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41.
Marc Garcia‐Borràs Dr. Josep M. Luis Prof. Dr. Marcel Swart Prof. Dr. Miquel Solà 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(14):4468-4479
One of the most important reactions in fullerene chemistry is the Diels–Alder (DA) reaction. In two previous experimental studies, the DA cycloaddition reactions of cyclopentadiene (Cp) and 1,2,3,4,5‐pentamethylcyclopentadiene (Cp*) with La@C2v‐C82 were investigated. The attack of Cp was proposed to occur on bond 19 , whereas that of Cp* was confirmed by X‐ray analysis to be over bond o . Moreover, the stabilities of the Cp and Cp* adducts were found to be significantly different, that is, the decomposition of La@C2v‐C82Cp was one order of magnitude faster than that of La@C2v‐C82Cp*. Herein, we computationally analyze these DA cycloadditions with two main goals: First, to compute the thermodynamics and kinetics of the cycloadditions of Cp and Cp* to different bonds of La@C2v‐C82 to assess and compare the regioselectivity of these two reactions. Second, to understand the origin of the different thermal stabilities of the La@C82Cp and La@C82Cp* adducts. Our results show that the regioselectivity of the two DA cycloadditions is the same, with preferred attack on bond o . This result corrects the previous assumption of the regioselectivity of the Cp attack that was made based only on the shape of the La@C82 singly occupied molecular orbital. In addition, we show that the higher stability of the La@C82Cp* adduct is not due to the electronic effects of the methyl groups on the Cp ring, as previously suggested, but to higher long‐range dispersion interactions in the Cp* case, which enhance the stabilization of the reactant complex, transition state, and products with respect to the separated reactants. This stabilization for the La@C82Cp* case decreases the Gibbs reaction energy, thus allowing competition between the direct and retro reactions and making dissociation more difficult. 相似文献
42.
Voltammetric Determination of Pb(II) and Cd(II) Ions in Well Water Using a Sputtered Bismuth Screen‐Printed Electrode 下载免费PDF全文
Velia Sosa Núria Serrano Cristina Ariño José Manuel Díaz‐Cruz Miquel Esteban 《Electroanalysis》2014,26(10):2168-2172
A method using commercially available sputtered bismuth screen‐printed electrodes (BispSPE), as substitute to mercury electrodes, for the determination of trace Pb(II) and Cd(II) ions in drinking well water samples collected in a contaminated area in The Republic of El Salvador by means of differential pulse anodic stripping voltammetry (DPASV) has been proposed. The comparable detection and quantification limits obtained for both BispSPE and hanging mercury drop electrode (HMDE), together with the similar results with a high reproducibility obtained in these water samples analyses recommend the applicability of BispSPE for the determination of low level of metal concentrations in natural samples. 相似文献
43.
Dr. Jordi Poater Prof. Dr. Miquel Solà Prof. Dr. Clara Viñas Prof. Dr. Francesc Teixidor 《Angewandte Chemie (International ed. in English)》2014,53(45):12191-12195
A bridge between classical organic polycyclic aromatic hydrocarbons (PAH) and closo borohydride clusters is established by showing that they share a common origin regulated by the number of valence electrons in an electronic confined space. Application of the proposed electronic confined space analogy (ECSA) method to archetypal PAHs leads to the conclusion that the 4n+2 Wade–Mingos rule for three‐dimensional closo boranes is equivalent to the (4n+2)π Hückel rule for two‐dimensional PAHs. More importantly, use of ECSA allows design of new interesting fused closo boranes which can be a source of inspiration for synthetic chemists. 相似文献
44.
Giorgio Olivo Giorgio Capocasa Barbara Ticconi Osvaldo Lanzalunga Stefano Di Stefano Miquel Costas 《Angewandte Chemie (International ed. in English)》2020,59(31):12703-12708
Predictability is a key requirement to encompass late‐stage C?H functionalization in synthetic routes. However, prediction (and control) of reaction selectivity is usually challenging, especially for complex substrate structures and elusive transformations such as remote C(sp3)?H oxidation, as it requires distinguishing a specific C?H bond from many others with similar reactivity. Developed here is a strategy for predictable, remote C?H oxidation that entails substrate binding to a supramolecular Mn or Fe catalyst followed by elucidation of the conformation of the host‐guest adduct by NMR analysis. These analyses indicate which remote C?H bonds are suitably oriented for the oxidation before carrying out the reaction, enabling prediction of site selectivity. This strategy was applied to late‐stage C(sp3)?H oxidation of amino‐steroids at C15 (or C16) positions, with a selectivity tunable by modification of catalyst chirality and metal. 相似文献
45.
46.
The optimization of P,S-ligands derived from enantiopure (2S,3S)-phenylglycidol for asymmetric rhodium-catalyzed hydrogenation of dehydroamino esters is described. The exceptionally high modular character of the (2S,3S)-phenylglycidol platform is demonstrated by systematic modification of the ether and thioether moieties, the skeletal aryl substituent and the stereo and regiochemistry of the ligands. An experimentally useful method for the monitoring of hydrogenation reactions is also described and used for obtaining relevant data of the catalytic systems studied. 相似文献
47.
48.
Dr. Carmen E. Castillo Dr. Ilaria Gamba Laia Vicens Dr. Martin Clémancey Prof. Dr. Jean-Marc Latour Prof. Dr. Miquel Costas Prof. Dr. Manuel G. Basallote 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(15):4946-4954
Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non-heme iron-dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes [FeII(CH3CN)2L](SbF6)2 ( [1](SbF6)2 and [2](SbF6)2 ) and [FeII(CF3SO3)2L] ( [1](OTf)2 and [2](OTf)2 ( 1 , L=Me,HPytacn; 2 , L=nP,HPytacn; R,R′Pytacn=1-[(6-R′-2-pyridyl)methyl]-4,7- di-R-1,4,7-triazacyclononane) with Bu4NIO4 to form the corresponding [FeIV(O)(CH3CN)L]2+ ( 3 , L=Me,HPytacn; 4 , L=nP,HPytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl− as entering ligand, which indicates that formation of [FeIV(O)(CH3CN)L]2+ is kinetically controlled by substitution in the starting complex to form [FeII(IO4)(CH3CN)L]+ intermediates that are converted rapidly to oxo complexes 3 and 4 . The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin-crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high-valent iron oxo species. 相似文献
49.
In abstract algebraic logic, the general study of propositional non-classical logics has been traditionally based on the abstraction
of the Lindenbaum-Tarski process. In this process one considers the Leibniz relation of indiscernible formulae. Such approach
has resulted in a classification of logics partly based on generalizations of equivalence connectives: the Leibniz hierarchy. This paper performs an analogous abstract study of non-classical logics based on the kind of generalized implication connectives
they possess. It yields a new classification of logics expanding Leibniz hierarchy: the hierarchy of implicational logics. In this framework the notion of implicational semilinear logic can be naturally introduced as a property of the implication, namely a logic L is an implicational semilinear logic iff it
has an implication such that L is complete w.r.t. the matrices where the implication induces a linear order, a property which
is typically satisfied by well-known systems of fuzzy logic. The hierarchy of implicational logics is then restricted to the
semilinear case obtaining a classification of implicational semilinear logics that encompasses almost all the known examples
of fuzzy logics and suggests new directions for research in the field. 相似文献