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- ItemMechanistic Understanding of the Heterogeneous, Rhodium-Cyclic (Alkyl)(Amino)Carbene-Catalyzed (Fluoro-)Arene Hydrogenation(Washington, DC : American Chemical Society, 2020) Moock D.; Wiesenfeldt M.P.; Freitag M.; Muratsugu S.; Ikemoto S.; Knitsch R.; Schneidewind J.; Baumann W.; Schäfer A.H.; Timmer A.; Tada M.; Hansen M.R.; Glorius F.Recently, chemoselective methods for the hydrogenation of fluorinated, silylated, and borylated arenes have been developed providing direct access to previously unattainable, valuable products. Herein, a comprehensive study on the employed rhodium-cyclic (alkyl)(amino)carbene (CAAC) catalyst precursor is disclosed. Mechanistic experiments, kinetic studies, and surface-spectroscopic methods revealed supported rhodium(0) nanoparticles (NP) as the active catalytic species. Further studies suggest that CAAC-derived modifiers play a key role in determining the chemoselectivity of the hydrogenation of fluorinated arenes, thus offering an avenue for further tuning of the catalytic properties. Copyright © 2020 American Chemical Society.
- ItemCobalt-catalysed reductive C-H alkylation of indoles using carboxylic acids and molecular hydrogen(Cambridge : RSC, 2017) Cabrero-Antonino, Jose R.; Adam, Rosa; Junge, Kathrin; Beller, MatthiasThe direct CH-alkylation of indoles using carboxylic acids is presented for the first time. The catalytic system based on the combination of Co(acac)3 and 1,1,1-tris(diphenylphosphinomethyl)-ethane (Triphos, L1), in the presence of Al(OTf)3 as co-catalyst, is able to perform the reductive alkylation of 2-methyl-1H-indole with a wide range of carboxylic acids. The utility of the protocol was further demonstrated through the C3 alkylation of several substituted indole derivatives using acetic, phenylacetic or diphenylacetic acids. In addition, a careful selection of the reaction conditions allowed to perform the selective C3 alkenylation of some indole derivatives. Moreover, the alkenylation of C2 position of 3-methyl-1H-indole was also possible. Control experiments indicate that the aldehyde, in situ formed from the carboxylic acid hydrogenation, plays a central role in the overall process. This new protocol enables the direct functionalization of indoles with readily available and stable carboxylic acids using a non-precious metal based catalyst and hydrogen as reductant.
- ItemOrigins of high catalyst loading in copper(i)-catalysed Ullmann-Goldberg C-N coupling reactions(Cambridge : RSC, 2017) Sherborne, Grant J.; Adomeit, Sven; Menzel, Robert; Rabeah, Jabor; Brückner, Angelika; Fielding, Mark R.; Willans, Charlotte E.; Nguyen, Bao N.A mechanistic investigation of Ullmann-Goldberg reactions using soluble and partially soluble bases led to the identification of various pathways for catalyst deactivation through (i) product inhibition with amine products, (ii) by-product inhibition with inorganic halide salts, and (iii) ligand exchange by soluble carboxylate bases. The reactions using partially soluble inorganic bases showed variable induction periods, which are responsible for the reproducibility issues in these reactions. Surprisingly, more finely milled Cs2CO3 resulted in a longer induction period due to the higher concentration of the deprotonated amine/amide, leading to suppressed catalytic activity. These results have significant implications on future ligand development for the Ullmann-Goldberg reaction and on the solid form of the inorganic base as an important variable with mechanistic ramifications in many catalytic reactions.
- ItemGeneral and selective synthesis of primary amines using Ni-based homogeneous catalysts(Cambridge : RSC, 2020) Murugesan, Kathiravan; Wei, Zhihong; Chandrashekhar, Vishwas G.; Jiao, Haijun; Beller, Matthias; Jagadeesh, Rajenahally V.The development of base metal catalysts for industrially relevant amination and hydrogenation reactions by applying abundant and atom economical reagents continues to be important for the cost-effective and sustainable synthesis of amines which represent highly essential chemicals. In particular, the synthesis of primary amines is of central importance because these compounds serve as key precursors and central intermediates to produce value-added fine and bulk chemicals as well as pharmaceuticals, agrochemicals and materials. Here we report a Ni-triphos complex as the first Ni-based homogeneous catalyst for both reductive amination of carbonyl compounds with ammonia and hydrogenation of nitroarenes to prepare all kinds of primary amines. Remarkably, this Ni-complex enabled the synthesis of functionalized and structurally diverse benzylic, heterocyclic and aliphatic linear and branched primary amines as well as aromatic primary amines starting from inexpensive and easily accessible carbonyl compounds (aldehydes and ketones) and nitroarenes using ammonia and molecular hydrogen. This Ni-catalyzed reductive amination methodology has been applied for the amination of more complex pharmaceuticals and steroid derivatives. Detailed DFT computations have been performed for the Ni-triphos based reductive amination reaction, and they revealed that the overall reaction has an inner-sphere mechanism with H2metathesis as the rate-determining step. © The Royal Society of Chemistry 2020.
- ItemDesign of a core-shell catalyst : an effective strategy for suppressing side reactions in syngas for direct selective conversion to light olefins(Cambridge : RSC, 2020) Tan, Li; Wang, Fan; Zhang, Peipe; Suzuki, Yuichi; Wu, Yingquan; Chen, Jiangang; Yang, Guohui; Tsubaki, NoritatsuAn elegant catalyst is designedviathe encapsulation of metallic oxide Zn-Cr inside of zeolite SAPO34 as a core-shell structure (Zn-Cr@SAPO) to realize the coupling of methanol-synthesis and methanol-to-olefin reactions. It can not only break through the limitation of the Anderson-Schulz-Flory distribution but can also overcome the disadvantages of physical mixture catalysts, such as excessive CO2formation. The confinement effect, hierarchical structure and extremely short distance between the two active components result in the Zn-Cr@SAPO capsule catalyst having better mass transfer and diffusion with a boosted synergistic effect. Due to the difference between the adsorption energies of the Zn-Cr metallic oxide/SAPO zeolite physical mixture and capsule catalysts, the produced water and light olefins are easily removed from the Zn-Cr@SAPO capsule catalyst after formation, suppressing the side reactions. The light olefin space time yield (STY) of the capsule catalyst is more than twice that of the typical physical mixture catalyst. The designed capsule catalyst has superior potential for scale-up in industrial applications while simultaneously extending the capabilities of hybrid catalysts for other tandem catalysis reactions through this strategy. © The Royal Society of Chemistry 2020.
- ItemSelective cobalt nanoparticles for catalytic transfer hydrogenation of N-heteroarenes(Cambridge : RSC, 2017) Chen, Feng; Sahoo, Basudev; Kreyenschulte, Carsten; Lund, Henrik; Zeng, Min; He, Lin; Junge, Kathrin; Beller, MatthiasNitrogen modified cobalt catalysts supported on carbon were prepared by pyrolysis of the mixture generated from cobalt(ii) acetate in aqueous solution of melamine or waste melamine resins, which are widely used as industrial polymers. The obtained nanostructured materials catalyze the transfer hydrogenation of N-heteroarenes with formic acid in the absence of base. The optimal Co/Melamine-2@C-700 catalyst exhibits high activity and selectivity for the dehydrogenation of formic acid into molecular hydrogen and carbon dioxide and allows for the reduction of diverse N-heteroarenes including substrates featuring sensitive functional groups.
- ItemHydration of nitriles using a metal-ligand cooperative ruthenium pincer catalyst(Cambridge : RSC, 2019) Guo, Beibei; de Vries, Johannes G.; Otten, EdwinNitrile hydration provides access to amides that are important structural elements in organic chemistry. Here we report catalytic nitrile hydration using ruthenium catalysts based on a pincer scaffold with a dearomatized pyridine backbone. These complexes catalyze the nucleophilic addition of H2O to a wide variety of aliphatic and (hetero)aromatic nitriles in tBuOH as solvent. Reactions occur under mild conditions (room temperature) in the absence of additives. A mechanism for nitrile hydration is proposed that is initiated by metal-ligand cooperative binding of the nitrile. This journal is © The Royal Society of Chemistry.
- ItemSelective Earth-Abundant System for CO2 Reduction: Comparing Photo- and Electrocatalytic Processes(Washington, DC : American Chemical Society, 2019) Steinlechner C.; Roesel A.F.; Oberem E.; Päpcke A.; Rockstroh N.; Gloaguen F.; Lochbrunner S.; Ludwig R.; Spannenberg A.; Junge H.; Francke R.; Beller M.The valorization of CO2 via photo- or electrocatalytic reduction constitutes a promising approach toward the sustainable production of fuels or value-added chemicals using intermittent renewable energy sources. For this purpose, molecular catalysts are generally studied independently with respect to the photo- or the electrochemical application, although a unifying approach would be much more effective with respect to the mechanistic understanding and the catalyst optimization. In this context, we present a combined photo- and electrocatalytic study of three Mn diimine catalysts, which demonstrates the synergistic interplay between the two methods. The photochemical part of our study involves the development of a catalytic system containing a heteroleptic Cu photosensitizer and the sacrificial BIH reagent. The system shows exclusive selectivity for CO generation and renders turnover numbers which are among the highest reported thus far within the group of fully earth-abundant photocatalytic systems. The electrochemical part of our investigations complements the mechanistic understanding of the photochemical process and demonstrates that in the present case the sacrificial reagent, the photosensitizer, and the irradiation source can be replaced by the electrode and a weak Brønsted acid. © 2019 American Chemical Society.
- ItemConstruction of cost-effective bimetallic nanoparticles on titanium carbides as a superb catalyst for promoting hydrolysis of ammonia borane(London : RSC Publishing, 2018) Guo, Zhangwei; Liu, Tong; Wang, Qingtao; Gao, GuanhuiBimetallic cost-effective CoNi nanoparticles (NPs) are conveniently supported on titanium carbides (MXene) by a simple one-step wet-chemical method. The synthesized CoNi/MXene catalysts are characterized by XPS, TEM, STEM-HAADF and ICP-AES. The as-prepared CoNi NPs with a size of 2.8 nm are well dispersed on the MXene surface. It is found that among the CoNi bimetallic system, Co0.7Ni0.3 shows the best performance toward catalyzing ammonia borane (AB) decomposition with a turnover frequency value of 87.6 molH2 molcat−1 min−1 at 50 °C. The remarkable catalytic performance is attributed to the mild affiliation of MXene to NPs, which not only stabilizes NPs to maintain a good dispersion but also leaves sufficient surface active sites to facilitate the catalytic reaction.