- Concerted mechanisms of the reactions of 2,4,6-trinitrophenyl methyl carbonate and 2,4,6-trinitrophenyl acetate with secondary alicyclic amines
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The reactions of secondary alicyclic amines with 2,4,6-trinitrophenyl methyl carbonate (TNPMC) and 2,4,6-trinitrophenyl acetate (TNPA) are subjected to a kinetic study in aqueous solution, 25.0 °C, ionic strength 0.2 (KCl). The reactions are studied by following spectrophotometrically (360 nm) the release of the 2,4,6-trinitrophenoxide anion. Under amine excess, pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsd vs [amine] are linear, with the slope (kN) independent of pH. The Broensted-type plots (log kN vs pKa of the conjugate acid of the amines) are linear, with slopes β = 0.41 and β = 0.36 for the reactions of TNPA and TNPMC, respectively. The predicted breaks of the Broensted plots for stepwise mechanisms are pKa0 = 6.8 and 7.3, respectively. The lack of Broensted breaks for these reactions and the values of the Broensted slopes are consistent with concerted mechanisms. By comparison of the reactions under investigation among them and with similar aminolysis and pyridinolysis, the following conclusions can be drawn: (i) Secondary alicyclic amines react with TNPA and TNPMC by concerted mechanisms. (ii) TNPA is more reactive toward these amines than TNPMC due to the greater electron release of MeO from the latter substrate. (iii) The change of 2,4-dinitrophenoxy to 2,4,6-trinitrophenoxy in the zwitterionic tetrahedral intermediate (T± formed in the reactions of the title amines with 2,4-dinitrophenyl acetate greatly destabilizes T±. (iv) Secondary alicyclic amines destabilize T± relative to pyridines. (v) The intermediate T± formed in the reactions of the title amines with S-(2,4,6-trinitrophenyl) acetate is greatly destabilized by substitution of S-(2,4,6-trinitrophenyl) by O-(2,4,6-trinitrophenyl) as the leaving group.
- Castro,Cubillos,Santos
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- Regioselectivity of Meisenheimer complexation in reaction of oxygen-centred nucleophiles with picryl aryl ethers: Polar vs. SET mechanisms
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Picryl alkyl ethers react with hydroxide and methoxide ions to give regioisomeric Meisenheimer (anionic σ-) adducts; the C-3 adduct is kinetically favoured and the C-1 adduct is thermodynamically favoured (K3T1 behaviour). In the current 400 MHz NMR spectroscopic study of the reactions of two picryl aryl ethers, picryl phenyl ether (PicOPh, 1) and picryl mesityl ether (PicOMes, 2), the charge localized nucleophiles OH- and MeO- displayed the same K3 regioselectivity as found with picryl alkyl ethers; attachment at C-1 leads to SNAr displacement of the aryloxide. In contrast, phenoxide (PhO-) and the sterically demanding 2,4,6-trimethylphenoxide (mesitoxide, MesO-) react with 1 and 2 to form the C-1 O-adduct as the product of kinetic control (i.e., K1 behaviour). These reactions were studied at low temperature (-40°C in acetonitrile-d3:dimethoxyethane-d10 1:1) and as a function of increasing temperature (-40°C to ambient). On the thermodynamic side, the C-1 PhO- O-adduct of 1 is also the more stable of the possible phenoxide O-adducts; it shows T1 regioselectivity within the manifold of O-adducts (K1T1), but the C-3 C-adduct (via para-attack of PhO-) is the ultimate thermodynamic product. The C-1 O-adducts formed by MesO - with 1 or 2 give way with time (or temperature increase) in favour of their C-3 regioisomers or a C-1,3-O-diadduct. Mesitoxide, therefore, displays K1T3 regioselectivity. Stereoelectronic stabilization is discussed as a factor influencing T1 regioselectivity in O-adduct formation. Frontier molecular orbital (FMO) interactions between the HOMO of the nucleophile and the LUMO of the picryl ether may play a role in the K1 preference of aryloxides. An alternative argument is presented based on a single electron (radical) transfer (SET) pathway for the aryloxide nucleophiles rather than the polar (SNAr) pathway for hydroxide and methoxide. The SET pathway also predicts a kinetic preference for C-1, as the C-1 position is of higher spin density than C-3 in the radical anion of the picryl ether and thus should be the preferred site for coupling by the aryloxide radical.
- Buncel, Erwin,Dust, Julian M.,Manderville, Richard A.,Tarkka, Richard M.
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- Kinetics of the reaction of phenyl picrates with phenoxide ions in water. Concerted or stepwise?
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A kinetic study is reported of the exchange reactions of substituted phenoxide ions with some ring-substituted 2,4,6-trinitrophenyl ethers in water. The βronsted diagrams formed by plotting log k, where k is the second-order rate constant for reaction, ve
- Crampton, Michael R.,Robotham, Ian A.
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p. 1084 - 1089
(2014/01/06)
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- Concerted pyridinolysis of aryl 2,4,6-trinitrophenyl carbonates
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(Chemical Equation Presented) The Bronsted plots for the title reactions are linear with slopes of 0.53-0.56. The magnitude of the slopes and the fact that there are no breaks at the predicted pKa for stepwise mechanisms indicate that these rea
- Castro, Enrique A.,Ramos, Mariela,Santos, Jose G.
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supporting information; experimental part
p. 6374 - 6377
(2009/12/26)
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- Kinetics and mechanism of the benzenethiolysis of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates
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The reactions of 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (DNPC and TNPC, respectively) and S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) ethyl thiolcarbonates (DNPTC and TNPTC, respectively) with a series of benzenethiolate anions were subjected to a kinetic investigation in water, at 25.0 °C, and an ionic strength of 0.2 M (KCl). These reactions obey pseudo-first-order kinetics, under excess of benzenethiolate, and are first order in the latter reactant. However, comparable reactant concentrations were used in the reactions of 4-nitrobenzenethiolate anion with TNPC and TNPTC, which showed second-order kinetics. The nucleophilic rate constants are pH independent, except those for the reactions of TNPC with 4-methoxy- and pentafluorobenzenethiolates, and TNPTC with benzenethiolate and 4-chloro- and 3-chlorobenzenethiolates, which show acid dependence. The Bronsted-type plots for the nucleophilic rate constants are linear with slopes β = 0.9, 1.0, 0.9, and 0.9 for the reactions of DNPC, TNPC, DNPTC, and TNPTC, respectively. No break in the Bronsted plot was found for the reactions of DNPC and DNPTC at pKa ca. 4.1 and 3.4, respectively, consistent with concerted mechanisms. TNPC is more reactive toward benzenethiolate anions than DNPC, and TNPTC more than DNPTC due to the better leaving groups involved. Comparison of the kinetic results obtained in this work with those for the concerted phenolysis of the same substrates shows that benzenethiolate anions are better nucleophiles toward carbonates than isobasic phenoxide anions. This is explained by Pearson's hard and soft acids and bases principle.
- Castro, Enrique A.,Pavez, Paulina,Santos, Jose G.
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p. 3640 - 3645
(2007/10/03)
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- Amines as leaving groups in nucleophilic aromatic substitution reactions. Part 4. σ-adduct formation in the hydrolysis of 1-amino-2,4,6-trinitrobenzenes
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The kinetic study of the reaction of 2,4,6-trinitro-1-pyrrolidinobenzene 3,2,4,6-trinitro-1-piperidinobenzene 1 and 1-morpholino-2,4,6-trinitrobenzene 2 was made in 1,4-dioxane-water mixtures at 25°C. In all cases, several processes were observed, the slowest of them leading to the formation of picrate ion. The fastest processes involved the formation of σ complexes by addition of one or two HO- to unsubstituted ring positions and the ionisation of the 1:1 complex. Besides, for compounds 1 and 3, cis-trans isomerisation of 1:2 complexes was kinetically detected. Substitution occurred by displacement of the amino group on the substrate and the 1:1 complex. The reaction pathway for the formation of phenol which involves the formation of these complexes has lower energy than that which results from addition to the 1 position of the substrate.
- Bujan, Elba I.,Remedi, M. Virginia,De Rossi, Rita H.
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p. 969 - 975
(2007/10/03)
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- Mechanism of the alkaline hydrolysis of O-ethyl S-(2,4,6-trinitrophenyl) thio-and dithiocarbonates
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The alkaline hydrolysis of O-ethyl S-(2,4,6-trinitrophenyl) thio-and dithiocarbonates, 1 and 2, respectively, were studied kinetically in 5% dioxane in water, at 25.0°C, and ionic strength 0.2 mol dm-3 (KCl). Two kinetic processes, well separated in time, were detected. The fast process involves the formation of a σ-complex by addition of HO- to one of the unsubstituted positions of the aromatic ring, followed by fast ionisation of this complex. The slow process leads to the formation of a mixture of 2,4,6-trinitrophenoxide and 2,4,6-trinitrobenzenethiolate ions in a 10:1 ratio, respectively, in the reaction of 2, and to a mixture in a 2:1 ratio in the reaction of 1, independent of KOH concentration. Although the substrates show different kinetic behaviour with KOH concentration, the results can be discussed on the basis of a common reaction mechanism.
- Castro, Enrique A.,Cubillos, Maria,Santos, Jose G.,Bujan, Elba I.,Remedi, M. Virginia,Fernandez, Mariana A.,De Rossi, Rita H.
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p. 2603 - 2607
(2007/10/03)
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- Reaction pathways for ambident aryloxide O- and C-nucleophiles in SNAr displacement versus Meisenheimer complex formation with picryl halides. Stereoelectronic effects on regioselectivity
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To probe regioselectivity in Meisenheimer complexation, the reaction of two picryl halides (PiX where X = F, Cl) with a series of aryloxide nucleophiles (phenoxide, 2,4,6-trimethylphenoxide and 2,6-di-t-butylphenoxide) were monitored by 1H NMR spectroscopy in dimethyl sulphoxide at ambient temperature and in acetonitrile-dimethoxyethane(MeCN-DME) at low temperature (-40°C). The reactions of both picryl halides with the ambident (oxygen versus carbon) nucleophile, phenoxide ion (PhO-), and 2,4,6-trimethylphenoxide (mesitoxide, MesO-) leads to clean SNAr displacement of X via the oxygen site of the nucleophile to form the respective aryl picryl ethers, i.e. phenyl picryl ether (3a) and mesityl picryl ether (3b). Meisenheimer complex formation at C-1 or C-3 was not detected in these systems. With 2,6-di-t-butylphenoxide (2,6-DTBPhO-), where oxygen attachment of the aryloxide is precluded by the bulky ortho t-butyl groups, para-carbon attachment was found to occur at C-1 to give picryl 2,6-di-t-butylphenol (3d) in competition with C-attack at C-3 to give the respective carbon-bonded Meisenheimer complexes [X = Cl (4) and X = F (5)]. For both picryl halides, the ratio of 3d, the product of C-1 attack, to the product of C-3 attack, 4 or 5, was roughly 7:1. These findings are considered with regard to the nucleofugality of the halide, X, steric hindrance (F-strain) to attack by the aryloxides at the various positions and stereoelectronic stabilization of C-1 adducts afforded by n → σ* donation.
- Manderville, Richard A.,Dust, Julian M.,Buncel, Erwin
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p. 515 - 528
(2007/10/03)
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- 1-(2-Acetoxyethoxy)-2,4,6-trinitrobenzene. A New Substrate for Enzyme Catalysed Hydrolysis
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The preparation of the title compound (3) is described.Hydrolysis of the ester group of (3) is readily detected spectrophotometrically since the 1-(2-hydroxyethoxy)-2,4,6-trinitrobenzene produced spontaneously cyclises in water at pH > 7 to give a strongl
- Crampton, Michael R.,Holt, Karen E.,Percy, Jonathan M.
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p. 1701 - 1704
(2007/10/02)
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- SNAr, SN2, and Aromatic Addition Processes in the Reactions of Picryl Ethers with Nitrogen and Carbon Bases
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The reactions of methyl, cyclohexyl, and phenyl picryl ethers with diethyl- and triethylamine in chloroform, acetone, and 1,3-dicarbomethoxyacetone have been studied.A number of different processes were observed, depending on substrate structure.Both amine nitrogen and enolate carbon act as nucleophiles in these reactions.With unhindered picryl ethers like 2,4,6-trinitroanisole, dealkylation often occurs via SN2 attack on the methyl group.With more hindered picryl ethers, addition to the ring is more common, resulting in covalent ? complexes, substituted picramides, or bicyclo nitropropenenitronates.In this paper, structural features that influence reaction path are discussed.
- Strauss, Michael,Torres, Ruben
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p. 756 - 760
(2007/10/02)
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- Eliminations from (E)-O-Arylbenzaldoximes Promoted by Tertiary Amines in Acetonitrile. Effects of Aryl Substituents, Base Strength, and Leaving Group upon the Nitrile-Forming Transition State
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Elimination reactions of (E)-O-(2,4-dinitrophenyl)benzaldoximes (1) and (E)-O-picrylbenzaldoximes (2) with R3N in MeCN have been investigated.The reactions are quantitative, producing only benzonitriles and aryloxides.The observation of second-order kinetics, β = 0.43 - 0.81, and 1g> = 0.39 - 0.73 is consistent with an E2 mechanism.For reactions of 1 with R3N in MeCN, the β and ρ values increased significantly with electron-withdrawing aryl substituents and stronger bases respectively.The corresponding changes in the β and ρ values for 2 were in the same direction but much less.When the leaving group was changed from 2,4-dinitrophenoxide to picrate, however, β increased and ρ decreased.Changes in transition-state parameters with alteration in the reactant structure are interpreted as resulting from variation in a nitrile-forming transition state.
- Cho, Bong Rae,Kim, Kee Dong,Lee, Jong Chan,Cho, Nam Soon
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p. 6145 - 6148
(2007/10/02)
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- The Acid-Base Behaviour of Hexamine and its N-Acetyl Derivatives
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The protonation equilibria and decomposition reactions in aqueous hydrochloric acid of hexamine and its acylated derivatives (2), (3), and (4) have been examined by u.v. and 1H n.m.r. spectroscopy. pKa Values at 25 deg C are: hexamine 4.89, 3,7-diacetyl-1,3,5,7-tetra-azabicyclo TAT > hexamine.Reaction of hexamine with picryl acetate may involve nucleophilic catalysis via the N-acetylhexaminium cation (6).
- Cooney, Aidan P.,Crampton, Michael R.,Golding, Peter
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p. 835 - 840
(2007/10/02)
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- Effect of Molecular Interactions on Rate Constants of Protolytic Reactions: Phenol and Its Nitro Derivatives
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The forward and reverse rate constants of protonation-deprotonation equilibria of phenol and its nitro derivatives (2-6) have been calculated using a model proposed earlier .The influence of intermolecular and intramolecular hydrogen bonds, as well as the inductive and mesomeric effects, on the rate constants are discussed.The calculated rate constants are in good agreement with the experimental values available in the literature.An analysis has been made for isolated molecules, in which solute-solvent interactions are neglected.For this case a semi-empirical CNDO/2 molecular orbital calculation has been performed.
- Ramos, Mozart N.
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p. 513 - 515
(2007/10/02)
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- REACTION OF METHYL AND PHENYL PICRATES WITH NUCLEOPHILES
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The reactions of 2,4,6-trinitroanisole with various nucleophiles Me3SnM, Me3SiLi, BuLi, Me4NBBu4, PhMgI, CN-, lithium (potassium) succinimide, F-, Cl-, Br-, SCN-, NO2-, NO3-, HCO3-, PhSO2-,AcO-, S2- were investigated.It was found that the final products are the picrate and the corresponding methyl derivatives.The formation of intermediate ? complexes was detected in a series of the reactions (Me3SnLi, potassium succinimide, Bu4B-, CN-, NO2-, PhSO2-).Possible mechanisms for the reaction are examined.Reactions of 2,4,6-tricyanoanisole and phenyl picrate with certain nucleophiles were also investigated.
- Artamkina, G. A.,Egorov, M. P.,Beletskaya, I. P.,Reutov, O.A.
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- HEMITHIO-ANALOGUES OF MIESENHEIMER COMPLEXES AND SULPHIDE GROUP ACTIVATION OF AROMATIC SUBSTITUTION BY ALKOXIDES
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EtO(1-) in DMSO adds to ethylthiopicrate at C-3 to generate an ephemeral ?-adduct which ends into 1-ethylthio-4-ethoxy-2,5-dinitrobenzene, whilst ethyl picrate gives two adducts of attack of EtS(1-) at C-3 or C-1.
- Cavazza, Marino,Morganti, Gioia,Guerriero, Antonio,Pietra, Francesco
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p. 3703 - 3704
(2007/10/02)
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