Rhodium-Catalyzed Coupling of 2-Silylphenylboronic Acids with Alkynes Leading to Benzosiloles: Catalytic Cleavage of the Carbon−Silicon Bond in Trialkylsilyl Groups

Modular and Regioselective Synthesis of Benzo-Fused Five-Membered Rings Enabled by Co/Ti Synergism

The regiocontrol in constructing benzo-fused five-membered rings by C-H cyclization remains an important challenge. We report a highly general and regioselective methodology to access such heterocycles and indenones, where under the catalysis of CoBr2/bipyridine, aryl titanates, alkynes and EX2 (E = NR, S(O), RP(O), R2Si, CO, etc.) were assembled to various heterocycles and indenones in a modular manner. Unprecedented 1,2-Co/Ti heterobimetallic arylene and benzotitanole intermediates have played crucial roles in these syntheses.

Mechanism and Origins of Enantioselectivity in the Nickel-catalyzed Asymmetric Synthesis of Silicon-Stereogenic Benzosiloles

As important π-skeletons, benzosiloles often possess unique electronic and optical properties and have been widely used in semiconductor materials. Therefore, great attention has been drawn to the area of developing novel synthetic methods for various benzosiloles. However, the synthesis of enantioenriched silicon-stereogenic benzosiloles is still at an early stage and remains to be explored. Herein, we performed systematic density functional theory studies on the recently reported nickel-catalyzed asymmetric synthesis of silicon-stereogenic benosiloles, which was enabled by an enantioselective desymmetrization of (2-alkenyl)aryl-substituted silacyclobutanes. Our computational study shows that the reaction mechanism involves ligand exchange, oxidative addition, alkene insertion, and hydrogen-transfer coupled reductive-demetalation steps. The proposed transmetalation and β-hydride elimination mechanism was not found, which might be due to the unfavorable ring strain of the multicyclic intermediates. The novel hydrogen-transfer coupled reductive-demetalation mechanism was shown to be reasonable for the generation of the silicon-stereogenic benzosilole. Noncovalent interactions (including C-H···π and hydrogen bonding) in the rate-determining alkene insertion transition state account for the origins of the enantioselectivity. Our computational study sheds light on the detailed reaction mechanism and also provides insights for the development of novel approaches for synthesis of high-value silicon-stereogenic compounds.

Advances in disilylation reactions to access cis/trans-1,2-disilylated and gem-disilylated alkenes

Organosilane compounds are widely used in both organic synthesis and materials science. Particularly, 1,2-disilylated and gem-disilylated alkenes, characterized by a carbon-carbon double bond and multiple silyl groups, exhibit significant potential for subsequently diverse transformations. The versatility of these compounds renders them highly promising for applications in materials, enabling them to be valuable and versatile building blocks in organic synthesis. This review provides a comprehensive summary of methods for the preparation of cis/trans-1,2-disilylated and gem-disilylated alkenes. Despite notable advancements in this field, certain limitations persist, including challenges related to regioselectivity in the incorporation and chemoselectivity in the transformation of two nearly identical silyl groups. The primary objective of this review is to outline synthetic methodologies for the generation of these alkenes through disilylation reactions, employing silicon reagents, specifically disilanes, hydrosilanes, and silylborane reagents. The review places particular emphasis on investigating the practical applications of the C-Si bond of disilylalkenes and delves into an in-depth discussion of reaction mechanisms, particularly those reactions involving the activation of Si-Si, Si-H, and Si-B bonds, as well as the C-Si bond formation.

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Synthetic strategies to access silacycles

In comparison with all-carbon parent compounds, the incorporation of Si-element into carboskeletons generally endows the corresponding sila-analogues with unique biological activity and physical-chemical properties. Silacycles have recently shown promising application potential in biological chemistry, pharmaceuticals industry, and material chemistry. Therefore, the development of efficient methodology to assemble versatile silacycles has aroused increasing concerns in the past decades. In this review, recent advances in the synthesis of silacycle-system are briefly summarized, including transition metal-catalytic and photocatalytic strategies by employing arylsilanes, alkylsilane, vinylsilane, hydrosilanes, and alkynylsilanes, etc. as starting materials. Moreover, a clear presentation and understanding of the mechanistic aspects and features of these developed reaction methodologies have been high-lighted.

1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation

The palladium-catalyzed annulation reaction of alkynes enables an attractive approach to siloles. Their access from silirenes and terminal alkynes proved rather general, involving reactive intermediates that have remained elusive to date. Starting from 1,2-bis(3-thienyl)silirene as a source of photochromic siloles, the mechanism of the annulation reaction has been revisited, and palladasilacyclobutenes resulting from the activation of the silirene could be isolated and thoroughly characterized (NMR, X-ray, and DFT). Their role as reactive intermediates and their fate in the course of the reaction were also studied in situ. In combination with in-depth DFT calculations, a clearer picture of the mechanism and the reactive key species is disclosed.

Selective Si-C(sp3) bond cleavage of a silyl-bridged amido alkyl ligand in an yttrium complex

Compared with Si-C(sp² and sp) bonds bearing neighboring π-bond hyperconjugative interactions, the activation of robust Si-C(sp³) bonds has proved to be a challenge. Herein, two distinct Si-C(sp³) bond cleavages have been realized by rare-earth-mediated and nucleophilic addition of unsaturated substrates. The reactions of Tp^Me2Y[κ²-(C,N)-CH(SiH₂Ph)SiMe₂NSiMe₃](THF) (1) with CO or CS₂ gave two endocyclic Si-C bond cleavage products, Tp^Me2Y[κ²-(O,N)-OCCH(SiH₂Ph)SiMe₂NSiMe₃](THF) (2) and Tp^Me2Y[κ²-(S,N)-SSiMe₂NSiMe₃](THF) (3), respectively. However, 1 reacted with nitriles such as PhCN and p-R'C₆H₄CH₂CN in a 1 : 1 molar ratio to yield the exocyclic Si-C bond products Tp^Me2Y[κ²-(N,N)-N(SiH₂Ph)C(R)CHSiMe₂NSiMe₃](THF) (R = Ph (4); R = C₆H₅CH₂ (6H); R = p-F-C₆H₄CH₂ (6F); and R = p-MeO-C₆H₄CH₂ (6MeO)), respectively. Moreover, complex 4 can continuously react with an excess of PhCN to form a Tp^Me2-supported yttrium complex with a novel pendant silylamido-substituted β-diketiminato ligand, Tp^Me2Y[κ³-(N,N,N)-N(SiH₂Ph)C(Ph)CHC(Ph)N-SiMe₂NSiMe₃](PhCN) (5).

Sila-spirocyclization involving unstrained C(sp3)-Si bond cleavage

C - Si Bond cleavage is one of the key elemental steps for a wide variety of silicon-based transformations. However, the cleavage of unstrained Si-C(sp3) bonds catalyzed by transition metal are still in their infancy. They generally involve the insertion of a M - C(sp2) species into the C - Si bond and consequent intramolecular C(sp2)‒Si coupling to exclusively produce siloles. Here we report the Pd-catalyzed sila-spirocyclization, in which the Si-C(sp3) bond is activated by the insertion of a M - C(sp3) species and followed by the formation of a new C(sp3)‒Si bond, allowing the construction of diverse spirosilacycles. This reactivity mode, which is strongly supported by DFT calculations may open an avenue for the Si-C(sp3) bond cleavage and silacycle synthesis.

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.

Recent advance in the reactions of silacyclobutanes and their applications

Silacyclobutanes (SCBs), as a key member of organosilicon family, have received considerable attention in synthetic chemistry since the silicon-carbon bond can be activated. Followed by ring-opening and ring expansion process,...

(3 + 2)-Annulation of 1,3-N,Si-tetraorganosilane reagents TsHNCH2SiBnR1R2 with arynes for efficient synthesis of 3-silaindolines

1,3-N,Si-Tetraorganosilane reagents TsHNCH2SiBnR1R2 were developed as robust synthons to prepare 3-silaindolines via a Cs2CO3-promoted (3 + 2)-annulation reaction with arynes.

Nickel(0)-Catalyzed Asymmetric Ring Expansion Toward Enantioenriched Silicon-Stereogenic Benzosiloles

The development of a straightforward strategy to obtain enantioenriched silicon-stereogenic benzosiloles remains a challenging yet appealing synthesis venture due to their potential future application in chiral electronic and optoelectronic devices. In this context, all of the existing methods rely on Rh-catalyzed systems and are somewhat limited in scope. Herein, we disclose the first Ni⁰ -catalyzed ring expansion process that enables the preparation of benzosiloles possessing tetraorganosilicon stereocenters in excellent yields and enantioselectivities. The presented catalysis strategy is further applied to the asymmetric synthesis of silicon-stereogenic bis-silicon-bridged π-extended systems. Preliminary studies reveal that such compounds exhibit fluorescence emission, Cotton effects and circularly polarized luminescence (CPL) activity.

Me3SiSiMe2(O n Bu): a disilane reagent for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement

Herein, a readily available disilane Me₃SiSiMe₂(OⁿBu) has been developed for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement. This protocol enables the incorporation of a silylene into different starting materials, including acrylamides, alkene-tethered 2-(2-iodophenyl)-1H-indoles, and 2-iodobiaryls, via the cleavage of Si–Si, Si–C, and Si–O bonds, leading to the formation of spirobenzosiloles, fused benzosiloles, and π-conjugated dibenzosiloles in moderate to good yields. Preliminary mechanistic studies indicate that this transformation is realized by successive palladium-catalyzed bis-silylation and Brook- and retro-Brook-type rearrangement of silane-tethered silanols.

A readily available disilane Me₃SiSiMe₂(OⁿBu) as a silylene source has been developed for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement.

Ruthenium-Catalyzed Isomerization of ortho-Silylanilines to Their para Isomers

The catalytic ortho to para transposition of a silyl group in aniline derivatives is described. [RuCl₂(p-cymene)]₂/BINAP in conjunction with a Cu(OAc)₂ additive serves as a potent catalytic system. This method is also applicable to the isomerization of 2-silylpyrrole derivatives to the corresponding 3-silyl isomers.

3-Silaazetidine: An Unexplored yet Versatile Organosilane Species for Ring Expansion toward Silaazacycles

Small-ring silacycles are important organosilane species in main-group chemistry and have found numerous applications in organic synthesis. 3-Silaazetidine, a unique small silacycle bearing silicon and nitrogen atoms, has not been adequately explored due to the lack of a general synthetic scheme and its sensitivity to air. Here, we describe that 3-silaazetidine can be easily prepared in situ from diverse air-stable precursors (RSO₂NHCH₂SiR¹₂CH₂Cl). 3-Silaazetidine shows excellent functional group tolerance in a palladium-catalyzed ring expansion reaction with terminal alkynes, giving 3-silatetrahydropyridines and diverse silaazacycle derivatives, which are promising ring frameworks for the discovery of Si-containing functional molecules.

A Combined Computational and Experimental Study of Rh-Catalyzed C-H Silylation with Silacyclobutanes: Insights Leading to a More Efficient Catalyst System

The study of new C-H silylation reagents and reactions remains an important topic. We reported that under Rh catalysis, silacyclobutanes (SCBs) for the first time were able to react with C(sp²)-H and C(sp³)-H bonds, however the underlying reasons for such a new reactivity were not understood. Through this combined computational and experimental study on C-H silylation with SCBs, we not only depict a reaction pathway that fully accounts for the reactivity and all the experimental findings but also streamline a more efficient catalyst that significantly improves the reaction rates and yields. Our key findings include: (1) the active catalytic species is a [Rh]-H as opposed to the previously proposed [Rh]-Cl; (2) the [Rh]-H is generated via a reductive elimination/β-hydride (β-H) elimination sequence, as opposed to previously proposed endocyclic β-H elimination; (3) the regio- and enantio-determining steps are identified; (4) and of the same importance, the discretely synthesized [Rh]-H is shown to be a more efficient catalyst. This work suggests that the [Rh]-H/diphosphine system should find further applications in C-H silylations involving SCBs.

Enantioselective construction of six- and seven-membered triorgano-substituted silicon-stereogenic heterocycles

The exploitation of chirality at silicon in asymmetric catalysis is one of the most intriguing and challenging tasks in synthetic chemistry. In particular, construction of enantioenriched mediem-sized silicon-stereogenic heterocycles is highly attractive, given the increasing demand for the synthesis of novel functional-materials-oriented silicon-bridged compounds. Here, we report a rhodium-catalyzed enantioselective construction of six- and seven-membered triorgano-substituted silicon-stereogenic heterocycles. This process undergoes a direct dehydrogenative C-H silylation, giving access to a wide range of triorgano-substituted silicon-stereogenic heterocycles in good to excellent yields and enantioselectivities, that significantly enlarge the chemical space of the silicon-centered chiral molecules. Further elaboration of the chiral monohydrosilane product delivers various corresponding tetraorgano-substituted silicon-stereogenic heterocycles without the loss of enantiopurity. These silicon-bridged heterocycles exhibit bright blue fluorescence, which would have potential application prospects in organic optoelectronic materials.

Catalytic asymmetric C-Si bond activation via torsional strain-promoted Rh-catalyzed aryl-Narasaka acylation

Atropisomers are important organic frameworks in bioactive natural products, drugs as well as chiral catalysts. Meanwhile, silanols display unique properties compared to their alcohol analogs, however, the catalytic synthesis of atropisomers bearing silanol groups is challenging. Here, we show a rhodium-catalyzed torsional strain-promoted asymmetric ring-opening reaction for the synthesis of α-silyl biaryl atropisomers. The reaction features a dynamic kinetic resolution of C(Ar)-Si bond cleavage, whose stereochemistry was controlled by a phosphoramidite ligand derived from (S)-3-methyl-1-((2,4,6-triisopropylphenyl)sulfonyl)piperazine. This work is a demonstration of an aryl-Narasaka acylation, where the C(Ar)-Si bond cleavage is promoted by the torsional strain of α, α’-disubstituted silafluorene.

Methods to access atropisomers bearing silanol groups are of value in organic synthesis. Here, the authors report a rhodium-catalyzed torsional strain-promoted asymmetric ring-opening reaction for the synthesis of α-silyl biaryl atropisomers.

Benzosiloles with Crystallization-Induced Emission Enhancement of Electrochemiluminescence: Synthesis, Electrochemistry, and Crystallography

Crystallization-induced emission enhancement (CIEE) was demonstrated for the first time for electrochemilunimescence (ECL) with two new benzosiloles. Compared with their solution, the films of the two benzosiloles gave CIEE of 24 and 16 times. The mechanism of the CIEE-ECL was examined by spooling ECL spectroscopy, X-ray crystal structure analysis, photoluminescence, and DFT calculations. This CIEE-ECL system is a complement to the well-established aggregation-induced emission enhancement (AIEE) systems. Unique intermolecular interactions are noted in the crystalline chromophore. The first heterogeneous ECL system is established for organic compounds with highly hydrophobic properties.

Catalytic Asymmetric trans-Selective Hydrosilylation of Bisalkynes to Access AIE and CPL-Active Silicon-Stereogenic Benzosiloles

Chirality widely exists in a diverse array of biologically active molecules and life forms, and the catalytic constructions of chiral molecules have triggered a heightened interest in the fields of chemistry and materials and pharmaceutical sciences. However, the synthesis of silicon-stereogenic organosilicon compounds is generally recognized as a much more difficult task than that of carbon-stereogenic centers because of no abundant organosilicon-based chiral sources in nature. Herein, we reported a highly enantioselective rhodium-catalyzed trans-selective hydrosilylation of silicon-tethered bisalkynes to access chiral benzosiloles bearing a silicon-stereogenic center. This protocol featured with chiral Ar-BINMOL-Phos bearing hydrogen-bond donors as a privileged P-ligand for catalytic asymmetric hydrosilylation that is operationally simple and has 100% atom-economy with good functional group tolerability as well as high enantioselectivity (up to >99:1 er). Benefiting from the trans-selective hydrosilylation with the aid of Rh/Ar-BINMOL-Phos-based asymmetric catalysis, the Si-stereogenic benzosiloles exhibited pronounced aggregation-induced emission (AIE) and circularly polarized luminescence (CPL) activity.

Si-C(sp3) bond activation through oxidative addition at a Rh(i) centre

An easy, direct and room temperature silicon-carbon bond activation is reported. The reaction of [RhCl(coe)₂]₂ with the silane Si(Me)₂(o-C₆H₄SMe)₂ in the presence of an halide extractor provokes a Si-CH₃ bond cleavage yielding a cationic silyl-methyl-Rh(iii). In contrast, if the reaction is performed using the Rh(i) bis-alkene dimers, [RhCl(cod)]₂ or [RhCl(nbd)]₂, the Si-CH₃ bond activation does not occur.

Advances in Synthesis of π-Extended Benzosilole Derivatives and Their Analogs

Benzosiloles and their π-extended derivatives are present in many important advanced materials due to their excellent physical properties. Especially, they have found many potential applications in the development of novel electronic materials such as OLEDs, semiconductors and solar cells. In this review, we have summarized several main approaches to construct (di)benzosilole derivatives and (benzo)siloles fused to aromatic five- and six-membered 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.

Rhodium-Catalyzed Carbonylative Synthesis of Benzosilinones

In this work, a novel and practical procedure for the synthesis of benzosilinones by carbonylative cyclization has been developed. Various benzosilinones were isolated in moderate to good yields, using rhodium as the catalyst with good functional group tolerance. Not only symmetric alkynes but also nonsymmetric alkynes are applicable with excellent regioselectivity and good yields. Remarkably, this is the first procedure for benzosilinone synthesis which is general and practical.

Catalytic Generation of Rhodium Silylenoid for Alkene-Alkyne-Silylene [2 + 2 + 1] Cycloaddition

An alkene-alkyne-silylene [2 + 2 + 1] cycloaddition takes place in the rhodium-catalyzed reaction of 1,6-enynes with borylsilanes bearing an alkoxy group on the silicon atoms, which react as synthetic equivalents of silylene. The reaction proceeds efficiently in 1,2-dichloroethane at 80-110 °C in the presence of a rhodium catalyst bearing bis(diphenylphosphino)methane (DPPM) as a ligand to afford 1-silacyclopent-2-enes in good to high yields.

Acceptorless dehydrogenative construction of CN and CC bonds through catalytic aza-Wittig and Wittig reactions in the presence of an air-stable ruthenium pincer complex

Acceptorless dehydrogenative construction of CN and CC bonds catalysed by air-stable ruthenium complexes.

An Additive-Free, Base-Catalyzed Protodesilylation of Organosilanes

We report an additive-free, base-catalyzed C-, N-, O-, and S-Si bond cleavage of various organosilanes in mild conditions. The novel catalyst system exhibits high efficiency and good functional group compatibility, providing the corresponding products in good to excellent yields with low catalyst loadings. Overall, this transition-metal-free process may offer a convenient and general alternative to current employing excess bases, strong acids, or metal-catalyzed systems for the protodesilylation of organosilanes.

Rhodium-catalyzed intramolecular carbosilylation of alkynes via C(sp3)–Si bond cleavage

A Rh(i)-catalyzed intramolecular cis-carbosilylation of alkynes was achieved to afford silatricyclic compounds in good yields.

Oxidative radical divergent Si-incorporation: facile access to Si-containing heterocycles

Oxidative radical cleavage of Si–H/silyl C(sp³)–H bonds, dual Si–H bonds and Si–H/Si–Si bonds toward Si-heterocycles is presented.

Synthesis of Benzosiloles by Intramolecular anti-Hydroarylation via ortho-C-H Activation of Aryloxyethynyl Silanes

Straightforward synthesis of benzosiloles was achieved by the invention of Pd/acid-catalyzed intramolecular anti-hydroarylation of aryloxyethynyl(aryl)silanes via ortho-C-H bond activation. The aryloxy group bound to the ethynyl carbon is the key factor for this transformation.

Versatile telluracycle synthesis via the sequential electrophilic telluration of C(sp2)-Zn and C(sp2)-H bonds

We report herein a new approach for the synthesis of tellurium-bridged aromatic compounds based on the sequential electrophilic telluration of C(sp2)-Zn and C(sp2)-H bonds with tellurium(iv) chlorides. A combination of transition metal-catalyzed (migratory) arylmetalation of alkynes and sequential telluration allows for the expedient construction of a library of functionalized benzo[b]tellurophenes. Furthermore, a variety of heteroarene-fused benzotellurophenes and other novel tellurium-embedded polycyclic aromatics can be readily synthesized from the corresponding 2-iodoheterobiaryls.