Enantiospecific Trifluoromethyl-Radical-Induced Three-Component Coupling of Boronic Esters with Furans

Influence of steric hindrance on the 1,4- versus 1,6-Michael addition: synthesis of furans and pentasubstituted benzenes

We described the influence of steric hindrance on the 1,4- versus 1,6-Michael addition reaction on 2-(3,3-bis(methylthio)-1-arylallylidene)malononitriles. An efficient and direct synthesis of trisubstituted furans was achieved through the reaction of 2-(3,3-bis(methylthio)-1-arylallylidene)malononitriles and acetone under mild conditions in good to moderate yield by the 1,4-Michael addition. Further exploration of the reaction with a sterically hindered aryl group containing 2-(3,3-bis(methylthio)-1-arylallylidene)malononitriles afforded biaryls by an in situ generated nucleophile through the 1,6-Michael addition. The synthetic utility of furan is further explored. These precursors are easily accessible from aryl methyl ketones. Various functional groups like alkyl, aryl, nitrile, amine, aroyl, and thiomethyl can be directly installed in the benzene and furan rings. A one-pot approach for the construction of a benzene nucleus was also developed. The structure of two compounds was confirmed by X-ray crystallography.

Thiophenium Salts as New Oxidant for Redox Polymerization under Mild- and Low-Toxicity Conditions

In mild conditions (under air, room temperature, no monomer purification and without any energy activation), redox free radical polymerization (RFRP) is considered as one of the most effective methods to polymerize (meth)acrylate monomers. In the past several years, there has been a growing interest in research on the development of new redox initiating systems (RISs), thanks mainly to the evolution of toxicity labeling and the stability issue of the current RIS based on peroxide and aromatic amine. In this study, a new, low-toxicity RIS based on thiophenium salt as the oxidant species is presented with various reductive species. The reactivity and the stability of the proposed RISs are investigated and the synthesis of new thiophenium salts reported.

Chiral Benzothiophene Synthesis via Enantiospecific Coupling of Benzothiophene S-Oxides with Boronic Esters

Benzothiophenes are valuable heterocycles that are widely used in medicines, agrochemicals, and materials science. Herein, we report a general method for the synthesis of enantioenriched 2,3-disubstituted benzothiophenes via a transition-metal-free C2-alkylation of benzothiophenes with boronic esters. The reactions utilize benzothiophene S-oxides in lithiation-borylations to generate intermediate arylboronate complexes, and subsequent Tf₂ O-promoted S-O bond cleavage to trigger a Pummerer-type 1,2-metalate shift, which gives the coupled products with complete enantiospecificity. Primary, secondary and tertiary alkyl boronic esters and aryl boronic esters are successfully coupled with a range of C3-substituted benzothiophenes. Importantly, this transformation does not require the use of C3 directing groups, therefore it overcomes a major limitation of previously developed transition-metal-mediated C2 alkylations of benzothiophenes.

Direct Observation of Reactive Intermediates by Time-Resolved Spectroscopy Unravels the Mechanism of a Radical-Induced 1,2-Metalate Rearrangement

Radical-induced 1,2-metalate rearrangements of boronate complexes are an emerging and promising class of reactions that allow multiple new bonds to be formed in a single, tunable reaction step. These reactions involve the addition of an alkyl radical, typically generated from an alkyl iodide under photochemical activation, to a boronate complex to produce an α-boryl radical intermediate. From this α-boryl radical, there are two plausible reaction pathways that can trigger the product forming 1,2-metalate rearrangement: iodine atom transfer (IAT) or single electron transfer (SET). Previous steady-state techniques have struggled to differentiate these pathways. Here we apply state-of-the-art time-resolved infrared absorption spectroscopy to resolve all the steps in the reaction cycle by mapping production and consumption of the reactive intermediates over picosecond to millisecond time scales. We apply this technique to a recently reported reaction involving the addition of an electron-deficient alkyl radical to the strained σ-bond of a bicyclo[1.1.0]butyl boronate complex to form a cyclobutyl boronic ester. We show that the previously proposed SET mechanism does not adequately account for the observed spectral and kinetic data. Instead, we demonstrate that IAT is the preferred pathway for this reaction and is likely to be operative for other reactions of this type.

Stereoselective synthesis of 2-trifluoromethylated and 2-difluoromethylated dihydrofurans via organocatalytic cascade Michael/alkylation reaction

A highly stereoselective cascade Michael/alkylation process has been established to construct 2-trifluoromethylated and 2-difluoromethylated dihydrofurans.

Transition-Metal-Free Oxidative Cross-Coupling of Tetraarylborates to Biaryls Using Organic Oxidants

Readily prepared tetraarylborates undergo selective (cross)-coupling through oxidation with Bobbitt's salt to give symmetric and unsymmetric biaryls. The organic oxoammonium salt can be used either as a stoichiometric oxidant or as a catalyst in combination with in situ generated NO2 and molecular oxygen as the terminal oxidant. For selected cases, oxidative coupling is also possible with NO2 /O2 without any additional nitroxide-based cocatalyst. Transition-metal-free catalytic oxidative ligand cross-coupling of tetraarylborates is unprecedented and the introduced method provides access to various biaryl and heterobiaryl systems.

Radical-Induced 1,2-Migrations of Boron Ate Complexes

1,2-Boron ate rearrangements represent a fundamental class of transformations to establish new C-C bonds while retaining the valuable boron moiety in the product. In established ionic processes, the boron ate complex is activated by an external electrophile to induce a 1,2-migration from boron to an adjacent sp 3 or sp 2 carbon atom. Recently, two complementary radical polar crossover approaches have been explored for both classes, 1,2-migrations to sp 2 and sp 3 carbon centers. This review describes the general concepts in this emerging research field and summarizes recent developments of radical-induced 1,2-migrations from boron to carbon.

Recent Advances in Radical Addition to Alkenylboron Compounds

The addition of carbon-centered radicals to alkenylboron compounds provides a useful method for the construction of organoboron reagents which are versatile reagents in chemical synthesis. While the first examples of this type or process appeared 70 years ago, until recently, attention to this type of reaction has been limited. A number of examples of this reactivity have been demonstrated recently, and strategies are emerging that allow for tuning the reagent structure in order to modulate reactivity. This review surveys recent advances in this class of reaction.

Stereoselective synthesis of trifluoromethyl-substituted 2H-furan-amines from enaminones

A straightforward strategy for synthesis of highly functionalized trifluoromethyl 2H-furans is described.

α-Borylalkyl radicals: their distinctive reactivity in modern organic synthesis

Organoborons are extremely important for synthetic organic chemistry; they can serve as advanced intermediates for a variety of transformations. Such a well-known transformation involves the loss of the boron moiety, creating alkyl radicals. Although these originally developed protocols for alkyl radical generation remain in active use today, in recent years their α-boryl carbon-centred radicals have been joined by a new array of radical generation strategies that offer a unique reactivity to forge a wider diversity of organoborons that often operate under mild and benign conditions. Herein, we will highlight the stability and reactivity of α-borylalkyl radicals and their remarkably recent advances in order to further utilise them for C-C and C-heteroatom bond formation. Their use for this purpose has been reported over the last decade in an attempt to guide the synthetic community. Various transition-metal and metal-free methods for their generation are presented, and more advanced photoredox approaches are discussed, mainly for the period of 2009-2019.

Catalytic Enantioselective Synthesis of β-Allenyl Boronic Esters by Conjunctive Cross-Coupling with Propargylic Carbonates

Enantioselective conjunctive cross-coupling with propargylic carbonates affords (β-boryl allenes as the reaction product. The reaction is found to proceed through the intermediacy of dimethoxyboronate complexes that are generated in situ by a strain-induced ligand exchange reaction.

An Olefinic 1,2-Boryl-Migration Enabled by Radical Addition: Construction of gem-Bis(boryl)alkanes

A series of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B₂ Pin₂ (bis(pinacolato)diboron) react with diverse alkyl halides by a Ru photocatalyst to give various gem-bis(boryl)alkanes. Alkyl radicals add efficiently to the alkenyl diboronate complexes, and the adduct radical anions undergo radical-polar crossover, specifically, a 1,2-boryl-anion shift from boron to the α-carbon sp² center. This transformation shows good functional-group compatibility and can serve as a powerful synthetic tool for late-stage functionalization in complex compounds. Measurements of the quantum yield reveal that a radical-chain mechanism is operative in which the alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.

Catalytic Conjunctive Coupling of Carboxylic Acid Derivatives with 9-BBN-Derived Ate Complexes

β-Boryl carbonyl compounds are produced by a Ni-catalyzed cross-coupling of vinylboron "ate" complexes and acid chloride or acid anhydride electrophiles. The reactions are efficient, being complete in as little as two minutes, and can be applied to a broad range of substrates.

Photoredox-Catalyzed Cyclobutane Synthesis by a Deboronative Radical Addition-Polar Cyclization Cascade

Photoredox‐catalyzed methylcyclobutanations of alkylboronic esters are described. The reactions proceed through single‐electron transfer induced deboronative radical addition to an electron‐deficient alkene followed by single‐electron reduction and polar 4‐exo‐tet cyclization with a pendant alkyl halide. Key to the success of the methodology was the use of easily oxidizable arylboronate complexes. Structurally diverse cyclobutanes are shown to be conveniently prepared from readily available alkylboronic esters and a range of haloalkyl alkenes. The mild reactions display excellent functional group tolerance, and the radical addition‐polar cyclization cascade also enables the synthesis of 3‐, 5‐, 6‐, and 7‐membered rings.

Insights into the mechanisms of Cu(i)-catalyzed heterocyclization of α-acyl-α-alkynyl ketene dithioacetals to form 3-cyanofurans: the roles of NH4OAc

NH₄OAc is decomposed into NH₃ and HOAc, and both NH₃ and HOAc as the proton shuttle can prompt catalytic reactions.

Diastereoselective and Enantioselective Conjunctive Cross-Coupling Enabled by Boron Ligand Design

Enantio- and diastereoselective conjunctive cross-coupling of β-substituted alkenylboron "ate" complexes is studied. Whereas β-substitution shifts the chemoselectivity of the catalytic reaction in favor of the Suzuki-Miyaura product, use of a boronic ester ligand derived from acenaphthoquinone allows the process to favor the conjunctive product, even with substituted substrates.

Radical-Triggered Three-Component Coupling Reaction of Alkenylboronates, α-Halocarbonyl Compounds, and Organolithium Reagents: The Inverse Ylid Mechanism

An operationally simple protocol to affect a radical addition to alkenylboronates that spontaneously undergo a [1,2]-metalate shift is described. Overall, the reaction is a three-component coupling of an organolithium, alkenylboronic ester, and halide which takes place with broad scope and good to excellent yields. Experimental mechanistic investigations support the formation of a boron inverse ylid intermediate.

1,2-Gold Carbene Transfer Empowers Regioselective Synthesis of Polysubstituted Furans

A gold-catalyzed cascade cycloisomerization/ring-opening reaction of allenyl ketones bearing a cyclopropyl moiety with a wide variety of alcohols or ketones is developed. This reaction involves an unprecedented 1,2-gold carbene transfer to provide regioselective and modular access to tri- or tetrasubstituted furans in moderate to high yields and with broad substrate tolerability.

Reactivity of fluorographene is triggered by point defects: beyond the perfect 2D world

Preparation of graphene derivatives using fluorographene (FG) as a precursor has become a key strategy for the large-scale synthesis of new 2-D materials (e.g. graphene acid, cyanographene, allyl-graphene) with tailored physicochemical properties. However, to gain full control over the derivatization process, it is essential to understand the reaction mechanisms and accompanying processes that affect the composition and structure of the final products. Despite the strength of C-F bonds and high chemical stability of perfluorinated hydrocarbons, FG is surprisingly susceptible to reactions under ambient conditions. There is clear evidence that nucleophilic substitution on FG is accompanied by spontaneous defluorination, and solvent-induced defluorination can occur even in the absence of any nucleophilic agent. Here, we show that distributed radical centers (fluorine vacancies) on the FG surface need to be taken into account in order to rationalize the defluorination mechanism. Depending on the environment, these radical centers can react as electron acceptors, electrophilic sites and/or cause homolytic bond cleavages. We also propose a new radical mechanism of FG defluorination in the presence of N,N'-dimethylformamide (DMF) solvent. Spin-trap experiments as well as 19F NMR measurements unambiguously confirmed formation of N,N'-dimethylformyl radicals and also showed that N,N'-dimethylcarbamoyl fluoride plays a key role in the proposed mechanism. These findings imply that point defects in 2D materials should be considered as key factor determining their chemical properties and reactivity.

Conjunctive functionalization of vinyl boronate complexes with electrophiles: a diastereoselective three-component coupling

A method for the conjunctive functionalization of vinyl boronate complexes with electrophiles is described. The overall process represents a three-component coupling between a vinyl boronic ester, carbon nucleophile and an electrophile, thus affording complex multifunctionalized products from simple starting materials. The diastereoselectivity (syn or anti) of this process is strongly dependent upon the nature of the electrophile.

Enantiospecific Three-Component Alkylation of Furan and Indole

Furan‐ and indole‐derived boronate complexes react with alkyl iodides under radical (photoredox) or polar (S_N2) conditions to generate three‐component alkylation products with high efficiency and complete stereospecificity. The methodology allows the incorporation of versatile functional groups such as nitriles, ketones, esters, sulfones, and amides, providing rapid access to complex chiral heteroaromatic molecules in enantioenriched form. Interestingly, while indolyl boronate complexes react directly with alkyl halides in a polar pathway, furyl boronates require photoredox catalysis. Careful mechanistic analysis revealed that the boronate complex not only serves as a substrate in the reaction but also acts as a reductive quencher for the excited state of the photocatalyst.

Synthesis of α-Chiral Ketones and Chiral Alkanes Using Radical Polar Crossover Reactions of Vinyl Boron Ate Complexes

Vinyl boron ate complexes of enantioenriched secondary alkyl pinacolboronic esters undergo stereospecific radical‐induced 1,2‐migration in radical polar crossover reactions. In this three‐component process various commercially available alkyl iodides act as radical precursors and light is used for chain initiation. Subsequent oxidation and protodeborylation leads to valuable α‐chiral ketones and chiral alkanes, respectively, with excellent enantiopurity.

An ab initio and DFT study of trifluoromethylation using Umemoto's reagent

Theoretical calculations revealed that trifluoromethylation using Umemoto's reagent would occur through the ionic backside mechanism.

Intermolecular Formation of Two C-C Bonds across Olefins Enabled by Boron-Based Relay Strategies

Smooth handoff in the relay: Vinyl boronates enable the direct addition of nucleophilic and electrophilic or nucleophilic and radical-generating carbon reagents across the double bond with retention of the valuable boronate group. The key to the success of this difficult twofold C-C bond-formation strategy is the initial relay of the nucleophilic addition to boron and the rearrangement of a 1,2-metalate rearrangement, shuttling it to the carbon atom.

Powerful, Thermally Stable, One-Pot-Preparable, and Recyclable Electrophilic Trifluoromethylating Agents: 2,8-Difluoro- and 2,3,7,8-Tetrafluoro-S-(trifluoromethyl)dibenzothiophenium Salts

Although many electrophilic trifluoromethylating agents have been reported to date, practically useful reagents have yet to be developed. S-(Trifluoromethyl)dibenzothiophenium salts, known as Umemoto's reagents, have two significant drawbacks that have hampered their practical application: (1) synthesis involving many steps and (2) the formation of large amounts of dibenzothiophene as waste after trifluoromethylation. Our idea to substitute fluorine at specific positions on the dibenzothiophenium rings has resulted in massive improvements in the synthesis, properties, reactivity, and applications of these compounds. On the basis of this idea, 2,8-difluoro- and 2,3,7,8-tetrafluoro-S-(trifluoromethyl)dibenzothiophenium triflates and other salts were developed as powerful, thermally stable, one-pot-preparable, and recyclable reagents for the trifluoromethylation of various types of nucleophilic substrates, such as carbanions, (hetero)aromatics, alkenes, alkynes, thiols, sulfinates, and phosphines. This one-pot and recycled production tremendously decreases the chemical and environmental costs of this process. Because of their higher reactivity and thermal stability, these new reagents may have wider applications than Umemoto's reagents. Therefore, these new versions of Umemoto's reagents could be widely used as the first practically useful electrophilic trifluoromethylating agents for the production of many types of trifluoromethyl-containing compounds in academic and industrial applications.

Merging Photoredox with 1,2-Metallate Rearrangements: The Photochemical Alkylation of Vinyl Boronate Complexes

Vinyl boronates react with electron-deficient alkyl iodides in the presence of visible light to give boronic esters in which two new C-C bonds have been created. The reaction occurs by radical addition of an electron-deficient alkyl radical to the vinyl boronate followed by electron transfer with another molecule of alkyl iodide, continuing the chain, and triggering a 1,2-metalate rearrangement. In a number of cases, the use of a photoredox catalyst enhances yields significantly. The scope of the radical precursor includes α-iodo ketones, esters, nitriles, primary amides, α-fluorinated halo-acetates and perfluoroalkyl iodides.

Enantiospecific Trifluoromethyl-Radical-Induced Three-Component Coupling of Boronic Esters with Furans

In the presence of trifluoromethylsulfonium reagents, boronate complexes derived from 2-lithio furan and non-racemic secondary and tertiary alkyl or aryl boronic esters undergo deborylative three-component coupling to give the corresponding 2,5-disubstituted furans with excellent levels of enantiospecificity. The process proceeds via the reaction of boronate complexes with a trifluoromethyl radical, which triggers 1,2-metallate rearrangement upon single-electron oxidation. Alternative electrophiles can also be used in place of trifluoromethylsulfonium reagents to effect similar three-component coupling reactions.

Stereospecific functionalizations and transformations of secondary and tertiary boronic esters

This feature article discusses the range of stereospecific transformations available to enantioenriched boronic esters, and their applications in synthesis.