Phenoxaphosphino‐Modified Xantphos‐Type Ligands in the Rhodium‐Catalysed Hydroformylation of Internal and Terminal Alkenes

Abstract

AbstractThe solubility of the modifying ligand is an important parameter for the efficiency of a rhodium‐catalysed hydroformylation system. A facile synthetic procedure for the preparation of well‐defined xanthene‐type ligands was developed in order to study the influence of alkyl substituents at the 2‐, and 7‐positions of the 9,9‐dimethylxanthene backbone and at the 2‐, and 8‐positions of the phenoxaphosphino moiety of ligands 1–16 on solubility in toluene and the influence of these substituents on the performance of the ligands in the rhodium‐catalysed hydroformylation. An increase in solubility from 2.3 mmol⋅L−1 to >495 mmol⋅L−1 was observed from the least soluble to the most soluble ligand. A solubility of at least 58 mmol⋅L−1 was estimated to be sufficient for a large‐scale application of these ligands in hydroformylation. Highly active and selective catalysts for the rhodium‐catalysed hydroformylation of 1‐octene and trans‐2‐octene to nonanal, and for the hydroformylation of 2‐pentene to hexanal were obtained by employing these ligands. Average rates of >1600 (mol aldehyde) × (mol Rh)−1×h−1 {conditions: p(CO/H2) = 20 bar, T = 353 K, [Rh] = 1 mM, [alkene] = 637 mM} and excellent regio‐selectivities of up to 99% toward the linear product were obtained when 1‐octene was used as substrate. For internal olefins average rates of >145 (mol aldehyde)×(mol Rh)−1×h−1 {p(CO/H2) = 3.6–10 bar, T = 393 K, [Rh] = 1 mM, [alkene] = 640–928 mM} and high regio‐selectivities up to 91% toward the linear product were obtained.