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.

Photoredox-catalyzed branch-selective pyridylation of alkenes for the expedient synthesis of Triprolidine

Alkenylpyridines are important pharmaceutical cores as well as versatile building blocks in organic synthesis. Heck reaction represents one of the most powerful platform for the construction of aryl-substituted alkenes, nevertheless, examples for Heck type coupling of alkenes with pyridines, particularly with branched selectivity, remain elusive. Here we report a catalytic, branch-selective pyridylation of alkenes via a sulfinate assisted photoredox catalysis. This reaction proceeds through a sequential radical addition/coupling/elimination, by utilizing readily available sodium sulfinates as reusable radical precursors as well as traceless elimination groups. This versatile protocol allows for the installation of important vinylpyridines with complete branched selectivity under mild conditions. Furthermore, this catalytic manifold is successfully applied to the expedient synthesis of Triprolidine.

Alkenylpyridines are versatile building blocks for the synthesis of drugs and other complex molecular structures. Here, the authors show a branch-selective pyridylation of alkenes via sulfinate-assisted photoredox catalysis and showcase its utility in an expedient synthesis of Triprolidine.

Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

The past decades have witnessed rapid development in organic synthesis via catalysis, particularly the reactions through C–H bond functionalization. Transition metals such as Pd, Rh and Ru constitute a crucial catalyst in these C–H bond functionalization reactions. This process is highly attractive not only because it saves reaction time and reduces waste,but also, more importantly, it allows the reaction to be performed in a highly region specific manner. Indeed, several organic compounds could be readily accessed via C–H bond functionalization with transition metals. In the recent past, tremendous progress has been made on C–H bond functionalization via ruthenium catalysis, including less expensive but more stable ruthenium(II) catalysts. The ruthenium-catalysed C–H bond functionalization, viz. arylation, alkenylation, annulation, oxygenation, and halogenation involving C–C, C–O, C–N, and C–X bond forming reactions, has been described and presented in numerous reviews. This review discusses the recent development of C–H bond functionalization with various ruthenium-based catalysts. The first section of the review presents arylation reactions covering arylation directed by N–Heteroaryl groups, oxidative arylation, dehydrative arylation and arylation involving decarboxylative and sp3-C–H bond functionalization. Subsequently, the ruthenium-catalysed alkenylation, alkylation, allylation including oxidative alkenylation and meta-selective C–H bond alkylation has been presented. Finally, the oxidative annulation of various arenes with alkynes involving C–H/O–H or C–H/N–H bond cleavage reactions has been discussed.

Ruthenium-catalyzed C–H allylation of arenes with allylic amines

The Ru-catalyzed pyridyl-directed C–H allylation of arenes with allylic amines was carried out in the presence of 5 mol% of [Ru(p-cymene)Cl₂]₂ and 0.5 equiv. of AgOAc in CF₃CH₂OH.

Ruthenium-catalysed one-pot regio- and diastereoselective synthesis of pyrrolo[1,2-a]indoles via cascade C–H functionalization/annulation

Catalytic, regioselective and diastereoselective synthesis of pyrrolo[1,2-a]indoles through a cascade C–H activation and cyclization process.

Ru-Catalyzed selective C–H oxidative olefination with N-heteroarenes directed by pivaloyl amide

Ruthenium-catalysed C7-position alkynylation for indoline derivatives with various olefins was reported.

Ruthenium-catalyzed direct arylations with aryl chlorides

Aryl chlorides are readily available at lower cost than the corresponding bromides and iodides, but are much more challenging as substrates in metal-catalyzed cross-couplings.

Active chemisorption sites in functionalized ionic liquids for carbon capture

Carbon capture with site-containing ionic liquids is reviewed with particular attention on the activation and design of the interaction sites.

Temperature-controlled redox-neutral ruthenium(ii)-catalyzed regioselective allylation of benzamides with allylic acetates

Temperature controlled regioselective synthesis of ortho allyl and vinyl benzamides under redox-free conditions was reported. Detailed mechanistic investigation of the C–H bond activation and isomerization reactions was carried out.

Ruthenium-Catalyzed Oxidant-Free Allylation of Aromatic Ketoximes with Allylic Acetates at Room Temperature

Substituted aromatic ketoximes reacted efficiently with allylic acetates in the presence of {[RuCl2(p-cymene)]2} and AgSbF6 in 1,2-dichloroethane at ambient temperature, providing ortho-allyl aromatic ketoximes in a highly regioselective manner without an oxidant. In the reaction, the acetate group of allyl acetate acts as a base to activate the C-H bond of aromatics. Later, ortho-allyl aromatic ketoximes were converted into ortho-allyl aromatic ketones in the presence of HCl.

Synthesis of 2,4-diarylsubstituted-pyridines through a Ru-catalyzed four component reaction

2,4-Diarylsubstituted pyridines were prepared from acetophenones, ammonium acetate and DMF under an oxygen atmosphere.