Silyl Anions Research Articles

Isolable T‐Shaped Planar Silyl Anion

AbstractSilyl anions have garnered significant attention due to their synthetic abilities. However, previously reported silyl anions have been limited to either trigonal‐pyramidal or trigonal‐planar geometries, which confine them primarily as nucleophiles in substitution reactions. Herein, we report the isolation of the unprecedented T‐shaped planar silyl anion salt 2 by employment of a geometrically constrained triamido pincer ligand. Theoretical calculations disclosed that the silicon centre in 2 possesses both a lone pair of electrons and an empty 3pz orbital. In addition to nucleophilic substitution reactions with Ph3PAuCl and W(CO)6, 2 readily undergoes oxidative additions with CO2 and 2,6‐dimethylphenylisonitrile at room temperature. Furthermore, under mild conditions, compound 2 cleaves Csp2−H, Csp2−H, and H−H bonds in 1,2,4,5‐tetrafluorobenzene, an intramolecular iPr group, and dihydrogen, representing the first examples of C−H and H−H activations mediated by a silyl anion, respectively. This work unveils new reactivity of silyl anions owing to the non‐classical geometry and electronic structure.

Isolable T-Shaped Planar Silyl Anion.

Silyl anions have garnered significant attention due to their synthetic abilities. However, previously reported silyl anions have been limited to either trigonal-pyramidal or trigonal-planar geometries, which confine them primarily as nucleophiles in substitution reactions. Herein, we report the isolation of the unprecedented T-shaped planar silyl anion salt 2 by employment of a geometrically constrained triamido pincer ligand. Theoretical calculations disclosed that the silicon centre in 2 possesses both a lone pair of electrons and an empty 3pz orbital. In addition to nucleophilic substitution reactions with Ph3PAuCl and W(CO)6, 2 readily undergoes oxidative additions with CO2 and 2,6-dimethylphenylisonitrile at room temperature. Furthermore, under mild conditions, compound 2 cleaves Csp2-H, Csp2-H, and H-H bonds in 1,2,4,5-tetrafluorobenzene, an intramolecular iPr group, and dihydrogen, representing the first examples of C-H and H-H activations mediated by a silyl anion, respectively. This work unveils new reactivity of silyl anions owing to the non-classical geometry and electronic structure.

Preparations of Silyl Anions

AbstractThis review collates synthetic procedures for the synthesis of different silyl anions, which are valuable reagents for chemists, particularly in the field of organic synthesis and organometallic chemistry. This includes the preparation of silyl anions of lithium, sodium, potassium, rubidium, and cesium from group I metals and silyl anions of beryllium, magnesium, calcium, strontium, and barium from group II metals. Synthetic procedures for silyl anions of zirconium, hafnium, cadmium, and zinc are also covered. The synthesis of group XIII silyl anions of boron and aluminum is also highlighted.1 Introduction2 Silyl Anions of Group I Metals2.1 Silyllithium2.2 Silylsodium2.3 Silylpotassium2.4 Silylrubidium and Silylcesium3 Silyl Anions of Group II Metals3.1 Silylberyllium3.2 Silylmagnesium3.3 Silylcalcium, -strontium, and -barium4 Silyl Anions of Transition Metals4.1 Silylzirconium and -hafnium4.2 Silylcopper and -cuprates4.3 Silylzinc4.4 Disilylzinc4.5 Silylzincate4.6 Silylcadmium5 Silyl Pronucleophiles of Group XIII5.1 Silylboranes5.2 Silylaluminum6 Silyl Anions of Group XIV6.1 Disilane Synthesis7 Conclusions

Aldol/Brook/Carbon Skeletal Rearrangement Cascade Reactions of β‐Silyl Ketones with Aldehydes

Abstractβ‐Silyl ketones reacted with aldehydes by treatment with KMMDS in the presence of 18‐crown‐6 to give β,γ‐unsaturated ketones accompanied with a skeletal rearrangement. The reactions proceeded by aldol reaction of β‐silyl ketones with aldehydes followed by [1,4]‐Brook rearrangement and intramolecular 1,2‐addition to form cyclopropanol derivatives, in which carbon‐carbon bond cleavage took place to afford β,γ‐unsaturated ketones. The β,γ‐unsaturated ketones were prepared in a one‐pot manner by conjugate addition of silyl anions to α,β‐unsaturated ketones followed by reactions with aldehydes. The Brook rearrangement proceeded with complete inversion of configuration at the carbon center.

Stereospecific synthesis of silicon-stereogenic optically active silylboranes and general synthesis of chiral silyl Anions

Silicon-stereogenic optically active silylboranes could potentially allow the formation of chiral silyl nucleophiles as well as the synthesis of various chiral silicon compounds. However, the synthesis of such silicon-stereogenic silylboranes has not been achieved so far. Here, we report the synthesis of silicon-stereogenic optically active silylboranes via a stereospecific Pt(PPh3)4-catalyzed Si–H borylation of chiral hydrosilanes, which are synthesized by stoichiometric and catalytic asymmetric synthesis, in high yield and very high or perfect enantiospecificity (99% es in one case, and >99% es in the others) with retention of the configuration. Furthermore, we report a practical approach to generate silicon-stereogenic silyl nucleophiles with high enantiopurity and configurational stability using MeLi activation. This protocol is suitable for the stereospecific and general synthesis of silicon-stereogenic trialkyl-, dialkylbenzyl-, dialkylaryl-, diarylalkyl-, and alkylary benzyloxy-substituted silylboranes and their corresponding silyl nucleophiles with excellent enantiospecificity (>99% es except one case of 99% es). Transition-metal-catalyzed C–Si bond-forming cross-coupling reactions and conjugate-addition reactions are also demonstrated. The mechanisms underlying the stability and reactivity of such chiral silyl anion were investigated by combining NMR spectroscopy and DFT calculations.

An Electrochemical Strategy to Synthesize Disilanes and Oligosilanes from Chlorosilanes.

Silanes are important compounds in industrial and synthetic chemistry. Here, we develop a general approach for the synthesis of disilanes as well as linear and cyclic oligosilanes via the reductive activation of readily available chlorosilanes. The efficient and selective generation of silyl anion intermediates, which are arduous to achieve by other means, allows for the synthesis of various novel oligosilanes by heterocoupling. In particular, this work presents a modular synthesis for a variety of functionalized cyclosilanes, which may give rise to materials with distinct properties from linear silanes but remain challenging synthetic targets. In comparison to the traditional Wurtz coupling, our method features milder conditions and improved chemoselectivity, broadening the functional groups that are compatible in oligosilane preparation. Computational studies support a mechanism whereby differential activation of sterically and electronically distinct chlorosilanes are achieved in an electrochemically driven radical-polar crossover mechanism.

An Electrochemical Strategy to Synthesize Disilanes and Oligosilanes from Chlorosilanes**

AbstractSilanes are important compounds in industrial and synthetic chemistry. Here, we develop a general approach for the synthesis of disilanes as well as linear and cyclic oligosilanes via the reductive activation of readily available chlorosilanes. The efficient and selective generation of silyl anion intermediates, which are arduous to achieve by other means, allows for the synthesis of various novel oligosilanes by heterocoupling. In particular, this work presents a modular synthesis for a variety of functionalized cyclosilanes, which may give rise to materials with distinct properties from linear silanes but remain challenging synthetic targets. In comparison to the traditional Wurtz coupling, our method features milder conditions and improved chemoselectivity, broadening the functional groups that are compatible in oligosilane preparation. Computational studies support a mechanism whereby differential activation of sterically and electronically distinct chlorosilanes are achieved in an electrochemically driven radical‐polar crossover mechanism.

Dearomative triple elementalization of quinolines driven by visible light

Organoboron and organosilicon compounds are used not only as synthetic building blocks but also as functional materials and pharmaceuticals, and compounds with multiple boryl and silyl groups are beginning to be used for these purposes. Especially in drug discovery, methodology providing easy stereoselective access to aliphatic nitrogen heterocycles bearing multiple boryl or silyl groups from readily available aromatic nitrogen heterocycles would be attractive. However, such transformations remain challenging, and available reactions have been mostly limited to dearomative hydroboration or hydrosilylation reactions. Here, we report the dearomative triple elementalization (carbo-sila-boration) of quinolines via the addition of organolithium followed by photo-boosted silaboration, affording the desired products with complete chemo-, regio-, and stereoselectivity. The reaction proceeds via the formation of silyl radicals instead of silyl anions. We also present preliminary studies to illustrate the potential of silaboration products as synthetic platforms.

Reversible C-H bond silylation with a neutral silicon Lewis acid.

The silicon-carbon bond is a valuable linchpin for synthetic transformations. However, installing Si-C functionalities requires metalated C-nucleophiles, activated silicon reagents (silylium ions, silyl radicals, and silyl anions), or transition metal catalysis, and it occurs irreversibly. In contrast, spontaneous C-H silylations with neutral silanes leading to anionic silicates, and their reversible deconstruction, are elusive. Herein, the CH-bond silylation of heterocycles or a terminal alkyne is achieved by reaction with bis(perfluoro(N-phenyl-ortho-amidophenolato))silane and 1,2,2,6,6-pentamethylpiperidine. Computational and experimental insights reveal a frustrated Lewis pair (FLP) mechanism. Adding a silaphilic donor to the ammonium silicate products induces the reformation of the C-H bond, thus complementing previously known irreversible C-H bond silylation protocols. Interestingly, the FLP "activated" N-methylpyrrole exhibits "deactivated" features against electrophiles, while a catalytic functionalization is found to be effective only in the absence of a base.

Formal Hydrotrimethylsilylation of Styrenes with Anti-Markovnikov Selectivity Using Hexamethyldisilane.

A combination of CsF and hexamethyldisilane in DMSO enabling an anti-Markovnikov formal hydrotrimethylsilylation of styrenes is reported. Mechanistic investigations detail the reaction pathways, including in situ generation of a silyl anion, the addition of this silyl anion onto the alkene to form a carbanion, and this carbanion is then protonated by DMSO. The choice of the solvent to match its reactivity with the carbanion and silyl anion is key to the success of this reaction.

Identification and characterization of selected silyl substituted silyl anions by liquid injection field desorption ionization mass spectrometry.

Silyl anions are crucial building blocks in silicon chemistry and are frequently used in organosilicon chemistry. These so-called silanides are negatively charged three-coordinate species, isoelectronic to carbanions. In this contribution, we synthesized already literature known and unknown anionic silicon species. Here we focused on silyl substituted silyl anions with different substituted silicon atoms like hydrogen, methyl, and methoxy groups. Furthermore, we investigated these species with liquid injection field desorption ionization mass spectrometry.

Synthesis and Unusual Reactivity of Acyl-Substituted 1,4-Disilacyclohexa-2,5-dienes.

In continuation of our recent studies on group 14 rings with exocyclic silicon-carbon double bonds, we report here on the synthesis and reactivity of previously unknown acyl-substituted 1,4-disilacyclohexa-2,5-dienes. 1,1,4,4-Tetrakistrimethylsilyl-1,4-disilacyclohexa-2,5-diene 1 cleanly afforded the silyl anion 1-K after addition of 1 equiv of KO t Bu. 1-K subsequently could be reacted with various electrophiles to the expected substitution products including compounds 4 and 5. When photolyzed with λ > 300 nm radiation, 4 and 5 undergo Brook-type 1,3-Si → O migration reactions to generate the corresponding 1,4-disilacyclohexadienes with exocyclic Si=C bonds as the primary products. These metastable silenes only could be characterized in form of appropriate quenching products. The reaction of compound 4 with KO t Bu followed by the addition of 1 equiv of PhMe2SiCl surprisingly gave the silylated 1,4-disilanorbornadiene cages 8 and 9 instead of the expected exocyclic silene. The responsible sila-Peterson-type mechanism could be elucidated by density functional theory calculations at the conductor-like polarizable continuum model (THF) B3LYP-GD3/6-31 + G(d) level and by the isolation and characterization of unstable intermediate products after proper derivatization.

Cleavage of Si–H and Si–C Bonds by Metal–Ligand Cooperation: Formation of Silyl Anion and Silylene Equivalents from Tertiary Silanes

The cleavage of silicon–hydrogen bonds to generate silicon-containing metal complexes has played an essential role in the preparation of organosilicon compounds. Herein, we report the reaction of cyclopentadienone iridium complex 1 with hydrosilanes to afford hydroxycyclopentadienyl iridium silyl complexes 4 via the umpolung of the Si–H bond. The nucleophilicity of the Ir–Si bond in the resulting complex was demonstrated in the reaction with an allyl halide and ethyl acrylate. Furthermore, a silylene complex was formed from hydroxycyclopentadienyl silyliridium complex 4 via the formal reductive elimination of the C–H bond at the silicon center to release benzene. The silylene complex synthesized from tricarbylsilane showed ambiphilic reactivities, which are consistent with the classical reactivities of silylenes.

Control of higher-silane generation by dilution gases in SiH4 plasmas

Higher-silane related reactive species (HSRS) is considered as a key factor in increasing the degree of photo-induced degradation, which is a serious problem for hydrogenated amorphous silicon (a-Si:H) film. Dilution gases (H2, He, Ar) are used to control the generation of HSRS. A global model for SiH4-containing discharge is developed to investigate the concentration of HSRS. The model calculations are compared with the available measurements, presenting a reasonable agreement. Increasing pressure contributes to more power loss via volumetric processes, which brings about the decline in the electron density and temperature with the same absorbed power. The average electronegativity rises with the silane content or pressure log-linearly, but decreases with the power log-linearly. As the main precursor of the anion formation, is the starting point for the formation of large silyl anions (). The HSRS density rises first and then falls with the increase of the absorbed power, which can be explained by the decomposition of SiH4. The dilution of H2 promotes the generation of H atomics, enhancing the production of SiH2 with abstraction reaction (H + SiH3 ⇒ H2 + SiH2). However, due to the declined content of SiH4, the insertion reaction (SiH2 + SiH4 ⇒ Si2H6) is inhibited. A higher Ar fraction results in more total ionization and a larger ion density. Due to the high threshold energies of He for both excitation and ionization, the dilution of He does not contribute much to the variation of the neutral radical density.

Synthesis of Stable Dianionic Cyclic Silenolates and Germenolates.

In this contribution a convenient synthetic method to obtain the previously unknown dianionic cyclic silenolates and germenolates is described. These dianions 2a,b and 4a,b are easily accessible via a one-pot synthetic protocol in high yields. Their structural properties were analyzed by a combination of NMR, single-crystal X-ray crystallography, and DFT quantum mechanical calculations. Moreover, the reactivity of 2a,b and 4a,b with selected examples of electrophiles was investigated. 2a and 4a were reacted with ClSiiPr3 to give new examples of polysilanes and polygermanes with exocyclic double bonds. The reaction of 2b with ClSiMe2SiMe2Cl led to the formation of the acyl bicyclo[2.2.2]octasilane 6. Moreover, the reaction of 2a,b and 4a,b with MeI, as an example of a carbon-centered electrophile, led to selective alkylation reactions at the negatively charged silicon and germanium atoms. The corresponding methylated structures 9a,b and 10a,b were formed in nearly quantitative yields. The competitive reactivity of the silyl and silenolate anion toward 1 equiv of ClSiMe3 showed that the outcome of the reaction was strongly influenced by the substituent at the carbonyl moiety. 2a reacted with 1 equiv of ClSiMe3 to give the corresponding cyclic silenolate S1a, which demonstrated that the silyl anion is more nucleophilic than the silenolate with attached aromatic groups. 2b, on the other hand, reacted with 1 equiv of ClSiMe3 to give the bicyclic compound 11via an intramolecular sila-Peterson alkenation reaction. These findings clearly showed that the alkyl-substituted silenolate is more nucleophilic than the silyl anion. This paper demonstrates that 2a,b and 4a,b have the potential to be used as unique building blocks for complex polysilane and polygermane frameworks.

Silyl Anion Initiated Hydroboration of Aldehydes and Ketones.

Hydroboration is an emerging method for mild and selective reduction of carbonyl compounds. Typically, transition‐metal or reactive main‐group hydride catalysts are used in conjunction with a mild reductant such as pinacolborane. The reactivity of the main‐group catalysts is a consequence of the nucleophilicity of their hydride ligands. Silicon hydrides are significantly less reactive and are therefore not efficient hydroboration catalysts. Here, a readily prepared silyl anion is reported to be an effective initiator for the reduction of aldehydes and ketones requiring mild conditions, low catalyst loadings and with a good substrate scope. The silyl anion it is shown to activate HBpin to generate a reactive borohydride in situ which reacts with aldehydes and ketones to afford the hydroboration product.

Catalytic Metal-Free Deoxygenation of Nitrous Oxide with Disilanes

Because of its high kinetic stability, conditions to reduce the greenhouse gas N2O are limited; therefore, a better understanding of N2O chemistry and N–O bond cleavage is required. In this work, N2O was deoxygenated under metal-free conditions. Using disilanes as reducing agents and a catalytic amount of fluoride anions or alkoxides allowed a mild reduction at ambient pressure and temperature. DFT calculations unveiled the mechanism, which shows a nucleophilic addition of a silyl anion to the central N atom of N2O and release of N2 from a pseudo-Brook rearrangement.

Cover Picture: Transition‐Metal‐Free Defluorosilylation of Fluoroalkenes with Silylboronates* (Chin. J. Chem. 10/2019)

The cover picture shows a simple, inexpensive and robust method on defluorosilylation of diverse fluoroalkenes with silylboronates in the presence of alkoxy base to directly synthesize various silylated fluoroalkenes. Density functional theory calculations revealed that transient silyl anion complex undergoes SN2’ or SNV substitution, which is responsible for this base‐mediated defluorosilylation reaction, thus obviating the need for copper salts. More details are discussed in the article by Shi et al. on page 1009–1014. image

Transition‐Metal‐Free Defluorosilylation of Fluoroalkenes with Silylboronates

Summary of main observation and conclusionSilylated fluoroalkenes are important synthetic intermediates with complementary reactivity, which play a key role in the construction of natural products, pharmaceuticals, and manmade materials. Converting the normally highly stable fluoroalkenes into silylated fluoroalkenes by selective defluorosilylation is a challenging task. Herein, we report a simple, inexpensive and robust defluorosilylation of a variety of fluoroalkenes with silylboronates in the presence of alkoxy base to directly synthesize various silylated fluoroalkenes. The protocol features mild and safe reaction conditions that avoid a catalyst, a transition metal, a ligand, and high reaction temperature and tolerates a wide scope of fluoroalkene substrates without compromising the efficiency. Density functional theory calculations show that transient silyl anion complex undergoes an SN2’ or SNV substitution, which is responsible for this base‐mediated defluorosilylation.

In Situ Generation of Silyl Anion Species through Si-B Bond Activation for the Concerted Nucleophilic Aromatic Substitution of Fluoroarenes.

In situ generated silyl anion species enable the concerted nucleophilic aromatic substitution of fluoroarenes. Model DFT calculations indicated that addition of a base to a silylborane would thermodynamically form a silyl borate complex and then kinetically release a silyl anion species through Si-B bond cleavage, and that the in situ generated silyl anion equivalent would further react with a fluoroarene through a concerted nucleophilic aromatic substitution pathway with an activation barrier of ca. 20 kcal/mol to afford the silylated product with a large energy gain. Experiments confirmed that the defluorosilylation reaction took place smoothly at room temperature simply upon mixing fluoroarenes with commercially available silylborane and NaOt Bu. Radical scavenger and radical clock reaction experiments provide further evidence for the in situ generation of the silyl anion.