Search

The transition-metal-catalyzed cyanations of aryl halides are among the most used methods for synthesizing aryl nitriles. Despite tremendous advances, cyanating an aryl halide in a facile and benig...

Significant attention has been given in the past few years to the selective transformations of N-tosylhydrazones to various useful compounds. However, the development of enantioselective versions poses considerable challenges. Herein we report a Pd-catalyzed enantioselective three-component coupling of N-tosylhydrazone, aryl halide, and terminal alkyne under mild conditions utilizing a novel chiral sulfinamide phosphine ligand (GF-Phos), which provides a facile access to chiral diarylmethyl alkynes, which are useful synthons in organic synthesis as well as exist as the skeleton in many bioactive molecules. A pair of enantiomers of the product could be easily prepared using the same chiral ligand by simply changing the aryl substituents of the N-tosylhydrazone and aryl halide. The salient features of this reaction include the readily available starting materials, general substrate scope, high enantioselectivity, ease of scale-up, mild reaction conditions, and versatile transformations.

Density functional theory computation indicates that bridge splitting of [PtII R2 (μ-SEt2 )]2 proceeds by partial dissociation to form R2 Pta (μ-SEt2 )Ptb R2 (SEt2 ), followed by coordination of N-donor bromoarenes (L-Br) at Pta leading to release of Ptb R2 (SEt2 ), which reacts with a second molecule of L-Br, providing two molecules of PtR2 (SEt2 )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH2 )2 C6 H3 Br (pz=pyrazol-1-yl) and 2,6-(Me2 NCH2 )2 C6 H3 Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via PtII Tol2 (L-N,Br) and reductive elimination from PtIV intermediates gives mer-PtII (L-N,C,N)Br and Tol2 . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH2 )2 C6 H3 Br, a stable PtIV product, fac-PtIV Me2 {2,6-(pzCH2 )2 C6 H3 -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable PtIV Tol2 {L-N,C,N}Br undergoing facile Tol2 reductive elimination. The mechanisms reported herein enable the synthesis of PtII pincer reagents with applications in materials and bio-organometallic chemistry.

We report the influence of substituents and physical conditions on activation energies for the noncatalyzed amination (C-N cross-coupling reactions) of aryl halides. We uncover a significant correlation between the barrier heights of the C-N bond formation and Hammett σ parameters-a formal measure of the electron-withdrawing or -donating ability of substituents on the aryl halides. Our results indicate that such correlations are useful predictive tools for the amination of aryl halides over a wide range of substituent types. From 54 cases studied (six substituents occupying specific positions relative to halogen atoms), the 2-COOHPhI + NH2n Pr amination reaction is predicted to possess the lowest noncatalyzed activation free energy (135.6 kJ mol-1) using the B3LYP method. The lower barriers for the 2-COOHPhX (for X = Cl, Br, and I) compounds are shown to originate from collusion between steric and electronic effects-specifically, the momentary formation of a hydrogen bond between an oxygen site on the ortho-COOH and the lone pair of the entering amine. Internal reaction coordinate (IRC) path calculations afforded us these and other key insights into the nature of the reactions. The control exerted by substituents on the arrangement of the transition state structure, as well as the sensitivity of the reaction barriers to temperature and solvent polarity, are discussed. These results offer new perspectives from which to assess the nature of the C-N bond formation and suggest new avenues for future exploration, especially in progress toward the metal-free amination of aryl compounds.

We report a catalytic method to access secondary alcohols by the coupling of aryl iodides. Either aldehydes or alcohols can be used as reaction partners, making the transformation reductive or redox-neutral, respectively. The reaction is mediated by a Ni catalyst and a 1,5-diaza-3,7-diphosphacyclooctane. This P2N2 ligand, which has previously been unrecognized in cross-coupling and related reactions, was found to avoid deleterious aryl halide reduction pathways that dominate with more traditional phosphines and NHCs. An interrupted carbonyl-Heck type mechanism is proposed to be operative, with a key 1,2-insertion step forging the new C-C bond and forming a nickel alkoxide that may be turned over by an alcohol reductant. The same catalyst was also found to enable synthesis of ketone products from either aldehydes or alcohols, demonstrating control over the oxidation state of both the starting materials and products.

The generally accepted mechanism of the Suzuki—Miyaura reaction suggests a sequential activation of the substrate (aryl halide) and the reagent (arylboronic acid) by a palladium catalyst with the formation of unsymmetric biaryl as a result of a single turnover of the catalytic cycle, i.e., it is linear from the kinetic point of view. At the same time, the use of an unconventional kinetic approach based on the analysis of the differential selectivity of the reaction, rather than the regularities of catalytic activity, indicates the inadequacy of the linear mechanism, that is consistent with the hypothesis of a nonlinear (the so-called cooperative) mechanism of catalysis, in which the product is formed as a result of the substrate and reagent activation by two different palladium-containing intermediates in two parallel catalytic cycles. The experimentally observed low kinetic orders of the Suzuki—Miyaura reaction with respect to the concentration of the palladium catalyst precursor under the ligand-free conditions of catalysis are also consistent with the cooperative mechanism and can be due to the changes in the relative amount of the catalyst in two parallel catalytic cycles and/or to the process of catalyst deactivation.

Phenols are important components of pharmaceuticals, biologically active natural products, and materials. Here, we briefly discuss recent advances in catalytic hydroxylation reactions for the synthesis of phenols, with particular attention to our recent work. H2O is proved to be an efficient hydroxide reagent in converting (hetero)aryl halides into the corresponding phenols under synergistic organophotoredox and nickel catalysis. Aryl bromides as well as less reactive aryl chlorides show high reactivity in this catalytic system. This methodology can be applied to the efficient synthesis of diverse phenols and allows the hydroxylation of multifunctional pharmaceutically relevant aryl halides.1 Introduction2 Representative Methods for Transition-Metal-Catalyzed Hydroxylation of (Hetero)Aryl Halides3 Organophotoredox/Ni Dual Catalytic Hydroxylation of Aryl Halides with Water4 Summary and Outlook

Graphical abstract Metal-catalyzed reactions play a vital part to construct a variety of pharmaceutically important scaffolds from past few decades. To carry out these reactions under mild conditions with low-cost easily available precursors, various new methodologies have been reported day by day. Sandmeyer reaction is one of these, first discovered by Sandmeyer in 1884. It is a well-known reaction mainly used for the conversion of an aryl amine to an aryl halide in the presence of Cu(I) halide via formation of diazonium salt intermediate. This reaction can be processed with or without copper catalysts for the formation of C–X (X = Cl, Br, I, etc.), C-CF3/CF2, C–CN, C–S, etc., linkages. As a result, corresponding aryl halides, trifluoromethylated compounds, aryl nitriles and aryl thioethers can be obtained which are effectively used for the construction of biologically active compounds. This review article discloses various literature reports about Sandmeyer-related transformations developed during 2000–2021 which give different ideas to synthetic chemists about further development of new and efficient protocols for Sandmeyer reaction. An updated compilation of new approaches for Sandmeyer reaction is described in this review to construct a variety of carbon-halogen, carbon-phosphorous, carbon-sulfur, carbon-boron etc. linkages.

In this paper, we report a novel magnetically separable silica coated copper nano-magnetite NHC-benzimi@Cu complex as heterogeneous catalyst for the multicomponent click reaction via Huisgen 1,3-dipolar cycloaddition reaction of alkyl or aryl halide, sodium azide and terminal alkyne, which affords various1,4-disubstituted 1,2,3-triazoles. The multistep prepared nano catalyst has been characterized by various spectroscopic methods such as FT-IR, TGA, EDX, XRD, TEM and VSM. The heterogeneous nano catalyst structures coated on the copper surface are responsible for the excellent catalyst performances in the reaction. The reusability of the catalyst makes the present protocol more fascinating from an environmental and economic point of view.

Designing of eco-friendly, low cost, and thermally stable stabilizing/supporting agents are always desired for production of catalyst systems which provide good catalytic performance in organic reactions. In this study, a novel, green, and efficient stabilizer containing chitosan/Co3O4 microspheres (CS/Co3O4) was developed. Palladium nanoparticles (Pd NPs) were then successfully immobilized on CS/Co3O4 as a heterogeneous nanocatalyst (Pd NPs/CS/Co3O4). Characterization of the designed materials were performed by FT-IR, TEM, FE-SEM, XRD, and EDS and it was determined that Pd NPs formed as approximately 20 nm. Catalytic behavior of Pd NPs/CS/Co3O4 was investigated in the production of different substituted benzonitriles via aryl halide cyanation. Catalytic studies indicate that electron-rich or poor aromatic halides were smoothly cyanated with good reaction yields by Pd NPs/CS/Co3O4 nanocatalyst by using K4[Fe(CN)6] as the cyanating agent. Moreover, it was found that Pd NPs/CS/Co3O4 nanocatalyst provided not only good reaction yields and but also good recovery/reusability for six times in the aryl halide cyanations. This paper displays that Pd NPs/CS/Co3O4 nanocatalyst has a great catalytic and recycling potential for aryl halide cyanations.

Bipyridine- and benzimidazole-based ligands for the self-assembly of Co4L6 cages were synthesised in short reaction times and high isolated yields directly from aryl halide precursors using a copper-free one-pot Sonogashira-type coupling. This one-pot method circumvents the often time-consuming and challenging ligand synthesis for the preparation and application of cages.

Highly active natural pandanus-extracted cellulose-supported poly(hydroxamic acid)−Cu(II) complex 4 was synthesized. The surface of pandanus cellulose was modified through graft copolymerization using purified methyl acrylate as a monomer. Then, copolymer methyl acrylate was converted into a bidentate chelating ligand poly(hydroxamic acid) via a Loosen rearrangement in the presence of an aqueous solution of hydroxylamine. Finally, copper species were incorporated into poly- (hydroxamic acid) via the adsorption process. Cu(II) complex 4 was fully characterized by Fourier transform infrared (FTIR), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses. The cellulose-supported Cu(II) complex 4 was successfully applied (0.005 mol %) to the Ullmann etherification of aryl, benzyl halides, and phenacyl bromide with a number of aromatic phenols to provide the corresponding ethers with excellent yield [benzyl halide (70−99%); aryl halide (20−90%)]. Cu(II) complex 4 showed high stability and was easily recovered from the reaction mixture. It could be reused up to seven times without loss of its original catalytic activity. Therefore, Cu(II) complex 4 can be commercially utilized for the preparation of various ethers, and this synthetic technique could be a part in the synthesis of natural products and medicinal compounds.

We herein disclose the first Ni-catalyzed Hiyama coupling of aryl halides with vinylsilanes. This protocol uses cheap, nontoxic, and stable vinyltrimethoxysilane as the vinyl donor, proceeds under mild and ligand-free conditions, furnishing a diverse variety of styrene derivatives in high yields with excellent functional group compatibility.

The formation of diaryl ethers is generally achieved via transition metal catalyzed etherification reactions (Ullmann, Chan–Lam, Buchwald–Hartwig) with prefunctionalized aryl halide substrates at elevated temperatures. Herein, we report a protocol for electrochemical C(sp2)–H/O–H cross-dehydrogenative coupling of phenols and tertiary anilines to synthesize diaryl ethers. The C(sp2) H/O–H coupling product can be obtained under metal- and oxidant-free conditions at room temperature in moderate to excellent yield (up to 83% yield) with high regioselectivity (>99% for para-substitution) and with a broad substrate scope (22 examples).

Biochar is a carbon-rich solid, the surface of which is covered with a high density of functional carbonyl, hydroxyl and carboxylic acid groups. In this work, palladium nanoparticles were embedded on magnetic biochar and a new reusable and environmentally-friendly catalyst was developed and applied for the promotion of Suzuki-Miyaura C-C coupling and cyanation reactions. The high-carbon (77%), low-ash content (5.8%) and the relatively high surface area (266 m(2) g(-1)) of pine tree biochar (PTB) suggested that it might be highly suitable as a catalyst substrate. The Fe3O4-Pd-biochar nanocomposite was successfully characterized using SEM, TEM, EDX, FT-IR, BET and XRD. Its catalytic role was initially evaluated using p-NO2C6H4I as a model reactant (for both types of reactions) and later for the production of biaryls and benzonitriles from a wide range of aryl halides under mild reaction conditions. Biaryls and benzonitriles were characterized using GC-MS. In the case of the Suzuki-Miyaura reaction, the optimum yield of 98% was obtained with a catalyst concentration of 0.04 mol%, microwave irradiation of 400 W, and a residence time of 5 min, using K2CO3 as the base. With respect to the cyanation reaction, dimethylformamide, Na2CO3 and 6 h were the optimum solvent, base and reaction duration, respectively. Subsequently, the nanocatalyst showed excellent catalytic activity in both reactions, achieving >88% yields in most cases, regardless of the aryl iodide or bromide used and the type of substitution.

Recently, the dynamic nature of the metal-NHC bond has been proposed and the key role of chemical evolution in changing the nature of catalytically active sites is now an emerging topic. A comparative analysis of the ketone α-arylation reaction with aryl halides, catalyzed by M/NHC complexes, was carried out in the present study and showed a fundamental difference in the behavior of the catalytic system for M = Ni and Pd. In situ evolution of Ni/NHC complexes with cleavage of the Ni-NHC bond leads to complete deactivation of catalytic systems, regardless of the nature of the aryl halide ArX (X = Cl, Br, I). However, upon Pd/NHC catalysis, the cleavage of the Pd-NHC bond causes deactivation only in the case of aryl chlorides. In the reactions of more active aryl iodides and aryl bromides, NHC-disconnected Pd species, formed as a result of the chemical transformation of Pd/NHC complexes, can provide effective catalysis in the arylation reaction under study. New catalytic systems based on Pd/NHC and Ni/NHC complexes generated in situ from stable imidazolium salts, IPr·HCl and IPr*OMe·HCl, and Pd(OAc)2 (0.1 mol%) or NiCl2Py2 (5 mol%) were developed for the selective α-arylation of methylaryl ketones (Pd-catalysis) and other ketones less prone to aldol-crotonic condensation (Ni-catalysis). The present study has shown that the different effects of the metal-NHC bond cleavage should be taken into account for the efficient choice and optimization of catalytic systems to carry out arylation reaction with various aryl halides.

Mixed metal–metal oxide/C (Ag–Cu–Cu2O/C) nanocomposites were synthesized by the heat treatment of a metal–organic framework under a N2 flow using the one-pot synthesis method. The as-prepared nanocomposites were characterized using a range of techniques, such as TEM, elemental mapping, XRD, N2 sorption, UV-Vis DRS, and XPS. The nanoparticles were successfully formed with high dispersion in porous carbon materials and high crystallinity based on the analysis results. The Ag–Cu–Cu2O/C nanocomposites (35 nm) showed high photocatalytic activity and good recyclability toward the borylation of aryl halides under a xenon arc lamp. This result can enhance the interest in photocatalysis for various applications, particularly in organic reactions, using a simple and efficient synthesis method.

Three-component cross-coupling in the system aryl halide–arylacetylene–alkene in the presence of simplest ligand-free palladium catalysts gave products of both 1+1+1-coupling and cross-dimerization of arylacetylene with alkene. The possibility of controlling the reaction selectivity has been demonstrated.

We report a facile solution-phase sonochemical route for the glucose-template mediated synthesis of Cu2O nanocrystals with well-defined sizes and shapes. This approach offers a superficial and cost-effective route for the synthesis of Cu2O nanocrystals. The formation of different nanostructures (i.e., stars, cubes, and octahedra) was tuned by varying the reaction conditions as well as the concentration of template. The morphology-controlled Cu2O nanocrystals show high activity for the external base-free Ullmann homocoupling reactions of aryl halides. 61.6% conversion of chlorobenzene was achieved over octagonal Cu2O nanocrystals with a turn-over-frequency (TOF) as high as 1696 h−1. This result demonstrates that the size and shape-dependent properties of Cu2O nanocrystals are critical to the activity of aryl halide and highlights the importance of shape engineering in the design and development of an efficient catalyst for the Ullmann reaction.

Density functional theory calculations were performed to understand the distinctly different reactivities of o-carboxylate-substituted aryl halides and pristine aryl halides toward the PdII-catalyzed γ-C(sp3)-H arylation of secondary alkylamines. It is found that, when 2-iodobenzoic acid (a representative of o-carboxylate-substituted aryl halides) is used as an aryl transfer agent, the arylation reaction is energetically favorable, while when the pristine aryl halide iodobenzene is used as the aryl transfer reagent, the reaction is kinetically difficult. Our calculations showed an operative PdII/PdIV/PdII redox cycle, which differs in the mechanistic details from the cycle proposed by the experimental authors. The improved mechanism emphasizes that (i) the intrinsic role of the o-carboxylate group is facilitating the C(sp3)-C(sp2) bond reductive elimination from PdIV rather than facilitating the oxidative addition of the aryl iodide on PdII, (ii) the decarboxylation occurs at the PdII species instead of the PdIV species, and (iii) the 1,2-arylpalladium migration proceeds via a stepwise mechanism where the reductive elimination occurs before decarboxylation, not via a concerted mechanism that merges the three processes decarboxylation, 1,2-arylpalladium migration, and C(sp3)-C(sp2) reductive elimination into one. The experimentally observed exclusive site selectivity of the reaction was also rationalized well.

Reported here are conditions for the construction of spirocyclic piperidines from linear aryl halide precursors. These conditions employ a strongly reducing organic photoredox catalyst in combination with a trialkylamine reductant to achieve formation of aryl radical species. Regioselective cyclization followed by hydrogen-atom transfer affords a range of complex spiropiperidines. This system operates efficiently under mild conditions without the need for toxic reagents or precious metals.

A palladium-catalyzed cross-coupling reaction with aryl halide functionalities has recently emerged as a valuable tool for protein modification. Herein, a new fluorogenic modification methodology for proteins, with genetically encoded fluorosulfate-l-tyrosine, which exhibits high efficiency and biocompatibility in bacterial cells as well as in aqueous medium, is described. Furthermore, the cross-coupling of 4-cyanophenylboronic acid on green fluorescent protein was shown to possess a unique fluorogenic property, which could open up the possibility of a responsive "off/on" switch with great potential to enable spectroscopic imaging of proteins with minimal background noise. Taken together, a convenient and efficient catalytic system has been developed that may provide broad utilities in protein visualization and live-cell imaging.

The copper-catalyzed arylation of unsaturated nitrogen heterocycles, known as the Ullmann–Goldberg coupling, is a valuable transformation for medicinal chemists, providing a modular disconnection for the rapid diversification of heteroaromatic cores. The utility of the coupling, however, has established limitations arising from a high-barrier copper oxidative addition step, which often necessitates the use of electron-rich ligands, elevated temperatures, and/or activated aryl electrophiles. Herein, we present an alternative aryl halide activation strategy, in which the critical oxidative addition (OA) mechanism has been replaced by a halogen abstraction–radical capture (HARC) sequence that allows the generation of the same Cu(III)-aryl intermediate albeit via a photoredox pathway. This alternative mechanistic paradigm decouples the bond-breaking and bond-forming steps of the catalytic cycle to enable the use of many previously inert aryl bromides. Overall, this mechanism allows access to both traditional C–N adducts at room temperature as well as a large range of previously inaccessible Ullmann–Goldberg coupling products including sterically demanding ortho-substituted heteroarenes.

In this work, Pd nanoparticles (Pd-NPs) were decorated on modified magnetic nanoparticles (MNPs) and used as an efficient and recyclable catalyst for the Suzuki cross-coupling reaction of aryl halides with phenylboronic acid (PhB(OH)2) in a green solvent. The prepared nanocomposite was characterized by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Fourier transforms infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis/differential thermal analysis, and vibrating sample magnetometry. All analyses confirmed the successful modification of MNPs and immobilization of Pd on modified MNPs. This catalyst exhibited superior catalytic activity and stability in the suzuki cross-coupling reaction of PhB(OH)2 and aryl halide derivatives. This protocol includes some advantages, such as magnetically reusability of the catalyst, mild experimental conditions, green solvent, excellent yields of the product (52–98%), and short reaction times (4–33 min). The catalyst could be reused for six successive runs without any significant loss of its efficiency.

The monoarylation of acetone is a powerful transformation, but is typically performed at temperatures significantly in excess of its boiling point. Conditions described for performing the reaction at ambient temperatures led to significant dehalogenation when applied to a complex aryl halide. We describe our attempts to overcome both issues in the context of our drug-discovery program.

A new free radical/palladium cooperative catalyzed formylation of aryl halides with glyoxylic acid as the formyl source under oxygen conditions has been developed. Various aromatic and heteroaromatic aldehydes were produced in medium to good yields.

Herein, we report the development of aryl halide-dependent chemoselective reactions, viz., the Buchwald-Hartwig type coupling reaction of an aryl iodide with an arylboronic acid and an aryl amine in the presence of a heterogeneous and reusable nickel catalyst and the Suzuki-Miyaura type coupling of an aryl chloride under similar conditions. Control experiments revealed that the presence of stoichiometric amounts of the phenylboronic acid/ester and aryl amine are essential for both reactions. NMR and XAFS studies suggested the formation of a boron-amine "ate" complex.

A photocatalytic system for the dearomative hydroarylation of benzene derivatives has been developed. Using a combination of an organic photoredox catalyst and an amine reductant, this process operates through a reductive radical-polar crossover mechanism where aryl halide reduction triggers a regioselective radical cyclization event, followed by anion formation and quenching to produce a range of complex spirocyclic cyclohexadienes. This light-driven protocol functions at room temperature in a green solvent system (aq. MeCN) without the need for precious metal-based catalysts or reagents or the generation of stoichiometric metal byproducts.

A new Schiff base catalyst (Pd(II) chemically mounted on the cellulose nanocrystals surface (CNC-APTES-IS-Pd) was developed for Ulmann and Suzuki cross-coupling reactions. The catalyst was applied for Ulmann reaction using s series of aryl halide and phenol derivatives in DMSO and preparation of biaryls via Suzuki C-C reactions between aryl halides and phenyl boronic acid. The Catalyst was characterized by FT-IR, XRD, SEM, ICP-AES and TGA techniques. The catalyst demonstrated high reaction efficiency with more than 90% reaction yield. The catalyst indicated good performance after several times recovery and reuse.

Palladium-catalyzed alkene-directed cross-coupling of aryl iodide with another aryl halide through C-H arylation opens a unique avenue for unsymmetrical biaryl-derived molecules. However, homo-coupling of aryl iodides often erodes the overall synthetic efficiency. Reported herein is a highly chemoselective Pd0 -catalyzed alkyne-directed cross-coupling of aryl iodides with bromophenols, which was subsequently followed by phenol dearomatization to furnish a very attractive [2+2+1] spiroannulation. Notably, possible homo-coupling of aryl iodides was not observed at all. Mechanistic studies indicated that a five-membered aryl/vinyl palladacycle most likely accounts for promoting the key step of biaryl cross-coupling.

The combination of metal- and biocatalysis is a challenging but forward-looking topic in synthetic chemistry. The unique selectivity of enzymes paired with the broad range of applications of chemical catalysts enables an undreamed-of number of novel processes. Herein, we describe the application of immobilized phenolic acid decarboxylase (PAD) for the decarboxylation of para-coumaric acid and subsequent Pd-catalyzed Heck cross-coupling with an aryl halide in a fully integrated two-step continuous flow process to synthesize (E)-4-hydroxy-stilbene. The application of a choline chloride-based deep eutectic solvent (DES) proved to be crucial to overcome the problem of solvent compatibility and enabled an increase in substrate concentration (from 5 mM in buffer to 20 mM in DES) as well as a process with a homogeneous starting solution. The two-step process was successfully operated for more than 16 h in continuous flow and full conversion was achieved. The results underline the usefulness of DES to overcome compatibility problems in tandem-catalytic processes. The system benefits from its simplicity due to increased substrate solubility, the possibility to conduct both reactions at their optimal temperatures and the elimination of isolating the reaction intermediate, which is prone to polymerization.

As a green synthetic approach, visible light-driven photosynthesis is highly desirable in arylation of inert alkyl halides, as they are important precursors in the total synthesis of natural products and pharmaceuticals. However, the high bond dissociation energy of aryl halides is typically out of the range of a single visible-light photon. Here, we propose an essential initiation and subsequent electron-transfer step process for visible light-driven aryl halide reduction, and identify the key pyrrole radical anion intermediate, that acts as the strong reduction species. We propose a photoinduced disproportionation (PDP) approach without the addition of any photocatalysts or additives to afford radical anions of pyrrole derivatives, which have enough reduction power to transfer an electron to aryl halide, giving rise to the corresponding aryl radical to afford the desired C–H arylated heterocyclic product. Once generated, the heterocyclic product can undergo the same photoinduced disproportionation (PDP) process to activate aryl halides, thereby promoting the reaction rate. This unprecedented initiation step, which was carried out in the absence of photocatalysts and additives under ambient conditions, can also be used for coupling a wide range of (hetero)aryl halides and pyrrole derivatives, as well as the synthesis of drug intermediates and biorelevant compounds.

This report introduces a new approach to high sulfur-content materials. This route, RASP (radical-induced aryl halide/sulfur polymerization), expands the substrate scope beyond olefins required for the traditional inverse vulcanization route to such materials. RASP allows direct reaction of two unmodified industrial waste products to give lignin–sulfur composites.

High sulfur-content materials (HSMs) have been investigated for a plethora of applications owing to a combination of desirable properties and the low cost of waste sulfur as a starting monomer. Whereas extended sulfur catenates are unstable with respect to orthorhombic sulfur (S8 rings) at STP, oligomeric/polymeric sulfur chains can be stabilized when they are confined in a supporting matrix. The vast majority of reported HSMs have been made by inverse vulcanization of sulfur and olefins. In the current case, a radical aryl halide–sulfur polymerization (RASP) route was employed to form an HSM (XS81) by copolymerizing elemental sulfur with the xylenol derivative 2,4-dimethyl-3,5-dichlorophenol (DDP). XS81 is a composite of which 81 wt% is sulfur, wherein the sulfur is distributed between cross-linking chains averaging four sulfur atoms in length and trapped sulfur that is not covalently attached to the network. XS81 (flexural strength = 2.0 MPa) exhibits mechanical properties on par with other HSMs prepared by inverse vulcanization. Notably, XS81 retains mechanical integrity over many heat-recast cycles, making it a candidate for facile recyclability. This is the first report of an HSM comprising stabilized polymeric sulfur that has been successfully prepared from a small molecular comonomer by RASP. Preparation of XS81 thus demonstrates a new route to access HSMs using small molecular aryl halides, a notable expansion beyond the olefins required for the well-studied inverse vulcanization route to HSMs from small molecular comonomers.

Ni 2,2’–bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited state Ni(II)–C bond homolysis. Here we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(Rbpy)(R′Ph)Cl (R = MeO, t-Bu, H, MeOOC; R′ = CH3, H, OMe, F, CF3), to illuminate the mechanism of excited state bond homolysis. At given excitation wavelengths, photochemical homolysis rates span two orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) excited state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited state barriers (ΔH‡ ~4 kcal mol-1) and a minimum energy excitation threshold (~55 kcal mol-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited state PESs provides a rational approach to utilize photonic energy and leverage excited state bond homolysis processes in synthetic chemistry.

DNA-encoded libraries are a very efficient means of identifying ligands for protein targets in high throughput. To fully maximize their use, it is essential to be able to carry out efficient reactions on DNA-conjugated substrates. Arylamines are privileged motifs in druglike molecules, and methods for their incorporation into DNA-encoded libraries are highly desirable. One of the preferred methods for their preparation, the Buchwald-Hartwig coupling, does not perform well on DNA conjugates using current approaches. We report the application of our recently developed micellar technology for on-DNA chemistry to the Buchwald-Hartwig reaction. Optimization of conditions led to a robust, high-yielding method for the synthesis of DNA-conjugated aryl and heteroarylamines, which is broad in substrate scope for both the arylamine and the DNA-conjugated aryl halide and is fully compatible with DNA-encoding and decoding procedures. This method will enable the preparation of diverse, high-fidelity libraries of biarylamines.

Although alcohols represent one of the largest pools of commercially available alkyl substrates, approaches to di-rectly utilize them in cross-coupling and cross-electrophile coupling are limited. We report the use of alcohols in cross-electrophile coupling with aryl and vinyl halides to form C(sp3)–C(sp2) bonds in a one-pot strategy. This strategy allows the use of primary and secondary alcohols through their very fast (

Photoredox nickel catalysis has emerged as a powerful strategy for cross-coupling reactions. Although the involvement of paramagnetic Ni(I)/Ni(III) species as active intermediates in the catalytic cycle has been proposed, a thorough spectroscopic investigation of these species is lacking. Herein, we report the tridentate pyridinophane ligands RN3 that allow for detailed mechanistic studies of the photocatalytic C–O coupling reaction. The derived (RN3)Ni complexes are active catalysts under mild conditions and without an additional photocatalyst. We also provide direct evidence for the key steps involving paramagnetic Ni species in the proposed catalytic cycle: the oxidative addition of an aryl halide to a Ni(I) species, the ligand exchange/transmetalation at a Ni(III) center, and the C–O reductive elimination from a Ni(III) species. Overall, the present work suggests the RN3 ligands are a practical platform for mechanistic studies of Ni-catalyzed reactions and for the development of new catalytic applications.

An efficient nickel-catalyzed regioselective hydroarylation of 1,3-dienes with aryl halides and a silane has been developed, affording a range of allylic arenes in good to excellent yields under mild conditions. This method exhibits broad substrate scope, and excellent functional group tolerance. Late-stage modification of complex architectures was demonstrated.

A highly regioselective Ni-catalyzed electrochemical reductive relay cross-coupling between an aryl halide and an alkyl halide has been developed in an undivided cell. Various functional groups are tolerated under these mild reaction conditions, which provides an alternative approach for the synthesis of 1,1-diarylalkanes.

A series of new monoligated arylpalladacyclic complexes with amide, urea, and carbamate frameworks have been developed and were found to be highly active for Suzuki–Miyaura cross-couplings. These palladacycle precatalysts are derived from simple, inexpensive starting materials and are air- and moisture-stable. The most active congeners among those tested are the urea-based palladacycles, which in conjunction with a tBu3P ligand induce high conversions for a range of aryl halide and boronate coupling partners. Notably, aryl chlorides are viable coupling partners even under relatively mild conditions at short reaction times (e.g., 1 h at 60 °C). One of the best catalysts described here exhibits improved turnover frequency for a particularly difficult coupling reaction involving an aryl chloride and a sterically congested boronic ester.

A practically convenient protocol has been developed to convert a mixture of an aldehyde, aryl halide, and the Bestmann-Ohira reagent into disubstituted acetylene via a successive addition of base (Cs2CO3) and a Pd(II) catalyst, allowing sufficient time after addition of each of these reagents for the tandem processes (Seyferth-Gilbert homologation and Sonogashira coupling) to occur. Notably, for the latter reaction, no copper catalyst was required.

GSK2981278A (1) is a RORγ inverse agonist used as a potential topical nonsteroidal therapy for psoriasis. New synthesis of 1 was developed based on a SNAr reaction of (tetrahydro-2H-pyran-4-yl)methanol with an aryl halide intermediate, prepared from 2-halobenzoic acids. The dianion underwent in situ N,O-bis-isobutylation, followed by a reduction to provide 1. The new route eliminated a genotoxic tosylate of (tetrahydro-2H-pyran-4-yl)methanol and a difficult reductive amination from the original synthesis starting from methyl salicylate. A primary version of the route has been scaled up to deliver 125 kg of 1. However, the heating of a strong base in DMSO for an extended period during the bis-alkylation was found to be a safety concern for manufacturing. A safer and greener process was then developed utilizing a facile N,O-bis-alkylation, which was conducted under phase transfer conditions with mild bases such as potassium carbonate and in green solvents such as water. The concise four stage sequence from ...

We report a well-defined example of intermolecular aryl halide oxidative addition (OA) to Pt(II). Complexes of the type (IMes)PtMe2(L) and (IMes′)PtMe(L) (L = SMe2, pyridine; IMes = N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; IMes′ = cyclometalated IMes) undergo intermolecular OA of phenyl iodide (PhI) at 60 °C, producing toluene via reductive elimination from a proposed Pt(IV) phenyl species. Isolation of a model Pt(IV) OA product provides evidence for a Pt(II)/Pt(IV) pathway. The OA of PhI is not limited to Pt(II) IMes complexes; analogous reactions also proceed with phosphine-ligated Pt(II) dialkyl complexes, demonstrating that this reaction is feasible for a variety of electron-rich Pt(II) complexes bearing labile ligands.

A rare reductive coupling of nitro compounds with organohalides has been realized. The reaction is initiated by a partial reduction of the nitro group to a nitrenoid intermediate. Therefore, not only aromatic but also aliphatic nitro compounds are efficiently transformed into monoalkylated amines, with organohalides as the alkylating agent. Given the innate reactivity of the nitrenoid, a catalyst is not required, resulting in a high tolerance for aryl halide substituents in both starting materials.