Palladium-Catalyzed C-O Cross-Coupling of Primary Alcohols

Synthesis of Biaryl-Based Phosphasilinanes via Giese-Type Addition for Palladium-Catalyzed Cross-Coupling Reactions

Herein, we report a method for synthesizing bench-stable phosphasilinane-based Phos ligands. Utilizing a Giese-type addition between biaryl phosphines and divinyl silanes, this strategy enables the efficient preparation of phosphasilinane-based ligands. The method is highly tunable, as demonstrated by the reaction with various divinylsilane derivatives, allowing fine-tuning of steric properties. These newly developed ligands outperform previously reported phosphasilinanes and SPhos in demanding cross-coupling reactions of heteroaryl chlorides with piperidine and phenylboronic acid.

Palladium-Catalyzed Functionalization of Shortwave Infrared Heptamethine Fluorophores Expands Their In Vivo Utility

Fluorescence imaging in the near-infrared (NIR, 700-1000 nm) and shortwave infrared (SWIR, 1000-2000 nm) regions is advantageous for studying mammals. This work applies palladium-catalyzed coupling methods to functionalize flavylium and chromenylium SWIR polymethine fluorophores, which are challenging substrates due to their small HOMO-LUMO gaps. These chemistries include Suzuki-Miyaura and Sonogashira couplings as well as an unprecedented coupling of alcohol substrates to ultimately achieve a panel of C-CAr, C-Csp, and C-O-alkyl functionalized SWIR fluorescent heptamethine dyes. The photophysical properties of the resulting fluorophores are analyzed against Hammett parameters to produce predictive metrics for absorption maxima. These metrics are strategically applied in the design of laser-matched, SWIR-emissive, chromenylium heptamethine dyes. Added functionalities advance the utility of SWIR fluorophores by increasing brightness in micelle formulations, modulating lipophilicity for alternative delivery vehicles, and enabling bioconjugation to targeting moieties. Ultimately, three functionalized fluorophores are employed in concert to achieve multicolor excitation-multiplexed imaging in murine cancer models.

Photochemistry of Ni(II) Tolyl Chlorides Supported by Bidentate Ligand Frameworks

Herein, we investigate the photoactivity of four NiII tolyl chloride complexes supported by either the new bidentate [2.2]pyridinophane (HN2) ligand or the traditional 4,4'-di-tert-butyl-2,2'-dipyridyl (tBubpy) ligand. Despite a change in the ligand framework, we observe similar quantum yields for the photodegradation of all four NiII complexes, while noting changes in their affinity for radical side reactivity and ability to stabilize the photogenerated mononuclear NiI species. Furthermore, changing from an ortho-tolyl to a para-tolyl group affects the geometry of the complexes and makes the Ni center more susceptible to side reactivity. By leveraging the newly developed HN2 ligand, a bidentate ligand that hinders axial interactions with the Ni center, we limit the radical side reactivity. Time-dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) calculations predict that all four complexes have accessible MLCTs that excite an electron from a Ni-aryl bonding orbital into a Ni-aryl antibonding orbital, initiating photolysis. By decreasing this energy gap and stabilizing the tetrahedral triplet excited state, we increase quantum yields of photoexcitation. Importantly, we characterize the photogenerated mononuclear NiI chloride species using X-band EPR spectroscopy and show that the HN2-supported NiI complexes do not undergo the deleterious dimerization and tetramerization observed for the (bpy)NiICl species. Overall, this study provides valuable insight into how the steric environment around the Ni center affects its photoactivity and demonstrates that such photoactivity is not unique to bipyridyl-supported Ni compounds.

Minimal Structural Variation of GPR84 Full Agonist Causes Functional Switch to Inverse Agonism

GPR84 is an orphan GPCR that is expressed primarily in immune cells such as neutrophils and macrophages, and that modulates immune responses during inflammation. The receptor has appeared as a promising drug target, and accumulating evidence indicates that GPR84 inhibition is a viable approach for treatment of various inflammatory and fibrotic disorders. Herein, we report the discovery of a minor structural modification resulting in functional switch of agonists to inverse agonists. Subsequent SAR explorations led to the identification of low-nanomolar potency inverse agonists and antagonists, as exemplified by TUG-2181 (40g). Representative compounds exhibited good physicochemical properties, selectivity over other free fatty acid receptors, and the ability to fully inhibit GPR84-mediated neutrophil activation.

Total Synthesis of Waltherione Alkaloids: A Strategic Molecular Diversity Approach

We report the first total synthesis of rac-8-deoxoantidesmone and rac-waltherione M, key intermediates in the unexplored 5,6,7,8-tetrahydro-1H-quinolin-4-one waltherione (THQW) alkaloid family. These compounds have been synthesized in three steps via a diversity-oriented approach utilizing a versatile intermediate 8-bromo-5-fluoro-3-methoxy-2-methyl-1H-quinolin-4-one, which was obtained through a one-step synthesis between 2-bromo-5-fluoro-aniline and ethyl 2-methoxy-3-oxo-butanoate. This intermediate underwent a MgCl2-mediated SNAr reaction using alkyl Grignard reagents to provide 8-bromo-3-methoxy-2-methyl-5-alkyl-1H-quinolin-4-one. Furthermore, leveraging these common intermediates, we achieved the first total syntheses of waltherione R, 8-demethoxywaltherione F, 8-demethoxywaltherione R, walindicaone C, and walindicaone D, as well as developing a concise three-step synthesis of waltherione F.

Recent Advances in Visible Light-Induced C-H Functionalization of Imidazo[1,2-a]pyridines

The imidazo[1,2-a]pyridine skeleton is widely present in many natural products and pharmaceutical agents. Due to its impressive and significant biological activities, such as analgesic, anti-tumor, antiosteoporosis, and anxiolytic properties, the derivatization of imidazo[1,2-a]pyridine skeleton has attracted widespread attention from chemists. In recent years, significant progress has been made in the derivatization of imidazo[1,2-a]pyridines through direct C-H functionalization, especially through visible light induction. This review highlights recent advances in visible light-induced C-H functionalization of imidazo[1,2-a]pyridines during the past ten years, and some reaction mechanisms are also discussed.

Cross-Coupling Reactions with Nickel, Visible Light, and tert-Butylamine as a Bifunctional Additive

Transition metal catalysis is crucial for the synthesis of complex molecules, with ligands and bases playing a pivotal role in optimizing cross-coupling reactions. Despite advancements in ligand design and base selection, achieving effective synergy between these components remains challenging. We present here a general approach to nickel-catalyzed photoredox reactions employing tert-butylamine as a cost-effective bifunctional additive, acting as the base and ligand. This method proves effective for C-O and C-N bond-forming reactions with a diverse array of nucleophiles, including phenols, aliphatic alcohols, anilines, sulfonamides, sulfoximines, and imines. Notably, the protocol demonstrates significant applicability in biomolecule derivatization and facilitates sequential one-pot functionalizations. Spectroscopic investigations revealed the robustness of the dynamic catalytic system, while elucidation of structure-reactivity relationships demonstrated how computed molecular properties of both the nucleophile and electrophile correlated to reaction performance, providing a foundation for effective reaction outcome prediction.

Migratory Aryl Cross-Coupling

A fundamental property of cross-coupling reactions is regiospecificity, meaning that the site of bond formation is determined by the leaving group's location on the electrophile. Typically, achieving a different substitution pattern requires the synthesis of a new, corresponding starting-material isomer. As an alternative, we proposed the development of cross-coupling variants that would afford access to multiple structural isomers from the same coupling partners. Here, we first demonstrate that a bulky palladium catalyst can facilitate the efficient, reversible transposition of aryl halides by temporarily forming metal aryne species. Despite the nearly thermoneutral equilibrium governing this process, combining it with the gradual addition of a suitable nucleophile results in dynamic kinetic resolution of the isomeric intermediates and high yields of unconventional product isomers. The method accommodates a range of oxygen- and nitrogen-centered nucleophiles and tolerates numerous common functional groups. A Curtin-Hammett kinetic scheme is supported by computational and experimental data, providing a general mechanistic framework for extending this migratory cross-coupling concept.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Oxalamide ligands with additional coordinating groups for Cu-catalyzed arylation of alcohols and phenols

A novel class of chain-like multidentate oxalamide ligands with additional coordinating groups was developed for the coupling of (hetero)aryl bromides with both alcohols and phenols under mild conditions. Introduction of oxygen atoms in N-alkyl chains is pivotal for the high catalytic efficiency and broad substrate versatility.

Palladium-catalyzed selective C-C bond cleavage of keto-vinylidenecyclopropanes: construction of structurally rich dihydrofurans and tetrahydrofurans

Palladium-catalyzed selective cleavage of the distal C-C bond and proximal C-C bond of keto-vinylidenecyclopropanes by altering the sterically bulky phosphine ligands has been realized. The proximal C-C bond cleavage can be achieved by using dtbpf as a phosphine ligand, affording bicyclic products containing dihydrofuran skeletons in good yields along with broad substrate scope. In proximal C-C bond cleavage reactions, the eight-membered cyclic palladium intermediate plays a key role in the reaction. The [3 + 2] cycloaddition of keto-vinylidenecyclopropanes through the distal C-C bond cleavage can be effectively accomplished with t BuXPhos as a phosphine ligand and ZnCl2 as an additive, delivering bicyclic products containing tetrahydrofuran skeletons in good yields. The further transformation of these bicyclic products has been demonstrated, and the reaction mechanisms of two different C-C bond cleavage reactions have been investigated by control experiments and DFT calculations.

DFT investigation of the DDQ-catalytic mechanism for constructing C-O bonds

In this study, we investigated the photo-catalytic mechanisms for the construction of C-O bonds from arenes (benzene, 2',6'-dimethyl-[1,1'-biphenyl]-2-carboxylic acid, or 2,4-dichloro-1-fluorobenzene), catalyzed by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). All the structures for the Gibbs free surfaces were calculated at the M06-2X-D3/ma-def2-SVP level in the SMD solvent model. Also, TDDFT calculations of DDQ were performed at the PBE1PBE-D3/ma-def2-SVP level in the SMD solvent model. The computational results indicated that DDQ, serving as a photo-catalyst, would be excited under visible light of 450 nm, aligning well with experimental observations as reflected in the UV-vis spectrum. Gibbs free energy surface analyses of the three reactions suggested that the path involving 3DDQ* activating the reactant (-COOH, H2O, or CH3OH) is favorable. Additionally, the role of O2 was investigated, revealing that it could facilitate the recycling of DDQ by lowering the energy barrier for the conversion of the DDQH˙ radical (not DDQH2) into DDQ. The use of ρhole and ρele can reveal the photo-catalytic reaction and charge transfer processes, while localized orbital locator isosurfaces and electron spin density isosurface graphs were employed to analyze structures and elucidate the single electron distribution. These computational results offer valuable insights into the studied interactions and related processes, shedding light on the mechanisms governing C-O bond formation from arenes catalyzed by DDQ.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Room-Temperature Copper-Catalyzed Etherification of Aryl Bromides

We disclose the development of a Cu-catalyzed C-O coupling method utilizing a new N1,N2-diarylbenzene-1,2-diamine ligand, L8. Under optimized reaction conditions, structurally diverse aryl and heteroaryl bromides underwent efficient coupling with a variety of alcohols at room temperature using an L8-based catalyst. Notably, the L8-derived catalyst exhibited enhanced activity when compared to the L4-based system previously disclosed for C-N coupling, namely the ability to functionalize aryl bromides containing acidic functional groups. Mechanistic studies demonstrate that C-O coupling utilizing L8 ⋅ Cu involves rate-limiting alkoxide transmetallation, resulting in a mechanism of C-O bond formation that is distinct from previously described Pd-, Cu-, or Ni-based systems. This lower energy pathway leads to rapid C-O bond formation; a 7-fold increase relative to what is seen with other ligands. The results presented in this report overcome limitations in previously described C-O coupling methods and introduce a new ligand that we anticipate may be useful in other Cu-catalyzed C-heteroatom bond-forming reactions.

Design, synthesis, and evaluation of novel quindoline derivatives with fork-shaped side chains as RNA G-quadruplex stabilizers for repressing oncogene NRAS translation

NRAS mutation is the second most common oncogenic factor in cutaneous melanoma. Inhibiting NRAS translation by stabilizing the G-quadruplex (G4) structure with small molecules seems to be a potential strategy for cancer therapy due to the NRAS protein's lack of a druggable pocket. To enhance the effects of previously reported G4 stabilizers quindoline derivatives, we designed and synthesized a novel series of quindoline derivatives with fork-shaped side chains by introducing (alkylamino)alkoxy side chains. Panels of experimental results showed that introducing a fork-shaped (alkylamino)alkoxy side chain could enhance the stabilizing abilities of the ligands against NRAS RNA G-quadruplexes and their anti-melanoma activities. One of them, 10b, exhibited good antitumor activity in the NRAS-mutant melanoma xenograft mouse model, showing the therapeutic potential of this kind of compounds.

Structure-Activity Relationship Studies and Optimization of 4-Hydroxypyridones as GPR84 Agonists

GPR84 is a putative medium-chain fatty acid receptor that is implicated in regulation of inflammation and fibrogenesis. Studies have indicated that GPR84 agonists may have therapeutic potential in diseases such as Alzheimer's disease, atherosclerosis, and cancer, but there is a lack of quality tool compounds to explore this potential. The fatty acid analogue LY237 (4a) is the most potent GPR84 agonist disclosed to date but has unfavorable physicochemical properties. We here present a SAR study of 4a. Several highly potent agonists were identified with EC50 down to 28 pM, and with SAR generally in excellent agreement with structure-based modeling. Proper incorporation of rings and polar groups resulted in the identification of TUG-2099 (4s) and TUG-2208 (42a), both highly potent GPR84 agonists with lowered lipophilicity and good to excellent solubility, in vitro permeability, and microsomal stability, which will be valuable tools for exploring the pharmacology and therapeutic prospects of GPR84.

DMSO-Assisted K3 PO4 -Catalyzed Cooperative Metal-Free, Base-Free Etherification of Chloroheteroarenes at Low Temperature

Etherification of chloroheteroarenes was performed at low temperatures under metal-free, ligand-free and base-free conditions, that is, the reaction is promoted by the cooperative effect of DMSO (solvent) as a promoter and K3 PO4 providing the catalytic surface (rather than a base). The protocol exhibits good substrate scope under mild reaction conditions and has also been explored mechanistically.

Synthesis of mono-, di- and tripalladated 1,3,5-benzenetristyryl complexes. CO insertion to give a dipalladated indenone

The tribrominated arenes 1,3,5-C₆(E-CHCHAr)₃Br₃ (Ar = Ph, (I), p-To (I')), add oxidatively to [Pd(dba)₂] ([Pd₂(dba)₃]·dba) in the presence of two equivalents of a phosphine (PPh₃ or PMe₂Ph) to form the monopalladated complexes trans-[Pd{C₆(E-CHCHAr)₃Br₂}Br(L)₂] (Ar = Ph, L = PPh₃ (1a), Ar = p-To, L = PPh₃ (1a'), Ar = Ph, L = PMe₂Ph (1b)), while the reaction in a 1 : 2 : 4 arene : Pd : PMe₂Ph molar ratio affords the dipalladated complex [{trans-PdBr(PMe₂Ph)₂}₂{μ₂-C₆(E-CHCHPh)₃Br}] (2b). Both I and I' add oxidatively to 3 equivalents of [Pd(dba)₂] in the presence of the chelating N-donor ligand tmeda (N,N,N',N'-tetramethylethylenediamine) to form the tripalladated complexes [{PdBr(tmeda)}₃{μ₃-C₆(E-CHCHAr)₃}] (Ar = Ph, (3c), p-To (3c')). Complex 3c reacts with PMe₃ to form [{trans-PdBr(PMe₃)₂}₃{μ₃-C₆(E-CHCHPh)₃}] (3d). Compound 3c also reacts with CO to give the novel dipalladated indenone [2-Ph-4,6-{PdBr(tmeda)}₂-5,7-(E-CHCHPh)₂-inden-1-one] (4). The crystal structures of 1a' and 1b were determined by X-ray diffraction studies.

Synthesis of Aryl-methylene Ethers through Pd(0)-Catalyzed Coupling between Hydrobenzoxazoles and Dichloromethane

Using dichloromethane (DCM) as the C1 feedstock, the first Pd(0)-catalyzed synthesis of aryl-methylene ether motifs from hydrobenzoxazoles (hydrothiazoles) has been reported. This protocol has exhibited an ample substrate scope (38 examples) and wide functional group tolerance. Aryl-methylene ether adducts have been successfully engaged in coupling with isocyanate to provide bis-iminoisoindolinones. In this reaction, DCM has served as a twofold electrophile for reaction with hydrobenzoxazoles via double C-Cl bond cleavage.

Synthesis of Palladium-Catalysed C-C Bond Forming 5-Chloro Quinolines via Suzuki-Miyaura Coupling; Anti-Pancreatic Cancer Screening on PANC-1 Cell Lines

Pancreatic cancer is the most severe among other cancers due to its late detection and less chance of survivability. Heterocycles are proven ring systems in the treatment of various cancers and this is due to the presence of two biodynamic molecules combined, which have a greater synergistic efficacy in many anticancer drugs. Quinoline and pyridine ring systems are brought together to obtain greater potency and this is achieved by coupling both using Pd-catalyst, and in the present investigation, Suzuki-Miyaura coupling (SMC) reactions are adopted to generate potent molecular entities. Pancreatic cancer is difficult to treat due to overexpression of the VEGFR2 protein. VEGFR2 is targeted to design the molecules of quinoline-coupled pyridine moieties and is docked to evaluate the protein-ligand interaction at the binding site. The binding affinity of conjugates revealed the potency and capability of ligands to inhibit the VEGFR2 pathway. The in-silico ADMET properties determined their inherent pharmacokinetic feasibility. The synthesized conjugates have been evaluated by MTT assay against the human pancreatic cancer cell lines (PANC-1). Among the series, compounds 5d, 5e, and 5h exhibited a greater inhibitory activity against the cell lines with an IC₅₀ value of 82.32±1.38, 54.74±1.18 and 80.35±1.68 μM. In the present exploration, 5e exhibited greater inhibitory activity and it could be a promising lead for the development of new chemotherapeutics against pancreatic cancer.

Access to Enantiomerically Pure P-Stereogenic Primary Aminophosphine Sulfides under Reductive Conditions

Stereochemically pure phosphines with phosphorus-heteroatom bonds and P-centered chirality are a promising class of functional building blocks for the design of chiral ligands and organocatalysts. A route to enantiomerically pure primary aminophosphine sulfides was opened through stereospecific reductive C-N bond cleavage of phosphorus(V) precursors by lithium in liquid ammonia. The chemoselectivity of the reaction as a function of reaction time, substrate pattern, and chiral auxiliary was investigated. In the presence of exclusively aliphatic groups bound to the phosphorus atom, all competing reductive side reactions are totally prevented. The absolute configurations of all P-stereogenic compounds were determined by single-crystal X-ray diffraction analysis. Their use as synthetic building blocks was demonstrated. The lithium salt of (R)-BINOL-dithiophosphoric acid proved to be a useful stereochemical probe to determine the enantiomeric purity. Insights into the coordination mode of the lithium-based chiral complex formed in solution was provided by NMR spectroscopy and DFT calculations.

Transient directing group enabled Pd-catalyzed C-H oxygenation of benzaldehydes and benzylic amines

We report a general protocol for ortho-C-H fluoroalkoxylation of benzaldehydes and benzylic amines utilizing an inexpensive amino amide as a transient directing group. In the presence of an electrophilic fluorinating bystanding oxidant and fluorinated alcohols, a wide range of benzaldehydes and benzylic amines could be oxygenated selectively at the ortho positions to afford fluoroalkyl aryl ethers. This elegant approach would provide appealing strategies for synthesis of drug molecules and natural products.

Predicting reaction conditions from limited data through active transfer learning

Transfer and active learning have the potential to accelerate the development of new chemical reactions, using prior data and new experiments to inform models that adapt to the target area of interest. This article shows how specifically tuned machine learning models, based on random forest classifiers, can expand the applicability of Pd-catalyzed cross-coupling reactions to types of nucleophiles unknown to the model. First, model transfer is shown to be effective when reaction mechanisms and substrates are closely related, even when models are trained on relatively small numbers of data points. Then, a model simplification scheme is tested and found to provide comparative predictivity on reactions of new nucleophiles that include unseen reagent combinations. Lastly, for a challenging target where model transfer only provides a modest benefit over random selection, an active transfer learning strategy is introduced to improve model predictions. Simple models, composed of a small number of decision trees with limited depths, are crucial for securing generalizability, interpretability, and performance of active transfer learning.

Organosulphur and organoselenium compounds as emerging building blocks for catalytic systems for O-arylation of phenols, a C-O coupling reaction

Diaryl ethers form an important class of organic compounds. The classic copper-mediated Ullmann diaryl ether synthesis has been known for many years and involves the coupling of phenols with aryl halides. However, the use of high reaction temperature, high catalyst loading and expensive ligands has created a need for the development of alternative catalytic systems. In the recent past, organosulphur and organoselenium compounds have been used as building blocks for developing homogeneous, heterogeneous and nanocatalysts for this C-O coupling reaction. Homogeneous catalytic systems include preformed complexes of metals with organosulphur and organoselenium ligands. The performance of such complexes is influenced dramatically by the nature of the chalcogen (S or Se) donor site of the ligand. Nanocatalytic systems (including Pd₁₇Se₁₅, Pd₁₆S₇ and Cu_1.8S) have been designed using a single-source precursor route. Heterogeneous catalytic systems contain either metal (Cu or Pd) or metal chalcogenides (Pd₁₇Se₁₅ or Cu_1.8S) as catalytically active species. This article aims to cover the simple and straightforward methodologies and approaches that are adopted for developing catalytically relevant organosulfur and organoselenium ligands, their homogeneous metal complexes, heterogeneous and nanocatalysts. The effects of chalcogen (S or Se) donor, halogen (Cl/Br/I) of aryl halide, nature (electron withdrawing or electron donating) of substituents present on the aromatic ring of aryl halides or substituted phenols and position (ortho or para) of substitution on the results of catalytic reactions have been critically analyzed and summarized. The effect of composition (Pd₁₇Se₁₅ or Pd₁₆S₇) on the performance of nanocatalytic systems is also highlighted. Substrate scope has also been discussed in all three types of catalysis. The superiority of heterogeneous catalytic systems (e.g., Pd₁₇Se₁₅ immobilised on graphene oxide) indicates the bright future possibilities for the development of efficient catalytic systems using similar or tailored ligands for this reaction.

Pd-Catalyzed Heteroannulation Using N-Arylureas as a Sterically Undemanding Ligand Platform

We report the development of ureas as sterically undemanding pro-ligands for Pd catalysis. N-Arylureas outperform phosphine ligands for the Pd-catalyzed heteroannulation of N-tosyl-o-bromoanilines and 1,3-dienes, engaging diverse coupling partners for the preparation of 2-subsituted indolines, including sterically demanding substrates that have not previously been tolerated. Experimental and computational studies on model Pd-urea and Pd-ureate complexes are consistent with monodentate binding through the nonsubstituted nitrogen, which is uncommon for metal-ureate complexes.

Deciphering the mechanism of the Ni-photocatalyzed C‒O cross-coupling reaction using a tridentate pyridinophane ligand

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.

Mechanistic knowledge of photocatalytic nickel reactions is lacking, particularly with regards to the identities and oxidation states of key intermediates. Here the authors report a class of tridentate ligands that enables in-depth study of a representative cross-coupling reaction, wherein evidence for multiple intermediates in a Ni(I/III) cycle is presented.

Rh-Catalyzed Oxidation and Trifluoroethoxylation of N-Aryl-pyrrolidin-2-ones : A Domino Approach for the Synthesis of N-Aryl-5-(2,2,2-trifluoroethoxy)-1,5- dihydro-2H-pyrrol-2-ones

The synthesis of trifluoroethoxylated dihydropyrrolidones via rhodium-catalyzed oxidation and trifluoroethoxylation of pyrrolidones is presented in this paper. This process realized trifluoroethoxylation of non-activated sp3 C-H by domino approach for the...

Catalytic methoxylation of aryl halides using 13C- and 14C-labeled CO2

A late-stage carbon isotope strategy, which allows methoxylation from CO2, is reported. This catalytic process, that relies on the formation of BBN-OCH3, enabled 13C and 14C labeling of a series of substrates, including pharmaceuticals.

The Formation of P-C Bonds Utilizing Organozinc Reagents for the Synthesis of Aryl- and Heteroaryl-Dichlorophosphines

Aryl- and heteroaryl-dichlorophosphines are mildly and selectively made in a one-pot synthesis in moderate to good yields starting from the respective aryl bromides or five-membered heterocycles, following lithiation with nBuLi, transmetalation with ZnCl₂, and subsequently the reaction with PCl₃. Selected aryl- and heteroaryl-dichlorophosphines were successfully synthesized using this reaction method and could easily be purified after isolation. The intermediate formation of the organozinc species is essential, as it prevents the formation of multiple substitution products. Important are also the reaction conditions: the usage of the proper solvent for the respective aromatic precursors and removal of the remaining salts by addition of a dioxane/pentane mixture. Depending on the solvent and steric demand of the substituent, mono- and bis-substitution products can be formed but formation also prevented. Hereby, different organozinc species might play an important role.

The Chosen Few: Parallel Library Reaction Methodologies for Drug Discovery

Parallel library synthesis is an important tool for drug discovery because it enables the synthesis of closely related analogues in parallel via robust and general synthetic transformations. In this perspective, we analyzed the synthetic methodologies used in >5000 parallel libraries representing 15 prevalent synthetic transformations. The library data set contains complex substrates and diverse arrays of building blocks used over the last 14 years at AbbVie. The library synthetic methodologies that have demonstrated robustness and generality with proven success are described along with their substrate scopes. The evolution of the synthetic methodologies for library synthesis over the past decade is discussed. We also highlight that the combination of parallel library synthesis with high-throughput experimentation will continue to facilitate the discovery of library-amenable synthetic methodologies in drug discovery.

Copper-Catalyzed Methoxylation of Aryl Bromides with 9-BBN-OMe

A Cu-catalyzed cross-coupling reaction between aryl bromides and 9-BBN-OMe to provide aryl methyl ethers under mild conditions is reported. The oxalamide ligand BHMPO plays a key role in the transformation. Various functional groups on bromobenzenes are well tolerated, providing the desired anisole products in moderate to high yields.

Chemoselective, Scalable Nickel-Electrocatalytic O-Arylation of Alcohols

The formation of aryl-alkyl ether bonds through cross coupling of alcohols with aryl halides represents a useful strategic departure from classical SN 2 methods. Numerous tactics relying on Pd-, Cu-, and Ni-based catalytic systems have emerged over the past several years. Herein we disclose a Ni-catalyzed electrochemically driven protocol to achieve this useful transformation with a broad substrate scope in an operationally simple way. This electrochemical method does not require strong base, exogenous expensive transition metal catalysts (e.g., Ir, Ru), and can easily be scaled up in either a batch or flow setting. Interestingly, e-etherification exhibits an enhanced substrate scope over the mechanistically related photochemical variant as it tolerates tertiary amine functional groups in the alcohol nucleophile.

CuI-Catalyzed Coupling Reactions of 4-Iodopyrazoles and Alcohols: Application toward Withasomnine and Homologs

The direct 4-alkoxylation of 4-iodo-1H-pyrazoles with alcohols was achieved by a CuI-catalyzed coupling protocol. The optimal reaction conditions employed excess alcohol and potassium t-butoxide (2 equiv) in the presence of CuI (20 mol%) and 3,4,7,8-tetramethyl-1,10-phenanthroline (20 mol%) at 130 °C for 1 h under microwave irradiation. The present method was efficiently applied to the synthesis of withasomnine and its six- and seven-membered cyclic homologs.

Oxalohydrazide Ligands for Copper-Catalyzed C-O Coupling Reactions with High Turnover Numbers

Here, we report a class of ligands based on oxalohydrazide cores and N-amino pyrrole and N-amino indole units that generates long-lived copper catalysts for couplings that form the C-O bonds in biaryl ethers. These Cu-catalyzed coupling of phenols with aryl bromides occurred with turnovers up to 8000, a value which is nearly two orders of magnitude higher than those of prior couplings to form biaryl ethers and nearly an order of magnitude higher than those of any prior copper-catalyzed coupling of aryl bromides and chlorides. This ligand also led to copper systems that catalyze the coupling of aryl chlorides with phenols and the coupling of aryl bromides and iodides with primary benzylic and aliphatic alcohols. A wide variety of functional groups including nitriles, halides, ethers, ketones, amines, esters, amides, vinylarenes, alcohols and boronic acid esters were tolerated, and reactions occurred with aryl bromides in pharmaceutically related structures.

Dialkyl Ether Formation at High-Valent Nickel

In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)-OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>-10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)-I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)-OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C-O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.

Synthesis of Fluorinated Alkyl Aryl Ethers by Palladium-Catalyzed C-O Cross-Coupling

Herein, we report a highly effective protocol for the cross-coupling of (hetero)aryl bromides with fluorinated alcohols using the commercially available precatalyst ^tBuBrettPhos Pd G3 and Cs₂CO₃ in toluene. This Pd-catalyzed coupling features a short reaction time, excellent functional group tolerance, and compatibility with electron-rich and -poor (hetero)arenes. The method provides access to ¹⁸F-labeled trifluoroethyl ethers by cross-coupling with [¹⁸F]trifluoroethanol.

Improved Process for the Palladium-Catalyzed C-O Cross-Coupling of Secondary Alcohols

An improved protocol for the Pd-catalyzed C-O cross-coupling of secondary alcohols is described. The use of biaryl phosphine L2 as the ligand was key to achieving efficient cross-coupling of (hetero)aryl chlorides with only a 20% molar excess of the alcohol. Additionally, we observed an unusual reactivity difference between an electron-rich aryl bromide and the analogous aryl chloride, and deuterium-labeling suggested that currently unidentified pathways for reduction play an important role in explaining this disparity.