Synthesis and Lewis acid properties of a ferrocene-based planar-chiral borenium cation

The first planar chiral ferrocenylborenium species (pR)-3(+) is obtained in the enantiopure form by halide abstraction from the corresponding chloroborane adduct (pR)-2 using Krossing's salt. Competition experiments suggest that the Lewis acidity of (pR)-3(+) is higher than that of B(C6F5)(3) towards anions and slightly lower towards neutral Lewis bases. The ferrocenylborenium species (pR)-3(+) is examined as a catalyst for the stereoselective hydrosilylation of ketones.