Preparation of allyl and vinyl silanes by the palladium-catalyzed silylation of terminal olefins: a silyl-Heck reaction

Suzuki-Miyaura Cross-Coupling Reaction Using Palladium Catalysts Supported on Phosphine Periodic Mesoporous Organosilica

Phosphine periodic mesoporous organosilicas (R-P-PMO-TMS: R=Ph, tBu), which possess electron-donating alkyl substituents on the phosphorus atom, were synthesized using bifunctional compounds with alkoxysilyl- and phosphino groups, bis[3-(triethoxysilyl)propyl]phenylphosphine borane (1 a) and bis[3-(triethoxysilyl)propyl]-tert-butylphosphine borane (1 b). Immobilization of Pd(0) species was performed to give R-P-Pd-PMO-TMS: R=Ph (2 a), tBu (3 a), respectively. The Pd(0) immobilized 2 a and 3 a were applicable as catalysts for Suzuki-Miyaura cross-coupling reactions of aryl chlorides with phenylboronic acid. It was revealed that 3 a bearing more electron-donating tBu groups exhibited higher catalytic activity. Various functional groups including both electron withdrawing and donating substituents were compatible in the system. The recyclability of 3 a was examined to support its moderate utility for the recycle use.

Palladium-Catalyzed Cyclization/Alkenylation of Ynone Oximes with Vinylsilanes for the Assembly of Isoxazolyl Vinylsilanes

A palladium-catalyzed cascade cyclization/alkenylation for the assembly of synthetically valuable isoxazolyl vinylsilane derivative has been accomplished. Easily accessible ynone oximes, and available vinylsilane agents were used as the reaction starting materials This protocol features broad substrate scope, good functional group tolerance, and good step- and atom-economy. Remarkably, this approach provides a new approach for the construction of structurally diverse isoxazolyl-containing vinylsilanes with high molecular complexity, showing a promising application in synthetic and pharmaceutical chemistry.

Photopromoted Free Radical Silylation of 2-Aryl-2H-indazoles with Silanes

Photoinduced silylation of silanes with 2-aryl-2H-indazoles was developed under mild conditions, which could efficiently result in diverse 3-silylated 2H-indazoles with good substrate scopes. A series of scaled-up to gram level and radical capture operations were performed in this system. Meanwhile, a bioactive molecule was tolerated well under typical conditions.

Ruthenium-Catalyzed Dehydrogenative Intermolecular O-H/Si-H/C-H Silylation: Synthesis of (E)-Alkenyl Silyl-Ether and Silyl-Ether Heterocycle

Selective dehydrogenative silylation is one of the most valuable tools for synthesizing organosilicon compounds. In this study, a regio- and stereoselective ruthenium-catalyzed dehydrogenative intermolecular silylation was firstly developed to access (E)-alkenyl silyl-ether derivatives and silyl-ether heterocycles with good functional group tolerance. Furthermore, two pathways for RuH2(CO)(PPh3)3/NBE-catalyzed dehydrogenative intermolecular silylation of alcohols and alkenes as well as intermolecular silylation of naphthol derivatives were investigated with H2SiEt2 as the hydrosilane reagent.

Decarboxylative Allylation of Silanecarboxylic Acids Enabled by Organophotocatalysis

Herein we present a visible-light-facilitated transition-metal-free allylic silylation reaction under mild conditions. This protocol is enabled by an inexpensive organophotocatalyst and provides efficient and concise synthetic routes to substituted allylsilanes, particularly from readily available allyl sulfones and stable silanecarboxylic acids as silyl radical precursors. Further investigations reveal that this strategy is also generally compatible with vinyl sulfones to access vinylsilanes. The silver catalytic system opens up an alternative entry to realize the decarboxylative allylation of silanecarboxylic acids.

1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation

The palladium-catalyzed annulation reaction of alkynes enables an attractive approach to siloles. Their access from silirenes and terminal alkynes proved rather general, involving reactive intermediates that have remained elusive to date. Starting from 1,2-bis(3-thienyl)silirene as a source of photochromic siloles, the mechanism of the annulation reaction has been revisited, and palladasilacyclobutenes resulting from the activation of the silirene could be isolated and thoroughly characterized (NMR, X-ray, and DFT). Their role as reactive intermediates and their fate in the course of the reaction were also studied in situ. In combination with in-depth DFT calculations, a clearer picture of the mechanism and the reactive key species is disclosed.

Nickel-Catalyzed Reductive Coupling of Chlorosilanes

Organosilanes play essential roles in many important research areas. The use of readily available chlorosilanes to catalytically access these compounds is synthetically appealing but remains a long-standing challenge. Nickel-catalyzed reductive cross-coupling reaction has recently emerged as a promising protocol to arrive at this goal. This strategy allows the chlorosilanes to be coupled with various carbon electrophiles under mild conditions. These reactions afford organosilanes with improved molecular diversity, structural complexity, and functional group compatibility. This Concept article summarizes the recent advance on nickel-catalyzed reductive C-Si couplings of chlorosilanes.

Oxidative Addition of Silicon-Chloride Bonds to a Zerovalent Ruthenium Center and Direct Generation of an Ethylene Insertion Complex

The oxidative addition of a silicon-chloride (Si-Cl) bond to a metal center can be a key reaction step in coordinative silicon chemistry, but this reaction is seldom observed. Herein, we report direct oxidative addition of the Si-Cl bonds of dimethyldichlorosilane (Me₂SiCl₂) and cyclotrimethylenedichlorosilane [(CH₂)₃SiCl₂] to low-valent ruthenium complexes, yielding the 16e^- chloro(organosilyl)ruthenium complexes [N₃]Ru(Cl)(SiMe₂Cl) (4a) and [N₃]Ru(Cl)(SiCl(CH₂)₃) (4b) ([N₃] = 2,6-(MesN═CMe)₂C₅H₃N; Mes = 1,3,5-trimethylphenyl; Me = methyl). The reversible reaction of 4a with ethylene yields an 18e^- ethylene adduct, in which an ethylene is subsequently inserted into a ruthenium-silicon (Ru-Si) bond to produce the 16e^- complex [N₃]Ru(Cl)(CH₂CH₂SiMe₂Cl) (7). This study provides a good example of the direct generation of an ethylene insertion product, which is an important intermediate in the catalytic reduction of unsaturated molecules.

Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)-carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)-metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM-E and TM-H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid-hydrogen and metalloid-metalloid bonds to unsaturated compounds; activation and functionalization of C-H bonds and C-X bonds with hydrometalloids and bismetalloids; activation and functionalization of C-H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

NiH-catalyzed C(sp3)–Si coupling of alkenes with vinyl chlorosilanes

A novel NiH-catalyzed highly selective cross-coupling of alkenes with vinyl chlorosilanes is developed. Using this practical chemistry, various benzyl organosilanes could be produced with good functional group tolerance.

Stereo-divergent synthesis of silyl-enynes via palladium-catalyzed coupling of alkynes and iodosilanes

We report a palladium-catalyzed coupling reaction of alkynes and silicon electrophiles, affording stereodefined silyl-enynes. Either E- or Z-enynes can be formed in high yields and in a highly stereoselective manner...

A library of new bifunctional alkenes obtained by a highly regiodivergent silylation of 1,5-hexadiene

An efficient methodology for the synthesis of two groups of silicon-containing alkenes is reported. It includes a highly regioselective functionalization of 1,5-hexadiene through hydrosilylation and dehydrogenative silylation with organofunctional silanes and siloxanes. The established conditions enable selective monofunctionalization of 1,5-hexadiene regardless of the organosilicon modifier used as well as the type of functional group bonded to the silicon-based compound. All products were isolated and fully characterized by NMR spectroscopy and MS techniques.

Nickel-Catalyzed Cross-Electrophile C(sp3)-Si Coupling of Unactivated Alkyl Bromides with Vinyl Chlorosilanes

Cross-electrophile C-Si coupling has emerged as a promising tool for the construction of organosilanes, but the potential of this method remains largely unexplored. Herein, we report a C(sp³)-Si coupling of unactivated alkyl bromides with vinyl chlorosilanes. The reaction proceeds under mild conditions, and it offers a new approach to alkylsilanes. Functionalities such as Grignard-sensitive groups (e.g., acid, amide, alcohol, ketone, and ester), acid-sensitive groups (e.g., ketal and THP protection), alkyl fluoride and chloride, aryl bromide, alkyl tosylate and mesylate, silyl ether, and amine were tolerated. Incorporation of the -Si(vinyl)R₂ moiety into complex molecules and the immobilization of a glass surface by formed organosilanes were demonstrated.

Synthesis of Structurally Diverse Allylsilanes via Copper-Catalyzed Regiodivergent Hydrosilylation of 1,3-Dienes

Unprecedented copper-catalyzed regiodivergent hydrosilylation reactions of substituted 1,3-dienes with hydrosilanes have been developed. The 1,2- and 1,4-hydrosilylations of 1-(hetero)aryl-substituted 1,3-dienes were highly selectively controlled via variation of the catalytic systems. Meanwhile, the 1,4-hydrosilylation reaction of 2-aryl-substituted 1,3-dienes with diphenylsilane was also successfully realized for the first time. These methods provide convenient and efficient approaches for the synthesis of structurally diverse allylsilanes.

Synergistic Ni/Cu catalyzed migratory arylsilylation of terminal olefins

Synthesis of organosilanes from alkenes is a very important topic owing to their wide applications. A Ni/Cu dual metal-catalyzed arylsilylation of terminal alkenes, featuring migratory selectivity, has been developed. A wide diversity of aliphatic silanes have been prepared from terminal alkenes, aryl halides and Suginome's reagent. This protocol is highlighted by excellent regioselectivity, mild reaction conditions and good functional group tolerance. In addition to benzylic positions, carbon-carbon bonds can also be constructed at allylic positions. Preliminary mechanistic studies suggest that the copper cocatalyst promotes the transmetalation of Suginome's reagent, and the addition of a PyrOx ligand inhibits the formation of side-products from the carbon-Heck pathway. Moreover, studies toward the nature of the PyrOx ligand revealed that the steric hindrance of the oxazoline moiety greatly affects the chain-walking process, but not the arylation step.

Visible Light-Induced Pd-Catalyzed Alkyl-Heck Reaction of Oximes

A visible light-induced palladium-catalyzed oxidative C-H alkylation of oximes has been developed. This mild protocol allows for an efficient atom economical C-C bond construction of alkyl-substituted oximes. A broad range of primary, secondary, and tertiary alkyl bromides and iodides, as well as a range of different formaldoximes, can efficiently undergo this transformation. The method features visible light-induced generation of nucleophilic hybrid alkyl Pd radical intermediates, which upon radical addition at the imine moiety and a subsequent β-hydrogen elimination deliver substituted imines.

Ruthenium-catalyzed formal sp3 C-H activation of allylsilanes/esters with olefins: efficient access to functionalized 1,3-dienes

Ru-catalysed oxidative coupling of allylsilanes and allyl esters with activated olefins has been developed via isomerization followed by C(allyl)-H activation providing efficient access to stereodefined 1,3-dienes in excellent yields. Mild reaction conditions, less expensive catalysts, and excellent regio- and diastereoselectivity ensure universality of the reaction. In addition, the unique power of this reaction was illustrated by performing the Diels-Alder reaction, and enantioselective synthesis of highly functionalized cyclohexenone and piperidine and finally synthetic utility was further demonstrated by the efficient synthesis of norpyrenophorin, an antifungal agent.

A General and Selective Synthesis of Methylmonochlorosilanes from Di-, Tri-, and Tetrachlorosilanes

Direct catalytic transformation of chlorosilanes into organosilicon compounds remains challenging due to difficulty in cleaving the strong Si-Cl bond(s). We herein report the palladium-catalyzed cross-coupling reaction of chlorosilanes with organoaluminum reagents. A combination of [Pd(C₃H₅)Cl]₂ and DavePhos ligand catalyzed the selective methylation of various dichlorosilanes 1, trichlorosilanes 5, and tetrachlorosilane 6 to give the corresponding monochlorosilanes.

Palladium-catalyzed domino Heck-disilylation and Heck-monosilylation of alkene-tethered carbamoyl chlorides: synthesis of versatile silylated oxindoles

We report efficient domino Heck-disilylation and Heck-monosilylation of alkene-tethered carbamoyl chlorides with hexamethyldisilane under mild reaction conditions.

Asymmetric synthesis of 3-benzyl and allyl isoindolinones by Pd-catalyzed dicarbofunctionalization of 1,1-disubstituted enamides

The Pd-catalyzed enantioselective Heck/Suzuki reaction of 1,1-disubstituted enamides with aryl/vinyl boronic acids has been developed to access 3-benzyl/allyl substituted isoindolinones bearing a tetrasubstituted stereogenic carbon center.

Iron-catalyzed para-selective C–H silylation of benzamide derivatives with chlorosilanes

This paper developed the para-selective silylation of benzamide derivatives with chlorosilanes using FeCl₂ catalysis.

Alkenes as hydrogen trappers to control the regio-selective ruthenium(ii) catalyzed ortho C–H silylation of amides and anilides

A convenient and practical pathway to versatile silylated amides and anilides is described via efficient and selective ruthenium(ii) catalyzed ortho C–H silylation with different alkenes as the hydrogen acceptors.

Cross-Electrophile C(sp2 )-Si Coupling of Vinyl Chlorosilanes

The cross-electrophile coupling has become a powerful tool for C-C bond formation, but its potential for forging the C-Si bond remains unexplored. Here we report a cross-electrophile Csp² -Si coupling reaction of vinyl/aryl electrophiles with vinyl chlorosilanes. This new protocol offers an approach for facile and precise synthesis of organosilanes with high molecular diversity and complexity from readily available materials. The reaction proceeds under mild and non-basic conditions, demonstrating a high step economy, broad substrate scope, wide functionality tolerance, and easy scalability. The synthetic utility of the method is shown by its efficient accessing of silicon bioisosteres, the design of new BCB-monomers, and studies on the Hiyama cross-coupling of vinylsilane products.

Fluorosilane Activation by Pd/Ni→Si-F→Lewis Acid Interaction: An Entry to Catalytic Sila-Negishi Coupling

A new mode of bond activation involving M→Z interactions is disclosed. Coordination to transition metals as σ-acceptor ligands was found to enable the activation of fluorosilanes, opening the way to the first transition-metal-catalyzed Si-F bond activation. Using phosphines as directing groups, sila-Negishi couplings were developed by combining Pd and Ni complexes with external Lewis acids such as MgBr₂. Several key catalytic intermediates have been authenticated spectroscopically and crystallographically. Combined with DFT calculations, all data support cooperative activation of the fluorosilane via Pd/Ni→Si-F→Lewis acid interaction with conversion of the Z-type fluorosilane ligand into an X-type silyl moiety.

Stereoselective Synthesis of Cis- and Trans-Tetrasubstituted Vinyl Silanes Using a Silyl-Heck Strategy and Hiyama Conditions for Their Cross-Coupling

We report a palladium-catalyzed, three-component carbosilylation reaction of internal symmetrical alkynes, silicon electrophiles, and primary alkyl zinc iodides. Depending on the choice of ligand, stereoselective synthesis of either cis- or trans-tetrasubstituted vinyl silanes is possible. We also demonstrate conditions for the Hiyama cross-coupling of these products to prepare geometrically defined tetrasubstituted alkenes.

Regio- and Stereoselective Thianthrenation of Olefins To Access Versatile Alkenyl Electrophiles

Herein, we report a regioselective alkenyl electrophile synthesis from unactivated olefins that is based on a direct and regioselective C-H thianthrenation reaction. The selectivity is proposed to arise from an unusual inverse-electron-demand hetero-Diels-Alder reaction. The alkenyl sulfonium salts can serve as electrophiles in palladium- and ruthenium-catalyzed cross-coupling reactions to make alkenyl C-C, C-Cl, C-Br, and C-SCF3 bonds with stereoretention.

Palladium-Catalyzed Synthesis of α-Carbonyl-α'-(hetero)aryl Sulfoxonium Ylides: Scope and Insight into the Mechanism

Despite recent advances, a general method for the synthesis of α-carbonyl-α'-(hetero)aryl sulfoxonium ylides is needed to benefit more greatly from the potential safety advantages offered by these compounds over the parent diazo compounds. Herein, we report the palladium-catalyzed cross-coupling of aryl bromides and triflates with α-carbonyl sulfoxonium ylides. We also report the use of this method for the modification of an active pharmaceutical ingredient and for the synthesis of a key precursor of antagonists of the neurokinin-1 receptor. In addition, the mechanism of the reaction was inferred from several observations. Thus, the oxidative addition complex [(XPhos)PhPdBr] and its dimer were observed by ³¹P{¹H} NMR, and these complexes were shown to be catalytically and kinetically competent. Moreover, a complex resulting from the transmetalation of [(XPhos)ArPdBr] (Ar = p-CF₃-C₆H₄) with a model sulfoxonium ylide was observed by mass spectrometry. Finally, the partial rate law suggests that the transmetalation and the subsequent deprotonation are rate-determining in the catalytic cycle.

Iron-promoted free radical cascade difunctionalization of unsaturated benzamides with silanes

An efficient cascade difunctionalization of unsaturated benzamides with silanes was developed for the synthesis of various silylated dihydroisoquinolinones and 1,3-isoquinolinediones.

Ruthenium(ii)/acetate catalyzed intermolecular dehydrogenative ortho C-H silylation of 2-aryl N-containing heterocycles

The first application of a RuHCl(CO)(PPh3)3-OAc catalytic system on the selective intermolecular mono C-H silylation of 2-aryl N-heterocycles using HSiEt3 as the silylating reagent has been described. This protocol features good functional group tolerance and high regioselectivity, and has potential for gram scale-up, which provides a convenient and practical pathway for the synthesis of versatile organosilane compounds. This catalytic system can also be applied to the silylation of challenging sp3 C-H bonds.

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.

Mechanochemistry allows carrying out sensitive organometallic reactions in air: glove-box-and-Schlenk-line-free synthesis of oxidative addition complexes from aryl halides and palladium(0)

Organic reactions that employ moisture- and/or oxygen-sensitive reagents or intermediates usually involve the use of glove-box or Schlenk-line techniques as well as dry and degassed solvents. Unfortunately, these requirements may greatly reduce the utility of the targeted organic molecules. Herein, we demonstrate that solvent-free mechanochemical synthetic techniques allow using highly oxygen-sensitive palladium(0) species in air for the stoichiometric oxidative addition of aryl halides. The low diffusion efficiency of gaseous oxygen in crystalline or amorphous solid-state reaction mixtures should be the main reason for the low impact of the presence of atmospheric oxygen on the sensitive oxidative addition reactions under the applied conditions. This study thus illustrates the outstanding potential of mechanochemistry to serve as an operationally simple, glove-box-and-Schlenk-line-free synthetic route to organometallic compounds and other valuable synthetic targets, even when sensitive reagents or intermediates are involved.

Cross-Coupling of Heteroatomic Electrophiles

At the advent of cross-coupling chemistry, carbon electrophiles based on halides or pseudohalides were the only suitable electrophilic coupling partners. Almost two decades passed before the first cross-coupling reaction of heteroatom-based electrophiles was reported. Early work by Murai and Tanaka initiated investigations into silicon electrophiles. Narasaka and Johnson pioneered the way in the use of nitrogen electrophiles, while Suginome began the exploration of boron electrophiles. The chemistry reviewed within provides perspective on the use of heteroatomic electrophiles, specifically silicon-, nitrogen-, boron-, oxygen-, and phosphorus-based electrophiles in transition-metal catalyzed cross-coupling. For the purposes of this review, a loose definition of cross-coupling is utilized; all reactions minimally proceed via an oxidative addition event. Although not cross-coupling in a traditional sense, we have also included catalyzed reactions that join a heteroatomic electrophile with an in situ generated nucleophile. However, for brevity, those involving hydroamination or C-H activation as a key step are largely excluded. This work includes primary references published up to and including October 2018.

Ruthenium(II) Acetate Catalyzed Synthesis of Silylated Oxazoles via C-H Silylation and Dehalogenation

An efficient ruthenium(II)-catalyzed intermolecular selective ortho C-H silylation of 2-aryloxazoles has been described for the first time, which provides a convenient and practical pathway for the synthesis of versatile organosilane compounds with good functional group tolerance and regioselectivity. This catalytic system could be also applied to the dehalogenation of Cl or Br group.

Pharmaceutical diversification via palladium oxidative addition complexes

Palladium-catalyzed cross-coupling reactions have transformed the exploration of chemical space in the search for materials, medicines, chemical probes, and other functional molecules. However, cross-coupling of densely functionalized substrates remains a major challenge. We devised an alternative approach using stoichiometric quantities of palladium oxidative addition complexes (OACs) derived from drugs or drug-like aryl halides as substrates. In most cases, cross-coupling reactions using OACs proceed under milder conditions and with higher success than the analogous catalytic reactions. OACs exhibit remarkable stability, maintaining their reactivity after months of benchtop storage under ambient conditions. We demonstrated the utility of OACs in a variety of experiments including automated nanomole-scale couplings between an OAC derived from rivaroxaban and hundreds of diverse nucleophiles, as well as the late-stage derivatization of the natural product k252a.

Palladium-catalyzed olefination of aryl/alkyl halides with trimethylsilyldiazomethane via carbene migratory insertion

The direct olefination of aryl/alkyl halides with trimethylsilyldiazomethane (TMSD) as a C1- or C2-unit was achieved successfully via a metal carbene migratory insertion process, which offered a new access to afford (E)-vinyl silanes and (E)-silyl-substituted α,β-unsaturated amides in good yields and high chemoselectivity.