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1. An Overview of α‐Aminoalkyl Radical Mediated Halogen‐Atom Transfer

   https://doi.org/10.1002/cctc.202300860  

   Sachidanandan, Krishnakumar; Niu, Ben; Laulhé, Sébastien (September 2023, ChemCatChem)

   Abstract The merging of photocatalysis with halogen‐atom transfer (XAT) processes has proven to be a versatile tool for the generation of carbon‐centered radicals in organic synthesis. XAT processes are unique in that they generate radicals without requiring the use of strong reductants necessary for the traditional single electron transfer (SET) activation of halides. Pathways to achieve XAT in synthetic applications can be categorized into three major sections: i) heteroatom‐based activators, ii) metal‐based activators, and iii) carbon‐based activators among which α‐aminoalkyl radicals have taken the center stage. Access to these α‐aminoalkyl radicals as XAT reagents has gained significant attention in the past few years due to the robustness of the reactions, the simplicity of the reagents required, and the broadness of their applications. Generation of these α‐aminoalkyl radicals is simply achieved through the single electron oxidation of tertiary amines, which after deprotonation at the α‐position generates the α‐aminoalkyl radicals. Due to the wide scope of tertiary amines available and the tunable nucleophilicity of α‐aminoalkyl radical formed, this strategy has become an attractive alternative to heteroatom/metal‐based radicals for XAT. In this minireview, we focus our attention on recent (2020–2023) developments and uses of this robust technology to mediate XAT processes. 

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2. Visible-light-driven net-1,2-hydrogen atom transfer of amidyl radicals to access β-amido ketone derivatives

   https://doi.org/10.1039/D4SC04997G  

   Jiang, Yonggang; Li, Hui; Tang, Haoqin; Zhang, Qingyue; Yang, Haitao; Pan, Yu; Zou, Chenggang; Zhang, Hongbin; Walsh, Patrick J; Yang, Xiaodong (January 2025, Chemical Science)

   Visible-light-driven N-centered radicals lead to C-centered α-amino radicals through rare net-1,2-HAT processes, with trapping by silyl enol ethers to access β-amido ketone derivatives. 

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3. Reactive halogen radicals in saline water promote photochemically-assisted formation of manganese oxide nanosheets

   https://doi.org/10.1039/D2EN00410K  

   Gao, Zhenwei; Skurie, Charlie; Jun, Young-Shin (October 2022, Environmental Science: Nano)

   Halide ions are naturally abundant in oceans and estuaries. Large amounts of highly saline efflux are also made and discharged to surface water from desalination processes and from unconventional oil and gas recovery. These highly concentrated halides can generate reactive halogen radicals. However, the redox reactions of halogen radicals with heavy metals or transition metals have received little attention. Here, we report undiscovered fast oxidation of manganese ions (Mn2+) by reactive halogen radicals. Hydroxyl radicals (˙OH) are produced by nitrate photolysis. While ˙OH radicals play a limited role in the direct oxidation of Mn2+, ˙OH can react with halide ions to generate reactive halogen radicals to oxidize Mn2+. In addition, more Mn2+ was oxidized by bromide (Br) radicals generated from 1 mM Br− than by chloride (Cl) radicals generated from 500 mM Cl−. In the presence of Br radicals, the abiotic oxidation rate of Mn2+ to Mn(IV)O2 nanosheets is greatly promoted, showing a 62% increase in Mn2+ (aq) oxidation within 6 h of reaction. This study advances our understanding of natural Mn2+ oxidation processes and highlights unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding nanoscale solid mineral formation in brine. This work suggests a new, environmentally-friendly, and facile pathway for synthesizing MnO2 nanosheets. 

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4. Organocatalytic synthesis of β-enaminyl radicals as the single-electron donors for phenyliodine(III) dicarboxylates: Direct one-pot alkylation-aminoxidation of styrenes

   https://doi.org/10.1039/D2OB01826H  

   Sakkani, Nagaraju; Jha, Dhiraj Kumar; Sadiq, Nouraan; Zhao, John C.-G. (April 2023, Organic & Biomolecular Chemistry)

   A direct one-pot alkylation-aminoxidation of styrene derivatives was achieved by using in-situ-generated alkyl and N-oxyl radicals. The corresponding O-alkylated hydroxylamine derivatives were obtained in moderate to good yields. The reaction features the generation of the alkyl radicals from phenyliodine(III) dicarboxylates via an organocatalytic process, the use of phenyliodine(III) dicarboxylates as the source of the alkyl radicals and the oxidant for the generation of the N-oxyl radicals, and the first generation of the β-enaminyl radicals via a HAT process and their use as the single-electron donors. 

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5. Generation of Diazomethyl Radicals by Hydrogen Atom Abstraction and Their Cycloaddition with Alkenes

   https://doi.org/10.1002/anie.202105472  

   Su, Yong‐Liang; Dong, Kuiyong; Zheng, Haifeng; Doyle, Michael P. (July 2021, Angewandte Chemie International Edition)

   Abstract A general catalytic methodology for the synthesis of pyrazolines from α‐diazo compounds and conjugated alkenes is reported. The direct hydrogen atom transfer (HAT) process of α‐diazo compounds promoted by thetert‐butylperoxy radical generates electrophilic diazomethyl radicals, thereby reversing the reactivity of the carbon atom attached with the diazo group. The regiocontrolled addition of diazomethyl radicals to carbon‐carbon double bonds followed by intramolecular ring closure on the terminal diazo nitrogen and tautomerization affords a diverse set of pyrazolines in good yields with excellent regioselectivity. This strategy overcomes the limitations of electron‐deficient alkenes in traditional dipolar [3+2]‐cycloaddition of α‐diazo compounds with alkenes. Furthermore, the straightforward formation of the diazomethyl radicals provides umpolung reactivity, thus opening new opportunities for the versatile transformations of diazo compounds. 

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