Dehydrogenative Annulation of Silylated 1H-Indoles with Alkynes via Silyl Migration

We investigated the dehydrogenative annulation of silylated 1H-indole derivatives with alkynes to synthesize a silole-fused indole. The addition of the in situ generated silylium ion to alkynes was followed by the sila-Friedel-Crafts reaction via silyl migration, realizing regioselective dehydrogenative annulation controlled by the steric bulkiness of a base. The optical properties of the obtained siloloindoles indicated fluorescence of which the intensity depends on the location of the fused silole.

Rhodium hydride enabled enantioselective intermolecular C–H silylation to access acyclic stereogenic Si–H

The tremendous success of stereogenic carbon compounds has never ceased to inspire researchers to explore the potentials of stereogenic silicon compounds. Intermolecular C–H silylation thus represents the most versatile and straightforward strategy to construct C–Si bonds, however, its enantioselective variant has been scarcely reported to date. Herein we report a protocol that allows for the enantioselective intermolecular C–H bond silylation, leading to the construction of a wide array of acyclic stereogenic Si–H compounds under simple and mild reaction conditions. Key to the success is (1) a substrate design that prevents the self-reaction of prochiral silane and (2) the employment of a more reactive rhodium hydride ([Rh]-H) catalyst as opposed to the commonly used rhodium chloride ([Rh]-Cl) catalyst. This work unveils opportunities in converting simple arenes into value-added stereogenic silicon compounds.

Construction of chiral organosilicon compounds could have implications in photophysical, biological, and chemical fields, as silicon is isoelectronic with carbon, and can mimic carbon atoms while providing slightly different properties. Here the authors present an intermolecular, enantioselective C–H silylation of heterocycles via rhodium catalysis.

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.

Electron-deficient boron-based catalysts for C-H bond functionalisation

In contrast to transition metal-catalysed C-H functionalisation, highly efficient construction of C-C and C-X (X = N, O, S, B, Si, etc.) bonds through metal-free catalytic C-H functionalisation remains one of the most challenging tasks for synthetic chemists. In recent years, electron-deficient boron-based catalyst systems have exhibited great potential for C-H bond transformations. Such emerging systems may greatly enrich the chemistry of C-H functionalisation and main-group element catalysis, and will also provide enormous opportunities in synthetic chemistry, materials chemistry, and chemical biology. This article aims to give a timely comprehensive overview to recognise the current status of electron-deficient boron-based catalysis in C-H functionalisation and stimulate the development of more efficient catalytic systems.

Facile synthesis of tribenzosilepins from terphenyls and dihydrosilanes by electrophilic double silylation

Tribenzosilepins were synthesized from terphenyls and dihydrosilanes via a facile approach using a double sila-Friedel–Crafts reaction.

B(C6F5)3-Catalyzed cyclization of alkynes: direct synthesis of 3-silyl heterocyclic compounds

An efficient one-pot strategy for easy access to 3-silyl heterocyclic compounds was developed via a B(C₆F₅)₃-catalyzed cycloaddition reaction of o-(1-alkynyl)(thio)anisoles or o-(1-alkynyl)-N-methylaniline. In this reaction, benzenethiophene, benzofuran or indole skeletons could be constructed by an intermolecular cyclization with diphenylsilane. This protocol elicited moderate-to-good yields with metal-free reaction systems.

Dual reactivity of B(C6F5)3 enables the silylative cascade conversion of N-aryl piperidines to sila-N-heterocycles: DFT calculations

A theoretical study reveals that the dual reactivity of B(C₆F₅)₃ enables the unique silylative cascade conversion of N-aryl piperidines to bridged sila-N-heterocycles.

Comparative DFT study of metal-free Lewis acid-catalyzed C–H and N–H silylation of (hetero)arenes: mechanistic studies and expansion of catalyst and substrate scope

Direct selective dehydrogenative silylation of thiophenes, pyridines, indoles and anilines to synthesize silyl-substituted aromatic compounds catalyzed by metal-free Lewis acids was achieved recently. However, there is still insufficient mechanistic data for these transformations. Using density functional theory calculations, we conducted a detailed investigation of the mechanism of the B(C₆F₅)₃-catalyzed dehydrogenative silylation of N-methylindole, N,N-dimethylaniline and N-methylaniline. We successfully located the most favourable reaction pathways that can explain the experimental observations notably well. The most favourable pathway for B(C₆F₅)₃-catalyzed C–H silylation of N-methylindole includes nucleophilic attack, proton abstraction and hydride migration. The C–H silylation of N,N-dimethylaniline follows a similar pathway to N-methylindole rather than that proposed by Hou's group. Our mechanism successfully explains that the transformations of N-methylindoline to N-methylindole produce different products at different temperatures. For N-methylaniline bearing both N–H and para-phenyl C–H bonds, the N–H silylation reaction is more facile than the C–H silylation reaction. Our proposed mechanism of N–H silylation of N-methylaniline is different from that proposed by the groups of Paradies and Stephan. Lewis acids Al(C₆F₅)₃, Ga(C₆F₅)₃ and B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂ can also catalyze the C–H silylation of N-methylindole like B(C₆F₅)₃, but the most favourable pathways are those promoted by N-methylindoline. Furthermore, we also found several other types of substrates that would undergo C–H or N–H silylation reactions under moderate conditions. These findings may facilitate the design of new catalysts for the dehydrogenative silylation of inactivated (hetero)arenes.

We investigated the mechanism of the dehydrosilylation of (hetero)arenes and extended the scope of the silylation catalysts and substrates.

B(C6F5)3-Catalyzed redox-neutral β-alkylation of tertiary amines using p-quinone methides via borrowing hydrogen

A transition metal-free intermolecular redox-neutral β-alkylation of acyclic tertiary amines via borrowing hydrogen catalyzed by commercially available B(C₆F₅)₃ was achieved.

NHC-Copper-Catalyzed Asymmetric Dearomative Silylation of Indoles

We report an asymmetric dearomative silylation reaction of 3-acylindoles using a chiral NHC-copper(I) complex as catalyst to afford a range of cyclic α-aminosilanes with high regio- and enantioselectivity. Initial mechanistic studies revealed that the observed high diastereoselectivity was attributed to the facile epimerization of the 3-acyl group. We also demonstrated that the product could be used as a versatile precursor for the synthesis of functionalized indolines in high enantiomeric purity.

Catalytic Access to Bridged Sila- N-heterocycles from Piperidines via Cascade sp3 and sp2 C-Si Bond Formation

Described herein is the development of an unprecedented route to bridged sila- N-heterocycles via B(C₆F₅)₃-catalyzed cascade silylation of N-aryl piperidines with hydrosilanes. Mechanistic studies indicated that an outer-sphere ionic path is operative to involve three sequential catalytic steps having N-silyl piperidinium borohydride as a resting species: (i) dehydrogenation of the piperidine ring, (ii) β-selective hydrosilylation of a resultant enamine intermediate, and (iii) intramolecular dehydrogenative sp² C-H silylation.

Boron-Catalyzed O-H Bond Insertion of α-Aryl α-Diazoesters in Water

A catalytic, metal-free O-H bond insertion of α-diazoesters in water in the presence of B(C₆F₅)₃· nH₂O (2 mol %) was developed, affording a series of α-hydroxyesters in good to excellent yields. The reaction features easy operation and wide substrate scope, and importantly, no metal is needed as compared with the conventional methods. Significantly, this approach further expands the applications of B(C₆F₅)₃ under water-tolerant conditions.

Transition-Metal-Free Ring-Opening Silylation of Indoles and Benzofurans with (Diphenyl-tert-butylsilyl)lithium

A practical method is presented for ring opening various indoles and benzofurans with concomitant stereoselective silylation using readily generated (diphenyl-tert-butylsilyl)lithium to afford ortho-β-silylvinylanilines or -phenols. Dearomatization of the heteroarene core proceeds in the absence of any transition-metal catalyst through addition of a silyl anion and a subsequent stereoselective β-elimination. DFT calculations provide insight into the mechanism. Functionalizing C-X bond cleavage of heteroarenes is rare and generally requires transition-metal catalysts.

Deprotonative Silylation of Aromatic C-H Bonds Mediated by a Combination of Trifluoromethyltrialkylsilane and Fluoride

A method for the deprotonative silylation of aromatic C-H bonds has been developed using trifluoromethyltrimethylsilane (CF₃SiMe₃, Ruppert-Prakash reagent) and a catalytic amount of fluoride. In this reaction, CF₃SiMe₃ is considered to act as a base and a silicon electrophile. This process is highly tolerant to various functional groups on heteroarenes and benzenes. Furthermore, this method can be applied to the synthesis of trimethylsilyl group-containing analogs of TAC-101, which is a bioactive synthetic retinoid with selective affinity for retinoic acid receptor α (RAR-α) binding. We also report further transformations of the silylated products into useful derivatives.

B(C6F5)3-Promoted hydrogenations of N-heterocycles with ammonia borane

A transition-metal-free method for the B(C₆F₅)₃-promoted hydrogenations of N-heterocycles using ammonia borane under mild reaction conditions has been developed. The reaction affords a broad range of hydrogenated products in moderate to good yields. The enantioselective versions for the corresponding products were also investigated via our approach, showing good feasibility.

Transition metal-catalyzed site- and regio-divergent C–H bond functionalization

The regioselectivity of C–H functionalization reactions can be redirected to obtain regioisomeric products form the same starting materials.

Electrophilic Aromatic Substitution with Silicon Electrophiles: Catalytic Friedel-Crafts C-H Silylation

Electrophilic aromatic substitution is a fundamental reaction in synthetic chemistry. It converts C-H bonds of sufficiently nucleophilic arenes into C-X and C-C bonds using either stoichiometrically added or catalytically generated electrophiles. These reactions proceed through Wheland complexes, cationic intermediates that rearomatize by proton release. Hence, these high-energy intermediates are nothing but protonated arenes and as such strong Brønsted acids. The formation of protons is an issue in those rare cases where the electrophilic aromatic substitution is reversible. This situation arises in the electrophilic silylation of C-H bonds as the energy of the intermediate Wheland complex is lowered by the β-silicon effect. As a consequence, protonation of the silylated arene is facile, and the reverse reaction usually occurs to afford the desilylated arene. Several new approaches to overcome this inherent challenge of C-H silylation by S_E Ar were recently disclosed, and this Minireview summarizes this progress.

A Pincer Ruthenium Complex for Regioselective C-H Silylation of Heteroarenes

A pincer Ru(II) catalyst for the highly efficient undirected silylation of O- and S-heteroarenes with (TMSO)₂MeSiH and Et₃SiH is described, producing heteroarylsilanes with exclusive C2-regioselectivity, good functional-group tolerance, and high turnover numbers (up to 1960). The synthetic utility of the silylated products is demonstrated by Pd-catalyzed Hiyama-Denmark cross-coupling under mild conditions. One-pot, two-step silylation and coupling procedures have been also developed.

Brønsted Acid-Promoted Formation of Stabilized Silylium Ions for Catalytic Friedel-Crafts C-H Silylation

A counterintuitive approach to electrophilic aromatic substitution with silicon electrophiles is disclosed. A strong Brønsted acid that would usually promote the reverse reaction, i.e., protodesilylation, was found to initiate the C-H silylation of electron-rich (hetero)arenes with hydrosilanes. Protonation of the hydrosilane followed by liberation of dihydrogen is key to success, fulfilling two purposes: to generate the stabilized silylium ion and to remove the proton released from the Wheland intermediate.

Boron-Catalyzed Aromatic C-H Bond Silylation with Hydrosilanes

Metal-free catalytic C-H silylation of a series of aromatic compounds such as N,N-disubstituted anilines with various hydrosilanes has been achieved for the first time using commercially available B(C6F5)3 as a catalyst. This protocol features simple and neutral reaction conditions, high regioselectivity, wide substrate scope (up to 40 examples), Si-Cl bond compatibility, and no requirement for a hydrogen acceptor.

Friedel-Crafts-Type Intermolecular C-H Silylation of Electron-Rich Arenes Initiated by Base-Metal Salts

An electrophilic aromatic substitution (SE Ar) with a catalytically generated silicon electrophile is reported. Essentially any commercially available base-metal salt acts as an initiator/catalyst when activated with NaBAr(F)4. The thus-generated Lewis acid then promotes the SE Ar of electron-rich arenes with hydrosilanes but not halosilanes. This new C-H silylation was optimized for FeCl2/NaBAr(F)4, affording good yields at catalyst loadings as low as 0.5 mol %. The procedure is exceedingly straightforward and comes close to typical Friedel-Crafts methods, where no added base is needed to absorb the released protons.