Element−Element Additions to Unsaturated Carbon−Carbon Bonds Catalyzed by Transition Metal Complexes

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.

Tropylium-Promoted Hydroboration Reactions: Mechanistic Insights Via Experimental and Computational Studies

Hydroboration reaction of alkynes is one of the most synthetically powerful tools to access organoboron compounds, versatile precursors for cross-coupling chemistry. This type of reaction has traditionally been mediated by transition-metal or main group catalysts. Herein, we report a novel method using tropylium salts, typically known as organic oxidants and Lewis acids, to promote the hydroboration reaction of alkynes. A broad range of vinylboranes can be easily accessed via this metal-free protocol. Similar hydroboration reactions of alkenes and epoxides can also be efficiently catalyzed by the same tropylium catalysts. Experimental studies and DFT calculations suggested that the reaction follows an uncommon mechanistic pathway, which is triggered by the hydride abstraction of pinacolborane with tropylium ion. This is followed by a series of in situ counterion-activated substituent exchanges to generate boron intermediates that promote the hydroboration reaction.

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.

Photoinduced Borylation for the Synthesis of Organoboron Compounds

Organoboron compounds have important synthetic value and can be applied in numerous transformations. The development of practical and convenient ways to synthesize boronate esters has thus attracted significant interest. Photoinduced borylations originated from stoichiometric reactions of alkanes and arenes with well-defined metal-boryl complexes. Now, photoredox-initiated borylations, catalyzed by either transition metal or organic photocatalysts, and photochemical borylations with high efficiency have become a burgeoning area of research. In this Focus Review, we summarize research on photoinduced borylations, especially emphasizing recent developments and trends. This includes the photoinduced borylation of arenes, alkanes, aryl/alkyl halides, activated carboxylic acids, amines, alcohols, and so on based on transition metal catalysis, metal-free organocatalysis, and direct photochemical activation. We focus on reaction mechanisms involving single-electron transfer, triplet-energy transfer, and other radical processes.

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.

Activation of the Si–B interelement bond related to catalysis

Covering the past seven years, this review comprehensively summarises the latest progress in the preparation and application of Si–B reagents, including the discussion of relevant reaction mechanisms.

Mechanistic insights into the dearomative diborylation of pyrazines: a radical or non-radical process?

The mechanisms of the dearomative diborylation of pyrazines were investigated via a combination of density functional theory calculations and experimental studies.

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.

Photoinduced Syntheses and Reactivities of Phosphorus-Containing Interelement Compounds

The photoinduced reactions of tetraphenyldiphosphine disulfide with a range of organic dichalcogenides successfully afforded a series of phosphorus(V)-chalcogen interelement compounds via a radical process. The relative reactivities of the organic dichalcogenides (i.e., (PhS)₂, (PhSe)₂, and (PhTe)₂) toward the P^III or P^V groups in the diphosphine analogues under light were investigated in detail, and a convenient method was developed to form P-S or P-Se interelement compounds from tetraphenyldiphosphine disulfide and (PhS)₂ or (PhSe)₂ upon photoirradiation. Furthermore, the relative photochemical properties and reactivities of tetraphenyldiphophine (P-P interelement compound) and its analogues toward photoinduced radical addition reactions were also discussed. The formed P-E (E = S, Se) interelement compounds could be utilized for ionic reactions, and they could be transformed into various phosphine reagents via one-pot processes.

Reversible carbon-boron bond formation at platinum centers through σ-BH complexes

A reversible carbon-boron bond formation has been observed in the reaction of the coordinatively unsaturated, cyclometalated, Pt(ii) complex [Pt(I ^t BuⁱPr')(I ^t BuⁱPr)][BAr^F], 1, with tricoordinated boranes HBR₂. X-ray diffraction studies provided structural snapshots of the sequence of reactions involved in the process. At low temperature, we observed the initial formation of the unprecedented σ-BH complexes [Pt(HBR₂)(I ^t BuⁱPr')(I ^t BuⁱPr)][BAr^F], one of which has been isolated. From -15 to +10 °C, the σ-BH species undergo a carbon-boron coupling process leading to the platinum hydride derivative [Pt(H)(I ^t BuⁱPr-BR₂)(I ^t BuⁱPr)][BAr^F], 4. Surprisingly, these compounds are thermally unstable undergoing carbon-boron bond cleavage at room temperature that results in the 14-electron Pt(ii) boryl species [Pt(BR₂)(I ^t BuⁱPr)₂][BAr^F], 2. This unusual reaction process has been corroborated by computational methods, which indicate that the carbon-boron coupling products 4 are formed under kinetic control whereas the platinum boryl species 2, arising from competitive C-H bond coupling, are thermodynamically more stable. These findings provide valuable information about the factors governing productive carbon-boron coupling reactions at transition metal centers.

Diboration of 3-substituted propargylic alcohols using a bimetallic catalyst system: access to (Z)-allyl, vinyldiboronates

The diboration of substituted propargylic alcohols has been achieved using a bimetallic Pd/Cu catalyst system. The in situ formation of a pentrafluoroboronic acid intermediate sufficiently activates the C-O bond towards dual catalysis affording (Z)-allyl, vinyldiboronates stereoselectively.

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.

Hexafluoroisopropanol-Promoted Disulfidation and Diselenation of Alkyne, Alkene, and Allene

Hexafluoroisopropanol (HFIP)-promoted disulfidation and diselenation of C-C unsaturated bonds is reported. Reactions of unactivated alkyne, alkene, and allene, respectively, with disulfides or diselenides in HFIP led to desired products in good to excellent yields (up to 96%). In contrast, other solvents, such as isopropanol and dichloroethane, could not promote the same reaction. This method revealed an example of HFIP-promoted transformations under the mild conditions, which greatly highlighted the unique reactivity of this special solvent.

High-Loading Single-Atom Copper Catalyst Supported on Coordinatively Unsaturated Al2 O3 for Selective Synthesis of Homoallylboronates

Single-atom catalysts (SACs) as a bridge between hetero- and homogeneous catalysis have attracted much attention. However, it is still challenging to generate stable single atoms with high metal loadings, and the application of SACs in traditionally homogeneous catalytic reactions is highly desirable. Herein, a Cu SAC with a high Cu loading of 8.7 wt % supported on coordinatively unsaturated Al₂ O₃ was prepared and used in the amine-free synthesis of homoallylboranes. Up to 99 % conversion, 95 % 1,4-selective boration of the enals, and 48-68 % isolated yields of homoallylboranes were achieved, equaling the results of reported homogenous catalysts, and the system was more efficient and stable than nano Cu/γ-Al₂ O₃ . Mechanistic investigation indicated that Cu-Bpin species are the active intermediates of selective boration. The superior catalytic and recycling performance of Cu SAC paves an efficient and green path toward selective synthesis of homoallyborane fine chemicals.

Catalytic Enantioselective Synthesis of anti-Vicinal Silylboronates by Conjunctive Cross-Coupling

Chiral 1,2-bimetallic reagents are useful motifs in synthetic chemistry. Although syn-1,2-bimetallic compounds can be prepared by alkene dimetallation, anti-1,2-bimetallics are still rare. The stereospecific 1,2-metallate shift that occurs during conjunctive cross-coupling is shown to enable a practical and modular approach to the catalytic synthesis of enantioenriched anti-1,2-borosilanes. In addition to reaction development, the synthetic utility of anti-1,2-borosilanes was investigated, including applications to the synthesis of anti-1,2-diols and anti-1,2-amino alcohols.

Proximity enforced oxidative addition of a strong unpolar σ-Si-Si bond at rhodium(i)

The new bidentate bisphosphino ligand (5-Ph₂P-Ace-6-SiMe₂)₂ (1) binds rhodium(i) chloride and brings it into close proximity to a strong unpolar σ-Si-Si bond, in which it immediately inserts. In the spirocyclic Rh(iii) product of the oxidative addition, (5-Ph₂P-Ace-6-SiMe₂)₂RhCl (2), the two Si atoms are still close enough to engage in weak non-covalent interactions.

Regioselective Chlorothiolation of Alkenes with Sulfonyl Chlorides

Newly developed sulfonyl chloride-based regioselective chlorothiolation of alkenes has been disclosed; the reaction is compatible with a variety of functional groups and can be scaled up to the gram scale with no loss in yield. The employment of readily available reactants, mild reaction conditions, and high regioselectivity makes this process very practical. Mechanistic studies revealed a possible free radical reaction pathway.

Catalytic Performance of Nanoporous Metal Skeleton Catalysts for Molecular Transformations

Nanoporous metal (MNPore) skeleton catalysts have attracted increasing attention in the field of green and sustainable heterogeneous catalysis owing to their unique three-dimensional nanopore structural features. In general, MNPores are fabricated through chemical or electrochemical corrosive dealloying of monolithic alloys. The dealloying process produces various MNPores with an open nanoporous network structure by formation of concave and convex hyperboloid-like ligaments. The large surface-to-volume ratio compared to bulk metals and high density of steps and kinks on ligaments of the unsupported MNPores make them promising heterogeneous catalyst candidates for highly active and selective molecular transformations. In this context, a variety of heterogeneous catalytic reactions using MNPores as nanocatalysts under gas- and liquid-phase conditions were developed over the last decade. In addition, the bulk metallic shape and mechanistic rigidity of the MNPore catalysts make the processes of catalyst recovery and reuse more facile and greener. This Minireview mainly focuses on the catalytic performance of nanoporous Au, Pd, Cu, and AuPd with respect to the achievements on catalytic applications in various molecular transformations.

Palladium-catalyzed intramolecular carboborylation of 1,3-diene and synthesis of ABCD ring of communesins

A palladium-catalyzed intramolecular carboborylation of 1,3-diene has been developed for the synthesis of iminoindolines with a quaternary carbon centre. This method was applied to a substrate bearing several functional groups to afford a complex iminoindoline, which was subsequently converted into an ABCD ring model compound of communesins via an intramolecular Friedel-Crafts-type reaction.

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.

Copper-mediated direct thiolation of aryl C–H bonds with disulfides

A directing group-assisted copper-mediated thiolation of aromatic amides with disulfides via direct C(sp²)–H activation has been developed.