Catalytic C-H bond silylation of aromatic heterocycles

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

p-Silylation of Arenes via Organic Photoredox Catalysis: Use of p-Silylated Arenes for Exclusive o-Silylation, o-Acylation, and o-Alkylation Reactions

Photocatalytic regiospecific p-silylation of arenes has been achieved by the coupling of in situ generated silyl radical with arene radical cation. The strategy involves reductive activation of PhSe-SiR₃ and single electron transfer from the electron rich arene to 9,10-dimethoxyanthracene radical cation (DMA^•+). p-Silyl arenes, thus formed, are further utilized for exclusive o-silylation reaction and for regiospecific o-acylation as well as o-alkylation reaction.

An effective and versatile strategy for the synthesis of structurally diverse heteroarylsilanes via Ir(iii)-catalyzed C-H silylation

A versatile silylation of heteroaryl C–H bonds is accomplished under the catalysis of a well-defined spirocyclic NHC Ir(iii) complex (SNIr), generating a variety of heteroarylsilanes. A significant advantage of this catalytic system is that multiple types of intermolecular C–H silylation can be achieved using one catalytic system at α, β, γ, or δ positions of heteroatoms with excellent regioselectivities. Mechanistic experiments and DFT calculations indicate that the polycyclic ligand of SNIr can form an isolable cyclometalated intermediate, which leaves a phenyl dentate free and provides a hemi-open space for activating substrates. In general, favorable silylations occur at γ or δ positions of chelating heteroatoms, forming 5- or 6-membered C–Ir–N cyclic intermediates. If such an activation mode is prohibited sterically, silylations would take place at the α or β positions. The mechanistic studies would be helpful for further explaining the reactivity of the SNIr system.

A versatile silylation of heteroaryl C–H bonds is accomplished under the catalysis of a well-defined spirocyclic NHC Ir(iii) complex (SNIr), generating a variety of heteroarylsilanes.

Phosphorus(III)-assisted regioselective C-H silylation of heteroarenes

Heteroarenes containing carbon-silicon (C-Si) bonds are important building blocks that play an important role in the construction of natural products, pharmaceuticals, and organic materials. In this context, the C-H silylation of heteroarenes is a topic of intense interest. Indole C-H silylation can preferentially occur at the nucleophilic C3 and C2 position (pyrrole core), while accessing the C4-C7 positions (benzene core) of the indole remains highly challenging. Here, we show a general strategy for the regioselective C7-H silylation of indole derivatives. Mainly, the regioselectivity is determined by strong coordination of the palladium catalyst with phosphorus (III) directing group. Using this expedient synthetic strategy, the diverse C7-silylated indoles are synthesized effectively which exhibits the broad functional group compatibility. Moreover, this protocol also been extended to other heteroarenes such as carbazoles. The obtained silylated indoles have been employed in various transformations to enable the corresponding differently functionalized indole derivatives. Significantly, a cyclopalladated intermediate is successfully synthesized to test the hypothesis about the P(III)-directed C-H metalation event. A series of mechanistic experiments and density functional theory (M06-2X) calculations has shown the preferred pathway of this directed C-H silylation process.

Et3SiH + KO t Bu provide multiple reactive intermediates that compete in the reactions and rearrangements of benzylnitriles and indolenines

The combination of potassium tert-butoxide and triethylsilane is unusual because it generates multiple different types of reactive intermediates simultaneously that provide access to (i) silyl radical reactions, (ii) hydrogen atom transfer reactions to closed shell molecules and to radicals, (iii) electron transfer reductions and (iv) hydride ion chemistry, giving scope for unprecedented outcomes. Until now, reactions with this reagent pair have generally been explained by reference to one of the intermediates, but we now highlight the interplay and competition between them.

B(C6F5)3-Catalyzed cyclization of alkynes: direct synthesis of 3-silyl heterocyclic compounds

An efficient one-pot strategy for easy access to 3-silyl heterocyclic compounds was developed via a B(C₆F₅)₃-catalyzed cycloaddition reaction of o-(1-alkynyl)(thio)anisoles or o-(1-alkynyl)-N-methylaniline. In this reaction, benzenethiophene, benzofuran or indole skeletons could be constructed by an intermolecular cyclization with diphenylsilane. This protocol elicited moderate-to-good yields with metal-free reaction systems.

New reductive rearrangement of N-arylindoles triggered by the Grubbs-Stoltz reagent Et3SiH/KO t Bu

N-Arylindoles are transformed into dihydroacridines in a new type of rearrangement, through heating with triethylsilane and potassium tert-butoxide. Studies indicate that the pathway involves (i) the formation of indole radical anions followed by fragmentation of the indole C2–N bond, and (ii) a ring-closing reaction that follows a potassium-ion dependent hydrogen atom transfer step. Unexpected behaviors of ‘radical-trap’ substrates prove very helpful in framing the proposed mechanism.

N-Arylindoles are transformed into dihydroacridines in a new type of rearrangement, through heating with triethylsilane and potassium tert-butoxide.

KOtBu-Catalyzed Michael Addition Reactions Under Mild and Solvent-Free Conditions

Designed transition metal complexes predominantly catalyze Michael addition reactions. Inorganic and organic base-catalyzed Michael addition reactions have been reported. However, known base-catalyzed reactions suffer from the requirement of solvents, additives, high pressure and also side-reactions. Herein, we demonstrate a mild and environmentally friendly strategy of readily available KO^t Bu-catalyzed Michael addition reactions. This simple inorganic base efficiently catalyzes the Michael addition of underexplored acrylonitriles, esters and amides with (oxa-, aza-, and thia-) heteroatom nucleophiles. This catalytic process proceeds under solvent-free conditions and at room temperature. Notably, this protocol offers an easy operational procedure, broad substrate scope with excellent selectivity, reaction scalability and excellent TON (>9900). Preliminary mechanistic studies revealed that the reaction follows an ionic mechanism. Formal synthesis of promazine is demonstrated using this catalytic protocol.

Photocatalytic C-H silylation of heteroarenes by using trialkylhydrosilanes

The efficient and selective C-H silylation of heteroarenes, especially the pharmaceutically relevant electron-deficient heteroarenes, represents a great challenge in organic synthesis. Herein we wish to report a distinctive visible light-promoted photocatalytic C-H silylation approach that enables the direct coupling of trialkylhydrosilanes with both electron-deficient and -rich heteroarenes as well as with cyano-substituted arenes in moderate to high yields and with good regioselectivity. The protocol features operational simplicity, mild reaction conditions, and the use of safe and readily available Na2S2O8, bis(trimethylsilyl) peroxide (BTMSPO) or iPr3SiSH as the radical initiators. Notably, the challenging bulky and inert trialkylhydrosilanes, such as (t-butyldimethyl)silane ( t BuMe2SiH) and (triisopropyl)silane (iPr3SiH), work smoothly with the protocol. Moreover, despite the higher stability of t BuMe2Si silylation products, our studies revealed their great reactivity and versatility in diverse C-Si-based chemical transformations, providing an operationally simple, low-cost, and environmentally benign synthetic technology for molecule construction and elaboration.

Catalytic Access to Bridged Sila- N-heterocycles from Piperidines via Cascade sp3 and sp2 C-Si Bond Formation

Described herein is the development of an unprecedented route to bridged sila- N-heterocycles via B(C₆F₅)₃-catalyzed cascade silylation of N-aryl piperidines with hydrosilanes. Mechanistic studies indicated that an outer-sphere ionic path is operative to involve three sequential catalytic steps having N-silyl piperidinium borohydride as a resting species: (i) dehydrogenation of the piperidine ring, (ii) β-selective hydrosilylation of a resultant enamine intermediate, and (iii) intramolecular dehydrogenative sp² C-H silylation.

Alkali metal complexes as efficient catalysts for hydroboration and cyanosilylation of carbonyl compounds

We report here reactions between the N-adamantyliminopyrolyl ligand 2-(AdN[double bond, length as m-dash]CH)-C4H3NH (L-H) and alkali metal hexamethyldisilazides [MN(SiMe3)2] (M = Li, Na and K) to afford the dimeric [{2-(AdN[double bond, length as m-dash]CH)-C4H3NLi(THF)}2] (1), [{2-(AdN[double bond, length as m-dash]CH)-C4H3N}{Na(THF)1.5}2] (2) and polymeric [{2-(AdN[double bond, length as m-dash]CH)-C4H3NK(THF)}n] (3) complexes at ambient temperature. A one-pot reaction between L-H, [KN(SiMe3)2] and alkaline earth metal diiodide (AeI2) in a 2 : 2 : 1 molar ratio, which resulted in the formation of a heteroleptic Ae metal complex [κ2-{2-(AdN[double bond, length as m-dash]CH)-C4H3N}2Ae(THF)2] [Ae = Mg (4), Ca (5)], is also reported. The solid-state structures of complexes 1, 3 and 4 were established through single-crystal X-ray diffraction analysis. The alkali and alkaline earth metal complexes 1-5 were utilised as precatalysts for the catalytic hydroboration of pinacolborane (HBpin) with aldehydes and ketones, and potassium complex 3 was identified as a competent catalyst under mild conditions. Additionally, cyanosilylation of carbonyl compounds was explored with trimethylsilyl cyanide and aldehydes/ketones, using the alkali metal precatalyst 3 under mild conditions. In both catalytic processes, the potassium catalyst 3 exhibited high tolerance towards a number of functional groups.

Silylation of Aryl Halides with Monoorganosilanes Activated by Lithium Alkoxide

Lithium alkoxide activates a monoorganosilane to generate a transient LiH/alkoxysilane complex, which quickly reacts with aryl and alkenyl halides at 25 °C to deliver a diorganosilane product. Experimental and theoretical studies suggest that the reaction includes nucleophilic attack of LiH on the halogen atom of the organic halide to generate a transient organolithium/alkoxysilane intermediate, which undergoes quick carbon-silicon bond formation within the complex.

Direct Reductive N-Functionalization of Aliphatic Nitro Compounds

The first general protocol for the direct reductive N-functionalization of aliphatic nitro compounds is presented. The nitro group is partially reduced to a nitrenoid, with a mild and readily available combination of B₂ pin₂ and zinc organyls. Thereby, the formation of an unstable nitroso intermediate is avoided, which has so far severely limited reductive transformations of aliphatic nitro compounds. The reaction is concluded by an electrophilic amination of zinc organyls.

Electron-Transfer and Hydride-Transfer Pathways in the Stoltz-Grubbs Reducing System (KOtBu/Et3 SiH)

Recent studies by Stoltz, Grubbs et al. have shown that triethylsilane and potassium tert-butoxide react to form a highly attractive and versatile system that shows (reversible) silylation of arenes and heteroarenes as well as reductive cleavage of C-O bonds in aryl ethers and C-S bonds in aryl thioethers. Their extensive mechanistic studies indicate a complex network of reactions with a number of possible intermediates and mechanisms, but their reactions likely feature silyl radicals undergoing addition reactions and S_H 2 reactions. This paper focuses on the same system, but through computational and experimental studies, reports complementary facets of its chemistry based on a) single-electron transfer (SET), and b) hydride delivery reactions to arenes.

Transition-Metal-Free Ring-Opening Silylation of Indoles and Benzofurans with (Diphenyl-tert-butylsilyl)lithium

A practical method is presented for ring opening various indoles and benzofurans with concomitant stereoselective silylation using readily generated (diphenyl-tert-butylsilyl)lithium to afford ortho-β-silylvinylanilines or -phenols. Dearomatization of the heteroarene core proceeds in the absence of any transition-metal catalyst through addition of a silyl anion and a subsequent stereoselective β-elimination. DFT calculations provide insight into the mechanism. Functionalizing C-X bond cleavage of heteroarenes is rare and generally requires transition-metal catalysts.

Stereoselective Synthesis of Z-Vinylsilanes via Palladium-Catalyzed Direct Intermolecular Silylation of C(sp2)-H Bonds

An efficient and convenient palladium-catalyzed direct intermolecular silylation of C(sp²)-H bonds by using disilanes as the silicon source with the assistance of a readily removable bidentate directing group is reported. This strategy provided a regio- and stereoselective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility. Silylation of the isolated palladacycle intermediate revealed the Z-stereoselective pathway. Moreover, the practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product.

Mechanistic Study of Arylsilane Oxidation through 19F NMR Spectroscopy

The mechanism of the oxidation of arylsilanes to phenols has been investigated using ¹⁹F NMR spectroscopy. The formation of silanols in these reactions results from a rapid background equilibrium between silanol and alkoxysilane; the relative rates of reaction of these species was evaluated by modeling of concentration profiles obtained through ¹⁹F NMR spectroscopic reaction monitoring. Combining these results with a study of initial rates of phenol formation, and of substituent electronic effects, a mechanistic picture involving rapid and reversible formation of a pentavalent peroxide ate complex, prior to rate-limiting aryl migration, has evolved.

Electrophilic Aromatic Substitution with Silicon Electrophiles: Catalytic Friedel-Crafts C-H Silylation

Electrophilic aromatic substitution is a fundamental reaction in synthetic chemistry. It converts C-H bonds of sufficiently nucleophilic arenes into C-X and C-C bonds using either stoichiometrically added or catalytically generated electrophiles. These reactions proceed through Wheland complexes, cationic intermediates that rearomatize by proton release. Hence, these high-energy intermediates are nothing but protonated arenes and as such strong Brønsted acids. The formation of protons is an issue in those rare cases where the electrophilic aromatic substitution is reversible. This situation arises in the electrophilic silylation of C-H bonds as the energy of the intermediate Wheland complex is lowered by the β-silicon effect. As a consequence, protonation of the silylated arene is facile, and the reverse reaction usually occurs to afford the desilylated arene. Several new approaches to overcome this inherent challenge of C-H silylation by S_E Ar were recently disclosed, and this Minireview summarizes this progress.

A Pincer Ruthenium Complex for Regioselective C-H Silylation of Heteroarenes

A pincer Ru(II) catalyst for the highly efficient undirected silylation of O- and S-heteroarenes with (TMSO)₂MeSiH and Et₃SiH is described, producing heteroarylsilanes with exclusive C2-regioselectivity, good functional-group tolerance, and high turnover numbers (up to 1960). The synthetic utility of the silylated products is demonstrated by Pd-catalyzed Hiyama-Denmark cross-coupling under mild conditions. One-pot, two-step silylation and coupling procedures have been also developed.