Stereodivergent Synthesis of Alkenylpyridines via Pd/Cu Catalyzed C-H Alkenylation of Pyridinium Salts with Alkynes

Divergent Synthesis of Cyclopropanated Tetrahydroquinolines by Tandem Functionalization of Quinoline Derivatives

Here, reported is a new method for divergent synthesis of functionalized tetrahydroquinolines (THQs), featuring a biomedically interesting azabicyclo[4.1.0]heptane core, proceeding with mild conditions, good substrate and functionality tolerance, and operational simplicity. Mechanistic studies suggest that the products are formed via carbonucleophilic 1,4-addition-induced dearomatization of quinolinium salts and intramolecular cyclopropanation with α-halo ketones followed by α-nucleophilic addition with different nucleophiles. The present work lays a foundation to access new N-heterocycles via the dearomative tandem functionalization of azaarenes.

Z-selective radical difunctionalization of aromatic alkynes: synthesis of multi-substituted triarylethenes

An efficient method for the radical difunctionalization of aromatic alkynes has been developed, resulting in the synthesis of a range of valuable triarylethenes. This approach utilizes strategically designed aryldiazonium salts with tertiary alcohol substitution as bifunctional reagents, along with cost-effective cuprous chloride as a catalyst. The method demonstrates remarkable Z-selectivity and is capable of gram-scale preparation. Additionally, a novel spin-trapping reagent has been developed based on the synthesized product.

Reductive Functionalization of Pyridine-Fused N-Heteroarenes

ConspectusThe selective functionalization/transformation of ubiquitous pyridine-fused N-heteroarenes is a practical method to synthesize structurally novel N-heterocycles, which is important for the development of medicines, bioactive agents, agrochemicals, materials, ligands, sensors, pigments, dyes, etc. However, owing to thermodynamic stability, kinetic inertness, and lone electron pair-induced catalyst deactivation of the pyridine-fused N-heteroarenes, limited strategies (e.g., C-H activation/functionalization, electrophilic substitution, and the Minisci reaction) are available to realize the synthetic purpose and maintain the aromaticity of the final products. Moreover, the relevant transformations have limitations such as needing harsh reaction conditions, requiring the preinstallation of specific coupling agents containing transformable functionalities or directing groups, using less environmentally benign oxidants and/or acidic activators, and poor selectivity. Herein, considering that imines, enamines, radicals, and cyclic amines are generated during the reduction of pyridine-fused N-heteroarenes, the precise transformation of these reductive intermediates offers a fundamental basis for developing novel tandem reactions. Our group revealed that a slow reduction rate, synergistic catalysis, and controlled electroreduction are effective strategies for fulfilling the selective reductive functionalization of pyridine-fused N-heteroarenes. Thus, we established a series of new synthetic methods that provide diverse construction modalities for functionalized N-heterocycles. The striking features of these synthetic methods include high efficiency, atom economy, and the use of readily accessible N-heteroarenes as transformable feedstocks in the absence of flammable and pressurized H2 gas, alongside a promising potential of the obtained N-heterocyclic products. The present study would be appealing to the fields of synthetic organic chemistry, catalysis, biomedical chemistry, and functional materials. This Account describes the application of reductive dearomatization as substrate-activating and tandem reaction-initiating modes and summarizes the reductive functionalization of pyridine-fused N-heteroarenes via selective alkylation, arylation, and annulation at nitrogen, α, β, and other remote carbon sites achieved over the past 8 years. Details regarding the development of new reactions and their plausible mechanisms and perspectives are discussed. We hope our contributions to this field will aid in the further development of novel strategies for the functionalization/transformation of pyridine-fused N-heteroarenes and tackle the intractable challenges in this area.

Diastereoselective construction of carbo-bridged polyheterocycles by a three-component tandem annulation reaction

By a hydroamination-induced tandem annulation process, we herein report a new three-component reaction for room temperature construction of carbo-bridged polyheterocycles with exclusive diastereoselectivity, which features readily available feedstocks, catalyst-free conditions, good substrate and functionality compatibility, no need for transition metal catalysts, and high step and atom efficiency. The products are formed via initial formation of 1,2-dihydro-3H-pyrazol-3-one nucleophiles from but-2-ynedioates and hydrazine followed by 2,4-difunctionalization of N-heteroarenium salts. Given that the obtained products possess structurally important tetrahydroquinoline and pyranopyrazole motifs, the developed chemistry is anticipated to be further applied to the discovery of functional molecules including biomedical ones.

Pyridine C(sp2)-H bond functionalization under transition-metal and rare earth metal catalysis

Pyridine is a crucial heterocyclic scaffold that is widely found in organic chemistry, medicines, natural products, and functional materials. In spite of the discovery of several methods for the synthesis of functionalized pyridines or their integration into an organic molecule, new methodologies for the direct functionalization of pyridine scaffolds have been developed during the past two decades. In addition, transition-metal-catalyzed C-H functionalization and rare earth metal-catalyzed reactions have flourished over the past two decades in the development of functionalized organic molecules of concern. In this review, we discuss recent achievements in the transition-metal and rare earth metal-catalyzed C-H bond functionalization of pyridine and look into the mechanisms involved.

Tandem Ring-Contraction/Regioselective C-H Iodination Reaction of Pyridinium Salts

A facile route for direct access to the 4-iodopyrrole-2-carbaldehydes from pyridinium salts has been successfully developed, which undergoes cascade pyrrole-2-carbaldehydes construction/selective C4 position iodination process. Using Na₂S₂O₈ as an oxidant and readily available sodium iodide as an iodine source, a variety of 4-iodopyrrole-2-carbaldehydes were obtained in good to excellent yields. Atom- and step-economy, good functional group tolerance, high regioselectivity, as well as mild conditions entail this transformation an alternative strategy for enriching pyrroles library.

Ir and NHC Dual Chiral Synergetic Catalysis: Mechanism and Stereoselectivity in γ-Butyrolactone Formation

Cooperative dual catalysis is a powerful strategy for achieving unique reactivity by combining catalysts with orthogonal modes of action. This approach allows for independent control of the absolute and relative stereochemistry of the product. Despite its potential utility, the combination of N-heterocyclic carbene (NHC) organocatalysis and transition metal catalysis has remained a formidable challenge as NHCs readily coordinate metal centers. This characteristic also makes it difficult to rationalize or predict the stereochemical outcomes of these reactions. Herein, we use quantum mechanical calculations to investigate formation of γ-butyrolactones from aldehydes and allyl cyclic carbonates by means of an NHC organocatalyst and an iridium catalyst. Stereoconvergent activation of the racemic allyl cyclic carbonate forms an Ir-π-allyl intermediate and activation of an unsaturated aldehyde forms an NHC enolate, the latter of which is rate-limiting. Union of the two fragments leads to stereodetermining C-C bond formation and ultimately ring closure to generate the product lactone. Notably, CO2 loss occurs after formation of the C-C bond and Et3NH+ plays a key role in stabilizing carboxylate intermediates and in facilitating proton transfer to form the NHC enolate. The computed pathways agree with the experimental findings in terms of the absolute configuration, the enantiomer excess, and the different diastereomers seen with the (R)- and (S)-spiro-phosphoramidite combined with the NHC catalyst. Calculations reveal the lowest energy pathway includes both an NHC ligand and a phosphoramidite ligand on the iridium center. However, the stereochemical features of this Ir-bound NHC were found to not contribute to the selectivity of the process.

Recent Applications on Dual-Catalysis for C–C and C–X Cross-Coupling Reactions

Coupling reactions stand amid the most significant reactions in synthetic organic chemistry. Of late, these coupling strategies are being viewed as a versatile synthetic tool for a wide range of organic transformations in many sectors of chemistry, ranging from indispensable synthetic scaffolds and natural products of biological significance to novel organic materials. Further, the use of dual-catalysis in accomplishing various interesting cross-coupling transformations is an emerging field in synthetic organic chemistry, owing to their high catalytic performance rather than the use of a single catalyst. In recent years, synthetic organic chemists have given considerable attention to hetero-dual catalysis; wherein these catalytic systems have been employed for the construction of versatile carbon–carbon [C(sp 3)–C(sp 3), C(sp 3)–C(sp 2), C(sp 2)–C(sp 2)] and carbon–heteroatom (C–N, C–O, C–P, C–S) bonds. Therefore, in this mini-review, we are emphasizing recently developed various cross-coupling reactions catalysed by transition-metal dual-catalysis (i.e., using palladium and copper catalysts, but omitting the reports on photoredox/metal catalysis).

1 Introduction

2 Cu/Pd-Catalysed Bond Formation

2.1 Pd/Cu-Catalysed C(sp 3)–C(sp 2) Bond Formation

2.2 Pd/Cu-Catalysed C(sp 2)–C(sp 2) Bond Formation

2.3 Pd/Cu-Catalysed C(sp)–C(sp 2) Bond Formation

2.4 Pd/Cu-Catalysed C(sp 3)–C(sp 3) Bond Formation

2.5 Pd/Cu-Catalysed C–X (X = B, N, P, S, Si) Bond Formation

3 Conclusion

Copper-catalyzed C2 alkenylation of pyridine-N-oxides with alkynes

A general and expedient method to construct the scaffolds of 2-alkenylpyridines, through copper-catalyzed C2 alkenylation of pyridine-N-oxides with alkynes, has been disclosed. This protocol shows operational simplicity, good functional group compatibility and broad substrate scope.

Mechanochemical Ruthenium-Catalyzed Ortho-Alkenylation of N-Heteroaryl Arenes with Alkynes under Ball-Milling Conditions

The mechanochemical, solvent-free Ru(II)-catalyzed alkenylation of N-heteroaryl arenes with alkynes has been successfully described. A wide spectrum of arenes bearing N-heteroaryl moieties such as imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine, benzo[d]imidazo[2,1-b]thiazole, imidazo[2,1-b]thiazole, 2H-indazole, 1H-indazole, 1H-pyrazole, and 1,2,4-oxadiazol-5(4H)-one as a directing group reacted with various substituted alkynes under ball milling in the presence of [Ru(p-cymene)Cl₂]₂, affording dialkenylated products in moderate to good yields. The reaction of 2,3-dihydrophthalazine-1,4-dione with 1-phenyl-1-propyne afforded a monoalkenylated product. Similarly, reaction of 2-phenylimidazo[1,2-a]pyridine with aliphatic terminal alkynes produced a monoalkenylated derivative as the major product along with minor amount of dialkenylated product. The developed method exhibited excellent functional group compatibility, broad substrate scope, shorter reaction times, and no external heating. Moreover, the method can be readily scaled-up as demonstrated by gram-scale synthesis of 2-(2,6-bis((E)1-phenylprop-1-en-2-yl)phenyl)imidazo[1,2-a]pyridine.

Direct C-H Sulfonylimination of Pyridinium Salts

A direct pyridinium C-H sulfonylimination has been developed for the synthesis of sulfonyl iminopyridine derivatives with high efficiency. This transformation features the direct and efficient formation of a C═N bond with a high functional group tolerance under metal-free conditions. The spectroscopic properties potentially enable these sulfonyl iminopyridine compounds to be useful new emitting materials.

The regioselective annulation of N-methylpyridinium ylides with alkenes enabled by palladium catalysis: access to 3-unsubstituted indolizine derivatives

The first catalytic protocol for the regioselective [3 + 2] annulation of N-methyl pyridinium ylides with alkenes to establish various valuable 3-unsubstituted indolizine derivatives is accomplished via palladium catalysis at the unactivated position.

Recent advances in Cu-catalyzed transformations of internal alkynes to alkenes and heterocycles

Numerous metal-catalyzed reactions involving internal alkynes and aimed towards synthetically and pharmacologically important alkenes and heterocycles have appeared in the literature. Among these, Cu-catalyzed reactions have a special place, which has prompted the investigation and development of carbon-carbon and carbon-heteroatom bond-forming reactions. These reactions possess wide scope, and during the paths of these reactions, either stable or in situ intermediates are formed via the addition of Cu as a core catalyst or synergistic catalyst. In this review, we aim to report different contributions relating to Cu-catalyzed reactions of internal alkynes for the synthesis of different valuable alkenes and heterocycles which have appeared in the literature in the last decade. We anticipate that this appraisal will deliver basic insights for the further advancement of Cu-catalyzed reactions in organic chemistry.

Facile construction of peptidomimetics by sequential C–S/C–N bond activation of Ugi-adducts

Diverse peptidomimetics containing a primary amide are prepared via the integration of an Ugi-4CR and sequential C–S/C–N bond activation.