The Power of the Proton: From Superacidic Media to Superelectrophile Catalysis

Abstract

Superacidic media became famous in connection with carbocations. Yet not all reactive intermediates can be generated, characterized, and eventually isolated from these Brønsted acid/Lewis acid cocktails. The counteranion, that is the conjugate base, in these systems is often too nucleophilic and/or engages in redox chemistry with the newly formed cation. The Brønsted acidity, especially superacidity, is in fact often not even crucial unless protonation of extremely weak bases needs to be achieved. Instead, it is the chemical robustness of the aforementioned counteranion that determines the success of the protolysis. The advent of molecular Brønsted superacids derived from weakly coordinating, redox-inactive counteranions that do withstand the enormous reactivity of superelectrophiles such as silicon cations completely changed the whole field. This Perspective summarizes general aspects of medium and molecular Brønsted acidity and shows how applications of molecular Brønsted superacids have advanced from stoichiometric reactions to catalytic processes involving protons and in situ generated superelectrophiles.

Silylium-Ion Regeneration by Protodesilylation Enables Friedel–Crafts Alkylation with Less Isomerization and No Defunctionalization

“…The found reaction performance as a function of the nucleofuge (allyl ≈ Ph > H ≫ Me) is in accordance with the generally observed ease of Si−Nu bond protolysis. 14,19 Variation of the leaving group X in the alkyl halide under the optimized conditions for X = Br (entry 12) revealed that the method is not limited to alkyl bromides and all other alkyl halides also participate in the reaction (entries 17−19). A competition experiment demonstrated the selective conversion of alkyl fluoride 1a in the presence of alkyl bromide 1c.…”

“…The allylsilane 6g performed equally well as phenylsilane 6a (entry 15), whereas tetramethylsilane ( 6h ) was less efficient (entry 16). Although Me 4 Si is known to undergo protolysis in the presence of strong acids, this is an elusive example where this fully alkylated silane is intimately involved in a catalytic process. The found reaction performance as a function of the nucleofuge (allyl ≈ Ph > H ≫ Me) is in accordance with the generally observed ease of Si–Nu bond protolysis. , Variation of the leaving group X in the alkyl halide under the optimized conditions for X = Br (entry 12) revealed that the method is not limited to alkyl bromides and all other alkyl halides also participate in the reaction (entries 17–19).…”

“…As part of our ongoing research program directed toward the chemistry of silylium ions (R 3 Si + ), we aimed to systematically study the Friedel–Crafts alkylation using preformed counteranion-stabilized silylium ions of type [R 3 Si­(HCB 11 H 5 Br 6 )] as catalytic promotors (Scheme , bottom). By means of the protolysis approach to (re)­generate a silylium ion from a suitable tetraorganosilane R 3 Si–Nu (Nu = nucleofuge), we envisioned to circumvent the use of hydrosilanes as silylium ion precursors. This strategy was successfully applied by Siegel and co-workers to the intramolecular C­(sp 2 )–C­(sp 2 ) coupling reaction of aryl fluorides and later elaborated by our laboratory as a general route to counteranion-stabilized silylium ions …”

Isolable fluorinated triphenylmethyl cation salts of [HCB11Cl11]−: demonstration of remarkable hydride affinity

“…The extent of H-D exchange was analyzed via 1 H, 13 C, or 19 F 46 NMR spectroscopy (see ESI † for details). The mechanism of the H/D exchange likely involves the generation of superacidic protonated arenes in situ, 47 which should enable rapid H/D exchange via H + /D + shuttling (Fig. S4 †).…”

“…47,48 The product of addition of either Et 3 Si + or F 6 Tr + to a neutral arene can be alternatively viewed as a protonated arene. 47 It is also possible that analogous cations are accessed via reactions involving the minor components of the mixture. The Oestreich group recently examined this type of H/D exchange catalysis in greater detail.…”