A Catalytic SEAr Approach to Dibenzosiloles Functionalized at Both Benzene Cores

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.

Heterolytic cleavage of a Si-H bond by a metal-ligand cooperation of a cationic iridium amido complex and hydrosilylation of aldehydes

Heterolytic cleavage of a Si-H bond was achieved mediated by a metal-ligand cooperation of a cationic iridium amido complex. The reaction was applied to the catalytic hydrosilylation of benzaldehyde and its derivatives, affording the corresponding hydrosilylated products in moderate to good yields.

Activation of Si-H and B-H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si-H and B-H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E-H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η1 or η2) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

Comparative DFT Study on Dehydrogenative C(sp)-H Elementation (E = Si, Ge, and Sn) of Terminal Alkynes Catalyzed by a Cationic Ruthenium(II) Thiolate Complex

Described herein is a comparative theoretical study of dehydrogenative C(sp)-H functionalizations of a terminal alkyne with group-14-based hydrides (HEEt₃; E = Si, Ge, Sn) catalyzed by an Ohki-Tatsumi complex-a cationic Ru(II) complex with a tethered thiolate ligand ([Ru-S] = [(DmpS)Ru(PiPr₃)][BAr₄^F]; Dmp = 2,6-(dimesityl)₂C₆H₃; Ar^F = 3,5-(CF₃)₂C₆H₃). The calculations indicate that the energy barriers for heterolytic cleavage of the H-EEt₃ bonds at the Ru-S sites of the Ohki-Tatsumi complex highly vary depending on the group 14 elements from 3.8 kcal/mol (E = Sn) to 10.5 kcal/mol (E = Ge) and 18.5 kcal/mol (E = Si), where Ru and S elements cooperatively serve as the Lewis acid and base, respectively. Likewise, the transfer of the group 14 cation (Et₃E⁺) to the C-C triple bond to generate the β-element-stabilized vinyl cations-the rate-determining step (RDS) of the overall reaction-is predicted to be susceptible to the element's identity [E_a = 36.8 for Sn < 42.9 and Ge < 50.7 for Si (kcal/mol)]. The key transition states involved in the RDS are compared in terms of energy and structure within each system of the group 14 hydrides. The distortion/interaction-activation strain (DIAS) model analysis of the transition states responsible for dehydrogenative stannylation and hydrostannation of a terminal alkyne sheds light on the origin of the experimentally observed kinetic preference toward dehydrogenative C-H stannylation over hydrostannation.

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.

Synthesis of six-membered silacycles by borane-catalyzed double sila-Friedel-Crafts reaction

We have developed a catalytic synthetic method to prepare phenoxasilins. A borane-catalyzed double sila-Friedel–Crafts reaction between amino group-containing diaryl ethers and dihydrosilanes can be used to prepare a variety of phenoxasilin derivatives in good to excellent yields. The optimized reaction conditions were also applicable for diaryl thioethers to afford their corresponding six-membered silacyclic products. The gram-scale synthesis of a representative bis(dimethylamino)phenoxasilin and the transformation of its amino groups have also been demonstrated.

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.

Preparation of Polyfunctional Biaryl Derivatives by Cyclolanthanation of 2-Bromobiaryls and Heterocyclic Analogues Using nBu2 LaCl⋅4 LiCl

Various aryl- and heteroaryl-substituted 2-bromobiaryls are converted to cyclometalated lanthanum intermediates by reaction with nBu2 LaCl⋅4 LiCl. These resulting lanthanum heterocycles are key intermediates for the facile preparation of functionalized 2,2'-diiodobiaryls, silafluorenes, fluoren-9-ones, phenanthrenes, and their related heterocyclic analogues. X-ray absorption fine structure (XAFS) spectroscopy was used to rationalize the proposed structures of the involved organolanthanum species.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

Lewis acid-catalyzed synthesis of silafluorene derivatives from biphenyls and dihydrosilanes via a double sila-Friedel–Crafts reaction

The synthesis of silafluorene derivatives from aminobiphenyl compounds and dihydrosilanes via a double sila-Friedel–Crafts reaction using a borane catalyst has been achieved.

The electrophilic aromatic substitution approach to C–H silylation and C–H borylation

Several approaches toward electrophilic C–H silylation of electron-rich arenes are discussed, comprising transition-metal-catalyzed processes as well as Lewis-acid- and Brønsted-acid-induced protocols. These methods differ in the catalytic generation of the silicon electrophile but share proton removal in form of dihydrogen. With slight modifications, these methods are often also applicable to the related electrophilic C–H borylation.

Ruthenium-Catalyzed Site-Selective Intramolecular Silylation of Primary C-H Bonds for Synthesis of Sila-Heterocycles

Incorporating the silicon element into bioactive organic molecules has received increasing attention in medicinal chemistry. Moreover, organosilanes are valuable synthetic intermediates for fine chemicals and materials. Transition metal-catalyzed C-H silylation has become an important strategy for C-Si bond formations. However, despite the great advances in aromatic C(sp²)-H bond silylations, catalytic methods for aliphatic C(sp³)-H bond silylations are relatively rare. Here we report a pincer ruthenium catalyst for intramolecular silylations of various primary C(sp³)-H bonds adjacent to heteroatoms (O, N, Si, Ge), including the first intramolecular silylations of C-H bonds α to O, N, and Ge. This method provides a general, synthetically efficient approach to novel classes of Si-containing five-membered [1,3]-sila-heterocycles, including oxasilolanes, azasilolanes, disila-heterocycles, and germasilolane. The trend in the reactivity of five classes of C(sp³)-H bonds toward the Ru-catalyzed silylation is elucidated. Mechanistic studies indicate that the rate-determining step is the C-H bond cleavage involving a ruthenium silyl complex as the key intermediate, while a η²-silene ruthenium hydride species is determined to be an off-cycle intermediate.

Cooperative Catalysis at Metal-Sulfur Bonds

Cooperative catalysis has attracted tremendous attention in recent years, emerging as a key strategy for the development of novel atom-economic and environmentally more benign catalytic processes. In particular, Noyori-type complexes with metal-nitrogen bonds have been extensively studied and evolved as privileged catalysts in hydrogenation chemistry. In contrast, catalysts containing metal-sulfur bonds as the reactive site are out of the ordinary, despite their abundance in living systems, where they are assumed to play a key role in biologically relevant processes. For instance, the heterolysis of dihydrogen catalyzed by [NiFe] hydrogenase is likely to proceed through cooperative H-H bond splitting at a polar nickel-sulfur bond. This Account provides an overview of reported metal-sulfur complexes that allow for cooperative E-H bond (E = H, Si, and B) activation and highlights the potential of this motif in catalytic applications. In recent years, our contributions to this research field have led to the development of a broad spectrum of synthetically useful transformations catalyzed by cationic ruthenium(II) thiolate complexes of type [(DmpS)Ru(PR₃)]⁺BAr^F₄^- (DmpS = 2,6-dimesitylphenyl thiolate, Ar^F = 3,5-bis(trifluoromethyl)phenyl). The tethered coordination mode of the bulky 2,6-dimesitylphenyl thiolate ligand is crucial, stabilizing the coordinatively unsaturated ruthenium atom and also preventing formation of binuclear sulfur-bridged complexes. The ruthenium-sulfur bond of these complexes combines Lewis acidity at the metal center and Lewis basicity at the adjacent sulfur atom. This structural motif allows for reversible heterolytic splitting of E-H bonds (E = H, Si, and B) across the polar ruthenium-sulfur bond, generating a metal hydride and a sulfur-stabilized E⁺ cation. Hence, this activation mode provides a new strategy to catalytically generate silicon and boron electrophiles. After transfer of the electrophile to a Lewis-basic substrate, the resulting neutral ruthenium(II) hydride can either act as a hydride donor (reductant) or as a proton acceptor (Brønsted base); the latter scenario is followed by dihydrogen release. On the basis of this concept, the tethered ruthenium(II) thiolate complexes emerged as widely applicable catalysts for various transformations, which can be categorized into (i) dehydrogenative couplings [Si-C(sp²), Si-O, Si-N, and B-C(sp²)], (ii) chemoselective reductions (hydrogenation and hydrosilylation), and (iii) hydrodefluorination reactions. All reactions are promoted by a single catalyst motif through synergistic metal-sulfur interplay. The most prominent examples of these transformations are the first catalytic protocols for the regioselective C-H silylation and borylation of electron-rich heterocycles following a Friedel-Crafts mechanism.

Mechanistic Study of Arylsilane Oxidation through 19F NMR Spectroscopy

The mechanism of the oxidation of arylsilanes to phenols has been investigated using ¹⁹F NMR spectroscopy. The formation of silanols in these reactions results from a rapid background equilibrium between silanol and alkoxysilane; the relative rates of reaction of these species was evaluated by modeling of concentration profiles obtained through ¹⁹F NMR spectroscopic reaction monitoring. Combining these results with a study of initial rates of phenol formation, and of substituent electronic effects, a mechanistic picture involving rapid and reversible formation of a pentavalent peroxide ate complex, prior to rate-limiting aryl migration, has evolved.

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.

Intramolecular Chain Hydrosilylation of Alkynylphenylsilanes Using a Silyl Cation as a Chain Carrier

Diorganyl[2-(trimethylsilylethynyl)phenyl]silanes 1a-c and methyl-substituted phenylsilanes 1d and 1e were treated with a small amount of trityl tetrakis(pentafluorophenyl)borate (TPFPB) as an initiator in benzene to afford the corresponding benzosiloles (2a-e) in moderate to good yields. However, no reaction was observed for the reaction using [2-(1-hexynyl)phenyl]diisopropylsilane lf. The methyl substituent was tolerated under the reaction conditions and increased the yield of the corresponding benzosilole depending on the substitution position. From the result using 1f, the current reaction was found to require the trimethylsilyl group, which can stabilize intermediary alkenyl carbocations by the β-silyl effect. The current reaction can be considered an intramolecular chain hydrosilylation of alkynylarylsilanes involving silyl cations as chain carriers. Therefore, the silyl cations generated by hydride abstraction from hydrosilanes 1 with the trityl cation causes intramolecular electrophilic addition to the C-C triple bond to form ethenyl cations, which abstract a hydride from 1 to afford benzosiloles 2 with the regeneration of the silyl cations.

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.

Rhodium-Catalyzed Intramolecular C-H Silylation by Silacyclobutanes

Silacyclobutane was discovered to be an efficient C-H bond silylation reagent. Under the catalysis of Rh(I) /TMS-segphos, silacyclobutane undergoes sequential C-Si/C-H bond activations, affording a series of π-conjugated siloles in high yields and regioselectivities. The catalytic cycle was proposed to involve a rarely documented endocyclic β-hydride elimination of five-membered metallacycles, which after reductive elimination gave rise to a Si-Rh(I) species that is capable of C-H activation.

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.

Ru-catalysed C–H silylation of unprotected gramines, tryptamines and their congeners

Directed and undirected Ru-catalysed C–H silylation of unprotected heteroarenes is presented which requires no protecting groups.