Silyl Group Research Articles

A Distinct Mode of Strain-Driven Cyclic Allene Reactivity: Group Migration to the Central Allene Carbon Atom.

Strained cyclic allenes are reactive species that can be trapped in a variety of complementary fashions that capitalize on their inherent high potential energy. 1,2,4-Cyclohexatrienes represent a subclass of allenes that, notably, can be conveniently generated by a net [4 + 2] cycloaddition within a 1,3-enyne bearing a tethered alkyne via a tetradehydro-Diels-Alder reaction. A limitation to the use of this type of thermally generated cyclic allene as a construct for the introduction of molecular complexity is their propensity to isomerize to benzenoids via a simple net 1,5-hydrogen atom migration. We have discovered that when the enyne component of the substrate is modified as an enol silyl ether (or an enol ester), migration of the silyl (or acyl) group can become the predominant event. Specifically, an appropriately electrophilic group can migrate from the O atom to the central allene carbon adjacent to the 1-siloxy(acyloxy) substituent. This process leads to highly substituted phenolic products (e.g., o-silyl phenols) following tautomerization of the intermediate cyclohexa-2,4-dienone. Experimental studies show that this novel mode of reactivity is general; DFT studies reveal the unimolecular nature of the group migration.

Palladium-Catalyzed Oxidative Coupling of Dibenzosiloles with α-Diazo Esters: Formal Replacement of the Silyl Group with Carbenes

We report herein a Pd(II)-catalyzed oxidative coupling reaction of dibenzosiloles with aryl diazoacetates via a metal carbene migratory insertion process. In this reaction, carbenic carbon formally displaces silicon atoms through continuous cleavage of two C(sp2)–Si bonds and successive formation of two C(sp2)–C(sp3) bonds on a carbenic carbon. Density functional theory (DFT) studies reveal that the reaction proceeds through a Pd(II)/Pd(IV) catalytic cycle, in which the two C–Si bonds are cleaved by two successive oxidative addition/reductive elimination processes rather than traditional transmetalation steps. Additive o-chloranil was necessary for this reaction, which significantly facilitated the reaction by acting as both a ligand and an oxidant, and it played different roles in different reaction stages.

Electrochemical Silylation of Electron‐Deficient Heterocycles Using N‐Hydroxyphthalimide as HAT Catalyst

AbstractA practical electrochemical Minisci‐type reaction for the synthesis of silylated heterocycles is described. Using N‐hydroxyphthalimide (NHPI) as the hydrogen atom transfer (HAT) catalyst and trialkylsilanes as the silyl radical sources, a wide range of electron‐deficient heterocycles in‐cluding quinoline, quinoxaline, phthalazine, 3,6‐dichloropyridazine, phenanthridine, 6‐chloroimidazo[1,2‐b]pyridazine and coumarin, etc. were easily converted to the corresponding silylated products in 37–83% yields.magnified image

Glycoconjugates of Mucochloric Acid-Synthesis and Biological Activity.

The pharmacological effects of the presence of a sugar moiety, 1,2,3-triazole ring and silyl groups in the structure of biologically active compounds have been extensively studied in drug design and medicinal chemistry. These components can be useful tools to tailoring the bioavailability of target molecules. Herein we present the study on the impact of the sugar substituent structure and triisopropylsilyl group presence on the anticancer activity of mucochloric acid (MCA) derivatives containing the furan-2(5H)-one or 2H-pyrrol-2-one core. The obtained results clearly indicated that tested compounds caused a significant decrease in cell viability of HCT116 and MCF-7 cell lines. MCF-7 cells indicate serious resistance toward investigated compounds in comparison with HCT116 cell line, it suggests that estrogen-dependent breast cancer cells are significantly less sensitive to the tested derivatives. Depending on the structure of the sugar, the type and site of connection with the furanone or 2H-pyrrol-2-one derivative and the presence of the silyl group, the selectivity of the compound towards cancer cells can be controlled. The obtained results may have an impact on the design of new furanone-based anticancer compounds.

Sulfonamide as Photoinduced Hydrogen Atom Transfer Catalyst for Organophotoredox Hydrosilylation and Hydrogermylation of Activated Alkenes

AbstractReadily available, sterically, and electronically tunable sulfonamides have been developed as effective photoinduced hydrogen atom transfer (HAT) catalysts for selective Si−H functionalizations of a broad range of silanes. N‐centered radicals, catalytically generated from sulfonamides by photoredox catalyzed single‐electron oxidation, are the key intermediates that enable an effective HAT process for silyl radical generation to achieve hydrosilylation of activated alkenes. Additionally, this catalytic system can also be applied for the activation of Ge−H bond for hydrogermylation of activated alkenes through hydrogen atom transfer by sulfonamide derived nitrogen centered radical. The ability to generate silyl and germyl radical using this photochemical HAT process offers new avenue towards the sustainable synthesis of organosilicon and organogermanium compounds.magnified image

β-Boron Effect Enables Regioselective and Stereospecific Electrophilic Addition to Alkenes.

Electrophilic addition to alkenes is a textbook-taught reaction, yet it is not always possible to control the regioselectivity of addition to unsymmetrical 1,2-disubstituted substrates. We report the observation and applications of the β-boron effect that accounts for high regioselectivity in electrophilic addition reactions to allylic MIDA (N-methyliminodiacetic acid) boronates. While the well-established β-silicon effect bears partial resemblance to the observed reactivity, the silyl group is typically lost during functionalization. In contrast, the boryl moiety is retained in the product when B(MIDA) is used as the nucleophilic stabilizer. Mechanistic studies elucidate the origin of this effect and demonstrate how σ(C-B) hyperconjugation helps stabilize the incipient carbocation. This transformation represents a rare example of the stereospecific hydrohalogenation of secondary allyl MIDA-boronates that proceeds in a syn-fashion.

Synthesis of silylamine and siloxyamine compounds: A novel approach to flame retardancy of polypropylene and Epoxy resins

A series of innovative flame retardants containing N-Si and N-O-Si bonds have been synthesized. Their chemical structures were verified by nuclear magnetic resonance (NMR) spectroscopy and thermal stabilities determined by thermogravimetric analysis (TGA). The results indicate that their thermal and hydrolytic stabilities could be adjusted by fine tuning their silylamine (N-Si) and siloxyamine (N-O-Si) skeletons. Thus, the substituents bonded to the phthalimides, such as diphenylmethylsilyl, dimethylphenylsilyl, diphenyl‑tert-butylsilyl, triisopropylsilyl or diphenylmethylsiloxy; whichever was attached to the imide played a significant role on its thermal and hydrolytic properties. Upon thermal dissociation, the N-Si and N-O-Si compounds were shown to yield aminyl, silyl and oxygen-centered radicals that provided fire proofing effect in polypropylene (PP) films alone and a synergistic effect in combination with conventional phosphorous flame retardant in PP, linear low-density polyethylene (LLDPE) and epoxy resins (EP). Their efficacy as flame retardants alone in thin PP films were evaluated according to the DIN4102-B2 test. Both N-Si and N-O-Si significantly slowed down the burning of PP films. Moreover, their potential as adjuvants with a conventional phosphorous based flame retardant, i.e., spirocyclic phosphonate (AFLAMMIT® PCO 900) in PP and EP resins was evaluated according to the UL-94 V fire standard. A synergistic effect was observed when the phosphorous flame retardant was combined with either N-Si or N-O-SI containing compounds, since none of the flame retardants by themselves could provide V-0 rating in the UL94-V test in PP, LLDPE or EP resins even at much higher loadings. The thermogravimetric analysis carried out under inert atmosphere revealed enhanced and earlier onset of decomposition and thermal activation of phosphorous based flame retardant in the presence of N-Si or N-O-Si in comparison to spirocyclic phosphonate, N-Si or N-O-Si used alone, respectively. The interaction of the phosphorus flame retardant with N-Si was verified by 31P NMR.

Stepwise on-demand functionalization of multihydrosilanes enabled by a hydrogen-atom-transfer photocatalyst based on eosin Y

Organosilanes are of vital importance for modern human society, having found widespread applications in functional materials, organic synthesis, drug discovery and life sciences. However, their preparation remains far from trivial, and on-demand synthesis of heteroleptic substituted silicon reagents is a formidable challenge. The generation of silyl radicals from hydrosilanes via direct hydrogen-atom-transfer (HAT) photocatalysis represents the most atom-, step-, redox- and catalyst-economic pathway for the activation of hydrosilanes. Here, in view of the green characteristics of neutral eosin Y (such as its abundance, low cost, metal-free nature, absorption of visible light and excellent selectivity), we show that using it as a direct HAT photocatalyst enables the stepwise custom functionalization of multihydrosilanes, giving access to fully substituted silicon compounds. By exploiting this strategy, we realize preferable hydrogen abstraction of Si-H bonds in the presence of active C-H bonds, diverse functionalization of hydrosilanes (for example, alkylation, vinylation, allylation, arylation, deuteration, oxidation and halogenation), and remarkably selective monofunctionalization of di- and trihydrosilanes.

Visible-Light-Driven Silyl or Germyl Radical Generation via Si-C or Ge-C Bond Homolysis.

We report a simple, rapid, and selective protocol for visible-light-driven generation of silyl radicals through photoredox-induced Si-C bond homolysis. Irradiating 3-silyl-1,4-cyclohexadienes with blue light in the presence of a commercially available photocatalyst smoothly generated silyl radicals bearing various substituents within 1 h, and these radicals were trapped by a broad range of alkenes to afford products in good yields. This process is also available for efficient generation of germyl radicals.

Unraveling the polishing processes and mechanisms of polyacrylate silyl ester based antifouling coating: A molecular dynamic study

Self-polishing coatings (SPCs) can renew its surface and consequently prevent the adhesion of marine fouling. In order to unravel the molecular mechanism, in this work, molecular dynamics (MD) simulations were conducted to simulate the polishing process of typical triisopropyl silyl acrylate (TIPSA). The results show that the poor static antifouling performance of TIPSA is due to the spontaneous adsorption of hydrolyzed side groups (silyl groups) onto the coating surface undertheir strong Van der Waals (vdW) nonbonding interactions. The infiltration of water molecules at the interface between hydrolyzed silyl groups and coating surface promotes the polishing process. However, when the number of permeated water molecules is not enough to screen the nonbonding interactions, the hydrolyzed silyl groups can be adsorbed on the resin matrix again. The formation of water channel in the hydrolyzed silyl layer further restrains the polishing process. In addition, the uneven distribution of hydrophilic groups in resin matrix surface also plays an important role in the polishing process of the TIPSA coating. This work deepens our understanding of the polishing process and mechanism of SPCs and might be able to open a new direction to further comprehend the antifouling mechanism.

Enhanced oxidative desulfurization of dibenzothiophene in the presence of cyclohexene, indole, quinoline, and p-xylene using surface-silylated titania-silica catalyst

Heterogeneously catalyzed oxidative desulfurization (ODS) offers a mild pathway to remove refractory sulfur compounds from liquid hydrocarbon fuels. However, adsorption of polar oxidation byproducts on the catalyst surface decreases desulfurization efficiency at the low ODS reaction temperature. Here, we report that high-performance surface-silylated titania-silica catalysts can be prepared by a silylation method using hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) as the silane reagents. The bulk structure and surface properties of the catalysts before and after surface silylation were characterized by N2 adsorption–desorption, XRD, FTIR, TGA, UV–vis DRS, FESEM, EDS, and water contact angle measurement to establish the structure–function relationship. The effects of silyl groups loading, reaction temperature, type of oxidant, and inhibiting compounds on the ODS performance of the catalysts were systematically evaluated. The results revealed that TS-H-2 catalyst with the optimum loading of silyl groups was an excellent catalyst capable of ultra-fast oxidizing ∼100 % of DBT, after only 5 min, which was due to increased surface hydrophobicity and decreased accumulation of poisoning polar byproducts on the active sites. The ODS performance of TS-H-2 was better preserved compared to unmodified TS-nonsil catalyst in the presence of inhibiting compounds such as cyclohexene (CH), indole (IND), and quinoline (QU), while it increased in the presence of p-xylene (PX). The negative effects of the three former inhibiting compounds were resulted from their competition with the sulfur compound for the oxidant and poisoning of the active sites. The positive effect of PX on the ODS rate was due to solubilization of the strongly adsorbed molecules on the surface of catalyst.

Photocatalytic C-Si Bond Formations Using Pentacoordinate Silylsilicates as Silyl Radical Precursors: Synthetic Tricks Using Old Reagents.

A visible-light-induced photocatalytic C-Si formation strategy has been disclosed by uncovering the reactivity of Martin's spirosilane-derived pentacoordinate silylsilicates as silyl radical precursors. The hydrosilylation of a broad spectrum of alkenes and alkynes, as well as the C-H silylation of heteroarenes, has been demonstrated. Remarkably, Martin's spirosilane was stable and could be recovered via a simple workup process. Furthermore, the reaction proceeded well using water as the solvent or low-energy green LEDs as an alternative energy source.

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.

Exploring Silyl Protecting Groups for the Synthesis of Carbon Nanohoops

AbstractThe synthesis of topological molecular nanocarbons, such as hoop-like [n]cycloparaphenylenes, requires the use of spatially prearranged, pro-aromatic units to overcome a build-up of large molecular strain in their curved structures. The used cyclohexadienyl units, however, contain tertiary alcohols that need protection to prevent side reactions until the aromatization step that affords the final curved hydrocarbon. Although alkyl and triethylsilyl groups have been successfully applied as protecting groups for this purpose, each suffers from specific drawbacks. Here, we explore the potential of sterically more crowded silyl groups, namely, tert-butyldimethylsilyl and triisopropylsilyl, as alternatives to the established protection strategies. We show that tert-butyldimethylsilyl can be easily installed and removed under mild conditions, displaying markedly higher resistance towards acids or bases than the triethylsilyl group used to date. Unlike in the case of alkyl groups, tert-butyldimethylsilyl also preserves a high stereoselectivity during the nucleophilic additions of ArLi. Furthermore, we demonstrate that both tert-butyldimethylsilyl and triethylsilyl groups can be installed on the same substrate, and that the latter be selectively deprotected. Thus, the high stereoselectivity, improved stability, and easy deprotection make tert-butyldimethylsilyl an excellent protecting group for the synthesis of carbon nanohoops.

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.

Photoredox/nickel cooperatively catalyzed radical allylic silylation of allyl acetates – Scope and mechanism

The compatibility between photogenerated silyl radicals and transition metal catalysis remains one of the foremost challenges in modern synthetic organic chemistry. Herein, we present the cooperation of photoredox/nickel catalyzed allylic silylation of diverse allyl acetates (even with multiple alkenyl groups) via a silyl radical intermediate. This protocol successfully surmounts the long-standing challenge and provides straightforward, modular synthetic access to substituted allylsilanes from silanecarboxylic acids. Moreover, this strategy has advantages of green and mild conditions over established nucleophilic substitution methods via a silanion pathway. Mechanistic study shows that the formation of silyl radical initiated by photo-induced decarboxylation of silanecarboxylic acids, and the nickel-catalyzed oxidative addition of allyl acetate, followed by silyl radical addition are critical processes. DFT computation reveals that the regioselectivity is controlled by the silyl radical attack of the sterically less hindered or less substituted terminal allylic carbon atom.

Visible Light Thiyl Radical-Mediated Desilylation of Arylsilanes.

A straightforward, visible-light triggered desilylation of arylsilanes by thiyl radicals is presented. Silyl groups are often used to block a reactive position in multi-step organic synthesis, for which a mild cleavage at a late-stage will provide new possibilities and disconnection routes by CAr -Si cleavage/deprotection. In this work, commercially available and cheap disulfides are employed for the first time in this type of C(sp2 )-Si bond cleavage reactions. Thus, upon irradiation with visible-light, homolytic cleavage of the disulfide give rise to the corresponding thiyl radical that allows for a radical chain mechanism. This methodology represents a mild, fast and simple approach suitable for a broad variety of simply substituted arylsilanes. Moreover, the procedure could be easily extended to natural products and therapeutic derivatives, showing its robustness and synthetic application potential.

Electroreductively Induced Radicals for Organic Synthesis.

Organic electrochemistry has attracted tremendous interest within the novel sustainable methodologies that have not only reduced the undesired byproducts, but also utilized cleaner and renewable energy sources. Particularly, oxidative electrochemistry has gained major attention. On the contrary, reductive electrolysis remains an underexplored research direction. In this context, we discuss advances in transition-metal-free cathodically generated radicals for selective organic transformations since 2016. We highlight the electroreductive reaction of alkyl radicals, aryl radicals, acyl radicals, silyl radicals, fluorosulfonyl radicals and trifluoromethoxyl radicals.

Iridium-Catalyzed Branch-Selective Hydroalkylation of Simple Alkenes with Malonic Amides and Malonic Esters

We report the iridium-catalyzed branch-selective hydroalkylation of simple alkenes such as aliphatic alkenes and aromatic alkenes with malonic amides and malonic esters under neutral reaction conditions. A variety of aliphatic alkenes and aromatic alkenes bearing bromine, chlorine, ester, 2-thienylcarboxylate, silyl, and phthalimide groups were all found to be suitable for this hydroalkylation. The combination of this method with Krapcho dealkoxycarbonylation realized a one-pot synthesis of β-substituted amide and ester from β-amide ester and malonic ester. The hydroalkylated products derived from malonic amides are suitable for further transformation. The finely tuned reaction conditions realized the selective transformation of hydroalkylated products to 1,3-diamines or monoamides with the same reagent. Deuterium labeling experiments and measurement of the kinetic isotope effect indicated that the catalytic cycle involves a reversible step and cleavage of the C-H bond is not a rate-determining step. Density functional theory calculations provided insight into the reaction mechanism, where the carboiridation step is followed by C-H reductive elimination.

Copper(I)-Catalyzed Regio- and Stereoselective Silaboration of Terminal Allenes.

Organic compounds bearing both silyl and boryl groups are important building blocks in organic synthesis because of the adequate reactivity of the silyl and boryl groups and high stereospecificity in their derivatization reactions. The difference in reactivity between the silyl and boryl groups enables stepwise derivatization of these groups to afford complex molecules. Here, we report the copper(I)-catalyzed silaboration of terminal allenes to produce multisubstituted allylic boronates embedded with an alkenyl silane structure. The reaction can proceed with a variety of allenes and silylboranes. Furthermore, the silyl and boryl groups were successfully converted into other functional groups, while retaining the stereochemistry of the alkene moiety.