Ag/Pd Cocatalyzed Direct Arylation of Fluoroarene Derivatives with Aryl Bromides

Umpolung-Enabled Divergent Dearomative Carbonylations

Although dearomative functionalizations enable the direct conversion of flat aromatics into precious three-dimensional architectures, the case for simple arenes remains largely underdeveloped owing to the high aromatic stabilization energy. We herein report a dearomative sequential addition of two nucleophiles to arene π-bonds through umpolung of chromium-arene complexes. This mode enables divergent dearomative carbonylation reactions of benzene derivatives by tolerating various nucleophiles in combination with alcohols or amines under CO-gas-free conditions, thus providing modular access to functionalized esters or amides. The tunable synthesis of 1,3- or 1,4-cyclohexadienes as well as the construction of carbon quaternary centers further highlight the versatility of this dearomatization. Diverse late-stage modifications and derivatizations towards synthetically challenging and bioactive molecules reveal the synthetic utility. A possible mechanism was proposed based on control experiments and intermediate tracking.

Control over Anion Coordination on Pd(II), Cu(I), and Ag(I) with Regioisomeric Phosphine-Carboxylate Ligands

The coordination of anionic donors is involved at various stages of catalytic cycles in transition-metal catalysis, but control over the spatial positioning of anions around a metal center is a challenge in coordination chemistry. Here we show that regioisomeric phosphine-carboxylate ligands provide spatial anion control on palladium(II) centers by favoring either κ2, cis-κ1, or trans-κ1 coordination of the carboxylate donor. Additionally, the palladium(II) carboxylates, which contain a methyl donor, upon protonation, deliver metal-alkyl complexes that feature a coordinated carboxylic acid. Such complexes can be considered as models for the minima that follow the concerted metalation-deprotonation transition state for C-H activation. The predictability of the coordination modes is further demonstrated on silver(I) and copper(I) centers, for which less common structures of mononuclear and dinuclear complexes can be obtained by using spatial anion control. Our results demonstrate the potential for spatial control over carboxylate anions in coordination chemistry.

C-H arylation of thiopyran derivatives with aryl halides

A C-H arylation of thiopyran derivatives with aryl halides has been developed. Under the catalysis of Pd(OAc)2/Ag2CO3, the C-H arylation takes place at the α-position of the thiopyran ring. When dibromo-substituted compounds are used as reactants, double C-H arylations may occur on the same thiopyran ring at its α- and β-positions.

Applications of Transition Metal-Catalyzed ortho-Fluorine-Directed C-H Functionalization of (Poly)fluoroarenes in Organic Synthesis

The synthesis of organic compounds efficiently via fewer steps but in higher yields is desirable as this reduces energy and reagent use, waste production, and thus environmental impact as well as cost. The reactivity of C-H bonds ortho to fluorine substituents in (poly)fluoroarenes with metal centers is enhanced relative to meta and para positions. Thus, direct C-H functionalization of (poly)fluoroarenes without prefunctionalization is becoming a significant area of research in organic chemistry. Novel and selective methodologies to functionalize (poly)fluorinated arenes by taking advantage of the reactivity of C-H bonds ortho to C-F bonds are continuously being developed. This review summarizes the reasons for the enhanced reactivity and the consequent developments in the synthesis of valuable (poly)fluoroarene-containing organic compounds.

Direct Heterocycle C-H Alkenylation via Dual Catalysis Using a Palladacycle Precatalyst: Multifactor Optimization and Scope Exploration Enabled by High-Throughput Experimentation

One of the major challenges in developing catalytic methods for C-C bond formation is the identification of generally applicable reaction conditions, particularly if multiple substrate structural classes are involved. Pd-catalyzed direct arylation reactions are powerful transformations that enable direct functionalization of C-H bonds; however, the corresponding direct alkenylation reactions, using vinyl (pseudo) halide electrophiles, are less well developed. Inspired by process development efforts toward GSK3368715, an investigational active pharmaceutical ingredient, we report that a Pd(II) palladacycle derived from tri-tert-butylphosphine and Pd(OAc)2 is an effective single-component precatalyst for a variety of direct alkenylation reactions. High-throughput experimentation identified optimal solvent/base combinations for a variety of HetAr-H substrate classes undergoing C-H activation without the need for cocatalysts or stoichiometric silver bases (e.g., Ag2CO3). We propose this reaction proceeds via a dual cooperative catalytic mechanism, where in situ-generated Pd(0) supports a canonical Pd(0)/(II) cross-coupling cycle and the palladacycle effects C-H activation via CMD in a redox-neutral cycle. In all, 192 substrate combinations were tested with a hit rate of approximately 40% and 24 isolated examples. Importantly, this method was applied to prepare a key intermediate in the synthesis of GSK3368715 on multigram scale.

Opening a Pandora's Flask on a Prototype Catalytic Direct Arylation Reaction of Pentafluorobenzene: The Ag2CO3/Pd(OAc)2/PPh3 System

Direct C-H functionalization reactions have opened new avenues in catalysis, removing the need for prefunctionalization of at least one of the substrates. Although C-H functionalization catalyzed by palladium complexes in the presence of a base is generally considered to proceed by the CMD/AMLA-6 mechanism, recent research has shown that silver(I) salts, frequently used as bases, can function as C-H bond activators instead of (or in addition to) palladium(II). In this study, we examine the coupling of pentafluorobenzene 1 to 4-iodotoluene 2a (and its analogues) to form 4-(pentafluorophenyl)toluene 3a catalyzed by palladium(II) acetate with the commonplace PPh3 ligand, silver carbonate as base, and DMF as solvent. By studying the reaction of 1 with Ag2CO3/PPh3 and with isolated silver (triphenylphosphine) carbonate complexes, we show the formation of C-H activation products containing the Ag(C6F5)(PPh3)n unit. However, analysis is complicated by the lability of the Ag-PPh3 bond and the presence of multiple species in the solution. The speciation of palladium(II) is investigated by high-resolution-MAS NMR (chosen for its suitability for suspensions) with a substoichiometric catalyst, demonstrating the formation of an equilibrium mixture of Pd(Ar)(κ1-OAc)(PPh3)2 and [Pd(Ar)(μ-OAc)(PPh3)]2 as resting states (Ar = Ph, 4-tolyl). These two complexes react stoichiometrically with 1 to form coupling products. The catalytic reaction kinetics is investigated by in situ IR spectroscopy revealing a two-term rate law and dependence on [Pdtot/nPPh3]0.5 consistent with the dissociation of an off-cycle palladium dimer. The first term is independent of [1], whereas the second term is first order in [1]. The observed rates are very similar with Pd(PPh3)4, Pd(Ph)(κ1-OAc)(PPh3)2, and [Pd(Ph)(μ-OAc)(PPh3)]2 catalysts. The kinetic isotope effect varied significantly according to conditions. The multiple speciation of both AgI and PdII acts as a warning against specifying the catalytic cycles in detail. Moreover, the rapid dynamic interconversion of AgI species creates a level of complexity that has not been appreciated previously.

Non-directed C-H arylation of electron-deficient arenes by synergistic silver and Pd3 cluster catalysis

Transition-metal clusters have attracted great attention in catalysis due to their unique reactivity and electronic properties, especially for novel substrate binding and activation modes at the bridging coordination sites of metal clusters. Although palladium complexes have demonstrated outstanding catalytic performance in various transformations, the catalytic behaviors of polynuclear palladium clusters in many important synthetic methodologies remain much less explored so far. Herein, we disclose the use of an atomically defined tri-nuclear palladium (Pd₃Cl) species as a catalyst precursor in Ag(I)-assisted direct C-H arylation with aryl iodides under mild conditions. This catalyst system leads to the formation of synthetically important biaryls in good yields with high site selectivities without the assistance of directing groups.

Multifaceted role of silver salts as ligand scavengers and different behavior of nickel and palladium complexes: beyond halide abstraction

The reaction of [NiArBr(PPh₃)₂] with AgBF₄ brings about the abstraction of both the halide and phosphine from the nickel center by silver. When the reaction is carried out in CH₂Cl₂/toluene a mixture of the cationic aquo derivatives [NiAr(H₂O)(PPh₃)₂]BF₄ (2) and [NiAr(H₂O)₂(PPh₃)]BF₄ (3) is formed, along with AgBr and [Ag(PPh₃)_n]BF₄. When the same reaction is carried out in acetone as the solvent, it leads to the completely different complex [NiAr(κ²-O, O-MeC(O)CH₂C(OH)Me₂)(PPh₃)] (5), bearing a chelating ligand formed by the aldol self-condensation of acetone. Phosphine abstraction by silver is less favorable for the analogous palladium(II) complexes and only occurs if a large excess of AgBF₄ is used. Thus, silver salts can be safely used as halide scavengers for palladium derivatives. However, the generation of cationic Ni complexes from neutral precursors by halide extraction with a silver salt may produce naked species, different than those expected, and highly reactive in certain media.

Site-Selective C-H Arylation of Diverse Arenes Ortho to Small Alkyl Groups

Catalytic systems for direct C-H activation of arenes commonly show preference for electronically activated and sterically exposed C-H sites. Here we show that a range of functionally rich and pharmaceutically relevant arene classes can undergo site-selective C-H arylation ortho to small alkyl substituents, preferably endocyclic methylene groups. The C-H activation is experimentally supported as being the selectivity-determining step, while computational studies of the transition state models indicate the relevance of non-covalent interactions between the catalyst and the methylene group of the substrate. Our results suggest that preference for C(sp² )-H activation next to alkyl groups could be a general selectivity mode, distinct from common steric and electronic factors.

Silver Ions Promoted Palladium-Catalyzed Inactive β-C(sp3)-H Bond Arylation in Batch and Continuous-Flow Conditions

A palladium(II)-catalyzed protocol for inactive β-C(sp³)-H bond functionalization has been first accomplished. The reaction proceeds through five-membered carbocycles for the formation of C-C bonds via the Pd(II)/Pd(IV) cycle. This reaction was carried out with various aryl iodides and benzothiazoles/benzoxazoles/benzimidazoles, which were well-tolerated in this reaction and successfully generated β-C(sp³)-H arylated products. Further implementation of this batch protocol to continuous flow by utilizing a PTFE (polytetrafluoroethylene) capillary reactor enhanced the reaction efficiency and decreased the reaction time (18.4 min) as compared to batch conditions (8 h). Even on the gram scale, the process produced excellent yield with negligible diarylations. Functional group tolerance, a continuous-flow approach, and easy-to-handle reaction conditions make this inactive β-C(sp³)-H bond functionalization protocol very attractive.

Circular Discovery in Small Molecule and Conjugated Polymer Synthetic Methodology

Simple and efficient methods are a key consideration for small molecule and polymer syntheses. Direct arylation polymerization (DArP) is of increasing interest for preparing conjugated polymers as an effective approach compared to conventional cross-coupling polymerizations. As DArP sees broader utilization, advancements are needed to access materials with improved properties and different monomer structures and to improve the scalability of conjugated polymer synthesis. Presented herein are considerations for developing new methods of conjugated polymer synthesis from small molecule transformations, exploring how DArP has successfully used this approach, and presenting how emerging polymerization methodologies are developing similarly. While it is common to adapt small molecule methods to polymerizations, we demonstrate the ways in which information gained from studying polymerizations can inform and inspire greater advancements in small molecule transformations. This circular approach to organic synthetic method development underlines the value of collaboration between small molecule and polymer-based synthetic research groups.

Introduction to Spatial Anion Control for Direct C–H Arylation

C–H activation of functionally rich molecules without the need for directing groups promises shorter organic syntheses and late-stage diversification of molecules for drug discovery. We highlight recent examples of palladium-catalyzed nondirected functionalization of C–H bonds in arenes as limiting substrates with a focus on the development of the concept of spatial anion control for direct C–H arylation.

1 C–H Activation and the CMD Mechanism

2 Nondirected C–H Functionalizations of Arenes as Limiting Substrates

3 Nondirected C–H Arylation

4 Spatial Anion Control for Direct C–H Arylation

5 Coordination Chemistry with Spatial Anion Control

6 Conclusion

Computational Study on Homolytic Bond Energies of the Ag-X (X = C, O, and H) Complexes and Hammett-Type Analysis of Reactivity

Thirty-seven calculation methods were benchmarked against the available experimental bond lengths and energies data regarding the Ag-X bonds. The theoretical protocol PBE0/VDZ//ωB97x-D/mVTZ was found to be capable of accurately predicting the homolytic bond dissociation energies (BDEs) of Ag-X complexes with a precision of 1.9 kcal/mol. With the available method in hand, a wide range of different Ag-X BDEs were estimated. BDE(Ag-CH₂X), BDE(Ag-PhX), BDE(Ag-OPhX), and BDE(Ag-OCOPhX) (X = NH₂, OMe, Me, H, Cl, and NO₂) were found to be in the ranges of 27-47, 51-54, 19-39, and 64-70 kcal/mol, respectively. Subsequently, Hammett-type analysis was carried out with reactivity parameters. Good positive linear relationships were found for BDE of Ag-O bands and decarboxylation barriers of Ag-OCOPhX with the Hammett constant σ. It suggested that electron-donating substituents could promote either the homolytic cleavage of the Ag-OPhX bond to undergo a radical process or Ag-OCOPhX decarboxylation. Moreover, ligand effects on Ag-H bonds were investigated using BDE(Ag-H) and related NPA charges on Ag. In the case of P-ligands, carbene ligands, and other small molecule ligands (i.e., CO, CO₂, and H₂O), a good negative linear relationship was found. In contrast, N-ligands could have a reverse effect. Understanding the intrinsic relationships of BDE(Ag-X) with related reactivity parameters might help gain insights into the structure-reactivity relationships in Ag-X-assisted C-H activation/decarboxylation.

Magnetically recoverable nickel-palladium alloy nanocatalysts for direct C-H arylation reactions

Novel magnetically recoverable nanocatalyst comprising nickel-palladium (NiPd) alloy nanoparticles (NPs) supported on reduced graphene oxide (rGO) modified with cobalt ferrite (CoFe₂O₄) NPs was fabricated for the direct C-H arylation of imidazopyridine, imidazole, indolizine and furan with aryl halides. To prepare the presented catalyst, rGO nanosheets were first modified with as-synthesized CoFe₂O₄ NPs and then the obtained CoFe₂O₄-rGO nanocomposites served as a support material for the synthesis of bimetallic NiPd alloy NPs at various compositions. The obtained CoFe₂O₄-rGO/NiPd nanocatalysts were characterized by many advanced analytical techniques including TEM, STEM-EDS, XRD, XPS, and ICP-MS. Next, to optimize the reaction conditions, CoFe₂O₄-rGO/NiPd nanocatalysts with different alloy compositions and their monometallic counterparts (CoFe₂O₄-rGO/Ni and CoFe₂O₄-rGO/Pd) were initially tested in the direct C-H arylation of imidazopyridine with bromobenzene. Among all tested nanocatalysts under the optimum reaction conditions, CoFe₂O₄-rGO/Ni₂₀Pd₈₀ showed the best catalytic activity in terms of the isolated product yields. The C-H arylation reactions were studied over a broad substrate scope (35 examples from 36 substrates) and gave the related biaryl products in good to excellent yields. Besides a broad substrate scope, the late-stage C-H arylation of zolimidine, a gastroprotective drug, was realized under the optimized reaction conditions. Moreover, the CoFe₂O₄-rGO/Ni₂₀Pd₈₀ nanocatalysts were recovered from the reaction medium using a simple magnet and reused in the C-H arylation reactions up to five consecutive runs without a significant drop in the product yield. This study shows that magnetically recoverable Pd nanoalloys are promising heterogeneous catalysts to be used in sustainable C-H functionalization reactions.

Direct Arylation of Simple Arenes with Aryl Bromides by Synergistic Silver and Palladium Catalysis

The direct, catalytic arylation of simple arenes in small excess with aryl bromides is disclosed. The developed method does not require the assistance of directing groups and relies on a synergistic catalytic cycle in which phosphine-ligated silver complexes cleave the aryl C-H bond, while palladium catalysts enable the formation of the biaryl products. Mechanistic experiments, including kinetic isotope effects, competition experiments, and hydrogen-deuterium exchange, support a catalytic cycle in which cleavage of the C-H bond by silver is the rate-determining step.

Site-Selective Silver-Catalyzed C-H Bond Deuteration of Five-Membered Aromatic Heterocycles and Pharmaceuticals

Catalytic methods for the direct introduction of hydrogen isotopes into organic molecules are essential to the development of improved pharmaceuticals and to the alteration of their absorption, distribution, metabolism, and excretion (ADME) properties. However, the development of homogeneous catalysts for selective incorporation of isotopes in the absence of directing groups under practical conditions remains a long-standing challenge. Here, we show that a phosphine-ligated, silver-carbonate complex catalyzes the site-selective deuteration of C-H bonds in five-membered aromatic heterocycles and active pharmaceutical ingredients that have been resistant to catalytic H/D exchange. The reactions occur with CH₃OD as a low-cost source of the isotope. The silver catalysts react with five-membered heteroarenes lacking directing groups, tolerate a wide range of functional groups, and react in both polar and nonpolar solvents. Mechanistic experiments, including deuterium kinetic isotope effects, determination of kinetic orders, and identification of the catalyst resting state, support C-H bond cleavage from a phosphine-ligated, silver-carbonate intermediate as the rate-determining step of the catalytic cycle.

Faster palladium-catalyzed arylation of simple arenes in the presence of a methylketone: beneficial effect of an a priori interfering solvent in C–H activation

A methyl ketone accelerates the Pd-catalyzed C–H activation of simple arenes. A noninnocent solvent, it reacts in a reversible way without affecting the outcome of the reaction.

DFT study of Ni/NHC-catalyzed C–H alkylation of fluoroarenes with alkenes to synthesize fluorotetralins: mechanism, chemoselectivity of C–H vs. C–F bond activation, and regio- and enantioselectivities of C–H bond activation

DFT calculations clarified the mechanism of Ni/NHC-catalyzed C–H alkylation of alkene tethered fluoroarene and rationalized enantio-, regio- and chemoselectivities.

Spatial Anion Control on Palladium for Mild C-H Arylation of Arenes

C-H arylation of arenes without the use of directing groups is a challenge, even for simple molecules, such as benzene. We describe spatial anion control as a concept for the design of catalytic sites for C-H bond activation, thereby enabling nondirected C-H arylation of arenes at ambient temperature. The mild conditions enable late-stage structural diversification of biologically relevant small molecules, and site-selectivity complementary to that obtained with other methods of arene functionalization can be achieved. These results reveal the potential of spatial anion control in transition-metal catalysis for the functionalization of C-H bonds under mild conditions.

Temperature-modulated selective C(sp3)-H or C(sp2)-H arylation through palladium catalysis

Transition metal-catalysed C–H bond functionalisations have been extensively developed in organic and medicinal chemistry. Among these catalytic approaches, the selective activation of C(sp³)–H and C(sp²)–H bonds is particularly appealing for its remarkable synthetic versatility, yet it remains highly challenging. Herein, we demonstrate the first example of temperature-dependent selective C–H functionalisation of unactivated C(sp³)–H or C(sp²)–H bonds at remote positions through palladium catalysis using 7-pyridyl-pyrazolo[1,5-a]pyrimidine as a new directing group. At 120 °C, C(sp³)–H arylation was triggered by the chelation of a rare [6,5]-fused palladacycle, whereas at 140 °C, C(sp²)–H arylation proceeded instead through the formation of a 16-membered tetramer containing four 7-pyridyl-pyrazolo[1,5-a]pyrimidine–palladium chelation units. The subsequent mechanistic study revealed that both C–H activations shared a common 6-membered palladacycle intermediate, which was then directly transformed to either the [6,5]-fused palladacycle for C(sp³)–H activation at 120 °C or the tetramer for C(sp²)–H arylation at 140 °C with catalytic amounts of Pd(OAc)₂ and AcOH. Raising the temperature from 120 °C to 140 °C can also convert the [6,5]-fused palladacycle to the tetramer with the above-mentioned catalysts, hence completing the C(sp²)–H arylation ultimately.

Unprecedented 16-membered tetramer or [6,5]-fused palladacycle, mutually shadowboxing-like transformed from the shared common intermediate, accomplishes the Pd-catalysed temperature-dependent selective arylation of C(sp²)–H or C(sp³)–H.