Site-selective Suzuki-Miyaura coupling of heteroaryl halides - understanding the trends for pharmaceutically important classes

Electrochemistry-Enabled C-Heteroatom Bond Formation of Alkyl Germanes

Because of their robustness and orthogonal reactivity features, alkyl germanes bear significant potential as functional handles for the construction of C(sp3)-rich scaffolds, especially in the context of modular synthetic approaches. However, to date, only radical-based reactivity has been accessible from these functional handles, which limits the types of possible decorations. Here, we describe the first general C(sp3)-heteroatom bond formation of alkyl germanes (-GeEt3) by leveraging electrochemistry to unlock polar reactivity. This approach allowed us to couple C(sp3)-GeEt3 with a variety of nucleophiles to construct ethers, esters, amines, amides, sulfonamides, sulfides, as well as C-P, C-F, and C-C bonds. The compatibility of the electrochemical approach with a modular synthetic strategy of a C1 motif was also showcased, involving the sequential functionalization of Cl, Bpin, and ultimately GeEt3 via electrochemistry.

Achieving efficient and stable blue thermally activated delayed fluorescence organic light-emitting diodes based on four-coordinate fluoroboron emitters by simple substitution molecular engineering

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor-acceptor (D-A) type TADF materials featuring a four-coordinated boron with 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1-8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (3CT) and the triplet local excited (3LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ EL) at 460-492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT50) of 9113 hours for 8 than that for 7 as well as satisfactory LT50 reaching 26 643 hours for 6 at an initial luminance of 100 cd m-2 have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

Oxidative Addition of (Hetero)aryl (Pseudo)halides at Palladium(0): Origin and Significance of Divergent Mechanisms

Two limiting mechanisms are possible for oxidative addition of (hetero)aryl (pseudo)halides at Pd(0): a 3-centered concerted and a nucleophilic displacement mechanism. Until now, there has been little understanding about when each mechanism is relevant. Prior investigations to distinguish between these pathways were limited to a few specific combinations of the substrate and ligand. Here, we computationally evaluated over 180 transition structures for oxidative addition in order to determine mechanistic trends based on substrate, ligand(s), and coordination number. Natural abundance 13C kinetic isotope effects provide experimental results consistent with computational predictions. Key findings include that (1) differences in highest occupied molecular orbital (HOMO) symmetries dictate that, although 12e- PdL is strongly biased toward a 3-centered concerted mechanism, 14e- PdL2 often prefers a nucleophilic displacement mechanism; (2) ligand electronics and sterics, including ligand bite angle, influence the preferred mechanism of the reaction at PdL2; (3) phenyl triflate always reacts through a displacement mechanism regardless of the catalyst structure due to the stability of a triflate anion and the inability of oxygen to effectively donate electron density to Pd; and (4) the high reactivity of C-X bonds adjacent to nitrogen in pyridine substrates relates to stereoelectronic stabilization of a nucleophilic displacement transition state. This work has implications for controlling rate and selectivity in catalytic couplings, and we demonstrate application of the mechanistic insight toward chemodivergent cross-couplings of bromochloroheteroarenes.

Cu(II)/PTABS-Promoted, Chemoselective Amination of HaloPyrimidines

Chemoselective amination is a highly desired synthetic methodology, given its importance as a possible strategy to synthesize various drug molecules and agrochemicals. We, herein, disclose a highly chemoselective Cu(II)-PTABS-promoted amination of pyrimidine structural feature containing different halogen atoms.

Nickel-Based Catalysts for the Selective Monoarylation of Dichloropyridines: Ligand Effects and Mechanistic Insights

This report describes a detailed study of Ni phosphine catalysts for the Suzuki-Miyaura coupling of dichloropyridines with halogen-containing (hetero)aryl boronic acids. With most phosphine ligands these transformations afford mixtures of mono- and diarylated cross-coupling products as well as competing oligomerization of the boronic acid. However, a ligand screen revealed that PPh2Me and PPh3 afford high yield and selectivity for monoarylation over diarylation as well as minimal competing oligomerization of the boronic acid. Several key observations were made regarding the selectivity of these reactions, including: (1) phosphine ligands that afford high selectivity for monoarylation fall within a narrow range of Tolman cone angles (between 136° and 157°); (2) more electron-rich trialkylphosphines afford predominantly diarylated products, while less-electron rich di- and triarylphosphines favor monoarylation; (3) diarylation proceeds via intramolecular oxidative addition; and (4) the solvent (MeCN) plays a crucial role in achieving high monoarylation selectivity. Experimental and DFT studies suggest that all these data can be explained based on the reactivity of a key intermediate: a Ni0-π complex of the monoarylated product. With larger, more electron-rich trialkylphosphine ligands, this π complex undergoes intramolecular oxidative addition faster than ligand substitution by the MeCN solvent, leading to selective diarylation. In contrast, with relatively small di- and triarylphosphine ligands, associative ligand substitution by MeCN is competitive with oxidative addition, resulting in selective formation of monoarylated products. The generality of this method is demonstrated with a variety of dichloropyridines and chloro-substituted aryl boronic acids. Furthermore, the optimal ligand (PPh2Me) and solvent (MeCN) are leveraged to achieve the Ni-catalyzed monoarylation of a broader set of dichloroarene substrates.

Mechanistic Origin of Ligand Effects on Exhaustive Functionalization During Pd-Catalyzed Cross-Coupling of Dihaloarenes

We describe a detailed investigation into why bulky ligands-those that enable catalysis at "12e -" Pd0-tend to promote overfunctionalization during Pd-catalyzed cross-couplings of dihalogenated substrates. After one cross-coupling event takes place, PdL initially remains coordinated to the π system of the nascent product. Selectivity for mono- vs. difunctionalization arises from the relative rates of π-decomplexation versus a second oxidative addition. Under the Suzuki coupling conditions in this work, direct dissociation of 12e - PdL from the π-complex cannot outcompete oxidative addition. Instead, Pd must be displaced from the π-complex as 14e - PdL(L') by a second incoming ligand L'. The incoming ligand is another molecule of dichloroarene if the reaction conditions do not include π-coordinating solvents or additives. More overfunctionalization tends to result when increased ligand or substrate sterics raises the energy of the bimolecular transition state for separating 14e - PdL(L') from the mono-cross-coupled product. This work has practical implications for optimizing selectivity in cross-couplings involving multiple halogens. For example, we demonstrate that small coordinating additives like DMSO can largely suppress overfunctionalization and that precatalyst structure can also impact selectivity.

Triply Selective & Sequential Diversification at Csp 3: Expansion of Alkyl Germane Reactivity for C-C & C-Heteroatom Bond Formation

We report the triply selective and sequential diversification of a single Csp 3 carbon carrying Cl, Bpin and GeEt3 for the modular and programmable construction of sp3-rich molecules. Various functionalizations of Csp 3-Cl and Csp 3-BPin (e.g. alkylation, arylation, homologation, amination, hydroxylation) were tolerated by the Csp 3-GeEt3 group. Moreover, the methodological repertoire of alkyl germane functionalization was significantly expanded beyond the hitherto known Giese addition and arylation to alkynylation, alkenylation, cyanation, halogenation, azidation, C-S bond formation as well as the first demonstration of stereo-selective functionalization of a Csp 3-[Ge] bond.

Direct C-H Hydroxylation of N-Heteroarenes and Benzenes via Base-Catalyzed Halogen Transfer

Hydroxylated (hetero)arenes are valued in many industries as both key constituents of end products and diversifiable synthetic building blocks. Accordingly, the development of reactions that complement and address the limitations of existing methods for the introduction of aromatic hydroxyl groups is an important goal. To this end, we apply base-catalyzed halogen transfer (X-transfer) to enable the direct C-H hydroxylation of mildly acidic N-heteroarenes and benzenes. This protocol employs an alkoxide base to catalyze X-transfer from sacrificial 2-halothiophene oxidants to aryl substrates, forming SNAr-active intermediates that undergo nucleophilic hydroxylation. Key to this process is the use of 2-phenylethanol as an inexpensive hydroxide surrogate that, after aromatic substitution and rapid elimination, provides the hydroxylated arene and styrene byproduct. Use of simple 2-halothiophenes allows for C-H hydroxylation of 6-membered N-heteroarenes and 1,3-azole derivatives, while a rationally designed 2-halobenzothiophene oxidant extends the scope to electron-deficient benzene substrates. Mechanistic studies indicate that aromatic X-transfer is reversible, suggesting that the deprotonation, halogenation, and substitution steps operate in synergy, manifesting in unique selectivity trends that are not necessarily dependent on the most acidic aryl position. The utility of this method is further demonstrated through streamlined target molecule syntheses, examples of regioselectivity that contrast alternative C-H hydroxylation methods, and the scalable recycling of the thiophene oxidants.

Aminative Suzuki-Miyaura coupling

The Suzuki-Miyaura and Buchwald-Hartwig coupling reactions are widely used to form carbon-carbon (C-C) and carbon-nitrogen (C-N) bonds, respectively. We report the incorporation of a formal nitrene insertion process into the Suzuki-Miyaura reaction, altering the products from C-C-linked biaryls to C-N-C-linked diaryl amines and thereby joining the Suzuki-Miyaura and Buchwald-Hartwig coupling pathways to the same starting-material classes. A combination of a bulky ancillary phosphine ligand on palladium and a commercially available amination reagent enables efficient reactivity across aryl halides and pseudohalides, boronic acids and esters, and many functional groups and heterocycles. Mechanistic insights reveal flexibility on the order of bond-forming events, suggesting potential for expansion of the aminative cross-coupling concept to encompass diverse nucleophiles and electrophiles as well as four-component variants.

Evidence for Suzuki-Miyaura cross-couplings catalyzed by ligated Pd3-clusters: from cradle to grave

Pdn clusters offer unique selectivity and exploitable reactivity in catalysis. Understanding the behavior of Pdn clusters is thus critical for catalysis, applied synthetic organic chemistry and greener outcomes for precious Pd. The Pd3 cluster, [Pd3(μ-Cl)(μ-PPh2)2(PPh3)3][Cl] (denoted as Pd3Cl2), which exhibits distinctive reactivity, was synthesized and immobilized on a phosphine-functionalized polystyrene resin (denoted as immob-Pd3Cl2). The resultant material served as a tool to study closely the role of Pd3 clusters in a prototypical Suzuki-Miyaura cross-coupling of 4-fluoro-1-bromobenzene and 4-methoxyphenyl boronic acid at varying low Pd ppm concentrations (24, 45, and 68 ppm). Advanced heterogeneity tests such as Hg poisoning and the three-phase test showed that leached mononuclear or nanoparticulate Pd are unlikely to be the major active catalyst species under the reaction conditions tested. EXAFS/XANES analysis from (pre)catalyst and filtered catalysts during and after catalysis has shown the intactness of the triangular structure of the Pd3X2 cluster, with exchange of chloride (X) by bromide during catalytic turnover of bromoarene substrate. This finding is further corroborated by treatment of immob-Pd3Cl2 after catalyzing the Suzuki-Miyaura reaction with excess PPh3, which releases the cluster from the polymer support and so permits direct observation of [Pd3(μ-Br)(μ-PPh2)2(PPh3)3]+ ions by ESI-MS. No evidence is seen for a proposed intermediate in which the bridging halogen on the Pd3 motif is replaced by an aryl group from the organoboronic acid, i.e. formed by a transmetallation-first process. Our findings taken together indicate that the 'Pd3X2' motif is an active catalyst species, which is stabilized by being immobilized, providing a more robust Pd3 cluster catalyst system. Non-immobilized Pd3Cl2 is less stable, as is followed by stepwise XAFS of the non-immobilized Pd3Cl2, which gradually changes to a species consistent with 'Pdx(PPh3)y' type material. Our findings have far-reaching future implications for Pd3 cluster involvement in catalysis, showing that immobilization of Pd3 cluster species offers advantages for rigorous mechanistic examination and applied chemistries.

Zwitterionic Aqua Palladacycles with Noncovalent Interactions for meta-Selective Suzuki Coupling of 3,4-Dichlorophenol and 3,4-Dichlorobenzyl Alcohol in Water

The site-selective reaction of substrates with multiple reactive sites has been a focus of the current synthetic chemistry. The use of attractive noncovalent interactions between the catalyst and substrate is emerging as a versatile approach to address site-selectivity challenges. Herein, we designed and synthesized a series of palladacycles, to control meta-selective Suzuki coupling of 3,4-dichlorophenol and 3,4-dichlorobenzyl alcohol. Noncovalent interactions directed zwitterionic aqua palladacycles catalyzed meta-selective Suzuki couplings of 3,4-dichloroarenes bearing hydroxyl in water have been developed. Experiments and density functional theory (DFT) calculations demonstrated that the electrostatic interactions play a critical role in meta-selective coupling of 3,4-dichlorophenol, while meta-selective coupling of 3,4-dichlorobenzyl alcohol arises due to the hydrogen-bonding interactions.

Orthogonal Olefination with Organogermanes

Reported herein is a fully orthogonal olefination, which involves the site- and E-selective coupling of aryl germanes with alkenes, tolerating otherwise widely employed coupling handles such as aromatic (pseudo)halogens (C-I, C-Br, C-Cl, C-F, C-OTf, C-OSO2 F), silanes and boronic acid derivatives as well as alternative functionalities. This unprecedented [Ge]-based oxidative Heck coupling proceeds at room temperature with high speed (10 min to 2 hours) and operational simplicity owing to its base-free and air-tolerant features.

Cu(II)/PTABS-Promoted, Regioselective SNAr Amination of Polychlorinated Pyrimidines with Mechanistic Understanding

Regioselective amination of polyhalogenated heteroarenes (especially pyrimidines) has extensive synthetic and commercial relevance for drug synthesis applications but is plagued by the lack of effective synthetic strategies. Herein, we report the Cu(II)/PTABS-promoted highly regioselective nucleophilic aromatic substitution (S_NAr) of polychlorinated pyrimidines assisted by DFT predictions of the bond dissociation energies of different C-Cl bonds. The unique reactivity of Cu(II)-PTABS has been attributed to the coordination/activation mechanism that has been known to operate in these reactions, but further insights into the catalytic species have also been provided.

Mesitylated trityl radicals, a platform for doublet emission: symmetry breaking, charge-transfer states and conjugated polymers

Neutral π-radicals have potential for use as light emitters in optoelectronic devices due to the absence of energetically low-lying non-emissive states. Here, we report a defect-free synthetic methodology via mesityl substitution at the para-positions of tris(2,4,6-trichlorophenyl)methyl radical. These materials reveal a number of novel optoelectronic properties. Firstly, mesityl substituted radicals show strongly enhanced photoluminescence arising from symmetry breaking in the excited state. Secondly, photoexcitation of thin films of 8 wt% radical in 4,4'-bis(carbazol-9-yl)-1,1'-biphenyl host matrix produces long lived (in the order of microseconds) intermolecular charge transfer states, following hole transfer to the host, that can show unexpectedly efficient red-shifted emission. Thirdly, covalent attachment of carbazole into the mesitylated radical gives very high photoluminescence yield of 93% in 4,4'-bis(carbazol-9-yl)-1,1'-biphenyl films and light-emitting diodes with maximum external quantum efficiency of 28% at a wavelength of 689 nm. Fourthly, a main-chain copolymer of the mesitylated radical and 9,9-dioctyl-9H-fluorene shows red-shifted emission beyond 800 nm.

Machine Learning-Guided Development of Trialkylphosphine Ni(I) Dimers and Applications in Site-Selective Catalysis

Owing to the unknown correlation of a metal's ligand and its resulting preferred speciation in terms of oxidation state, geometry, and nuclearity, a rational design of multinuclear catalysts remains challenging. With the goal to accelerate the identification of suitable ligands that form trialkylphosphine-derived dihalogen-bridged Ni^(I) dimers, we herein employed an assumption-based machine learning approach. The workflow offers guidance in ligand space for a desired speciation without (or only minimal) prior experimental data points. We experimentally verified the predictions and synthesized numerous novel Ni^(I) dimers as well as explored their potential in catalysis. We demonstrate C-I selective arylations of polyhalogenated arenes bearing competing C-Br and C-Cl sites in under 5 min at room temperature using 0.2 mol % of the newly developed dimer, [Ni^(I)(μ-Br)PAd₂(n-Bu)]₂, which is so far unmet with alternative dinuclear or mononuclear Ni or Pd catalysts.

A fruitful century for the scalable synthesis and reactions of biphenyl derivatives: applications and biological aspects

This review provides recent developments in the current status and latest synthetic methodologies of biphenyl derivatives. Furthermore, this review investigates detailed discussions of several metalated chemical reactions related to biphenyl scaffolds such as Wurtz-Fittig, Ullmann, Bennett-Turner, Negishi, Kumada, Stille, Suzuki-Miyaura, Friedel-Crafts, cyanation, amination, and various electrophilic substitution reactions supported by their mechanistic pathways. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed. Furthermore, atropisomerism as a type of axial chirality in biphenyl molecules is discussed. Additionally, this review covers a wide range of biological and medicinal applications of the synthesized compounds involving patented approaches in the last decade corresponding to investigating the crucial role of the biphenyl structures in APIs.

Regulating supramolecular interactions in dimeric macrocycles

Supramolecular behavior is highly dependent on many factors, including complicated microenvironments and weak interactions. Herein, we describe tuning supramolecular architectures of rigid macrocycles by synergistic effects of their geometric configurations, sizes, and guests. Two paraphenylene-based macrocycles are anchored onto different positions in a triphenylene derivative, resulting in dimeric macrocycles with different shapes and configurations. Interestingly, these dimeric macrocycles show tunable supramolecular interactions with guests. In solid state, a 2 : 1 host-guest complex was observed between 1a and C₆₀/C₇₀, while an unusual 2 : 3 host-guest complex 3C₆₀@(1b)₂ can be observed between 1b and C₆₀. This work expands the scope of the synthesis of novel rigid bismacrocycles and provides a new strategy to construct different supramolecular systems.

Synthesis of C3,C6-Diaryl 7-Azaindoles via One-Pot Suzuki-Miyaura Cross-Coupling Reaction and Evaluation of Their HIV-1 Integrase Inhibitory Activity

There is a continuing demand of new inhibitors of HIV-1 Integrase (HIV-1 IN) due to mutations of HIV-1. This study aims to develop the synthesis of 3,6-diaryl 7-azaindoles and introspect the role of aryl groups on the strand transfer (ST) inhibition of HIV-1 IN. An efficient and chemo-selective one-pot method is established for the synthesis of the unexplored diverse C3 → C6 diaryl 7-azaindoles starting from 6-chloro-3-iodo-N-protected 7-azaindoles. Here we report Pd₂dba₃/SPhos catalyzed synthesis of eight selective C3 monoaryl 7-azaindoles (10a-h) and eight C3,C6-diaryl 7-azaindoles (11a-f, 12a,b) with yields in the ranges of 67-93% and 43-88% respectively. The synthesized derivatives inhibit the strand transfer (ST) activity of HIV-1 IN enzyme at 10 μM dose with 11d and 11f exhibiting %ST inhibitions of 72% and 71%, respectively. SAR studies indicate the para-substitution on the C3 aryl ring and C6 aryl is essential for enhanced %ST inhibition. 11b,c, 11e-f, and 12b showed lower cytotoxicity (IC₅₀ > 200 μM) against TZM-bl cells. Molecular docking of the diaryl 7-azaindoles and Raltegravir (RAL), to the PFV-integrase revealed favorable binding interactions.

Heterocyclic Suzuki-Miyaura coupling reaction of metalla-aromatics and mechanistic analysis of site selectivity

Pd-catalyzed Suzuki-Miyaura cross-coupling is one of the most straightforward and versatile methods for the construction of functionalized arenes and heteroarenes but site-selective cross-coupling of polyhalogenated (hetero)arenes containing identical halogen substituents remains a challenging problem. Herein, we report a new candidate for heterocyclic Suzuki-Miyaura coupling reaction. This candidate has been applied in organometallic systems by combining classical aryl boronic acid reagents with non-classical heteroarenes. Experimental and computational studies of the mechanism of the reactions were performed, with an emphasis on the identity of the reactive species in the oxidative addition step and the nature of the precise site selectivity. The influence of both the aromaticity of the metalla-aromatic substrates and the steric and electronic properties of the halogenated sites are studied in detail.

A Biaryl-Cyclohexenone Photoelectrocyclization/Dearomatization Sequence to Substituted Terpenes

Described here is the development of sequential cross-coupling, photoelectrocyclizations, and reductive dearomatizations of biaryl cyclohexenones as a means of synthesizing terpene skeletons. This methodology promises to provide insight that will enable us and others to use this approach to generate a variety of biologically active small molecules, including members of the abietane and morphinan skeletons.

Highly emissions of TPA-linear based pyrazine derivatives with different mechanochromic luminosity

We designed the TPA-based linear pyrazine derivatives of PP-1 and PP-2, synthesized using the conventional Suzuki cross-linking reaction. It was followed by photophysical studies such as aprotic solvent (Haxene to DMF). A red-shift was observed from the non-polar aprotic solvent to the polar aprotic solvent, and the emission intensity was gradually decreased. In addition, the Aggregation-induced emission (AIE) effect has been studied against the DMF/water addition of linear pyrazine compounds. It showed a classic aggregation-caused quenching effect (ACQ) and red-shifted at an increase of (f_w) 0 to 40%. After this case, when the water fraction in these studies was increased by (fw) 50 to 90%, a blue shift and a mild AIE effect has occurred. And also, was investigated acidochromic effect of compounds PP-1 and PP-2 using TFA acid. Absorption and emission intensity were gradually reduced as the acid concentration increased for these studies, while the new peaks appeared red-shifted in the absorption spectrum. They were examined before and after exposure to UV light irradiation in the synthesized dye compounds.

The Road Less Traveled: Unconventional Site Selectivity in Palladium-Catalyzed Cross-Couplings of Dihalogenated N-Heteroarenes

The vast majority (≥90%) of literature reports agree on the regiochemical outcomes of Pd-catalyzed cross-coupling reactions for most classes of dihalogenated N-heteroarenes. Despite a well-established mechanistic rationale for typical selectivity, several examples reveal that changes to the catalyst can switch site selectivity, leading to the unconventional product. In this Perspective, we survey these unusual cases in which divergent selectivity is controlled by ligands or catalyst speciation. In some cases, the mechanistic origin of inverted selectivity has been established, but in others the mechanism remains unknown. This Perspective concludes with a discussion of remaining challenges and opportunities for the field of site-selective cross-coupling. These include developing a better understanding of oxidative addition mechanisms, understanding the role of catalyst speciation on selectivity, establishing an explanation for the influence of ring substituents on regiochemical outcome, inverting selectivity for some "stubborn" classes of substrates, and minimizing unwanted over-reaction of di- and polyhalogenated substrates.

Different Oxidative Addition Mechanisms for 12- and 14-Electron Palladium(0) Explain Ligand-Controlled Divergent Site Selectivity

In cross-coupling reactions, dihaloheteroarenes are usually most reactive at C─halide bonds adjacent to a heteroatom. This selectivity has been previously rationalized. However, no mechanistic explanation exists for anomalous reports in which specific ligands effect inverted selectivity with dihalopyridines and -pyridazines. Here we provide evidence that these ligands uniquely promote oxidative addition at 12e- Pd(0). Computations indicate that 12e- and 14e- Pd(0) can favor different mechanisms for oxidative addition due to differences in their HOMO symmetries. These mechanisms are shown to lead to different site preferences, where 12e- Pd(0) can favor oxidative addition at an atypical site distal to nitrogen.

Unconventional Site Selectivity in Palladium-Catalyzed Cross-Couplings of Dichloroheteroarenes under Ligand-Controlled and Ligand-Free Systems

Halides adjacent to nitrogen are conventionally more reactive in Pd-catalyzed cross-couplings of dihalogenated N-heteroarenes. However, a very sterically hindered N-heterocyclic carbene ligand is shown to promote room-temperature cross-coupling at C4 of 2,4-dichloropyridines with high selectivity (∼10:1). This work represents the first highly selective method with a broad scope for C4-coupling of these substrates where selectivity is clearly under ligand control. Under the optimized conditions, diverse substituted 2,4-dichloropyridines and related compounds undergo cross-coupling to form C4-C_(sp2) and C4-C_(sp3) bonds using organoboron, -zinc, and -magnesium reagents. The synthetic utility of this method is highlighted in multistep syntheses that combine C4-selective cross-coupling with subsequent nucleophilic aromatic substitution reactions. The majority of the products herein (71%) have not been previously reported, emphasizing the ability of this methodology to open up underexplored chemical space. Remarkably, we find that ligand-free "Jeffery" conditions enhance the C4 selectivity of Suzuki coupling by an order of magnitude (>99:1). These ligand-free conditions enable the first C5-selective cross-couplings of 2,5-dichloropyridine and 2,5-dichloropyrimidine.

Modular Generation of (Iodinated) Polyarenes Using Triethylgermane as Orthogonal Masking Group

While the modular construction of molecules from suitable building blocks is a powerful means to more rapidly generate a diversity of molecules than through customized syntheses, the further evolution of the underlying coupling methodology is key to realize widespread applications. We herein disclose a complementary modular coupling approach to the widely employed Suzuki coupling strategy of boron containing precursors, which relies on organogermane containing building blocks as key orthogonal functionality and an electrophilic (rather than nucleophilic) unmasking event paired with air-stable PdI dimer based bond construction. This allows to significantly shorten the reaction times for the iterative coupling steps and/or to close gaps in the accessible compound space, enabling straightforward access also to iodinated compounds.

Palladium-catalyzed chemoselective direct α-arylation of carbonyl compounds with chloroaryl triflates at the C-Cl site

This study described palladium-catalyzed chemoselective direct α-arylation of carbonyl compounds with chloroaryl triflates in the Ar–Cl bond. The Pd/SelectPhos system showed excellent chemoselectivity toward the Ar–Cl bond in the presence of the Ar–OTf bond with a broad substrate scope and excellent product yields. The electronic and steric hindrance offered by the –PR₂ group of the ligand with the C2-alkyl group was found to be the key factor affecting the reactivity and chemoselectivity of the α-arylation reaction. The chemodivergent approach was also successfully employed in the synthesis of flurbiprofen and its derivatives (e.g., –OMe and –F).

Palladium-catalyzed chemoselective direct α-arylation of carbonyl compounds with chloroaryl triflates in the Ar–Cl bond is reported. The effects of –PR₂ and C2-alkyl groups of the ligands are investigated using experimental and computational methods.

Stereodefined alkenes with a fluoro-chloro terminus as a uniquely enabling compound class

Trisubstituted alkenyl fluorides are important compounds for drug discovery, agrochemical development and materials science. Despite notable progress, however, many stereochemically defined trisubstituted fluoroalkenes either cannot be prepared efficiently or can only be accessed in one isomeric form. Here we outline a general solution to this problem by first unveiling a practical, widely applicable and catalytic strategy for stereodivergent synthesis of olefins bearing a fluoro-chloro terminus. This has been accomplished by cross-metathesis between two trisubstituted olefins, one of which is a purchasable but scarcely utilized trihaloalkene. Subsequent cross-coupling can then be used to generate an assortment of trisubstituted alkenyl fluorides. The importance of the advance is highlighted by syntheses of, among others, a fluoronematic liquid-crystal component, peptide analogues bearing an E- or a Z-amide bond mimic, and all four stereoisomers of difluororumenic ester (an anti-cancer compound).

Solvent coordination to palladium can invert the selectivity of oxidative addition

Reaction solvent was previously shown to influence the selectivity of Pd/P t Bu3-catalyzed Suzuki-Miyaura cross-couplings of chloroaryl triflates. The role of solvents has been hypothesized to relate to their polarity, whereby polar solvents stabilize anionic transition states involving [Pd(P t Bu3)(X)]- (X = anionic ligand) and nonpolar solvents do not. However, here we report detailed studies that reveal a more complicated mechanistic picture. In particular, these results suggest that the selectivity change observed in certain solvents is primarily due to solvent coordination to palladium. Polar coordinating and polar noncoordinating solvents lead to dramatically different selectivity. In coordinating solvents, preferential reaction at triflate is likely catalyzed by Pd(P t Bu3)(solv), whereas noncoordinating solvents lead to reaction at chloride through monoligated Pd(P t Bu3). The role of solvent coordination is supported by stoichiometric oxidative addition experiments, density functional theory (DFT) calculations, and catalytic cross-coupling studies. Additional results suggest that anionic [Pd(P t Bu3)(X)]- is also relevant to triflate selectivity in certain scenarios, particularly when halide anions are available in high concentrations.

An environment-friendly methodology for the construction of diversified bicycloacenaphtho[1,2-d]imidazole-8-thione scaffolds using spinel NiFe2O4 nanoparticles as a sustainable catalyst

Herein, we successfully developed an easy access to bicycloacenaphtho[1,2-d]imidazole-8-thione by one-pot three-component MCRs of acenaphthoquinone, aryl or alkyl isothiocyanates and amines using environmentally benevolent and recyclable spinel NiFe₂O₄ nanocatalyst in aqueous ethanol. A broad number of products have been synthesized with both EDGs and EWGs present in the ring which increases the diversity of the protocol. The NiFe₂O₄ nanopowder has been synthesized and thoroughly characterized by powdered XRD, HRTEM, EDX, BET and ICP-AES analysis. The protocol to this bicyclic-heterocycle is noteworthy due to good to excellent yields, practical simplicity and high regioselectivity without any troublesome or hazardous by-products and its easy recovery and reusability of the catalyst. Spinel NiFe₂O₄ NPs-catalysed synthesis of various bicycloacenaphtho[1,2-d]imidazole-8-thione scaffolds under mild and sustainable conditions.

Catalyst-controlled regioselective Sonogashira coupling of 9-substituted-6-chloro-2,8-diiodopurines

We have established a catalyst-dependent regioselective Sonogashira coupling methodology where both regioisomeric products can be obtained independently with remarkably high selectivity.

Electroreductive 4-pyridylation of unsaturated compounds using gaseous ammonia as a hydrogen source

By using ammonia as a hydrogen source, electrochemical pyridylation of unsaturated compounds is achieved with more than 50 examples. In particular, the β-keto ester could be converted to the corresponding tertiary β-hydroxyl ester for the first time.

A reactivity model for oxidative addition to palladium enables quantitative predictions for catalytic cross-coupling reactions

Making accurate, quantitative predictions of chemical reactivity based on molecular structure is an unsolved problem in chemical synthesis, particularly for complex molecules. We report an approach to reactivity prediction for...

A General, Multimetallic Cross-Ullmann Biheteroaryl Synthesis from Heteroaryl Halides and Heteroaryl Triflates

Despite their importance to medicine and materials science, the synthesis of biheteroaryls by cross-coupling remains challenging. We describe here a new, general approach to biheteroaryls: the Ni- and Pd-catalyzed multimetallic cross-Ullmann coupling of heteroaryl halides with triflates. An array of 5-membered, 6-membered, and fused heteroaryl bromides and chlorides, as well as aryl triflates derived from heterocyclic phenols, proved to be viable substrates in this reaction (62 examples, 63 ± 17% average yield). The generality of this approach to biheteroaryls was further demonstrated in 96-well plate format at 10 μmol scale. An array of 96 possible products provided >90% hit rate under a single set of conditions. Further, low-yielding combinations could be rapidly optimized with a single "Toolbox Plate" of ligands, additives, and reductants.

Enantioselective Construction of Modular and Asymmetric Baskets

The precise positioning of functional groups about the inner space of abiotic hosts is a challenging task and of interest for developing more effective receptors and catalysts akin to those found in nature. To address it, we herein report a synthetic methodology for preparing basket-like cavitands comprised of three different aromatics as side arms with orthogonal esters at the rim for further functionalization. First, enantioenriched A (borochloronorbornene), B (iodobromonorbornene), and C (boronorbornene) building blocks were obtained by stereoselective syntheses. Second, consecutive A-to-B and then AB-to-C Suzuki-Miyaura (SM) couplings were optimized to give enantioenriched ABC cavitand as the principal product. The robust synthetic protocol allowed us to prepare (a) an enantioenriched basket with three benzene sides and each holding either tBu, Et, or Me esters, (b) both enantiomers of a so-called "spiral staircase" basket with benzene, naphthalene, and anthracene groups surrounding the inner space, and (c) a photo-responsive basket bearing one anthracene and two benzene arms.

Automated High-Throughput System Combining Small-Scale Synthesis with Bioassays and Reaction Screening

The Purdue Make It system is a unique automated platform capable of small-scale in situ synthesis, screening small-molecule reactions, and performing direct label-free bioassays. The platform is based on desorption electrospray ionization (DESI), an ambient ionization method that allows for minimal sample workup and is capable of accelerating reactions in secondary droplets, thus conferring unique advantages compared with other high-throughput screening technologies. By combining DESI with liquid handling robotics, the system achieves throughputs of more than 1 sample/s, handling up to 6144 samples in a single run. As little as 100 fmol/spot of analyte is required to perform both initial analysis by mass spectrometry (MS) and further MSⁿ structural characterization. The data obtained are processed using custom software so that results are easily visualized as interactive heatmaps of reaction plates based on the peak intensities of m/z values of interest. In this paper, we review the system's capabilities as described in previous publications and demonstrate its utilization in two new high-throughput campaigns: (1) the screening of 188 unique combinatorial reactions (24 reaction types, 188 unique reaction mixtures) to determine reactivity trends and (2) label-free studies of the nicotinamide N-methyltransferase enzyme directly from the bioassay buffer. The system's versatility holds promise for several future directions, including the collection of secondary droplets containing the products from successful reaction screening measurements, the development of machine learning algorithms using data collected from compound library screening, and the adaption of a variety of relevant bioassays to high-throughput MS.

Site-Selective Cross-Coupling of Polyhalogenated Arenes and Heteroarenes with Identical Halogen Groups

Methods to functionalize arenes and heteroarenes in a site-selective manner are highly sought after for rapidly constructing value-added molecules of medicinal, agrochemical, and materials interest. One effective approach is the site-selective cross-coupling of polyhalogenated arenes bearing multiple, but identical, halogen groups. Such cross-coupling reactions have proven to be incredibly effective for site-selective functionalization. However, they also present formidable challenges due to the inherent similarities in the reactivities of the halogen substituents. In this Review, we discuss strategies for site-selective cross-couplings of polyhalogenated arenes and heteroarenes bearing identical halogens, beginning first with an overview of the reaction types that are more traditional in nature, such as electronically, sterically, and directing-group-controlled processes. Following these examples is a description of emerging strategies, which includes ligand- and additive/solvent-controlled reactions as well as photochemically initiated processes.