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- ItemTheoretical mechanistic investigation of zinc(ii) catalyzed oxidation of alcohols to aldehydes and esters(London : RSC Publishing, 2016) Nisa, Riffat Un; Mahmood, Tariq; Ludwig, Ralf; Ayub, KhurshidThe mechanism of the Zn(II) catalyzed oxidation of benzylic alcohol to benzaldehyde and ester by H2O2 oxidant was investigated through density functional theory methods and compared with the similar oxidation mechanisms of other late transition metals. Both inner sphere and intermediate sphere mechanisms have been analyzed in the presence and absence of pyridine-2-carboxylic acid (ligand). An intermediate sphere mechanism involving the transfer of hydrogen from alcohol to H2O2 was found to be preferred over the competitive inner sphere mechanism involving β-hydride elimination. Kinetic barriers associated with the intermediate sphere mechanism are consistent with the experimental observations, suggesting that the intermediate sphere mechanism is a plausible mechanism under these reaction conditions. The oxidation of alcohols to aldehydes (first step) is kinetically more demanding than the oxidation of hemiacetals to esters (second step). Changing the oxidant to tert-butyl hydrogen peroxide (TBHP) increases the activation barrier for the oxidation of alcohol to aldehyde by 0.4 kcal mol−1, but decreases the activation barrier by 3.24 kcal mol−1 for oxidation of hemiacetal to ester. Replacement of zinc bromide with zinc iodide causes the second step to be more demanding than the first step. Pyridine-2-carboxylic acid ligand remarkably decreases the activation barriers for the intermediate sphere pathway, whereas a less pronounced inverse effect is estimated for the inner sphere mechanism.
- 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.
- ItemManganese-catalyzed selective C–H activation and deuteration by means of a catalytic transient directing group strategy(London : Royal Society of Chemistry (RSC), 2021) Kopf, Sara; Neumann, Helfried; Beller, MatthiasA novel manganese-catalyzed C-H activation methodology for selective hydrogen isotope exchange of benzaldehydes is presented. Using D2O as a cheap and convenient source of deuterium, the reaction proceeds with excellent functional group tolerance. Highortho-selectivity is achieved in the presence of catalytic amounts of specific amines, whichin situform a transient directing group. © The Royal Society of Chemistry 2021.
- ItemNickel-Catalyzed Carbonylative Synthesis of Functionalized Alkyl Iodides(Amsterdam : Elsevier B.V., 2018) Peng, J.-B.; Wu, F.-P.; Xu, C.; Qi, X.; Ying, J.; Wu, X.-F.Chemistry; Catalysis; Organic Synthesis © 2018 The Author(s)Functionalized alkyl iodides are important compounds in organic chemistry and biology. In this communication, we developed an interesting nickel-catalyzed carbonylative synthesis of functionalized alkyl iodides from aryl iodides and ethers. With Mo(CO)6 as the solid CO source, both cyclic and acyclic ethers were activated, which is also a challenging topic in organic synthesis. Functionalized alkyl iodides were prepared in moderate to excellent yields with outstanding functional group tolerance. Besides the high value of the obtained products, all the atoms from the starting materials were incorporated in the final products and the reaction had high atom efficiency as well.
- ItemEnantio- and diastereoselective synthesis of γ-amino alcohols(Cambridge : Soc., 2015) Verkade, Jorge M. M.; Quaedflieg, Peter J. L. M.; Verzijl, Gerard K. M.; Lefort, Laurent; van Delft, Floris L.; de Vries, Johannes G.; Rutjes, Floris P. J. T.The γ-amino alcohol structural motif is often encountered in drugs and natural products. We developed two complementary catalytic diastereoselective methods for the synthesis of N-PMP-protected γ-amino alcohols from the corresponding ketones. The anti-products were obtained through Ir-catalyzed asymmetric transfer hydrogenation, the syn-products via Rh-catalyzed asymmetric hydrogenation.
- ItemNovel naphthylpyridines from cobalt-catalyzed cyclotrimerization of a chiral diyne(Wien [u.a.] : Springer, 2017-11-28) Trommer, Volkmar; Fischer, Fabian; Hapke, MarkoThe concise synthesis of a novel chiral diyne substrate for the assembly of chiral naphthylpyridines was described and different conditions for the cobalt-catalyzed co-cyclotrimerization with nitriles investigated. The products are novel naphthylpyridines possessing configurationally stable biaryl axes.
- ItemAldehydes and ketones influence reactivity and selectivity in nickel-catalysed Suzuki-Miyaura reactions(Cambridge : RSC, 2020) Cooper, Alasdair K.; Leonard, David K.; Bajo, Sonia; Burton, Paul M.; Nelson, David J.The energetically-favorable coordination of aldehydes and ketones-but not esters or amides-to Ni0 during Suzuki-Miyaura reactions can lead either to exquisite selectivity and enhanced reactivity, or to inhibition of the reaction. Aryl halides where the C-X bond is connected to the same π-system as an aldehyde or ketone undergo unexpectedly rapid oxidative addition to [Ni(COD)(dppf)] (1), and are selectively cross-coupled during competition reactions. When aldehydes and ketones are present in the form of exogenous additives, the cross-coupling reaction is inhibited to an extent that depends on the strength of the coordination of the pendant carbonyl group to Ni0. This work advances our understanding of how common functional groups interact with Ni0 catalysts and how these interactions affect workhorse catalytic reactions in academia and industry. This journal is © The Royal Society of Chemistry.
- ItemUnprecedented selective homogeneous cobalt-catalysed reductive alkoxylation of cyclic imides under mild conditions(Cambridge : RSC, 2017) Cabrero-Antonino, Jose R.; Adam, Rosa; Papa, Veronica; Holsten, Mattes; Junge, Kathrin; Beller, MatthiasThe first general and efficient non-noble metal-catalysed reductive C2-alkoxylation of cyclic imides (phthalimides and succinimides) is presented. Crucial for the success is the use of [Co(BF4)2·6H2O/triphos (L1)] combination and no external additives are required. Using the optimal cobalt-system, the hydrogenation of the aromatic ring of the parent phthalimide is avoided and only one of the carbonyl groups is selectively functionalized. The resulting products, N- and aryl-ring substituted 3-alkoxy-2,3-dihydro-1H-isoindolin-1-one and N-substituted 3-alkoxy-pyrrolidin-2-one derivatives, are prepared under mild conditions in good to excellent isolated yields. Intramolecular reductive couplings can also be performed affording tricyclic compounds in a one-step process. The present protocol opens the way to the development of new base-metal processes for the straightforward synthesis of functionalized N-heterocyclic compounds of pharmaceutical and biological interest.
- ItemZnO Nanoparticles Encapsulated in Nitrogen-Doped Carbon Material and Silicalite-1 Composites for Efficient Propane Dehydrogenation(Amsterdam [u.a.] : Elsevier, 2019) Zhao, Dan; Li, Yuming; Han, Shanlei; Zhang, Yaoyuan; Jiang, Guiyuan; Wang, Yajun; Guo, Ke; Zhao, Zhen; Xu, Chunming; Li, Ranjia; Yu, Changchun; Zhang, Jian; Ge, Binghui; Kondratenko, Evgenii V.Chemistry; Catalysis; Nanoparticles © 2019 The Author(s)Non-oxidative propane dehydrogenation (PDH)is an attractive reaction from both an industrial and a scientific viewpoint because it allows direct large-scale production of propene and fundamental analysis of C-H activation respectively. The main challenges are related to achieving high activity, selectivity, and on-stream stability of environment-friendly and cost-efficient catalysts without non-noble metals. Here, we describe an approach for the preparation of supported ultrasmall ZnO nanoparticles (2–4 nm, ZnO NPs)for high-temperature applications. The approach consists of encapsulation of NPs into a nitrogen-doped carbon (NC)layer in situ grown from zeolitic imidazolate framework-8 on a Silicalite-1 support. The NC layer was established to control the size of ZnO NPs and to hinder their loss to a large extent at high temperatures. The designed catalysts exhibited high activity, selectivity, and on-stream stability in PDH. Propene selectivity of about 90% at 44.4% propane conversion was achieved at 600°C after nearly 6 h on stream. © 2019 The Author(s)
- ItemMechanistic insight of TiCl4catalyzed formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles(London : RSC Publishing, 2015) Nisa, Riffat Un; Maria, Maria; Wasim, Fatima; Mahmood, Tariq; Ludwig, Ralf; Ayub, KhurshidThe mechanism of TiCl4 mediated formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles is studied at the B3LYP level of density functional theory (DFT) to rationalize the experimental regioselectivity. Methyl and trifluoromethyl substituted 1,3 dielectrophiles are studied theoretically since they show different regioselectivities. Two different mechanisms involving 1,2 and 1,4 addition of 1,3-bis(silyl enol ethers) on 1,3-dielectrophiles are studied for each dienophile. The intramolecular transition metal catalyzed and non-catalyzed dynamic shift of the silyl moiety is also studied. The structure of the 1,3 dienophile and the associated Mulliken charges are the driving forces for different regioselectivities in methyl and trifluoromethyl dienophiles.