The solubility of the modifying ligand is an important parameter for the efficiency of a rhodiumcatalysed 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 À 1 to > 495 mmol ¥ L À 1 was observed from the least soluble to the most soluble ligand. A solubility of at least 58 mmol ¥ L À 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) Â (mol Rh)¼ 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) Â (mol Rh)À 1 Â h À 1 {p(CO/ H 2 ) ¼ 3.6 ± 10 bar, T ¼ 393 K, [Rh] ¼ 1 mM, [alkene] ¼ 640 ± 928 mM} and high regio-selectivities up to 91% toward the linear product were obtained.

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