Ketene-forming eliminations from aryl bis(4'-chlorophenyl)acetates promoted by R2NH-R2NH2+ in aqueous MeCN. Change of mechanism

Elimination reactions of (4'-ClC6H4)2CHCO2C6H3-2-X-4-NO2 promoted by R2NH-R2NH2+ in 70 mol% MeCN (aq.) have been studied kinetically. The reactions are second-order and exhibit Br?nsted beta = 0.44-0.86 and /beta(lg)/ = 0.41-0.71. The Br?nsted beta decreased with a poorer leaving group and /beta(lg)/ increased with a weaker base. The results are consistent with an E2 mechanism. When X=H, the reaction proceeded by the concurrent E2 and Elcb mechanism.     

Ketene-forming elimination reactions from aryl thienylacetates promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq). Effect of the beta-aryl group

Ketene-forming eliminations from ArCH2CO2C6H3-2-X-4-NO2 (Ar = thienyl, 1) promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq) have been studied kinetically. When X = CF3 and NO2, the reactions exhibit second-order kinetics as well as beta = 0.30-0.64 and |betalg| = 0.31-0.52 that decrease with a better leaving group. Hence, an E2 mechanism is evident. As the leaving group is made poorer (X = H, OCH3, and Cl), E2 transition state becomes more skewed toward the proton transfer, as revealed by the increase in Br?nsted beta to 0.5-0.64, and the E1cb mechanism competes. The changes in the k1 and k-1/k2 values with the reactant structure variation provide additional support for the competing E1cb mechanism. By comparing with existing data for 4-YC6H4CH2CO2C6H3-2-X-4-NO2, the effect of beta-aryl group on ketene-forming elimination is assessed.     

Elimination reactions of (E)-2,4-dinitrobenzaldehyde O-benzoyloximes promoted by R2NH/R2NH2(+) in 70 mol % MeCN(aq). Change of reaction mechanism

Elimination reactions of (E)-2,4-(NO(2))(2)C(6)H(3)CHNOC(O)C(6)H(4)X (1) promoted by R(2)NH/R(2)NH(2)(+) in 70 mol % MeCN(aq) have been studied kinetically. The reactions are second-order and exhibit Bronsted beta = 0.27-0.32 and |beta(lg)| = 0.28-0.32. The result can be described by a negligible p(xy) interaction coefficient, p(xy) = partial differential beta/partial differential pK(lg) = partial differential beta(lg)/partial differential pK(BH) approximately = 0, which describes the interaction between the base catalyst and the leaving group. The negligible p(xy) coefficients are consistent with the (E1cb)(irr) mechanism.     

Dynamics effects on an E2/E1cb borderline mechanism: unimolecular elimination of 2-aryl-3-chloro-2-R-propanols

The mechanistic dichotomy between concerted E2 and stepwise E1cb of the base-promoted elimination of 2-aryl-3-chloro-2-R-propanols was examined computationally at the HF, M05-2X, and MP2 levels of theory. Optimizations of transition states (TSs) and reaction intermediates, and intrinsic reaction coordinates (IRC) calculations showed that there was a single reaction route for each substrate, and that the mechanism could be changed from E2 to E1cb by making a carbanion intermediate more stable through the introduction of electron-withdrawing substituents. Molecular dynamics simulations revealed that trajectories started at a single TS led directly to two product regions; the carbanion intermediate region in the E1cb mechanism, and the alkene product region in the E2 mechanism, through path bifurcation after the TS. The present system is a new example of bifurcation in reactions of closed-shell molecules. The overall reaction mechanism changes dynamically from E2 to E1cb by a gradual change in the ratio of E2 and E1cb trajectories, rather than a path switch in concurrent pathways.     

Synthesis of aryl azides from aryl halides promoted by Cu2O/tetraethylammonium prolinate

An efficient approach to aryl azides, in short reaction times and good to excellent yields, has been developed via the reaction of aryl halides with sodium azide under Cu₂O/tetraethylammonium prolinate catalysis.     

Ab initio molecular dynamics simulations of elimination reactions in water solution: exploring the borderline region between the E1cb and E2 reaction mechanisms

We report a theoretical study, based on ab initio molecular dynamics simulations in water solution, of the mechanism of base-induced beta-elimination reactions in systems activated by the pyridyl ring, with halogen leaving groups. The systems investigated represent borderline cases, where it is uncertain whether the reaction proceeds via a carbanion intermediate (E1cb, A(xh)D(H) + D(N)) or via the concerted loss of a proton and the halide (E2, A(N)D(E)D(N)) upon base attack. Recent theoretical and experimental evidence points toward the lack of a net distinction between the E1cb and E2 reaction paths, which seem to merge smoothly into each other in these borderline cases (Alunni, S.; De Angelis, F.; Ottavi, L.; Papavasileiou, M.; Tarantelli, F. J. Am.Chem. Soc. 2005, 127, 15151-15160). In this study, we explore the dynamics on the potential energy surface for the reaction of 2-(2-fluoroethyl)-1-methyl pyridinium with OH- by means of Car-Parrinello simulations in water solution. Our results indicate that the reaction mechanism effectively evolves through the potential energy region of the carbanion: the carbon-fluoride bond breaks only after the carbon-hydrogen bond, confirming the conclusions of a recently reported study of the potential energy surface for this system.     

Evidence of a borderline region between E1cb and E2 elimination reaction mechanisms: a combined experimental and theoretical study of systems activated by the pyridine ring

We report a combined experimental and theoretical study to characterize the mechanism of base-induced beta-elimination reactions in systems activated by the pyridyl ring, with halogen leaving groups. The systems investigated represent borderline cases, where it is uncertain whether the reaction proceeds via a carbanion intermediate (E1cb, A(xh)D(H) + D(N)) or via the concerted loss of a proton and the halide (E2, A(N)D(E)D(N)) upon base attack. Experimentally, the Taft correlation for H/D exchange, in OD(-)/D(2)O with noneliminating substrates (1-methyl-2-(2-Xethyl)pyridinium iodide), is used to predict the expected values of the rate constants for the elimination reactions with N-methylated substrates and F, Cl, Br as the leaving group. The comparison indicates an E1cb irreversible mechanism with F, but the deviation observed with Cl and Br does not allow a conclusive assignment. The theoretical calculations show that for the N-methylated substrate with a fluoride leaving group the elimination proceeds via formation of a moderately stable carbanion. No stable anionic intermediate is instead found when the leaving group is Cl or Br, as well as for any of the nonmethylated species, indicating a concerted elimination. The methylated substrate with Cl shows however only a moderate increase in reactivity compared to the fluorinated substrate, despite the change in mechanism. Very interestingly, our analysis of the computed two-dimensional potential energy surface for the reaction with a F leaving group indeed evidences the lack of a net distinction between the E1cb and E2 reaction paths, which appear to merge smoothly into each other in these borderline cases.     

Homocoupling of aryl halides promoted by an NiCl2/bpy/Mg system in DMF

Homocoupling of aryl halides (2 ArX → Ar―Ar) promoted by NiCl₂/2,2′‐bipyridine (bpy)/Mg mixtures in DMF has been studied. Mixtures of NiCl₂, bpy and Mg in DMF promoted homocoupling of aryl halides such as phenyl bromide and p‐tolyl bromide to give the coupling products in good (e.g. approximately 60–75%) yields, and the homocoupling products were easily isolated from the reaction mixtures. Application of this homocoupling to dibromo‐aromatic compounds (Br–arylene–Br: 2,5‐dibromopyridine, 2,7‐dibromo‐9.9‐dioctylfluorene and 2,7‐dibromo‐9,10‐dioctyl‐9,10‐dihydrophenanthrene) gave the corresponding π‐conjugated polymers, –(arylene)_n–, in good yields. Organometallic processes for the homocoupling are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Catalysis of the beta-elimination of HF from isomeric 2-fluoroethylpyridines and 1-methyl-2-fluoroethylpyridinium salts. Proton-activating factors and methyl-activating factors as a mechanistic test to distinguish between concerted E2 and E1cb irreversible mechanisms

Second-order rate constants, k(OH)(N), M(-)(1) s(-)(1), for the beta-elimination reactions of HF with 2-(2-fluoroethyl)pyridine (2), 3-(2-fluoroethyl)pyridine (3), and 4-(2-fluoroethyl)pyridine (4) in OH(-)/H(2)O, at 50 degrees C and mu = 1 M KCl, are = 0.646 x 10(-)(4) M(-)(1) s(-)(1), = 2.97 x 10(-)(6) M(-)(1) s(-)(1), and = 5.28 x 10(-)(4) M(-)(1) s(-)(1), respectively. When compared with the second-order rate constants for the same processes with the nitrogen-methylated substrates 1-methyl-2-(2-fluoroethyl)pyridinium iodide (5), 1-methyl-3-(2-fluoroethyl)pyridinium iodide (6), and 1-methyl-4-(2-fluoroethyl)pyridinium iodide (7), the methyl-activating factor (MethylAF) can be calculated from the ratio k(OH)(NCH)3/, and a value of 8.7 x 10(5) is obtained with substrates 5/2, a value of 1.6 x 10(3) with 6/3, and a value of 2.1 x 10(4) with 7/4. The high values of MethylAF are in agreement with an irreversible E1cb mechanism (A(N)D(E) + D(N)) for substrates 5 and 7 and with the high stability of the intermediate carbanion related to its enamine-type structure. In acetohydroxamate/acetohydroxamic acid buffers (pH 8.45-9.42) and acetate/acetic acid buffers (pH 4.13-5.13), the beta-elimination reactions of HF, with substrates 2 and 4, occur at NH(+), the substrates protonated at the nitrogen atom of the pyridine ring, even when the [NH(+)] is much lower than the [N], the unprotonated substrate, due to the high proton-activating factor (PAF) value observed: 3.6 x 10(5) for 2 and 6.5 x 10(4) for 4 with acetohydroxamate base. These high PAF values are indicative of an irreversible E1cb mechanism rather than a concerted E2 (A(N)D(E)D(N)) mechanism. Finally, the rate constant for carbanion formation from NH(+) with 2 is k(B)(NH)+ = 0.35 M(-)(1) s(-)(1), which is lower than when chlorine is the leaving group ( = 1.05 M(-)(1) s(-)(1); Alunni, S.; Busti, A. J. Chem. Soc., Perkin Trans. 2 2001, 778). This is direct experimental evidence that some lengthening of the carbon-leaving group bond can occur in the intermediate carbanion. This is a point of interest for interpreting a heavy-atom isotope effect.     

E.S.R. and optical study of VO2+ in lithium zinc-phosphate glasses

The E.S.R. and optical spectra of glasses of basic composition 30% Li₂ O-30% ZnO-40% P₂ O₅ doped with vanadium have been studied.The partial substitution of ZnO with Al₂O₃ and PbO has a great effect on the chemical durability of the glasses but does not affect their magnetic and optical properties.The E.S.R. and optical data suggest that the dxy orbital of the umpaired spin of the V(IV) specie is not bonded with the π orbitals of the in plane ligands while there is a strong σ bonding with the same ligands. The bonding between the vanadium and the vanadyl oxygen is only slightly covalent. The E.S.R. spectra show a large reduction of the line-width when the concentration of the V₂O₅ goes from 1% to 0.1% showing that the dipolar broadening gives the larger contribution to the E.S.R. line-width.     

Mechanistic studies of La3+- and Zn2+-catalyzed methanolysis of aryl phosphate and phosphorothioate triesters. Development of artificial phosphotriesterase systems

The methanolyses of a series of O,O-diethyl O-aryl phosphates (2,5) and O,O-diethyl S-aryl phosphorothioates (6) promoted by methoxide and two metal ion systems, (La3+)2(-OCH3)2 and 4:Zn2+:-OCH3 (4 = 1,5,9-triazacyclododecane) has been studied in methanol at 25 degrees C. Br?nsted plots of the logk2 values vs. pKa for the phenol leaving groups give beta(lg) values of -0.70, -1.43 and -1.12 for the methanolysis of the phosphates and -0.63, -0.87 and -0.74 for the methanolysis of the phosphorothioates promoted by the methoxide, La3+ and Zn2+ systems respectively. The kinetic data for the metal-catalyzed reactions are analyzed in terms of a common mechanism where there is extensive cleavage of the P-XAr bond in the rate-limiting transition state. The relevance of these findings to the mechanism of action of the phosphotriesterase enzyme is discussed.     

Unique selectivity of iodohydroxylation reaction of allenyl phenyl sulfoxides in aqueous MeCN. A stereodefined synthesis of (E)-2-iodo-3-hydroxy-1-alkenyl sulfoxides

Iodohydroxylation reaction of allenyl phenyl sulfoxides with I(2) can smoothly proceed to generate (E)-2-iodo-3-hydroxy-1-alkenyl sulfoxides with excellent regio- and stereoselectivity in high or excellent yields. The configuration of E-2a was determined by the X-ray diffraction study.     

Alkynylation of N-tosylimines with aryl acetylenes promoted by ZnBr2 and N,N-diisopropylethylamine in acetonitrile

We found a suitable condition for the effective alkynylation of N-tosylimines with aryl acetylenes. The reaction of N-tosylimines and aryl acetylenes in the presence of ZnBr₂ and DIEA (N,N-diisopropylethylamine) in CH₃CN afforded the desired N-tosyl propargylamines in moderate to good yields.     

Energy of the reaction between NH3 and Mg2+ and Na+ in NaMgA zeolite

Russian Chemical Bulletin -     

Competitive uptake of Cs+, Ba2+, and Zn2+ ions from binary aqueous mixtures by zeolite-13X

The preferential exchange uptake of the cations Cs⁺, Ba²⁺ and Zn²⁺ from pure solutions by zeolite-13X follows the order Q [Cs⁺]>Q [Ba²⁺]>Q [Zn²⁺], while in case of binary mixtures the order is Q [Ba²⁺(Zn²⁺)]>Q [Ba²⁺(Cs⁺)]>Q [Cs⁺(Zn²⁺)]>Q [Cs⁺(Ba²⁺)]>Q [Zn²⁺(Cs⁺)]>Q [Zn²⁺(Ba²⁺)]. Ba²⁺ uptake from mixtures shows the least suppression effect.

---

Cs⁺, Ba⁺² Zn⁺² -13 Q[Cs⁺]>Q[Ba⁺²]>Q[Zn⁺²] Q[Ba⁺²(mix Zn⁺²)]>Q[Ba⁺²(mix Cs⁺)]>Q[Cs⁺(mix Zn⁺²)]>Q[Cs⁺(mix Ba⁺²)]>Q[Zn⁺²(mix Cs⁺)]>Q[Zn⁺²(mix Ba⁺²)]. Ba⁺² .

     

Mechanism of ascorbic acid oxidation by molecular oxygen in aqueous pyridine catalyzed by CO2+, Ni2+, Mn2+ and Zn2+

Data on the radical non-chain mechanism of ascorbic acid oxidation by molecular oxygen catalyzed by Co²⁺, Ni²⁺, Mn²⁺ and Zn²⁺ ions are reported.

---

Co²⁺, Ni²⁺, Mn²⁺ Zn²⁺.