Silylium ions in catalysis.

The controversy over "how free the trivalent silicon cation is" placed the emphasis on their spectroscopic and crystallographic characterisation. Those investigations contributed substantially to the understanding of the role of σ or π Lewis basic molecules in the chemistry of extremely potent Lewis acids, culminating in the discovery of innocent, that is non-coordinating, anions and an X-ray molecular structure. Synthetic uses of such reactive intermediates are however underrepresented yet a few recent contributions offer a glimpse of their true potential. This perspective provides a concise overview of known silylium ions, followed by examples of their uses as catalysts in C-F bond activation and C-C bond formation, namely challenging Diels-Alder reactions.