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1. 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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2. A Macrocyclic Ruthenium Carbene for Size-Selective Alkene Metathesis

   https://doi.org/10.1021/jacs.0c00081  

   Zhang, Y; Diver, ST. (February 2020, Journal of the American Chemical Society)

   The synthesis of a macrocyclic Ru carbene catalyst for selective cross alkene metathesis is reported. The new catalyst showed different reactivity for various type 1 alkenes in homodimerization which correlated with the aggregrate size of the allylic substituent. The altered reactivity profile allowed for selective product formation in competition cross alkene metathesis between two different type 1 alkenes and tert-butyl acrylate. Selectivity in these reactions is attributed to the ability of the macrocyclic catalyst to differentiate alkenes based on their size. Two preparative examples of cross metathesis with the macrocyclic catalyst are also provided. 

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3. 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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4. Catalytic Amidomethylative [2+2+2] Cycloaddition of Formaldimine and Styrenes toward N-Heterocycles

   https://doi.org/10.1055/a-1710-7256  

   Chaminda Lakmal, Hetti Handi; Istre, Jacob; Qian, Xiaolin; Zhou, Hui; Valle, Henry U.; Xu, Xue; Cui, Xin (April 2022, Synthesis)

   Abstract Chemo-switchable catalytic [2+2+2] cycloaddition of alkenes with formaldimines is reported. Bis(tosylamido)methane (BTM) and 1,2-ditosyl-1,2-diazetidine (DTD), two bench-stable precursors for highly reactive tosylformaldimine, have been identified to be effective. BTM worked as a selective releaser of the formaldimine for catalytic [2+2+2] reactions toward hexahydropyrimidine products via a presumable ‘imine–alkene–imine’ addition. A unique catalytic retro-[2+2] reaction of DTD was used and has enabled a proposed ‘imine–alkene–alkene’ pathway with high chemoselectivity for the synthesis of 2,4-di­arylpiperidine derivatives. The two alternative processes are catalyzed by the simple and environmentally benign catalysts InCl3 and FeBr2, respectively. 

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5. Multi-component heteroarene couplings via polarity-reversed radical cascades

   https://doi.org/10.1039/c9cc03498f  

   Lear, Jeremy M.; Buquoi, J. Quentin; Gu, Xin; Pan, Kui; Mustafa, Darsheed N.; Nagib, David A. (July 2019, Chemical Communications)

   null (Ed.)

   A multi-component radical addition strategy enables difunctionalization of alkenes with heteroarenes and a variety of radical precursors, including N 3 , P(O)R 2 , and CF 3 . This unified approach for coupling diverse classes of electrophilic radicals and heteroarenes to vinyl ethers allows for direct, vicinal C–C as well as C–N, C–P, and C–R f bond formation. 

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