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1. IPr* ^Oxa – a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

   https://doi.org/10.1039/D3DT02255B  

   Podchorodecka, Pamela; Dziuk, Błażej; Szostak, Roman; Szostak, Michal; Bisz, Elwira (October 2023, Dalton Transactions)

   We report a new class of sterically-unsymmetrical NHC ligands. The evaluation of steric, electronic properties and coordination chemistry is described. Studies of catalytic activity in Ag, Pd and Rh-catalyzed reactions are presented. 

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2. Rhodium/Gold Dual Catalysis in Carbene sp ² C−H Functionalization/Conia‐ene Cascade for the Stereoselective Synthesis of Diverse Spirocarbocycles

   https://doi.org/10.1002/adsc.201900079  

   Hunter, Arianne_C; Chinthapally, Kiran; Bain, Anae_I; Stevens, Joseph_C; Sharma, Indrajeet (April 2019, Advanced Synthesis & Catalysis)

   Abstract A Rh(II)/Au(I) catalyzed carbene cascade approach for the stereoselective synthesis of diverse spirocarbocycles is described. The cascade reaction involves a rhodium carbene initiatedsp²C−H functionalization followed by a gold catalyzed Conia‐ene cyclization. The cascade reaction accommodates a variety of aryl substituents as well as ring sizes and proceeds with high diastereoselectivity providing access to diverse spirocarbocycles. magnified image 

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3. Nitrene C−H Bond Insertion Approach to Carbazolones and Indolones, and a Reactivity Departure for 7‐Membered Analogues**

   https://doi.org/10.1002/chem.202302995  

   Lakshman, Mahesh_K; Sebastian, Dellamol; Pradhan, Padmanava; Neary, Michelle_C; Piette, Alexis_M; Trzebiatowski, Samuel_P; Henriques, Alexander_E_K; Willoughby, Patrick_H (November 2023, Chemistry – A European Journal)

   Abstract A modular platform for facile access to 1,2,3,9‐tetrahydro‐4H‐carbazol‐4‐ones (H₄‐carbazolones) and 3,4‐dihydrocyclopenta[b]indol‐1(2H)‐ones (H₂‐indolones) is described. The requisite 6‐ and 5‐membered 2‐arylcycloalkane‐1,3‐dione precursors were readily obtained through a Cu‐catalyzed arylation of 1,3‐cyclohexanediones or by a ring expansion of aryl succinoin derivatives. Enolization of one carbonyl group in the diones, conversion to a leaving group, and subsequent azidation gave 2‐aryl‐3‐azidocycloalk‐2‐en‐1‐ones. This two‐step, one‐pot azidation is highly regioselective with unsymmetrically substituted 2‐arylcyclohexane‐1,3‐diones. The regioselectivity, which is important for access to single isomers of 3,3‐disubstituted carbazolones, was analyzed mechanistically and computationally. Finally, a Rh‐catalyzed nitrene/nitrenoid insertion into theorthoC−H bond of the aryl moiety gave the H₄‐carbazolones and H₂‐indolones. One carbazolone was elaborated to an intermediate reported in the total synthesis ofN‐decarbomethoxychanofruticosinate, (−)‐aspidospermidine, (+)‐kopsihainanine A. With 2‐phenylcycloheptane‐1,3‐dione, prepared from cyclohexanone and benzaldehyde, the azidation reaction was readily accomplished. However, the Rh‐catalyzed reaction unexpectedly led to a labile but characterizable azirine rather than the indole derivative. Computations were performed to understand the differences in reactivities of the 5‐ and 6‐membered 2‐aryl‐3‐azidocycloalk‐2‐en‐1‐ones in comparison to the 7‐membered analogue, and to support the structural assignment of the azirine. 

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4. Light‐Driven Hydrodefluorination of Electron‐Rich Aryl Fluorides by an Anionic Rhodium‐Gallium Photoredox Catalyst

   https://doi.org/10.1002/anie.202205575  

   Moore, James T.; Dorantes, Michael J.; Pengmei, Zihan; Schwartz, Timothy M.; Schaffner, Jacob; Apps, Samantha L.; Gaggioli, Carlo A.; Das, Ujjal; Gagliardi, Laura; Blank, David A.; et al (September 2022, Angewandte Chemie International Edition)

   Abstract An anionic Rh−Ga complex catalyzed the hydrodefluorination of challenging C−F bonds in electron‐rich aryl fluorides and trifluoromethylarenes when irradiated with violet light in the presence of H₂, a stoichiometric alkoxide base, and a crown‐ether additive. Based on theoretical calculations, the lowest unoccupied molecular orbital (LUMO), which is delocalized across both the Rh and Ga atoms, becomes singly occupied upon excitation, thereby poising the Rh−Ga complex for photoinduced single‐electron transfer (SET). Stoichiometric and control reactions support that the C−F activation is mediated by the excited anionic Rh−Ga complex. After SET, the proposed neutral Rh⁰intermediate was detected by EPR spectroscopy, which matched the spectrum of an independently synthesized sample. Deuterium‐labeling studies corroborate the generation of aryl radicals during catalysis and their subsequent hydrogen‐atom abstraction from the THF solvent to generate the hydrodefluorinated arene products. Altogether, the combined experimental and theoretical data support an unconventional bimetallic excitation that achieves the activation of strong C−F bonds and uses H₂and base as the terminal reductant. 

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5. Ligand‐Promoted Rh ^I ‐Catalyzed C2‐Selective C−H Alkenylation and Polyenylation of Imidazoles with Alkenyl Carboxylic Acids

   https://doi.org/10.1002/chem.202200441  

   Zhao, Haoqiang; Luo, Zhenli; Yang, Ji; Li, Bohan; Han, Jiahong; Xu, Lijin; Lai, Wenzhen; Walsh, Patrick_J (May 2022, Chemistry – A European Journal)

   Abstract The first Rh^I‐catalyzed, directed decarbonylative C2−H alkenylation of imidazoles with readily available alkenyl carboxylic acids is reported. The reaction proceeds in a highly regio‐ and stereoselective manner, providing efficient access to C2‐alkenylated imidazoles that are generally inaccessible by known C−H alkenylation methods. This transformation accommodates a wide range of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, and diversely decorated imidazoles with high functional group compatibility. The presence of a removable pyrimidine directing group and the use of a bidentate phosphine ligand are pivotal to the success of the catalytic reaction. This process is also suitable for benzimidazoles. Importantly, the scalability and diversification of the products highlight the potential of this protocol in practical applications. Detailed experimental and computational studies provide important insights into the underlying reaction mechanism. 

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