Silyl Cations Research Articles

Heavier Group 14 Congeners of Metalla‐isobenzenes: Access to Metalla‐isosilabenzene and Metalla‐isogermabenzene by One‐Pot Reactions

Comprehensive SummaryMetalla‐isosilabenzenes and metalla‐isogermabenzenes have been successfully synthesized by the formal [5+1]‐cycloaddition of diethynylsilane or diethynylgermane with simple metal complexes. This is the first example of a heavier Group 14 metalla‐isobenzene isomer bearing a cumulative double bond motif within a metallacycle. These novel complexes were fully characterized by NMR spectroscopy and single‐crystal X‐ray diffraction analysis. The stabilization of the cyclic metal‐vinylidene complexes has been analyzed using density functional theory (DFT) calculations. When the metalla‐isosilabenzenes bearing Si—H bond were treated with the trityl salt as a hydride scavenger, the formation of silylium cation was observed spectroscopically. Both of metalla‐isosilabenzenes and metalla‐isogermabenzenes can readily undergo migratory insertion reactions to furnish siloles or germoles.

P(V)-bis(amidophenolate) ligand cooperation: stoichiometric CO-bond cleavage in aldehydes and ketones.

The cooperation between a geometrically constrained, highly electrophilic phosphorus(V) center, and an electronically rich tetradentate bis(amidophenolate) ligand enables the cleavage of the CO bond from typical aldehydes and ketones delivering iminio phosphoramidate species. The amphiphilic nature of these products, which is demonstrated through their reaction with typical Lewis acids and bases, enables their use as a mild source of silylium cations from silanes, allowing the selective reductive coupling of aldehydes to ethers under catalytic conditions.

Mechanism of Nitrogen-Carbon Bond Formation from Iron(IV) Disilylhydrazido Intermediates during N2 Reduction.

We recently reported a reaction sequence that activates C-H bonds in simple arenes as well as the N-N triple bond in N2, delivering the aryl group to N2 to form a new N-C bond (Nature 2020, 584, 221). This enables the transformation of abundant feedstocks (arenes and N2) into N-containing organic compounds. The key N-C bond forming step occurs upon partial silylation of N2. However, the pathway through which reduction, silylation, and migration occurred was unknown. Here, we describe synthetic, structural, magnetic, spectroscopic, kinetic, and computational studies that elucidate the steps of this transformation. N2 must be silylated twice at the distal N atom before aryl migration can occur, and sequential silyl radical and silyl cation addition is a kinetically competent pathway to a formally iron(IV)-NN(SiMe3)2 intermediate that can be isolated at low temperature. Kinetic studies show its first-order conversion to the migrated product, and DFT calculations indicate a concerted transition state for migration. The electronic structure of the formally iron(IV) intermediate is examined using DFT and CASSCF calculations, which reveal contributions from iron(II) and iron(III) resonance forms with oxidized NNSi2 ligands. The depletion of electron density from the Fe-coordinated N atom makes it electrophilic enough to accept the incoming aryl group. This new pathway for the N-C bond formation offers a method for functionalizing N2 using organometallic chemistry.

Photoinitiated Cationic Ring-Opening Polymerization of Octamethylcyclotetrasiloxane.

Photochemical techniques have recently been revitalized as they can readily be adapted to different polymerization modes to yield a wide range of complex macromolecular structures. However, the implementation of the photoinduced cationic methods in the polymerization of cyclic siloxane monomers has scarcely been investigated. Octamethylcyclotetrasiloxane (D4) is an important monomer for the synthesis of polydimethylsiloxane (PDMS) and its copolymers. In this study, the cationic ring-opening polymerization (ROP) of D4, initiated by diphenyl iodonium hexafluorophosphate (DPI), has been studied. Both direct and indirect initiating systems acting at broad wavelength using benzophenone and pyrene were investigated. In both systems, photochemically generated protonic acids and silylium cations are responsible for the polymerization. The kinetics of the polymerization are followed by viscosimetry and GPC analyses. The reported approach may overcome the problems associated with conventional methods and therefore represents industrial importance for the fabrication of polysiloxanes.

Synthesis, Characterization, and Properties of Three-Dimensional Analogues of 9-Borafluorenes.

Three-dimensional (3D) analogues of 9-borafluorenes, (C2B10H10)2BR (R = Cl (1), Br (2), H (3, merely as ether or silane adduct), phenyl (4), mesityl (5)), were synthesized and fully characterized. Gutmann-Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies confirmed the Lewis superacidity of 1-4, with the Lewis acidity of 1-3 being higher than that of the corresponding 3D analogues of 9,10-diboraanthracenes (DBA). The phenyl group and the C4B borole unit are nearly coplanar in the solid state, while the rotation of the mesityl group of 5 is hindered, forcing 5 to adopt a large C4B/Mes dihedral angle, which leads to distinct properties: air-stable and bluish-violet fluorescence arising from the intramolecular charge-transfer (ICT) transition. The 29Si NMR spectra indicated a high silylium cation character of 3·Et3SiH. The reaction of 4 with Ph3CBr yielded (Ph3C)+[(C2B10H10)2B(Ph)Br]- (6). The 4/PPh3 Lewis pair was capable of splitting dihydrogen and Si-H bond of triethylsilane.

A Reductive Benzylation for Benzenes Using Aroyl Chlorides and Triethylsilane Catalyzed by Aluminosilicate-Stabilized Silyl Cations on Montmorillonite

AbstractWe discovered that the aluminosilicate-stabilized silyl cations, which were created from a solid-acid catalyst, the proton-exchanged montmorillonite, and Et3SiH, efficiently promoted the reductive benzylation of benzenes with aromatic carboxylic acid chlorides and Et3SiH in one pot.

Silyl Cation-Initiated, Brønsted Acid-Catalyzed Strategy toward Unsymmetrical 3,3-Disubstituted 2-Oxindoles and Azonazine Cores.

Herein, a mild, metal-free, robust approach for synthesizing valuable and sterically demanding unsymmetrical 3,3-disubstituted 2-oxindoles via reductive cyclization of α-ketoamides is reported. This operationally simple protocol is initiated by a silyl cation and further catalyzed by a Brønsted acid. We have utilized a wide range of arenes, amines, and thiols as coupling partners with various α-ketoamides. The products were afforded in excellent regioselectivity and good functional group tolerance. This procedure provides easy access to the scaffolds of azonazine and its derivatives with an excellent syn-diastereoselectivity bearing all-carbon quaternary stereocenters.

Trityl Cation-Catalyzed Hosomi-Sakurai Reaction of Allylsilane with β,γ-Unsaturated α-Ketoester to Form γ,γ-Disubstituted α-Ketoesters.

(Ph3C)[BPh(F)4]-catalyzed Hosomi-Sakurai allylation of allylsilanes with β,γ-unsaturated α-ketoesters has been developed to give γ,γ-disubstituted α-ketoesters in high yields with excellent chemoselectivity. Preliminary mechanistic studies suggest that trityl cation dominates the catalysis, while the silyl cation plays a minor role.

Front Cover: Electrophilic C(sp2)−H Silylation by Supported Gold Catalysts (ChemCatChem 22/2021)

The Front Cover illustrates the gold nanoparticles-catalyzed silylation of C(sp2)−H bond of heteroarenes and electron-rich arenes to give the corresponding aryl- and heteroarylsilanes. Activation of Au nanoparticles by molecular O2 and stabilization of highly reactive silyl cation by ethers are the keys for efficient C−H silylation. In their Full Paper, H. Miura et al. show the high activity, reusability and environmentally-friendly nature of the heterogeneous gold catalysts as well as the scalability of the reaction system, which enable the sustainable synthesis of aryl and heteroarylsilanes. More information can be found in the Full Paper by H. Miura et al.

Electrophilic C(sp2)−H Silylation by Supported Gold Catalysts

AbstractHeterogeneous gold‐catalyzed C(sp2)−H silylation is described. A variety of heteroarenes and electron‐rich arenes participated in gold‐catalyzed silylation to afford the corresponding heteroaryl and arylsilanes in good to excellent yields. A detailed mechanistic investigation corroborated that the cooperative catalysis of ether and O2‐activated gold nanoparticles realized heterolysis of the Si−H bond to generate silyl cations, which enabled subsequent electrophilic C−Si bond formation. The high activity, reusability and environmentally‐friendly nature of the heterogeneous gold catalysts as well as the scalability of the reaction system realize the sustainable synthesis of aryl and heteroarylsilanes.

Bipyridinium and Phenanthrolinium Dications for Metal-Free Hydrodefluorination: Distinctive Carbon-Based Reactivity.

The development of novel Lewis acids derived from bipyridinium and phenanthrolinium dications is reported. Calculations of Hydride Ion Affinity (HIA) values indicate high carbon-based Lewis acidity at the ortho and para positions. This arises in part from extensive LUMO delocalization across the aromatic backbones. Species [C10 H6 R2 N2 CH2 CH2 ]2+ (R=H [1 a]2+ , Me [1 f]2+ , tBu [1 g]2+ ), and [C12 H4 R4 N2 CH2 CH2 ]2+ (R=H [2 a]2+ , Me [2 b]2+ ) were prepared and evaluated for use in the initiation of hydrodefluorination (HDF) catalysis. Compound [2 a]2+ proved highly effective towards generating catalytically active silylium cations via Lewis acid-mediated hydride abstraction from silane. This enabled the HDF of a range of aryl- and alkyl- substituted sp3 (C-F) bonds under mild conditions. The protocol was also adapted to effect the deuterodefluorination of cis-2,4,6-(CF3 )3 C6 H9 . The dications are shown to act as hydride acceptors with the isolation of neutral species C16 H14 N2 (3 a) and C16 H10 Me4 N2 (3 b) and monocationic species [C14 H13 N2 ]+ ([4 a]+ ) and [C18 H21 N2 ]+ ([4 b]+ ). Experimental and computational data provide further support that the dications are initiators in the generation of silylium cations.

Quinoline‐Based Silylium Ions: Synthesis, Structure and Lewis Acidity

AbstractHeteroarylsilyl cations elaborated from a quinoline skeleton have been prepared from the corresponding silanes, and their structures were studied by 1H, 29Si, and 15N NMR. DOSY experiments and DFT calculations were also carried out which showed that the silyl cation center is stabilized intramolecularly by the quinoline nitrogen atom (N1), forming a highly strained and quasi planar 4‐membered ring. Oxygenated and nitrogenated substituents at the C4 position on the quinoline ring were shown to reinforce the Si−N1 interaction through a p‐π conjugation. The Lewis acidity of this class of silylium was finally determined using Müller's nitrilium method.

Reactivities of Silaimines with Boranes: From Cooperative B–H Bond Activation to Donor Stabilized Silyl Cation

Reactivities of Silaimines with Boranes: From Cooperative B–H Bond Activation to Donor Stabilized Silyl Cation

The Trityl‐Cation Mediated Phosphine Oxides Reduction

AbstractReduction of phosphine oxides into the corresponding phosphines using PhSiH3 as a reducing agent and Ph3C+[B(C6F5)4]− as an initiator is described. The process is highly efficient, reducing a broad range of secondary and tertiary alkyl and arylphosphines, bearing various functional groups in generally good yields. The reaction is believed to proceed through the generation of a silyl cation, which reaction with the phosphine oxide provides a phosphonium salt, further reduced by the silane to afford the desired phosphine along with siloxanes.magnified image

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.

Intramolecular Halo Stabilization of Silyl Cations-Silylated Halonium- and Bis-Halo-Substituted Siliconium Borates.

The stabilizing neighboring effect of halo substituents on silyl cations was tested for a series of peri‐halo substituted acenaphthyl‐based silyl cations 3. The chloro‐ (3 b), bromo‐ (3 c), and iodo‐ (3 d) stabilized cations were synthesized by the Corey protocol. Structural and NMR spectroscopic investigations for cations 3 b–d supported by the results of density functional calculations, which indicate their halonium ion nature. According to the fluorobenzonitrile (FBN) method, the silyl Lewis acidity decreases along the series of halonium ions 3, the fluoronium ion 3 a being a very strong and the iodonium ion 3 d a moderate Lewis acid. Halonium ions 3 b and 3 c react with starting silanes in a substituent redistribution reaction and form siliconium ions 4 b and 4 c. The structure of siliconium borate 4 c 2[B12Br12] reveals the trigonal bipyramidal coordination environment of the silicon atom with the two bromo substituents in the apical positions.

Hydride Affinities for Main-Group Hydride Reductants: Assessment of Density Functionals and Trends in Reactivities.

In the present study, we have examined hydride affinities relevant to a range of group 13 and group 14 reductants. We use the high-level W1X-G0, G4(MP2)-XK, and DSD-PBEP86 methods to obtain the RHA42 set of accurate reductant hydride affinities. Assessment of DFT methods with the RHA42 set shows that all functionals that we have examined are fairly accurate. Overall, we find ωB97X-V to be the most accurate. The MN12-SX screened-exchange functional and the nonhybrid B97-D3BJ method also perform well, and they may provide a lower-cost means for obtaining hydride affinities. The trend in the hydride affinities suggests an increased reducing power when one moves down the periodic table, e.g., with TlH3 being a stronger reductant than BH3. We also find that group 13 hydrides are stronger reductants than the group 13 analogues. In general, substitution of a hydrogen, e.g., BH2+ → BHMe+, and the formation of dimer, e.g., BH2+ → B2H5+, also lead to stronger reductants. A notable observation is the small hydride affinities for silyl cations, which are indicative of the potential of silanes as strong reducing agents. In particular, poly(methylhydrosiloxane) (PMHS) cations are associated with especially small hydride affinities owing to the presence of intramolecular oxygen atoms that can stabilize the cation center. We have further found the germanium analogues of the silanes to be more reactive, and they may further widen the scope of main-group hydride reducing agents.

Activation of CS2 and CO2 by Silylium Cations.

The hydride‐bridged silylium cation [Et3Si−H−SiEt3]+, stabilized by the weakly coordinating [Me3NB12Cl11]− anion, undergoes, in the presence of excess silane, a series of unexpected consecutive reactions with the valence‐isoelectronic molecules CS2 and CO2. The final products of the reaction with CS2 are methane and the previously unknown [(Et3Si)3S]+ cation. To gain insight into the entire reaction cascade, numerous experiments with varying conditions were performed, intermediate products were intercepted, and their structures were determined by X‐ray crystallography. Besides the [(Et3Si)3S]+ cation as the final product, crystal structures of [(Et3Si)2SMe]+, [Et3SiS(H)Me]+, and [Et3SiOC(H)OSiEt3]+ were obtained. Experimental results combined with supporting quantum‐chemical calculations in the gas phase and solution allow a detailed understanding of the reaction cascade.

Selective Electrochemical Hydrolysis of Hydrosilanes to Silanols via Anodically Generated Silyl Cations.

The first electrochemical hydrolysis of hydrosilanes to silanols under mild and neutral reaction conditions is reported. The practical protocol employs commercially available and cheap NHPI as a hydrogen-atom transfer (HAT) mediator and operates at room temperature with high selectivity, leading to various valuable silanols in moderate to good yields. Notably, this electrochemical method exhibits a broad substrate scope and high functional-group compatibility, and it is applicable to late-stage functionalization of complex molecules. Preliminary mechanistic studies suggest that the reaction appears to proceed through a nucleophilic substitution reaction of an electrogenerated silyl cation with H2 O.

Selective Electrochemical Hydrolysis of Hydrosilanes to Silanols via Anodically Generated Silyl Cations

AbstractThe first electrochemical hydrolysis of hydrosilanes to silanols under mild and neutral reaction conditions is reported. The practical protocol employs commercially available and cheap NHPI as a hydrogen‐atom transfer (HAT) mediator and operates at room temperature with high selectivity, leading to various valuable silanols in moderate to good yields. Notably, this electrochemical method exhibits a broad substrate scope and high functional‐group compatibility, and it is applicable to late‐stage functionalization of complex molecules. Preliminary mechanistic studies suggest that the reaction appears to proceed through a nucleophilic substitution reaction of an electrogenerated silyl cation with H2O.