Liquid Crystalline Organisation and Reaction Rate in the Formation of Liquid Crystal Thermoset Networks from Diepoxides and Dicyanates

Summary: The influence of the type of mesophase on the rate of reaction in amine curing or anionic polymerisation of mesogenic diepoxides, and in the cyclotrimerisation of mesogenic dicyanates has been investigated by isothermal DSC and IR-spectroscopy. Epoxide/amine systems were found to react faster in the nematic than in the isotropic phase. Anionic polymerisation of epoxides is an example where increase of the reaction rate occurs with the transition from a mesophase of higher order (smectic) to phases with lower order (nematic, isotropic). For triaromatic dicyanates with one or three methyl groups at the central ring cyclotrimerisation is faster in the isotropic than in the nematic phase showing an increase of activation energy in the nematic phase with the number of methyl groups. A pronounced rate increase has been observed in the smectic phase as compared to the nematic phase for the unsubstituted triaromatic dicyanate. In this case activation energies in the two phases are comparable with those of non liquid crystalline dicyanates.     

β羟肟萃取钯(Ⅱ)取代反应中的动力学催化作用

β羟肟、硫醚等在盐酸介质中萃取钯(Ⅱ)的配位取代反应具有较高的选择性,有实用意义^[1-5]。Cleare等^[1]曾提出加入胺类化合物以加快β羟肟萃钯的速度。本文在研究β羟肟萃钯(Ⅱ)机理^[2,3]的基础上考察了加入伯胺后2-羟基-4-(1-甲基庚氧基)二苯甲酮肟(HL)萃取钯(Ⅱ)的热力学和动力学行为以及界面特性,试图阐明伯胺对该取代反应的加速机理。试验所用伯胺N1923(Am)为混合支链伯胺,上海有机所实验厂生产。水相中伯胺含量用溴酚兰分光光度法测定^[6],其它实验方法参见[3]。     

Kinetics of the reaction of carbon dioxide with blends of amines in aqueous media using the stopped‐flow technique

The effect of mixing 2‐amino‐2‐methyl‐1‐propanol (AMP) with a primary amine, monoethanolamine (MEA), and a secondary amine, diethanolamine (DEA), on the kinetics of the reaction with carbon dioxide in aqueous media has been studied at 298, 303, 308, and 313 K over a range of blend composition and concentration. The direct stopped‐flow conductimetric method has been used to measure the kinetics of these reactions. The proposed model representing the reaction of CO₂ with either of the blends studied is found to be satisfactory in determining the kinetics of the involved reactions. This model is based on the zwitterion mechanism for all the amines involved (AMP, MEA, and DEA). Blending AMP with either of the amines results in observed pseudo‐first‐order reaction rate constant values (k_o) that are greater than the sum of the k_o values of the respective pure amines. This is due to the role played by one amine in the deprotonation of the zwitterion of the other amine. Steric factor and basicity of the formed zwitterion and the deprotonating species have a great bearing in determining the rate of the reactions studied. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 391–405, 2005     

Kinetics and mechanism of base catalysed ethyl cyanoformate addition to aldehydes

The mechanism by which tertiary amines catalyse the formation of cyanohydrin carbonates from aldehydes and alkyl cyanoformates is investigated by means of a kinetic study. The reaction rate shows a second order dependence on amine concentration unless the amine is sterically hindered, when the rate has a first order dependence on amine concentration. The catalytic activity of the amine correlated with its pK_aH. On the basis of these results, a mechanism is proposed in which the amine acts as a base to activate a water molecule, which reacts with the ethyl cyanoformate generating cyanide in situ.     

Correlation analysis of reactivity in the addition of substituted benzylamines to benzylidenemalononitrile

The kinetics of addition of a number of ortho‐, meta‐, and para‐substituted benzylamines to benzylidenemalononitrile (BMN) in acetonitrile have been studied. The reaction is first‐order with respect to BMN. The order with respect to the amine is more than one. It has been shown that the reaction followed two mechanistic pathways, uncatalyzed and catalyzed by the amine. The enthalpy of activation for the catalyzed path is negative indicating the presence of a preequilibrium (k₁, k₋₁) leading to the formation of a zwitterion. The values of rate constant, k₁, for the nucleophilic attack have been determined for twenty‐eight benzylamines. The rate constant, k₁ was subjected to correlation analyses using various single‐ and multi‐parametric equations. The best correlation is obtained in terms of Charton's LDR and LDRS equations. The polar regression coefficients are negative indicating the formation of a cationic species in the transition state. The reaction is subject to steric hindrance by ortho‐substituents. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 245–252, 1999     

The mechanism of hydroaminoalkylation catalyzed by group 5 metal binaphtholate complexes

The intermolecular hydroaminoalkylation of unactivated alkenes and vinyl arenes with secondary amines occurs readily in the presence of tantalum and niobium binaphtholate catalysts with high regio- and enantioselectivity (up to 98% ee). Mechanistic studies have been conducted in order to determine the kinetic order of the reaction in all reagents and elucidate the rate- and stereodetermining steps. The effects of substrate steric and electronic properties on the overall reaction rate have been evaluated. The reaction is first order in amine and the catalyst, while exhibiting saturation in alkene at high alkene concentration. Unproductive reaction events including reversible amine binding and arene C-H activation have been observed. The formation of the metallaaziridine is a fast reversible nondissociative process and the overall reaction rate is limited either by amide exchange or alkene insertion, as supported by reaction kinetics, kinetic isotope effects, and isotopic labeling studies. These results suggest that the catalytic activity can be enhanced by employing a more electron-deficient ligand backbone.     

Insights into the Oxidative Dehydrogenation of Amines with Nanoparticulate Iridium Oxide

The aerobic oxidation of amines offers a promising route towards many versatile chemical compounds. Within this contribution, we extend our previous investigations of iridium oxide‐catalyzed alcohol oxidation to amine substrates. In addition to demonstrating the versatility of this catalyst, particular attention is focused on the mechanisms of the reaction. Herein, we demonstrate that although amines are oxidized slower than the corresponding alcohols, the catalyst has a preference for amine substrates, and oxidizes various amines at turnover frequencies greater than other systems found in the open literature. Furthermore, the competition between double amine dehydrogenation, to yield the corresponding nitrile, and amine–imine coupling, to yield the corresponding coupled imine, has been found to arise from a competitive reaction pathway, and stems from an effect of substrate‐to‐metal ratio. Finally, the mechanism responsible for the formation of N‐benzylidene‐1‐phenylmethanamine was examined, and attributed to the coupling of free benzyl amine substrate and benzaldehyde, formed in situ through hydrolysis of the primary reaction product, benzyl imine.     

Exploring the mechanism for the amine-catalyzed isomerization of dimethyl maleate. A computational study

The main objective of this study was to investigate the amine-catalyzed isomerization of dimethyl maleate into dimethyl fumarate in order to utilize the former as a prodrug for the latter. Mechanistic study of this reaction using DFT at B3LYP/6-31G(d,p) level revealed that the reaction is first order in dimethyl maleate, second order in the amine, and overall third order. Moreover, the calculations revealed the existence of a linear correlation between the basicity of the amine catalyst and the isomerization rate.     

A mechanistic investigation of the three‐component radical photoinitiator system Eosin Y spirit soluble,N‐methyldiethanolamine,and diphenyliodonium chloride

Photo‐DSC and in situ, time‐resolved, laser‐induced, steady‐state fluorescence spectroscopy were used to study the initiation mechanism of the three‐component system: Eosin Y spirit soluble (EYss), N‐methyldiethanolamine, and diphenyliodonium chloride. Kinetic studies based on photo‐DSC revealed that the fastest polymerization occurred when all three components were present (the next fastest was with the dye/amine pair, and the slowest was with the dye/iodonium pair). However, the laser‐induced fluorescence experiments showed that the pairwise reaction between the eosin and iodonium bleaches the dye much more rapidly than does the reaction between the eosin and amine. We concluded that although a direct eosin/amine reaction can produce active radicals in the three‐component system, this reaction is largely overshadowed by the eosin/iodonium reaction, which does not produce active radicals as effectively. We proposed that the amine reduces the oxidized dye radical formed in the eosin/iodonium reaction back to its original state as well as the simultaneous production of an active initiating amine‐based radical. Because of the difference in the pairwise reaction rates for eosin/amine and eosin/iodonium, it is likely that this regeneration reaction was the primary source of active radicals in the three‐component eosin/amine/iodonium system. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 715–723, 2001     

Dramatic acceleration of the Menschutkin reaction and distortion of halide leaving-group order

Compared to structurally related linear trialkylamines, a simple macrocyclic amine with an anion-binding cavity exhibits very large rate enhancements (>10(5)) for stoichiometric N-alkylation with primary alkyl, allyl, and benzyl halides in the weakly polar solvent CDCl3. There is also a major distortion of the halide leaving-group order. For example, with benzyl halides the relative leaving-group order with a control amine is Cl (1) < Br (71) < I (160), whereas the leaving-group order with the macrocyclic amine is I (0.4) < Cl (1) < Br (8.5). Reaction with the macrocyclic amine is inhibited by the addition of DMSO, which is unusual because the Menschutkin reaction is normally enhanced by the presence of a polar aprotic solvent. Competitive inhibition studies indicate that the reaction proceeds through a prereaction complex. Effective molarities for the subsequent unimolecular N-alkylation step with 4-t-butylbenzyl halides are 4-t-BuBnCl (62,000 M) > 4-t-BuBnBr (2200 M) > 4-t-BuBnI (35 M); thus, the free energy of activation is selectively decreased for organohalides having smaller and more charge dense leaving groups. Likely reasons for this selective enhancement effect are: (a) increased transition-state stabilization due to hydrogen bonding in the macrocyclic pocket and (b) reduced entropic penalty in the transition state due to an increased fraction of prereaction complexes that are oriented in a near attack conformation. The study suggests that it should be possible to develop highly reactive macrocyclic amines that selectively sense or scavenge carcinogenic haloalkanes from the atmosphere.     

Importance of the diamine reactant in the production of polyphosphonamides by the interfacial technique

The stirred interfacial polycondensation of phenylphosphonic dichloride and 1,6-hexanediamine has been studied as a function of several reaction variables. The reaction is rapid, being completed in less than 1 min. When organic solvent is varied and reactant molar ratio is varied with an excess of the acid chloride, yield is constant. When reactant molar ratio is varied polymer yield increases with increase in amine concentration. When reactant concentration is increased yield increases. With the addition of a soluble salt in the aqueous phase yield is increased. The above indicates that the diffusion of the amine to the reaction zone is of primary importance in determining polymerization rate and that the diffusion of the acid chloride is relatively unimportant. Polymer yield was found to be dependent on the pH of the amine in the aqueous phase. The observed trend is related to the apparent solubility of the amine in the aqueous phase such that the greater the apparent solubility of the amine, the less the polymer yield. Polymer molecular weight is found to be independent of reaction variables tested. Polymer was also formed from the condensation of phenylphosphonic dichloride with p-phenylenediamine, H₂N-D-NH₂ (where D is a 36-carbon hydrocarbon chain), 1,3-di-4-piperidylpropane, and 4-aminomethylpiperidine; phenyl phosphorodichloridate with 1,6-hexanediamine; chloromethyl phosphonic dichloride with 1,6-hexanediamine.     

Rapid SmI2-mediated reductions of alkyl halides and electrochemical properties of SmI2/H2O/amine

Mixtures of SmI(2)/H(2)O/amine have been found to reduce alkyl halides more efficiently than SmI(2)/HMPA/alcohol mixtures at room temperature. Alkyl and aryl iodides were quantitatively reduced in <1 min and alkyl bromides in 10 min, while alkyl and aryl chlorides required more than 5 h for completion. Determination of the reaction order of Et(3)N in the reduction of 1-chlorodecane showed that the reaction order is one. Water was shown not to participate in the rate-determining step of this reduction. There was a significant change of the UV-vis spectrum and color of SmI(2) upon addition of either PMDTA or water, while no effect was observed with the addition of Et(3)N or TMEDA. Although the combination of SmI(2), water, and amines produces a very efficient reducing system, cyclic voltammetric experiments showed that the redox potential is nearly identical with that of SmI(2) alone. These results are consistent with precipitation providing the driving force for reduction. Taken together, the results of these experiments show that the combination of SmI(2)/H(2)O/amine provides a fundamentally novel and useful approach to enhance the reactivity of SmI(2).     

Effect of stoichiometry and diffusion on an epoxy/amine reaction mechanism

The bulk phase kinetics of an epoxy (DGEBA) /amine (DDS) thermoset have been studied using DSC, FTIR, and ¹³C-NMR. In the absence of catalyst, the reaction was found to involve a main exothermic reaction between epoxide and amine hydrogen and a side reaction between tertiary amine formed in the main reaction and epoxide. The main reaction was exothermic while the side reaction had no discernable exotherm. Etherification did not occur to any significant extent. Since only the main reaction is exothermic, DSC was very useful for studying the main reaction kinetics. FTIR was used for determining whether epoxide and amine hydrogen were consumed at different rates as a way of following the side reaction. An IR band previously unused by other investigators was used to monitor the amine hydrogen concentration. NMR confirmed the above mechanism by identifying the formation of a quaternary ammonium ion/alkoxide ion pair as a reaction product of tertiary amine and epoxide. This mechanism has been successfully fit to a rate law valid over the entire extent of reaction. The rate constant for the epoxy/amine addition reaction was found to depend on hydroxide concentration (extent), reaction temperature, and glass transition temperature and included contributions from uncatalyzed and autocatalyzed parts. The side reaction (quaternary ammonium ion formation) formed weak bonds which did not affect the overall system T_g. Both reactions were second order. The rate constants for the main reaction first increase with increasing extent due to autocatalysis by hydroxide before decreasing due to the diffusion limit caused by gelation and vitrification. © 1995 John Wiley & Sons, Inc.     

Selective N-Monomethylation of Primary Amines via N-Trialkylsilyl-Lithioamines

Transformation of a primary amine to a secondary amine can be achieved by reaction with an alkyl halide in the presence of an excess of the primary amine to suppress multiple alkylation, but, in general, direct alkylation is not the preferred method when the product alkylated amine is more nucleophilic than the startirg amine. More efficient procedures involve a relative vitiation of the nucleophilicity of the monoalkylated product with respect to that of the unalkylated substrate. Such an effect may be realised by the incorporation of monovalent activating groups (Z), which increase the acidity of the amido-proton and allow alkylation of a stabilised amidoanion, often generated in situ, this species being more reactive than a simple amine. Since in the conversion of a primary to a secondary amine only one amido-proton remains after incorporation of the activating group, further N-alkylation is obviated (Scheme 1.)     

Polymerization photoinitiated by carbonyl compounds. X. Methyl methacrylate polymerization photoinitiated by fluorenone in the presence of triethylamine

The photoinitiation efficiency of the fluorenone/triethylamine (TEA) system in the polymerization of methylmethacrylate (MMA) has been evaluated as a function of the monomer concentration, the amine concentration, and the polarity of the reaction medium. The polymerization proceeds readily in low polarity media (benzene/monomer), but it is negligible in more polar solvents (acetonitrile/monomer). The polymerization rate increases with the amine concentration up to 0.01 M TEA. Further increase in amine concentration produces a decrease in the polymerization rate. A similar behavior was observed for the fluorenone photoreduction yield and the yield of fluorenone derived radicals. All these processes are considered to involve the excited triplet, while quenching of the excited singlet by the amine decreases the rate of these processes. However, the decrease in photoinitiation efficiency observed at high amine concentration is larger than that expected from the singlet quenching extent, as estimated from the effect of the amine on the fluorescence yield under the same experimental conditions. This discrepancy indicates that other process(es) must contribute to the protection afforded by high amine concentrations. Quenching of the charge transfer intermediate by the amine is postulated as a competitive process that could explain the above mentioned effects. © 1994 John Wiley & Sons, Inc.     

Synchrotron X-ray Electron Density Analysis of Chemical Bonding in the Graphitic Carbon Nitride Precursor Melamine

Melamine is a precursor and building block for graphitic carbon nitride (g-CN) materials, a group of layered materials showing great promise for catalytic applications. The synthetic pathway to g-CN includes a polycondensation reaction of melamine by evaporation of ammonia. Melamine molecules in the crystal organize into wave-like planes with an interlayer distance of 3.3 Å similar to that of g-CN. Here we present an extensive investigation of the experimental electron density of melamine obtained from modelling of synchrotron radiation X-ray single-crystal diffraction data measured at 25 K with special focus on the molecular geometry and intermolecular interactions. Both intra- and interlayer structures are dominated by hydrogen bonding and π-interactions. Theoretical gas-phase optimizations of the experimental molecular geometry show that bond lengths and angles for atoms in the same chemical environment (C−N bonds in the ring, amine groups) differ significantly more for the experimental geometry than for the gas-phase-optimized geometries, indicating that intermolecular interactions in the crystal affects the molecular geometry. In the experimental crystal geometry, one amine group has significantly more sp³-like character than the others, hinting at a possible formation mechanism of g-CN. Topological analysis and energy frameworks show that the nitrogen atom in this amine group participates in weak intralayer hydrogen bonding. We hypothesize that melamine condenses to g-CN within the layers and that the unique amine group plays a key role in the condensation process.     

Ugi反应在环肽生物碱和小肽衍生物抗生素及细胞毒素类等天然产物合成中的应用

Ugi反应是由1分子醛或酮、1分子胺、1分子异腈和1分子羧酸4个组分通过缩聚反应生成α- 酰氨基酰胺类化合物的反应。 具有反应条件温和、产率高和原子经济性好等特点。 该反应还可与Suzuki、Heck和Smiles等经典反应偶联,使得其在天然产物合成方面得到越来越多的关注。 本文概括总结了近几十年来Ugi反应在一些天然产物合成中的应用。     

On the formation and properties of a high-temperature resin from a bisphthalonitrile

When 4,4'-bis (3,4-dicyanophenoxy) biphenyl is heated with small amounts of aromatic amine or amidine salts, a highly crosslinked polytriazine is obtained. This polymer has been shown to be the same as that reported when bisphthalonitriles are heated with amines. Salts promote this reaction more readily and the glass transition temperature of the polymer after post-cure at 315°C is generally 30°C higher than when free bases are used. The fracture properties and elasticity of the salt-cured polymer have been measured at temperatures up to 250°C. As a model system, the self reaction of phthalonitrile promoted by amines and their salts has been studied. In both cases, poly[4-(2-cyanophenyl)-1,3,5-triazine-2,6-diyl-1,2-phenylene] is produced, and more efficiently using the salts. A reaction mechanism for this polymerization has been proposed. © 1994 John Wiley & Sons, Inc.     

Mechanism and application of a microcapsule enabled multicatalyst reaction

In this paper, we describe the development and application of a multistep one-pot reaction that is made possible by the site isolation of two otherwise incompatible catalysts. We prepared a microencapsulated amine catalyst by interfacial polymerization and used it in conjunction with a nickel-based catalyst for the transformation of an aldehyde to a Michael adduct via a nitroalkene intermediate. The amine-catalyzed conversion of an aldehyde to a nitroalkene was found to proceed through an imine rather than a nitroalcohol. Kinetic studies indicated that the reaction is first order in both the nickel catalyst and the shell of the encapsulated amine catalyst. Furthermore, we provide evidence against interaction between amine and nickel catalysts and present kinetic data that demonstrates that there is a rate enhancement of the Michael addition due to the urea groups on the surface of the microencapsulated catalyst. We applied our one-pot reaction to the development of a new synthetic route for pregabalin that proceeds with an overall yield of 74%.     

Are amines basic or nucleophilic catalysts for oxirane ring opening by proton-donating nucleophiles?

The behavior of amines as catalysts for oxirane acidolysis and phenolysis has been studied using kinetic methods. The apparent catalytic and noncatalytic reaction rate constants have been estimated. It has been demonstrated that the noncatalytic pathway has almost no effect on the apparent reaction rate constant. In order to determine the character of the behavior of amines (bases/nucleophiles) in this reaction, their reactivity has been analyzed within the conceptions of basic and nucleophilic mechanisms of catalysis. Based on the quantitative amine structure—catalytic activity correlation, it has been shown by comparing the values of correlation coefficients (r) of equations describing mechanisms for various reaction systems that, in the reactions of oxiranes with proton donors (carboxylic acids and phenols), the catalytic activity of tertiary amines/pyridines is determined by their nucleophilicity rather than basicity.