Planar Chiral Organoborane Lewis Acids Derived from Naphthylferrocene

Enantiomerically pure metalated 2-(1-naphthyl)ferrocene (NpFc) derivatives NpFcM (M=SnMe3, HgCl) were prepared and characterized by multinuclear NMR and UV/Vis spectroscopy, cyclic voltammetry, and elemental analysis. Optical rotation measurements were performed and the absolute configuration of the new planar chiral ferrocene species was confirmed by single-crystal X-ray diffraction analysis. The mercuriated species NpFcHgCl proved suitable as a reagent for the preparation of the chiral organoborane Lewis acid NpFcBCl(2), which can in turn be converted to other ferrocenylboranes by replacement of Cl with nucleophiles. The highly Lewis acidic perfluoroarylborane derivatives NpFcB(C6F5)Cl and NpFcB(C6F5)(2) were successfully prepared by treatment with CuC6F5. The structures were studied by single-crystal X-ray diffraction and variable-temperature F-19 NMR spectroscopy, which suggested that pi stacking of a C6F5 group on boron with the adjacent naphthyl group is energetically favorable. UV/Vis absorption spectroscopy and cyclic voltammetry measurements were performed to examine the electronic properties of these novel redox-active chiral Lewis acids.