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1. Esters as Radical Acceptors: β‐NHC‐Borylalkenyl Radicals Induce Lactonization by C−C Bond Formation/Cleavage on Esters

   https://doi.org/10.1002/anie.201902001  

   Shimoi, Masaki; Maeda, Katsuhiro; Geib, Steven J.; Curran, Dennis P.; Taniguchi, Tsuyoshi (April 2019, Angewandte Chemie International Edition)

   Abstract Substituted propargyl acetates are converted into 4‐boryl‐2(5H)‐furanones upon thermolysis in the presence of an N‐heterocyclic carbene borane (NHC‐borane) and di‐tert‐butyl peroxide. The acetyl methyl group is lost during the reaction as methane. Evidence suggests that the reaction proceeds by a sequence of radical events including: 1) addition of an NHC‐boryl radical to the triple bond; 2) cyclization of the resultant β‐borylalkenyl radical to the ester carbonyl group; 3) β‐scission of the so‐formed alkoxy radical to provide the 4‐boryl‐2(5H)‐furanone and a methyl radical; and 4) hydrogen abstraction from the NHC‐borane to return the initial NHC‐boryl radical and methane. 

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2. Enantioselective and Regiodivergent Synthesis of Propargyl‐ and Allenylsilanes through Catalytic Propargylic C−H Deprotonation

   https://doi.org/10.1002/anie.202318040  

   Zhu, Jin; Xiang, Hengye; Chang, Hai; Corcoran, James_C; Ding, Ruiqi; Xia, Yue; Liu, Peng; Wang, Yi‐Ming (March 2024, Angewandte Chemie International Edition)

   Abstract We report a highly enantioselective intermolecular C−H bond silylation catalyzed by a phosphoramidite‐ligated iridium catalyst. Under reagent‐controlled protocols, propargylsilanes resulting from C(sp³)−H functionalization, as well the regioisomeric and synthetically versatile allenylsilanes, could be obtained with excellent levels of enantioselectivity and good to excellent control of propargyl/allenyl selectivity. In the case of unsymmetrical dialkyl acetylenes, good to excellent selectivity for functionalization at the less‐hindered site was also observed. A variety of electrophilic silyl sources (R₃SiOTf and R₃SiNTf₂), either commercial or in situ‐generated, were used as the silylation reagents, and a broad range of simple and functionalized alkynes, including aryl alkyl acetylenes, dialkyl acetylenes, 1,3‐enynes, and drug derivatives were successfully employed as substrates. Detailed mechanistic experiments and DFT calculations suggest that an η³‐propargyl/allenyl Ir intermediate is generated upon π‐complexation‐assisted deprotonation and undergoes outer‐sphere attack by the electrophilic silylating reagent to give propargylic silanes, with the latter step identified as the enantiodetermining step. 

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3. An Efficient Route to Isochromene Derivatives via Cascade Radical Cyclization and Radical‐Radical Coupling

   https://doi.org/10.1002/adsc.201900497  

   Yu, Kaili; Li, Minyan; Deng, Guogang; Liu, Chunxiang; Wang, Jing; Liu, Zhengfen; Zhang, Hongbin; Yang, Xiaodong; Walsh, Patrick_J (July 2019, Advanced Synthesis & Catalysis)

   Abstract Isochromenes are important pharmacophores present in biologically active molecules and natural products. Their synthesis is generally limited to cyclization of phenyl propargyl ether precursors under transition metal catalyzed conditions. Herein, we present a novel disconnection that rapidly constructs isochromene derivatives through a cascade radical cyclization strategy. Generation of aryl radicals by SET reduction of 2‐iodo benzyl allenyl ethers is followed by radical cyclization to construct the isochromene core with formation of an allylic radical. The allylic radical then undergoes coupling with the azaallyl radical to give products in good to excellent yields. The elaborated 2‐iodo phenyl propargyl ether precursors can be used to construct isochromenes bearing various functional groups. magnified image 

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4. Enantioselective regiospecific addition of propargyltrichlorosilane to aldehydes catalyzed by biisoquinoline N , N’ -dioxide

   https://doi.org/10.3762/bjoc.20.255  

   Brako, Noble; Narayanan, Sreerag Moorkkannur; Burns, Amber; Auter, Layla; Cesiliano, Valentino; Prabhakar, Rajeev; Takenaka, Norito (January 2024, Beilstein Journal of Organic Chemistry)

   Distilled propargyltrichlorosilane with >99% isomeric purity was prepared for the first time, and its asymmetric catalytic regiospecific addition reaction to aldehydes was developed through a systematic catalyst structure–reactivity and selectivity relationship study. The observed catalyst structure–enantioselectivity relationship of the present allenylation reaction was found exactly opposite to that of the analogous allylation reaction. The method provided eleven α-allenic alcohols in 22–99% yield with 61:39–92:8 enantiomeric ratios. Furthermore, possible mechanisms of propargyl–allenyl isomerization of propargyltrichlorosilane were computationally investigated. 

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5. Melding of Experiment and Theory Illuminates Mechanisms of Metal-Catalyzed Rearrangements: Computational Approaches and Caveats

   https://doi.org/10.1055/s-0040-1720451  

   Laconsay, Croix J.; Tantillo, Dean J. (October 2021, Synthesis)

   Abstract This review summarizes approaches and caveats in computational modeling of transition-metal-catalyzed sigmatropic rearrangements involving carbene transfer. We highlight contemporary examples of combined synthetic and theoretical investigations that showcase the synergy achievable by integrating experiment and theory. 1 Introduction 2 Mechanistic Models 3 Theoretical Approaches and Caveats 3.1 Recommended Computational Tools 3.2 Choice of Functional and Basis Set 3.3 Conformations and Ligand-Binding Modes 3.4 Solvation 4 Synergy of Experiment and Theory – Case Studies 4.1 Metal-Bound or Free Ylides? 4.2 Conformations and Ligand-Binding Modes of Paddlewheel Complexes 4.3 No Metal, Just Light 4.4 How To ‘Cope’ with Nonstatistical Dynamic Effects 5 Outlook 

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