Effective Tools for the Metal-Catalyzed Regiodivergent Direct Arylations of (Hetero)arenes

Photochemical direct alkylation of heteroarenes with alkanes, alcohols, amides, and ethers

Direct functionalization of heteroarenes with simple alkanes utilizing anthracene as a photoredox catalyst has been established. This approach provides a sustainable alternative, avoiding costly reagents or peroxides. The method demonstrates a broad substrate scope, enabling regioselective alkylation of various heteroarenes, including azoles, pyridines, quinolines, isoquinolones, and quinoxalinones under mild conditions. A range of alkyl sources, such as alkanes, ethers, dioxane, trioxane, alcohol, and alkylamides were viable substrates. A plausible catalytic cycle was proposed based on the preliminary mechanistic evidence.

Mono or double Pd-catalyzed C-H bond functionalization for the annulative π-extension of 1,8-dibromonaphthalene: a one pot access to fluoranthene derivatives

The Pd-catalyzed annulative π-extension of 1,8-dibromonaphthalene for the preparation of fluoranthenes in a single operation has been investigated. With specific arenes such as fluorobenzenes, the Pd-catalyzed double functionalization of C-H bonds yields the desired fluoranthenes. The reaction proceeds via a palladium-catalyzed direct intermolecular arylation, followed by a direct intramolecular arylation step. As the C-H bond activation of several benzene derivatives remains very challenging, the preparation of fluoranthenes from 1,8-dibromonaphthalene via Suzuki coupling followed by intramolecular C-H activation has also been investigated to provide a complementary method. Using the most appropriate synthetic route and substrates, it is possible to introduce the desired functional groups at positions 7-10 on fluoranthenes.

Palladium-Catalyzed Regiodivergent C-H Olefination of Imidazo[1,2a]pyridine Carboxamide and Unactivated Alkenes

Despite remarkable successes in linear and branched vinyl (hetero) arene synthesis, regiodivergent C-H olefination with a single catalytic system has remained underdeveloped. Overcoming this limitation, a Pd/MPAA-catalyzed regiodivergent C-H olefination of imidazo[1,2a] pyridine carboxamides with unactivated terminal alkenes to generate branched and linear olefinated products depending upon the electronic nature of alkenes is reported herein. Moreover, this protocol can be applied for C-H deuteriation of the corresponding heteroarenes with D2 O as deuterium source. Preliminary experimental studies combined with computational investigations (DFT studies) suggest that regiodivergent olefination can be controlled by olefin insertion and β-hydride elimination steps.

A new transient directing group diethoxyethyl-L-proline facilitates ortho-arylation of aryl-amines/-amino acids via Pd-catalyzed C(sp2)-H activation

Mono-ortho-arylated arylamines are constituents of various natural products but their syntheses are challenging. This report describes a new synthetic methodology for the ortho-arylation of arylamines and α-aromatic amino acids (phenylglycine and phenylalanine) through a Pd-catalyzed C(sp²)-H activation using the synthetic transient directing group diethoxyethyl-L-proline (DEP). A catalytic amount of diethoxyethyl-L-proline is sufficient to form mono-arylated arylamines as the major products using aryliodides. This method could be useful for the synthesis of various biphenyl amines and novel peptidomimetics.

Palladium-catalysed C–H arylation of benzophospholes with aryl halides

A palladium-catalysed C–H arylation of benzophospholes with aryl halides has been developed. The reaction with aryl iodides and bromides proceeds well even under phosphine ligand-free Pd(OAc)₂ catalysis whereas the Pd(PCy₃)₂ is effective for the coupling with less reactive aryl chlorides. The optimal conditions are also applicable to the double arylations with organic dihalides and annulation reaction with ortho-dihalogenated benzenes, making the corresponding benzophosphole-based acceptor–donor–acceptor-type molecules and highly condensed heteroacene-type molecules of potent interest in materials chemistry. Although there are many reports of catalytic C–H functionalisations of related benzoheteroles such as indoles, benzothiophenes, and benzofurans, this is the first successful example of the catalytic direct C–H transformation of benzophospholes, to the best of our knowledge. The preliminary optoelectronic properties of some newly synthesized benzophosphole derivatives are also investigated.

A palladium-catalysed C–H arylation of benzophospholes with aryl halides has been developed to form the corresponding acceptor–donor–acceptor-type molecules and highly condensed heteroacene-type molecules of potent interest in material chemistry.

A General, Multimetallic Cross-Ullmann Biheteroaryl Synthesis from Heteroaryl Halides and Heteroaryl Triflates

Despite their importance to medicine and materials science, the synthesis of biheteroaryls by cross-coupling remains challenging. We describe here a new, general approach to biheteroaryls: the Ni- and Pd-catalyzed multimetallic cross-Ullmann coupling of heteroaryl halides with triflates. An array of 5-membered, 6-membered, and fused heteroaryl bromides and chlorides, as well as aryl triflates derived from heterocyclic phenols, proved to be viable substrates in this reaction (62 examples, 63 ± 17% average yield). The generality of this approach to biheteroaryls was further demonstrated in 96-well plate format at 10 μmol scale. An array of 96 possible products provided >90% hit rate under a single set of conditions. Further, low-yielding combinations could be rapidly optimized with a single "Toolbox Plate" of ligands, additives, and reductants.

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.

Recent Advances in Transition-Metal-Free Late-Stage C-H and N-H Arylation of Heteroarenes Using Diaryliodonium Salts

Transition-metal-free direct arylation of C-H or N-H bonds is one of the key emerging methodologies that is currently attracting tremendous attention. Diaryliodonium salts serve as a stepping stone on the way to alternative environmentally friendly and straightforward pathways for the construction of C-C and C-heteroatom bonds. In this review, we emphasize the recent synthetic advances of late-stage C(sp2)-N and C(sp2)-C(sp2) bond-forming reactions under metal-free conditions using diaryliodonium salts as arylating reagent and its applications to the synthesis of new arylated bioactive heterocyclic compounds.