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.     

Ketene-forming eliminations from aryl phenylacetates promoted by R2NH/R2NH2+ in aqueous MeCN. Mechanistic borderline between E2 and E1cb

Elimination reactions of 2-X-4-NO2C6H3CH2C(O)OC6H3-2-Y-4-NO2 [X = H (1), NO2 (2)] promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq) have been studied kinetically. The base-promoted eliminations from 1 proceeded by the E2 mechanism when Y = Cl, CF3, and NO2. The mechanism changed to the competing E2 and E1cb mechanisms by the poorer leaving groups (Y = H, OMe) and to the E1cb extreme by the strongly electron-withdrawing beta-aryl group (2, X = NO2). The values of beta = 0.14 and beta(lg) = 0.10-0.21 calculated for elimination from 1 (Y = NO2) indicate a reactant-like transition state with small extents of proton transfer and C(alpha)-OAr bond cleavage. The extent of proton transfer increased with a poorer leaving group, and the degree of leaving group bond cleavage increased with a weaker base. Also, the changes in the k(1) and k(-1)/k(2) values with the reactant structure variation are consistent with the E1cb mechanism. From these results, a plausible pathway of the change of the mechanism from E2 to the E1cb extreme is proposed.     

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.     

Elimination reactions of N-alkyl-N-chlorothenylamines promoted by MeONa-MeOH and Et(2)NH-MeCN. Effect of the beta-aryl group on the imine-forming transition state

Elimination reactions of N-alkyl-N-chlorothenylamines 1-4 with MeONa-MeOH and Et(2)NH-MeCN have been studied kinetically. The elimination reactions are regiospecific, producing only the conjugated imines. The reactions are second order and exhibit substantial values of Hammett rho and k(H)/k(D), and an E2 mechanism is evident. The relative rates of elimination for Me/Et/i-Pr/t-Bu substituents are 1/0.5/0.2/0.02 with MeONa-MeOH and 1/0.4/0.2/0.06 with Et(2)NH-MeCN. The transition state structure changes toward more product-like as the base is changed from MeONa-MeOH to Et(2)NH-MeCN. Comparison with existing data reveals that the structure of the transition state is relatively insensitive to the beta-aryl group variation.     

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.     

Ethanol‐promoted reductive homocoupling reactions of aryl halides catalyzed by palladium on carbon (Pd/C)

Homocoupling reactions of aryl bromides or iodides proceeded smoothly with palladium on carbon (Pd/C) catalyst, ethanol and base in dimethyl sulfoxide (DMSO) to afford exclusively symmetric biaryls in good to excellent yields. Ethanol was first used as a reducing agent in situ to reduce the Pd²⁺/C species into Pd⁰/C active species to complete the catalytic redox cycle. It was found that ethanol can promote the Pd/C‐catalyzed reductive homocoupling of aryl iodides and bromides efficiently in the presence of base. A reaction mechanism has been put forward and discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Luminescence of UO2(aq)2+ + FUO2(aq)+ and evidence for the formation of a new inorganic radiative exciplex in aqueous solution

Steady-state and time-resolved luminescence measurements, as well as decays of excited UO₂²⁺ + FUO₂⁺ in aqueous solution, show the formation of a radiative exciplex, resulting from the interaction of ¹FUO₂H⁺ with UO₂²⁺, similarly to the ¹U₂O₄H⁴⁺ exciplex already reported. Lifetimes and evaluations of the luminescence yields of the ¹UO₂²⁺, ¹FUO₂⁺ and of the exciplex of probable ¹{0(F)UOHOUO}³⁺ composition, are given. The overall heat the exciplex formation is estimated.     

Direct dynamics study on the hydrogen abstraction reactions N2H4 + R-->N2H3 + RH (R=NH2, CH3)

We present a direct ab initio dynamics study on the hydrogen abstraction reactions N(2)H(4)+R-->N(2)H(3)+RH (R=NH(2),CH(3)), which are predicted to have six possible reaction channels for NH(2) abstraction and four for CH(3) abstraction caused by the different N(2)H(4) isomers and various attacking orientations of foreign radical to N(2)H(4). The structures and frequencies at the stationary points and the points along the minimum energy paths (MEPs) of all reaction channels are obtained at the UMP2(full)6-31+G(d,p) level of theory. Energetic information of stationary points and the points along the MEPs is further refined by means of MC-QCISD method. The rate constants of these channels are calculated using the improved canonical variational transition-state theory with the small-curvature tunneling correction (ICVT/SCT) method. The calculated results show that the favorable reaction channels are channels (n1) and (n4) as well as (c1) and (c3) (refer to Scheme 1) in the whole temperature range. The total ICVT/SCT rate constants of all channels for the two reactions at the MC-QCISDUMP2(full)6-31+G(d,p) level are both in good agreement with the available experimental data, and corresponding three-parameter expressions of k(ICVTSCT) in 220-3000 K are fitted as 6.46 x 10(-15)(T298)(3.60) exp(-386T) cm(3) mol(-1) s(-1) for NH(2) abstraction and 1.04 x 10(-14)(T298)(4.00) exp(-2037T) cm(3) mol(-1) s(-1) for CH(3) abstraction. Additionally, the long range interaction between the H atom of X-H bond in foreign radicals and the lone pair on the nonreactive N atom of the transition states is further discussed to explain the various transition-state numbers of the two similar hydrogen abstraction reactions.     

ortho-Difunctionalization of arynes by LiZnEt2(TMP)-mediated deprotonative zincation/elimination of aryl triflates

Generation of arynes from aryl triflates has been achieved using lithium diethyl(tetramethylpiperidyl)-zincate base LiZnEt₂(TMP), via a directed ortho-deprotonative zincation and subsequent elimination of the triflate group. The aryne formation has been demonstrated by the cycloaddition reaction with diene and insertion reactions with ureas. Furthermore, the nucleophilic addition of LiZnEt₂(TMP) to arynes was observed in the absence of external aryne partners, offering a new cascade strategy for diverse difunctionalization of arynes.     

A novel photochromic system of 4,5-dialkenylthiophenes constructed by the smarium diiodide promoted coupling reactions of thiophene-2-carboxylate with aryl ketones

[reaction: see text] The SmI2-promoted coupling reaction of ethyl thiophene-2-carboxylate with aryl ketones (2 equiv), followed by acid-catalyzed dehydration and oxidative aromatization, gave dialkenylthiophenes 1b-d, which underwent electrocyclizations upon irradiation with 300-nm light in CH3CN solution to give the corresponding closed-ring species with absorption lambda(max) approximately 425 nm. The interconversion between dialkenylthiophenes and their corresponding closed-ring species constitutes a novel photochromic system bearing an ester group for potential uses in linkage and wavelength tuning.     

"Oxidatively induced" reductive elimination of dioxygen from an eta2-peroxopalladium(II) complex promoted by electron-deficient alkenes

The first example of associative displacement of dioxygen from a peroxopalladium(II) complex is reported. Electron-deficient alkenes, p-X-trans-beta-nitrostyrene (X = OCH3, CH3, H, F, Br, CF3, NO2), react quantitatively with (bc)Pd(eta2-O2) (bc = bathocuproine) in dichloromethane at room temperature to form the corresponding palladium(0)-alkene complexes. Mechanistic studies indicate that ligand substitution proceeds through an associative mechanism, and the electronic characteristics of the reactions are consistent with an "oxidatively induced" reductive elimination pathway.     

Catalysis of acyl group transfer by a double-displacement mechanism: the cleavage of aryl esters catalyzed by calixcrown-Ba2+ complexes

The scope of the barium salt of p-tert-butylcalix[4]arene-crown-5 as a transacylation catalyst has been defined by evaluating its efficiency in the methanolysis of a series of aryl acetates at 25.0 degrees C in MeCN/MeOH 9:1 (v/v) under slightly basic conditions. In this system a phenolic hydroxyl is the acyl-receiving and -releasing unit in a double-displacement mechanism. The complexed barium ion acts both as a nucleophile carrier and a built-in Lewis acid in providing electrophilic assistance to the ester carbonyl both in the acylation and deacylation step (nucleophilic-electrophilic catalysis). Turnover capability is ensured by the acylated intermediate reacting with the solvent more rapidly than the original ester, but a serious drawback derives from the incursion of back-acylation of the liberated phenol. A gradual shift from rate-determining deacylation (p-nitrophenyl acetate) to rate-determining acylation (phenyl acetate) is observed along the investigated series. It is shown that the scope of the catalyst is restricted to acetate esters whose reactivity lies in the range approximately defined by the phenyl acetate-p-nitrophenyl acetate pair, with a maximum efficiency for p-chlorophenyl acetate. Moreover, the catalyst effectively promotes ester interchange between phenols, showing that its activity is not limited to solvolysis reactions. The very high sensitivity of the rate of acylation of the catalyst to leaving group basicity has been interpreted as due to rate-determining decomposition of the tetrahedral intermediate, which is believed to arise from the presumably low basicity of the metal ion stabilized nucleophile. The turnover frequency was in the range of 3.8 x 10(-4) min(-1) for phenyl acetate to 7.4 x 10(-3) min(-1) for p-nitrophenyl acetate ([ArOAc]0=4.0 mM]). A first attempt to enhance the rate of acylation of the catalyst through intramolecular general acid catalysis is also described.     

A re-determination of the standard potential of the Pb2+ (aq)|Pb(s) half cell

The standard reduction potential of the Pb²⁺ (aq)|Pb(s) half-cell at 298 K has been re-determined and found to be ? 126.7 ± 0.2 mV, 0.4 mV more negative than the currently accepted value. The new value is used to calculate a self-consistent set of solubility products for several sparingly soluble lead salts using the known E⁰s of the appropriate lead-lead salt electrodes. Activity. Activity coefficients of lead chloride in aqueous solution are also given.     

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.     

Cp2TiCl2/M(Zn,Mg) system promoted the addition reactions,enyne cyclization reactions and applied in the synthesis of (+)-Goniopypyrone

In the course of our ongoing projects. It has been focused on the studies of carbon-carbon formation using redox system such as Cp₂TiCl₂/M(Zn,Mg) and LnCl₃/Zn. This paper deals with our research progess on exploring reactions for the syntheses of useful building block, intermediates and natural products.     

The role of distonic ions in the formation of CH3NH 3 + and (CH3)2NH 2 + from the molecular ions of octopamine and synephrine

It is shown by mass-analyzed ion kinetic energy spectrometry that the metastably decomposing molecular ions of octopamine (p-HOC₆H₄CH(OH)CH₂NH₂) and synephrine (p-HOC₆H₄CH(OH)CH₂NHCH₃) yield only protonated methylamine and dimethylamine, respectively, as product ions. From deuterium labeling and variation of the internal energy of the molecular ions, experimental support has been obtained that these product ions are generated via the occurrence of a distonic ion-neutral complex. In the case of octopamine, this complex would consist of a nitrogen-protonated aminomethyl radical and p-hydroxylbenzaldehyde in which the former species abstracts the aldehydic or phenolic hydrogen atom from the latter to give protonated dimethylamine.     

Cooperativity in the counterion catalysis of Morita/Baylis/Hillman reactions promoted by enantioselective trifunctional organocatalysts

New trifunctional organocatalysts with a NHTs Brønsted acid were prepared and tested in their ability to promote the counterion catalysis of generic and aza-Morita/Baylis/Hillman reactions. The cooperativity between the counterion and the NHTs Brønsted acid of the trifunctional catalyst was required for good enantioselectivity and rate enhancement. Better enantioselectivity was observed for aza-MBH reactions at relatively low catalyst loading (2-5 mol %) under facile conditions.