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ACS Catalysis

Mercedes Boronat, Avelino Corma
Based on theoretical calculations of CO, NH3 , and pyridine adsorption at different sites in MOR and MFI zeolites, we analyze how confinement effects influence the measurement of acidity based on the interaction of probe molecules with Brönsted acid sites. Weak bases, such as CO, form neutral ZH-CO adducts with a linear configuration that can be distorted by spatial restrictions associated with the dimensions of the pore, leading to weaker interaction, but can also be stabilized by dispersion forces if a tighter fitting with the channel void is allowed...
February 1, 2019: ACS Catalysis
Mohamed Ismail, Lea Schroeder, Marcel Frese, Tilman Kottke, Frank Hollmann, Caroline E Paul, Norbert Sewald
Flavin-dependent halogenases are known to regioselectively introduce halide substituents into aromatic moieties, for example, the indole ring of tryptophan. The process requires halide salts and oxygen instead of molecular halogen in the chemical halogenation. However, the reduced cofactor flavin adenine dinucleotide (FADH2 ) has to be regenerated using a flavin reductase. Consequently, coupled biocatalytic steps are usually applied for cofactor regeneration. Nicotinamide adenine dinucleotide (NADH) mimics can be employed stoichiometrically to replace enzymatic cofactor regeneration in biocatalytic halogenation...
February 1, 2019: ACS Catalysis
Bas Venderbosch, Jean-Pierre H Oudsen, Lukas A Wolzak, David J Martin, Ties J Korstanje, Moniek Tromp
1-Hexene is an important α-olefin for polyethylene production and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixture of chromium 2-ethylhexanoate, AlEt3 , AlEt2 Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic experiments (XAS, EPR, and UV-vis) under conditions employed in industry...
February 1, 2019: ACS Catalysis
Sébastien J-P Willot, Elena Fernández-Fueyo, Florian Tieves, Milja Pesic, Miguel Alcalde, Isabel W C E Arends, Chan Beum Park, Frank Hollmann
Peroxygenases require a controlled supply of H2 O2 to operate efficiently. Here, we propose a photocatalytic system for the reductive activation of ambient O2 to produce H2 O2 which uses the energy provided by visible light more efficiently based on the combination of wavelength-complementary photosensitizers. This approach was coupled to an enzymatic system to make formate available as a sacrificial electron donor. The scope and current limitations of this approach are reported and discussed.
February 1, 2019: ACS Catalysis
Lieuwe Biewenga, Thangavelu Saravanan, Andreas Kunzendorf, Jan-Ytzen van der Meer, Tjaard Pijning, Pieter G Tepper, Ronald van Merkerk, Simon J Charnock, Andy-Mark W H Thunnissen, Gerrit J Poelarends
Chiral γ-aminobutyric acid (GABA) analogues represent abundantly prescribed drugs, which are broadly applied as anticonvulsants, as antidepressants, and for the treatment of neuropathic pain. Here we report a one-pot two-step biocatalytic cascade route for synthesis of the pharmaceutically relevant enantiomers of γ-nitrobutyric acids, starting from simple precursors (acetaldehyde and nitroalkenes), using a tailor-made highly enantioselective artificial "Michaelase" (4-oxalocrotonate tautomerase mutant L8Y/M45Y/F50A), an aldehyde dehydrogenase with a broad non-natural substrate scope, and a cofactor recycling system...
February 1, 2019: ACS Catalysis
Roderigh Y Rohling, Ionut C Tranca, Emiel J M Hensen, Evgeny A Pidko
The Diels-Alder cycloaddition (DAC) is a powerful tool to construct C-C bonds. The DAC reaction can be accelerated in several ways, one of which is reactant confinement as observed in supramolecular complexes and Diels-Alderases. Another method is altering the frontier molecular orbitals (FMOs) of the reactants by using homogeneous transition-metal complexes whose active sites exhibit d-orbitals suitable for net-bonding orbital interactions with the substrates. Both features can be combined in first row d-block (TM) exchanged faujasite catalysts where the zeolite framework acts as a stabilizing ligand for the active site while confining the reactants...
January 4, 2019: ACS Catalysis
Alberto Naldoni, Marco Altomare, Giorgio Zoppellaro, Ning Liu, Štěpán Kment, Radek Zbořil, Patrik Schmuki
Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2 , it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure-activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity...
January 4, 2019: ACS Catalysis
Mathias Pickl, Sara Kurakin, Fabián G Cantú Reinhard, Philipp Schmid, Alexander Pöcheim, Christoph K Winkler, Wolfgang Kroutil, Sam P de Visser, Kurt Faber
The majority of cytochrome P450 enzymes (CYPs) predominantly operate as monooxygenases, but recently a class of P450 enzymes was discovered, that can act as peroxygenases (CYP152). These enzymes convert fatty acids through oxidative decarboxylation, yielding terminal alkenes, and through α- and β-hydroxylation to yield hydroxy-fatty acids. Bioderived olefins may serve as biofuels, and hence understanding the mechanism and substrate scope of this class of enzymes is important. In this work, we report on the substrate scope and catalytic promiscuity of CYP OleTJE and two of its orthologues from the CYP152 family, utilizing α-monosubstituted branched carboxylic acids...
January 4, 2019: ACS Catalysis
Xiaoming Huang, Jasper M Ludenhoff, Mike Dirks, Xianhong Ouyang, Michael D Boot, Emiel J M Hensen
Lignocellulosic biomass is the only renewable source of carbon for the chemical industry. Alkylmethoxyphenols can be obtained in good yield from woody biomass by reductive fractionation, but these compounds are of limited value for large-scale applications. We present a method to convert lignocellulose-derived alkylmethoxyphenols to phenol that can be easily integrated in the petrochemical industry. The underlying chemistry combines demethoxylation catalyzed by a titania-supported gold nanoparticle catalyst and transalkylation of alkyl groups to a low-value benzene-rich stream promoted by HZSM-5 zeolite...
December 7, 2018: ACS Catalysis
Maximilian J L J Fürst, Elvira Romero, J Rúben Gómez Castellanos, Marco W Fraaije, Andrea Mattevi
Detoxifying enzymes such as flavin-containing monooxygenases deal with a huge array of highly diverse xenobiotics and toxic compounds. In addition to being of high physiological relevance, these drug-metabolizing enzymes are useful catalysts for synthetic chemistry. Despite the wealth of studies, the molecular basis of their relaxed substrate selectivity remains an open question. Here, we addressed this issue by applying a cumulative alanine mutagenesis approach to cyclohexanone monooxygenase from Thermocrispum municipale , a flavin-dependent Baeyer-Villiger monooxygenase which we chose as a model system because of its pronounced thermostability and substrate promiscuity...
December 7, 2018: ACS Catalysis
Arno J F van Hoof, Ivo A W Filot, Heiner Friedrich, Emiel J M Hensen
The restructuring of a silver catalyst during ethylene epoxidation under industrially relevant conditions was investigated without and with vinyl chloride (VC) promotion. During non-VC-promoted ethylene epoxidation, the silver particles grow and voids are formed at the surface and in the bulk. Electron tomography highlighted the presence of voids below the Ag surface. A mechanism is proposed involving reconstruction of the silver lattice and defect sites induced by oxygen adsorption on the external surface and grain boundaries, which finally create pores...
December 7, 2018: ACS Catalysis
Haixia Li, Jennifer V Obligacion, Paul J Chirik, Michael B Hall
The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans -(iPr PNP)CoH2 (BPin) as the resting state in the borylation of five-membered heteroarenes, and 4-BPin-(iPr PNP)Co(N2 )BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-(iPr PNP)CoBPin (R=H, BPin), were generated by reductive elimination of H2 in the former, through Berry pseudorotation to the cis isomer, and N2 loss in the latter...
November 2, 2018: ACS Catalysis
Xin Cindy Yan, Anthony J Metrano, Michael J Robertson, Nadia C Abascal, Julian Tirado-Rives, Scott J Miller, William L Jorgensen
It is widely accepted that structural rigidity is required to achieve high levels of asymmetric induction in catalytic, enantioselective reactions. This fundamental design principle often does not apply to highly selective catalytic peptides that often exhibit conformational heterogeneity. As a result, these complex systems are particularly challenging to study both experimentally and computationally. Herein, we utilize molecular dynamics simulations to investigate the role of conformational mobility on the reactivity and selectivity exhibited by a catalytic, β-turn-biased peptide in an atroposelective bromination reaction...
November 2, 2018: ACS Catalysis
Nitinchandra Dahyabhai Patel, Joshua D Sieber, Sergei Tcyrulnikov, Bryan J Simmons, Daniel Rivalti, Krishnaja Duvvuri, Yongda Zhang, Donghong A Gao, Keith R Fandrick, Nizar Haddad, Kendricks So Lao, Hari Prasad Reddy Mangunuru, Soumik Biswas, Bo Qu, Nelu Grinberg, Scott Pennino, Heewon Lee, Jinhua J Song, B Frank Gupton, Neil K Garg, Marisa C Kozlowski, Chris H Senanayake
Metal-catalyzed cross-coupling reactions are extensively employed in both academia and industry for the synthesis of biaryl derivatives for applications to both medicine and material science. Application of these methods to prepare tetra- ortho -substituted biaryls leads to chiral atropisomeric products that introduces the opportunity to use catalyst-control to develop asymmetric cross-coupling procedures to access these important compounds. Asymmetric Pd-catalyzed Suzuki-Miyaura and Negishi cross-coupling reactions to form tetra- ortho -substituted biaryls were studied employing a collection of P -chiral dihydrobenzooxaphosphole (BOP) and dihydrobenzoazaphosphole (BAP) ligands...
November 2, 2018: ACS Catalysis
Tom W van Deelen, Jelle J Nijhuis, Nynke A Krans, Jovana Zečević, Krijn P de Jong
Colloidal synthesis of nanocrystals (NC) followed by their attachment to a support and activation is a promising route to prepare model catalysts for research on structure-performance relationships. Here, we investigated the suitability of this method to prepare well-defined Co/TiO2 and Co/SiO2 catalysts for the Fischer-Tropsch (FT) synthesis with high control over the cobalt particle size. To this end, Co-NC of 3, 6, 9, and 12 nm with narrow size distributions were synthesized and attached uniformly on either TiO2 or SiO2 supports with comparable morphology and Co loadings of 2-10 wt %...
November 2, 2018: ACS Catalysis
Jie Wang, Ming Shang, Helena Lundberg, Karla S Feu, Scott J Hecker, Tian Qin, Donna G Blackmond, Phil S Baran
A simple method for the conversion of carboxylic acids to boronic esters via redox-active esters (RAEs) is reported using copper catalysis. The scope of this transformation is broad, and compared with the known protocols available, it represents the most inexpensive, rapid, and operationally simple option. In addition to a full exploration of the scope, a kinetic study was performed to elucidate substrate and reagent concentration dependences.
October 5, 2018: ACS Catalysis
John A Gurak, Keary M Engle
The hydroarylation of alkenes is an attractive approach to construct carbon-carbon (C-C) bonds from abundant and structurally diverse starting materials. Herein we report a palladiumcatalyzed reductive Heck hydroarylation of unactivated and heteroatom-substituted terminal alkenes with an array of (hetero)aryl iodides. The reaction is anti-Markovnikov selective and tolerates a wide variety of functional groups on both the alkene and (hetero)aryl coupling partners. Additionally, applications of this method to complex molecule diversifications were demonstrated...
October 5, 2018: ACS Catalysis
Juan Chen, Apparao Draksharapu, Davide Angelone, Duenpen Unjaroen, Sandeep K Padamati, Ronald Hage, Marcel Swart, Carole Duboc, Wesley R Browne
The oxidation of the C-H and C=C bonds of hydrocarbons with H2 O2 catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive FeIV =O species such as [(N4Py)FeIV =O]2+ formed by homolytic cleavage of the O-O bond of an FeIII -OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)- N , N -bis(pyridin-2-ylmethyl)methanamine). We show here that at low H2 O2 concentrations the FeIV =O species formed is detectable in methanol. Furthermore, we show that the decomposition of H2 O2 to water and O2 is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H2 O2 are present...
October 5, 2018: ACS Catalysis
Pieter Cnudde, Kristof De Wispelaere, Louis Vanduyfhuys, Ruben Demuynck, Jeroen Van der Mynsbrugge, Michel Waroquier, Veronique Van Speybroeck
Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C4 -C8 alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number...
October 5, 2018: ACS Catalysis
Amanda C Garcia, Marc T M Koper
No abstract text is available yet for this article.
October 5, 2018: ACS Catalysis
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