"Strong Lewis Acids in Catalysis“

Tricoordinate silicon cations are exceptionally strong electron pair acceptors that react, either desired or undesired, with almost any σ and π basic molecule. One way of intramolecular attenuation of the Lewis acidity of these superelectrophiles is by installation of a ferrocene unit at the electrondeficient silicon atom. The structural characterization of our ferrocene‐stabilized silicon cation now reveals an unprecedented bonding motif different from its analogs. An extreme dip angle of the silicon atom toward the iron atom is explained by two three‐center‐two‐electron (3c2e) bonds through participation of both the upper and the lower aromatic rings of the ferrocene sandwich structure. The positive charge is still localized at the silicon atom that also retains a quasi‐planar configuration. The thus‐tamed silicon cation is nevertheless a potent Lewis acid, showing unusual reactivity in Lewis acid catalysis.
Merging cooperative Si–H bond activation and electrophilic aromatic substitution paves the way for C–3‐selective indole C–H functionalization under electronic and not conventional steric control. The Si–H bond is heterolytically split by the Ru–S bond of a coordinatively unsaturated cationic ruthenium (II) complex, thereby forming a sulfur‐stabilized silicon electrophile. The Wheland intermediate of the subsequent Friedel–Crafts‐type process is assumed to be deprotonated by the sulfur atom, no added base is required.
Clarification of the Si–H bond activation step in the B(C6F5)3‐catalyzed C=O reduction is presented. A silicon‐stereogenic silane is employed as a stereochemical probe. The reduction step is a conventional borohydride reduction, and an enantioselective variant would rely on a chiral borane that is sufficiently Lewis acidic to promote the Si–H bond activation. A novel motif of an electron‐deficient chiral borane is reported, including its full spectroscopic and crystallographic characterization. Representative examples of Si–H bond activation (C=O and C=N reduction) demonstrate the chemical stability of the borane and the synthetic potential of the new chiral boron‐based Lewis acid.