A remarkable anion effect on the enantioselectivity of the Pd-catalyzed allylic amination using ferrocenyl ligands

Abstract

The effect of several anions on the enantioselectivity of the Pd-catalyzed substitution reaction of 1,3-diphenylprop-2-enylethyl carbonate 1 with benzylamine, utilizing chiral ferrocenyl ligands such as 1-[( R)-1-[( S)-2-(diphenylphosphino)-ferrocenyl]ethyl]-3-(1-adamantyl)-1 H-pyrazole 3a, was investigated. Small hard anions such as F − and BH 4 −, added in co-catalytic amounts to the catalyst, were found to enhance the ee to >99.5%. On the other hand, non-coordinating anions, e.g. PF 6 −, had a detrimental effect on the enantioselectivity (<10% ee). The beneficial effect of fluoride is discussed in terms of its establishing Curtin-Hammett conditions, by virtue of its coordination to Pd.