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991.
Jia Liu Yunliang Zhao Hongliang Li Tianxing Chen 《Journal of Dispersion Science and Technology》2017,38(7):1035-1040
The aim of this study was to investigate the effects of inorganic cations (Na+, Ca2+, and Al3+) with different valences and concentrations on the stability of Na-montmorillonite (Na-MMT) suspension. The stability was studied by turbidity and zeta potential measurements and adsorption thermodynamics. The results showed that the Na-MMT particles coagulated at pH 3–4.8, the stability of which was much inhibited by cations with higher concentration and higher valence. This was caused by the adsorption of cations and the reduction of electrostatic repulsion. Adsorption of Ca2+ on Na-MMT surface fits the Freundlich curve model well. Furthermore, the adsorption reduced the zeta potential of Na-MMT and then the energy barrier between the particles which well obeyed the classic DLVO theory. 相似文献
992.
《Angewandte Chemie (International ed. in English)》2017,56(8):2198-2202
A bulky substituted stannane Ar*SnH3 (Ar*=2,6‐(2′,4′,6′‐triisopropylphenyl)phenyl) was treated with the well‐known frustrated Lewis pair (FLP) Pt Bu3/B(C6F5)3 in varying stoichiometries. To some degree, hydride abstraction and adduct formation is observed, leading to [Ar*SnH2(Pt Bu3)]+ which is rather unreactive toward further dehydrogenation. In a competing process, the FLP proved to be capable of completely striping‐off hydrogen and hydrides to generate the first cationic phosphonio‐stannylene [Ar*Sn(Pt Bu3)]+. This behavior provides insight into the activation/abstraction mechanism processes involved in these Group 14 hydride derivatives. 相似文献
993.
Electrophilic Aromatic Substitution with Silicon Electrophiles: Catalytic Friedel–Crafts C−H Silylation
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《Angewandte Chemie (International ed. in English)》2017,56(1):52-59
Electrophilic aromatic substitution is a fundamental reaction in synthetic chemistry. It converts C−H bonds of sufficiently nucleophilic arenes into C−X and C−C bonds using either stoichiometrically added or catalytically generated electrophiles. These reactions proceed through Wheland complexes, cationic intermediates that rearomatize by proton release. Hence, these high‐energy intermediates are nothing but protonated arenes and as such strong Brønsted acids. The formation of protons is an issue in those rare cases where the electrophilic aromatic substitution is reversible. This situation arises in the electrophilic silylation of C−H bonds as the energy of the intermediate Wheland complex is lowered by the β‐silicon effect. As a consequence, protonation of the silylated arene is facile, and the reverse reaction usually occurs to afford the desilylated arene. Several new approaches to overcome this inherent challenge of C−H silylation by SEAr were recently disclosed, and this Minireview summarizes this progress. 相似文献
994.
《Angewandte Chemie (International ed. in English)》2017,56(43):13270-13274
The selective, metal‐free generation of α‐carbonyl cations from simple internal alkynes was accomplished by the addition of a sulfoxide to a densely substituted vinyl cation. The high reactivity of the α‐carbonyl cations was found to efficiently induce hydrogen and even carbon shift reactions with unusual selecivities. Complex compounds with highly congested tertiary and all‐carbon‐substituted quartenary carbon centers can thus be accessed in a single step from simple precursors. Mechanistic analysis strongly supports the intermediacy of the title compounds and provides a simple predictive scheme for the migratory aptitude of different substituents. 相似文献
995.
Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach
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Elliot J. Lawrence Thomas J. Herrington Dr. Andrew E. Ashley Dr. Gregory G. Wildgoose 《Angewandte Chemie (International ed. in English)》2014,53(37):9922-9925
In order to use H2 as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H2 into 2 H+ and 2 e?. Herein, we report a metal‐free approach to catalyze the oxidation of H2 by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H2 with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC‐stabilized borenium cation [(IiPr2)(BC8H14)]+ (IiPr2=C3H2(NiPr)2, NHC=N‐heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H2 oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol?1. The NHC–borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C6F5)3, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach. 相似文献
996.
Dr. Daniel Franz Dr. Elisabeth Irran Prof. Dr. Shigeyoshi Inoue 《Angewandte Chemie (International ed. in English)》2014,53(51):14264-14268
The reaction of the bulky bis(imidazolin‐2‐iminato) ligand precursor (1,2‐(LMesNH)2‐C2H4)[OTs]2 ( 1 2+ 2[OTs]?; LMes=1,3‐dimesityl imidazolin‐2‐ylidene, OTs=p‐toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2‐(LMesN)2‐C2H4)BH2[OTs] ( 2 + [OTs]?). The boronium cation 2 + [OTs]? reacts with elemental sulfur to give the thioxoborane salt (1,2‐(LMesN)2‐C2H4)BS[OTs] ( 3 + [OTs]?). The hitherto unknown compounds 1 2+ 2[OTs]?, 2 + [OTs]?, and 3 + [OTs]? were fully characterized by spectroscopic methods and single‐crystal X‐ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2 + and 3 +. The theoretical, as well as crystallographic studies reveal that 3 + is the first example for a stable cationic complex of three‐coordinate boron that bears a B?S double bond. 相似文献
997.
Aryl or heteroaryl 5-substituted imidazo-benzothiadiazole derivatives were synthesized and shown to display remarkable solvatofluorochromism and selectively sense mercury(II) cations, acetate anions, and nitroaromatic derivatives, with discrimination between p-nitrophenol and picric acid. These novel sensors are of importance these days, as the detection of explosives is a high priority in issues of national security and environmental protection. To determine the ion binding properties of the sensors, their absorption and fluorescence emission spectra upon binding different cations and anions were compared. Significant shifts in the spectra were only observed for mercury(II) and acetate. The binding of these two ions was further studied using 1H NMR. The binding properties of different nitroaromatic compounds were also determined, and the results showed the importance of the presence of a phenol group in the guest molecule. Specifically, the two sensors were shown to discriminate between p-nitrophenol and picric acid. Finally, the mechanism of fluorescence quenching upon addition of nitrophenols was determined by computational methods. 相似文献
998.
Catalytic Behaviour of Transition Metal Cations In Electrosynthesis and Growth of Nanostructure Conducting Polypyrrole Films On/Between the Passive Cu Interdigital Electrodes: Application In Gas Sensors
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The electrosynthesis of nanostructured polypyrrole (PPy) on copper interdigital electrodes (Cu‐IDEs) surfaces was performed by anodic oxidation of pyrrole in the presence of oxalic acid in aqueous solution (passivation technique) by potentiostatic method. Some divalent transition metal ions (Fe(II), Co(II), Ni(II), Cu(II), Zn(II)) as catalyst can be used to polymers growth on/between spacing between passive Cu‐IDEs by using potentiostatic technique. The morphology of the conducting films was examined by field emission scanning electron microscope, indicating a dependence of the film morphology to catalyst type. The PPy films on Cu‐IDEs were used to investigate the properties of the gas sensing ability. 相似文献
999.
Julian C. Kwok 《Supramolecular chemistry》2014,26(3-4):182-191
The 1:1 and 2:1 host–guest complexation of a series of 1,n-bis(isoquinolinium)alkane dications (Iq(CH2)nIq2+, n = 2, 4, 5, 6, 8, 9, 10 and 12, and Iq(p-xylene)Iq2+) by cucurbit[7]uril (CB[7]) in aqueous solution has been investigated by 1H NMR spectroscopy and ESI mass spectrometry. The site of binding of the first CB[7] is dependent on the nature of the central linker group, with encapsulation of the p-xylene group or the polymethylene chain when n = 6–10.With shorter (n = 2–5) or longer (n = 12) chains, the first CB[7] binds over an isoquinolinium group. With a second CB[7], the binding of the central group is abandoned in favour of the CB[7] hosts encapsulating the two cationic isoquinolinium termini. The 1:1 and 2:1 host–guest stability constants are related to modes of binding and the nature of the central linkers, and are compared with dicationic guests bearing different terminal groups. 相似文献
1000.
M. Sc. Hans A. Laub Prof. Dr. Gwilherm Evano Prof. Dr. Herbert Mayr 《Angewandte Chemie (International ed. in English)》2014,53(19):4968-4971
Donor‐substituted diarylcarbenium ions Ar2CH+ react with ynamides to give 1‐amido‐substituted allyl cations (α,β‐unsaturated iminium ions). Kinetic studies show that these adducts, which correspond to the addition of a C? H bond across the C?C bond, are formed stepwise with initial formation of keteniminium ions and subsequent 1,3‐hydride shifts. The linear correlations between the second‐order rate constants (lg k2, 20 °C) with the electrophilicity parameters E of the diarylcarbenium ions allow us to include ynamides in our comprehensive nucleophilicity scale and thus predict potential electrophilic reaction partners. 相似文献