Mechanistic Studies of the Hydroformylation of 1-Alkenes Using a Monodentate Phosphorus Diamide Ligand

Abstract

The mechanism of the rhodium-catalyzed hydroformylation reaction using a monodentate phosphorus diamide ligand has been investigated. The system presents an ideal case to illustrate the basics of hydroformylation. A detailed kinetic study and (in situ) spectroscopic techniques revealed that several of the elementary reaction steps are involved in the hydroformylation rate control. Which step is rate-determining depends strongly on the conditions used. Deuterioformylation showed that alkene coordination followed by hydride migration is irreversible under the conditions studied. The rhodium hydride complex HRhL2(CO)2 and several rhodium−acyl complexes were observed during the hydroformylation reaction. The structures of the rhodium−acyl complexes have been characterized using 31P, 13C, and 103Rh NMR spectroscopy. The major rhodium−acyl complex formed, RC(O)RhL2(CO)2, has a trigonal-bipyramidal structure with the two phosphorus ligands coordinated in the equatorial plane. The exchange rates of the equatorial and apical carbonyl ligands with dissolved carbon monoxide differ significantly, the equatorial carbon monoxide being much more labile.