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The first neutral and cationic Mo imido alkylidene cyclic alkyl amino carbene (CAAC) complexes of the general formulae [Mo(N−Ar)(CHCMe2Ph)(X)2(CAAC)] and [Mo(N−Ar)(CHCMe2Ph)(X)(CAAC)][B(ArF)4] (X=Br, Cl, OTf, OC6F5; CAAC=1‐(2,6‐iPr2‐C6H3)‐3,3,5,5‐tetramethyltetrahydropyrrol‐2‐ylidene) have been synthesized from molybdenum imido bishalide alkylidene DME precursors. Different combinations of the imido and "X" ligands have been employed to understand synthetic peculiarities. Selected complexes have been characterized by single‐crystal X‐ray analysis. Due to the pronounced σ‐donor/π‐acceptor characteristics of CAACs, the corresponding neutral and cationic molybdenum imido alkylidene CAAC complexes do not require the presence of stabilizing donor ligands such as nitriles. Calculations on the PBE0‐D3BJ/def2‐TZVP level for PBE0‐D3BJ/def2‐SVP optimized geometries revealed partial charges at molybdenum similar to the corresponding molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes with a slightly higher polarization of the molybdenum alkylidene bond in the CAAC complexes. All cationic complexes have been tested in olefin metathesis reactions and showed improved activity compared to the analogous NHC complexes for hydrocarbon‐based substrates, allowing for turnover numbers (TONs) up to 9500 even at room temperature. Some Mo imido alkylidene CAAC complexes are tolerant towards functional groups like thioethers and sulfonamides. [ABSTRACT FROM AUTHOR]

Pyrazolones represent an important structural motif in active pharmaceutical ingredients. Their asymmetric synthesis is thus widely studied. Still, a generally highly enantio‐ and diastereoselective 1,4‐addition to nitroolefins providing products with adjacent stereocenters is elusive. In this article, a new polyfunctional CuII‐1,2,3‐triazolium‐aryloxide catalyst is presented which enables this reaction type with high stereocontrol. DFT studies revealed that the triazolium stabilizes the transition state by hydrogen bonding between C(5)−H and the nitroolefin and verify a cooperative mode of activation. Moreover, they show that the catalyst adopts a rigid chiral cage/pore structure by intramolecular hydrogen bonding, by which stereocontrol is achieved. Control catalyst systems confirm the crucial role of the triazolium, aryloxide and CuII, requiring a sophisticated structural orchestration for high efficiency. The addition products were used to form pyrazolidinones by chemoselective C=N reduction. These heterocycles are shown to be valuable precursors toward β,γ'‐diaminoamides by chemoselective nitro and N−N bond reductions. Morphological profiling using the Cell painting assay identified biological activities for the pyrazolidinones and suggest modulation of DNA synthesis as a potential mode of action. One product showed biological similarity to Camptothecin, a lead structure for cancer therapy. [ABSTRACT FROM AUTHOR]

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For decades there were many attempts to dispense with stoichiometric amounts of metal reagents for the synthesis of secondary alcohols. In 2021, the synthetic results of Newman and collaborators pioneered a synthesis still with metals, but not as reactants. Instead, they serverd as catalytic engines. Here we present a description by means of Density Functional Theory calculations of how this process can occur, and an attempt is made to shed light on the mechanism that facilitates the attainment of secondary alcohols, emphasizing the eternal cross‐coupling debate of whether the catalytically active species is Ni(0) or they are really taking shortcuts following the course of Ni(II). Effective Orbital analyses give a clear picture. Furthermore, this paper provides insight not only into the nature of the ligands of the metal catalyst but also the role of the base. [ABSTRACT FROM AUTHOR]

While ZrO2 is known to have a large effect on the activity and selectivity of the Cu/ZrO2 catalyst for methanol synthesis, its role in this process is poorly understood. Surface defects such as oxygen vacancies could play a role in the strong metal‐support interaction (SMSI) between Cu and ZrO2. However, due to the complexity of the surfaces, the exact molecular nature of this interaction is not at present known. Here, we make well‐defined models of both reduced and coordinatively unsaturated surface oxygen vacancies on ZrO2 using the molecular precursor [Cp2ZrCl]2(μ2‐O) (1). Complex 1 can be reduced to form a complex (2) containing one Zr(III) center and a bridging hydride ligand (according to EPR and IR spectroscopy) derived from C−H activation of either thf or the Cp ring. Complex 2 reacts with CO2 to largely produce CO, suggesting that surface defects with similar structures probably do not play a role in the industrial catalyst. Halide abstraction from complex 1 results in the Lewis acidic species 3, which has similar Lewis acid properties to acidic defects on the ZrO2 surface. Similarities of both of these model species to real surface oxygen vacancies and their role in the catalytic reaction are discussed. [ABSTRACT FROM AUTHOR]

A catalyst type is disclosed allowing for exceptional efficiency in direct 1,4‐additions. The catalyst is a zwitterionic entity, in which acetate binds to CuII, which is formally negatively charged and serving as counterion for benzimidazolium. All 3 functionalities are involved in the catalytic activation. For maleimides productivity was increased by a factor >300 compared to literature (TONs up to 6700). High stereoselectivity and productivity was attained for a broad range of other Michael acceptors as well. The polyfunctional catalyst is accessible in only 4 steps from N‐Ph‐benzimidazole with an overall yield of 96 % and robust during catalysis. This allowed to reuse the same catalyst multiple times with nearly constant efficiency. Mechanistic studies, in particular by DFT, give a detailed picture how the catalyst operates. The benzimidazolium unit stabilizes the coordinated enolate nucleophile and prevents that acetate/acetic acid dissociate from the catalyst. [ABSTRACT FROM AUTHOR]

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Sulfurized poly(acrylonitrile) (SPAN) is a prominent example of a highly cycle stable and rate capable sulfur/polymer composite, which is solely based on covalently bound sulfur. However, so far no in‐depth study on the influence of nitrogen in the carbonaceous backbone, to which sulfur in the form of thioketones and poly(sulfides) is attached, exists. Herein, we investigated the role of nitrogen by comparing sulfur/polymer composites derived from nitrogen‐containing poly(acrylonitrile) (PAN) and nitrogen‐free poly(vinylacetylene) (PVac). Results strongly indicate the importance of a nitrogen‐rich, aromatic carbon backbone to ensure full addressability of the polymer‐bound sulfur and its reversible binding to the aromatic backbone, even at high current rates. This study also presents key structures, which are crucial for highly cycle and rate stable S‐composites. [ABSTRACT FROM AUTHOR]

High‐temperature sodium‐sulfur battery (HT Na–S) technology has attracted substantial interest in the stationary energy storage sector due to its low cost and high energy density. However, the currently used solid electrolyte (ß‐alumina) is expensive and can only be operated at high temperatures, which compromises safety. On the other hand, liquid electrolytes in room temperature sodium‐sulfur batteries (RT Na–S) are susceptible to dendrite formation and polysulfide shuttle. Consequently, an electrolyte with both solid (shuttle blocking) and liquid (ionic conductivity) properties to overcome the above‐mentioned issues is highly desired. Herein, a high‐performance quasi‐solid state crosslinked gel polymer electrolyte (GPE) prepared in situ using pentaerythritol triacrylate (PETA) exhibiting high ionic conductivity of 2.33 mS cm−1 at 25 °C is presented. The GPE‐based electrolyte shows high stability resulting in a high discharge capacity of >600 mAh gs−1 after 2500 cycles with an average Coulombic efficiency of 99.91%. Density functional theory calculations reveal a weak interaction between the Na+ ions and the oxygen molecules of the PETA moiety, which leads to a facile cation movement. The crosslinked polymer network is tightly connected to the cathode and can confine sulfides, thereby facilitating the conversion process. [ABSTRACT FROM AUTHOR]

Microemulsions provide a unique opportunity to tailor the polarity and liquid confinement in asymmetric catalysis via nanoscale polar and nonpolar domains separated by a surfactant film. For chiral diene Rh complexes, the influence of counterion and surfactant film on the catalytic activity and enantioselectivity remained elusive. To explore this issue chiral norbornadiene Rh(X) complexes (X=OTf, OTs, OAc, PO2F2) were synthesized and characterized by X‐ray crystallography and theoretical calculations. These complexes were used in Rh‐catalyzed 1,2‐additions of phenylboroxine to N‐tosylimine in microemulsions stabilized either exclusively by n‐octyl‐β‐D‐glucopyranoside (C8G1) or a C8G1‐film doped with anionic or cationic surfactants (AOT, SDS and DTAB). The Rh(OAc) complex showed the largest dependence on the composition of the microemulsion, yielding up to 59 % (90 %ee) for the surfactant film doped with 5 wt% of AOT as compared to 52 % (58 %ee) for neat C8G1 at constant surfactant concentration. Larger domains, determined by SAXS analysis, enabled further increase in yield and selectivity while the reaction rate almost remained constant according to kinetic studies. [ABSTRACT FROM AUTHOR]

We compare three methods for quantitatively distinguishing the location of noble metal (NM) particles in mesopores from those found on the external support surface. MCM‐41 and SBA‐15 with NM located in mesopores or on the external surface were prepared and characterized by TEM. 31P MAS NMR spectroscopy was used to quantify arylphosphines in complexes with NM. Phosphine/NM ratios drop from 2.0 to 0.2 when increasing the probe diameter from 1.08 to 1.54 nm. The reaction between NM and triphenylphosphine (TPP) within 3.0 nm MCM‐41 pores takes due to confinement effects multiple weeks. In contrast, external NM react with TPP instantly. A promising method is filling the pores by using the pore volume impregnation technique with tetraethylorthosilicate (TEOS). TPP loading revealed that 66 % of NMs are located on the external surface of MCM‐41. The pore filling method can be used in association with any probe molecule, also for the quantification of acid sites. [ABSTRACT FROM AUTHOR]

Amines are a ubiquitous class of compounds found in a variety of functional organic building blocks. Within the past years, hydrogen autotransfer catalysis has evolved as a new concept for the synthesis of amines. A through understanding of the mechanism of these reactions is necessary to design optimal catalysts. We investigate secondary amine formation catalyzed by a NNNN(P)Ru‐complex and provide understanding on the three reaction steps involved. We find that the ligand has to open one coordination site in order to allow the formation of a metal hydride intermediate. In a second step, a condensation reaction, which could also happen uncatalyzed in solution, is significantly enhanced by the presence of the ruthenium complex. The back‐transfer of the hydride to the substrate in a third step regenerates the catalyst. [ABSTRACT FROM AUTHOR]

Enantiopure secondary alcohols are fundamental high‐value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5–3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working. [ABSTRACT FROM AUTHOR]

A trivial flaw in the utilization of artificial neural networks in interpolating chemical potential energy surfaces (PES) whose descriptors are Cartesian coordinates is their dependence on simple translations and rotations of the molecule under consideration. A different set of descriptors can be chosen to circumvent this problem, internuclear distances, inverse internuclear distances or z‐matrix coordinates are three such descriptors. The objective is to use an interpolated PES in instanton rate constant calculations, hence information on the energy, gradient, and Hessian is required at coordinates in the vicinity of the tunneling path. Instanton theory relies on smoothly fitted Hessians, therefore we use energy, gradients, and Hessians in the training procedure. A major challenge is presented in the proper back‐transformation of the output gradients and Hessians from internal coordinates to Cartesian coordinates. We perform comparisons between our method, a previous approach and on‐the‐fly rate constant calcuations on the hydrogen abstraction from methanol and on the hydrogen addition to isocyanic acid. © 2018Wiley Periodicals, Inc. The challenge of utilizing instanton theory on interpolated potential energy surfaces, trained using a deep neural network, where energies, gradients and Hessians are required, in coordinates invariant to rotational and translational degrees of freedom, has been examined in this contribution. One of the studied reactions, shows good agreement both with existing neural network based approaches and with on‐the‐fly rate calculations based on a UCCSD(T)‐pVTZ‐F12 potential energy surface. [ABSTRACT FROM AUTHOR]

Asymmetric 1,2‐additions of cyanide yield enantioenriched cyanohydrins as versatile chiral building blocks. Next to HCN, volatile organic cyanide sources are usually used. Among them, cyanoformates are more attractive on technical scale than TMSCN for cost reasons, but catalytic productivity is usually lower. Here, the development of a new strategy for cyanations is described, in which this activity disadvantage is overcome. A Lewis acidic Al center cooperates with an aprotic onium moiety within a remarkably robust bifunctional Al–F–salen complex. This allowed for unprecedented turnover numbers of up to 104. DFT studies suggest an unexpected unique trimolecular pathway in which the ammonium bound cyanide attacks the aldehyde, which itself is activated by the carbonyl group of the cyanoformate binding to the Al center. In addition, a novel practical carboxycyanation method was developed that makes use of KCN as the sole cyanide source. The use of a pyrocarbonate as carboxylating reagent provided the best results. Trimolecular? The evolution of a robust, highly active catalyst for asymmetric aldehyde cyanations is reported, which in the final synthetic protocol uses inorganic KCN instead of volatile organic cyanation reagents. DFT studies for the initial protocol using a cyanoformate reagent suggest a unique trimolecular reaction pathway. [ABSTRACT FROM AUTHOR]

Abstract: Fluoroacetate dehalogenase is able to cleavage a carbon–fluoride bond, the strongest carbon–halogen bond in nature, in a process initiated by a SN2 reaction. The role of the enzyme machinery and particularly of the halogen pocket in the SN2 reaction is thoroughly explored by using state‐of‐the‐art computational tools. A comparison between the non‐catalyzed versus enzyme‐catalyzed reaction, as well as with a mutant of the enzyme (Tyr219Phe), is presented. The energy barrier changes are rationalized by means of reaction force analysis and the activation strain model coupled with energy decomposition analysis. The catalysis is in part caused by the reduction of structural work from bringing the reactant species towards the proper reaction orientation, and the reduction of the electrostatic repulsion between the nucleophile and the substrate, which are both negatively charged. In addition, catalysis is also driven by an important reduction of the electronic reorganization processes during the reaction, where Tyr from the halogen pocket acts as a charge acceptor from the SN2 reaction axis therefore reducing the electronic steric repulsion between the reacting parts. [ABSTRACT FROM AUTHOR]

Abstract: Isoxazolinones are biologically and synthetically interesting densely functionalized heterocycles, which for a long time were not accessible in enantioenriched form by asymmetric catalysis. Next to the deficit of enantioselective methods, the functionalization of isoxazolinones is often plagued by regioselectivity issues due to the competition of various nucleophilic centers within the heterocycles. We report the first regio‐ and enantioselective C‐allylations of isoxazolinones. These occur with high regioselectivity in favor of the linear allylation products, although Ir phosphoramidite catalysts were used, which commonly results in branched isomers. Our studies suggest that this outcome is the result of a reaction cascade via an initial regio‐ and enantioselective N‐allylation to provide a branched allyl intermediate, followed by a spontaneous [3,3]‐rearrangement resulting in chirality transfer. [ABSTRACT FROM AUTHOR]

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Canonical instanton theory is known to overestimate the rate constant close to a system-dependent crossover temperature and is inapplicable above that temperature. We compare the accuracy of the reaction rate constants calculated using recent semi-classical rate expressions to those from canonical instanton theory. We show that rate constants calculated purely from solving the stability matrix for the action in degrees of freedom orthogonal to the instanton path is not applicable at arbitrarily low temperatures and use two methods to overcome this. Furthermore, as a by-product of the developed methods, we derive a simple correction to canonical instanton theory that can alleviate this known overestimation of rate constants close to the crossover temperature. The combined methods accurately reproduce the rate constants of the canonical theory along the whole temperature range without the spurious overestimation near the crossover temperature. We calculate and compare rate constants on three different reactions: H in the Müller-Brown potential, methylhydroxycarbene → acetaldehyde and H2 + OH → H + H2O. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A detailed analysis of the C(sp3)−H activation process by vinylidene AuI complexes is described based on an intrinsic bond orbital analysis. Based on our analysis this event can be divided into three phases: (i) hydride transfer, (ii) C−C bond formation, and (iii) σ to π rearrangement of the lone pair coordinated to Au. Small perturbations of the system lead to either a concerted asynchronous reaction, or a stepwise reaction featuring an intermediate with a C-H-C three-centre two-electron (3c-2e) bond. The role of π-donating substituents is highlighted and provides a way of controlling reactions of this type in future experimental studies. [ABSTRACT FROM AUTHOR]

The first synthetic protocol to high oxidation state molybdenum(VI) N-heterocyclic carbene (NHC) alkylidyne complexes (NHC=1,3-diisopropylimidazol-2-ylidene, 1,3-dimethyl-4,5-R2-imidazol-2-ylidene, R2=H, Cl, CN) is reported. Steric limitations of the NHCs and the benzylidyne are described. All novel complexes were characterized by single crystal X-ray diffraction and solution NMR techniques. It was shown that all complexes presented here show activity in the self-metathesis of 1-phenyl-1-propyne at room temperature. To identify mechanistic differences, an experimental sequence to detect dissociation of ligands was developed. Results reveal dissociation of less electron-donating NHCs in course of the reaction. Mechanistic and reactivity differences were attributed to electronic and steric effects through Tolman's electronic parameter and the percentage of buried volume. Furthermore, Mo-1 containing the 1,3-dimethylimidazol-2-ylidene ligand showed good activity in self-metathesis reactions of p-substituted 1-phenyl-1-propynes with electron-donating moieties at room temperature. [ABSTRACT FROM AUTHOR]

The potential of vinyl Au species to react either through a controlled π- or σ-pathway is demonstrated. This nomenclature is directly derived from the orbitals of the vinyl Au species leading to the newly formed bonds. When the π-bond of the vinyl Au intermediate is transformed into a σ-bond, we name it π-pathway, and a σ- to σ- transformation is named σ-pathway. Examples of reactions following these pathways are a Au-catalysed [3,3]-sigmatropic rearrangement and a protodeauration reaction. These reactions have been studied using intrinsic bond orbitals (IBOs) and allow for the clear identification of these pathways. Energies for the reaction path of the Au-catalysed [3,3]-sigmatropic rearrangement were in addition computed using CCSD(T)-F12. Analysis of the intrinsic reaction coordinate (IRC) of the [3,3]-sigmatropic rearrangement using IBOs further allows us to refine the previous mechanistic proposal and identifies a hidden intermediate along the reaction path. [ABSTRACT FROM AUTHOR]

Salicylate 1,2-dioxygenase (SDO) was the first enzyme discovered, in the family of iron dioxygenases, to catalyze the ring cleavage of a monohydroxylated aromatic compound, salicylate, without a proton donor. Salicylate is not electron-rich like the familiar dihydroxy or aromatic substrates with an electron-donating group that are utilized in the well-known dioxygenases. SDO carries out the intramolecular C−C bond cleavage in salicylate bearing the OH and COOH groups with high regioselectivity in comparison with the extradiol and intradiol dioxygenases. The catalytic cleavage of a nonactivated substrate like salicylate that lacks an electron-donating group, also in the absence of a proton source, raises many puzzling questions about the oxy intermediates in the reaction pathway of dioxygenase enzymes in general. To answer these fundamental queries, we have investigated the full catalytic mechanism of SDO by a combination of quantum mechanics and molecular mechanics (QM/MM) calculations. Herein, our QM/MM study has several unexpected and interesting implications for the mechanistic pathway of SDO in comparison to the experimental observations. Importantly, it unravels the basis for the unexpected 'intra'-cleavage regioselectivity in SDO. Ostensibly a similar alkylperoxo intermediate is formed in SDO much like in the extradiol and intradiol dioxygenases. In stark contrast to the two diol enzymes, the O−O bond breaking leads to an unprotonated gem-hydroxy carboxylate intermediate, a paradigm analogue of the elusive gem diol intermediate. This unprotonated gem-hydroxy carboxylate intermediate exclusively dictates the C−C cleavage regiospecificity in SDO, which is unprecedented in the family of dioxygenases. It forms a seven-membered lactone species, which eventually forms the ring-cleavage final product by incorporation of two oxygen atoms in the salicylate. Thus, our computational study unravels a detailed reaction pathway of the oxidative cleavage of salicylate without a proton source by identifying the hitherto elusive intermediates in the catalytic cycle of SDO with testable predictions. Moreover, we describe the atomistic origin of the new catalytic role of Arg127 in the catalysis and regioselectivity of SDO. It also rationalizes the formation of a side product without invoking a dioxetane intermediate. This study is important from a fundamental perspective and opens up a new window in the mechanism of the family of dioxygenase enzymes. [ABSTRACT FROM AUTHOR]

Al−F bonds are among the most stable σ bonds known, exhibiting an even higher bond energy than Si−F bonds. Despite a stability advantage and a potentially high Lewis acidity of Al−F complexes, they have not been described as structurally defined catalysts for enantioselective reactions. We show that Al−F salen complexes with appended ammonium moieties give exceptional catalytic activity in asymmetric carboxycyanations. In addition to aromatic aldehydes, enal and aliphatic substrates are well accepted. Turnover numbers up to around 104 were achieved, whereas with previous catalysts 101-102 turnovers were typically attained. In contrast to Al−Me and Al−Cl salen complexes, the analogous Al−F species are remarkably stable towards air, water, and heat, and can be recovered unchanged after catalysis. They possess a considerably increased Lewis acidity as shown by DFT calculations. [ABSTRACT FROM AUTHOR]

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Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom tunneling can be indirectly detected by temperature-independent rate constants at low temperature or by enhanced kinetic isotope effects. In contrast, the influence of tunneling on the reaction rates can be monitored directly through computational investigations. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes. [ABSTRACT FROM AUTHOR]

Der quantenmechanische Tunneleffekt von Atomen wird immer häufiger als wichtiger Aspekt in chemischen Reaktionen identifiziert. Experimentell kann man Atomtunneln nur indirekt nachweisen; beispielsweise durch temperaturunabhängige Reaktionsgeschwindigkeitskonstanten oder durch sehr große kinetische Isotopeneffekte. Im Gegensatz dazu können theoretische Methoden den Einfluss des Tunneleffekts auf die Reaktionsgeschwindigkeit direkt bestimmen. Der Tunneleffekt ändert beispielsweise Reaktionspfade und Verzweigungsverhältnisse, ermöglicht chemische Reaktionen in astrochemischer Umgebung, die thermisch nicht stattfinden, und beeinflusst auch biochemische Prozesse. Der quantenmechanische Tunneleffekt von Atomen wird zunehmend als wichtiger Aspekt chemischer Reaktionen wahrgenommen. Während er experimentell nur indirekt zugänglich ist, erlauben theoretische Simulationen eine direkte Beobachtung des Tunneleffekts. Hier wird sein Einfluss auf Reaktionspfade, Verzweigungsverhältnisse, biochemische Prozesse und die Erhöhung der Reaktionsgeschwindigkeit beleuchtet, die manche Reaktionen in der Astrochemie erst ermöglicht. [ABSTRACT FROM AUTHOR]

We herein report a computational study of the bonding in gold(I) vinylidene complexes and compare them to their carbene and CO analogues. The relevance of these intermediates is analysed for the intramolecular cyclisation leading to vinyl sulfonates. [ABSTRACT FROM AUTHOR]

Salicylate 1,2-dioxygenase (SDO) is the first enzyme to be discovered to catalyze the oxidative cleavage of a monohydroxylated aromatic compound, namely salicylate, instead of the well-known electron-rich substrates. We have investigated the mechanism of dioxygen activation in SDO by QM/MM calculations. Our study reveals that the non-heme FeII center in SDO activates salicylate and O2 synergistically through a strong covalent interaction to facilitate the reductive cleavage of O2. A covalent salicylate-FeII-O2 complex is the reactive oxygen species in this case, and its electronic structure is best described as being between the two limiting cases, FeII−O2 and FeII−O2.−, with partial electron transfer from the activated salicylate to O2 via the Fe center. Thus SDO employs a synergistic strategy of substrate and oxygen activation to carry out the catalytic reaction, which is unprecedented in the family of iron dioxygenases. Moreover, O2 activation in SDO happens without the assistance of a proton source. Our study essentially shows a new mechanistic possibility for O2 activation. [ABSTRACT FROM AUTHOR]

Salicylat ‐ 1,2 ‐ Dioxygenase (SDO) ist das erste bekannte Enzym, das die oxidative Spaltung eines einfach hydroxylierten Aromaten, Salicylat, statt der wohlbekannten elektronenreichen Substrate katalysiert. Wir haben den Mechanismus der Sauerstoffaktivierung in SDO mit QM/MM ‐ Simulationen untersucht. Unsere Ergebnisse zeigen, dass das Nicht ‐ Häm ‐ FeII ‐ Zentrum in SDO Salicylat und O2 gemeinsam durch eine starke kovalente Wechselwirkung aktiviert, um die reduktive Spaltung von O2 zu ermöglichen. Die reaktive Sauerstoffspezies ist ein kovalenter Salicylat ‐ FeII ‐ O2 ‐ Komplex, wobei die Elektronenstruktur zwischen den beiden Grenzfällen FeII ‐ O2 und FeII ‐ O2.− mit partiellem Elektronentransfer vom aktivierten Salicylat zu O2 über das Fe ‐ Zentrum liegt. Folglich verwendet SDO eine synergetische Strategie der Substrat ‐ und Sauerstoffaktivierung, um die katalytische Reaktion auszuführen, die einmalig in der Familie der Eisendioxygenasen ist. Darüber hinaus erfolgt die O2 ‐ Aktivierung in SDO ohne die Hilfe einer Protonenquelle. Unsere Studie zeigt eine bisher unbekannte Möglichkeit für den Mechanismus der O2 ‐ Aktivierung. Das Nicht ‐ Häm ‐ FeII ‐ Zentrum in Salicylat ‐ 1,2 ‐ Dioxygenase (SDO) aktiviert Salicylat und O2 synergetisch durch eine starke kovalente Wechselwirkung, wie QM/MM ‐ Simulationen der SDO zeigen. Die reaktive Sauerstoffspezies ist ein kovalenter Salicylat ‐ FeII ‐ O2 ‐ Komplex, und die O2 ‐ Aktivierung erfolgt ohne die Hilfe einer Protonenquelle. [ABSTRACT FROM AUTHOR]

Bonding and stabilizing effects in gold carbene complexes are investigated by using Kohn-Sham density functional theory (DFT) and the intrinsic bond orbital (IBO) approach. The π-stabilizing effects of organic substituents at the carbene carbon atom coordinated to the gold atom are evaluated for a series of recently isolated and characterized complexes, as well as intermediates of prototypical 1,6-enyne cyclization reactions. The results indicate that these effects are of particular importance for gold complexes especially because of the low π-backbonding contribution from the gold atom. [ABSTRACT FROM AUTHOR]

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Nitrogenase converts gaseous dinitrogen into biologically accessible ammonia. The binding of N2 to a reduced and protonated form of the FeMo-cofactor of nitrogenase including its central carbon ligand was investigated by means of density functional calculations. It was found the central ligand to stabilize the cluster. N2 can associate to iron or molybdenum with iron being the preferred binding site. While endo and exo binding modes were investigated the exo modes are more stable. Implications on the mechanism of N2 reduction are discussed. [ABSTRACT FROM AUTHOR]

A quantum chemical investigation of the Bu4N[Fe(CO)3(NO)]-catalyzed Cloke-Wilson rearrangement of vinyl cyclopropanes is reported. It was found that allylic CC bond activation can proceed through a SN2′ or SN2-type mechanism. The application of the recently reported intrinsic bond orbital (IBO) method for all structures indicated that one FeN π bond is directly involved. Further analysis showed that during the reaction oxidation occurs at the NO ligand exclusively. [ABSTRACT FROM AUTHOR]

Quantum mechanical tunneling of atoms allows chemical reactions to proceed through barriers too high for thermally activated processes. This causes hydroxycarbenes to decay rapidly and at a temperature-independent rate even at 11 K. In methylhydroxycarbene, tunneling causes decay through a mechanism that reveals a high but thin barrier rather than an alternative with a lower but broader barrier. No accurate estimates of the widths of such barriers and the lengths of tunneling paths were available. Herein, such a measure is provided by calculating the length of the tunneling paths by using instanton theory. Potential energies are provided by density functional theory verified by explicitly correlated coupled cluster CCSD(T) energies. Our results explain the decay efficiency in the known cases and suggest new substitutions to tune the effects of barrier widths and heights. Fluorination and replacement of the hydroxyl group by a thiol group change the qualitative character of the decay. Methylaminocarbene is predicted to be stable for thousands of years. [ABSTRACT FROM AUTHOR]

A well-studied periplasmic-binding protein involved in the abstraction of maltose is maltose-binding protein (MBP), which undergoes a ligand-induced conformational transition from an open (ligand-free) to a closed (ligand-bound) state. Umbrella sampling simulations have been us to estimate the free energy of binding of maltose to MBP and to trace the potential of mean force of the unbinding event using the center-of-mass distance between the protein and ligand as the reaction coordinate. The free energy thus obtained compares nicely with the experimentally measured value justifying our theoretical basis. Measurement of the domain angle (N-terminal-domain - hinge - C-terminal-domain) along the unbinding pathway established the existence of three different states. Starting from a closed state, the protein shifts to an open conformation during the initial unbinding event of the ligand then resides in a semi-open conformation and later resides predominantly in an open-state. These transitions along the ligand unbinding pathway have been captured in greater depth using principal component analysis. It is proposed that in mixed-model, both conformational selection and an induced-fit mechanism combine to the ligand recognition process in MBP. Proteins 2013. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The ring-opening reaction of the cyclopropylcarbinyl radical proceeds via heavy-atom tunneling at low temperature. We used instanton theory to calculate tunneling rates and kinetic isotope effects with on-the-fly calculation of energies by density functional theory (B3LYP). The accuracy was verified by explicitly correlated coupled-cluster calculations (UCCSD(T)-F12). At cryogenic temperatures, we found protium/deuterium KIEs up to 13 and inverse KIEs down to 0.2. We also studied an intramolecular tautomerization reaction. A simple and computationally efficient method is proposed to calculate KIEs with the instanton method: the instanton path is assumed to be independent of the atomic masses. This results in surprisingly good estimates of the KIEs for the cyclopropylcarbinyl radical and for the secondary KIEs of the tautomerization. Challenges and capabilities of the instanton method for calculating KIEs are discussed. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011 [ABSTRACT FROM AUTHOR]

Keywords: asymmetric catalysis; diene ligands; liquid confinement; microemulsions; rhodium-catalyzed 1,2-addition EN asymmetric catalysis diene ligands liquid confinement microemulsions rhodium-catalyzed 1,2-addition 1 1 1 12/09/21 20211206 NES 211206 B The influence of anionic and cationic co-surfactants b in nonionic microemulsions on the performance of chiral Rh-norbornadiene complexes with different counterions in the 1,2-addition of phenylboronic acids to I N i -tosylimines is described. Asymmetric catalysis, diene ligands, liquid confinement, microemulsions, rhodium-catalyzed 1,2-addition. [Extracted from the article]