meta-Selective C–H Bond Alkylation with Secondary Alkyl Halides

Experimental and Computational Exploration of para-Selective Silylation with a Hydrogen-Bonded Template

The regioselective conversion of C-H bonds into C-Si bonds is extremely important owing to the natural abundance and non-toxicity of silicon. Classical silylation reactions often suffer from poor functional group compatibility, low atom economy, and insufficient regioselectivity. Herein, we disclose a template-assisted method for the regioselective para silylation of toluene derivatives. A new template was designed, and the origin of selectivity was analyzed experimentally and computationally. An interesting substrate-solvent hydrogen-bonding interaction was observed. Kinetic, spectroscopic, and computational studies shed light on the reaction mechanism. The synthetic significance of this strategy was highlighted by the generation of a precursor of a potential lipophilic bioisostere of γ-aminobutyric acid (GABA), various late-stage diversifications, and by mimicking enzymatic transformations.

meta-Selective C-H Borylation of Benzylamine-, Phenethylamine-, and Phenylpropylamine-Derived Amides Enabled by a Single Anionic Ligand

Selective functionalization at the meta position of arenes remains a significant challenge. In this work, we demonstrate that a single anionic bipyridine ligand bearing a remote sulfonate group enables selective iridium-catalyzed borylation of a range of common amine-containing aromatic molecules at the arene meta position. We propose that this selectivity is the result of a key hydrogen bonding interaction between the substrate and catalyst. The scope of this meta-selective borylation is demonstrated on amides derived from benzylamines, phenethylamines and phenylpropylamines; amine-containing building blocks of great utility in many applications.

Meta-Selective C-H Arylation of Aromatic Alcohols with a Readily Attachable and Cleavable Molecular Scaffold

The first example of meta-selective C-H arylations of arene alcohol-based substrates is described. The strategy involves the combination of the transient norbornene strategy with the quinoline-based acetal scaffold to achieve the formation of biaryl compounds. Both a two-step meta-arylation/scaffold cleavage process and a total telescoping procedure are described, highlighting the convenient attributes of attachment, removal, and recovery of the acetal scaffold. Moreover, the meta-arylated compounds can be further derivatized via ortho-selective functionalizations. These processes establish a foundation for catalytic polyfunctionalization of alcohol-based compounds.

Experimental and Computational Development of a Conformationally Flexible Template for the meta-C-H Functionalization of Benzoic Acids

A conformationally flexible template for the meta-C-H olefination of benzoic acids was designed through both experimental and computational efforts. The newly designed template favors a silver-palladium heterodimer low barrier transition state, and demonstrates that it is feasible to lengthen templates so as to achieve meta-selectivity when the distance between the functional handle of the native substrate and target C-H bond decreases. Analysis of the ortho-, meta-, and para-C-H cleavage transition states determined that the new template conformation optimizes the interaction between the nitrile and palladium-silver dimer in the meta-transition state, enabling palladium to cleave meta-C-H bonds with moderate-to-good yields and generally high regioselectivity. Regioselectivity is governed exclusively by the template, and kinetic experiments reveal that there is a 4-fold increase in rate in the presence of monoprotected amino acid ligands. Using a Boltzmann distribution of all accessible C-H activation transition states, it is possible to computationally predict meta-selectivity in a number of investigated templates with reasonable accuracy. Structural and distortion energies reported may be used for the further development of templates for meta-C-H activation of hitherto unexplored arene substrates.

meta-C-H Arylation and Alkylation of Benzylsulfonamide Enabled by a Palladium(II)/Isoquinoline Catalyst

Palladium(II)-catalyzed meta-C-H arylation and alkylation of benzylsulfonamide using 2-carbomethoxynorbornene (NBE-CO2 Me) as a transient mediator are realized by using a newly developed electron-deficient directing group and isoquinoline as a ligand. This protocol features broad substrate scope and excellent functional-group tolerance. The meta-substituted benyzlsulfonamides can be readily transformed into sodium sulfonates, sulfonate esters, and sulfonamides, as well as styrenes by Julia-type olefination. The unique impact of the isoquinoline ligand underscores the importance of subtle matching between ligands and the directing groups.

A Modular Flow Design for the meta-Selective C-H Arylation of Anilines

Described herein is an effective and practical modular flow design for the meta-selective C-H arylation of anilines. The design consists of four continuous-flow modules (i.e., diaryliodonium salt synthesis, meta-selective C-H arylation, inline copper extraction, and aniline deprotection) which can be operated either individually or consecutively to provide direct access to meta-arylated anilines. With a total residence time of 1 hour, the desired product could be obtained in high yield and excellent purity without the need for column chromatography, and the residual copper content meets the standards for parenterally administered pharmaceutical substances.

Ruthenium(II)-catalysed remote C-H alkylations as a versatile platform to meta-decorated arenes

The full control of positional selectivity is of prime importance in C-H activation technology. Chelation assistance served as the stimulus for the development of a plethora of ortho-selective arene functionalizations. In sharp contrast, meta-selective C-H functionalizations continue to be scarce, with all ruthenium-catalysed transformations currently requiring difficult to remove or modify nitrogen-containing heterocycles. Herein, we describe a unifying concept to access a wealth of meta-decorated arenes by a unique arene ligand effect in proximity-induced ruthenium(II) C-H activation catalysis. The transformative nature of our strategy is mirrored by providing a step-economical entry to a range of meta-substituted arenes, including ketones, acids, amines and phenols-key structural motifs in crop protection, material sciences, medicinal chemistry and pharmaceutical industries.

Ligand-Enabled Auxiliary-Free meta-C-H Arylation of Phenylacetic Acids

The meta-C-H arylation of free phenylacetic acid was realized using 2-carbomethoxynorbornene (NBE-CO2 Me) as a transient mediator. Both the modified norbornene and the mono-protected 3-amino-2-hydroxypyridine type ligand are crucial for this auxiliary-free meta-C-H arylation reaction. A series of phenylacetic acids, including mandelic acid and phenylglycine, react smoothly with various aryl iodides to provide the meta-arylated products in high yields.

Copper-catalyzed direct alkylation of heteroarenes

An efficient and broadly applicable process is reported for the direct alkylation of C-H bonds in heteroarenes, privileged scaffolds in many areas of science. This reaction is based on the copper-catalyzed addition of alkyl radicals generated from activated secondary and tertiary alkyl bromides to a wide range of arenes, including furans, thiophenes, pyrroles, and their benzo-fused derivatives, as well as coumarins and quinolinones.

Ligand-Promoted meta-C-H Functionalization of Benzylamines

Meta-C-H functionalization of benzylamines has been developed using a PdII /transient mediator strategy. Using 2-pyridone ligands and 2-carbomethoxynorbornene (NBE-CO2 Me) as the mediator, arylation, amination, and chlorination of benzylamines are realized. This protocol features a broad substrate scope and is compatible with heterocylic coupling partners. Moreover, the loading of the Pd can be lowered to 2.5 mol % by using the optimal ligand.

Rhodium-Catalyzed meta-C-H Functionalization of Arenes

Rhodium-catalyzed ortho-C-H functionalization is well known in the literature. Described herein is the Xphos-supported rhodium catalysis of meta-C-H olefination of benzylsulfonic acid and phenyl acetic acid frameworks with the assistance of a para-methoxy-substituted cyano phenol as the directing group. Complete mono-selectivity is observed for both scaffolds. A wide range of olefins and functional groups attached to arene are tolerated in this protocol.

Remote site-selective C-H activation directed by a catalytic bifunctional template

In chemical syntheses, the activation of carbon-hydrogen (C-H) bonds converts them directly into carbon-carbon or carbon-heteroatom bonds without requiring any prior functionalization. C-H activation can thus substantially reduce the number of steps involved in a synthesis. A single specific C-H bond in a substrate can be activated by using a 'directing' (usually a functional) group to obtain the desired product selectively. The applicability of such a C-H activation reaction can be severely curtailed by the distance of the C-H bond in question from the directing group, and by the shape of the substrate, but several approaches have been developed to overcome these limitations. In one such approach, an understanding of the distal and geometric relationships between the functional groups and C-H bonds of a substrate has been exploited to achieve meta-selective C-H activation by using a covalently attached, U-shaped template. However, stoichiometric installation of this template has not been feasible in the absence of an appropriate functional group on which to attach it. Here we report the design of a catalytic, bifunctional nitrile template that binds a heterocyclic substrate via a reversible coordination instead of a covalent linkage. The two metal centres coordinated to this template have different roles: one reversibly anchors substrates near the catalyst, and the other cleaves remote C-H bonds. Using this strategy, we demonstrate remote, site-selective C-H olefination of heterocyclic substrates that do not have the necessary functional groups for covalently attaching templates.

Palladium-Catalyzed Pyrazole-Directed sp3 C-H Bond Arylation for the Synthesis of β-Phenethylamines

We have developed a method for palladium-catalyzed, pyrazole-directed sp³ C-H bond arylation by aryl iodides. The reaction employs a Pd(OAc)₂ catalyst at 5-10 mol % loading and silver(I) oxide as a halide-removal agent, and it proceeds in acetic acid or acetic acid/hexafluoroisopropanol solvent. Ozonolysis of the pyrazole moiety affords pharmaceutically important β-phenethylamines.

Rh(III)-Catalyzed meta-C-H Olefination Directed by a Nitrile Template

A range of Rh(III)-catalyzed ortho-C-H functionalizations have been developed; however, extension of this reactivity to remote C-H functionalizations through large-ring rhodacyclic intermediates has yet to be demonstrated. Herein we report the first example of the use of a U-shaped nitrile template to direct Rh(III)-catalyzed remote meta-C-H activation via a postulated 12-membered macrocyclic intermediate. Because the ligands used for Rh(III) catalysts are significantly different from those of Pd(II) catalysts, this offers new opportunities for future development of ligand-promoted meta-C-H activation reactions.