New phosphacyclic diphosphines for rhodium-catalyzed hydroformylation

The use of phosphacyclic diphosphines based on the xanthene backbone as lig ands in rhodium-catalyzed hydroformylation was studied. New phosphacyclic x antphos 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 moi eties on the coordination chemistry in the (diphosphine)Rh(CO)(2)H complexe s was investigated using NMR and IR spectroscopy. Both NMR and IR spectrosc opy showed that the phosphacyclic xantphos ligands exhibit an enhanced pref erence for diequatorial(ee) chelation compared to the diphenylphosphino-sub stituted parent compound. In the hydroformylation of 1-octene the introduct ion of the phosphacyclic moieties leads to higher reaction rates. More impo rtantly, the dibenzophospholyl- and phenoxaphosphino-substituted xantphos l igands exhibit an unprecedented high activity and selectivity in the hydrof ormylation 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. Th e extraordinary high activity of the phenoxaphosphino - substituted xantpho s ligand can be attributed to the 4- to 6 -fold higher rate of CO dissociat ion 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.