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 alpha,alpha-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 pi-allylpalladium intermediates formed by oxidative addition of allylic ester to a palladium(0) species. The rare of syn-anti isomerization was measured by the magnetization saturation transfer in H-1 NMR. (C) 2000 Elsevier Science Ltd. All rights reserved.