Palladium-Catalyzed Asymmetric Reduction of Racemic Allylic Esters with Formic Acid: Effects of Phosphine Ligands on Isomerization of π-Allylpalladium Intermediates and Enantioselectivity

A new MOP ligand (1b), (R)-(+)-2-(bis(3-trifluoromethylphenyl)phosphino)-2′-methoxy-1,1′-binaphthyl, was found to be more enantioselective than other MOP ligands for the palladium-catalyzed asymmetric reduction of α,α-disubstituted allylic esters with formic acid. The reduction of dl-2-(1-naphthyl)-3-buten-2-yl benzoate gave 3-(1-naphthyl)-1-butene of 90% ee. The higher enantioselectivity of 1b is ascribed to fast syn–anti isomerization of π-allylpalladium intermediates formed by oxidative addition of allylic ester to a palladium(0) species. The rate of syn–anti isomerization was measured by the magnetization saturation transfer in 1H NMR.