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.

Asymmetric allylic alkylation mediated by transition metals provides an efficient strategy to form quaternary stereogenic centers. While this transformation is dominated by the use of second- and third-row transition metals (e.g., Pd, Rh, and Ir), recent developments have revealed the potential of first-row transition metals, which provide not only a less expensive and potentially equally efficient alternative but also new mechanistic possibilities. This review summarizes examples for the assembly of quaternary stereocenters using prochiral allylic substrates and hard, achiral nucleophiles in the presence of copper complexes and highlights the complementary approaches with soft, prochiral nucleophiles catalyzed by chiral cobalt and nickel complexes.

An axially chiral, cyclic borane decorated with just one C6F5 group at the boron atom promotes the highly enantioselective hydrosilylation of acetophenone derivatives without assistance of an additional Lewis base (up to 99% ee). The reaction is an unprecedented asymmetric variant of Piers' B(C6F5)3-catalyzed carbonyl hydrosilylation. The steric congestion imparted by the 3,3'-disubstituted binaphthyl backbone of the borane catalyst as well as the use of reactive trihydrosilanes as reducing agents are keys to success.

The highly electrophilic fluorophosphonium cation [(C 6 F 5 ) 3 PF] + [B(C 6 F 5 ) 4 ] − is shown to catalyze Diels− Alder reactions of challenging dienophile/enophile combinations and Nazarov cyclizations of various precursors. Several other electrophilic phosphonium cations (EPCs) have been tested for comparison. This systematic study demonstrates the power of these Lewis acids to act as catalysts for synthetically useful pericyclic reactions.

Axially chiral [(C6F5)3PF]­[B­(C6F5)4] analogues based on dihydrophosphepines with a binaphthyl backbone were prepared and structurally characterized by X-ray diffraction analysis. Computational calculations of FIA and GEI values attest that these new fluorophosphonium cations have a higher Lewis acidity compared to the ubiquitous B­(C6F5)3. Furthermore, application of these highly electrophilic compounds in the catalytic hydrosilylation of ketones and an investigation of the mechanism lead to a refined picture of the role of highly electrophilic fluorophosphonium cations.

A chiral variant of B(C F ) with a 3,3'-disubstituted binaphthyl backbone is shown to catalyze Nazarov cyclizations with high levels of enantio- and diastereocontrol. The parent B(C F ) also promotes these ring closures efficiently. This electrocyclization is another example of the still small family of C-C bond formations mediated by B(C F ) as the catalyst.

Herein, we report a multistep one-pot reaction of substituted pyridines leading to N-protected tetrahydropyridines with outstanding enantioselectivity (up to 97% ee). An iridium(I)-catalyzed dearomative 1,2-hydrosilylation of pyridines enables the use of N-silyl enamines as a new type of nucleophile in a subsequent palladium-catalyzed asymmetric allylic alkylation. This telescoped process overcomes the intrinsic nucleophilic selectivity of pyridines to synthesize enantioenriched, C-3-substituted tetrahydropyridine products that have been otherwise challenging to access.

Es wird gezeigt, dass eine chirale Variante von B(C 6 F 5 ) 3 mit einem 3,3'-disubstituierten Binaphthylrückgrat Nazarov-Cyclisierungen mit hoher Enantio-und Diastereokontrolle katalysiert. Die Stammverbindung B(C 6 F 5 ) 3 vermittelt diese Ringschlüsse ebenfalls effizient. Diese Elektrocyclisierung ist ein weiteres Beispiel einer noch immer kleinen Gruppe an C-C-Bindungsknüpfungen, die von B(C 6 F 5 ) 3 als Katalysator bewerkstelligt werden.Schema 3. Grenzen der von (S)-3·DMS katalysierten, enantioselektiven Nazarov-Cyclisierung( cis:trans-Verhältnisse vor der Aufreinigung 1 H-NMR-spektroskopisch bestimmt;Gesamtausbeuten nach Aufreinigung mittels FlashchromatographieanKieselgel bestimmt;E nantiomerenüberschüsse mittels HPLC-Analyse an chiralen stationärenP hasen bestimmt).