Cleavage of Unactivated Si−C(sp3) Bonds with Reed's Carborane Acids: Formation of Known and Unknown Silylium Ions

Silylium-Ion-Promoted (3 + 2) Annulation of Allenylsilanes with Internal Alkynes Involving a Pentadienyl-to-Allyl Cation Electrocyclization

A (3 + 2) annulation of allenyl- and, after rapid isomerization, propargylsilanes with internal 1-aryl-1-alkynes to form 4-methylenecyclopentenes is reported. The reaction is initiated by a silylium ion, and the catalytic cycle is subsequently maintained by the self-regeneration of the silylium-ion promoter. Unlike the well-established Danheiser annulation, where the allenylsilane serves as a three-carbon synthon, the present transformation engages the allenylsilane as a two-carbon synthon. Experimental observations and DFT calculations unveil a reaction cascade involving various β-silicon-stabilized carbocations, where a pentadienyl-to-allyl cation electrocyclization is the key step.

Arenium-ion-catalysed halodealkylation of fully alkylated silanes

'Organic silicon' is not found in nature but modern chemistry is hard to imagine without silicon bound to carbon. Although silicon-containing commodity chemicals such as those emerging from the 'direct process'1-4 look simple, it is not trivial to selectively prepare aryl-substituted and alkyl-substituted (functionalized) silicon compounds, known as silanes. Chlorosilanes such as Me4-nSiCln (n = 1-3) as well as SiCl4 (n = 4) are common starting points for the synthesis of silicon-containing molecules. Yet these methods often suffer from challenging separation problems5. Conversely, silanes with four alkyl groups are considered synthetic dead ends. Here we introduce an arenium-ion-catalysed halodealkylation that effectively converts Me4Si and related quaternary silanes into a diverse range of functionalized derivatives. The reaction uses an alkyl halide and an arene (co)solvent: the alkyl halide is the halide source that eventually engages in a Friedel-Crafts alkylation with the arene to regenerate the catalyst6, whereas the arenium ion acts as a strong Brønsted acid for the protodealkylation step7. The advantage of the top-down halodealkylation methodology over reported bottom-up procedures is demonstrated, for example, in the synthesis of a silicon drug precursor. Moreover, chemoselective chlorodemethylation of the rather inert Me3Si group attached to an alkyl chain followed by oxidative degradation is shown to be an entry into Tamao-Fleming-type alcohol formation8,9.

Intramolecular 7-endo-dig-Selective Carbosilylation of Internal Alkynes Involving Silylium-Ion Regeneration

A catalytic silylium-ion-promoted intramolecular alkyne carbosilylation reaction is reported. The ring closure is initiated by electrophilic activation of the C-C triple bond by a silylium ion, and the catalytic cycle is then maintained by the protodesilylation of a stoichiometrically added allylsilane reagent. Exclusive 7-endo-dig selectivity is seen, leading to a series of silylated benzocycloheptene derivatives with a fully substituted vinylsilane. Control experiments showed that the catalytically active silylium ion can also be regenerated by protodesilylation of the vinylsilane product.

Catalytically Generated Meerwein's Salt-Type Oxonium Ions for Friedel-Crafts C(sp2)-H Methylation with Methanol

A catalytic protocol for a Friedel-Crafts-type direct C(sp²)-H methylation of various arenes with methanol is disclosed. The reaction is initiated by counteranion-stabilized silylium or arenium ions, which form Meerwein's salt-like oxonium ions with methanol as the active methylating agents. The silylated methyloxonium ions are stronger electrophiles than their protonated congeners, allowing the Friedel-Crafts alkylation to proceed more efficiently and at a lower reaction temperature. The regeneration of these superelectrophiles within the catalytic cycle is accomplished by the addition of a tetraorganosilane additive, i.e., trimethyl(phenyl)silane or tetraethylsilane, that releases a silylium ion through protodesilylation by the Brønsted acidic Wheland intermediate, thereby acting as a productive "proton-into-silylium ion" generator. By this method, even the C-H methylation of electronically deactivated aryl halides with methanol is achieved. The protocol is also applicable to nonactivated primary as well as π-activated benzylic alcohols. Dialkyl ethers are also competent alkylating agents in the presence of the quaternary phenylsilane additive.

The [3]Dendralene Motif as an Entry into Nazarov Cyclizations by Silylium-Ion Initiation

Geminal alkenes bearing an aryl and an allenyl group contain the motif of [3]dendralenes. The central alkene double bond in these cross-conjugated polyenes can be reacted with a silylium ion, thereby initiating a Nazarov cyclization. The cationic intermediate emerging from the electrocyclic ring closure is captured by hydride in the presence of excess hydrosilane. The resulting benzannulated methylenecyclopentene derivatives bearing a silylalkyl group then engage in silylium-ion regeneration followed by an unusual endo-selective intramolecular hydrosilylation. This cascade eventually leads to the formation of a silicon-containing bicyclo[3.2.1]octane skeleton.

Intermolecular Carbosilylation of α-Olefins with C(sp3 )-C(sp) Bond Formation Involving Silylium-Ion Regeneration

A regioselective addition of alkynylsilanes across unactivated, terminal alkenes is reported. The reaction is initiated by the capture of a sterically unhindered silylium ion by a silylated phenylacetylene derivative to form a bis(silylated) ketene-like carbocation. This in situ-generated key intermediate is the actual catalyst that maintains the catalytic cycle by a series of electrophilic addition reactions of silylium ions and β-silicon-stabilized carbocations. The computed reaction mechanism is fully consistent with the experimental findings. This unprecedented two-component carbosilylation establishes a C(sp3 )-C(sp) bond and a C(sp3 )-Si bond in atom-economic fashion.

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

Significantly fluorinated triarylmethyl cations have long attracted attention as potentially accessible highly reactive carbocations, but their isolation in a convenient form has proved elusive. We show that abstraction of chloride with a cationic silylium reagent leads to the facile formation of di-, tetra-, and hexafluorinated trityl cations, which could be isolated as analytically pure salts with the [HCB₁₁Cl₁₁]^- counterion and are compatible with (halo)arene solvents. The F₆Tr⁺ cation carrying six meta-F substituents was computationally predicted to possess up to 20% higher hydride affinity than the parent triphenylmethyl cation Tr⁺. We report that indeed F₆Tr⁺ displays reactivity unmatched by Tr⁺. F₆Tr⁺ at ambient temperature abstracts hydrides from the C-H bonds in tetraethylsilane, mesitylene, methylcyclohexane, and catalyzes Friedel-Crafts alkylation of arenes with ethylene, while Tr⁺ does none of these.

Perdeuteration of Deactivated Aryl Halides by H/D Exchange under Superelectrophile Catalysis

Superelectrophilic silylium/arenium ions are shown to be highly effective H/D exchange promoters for the exhaustive deuteration of electron-deficient aryl halides. Several of the resulting perdeuterated aryl halides have been previously inaccessible with existing deuterium-labeling procedures. Using inexpensive C₆D₆ as the deuterium source, excellent degrees of deuterium incorporation were achieved under ambient reaction conditions. Importantly, the perdeuteration remains unaffected on multigram scale, even at a reduced catalyst loading of 0.1 mol %. By this method, otherwise expensive or noncommercially available NMR solvents such as 1,2-dichloro- and 1,2-difluorobenzene can be prepared.

Protodesilylation of Arylsilanes by Visible-Light Photocatalysis

The first visible-light-mediated photocatalytic, metal- and base-free protodesilylation of arylsilanes is presented. The C(sp²)-Si bond cleavage process is catalyzed by a 5 mol % loading of a commercially available acridinium salt upon blue-light irradiation. Two simple approaches have been identified employing either aerobic or hydrogen atom transfer cocatalytic conditions, which enable the efficient and selective desilylation of a broad variety of simple and complex arylsilanes under mild conditions.

Taming the Lewis Superacidity of Non-Planar Boranes: C-H Bond Activation and Non-Classical Binding Modes at Boron

The rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It allowed the formation of non-classical electron deficient B-H-B type of bonding which was supported by spectroscopic and X-ray diffraction parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations such as Csp 2 -H and Csp 3 -H activation under frustrated Lewis pair regime and the cleavage of Csp 3 -Si bonds. The demonstration of such type of rich chemical behavior and flexibility on a single molecular compound make it a unique mediator of chemical transformations generally restricted to transition metals.

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

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 Ions: From Elusive Reactive Intermediates to Potent Catalysts

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.

Chemoselective Cleavage of Si-C(sp3) Bonds in Unactivated Tetraalkylsilanes Using Iodine Tris(trifluoroacetate)

Organosilanes are synthetically useful reagents and precursors in organic chemistry. However, the typical inertness of unactivated Si-C(sp³) bonds under conventional reaction conditions has hampered the application of simple tetraalkylsilanes in organic synthesis. Herein we report the chemoselective cleavage of Si-C(sp³) bonds of unactivated tetraalkylsilanes using iodine tris(trifluoroacetate). The reaction proceeds smoothly under mild conditions (-50 °C to room temperature) and tolerates various polar functional groups, thus enabling subsequent Tamao-Fleming oxidation to provide the corresponding alcohols. NMR experiments and density functional theory calculations on the reaction indicate that the transfer of alkyl groups from Si to the I(III) center and the formation of the Si-O bond proceed concertedly to afford an alkyl-λ³-iodane and silyl trifluoroacetate. The developed method enables the use of unactivated tetraalkylsilanes as highly stable synthetic precursors.

Intramolecular Friedel-Crafts alkylation with a silylium-ion-activated cyclopropyl group: formation of tricyclic ring systems from benzyl-substituted vinylcyclopropanes and hydrosilanes

A trityl-cation-initiated annulation of benzyl-substituted vinylcyclopropanes (VCPs) with hydrosilanes is reported. Two Si–C(sp³) bonds and one C(sp²)–C(sp³) bond are formed in this process where an intramolecular 6-endo-tet Friedel–Crafts alkylation of a silylium-ion-activated cyclopropane ring is the rate-determining key step. The reaction mechanism is proposed based on computations and is in agreement with experimental observations. The new reaction leads to an unprecedented silicon-containing 6/6/5-fused ring system. A phenethyl-substituted VCP derivative yields another unknown tricycle having 6/6/6 ring fusion by reacting in a related but different way involving a 6-exo-tet ring closure.

Downstream to alkene hydrosilylation, the opening of the cyclopropane ring in benzyl-substituted VCPs is interlinked with an S_EAr of the aryl group.

Silylium-Ion-Promoted (5+1) Cycloaddition of Aryl-Substituted Vinylcyclopropanes and Hydrosilanes Involving Aryl Migration

A transition‐metal‐free (5+1) cycloaddition of aryl‐substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self‐regeneration of the silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4‐ rather than 3‐aryl‐substituted silacyclohexane derivatives as major products. Various control experiments and quantum‐chemical calculations support a mechanistic picture where a silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane‐to‐cyclopropane rearrangement.

Characterization of hydrogen-substituted silylium ions in the condensed phase

Hydrogen-substituted silylium ions are long-sought reactive species. We report a protolysis strategy that chemoselectively cleaves either an Si-C(sp²) or an Si-H bond using a carborane acid to access the full series of [CHB₁₁H₅Br₆]^--stabilized R₂SiH⁺, RSiH₂ ⁺, and SiH₃ ⁺ cations, where bulky tert-butyl groups at the silicon atom (R = tBu) were crucial to avoid substituent redistribution. The crystallographically characterized molecular structures of [CHB₁₁H₅Br₆]^--stabilized tBu₂HSi⁺ and tBuH₂Si⁺ feature pyramidalization at the silicon atom, in accordance with that of tBu₃Si⁺[CHB₁₁H₅Br₆]^- Conversely, the silicon atom in the H₃Si⁺ cation adopts a trigonal-planar structure and is stabilized by two counteranions. This solid-state structure resembles that of the corresponding Brønsted acid.

Origin of the 29Si NMR chemical shift in R3Si–X and relationship to the formation of silylium (R3Si+) ions

The origin in deshielding of ²⁹Si NMR chemical shifts in R₃Si–X, where X = H, OMe, Cl, OTf, [CH₆B₁₁X₆], toluene, and O_X (O_X = surface oxygen), as well as ⁱPr₃Si⁺ and Mes₃Si⁺ were studied using DFT methods.

Catalytic Difunctionalization of Unactivated Alkenes with Unreactive Hexamethyldisilane through Regeneration of Silylium Ions

A metal-free, intermolecular syn-addition of hexamethyldisilane across simple alkenes is reported. The catalytic cycle is initiated and propagated by the transfer of a methyl group from the disilane to a silylium-ion-like intermediate, corresponding to the (re)generation of the silylium-ion catalyst. The key feature of the reaction sequence is the cleavage of the Si-Si bond in a 1,3-silyl shift from silicon to carbon. A central intermediate of the catalysis was structurally characterized by X-ray diffraction, and the computed reaction mechanism is fully consistent with the experimental findings.

Making Use of the Direct Process Residue: Synthesis of Bifunctional Monosilanes

The industrial production of monosilanes Me_n SiCl_4-n (n=1-3) through the Müller-Rochow Direct Process generates disilanes Me_n Si₂ Cl_6-n (n=2-6) as unwanted byproducts ("Direct Process Residue", DPR) by the thousands of tons annually, large quantities of which are usually disposed of by incineration. Herein we report a surprisingly facile and highly effective protocol for conversion of the DPR: hydrogenation with complex metal hydrides followed by Si-Si bond cleavage with HCl/ether solutions gives (mostly bifunctional) monosilanes in excellent yields. Competing side reactions are efficiently suppressed by the appropriate choice of reaction conditions.

Highly selective palladium-catalyzed one-pot, five-fold B-H/C-H cross coupling of monocarboranes with alkenes

Palladium-catalyzed dehydrogenative B-H/C-H cross coupling of monocarborane anions with alkenes is reported, allowing for the first time the isolation of selectively penta-alkenylated boron clusters. The reaction cascade is regioselective for the cage positions, leading directly to B2-6 functionalization. Under mild and convenient conditions, styrenes, benzylic alkenes and aliphatic alkenes are demonstrated to be viable coupling partners with exclusive vinyl-type B-C bond formation. Multiple subsequent transformations provide access to directing group-free products, chiral derivatives and penta-alkylated cages. The five-fold coupling, combined with the latter reactions, represents a powerful methodology for the straightforward synthesis of new classes of boron clusters.