Ligand hydrogenation during hydroformylation catalysis detected by in-situ high-pressure infra-red spectroscopic analysis of a rhodium/phospholene-phosphite catalyst

Abstract

Phospholane-phosphites are known to give highly unusual selectivity’s towards branched aldehydes in the hydroformylation of terminal alkenes. This paper describes the synthesis of hitherto unknown unsaturated phospholene-borane precursors and their conversion to the corresponding phospholene-phosphites. The relative stereochemistry of one of these ligands, and it’s Pd complex was assigned with the aid of X-ray crystal structure determinations. These ligands were able to approach the level of selectivity observed for phospholane-phosphites in the rhodium catalysed hydroformylation of propene. High-Pressure-Infra-Red (HPIR) spectroscopic monitoring of the catalyst formation revealed that whilst the catalysts show good thermal stability with respect to fragmentation, the C=C bond in the phospholene is slowly hydrogenated in the presence of rhodium and syngas. The ability of this spectroscopic tool to detect even subtle changes in structure, remote from the carbonyl ligands underlines the usefulness of HPIR spectroscopy in hydroformylation catalyst development.