Steric and electronic ligand perturbations in catalysis: Asymmetric allylic substitution reactions using C-2-symmetrical phosphorus-chiral (Bi)ferrocenyl donors

Three series of P-chiral diphosphines based on ferrocene (1a-f, 2a-c) and b iferrocenyl skeletons (3a-c), including novel ligands If and Sc, were emplo yed in palladium-catalyzed allylic substitution reactions. Steric effects i mposed by the phosphine residues were studied using C-2-symmetrical donors 1 (1 = 1, 1'-bis(arylphenylphosphino)ferrocene with aryl groups a = l-napht hyl, b = 2-naphthyl, c = 2-anisyl, d = 2-biphenylyl, e = 9-phenanthryl, and f = ferrocenyl), whereas paramethoxy- and/or para-trifluoromethyl substitu tion of the phenyl moieties in la enabled investigation of ligand electroni c effects applying ferrocenyl diphosphines 2a-c. Ligands 3 (3 = 2,2'-bis(ar ylphenylphosphino)-1,1'-biferrocenyls with aryl substituents a,e = 1-naphth yl (diastereomers) and b = 2-biphenylyl) allowed for comparison of backbone structure effects (bite angle variation) in catalysis. Linear and cyclic a llylic acetates served as substrates in typical test reactions; upon attack of soft carbon and nitrogen nucleophiles on (E)-1,3-diphenylprop-2-ene-1-y l acetate the respective malonate, amine, or imide products were obtained i n enantioselectivities of up to 99% ee. A crystal structure analysis of a p alladium 1,3-diphenyl-eta (3)-allyl complex incorporating ligand (S,S)-1a r evealed a marked distortion of the allyl fragment, herewith defining the re gioselectivity of nucleophile addition.