Silylenes, Silenes, and Disilenes: Novel Silicon-Based Reagents for Organic Synthesis?

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Wavelength-dependent rearrangements of an α-dione chromophore: a chemical pearl in a bis(hypersilyl) oyster

The symmetric bissilyl-dione 3 reveals two well-separated n → π* absorption bands at λmax = 637 nm (ε = 140 mol-1 dm3 cm-1) and 317 nm (ε = 2460 mol-1 dm3 cm-1). Whereas excitation of 3 at λ = 360/365 nm affords an isolable siloxyketene 4 in excellent yields, irradiation at λ = 590/630 nm leads to the stereo-selective and quantitative formation of the siloxyrane 5. These remarkable wavelength-dependent rearrangements are based on the electronic and steric properties provided by the hypersilyl groups. While the siloxyketene 4 is formed via a hitherto unknown 1,3-hypersilyl migration via the population of a second excited singlet state (S2, λmax = 317 nm, a rare case of anti-Kasha reactivity), the siloxyrane 5 emerges from the first excited triplet state (T1via S1λmax = 637 nm). These distinct reaction pathways can be traced back to specific energy differences between the S2, S1 and T1, an electronic consequence of the bissilyl substited α-dione (the "pearl"). The hypersilyl groups act as protective ''oyster shell", which are responsible for the clean formation of 4 and 5 basically omitting side products. We describe novel synthetic pathways to achieve hypersilyl substitution (3) and report an in-depth investigation of the photorearrangements of 3 using UV/vis, in situ IR, NMR spectroscopy and theoretical calculations.

Markovnikov-selective double hydrosilylation of challenging terminal aryl alkynes under cobalt and iron catalysis

Geminal bis(silanes) are unique compounds with interesting properties. The most straightforward way to access them is double hydrosilylation of alkynes, which was established only recently. Previous articles about transition metal-catalysed double hydrosilylation show that terminal aryl alkynes are a challenge. We report on cobalt(II) and iron(III) complexes with the easy-to-synthesise N,N,N-tridentate hydrazone ligand being active precatalysts in Markovnikov-selective double hydrosilylation of terminal aryl alkynes. The influence of the hydrazone ligand structure and the potential role of the sodium triethylborohydride activator were studied. Sets of geminal bis(silanes) with two identical or different silyl groups were synthesised, showing the applicability of the reported method.

Stereoselective Activation of Small Molecules by a Stable Chiral Silene

The reaction of the dilithium salt of the enantiopure (S)-BINOL (1,1'-bi-2-naphthol) with two equivalents of the amidinate-stabilized chlorosilylene [L^Ph SiCl] (L^Ph =PhC(NtBu)₂ ) led to the formation of the first example of a chiral cyclic silene species comprising an (S)-BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO₂ and HCl. The reaction with S₈ led to a Si=C bond cleavage and concomitantly to a ring-opened product with imine and silanethione functional groups. The reaction with CO₂ resulted in the cleavage of the CO₂ molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO₂ and HCl to a Si=C bond is unprecedented.

Accessing Cationic α-Silylated and α-Germylated Phosphorus Ylides

The synthesis and full characterization of α‐silylated (α‐SiCPs; 1–7) and α‐germylated (α‐GeCPs; 11–13) phosphorus ylides bearing one chloride substituent R₃PC(R¹)E(Cl)R²₂ (R=Ph; R¹=Me, Et, Ph; R²=Me, Et, iPr, Mes; E=Si, Ge) is presented. The molecular structures were determined by X‐ray diffraction studies. The title compounds were applied in halide abstraction studies in order to access cationic species. The reaction of Ph₃PC(Me)Si(Cl)Me₂ (1) with Na[B(C₆F₅)₄] furnished the dimeric phosphonium‐like dication [Ph₃PC(Me)SiMe₂]₂[B(C₆F₅)₄]₂ (8). The highly reactive, mesityl‐ or iPr‐substituted cationic species [Ph₃PC(Me)SiMes₂][B(C₆F₅)₄] (9) and [Ph₃PC(Et)SiiPr₂][B(C₆F₅)₄] (10) could be characterized by NMR spectroscopy. Carrying out the halide abstraction reaction in the sterically demanding ether iPr₂O afforded the protonated α‐SiCP [Ph₃PCH(Et)Si(Cl)iPr₂][B(C₆F₅)₄] (6 dec) by sodium‐mediated basic ether decomposition, whereas successfully synthesized [Ph₃PC(Et)SiiPr₂][B(C₆F₅)₄] (10) readily cleaves the F−C bond in fluorobenzene. Thus, the ambiphilic character of α‐SiCPs is clearly demonstrated. The less reactive germanium analogue [Ph₃PC(Me)GeMes₂][B{3,5‐(CF₃)₂C₆H₃}₄] (14) was obtained by treating 11 with Na[B{3,5‐(CF₃)₂C₆H₃}₄] and fully characterized including by X‐ray diffraction analysis. Structural parameters indicate a strong C_Ylide−Ge interaction with high double bond character, and consequently the C−E (E=Si, Ge) bonds in 9, 10 and 14 were analyzed with NBO and AIM methods.

Sila-Peterson Reaction of Cyclic Silanides

Sila-Peterson type reactions of the 1,4,4-tris(trimethylsilyl)-1-metallooctamethylcyclohexasilanes (Me₃Si)₂Si₆Me₈(SiMe₃)M (2a, M = Li; 2b, M = K) with various ketones were investigated. The obtained products strongly depend on the nature of the ketone component. With 2-adamantanone 2a,b afforded the moderately stable silene 3. 3 is the first example of an Apeloig–Ishikawa–Oehme-type silene with the tricoordinate silicon atom incorporated into a cyclopolysilane framework and could be characterized by NMR and UV spectroscopy as well as by trapping reactions with water, methanol, and MeLi. The reaction of 2b with aromatic ketones also follows a sila-Peterson type mechanism with formation of carbanionic species. With 1,2-diphenylcyclopropenone 2b reacted by conjugate 1,4-addition to give a spirocyclic carbanion. In most cases the underlying reaction mechanism could be elucidated by the isolation and characterization of unstable intermediates and final products after proper derivatization.

Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Interconversions between manganese silylene and silene complexes are reported, including those involving the first spectroscopically observed silene complexes with an SiH substituent, and their involvement in ethylene hydrosilylation is discussed.

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). Compounds 6^R,H reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6^Bu,H slowly isomerized to the silylene hydride complex [(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiEtⁿBu)] (3^Bu,Et). Reactions of 4^R with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu)} or silylene hydride {[(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu)} intermediates accessed from 4^R by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4^R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a–d: R = Ph or ⁿBu; R′ = o-xylyl or ^tBu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{ Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR(NHC)}] (8a–d: R = Ph or ⁿBu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.

Synthesis of Unsaturated Silyl Heterocycles via an Intramolecular Silyl-Heck Reaction

We report the synthesis of unsaturated silacycles via an intramolecular silyl-Heck reaction. Using palladium catalysis, silicon electrophiles tethered to alkenes cyclize to form 5- and 6-membered silicon heterocycles. The effects of alkene substitution and tether length on the efficiency and regioselectivity of the cyclizations are described. Finally, through the use of an intramolecular tether, the first examples of disubstituted alkenes in silyl-Heck reactions are reported.

Mechanism of the Thermal Z⇌E Isomerization of a Stable Silene; Experiment and Theory

The E and Z geometric isomers of a stable silene (tBu₂ MeSi)(tBuMe₂ Si)Si=CH(1-Ad) (1) were synthesized and characterized spectroscopically. The thermal Z to E isomerization of 1 was studied both experimentally and computationally using DFT methods. The measured activation parameters for the 1Z⇌1E isomerization are: E_a =24.4 kcal mol^-1 , ΔH^≠ =23.7 kcal mol^-1 , ΔS^≠ =-13.2 e.u. Based on comparison of the experimental and DFT calculated (at BP86-D3BJ/def2-TZVP(-f)//BP86-D3BJ/def2-TZVP(-f)) activation parameters, the Z⇌E isomerization of 1 proceeds through an unusual (unprecedented for alkenes) migration-rotation-migration mechanism (via a silylene intermediate), rather than through the classic rotation mechanism common for alkenes.

Reactivity of a Sterically Unencumbered α-Borylated Phosphorus Ylide towards Small Molecules

The influence of substituents on α‐borylated phosphorus ylides (α‐BCPs) has been investigated in a combined experimental and quantum chemical approach. The synthesis and characterization of Me₃PC(H)B(iBu)₂ (1), consisting of small Me substituents on phosphorous and iBu residues on boron, is reported. Compound 1 is accessible through a novel synthetic approach, which has been further elucidated through DFT studies. The reactivity of 1 towards various small molecules was probed and compared with that of a previously published derivative, Ph₃PC(Me)BEt₂ (2). Both α‐BCPs react with NH₃ to undergo heterolytic N−H bond cleavage. Different di‐ and trimeric ring structures were observed in the reaction products of 1 with CO and CO₂. With PhNCO and PHNCS, the expected insertion products [Me₃PC(H)(PhNCO)B(iBu)₂] and [Me₃PC(H)(PhNCS)B(iBu)₂], respectively, were isolated.

Geminal Dianions Stabilized by Main Group Elements

This review is dedicated to the chemistry of stable and isolable species that bear two lone pairs at the same C center, i.e., geminal dianions, stabilized by main group elements. Three cases can thus be considered: the geminal-dilithio derivative, for which the two substituents at C are neutral, the yldiide derivatives, for which one substituent is neutral while the other is charged, and finally the geminal bisylides, for which the two substituents are positively charged. In this review, the syntheses and electronic structures of the geminal dianions are presented, followed by the studies dedicated to their reactivity toward organic substrates and finally to their coordination chemistry and applications.

A Germacalicene: Synthesis, Structure, and Reactivity

The synthesis of the germacalicene 7 from the reaction of the dipotassium germole dianion K₂ [6] with 1,2-bis-diisopropylamino-3-chlorocyclopropenyl perchlorate is reported. Based on the crystal structure analysis and the results of DFT calculations, the germacalicene 7 can be viewed as a cyclopropenium germacyclopentadienide ylide that is isoelectronic to α-cationic phosphanes. First reactivity studies revealed its nucleophilic character and resulted in the isolation of the air- and moisture-stable carbonyl iron complex 15 and the cationic silver complex 20. One-electron oxidation of the germacalicene 7 was achieved by its reaction with [Ph₃ C][B(C₆ F₅ )₄ ] and the bis-cationic Ge-Ge-bonded dimer 22 was isolated.

Electrochemical Arylation Reaction

Arylated products are found in various fields of chemistry and represent essential entities for many applications. Therefore, the formation of this structural feature represents a central issue of contemporary organic synthesis. By the action of electricity the necessity of leaving groups, metal catalysts, stoichiometric oxidizers, or reducing agents can be omitted in part or even completely. The replacement of conventional reagents by sustainable electricity not only will be environmentally benign but also allows significant short cuts in electrochemical synthesis. In addition, this methodology can be considered as inherently safe. The current survey is organized in cathodic and anodic conversions as well as by the number of leaving groups being involved. In some electroconversions the reagents used are regenerated at the electrode, whereas in other electrotransformations free radical sequences are exploited to afford a highly sustainable process. The electrochemical formation of the aryl-substrate bond is discussed for aromatic substrates, heterocycles, other multiple bond systems, and even at saturated carbon substrates. This survey covers most of the seminal work and the advances of the past two decades in this area.

Cycloaddition of carbonyl compounds and alkynes to (di)silenes and (di)germenes: reactivity and mechanism

The cycloaddition reactions of carbonyl compounds and alkynes to (di)tetrelenes appear to follow Woodward–Hoffman rules.

Structures and electronic properties of B2Si6−/0/+: anion photoelectron spectroscopy and theoretical calculations

The lowest-energy structures of B₂Si₆^q(q= −1, 0, +1) clusters are a peculiar structure with a silicon atom hanging over a distorted bowl-like B₂Si₅framework. It is characterized with σ or π delocalization in chemical bonding.

Coordination complexes of thiazyl rings — Synthesis, structure, and density functional theory (DFT) computational analysis of CpCr(CO)x (x = 2, 3) complexes of fluorinated and nonfluorinated 1λ3-1,2,4,6-thiatriazinyls with differing Cr–S bond orders

The reaction of [3,5-Ph2-C2N3S]2 with [CpCr(CO)3]2 in toluene at room temperature forms an adduct via a Cr–S bond, formulated as CpCr(CO)3SN3C2Ph2, which has fitting NMR, IR, and combustion analysis data. The structure was determined by a single-crystal X-ray structure diffraction study (P21/n, a = 8.4611(17) Å, b = 20.509(4) Å, c = 11.757(2) Å, β = 104.453(7)°). The Cr–S bond length of 2.4908(11) Å corresponds to a bond order of 1.0 from >90 values for CpCr(CO)x or Cp*Cr(CO)x moieties (x = 2, 3) bonded to S, which are used to establish a Pauling-type bond order scale specific to this class of compounds. Similar reactions of fluorinated thiatriazinyls derived from [3-Ph-5-CF3-C2N3S]2 or [4-MeOC6H4-5-CF3-C2N3S]2 are accompanied by the loss of CO to produce CpCr(CO)2SN3C2PhCF3 (P1, a = 8.0929(8) Å, b = 10.3160(10) Å, c = 11.2405(11) Å, α = 70.032(2)°, β = 72.076(2)°, γ = 82.375(2)°) and CpCr(CO)2SN3(CCF3)(C6H4OCH3) (P21/c, a = 8.1311(7) Å, b = 24.284(2) Å, c = 9.1025(8) Å, β = 97.218(2)°), also fully characterized by spectroscopy and crystallography. Their measured Cr–S bond lengths, 2.2987(14) and 2.2965(11) Å, correspond to bond orders of 1.5. (U/R)B3PW91/6-311+G(2df,2p)//B3PW91/6-31G(2d,p) hybrid density functional theory (DFT) calculations show that the tricarbonyl complex has an unusual σ bond. However, the dicarbonyl complexes of the fluorinated thiatriazinyls are π bonded.

Mechanism of the addition of alkynes to silenes and germenes: A density functional study

Density functional theory (DFT) and the coupled cluster method have been used to study the mechanism of the cycloaddition of acetylene to silene, H2Si=CH2, and germene, H2Ge=CH2, at the B3LYP/6-311++G(d,p) and CCSD/6-311++G(d,p) levels of theory. Diradical, zwitterionic, and concerted pathways were located for both metallenes. The computational results were compared to experimental data to propose the most likely reaction pathway for each metallene.

Reactivity of an NHC-stabilized silylene towards ketones. Formation of silicon bis-enolates vs. bis-silylation of the CO bond

The reaction of NHC-stabilized silylaminosilylene 1 with enolizable ketones rapidly led to the regio- and stereoselective formation of silicon bis-enolates in excellent yields under mild conditions.

Stable Silenolates and Brook-Type Silenes with Exocyclic Structures

The first silenolates with exocyclic structures [(Me₃Si)₂Si(Si₂Me₄)₂SiC(R)O]⁻K⁺ (2a: R = 1-adamantyl; 2b: mesityl; 2c: o-tolyl) were synthesized by the reaction of the corresponding acylcyclohexasilanes 1a–c with KOtBu. NMR spectroscopy and single-crystal X-ray diffraction analysis suggest that the aryl-substituted silenolates 2b,c exhibit increased character of functionalized silenes as compared to the alkyl-substituted derivative 2a due to the different coordination of the K⁺ counterion to the SiC(R)O moiety. 2b,c, thus, reacted with ClSiiPr₃ to give the exocyclic silenes (Me₃Si)₂Si(Si₂Me₄)₂Si=C(OSiiPr₃)R (3b: R = Mes; 3c: o-Tol), while 2a afforded the Si-silylated acylcyclohexasilane 1d. The thermally remarkably stable compound 3b, which is the first isolated silene with the sp² silicon atom incorporated into a cyclopolysilane framework, could be fully characterized structurally and spectroscopically.

The addition of terminal alkynes to dimesitylfluorenylidenegermane

A variety of terminal alkynes were added to dimesitylfluorenylidenegermane, Mes2Ge=CR2 (where CR2 = fluorenylidene). The addition of phenylacetylene and 1-hexyne to Mes2Ge=CR2 gave a germacyclohexene via a cycloaddition where the germene acts as the 4π component and the alkyne as the 2π component. Through the use of a mechanistic probe, trans-(2-phenylcyclopropyl)-acetylene, the reaction was postulated to proceed through a concerted [2+4] cycloaddition. The addition of ethoxyacetylene to the germene produced both a [2+2] cycloadduct, a germacyclobutene, and a [2+4] cycloadduct, a germacyclohexene. The results of this study are compared to the results of the addition of alkynes to Mes2Ge=CHCH2-t-Bu.

The first intramolecular silene Diels-Alder reactions

The synthesis of silaheterocycles through the first examples of an intramolecular silene Diels-Alder reaction is described.

Photoinduced Brook-Type Rearrangement of Acylcyclopolysilanes

Previously unknown 1,1,4-tris(trimethylsilyl)-4-acyldodecamethylcyclohexasilanes (Me₃Si)₂Si₆Me₁₂(Me₃Si)COR (16a, R = tert-butyl; 16b, R = 1-adamantyl) have been synthesized by the reaction of the potassium silanides (Me₃Si)₂Si₆Me₁₂(Me₃Si)K with acid chlorides ClCOR, and their photochemical rearrangement reactions have been studied. The molecular structures of 16a,b as determined by single-crystal X-ray diffraction analysis exhibit an unusual twist-boat conformation of the cyclohexasilane ring. When 16a,b were photolyzed with λ >300 nm radiation, they underwent Brook type 1,3-Si → O migration reactions to generate the cyclohexasilanes 17a,b with exocyclic Si=C bonds along with smaller amounts of the ring-enlarged species 19a,b with endocyclic Si=C double bonds. While 17a,b were stable enough to allow characterization by NMR and UV absorption spectroscopy, the less stable products 19a,b could only be observed in the form of their methanol adducts.

New Class of Molecular Conductance Switches Based on the [1,3]-Silyl Migration from Silanes to Silenes

On the basis of first-principles density functional theory calculations, we propose a new molecular photoswitch which exploits a photochemical [1,3]-silyl(germyl) shift leading from a silane to a silene (a Si=C double bonded compound). The silanes investigated herein act as the OFF state, with tetrahedral saturated silicon atoms disrupting the conjugation through the molecules. The silenes, on the other hand, have conjugated paths spanning over the complete molecules and thus act as the ON state. We calculate ON/OFF conductance ratios in the range of 10-50 at a voltage of +1 V. In the low bias regime, the ON/OFF ratio increases to a range of 200-1150. The reverse reaction could be triggered thermally or photolytically, with the silene being slightly higher in relative energy than the silane. The calculated activation barriers for the thermal back-rearrangement of the migrating group can be tuned and are in the range 108-171 kJ/mol for the switches examined herein. The first-principles calculations together with a simple one-level model show that the high ON/OFF ratio in the molecule assembled in a solid state device is due to changes in the energy position of the frontier molecular orbitals compared to the Fermi energy of the electrodes, in combination with an increased effective coupling between the molecule and the electrodes for the ON state.