Palladium-catalyzed cross-coupling reactions of aryl mesylates

Palladium-Catalyzed Domino Conversion of Aryl-Thianthreniums with Anhydrides: Rapidly Building Highly Functionalized Fluorenones

As a class of rising electrophilic coupling reagents, aryl-thianthreniums (aryl-TTs) have been gaining immense attention. Herein, a novel palladium-catalyzed domino annulation of aryl-TTs with anhydrides is proposed to rapidly assemble a collection of highly functionalized fluorenones. This finding presents an innovative reaction pattern of aryl-TTs wherein the domino annulation version is first involved. Heavily compared with the existing conversions with aryl-TTs, this identified process successively functions as four aryl C-H bonds.

Organophosphates as Versatile Substrates in Organic Synthesis

This review summarizes the applications of organophosphates in organic synthesis. After a brief introduction, it discusses cross-coupling reactions, including both transition-metal-catalyzed and transition-metal-free substitution reactions. Subsequently, oxidation and reduction reactions are described. In addition, this review highlights the applications of organophosphates in the synthesis of natural compounds, demonstrating their versatility and importance in modern synthetic chemistry.

Fe3O4@WO3-E-SMTU-NiII: as an environmentally-friendly, recoverable, durable and noble-free nanostructured catalyst for C-C bond formation reaction in green media

In the present study, NiII immobilized on Fe3O4@WO3 functionalized by aminated epichlorohydrin using S-methylisothiourea (Fe3O4@WO3-E-SMTU-NiII) as a novel magnetically separable nanostructured catalyst was successfully synthesized and characterized using FT-IR, XRD, TEM, FE-SEM, EDX, EDX mapping, VSM, TGA, H2-TPR, ICP-OES and CHNS techniques. Characterization results revealed the spherical morphology and superparamagnetic behaviour of the as-synthesized catalyst with mean diameters of 19-31 nm as well as uniform distributions of the desired elements (Fe, O, W, C, N, S and Ni). The antibacterial activity of Fe3O4@WO3-E-SMTU-NiII was evaluated against a set of Gram positive and Gram negative bacteria, and the catalyst showed considerable activity against the Staphylococcus aureus strain. The aforementioned nanostructured catalyst exhibited perfect catalytic efficiency in the Heck-Mizoroki and Suzuki-Miyaura reactions under mild conditions without using toxic solvents (EtOH as a green solvent and WEB as a benign base). Desired coupled products were obtained from the reaction of different Ar-X (X = I, Br, Cl) with alkyl acrylates and arylboronic acids. A high nickel content with negligible metal leaching during the course of reactions led to the high catalytic performance and stability of Fe3O4@WO3-E-SMTU-NiII under optimized reaction conditions. The magnetically separation and ease of recovery and reusability of up to six cycles without a discernible decrease in catalytic activity or metal leaching are the most important features of the catalytic system from both industrial and environmental viewpoints.

Recent progress in reductive carboxylation of C-O bonds with CO2

Transformation of carbon dioxide (CO2) into value-added organic compounds has attracted increasing interest of scientific community in the last few decades, not only because CO2 is the primary greenhouse gas that drives global climate change and ocean acidification, but also because it has been regarded as a plentiful, nontoxic, nonflammable and renewable one-carbon (C1) feedstock. Among the various CO2-conversion processes, carboxylation reactions represent one of the most beautiful and attractive research topics in the field, since it offers the possibility for the construction of synthetically and biologically important carboxylic acids from various easily accessible (pseudo)halides, organosilicon, and organoboron compounds. The purpose of this review is to summarize the available literature on deoxygenative carboxylation of alcohols and their derivatives utilizing CO2 as a carboxylative reagent. Depending on the C-O compounds employed, the paper is divided into five major sections. The direct dehydroxylative carboxylation of free alcohols is discussed first. This is followed by reductive carboxylation of carboxylates, triflates, and tosylates. In the final section, the only reported example on catalytic carboxylation of fluorosulfates will be covered. Notably, special attention has been paid on the mechanistic aspects of the reactions that may provide new insights into catalyst improvement and development, which currently mainly relies on the use of transition metal catalysts.

Dichotomic Dearomatizations of Benzene vs Pyridine Rings of Sulfonyloxypyridine via (3 + 2) Cycloaddition

Electron-poor arenesulfonyloxypyridines are selectively dearomatized whether on the pyridine or on the phenyl group through 1,3-dipolar cycloaddition (1,3-DC) involving non-stabilized azomethine ylides (AMY). Electronic effects of substituents on the aromatic rings allow to induce the regioselectivity of the transformation. Novel pyrrolidinic polycyclic heterocycles are thereby produced under mild acidic conditions at room temperature.

Palladium-catalyzed and norbornene-mediated C-H amination and C-O alkenylation of aryl triflates

The C-H amination and C-O alkenylation of aryl triflates was achieved through Pd/norbornene (NBE) cooperative catalysis. By this strategy, various ortho-alkenyl tertiary anilines including those bearing functional groups were produced in good to excellent yields. This reaction represents a new conversion model for phenoxides. It expands the scope of Catellani-type reactions and the application of phenoxides in organic synthesis.

Aryl fluorosulfates: powerful and versatile partners in cross-coupling reactions

Aryl fluorosulfates are versatile building blocks in organic synthesis and have gained increasing attention in SuFEx (Sulfur Fluoride Exchange) click chemistry. They are easily and conveniently prepared from phenols using sulfuryl fluoride SO₂F₂ as a low-cost sulfonyl fluoride provider. Recently, they served as less toxic and more atom economical alternatives to triflates in an impressive number of carbon-carbon and carbon-heteroatom cross-coupling reactions. In this review, we summarize the current advances and developments in applying aryl fluorosulfates as electrophilic partners in cross-coupling reactions.

α-Arylsulfonyloxyacrylates: attractive O-centered electrophiles for synthesis of α-substituted acrylates via Pd-catalysed Suzuki reactions

We herein report α-arylsulfonyloxyacrylates as a kind of useful and attractive O-centered electrophiles for Suzuki cross-coupling reactions. A range of α-(hetero)aryl substituted acrylates has been prepared via the palladium-catalysed C-C cross-coupling reactions between potassium (hetero)aryltrifluoroborates and α-arylsulfonyloxyacrylates. Moreover, α-arylsulfonyloxyacrylate could also react with B-alkyl-9-BBN to produce α-alkyl substituted acrylates. The synthetic application of this new method was demonstrated by the preparation of the intermediate for synthesis of retinoid X receptors-selective retinoids. These Suzuki reaction-based protocols feature broad substrate scope, generality, and mild reaction conditions.

Facile Assembly of Modular-Type Phosphines for Tackling Modern Arylation Processes

This Account presents an overview of a promising collection of phosphine ligands simply made from the modular Fischer indolization process and their applications in modern arylation processes. Using one easily accessible 2-arylindole scaffold, three major phosphino-moiety-positioned ligand series can be readily generated. We have attempted to explore challenging electrophilic and nucleophilic partners for the coupling reaction using the modular ligand tool. For the electrophilic partner study, CM-phos-type ligands, where the phosphino group is located at the 2-arene position of 2-arylindole, allow the successful cross-coupling of aryl mesylates. The CM-phos ligand forms a palladacycle before entering the cross-coupling catalytic cycle. For the nucleophilic partner investigation, the indole C3-positioned phosphines show the first accomplishment of Pd-catalyzed organotitanium nucleophile arylation. Indeed, the aryl-titanium nucleophile undergoes cross-coupling more efficiently than does the organoboron coupling partner in particular cases. Moreover, in the indole C3-positioned phosphine series, the -PPh₂-containing ligands perform better in the highly sterically hindered cross-coupling of aryl chlorides than do ligands containing the -PCy₂ moiety. The catalyst loading can even be reduced to 0.2 mol % Pd for tetra-ortho-substituted biaryl synthesis. This finding offers a new perspective on the next-generation design of phosphine ligands in which the sterically bulky and electron-rich -PR₂ group (R = alkyl) may not be necessary for the cross-coupling of aryl chlorides. In general, we hypothesize that a good balance of steric and electronic properties for entertaining the oxidative addition and reductive elimination steps is crucial to the success of the reaction. For the steric factor, the highly sterically congested -PR₂ group normally favors the reductive elimination, yet we conjecture that this sterically bulky group would serve as an obstacle for the incoming aryl halides. For the electronic factor, the electron rich -PR₂ group is believed to support the oxidative cleavage of the C(Ar)-Cl bond by donating more electron density to the corresponding σ* orbital. Nevertheless, the high electron richness of the -PR₂ group may disfavor the reductive elimination electronically. Overall, an appropriate balance of both electron density and steric bulkiness is suggested to allow the sterically hindered cross-coupling to proceed smoothly. We have found that the -PPh₂-containing ligand is a good starting point for this investigation. The formation of aromatic carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds from aryl chlorides was successfully realized using our proprietary phosphines.In addition to the indole-core-bearing ligand skeleton, we also explored the relevant imidazolyl and carbazolyl phosphines for their unique applications. Interestingly, the carbazolyl ligand, having more flexible C-N axial chirality, displays particular interchangeable Pd-N and Pd-arene coordination, which facilitates both oxidative addition and reductive elimination processes. Moreover, this C-N axially chiral ligand allows the successful asymmetric Suzuki-Miyaura coupling for attaining the most sterically hindered tetra-ortho-substituted biaryls with excellent enantioselectivity. The rationale behind these scientifically interesting findings is presented in detail.

Palladium-Catalyzed Sequential Three-Component Cross-Coupling to 1,3-Dienes: Employing Alkenes as Hydride and Alkenyl Donors

This report discloses a novel Pd-catalyzed sequential three-component multiple reaction of alkenes, bromoalkynes, and boronic acids using alkenes as hydride and alkenyl donors, leading to highly stereoselective assembly of (Z,E)-1,3-diene derivatives. Mechanistic studies demonstrate that the generation and reutilization of palladium hydride species are critical to the success of this transformation. In addition, the good functional group compatibility, late-stage modification, and investigation of photophysical properties of 1,3-diene products illustrate the synthetic value of this strategy.

Visible-light-mediated tandem phosphorylation/cyclization for the synthesis of phosphorylated oxindoles

The visible-light-mediated tandem phosphorylation/cyclization of N-arylacrylamides with H-phosphine oxides has been developed for the synthesis of phosphorylated oxindoles. This efficient and facile process was useful for the construction of a C-P bond and triggered the formation of a C-C bond with good compatibility with functional groups undermild reaction conditions.

Sulfonyl fluorides as targets and substrates in the development of new synthetic methods

The advent of sulfur(VI)-fluoride exchange (SuFEx) processes as transformations with click-like reactivity has invigorated research into electrophilic species featuring a sulfur-fluorine bond. Among these, sulfonyl fluorides have emerged as the workhorse functional group, with diverse applications being reported. Sulfonyl fluorides are used as electrophilic warheads by both medicinal chemists and chemical biologists. The balance of reactivity and stability that is so attractive for these applications, particularly the resistance of sulfonyl fluorides to hydrolysis under physiological conditions, has provided opportunities for synthetic chemists. New synthetic approaches that start with sulfur-containing substrates include the activation of sulfonamides using pyrilium salts, the deoxygenation of sulfonic acids, and the electrochemical oxidation of thiols. Employing non-sulfur-containing substrates has led to the development of transition-metal-catalysed processes based on palladium, copper and nickel, as well as the use of SO₂F₂ gas as an electrophilic hub. Selectively manipulating molecules that already contain a sulfonyl fluoride group has also proved to be a popular tactic, with metal-catalysed processes again at the fore. Finally, coaxing sulfonyl fluorides to engage with nucleophiles, when required, and under suitable reaction conditions, has led to new activation methods. This Review provides an overview of the challenges in the efficient synthesis and manipulation of these intriguing functional groups.

Transition metal complexes with functionalized indenyl phosphine ligands: structures and catalytic properties

Transition-metal indenyl complexes usually exhibit different reactivities compared with their cyclopentadienyl analogues. Up to now, at least 10 metal-indenyl bonding modes have been reported. Because of the "indenyl effect", transition-metal indenyl complexes usually show enhanced reactivity in substitution and related reactions. This review provides an overview on the use and impact of indenyl phosphines in organometallic chemistry and transition-metal-catalysed reactions in the recent two decades. Some cationic and zwitterionic metal complexes supported by P,N-substituted indene or indenide ligands are described. They have been reported to induce the cleavage of E-H (E = H, Si and B) bonds and can be used as catalysts for addition of E-H bonds to unsaturated substrates. 2-Aryl indenyl phosphine ligands L3-L11 have been proven to be a class of versatile ligands for palladium-catalysed C-C and C-N cross-coupling reactions. Moreover, optically active tethered indenyl phosphine ligands can have better stereoselective control over the chirality arising at the metal center in the oxidative addition of their rhodium complexes with alkyl halides.

A mechanistic investigation of the Pd-catalyzed cross-coupling between N-tosylhydrazones and aryl halides

The cross-coupling of N-tosylhydrazones and aryl halides forms carbon-carbon bonds, producing 1,1-disubstituted alkenes. Though it has proven extremely useful in several fields of chemistry, its mechanism remains experimentally unexplored. Combining benchtop NMR and real-time mass spectrometry afforded the ability to monitor the catalytic intermediates as well as the rate of product formation.

Palladium-Catalyzed Aryl-Furanylation of Alkenes: Synthesis of Benzofuran-Containing 3,3-Disubstituted Oxindoles

A novel palladium-catalyzed aryl-furanylation of alkenes is described. This protocol provided a straightforward route to the synthesis of various benzofuran-containing 3,3-disubstitutedoxindole derivatives bearing a quarternary carbon center. In the cascade process, one C(sp²)-O bond, two C(sp²)-C(sp³) bonds, an oxindole, and a furan ring are formed in a single chemical operation.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Recent Advances in the Pd-Catalyzed Coupling of Arylhydrazines and Ammonium Salts via C-N Bond Cleavage

Both arylhydrazines and quaternary ammonium salts are readily accessible or commercially available chemicals that show versatile reactivity in Pd-catalyzed coupling reactions via C-N bond cleavage. A tremendous array of coupling reactions involving reaction partners such as organoborons, aryl silanes, alkenes, alkynes, arylation or alkylation reagents in C-H functionalization and carbonylation reactions are summarized, in which arylhydrazines or quaternary ammonium salts function as aryl or alkyl donors. This account mainly focuses on recent advances in Pd-catalyzed coupling reactions with arylhydrazines or quaternary ammonium salts via C-N bond cleavage, including mechanistic elucidations.

Suzuki-Miyaura Cross-Coupling of Esters by Selective O-C(O) Cleavage Mediated by Air- and Moisture-Stable [Pd(NHC)(μ-Cl)Cl]2 Precatalysts: Catalyst Evaluation and Mechanism

The cross-coupling of aryl esters has emerged as a powerful platform for the functionalization of otherwise inert acyl C-O bonds in chemical synthesis and catalysis. Herein, we report a combined experimental and computational study on the acyl Suzuki-Miyaura cross-coupling of aryl esters mediated by well-defined, air- and moisture-stable Pd(II)-NHC precatalysts [Pd(NHC)(μ-Cl)Cl]2. We present a comprehensive evaluation of [Pd(NHC)(μ-Cl)Cl]2 precatalysts and compare them with the present state-of-the-art [(Pd(NHC)allyl] precatalysts bearing allyl-type throw-away ligands. Most importantly, the study reveals [Pd(NHC)(μ-Cl)Cl]2 as the most reactive precatalysts discovered to date in this reactivity manifold. The unique synthetic utility of this unconventional O-C(O) cross-coupling is highlighted in the late-stage functionalization of pharmaceuticals and sequential chemoselective cross-coupling, providing access to valuable ketone products by a catalytic mechanism involving Pd insertion into the aryl ester bond. Furthermore, we present a comprehensive study of the catalytic cycle by DFT methods. Considering the clear advantages of [Pd(NHC)(μ-Cl)Cl]2 precatalysts on several levels, including facile one-pot synthesis, superior atom-economic profile to all other Pd(II)-NHC catalysts, and versatile reactivity, these should be considered as the 'first-choice' catalysts for all routine applications in ester O-C(O) bond activation.

Photohomolysis and Photoheterolysis in Aryl Sulfonates and Aryl Phosphates

The photochemical behaviour of selected aryl sulfonates and phosphates (ArOX) in polar and nonpolar media has been investigated by laser flash photolysis (LFP) experiments. Two main pathways have been identified, namely the photohomolysis of the ArO-X bond or the photoheterolysis of the Ar-OX bond depending on the nature of the leaving group (OX) and on the nature of the substituents on the aromatic ring. In nonpolar solvents the esters are quite photostable due to an efficient triplet deactivation. In polar solvents, the homolytic fragmentation of the ArO-S bond from the exited singlets was found in aryl sulfonates bearing moderately electron-donating groups as well as electron-withdrawing groups. In electron-rich aryl phosphates and sulfonates photoheterolysis of the Ar-OP/Ar-OS bond took place as the exclusive pathway.

Convergent Synthesis of Fluoroalkenes Using a Dual-Reactive Unit

Fluoroalkenes have shown importance as a metabolically stable isostere of amide compounds. To expedite the synthesis of diverse fluoroalkenes, we have developed a dual-reactive C2-unit, (Z)-1-boryl-1-fluoro-2-tosyloxyethene, containing nucleophilic and electrophilic moieties. Consecutive palladium-catalyzed cross-coupling reactions of this unit with aryl bromides and aryl boronic acids allow for the convergent synthesis of diverse trans-1,2-diaryl-substituted fluoroethenes in a chemoselective and stereoretentive manner.

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.

C(acyl)–C(sp2) and C(sp2)–C(sp2) Suzuki–Miyaura cross-coupling reactions using nitrile-functionalized NHC palladium complexes

Application of N-heterocyclic carbene (NHC) palladium complexes has been successful for the modulation of C–C coupling reactions. For this purpose, a series of azolium salts (1a–f) including benzothiazolium, benzimidazolium, and imidazolium, bearing a CN-substituted benzyl moiety, and their (NHC)₂PdBr₂ (2a–c) and PEPPSI-type palladium (3b–f) complexes have been systematically prepared to catalyse acylative Suzuki–Miyaura coupling reaction of acyl chlorides with arylboronic acids to form benzophenone derivatives in the presence of potassium carbonate as a base and to catalyse the traditional Suzuki–Miyaura coupling reaction of bromobenzene with arylboronic acids to form biaryls. All the synthesized compounds were fully characterized by Fourier Transform Infrared (FTIR), and ¹H and ¹³C NMR spectroscopies. X-ray diffraction studies on single crystals of 3c, 3e and 3f prove the square planar geometry. Scanning Electron Microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), metal mapping analyses and thermal gravimetric analysis (TGA) were performed to get further insights into the mechanism of the Suzuki–Miyaura cross coupling reactions. Mechanistic studies have revealed that the stability and coordination of the complexes by the CN group are achieved by the removal of pyridine from the complex in catalytic cycles. The presence of the CN group in the (NHC)Pd complexes significantly increased the catalytic activities for both reactions.

Nitrile-functionalized Pd(ii) complexes have evaluated for the Suzuki–Miyaura cross-coupling reactions. The highest TON value was reached for the acylative Suzuki–Miyaura cross-coupling reaction of acyl chlorides with phenylboronic acids.

1,3,5-Triaza-7-phosphaadamantane (PTA) Derived Caged Phosphines for Palladium-Catalyzed Selective Functionalization of Nucleosides and Heteroarenes

Phosphines have, in combination with transition metals, played a pivotal role in the rapid development of efficient catalytic processes. Caged phosphines constitute a class of three-dimensional scaffolds providing unique control over steric and electronic properties. The versatility of the caged phosphine ligands has been demonstrated elegantly by the groups of Verkade, Gonzalvi as well as Stradiotto. Our research group has also been working extensively for the past several years in the development of 1,3,5-triaza-7-phosphaadamantane-based caged ligands and in this personal note we have summarized these applications pertaining to the modification of biologically useful nucleosides and heteroarenes.

C–O Bond Activation as a Strategy in Palladium-Catalyzed Cross-Coupling

The activation of strong C–O bonds in cross-coupling catalysis can open up new oxygenate-based feedstocks and building blocks for complex-molecule synthesis. Although Ni catalysis has been the major focus for cross-coupling of carboxylate-based electrophiles, we recently demonstrated that palladium catalyzes not only difficult C–O oxidative additions but also Suzuki-type cross-couplings of alkenyl carboxylates under mild conditions. We propose that, depending on the reaction conditions, either a typical Pd(0)/(II) mechanism or a redox-neutral Pd(II)-only mechanism can operate. In the latter pathway, C–C bond formation occurs through carbopalladation of the alkene, and C–O cleavage by β-carboxyl elimination.

1 Introduction

2 A Mechanistic Challenge: Activating Strong C–O Bonds

3 Exploiting Vinylogy for C–Cl and C–O Oxidative Additions

4 An Alternative Mechanism for Efficient Cross-Coupling Catalysis

5 Conclusions and Outlook

Practical and scalable synthesis of bench-stable organofluorosilicate salts

Silanes have enjoyed significant success as synthetic tools in the last few decades. In many of the reactions that use silanes, a pentacoordinate silicate is proposed as the reactive intermediate. Despite this, there is no general method to synthesize pentacoordinate fluorosilicates and use them as reagents instead of organo- or alkoxysilanes. Herein, we report the first practical synthesis of organotetrafluorosilicates. The method is tolerant of a number of different functional groups including electrophiles with preferential attack of the fluoride on the silane rather than the electrophile. This transformaton is generally high yielding, even at the mole scale. Furthermore, we demonstrate that organotetrafluorosilicates are both more reactive than the corresponding trialkoxysilanes and more stable under solvolytic conditions. Organotetrafluorosilicates can be used as substrates for a variety of coupling reactions, oxidations, and radical reactions. Overall, organotetrafluorosilicates represent a new platform on which to develop challenging transformations.

A Novel Modified Cross-Coupling of Phenols and Amines Using Dichloroimidazolidinedione (DCID)

Phenols are considered as an ideal alternative to aryl halides as coupling partners in cross-coupling reactions. In the present work a copper-catalyzed cross-coupling of phenols with various aromatic and aliphatic amines for the synthesis of secondary aryl amines using dichloroimidazolidinedione (DCID) as a new and efficient activating agent has been developed. Substituted phenols were compatible with the standard reaction conditions. The two proposed mechanisms, which are based on the oxidation addition of copper with Ar-OMCID (MCID: Monochloroimidazolidinedione), are also discussed.

Palladium-Catalyzed Cross-Couplings by C-O Bond Activation

Although palladium-catalyzed cross-coupling of aryl halides and reactive pseudohalides has revolutionized the way organic molecules are constructed today across various fields of chemistry, comparatively less progress has been made in the palladium-catalyzed cross-coupling of less reactive C-O electrophiles. This is despite the fact that the use of phenols and phenol derivatives as bench-stable cross-coupling partners has been well-recognized to bring about major advantages over aryl halides, such as (1) natural abundance of phenols, (2) avoidance of toxic halides, (3) orthogonal cross-coupling conditions, (4) prefunctionalization of phenolic substrates by electrophilic substitution or C-H functionalization, (5) ready availability of phenols from a different pool of precursors than aryl halides. In this review, we present an overview of recent advances made in the field of palladium-catalyzed cross-coupling of C-O electrophiles with a focus on (1) catalytic systems, (2) reaction type, and (3) class of C-O coupling partners. Although the field has been historically dominated by nickel catalysis, it is now evident that the use of more versatile, more functional group tolerant and highly active palladium catalysts supported by appropriately designed ancillary ligands enables the cross-coupling with improved substrate scope and generality, and likely represents a practical solution to the broadly applicable cross-coupling of various C-O bonds across diverse chemical disciplines. The review covers the period through June 2020.

Transition-metal-free formal cross-coupling of aryl methyl sulfoxides and alcohols via nucleophilic activation of C-S bond

Employment of sulfoxides as electrophiles in cross-coupling reactions remains underexplored. Herein we report a transition-metal-free cross-coupling strategy utilizing aryl(heteroaryl) methyl sulfoxides and alcohols to afford alkyl aryl(heteroaryl) ethers. Two drug molecules were successfully prepared using this protocol as a key step, emphasizing its potential utility in medicinal chemistry. A DFT computational study suggests that the reaction proceeds via initial addition of the alkoxide to the sulfoxide. This adduct facilitates further intramolecular addition of the alkoxide to the aromatic ring wherein charge on the aromatic system is stabilized by the nearby potassium cation. Rate-determining fragmentation then delivers methyl sulfenate and the aryl or heteroaryl ether. This study establishes the feasibility of nucleophilic addition to an appended sulfoxide as a means to form a bond to aryl(heteroaryl) systems and this modality is expected to find use with many other electrophiles and nucleophiles leading to new cross-coupling processes.

Cross-coupling processes without the use of transition metals are challenging to achieve. Here, the authors show a transition-metal-free cross-coupling utilizing aryl(heteroaryl) methyl sulfoxides and alcohols to afford alkyl aryl(heteroaryl) ethers and propose a nucleophilic addition mechanism based on experiments and theory.

Pd-Catalyzed Cross-Coupling of Highly Sterically Congested Enol Carbamates with Grignard Reagents via C-O Bond Activation

The palladium-catalyzed cross-coupling reaction of enol carbamates to construct highly sterically congested alkenyl compounds is presented for the first time. This protocol demonstrates the potential of using thermally stable and highly atom-economic enol electrophiles as building blocks in bulky alkene synthesis. This reaction accommodates a broad substrate scope with excellent Z/E isomer ratios, which also provides a new synthetic pathway for accessing Tamoxifen.

Oxidative addition of activated aryl-carboxylates to Pd(0): divergent reactivity dependant on temperature and structure

With the exception of activated sulfonate esters, oxidative addition of Ar-O bonds to Pd(0) complexes is extremely rare. This has led to a general perception that Pd-catalyzed cross-coupling is not feasible with O-based electrophiles such as aryl esters. We report that pyrone and coumarin esters do undergo oxidative addition to Pd(PCy3)2, with Pd insertion into either the acyl-O or Ar-O bond. Addition of the acyl-O bond to Pd(0) is kinetically favoured and reversible, while addition of the Ar-O bond is thermodynamically favoured. Using a larger and more electron-rich pivalate derivative disfavours acyl-O cleavage, enabling selective oxidative addition of the Ar-O bond and corresponding catalytic cross-coupling.

Palladium-catalyzed cross-coupling of (hetero)aryl or alkenyl sulfonates with aryl titanium as the multi-functional reagent

The first palladium-catalyzed cross-coupling reaction of aryl/heteroaryl and alkenyl mesylates and tosylates with aryl titanium as the multi-functional reagent is reported.

Synthesis of polysubstituted arenes through organocatalytic benzannulation

Polysubstituted arenes serve as ubiquitous structural cores of aromatic compounds with significant applications in chemistry, biological science, and materials science. Among all the synthetic approaches toward these highly functionalized arenes, organocatalytic benzannulation represents one of the most efficient and versatile transformations in the assembly of structurally diverse arene architectures under mild conditions with exceptional chemo-, regio- or stereoselectivities. Thus, the development of new benzannulation reactions through organocatalysis has attracted much attention in the past ten years. This review systemically presents recent advances in the organocatalytic benzannulation strategies, categorized as follows: (1) Brønsted acid-catalysis, (2) secondary amine catalysis, (3) primary amine catalysis, (4) tertiary amine catalysis, (5) tertiary phosphine catalysis, and (6) N-heterocyclic carbene catalysis. Each part is further classified into several types according to the number of carbon atoms contributed by different synthons participating in the cyclization reaction. The reaction mechanisms involved in different benzannulation strategies were highlighted.

Organocatalytic benzannulation represents one of the most efficient transformations for assembling polysubstituted arenes, this review presents recent advances in organocatalytic benzannulation strategies to construct functionalized benzenes.

Synthetic and mechanistic aspects of sulfonyl migrations

Sulfonyl migrations, frequently described as ‘unusual’ or ‘unexpected’, from the last 20 years, including 1,2-, 1,3-, 1,4-, 1,5-, 1,6- and 1,7-sulfonyl shifts, through either radical or polar processes, either inter- or intramolecularly are reviewed.