Practical Palladium Catalysts for C-N and C-O Bond Formation

Adaptive Photochemical Amination via Co(II) Catalysis

Transition-metal-catalyzed amination of aryl halides is one of the most employed methods for constructing N-arylation adducts. However, the broad success of these reactions largely relies on the screening of precatalysts, elaborated ligands, and case-by-case optimization of reaction conditions (solvent, base, additive, temperature, etc.) for electronically or structurally diverse nucleophiles. Herein, we report an adaptive photochemical C-N coupling of aryl halides with various nitrogen nucleophiles (aliphatic and aromatic amines, amides, sulfonamides, pyrazoles, and ammonium salts) by Co(II) catalysis under the same reaction conditions (same precatalyst, same ligand, same base, same solvent, same temperature) without the addition of any exogenous photocatalyst. This photochemical amination features a wide substrate scope (>130 examples, up to 95% yield) with excellent functional group tolerance. Mechanistic studies indicate that these C-N coupling reactions may proceed via a Co(I)/Co(III) catalytic cycle.

Heterogeneous Iron-Based Catalysts for Organic Transformation Reactions: A Brief Overview

Iron (Fe) is considered to be one of the most significant elements due to its wide applications. Recent years have witnessed a burgeoning interest in Fe catalysis as a sustainable and cost-effective alternative to noble metal catalysis in organic synthesis. The abundance and low toxicity of Fe, coupled with its competitive reactivity and selectivity, underscore its appeal for sustainable synthesis. A lot of catalytic reactions have been performed using heterogeneous catalysts of Fe oxide hybridized with support systems like aluminosilicates, clays, carbonized materials, metal oxides or polymeric matrices. This review provides a comprehensive overview of the latest advancements in Fe-catalyzed organic transformation reactions. Highlighted areas include cross-coupling reactions, C-H activation, asymmetric catalysis, and cascade processes, showcasing the versatility of Fe across a spectrum of synthetic methodologies. Emphasis is placed on mechanistic insights, elucidating the underlying principles governing iron-catalyzed reactions. Challenges and opportunities in the field are discussed, providing a roadmap for future research endeavors. Overall, this review illuminates the transformative potential of Fe catalysis in driving innovation and sustainability in organic chemistry, with implications for drug discovery, materials science, and beyond.

Trivalent Organostibines: Sb,N Ligands in Double N-Arylation of Primary Amines toward Functionalized Carbazoles

A Sb,N ligand (L-Sb) for Pd-catalyzed double N-arylation of primary amines was developed. This trivalent ligand L-Sb, containing a 5,6,7,12-tetrahydrodibenzo[c,f][1,5]azastibocine skeleton and stable under air and moisture, could be synthesized facilely on a gram scale from chlorostibine (1) and cyclopentylmagnesium bromide. L-Sb showed excellent catalytic performance in Pd2(dba)3-catalyzed double N-arylation of 2,2'-dibromo-1,1'-biphenyl (2) with primary amines (3), affording functionalized carbazoles in good yields. This Pd2(dba)3/L-Sb-catalyzed double N-arylation, the first example of the application of trivalent organostibines as a ligand in N-arylation, featured the following advantages: small catalyst loading, wide functional group tolerance, good yields, and ease of gram-scale synthesis.

N-Functionalization of 1,2-Azaborines

General protocols for the N-functionalization of 1,2-azaborines with C(sp3), C(sp2), or C(sp) electrophiles are described. The syntheses of a new parental BN isostere of trans-stilbene and a BN isostere of a lisdexamfetamine derivative were accomplished with the developed methodology.

A Second-Generation Palladacycle Architecture Bearing a N-Heterocyclic Carbene and Its Catalytic Behavior in Buchwald–Hartwig Amination Catalysis

Palladacyclic architectures have been shown as versatile motifs in cross-coupling reactions. NHC-ligated palladacycles possessing unique electronic and steric properties have helped to stabilize the catalytically active species and provide additional control over reaction selectivity. Here, we report on a synthetic protocol leading to palladacycle complexes using a mild base and an environmentally desirable solvent, with a focus on complexes bearing backbone-substituted N-heterocyclic carbene ligands. The readily accessible complexes exhibit high catalytic activity in the Buchwald–Hartwig amination. This is achieved using low catalyst loading and mild reaction conditions in a green solvent.

Steric and electronic effects of arsa-Buchwald ligands on Suzuki-Miyaura coupling reaction

The Suzuki-Miyaura coupling (SMC) reaction is one of the most commonly used cross-coupling reactions. Bulky biaryldialkyl monophosphine ligands, i.e., Buchwald ligands, are beneficial for the SMC reaction. We recently developed a synthetic procedure for arsa-Buchwald ligands, arsenic analogs of Buchwald ligands, and found that these ligands are effective for sterically hindered substrates because of facilitating the transmetalation step owing to the longer arsenic-palladium bond. However, the relationship between the structure and steric/electronic properties of the arsa-Buchwald ligands has not yet been studied in detail. In this study, a series of arsa-Buchwald ligands with various alkyl substituents were synthesized. The cyclopentyl group afforded the highest catalytic activity for the SMC reaction, particularly with sterically hindered substrates. Furthermore, the steric/electronic properties of the arsa-Buchwald ligands were computationally analyzed.

Zn(II)-Catalyzed Selective N-Alkylation of Amines with Alcohols Using Redox Noninnocent Azo-Aromatic Ligand as Electron and Hydrogen Reservoir

We report a sustainable and eco-friendly approach for selective N-alkylation of various amines by alcohols, catalyzed by a well-defined Zn(II)-catalyst, Zn(L^a)Cl₂ (1a), bearing a tridentate arylazo scaffold. A total of 57 N-alkylated amines were prepared in good to excellent yields, out of which 17 examples are new. The Zn(II)-catalyst shows wide functional group tolerance, is compatible with the synthesis of dialkylated amines via double N-alkylation of diamines, and produces the precursors in high yields for the marketed drugs tripelennamine and thonzonium bromide in gram-scale reactions. Control reactions and DFT studies indicate that electron transfer events occur at the azo-chromophore throughout the catalytic process, which shuttles between neutral azo, one-electron reduced azo-anion radical, and two-electron reduced hydrazo forms acting both as electron and hydrogen reservoir, enabling the Zn(II)-catalyst for N-alkylation reaction.

One-Pot Synthesis of N,N-Diphenyl-2-benzothiazolamines from 1-(2-Iodophenyl)-3-phenylthioureas and Iodobenzenes

An efficient copper-catalyzed synthesis of a variety of N,N-diphenyl-2-benzothiazolamines was developed. Starting from substituted 1-(2-iodophenyl)-3-phenylthioureas and substituted iodobenzenes, the reaction proceeded smoothly via a tandem manner in the presence of CuI to afford the corresponding N,N-diphenyl-2-benzothiazolamine derivatives with good functional group tolerance. The protocol features simple performance, easily available starting materials, a one-pot manner, and good functional group tolerance, providing a practical strategy for the preparation of poly-functionalized amines.

Mechanochemical halogenation of unsymmetrically substituted azobenzenes

The direct and selective mechanochemical halogenation of C–H bonds in unsymmetrically substituted azobenzenes using N-halosuccinimides as the halogen source under neat grinding or liquid-assisted grinding conditions in a ball mill has been described. Depending on the azobenzene substrate used, halogenation of the C–H bonds occurs in the absence or only in the presence of Pd^II catalysts. Insight into the reaction dynamics and characterization of the products was achieved by in situ Raman and ex situ NMR spectroscopy and PXRD analysis. A strong influence of the different 4,4’-substituents of azobenzene on the halogenation time and mechanism was found.

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.

Visible light-induced Ni-catalyzed C–heteroatom cross-coupling of aryl halides via LMCT with DBU to access a Ni(I)/Ni(III) cycle

Cross-coupling of aryl halides with nucleophiles is a synthetically attractive strategy to construct C–heteroatom bonds. Here we report a highly efficient photoinduced Ni-catalyzed method for the C–heteroatom cross-coupling of aryl...

Discovery of AL-GDa62 as a Potential Synthetic Lethal Lead for the Treatment of Gastric Cancer

Diffuse gastric cancer and lobular breast cancer are aggressive malignancies that are frequently associated with inactivating mutations in the tumor suppressor gene CDH1. Synthetic lethal (SL) vulnerabilities arising from CDH1 dysfunction represent attractive targets for drug development. Recently, SLEC-11 (1) emerged as a SL lead in E-cadherin-deficient cells. Here, we describe our efforts to optimize 1. Overall, 63 analogues were synthesized and tested for their SL activity toward isogenic mammary epithelial CDH1-deficient cells (MCF10A-CDH1^-/-). Among the 26 compounds with greater cytotoxicity, AL-GDa62 (3) was four-times more potent and more selective than 1 with an EC₅₀ ratio of 1.6. Furthermore, 3 preferentially induced apoptosis in CDH1^-/- cells, and Cdh1^-/- mammary and gastric organoids were significantly more sensitive to 3 at low micromolar concentrations. Thermal proteome profiling of treated MCF10A-CDH1^-/- cell protein lysates revealed that 3 specifically inhibits TCOF1, ARPC5, and UBC9. In vitro, 3 inhibited SUMOylation at low micromolar concentrations.

Copper(II) mediated ortho C-H alkoxylation of aromatic amines using organic peroxides: efficient synthesis of hindered ethers

Synthesis of hindered alkyl aryl ether derivatives (R-O-Ar) remains a huge challenge and highly desirable in organic and medicinal chemistry because extensive substitution on the ether bond prevents the undesired metabolic process and thus avoids rapid degradation in vivo. Herein, we report an unprecedented hindered alkoxylation of picolinamide attached aromatic amines using economic copper salt and organic peroxide to get highly desirable α-tertiary alkyl aryl ethers.

Nickel-Catalyzed C-Heteroatom Cross-Coupling Reactions under Mild Conditions via Facilitated Reductive Elimination

The formation of C-heteroatom bonds represents an important type of bond-forming reaction in organic synthesis and often provides a fast and efficient access to privileged structures found in pharmaceuticals, agrochemical and materials. In contrast to conventional Pd- or Cu-catalyzed C-heteroatom cross-couplings under high-temperature conditions, recent advances in homo- and heterogeneous Ni-catalyzed C-heteroatom formations under mild conditions are particularly attractive from the standpoint of sustainability and practicability. The generation of Ni^III and excited Ni^II intermediates facilitate the reductive elimination step to achieve mild cross-couplings. This review provides an overview of the state-of-the-art approaches for mild C-heteroatom bond formations and highlights the developments in photoredox and nickel dual catalysis involving SET and energy transfer processes; photoexcited nickel catalysis; electro and nickel dual catalysis; heterogeneous photoredox and nickel dual catalysis involving graphitic carbon nitride (mpg-CN), metal organic frameworks (MOFs) or semiconductor quantum dots (QDs); as well as more conventional zinc and nickel dual catalyzed reactions.

Sodium Butylated Hydroxytoluene: A Functional Group Tolerant, Eco-Friendly Base for Solvent-Free, Pd-Catalysed Amination

NaBHT (sodium 2,6-di-tert-butyl-4-methylphenolate), a strong, but hindered and lipophilic base, has been effectively paired with similarly lipophilic, high-reactivity Pd-NHC (N-heterocyclic carbene) catalysts to produce an ideal combination for performing solvent-free (melt) cross-coupling amination. The mild nucleophilicity of NaBHT, coupled with the anti-oxidant properties of its conjugate acid byproduct, BHT means the process seems to have no functional group incompatibilities. Highly effective coupling of base-sensitive and redox-active functional groups was observed in all cases with only 0.1-0.2 mol percent catalyst. Comparisons using the standard base for this reaction, KOtBu, led to poor couplings or complete degradation in most applications - only NaBHT works.

Redox-Neutral Cross-Coupling Amination with Weak N-Nucleophiles: Arylation of Anilines, Sulfonamides, Sulfoximines, Carbamates, and Imines via Nickelaelectrocatalysis

A nickel-catalyzed cross-coupling amination with weak nitrogen nucleophiles is described. Aryl halides as well as aryl tosylates can be efficiently coupled with a series of weak N-nucleophiles, including anilines, sulfonamides, sulfoximines, carbamates, and imines via concerted paired electrolysis. Notably, electron-deficient anilines and sulfonamides are also suitable substrates. Interestingly, when benzophenone imine is applied in the arylation, the product selectivity toward the formation of amine and imine product can be addressed by a base switch. In addition, the alternating current mode can be successfully applied. DFT calculations support a facilitated reductive elimination pathway.

A palladium-catalyzed approach to allenic aromatic ethers and first total synthesis of terricollene A

A palladium-catalyzed C-O bond formation reaction between phenols and allenylic carbonates to give 2,3-allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A variety of synthetically useful functional groups are tolerated and the synthetic utility of this method has been demonstrated through a series of transformations of the allene moiety. By applying this reaction as the key step, the total syntheses of naturally occurring allenic aromatic ethers, eucalyptene and terricollene A (first synthesis; 4.5 g gram scale), have been accomplished.

Oxidative cross-dehydrogenative coupling between iodoarenes and acylanilides for C–N bond formation under metal-free conditions

A metal-free oxidative cross-dehydrogenative coupling between iodoarenes and acylanilides was developed. It gave highly para-selectivie C–N coupling products with the retention of iodine atom which enables further transformations.

Synthesis of ortho-arylated and alkenylated benzamides by palladium-catalyzed denitrogenative cross-coupling reactions of 1,2,3-benzotriazin-4(3H)-ones with organoboronic acids

An efficient palladium-catalyzed denitrogenative cross-coupling reaction of 1,2,3-benzotriazin-4(3H)-ones with organoboronic acids is described. The reaction affords various ortho-aryl and alkenylated benzamides in good to high yields.

Sustainability in Ru- and Pd-based catalytic systems using N-heterocyclic carbenes as ligands

This review is a critical presentation of catalysts based on palladium and ruthenium bearing N-heterocyclic carbene ligands that have enabled a more sustainable approach to catalysis and to catalyst uses.

Copper(ii) ions supported on functionalized graphene oxide: an organometallic nanocatalyst for oxidative amination of azoles via C–H/C–N bond activation

Graphene oxide (GO) was chemically modified with para-aminobenzoic acid (PABA) to immobilize copper(ii) ions on its surface and used as a nanocatalyst for the oxidative C (sp²)–H bond amination reaction.

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation

Thermodynamic knowledge of the metal-ligand (M-L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd-X (X=OR and NHAr) bond heterolysis energies (ΔG_het (Pd-X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N',N'-tetramethylethylenediamine) as the model complexes. The ΔG_het (Pd-X) scales span a range of 2.6-9.0 kcal mol^-1 for ΔG_het (Pd-O) values and of 14.5-19.5 kcal mol^-1 for ΔG_het (Pd-N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X-H bond activation reveal that the M-X bond metathesis is dominated by differences of the X-H and Pd-X bond strengths, the former being more influential. The ΔG_het (Pd-X) and pK_a (X-H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd-X complexes.

Tuning of Structure Evolution and Electronic Properties through Palladium-Doped Boron Clusters: PdB16 as a Motif for Boron-Based Nanotubes

Transition metal-doped electronic deficiency boron clusters have led to a vast variety of electronic bonding properties in chemistry and materials science. We have determined the ground state structures of PdB_n^0/- (n = 10-20) clusters by performing CALYPSO search and density functional theory (DFT) optimization. The identified lowest energy structures for both neutral and anionic Pd-doped boron clusters follow the structure evolution from two dimensional (2D) planar configurations to 3D distorted Pd-centered drum-like or tubular structures. Photoelectron spectra are simulated by time-dependent DFT theoretical calculations, which is a powerful method to validate our obtained ground-state structures. More interestingly, two "magic" number clusters, PdB₁₂ and PdB₁₆, are found with enhanced stability in the middle size regime studied. Subsequently, molecular orbital and adaptive natural density partitioning analyses reveal that the high stability of the PdB₁₆ cluster originates from doubly σ π aromatic and bonding interactions of d-type atomic orbitals of the Pd atom with tubular B₁₆ units. The tubular C_8v PdB₁₆ cluster, with robust relative stability, is an ideal embryo for forming finite and infinite nanotube nanomaterials.

C4-Alkylamination of C4-Halo-1H-1-tritylpyrazoles Using Pd(dba)2 or CuI

Alkylamino coupling reactions at the C4 positions of 4-halo-1H-1-tritylpyrazoles were investigated using palladium or copper catalysts. The Pd(dba)₂ catalyzed C-N coupling reaction of aryl- or alkylamines, lacking a β-hydrogen atom, proceeded smoothly using ^tBuDavePhos as a ligand. As a substrate, 4-Bromo-1-tritylpyrazole was more effective than 4-iodo or chloro-1-tritylpyrazoles. Meanwhile, the CuI mediated C-N coupling reactions of 4-iodo-1H-1-tritylpyrazole were effective for alkylamines possessing a β-hydrogen atom.

Dual Nickel-/Palladium-Catalyzed Reductive Cross-Coupling Reactions between Two Phenol Derivatives

Cross-coupling between substrates that can be easily derived from phenols is highly attractive due to the abundance of phenols. Here, we report a dual nickel-/palladium-catalyzed reductive cross-coupling between aryl tosylates and aryl triflates; both substrates can be accessed in just one step from readily available phenols. The reaction has a broad functional group tolerance and substrate scope (>60 examples). Furthermore, it displays low sensitivity to steric effects demonstrated by the synthesis of a 2,2'-disubstituted biaryl and a fully substituted aryl product. The widespread presence of phenols in natural products and pharmaceuticals allows for straightforward late-stage functionalization, illustrated with examples such as ezetimibe and tyrosine.

Sulfone-Mediated SNAr Reaction as a Powerful Tool for the Synthesis of 4-Quinolinyl Ethers and More-Application to the Synthesis of HCV NS3/4a Protease Inhibitor

An efficient general methodology for the synthesis of 4-quinolinyl ethers is demonstrated via a highly reactive S_NAr reaction of 4-quinolinyl sulfones with a range of structurally diversified 1°, 2°, and 3° alcohols with a wide substrate scope and high yields. By adapting this methodology, a convergent synthesis of a complex target of HCV NS3/4a protease inhibitor BI 201420 was accomplished.

Copper-Catalyzed Electrophilic Etherification of Arylboronic Esters with Isoxazolidines

A copper-catalyzed electrophilic etherification of arylboronic esters is reported. Isoxazolidines are utilized as easily available and stable [RO]⁺ surrogates to give 1,3-amino aryl ethers. The O-selective arylation of isoxazolidines takes place without causing competitive N-arylation. In contrast to previously reported anionic conditions, our copper-catalyzed conditions are mild enough to achieve high functional group tolerance. Preliminary mechanistic studies and DFT calculations support that the reaction proceeds via a transmetalation/oxidative addition pathway, followed by a Lewis acid-promoted reductive elimination to induce the crucial O-selectivity.

Isolation and synthesis of cryptosanguinolentine (isocryptolepine), a naturally-occurring bioactive indoloquinoline alkaloid

Cryptosanguinolentine (isocryptolepine) is one of the minor naturally-occurring monomeric indoloquinoline alkaloids, isolated from the West African climbing shrub Cryptolepis sanguinolenta. The natural product displays such a simple and unique skeleton, which chemists became interested in well before it was found in Nature. Because of its structure and biological activity, the natural product has been targeted for synthesis on numerous occasions, employing a wide range of different strategies. Hence, discussed here are aspects related to the isolation of isocryptolepine, as well as the various approaches toward its total synthesis.