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1. Chemoselective bond activation by unidirectional and asynchronous PCET using ketone photoredox catalysts

   https://doi.org/10.1039/d3sc04362b  

   Sun, Rui; Ruccolo, Serge; Nascimento, Daniel L; Qin, Yangzhong; Hibbert, Nathaniel; Nocera, Daniel G (December 2023, Chemical Science)

   The triplet excited states of ketones effect selective H-atom abstraction from amide N–H bonds in the presence of weaker C–H bondsviaa proton-coupled electron transfer (PCET) pathway in which the electron and proton transfers are asynchronous. 

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2. Photooxidation of Polyolefins to Produce Materials with In‐Chain Ketones and Improved Materials Properties

   https://doi.org/10.1002/anie.202418411  

   Liu, Xin; Hu, Zhitao; Portela, Brandon_S; Rettner, Emma_M; Pineda, Agustin; Miscall, Joel; Rorrer, Nicholas_A; Krummel, Amber_T; Paton, Robert_S; Miyake, Garret_M (November 2024, Angewandte Chemie International Edition)

   Abstract Herein, we report a selective photooxidation of commodity postconsumer polyolefins to produce polymers with in‐chain ketones. The reaction does not involve the use of catalyst, metals, or expensive oxidants, and selectively introduces ketone functional groups. Under mild and operationally simple conditions, yields up to 1.23 mol % of in‐chain ketones were achieved. Installation of in‐chain ketones resulted in materials with improved adhesion of the materials and miscibility of mixed plastics relative to the unfunctionalized plastics. The introduction of ketone groups into the polymer backbone allows these materials to react with diamines, forming dynamic covalent polyolefin networks. This strategy allows for the upcycling of mixed plastic waste into reprocessable materials with enhanced performance properties compared to polyolefin blends. Mechanistic studies support the involvement of photoexcited nitroaromatics in consecutive hydrogen and oxygen atom transfer reactions. 

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3. Red Light–Blue Light Chromoselective C(sp ² )–X Bond Activation by Organic Helicenium-Based Photocatalysis

   https://doi.org/10.1021/jacs.3c13380  

   Hossain, Md Mubarak; Shaikh, Aslam C; Kaur, Ramandeep; Gianetti, Thomas L (March 2024, Journal of the American Chemical Society)

   Chromoselective bond activation has been achieved in organic helicenium (nPr-DMQA+)-based photoredox catalysis. Consequently, control over chromoselective C(sp2)–X bond activation in multihalogenated aromatics has been demonstrated. nPr-DMQA+ can only initiate the halogen atom transfer (XAT) pathway under red light irradiation to activate low-energy-accessible C(sp2)–I bonds. In contrast, blue light irradiation initiates consecutive photoinduced electron transfer (conPET) to activate more challenging C(sp2)–Br bonds. Comparative reaction outcomes have been demonstrated in the α-arylation of cyclic ketones with red and blue lights. Furthermore, red-light-mediated selective C(sp2)–I bonds have been activated in iodobromoarenes to keep the bromo functional handle untouched. Finally, the strength of the chromoselective catalysis has been highlighted with two-fold functionalization using both photo-to-transition metal and photo-to-photocatalyzed transformations. 

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4. Branched‐Selective Direct α‐Alkylation of Cyclic Ketones with Simple Alkenes

   https://doi.org/10.1002/anie.201900301  

   Xing, Dong; Qi, Xiaotian; Marchant, Daniel; Liu, Peng; Dong, Guangbin (February 2019, Angewandte Chemie International Edition)

   Abstract Herein, we describe an intermolecular direct branched‐selective α‐alkylation of cyclic ketones with simple alkenes as the alkylation agents. Through an enamine‐transition metal cooperative catalysis mode, the α‐alkylation is realized in an atom‐ and step‐economic manner with excellent branched selectivity for preparing β‐branched ketones. Employment of a pair of bulky Brønsted acid and base as additives is responsible for enhanced efficiency. Promising enantioselectivity (74 % ee) has been obtained. Experimental and computational mechanistic studies suggest that a pathway through alkene migratory insertion into the Ir−C bond followed by C−H reductive elimination is involved for the high branched selectivity. 

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5. C–H functionalization reactions enabled by hydrogen atom transfer to carbon-centered radicals

   https://doi.org/10.1039/D0SC04881J  

   Sarkar, Sumon; Cheung, Kelvin Pak; Gevorgyan, Vladimir (December 2020, Chemical Science)

   null (Ed.)

   Selective functionalization of ubiquitous unactivated C–H bonds is a continuous quest for synthetic organic chemists. In addition to transition metal catalysis, which typically operates under a two-electron manifold, a recent renaissance in the radical approach relying on the hydrogen atom transfer (HAT) process has led to tremendous growth in the area. Despite several challenges, protocols proceeding via HAT are highly sought after as they allow for relatively easy activation of inert C–H bonds under mild conditions leading to a broader scope and higher functional group tolerance and sometimes complementary reactivity over methods relying on traditional transition metal catalysis. A number of methods operating via heteroatom-based HAT have been extensively reported over the past few years, while methods employing more challenging carbon analogues have been less explored. Recent developments of mild methodologies for generation of various carbon-centered radical species enabled their utilization in the HAT process, which, in turn, led to the development of remote C(sp 3 )–H functionalization reactions of alcohols, amines, amides and related compounds. This review covers mostly recent advances in C–H functionalization reactions involving the HAT step to carbon-centered radicals. 

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