A nitrate-selective electrode based on bis(triphenylphosphine)iminium salts

A nitrate-sensitive electrode based on the ionic associates between bis(triphenylphosphine)iminium and nitrate ions in nitrobenzene solution is proposed. The electrode can be used for determinations of 10(-5) 0.1M nitrate in aqueous solutions.     

The Strecker reaction: kinetic and equilibrium studies of cyanide addition to iminium ions

Kinetics studies are reported of the reactions of benzylidene benzylamine 4a, and of benzylidene allylamine 4b, with cyanide in aqueous buffers to give the corresponding [small alpha]-aminonitriles. The results allow the calculation of values of rate and equilibrium constants for reaction of the iminium ions formed from 4a and 4b with cyanide ions. These values are compared with those, obtained from the hydrolysis reactions, for reaction of the iminium ions with hydroxide ions and with water. Comparison with some other iminium ions reveals that those formed from 4a and 4b are relatively unreactive due to the possibilities of charge delocalisation.     

Aminocatalytic Cross‐Coupling Approach via Iminium Ions to Different CC Bonds

Given the attractive ability of iminium ions to functionalize molecules directly at ostensibly unreactive positions, the reactivity of iminium ions, in which an α CH₂ group is replaced by C?O was explored. Background studies on the ability of such iminium cations to promote reactions via an iminium‐catalyzed or iminium‐equivalent pathway are apparently unavailable. Previously, tandem cross‐coupling reactions were reported, in which an iminium ion undergoes nucleophilic 1,2‐addition to give a putative three‐component intermediate that abstracts a proton in situ and undergoes self‐deamination followed by unprecedented DMSO/aerobic oxidation to generate α‐ketoamides. However, later it was observed that iminium ions can generate valuable α‐ketoamides through simple aerobic oxidation. In all reactions, iminium ions were generated in situ by reaction of 2‐oxoaldehydes with secondary amines.     

Facile synthesis of N-dialkylaminomethyl-substituted heterocycles

Iminium ions are generated by treatment of aminals with succinic anhydride. These iminium ions are trapped by heterocycles, giving the corresponding N-dialkylamino-methyl-substituted heterocycles, which are easily separated from the succinic acid monoamide byproducts by means of an aqueous base wash. The heterocyclic products are obtained in good yield and in a high state of purity without need of recrystallization or distillation.     

Contrasting the denaturing effect of guanidinium chloride with the stabilizing effect of guanidinium sulfate

Guanidinium chloride, GdmCl, is a strong denaturing agent of globular proteins, whereas guanidinium sulfate, Gdm(2)SO(4), is a stabilizing agent of globular proteins. The stabilizing activity of Gdm(2)SO(4) is unexpected because the denaturant capability of GdmCl is due to direct interactions of Gdm(+) ions with protein surface groups. It is shown that the statistical thermodynamic approach devised to explain the molecular origin of cold denaturation [G. Graziano, Phys. Chem. Chem. Phys., 2010, 12, 14245-14252] can provide a rationalization of the different behaviour of GdmCl and Gdm(2)SO(4) towards globular proteins. The fundamental quantity is the reversible work to create in the aqueous solution a cavity suitable to host the D-state and a cavity suitable to host the N-state. In aqueous GdmCl solutions, this contribution is not large enough to overwhelm the conformational entropy gain upon unfolding and the direct attractions between Gdm(+) ions and protein surface groups; in aqueous Gdm(2)SO(4) solutions, it is so large that it overwhelms the two destabilizing contributions. Sulfate ions, due to their high charge density, interact strongly with water molecules producing a number density increase, that, in turn, renders the cavity creation process very costly, reversing the denaturing power of Gdm(+) ions and stabilizing the N-state of globular proteins.     

Nucleophilicity parameters of enamides and their implications for organocatalytic transformations

The kinetics of the reactions of eleven substituted enamides with benzhydrylium ions (diarylcarbenium ions) were determined in acetonitrile solution. The second-order rate constants follow the correlation log k(2) (20 °C)=s(N)(E+N), which allowed us to derive reactivity parameters N and s(N). With 4.6相似文献   

Intramolecular hetero-Diels-Alder reactions of imine and iminium dienophiles: quantum mechanical exploration of mechanisms and stereoselectivities

A series of intramolecular hetero-Diels-Alder reactions of iminium and imine dienophiles has been explored with density functional theory using the B3LYP functional and 6-31+G* basis set. Aqueous solvation energies were calculated with the CPCM method. DFT predicts that these reactions are concerted but involve highly asynchronous transition states. Stereochemical preferences of imine cycloaddition transition states arise from electron repulsion of the nitrogen lone pair with electron density from the butadiene moiety. Protonation of the nitrogen leads to a highly asynchronous transition state. The iminium dienophiles are predicted to have a 17 kcal/mol lower barrier than the corresponding imines, even in aqueous solution.     

NaCl溶液中纤连蛋白在TiO_2表面吸附的动力学模拟

对NaCl水溶液环境中,纤连蛋白(FNIII_(10))分子在金红石型TiO_2 (110)表面的吸附行为进行了分子动力学模拟.根据模拟溶液各成分与TiO_2表面原子之间的径向分布函数、离子在水溶液中的扩散系数及FNIII10和离子的吸附构象等相关参数发现,分布于TiO_2表面的稳定双层水分子是固液界面的主要特征,FNIII10分子通过天冬氨酸残基侧链末端的羧基基团(COO~-)同表面Ti原子之间的强相互作用,结合赖氨酸残基侧链及位于FNIII_(10)始端的精氨酸残基N端的氨基基团(NH_3~+)与表面桥氧原子之间的氢键作用,牢固地吸附在TiO_2表面.溶液中吸附在TiO_2表面的Na~+可与羧基氧原子配合形成稳定的吸附构型,而分布在第二水层外侧的Cl-对纤连蛋白分子在TiO_2(110)表面的吸附点位基本无影响.     

Iminium Ions as Dienophiles in Aza‐Diels–Alder Reactions: A Closer Look

This review highlights the state of the art of the use of iminium ions as dienophiles in Aza‐Diels–Alder (ADA) cycloadditions. An historical survey spanning the very first discovery of the reaction to modern developments, mechanistic studies and synthetic applications of the iminium variant of the ADA (iADA) reaction are presented. The discussion is focused on the intermolecular and intramolecular versions of the iADA reactions that are conducted in aqueous solutions to generate, in situ, the reactive dienophile from an amine hydrochloride and either aliphatic or aromatic aldehydes in the presence of a variety of dienes. The retro‐ADA reaction is also presented as an interesting method for the protection of amines. The use of Lewis acid catalysis in these reactions was thoroughly studied by the reactions of different amines and aldehydes conducted in the presence of lanthanide(III) complexes.     

Aza Diels-Alder reactions in water: Cyclocondensation of C-acyl iminium ions with cyclopentadiene

C-Acyl-N-alkyl iminium ions as well as C-acyl N-unsubstituted iminium ions react with cyclopentadiene in water at ambient temperature giving rise to aza Diels-Alder adducts.     

Solution-phase electronegativity scale: insight into the chemical behaviors of metal ions in solution

By incorporating the solvent effect into the Born effective radius, we have proposed an electronegativity scale of metal ions in aqueous solution with the most common oxidation states and hydration coordination numbers in terms of the effective ionic electrostatic potential. It is found that the metal ions in aqueous solution are poorer electron acceptors compared to those in the gas phase. This solution-phase electronegativity scale shows its efficiency in predicting some important properties of metal ions in aqueous solution such as the aqueous acidities of the metal ions, the stability constants of metal complexes, and the solubility product constants of the metal hydroxides. We have elaborated that the standard reduction potential and the solution-phase electronegativity are two different quantities for describing the processes of metal ions in aqueous solution to soak up electrons with different final states. This work provides a new insight into the chemical behaviors of the metal ions in aqueous solution, indicating a potential application of this electronegativity scale to the design of solution reactions.     

Generation of α,β-Unsaturated Iminium Ions by Laser Flash Photolysis

Two at a time: α,β-Unsaturated iminium ions can be generated by laser flash photolysis of enaminophosphonium ions. The rate constants of their reactions with nucleophiles provide the first direct comparison of the electrophilicities of iminium ions derived from MacMillan's first- and second-generation catalysts.     

Thermodynamic stability and reactivity of silylated bis(oxy)iminium ions

Silylation of various nitronates with trialkylsilyl triflates was investigated by applying NMR techniques. In several cases, a flexible nitronate-bis(oxy)iminium ion (as an ion pair with triflate as counterion) equlibrium was found, and its thermodynamic parameters were determined. Elevation of temperature or dilution shifts this equilibrium toward the reactants. Activation parameters for the C,C-coupling reaction of silylated bis(oxy)iminium ions with a series of reference nucleophiles were determined. Estimated electrophilicity of bis(oxy)iminium ions allows one to count on C,C-coupling when partner nucleophilicity is N > 4.     

Computational study of iminium ion formation: effects of amine structure

Density functional calculations are used to explore the formation of iminium ions from secondary amines and acrolein and the subsequent reactivity of the resulting iminium ions. After establishing a feasible profile for this reaction in simulated experimental conditions, we focus on the effect of variation in amine structure on calculated barriers. This analysis shows that incorporation of a heteroatom (N or O) in the alpha-position to the reactive amine results in significantly reduced energy barriers, as does an electron-withdrawing group (carbonyl or thiocarbonyl) in the beta-position. Electron density analysis is used to monitor reactions at a detailed level, and to identify important intermolecular interactions at both minima and transition states. Barriers to reaction are linked to calculated proton affinities of secondary amines, suggesting that the relative ease of protonation-deprotonation of the amine is a key property of effective catalysts. Moreover, barriers for subsequent Diels-Alder reaction of iminium ions with cyclopentadiene are lower than for their formation, suggesting that formation may be the rate determining step in the catalytic cycle.     

Oxidation of 5-aminouracil with molecular oxygen in aqueous solution in the presence of copper(II) chloride

5-Aminouracil was found to form a six-coordinate copper(II) complex in aqueous solution and undergo oxidation to 5,5,6-trihydroxypyrimidine-2,4(1H,3H)-dione in the presence of molecular oxygen. A scheme was proposed for the fixation and activation of molecular oxygen on six-coordinate copper(II) complex with 5-aminouracil. The effect of the ligand structure on the rate of its oxidation with molecular oxygen in aqueous solution in the presence of copper(II) ions was shown.     

Anhydride Functional Nanocoatings for Heavy Metal Cadmium Capture and Release

Pulsed plasmachemical deposited poly(maleic anhydride) nanolayers display high efficiency for the capture of toxic cadmium ions from aqueous solution (down to the low parts per billion range). Subsequent release of the isolated heavy metal species and host media regeneration is accomplished by rinsing in weak acid solution.     

Basicity‐dependent properties of anion conducting membranes consisting of iminium cations for alkaline fuel cells

To design novel anion‐conducting polymer electrolyte membranes (AEMs), this paper proposes a basicity index (BI) that is defined by the ion‐exchange ratios of AEMs from the OH^? to Cl^? forms in a neutral aqueous solution as a parameter for Arrhenius basicity (dissociation constant). Using a radiation‐induced graft polymerization technique, three iminium cations are introduced into fluorinated polymer films. The BI of the iminium‐containing AEMs is less than that of a conventional ammonium‐type AEM. The conductivity and water uptake correlate positively with the BI, whereas the thermal and chemical stabilities correlate negatively with the BI. The dependence on the BI stems from the stabilization of the iminium hydroxide in proportion to the basicity of the original diaza‐compounds, resulting in a decrease in conductivity and water uptake with keeping higher thermal and chemical stabilities. Notably, ion conductivity is sufficient and water uptake is less in AEMs with a medium BI. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 503–510     

A two-component polymeric optode membrane based on a multifunctional ionic liquid

This work details the use of a 2-component optode membrane which is capable of generating three distinct colours in the presence of Cu(2+) and Co(2+) ions. It has been found that the ionic liquid (IL) trihexyltetradecylphosphonium dicyanamide [P(6,6,6,14)][DCA] can act as plasticizer, ligand and transducer dye when used in poly(vinyl chloride) (PVC) membranes, which significantly simplifies the optode membrane cocktail. Upon exposure to an aqueous Cu(2+) solution, a yellow colour is generated within the membrane, while exposure to an aqueous Co(2+) solution generates a blue colour. Exposure to a solution containing both ions produces a green colour. Vibrational spectroscopy has been used to investigate the molecular basis of the IL-metal ion the binding mechanism. Analytical characteristics of the membranes including the effect of interfering ions, binding constants and the limit of detection for both ions have been estimated. Finally the case of simultaneous dual-analyte recognition is presented based on two distinct absorption maxima.     

A Theoretical and Experimental Study of the Effects of Silyl Substituents in Enantioselective Reactions Catalyzed by Diphenylprolinol Silyl Ether

The effect of silyl substituents in diphenylprolinol silyl ether catalysts was investigated. Mechanistically, reactions catalyzed by diphenylprolinol silyl ether can be categorized into three types: two that involve an iminium ion intermediate, such as for the Michael‐type reaction (type A) and the cycloaddition reaction (type B), and one that proceeds via an enamine intermediate (type C). In the Michael‐type reaction via iminium ions (type A), excellent enantioselectivity is realized when the catalyst with a bulky silyl moiety is employed, in which efficient shielding of a diastereotopic face of the iminium ion is directed by the bulky silyl moiety. In the cycloaddition reaction of iminium ions (type B) and reactions via enamines (type C), excellent enantioselectivity is obtained even when the silyl group is less bulky and, in this case, too much bulk reduces the reaction rate. In other cases, the yield increases when diphenylprolinol silyl ethers with bulky substituents are employed, presumably by suppressing side reactions between the nucleophilic catalyst and the reagent. The conformational behaviors of the iminium and enamine species have been determined by theoretical calculations. These data explain the effect of the bulkiness of the silyl substituent on the enantioselectivity and reactivity of the catalysts.     

Reaktionen von 2- und 3-phenylsubstituierten Alkylakyliden-iminium-Ionen in der Gasphase. 5. Mitteilung über stossinduzierte Fragmentierungen von felddesorbierten Kationen

Reactions of 2- and 3-Phenyl Substituted Alkylalkylidene Iminium Ions in the Gas Phase The collision-induced fragmentations of 2- and 3-phenyl substituted alkylalkylidene iminium ions are reported. Besides the homolytic cleavage of the azaallyl bond a nucleophilic attack of the unsubstituted phenyl group at the iminium function is observed in the gas phase, yielding carbonium ions such as cyclopropanspirobenzenium ( 3 ), indanylium ( 10 ) and indenylium ions ( 11 ).