Sidearm modified phosphine ligands for Rh complex-catalyzed hydroformylation: Mechanistic pathway and structure-activity relationship

Development of high-performance phosphine ligands is an effectual strategy to improve the homogeneous hydroformylation reaction. This study designed a series of amide/ester sidearms-modified phosphine ligands with different characteristics (e.g., proton affinity, steric hindrance) for homogeneous Rh-complex in hydroformylation of formaldehyde. The sidearms-modified ligands with the stronger proton affinity serve to transfer proton from the hydrido rhodium species to the activated formaldehyde via the sidearms to generate the critical hydroxymethyl rhodium species that favours the hydroformylation to glycolaldehyde, yielding significantly improved reaction rates (twice as much as PPh₃). The bulky sidearm with larger steric hindrance can stretch the hydrogen bond between the product and the sidearm, suppressing the by-product production and improving the target selectivity. A potential reaction mechanism involving sidearm-induced deprotonation and inner-molecule proton transfer was proposed for the sidearm-modified phosphine ligands coordinated Rh complex based on the DFT calculation and experimental study. This study can trigger the innovative phosphine ligand design with special functional sidearms for hydroformylation.