Aryl Silyl Research Articles

Copper-catalyzed silylation of aryl and alkenyl triflates with silylboronic esters avoiding base-mediated borylation.

Silylation of aryl and alkenyl triflates is found to occur readily with silylboronic esters as a silicon source under copper catalysis. The silyl moieties are exclusively installed into the organic frameworks through the preferential generation of a silylcopper species, wherein base-mediated direct borylation is totally suppressed. The combined use of tri-n-butylphosphine and 4,4'-diphenyl-2,2'-bipyridine as a ligand combination turned out to be indispensable for achieving the high catalytic activity.

Phosphorus fluoride exchange: Multidimensional catalytic click chemistry from phosphorus connective hubs

Phosphorus Fluoride Exchange (PFEx) represents a cutting-edge advancement in catalytic click-reaction technology. Drawing inspiration from Nature's phosphate connectors, PFEx facilitates the reliable coupling of P(V)-F loaded hubs with aryl alcohols, alkyl alcohols, and amines to produce stable, multidimensional P(V)-O and P(V)-N linked products. The rate of P-F exchange is significantly enhanced by Lewis amine base catalysis, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). PFEx substrates containing multiple P-F bonds are capable of selective, serial exchange reactions via judicious catalyst selection. In fewer than four synthetic steps, controlled projections can be deliberately incorporated along three of the four tetrahedral axes departing from the P(V) central hub, thus taking full advantage of the potential for generating three-dimensional diversity. Furthermore, late-stage functionalization of drugs and drug fragments can be achieved with the polyvalent PFEx hub, hexafluorocyclotriphosphazene (HFP), as has been demonstrated in prior research.

N-Arylation of primary and tertiary sulfonimidamides (SIAs) with benzyne precursors under transition metal-free conditions

Herein, we report an effective and efficient transition metal-free method wherein benzyne intermediates from silyl aryl triflates were successfully coupled with primary sulfonimidamides. The use of CsF in ACN proved to be the optimal condition for the N-arylation of various substituted primary sulfonimidamides, resulting in good to excellent yields. Further, the addition of 18-crown-6 to the present method extended the scope to tertiary sulfonimidamides, which were obtained in moderate to low yields.

Deoxyfluorination of Electron-Deficient Phenols.

In this study, we report a facile synthesis of 2-chloro-1,3-bis(2,6-diisopropylphenyl)imidazolium salts in aqueous media under ambient conditions using hypochlorite as a chlorinating agent. In addition, an air-stable and moisture-insensitive deoxyfluorination reagent based on poly[hydrogen fluoride] salt is presented, which is capable of converting electron-deficient phenols or aryl silyl ethers into the corresponding aryl fluorides in the presence of DBU as a base, with good to excellent yields and high tolerance to functional groups.

Atom-Transfer Radical Polymerization of a SuFExable Vinyl Monomer and Polymer Library Construction via SuFEx Click Reaction

The advancement in the synthesis of functional polymeric materials has greatly benefited from the development of functionalized monomers and effective polymerization methods. Here, inspired by the controlled radical polymerization and the new generation of click chemistry, sulfur(VI) fluoride exchange (SuFEx), we introduce a SuFExable vinyl monomer, 4-vinylbenzenesulfonyl fluoride (VBSF), which shows a good balance of stability and reactivity and can readily undergo organocatalytic atom-transfer radical polymerization under visible light with good controls and high end-group fidelity, while retaining the sulfonyl fluoride groups intact. Moreover, poly(VBSF) can be further modified with high efficiency via SuFEx reactions with various aryl silyl ethers, thus allowing a rapid construction of a functional polymer library in a click fashion.

Construction of C(sp2)-Si Bonds via Ligand-Promoted C-C Bonds Cleavage of Unstrained Ketones.

Herein, we report an efficient palladium-catalyzed silylation of aryl and alkenyl ketones via C-C bond cleavage and C-Si bond formation. This protocol features high efficiency and broad substrate scope. Further applications in the late-stage diversification of biologically important molecules demonstrate the synthetic utility of this method.

Transition-Metal-Free N-(o-Halo)arylation of Sulfoximines/Sulfonimidamides

AbstractA transition-metal-free novel route to access N-(o-halo)arylsulfoximines/sulfonimidamides is achieved by the reaction of sulfoximine/sulfonimidamide, aryne precursor, and CCl4/CBr4 in the presence of KF/18-crown-6. The in situ generated benzyne intermediate (from silyl aryl triflate) reacts with the nucleophile (sulfoximine/sulfonimidamide) and halide source (CCl4/CBr4) to yield the product in moderate to good yield. The protocol exhibits broad substrate scope. The regioisomers formed from the unsymmetric aryne precursor were separated effectively via column chromatography.

Chain-Growth Sulfur(VI) Fluoride Exchange Polycondensation: Molecular Weight Control and Synthesis of Degradable Polysulfates.

Sulfur(VI) fluoride exchange (SuFEx) click chemistry has offered a facile and reliable approach to produce polysulfates and polysulfonates. However, the current SuFEx polymerization methods lack precise control of target molecular weight and dispersity. Herein, we report the first chain-growth SuFEx polycondensation process by exploiting the unique reactivity and selectivity of S–F bonds under SuFEx catalysis. Given the higher reactivity of iminosulfur oxydifluoride versus fluorosulfate, the chain-growth SuFEx polycondensation is realized by using an iminosulfur oxydifluoride-containing compound as the reactive chain initiator and deactivated AB-type aryl silyl ether-fluorosulfates bearing an electron-withdrawing group as monomers. When 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was utilized as the polymerization catalyst, precise control over the polymer molecular weight and polydispersity was achieved. The resulting polymers possess great thermal stability but are easily degradable under mild acidic and basic conditions.

SuFExable polymers with helical structures derived from thionyl tetrafluoride.

Sulfur(VI) fluoride exchange (SuFEx) is a category of click chemistry that enables covalent linking of modular units through sulfur(VI) connective hubs. The efficiency of SuFEx and the stability of the resulting bonds have led to polymer chemistry applications. Now, we report the SuFEx click chemistry synthesis of several structurally diverse SOF4-derived copolymers based on the polymerization of bis(iminosulfur oxydifluorides) and bis(aryl silyl ethers). This polymer class presents two key characteristics. First, the [-N=S(=O)F-O-] polymer backbone linkages are themselves SuFExable and undergo precise SuFEx-based post-modification with phenols or amines to yield branched functional polymers. Second, studies of individual polymer chains of several of these new materials indicate helical polymer structures. The robust nature of SuFEx click chemistry offers the potential for post-polymerization modification, enabling the synthesis of materials with control over composition and conformation.

Transformations of Less-Activated Phenols and Phenol Derivatives via C–O Cleavage

Employing phenols and phenol derivatives as electrophiles for cross-coupling reactions has numerous advantages over commonly used aryl halides in terms of environmental-friendliness and sustainability. In the early stage of discovering such transformations, most efforts have been devoted to utilizing highly activated sulfonate types of phenol derivatives (e.g., OTf, OTs, etc.), which have similar reactivities to the corresponding aryl halides. However, a continuing scientific challenge is how to achieve the direct C-O functionalizations of relatively less-activated phenol derivatives more efficiently. In this review, we will focus on the recent updates on the C-O functionalizations of less-activated phenol derivatives, from aryl carboxylates (e.g., pivalates, acetates, etc.), aryl carbamates and carbonates, to aryl ethers (anisoles, diaryl ethers, aryl pyridyl ethers, aryl silyl ethers), to phenolate salts, and ultimately to simply unprotected phenols, sorted by the types of bond formations. Both transition-metal-catalyzed and transition-metal-free protocols will be covered and discussed in detail. Instead, the C-O functionalizations of aryl sulfonates will not be covered extensively unless they are closely related, due to their high reactivity. Since aryl ethers and phenols represent the main linkages or units in lignin biomass, the successes of such transformations will potentially make major contributions to the direct lignin biomass upgrading and depolymerization.

Mechanism of the Iridium-Catalyzed Silylation of Aromatic C-H Bonds.

Phenanthroline ligands and [Ir(cod)(OMe)]2 form complexes that catalyze the silylation of aromatic and aliphatic C-H bonds. However, no experimental data on the identity of complexes related to the mechanism of this process or the mechanisms by which they react to functionalize C-H bonds have been reported. Herein, we describe our studies on the mechanism of the iridium-catalyzed silylation of aryl C-H bonds. The resting state of the catalyst is an iridium disilyl hydride complex (phenanthroline)Ir(SiMe(OTMS)2)2(H)(L), in which L varies with the arene and additives. An iridium disilyl hydride complex was isolated, characterized, and allowed to react with arenes to form aryl silanes. The kinetics of the reactions of electron-rich and electron-poor arenes showed that the rate-limiting step varies with the electronic properties of the arene. Computational studies on related iridium silyl complexes revealed that the high activity of iridium complexes containing sterically encumbered phenanthroline ligands is due to a change in the number of silyl groups bound to iridium between the resting state of the catalyst containing the hindered phenanthroline and that containing less-hindered phenanthroline.

C-O Bond Silylation Catalyzed by Iron: A General Method for the Construction of Csp2-Si Bonds.

The iron-catalyzed construction of Csp2-Si bonds via unreactive C-O bonds possesses a challenging topic in organic chemistry. Herein we report an iron-catalyzed silylation of aryl and alkenyl carbamates via C-O bond activation. This protocol features high efficiency and a broad substrate scope, enabling the late-stage silylation of biorelevant compounds and thus providing a good method to access valuable motifs in medicinal chemistry. Moreover, this protocol enables orthogonal transformations of phenol derivatives and also allows for the synthesis of multisubstituted arenes through the carbamate group as the directing group.

Fluoride-Ion-Catalyzed Synthesis of Ladder-type Conjugated Benzobisbenzofurans via Intramolecular Nucleophilic Aromatic Substitution Reaction under Metal-free and Mild Conditions.

The fluoride-ion-catalyzed synthesis of benzobisbenzofuran derivatives is described. Fluorine-containing aryl silyl ethers were reacted with 5 mol % of Bu4NF to give desired compounds in high yield under mild conditions. Syn-selective cyclization reaction was discovered for a particular compound as a kinetic product. Computational analysis revealed that the fluorine substituents in the anti-type benzobisbenzofurans affect the order of the molecular orbitals.

Copper-Mediated Fluorination of Aryl Trisiloxanes with Nucleophilic Fluoride.

A method for the nucleophilic fluorination of heptamethyl aryl trisiloxanes to form fluoroarenes is reported. The reaction proceeds in the presence of Cu(OTf)2 and KHF2 as the fluoride source under mild conditions for a broad range of heptamethyltrisiloxyarenes with high functional group tolerance. The combination of this method with the silylation of aryl C-H bonds enables the regioselective fluorination of non-activated arenes controlled by steric effects following a two-step protocol.

NFSI/KF mediated mild and chemoselective interconversion of aryl TBDMS ethers to their benzene sulfonate

A one pot protocol for the transformation of aryl TBDMS ethers to corresponding benzene sulfonate esters using NFSI (N-flurobenzenesulfonimide)/KF is described. In situ generation of benzenesulfonyl fluoride directs chemoselective cleavage of aryl silyl ethers over alkyl silyl ethers. Electron withdrawing substituent’s on aryl ring provided better yield than donating groups. Protecting groups and sensitive functionalities are well tolerated in this methodology. Thus, commercially available inexpensive reagents, mild reaction conditions and step economy are the advantages of this method.

Nickel‐Catalyzed Reaction of Aryl 2‐Pyridyl Ethers with Silylzinc Chlorides: Silylation of Aryl 2‐Pyridyl Ethers via Cleavage of the Carbon−Oxygen Bond

AbstractNi‐catalyzed C−O(Py) bond activation and silylation of aryl 2‐pyridyl ethers with silylzinc chlorides were carried out. This protocol allowed the 2‐pyridyloxy group to be substituted by a silyl group with short reaction times, mild reaction conditions, and good compatibility of functional groups.magnified image

Iridium-Catalyzed Silylation of C-H Bonds in Unactivated Arenes: A Sterically Encumbered Phenanthroline Ligand Accelerates Catalysis.

We report a new system for the silylation of aryl C-H bonds. The combination of [Ir(cod)(OMe)]2 and 2,9-Me2-phenanthroline (2,9-Me2-phen) catalyzes the silylation of arenes at lower temperatures and with faster rates than those reported previously, when the hydrogen byproduct is removed, and with high functional group tolerance and regioselectivity. Inhibition of reactions by the H2 byproduct is shown to limit the silylation of aryl C-H bonds in the presence of the most active catalysts, thereby masking their high activity. Analysis of initial rates uncovered the high reactivity of the catalyst containing the sterically hindered 2,9-Me2-phen ligand but accompanying rapid inhibition by hydrogen. With this catalyst, under a flow of nitrogen to remove hydrogen, electron-rich arenes, including those containing sensitive functional groups, undergo silylation in high yield for the first time, and arenes that underwent silylation with prior catalysts react over much shorter times with lower catalyst loadings. The synthetic value of this methodology is demonstrated by the preparation of key intermediates in the synthesis of medicinally important compounds in concise sequences comprising silylation and functionalization. Mechanistic studies demonstrate that the cleavage of the aryl C-H bond is reversible and that the higher rates observed with the 2,9-Me2-phen ligand are due to a more thermodynamically favorable oxidative addition of aryl C-H bonds.

Modification of Carbon Fibre Surfaces by Sulfur-Fluoride Exchange Click Chemistry.

Technologies that enable surface modification are in high demand and are critical for the implementation of new functional materials and devices. Here, we describe the first modification of a carbon surface (in this case carbon fiber) using the sulfur-fluoride exchange (SuFEx) reaction. The parent sulfur (VI) fluoride moiety can be installed directly to the surface via electrochemical deposition of the fluorosulfate phenyldiazonium tetrafluoroborate salt, or by 'SuFExing' a phenol on the carbon surface followed by treatment of the material with SO2 F2 ; similar to a 'graft to' or 'graft from' functionalization approach. We demonstrate that these SuFEx-able surfaces readily undergo exchange with aryl silyl ethers, and that the subsequent sulfate linkages are themselves stable under electrochemical redox conditions. Finally, we showcase the utility of the SuFEx chemistry by installing a pendant amino group to the fiber surface resulting in interfacial shear strength improvements of up to 130 % in epoxy resin.

Intermediacy of Ni-Ni Species in sp2 C-O Bond Cleavage of Aryl Esters: Relevance in Catalytic C-Si Bond Formation.

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C-O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C-O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy3-catalyzed silylation of aryl pivalates with CuF2/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C-O bond cleavage and their relevance in C-Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C-O functionalization reactions employing monodentate phosphines.

Synthesis of Phenols and Aryl Silyl Ethers via Arylation of Complementary Hydroxide Surrogates.

Two transition-metal-free methods to access substituted phenols via the arylation of silanols or hydrogen peroxide with diaryliodonium salts are presented. The complementary reactivity of the two nucleophiles allows synthesis of a broad range of phenols without competing aryne formation, as illustrated by the synthesis of the anesthetic Propofol. Furthermore, silyl-protected phenols can easily be obtained, which are suitable for further transformations.