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1. Electrocatalytic redox neutral [3 + 2] annulation of N -cyclopropylanilines and alkenes

   https://doi.org/10.1039/d0sc05665k  

   Wang, Qi; Wang, Qile; Zhang, Yuexiang; Mohamed, Yasmine M.; Pacheco, Carlos; Zheng, Nan; Zare, Richard N.; Chen, Hao (January 2021, Chemical Science)

   null (Ed.)

   Although synthetic organic electrochemistry (EC) has advanced significantly, net redox neutral electrosynthesis is quite rare. Two approaches have been employed to achieve this type of electrosynthesis. One relies on turnover of the product by the reactant in a chain mechanism. The other involves both oxidation on the anode and reduction on the cathode in which the radical cation or the radical anion of the product has to migrate between two electrodes. Herein, a home-built electrochemistry/mass spectrometry (EC/MS) platform was used to generate an N -cyclopropylaniline radical cation electrochemically and to monitor its reactivity toward alkenes by mass spectrometry (MS), which led to the discovery of a new redox neutral reaction of intermolecular [3 + 2] annulation of N -cyclopropylanilines and alkenes to provide an aniline-substituted 5-membered carbocycle via direct electrolysis (yield up to 81%). A chain mechanism, involving the regeneration of the substrate radical cation and the formation of the neutral product, is shown to be responsible for promoting such a redox neutral annulation reaction, as supported by experimental evidence of EC/MS. 

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2. Cationic Co(I) Catalysts for Regiodivergent Hydroalkenylation of 1,6-Enynes: An Uncommon cis -β-C–H Activation Leads to Z -Selective Coupling of Acrylates

   https://doi.org/10.1021/acscatal.1c02530  

   Herbort, James H.; Lalisse, Remy F.; Hadad, Christopher M.; RajanBabu, T. V. (July 2021, ACS Catalysis)

   null (Ed.)

   ABSTRACT Two intermolecular hydroalkenylation reactions of 1,6-enynes are presented which yield substituted 5-membered carbo- and -heterocycles. This reactivity is enabled by a cationic bis-diphenylphosphinopropane (DPPP)CoI species which forms a cobaltacyclopentene intermediate by oxidative cyclization of the enyne. This key species interacts with alkenes in distinct fashion, depending on the identity of the coupling partner to give regiodivergent products. Simple alkenes undergo insertion reactions to furnish 1,3-dienes whereby one of the alkenes is tetrasubstituted. When acrylates are employed as coupling partners, the site of intermolecular C-C formation shifts from the alkyne to the alkene motif of the enyne, yield-ing Z-substituted-acrylate derivatives. Computational studies provide support for our experimental observations and show that the turnover-limiting steps in both reactions are the interactions of the alkenes with the cobaltacyclopentene intermediate via either a 1,2-insertion in the case of ethylene, or an unexpected b-C-H activation in the case of most acrylates. Thus, the H syn to the ester is activated through the coordination of the acrylate carbonyl to the cobaltacycle intermediate, which explains the uncommon Z-selectivity and regiodivergence. Variable time normalization analysis (VTNA) of the kinetic data reveals a dependance upon the concentration of cobalt, acrylate, and activator. A KIE of 2.1 was observed with methyl methacrylate in separate flask experiments, indicating that C-H cleavage is the turnover-limiting step in the catalytic cycle. Lastly, a Hammett study of aryl-substituted enynes yields a rho- value of -0.4, indicating that more electron-rich substituents accelerate the rate of the reaction. 

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3. Ligand-enabled ortho -C–H olefination of phenylacetic amides with unactivated alkenes

   https://doi.org/10.1039/c7sc04827k  

   Lu, Ming-Zhu; Chen, Xing-Rong; Xu, Hui; Dai, Hui-Xiong; Yu, Jin-Quan (January 2018, Chemical Science)

   Although chelation-assisted C–H olefination has been intensely investigated, Pd( ii )-catalyzed C–H olefination reactions are largely restricted to acrylates and styrenes. Here we report a quinoline-derived ligand that enables the Pd( ii )-catalyzed olefination of the C(sp 2 )–H bond with simple aliphatic alkenes using a weakly coordinating monodentate amide auxiliary. Oxygen is used as the terminal oxidant with catalytic copper as the co-oxidant. A variety of functional groups in the aliphatic alkenes are tolerated. Upon hydrogenation, the ortho -alkylated product can be accessed. The utility of this reaction is also demonstrated by the late-stage diversification of drug molecules. 

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4. Co-Catalyzed Hydrofluorination of Alkenes: Photocatalytic Method Development and Electroanalytical Mechanistic Investigation

   https://doi.org/10.1021/jacs.3c10989  

   Liu, Jinjian; Rong, Jian; Wood, Devin P; Wang, Yi; Liang, Steven H; Lin, Song (February 2024, Journal of the American Chemical Society)

   The hydrofluorination of alkenes represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)–F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and poor nucleophilicity of fluoride. In this study, we present a new Co(salen)-catalyzed hydrofluorination of simple alkenes, utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a polar- radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radi- opharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calcula- tions, unveiled the involvement of both carbocation and CoIV–alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene. 

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5. Stereodivergent, Kinetically Controlled Isomerization of Terminal Alkenes via Nickel Catalysis

   https://doi.org/10.1002/anie.202320081  

   Rubel, Camille_Z; Ravn, Anne_K; Ho, Hang_Chi; Yang, Shenghua; Li, Zi‐Qi; Engle, Keary_M; Vantourout, Julien_C (April 2024, Angewandte Chemie International Edition)

   Abstract Because internal alkenes are more challenging synthetic targets than terminal alkenes, metal‐catalyzed olefin mono‐transposition (i.e., positional isomerization) approaches have emerged to afford valuableE‐ orZ‐internal alkenes from their complementary terminal alkene feedstocks. However, the applicability of these methods has been hampered by lack of generality, commercial availability of precatalysts, and scalability. Here, we report a nickel‐catalyzed platform for the stereodivergentE/Z‐selective synthesis of internal alkenes at room temperature. Commercial reagents enable this one‐carbon transposition of terminal alkenes to valuableE‐ orZ‐internal alkenes via a Ni−H‐mediated insertion/elimination mechanism. Though the mechanistic regime is the same in both systems, the underlying pathways that lead to each of the active catalysts are distinct, with theZ‐selective catalyst forming from comproportionation of an oxidative addition complex followed by oxidative addition with substrate and theE‐selective catalyst forming from protonation of the metal by the trialkylphosphonium salt additive. In each case, ligand sterics and denticity control stereochemistry and prevent over‐isomerization. 

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