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1. Allylic C–H functionalization via group 9 π-allyl intermediates

   https://doi.org/10.1039/D0DT02313B  

   Nelson, Taylor A.; Hollerbach, Michael R.; Blakey, Simon B. (October 2020, Dalton Transactions)

   null (Ed.)

   Allylic C–H functionalization catalysed by group 9 Cp* transition-metal complexes has recently gained significant attention. These reactions have expanded allylic C–H functionalization to include di- and trisubstituted olefins, and a broad range of coupling partners. More specifically, several catalytic C–N, C–O, and C–C bond forming allylic C–H functionalization reactions have been reported, proceeding via MCp*-π-allyl intermediates. Herein we present an overview of these reactions by mechanistic paradigm. We also place this information in context of recent advances, as well as, limitations that remain for this class of reactions. 

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2. Catalytic Behavior of Mono‐ N ‐Protected Amino‐Acid Ligands in Ligand‐Accelerated C−H Activation by Palladium(II)

   https://doi.org/10.1002/anie.202002484  

   Salazar, Chase A.; Gair, Joseph J.; Flesch, Kaylin N.; Guzei, Ilia A.; Lewis, Jared C.; Stahl, Shannon S. (April 2020, Angewandte Chemie International Edition)

   Abstract Mono‐N‐protected amino acids (MPAAs) are increasingly common ligands in Pd‐catalyzed C−H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C−H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand‐accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand‐to‐metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C−H activation and catalytic C−H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50–100‐fold increase in rate when only 0.05 equivalents of MPAA are present relative to Pd^II. These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to Pd^IIenables a single MPAA to support C−H activation at multiple Pd^IIcenters. 

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3. Recent Strategies in Nickel-Catalyzed C–H Bond Functionalization for Nitrogen-Containing Heterocycles

   https://doi.org/10.3390/catal12101163  

   Yang, Ke; Li, Zhi; Hu, Qingyue; Elsaid, Mazen; Liu, Chong; Chen, Jun; Ge, Haibo (October 2022, Catalysts)

   N-heterocycles are ubiquitous in natural products, pharmaceuticals, organic materials, and numerous functional molecules. Among the current synthetic approaches, transition metal-catalyzed C–H functionalization has gained considerable attention in recent years due to its advantages of simplicity, high atomic economy, and the ready availability of starting materials. In the field of N-heterocycle synthesis via C–H functionalization, nickel has been recognized as one of the most important catalysts. In this review, we will introduce nickel-catalyzed intramolecular and intermolecular pathways for N-heterocycle synthesis from 2008 to 2021. 

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4. Recent Advances in Metal‐Catalyzed Functionalization of Indoles

   https://doi.org/10.1002/adsc.202100116  

   Urbina, Kelvin; Tresp, David; Sipps, Karli; Szostak, Michal (April 2021, Advanced Synthesis & Catalysis)

   Abstract Indole is one of the most important heterocycles in organic synthesis, natural products, and drug discovery. Recently, tremendous advances in the selective functionalization of indoles have been reported. Although the most important advances have been powered by transition metal catalysis, exceedingly useful methods in the absence of transition metals have also been reported. In this review, we provide an overview of functionalization reactions of indoles that have been published in the last years with a focus on the most recent advances, aims, and future trends. The review is organized by the positional selectivity and type of methods used for functionalization. In particular, we discuss major advances in transition‐metal‐catalyzed C−H functionalization at the classical C2/C3 positions, transition‐metal‐catalyzed C−H functionalization at the remote C4/C7 positions, transition‐metal‐catalyzed cross‐coupling, and transition‐metal‐free functionalization. magnified image 

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5. Palladium‐Catalyzed Dearomative syn ‐1,4‐Oxyamination

   https://doi.org/10.1002/anie.201909838  

   Tang, Conghui; Okumura, Mikiko; Deng, Hejun; Sarlah, David (September 2019, Angewandte Chemie International Edition)

   Abstract A palladium‐catalyzed dearomativesyn‐1,4‐oxyamination protocol using non‐activated arenes has been developed. This one‐pot procedure utilizes arenophile chemistry, and the correspondingpara‐cycloadducts are treated with oxygen nucleophiles via formal allylic substitution, providing direct access tosyn‐1,4‐oxyaminated products. The reaction conditions permit a range of arenes, as well as different O‐nucleophiles, such as oximes and benzyl alcohols. Moreover, this process was established in an asymmetric fashion, delivering products with high enantioselectivity. The dearomatized products are amenable to a multitude of further derivatizations ranging from olefin chemistry to C−H activation, giving rise to a diverse set of new functionalities. Overall, this dearomative functionalization offers rapid and controlled formation of molecular complexity, enabling straightforward access to functionalized small molecules from simple and readily available arenes. 

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