Regioselectivity of fluorine substitution by alkoxides on unsymmetrical difluoroarenes

An efficient approach to unsymmetrical halogenated resorcinol diethers has been developed. This synthesis consists of two subsequent nucleophilic aromatic substitutions (S_NAr) of unsymmetrical difluoroarenes by alkoxides. The novelty of this approach is its control of regioselectivity during the first S_NAr, which occurs at room temperature. Interestingly, the reactivity of competing fluorines was correlated to their chemical shift in ¹⁹F NMR.     

Theoretical study on activation mechanism of fluorine substitution reactions of Keggin-MAl12 in aqueous solutions

The fluorine substitution reactions of Keggin polymeric aluminum species K-MAl₁₂ (M = Al, Ga, Ge) were investigated using density functional theory. Two substitution pathways (associative and dissociative) were simulated. The results show that the replacement of charged tetrahedral center metals causes the change in energy barrier either for associative mechanism or dissociative mechanism. The preferred activation mechanisms are proposed by comparing associative and dissociative barriers for fluorine substitution reactions of three Keggin Al species. The fluorine substitution reactions of Al₁₃ and GaAl₁₂ are inclined to dissociative mechanism and GeAl₁₂ follows an associative mode, indicating a mechanistic variation induced by the alteration of tetrahedral metals.     

Kinetics and mechanism of the oxidation of benzyl alcohol by potassium chlorochromate

The kinetics of the oxidation of benzyl alcohol by potassium chlorochromate, KCrClO₃, has been studied in dimethyl sulfoxide-dichloromethane medium. The reaction is catalyzed by acid. The effects of temperature and solvent composition were studied and activation parameters evaluated. Probable mechanisms are discussed.

---

, KCrClO₃, . . . .

     

Platinum-like oxidation of nickel surfaces by rapidly switching voltage to generate highly active bifunctional catalysts

Recently the importance of catalyzing the water splitting step of the hydrogen evolution reaction (HER) was highlighted. We demonstrate here a treatment to modify a nickel surface into a highly effective bifunctional HER catalyst (i₀ = 0.18 A/m², Tafel Slope = 106 mV/dec) that has a good distribution of both water splitting sites and H_ads combination sites. The resulting surface is characterized electrochemically, and with SEM, EDX, XPS and AFM. The data is found to be consistent with the treatment oxidizing the Ni surface in a novel way creating the hypothesized “Ni(OH)_x” structure (x between 0 and 2).     

Kinetics and mechanism of oxidation of allyl alcohol byN-bromosuccinimide

The kinetics of oxidation of allyl alcohol byN-bromosuccinimide (NBS) has been studied at 35 °C in aqueous medium. The reaction shows first order dependence on bothNBS and allyl alcohol. In fairly high acid concentration, there is no change in the rate of the reaction but at low acid concentration, the rate is considerably enhanced. There is no primary salt effect. At varying mercuric acetate concentrations, the rate constant remains the same. But in the absence of mercuric acetate, the rate is enhanced. The kinetic parameters,E _a,Arrhenius factorA, H, G and S have been calculated. A rate law in agreement with experimental results has been derived. A mechanism is proposed.

---

Kinetik und Mechanismus der Oxidation von Allylalkohol mixN-Bromsuccinimid

Zusammenfassung Die Kinetik der Oxidation von Allylalkohol mitN-Bromsuccinimid (NBS) wurde bei 35 °C in wäßrigem Medium untersucht. Die Reaktion zeigt erste Ordnung gegenüberNBS und Allylalkohol. Bei relativ hoher Säurekonzentration zeigt sich keine Änderung der Reaktionsgeschwindigkeit, bei niedriger Säurekonzentration wird die Reaktionsgeschwindigkeit beträchtlich erhöht. Es wurde kein primärer Salzeffekt festgestellt. Bei varriierender Quecksilberacetatkonzentration bleibt die Reaktionsgeschwindigkeit gleich, bei Abwesenheit von Quecksilberacetat wird jedoch die Geschwindigkeitskonstante erhöht. Die kinetischen Parameter,E _a, derArrheniusfaktorA, H , G und S wurden bestimmt. Ein Geschwindigkeitsgesetz in Übereinstimmung mit den experimentellen Befunden wurde abgeleitet und ein Mechanismus vorgeschlagen.

     

Structural effects of fluorine substitution in proteins

The consequences of substitution of fluorine for the para hydrogen of a phenylalanine residue in ribonuclease-S were investigated by conformational energy calculations using the AMBER force field. Both the fluorine-containing protein and the corresponding nonfluorinated material were subjected to conformational adjustment through energy minimization and the minimum energy structures so defined were compared. Fluorine substitution leads to small alterations in many atomic positions in the protein, with adjustments at at sites more than 0.5 nm from the fluorine appearing to be somewhat larger than those within the immediate vicinity of the fluorine. Several atoms proximate to the fluorine atom were observed to move toward the fluorine while others in the same vicinity move away. The greater bulk of the fluorine atom and the strongly different electronic properties of fluorine compared to hydrogen thus appear to be insufficient to cause a consistent, unidirectional change in nearest-neighbor interactions upon introduction of a fluorine atom into a protein structure. Virtually all changes in atomic positions that are predicted by these calculations would be barely detectable by a crystallographic study with a resolution of 0.2 nm.     

Theoretical studies on the substitution effect of fluorine on ethylene

The substitution effect of fluorine on ethylene is investigated by means of studyingthe properties of the charge distribution at the bond critical points with the theory of atomsin molecules.It is found that fluorine atom acts not only as a σ electron acceptor,but also asa π electron donor,and these double effects are reflected in the quantity of ellipticity,Lap-lacian and the charge density of charge distribution at the bond critical points.For C—C,C—Fbonds,the major axis of elliptical contours is perpendicular to the molecular plane,but forC—H bond,it is parallel to the molecular plane.Other effects originating from the substi-tution have also been discussed.     

Kinetics and mechanism of catalytic partial oxidation reactions of alkanes on rhodium surfaces

The kinetics of the partial oxidation of isobutane with molecular oxygen on Rh(111) single-crystal surfaces were studied by using a collimated molecular beam under ultrahigh vacuum conditions. Both hydrogen and water were shown to form as primary products, not after secondary reforming or water-gas shift steps as it has been suggested in the past. The production of carbon monoxide (but not of carbon dioxide) was also detected. Water production reaches its steady-state rate in a slower fashion than the rest of the products, presumably because of the kinetics of formation and consumption of the hydroxo surface intermediate involved.     

Impact of the alkali cation on the electrocatalytic oxidation of urea and benzyl alcohol on nickel electrode

The effect of electrolyte alkali metal cations (Li⁺, Na⁺, or K⁺) on the electro-oxidation of urea and benzyl alcohol on NiOOH catalyst has been investigated by means of cyclic voltammetry and chronoamperometry in the presence of an electrolyte containing LiOH, NaOH, or KOH. The catalytic activity toward the electro-oxidation of urea and benzyl alcohol was found to increase in the sequence Li⁺ < Na⁺ < K⁺. This activity's difference is partly caused by different surface blockage abilities by OH–M⁺(H₂O)_x (M: Li, Na, K) clusters, which is similar to many electrocatalytic reactions on Pt reported previously, additionally, incorporation of various cations to the catalyst may induce the activities difference as well.     

Elucidation of the vicarious nucleophilic substitution of hydrogen mechanism via studies of competition between substitution of hydrogen, deuterium, and fluorine.

Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and S(N)Ar substitution of fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined from competitive experiments carried out at various concentrations of base. The observed dependence of the VNS/S(N)Ar rate ratio on the base concentration confirmed the two-step mechanism of the VNS, which consists of reversible formation of sigma(H) adducts of the alpha-chlorocarbanion to nitroarene, followed by base-induced beta-elimination of HCl. It was also evidenced that both of these processes can be the rate-limiting steps: the beta-elimination at low base concentration and the nucleophilic addition at high base concentration. Consistent with that conclusion is the finding that the kinetic isotope effect in the VNS reaction decreases from 4.2 (a value typical of a primary KIE) to 0.8 (a value typical of a secondary KIE) with increasing base concentration. Also reported is our discovery that the S(N)Ar substitution of the 2-fluoronitrobenzenes studied in this work was subject to base catalysis under some of the experimental conditions employed in our competitive experiments.     

The effect of fluorine substitution on the zero-field splittings of naphthalene

Zero-field splitting parameters of 1-fluoro- and 2-fluoronaphthalene in single crystal hosts of durene and biphenyl have been measured in experiments near zero magnetic field. Disagreements with previous reports concerning the effect of fluorine on the ZFS parameters are discussed and are shown to arise due to crystal field effects.     

Chemisorption of CO2 on nickel surfaces

CO2 chemisorption on the Ni(111), Ni(100), and Ni(110) surfaces was investigated at the level of density functional theory. It was found that the ability of CO2 chemisorption is in the order of Ni(110) > Ni(100) > Ni(111). CO2 has exothermic chemisorption on Ni(110) and endothermic chemisorption on Ni(111), while it is thermally neutral on Ni(100). It is also found that there is no significant lateral interaction between the adsorbed CO2 at 1/4 monolayer (ML) coverage, while there is stronger repulsive interaction at 1/2 ML. On all surfaces, the chemisorbed CO2 is partially negatively charged, indicating the enhanced electron transfer, and the stronger the electron transfer, the stronger the C=O bond elongation. The bonding nature of the adsorbed CO2 on nickel surfaces has been analyzed. The thermodynamics of CO2 dissociative chemisorption, compared with CO and O adsorption, has been discussed, and the thermodynamic preference is in the sequence Ni(100) > Ni(111) > Ni(110).     

碱性介质中二过碘酸合铜(III)配离子氧化四氢糠醇的动力学及机理

本文采用分光光度法研究了二过碘酸合铜(III)配离子在碱性介质中氧化四氢糠醇的动力学及机理. 结果表明反应对[Cu(III)]是一级, 对四氢糠醇是1.3级. 反应速率随体系中[OH^-]的增大而增大, 随过碘酸浓度的增大而减小, 反应体系加入硝酸钾盐时,速率增大, 有正盐效应. 在氮气保护下, 体系能够诱发丙烯酰胺聚合. 提出了一种含有自由基过程的反应机理, 据此导出了一个能够解释本文全部实验事实的速率方程. 求得了速率控制步骤的速率常数, 并给出了相应的活化参数.     

Kinetics and mechanism of the oxidation of some carboxylates by a nickel (III) oxime-imine complex

The kinetics of the oxidation of formate, oxalate, and malonate by |Ni^III(L¹)|²⁺ (where HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) were carried out over the regions pH 3.0–5.75, 2.80–5.50, and 2.50–7.58, respectively, at constant ionic strength and temperature 40°C. All the reactions are overall second-order with first-order on both the oxidant and reductant. A general rate law is given as - d/dt|Ni^III(L¹)²⁺| = k_obs|Ni^III(L¹)²⁺| = (k_d + nk_s |R|)|Ni^III(L¹)²⁺|, where k_d is the auto-decomposition rate constant of the complex, k_s is the electron transfer rate constant, n is the stoichiometric factor, and R is either formate, oxalate, or malonate. The reactivity of all the reacting species of the reductants in solution were evaluated choosing suitable pH regions. The reactivity orders are: k_HCOOH > k; k > k > k, and k > k < k for the oxidation of formate, oxalate, and malonate, respectively, and these trends were explained considering the effect of hydrogen bonded adduct formation and thermodynamic potential. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 225–230, 1997.     

Catalytic alcohol oxidation by an unsymmetrical 5-coordinate copper complex: electronic structure and mechanism

Density functional theory reveals the detailed mechanism of alcohol oxidation by a model copper complex, Cu(II)L, L = cis-1-(3',5'-dimethoxy-benzylideneamino)-3,5-[2-hydroxy-(3',5'-di-tert-butyl)benzylideneimino]cyclohexane. Despite the obvious structural and functional parallels between the title compound and the enzyme galactose oxidase, the details of the catalytic pathway are fundamentally different. In the enzyme, coordination of the substrate produces an active form containing a Cu(II) centre and a tyrosyl radical, the latter being responsible for the abstraction of hydrogen from the substrate. In the model system, in marked contrast, the active form contains a Cu(II) centre, but the ligand radical character is localised on the substrate (alcoholate) oxygen, rather than the phenolate ligand. The result is a significantly higher barrier to hydrogen-atom abstraction compared to the enzyme itself. The origin of these significant differences is traced to the rigid nature of the pentadentate ligand, which resists changes in coordination number during the catalytic cycle.