The 2-Pyridyl Problem: Challenging Nucleophiles in Cross-Coupling Arylations

Alkyl sulfones: discovery of novel structural types with differentiated opportunities for insect control

The discovery of novel chemical classes with novel modes of action for insect control form the backbone of innovation with the goal to deliver much-needed solutions into the hands of growers. Over the last decade, alkyl sulfones have emerged as one of the most versatile new classes and are under intensive investigation in many R&D programs in the industry, with Sumitomo Chemicals recently introducing oxazosulfyl as a first active ingredient to the market. In this review, we discuss some of our strategies to invent novel classes based upon ligand-based design, and also show how incorporation of physical chemical properties into our design enabled us to predictably control chewing and sucking pests. © 2024 Society of Chemical Industry.

Total Synthesis and Pharmacological Evaluation of Phochrodines A-C

The first syntheses of the Phomopsis-isolated natural products phochrodines A-C are reported. Functional group manipulations on a key 5H-chromeno[4,3-b]pyridine intermediate, itself synthesized from intramolecular Suzuki-Miyaura coupling, enabled facile and high-yielding syntheses of all three natural products. Additionally, sufficient material was generated to enable detailed pharmacological profiling of each compound. Preliminary drug metabolism and pharmacokinetic (DMPK) experiments and ancillary pharmacology screening revealed phochrodine C (3) as an attractive scaffold for further modification, particularly for medicinal chemists working in the antidepressant space.

Pd/NBE-Catalyzed One-Pot Modular Synthesis of Tetrahydro-γ-carbolines

Tetrahydro-γ-carbolines are especially outstanding fused heterocyclic ring systems possessing significant biological activities in the central nervous system. Here, using commercially available NBE derivatives (NBEs), we report an efficient protocol for the one-pot modular synthesis of 4-substituted tetrahydro-γ-carbolines via Catellani/aza-Michael addition cascade from easily available 3-iodo-1-tosyl-1H-indole, aziridines and olefins. This approach exhibits a wide substrate scope, good yields, scalability, and potential extension toward the synthesis of Mebhydroline analogues.

A Mechanistic Perspective on Photocatalytic EnT-Enabled C3-N-Heteroarylation of Aryl Quinoxaline via C(sp2)-C(sp2) Coupling

An EnT-mediated C3-N-heteroarylation of 2-aryl quinoxalines via decarboxylative radical-radical cross-coupling (C(sp2)-C(sp2)) with oxime esters is presented. Upon photoactivation, the triplet energy of the photocatalyst is transferred to both reacting partners (oxime ester and 2-phenylquinoxaline). The excited oxime ester undergoes decarboxylative fragmentation to a C-centered N-heteroaryl radical and an N-centered imine radical. The former attacks the C3 site of the quinoxaline diradical, while the latter acts as a hydrogen atom abstractor (HAA). Computational studies revealed that C-C bond formation with the heteroaryl radical is energetically more favorable than C-N bond formation.

Water-Stable 2-Pyridylboron Reagents: Pd-Catalyzed 2-Pyridylation Reaction of Aryl Halides

The stability of 2-pyridylation reagents is a long-standing issue in cross-coupling chemistry due to hydrolysis. However, as the use of pyridine-based pharmaceuticals continues to increase, there is a high demand for stable and reactive 2-pyridylation reagents. Herein, a general strategy to prepare water-stable 2-pyridylboron reagents has been developed. The application of the water-stable 2-pyridylboron reagents in a neutral Suzuki-Miyaura coupling with a halide scavenger enables an efficient 2-pyridylation reaction of aryl halides.

Copper-Catalyzed C-B(sp3) Bond Formation through the Intermediacy of Cu-B(sp3) Complex

The vast majority of transition metal (TM) catalyzed borylative transformations rely on TM-B(sp2) complexes. Contrastingly, the chemistry of TM-B(sp3) species and their potential in catalytic borylation remain surprisingly underdeveloped, due probably to the lack of suitable boron(sp3) reagents. Herein, we employ our recently developed sp2-sp3 diboron reagent to successfully enable a copper-catalyzed hydroboration of allenes for C-B(sp3) bond formation. A comprehensive mechanistic study, including the isolation and structural characterization of a Cu-B(sp3) complex, substantiates the presence of a Cu-B(sp3) intermediate in the catalytic cycle.

Late-Stage Cross-Electrophile Coupling of Arylthianthrenium Salts with (Hetero)aryl (Pseudo)halides via Palladium Catalysis

Herein, we present a cross-coupling reaction of arylthianthrenium salts at a late stage with diverse (hetero)aryl (pseudo)halides under reductive conditions, in which a palladium(0) catalyst differentiates between two aryl electrophiles based on the different rates of oxidative addition of arylthianthrenium salts and aryl halides for selective umpolung. A measured near-zero Hammett rho value is consistent with oxidative addition of the arylthianthrenium salts to palladium(0) being insensitive to substituent effects, which enables reaction with structurally and electronically diverse arylthianthrenium salts. In addition, we show the robustness of this method by coupling of two complex fragments that would otherwise be difficult to access in a single step.

Pyridyl Pyrimidylsulfones as Latent Pyridyl Nucleophiles in Palladium-Catalyzed Cross-Coupling Reactions

We introduce pyridyl pyrimidylsulfones as latent, efficient pyridyl nucleophiles for Pd-catalyzed cross-coupling reactions with (hetero)aryl bromides. The reaction proceeds via an SNAr process, generating sulfinates in situ, followed by desulfinative cross-coupling in the presence of Pd catalyst, phenol, and cesium carbonate. This method provides access to a wide range of (hetero)arylpyridine derivatives and enables late-stage functionalization, addressing limitations commonly associated with pyridylboron reagents in Pd-catalyzed cross-coupling. Additionally, the pyrimidylsulfones demonstrate compatibility with multistep substrate elaboration, as evidenced by the successful bidirectional functionalization of the pyridine ring, yielding teraryl compounds.

An I2/amine synergistic catalysis enables 2,2'-bipyridine synthesis from oxime esters and enals

A synergistic I2/secondary amine catalysis has been developed for the modular assembly of 2,2'-bipyridines, employing oxime esters and enals as readily available starting materials. This method demonstrates a good substrate scope and functional group tolerance and can easily afford a series of structurally new 2,2'-bipyridines in moderate to good yields.

Pd-Catalyzed Decarboxylative Negishi Coupling of Zinc Aryl Carboxylates with Arylthianthrenium Salts

We report a Pd-catalyzed decarboxylative Negishi coupling reaction for efficient biaryl synthesis from various zinc aryl carboxylates, including polyfluorobenzoates and heteroaryl carboxylates, using DMF as the solvent. This mild reaction exhibits a broad substrate scope and enables late-stage functionalization of bioactive molecules. Mechanistic studies show that DMF-assisted zinc catalyzes decarboxylation of polyfluorinated aryl carboxylates to generate arylzinc reagents in situ, which then undergo Negishi coupling catalyzed by palladium with arylthianthrenium salts to form biaryl compounds. Notably, this protocol represents a rare example of zinc-mediated decarboxylation and demonstrates a novel strategy for preparing arylzinc reagents from easily accessible (hetero)aryl carboxylic acids.

Preparation of 2-Heteroaryl-indolin-3-ones by Acid-Mediated Intramolecular Cyclization in Batch and Flow

An intramolecular cyclization strategy to synthesize nitrogen-rich and biologically important 2-heteroaryl-indolin-3-ones has been developed in both batch and flow. Using high-throughput screening, citric acid in a dimethylformamide/water mixture was found to facilitate the reaction under batch conditions, which can tolerate substituted phenyl rings, quinolines, and naphthalenes with a variety of electronic and steric properties. The reaction was reoptimized for flow conditions using design of experiments, complementing the batch conditions for some substrates with greatly reduced reaction times.

Selective Ni-Catalyzed Cross-Electrophile Coupling of Heteroaryl Chlorides and Aryl Bromides at 1:1 Substrate Ratio

Nickel-catalyzed cross-electrophile coupling (XEC) reactions of (hetero)aryl electrophiles represent appealing alternatives to palladium-catalyzed methods for biaryl synthesis, but they often generate significant quantities of homocoupling and/or proto-dehalogenation side products. In this study, an informer library of heteroaryl chloride and aryl bromide coupling partners is used to identify Ni-catalyzed XEC conditions that access high selectivity for the cross-product when using equimolar quantities of the two substrates. Two different catalyst systems are identified that show complementary scope and broad functional-group tolerance, and time-course data suggest that the two methods follow different mechanisms. A NiBr2/terpyridine catalyst system with Zn as the reductant converts the aryl bromide into an arylzinc intermediate that undergoes in situ coupling with 2-chloropyridines, while a NiBr2/bipyridine catalyst system with tetrakis(dimethylamino)ethylene as the reductant uses FeBr2 and NaI as additives to achieve selective cross-coupling.

Copper-Catalyzed Three-Component Tandem Cyclization for One-Pot Synthesis of Indole-Benzofuran Bis-Heterocycles

A one-pot, three-component synthesis of indole-benzofuran bis-heterocycles from terminal alkynes, salicylaldehydes, and indoles has been developed via copper-catalyzed tandem annulation. This catalytic system utilizes readily available starting materials, enabling predictable synthesis of indole-benzofuran bis-heterocycles with broad substrate versatility, excellent regiocontrol, and gram-scale amenability. The reaction proceeds via a sequential pathway involving A3 coupling, 1,4-conjugate addition, and 5-exo-dig cyclization.

Weak Base-Promoted Direct Cross-Coupling of Naphthalene-1,8-diaminato-substituted Arylboron Compounds

The indispensability of a base in Suzuki-Miyaura coupling (SMC) employing organoboronic acids/esters is well recognized, which occasionally induces competitive protodeborylation in organoboron reagents. This phenomenon is particularly pronounced in fluorine-substituted aryl and heteroaryl boron compounds. Here, we show that direct SMC of naphthalene-1,8-diaminato (dan)-substituted aryl boron compounds, Ar-B(dan), characterized by its remarkable stability toward protodeborylation due to their diminished boron-Lewis acidity, occurs utilizing a weak base in conjunction with a palladium/copper cooperative catalyst system. The approach delineated in this study enables the efficient incorporation of various perfluoroaryl- and heteroaryl-B(dan) reagents, while maintaining high functional group tolerance. Furthermore, the inherent inertness of the B(dan) moiety allowed sequential cross-coupling, where other metallic moieties chemoselectively undergo the reaction, thus leading to the concise, protection-free synthesis of oligoarenes. Our results provide a potent approach to a delicate dilemma between a protodeborylation-resistant property and SMC activity intimately linked to boron-Lewis acidity.

Controllable 1,4-Palladium Aryl to Aryl Migration in Fused Systems─Application to the Synthesis of Azaborole Multihelicenes

Herein, we report the first 1,4-Pd aryl to aryl migration/Miyaura borylation tandem reaction in fused systems. The Pd shift occurred in the bay region of the dibenzo[g,p]chrysene building blocks, giving rise to a thermodynamically controlled mixture of 1,8- and 1,9-borylated compounds that allowed the preparation of regioisomeric azaborole multihelicenes from the same starting material. The outcome of this synthesis can be controlled by the choice of reaction conditions, allowing the migration process to be turned off in the absence of an acetate additive and the target multiheterohelicenes to be prepared in a regioselective manner. The target compounds show bright green fluorescence in dichloromethane with emission quantum yields (Φ) of up to 0.29, |glum| values up to 2.7 × 10-3, and green or green-yellow emission in the solid state, reaching Φ of 0.22. Single crystal X-ray diffraction analyses gave insight into their molecular structures and the packing arrangement. Evaluation of aromaticity in these multihelicenes revealed a nonaromatic character of the 2H-1,2-azaborole constituent rings.

Modular Synthesis of Heterobenzylic Amines via Carbonyl Azinylative Amination

Transformations enabling the synthesis of α-alkyl, α'-2-azinyl amines by addition of 2-heteroaryl-based nucleophiles to in situ-generated and non-activated alkyl-substituted iminium ions are extremely rare. Approaches involving classical 2-azinyl organometallics, such as the corresponding Grignard reagents, often fail to produce the desired products. Here, we report an operationally straightforward solution to this problem through the development of a multicomponent coupling process wherein a soft 2-azinyl indium nucleophile, generated in situ from the corresponding 2-iodo heteroarene and indium powder, adds to an iminium ion that is also formed directly in the reaction. This modular carbonyl azinylative amination (CAzA) displays a broad scope and only a metal reductant is needed to generate a reactive 2-azinyl nucleophile. Beyond the addition to iminium ions, the 2-azinyl addition to polyfluoromethyl ketones forms the corresponding tertiary alcohols. Together, the products of these reactions possess a high degree of functionality, are typically challenging to synthesize by other methods, and contain motifs recognized as privileged in the context of pharmaceuticals and agrochemicals.

Diversification of Bipyridines and Azaheterocycles via Nucleophilic Displacement of Trimethylammoniums

Bipyridines and azaarenes are an important class of ligands that impart unique and tunable properties to transition metal complexes and catalysts. While some derivatives are commercially available, noncommercial analogues are often challenging to prepare and purify. Herein, we report a general nucleophilic aromatic substitution reaction that converts cationic trimethylaminated bipyridines into a series of functionalized bipyridines. Our method showcases a series of C-O, C-S, and C-F bond-forming reactions as well as a selective monodemethylation that converts the electron-deficient trimethylammonium to an electron-rich dimethylamine. The approach was further applied to diversification of pharmaceuticals and natural products and was applied to the total synthesis of Graveolinine and the preparation of Graveolinine derivatives.

Modulating Decarboxylative Oxidation Photocatalysis by Ligand Engineering of Atomically Precise Copper Nanoclusters

Copper nanoclusters (Cu NCs) characterized by their well-defined electronic and optical properties are an ideal platform for organic photocatalysis and exploring atomic-level behaviors. However, their potential as greener, efficient catalysts for challenging reactions like decarboxylative oxygenation under mild conditions remains unexplored. Herein, we present Cu13(Nap)3(PPh3)7H10 (hereafter Cu13Nap), protected by 1-naphthalene thiolate (Nap), which performs well in decarboxylative oxidation (90% yield) under photochemical conditions. In comparison, the isostructural Cu13(DCBT)3(PPh3)7H10 (hereafter Cu13DCBT), stabilized by 2,4-dichlorobenzenethiolate (DCBT), yields only 28%, and other previously reported Cu NCs (Cu28, Cu29, Cu45, Cu57, and Cu61) yield in the range of 6-18%. The introduction of naphthalene thiolate to the surface of Cu13 NCs influences their electronic structure and charge transfer in the ligand shell, enhancing visible light absorption and catalytic performance. Density functional theory (DFT) and experimental evidence suggest that the reaction proceeds primarily through an energy transfer mechanism. The energy transfer pathway is uncommon in the context of previous reports for decarboxylative oxidation reactions. Our findings suggest that strategically manipulating ligands holds significant potential for creating composite active sites on atomically precise copper NCs, resulting in enhanced catalytic efficacy and selectivity across various challenging reactions.

Catalytic Generation of Pyridyl Radicals via Electron Donor-Acceptor Complex Photoexcitation: Synthesis of 2-Pyridylindole-Based Heterobiaryls

We report the catalytic generation of pyridyl radicals through photoexcitation of the electron donor-acceptor (EDA) complex, which enables the C2-selective heteroarylation of indole under ambient conditions. In this manifold, catalytic triarylamine and chloropyridine aggregate into an EDA complex in the presence of an inorganic base, making readily available chloropyridines good precursors for the generation of diverse pyridyl radicals. Given the broad reaction scope, this catalytic EDA complex protocol provides robust access to heterobiaryl scaffolds that are widely present in biologically important molecules.

Synthesis and Suzuki-Miyaura Cross-Coupling of Alkyl Amine-Boranes. A Boryl Radical-Enabled Strategy

Alkyl organoborons are powerful materials for the construction of C(sp3)-C(sp2) bonds, predominantly via Suzuki-Miyaura cross-coupling. These species are generally assembled using 2-electron processes that harness the ability of boron reagents to act as both electrophiles and nucleophiles. Herein, we demonstrate an alternative borylation strategy based on the reactivity of amine-ligated boryl radicals. This process features the use of a carboxylic acid containing amine-ligated borane that acts as boryl radical precursor for photoredox oxidation and decarboxylation. The resulting amine-ligated boryl radical undergoes facile addition to styrenes and imines through radical-polar crossover manifolds. This delivers a new class of sp3-organoborons that are stable solids and do not undergo protodeboronation. These novel materials include unprotected α-amino derivatives that are generally unstable. Crucially, these aliphatic organoboron species can be directly engaged in Suzuki-Miyaura cross-couplings with structurally complex aryl halides. Preliminary studies suggest that they enable slow-release of the corresponding and often difficult to handle alkyl boronic acids.

Halide Salts Alleviate TMSOK Inhibition in Suzuki-Miyaura Cross-Couplings

The Suzuki-Miyaura cross-coupling (SMC) remains one of the most widely used transformations available to chemists. Recently, robust new conditions achieving rapid reactivity under homogeneous aprotic conditions enabled by the use of potassium trimethylsilanolate (TMSOK) as a base were reported. However, the strong inhibitory effect of TMSOK restricts the generality of such conditions. Moreover, the basic nature of TMSOK impedes the use of protic heterocycles as substrates, as these latter anionic species are even more potent catalyst inhibitors. Herein, we report a thorough mechanistic study of these novel SMC conditions. Halide salt additives were found to provide a dramatic rate acceleration and mitigate the inhibitory effect of TMSOK. NMR experiments revealed that this is largely achieved by impacting the unexpected formation of inactive [LnPd(Ar)(μ-OH)]2, favoring the formation of active LnPd(Ar)(X) instead. These findings enabled an impressive substrate scope even at low catalyst loadings (0.1 mol %). Finally, halide additives were observed to enable the use of protic heterocyclic substrates, which could otherwise completely inhibit reactivity.

Bent naphthodithiophenes: synthesis and characterization of isomeric fluorophores

Thiophene-containing heteroarenes are one of the most well-known classes of π-conjugated building blocks for photoactive molecules. Isomeric naphthodithiophenes (NDTs) are at the forefront of this research area due to their straightforward synthesis and derivatization. Notably, NDT geometries that are bent - such as naphtho[2,1-b:3,4-b']dithiophene (α-NDT) and naphtho[1,2-b:4,3-b']dithiophene (β-NDT) - are seldom employed as photoactive small molecules. This report investigates how remote substituents impact the photophysical properties of isomeric α- and β-NDTs. The orientation of the thiophene units plays a critical role in the emission: in the α(OHex)R2 series conjugation from the end-caps to the NDT core is apparent, while in the β(Oi-Pent)R2 series minimal change is observed unless strong electron acceptors, such as β(Oi-Pent)(PhCF3)2, are employed. This push-pull acceptor-donor-acceptor (A-D-A) fluorophore exhibits positive fluorosolvatochromism that correlates with increasing solvent polarity parameter, E T(30). In total, these results highlight how remote substituents are able to modulate the emission of isomeric bent NDTs.

Regioselective Homolytic C2-H Borylation of Unprotected Adenosine and Adenine Derivatives via Minisci Reaction

A Minisci-type borylation of unprotected adenosine, adenine nucleotide, and adenosine analogues was successfully achieved through photocatalysis or thermal activation. Despite the challenges posed by the presence of two potential reactive sites (C2 and C8) in the purine motif, the unique nucleophilic amine-ligated boryl radicals effortlessly achieved excellent C2 site selectivity and simultaneously avoided the formation of multifunctionalized products. This protocol proved effective for the late-stage borylation of some important biomolecules as well as a few antiviral and antitumor drug molecules, such as AMP, cAMP, Vidarabine, Cordycepin, Tenofovir, Adefovir, GS-441524, etc. Theoretical calculations shed light on the site selectivity, revealing that the free energy barriers for the C2-Minisci addition are further lowered through the chelation of additive Mg2+ to N3 and furyl oxygen. This phenomenon has been confirmed by an IGMH analysis. Preliminary antitumor evaluation, derivation of the C2-borylated adenosine to other analogues with high-value functionalities, along with the CuAAC click reactions, suggest the potential application of this methodology in drug molecular optimization studies and chemical biology.

Size Matters in Conjugated Polymer Chirality-Selective SWCNT Extraction

Carbon-based nanomaterials have catalyzed breakthroughs across various scientific and engineering disciplines. The key to unlocking a new generation of tailor-made nanomaterials based on single-walled carbon nanotubes (SWCNTs) lies in the precise sorting of raw material into individual chiralities, each possessing unique properties. This can be achieved using conjugated polymer extraction (CPE), but to a very limited extent since the process generates only a few chirality-enriched suspensions. Therefore, it is imperative to comprehend the mechanism of the wrapping of SWCNTs by polymers to unleash CPE's full potential. However, the lack of a diverse palette of chirality-selective polymers with varying macromolecular parameters has hindered a comprehensive understanding of how the nature of the polymer affects the performance and selectivity of SWCNT isolation. To address this gap, multiple batches of such polymers are synthesized to elucidate the impact of molecular weight and dispersity on the purity and concentrations of the generated SWCNT suspensions. The obtained results explain the inconsistent outcomes reported in the literature, greatly improving the application potential of this promising SWCNT sorting approach. Concomitantly, the discovered significant influence of the macromolecular characteristics of conjugated polymers on the SWCNT isolation efficacy sheds considerable insight into the unresolved mechanism of this sorting technique.

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Multiple neighboring active sites of an atomically precise copper nanocluster catalyst for efficient bond-forming reactions

Atomically precise copper nanoclusters (NCs) are an emerging class of nanomaterials for catalysis. Their versatile core-shell architecture opens the possibility of tailoring their catalytically active sites. Here, we introduce a core-shell copper nanocluster (CuNC), [Cu29(StBu)13Cl5(PPh3)4H10]tBuSO3 (StBu: tert-butylthiol; PPh3: triphenylphosphine), Cu29NC, with multiple accessible active sites on its shell. We show that this nanocluster is a versatile catalyst for C-heteroatom bond formation (C-O, C-N, and C-S) with several advantages over previous Cu systems. When supported, the cluster can also be reused as a heterogeneous catalyst without losing its efficiency, making it a hybrid homogeneous and heterogeneous catalyst. We elucidated the atomic-level mechanism of the catalysis using density functional theory (DFT) calculations based on the single crystal structure. We found that the cooperative action of multiple neighboring active sites is essential for the catalyst's efficiency. The calculations also revealed that oxidative addition is the rate-limiting step that is facilitated by the neighboring active sites of the Cu29NC, which highlights a unique advantage of nanoclusters over traditional copper catalysts. Our results demonstrate the potential of nanoclusters for enabling the rational atomically precise design and investigation of multi-site catalysts.

Regioselective Arylation of Amidoaryne Precursors via Ag-Mediated Intramolecular Oxy-Argentation

An unprecedented silver-mediated intramolecular oxy-argentation of 3-amidoaryne precursors that quickly generates a heteroarylsilver species is developed. AgF acts as both a stoichiometric fluoride source and a reagent for the formation of a benzoxazolylsilver intermediate via aryne generation. Pd-catalyzed coupling reactions of (hetero)aryl iodides with a silver species, generated in situ, allow for the synthesis of various C7-arylated benzoxazoles. As a result, an aryl group is selectively introduced into the meta-position of 3-amidobenzyne precursors. Mechanistic studies have indicated the presence of a benzoxazolylsilver intermediate and revealed that the reaction proceeds via an intramolecular oxy-argentation process, which is initiated by a direct fluoride attack on the silyl group.

Chelation enables selectivity control in enantioconvergent Suzuki-Miyaura cross-couplings on acyclic allylic systems

Asymmetric Suzuki-Miyaura cross-couplings with aryl boronic acids and allylic electrophiles are a powerful method to convert racemic mixtures into enantioenriched products. Currently, enantioconvergent allylic arylations are limited to substrates that are symmetrical about the allylic unit, and the absence of strategies to control regio-, E/Z- and enantioselectivity in acyclic allylic systems is a major restriction. Here, using a system capable of either conjugate addition or allylic arylation, we have discovered the structural features and experimental conditions that allow an acyclic system to undergo chemo- and regioselective, enantioconvergent allylic Suzuki-Miyaura-type arylation. A wide variety of boronic acid coupling partners can be used, and both alkyl and aromatic substituents are tolerated on the allylic unit so that a wide variety of structures can be obtained. Preliminary mechanistic studies reveal that the chelating ability of the ester group is crucial to obtaining high regio- and enantioselectivity. Using this method, we were able to synthesize the natural products (S)-curcumene and (S)-4,7-dimethyl-1-tetralone and the clinically used antidepressant sertraline (Zoloft).

Pd-NHC (NHC = N-Heterocyclic Carbene)-Catalyzed B-Alkyl Suzuki Cross-Coupling of 2-Pyridyl Ammonium Salts by N-C Activation: Application to the Discovery of Agrochemical Molecular Hybrids

2-Alkylpyridines are a privileged scaffold throughout the realm of organic synthesis and play a key role in natural products, pharmaceuticals, and agrochemicals. Herein, we report the first B-alkyl Suzuki cross-coupling of 2-pyridyl ammonium salts to access functionalized 2-alkylpyridines. The use of well-defined, operationally simple Pd-NHCs permits for an exceptionally broad scope of the challenging B-alkyl C-N cross-coupling with organoboranes containing β-hydrogen, representing a novel method for the discovery of highly sought-after molecules for plant protection.

Cathodically Coupled Electrolysis to Access Biheteroaryls

An electrochemical approach to biheteroaryls through the coupling of diverse N-heteroarenes with heteroaryl phosphonium salts is reported. The reaction features pH and redox-neutral conditions and excellent regioselectivity, as well as exogenous air or moisture tolerance. Additionally, a one-pot, two-step protocol can be established to realize formal C-H/C-H coupling of heteroarenes, thereby greatly expanding the substrate availability. The utility of this method is demonstrated through late-stage functionalization, the total synthesis of nitraridine, and antifungal activity studies.

Recent Progress on the Synthesis of Bipyridine Derivatives

Bipyridine and related compounds are starting materials or precursors for a variety of valuable substances such as biologically active molecules, ligands for catalysts, photosensitizers, viologens, and supramolecular architectures. Thus, it is important to classify their synthesis methods and understand their characteristics. Representative examples include methods using homo and heterocoupling of pyridine derivatives in the presence of a catalyst. Because bipyridine compounds strongly coordinate with metal centers, a decrease in catalytic activity and yield is often observed in the reaction system. To address this issue, this review provides insights into advances over the last ~30 years in bipyridine synthesis using metal complexes under both homogeneous and heterogeneous conditions. Moreover, strategies for bipyridine synthesis involving sulfur and phosphorous compounds are examined. These alternative pathways offer promising avenues for overcoming the challenges associated with traditional catalysis methods, providing a more comprehensive understanding of the synthesis landscape.

Palladium-Catalyzed Synthesis of Linked Bis-Heterocycles─Synthesis and Investigation of Photophysical Properties

A palladium-catalyzed domino C-N coupling/Cacchi reaction is reported. Design of photoluminescent bis-heterocycles, aided by density functional theory calculations, was performed with synthetic yields up to 98%. The photophysical properties of the products accessed via this strategy were part of a comprehensive study that led to broad emission spectra and quantum yields of up to 0.59. Mechanistic experiments confirmed bromoalkynes as competent intermediates, and a density functional theory investigation suggests a pathway involving initial oxidative addition into the cis C-Br bond of the gem-dihaloolefin.

Chemo- and Regioselective Alkylation of Pyridine N-Oxides with Titanacyclopropanes

While titanacyclopropanes are used to react mainly with ester, amide, and cyano to undergo cyclopropanation, herein they react preferentially with pyridine N-oxide to accomplish C2-H alkylation beyond these functionalities with double regioselectivity. After being pyridylated at the less hindered C-Ti bond, the remaining C-Ti bond of titanacyclopropanes can be further functionalized by various electrophiles, allowing facile introduction of complex alkyls onto the C2 of pyridines. Its synthetic potential has been demonstrated by late-stage diversification of drugs.

Photochemically Mediated Ring Expansion of Indoles and Pyrroles with Chlorodiazirines: Synthetic Methodology and Thermal Hazard Assessment

We demonstrate that arylchlorodiazirines serve as photo-activated halocarbene precursors for the selective one-carbon ring expansion of N-substituted pyrroles and indoles to the corresponding pyridinium and quinolinium salts. Preliminary investigations indicate that the same strategy also enables the conversion of N-substituted pyrazoles to pyrimidinium salts. The N-substituent of the substrate plays an essential role in: (1) increasing substrate scope by preventing product degradation, (2) enhancing yields by suppressing co-product inhibition, and (3) activating the azinium products towards subsequent synthetic manipulations. This latter point is illustrated by subjecting the quinolinium salts to four complementary partial reductions, which provide concise access to ring-expanded products with different degrees of increased C(sp3 ) character. Thermal analysis of the diazirines by differential scanning calorimetry (DSC) provides detailed insight into their energetic properties, and highlights the safety benefits of photolyzing-rather than thermolyzing-these reagents.

One-Pot Synthesis of Pentasubstituted Pyridines following the Gold(I)-Catalyzed Aza-Enyne Metathesis/6π-Electrocyclization-Aromatization Sequence

The one-pot de novo synthesis of pentasubstituted pyridines was realized following the process of Au(I)-autotandem catalysis and subsequent aromatization. The process involves aza-enyne metathesis with aryl propiolates to yield 1-azabutadienes and their addition/6π-electrocyclization sequence with the other propiolate units. The resultant 1,4-dihydropyridines were aromatized to furnish the pyridines in the presence of atmospheric oxygen. The aryl propiolates were regioselectively incorporated into the ring system to afford 2-arylpyridines as the sole product.

Efficient one-pot synthesis of dan-substituted organo- and silyl-boron compounds

Direct, transition metal-free B(dan)-installation into organic frameworks has been developed. Heteroaryl-H bonds were transformable into the respective heteroaryl-B(dan) bonds through deprotonation. The resulting heteroaryl-B(dan) compounds, which are otherwise difficult to access, can undergo the direct Suzuki-Miyaura coupling. The method was demonstrated to apply to a silicon nucleophile, giving Lewis acidity-diminished stable silyl-B(dan) and -B(aam) in one pot.

Suzuki-Miyaura Cross-Coupling of 2-Pyridyl Trimethylammonium Salts by N-C Activation Catalyzed by Air- and Moisture-Stable Pd-NHC Precatalysts: Application to the Discovery of Agrochemicals

We report the first Suzuki-Miyaura cross-coupling of 2-pyridyl ammonium salts by highly selective N-C activation catalyzed by air- and moisture-stable Pd(II)-NHC (NHC = N-heterocyclic carbene) precatalysts. The use of well-defined and highly reactive [Pd(IPr)(3-CF₃-An)Cl₂] (An = aniline) or [Pd(IPr)(cin)Cl] (cin = cinnamyl) Pd(II)-NHC catalysts permits an exceptionally broad scope of the cross-coupling to furnish valuable biaryl and heterobiarylpyridines that are ubiquitous in medicinal chemistry and agrochemistry research. The overall process leverages the Chichibabin C-H amination of pyridines with N-C activation to enable an attractive strategy to the 2-pyridyl problem. The utility of the method to the discovery of potent agrochemicals is presented. Considering the importance of 2-pyridines and the versatility of N-C activation methods, we envision that this new C-H/N-C activation strategy will find broad application.

Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Masuda Borylation–Suzuki Coupling (MBSC) Sequence: A One-Pot Process to Access Complex (hetero)Biaryls

The direct formation of (hetero)biaryls from readily available (hetero)aryl halides under mild reaction conditions can be efficiently achieved through the Masuda borylation–Suzuki coupling (MBSC) sequence. The MBSC sequence catenates Pd-catalyzed Masuda borylation and Suzuki coupling into a one-pot process, giving access to diverse symmetrically and unsymmetrically substituted scaffolds. (Hetero)biaryls are ubiquitous structural motifs that appear in natural products, pharmaceutically relevant scaffolds, functional dyes, and several other structures. This review summarizes the development of the MBSC sequence and its improvements over the past two decades.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

Identification of Triazolopyridines as Selective α5-GABAA Receptor Negative Allosteric Modulators by a Hybridization Approach

The identification and characterization of novel triazolopyridine derivatives with selective α5 subunit-containing GABA_A receptor negative allosteric modulator (NAM) activity are disclosed. As a result of in silico screening of our corporate compound deck, we identified a moderately potent hit that was converted to an advanced hit bearing better physicochemical and pharmacological properties using a hybridization approach. Subsequent optimization led to the identification of in vitro potent and subtype-selective α5-GABA_A receptor NAMs representing a new chemotype in this area.

Catalytic asymmetric synthesis of carbocyclic C-nucleosides

Access to carbocyclic C-nucleosides (CC-Ns) is currently restricted. The few methods available to make CC-Ns suffer from long syntheses and poor modularity, hindering the examination of potentially important chemical space. Here we report an approach to CC-Ns which uses an asymmetric Suzuki-Miyaura type reaction as the key C-C bond forming step. After coupling the densely functionalized racemic bicyclic allyl chloride and heterocyclic boronic acids, the trisubstituted cyclopentenyl core is elaborated to RNA analogues via a hydroborylation-homologation-oxidation sequence. We demonstrate that the approach can be used to produce a variety of enantiomerically enriched CC-Ns, including a carbocyclic derivative of Showdomycin.

Overcoming Limitations in Decarboxylative Arylation via Ag-Ni Electrocatalysis

A useful protocol for achieving decarboxylative cross-coupling (DCC) of redox-active esters (RAE, isolated or generated in situ) and halo(hetero)arenes is reported. This pragmatically focused study employs a unique Ag-Ni electrocatalytic platform to overcome numerous limitations that have plagued this strategically powerful transformation. In its optimized form, coupling partners can be combined in a surprisingly simple way: open to the air, using technical-grade solvents, an inexpensive ligand and Ni source, and substoichiometric AgNO3, proceeding at room temperature with a simple commercial potentiostat. Most importantly, all of the results are placed into context by benchmarking with state-of-the-art methods. Applications are presented that simplify synthesis and rapidly enable access to challenging chemical space. Finally, adaptation to multiple scale regimes, ranging from parallel milligram-based synthesis to decagram recirculating flow is presented.

Chemodivergent Organolanthanide-Catalyzed C-H α-Mono-Borylation of Pyridines

Chemodivergent synthetic methodologies enable the efficient introduction of structural diversity into high-value organic products via simple chemical alterations. In this regard, C-H activation and functionalization of pyridinoid azines are important transformations in the synthesis of many natural products, pharmaceuticals, and functional materials. Reflecting on azinyl nitrogen lone-pair steric repulsion, its tendency to irreversibly coordinate metal ion catalysts, and the electron deficiency of pyridine, C-H functionalization at the important α-position remains challenging. Thus, developing earth-abundant catalysts for α-selective azine mono-functionalization is an attractive target for chemical synthesis. Here, the selective organolanthanide-catalyzed α-mono-borylation of a diverse series of 18 pyridines is reported using Cp*₂LuCH(TMS)₂ (Cp* = η⁵-C₅Me₅) (TMS = SiMe₃) and affording valuable precursors for subsequent functionalization. Experimental and theoretical mechanistic data reported here support the intermediacy of a C-H-activated η²-lanthanide-azine complex, followed by intermolecular α-mono-borylation via σ-bond metathesis. Notably, varying the lanthanide identity and substrate substituent electronic character promotes marked chemodivergence of the catalytic selectivity: smaller/more electrophilic lanthanide³⁺ ions and electron-rich substrates favor selective α-C-H functionalization, whereas larger/less electrophilic lanthanide³⁺ ions and electron-poor substrates favor selective B-N bond-forming 1,2-dearomatization. Such lanthanide series catalytic chemodivergence is, to our knowledge, unprecedented.

Rhodium-Catalyzed Chemodivergent Pyridylation of Alkynes with Pyridylboronic Acids

The pyridylation of alkynes with pyridylboronic acids is realized under rhodium catalysis. Chemodivergent pyridylation products, including alkenylpyridines produced via the hydropyridylation pathway and cyclopenta[c]pyridines produced via the pyridylation/cyclization pathway, were selectively produced by fine-tuning the reaction conditions. A mechanistic study revealed that 1,4-rhodium migration to the pyridine ring was involved as the key step in the chemodivergent synthesis.

Visible-Light Copper Nanocluster Catalysis for the C-N Coupling of Aryl Chlorides at Room Temperature

Activation of aryl chlorides in cross-coupling reactions is a long-standing challenge in organic synthesis that is of great interest to industry. Ultrasmall (<3 nm), atomically precise nanoclusters (NCs) are considered one of the most promising catalysts due to their high surface area and unsaturated active sites. Herein, we introduce a copper nanocluster-based catalyst, [Cu₆₁(S^tBu)₂₆S₆Cl₆H₁₄] (Cu₆₁NC) that enables C-N bond-forming reactions of aryl chlorides under visible-light irradiation at room temperature. A range of N-heterocyclic nucleophiles and electronically and sterically diverse aryl/hetero chlorides react in this new Cu₆₁NC-catalyzed process to afford the C-N coupling products in good yields. Mechanistic studies indicate that a single-electron-transfer (SET) process between the photoexcited Cu₆₁NC complex and aryl halide enables the C-N-arylation reaction.