Elementminus signElement Addition to Alkynes Catalyzed by the Group 10 Metals

Solvent-Controlled, Atom-Economic, and Highly Regio- and Stereoselective Halo-Chalcogenations of Ynamides: Green Synthesis of Stereodefined Tetrasubstituted Alkenes Bearing Four Different Functional Groups

The synthesis of stereodefined tetrasubstituted alkenes bearing four different functional groups is challenging. Herein, we disclose a 100% atom-economic and highly regio- and stereoselective halo-chalcogenations, in particular, chlorosulfenylation, bromosulfenylation, chloroselenation, and bromoselenation, of ynamides in toluene at room temperature under an aerobic atmosphere for the synthesis of a wide variety of stereodefined tetrasubstituted alkenes bearing four different functional groups in excellent yields. Notably, all the reactions are highly efficient and furnished the desired products in excellent yield (average yield >96%) and stereoselectivity (Z/E = 90:10 to >99:1) within a short time (15-30 min). Interestingly, the high (Z)-stereoselectivity (syn-addition) is controlled by the solvent. The transformation does not require any catalyst, oxidizing or reducing reagent, or external energy. The products were obtained pure by evaporating the solvent after the reaction and washing the crude product with either pentane or ethanol (column-chromatography-free protocol). Moreover, the solvent toluene was recovered and reused in subsequent reactions, which makes the protocol highly sustainable. The protocol is efficiently scalable (96% yield) on a gram scale. Notably, the products were synthetically diversified to other new classes of stereodefined tetrasubstituted alkenes. Significantly, the green chemistry metrics of the protocol are found to be excellent.

Organosulphur and organoselenium compounds as ligands for catalytic systems in the Sonogashira coupling

Sonogashira coupling is a reaction of aryl/vinyl halides with terminal alkynes. It is used for the synthesis of conjugated enynes. Generally, copper (Cu) is required as a mediator for this reaction. It requires a long reaction time, high catalyst loading, or expensive ligands. Recently, homogeneous, heterogeneous, and nanocatalysts have been developed using organosulphur and organoselenium compounds as building blocks. Preformed complexes of metals with organosulphur and organoselenium ligands are used for homogeneous catalysis. Heterogeneous catalytic systems have also been developed using Cu, Pd, and Ni as metals. The nanocatalytic systems (synthesized using such ligands) include copper selenides and stabilized palladium(0) nanospecies. This article aims to cover the developments in the field of the processes and techniques used so far to generate catalytically relevant organic ligands having sulphur or selenium donor sites, the utility of such ligands in the syntheses of homogeneous, heterogeneous, and nanocatalytic systems, and critical analysis of their application in the catalysis of this coupling reaction. The results of catalysis are analyzed in terms of the effects of the S/Se donor, halogen atom of aryl halide, the effect of the presence/absence of electron-withdrawing or electron-donating groups or substituents on the aromatic ring of haloarenes/substituted phenylacetylenes, as well as the position (ortho or para) of the substitution. Substrate scope is discussed for all the kinds of catalysis. The supremacy of heterogeneous and nanocatalytic systems indicates promising future prospects.

Electrophilic Activation of S-Si Reagents by Silylium Ions for Their Regio- and Diastereoselective Addition Across C-C Multiple Bonds

A regioselective silylium-ion-promoted thiosilylation of internal C-C triple bonds with control over the double bond geometry is described. Both a C(sp2)-S and a C(sp2)-Si bond are formed with a trans relationship in this two-component reaction of an alkyne and a thiosilane. The resulting orthogonally functionalized C-C double bond can be chemoselectively defunctionalized or further processed by cross-coupling reactions with the alkene configuration retained. The procedure is also applicable to the regio- and diastereoselective thiosilylation of terminal allenes to arrive at allylic thioethers containing a vinylsilane unit. These reactions involve the electrophilic activation of the S-Si reagent, both a silylated thiophenol and even alkylthiol derivative, by an in situ-generated carbocation intermediate. The catalytic cycle is maintained by a bissilylated aryl- or alkylsulfonium ion as a shuttle for the cationic silicon electrophile. Its independent preparation and structural characterization by X-ray diffraction are also reported.

NHC-Stabilized Dialanes(4) of Al2 Mes4

The synthesis and characterization of novel N-heterocyclic carbene (NHC) stabilized dialanes Al2 Mes4 as well as first investigations concerning the reactivity of these compounds are reported. The synthesis of these compounds proceeds via the mesityl-substituted alanes (NHC)⋅AlHMes2 (NHC=IMeMe {=1,3,4,5-tetramethyl-imidazolin-2-ylidene}, IiPrMe {=1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene}) and iodo-alanes (NHC)⋅AlIMes2 (NHC=IMeMe , IiPrMe ). Metallic reduction of (NHC)⋅AlIMes2 afforded the new NHC-stabilized dialanes (NHC)2 ⋅Al2 Mes4 (NHC=IMeMe , IiPrMe ). The NHC-ligated dialanes are thermally robust and storable synthons for the dialane Al2 Mes4 . First reactivity studies on (IMeMe )2 ⋅Al2 Mes4 towards small molecules confirm this, as this compound shows controlled and selective reactions with several substrates. Reaction with CuCl leads to oxidation of the dialane and formation of (IMeMe )⋅AlClMes2 , reactions with pyridine N-oxide and t Bu-N=C=S, respectively, gave the chalcogenide-bridged dimers {(IMeMe )⋅AlMes2 }2 -μ-E (E=O, S), and reaction with acetylene afforded the dimetallaacetylide {(IMeMe )⋅AlMes2 }2 -μ-(C≡C).

Ru(OAc)3-Catalyzed Regioselective Difunctionalization of Alkynes: Access to (E)-2-Bromo-1-alkenyl Sulfonates

A new approach is proposed for the divergent and regioselective synthesis of (E)-2-bromo-1-phenylvinyl trifluoromethanesulfonates through alkyne difunctionalization by employing a compatible system of abundantly available alkynes, N-bromosuccinimide (NBS), and trimethylsilyl trifluoromethanesulfonate (TMSOTf) catalyzed by ruthenium(III) acetate [Ru(OAc)3]. It is a novel method for the preparation of vinyl triflate and it offers a fundamental basis for the development of advanced functional compounds, including drugs and organic functional materials. Unlike previously reported methods, the proposed protocol can tolerate a broad range of functional groups. Alkynes derived from bioactive molecules, such as l(-)-borneol, demonstrate the potential value of this new reaction in organic synthesis.

Synergistic Palladium/Lewis Acid-Catalyzed Regio- and Stereo-divergent Bissilylation of Alkynoates: Scope, Mechanism, and Origin of Selectivity

Transition metal-catalyzed bissilylation reactions of alkynes with disilane reagents have become one of the most straightforward and efficient protocols to rapidly produce structurally diverse alkenyl silicon derivatives. In these reactions, the utilization of unsymmetrical disilane reagents provided the possibilities for reactivity enhancement as well as the synthetic merits in contrast to symmetrical disilane reagents. However, a major yet challenging objective is achieving precise control over the selectivity including the regioselectivity and the cis/trans-selectivity. Herein we realized the first divergent bissilylation of alkynoates with our developed air-stable disilane reagent 8-(2-substituted-1,1,2,2-tetramethyldisilanyl)quinoline (TMDQ) by means of synergistic Pd/Lewis acid catalytic system. The catalytic system precisely dictates the selectivity, resulting in the divergent synthesis of 1,2-bissilyl alkenes. The power of these 1,2-bissilyl alkenes serving as the key synthetic intermediates has been clearly demonstrated by rapid construction of diverse motifs and densely functionalized biologically active compounds. In addition, the origins of the switchable selectivities were well elucidated by experimental and computational studies on the reaction mechanism and were mainly attributed to different ligand steric effects, the use of the specific disilane reagent TMDQ and the different coordination modes of different Lewis acid with alkynoates.

Rhodium-Catalyzed Trans-Bis-Silylation Reactions of 2-Ethynyl-3-pentamethyldisilanylpyridines

Rhodium-catalyzed reactions of 2-ethynyl-3-pentamethyldisilanylpyridine derivatives (1 and 2) are reported. The reactions of compounds 1 and 2 in the presence of catalytic amounts of rhodium complexes at 110 °C gave the corresponding pyridine-fused siloles (3) and (4) through intramolecular trans-bis-silylation cyclization. The reaction of 2-bromo-3-(1,1,2,2,2-pentamethyldisilanyl)pyridine with 3-phenyl-1-propyne in the presence of PdCl₂(PPh₃)₂-CuI catalysts afforded 1:2 bis-silylation adduct 6. DFT calculations were also performed to understand the reaction mechanism for the production of compound 3 from compound 1.

Synthesis of Pyridine-Fused Siloles by Palladium-Catalyzed Intramolecular Bis-Silylation

Silole derivatives are attracting significant attention as new functional materials with excellent electronic and photophysical properties. Thus, the development of synthesis methods to afford such derivatives is highly desirable. Herein, the synthesis of pyridine-fused siloles under the conditions of the Sonogashira coupling reaction is described. The reactions of 2-bromo-3-(pentamethyldisilanyl)pyridine (1) with ethynylbenzene derivatives in the presence of PdCl2(PPh3)2-CuI as a catalyst afforded the corresponding pyridine-fused siloles (2a-2c) through intramolecular trans-bis-silylation. DFT calculations were also performed to understand the reaction mechanism. This paper is the first to report on the successful use of palladium catalysts in the trans-bis-silylation of alkynes with disilanes.

Palladium-Catalyzed Highly Chemo- and Stereoselective Bisthiolation of Terminal Alkynes with Allyl Phenyl Sulfides via C-S Bond Cleavage

A facile and general method for palladium-catalyzed stereoselective bisthiolation of terminal alkynes with allyl phenyl sulfides has been developed. The scope and versatility of the reaction have been demonstrated, and a broad range of substrates bearing electron-donating and -withdrawing groups on the aromatic rings were all compatible with this reaction, providing the desired (Z)-1,2-dithio-1-alkenes in moderate to good yields. Preliminary mechanistic studies demonstrated that the sulfur source of the desired products may be successively incorporated into alkynes via C-S bond cleavage of two molecules of allyl phenyl sulfides and ruled out the possibility of vinyl sulfides, alkynyl sulfides, and disulfide intermediates being involved in this reaction.

Palladium-catalyzed hydroboration reaction of unactivated alkynes with bis (pinacolato) diboron in water

A highly efficient and mild palladium-catalyzed hydroboration of unactivated internal alkynes in water is described. Both aryl- and alkyl-substituted alkynes proceeded smoothly within the reaction time to afford the desired vinylboronates in moderate to high yields. Bis (pinacolato) diboron was used to afford α- and β-hydroborated products in the presence of HOAc. These reactions showed high reactivities and tolerance, thus providing a promising method for the synthesis of alkenyl boron compounds.

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.

Development of Methods to the Synthesis of β-Boryl Acyls, Imines and Nitriles

Organoboron compounds are highly important and versatile synthetic intermediates for the preparation of a wide range of organic molecules. Organoboron compounds have drawn significant attention among organic chemists due to their Lewis acidic property, non-toxicity, and commercial availability. Over the last several decades, there has been a substantial development of new organoboron compounds, useful in organic synthesis. Among all other organoboron compounds, β-boryl carbonyl compounds are the important ones. The β-boryl compounds have appeared as promising intermediates for various synthetic transformations. The 1,4-conjugate addition of diboron reagents to carbon-carbon double bond in the presence of different transition-metal catalysts has been extensively reported by various research groups across the globe. This mini-review outlines the numerous racemic as well as asymmetric β-borylation methods developed to date.

Direct synthesis and applications of solid silylzinc reagents

The increased synthetic utility of organosilanes has motivated researchers to develop milder and more practical synthetic methods. Silylzinc reagents, which are typically the most functional group tolerant, are notoriously difficult to synthesize because they are obtained by a pyrophoric reaction of silyllithium, particularly Me3SiLi which is itself prepared by the reaction of MeLi and disilane. Furthermore, the dissolved LiCl in silylzinc may have a detrimental effect. A synthetic method that can avoid silyllithium and involves a direct synthesis of silylzinc reagents from silyl halides is arguably the simplest and most economical strategy. We describe, for the first time, the direct synthesis of PhMe2SiZnI and Me3SiZnI reagents by employing a coordinating TMEDA ligand, as well as single crystal XRD structures. Importantly, they can be obtained as solids and stored for longer periods at 4 °C. We also demonstrate their significance in cross-coupling of various free alkyl/aryl/alkenyl carboxylic acids with broader functional group tolerance and API derivatives. The general applicability and efficiency of solid Me3SiZnI are shown in a wide variety of reactions including alkylation, arylation, allylation, 1,4-addition, acylation and more.

Clarification on the Reactivity of Diaryl Diselenides toward Hexacyclohexyldilead under Light

In this study, the reactivity of organochalcogen compounds toward a representative alkyl-lead bond compound under light was investigated in detail. Under light irradiation, the Cy-Pb bond of Cy6Pb2 (Cy = cyclohexyl) undergoes homolytic cleavage to generate a cyclohexyl radical (Cy•). This radical can be successfully captured by diphenyl diselenide, which exhibits excellent carbon-radical-capturing ability. In the case of (PhS)2 and (PhTe)2, the yields of the corresponding cyclohexyl sulfides and tellurides were lower than that of (PhSe)2. This probably occurred due to the low carbon-radical-capturing ability of (PhS)2 and the high photosensitivity of the cyclohexyl-tellurium bond.

2,2-Bis(phenylselanyl)-1-(p-tolyl)vinyl 2-Oxo-2-(p-tolyl)acetate

2,2-Bis(phenylselanyl)-1-(p-tolyl)vinyl 2-oxo-2-(p-tolyl)acetate was synthesized via the reaction of p-tolylacetylene with diphenyl diselenide and benzoyl peroxide in benzene under atmospheric conditions. The molecular structure of the synthesized compound was evaluated using single-crystal X-ray analysis and spectral analyses. The process reported here provides a rare example of the direct and selective transformation of a terminal alkyne to the corresponding geminal diseleno-substituted alkene.

Me3SiSiMe2(O n Bu): a disilane reagent for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement

Herein, a readily available disilane Me₃SiSiMe₂(OⁿBu) has been developed for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement. This protocol enables the incorporation of a silylene into different starting materials, including acrylamides, alkene-tethered 2-(2-iodophenyl)-1H-indoles, and 2-iodobiaryls, via the cleavage of Si–Si, Si–C, and Si–O bonds, leading to the formation of spirobenzosiloles, fused benzosiloles, and π-conjugated dibenzosiloles in moderate to good yields. Preliminary mechanistic studies indicate that this transformation is realized by successive palladium-catalyzed bis-silylation and Brook- and retro-Brook-type rearrangement of silane-tethered silanols.

A readily available disilane Me₃SiSiMe₂(OⁿBu) as a silylene source has been developed for the synthesis of diverse silacycles via Brook- and retro-Brook-type rearrangement.

A Crystalline B4N2 Dewar Benzene as a Building Block for Conjugated B,N-Chains

Dewar benzene, one of the isolable valence isomers of C₆H₆, has been extensively studied since its first synthesis in 1962. By contrast, the chemistry of inorganic congeners of Dewar benzene, which can be formally gained by replacing the skeletal carbon atoms with heteroatoms, has been less developed despite their peculiar structural and electronic features. Among them, the extant B,N-Dewar benzenes are limited to the B₃N₃ system. Herein, we report the development of the first example of an isolable B₄N₂ Dewar benzene, 3. As predicted by DFT calculations, a judicious selection of the substituents allows synthesizing 3. Single-crystal X-ray analysis, NMR, and computational studies confirmed that 3 possesses a high-lying B(sp³)-B(sp³) σ-bond at the bridgehead position. Reactions with ethylene and phenylacetylene proceeded smoothly under mild conditions, affording the fused B₄C₄N₂ ring systems (4 and 5). Structural characterization as well as DFT calculations revealed that compounds 4 and 5 involve a rigid and conjugated (BN)₄ tetraene scaffold. Formation of 4 and 5 demonstrates that 3 may serve as a building block for the construction of conjugated B,N-chains.

Palladium-Catalyzed Three-Component Silylalkoxylation of 1,3-Diene with Alcohol and Disilane via Oxidative Coupling

A regioselective and Z-selective three-component silylalkoxylation of 1,3-diene using various alcohols, disilane, and a catalytic Pd/Cu/1,4-benzoquinone/O₂ system is established in this Letter. The reaction generates tetra-substituted allyl silanes containing allyl ether moieties in up to 80% isolated yield and on a 1-10 mmol scale via oxidative coupling. A wide variety of substrates, including benzyl alcohol derivates, aliphatic alcohols, and bioactive compounds such as cholesterol, are suitable for use in the developed reaction system.

Recent advances in the stereoselective synthesis of acyclic all-carbon tetrasubstituted alkenes

Alkenes bearing four carbon-based groups are ubiquitous motifs in chemical sciences due to their various applications from medicinal to materials chemistry, and as chemical platforms for the synthesis of complex, chiral molecules. As such, tremendous research efforts are currently ongoing in order to develop general procedures for the challenging stereoselective synthesis of all-carbon tetrasubstituted alkenes, especially for acyclic structures. Since classical approaches to carbon-carbon double bonds are not suitable for the high steric demand around tetrasubstituted alkenes, a variety of unique approaches to access these privileged functional groups have been developed in recent years. This review article highlights the most significant developments in the field from 2007 to 2020, with an emphasis on the mechanisms and remaining limitations of these contemporary methods. Specifically, recent advances in internal alkyne carbofunctionalizations, in multicomponent couplings or other cross-couplings from nucleophilic or electrophilic alkenyl partners, and in the development of miscellaneous methods, are discussed.

Tetryl-Tetrylene Addition to Phenylacetylene

Phenylacetylene adds [Ar*GeH2 -SnAr'], [Ar*GeH2 -PbAr'] and [Ar'SnH2 -PbAr*] at rt in a regioselective and stereoselective reaction. The highest reactivity was found for the stannylene, which reacts immediately upon addition of one equivalent of alkyne. However, the plumbylenes exhibit addition to the alkyne only in reaction with an excess of phenylacetylene. The product of the germylplumbylene addition reacts with a second equivalent of alkyne and the product of a CH-activation, a dimeric lead acetylide, were isolated. In the case of the stannylplumbylene the trans-addition product was characterized as the kinetically controlled product which isomerizes at rt to yield the cis-addition product, which is stabilized by an intramolecular Sn-H-Pb interaction. NMR chemical shifts of the olefins were investigated using two- and four-component relativistic DFT calculations, as spin-orbit effects can be large. Hydride abstraction was carried out by treating [Ar'SnPhC=CHGeH2 Ar*] with the trityl salt [Ph3 C][Al(OC{CF3 })4 ] to yield a four membered ring cation.

Bis-silylation of internal alkynes enabled by Ni(0) catalysis

1,2-Bis-silyl alkenes have exciting synthetic potential for programmable sequential synthesis via manipulation of the two vicinal silyl groups. Transition metal-catalyzed bis-silylation of alkynes with disilanes is the most straightforward strategy to access such useful building blocks. However, this process has some limitations: (1) symmetric disilanes are frequently employed in most of the reactions to assemble two identical silyl groups, which makes chemoselective differentiation for stepwise downstream transformations difficult; (2) the main catalysts are low-valent platinum group transition metal complexes, which are expensive; and (3) internal alkynes remain challenging substrates with low inherent reactivity. Thus, the development of abundant metal-catalyzed bis-silylation of internal alkynes with unsymmetrical disilanes is of significance. Herein, we solve most of the aforementioned limitations in bis-silylation of unsaturated bonds by developing a strongly coordinating disilane reagent and a Ni(0) catalytic system. Importantly, we sufficiently realize the stepwise recognition of the two silyl groups, making this synthetic protocol of wide potential utility.

Bis-silylated alkenes offer the advantage of two functional handles with distinguished reactivity for downstream functionalization. Here, the authors report a nickel-catalyzed bis-silylation of internal alkynes to versatile silylated alkene intermediates which can be chemoselectively manipulated.

Radical 1,2-addition of bromoarenes to alkynes via dual photoredox and nickel catalysis

A 1,2-addition of aryl bromides to alkynes enabled by the photocatalytic generation of bromine radicals via photoredox and nickel catalysis is reported.

Cu-Catalyzed highly selective silylation and borylation of alkenylsulfonium salts

A Cu-catalyzed highly selective silylation and borylation of alkenylsulfonium salts under mild conditions is reported providing various alkenylsilanes and alkenylboranes.

Insertion of Ni(0) and Pd(0) precursors into the Si-Si bond of a disilane with two hypercoordinate silicon atoms

The activation of the silicon-silicon bond in disilane Ph2Si(μ-PzMe2)2SiPh2 (1), which possesses two five-coordinate silicon centers, was achieved by a reaction with Pd(0) or Ni(0) precursors of modest steric demand to afford a dinuclear Pd complex (2) or two types of mononuclear Ni complexes (3 and 4), respectively.

Chelation-assisted transition metal-catalysed C–H chalcogenylations

This review summarizes recent advances in C–S and C–Se formationsviatransition metal-catalyzed C–H functionalization utilizing directing groups to control the site-selectivity.

α-Silicon effect assisted Curtin-Hammett allylation using allylcopper reagents derived from 1,3-dienylsilanes

Cu-catalyzed stereoselective synthesis of (E)-δ-silyl-anti-homoallylic alcohols from 1,3-dienylsilane was developed. Mechanistic studies revealed that the borocupration of dienylsilane proceeded through a 1,2-addition pathway to give an allylcopper intermediate with Cu distal to the silyl group. However, the subsequent aldehyde allylation proceeded via Curtin-Hammett control to give (E)-δ-silyl-anti-homoallylic alcohols with high diastereoselectivities. This method was applied to the synthesis of the C1-9 fragment of a polyketide natural product, mycinolide IV.

Recent Advances in the Addition of Amide/Sulfonamide Bonds to Alkynes

The addition of amide/sulfonamide bonds to alkynes is not only one of the most important strategies for the direct functionalization of carbon⁻carbon triple bonds, but also a powerful tool for the downstream transformations of amides/sulfonamides. The present review provides a comprehensive summary of amide/sulfonamide bond addition to alkynes, including direct and metal-free aminoacylation, based-promoted aminoacylation, transition-metal-catalyzed aminoacylation, organocatalytic aminoacylation and transition-metal-catalyzed aminosulfonylation of alkynes up to December 2018. The reaction conditions, regio- and stereoselectivities, and mechanisms are discussed and summarized in detail.

Total Synthesis of Putative Chagosensine

The marine macrolide chagosensine is the only natural product known to date that embodies a Z,Z-configured chloro-1,3-diene unit. This distinguishing substructure was prepared by a sequence of palladium-catalyzed 1,2-distannation of an alkyne precursor, regioselective Stille cross-coupling at the terminus of the resulting bisstannyl alkene with an elaborated alkenyl iodide, followed by chloro-destannation of the remaining internal site. The preparation of the required substrates centered on cobalt-catalyzed oxidative cyclization reactions of hydroxylated olefin precursors, which allowed the 2,5-trans-disubstituted tetrahydrofuran rings, embedded into each building block, to be formed with excellent selectivity. The highly strained macrolactone could ultimately be closed under forcing Yamaguchi conditions. Comparison of the spectral data of the synthetic sample with those of authentic chagosensine methyl ester confirmed that the structure of this intriguing compound has been mis-assigned by the isolation team.