1-phosphino-2-sulfenylferrocenes as planar chiral ligands in enantioselective palladium-catalyzed allylic substitutions

The synthesis of a wide structural variety of enantiopure 1-phosphino-2-sulfenylferrocene ligands 1 possessing exclusively planar chirality is described. In the case of the readily available tert-butylsulfenyl derivatives very high enantioselectivities were obtained in the palladium-catalyzed allylic substitution of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate (ee's up to 97%) and nitrogen nucleophiles (ee's up to 99.5%). Palladium complexes of these ferrocenes were characterized by NMR and X-ray diffraction, revealing the P,S-bidentate character of the ligands 1 and the formation: of a single epimer on the stereogenic sulfur atom resulting from the complexation with palladium. A model justifying the observed asymmetric induction exerted by this novel family of chiral ferrocenes, supported by solution NMR studies on a palladium allylic complex, is discussed.