Recent advances in the transition metal catalyzed etherification reactions

Adaptive alcohols-alcohols cross-coupling via TFA catalysis: access of unsymmetrical ethers

Ethers are high value organic compounds widely applied in chemical industry, natural products, material, pharmaceuticals, argochemicals, as well as modern organic synthesis. Herein, we report an adaptive TFA-catalyzed cross-coupling of alcohols with various oxygen nucleophiles (nitro-, halogen-, sulfur-, nitrogen-, aryl-, and alkynyl-substituted aliphatic alcohols), delivering diverse unsymmetrical ethers under mild conditions and simple operation. This protocol features a broad range of substrate scope and high catalytic efficiency (54 examples, up to 99% yield). The decagram scale performance and one-step synthesis of drug molecules evidenced the potential industrial production and practicability of this protocol.

Electrochemical C-O and C-N Arylation using Alternating Polarity in flow for Compound Libraries

Etherification and amination of aryl halide scaffolds are commonly used reactions in parallel medicinal chemistry to rapidly scan structure-activity relationships with abundant building blocks. Electrochemical methods for aryl etherification and amination demonstrate broad functional group tolerance and extended nucleophile scope compared to traditional methods. Nevertheless, there is a need for robust and scale-transferable workflows for electrochemical compound library synthesis. Herein we describe a platform for automated electrochemical synthesis of C-X arylation (X=NH, OH) in flow to access compound libraries. A comprehensive Design of Experiment (DoE) study identifies an optimal protocol which generates high yields across>30 aryl halide scaffolds, diverse amines (including electron-deficient sulfonamides, sulfoximines, amides, and anilines) and alcohols (including serine residues within peptides). Reaction sequences are automated on commercially available equipment to generate libraries of anilines and aryl ethers. The unprecedented application of potentiostatic alternating polarity in flow is essential to avoid accumulating electrode passivation. Moreover, it enables reactions to be performed in air, without supporting electrolyte and with high reproducibility over consecutive runs. Our method represents a powerful means to rapidly generate nucleophile independent C-X arylation compound libraries using flow electrochemistry.

Nickel-Catalyzed O-Arylation of Primary or Secondary Aliphatic Alcohols with (Hetero)aryl Chlorides: A Comparison of Ni(I) and Ni(II) Precatalysts

A comparative experimental and computational study examining the interplay of the ancillary ligand structure and Ni oxidation state in the Ni-catalyzed C(sp2)-O cross-coupling of (hetero)aryl chlorides and primary or secondary aliphatic alcohols is presented, focusing on PAd-DalPhos (L1)-, CyPAd-DalPhos (L2)-, PAd2-DalPhos (L3)-, and DPPF (L4)-ligated [(L)NiCl]n (n = 1 or 2) and (L)Ni(o-tol)Cl precatalysts. Both L1 and L2 were found to outperform the other ligands examined, with the latter proving to be superior overall. While Ni(II) precatalysts generally outperformed Ni(I) species, in some instances the catalytic abilities of Ni(I) precatalysts were competitive with those of Ni(II). Density-functional theory calculations indicate the favorability of a Ni(0)/Ni(II) catalytic cycle featuring turnover-limiting C-O bond reductive elimination over a Ni(I)/Ni(III) cycle involving turnover-limiting C-Cl oxidative addition.

Efficient base- and ligand-free palladium catalysed O-arylation of phenols in choline chloride:triethanolamine as a reusable deep eutectic solvent

In this paper, we present a novel and sustainable approach using choline chloride:triethanolamine as a green, efficient and reusable deep eutectic solvent (DES) for Pd-catalysed O-arylation reactions with Pd/BaSO4 (10%). By using the unique properties of DESs, we successfully achieved C-O bond formation without the need for additional solvents, bases and ligands. This solvent/catalyst system ([ChCl][TEA]2) functioned as a dual catalyst and solvent system, enabling fast and environmentally friendly C-O bond formation from phenol derivatives and electron-deficient aryl halides, leading to remarkable yields under mild reaction conditions. To identify and characterize this DES, we employed differential scanning calorimetry, thermogravimetric analysis, Fourier-transform infrared spectroscopy, refractive index, viscosity, the potential of hydrogen (pH) and conductivity measurements. One of the remarkable advantages of this DES system is its exceptional stability. This solvent/catalyst system exhibited high stability throughout the reaction cycles, showing no significant loss of activity. As a result, this DES and catalyst (Pd/BaSO4 (10%)) can be easily recycled and re-used for up to three consecutive cycles, making it an economically and environmentally attractive option for organic reactions. Our approach offers several key benefits, including simple catalyst preparation, quick reaction times and excellent production efficiency.

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.

Identifying general reaction conditions by bandit optimization

Reaction conditions that are generally applicable to a wide variety of substrates are highly desired, especially in the pharmaceutical and chemical industries1-6. Although many approaches are available to evaluate the general applicability of developed conditions, a universal approach to efficiently discover these conditions during optimizations is rare. Here we report the design, implementation and application of reinforcement learning bandit optimization models7-10 to identify generally applicable conditions by efficient condition sampling and evaluation of experimental feedback. Performance benchmarking on existing datasets statistically showed high accuracies for identifying general conditions, with up to 31% improvement over baselines that mimic state-of-the-art optimization approaches. A palladium-catalysed imidazole C-H arylation reaction, an aniline amide coupling reaction and a phenol alkylation reaction were investigated experimentally to evaluate use cases and functionalities of the bandit optimization model in practice. In all three cases, the reaction conditions that were most generally applicable yet not well studied for the respective reaction were identified after surveying less than 15% of the expert-designed reaction space.

Reaction of Dioxazolones with Boronic Acids: Copper-Mediated Synthesis of N-Aryl Amides via N-Acyl Nitrenes

Dioxazolones, as direct amide sources, have been used with boronic acids in the presence of copper(I) chloride to access N-aryl amides at room temperature. The versatility of the developed reaction is proven by ample scope having a wide range of functional group tolerance. The reaction optimization conditions revealed that a fluorine additive demonstrated improved reactivity toward the intended transformation. The addition of triphenylphosphine resulted in N-acyl iminophosphorane, suggesting the involvement of an N-acyl nitrene intermediate.

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.

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.

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.

Light-Promoted Nickel Catalysis: Etherification of Aryl Electrophiles with Alcohols Catalyzed by a NiII -Aryl Complex

A highly effective C-O coupling reaction of (hetero)aryl electrophiles with primary and secondary alcohols is reported. Catalyzed by a Ni^II -aryl complex under long-wave UV (390-395 nm) irradiation in the presence of a soluble amine base without any additional photosensitizer, the reaction enables the etherification of aryl bromides and aryl chlorides as well as sulfonates with a wide range of primary and secondary aliphatic alcohols, affording synthetically important ethers. Intramolecular C-O coupling is also possible. The reaction appears to proceed via a Ni^I -Ni^III catalytic cycle.

Ultrasound accelerated synthesis of O-alkylated hydroximides under solvent- and metal-free conditions

A novel, sustainable, environmentally friendly, high substrate scope, efficient, solvent-free and metal catalyst-free method for the cross-dehydrogenative coupling (CDC) reaction between N-hydroxyphthalimide (NHPI) and benzyl/ether compounds is described. This coupling reaction proceeds through ultrasound acceleration. Compared to conventional heating conditions, the use of ultrasound techniques not only improves the reaction efficiency and enhances the reaction rate but also minimizes the side reactions.

Transition-metal free oxidative C-H etherification of acylanilines with alcohols through a radical pathway

A transition metal free approach for the synthesis of methyl/ethyl aryl ether via oxidative C-H etherification of acylanilines with alcohols has been developed. Various acylanilines are compatible under standard conditions, giving the corresponding products in moderate to good yields. This strategy avoids transition-metal catalyst and excessive alcohol, providing a simple and reliable alternative method for the synthesis of methyl/ethyl aryl ether. Control experiments reveal that a radical mechanism is involved in this transformation.

Phosphazene Base tBu-P4 Catalyzed Methoxy-Alkoxy Exchange Reaction on (Hetero)Arenes

The organic superbase tBu-P4 catalyzes methoxy-alkoxy exchange reactions on (hetero)arenes with alcohols. The catalytic reaction proceeded efficiently with electron-deficient methoxy(hetero)arenes as well as with a variety of alcohols, including 3-amino-1-propanol, β-citronellol, menthol, and cholesterol. An intramolecular version of this reaction furnished six- and seven-membered ring compounds.

Oxalic Diamides and tert-Butoxide: Two Types of Ligands Enabling Practical Access to Alkyl Aryl Ethers via Cu-Catalyzed Coupling Reaction

A robust and practical protocol for preparing alkyl aryl ethers has been developed, which relies on using two types of ligands to promote Cu-catalyzed alkoxylation of (hetero)aryl halides. The reaction scope is very general for a variety of coupling partners, particularly for challenging secondary alcohols and (hetero)aryl chlorides. In case of coupling with aryl chlorides and bromides, two oxalic diamides serve as the powerful ligands. The tert-butoxide is first demonstrated as a ligand for Cu-catalyzed coupling reaction, leading to alkoxylation of aryl iodides complete at room temperature. Additionally, a number of carbohydrate derivatives are applicable for this coupling reaction, affording the corresponding carbohydrate-aryl ethers in 29-98% yields.

Nickel-catalyzed reductive defunctionalization of esters in the absence of an external reductant: activation of C–O bonds

The nickel-catalyzed reductive cleavage of esters in the absence of an external reductant, which involves the cleavage of an inert acyl C–O bond in O-alkyl esters is reported.

Exploiting Ancillary Ligation To Enable Nickel-Catalyzed C-O Cross-Couplings of Aryl Electrophiles with Aliphatic Alcohols

The use of (L)Ni( o-tolyl)Cl precatalysts (L = PAd-DalPhos or CyPAd-DalPhos) enables the C( sp²)-O cross-coupling of primary, secondary, or tertiary aliphatic alcohols with (hetero)aryl electrophiles, including unprecedented examples of such nickel-catalyzed transformations employing (hetero)aryl chlorides, sulfonates, and pivalates. In addition to offering a competitive alternative to palladium catalysis, this work establishes the feasibility of utilizing ancillary ligation as a complementary means of promoting challenging nickel-catalyzed C( sp²)-O cross-couplings, without recourse to precious-metal photoredox catalytic methods.

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

Two catalyst systems are described, which together provide mild and general conditions for the Pd-catalyzed C-O cross-coupling of primary alcohols. For activated substrates, such as electron-deficient aryl halides, the commercially available ligand L2 promotes efficient coupling for a variety of alcohol nucleophiles. In the case of unactivated electrophiles, such as electron-rich aryl halides, the new ligand L8 was developed to improve these challenging C-O bond-forming reactions.

Metal-mediated C–O bond forming reactions in natural product synthesis

Metal catalyzed reactions for the formation of C(sp²)–O bonds have had a dramatic impact in natural product synthesis. They have enabled the emergence of new bond disconnections, which notably resulted in remarkably efficient and short synthetic pathways. The use of these reactions for the formation of C–O bonds in natural product synthesis is overviewed in this critical review.