Synthesis of Norbornene-Based Phosphine-Stabilized Silylium Ions Behaving as Masked Frustrated Lewis Pairs

A novel architecture of phosphine-stabilized silylium ions is described. The peculiar moderate flexible framework allows an efficient donation of the phosphine group toward the electron-deficient silicon center, thus providing a very good stabilization of the corresponding silylium ions. The P–Si interaction was studied in solution and in the solid state by NMR spectroscopy and X-ray diffraction analysis, respectively. The reactivity of silylium ions is easily improved using less donating phosphine ligands. Of particular interest, thanks to the flexible ligand framework, the phosphine-stabilized silylium ion 5f behaves as a frustrated Lewis pair instead of a simple Lewis acid and exhibits an original ambiphilic reactivity with carbonyl derivatives that allows the synthesis of seven- to nine-membered-ring heterocycles.