New phosphacyclic diphosphines for rhodium-catalyzed hydroformylation

The use of phosphacyclic diphosphines based on the xanthene backbone as ligands in rhodium-catalyzed hydroformylation was studied. New phosphacyclic xantphos ligands with wide natural bite angles were synthesized, and a short, efficient route toward the synthesis of 10-chlorophenoxaphosphine and 10-chlorophenothiaphosphine was developed. The effect of the phosphacyclic moieties on the coordination chemistry in the (diphosphine)Rh(CO)(2)H complexes was investigated using NMR and IR spectroscopy. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial(ee) chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. More importantly, the dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit an unprecedented high activity and selectivity in the hydroformylation of trans 2- and 4-octene to linear nonanal. The high activities of the phosphacyclic xantphos ligands are explained by the lower phosphine basicity and the wider natural bite angles of the phosphacyclic ligands. The extraordinary high activity of the phenoxaphosphino - substituted xantphos ligand can be attributed to the 4- to 6 -fold higher rate of CO dissociation compared to the other xantphos ligands. CO dissociation rates from the (diphosphine)Rh(Co)(2)H complexes were determined using (CO)-C-13 labeling in rapid-scan IR experiments.