需氧铜催化氮自由基4-exo-trig环化实现[1,3]-氮迁移

正Angew.Chem.Int.Ed.2017,56,15436~15440碳自由基串联环化反应被广泛地应用于多环结构的构建,然而氮自由基串联环化反应的研究却由于其引发方式缺乏以及稳定性方面的原因,其发展相对缓慢.另一方面,相对于自由基的五元及六元环合过程,经历四元环途径的自由基环合反应却难以进行,这主要是由于四元环的环张力造成其逆反应——自由基开环反应的速率更快.目前     

碳苷合成研究——3-β-D-吡喃木糖基-1,2,4-氧杂二唑类化合物的合成

本文研究了1-(2,3,4-三-O-苯甲酰-β-D-吡喃木糖基)-偕氨基肟与酸酐及酰氯的缩合反应,探讨了不同缩合剂、不同取代基对环合反应的影响,与酸酐反应可以一步完成,且收率较好;与酰氯反应分两步进行,第一步酰化,第二步脱水环合,芳环上取代基的电性效应,决定环合反应的难易,并证明了该环系的碳苷对酸、碱、热具有化学稳定性,本文合成了12个氧杂二唑类木糖碳苷及2个木糖基化的开环产物,通过光谱及元素分析,确定了它们的结构。     

热动力学方法的研究Ⅰ. 反应速率常数、活化能的量热法测定

用流动型微量量热法测定了在不同温度下乙酸和甲醇及乙醇的酯化反应的速率常数, 并籍助于阿累尼鸟斯公式测定了这些反应的活化能。为了检验此方法的可靠性, 测定了乙酸甲酯和乙酸乙酯水解反应(酯化反应的逆反应)的速率常数, 这一实验结果与根据酯化反应实验数据计算得到的结果相一致。     

吲哚生物碱(+)-Alsmaphorazine D的不对称全合成及其绝对构型更正

通过无痕手性转移策略实现了(+)-alsmaphorazine D的首次不对称全合成, 并对其绝对构型进行了更正. 该高效合成路径的关键步骤主要包括催化氧化环合反应、非对映选择性氧化缩醛胺化环合反应以及分子内自由基环合反应.     

自由基环化制备β-内酰胺的理论研究

使用密度泛函方法在UB3LYP/6-311＋＋G(3df, 2p)水平上对自由基环化合成β-内酰胺的四种反应途径进行理论研究. 结合Marcus理论对影响反应的热力学及动力学因素进行分析, 发现氨基甲酰基自由基4-exo环合反应是理想的动力学控制过程; 酰胺自由基的4-exo环合反应与5-endo环合反应相比是动力学有利的转化过程; 单取代的酰胺烷基自由基的4-exo环合反应是一类动力学和热力学都较为不利的反应; 羰基自由基加成亚胺N＝C双键的4-exo环合反应与5-endo环合反应相比动力学不利而热力学有利.     

2,3-二芳基马来酸酐环合反应机理的计算研究

在B₃LYP/6-311+G(d,p)//HF/3-21G水平上研究了2,3-二芳基马来酸酐环合反应机理.2,3-二芳基马来酸酐环合反应有两条反应途径:(1)反应过程中涉及的各种构型始终保持反式,即中心成环碳原子上的取代基团甲基或氢原子位于分子平面的异侧,这些构型具有C₂对称性;(2)反应过程中涉及的各种构型始终保持顺式,即中心成环碳原子上的取代基团甲基或氢原子位于分子平面的同一侧面,这些构型可具有Cs对称性.二芳基乙烯类化合物环合反应的位垒计算结果与Woodward-Hoffman原理一致,计算结果支持在环合碳原子上引入甲基能改善反应的热不可逆反应性能的实验结果.电荷分析表明,在环合碳原子上引入甲基,能增加二芳基乙烯类化合物体系对氧的稳定性,与实验结果一致.2,3-二芳基马来酸酐化合物的闭环结构的LUMO和HOMO能隙小于开环结构的能隙,说明闭环结构的最大吸收波长大于开环结构的最大吸收波长,与实验结果一致.     

表征固体超强酸性的新方法--正丁烷异构化反应的原位13C MAS NMR谱

室温下SO2 -4/ZrO2 催化剂 (SZ)上13 C标记的正丁烷异构化反应的原位13 CMASNMR谱研究结果表明 :其反应动力学符合Langmuir Hinshelwood一级可逆表面反应动力学公式 ,由该动力学公式计算得到的反应速率常数可以用于衡量固体催化剂的表面超强酸性 .这种新的表征方法显示采用一步 -醇热 -超临界干燥综合技术合成的SZ催化剂不仅比表面和硫酸根含量高 ,而且其超强酸性和异构化反应活性均明显优于常规法合成的催化剂 ,具有良好的应用前景 .     

表征固体超强酸性的新方法-正丁烷异构化反应的原位~(13)CMASNMR谱

室温下SO_4~(2-)/ZrO_2催化剂（SZ）上~(13)C标记的丁烷异构化反应的原位 ~(13)C MAS NMR谱研究结果表明：其反应动力学符合Langmuir-Hinshelwood一级可 逆表面反应动力学公式，由该动力学公式计算得到的反应速率常数可以用于衡量固 体催化剂的表面超强酸性。这种新的表征方法显示采用一步-醇热-超临界干燥综合 技术合成的SZ催化剂不仅比表面和硫酸根含量高，而且其超强酸性和异构化反应活 性均明显优于常规法合成的催化剂，具有良好的应用前景。     

化学反应速率的微观理论

本文针对反应位能面上存在低洼的普遍情况(当低洼深度为零时,便退化为Eyrjng所讨论的无低洼情况),用求解含时薛定格方程的方法导得具有高的能量分辨率的反应的微观速率常数公式;在反应物可用玻尔兹曼分布描写的假定下,导得与Eyring公式加形式类似的反应的宏观速率常数公式,给出穿透系数的表示式。在位能曲面无低洼并且穿透系数可视为与温度关系不大时,本文公式简化为Eyring公式,因而Eyring公式是本文的特例。     

固相物质热分解反应中活化能异常问题的研究

用热分析法研究固相物质分解反应时,往往因升温速率不同而得到不同的反应活化能。这一异常现象的本质原因在于反应本身的复杂性导致反应活化能服从非均匀分布。本文以此为前提,提出了一种解析热分析实验曲线,计算反应活化能的方法。其特点是将反应活化能视为反应转化率的函数,利用不同升温速率下的多条实验曲线,借助严格的理论处理进行计算,同时建立了反应活化能与反应转化率的定量关系式。文中以固体碳酸钙粉末的分解反应为例,对提出的方法进行了讨论。     

硫化环氧天然橡胶的热分析

环氧化天然橡胶;热重分析;动力学;硫化环氧天然橡胶的热分析     

正丁基锂、四氢呋喃引发的丁苯共聚合(Ⅱ)——动力学及活性种的反应性

本文提出了n·BuLi-THF引发的阴离子丁苯共聚体系中诸活性种之间的平衡及增长图式,导出了共聚组成方程,讨论了单体的表观竞聚率的含义。此外,还建立了表观增长速度常数k_B和kS与[THF]的关系式,估算出一络合活性种对形成聚丁二烯链节中乙烯基结构的几率。     

A Short Sequence for the Iterative Synthesis of Fused Polyethers

A simple and efficient four‐step sequence for the synthesis of fused polyether arrays has been developed. Cyclic ethers are installed by sequential alkynyl ether formation, carbocupration, ring‐closing metathesis and hydroboration with acidic workup. Crucially, the alkene required for the subsequent ring formation by ring‐closing metathesis is present in the substrate but is masked in the form of a vinylic silane, which prevents competitive metathesis of the side chain. Generation of the reactive alkene from the unreactive vinylic silane is accomplished by hydroboration and subsequent acid‐mediated Peterson elimination of the intermediate hydroxysilane.     

Regio- and stereo-selectivity of alkenyl radical ring closure: A theoretical study

A theoretical study has been undertaken of the relative rates and the regio- and stereo-chemistry of ring closure of a variety of alkenyl, alkenylaryl, alkenylvinyl and similar radicals. The method involves the application of MM2 force-field calculations to model transition structures for which the dimensions of the arrays of reactive centres have been obtained by MNDO-UHF techniques. The results, which generally accord with guidelines based on stereochemical considerations, show excellent qualitative and satisfactory quantitative agreement with experimental data. The method has been successfully applied to complex systems including ring closure of alkylperoxy radicals, and formation of the triquinane system by three consecutive cyclisations.     

Formation of reactive o-quinone methides from the reaction of trimethylsilyl(methyl)-substituted 1,4-benzoquinones with nucleophiles

o-Quinone methides are formed from the reaction of nucleophiles with trimethylsilyl(methyl)-1,4-benzoquinones. These reactive intermediates are trapped by excess nucleophile to form substituted quinones following oxidation. In addition, varying amounts of a symmetrical dimer and a xanthen derivative were observed. The influence of different nucleophiles and ring substituents on the rate of reaction have been studied, and are consistent with rate-limiting formation of a vinylogous enolate initiated by attack of the nucleophile on the silyl group.     

Amino substituted bisketenes: generation, structure, and reactivity

Hitherto unknown diamino-substituted bisketenes with both free (14) and tethered (16) amino substituents have been generated by using laser flash photolysis for ring opening of the corresponding cyclobutenediones. The time-resolved kinetics of ring closure of the amino bisketenes back to the cyclobutenediones were measured by IR or UV spectroscopy, and give first-order rate constants which vary by a factor of 7.5x10(4), and the bis(Me2N) bisketene 14 is the most reactive in ring closure that has been reported. Rate constants for ring closure of these and previously observed bisketenes vary by a factor of 10(13). The dialkylamino bisketenes 16 (R=Me, n-Bu) with tethered substituents and restricted geometries are less reactive than the bis(Me2N) bisketene 14 by factors of 1700 and 540, respectively. Computational results obtained with DFT methods suggest angle strain in the tethered cyclobutenediones 15 inhibits facile cyclization of bisketenes 16.     

The initiation of the free radical polymerization of vinyl monomers by hexakis(arylisocyano) derivatives of chromium (o)

The present paper describes studies carried out to determine the effect of structure on the free radical polymerization of vinyl monomers initiated by means of the hexakis(arylisocyano) derivatives of chromium (o) in which chlorine, methoxy, and methyl groups have been substituted into different positions of the benzene ring. The kinetics of the initiation reactions resembles that studied in detail for the parent compound hexakis(phenylisocyano) chromium (o). The observed changes in the initial rates of the polymerization of methyl methacrylate at 80°C in the presence of carbon tetrachloride can be accounted for by the electronic effects associated with these substituents. However, when steric overcrowding occurs, the initial rates of polymerization are reduced considerably. The results obtained confirm the mechanism of initiation proposed earlier in which the rate determining step is the S_n² displacement of an arylisocyano ligand by a monomer or reactive solvent molecule. An attempt has been made to explain the increased rate of radical formation observed when chlorine is substituted into the benzene ring.     

Fluorine reactivity in difluoromethylimidazoles

A difluoromethyl substituent attached directly to an imidazole ring is very reactive toward basic hydrolysis. A correlation of rate of fluoride loss with increasing pH is consistent with a mechanism that involves initial ionization of the imidazole NH, formation of an intermediate azafulvene by loss of HF, and reaction of the intermediate with solvent water.     

Exclusive formation of alpha-methyleneoxetanes in ketene-alkene cycloadditions. Evidence for intervention of both an alpha-methyleneoxetane and the subsequent 1,4-zwitterion

This paper describes a new mechanistic feature for the Staudinger ketene-alkene cycloaddition reactions to give cyclobutanones. Low-temperature NMR (13C, 19F, and 1H) monitoring of a reaction between bis(trifluoromethyl)ketene (1) and ethyl vinyl ether (2) has shown that the Staudinger reaction proceeds to form initially and exclusively an alpha-methyleneoxetane (3) by [2 + 2](C=O) cycloaddition across the ketene C=O bond. The initial intermediate 3 undergoes ring cleavage to produce a 1,4-zwitterion (4), which is converted to the final [2 + 2](C=C)-type product, cyclobutanone (5). The key intermediate 3 has been isolated in its pure form and was found to be converted to the final products 5 on warming, via the 1,4-zwitterion 4. The alpha-methyleneoxetane 3 is so reactive that it reacts with methanol rapidly even at -80 degrees C via solvolysis to afford an adduct 7. The ion 4 derived from the pure isolated oxetane 3 was intercepted with acetone by a 1,4-dipolar cycloaddition to give a 1,3-dioxane 8. An open-chain alpha,beta-enone (6) has been also obtained from 3. We conclude that the (1 + 2) reaction proceeds in a new three-step mechanism; formation of an alpha-methyleneoxetane 3, a [2 + 2]-type cycloadduct across the C=O bond of ketene, followed by ring cleavage to give the zwitterion 4 and by recombination to form the final product, cyclobutanone 5. The zwitterion 4 is not equilibrating with reactants 1 and 2 but comes from the alpha-methyleneoxetane 3. Exclusive formation of another oxetane 12 has been observed in a reaction between diphenylketene (9) and methyl isopropenyl ether (11). The selectivity of initial formation of cyclobutanone or oxetane has been generalized with aid of frontier-orbital theory and ab initio calculations.     

Intrinsic gas-phase electrophilic reactivity of cyclic N-alkyl- and N-acyliminium ions

The intrinsic gas-phase reactivity of cyclic N-alkyl- and N-acyliminium ions toward addition of allyltrimethylsilane (ATMS) has been compared using MS(2) and MS(3) pentaquadrupole mass spectrometric experiments. An order of electrophilic reactivity has been derived and found to agree with orders of overall reactivity in solution. The prototype five-membered ring N-alkyliminium ion 1a and its N-CH(3) analogue 1b, as well as their six-membered ring analogues 1c and 1d, lack N-acyl activation and they are, accordingly, inert toward ATMS addition. The five- and six-membered ring N-acyliminium ions with N-COCH(3) exocycclic groups, 3a and 3b, respectively, are also not very reactive. The N-acyliminium ions 2a and 2c, with s-trans locked endocyclic N-carbonyl groups, are the most reactive followed closely by 3c and 3d with exocyclic (and unlocked) N-CO(2)CH(3) groups. The five-membered ring N-acyliminium ions are more reactive than their six-membered ring analogues, that is: 2a > 2c and 3c > 3d. In contrast with the high reactivity of 2a, its N-CH(3) analogue 2b is inert toward ATMS addition. For the first time, the transient intermediates of a Mannich-type condensation reaction were isolated-the beta-silyl cations formed by ATMS addition to N-acyliminium ions-and their intrinsic gas-phase behavior toward dissociation and reaction with a nucleophile investigated. When collisionally activated, the beta-silyl cations dissociate preferentially by Grob fragmentation, that is, by retro-addition. With pyridine, they react competitively and to variable extents by proton transfer and by trimethylsilylium ion abstraction-the final and key step postulated for alpha-amidoalkylation. Becke3LYP/6-311G(d,p) reaction energetics, charge densities on the electrophilic C-2 site, and AM1 LUMO energies have been used to rationalize the order of intrinsic gas-phase electrophilic reactivity of cyclic iminium and N-acyliminium ions.