Silicenium ions electrophilicity

The high electrophilicity of the silicenium ion can be modified by intramolecular electron donation from remote substituents. Corriu and co-workers668 were the first to synthesize species 269 with the bidentate nitrogen ligand. Cations with other structural units and donor atoms (O, S, P) were subsequently generated.669-674 Due to the interaction called internal solvation,675 the silicon atom becomes pentacoordinated and largely loses its ionic character, that is, such ions differ in principle from the trivalent silicenium ions. 

The hydride in silanes ArSiR2H can be abstracted by the trityl cation, leaving principally a silicenium ion. This highly electrophilic species interacts with the anion associated with the trityl cation either by coordination (e.g., CIO4 ) or by halide abstraction (e.g., BF4 ) [3]. 

C. Cyclization Processes Involving Silicenium Ions as Electrophiles. 469... 

Thus in the aldol addition, just as in the case of epoxide-opening reactions, the chloride ion, formed as a necessary consequence of the mechanism of Lewis base catalysis with chlorosilanes, is not innocuous. In fact, it is a competent nucleophile that can attack an aldehyde or an epoxide activated by the Lewis base-coordinated silicenium cation in an intermolecular fashion. The desire to understand these two seemingly inconsistent results obtained in our study of the Lewis base-catalyzed reactions of trichlorosilanes presented an opportunity for the development of novel catalytic processes. For example, if a chloride ion can capture these activated electrophiles, could other exogenous nucleophiles be employed to intercept these reactive intermediates If so, a wide variety of bond-forming processes mediated by the phosphoramide-bound chiral Lewis acid [LB SiCls]" would be feasible. At this point it remained unclear if (1) an exogenous nucleophile could compete with the ion-paired chloride and (2) what kinds of nucleophiles could be compatible with the reaction conditions. 

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