L-缬氨酸旋光异构的两种光反应可能途径

利用密度泛函理论(DFT)和从头算分子轨道理论对L-缬氨酸的旋光异构光反应机理进行了研究. 分别用B3LYP和MP2方法在6-311++G(d, p)基组级别上全优化得到了S₀和T₁态反应路径上的反应物、产物、中间体以及过渡态结构的几何构型, 给出了反应能垒, 利用含时密度泛函理论(TD-DFT)中的B3LYP/6-311++G(d, p)方法优化得到了S₁态反应路径上的平衡态几何构型. 通过分析反应途径上各个驻点的几何构型特征, 确定了L-缬氨酸在激发态可能通过手性碳上的氢原子以羰基氧或氨基氮为中转媒介发生质子迁移来完成旋光异构反应. 进一步用自洽反应场理论中的极化连续模型(PCM)方法探讨了溶剂化效应对旋光异构反应机理的影响.     

四正丁基过硫酸铵氧化裂解2，4-二硝基苯腙衍生物的反应研究

四正丁基过硫酸铵在1,2二氯乙烷溶剂中可高产率地使醛和酮的2,4-二硝基苯腙衍生物氧化裂解成相应的羰基化合物,其分子中碳碳双键,氰基,酯基和缩醛基等基团均不受影响。同时探讨了可能的反应机理和反应溶剂对氧化裂解反应的影响。     

铱(Ⅲ)卟啉β-羟乙与基醛的碳氢键活化

铱(Ⅲ)卟啉β-羟乙基,Ir^Ⅲ(ttp)CH₂CH₂OH(ttp=5,10,15,20-二价阴离子tetratolylporphyrinato),发现选择性裂解芳醛的醛的碳-氢键和给铱酰基配合物。酰基铱配合物对羰基化和脱羰反应以及潜在的功能化研究的很好的候选材料。这碳-氢键的建议活化机理：通过初始的β-羟基消除Ir^Ⅲ(ttp)CH₂CH₂OH生成Ir^Ⅲ(ttp)OH,然后进行与醛的CH键的σ-键复分解发生。     

U原子和CO反应机理的第一性原理的理论研究

在有关实验结果的基础上提出了U原子和CO分子的各种可能反应通道, 然后采用第一性原理对反应通道上的各物种的几何构型、谐振频率以及总能量进行了计算和研究, 计算结果表明, 初级和次级反应的稳定产物分别为CUO和(η²-C₂)UO₂. 提出了最可能反应通道为U原子以C端或侧位进攻CO分子引起反应, 并用分子轨道理论解释了该反应机理.     

分子诱导效应指数与脂肪族醛酮的沸点

利用分子诱导效应指数，建立了三参数方法计算脂肪族醛酮沸点的关系式： ln(820．5—Tb)＝6．38327—1．23961×10^-1Nc+1．95353△I+6．68434×10^- 2N，式中Nc为脂肪族醛酮中烷基部分的有效碳链长度；△I为具有相同碳原子数目 的支链烷基与直链烷基的诱导效应指数的差值，它表示羟基对醛酮沸点的影响；N 为碳原子数．     

HAlMCM-41介孔分子筛催化1,3-苯并二噁茂烷合成的研究

研究了HAlMCM-41分子筛催化邻苯二酚与环己酮、丁酮、丙酮、丙醛、丁醛、异丁醛、戊醛、正己醛、正辛醛、苯甲醛、二苯甲酮等十余种醛(酮)的缩合反应. 考察了反应时间、酚与醛(酮)的配比、HAlMCM-41分子筛用量、硅铝比、催化剂重用次数等因素对邻苯二酚与醛(酮)反应的影响. 结果表明, 当邻苯二酚与醛(酮)物质的量比为1∶1.4, 催化剂用量为3.5 g/mol邻苯二酚, 反应4 h, 分子筛n(SiO₂)/n(Al₂O₃)为15时, 选择性一般在99.4%以上, 转化率也一般在85.6%以上, 因此, HAlMCM-41分子筛对邻苯二酚与醛(酮)的反应有较好的催化性能.     

双核三螺旋铁配合物的合成和结构

本文选择了2-羟基-1-萘醛与水合肼形成的席夫碱,[(HO)(C₁₀H₆)CH=N-N=CH(C₁₀H₆)(OH)],作为配体,设计组装了双核三螺旋的三价铁配合物。配合物中每一个铁离子以准八面体的配位方式分别与三个NO双齿单元配位, 三个配体分别桥联两个金属形成特定的三螺旋构型。分子内和分子间的π-π堆积作用对螺旋体的形成和堆积方式起着十分重要的作用。作为对照,本文还报道了配体的晶体结构。     

香豆素343敏化TiO2纳米粒子光致电子转移的荧光和拉曼光谱

通过对香豆素343(C343)染料敏化TiO₂纳米粒子光致电子转移的荧光和拉曼光谱特性的研究表明,C343染料敏化TiO₂纳米粒子稳态吸收光谱和稳态荧光光谱的红移归因于从被吸附的C343染料分子激发态和C343/TiO₂复合物到TiO₂纳米粒子导带的光致电子转移. 由时间分辨荧光光谱确定了C343染料敏化TiO₂纳米粒子的逆向电子转移速率常数为τ₁=31 ps. C343 染料敏化TiO₂纳米粒子体系拉曼光谱的研究表明, 被吸附在界面处的染料分子主链碳键的伸缩振动和碳环的呼吸运动的振动模式对超快界面光致电子转移有着重要的促进作用.     

高压下β-HMX热分解机理的ReaxFF反应分子动力学模拟

采用ReaxFF反应分子动力学方法研究了不同压缩态β-HMX晶体(ρ=1.89、2.11、2.22、2.46、2.80、3.20 g·cm^-3)在T=2500 K时的热分解机理, 分析了压力对初级和次级化学反应速率的影响、高压与低压下初始分解机理的区别以及造成反应机理发生变化的原因. 发现HMX的初始分解机理与压力(或密度)相关. 低压下(ρ<2.80 g·cm^-3)以分子内反应为主, 即N－NO₂键的断裂、HONO的生成以及分子主环的断裂(C－N键的断裂). 高压下(ρ≥2.80 g·cm^-3)分子内反应被显著地抑制, 而分子间反应得到促进, 生成了较多的O₂、HO等小分子和大分子团簇. 初始分解机理随压力的变化导致不同密度下的反应速率和势能也有所不同. 本文在原子水平对高压下HMX反应机理的深入研究对于认识含能材料在极端条件下的起爆、化学反应的发展以及爆轰等具有重要意义.     

钒(Ⅳ/Ⅴ)电对在碳纸电极上的反应机理研究

采用循环伏安、极化曲线和交流阻抗技术研究了在0.8 mol·L^-1 VOSO₄+3.0 mol·L^-1 H₂SO₄中,V(Ⅳ/Ⅴ)电对在碳纸电极上的反应机理及可能的速度控制步骤。研究结果表明：V(Ⅳ/Ⅴ)电对在碳纸电极上的反应属准可逆过程,且氧化过程包含有后置化学转化步骤,计算得到VO²⁺的扩散系数为4.5×10^-5 cm²·s相似文献   

Mechanistic studies on the Mukaiyama epoxidation

A detailed mechanistic study on the Mukaiyama epoxidation of limonene with dioxygen as oxidant, bis(acetylacetonato)nickel(II) as catalyst, and an aldehyde as co-reagent is reported. All major products of the reaction have been quantitatively identified, both with isobutyraldehyde and 2-methylundecanal as co-reacting aldehydes. Limonene epoxide is formed in good yield. The main products evolving from the aldehyde are carboxylic acid, CO(2), CO, and lower molecular weight ketone and alcohol (K + A). A mechanism is proposed in which an acylperoxy radical formed by the autoxidation of the aldehyde is the epoxidizing species. The observation of carbon dioxide and (K + A) in a 1:1 molar ratio supports this mechanism. CO(2) and (K + A) are formed in molar amounts of 50-60% with respect to the amount of epoxide produced, indicating that epoxidation takes place not only via acylperoxy radicals but also via a peracid route. Cyclohexene epoxidation was also investigated with a number of different metal complexes as catalysts. Cyclohexene is very sensitive for allylic oxidation, which provides information about the action of the catalyst, e.g., metals that form strongly oxidizing stable high-valence complexes are more likely to induce allylic oxidation. Color changes in the reaction mixture indicate the presence of such high-valence species. In the case of nickel, it was found that low-valence compounds predominate during the reaction, which is in line with the fact that this metal displays the highest selectivity for epoxide. A mechanism that accounts for the observations is presented.     

Mechanism of the Morita-Baylis-Hillman reaction: a computational investigation

Accurate calculations are presented on the mechanism of the MBH reaction, focusing on the reaction between methyl acrylate and benzaldehyde, catalyzed by a tertiary amine. We address the mechanism under protic solvent-free conditions, but also consider how the mechanism and rate-limiting step change in the presence of alcohols. We have carefully calibrated the DFT method used in the calculations by carrying out high-level G3MP2 calculations on a model system. All of our calculations also treat the effect of solvent, described as a dielectric continuum. In the absence of protic solvent, we predict that deprotonation of the alpha-position is the rate-determining step and occurs through a cyclic transition state, with proton transfer to a hemiacetal alkoxide formed by addition of a second equivalent of aldehyde to the intermediate alkoxide. As first suggested by McQuade, this mechanism explains the observed second-order kinetics with respect to aldehyde concentration in the absence of protic solvent. In contrast, in the presence of methanol, we find a slightly lower energy pathway, in which the alcohol serves as a shuttle to transfer the proton from carbon to oxygen. Overall, the barrier to reaction for the latter mechanism is of 24.6 kcal/mol with respect to reactants at the B3LYP level of theory. The relative energy for the addition transition state of the amine-acrylate betaine adduct to the aldehyde is much lower, at 16.0 kcal/mol relative to reactants, so C-C bond formation should not be rate-limiting, except perhaps for some aliphatic aldehydes or imines. We discuss the implications of this mechanism for the design of asymmetric versions of the MBH reaction, given the overwhelming importance of the proton-transfer step.     

Kinetics of the oxidation of aliphatic aldehydes by Os(VIII) in alkaline solution

Kinetics of oxidation of six aliphatic aldehydes by Os(VIII) in alkaline solutions have been studied. The reaction is of first order with respect to each of the aldehyde and Os(VIII). The pseudo-first order rate constants decreased with an increase in the concentration of hydroxyl ions. The oxidation of deuterioacetaldehyde (MeCDO) exhibited a substantial primary kinetic isotope effect. Separate rate constants for the oxidation of hydrate and free aldehyde forms have been evaluated. The aldehyde hydrate is postulated as the active reductant. Ionic strength has no noticable effect on the rate. The rate-determining step is, therefore, postulated to be a bimolecular reaction between the aldehyde hydrate and [OsO₄(OH)₂]^?2. The value of the limiting rate constant exhibited an excellent correlation with Taft σ* values; reaction constant being negative. A mechanism involving transfer of a hydride ion from the aldehyde hydrate to Os(VIII) has been proposed.     

Mechanistic investigation of the enantioselective intramolecular stetter reaction: proton transfer is the first irreversible step

A study on the mechanism of the asymmetric intramolecular Stetter reaction is reported. This investigation includes the determination of the rate law, kinetic isotope effects, and competition experiments. The reaction was found to be first order in aldehyde and azolium catalyst or free carbene. A primary kinetic isotope effect was found for the proton of the aldehyde. Taken together with a series of competition experiments, these results suggest that proton transfer from the tetrahedral intermediate formed upon nucleophilic attack of the carbene onto the aldehyde is the first irreversible step.     

Palladium-Nanoparticle-Catalysed Ullmann Reactions in Ionic Liquids with Aldehydes as the Reductants: Scope and Mechanism

An efficient Ullmann-type reductive homocoupling of aryl, vinyl and heteroaryl halides can be promoted by an aldehyde in tetraalkylammonium ionic liquids under very mild reaction conditions. This simple procedure generates symmetrical biaryls under relatively mild conditions. The ionic liquid is crucial for this process because it behaves simultaneously as a base, ligand and reaction medium. The role of the aldehyde is also discussed and a general mechanism for this unusual reaction is proposed. These results open the way to a new efficient method of Pd-catalysed dehydrogenation of carbonyl compounds.     

Kinetics and mechanism of cyclohexane oxidation with molecular oxygen in the presence of propionic aldehyde

The kinetics and mechanism of the liquid-phase oxidation of cyclohexane with molecular oxygen in the presence of the additives of propionic aldehyde are studied at 303.0, 322.5, and 341.5 K by measuring the rates of oxygen and propionic aldehyde consumption and the yields of the main reaction products (cyclohexanol (COL), cyclohexanone (CON), cyclohexyl hydroperoxide, and propionic acid and peracid). A kinetic scheme is proposed and rate constants of elementary reactions are estimated based on the analysis of their rates and the yields of the main cyclohexane products. The key reactions of the main steps (including chain initiation, propagation, and termination) are determined. An increase in the rate of cyclohexane oxidation and the yield of the target products (cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide) in the presence of propionic aldehyde suggests that highly active acylperoxy radicals participate in chain propagation. The [CON]/[COL] ratio indicates that these products are mainly formed in chain propagation. The strong effect of the Baeyer-Villiger rearrangement on both the rate of oxygen consumption and the yield of the target products at the initial stages of the process and at high propionic aldehyde concentrations is explained.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 28: Formal kinetics of aldehyde and carboxylic acid formation in the advanced stages

The rate of acid formation at high temperature is constantly increasing but temperature independent. Two main mechanisms can account for this behavior in the advanced stages of polyethylene processing. The first mechanism is based on free radical induced oxidation of aldehyde pairs that are formed on acid-catalyzed decomposition of allylic hydroperoxides. The last will be formed essentially on mechanical stress-induced oxygen addition to trans-vinylene groups. Peroxidation of one of the aldehydes might yield an acyl-peroxy radical that is likely to abstract the labile hydrogen atom from the second aldehyde. The acyl radical formed in the reaction will abstract a hydroxyl group from the peracid formed in the same reaction. This yields an acid and an acyl-oxy radical that will give a primary alkyl radical on decarboxylation. The second mechanism involves oxidation of ketones and alcohols that accumulate in the oxidizing melt. Acid-catalyzed decomposition of the α-keto-hydroperoxides yields simultaneously an acid and an aldehyde. Formal kinetics based on each mechanism shows that they do not involve significant activation energy, as it is required by the experimental data. The dependency on the oxygen concentration deduced from the formal kinetics for the oxidation of aldehyde pairs is in agreement with the experiments.     

Bu3P-CS2加合物与含不饱和键化合物的环化反应

Bu₃P-CS₂加合物与含不饱和键化合物(富电子炔类、磷酰基炔类、磷酸基烯类)以及醛进行一锅反应以较好的收率得到1,3-二硫环戊烯或1,3-二硫环戊烷衍生物.Bu₃P-CS₂加合物与偶氮化合杨和醛类进行类似反应却得到四氢噻二唑硫酮衍生物.对反应机理进行了讨论.     

Greatly Enhanced Fluorescence by Increasing the Structural Rigidity of an Imine: Enantioselective Recognition of 1,2‐Cyclohexanediamine by a Chiral Aldehyde

An aldehyde that is not fluorescent responsive toward a chiral diamine has been converted to a sensitive fluorescence enhancement sensor through incorporation of an additional hydrogen bonding unit to increase the structural rigidity of the reaction product of the aldehyde with the diamine. This new chiral aldehyde is synthesized in one step from the reaction of (S)‐3‐formylBINOL with salicyl chloride. When treated with trans‐1,2‐cyclohexanediamine in ethanol, it shows greatly enhanced fluorescence at λ=410 nm with good enantioselectivity. NMR and mass spectroscopic methods are used to investigate the reaction of the chiral aldehyde with the diamine. This study has revealed a two‐stage reaction mechanism including a fast imine formation and a slow ester cleavage.     

龙葵醛的新合成方法研究

龙葵醛是一种珍贵的香料 ,也是重要的化工原料 ,广泛应用于香料、医药、染料及农药等行业 .龙葵醛的工业生产以苯乙酮和一氯醋酸乙酯为原料 ,按 Darzens法合成[1,2 ] .苯乙酮与一氯醋酸乙酯在强碱作用下制得甲基苯基环氧丙酸酯 ,该酯经皂化、中和、水解成酸 ,再将该酸加热分解 ,得龙葵醛 .该方法操作比较复杂 ,产率和产品纯度也不高 .为了寻找产率高、纯度好且成本低的新制备方法已有广泛的研究 .Rivero等 [3]提出的 2 -甲基苯乙醇氧化法 ,尽管产率较高 ,但反应温度要在 -78℃ ,不利于工业化生产 .陈万之等[4 ] 利用各种铑螯合物作为催化剂…