Ligand effect in racemization and dynamic kinetic resolution of alcohols: Mechanism on cymene ruthenium complexes

• Ligand effect on cymene Ru complexes-catalyzed racemization of sec-alcohols.
• 16- and 18-electron complexes bearing a weak ligand are excellent catalysts.
• The dissociation of ligands to form vacant sites is crucial to 18-electron Ru complexes.
• The optimized Ru complexes showed efficiency in alcohols DKR.

A family of ruthenium complexes with different ligands was utilized in racemization of (R)-1-phenylethanol to investigate the potential influence of the ligands coordinated to the ruthenium center. Kinetic experiments showed that 16-electron cymene ruthenium complex with two chloro-bridge bonds and 18-electron ones with easily dissociative ligands are highly active for catalytic racemization of alcohols. Possible racemization mechanism for cymene ruthenium complexes was then proposed. Computational analysis of dissociation energy barrier, NBO analysis and reaction potential energy surface suggest that ligand-dissociation process is the vital step of the racemization catalyzed by cymene ruthenium complexes. Thereafter, these complexes were applied in the DKR of secondary alcohols to verify their efficiency and applicability.

Kinetic profiles show that 16-electron cymene ruthenium complex and 18-electron ones with easily dissociative ligands are highly active for catalytic racemazition of alcohols. Computational analyses provide the evidence of ligand-dissociation process for 18-electron complexes to transform into 16-electron ones. The optimized Ru complexes showed good efficiency in alcohols DKR.Figure optionsDownload as PowerPoint slide