Silylium Ion Catalysis in Organic Transformations

Silylium ions, formally R₃Si⁺ species paired with weakly coordinating anions, have emerged as exceptionally powerful electrophilic catalysts in a diverse array of organic transformations. Generated by hydride or halide abstraction from silanes, these cationic silicon centres display Lewis acidities comparable to or exceeding those of traditional main-group Lewis acids. Their unique ability to stabilise adjacent carbocationic intermediates via β-silicon effects underpins a range of highly selective bond-forming processes, including hydrosilylation of unsaturated substrates, intramolecular and intermolecular annulations, C–H and N–H silylations, thiosilylations and Prins-type desymmetrisation reactions. Key mechanistic features involve rapid self-regeneration of the silylium-ion catalyst, often through relay processes mediated by bissilylated sulfonium or oxocarbenium ions, and intramolecular hydride shifts that drive rearrangements. The metal-free nature of this catalysis, combined with operational simplicity and high functional-group tolerance, positions silylium ion methodologies as attractive alternatives for the synthesis of fine chemicals, pharmaceuticals and silicon-containing materials. Recent advances have extended the scope to include remote α-C–H functionalisation, orthogonal dual-bond construction and cascade cyclisations, highlighting the global significance of silylium-ion chemistry for sustainable and selective organic synthesis.

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Silylium Ion Catalysis in Organic Transformations publication trend

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Technical terms

Silylium ion: A tricoordinate silicon cation (R₃Si⁺) paired with a weakly coordinating anion, acting as a potent Lewis acid.

β-Silicon-stabilised carbocation: A carbocation adjacent to a silicon centre, stabilised through hyperconjugation with Si–C bonds.

Electrophilic activation: Enhancement of a reagent’s electrophilicity, often by coordination to a Lewis acid to facilitate bond formation.

Self-regeneration: Mechanistic feature whereby the active silylium-ion catalyst is continually reformed during a catalytic cycle.

References

1. Electrophilic Activation of S−Si Reagents by Silylium Ions for Their Regio‐ and Diastereoselective Addition Across C−C Multiple Bonds. Angewandte Chemie International Edition (2024).
2. Intramolecular Friedel–Crafts alkylation with a silylium-ion-activated cyclopropyl group: formation of tricyclic ring systems from benzyl-substituted vinylcyclopropanes and hydrosilanes. Chemical Science (2021).
3. Silylium-Ion-Promoted (3 + 2) Annulation of Allenylsilanes with Internal Alkynes Involving a Pentadienyl-to-Allyl Cation Electrocyclization. Journal of the American Chemical Society (2024).