Chiral 1-Phosphabarrelene-Pyridines as Suitable Ligands for the Rh/Ir-Catalyzed Asymmetric Hydrogenation of Olefins

Herein, we report the synthesis of chiral phosphabarrelene-pyridine ligands. Their synthesis benefit from modified reaction conditions to overcome the low yields normally associated with the [4+2] cycloaddition reaction of phosphinines with hexafluoro-2-butyne, which is a key to install the P-stereocenter in the phosphabarrelene. Their potential as chelating ligands in asymmetric catalysis was assessed in the Rh- and Ir-catalyzed hydrogenation of cyclic beta-enamides and beta-dehydroamino acid derivatives. The catalytic system containing a tert-butyl substituent at the ortho position of the phosphabarrelene moiety successfully hydrogenates a range of cyclic beta-enamides (ee's between 92% to 94%) and beta-dehydroamino acid derivatives (ee's between 93% to 95%). Moreover, the reactions can be carried out in the environmentally friendly 1,2-propylene carbonate as solvent with no loss of enantioselectivity. Mechanistic studies with the Rh/P,N catalytic systems agree with the Landis-Halpern mechanism and explain the influence of the substituent at the phosphabarrelene on enantioselectivity. Finally, the hydrogenation reactions can be carried out at large scale maintaining high enantioselectivities. +image