Competing elimination and substitution reactions of simple acyclic disulfides

MP2/aug-cc-pVDZ and B3LYP/cc-pVDZ calculations of the reactions of CH3SSR (R = H or CH3) with fluoride, hydroxide or allyl anion in the gas-phase were performed to determine the mechanism for both elimination and substitution reactions. The elimination reactions were shown to follow the E2 mechanism. The substitution reactions with hydroxide and fluoride proceed by the addition-elimination mechanism, but those with allyl anion proceed by the SN2 mechanism. The elimination reactions with F- and HO- are preferred to the substitution reactions, while allyl anion prefers the substitution route.     

High‐Fidelity Multistate Switching with Anion–Anion and Acid–Anion Dimers of Organophosphates in Cyanostar Complexes

The acid–base switching of complexes formed from anti‐electrostatic anion–anion homodimers of organophosphates and cyanostar macrocycles was investigated for the first time. High‐fidelity 2:2 complexes were selected by using suitably sized organo substituents. Reversible and direct switching occurs with triflic acid and hydroxide base. An unexpected acid⋅⋅⋅anion heterodimer was discovered with weaker picric acid, which helped reveal some of the elementary steps. Switching can also proceed in a cooperative (strong anion then weak acid) or stepwise manner (weak acid then strong anion).     

Isomer differentiation by charge inversion tandem mass spectrometry: an investigation into the structure of the ionic products from an SN(ANRORC) eeaction

Negative ion fast atom bombardment and negative ion chemical ionization tandem mass spectrometry combined with charge inversion reactions are used to confirm that 2-chloro-5nitropyridine reacts with the hydroxide ion by an S_N(ANRORC) mechanism in solution to form a ring-opened C₅H₃N₂O ₃ ^? anion that has the structure of the (M-H)^? anion of 2-nitro-4-cyano-2-butenal. The addition of excess hydroxide ion forms the expected 2-oxy5-nitropyridine anion. In the gas phase the same reaction forms only the 2-oxy-5-nitropyridine anion. High energy charge inversion is shown to be an excellent means of differentiating between these isomeric negative ions.     

Reductive openings of benzylidene acetals revisited: a mechanistic scheme for regio- and stereoselectivity

Despite the importance of regioselective reductive openings of cyclic acetals, mechanistic details are scarce. In this study 4,6-O-benzylidene acetals were used as model compounds for deciphering the mechanism of regioselective openings using a variety of reducing agents. Competitive isotopic studies aiming at primary and secondary isotope effects, as well as an electron-deficient substrate, were used to evaluate stereo- and regioselectivity. We show that there are three distinctly different mechanistic pathways. In nonpolar solvents, such as toluene, the acetal is activated by the very reactive naked Lewis acid to give a fully developed oxocarbenium ion that is then reduced by the borane, with low stereoselectivity. In THF the reactivity of the Lewis acid is moderated by complex formation with the solvent. These reactions are thus much slower and proceed through an intimate ion pair and thereby show high stereoselectivities. The regioselectivity in these reactions is directed by the interaction between the Lewis acid and the most nucleophilic oxygen of the acetal, thus yielding a free 6-hydroxyl group. Finally, boranes such as BH(3)·NMe(3) are activated by Lewis acid, which results in the borane being the most electrophilic species, and consequently the reaction shows inversed regioselectivity to give a free 4-hydroxyl group. These reactions proceed through an oxocarbenium ion and thus show low stereoselectivity.     

Kinetics and mechanisms of the catalytic reactions of formaldehyde with copper oxides and a copper ion complex in aqueous alkali

The kinetics of the autocatalytic reactions of formaldehyde with copper(II) and copper(I) oxides and with the Cu²⁺ ion of the copper EDTA complex, as well as formaldehyde disproportionation in the presence of copper metal, have been investigated in aqueous solutions of sodium hydroxide. Two likely reaction mechanisms are presented. The difference between these mechanisms does not alter the observed kinetics of the processes, whose rate is determined by their first, slow step, namely, the oxidation of the methylene glycol anion adsorbed on the copper surface into formic acid. In the slow step of the first mechanism, a hydride ion is abstracted from the methylene glycol anion and is transferred to copper. In the slow step of the second mechanism, the methylene glycol anion undergoes anodic oxidation, releasing a hydrogen atom and an electron. In the rapid steps of the first mechanism, the hydride ion undergoes anodic oxidation to hydrogen, the copper compound undergoes cathodic reduction to copper metal, and, simultaneously, the electron and hydrogen are transferred to a nonionized formaldehyde molecule to yield methanol. Mathematical models are suggested for the reactions. The effective rate constants and activation energies of the slow steps of the reactions have been determined. The effective rate constants of the noncatalytic reduction reactions of the copper compounds and the ratios of the rates of the rapid hydrogen and methanol formation reactions have been estimated.     

Oxygen anion (O−) and hydroxide anion (HO−) reactivity with a series of old and new refrigerants

The reactivity of a series of commonly used halogenated compounds (trihalomethanes, chlorofluorocarbon, hydrochlorofluorocarbon, fluorocarbons, and hydrofluoroolefin) with hydroxide and oxygen anion is studied in a compact Fourier transform ion cyclotron resonance. O⁻ is formed by dissociative electron attachment to N₂O and HO⁻ by a further ion‐molecule reaction with ammonia. Kinetic experiments are performed by increasing duration of introduction of the studied molecule at a constant pressure. Hydroxide anion reactions mainly proceed by proton transfer for all the acidic compounds. However, nucleophilic substitution is observed for chlorinated and brominated compounds. For fluorinated compounds, a specific elimination of a neutral fluorinated alkene is observed in our results in parallel with the proton transfer reaction. Oxygen anion reacts rapidly and extensively with all compounds. Main reaction channels result from nucleophilic substitution, proton transfer, and formal H₂⁺ transfer. We highlight the importance of transfer processes (atom or ion) in the intermediate ion‐neutral complex, explaining part of the observed reactivity and formed ions. In this paper, we present the first reactivity study of anions with HFO 1234yf. Finally, the potential of O⁻ and HO⁻ as chemical ionization reagents for trace analysis is discussed.     

The oxidation of cinnamaldehyde with alkaline hydrogen peroxide

The oxidation of cinnamaldehyde (3-phenyl-2-propenal) by alkaline peroxide results in epoxidation of the double bond to form cinnamaldehyde epoxide (3-phenyl-2,3-epoxy-propanal) which undergoes further reaction by ring opening and side chain cleavage to yield benzaldehyde and acidic fragments. The reactions are first-order in the organic substrates and perhydroxyl anion and second-order overall. In the presence of alkali alone, two further reactions take place in which cinnamaldehyde and cinnamaldehyde epoxide side chains are cleaved by reaction with hydroxide ion to form benzaldehyde and side chain fragments. These reactions are first-order in the organic substrates and hydroxide ion and second-order overall. Increasing solvent polarity accelerates the rates of reaction and reaction mechanisms have been proposed to describe the observed kinetic behavior. The stereoselectivity of the epoxidation reaction has been examined in terms of an existing model for epoxidation of α, β-unsaturated ketones by alkaline peroxide. © 1993 John Wiley & Sons, Inc.     

牛黄酸及水杨酸系药物对镁铝层状复合氢氧化物的插层组装

通过XRD和IR表征对镁铝层状复合氧氧化物(LDH)与水杨酸、乙酰氨基酚、乙酰水杨酸,以及谷氨酸、色氨酸、牛黄酸反应产物的比较分析,研究了不同药物对有关组装方式的适宜性.结果表明水杨酸类药物均可通过离子交换组装到LDH层间,晶胞参数c由2.3893 nm依次增大为2.4024、2.4110和2.4111nm,通道高度h由0.3194 nm增大为0.3238、0.3267和0.3268 nm;通过离子交换能将谷氨酸组装到LDH层间,产物的IR吸收、热分解行为及TEM形貌与前体有明显区别,晶胞参数c由2.3765nm增大为2.3851nm,h由0.3152nm增大为0.3180nm;共沉淀法适宜制备LDH-牛黄酸插层复合物,但简单的离子交换不能使色氨酸与LDH有效复合.     

Polymers with Tethered Anionic and Cationic Groups as Membranes for Fuel Cells

Ionic clustering, water binding, and ion conductivity were studied in polymers functionalized with sulfonic acid and quaternary ammonium hydroxide groups. Small-angle x-ray scattering showed that no clustering occurred in the quaternary ammonium containing anion exchange membranes, while evidence of ionic clusters was present in both sulfonated poly(phenylene) and in Nafion, a poly(perfluorosulfonic acid). Interestingly, the water self-diffusion coefficients of the anion exchange membranes were generally greater than those observed for the sulfonated poly- (phenylene)s, and moreover, the water self diffusion coefficients in anion exchange membranes were not a strong function of diffusion time. The water binding behavior lead to increased normalized conductivity in anion exchange membranes as compared to proton exchange membranes at the highest ion exchange capacities.     

Base-Catalyzed Dehydration of 3-Substituted Benzene cis-1,2-Dihydrodiols: Stabilization of a Cyclohexadienide Anion Intermediate by Negative Aromatic Hyperconjugation

Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic "negative hyperconjugation" is described. It complements an earlier inference of "positive" hyperconjugative aromaticity for the cyclohexadienyl cation. The anion is a reactive intermediate in the dehydration of benzene cis-1,2-dihydrodiol to phenol. Rate constants for 3-substituted benzene cis-dihydrodiols are correlated by σ(-) values with ρ = 3.2. Solvent isotope effects for the reactions are k(H(2)O)/k(D(2)O) = 1.2-1.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a "carbanion-like" transition state. These and other experimental results confirm that the reaction proceeds by a stepwise mechanism, with a change in rate-determining step from proton transfer to the loss of hydroxide ion from the intermediate. Hydrogen isotope exchange accompanying dehydration of the parent benzene cis-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of ～10(11) s(-1), corresponding to rotational relaxation of the aqueous solvent, is assigned to loss of hydroxide ion from the intermediate. The rate constant for internal return therefore falls in the range 10(11)-10(12) s(-1). From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pK(a) of 30.8 ± 0.5 was determined for formation of the anion. Although loss of hydroxide ion is hugely exothermic, a concerted reaction is not enforced by the instability of the intermediate. Stabilization by negative hyperconjugation is proposed for 1,2-dihydroxycyclohexadienide and similar anions, and this proposal is supported by additional experimental evidence and by computational results, including evidence for a diatropic ("aromatic") ring current in 3,3-difluorocyclohexadienyl anion.     

Synthesis of α‐alkenyl‐β‐hydroxy adducts by α‐addition of unprotected 4‐bromocrotonic acid and amides with aldehydes and ketones by chromium(II)‐mediated reactions

The regioselective and diastereoselective chromium(II)‐mediated reactions of 4‐bromocrotonic acid or amides with aldehydes and ketones can proceed without the need to protect protic sites to generate the respective α‐alkenyl‐β‐hydroxy adducts, i.e. formally the addition of the α‐anion of a carboxylic acid or amide to an oxo‐compound is featured. Copyright © 2016 John Wiley & Sons, Ltd.     

Electro-nuclear double resonance spectroscopic evidence for a hydroxo-bridge nucleophile involved in catalysis by a dinuclear hydrolase

Despite the current availability of several crystal structures of purple acid phosphatases, to date there is no direct evidence for solvent-derived ligands occupying terminal positions in the active enzyme. This is of central importance, because catalysis has been shown to proceed through the direct attack on a metal-bound phosphate ester by a metal-activated solvent-derived moiety, which has been proposed to be either (i) a hydroxide ligand terminally bound to the ferric center or (ii) a bridging hydroxide. In this work we use (2)H Q-band (35 GHz) pulsed electron-nuclear double resonance (ENDOR) spectroscopy to identify solvent molecules coordinated to the active mixed-valence (Fe(3+)Fe(2+)) form of the dimetal center of uteroferrin (Uf), as well as to its complexes with the anions MoO(4), AsO(4), and PO(4). The solvent-derived coordination of the dinuclear center of Uf as deduced from ENDOR data includes a bridging hydroxide and a terminal water/hydroxide bound to Fe(2+) but no terminal water/hydroxide bound to Fe(3+). The terminal water is lost upon anion binding while the hydroxyl bridge remains. These results are not compatible with a hydrolysis mechanism involving a terminal Fe(3+)-bound nucleophile, but they are consistent with a mechanism that relies on the bridging hydroxide as the nucleophile.     

Studies on the preparation and exchange reactions of 5-deuterated uracils

The mechanism of base catalyzed proton exchange at the 5-position of uracil and its N-methylated derivatives has been studied. These reactions proceed by addition — elimination across the 5,6-double bond when the 1-nitrogen is substituted with a methyl group, or with anchimeric assistance of the N-1 anion if the 1-position is unsubstituted. The base catalyzed hydrolyses of 1,3-dimethyluracil and 3-methyluracil also appear to proceed through hydrated intermediates. A facile method for an acid catalyzed preparation of 5-deuterated uracils is described as well as a simple and accurate method for analysis of deuterium content.     

New suppressor technology improves trace level anion analysis with carbonate-hydrogencarbonate mobile phases

Sodium carbonate-hydrogencarbonate mobile phases are preferred over sodium hydroxide for anion analysis by suppressor-based ion chromatography (IC). Unlike hydroxide, carbonate-hydrogencarbonate has strong eluting power and its buffering capacity can be used as a selectivity tool for controlling separations. However, carbonate-hydrogencarbonate mobile phases fell out of favor for trace level analysis because the carbonic acid suppressor effluent has some background conductivity, which reduces sensitivity compared to sodium hydroxide. This paper describes a new suppressor technology that improved anion analysis with carbonate-hydrogencarbonate mobile phases. In addition to converting the carbonate-hydrogencarbonate buffer to carbonic acid like other traditional IC suppressors, the new DS-Plus suppressor also removed carbonic acid from the suppressor effluent. Anions are now detected in water background, just like when using sodium hydroxide as the mobile phase. The lower background conductivity improves sensitivity and reduces detection limits. The water-dip often seen with conventional suppressors is greatly reduced, improving fluoride quantification.     

Alkynes to (E)-enolates using tandem catalysis: stereoselective anti-aldol and syn-[3,3]-rearrangement reactions

A new tandem catalysis strategy that transforms alkyne derivatives to (E)-enol-equivalents followed by stereoselective anti-selective aldol coupling or syn-selective [3,3]-rearrangement transformations is reported. The mechanism is thought to proceed through an interchanging series of Lewis acid and Brønsted acid catalyzed reactions via the intermediacy of a ketiminum ion species.     

Production of high purity solution of tetraalkylammonium hydroxide using ion-exchange process

The possibility of obtaining high-purity solutions of tetramethylammonium hydroxide and tetra-nbutylammonium hydroxide was examined by passing of appropriate tetraalkylammonium halide solution through a column of an anion exchanger in the base form. Optimizing a ratio of an amount of used resin to degree of conversion determined by the residual concentration of halogen ion was demonstrated.     

静态涂覆液态离子交换剂柱的离子色谱——十六烷基吡啶季铵盐涂覆柱的性能和应用

Small和Gjerde发展了离子色谱和单柱离子色谱以后,在检测系统和高效固定相方面也有令人注目的进展。例如:近年来利用烷基键合相或中性聚合物和离子对试剂相结合的离子色谱技术,使固定相材料     

A new method for fluorination of sterols

Diethylaminosulfurtrifluoride (DAST) is a good fluorinating reagent for sterols. The fluorinating reactions proceed through SN₂ mechanism, while reactions through free carbonium ion are responsible for rearrangements and eliminations.     

胶束介质中的Wittig—Horner反应研究

近年来手性胶束及混合胶束的出现,使得胶束在模拟酶研究方面取得了长足的进展。胶束作为酶的一个简单模型,可催化多种复杂有机反应,已有的胶束催化有机反应大多数与生化反应有关,如酯的水解反应等,胶束催化碳负离子反应的研究却很少,非离子型胶束催化的碳负离子反应就更少,本文报导用聚乙二醇类表面活性剂形成的胶束来     

Dissociation rates of urea in the presence of NiOOH catalyst: a DFT analysis

Single molecule reactions have been studied between nickel oxyhydroxide, urea, and the hydroxide ion to understand the process of urea dissociation into ammonia, isocyanic acid, cyanate ion, carbon dioxide, and nitrogen. In the absence of hydroxide ions, nickel oxyhydroxide will catalyze urea to form ammonia and isocyanic acid with the rate-limiting step being the formation of ammonia with a rate constant of 1.5 × 10?? s?1. In the presence of hydroxide, the evolution of ammonia was also the rate-limiting step with a rate constant of 1.4 × 10?2? s?1. In addition, desorption of the cyanate ion presented an energy barrier of 6190 kJ mol?1 suggesting that the cyanate ion cannot be separated from NiOOH unless further reactions occurred. Finally, elementary dissociation reactions with hydroxide ions deprotonating urea to produce nitrogen and carbon dioxide were analyzed. These elementary reactions were investigated along three paths differing in the order that protons were removed and the nitrogen atoms were rotated. The rate-limiting step was found to be the removal of carbon dioxide with a rate constant of 4.3 × 10??? s?1. Therefore, the catalyst could be deactivated by the surface blockage caused by carbon dioxide adsorption.