Rhodium complexes based on phosphorus diamide ligands: Catalyst structure versus activity and selectivity in the hydroformylation of alkenes

The rhodium-catalyzed hydroformylation reaction of 1-octene with phosphorus diamide ligands has been investigated. Four monodentate phosphorus diamide ligands and six bidentate phosphorus diamide ligands derived from a 1,3,5-trisubstituted biuret structure have been synthesized. These types of ligands combine steric bulk with,pi-acidity. The rhodium complexes formed under CO/H-2 have been characterized by high-pressure spectroscopic techniques. Spectroscopic experiments revealed that the monodentate ligands form mixtures of HRhL2(CO)(2) and HRhL(CO)(3). The ratio HRhL2(CO)(2):HRhL(CO)(3) depends on the ligand concentration and its bulkiness. The bidentate ligands form stable, well-defined catalysts with the structure HRhL boolean AND L(CO)(2) under hydroformylation conditions. Both monodentate and bidentate ligands have been tested in the hydroformylation reaction of 1-octene. The monodentate ligands form very active catalysts, but the linear-to-branched ratio of the product is low. The bidentate ligands show improved selectivity compared to the monodentate ligands and an activity that is only slightly lower.