亲电反应中的缺电子中心

对于不饱和烃类化合物的亲电反应,在反应类型上,有亲电加成和亲电取代;在反应产物上,有取代产物和加成产物,而且在加成产物中既有马氏加成、又有反马氏加成产物。知识点多而且复杂,学生学习记忆往往比较困难。本文以缺电子中心为主线,系统分析了亲电反应中的4种主要缺电子中心(包括正离子、自由基、卡宾和中性分子),从其结构特点和反应机理归纳总结了各种反应底物的亲电反应,以期对亲电反应有一个更深入、系统的认识。     

Microemulsions as microreactors for food applications

Major recent advances. Structured self-assembled liquids have been considered as efficient microreactors for organic and enzymatic reactions. Only recently scientists learned to use food-grade cosolvents and coemulsifiers together with hydrophilic non-ionic surfactants and to construct U-type phase diagrams with large isotropic regions ranging continuously from the oil-rich corner to the water-rich corner without any phase separation. The U-type microemulsions facilitate triggering and control of certain reactions by changing water activities. Maillard thermal degradation between sugars and amino acids is the main, and almost the only, chemical reaction that has been studied in food-grade microemulsions. Some examples of recent studies include: Maillard processes in binary structured fluids composed of monoglycerides of fatty acids and water forming microemulsions and lyotropic liquid crystalline structures; pseudoternary and pseudoquaternary W/O microemulsions; U-type microemulsions (W/O, O/W and bicontinuous microemulsions); enzymatic reactions aimed to prepare other surfactants such as sugar esters, monoglycerides and lysolecithins or triglycerides. Reactions in microreactors lead to unique new products. The reaction products and rates are controlled by the hydrophilicity/lipophilicity of the reagents (guest molecules), their molar ratios, type of oil phase, nature of surfactants and oil/surfactant ratios, nature of curvature and its elasticity (adjusted by cosolvent and coemulsifier) and by the water activity. The field is in its infancy and will need work of many more model reactions before it will be used in industrial food applications. Enzymatic reactions in non-food microemulsions are common practice but only few examples of food microemulsions as enzymatic microreactors have been extensively studied.     

Multistep small-molecule synthesis programmed by DNA templates

The translation of DNA sequences into synthetic products is a key requirement of our approach to evolving synthetic molecules through iterated cycles of translation, selection, and amplification. Here we report general linker and purification strategies for sequence-specific DNA-templated synthesis that collectively enable the product of a DNA-templated reaction to be isolated and to undergo subsequent DNA-templated reactions. Using these strategies, we have achieved the first multistep nucleic acid-templated small-molecule syntheses to generate two different molecules. In addition to representing a method for translating DNA templates sequence-specifically into corresponding multistep synthetic products, our findings also provide experimental support for previously proposed models invoking multistep nucleic acid-templated synthesis as mediating the prebiotic translation of replicable information into the earliest functional molecules.     

Novel approaches to the analysis of the Maillard reaction of proteins

The Maillard reaction comprises a complex network of reactions which has proven to be of great importance in both food science and medicine. The majority of methods developed for studying the Maillard reaction in food have focused on model systems containing amino acids and monosaccharides. In this study, a number of electrophoretic techniques, including two-dimensional gel electrophoresis and capillary electrophoresis, are presented. These have been developed specifically for the analysis of the Maillard reaction of food proteins, and are giving important insights into this complex process.     

生物质热解液化与美拉德反应

对生物质热解液化和美拉德反应进行了介绍,指出美拉德反应不仅存在于生物质热解液化过程中,而且通过引入适量氨等调控措施,可以促进美拉德反应有选择性地生成吡嗪类杂环化合物等高值化学品,然后再通过分级冷凝将生物油分为化工生物油和燃料生物油,前者用于分离提取高值化学品,后者用于锅炉和窑炉的燃料.引入美拉德反应后,生物质热解液化技术经济性将会得到根本性的改善.     

The absorption and metabolism of modified amino acids in processed foods

The chemical reactions involved in the modifications of amino acids in processed food proteins are described. They concern the Maillard reaction, reaction with polyphenols and tannins, formation of lysinoalanine during alkaline and heat treatments, formation of isopeptides, oxidation reaction of the sulfur amino acids, and isomerization of the L-amino acids into their D-form. Information on the digestion, absorption, and urinary excretion of the reaction products obtained by using conventional nutritional tests is given. The studies that have been made on the metabolism of these molecules by using a radioisotopic approach to follow their kinetics in the organism after ingestion are also reviewed. This approach provides unique data on the quantitation of the metabolic pathways and on the kinetics of the metabolic processes involved.     

Automated derivation of reaction rules for the EROS 6.0 system for reaction prediction

The purpose of the EROS 6.0 system is to predict the products of chemical reactions and to model reaction mechanisms. This is accomplished by elementary reaction steps that are selected through the rules that constitute the knowledge base of the EROS 6.0 system. These rules are derived by methods of machine learning. The learning process is based on reaction in data bases. An overview of the EROS 6.0 system is given and the structure of the knowledge as well as the generation of reaction rules are described.     

Methylation reaction for four DNA base molecules by methanediazonium ions

The methylation reactions at ten nucleophilic sites in four DNA base molecules with methanediazonium ions (CH₃N₂ ⁺) have been theoretically investigated including solvent effects at the B3LYP/6-31G^** and MP2/6-31G^** levels. The results show that all the methylation reactions have relatively small activation energy (<33.5 kJ/mol), and the methylation process is exothermic reaction and easy to occur. This study shows that the ultimate carcinogen CH₃N₂ ⁺ by NDMA can easily methylate DNA base molecules and form carcinogenic products. Supported by the Shanghai Municipal Education Commission (Grant No. YYY-07015) and Shanghai Institute of Technology (Grant No. YJ2007-36)     

A chemically consistent graph architecture for massive reaction networks applied to solid-electrolyte interphase formation

Modeling reactivity with chemical reaction networks could yield fundamental mechanistic understanding that would expedite the development of processes and technologies for energy storage, medicine, catalysis, and more. Thus far, reaction networks have been limited in size by chemically inconsistent graph representations of multi-reactant reactions (e.g. A + B → C) that cannot enforce stoichiometric constraints, precluding the use of optimized shortest-path algorithms. Here, we report a chemically consistent graph architecture that overcomes these limitations using a novel multi-reactant representation and iterative cost-solving procedure. Our approach enables the identification of all low-cost pathways to desired products in massive reaction networks containing reactions of any stoichiometry, allowing for the investigation of vastly more complex systems than previously possible. Leveraging our architecture, we construct the first ever electrochemical reaction network from first-principles thermodynamic calculations to describe the formation of the Li-ion solid electrolyte interphase (SEI), which is critical for passivation of the negative electrode. Using this network comprised of nearly 6000 species and 4.5 million reactions, we interrogate the formation of a key SEI component, lithium ethylene dicarbonate. We automatically identify previously proposed mechanisms as well as multiple novel pathways containing counter-intuitive reactions that have not, to our knowledge, been reported in the literature. We envision that our framework and data-driven methodology will facilitate efforts to engineer the composition-related properties of the SEI – or of any complex chemical process – through selective control of reactivity.

A chemically consistent graph architecture enables autonomous identification of novel solid-electrolyte interphase formation pathways from a massive reaction network.     

Theoretical investigation of the gas-phase reaction of NiO+ with ethane

To explore the mechanisms for Ni-based oxide-catalyzed oxidative dehydrogenation (ODH) reactions, we investigate the reactions of C₂H₆ with NiO⁺ using density functional calculations. Two possible reaction pathways are identified, which lead to the formation of ethanol (path 1), ethylene and water (path 2). The proportion of products is discussed by Curtin-Hammett principle, and the result shows that path 2 is the main reaction channel and the water and ethylene are the main products. In order to get a deeper understanding of the titled reaction, numerous means of analysis methods including the atoms in molecules (AIM), electron localization function (ELF), natural bond orbital (NBO), and density of states (DOS) are used to study the properties of the chemical bonding evolution along the reaction pathways.

     

Dextran oxidized by a malaprade reaction shows main chain scission through a maillard reaction triggered by schiff base formation between aldehydes and amines

Although periodate‐oxidized dextran is widely used in biomedical applications, the degradation mechanism of oxidized dextran has not yet been elucidated. Herein, we propose a novel main chain scission mechanism of oxidized dextran triggered by reaction with amine. NMR analysis revealed four hemiacetal substructures during oxidation by periodate. Kinetic analysis showed that the degradation time constant of the C3‐removed substructure and increasing time constant of the reducing end protons are consistent with the decrease in molecular weight determined by gel permeation chromatography. A methylene group is generated at the same time constant of degradation, indicating that oxidized dextran degradation proceeds via a Maillard reaction. Oxidized dextran does not degrade in saline solution without reactive amine species. Thus, we conclude that oxidized dextran is degraded in the main chain via Schiff base formation through a Maillard reaction, depending on the oxidation ratio and amino acid concentration. These findings help to elucidate the reaction mechanism of polysaccharide degradation and develop novel biodegradable polysaccharide materials for biomedical applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2254–2260     

Fluctuation dynamics of chemiluminescence in the maillard reaction

The general problems of information deriving from the oscillation kinetics of Belousov-Zhabotinsky reactions were discussed. The discussion was organized around a particular example of the complex fluctuation dynamics of the Maillard reaction, i.e., a multistage reaction between a carbonylcontaining compound and the nucleophilic amino group of an amino-containing compound at different temperatures, as visualized by chemiluminescence. Flicker-noise spectroscopy served as methodological background of the fluctuation analysis comprising the two main steps: separation of low-frequency components attributed to resonances and their interference, from the source signals and analysis of the remaining chaotic components. Possibility of modeling the chaotic dynamics of fluctuations of the measured dynamic variables in terms of anomalous diffusion was shown.     

Automated discovery of chemically reasonable elementary reaction steps

Due to the significant human effort and chemical intuition required to locate chemical reaction pathways with quantum chemical modeling, only a small subspace of possible reactions is usually investigated for any given system. Herein, a systematic approach is proposed for locating reaction paths that bypasses the required human effort and expands the reactive search space, all while maintaining low computational cost. To achieve this, a range of intermediates are generated that represent potential single elementary steps away from a starting structure. These structures are then screened to identify those that are thermodynamically accessible, and then feasible reaction paths to the remaining structures are located. This strategy for elementary reaction path finding is independent of atomistic model whenever bond breaking and forming are properly described. The approach is demonstrated to work well for upper main group elements, but this limitation can easily be surpassed. Further extension will allow discovery of multistep reaction mechanisms in a single computation. The method is highly parallel, allowing for effective use of modern large‐scale computational clusters. © 2013 Wiley Periodicals, Inc.     

Proton-block strategy for the synthesis of oligodeoxynucleotides without base protection,capping reaction,and P-N bond cleavage reaction

A new N-unprotected phosphoramidite method called the "proton-block" approach was developed for the chemical synthesis of oligodeoxynucleotides based on the hitherto simplest and rational principle of acid-base reactions. This concept involves protection of the nucleobases of deoxycytidine and deoxyadenosine with "protons" to convert them to unreactive protonated bases during condensation by use of promoters having pK(a) values lower than 2.8. This strategy was applied to the synthesis of d[CpT] and d[ApT] to check the side reactions associated with the base residues. In this "proton-block" method, 5-nitrobenzimidazolium triflate (NBT) was found to be the best promoter, and THF was superior to CH(3)CN as the solvent so that the concomitant detritylation due to the inherent acidity of the promoter could be greatly suppressed. Application of this strategy to the solid-phase synthesis gave d[CpT], d[ApT], d[ApA], d[CpC], and d[GpT] as almost single peaks in HPLC analysis. Similarly, d[ApApApT] and d[CpCpCpT] were successfully synthesized without significant side reactions. Finally, d[CpCpCpCpCpCpT] and d[ApApApApApApT] were obtained as the main products. In the case of a longer oligomer, d[CpApGpTpCpApGpTpCpApGpT], a mixed solvent of CH(3)CN-N-methylpyrrolidone (1:1, v/v) was superior to THF so that the desired oligodeoxynucleotide could be isolated in a satisfactory yield. These results suggest that DNA synthesis can be carried out simply by using the protonated bases at the oligomer level not only without base protection but also without the capping reaction and the posttreatment of branched chains with MeOH-benzimidazolium triflate that previously was requisite. It is concluded that most of the reactions and solvent effects involved in this strategy can be explained in terms of simple acid-base reactions. Some problems associated with the previous posttreatment are also discussed with our own results.     

Characterizing complex reaction mechanisms using machine learning clustering techniques

A machine learning conceptual clustering method applied to reaction mechanisms provides an automatic and, hence, unbiased means to differentiate between reactive phases within a total reactive process. Similar reactive phases were defined by means of local reaction sensitivity values. The method was applied to the Hochgreb and Dryer aldehyde combustion mechanism of 36 reactions. Three major time ranges were found and characterized: an initial phase of aldehyde reaction, an intermediate phase where only a small amount of aldehyde is left, and an end phase of reactions to final products. Further refinements of these phases into subtime intervals were found. All ranges found could be chemically justified. This method is meant as a supplement to existing methods of mechanism analysis and its main purpose is the automatic characterization of existing mechanisms and can potentially be used for mechanism reduction. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 107–118, 2004     

CE: a useful analytical tool for the characterization of Maillard reaction products in foods

The Maillard reaction (MR) is a complex series of nonenzymatic reactions between reducing compounds and amines, amino acids, peptides, or proteins that play an important role in the formation of flavors and colors in foods during processing and storage. Also, the antioxidant properties of some Maillard reaction products (MRP) was an additional benefit reported. On the other hand, these reactions decrease the nutritional quality of foods and may result in the formation of toxic MRP. Although, research to assess the risks and benefits associated with the consumption of MRP in the diet is still awaiting for new analytical methodologies to be developed. Structural characterization of MRP has been very challenging due to the chemical diversity of these compounds which present a wide range of polarities and molecular weights, making analyses difficult. CE is a technique that has gained popularity for the separation of complex mixtures that have otherwise proved difficult to analyze. Thus, the purpose of this overview is to give the reader an appreciation of some of the CE analytical developments on the analysis of MRP in model systems and foods, and to address the potential of CE on the characterization of this complex group of compounds.     

A reversible reaction inside a self-assembled capsule

Reversible encapsulation allows the direct observation of the isolated molecules under ambient conditions, at equilibrium and in the liquid phase. Here we show that capsules can amplify and stabilize molecules that are present in only trace concentrations in solution. Evidence is given that reversible chemical reactions take place within the capsule. Stabilization of reaction intermediates is a characteristic property of enzymes and is widely regarded as an essential feature of catalytic activity. Reactive molecules can also be stabilized by encapsulation, a process that involves completely surrounding the reactive species within synthetic receptors. Here, we show that self-assembled capsules can isolate and stabilize molecules that are present in only trace amounts in solution. The system amplifies the concentrations of high-energy species with reduced entropies.     

Asymmetric Kinugasa reaction involving six-membered cyclic nitrones

Kinugasa reactions between terminal acetylenes and six-membered ring nitrones when one or both components are chiral, proceed in a low to moderate yield and with a high diastereoselectivity affording mostly one, dominant β-lactam product. The first step of the reaction is controlled by the configuration of the nitrone, whereas the protonation of the C-7 center of the carbacepham skeleton in the second step depends on: a) the configuration of the bridgehead carbon atom formed in the first step, b) epimerization process in the presence of a base, and c) on the configuration of the stereogenic center in the acetylenic partner. In the case of the nitrone derived from dihydroisoquinoline, the reaction proceeds in a more complex way affording not only β-lactams, but also products derived from the alternative regio-1,3-cycloaddition, or nucleophilic addition of the acetylene to the double bond of the nitrone.     

VET: a tool for reaction plausibility checking

Production of chemical reaction databases is a multistep process, with the possibility of errors at each of these steps. VET is a tool developed to trap errors in the chemical reactions identified as a part of this process. VET has been designed to minimize the acceptance of incorrect reactions, while still supporting various common practices in reaction depiction, including unbalanced reactions, suppressed components, and reactions with alternative products. We discuss the assumptions made in its construction, a general overview of its structure, and some performance characteristics.