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1. Cycloisomerization of Alkyne‐Tethered N ‐Acyloxycarbamates to 2‐( 3H )Oxazolones through Nitrenoid‐Mediated Carboxyamidation

   https://doi.org/10.1002/chem.202203371  

   Su, Siyuan; Wu, Tongtong; Xia, Yuanzhi; Wink, Donald_J; Lee, Daesung (March 2023, Chemistry – A European Journal)

   Abstract The cycloisomerization of alkyne‐tetheredN‐benzoyloxycarbamates to 2‐(3H)oxazolones is described. Two catalytic systems are tailored for intramolecular 5‐exo‐alkyne carboxyamidation and concomitant alkene isomerization. PtCl₂/CO (5 mol%, toluene, 100 °C) promotes both carboxyamidation and alkene isomerization but has a limited substrate scope. On the other hand, FeCl₃(5 mol%, CH₃CN, 100 °C) promotes carboxyamidation effectively but a cocatalyst is required for the exocyclic alkene isomerization. Thus, a two‐step one‐pot protocol has been developed for a broader reaction scope, which involves FeCl₃‐catalyzed carboxyamidation and base‐induced alkene isomerization. Crossover experiments suggest that these reactions proceed mainly through a mechanism involving acylnitrenoid intermediates rather than carbenoid intermediates. 

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2. Methods to Investigate the Kinetic Profile of Cysteine Desulfurases

   https://doi.org/https://doi.org/10.1007/978-1-0716-1605-5_10  

   Addo, M.A.; Edwards, A. E.; Dos Santos, P.C. (July 2021, Methods in molecular biology)

   Dos Santos, P.C. (Ed.)

   Biological iron-sulfur (Fe-S) clusters are essential protein prosthetic groups that promote a range of biochemical reactions. In vivo, these clusters are synthesized by specialized protein machineries involved in sulfur mobilization, cluster assembly, and cluster transfer to their target proteins. Cysteine desulfurases initiate the first step of sulfur activation and mobilization in cluster biosynthetic pathways. The reaction catalyzed by these enzymes involves the abstraction of sulfur from the amino acid l-cysteine, with concomitant formation of alanine. The presence and availability of a sulfur acceptor modulate the sulfurtransferase activity of this class of enzymes by altering their reaction profile and catalytic turnover rate. Herein, we describe two methods used to probe the reaction profile of cysteine desulfurases through quantification of alanine and sulfide production in these reactions. 

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3. Protonated methylcyclopropane is an intermediate providing complete 13C-label scrambling at C4 olefin isomerization in zeolite

   https://doi.org/10.1016/j.checat.2022.100503  

   Chen, Wei; Huang, Mengdi; Yi, Xianfeng; Hui, Yu; Gao, Pan; Hou, Guangjin; Stepanov, Alexander G.; Qin, Yucai; Song, Lijuan; Liu, Shang-Bin; et al (February 2023, Chem Catalysis)

   Carbocations play crucial roles during catalytic reactions by dictating the reaction pathways and genuine mechanisms, but the instability of carbocations prevents thorough observations. The stabilization of carbocations would greatly help us gain a deep understanding of the reaction mechanisms. By means of ab initio molecular dynamics (AIMD) simulations and an in situ experimental approach, a complete scrambling of 13C-labeled C4 = products was observed during the isomerization reaction in the H-ZSM5 zeolite at room temperature, and the corner-protonated methyl cyclopropanes (as a non-classical carbocation) featuring the three center two-electron (3c–2e) bonds were confirmed to be the highly active metastable intermediates of C4 isomerization. Our results not only uncover the nature of facile C shift in carbocations during zeolite-catalyzed reactions but also bring some fundamental understandings to carbocation chemistry in a zeolite confined environment 

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4. Uncovering a superfamily of nickel‑dependent hydroxyacid racemases and epimerases

   https://doi.org/10.1038/s41598-020-74802-6  

   Desguin, Benoît; Urdiain‑ Arraiza, Julian; Da Costa, Matthieu; Fellner, Matthias; Hu, Jian; Hausinger, Robert P.; Desmet, Tom; Hols, Pascal; Soumillion, Patrice (October 2020, Scientific reports)

   null (Ed.)

   Isomerization reactions are fundamental in biology. Lactate racemase, which isomerizes L- and D-lactate, is composed of the LarA protein and a nickel-containing cofactor, the nickel-pincer nucleotide (NPN). In this study, we show that LarA is part of a superfamily containing many different enzymes. We overexpressed and purified 13 lactate racemase homologs, incorporated the NPN cofactor, and assayed the isomerization of different substrates guided by gene context analysis. We discovered two malate racemases, one phenyllactate racemase, one α-hydroxyglutarate racemase, two D-gluconate 2-epimerases, and one short-chain aliphatic α-hydroxyacid racemase among the tested enzymes. We solved the structure of a malate racemase apoprotein and used it, along with the previously described structures of lactate racemase holoprotein and D-gluconate epimerase apoprotein, to identify key residues involved in substrate binding. This study demonstrates that the NPN cofactor is used by a diverse superfamily of α-hydroxyacid racemases and epimerases, widely expanding the scope of NPN-dependent enzymes. 

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5. Using Experimental Spectroscopic Data to Guide and Validate Mechanisms in Catalyzed Aldol Reactions

   https://doi.org/10.1021/acs.jchemed.2c00185  

   Weaver, Kyle; Reeves, Jennifer A.; Konkolewicz, Dominik (August 2022, Journal of Chemical Education)

   Organic chemistry students often struggle with reaction mechanisms, particularly in how they are proposed and justified. In this activity targeting second year organic undergraduates, students used infrared spectroscopy (IR) to track the reaction progress of two distinct aldol reactions and used polarimetry to analyze the stereoselectivity of aldol catalysts. Students worked in two pairs, with one focusing on the traditional hydroxide-catalyzed aldol reaction (two units of propionaldehyde combining via an enolate intermediate) and the other focusing on the enantioselective l-proline-catalyzed aldol reaction (propionaldehyde catalyzed by l-proline, showing an iminium intermediate). During the course of the lab period, students used IR spectra showing kinetic data and guided questions to propose and validate the reaction mechanisms. After the pairs of students analyzed their individual reactions, they formed groups of four to further analyze and compare the two mechanisms. This comparison of IR and polarimetry data allowed students to discuss both pathways and consider why chemists use different reaction conditions to reach the same product. The focus of this experiment is to improve the understanding of reaction mechanisms and the process by which scientists propose and justify mechanisms, while giving students practical experience with IR spectroscopy, polarimetry, and intermediate analysis. 

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