Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

Abstract

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Quinoline‐Based Silylium Ions: Synthesis, Structure and Lewis Acidity

“…Silylium ions have been of growing interest for several years in organic catalysis due to their exceptional Lewis acidity. [1] They have thus been used as Lewis acids for example in Diels-Alder or Mukaiyama reactions, but also to activate small molecules such as CO, H 2 or in defluorination processes. [1,2] Their synthesis has however been a long-standing problem due to their high reactivity and instability in the presence of all types of nucleophiles present in condensed phase, whether solvents or counter-anions.…”

“…[1] They have thus been used as Lewis acids for example in Diels-Alder or Mukaiyama reactions, but also to activate small molecules such as CO, H 2 or in defluorination processes. [1,2] Their synthesis has however been a long-standing problem due to their high reactivity and instability in the presence of all types of nucleophiles present in condensed phase, whether solvents or counter-anions. These electronically deficient species can be classified in different categories, depending on whether they are free (i. e. I, Figure 1), thus tricoordinated and perfectly planar, or coordinated by internal or external Lewis base donors.…”

“…The latter exhibit pyramidal geometries as shown by XRDS and can be distinguished as σ or π-stabilized silyl cations II and III respectively. [1] Free silyl ions signals in 29 Si NMR generally appear at chemical shifts higher than 200 ppm, while inter-and intramolecularly stabilized silyl ions exhibit chemical shifts in the 40-120 ppm range. Several acidity scales have appeared over time to evaluate their Lewis acidity.…”

Silylated Sulfuric Acid: Preparation of a Tris(trimethylsilyl)oxosulfonium [(Me3Si−O)3SO]+ Salt

“…Almost 50 years of silylium ion chemistry have shown that many applications for silylium ions in the field of catalysis have emerged from the pure basic research of the first decades. [1][2][3][4][5][6][7][8][9] The development of silylium ion chemistry is closely related to carbenium ion chemistry, and it is no coincidence that silicon is also called the "kissing cousin" of carbon. [2] And while we are on the subject of relationships: The [Me 3 Si] + ion (T + ) can also be understood as the "big brother" of the proton (Scheme 1).…”

“…However, protonated sulfuric acid, [H 3 SO 4 ] + , was isolated by Minkwitz et al in a super acidic system (HF/SbF 5 ) as [SbF 6 ] À salt. [40] As early as 1945, Patnode and Schmid reported on the synthesis of bis(trimethylsilyl)sulfate, T 2 SO 4 (1), which they obtained in the reaction of TCl with H 2 SO 4 (Scheme 2, Eq. 1).…”