Electronic Effects on the Synthesis, Structure, Reactivity, and Selectivity of Rhodium Hydroformylation Catalysts

Steric effects of phosphine and phosphite ligands on carbonylhydridorhodium catalysts have been considered. However, the electronic effects were not considered systematically. We studied rhodium complexes of phosphorus ligands of varying strengths of coordination via sigma donation and pi back-donation. For example, we compared complexes of Ph3P, Ph2PEt, PhPEt2, Et3P, and (EtO)3P. In general, complexes of strongly σ-donating aliphatic phosphine ligands required a high temperature for their activation by ligand dissociation and showed a high total (normal plus iso) aldehyde selectivity. In contrast, pi back-donation by triarylphosphines and particularly by phosphite esters led to a very high normal-to-iso ratio of aldehydes derived from α-olefins but also led to more undesired olefin hydrogenation and isomerization. The results can be explained with a consistent, overall framework of rhodium hydroformylation mechanisms involving complexes of different carbonylation degrees.