Deuteration of Arenes via Pd-Catalyzed C-H Activation: A Lesson in Nondirected C-H Activation, Isotopic Labeling, and NMR Characterization.

Isotopic labeling is an important tool in medicinal research, metabolomics, and for understanding reaction mechanisms. In this context, transition metal-catalyzed C-H activation has emerged as a key technology for deuterium incorporation via hydrogen isotope exchange. A detailed and easy-to-implement experimental procedure for a nondirected arene deuteration has been developed that exclusively uses commercial equipment and chemicals.

Palladium(II)-Catalyzed Nondirected Late-Stage C(sp)-H Deuteration of Heteroarenes Enabled Through a Multi-Substrate Screening Approach.

The importance of deuterium labelling in a variety of applications, ranging from mechanistic studies to drug-discovery, has spurred immense interest in the development of new methods for its efficient incorporation in organic, and especially in bioactive molecules. The five-membered heteroarenes at the center of this work are ubiquitous motifs in bioactive molecules and efficient methods for the deuterium labelling of these compounds are therefore highly desirable. However, the profound differences in chemical properties encountered between different heteroarenes hamper the development of a single set of broadly applicable reaction conditions, often necessitating a separate optimization campaign for a given type of heteroarene.

Controlling Reactivity and Selectivity in the Nondirected C-H Activation of Arenes with Palladium.

ConspectusAromatic structures are widespread motifs throughout organic chemistry, and C-H activation has been recognized as a major tool for enabling their sustainable and efficient functionalization. Through C-H activation, arenes can be modified without the need for prefunctionalization, leading to inherent atom- and step-economic advantages over traditional methods. However, for the development of synthetically useful methods, several hurdles have to be overcome.

Preparative Scale Applications of C-H Activation in Medicinal Chemistry.

C-H activation is an attractive methodology to increase molecular complexity without requiring substrate prefunctionalization. In contrast to well-established cross-coupling methods, C-H activation is less explored on large scales and its use in the production of pharmaceuticals faces substantial hurdles. However, the inherent advantages, such as shorter synthetic routes and simpler starting materials, motivate medicinal chemists and process chemists to overcome these challenges, and exploit C-H activation steps for the synthesis of pharmaceutically relevant compounds.

Charge-controlled Pd catalysis enables the -C-H activation and olefination of arenes.

The regioselective C-H activation of arenes remains one of the most promising techniques for accessing highly important functionalized motifs. Such functionalizations can generally be achieved through directed and non-directed processes. The directed approach requires a covalently attached directing group (DG) on the substrate to induce reactivity and selectivity and therefore intrinsically leaves a functional group at the point of attachment within the molecule, even after the tailored DG has been removed.

Sterically controlled isodesmic late-stage C-H iodination of arenes.

Aryl iodides are key motifs in organic chemistry due to their versatility as linchpins in metal-mediated cross-coupling reactions for synthesis and drug discovery. These scaffolds are typically prepared indirectly from prefunctionalized starting materials or electrophilic aromatic iodination protocols. These methods are limited to specific regioisomers by their inherent selectivities and/or the availability of the required starting materials.

A Modular Olefination of Aldehydes with Thiols as Coupling Partners.

Olefins range amongst the most important motifs in organic chemistry. Hence, the development of novel olefin syntheses has remained a constant field of research in synthetic chemistry to date. Herein, we report the development of a modular olefination that converts aldehydes into olefins with thiols as reaction partners.

Silver-Free C-H Activation: Strategic Approaches towards Realizing the Full Potential of C-H Activation in Sustainable Organic Synthesis.

The activation of carbon-hydrogen bonds is considered as one of the most attractive techniques in synthetic organic chemistry because it bears the potential to shorten synthetic routes as well as to produce complementary product scopes compared to traditional synthetic strategies. However, many current methods employ silver salts as additives, leading to stoichiometric metal waste and thereby preventing the full potential of C-H activation to be exploited. Therefore, the development of silver-free protocols has recently received increasing attention.

Palladium-Catalyzed Nondirected Late-Stage C-H Deuteration of Arenes.

We describe a palladium-catalyzed nondirected late-stage deuteration of arenes. Key aspects include the use of DO as a convenient and easily available deuterium source and the discovery of highly active N,N-bidentate ligands containing an -acylsulfonamide group. The reported protocol enables high degrees of deuterium incorporation via a reversible C-H activation step and features extraordinary functional group tolerance, allowing for the deuteration of complex substrates.

Late-Stage β-C(sp)-H Deuteration of Carboxylic Acids.

Carboxylic acids are highly abundant in bioactive molecules. In this study, we describe the late-stage β-C(sp)-H deuteration of free carboxylic acids. On the basis of the finding that C-H activation with our catalysts is reversible, the de-deuteration process was first optimized.

Mechanism of the Arene-Limited Nondirected C-H Activation of Arenes with Palladium*.

Recently palladium catalysts have been discovered that enable the directing-group-free C-H activation of arenes without requiring an excess of the arene substrate, thereby enabling methods for the late-stage modification of complex organic molecules. The key to success has been the use of two complementary ligands, an N-acyl amino acid and an N-heterocycle. Detailed experimental and computational mechanistic studies on the dual-ligand-enabled C-H activation of arenes have led us to identify the catalytically active species and a transition state model that explains the exceptional activity and selectivity of these catalysts.

Direct Synthesis of Unsymmetrical Dithioacetals.

Sabine Bognar and Manuel van Gemmeren from the WWU Münster have been invited to contribute the cover of this issue. The image depicts a volcano to symbolize the general theme of sulfur chemistry, whereas the piles of rubble reflect the selectivity of the reaction. Read the full text of the article at 10.

Direct Synthesis of Unsymmetrical Dithioacetals.

Dithioacetals are a frequently used motif in synthetic organic chemistry and have recently seen increasing attention as structural motif in promising antiviral agents against plant pathogens. Most existing reports, however, only discuss symmetrical dithioacetals. Examples of mixed dithioacetals are scarce and no general method for the selective synthesis of these compounds exists.

Catalyst-Controlled Regiodivergent C-H Alkynylation of Thiophenes*.

Alkynes are highly attractive motifs in organic synthesis due to their presence in natural products and bioactive molecules as well as their versatility in a plethora of subsequent transformations. A common procedure to insert alkynes into (hetero)arenes, such as the thiophenes studied herein, consists of a halogenation followed by a Sonogashira cross-coupling. The regioselectivity of this approach depends entirely on the halogenation step.

Direct β- and γ-C(sp )-H Alkynylation of Free Carboxylic Acids*.

In this study we report the identification of a novel class of ligands for palladium-catalyzed C(sp )-H activation that enables the direct alkynylation of free carboxylic acid substrates. In contrast to previous synthetic methods, no introduction/removal of an exogenous directing group is required. A broad scope of acids including both α-quaternary and challenging α-non-quaternary can be used as substrates.

Sterically Controlled C-H Olefination of Heteroarenes.

The regioselective functionalization of heteroarenes is a highly attractive synthetic target due to the prevalence of multiply substituted heteroarenes in nature and bioactive compounds. Some substitution patterns remain challenging: While highly efficient methods for the C2-selective olefination of 3-substituted five-membered heteroarenes have been reported, analogous methods to access the 5-olefinated products have remained limited by poor regioselectivities and/or the requirement to use an excess of the valuable heteroarene starting material. Herein we report a sterically controlled C-H olefination using heteroarenes as the limiting reagent.

Ligand-Enabled γ-C(sp )-H Olefination of Free Carboxylic Acids.

We report the ligand-enabled C-H activation/olefination of free carboxylic acids in the γ-position. Through an intramolecular Michael addition, δ-lactones are obtained as products. Two distinct ligand classes are identified that enable the challenging palladium-catalyzed activation of free carboxylic acids in the γ-position.

Sterically Controlled Late-Stage C-H Alkynylation of Arenes.

Phenylacetylenes are key structural motifs in organic chemistry, which find widespread applications in bioactive molecules, synthetic intermediates, functional materials, and reagents. These molecules are typically prepared from prefunctionalized starting materials, e.g.

Direct β-C(sp)-H Acetoxylation of Aliphatic Carboxylic Acids.

The controlled construction of defined oxidation patterns is one of the key aspects in the synthesis of natural products and bioactive molecules. Towards this goal, we herein report a novel protocol for the Pd-catalyzed direct β-C(sp)-H acetoxylation of aliphatic carboxylic acids. The protocol enables the use of free carboxylic acids in one step and without the need of introducing specialized strong directing groups.

The Direct Conversion of α-Hydroxyketones to Alkynes.

Alkynes are highly important functional groups in organic chemistry, both as part of target structures and as versatile synthetic intermediates. In this study, a protocol for the direct conversion of α-hydroxyketones to alkynes is reported. In combination with the variety of synthetic methods that generate the required starting materials by forming the central C-C bond, it enables a highly versatile fragment coupling approach toward alkynes.

Arene-Limited Nondirected C-H Activation of Arenes.

The nondirected C(sp )-H activation of simple arenes has advanced significantly in recent years through the discovery of new catalyst systems that are able to perform transformations with the arene as the limiting reagent. Important developments in catalyst and ligand design that have improved reactivity and selectivity are reviewed.

Dual Ligand-Enabled Nondirected C-H Olefination of Arenes.

The application of the Pd-catalyzed oxidative C-H olefination of arenes, also known as the Fujiwara-Moritani reaction, has traditionally been limited by the requirement for directing groups on the substrate or the need to use the arene in large excess, typically as a (co)solvent. Herein the development of a catalytic system is described that, through the combined action of two complementary ligands, makes it possible to use directing-group-free arenes as limiting reagents for the first time. The reactions proceed under a combination of both steric and electronic control and enable the application of this powerful reaction to valuable arenes, which cannot be utilized in excess.

Pd-Catalyzed β-C(sp )-H Arylation of Propionic Acid and Related Aliphatic Acids.

A generally applicable Pd-catalyzed protocol for the β-C(sp )-H arylation of propionic acid and related α-branched aliphatic acids is reported. Enabled by the use of N-acetyl-β-alanine as ligand our protocol delivers a broad range of arylation products. Notably, the highly challenging substrate, propionic acid, which lacks any acceleration through the Thorpe-Ingold effect, can be employed as substrate with synthetically useful yields.

Switchable Site-Selective Catalytic Carboxylation of Allylic Alcohols with CO.

A switchable site-selective catalytic carboxylation of allylic alcohols has been developed in which CO is used with dual roles, both facilitating C-OH cleavage and as a C source. This protocol is characterized by its mild reaction conditions, absence of stoichiometric amounts of organometallic reagents, broad scope, and exquisite regiodivergency which can be modulated by the type of ligand employed.

1,1,3,3-Tetratriflylpropene (TTP): A Strong, Allylic C-H Acid for Brønsted and Lewis Acid Catalysis.

Tetratrifylpropene (TTP) has been developed as a highly acidic, allylic C-H acid for Brønsted and Lewis acid catalysis. It can readily be obtained in two steps and consistently shows exceptional catalytic activities for Mukaiyama aldol, Hosomi-Sakurai, and Friedel-Crafts acylation reactions. X-ray analyses of TTP and its salts confirm its designed, allylic structure, in which the negative charge is delocalized over four triflyl groups.