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1. Mechanistic investigations of alcohol silylation with isothiourea catalysts

   https://doi.org/10.1039/d1ob01732b  

   Redden, Brandon K.; Clark, Robert W.; Gong, Ziyuan; Rahman, Md. Mamdudur; Peryshkov, Dmitry V.; Wiskur, Sheryl L. (December 2021, Organic & Biomolecular Chemistry)

   The mechanism of the asymmetric silylation of alcohols with isothiourea catalysts was studied by employing reaction progress kinetic analysis. These reactions were developed by the Wiskur group, and use triphenyl silyl chloride and chiral isothiourea catalysts to silylate the alcohols. While the order of most reaction components was as expected (catalyst, amine base, alcohol), the silyl chloride was determined to be a higher order. This suggested a multistep mechanism between the catalyst and silyl chloride, with the second equivalent of silyl chloride assisting in the formation of the reactive intermediate leading to the rate-determining step. Through the addition of additives and investigating changes in the silyl chloride, an understanding of the catalyst equilibrium emerged for this reaction and provided pathways for further reaction development. 

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2. Mechanistic investigations on a homogeneous ruthenium Guerbet catalyst in a flow reactor

   https://doi.org/10.1039/D1RE00551K  

   Wang, Nicholas M.; Dillon, Sam; Guironnet, Damien (March 2022, Reaction Chemistry & Engineering)

   A mechanistic investigation on the ethanol self-condensation reaction (Guerbet reaction) catalyzed by a bis(pyridylimino)isoindolate Ru( ii ) catalyst was performed using a specifically designed continuously-stirred tank reactor (CSTR). Leveraging vapor–liquid equilibrium, the homogeneous catalyst was maintained in the reactor at a constant concentration by dissolving it in a non-volatile solvent while volatile substrates were fed continuously. The activity of the catalyst was monitored by analyzing the vapor exiting the reactor (reagents and products) using an in-line gas chromatograph. The formation of C 6 products demonstrates the catalyst's reactivity towards butanol, and the detection of solely saturated products implies that hydrogenation is fast under the reaction conditions. These observations led us to perform a detailed study of the hydrogenation step that provided evidence for a hydrogen-transfer pathway. The corresponding reaction mechanism for the Guerbet reaction was established. 

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3. Immobilization of molecular catalysts on solid supports via atomic layer deposition for chemical synthesis in sustainable solvents

   https://doi.org/10.1039/D1GC02024B  

   Ayare, Pooja J.; Gregory, Shawn A.; Key, Ryan J.; Short, Andrew E.; Tillou, Jake G.; Sitter, James D.; Yom, Typher; Goodlett, Dustin W.; Lee, Dong-Chan; Alamgir, Faisal M.; et al (November 2021, Green Chemistry)

   Homogeneous molecular catalysts are valued for their reaction specificity but face challenges in manufacturing scale-up due to complexities in final product separation, catalyst recovery, and instability in the presence of water. Heterogenizing these molecular catalysts, by attachment to a solid support, could transform the practical utility of molecular catalysts, simplify catalyst separation and recovery, and prevent catalyst decomposition by impeding bimolecular catalyst interactions. Previous strategies to heterogenize molecular catalysts via ligand-first binding to supports have suffered from reduced catalytic activity and leaching (loss) of catalyst, especially in environmentally friendly solvents like water. Herein, we describe an approach in which molecular catalysts are first attached to a metal oxide support through acidic ligands and then “encapsulated” with a metal oxide layer via atomic layer deposition (ALD) to prevent molecular detachment from the surface. For this initial report, which is based upon the well-studied Suzuki carbon–carbon cross-coupling reaction, we demonstrate the ability to achieve catalytic performance using a non-noble metal molecular catalyst in high aqueous content solvents. The catalyst chosen exhibits limited catalytic reactivity under homogeneous conditions due to extremely short catalyst lifetimes, but when heterogenized and immobilized with an optimal ALD layer thickness product yields >90% can be obtained in primarily aqueous solutions. Catalyst characterization before and after ALD application and catalytic reaction is achieved with infrared, electron paramagnetic resonance, and X-ray spectroscopies. 

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4. Tailored quinones support high-turnover Pd catalysts for oxidative C–H arylation with O ₂

   https://doi.org/10.1126/science.abd1085  

   Salazar, Chase A.; Flesch, Kaylin N.; Haines, Brandon E.; Zhou, Philip S.; Musaev, Djamaladdin G.; Stahl, Shannon S. (December 2020, Science)

   null (Ed.)

   Palladium(II)-catalyzed C–H oxidation reactions could streamline the synthesis of pharmaceuticals, agrochemicals, and other complex organic molecules. Existing methods, however, commonly exhibit poor catalyst performance with high Pd loading (e.g., 10 mol %) and a need for (super)stoichiometric quantities of undesirable oxidants, such as benzoquinone and silver(I) salts. The present study probes the mechanism of a representative Pd-catalyzed oxidative C–H arylation reaction and elucidates mechanistic features that undermine catalyst performance, including substrate-consuming side reactions and sequestration of the catalyst as inactive species. Systematic tuning of the quinone co-catalyst overcomes these deleterious features. Use of 2,5-di- tert -butyl- p -benzoquinone enables efficient use of molecular oxygen as the oxidant, high reaction yields, and >1900 turnovers by the palladium catalyst. 

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5. Self-Assembled Bimetallic Aluminum-Salen Catalyst for the Cyclic Carbonates Synthesis

   https://doi.org/10.3390/molecules26134097  

   Seong, Wooyong; Hahm, Hyungwoo; Kim, Seyong; Park, Jongwoo; Abboud, Khalil A.; Hong, Sukwon (July 2021, Molecules)

   null (Ed.)

   Bimetallic bis-urea functionalized salen-aluminum catalysts have been developed for cyclic carbonate synthesis from epoxides and CO2. The urea moiety provides a bimetallic scaffold through hydrogen bonding, which expedites the cyclic carbonate formation reaction under mild reaction conditions. The turnover frequency (TOF) of the bis-urea salen Al catalyst is three times higher than that of a μ-oxo-bridged catalyst, and 13 times higher than that of a monomeric salen aluminum catalyst. The bimetallic reaction pathway is suggested based on urea additive studies and kinetic studies. Additionally, the X-ray crystal structure of a bis-urea salen Ni complex supports the self-assembly of the bis-urea salen metal complex through hydrogen bonding. 

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