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

Identifier:  imarina:9333967

Handle: https://hdl.handle.net/20.500.11797/imarina9333967

Authors:  Saltó, J; Tarr, D; Petrov, A; Reich, D; Weber, M; Biosca, M; Pamies, O; Müller, C; Diéguez, M

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.