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

Abstract

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 β‐enamides and β‐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 β‐enamides (ee's between 92% to 94%) and β‐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.