Biaryl Synthesis via C–H Bond Activation

Synthesis of 1-(4-Bromobenzoyl)-1,3-dicyclohexylurea and Its Arylation via Readily Available Palladium Catalyst-Their Electronic, Spectroscopic, and Nonlinear Optical Studies via a Computational Approach

In this study, we reported the synthesis of 1-(4-bromobenzoyl)-1,3-dicyclohexylurea by the reaction of DCC (N,N'-dicyclohexylcarbodiimide) with 4-bromobenzoic acid. Subsequently, we further synthesized a new series of 1-(4-arylbenzoyl)-1,3-dicyclohexylurea (5a-g) derivatives using a Suzuki cross-coupling reaction between 1-(4-bromobenzoyl)-1,3-dicyclohexylurea (3) and various aryl/heteroaryl boronic acids (4). Thus, density functional theory (DFT) calculations have been performed to examine the electronic structure of the synthesized compounds (3, 5a-g) and to calculate their spectroscopic data. Moreover, optimized geometries and thermodynamic properties, such as frontier molecular orbitals (HOMO, LUMO), molecular electrostatic potential surfaces, and reactivity descriptors, were also calculated at the PBE0-D3BJ/def2-TZVP/SMD1,4-dioxane level of theory to validate the structures of the synthesized compounds.

Electrochemical synthesis of biaryls by reductive extrusion from N,N'-diarylureas

The synthesis of biaryl compounds by the transition-metal free coupling of arenes is an important contemporary challenge, aiming to avoid the toxicity and cost profiles associated with the metal catalysts commonly used in the synthesis of these pharmaceutically relevant motifs. In this paper, we describe an electrochemical approach to the synthesis of biaryls in which aniline derivatives are coupled through the formation and reduction of a temporary urea linkage. The conformational alignment of the arenes in the N,N'-diaryl urea intermediates promotes C-C bond formation following single-electron reduction. Our optimized conditions are suitable for the synthesis of a variety of biaryls, including sterically hindered examples carrying ortho-substituents, representing complementary reactivity to most metal catalysed methods.

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.

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.

Synthesis of Indolo[2,3-c]quinolin-6(7H)-ones and Antimalarial Isoneocryptolepine. Computational Study on the Pd-Catalyzed Intramolecular C-H Arylation

The synthesis of variously substituted indolo[2,3-c]quinolin-6(7H)-ones was developed via Pd-catalyzed intramolecular C-H arylation. This method highlights a strategy for preparing indoloquinoline precursors bearing versatile functional groups and provides a new approach for the synthesis of antimalarial isoneocryptolepine analogues. The plausible ring closure mechanism was examined with quantum chemical calculations, where a trigonal bipyramidal concerted metalation-deprotonation transition state is presumable.

meta-Selective olefination of fluoroarenes with alkynes using CO2 as a traceless directing group

Over the last few decades C-H olefination has received significant interest, due to the importance and usefulness of aryl olefins both as synthetic targets and intermediates. While a wide range of ortho-olefination protocols have been developed, only a small number of meta-olefinations are currently available. Importantly, the most common approach to meta-olefination, using a large meta-directing template, is not suitable for substrates such as fluorobenzenes, which cannot be derivatised. We report that the meta-selective olefination of fluoroarenes can be achieved via the use of CO2 as a traceless directing group, which can be easily installed and removed in a one-pot process. Furthermore, this approach avoids the use of stoichiometric Ag(i)-salts, commonly used in C-H olefinations, and affords complete meta- over ortho/para-regioselectivity.

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

Diverse C-H functionalizations catalyzed by Pd employ Ag(I) salts added as halide abstractors or oxidants. Recent reports have shown that Ag can also perform the crucial C-H activation step in several of these functionalizations. However, all of these processes are limited by the wasteful requirement for (super)stoichiometric Ag(I) salts. Herein, we report the development of a Ag/Pd cocatalyzed direct arylation of (fluoroarene) chromium tricarbonyl complexes with bromoarenes. The small organic salt, NMe₄OC(CF₃)₃, added as a halide abstractor, enables the use of a catalytic amount of Ag, reversing the rapid precipitation of AgBr. We have shown through H/D scrambling and kinetic studies that a (PR₃)Ag-alkoxide is responsible for C-H activation, a departure from previous studies with Ag carboxylates. Furthermore, the construction of biaryls directly from the simple arene is achieved via a one-pot chromium tricarbonyl complexation/C-H arylation/decomplexation sequence using (pyrene)Cr(CO)₃ as a Cr(CO)₃ donor.

Using sodium acetate for the synthesis of [Au(NHC)X] complexes

Sodium acetate enables the synthesis of [Au(NHC)Cl] complexes, as well as their Au-alkynyl and -thiolato derivatives in high yields, under air and in technical grade, green solvents. The mild synthetic methods are also investigated computationally.

Transition metal-free cross-dehydrogenative arylation of unactivated benzylic C–H bonds

The cross-dehydrogenative arylation of benzylic C–H bonds with arenes provides straightforward access to synthetically useful 1,1-diarylmethanes, from readily available starting materials.

Benzoate Cyclometalation Enables Oxidative Addition of Haloarenes at a Ru(II) Center

The first Ru(II)-catalyzed arylation of substrates without a directing group was recently developed. Remarkably, this process only worked in the presence of a benzoate additive, found to be crucial for the oxidative addition step at Ru(II). However, the exact mode of action of the benzoate was unknown. Herein, we disclose a mechanistic study that elucidates the key role of the benzoate salt in the C–H arylation of fluoroarenes with aryl halides. Through a combination of rationally designed stoichiometric experiments and DFT studies, we demonstrate that the aryl–Ru(II) species arising from initial C–H activation of the fluoroarene undergoes cyclometalation with the benzoate to generate an anionic Ru(II) intermediate. The enhanced lability of this intermediate, coupled with the electron-rich anionic Ru(II) metal center renders the oxidative addition of the aryl halide accessible. The role of an additional (NMe₄)OC(CF₃)₃ additive in facilitating the overall arylation process is also shown to be linked to a shift in the C–H pre-equilibrium associated with benzoate cyclometalation.

Cyclometallated ruthenium catalyst enables late-stage directed arylation of pharmaceuticals

Biaryls are ubiquitous core structures in drugs, agrochemicals and organic materials that have profoundly improved many aspects of our society. Although traditional cross-couplings have made practical the synthesis of many biaryls, C-H arylation represents a more attractive and cost-effective strategy for building these structural motifs. Furthermore, the ability to install biaryl units in complex molecules via late-stage C-H arylation would allow access to valuable structural diversity, novel chemical space and intellectual property in only one step. However, known C-H arylation protocols are not suitable for substrates decorated with polar and delicate functionalities, which are commonly found in molecules that possess biological activity. Here we introduce a class of ruthenium catalysts that display a unique efficacy towards late-stage arylation of heavily functionalized substrates. The design and development of this class of catalysts was enabled by a mechanistic breakthrough on the Ru(II)-catalysed C-H arylation of N-chelating substrates with aryl (pseudo)halides, which has remained poorly understood for nearly two decades.

Transition-Metal-Free Decarboxylative Iodination: New Routes for Decarboxylative Oxidative Cross-Couplings

Constructing products of high synthetic value from inexpensive and abundant starting materials is of great importance. Aryl iodides are essential building blocks for the synthesis of functional molecules, and efficient methods for their synthesis from chemical feedstocks are highly sought after. Here we report a low-cost decarboxylative iodination that occurs simply from readily available benzoic acids and I₂. The reaction is scalable and the scope and robustness of the reaction is thoroughly examined. Mechanistic studies suggest that this reaction does not proceed via a radical mechanism, which is in contrast to classical Hunsdiecker-type decarboxylative halogenations. In addition, DFT studies allow comparisons to be made between our procedure and current transition-metal-catalyzed decarboxylations. The utility of this procedure is demonstrated in its application to oxidative cross-couplings of aromatics via decarboxylative/C–H or double decarboxylative activations that use I₂ as the terminal oxidant. This strategy allows the preparation of biaryls previously inaccessible via decarboxylative methods and holds other advantages over existing decarboxylative oxidative couplings, as stoichiometric transition metals are avoided.