Phenoxaphosphino-modiried xantphos-type Ligands in the rhodium-catalysed hydroformylation of internal and terminal alkenes

The 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 I - 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 58mmol (.) 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) x (mol Rh)(-1) x h(-1) [conditions: p(CO/H-2) = 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 I-octene was used as substrate. For internal olefins average rates of > 145 (mol aldehyde)x(mol Rh)(-1) x h(-1) {p(CO/H-2) = 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.