Sterically controlled C-H alkenylation of pyrroles and thiophenes

Palladium Catalyzed C3-(sp2)-H Alkenylation of Pyrroles via a Benzothiazole Directing Group: A Direct Access to Organic Thermally Activated Delayed Fluorescence Materials

A Pd (II)-catalyzed direct C3-(sp2)-H alkenylation of heteroarenes using benzothiazole as a directing group was successfully achieved. A wide range of 2-N-alkylpyrroles undergo an oxidative coupling with a variety of acrylates to furnish highly regio- and chemoselective E-alkenylation products at the C3 position. An important intermediate complex has been isolated and characterized so as to have an insight into the mechanism. This convenient protocol proved to be practical to access thermally activated delayed fluorescence materials (TADF). These molecules proved to be blue-emitting TADF materials (∼ms lifetime). A detailed and systematic investigation has been carried out to study the photophysical properties, and this has been further validated by the time-dependent density functional theory calculations.

Palladium(II)-Catalyzed Nondirected Late-Stage C(sp2)-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. In this study we describe the use of a multi-substrate screening approach to identify optimal reaction conditions for different classes of heteroarenes from a minimal number of screening reactions. Using this approach, four sets of complementary reaction conditions derived from our dual ligand-based palladium catalysts for nondirected C(sp2)-H activation were identified, that together enable the deuteration of structurally diverse heteroarenes, including bioactive molecules.

Sulfur in Waste-Free Sustainable Synthesis: Advancing Carbon-Carbon Coupling Techniques

This review explores the pivotal role of sulfur in advancing sustainable carbon-carbon (C-C) coupling reactions. The unique electronic properties of sulfur, as a soft Lewis base with significant mesomeric effect make it an excellent candidate for initiating radical transformations, directing C-H-activation, and facilitating cycloaddition and C-S bond dissociation reactions. These attributes are crucial for developing waste-free methodologies in green chemistry. Our mini-review is focused on existing sulfur-directed C-C coupling techniques, emphasizing their sustainability and comparing state-of-the-art methods with traditional approaches. The review highlights the importance of this research in addressing current challenges in organic synthesis and catalysis. The innovative use of sulfur in photocatalytic, electrochemical and metal-catalyzed processes not only exemplifies significant advancements in the field but also opens new avenues for environmentally friendly chemical processes. By focusing on atom economy and waste minimization, the analysis provides broad appeal and potential for future developments in sustainable organic chemistry.

Photoredox-Cobaloxime Catalysis for Selective Oxidative Dehydrogenative [4+2] Annulation of Imidazo-Fused Heterocycles with Alkenes

A selective oxidative [4+2] annulation of alkenes with imidazo-fused heterocycles has been developed by using the synergistic combination of photoredox and cobaloxime catalysts. It allows facile access to various imidazole-fused polyaromatic scaffolds accompanied by H2 evolution. This protocol features high regioselectivity as well as a broad substrate scope. Detailed mechanistic studies indicate that twice the electron/H transfer processes facilitated by this catalytic system achieve the annulation π-extension of imidazo-fused heterocycles with alkenes.

Late-Stage C-H Activation of Drug (Derivative) Molecules with Pd(ll) Catalysis

This review comprehensively analyses representative examples of Pd(II)-catalyzed late-stage C-H activation reactions and demonstrates their efficacy in converting C-H bonds at multiple positions within drug (derivative) molecules into diverse functional groups. These transformative reactions hold immense potential in medicinal chemistry, enabling the efficient and selective functionalization of specific sites within drug molecules, thereby enhancing their pharmacological activity and expanding the scope of potential drug candidates. Although notable articles have focused on late-stage C-H functionalization reactions of drug-like molecules using transition-metal catalysts, reviews specifically focusing on late-stage C-H functionalization reactions of drug (derivative) molecules using Pd(II) catalysts are required owing to their prominence as the most widely utilized metal catalysts for C-H activation and their ability to introduce a myriad of functional groups at specific C-H bonds. The utilization of Pd-catalyzed C-H activation methodologies demonstrates impressive success in introducing various functional groups, such as cyano (CN), fluorine (F), chlorine (Cl), aromatic rings, olefin, alkyl, alkyne, and hydroxyl groups, to drug (derivative) molecules with high regioselectivity and functional-group tolerance. These breakthroughs in late-stage C-H activation reactions serve as invaluable tools for drug discovery and development, thereby offering strategic options to optimize drug candidates and drive the exploration of innovative therapeutic solutions.

Catalyst-controlled regiodivergent C-H bond alkenylation of 2-pyridylthiophenes

A novel and effective Rh^III- and Pd^II-controlled switchable C-H alkenylation of 2-pyridylthiophenes with alkenes is realized. The alkenylation reactions proceeded smoothly in a highly regio- and stereo-selective manner to afford a broad range of C3- and C5-alkenylated products. Depending on the catalyst employed, the reactions involve two typical approaches: C3-alkenylation via chelation-assisted rhodation and C5-alkenylation via electrophilic palladation. This regiodivergent synthetic protocol was successfully applied for the straightforward building of π-conjugated difunctionalized 2-pyridylthiophenes, which may show great potential in organic electronic materials.

Nondirected Pd-catalyzed aerobic C-H alkenylation of ruthenocene and ferrocene

Pd-catalyzed alkenylations of metallocenes via C-H activation were developed using electronically tunable pyrazolonaphthyridine (PzNPy) ligands. Ferrocene was alkenylated using the most electron-deficient ligand in the series, whereas the less reactive ruthenocene needed balancing of the electrophilicity and stability of catalysts. Various alkenes were installed, allowing fine-tuning of redox potentials.

Transition-Metal-Catalyzed Divergent C-H Functionalization of Five-Membered Heteroarenes

Conversion of common reactants to diverse products is a key objective of organic syntheses. Recent developments in transition-metal-catalyzed C-H functionalization have increased the interest in such conversions. Both the position of functionalization and the type of the substituent can be varied, allowing systematic diversification of common structural cores. Because five-membered heteroarenes (pyrazole, imidazole, thiazole, pyrrole, and thiophene) are ubiquitous in pharmaceuticals and organic functional materials, the selective C-H functionalization of these heterocyclic cores facilitates both the optimization of their physicochemical properties and streamlining of their preparation. In addition, the parent forms of these heterocycles are more readily available and inexpensive than any other derivatives of their families. Hence, their nondirected C-H functionalization is highly desirable. Although various regioselective reactions have been developed, many of them target the most reactive site; hence, except for some extensively studied arylation reactions, regiodivergent functionalization of two or more sites has been limited.This Account summarizes our work on the regiodivergent, nondirected C-H functionalization of five-membered heteroarenes with alkenes and alkynes. These unsaturated hydrocarbons are readily available, and all the composing atoms can be incorporated into products with high atom efficiency. Furthermore, the installed alkenyl groups can be transformed to other useful functional groups. To achieve comparable selectivity to that observed in the traditional reactions of these heteroarenes with highly electrophilic reagents and strong bases, a transition metal catalytic system was carefully devised with a more streamlined synthesis. A judicious choice of metals, ligands, acid and base additives, and solvents orchestrates divergent transformations using electronic and steric effects of the heteroarenes. Although C-H cleavage is a rate- and site-selectivity-determining step in most cases, the subsequent steps involving the formation of C-C bonds are often more critical than the other steps. For the C-H cleavage step, modulating the electronic properties of catalysts to make them electrophilic allows preferential alkenylation at the nucleophilic position. In addition, the presence of an internal base that can be exploited for concerted metalation-deprotonation of the acidic C-H bond offers alternative regioselectivity. Furthermore, we developed our own ligand system based on a conformationally rigid pyrazolonaphthyridine scaffold that enables aerobic C-H alkenylation reactions with steric control. We showed that the electronic and steric effects of heteroarenes can be further extended to chemodivergent reactions with norbornene derivatives. Depending on whether the palladacycle is formed, heteroarenes selectively undergo 1:2 annulation with norbornene derivatives and three-component reactions with other azoles through the Pd-norbornene adducts or Catellani and 2:1 annulation reactions through the palladacycle intermediates.Other research groups have also contributed to the development of divergent reactions, in investigations ranging from the pioneering studies in the early days of research on C-H functionalization to recent studies with new ligands. We have also discussed these studies in context. These approaches provide access to many heteroarenes with systematically varied substituents. We believe that new ligand systems and mechanistic insights gained through these studies will enrich fields beyond C-H functionalization of five-membered heteroarenes.