Highly Enantioselective Rhodium-Catalyzed Hydrogenation with Monodentate Ligands

Abstract

The homogeneous enantioselective hydrogenation of functionalized prochiral olefins is one of the most frequently studied and most efficient transition metal-catalyzed reactions. In the first reports using chiral Wilkinson type catalysts, low enantiomeric excesses (ee’s) were obtained using monodentate phosphines as ligands. All attempts to develop monodentate ligands which would afford high ee’s in this reaction met with limited success, the best result being reached with CAMP, already published in 1972, giving ee’s up to 90% in the hydrogenation of dehydroamino acids. Although new monodentate phosphorus ligands play a significant role in other transition metal-catalyzed reactions, highly enantioselective hydrogenations are exclusively based on bidentate phosphorus ligands. Starting with Kagan’s diop a large number of bidentate ligands with excellent selectivities was designed. Among the most successful are DIPAMP, which gives superior results compared to its monodentate analogue PAMP, the frequently used BINAP ligand, the ferrocenyl-based ligands and the DuPHOS, BPE, and FerroTANE ligands, the latter showing extremely high enantioselectivities and broad scope. To date, not only phosphines (phosphanes) are used but also bidentate aminophosphines, phosphites, phosphinites, phosphonites, and hybrid ligands such as phosphine-phosphite, aminophosphine-phosphinite, and phosphine-phosphonite,