Highly Efficient Supramolecular Catalysis by Endowing the Reaction Intermediate with Adaptive Reactivity

A new strategy of highly efficient supramolecular catalysis is developed by endowing the reaction intermediate with adaptive reactivity. The supramolecular catalyst, prepared by host–guest complexation between 2,2,6,6‐tetramethylpiperidin‐1‐oxyl (TEMPO) and cucurbit[7]uril (CB[7]), was used for biphasic oxidation of alcohols. Cationic TEMPO⁺, the key intermediate, was stabilized by the electrostatic effect of CB[7] in aqueous phase, thus promoting the formation of TEMPO⁺ and inhibiting side reactions. Moreover, through the migration into the organic phase, TEMPO⁺ was separated from CB[7] and recovered the high reactivity to drive a fast oxidation of substrates. The adaptive reactivity of TEMPO⁺ induced an integral optimization of the catalytic cycle and greatly improved the conversion of the reaction. This work highlights the unique advantages of dynamic noncovalent interactions on modulating the activity of reaction intermediates, which may open new horizons for supramolecular catalysis.     

探析反应机理与理解反应条件——中英高中化学“烯烃与溴的加成反应”内容比较

有机化学模块是高中化学的重要组成部分,了解有机化学反应机理有利于从微观角度探析物质的结构与性质。目前国内各版本高中化学教材对烯烃与溴加成反应的描述过于简洁。参考了英国高中化学教材和我国的《基础有机化学》,综合分析不同教材的特点,以期更合理地讲解加成反应过程。     

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide,an Isoprene-Derived Criegee Intermediate,with Formic Acid: Experiment and Theory

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO₂ upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.     

Organic chemistry on cold molecular films: kinetic stabilization of SN1 and SN2 intermediates in the reactions of ethanol and 2-methylpropan-2-ol with hydrogen bromide

We prepared thin molecular films of ethanol and 2-methylpropan-2-ol on Ru(001) substrates at temperature of 100-150 K and examined their reactivity toward HBr. The reaction intermediates and products formed at the surfaces were unambiguously identified by the techniques of Cs(+) reactive ion scattering (RIS) and low-energy sputtering. The reaction on the ethanol surface produced protonated ethanol, which is stabilized on the surface and does not proceed to further reactions. On the 2-methylpropan-2-ol surface, protonated alcohol [(CH(3))(3)COH(2) (+)] and carbocation [(CH(3))(3)C(+)] were formed with the respective yield of 20 and 78 %. Alkyl bromides, which are the final products of the corresponding reactions in liquid solvents, have extremely small yields on these surfaces (< 0.3 % for ethyl bromide and 2 % for tert-butyl bromide). The results indicate that the reactions on frozen films are characterized by kinetic control, stabilization of ionic intermediates (protonated alcohols and tert-butyl cation), and effective blocking of the charge recombination steps in S(N)1 and S(N)2 paths. The implication of these findings for the molecular evolution process in interstellar medium is also discussed.     

以反应机理为主线的芳烃亲电取代反应教学设计

在缩减大学课时而不降低教学效果的背景下,如何保证有机化学的教学质量是新时期亟需研究和解决的课题之一。在大学有机化学知识中,不同反应具备相似的反应机理的例子很多,如Friedel-Crafts（烷基化、酰基化）反应、卤化反应、磺化反应、硝化反应、Hoesch反应等都具有相似的亲电取代机理,论文展示了以反应机理为抓手,可运用较少课时讲授的该部分内容的教学设计。同时总结了芳烃亲电取代反应知识的学习方法。     

含分子内氢键的苯硫酚在水相中的Michael加成反应

苯硫酚的巯基是一个较强的亲核基团,对α,β-不饱和双键的Michael加成反应已有报道．但是当巯基的邻位存在与其形成氢键的基团时,如果按文献条件进行反应,或者反应无法进行,或者要使用反应条件苛刻的丁基锂,或者使用吡啶等毒性较大的反应溶剂,而且产率较低．本文选择     

硅炔氢加成生成甲硅烷基硅烯的量子化学研究

利用量子化学从头算和密度泛函理论(DFT), 研究了硅炔和氢气分子加成生成甲硅烷基硅烯的反应机理. 在B3LYP/6-311G**水平上, 全参数优化了反应通道上各驻点(反应物、过渡态和产物)的几何构型, 计算出了它们的振动频率和零点振动能(ZPVE), 并对它们进行了振动分析, 以确定过渡态的真实性. 各物质总能量由QCISD(T)/6-311G**// B3LYP/6-311G**给出, 并对能量进行了零点能校正. 计算表明, 硅炔与氢分子加成反应可生成稳定的甲硅烷基硅烯. 热力学与动力学计算表明, 反应过程是一个放热、熵减少而自发趋势和反应程度较大的反应.     

Metal‐Promoted Nucleophilic Addition and Cyclization of Diamines with Dicyanonitrosomethanide, [C(CN)2(NO)]−

Additional cyclization : Dicyanonitrosomethanide, [C(CN)₂(NO)]^? undergoes nucleophilic addition and cyclization of 1,2‐diaminoethane and 1,3‐diaminopropane on the nitrile groups to form imidazolinyl and 1,4,5,6‐tetrahydropyrimidinyl groups, respectively. Ethanolamine has lower reactivity and fails to cyclize.

     

The Addition of Nitriles to a Molecular Digermene: Reversible Addition and Comparison to Surface Reactivity

The addition of acetonitrile, propionitrile, and acrylonitrile to tetramesityldigermene was investigated and compared to the addition of acetonitrile and acrylonitrile to germanium dimers on the Ge(100)‐2×1 surface. In each case, a 1,2,3‐azadigermetine was formed as the major product. As on the surface, the addition of nitriles to digermenes was found to be reversible, providing the first example of a reversible cycloaddition of a ditetrelene. No evidence for a six‐membered cyclic ketenimine was observed as noted in the surface chemistry, suggesting that the surface ketenimine might only form between two adjacent dimers rather than on a single dimer. The comparative chemistry provides important insights that are not possible by the independent study of each system.     

Ti-MWW催化氯丙烯环氧化反应动力学行为

以Ti-MWW分子筛为催化剂,以H2O2为氧化剂,系统研究了氯丙烯环氧化反应的动力学行为.结果表明,该反应速率与Ti-MWW分子筛的用量成正比,是1级反应.当H2O2浓度小于0.67 mol/L时,环氧化反应为1级反应;大于2 mol/L时,为0级反应.随着氯丙烯浓度的增加,环氧化反应级数从1级向0级转变;且只有当其浓...     

Addition of Isocyanides to Tetramesityldigermene: A Comparison of the Reactivity between Surface and Molecular Digermenes

The reaction of benzyl isocyanide, tert‐butyl isocyanide, and 2,6‐dimethylphenyl isocyanide with tetramesityldigermene (Mes₂Ge=GeMes₂) was examined. Whereas the addition of benzyl isocyanide gave the C?NC activation product, Mes₂Ge(CH₂Ph)Ge(CN)Mes₂, tert‐butyl isocyanide, and 2,6‐dimethylphenyl isocyanide did not give stable adducts, rather the rate of conversion of the digermene to the corresponding cyclotrigermane was accelerated. A comparison between the reactivity of the isocyanides with Mes₂Ge=GeMes₂ and the Ge(100)‐2×1 surface was made and some insights into the surface chemistry are offered.     

用卤代乙酸酯与酰卤的格氏偶合反应合成新戊酰乙酸乙酯

卤代乙酸酯与Mg反应形成格氏试剂后,易与酯基发生自身偶合,格氏试剂难以分离,因而很少应用于合成。M0riwake报道用卤代乙酸的叔丁酯代替乙酯,并与酮一起加入到Mg/     

钛硅分子筛TS-1催化氯丙烯环氧化反应动力学研究

 摘要：研究了钛硅分子筛催化氯丙烯环氧化反应的条件及动力学行为．\r\n结果表明，以钛硅分子筛为催化剂，氯丙烯可被高选择性地氧化为环氧\r\n氯丙烷．环氧化反应速度与分子筛中骨架钛的含量及分子筛的用量呈正\r\n比关系，是一级反应．对于氧化剂H2O2，只有当c（H2O2）＜0．4mol／\r\nL时，环氧化反应为一级反应；而c（H2O2）＞1．0mol／L时，为零级反\r\n应．对于氯丙烯，随着其浓度的变化，环氧化反应的级数在1和0之间．\r\n然而，只有当氯丙烯浓度很高时，环氧化反应的级数才有明显的降低．\r\n根据实验结果和Eley－Rideal单分子吸附方程，提出了氯丙烯环氧化反\r\n应的动力学模型．\r\n关键词：钛硅分子筛，氯丙烯，过氧化氢，环氧化，环氧氯丙烷，反应\r\n动力学     

表面金属有机化学:SnMe4在HY沸石超笼表面的接枝反应

研究了高真空条件下SnMe4在HY沸石超笼的接枝反应,并用元素分析,ICP, GC, XRD, FTIR, DTG, DTA, UV-vis, DRS, N2吸附等方法对产物的组成、结构和性质进行了详细表征.研究结果表明,两者可以定量地、有选择性地进行化学反应,将确定数目的三甲基锡基团接枝在沸石的超笼中.反应可以在非常低的温度下快速进行,表观活化能为10.4 kJ·mol-1.经SnMe4改性后的HY沸石分子筛BET比表面积降低,孔体积变小,对不同尺寸的烃分子表现出明显的吸附择形性.     

Time‐Resolved Crystallography of the Reaction Intermediate of Nitrile Hydratase: Revealing a Role for the Cysteinesulfenic Acid Ligand as a Catalytic Nucleophile

The reaction mechanism of nitrile hydratase (NHase) was investigated using time‐resolved crystallography of the mutant NHase, in which βArg56, strictly conserved and hydrogen bonded to the two post‐translationally oxidized cysteine ligands, was replaced by lysine, and pivalonitrile was the substrate. The crystal structures of the reaction intermediates were determined at high resolution (1.2–1.3 Å). In combination with FTIR analyses of NHase following hydration in H₂¹⁸O, we propose that the metal‐coordinated substrate is nucleophilically attacked by the O(SO^?) atom of αCys114‐SO^?, followed by nucleophilic attack of the S(SO^?) atom by a βArg56‐activated water molecule to release the product amide and regenerate αCys114‐SO^?.     

Reaction of alkenes with hydrogen peroxide and sodium iodide: a nonenzymatic biogenic-like approach to iodohydrins

An efficient protocol to synthesize iodohydrins from alkenes is presented. Reactions were conducted in aqueous media using safe and readily available sodium iodide (the most abundant form of the element), and a highly convenient oxidant such as hydrogen peroxide. Addition of a protic acid triggers a faster and efficient process, a role formally related to that played by haloperoxidase enzymes in naturally occurring transformations. The successful application of these conditions to multigram scale preparations and over natural products derivatives is also discussed.     

四甲基锡在MCM-41分子筛表面的接枝反应及产物性能

研究了真空条件下SnMe4在MCM-41分子筛表面的接枝反应，并用元素分析、ICP、GC-MS、XRD、FTIR、DRS、13C及119Sn MAS NMR、XPS、BET、TPD、TPR等方法对产物的组成、结构和性质进行了表征.结果表明, 两者可以定量地进行化学反应，将确定数目的甲基锡基团接枝在分子筛表面, 形成SnMe3/MCM-41物种；接枝反应发生在分子筛表面上, 不破坏MCM-41分子筛骨架结构；改性分子筛的BET比表面积有所降低，孔体积变小, 表面性质发生变化.四甲基锡在MCM-41上接枝反应的温度为343 K,比它与HY沸石的反应温度(193 K)高得多，并且产物SnMe3/MCM-41的热稳定性也高于SnMe3/HY.