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971.
Conchín Meliá Dr. Silvia Ferrer Dr. Jan Řezáč Dr. Olivier Parisel Prof. Olivia Reinaud Prof. Vicent Moliner Dr. Aurélien de la Lande 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(51):17328-17337
In Nature, the family of copper monooxygenases comprised of peptidylglycine α‐hydroxylating monooxygenase (PHM), dopamine β‐monooxygenase (DβM), and tyramine β‐monooxygenase (TβM) is known to perform dioxygen‐dependent hydroxylation of aliphatic C? H bonds by using two uncoupled metal sites. In spite of many investigations, including biochemical, chemical, and computational, details of the C? H bond oxygenation mechanism remain elusive. Herein we report an investigation of the mechanism of hydroxylation by PHM by using hybrid quantum/classical potentials (i.e., QM/MM). Although previous investigations using hybrid QM/MM techniques were restricted to geometry optimizations, we have carried out ab initio molecular dynamics simulations in order to include the intrinsic flexibility of the active sites in the modeling protocol. The major finding of this study is an extremely fast rebound step after the initial hydrogen‐abstraction step promoted by the cupric–superoxide adduct. The hydrogen‐abstraction/rebound sequence leads to the formation of an alkyl hydroperoxide intermediate. Long‐range electron transfer from the remote copper site subsequently triggers its reduction to the hydroxylated substrate. We finally show two reactivity consequences inherent in the new mechanistic proposal, the investigation of which would provide a means to check its validity by experimental means. 相似文献
972.
Prof. James H. Clark Dr. Duncan J. Macquarrie James Sherwood 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(16):5174-5182
The traditional Biginelli reaction is a three‐component condensation between urea, benzaldehyde and an acetoacetate ester to give a dihydropyrimidinone. An investigation into catalytic and solvent effects has returned the conclusion that the diketo–enol tautomerisation equilibrium of the dicarbonyl reactant dictates the yield of the reaction. Whereas the solvent is responsible for the tautomerisation equilibrium position, the catalyst only serves to eliminate kinetic control from the reaction. Generally, to preserve reaction efficiency and improve sustainability, bio‐derivable p‐cymene was found to be a useful solvent. The metal–enolate intermediate that results from the application of a Lewis acidic catalyst often cited as promoting the reaction appears to hinder the reaction. In this instance, a Brønsted acidic solvent can be used to return greater reactivity to the dicarbonyl reagent. 相似文献
973.
Alexander S. Novikov Dr. Maxim L. Kuznetsov Prof. Armando J. L. Pombeiro 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(8):2874-2888
The theoretical background of the formation of N‐heterocyclic oxadiazoline carbenes through a metal‐assisted [2+3]‐dipolar cycloaddition (CA) reaction of nitrones R1CH?N(R2)O to isocyanides C?NR and the decomposition of these carbenes to imines R1CH?NR2 and isocyanates O?C?NR is discussed. Furthermore, the reaction mechanisms and factors that govern these processes are analyzed in detail. In the absence of a metal, oxadiazoline carbenes should not be accessible due to the high activation energy of their formation and their low thermodynamic stability. The most efficient promotors that could assist the synthesis of these species should be “carbenophilic” metals that form a strong bond with the oxadiazoline heterocycle, but without significant involvement of π‐back donation, namely, AuI, AuIII, PtII, PtIV, ReV, and PdII metal centers. These metals, on the one hand, significantly facilitate the coupling of nitrones with isocyanides and, on the other hand, stabilize the derived carbene heterocycles toward decomposition. The energy of the LUMOCNR and the charge on the N atom of the C?N group are principal factors that control the cycloaddition of nitrones to isocyanides. The alkyl‐substituted nitrones and isocyanides are predicted to be more active in the CA reaction than the aryl‐substituted species, and the N,N,C‐alkyloxadiazolines are more stable toward decomposition relative to the aryl derivatives. 相似文献
974.
Dr. Bo Liu Dr. Thierry Roisnel Prof. Dr. Jean‐François Carpentier Dr. Yann Sarazin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(8):2784-2802
The behavior of the first aminophenolate catalysts of the large alkaline earth metals (Ae) [(LOi)AeN(SiMe2R)2(thf)x] (i=1–4; Ae=Ca, Sr, Ba; R=H, Me; x=0–2) for the cyclohydroamination of terminal aminoalkenes is discussed. The complexes [(BDI)AeN(SiMe2H)2(thf)x] (Ae=Ca, Sr, Ba, x=1–2; (BDI)H=H2C[C(Me)N‐2,6‐(iPr)2C6H3]2)) and [(BDI)CaN(SiMe3)2(thf)] supported by the β‐diketiminate (BDI)? ligand have also been employed for comparative and mechanistic considerations. The catalytic performances decrease in the order Ca>Sr?Ba, which is the opposite trend to that previously observed during the intermolecular hydroamination of activated alkenes catalyzed by the same alkaline‐earth metal complexes. Catalyst efficacy increases when the chelating and donating ability of the aminophenolate ligands decreases. For given metals and ancillary scaffolds, disilazide catalysts that incorporate the N(SiMe3)2? amido group outclass their congeners containing the N(SiMe2H)2? amide owing to the lower basicity of the N(SiMe2H)2? with respect to the N(SiMe3)2? group, and also because Ae–N(SiMe2H)2 catalysts suffer from irreversible deactivation through the dehydrogenative coupling of amine and hydrosilane moieties. This deactivation process takes place at 25 °C in the case of [(LOi)AeN(SiMe2H)2(thf)x] phenolate complexes and occurs even with the related [(BDI)AeN(SiMe2H)2(thf)x] complex, albeit under conditions harsher than those required for effective cyclohydroamination catalysis. A mechanistic scenario for cyclohydroamination catalyzed by [(LX)AeN(SiMe2H)2(thf)x] complexes ((LX)?=(LOi)? or (BDI)?) is proposed. Although beneficial for the synthesis of Ae heteroleptic complexes able to resist deleterious Schlenk‐type equilibria, the use of the N(SiMe2H)2? is prejudicial to catalytic activity in the case of catalyzed transformations that involve reactive amine (and potentially other) substrates. Mechanistic and kinetic investigations further illustrate the interplay between the catalytic activity, operative mechanism, and identity of the metal, ancillary ligand, and amido group. These studies suggest that the widely accepted mechanism for cyclohydroamination reactions cannot be extended systematically to all alkaline‐earth catalysts. The [(BDI)CaN(SiMe2H)2{H2NCH2C(CH3)2CH2CH?CH2}2] complex, the first Ca–aminoalkene adduct structurally characterized, was prepared quantitatively and essentially behaves like [(BDI)CaN(SiMe2H)(thf)], thus serving as a model compound for mechanistic studies, as illustrated during stoichiometric reactions monitored by 1H NMR spectroscopy. 相似文献
975.
Dr. Patrick Hemberger Dr. Andras Bodi Dr. Thomas Gerber Dr. Max Würtemberger Prof. Dr. Udo Radius 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(22):7090-7099
The photoionization and dissociative photoionization of Im(iPr)2, 1,3‐diisopropylimidazolin‐2‐ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone‐pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IEad=7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate‐determining step as hydrogen‐atom migration from the isopropyl group to the carbene carbon center forming a resonance‐stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene‐loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance‐stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate‐sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry. 相似文献
976.
Dr. Igor E. Soshnikov Dr. Nina V. Semikolenova Prof. Vladimir A. Zakharov Prof. Dr. Heiko M. Möller Franz Ölscher Anna Osichow Dr. Inigo Göttker‐Schnettmann Prof. Dr. Stefan Mecking Prof. Dr. Evgenii P. Talsi Prof. Dr. Konstantin P. Bryliakov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(34):11409-11417
Formation of Ni–polymeryl propagating species upon the interaction of three salicylaldiminato nickel(II) complexes of the type [(N,O)Ni(CH3)(Py)] (where (N,O)=salicylaldimine ligands, Py=pyridine) with ethylene (C2H4/Ni=10:30) has been studied by 1H and 13C NMR spectroscopy. Typically, the ethylene/catalyst mixtures in [D8]toluene were stored for short periods of time at +60 °C to generate the [(N,O)Ni(polymeryl)] species, then quickly cooled, and the NMR measurements were conducted at ?20 °C. At that temperature, the [(N,O)Ni(polymeryl)] species are stable for days; diffusion 1H NMR measurements provide an estimate of the average length of polymeryl chain (polymeryl=(C2H4)nH, n=6–18). At high ethylene consumptions, the [(N,O)Ni(polymeryl)] intermediates decline, releasing free polymer chains and yielding [(N,O)Ni(Et)(Py)] species, which also further decompose to form the ultimate catalyst degradation product, a paramagnetic [(N,O)2Ni(Py)] complex. In [(N,O)2Ni(Py)], the pyridine ligand is labile (with activation energy for its dissociation of (12.3±0.5) kcal mol?1, ΔH≠298=(11.7±0.5) kcal mol?1, ΔS≠298 =(?7±1) cal K?1 mol?1). Upon the addition of nonpolar solvent (pentane), the pyridine ligand is lost completely to yield the crystals of diamagnetic [(N,O)2Ni] complex. NMR spectroscopic analysis of the polyethylenes formed suggests that the evolution of chain‐propagating species ends up with formation of polyethylene with predominately internal and terminal vinylene groups rather than vinyl groups. 相似文献
977.
Dr. Bo Liu Dr. Thierry Roisnel Prof. Dr. Jean‐François Carpentier Dr. Yann Sarazin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(40):13445-13462
[{N^N}M(X)(thf)n] alkyl (X=CH(SiMe3)2) and amide (X=N(SiMe3)2) complexes of alkaline earths (M=Ca, Sr, Ba) and divalent rare earths (YbII and EuII) bearing an iminoanilide ligand ({N^N}?) are presented. Remarkably, these complexes proved to be kinetically stable in solution. X‐ray diffraction studies allowed us to establish size–structure trends. Except for one case of oxidation with [{N^N}YbII{N(SiMe3)2}(thf)], all these complexes are stable under the catalytic conditions and constitute effective precatalysts for the cyclohydroamination of terminal aminoalkenes and the intermolecular hydroamination and intermolecular hydrophosphination of activated alkenes. Metals with equal sizes across alkaline earth and rare earth families display almost identical apparent catalytic activity and selectivity. Hydrocarbyl complexes are much better catalyst precursors than their amido analogues. In the case of cyclohydroamination, the apparent activity decreases with metal size: Ca>Sr>Ba, and the kinetic rate law agrees with RCHA=k[precatalyst]1[aminoalkene]1. The intermolecular hydroamination and hydrophosphination of styrene are anti‐Markovnikov regiospecific. In both cases, the apparent activity increases with the ionic radius (Ca<Sr<Ba) but the rate laws are different, and obey RHA=k[styrene]1[amine]1[precatalyst]1 and RHP=k[styrene]1[HPPh2]0[precatalyst]1, respectively. Mechanisms compatible with the rate laws and kinetic isotopic effects are proposed. [{N^N}Ba{N(SiMe3)2}(thf)2] ( 3 ) and [{N^N}Ba{CH(SiMe3)2}(thf)2] ( 10 ) are the first efficient Ba‐based precatalysts for intermolecular hydroamination and hydrophosphination, and display activity values that are above those reported so far. The potential of the precatalysts for C? N and C? P bond formation is detailed and a rare cyclohydroamination–intermolecular hydroamination “domino” sequence is presented. 相似文献
978.
Mette Ø. Filsø Dr. Michael J. Turner Prof. Gerald V. Gibbs Prof. Stefan Adams Prof. Mark A. Spackman Prof. Bo B. Iversen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(46):15535-15544
The understanding of lithium‐ion migration through the bulk crystal structure is crucial in the search for novel battery materials with improved properties for lithium‐ion conduction. In this paper, procrystal calculations are introduced as a fast, intuitive way of mapping possible migration pathways, and the method is applied to a broad range of lithium‐containing materials, including the well‐known battery cathode materials LiCoO2, LiMn2O4, and LiFePO4. The outcome is compared with both experimental and theoretical studies, as well as the bond valence site energy approach, and the results show that the method is not only a strong, qualitative visualization tool, but also provides a quantitative measure of electron‐density thresholds for migration, which are correlated with theoretically obtained activation energies. In the future, the method may be used to guide experimental and theoretical research towards materials with potentially high ionic conductivity, reducing the time spent investigating nonpromising materials with advanced theoretical methods. 相似文献
979.
Weimin Hu Hua Qin Prof. Yuxin Cui Prof. Yanxing Jia 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(9):3139-3147
In this article, the total syntheses of antimalarial compound decursivine and its biologically inactive sibling serotobenine are presented. The biomimetic synthesis of (±)‐serotobenine was investigated first, but failed. During the subsequent investigation of other synthetic routes, we discovered a new cascade Witkop photocyclization/elimination/addition sequence, which enabled the expedient synthesis of not only racemic decursivine and serotobenine, but also enantiopure (+)‐ and (?)‐decursivine and a variety of their analogues. The present syntheses represent the shortest pathway for the total synthesis of decursivine and serotobenine to date. Moreover, the newly developed cascade sequence for the total synthesis of decursivine does not need any protecting steps. The scope and the reaction mechanism of the cascade sequence were also studied. A rational mechanism for the cascade sequence is proposed, which is consistent with the previous studies and our current experimental results. 相似文献
980.
C. Kohrt Dr. W. Baumann Dr. A. Spannenberg Dr. H.‐J. Drexler Prof. Dr. I. D. Gridnev Prof. Dr. D. Heller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(23):7443-7451
Recently described and fully characterized trinuclear rhodium‐hydride complexes [{Rh(PP*)H}3(μ2‐H)3(μ3‐H)][anion]2 have been investigated with respect to their formation and role under the conditions of asymmetric hydrogenation. Catalyst–substrate complexes with mac (methyl (Z)‐ N‐acetylaminocinnamate) ([Rh(tBu‐BisP*)(mac)]BF4, [Rh(Tangphos)(mac)]BF4, [Rh(Me‐BPE)(mac)]BF4, [Rh(DCPE)(mac)]BF4, [Rh(DCPB)(mac)]BF4), as well as rhodium‐hydride species, both mono‐([Rh(Tangphos)‐ H2(MeOH)2]BF4, [Rh(Me‐BPE)H2(MeOH)2]BF4), and dinuclear ([{Rh(DCPE)H}2(μ2‐H)3]BF4, [{Rh(DCPB)H}2(μ2‐H)3]BF4), are described. A plausible reaction sequence for the formation of the trinuclear rhodium‐hydride complexes is discussed. Evidence is provided that the presence of multinuclear rhodium‐hydride complexes should be taken into account when discussing the mechanism of rhodium‐promoted asymmetric hydrogenation. 相似文献