Stereochemistry

Background:

• Palladium-catalyzed allylic alkylations are crucial C-C bond-forming reactions.
• Oxazoline ligands are widely used in asymmetric catalysis.
• Understanding ligand conformation is key to controlling enantioselectivity.

Purpose of the Study:

• To investigate the role of hydroxymethyl substituents on oxazoline ligands in palladium-catalyzed allylic alkylations.
• To elucidate the mechanism by which ligand conformation affects enantioselectivity.
• To explore the influence of hydrogen bonding on catalytic stereochemistry.

Main Methods:

• Density functional theory (DFT) computations were employed to model catalytic cycles.
• Analysis of ligand conformations and transition states.
• Comparison with O-alkylated analogs to isolate the effect of the hydroxymethyl group.

Main Results:

• Catalysts with 4-hydroxymethyl oxazoline ligands undergo a conformational change upon nucleophilic attack, involving Pd(II) to Pd(0) reduction.
• The conformation of Pd(0) complexes is stabilized by intramolecular hydrogen bonding, with the metal center as the acceptor.
• This conformational change, absent in O-alkylated analogs, significantly impacts the enantioselectivity of the alkylation reaction.

Conclusions:

• A previously uninvestigated hydrogen bonding interaction governs catalyst conformation and stereochemical outcome.
• The hydroxymethyl substituent plays a critical role in enabling this hydrogen bond and subsequent conformational change.
• This finding offers new insights into catalyst design for enhanced enantioselectivity in palladium-catalyzed reactions.