Intermediacy of silylene and germylene in direct synthesis of organosilanes and organogermanes

In the direct synthesis of silicon compounds by reactions of elemental silicon with methyl chloride, methanol and hydrogen chloride, silylene formed on surface of silicon grains during the reaction is an intermediate. The reaction of surface silylene with a variety of unsaturated hydrocarbons provides new direct synthesis of organosilanes. In the direct synthesis of methylchlorogermanes from elemental germanium, surface germylene is not an intermediate, while tetrachlorogermane is synthesized by the direct reaction of germanium with hydrogen chloride via dichlorogermylene intermediate. Various unsaturated hydrocarbons or organic chlorides added to the system of tetrachlorogermane synthesis give new methods for the synthesis of organogermanes.     

The direct synthesis of methyldichlorosilane and dimethylchlorosilane

The synthesis of methylchlorosilanes with a siliconhydrogen bond, based on the reaction of silicon and methyl chloride with copper as a catalyst, has been investigated at a temperature of 332°C and a pressure of 1 atmosphere. By adding hydrogen to the gas phase, an overall selectivity of methyldichlorosilane and dimethylchlorosilane of over 80 mol% has been achieved together with a small quantity of by-products. The action of hydrogen consists of a reaction with the CuCl reaction intermediates; reaction of hydrogen with CuCl and silicon or with CuCl and chemisorbed methyl chloride also takes place. Metal chlorides such as CdCl₂ and ZnCl₂, which usually are promoters in the synthesis of methylchlorosilanes, do not promote the formation of the hydrogen-containing products.     

Intermediacy of silylene and germylene in direct synthesis of organosilanes and organogermanes

In the direct synthesis of silicon compounds by reactions of elemental silicon with methyl chloride, methanol and hydrogen chloride, silylene formed on surface of silicon grains during the reaction is an intermediate. The reaction of surface silylene with a variety of unsaturated hydrocarbons provides new direct synthesis of organosilanes. In the direct synthesis of methylchlorogermanes from elemental germanium, surface germylene is not an intermediate, while tetrachlorogermane is synthesized by the direct reaction of germanium with hydrogen chloride via dichlorogermylene intermediate. Various unsaturated hydrocarbons or organic chlorides added to the system of tetrachlorogermane synthesis give new methods for the synthesis of organogermanes.     

次甲基硅SiH与HF反应的热力学及动力学性质研究

在量子化学对SiH与HF反应计算的基础上,运用统计热力学和Wigner校正的Eyring过渡态理论计算了该反应在200～2000K温度范围内的热力学函数、平衡常数、频率因子A和速率常数随温度的变化。计算结果表明该反应在低温下具有热力学优势,而在高温下具有动力学优势。该反应在研究的温度范围内是一放热、熵减少的反应,反应的速率常数随温度的升高而增大,且服从Arrhenius定律。     

Theoretical Analysis of the Heterocyclic [4+2] Cycloaddition Between Pyridinium Ion and Enol Ether

Dearomative heterocyclic [4+2] cycloaddition between the N-(2,4-dinitrophenyl)pyridinium ion of nicotinamide and an enol ether was analyzed by Density Functional Theory (DFT) calculations. The calculation revealed that the reaction undergoes stepwise bond formation rather than occurring in a concerted manner. The experimental products were found to be both kinetically and thermodynamically favored. The calculated transition states and intermediate suggested that the high diastereoselectivity is derived from the electrostatic interaction between the 2-nitro group of the pyridinium ion and the hydrogen of the enol ether.     

单重态CCl~2与O~3反应的热力学及动力学性质研究

在量化计算的基础上运用统计热力学和Wigner校正的Eyring过渡态理论研究了不同温度下单重态CCl~2和臭氧O~3反应的热力学及动力学性质。计算结果表明该反应在低温下具有热力学优势,而在高温下具有动力学优势。     

Methane formation route in the conversion of methanol to hydrocarbons

The influence factors and paths of methane formation during methanol to hydrocarbons （MTH） reaction were studied experimentally and thermodynamically. The fixed-bed reaction results show that the formation of methane was favored by not only high temperature, but also high feed velocity, low pressure, as well as weak acid sites dominated on deactivated catalyst. The thermodynamic analysis results indicate that methane would be formed via the decomposition reactions of methanol and DME, and the hydrogenolysis reactions of methanol and DME. The decomposition reactions are thermal chemistry processes and easily occurred at high temperature. However, they are influenced by catalyst and reaction conditions through DME intermediate. By contrast, the hydrogenolysis reactions belong to catalytic processes. Parallel experiments suggest that, in real MTH reactions, the hydrogenolysis reactions should be mainly enabled by surface active H atom which might come from hydrogen transfer reactions such as aromatization. But H2 will be involved if the catalyst has active components like NiO.     

微结构的纳米设计膜反应器中的催化

For thermodynamically and kinetically controlled catalytic reactions,the influence of a membrane is discussed. For reactions operating near to the thermodynamic equilibrium,the conversion can be increased if one/all of the products is/are selectively removed in an extractor type membrane reactor. Examples are esterifications,dehydrogenations,and water dissociation using water,hydrogen,and oxygen selective membranes,respectively. For kinetically controlled reactions,i.e. reactions with a very negative free enthalpy,mainly the selectivity can be increased via the control of the partial pressure of the educts by dosing effects using distributor/contactor membrane reactors. Examples are partial oxidations and hydrogenations. In detail,the application of an oxygen transporting perovskite hollow fiber membrane with a nano-designed grain boundary structure in the hydrogen production by thermal water splitting and in the partial oxidation of hydrocarbons as case studies for thermodynamically and kinetically controlled reactions is discussed.     

Understanding selectivity in the oxidative addition of the carbon-sulfur bonds of 2-cyanothiophene to Pt(0)

The reaction of 2-cyanothiophene with a zerovalent platinum bisalkylphosphine fragment yields two thiaplatinacycles derived from the cleavage of the substituted and unsubstituted C-S bonds. While cleavage away from the cyano group is preferred kinetically, cleavage adjacent to the cyano group is preferred thermodynamically. Density functional theory using B3LYP level of theory on a model of this system is consistent with experimental results in that the transition state energy leading to the formation of the kinetically favored C-S bond cleavage product is lower by 5.3 kcal mol(-1) than the barrier leading to the thermodynamically favored product. There is a 6.7 kcal mol(-1) difference between these two products. The cyano substituent at the 2- position of thiophene did not substantially change the mechanism involved in the C-S bond cleavage of thiophene previously reported.     

NMR and calculational studies on the regioselective lithiation of 1-methoxynaphthalene

1-Methoxynaphthalene (1) undergoes regioselective lithiation in position 2 (n-BuLi/TMEDA) or in position 8 (t-BuLi), respectively. The detected formation of a n-BuLi/1 complex (1:1 n-BuLi/1 mixture) appears to have only minor influence on the regioselectivity (both products are obtained). The exchange of hydrogen atom H2 by deuterium results in a remarkably reduced reaction rate for the lithiation with n-BuLi in THF-d(8). This isotope effect and the formation of the thermodynamically less favorable 2-lithio compound suggest a kinetically controlled mechanism. The lack of an isotope effect for the reaction of 8-deuterio-1-methoxynaphthalene with t-BuLi and the formation of the thermodynamically preferred 8-lithiated product indicate a thermodynamically controlled mechanism. Slow conversion of the 2- into the 8-lithiated species (at higher temperatures) gives further evidence that n-BuLi and t-BuLi afford the kinetically and thermodynamically preferred products, respectively.     

Cyanuric chloride: decent dehydrating agent for an exclusive and efficient synthesis of kinetically controlled isomaleimides

Starting from maleanilic and maleamic acids, a facile general approach to kinetically controlled isomaleimides has been described for the first time using cyanuric chloride as a dehydrating agent with 85-98% yields. The effect of a variety of substituents present on the aromatic ring in amine and maleic anhydride moiety, on these kinetic/thermodynamic dehydration processes of anilic acids has been also described. Under the same set of reaction conditions the phthalanilic acid gave kinetically controlled product, isophtalimide in 91% yield, while the corresponding succinanilic acid furnished the thermodynamically more stable succinimide in high yield.     

胆汁酸盐对胆红素钙沉淀过程的影响

研究了六种人胆汁主要胆汁酸盐对胆红素钙沉淀形成过程的影响。用曲线拟合法处理数据取得热力学及动力学参数。结果表明,六种胆汁酸盐都表现动力学和热力学抑制,而且抑制作用有浓度依赖关系。在一定条件下出现诱导期。虽然不同胆汁酸盐的作用结果相似,但在相同条件下,二羟基胆汁酸和三羟基胆汁酸影响程度不同。     

Thermodynamics and kinetics of glyoxal dimer formation: a computational study

Density functional theory (B3LYP//6-311+G) calculations including Poisson-Boltzmann implicit solvent were used to study the hydration of glyoxal and subsequent formation of dimeric species in solution. Our calculations show that the dioxolane ring dimer is the thermodynamic sink among all monomers and dimers with varying degrees of hydration. Although fully hydrated species are thermodynamically favored over their less hydrated counterparts, we find that a preliminary dehydration step precedes dimerization and ring closure. Ring closure of the open dimer monohydrate to the dioxolane ring dimer is kinetically favored over both hydration to the open dimer dihydrate and ring closure to form the dioxane ring dimer. The kinetic barriers for different geometric approaches for dimerization suggest an explanation why oligomerization stops after the formation of a dioxolane ring trimer as observed experimentally.     

Access to Metastable Gel States Using Seeded Self‐Assembly of Low‐Molecular‐Weight Gelators

Here we report on how metastable supramolecular gels can be formed through seeded self‐assembly of multicomponent gelators. Hydrazone‐based gelators decorated with non‐ionic and anionic groups are formed in situ from hydrazide and aldehyde building blocks, and lead through multiple self‐sorting processes to the formation of heterogeneous gels approaching thermodynamic equilibrium. Interestingly, the addition of seeds composing of oligomers of gelators bypasses the self‐sorting processes and accelerates the self‐assembly along a kinetically favored pathway, resulting in homogeneous gels of which the network morphologies and gel stiffness are markedly different from the thermodynamically more stable gel products. Importantly, over time, these metastable homogeneous gel networks are capable of converting into the thermodynamically more stable state. This seeding‐driven formation of out‐of‐equilibrium supramolecular structures is expected to serve as a simple approach towards functional materials with pathway‐dependent properties.     

A theoretical study of the mechanism and stereoselectivity of the Diels-Alder cycloaddition between difluoro-2-methylencyclopropane and furan

A theoretical study of the molecular mechanism and stereoselectivity of the Diels-Alder cycloaddition reaction between difluoro-2-methylencyclopropane and furan has been carried out at the B3LYP/6-31G+^∗∗ level of theory. The calculation of activation and reaction energies indicates that the 3-endo cycloadduct is favored both kinetically and thermodynamically, which is in agreement with the experimental data. Analysis of the bond order and charge transfer in the transition states shows that this reaction takes place via a synchronous-concerted mechanism.     

Palladium-Catalyzed Branch- and Z-Selective Allylic C−H Amination with Aromatic Amines

Allylamines are important building blocks in the synthesis of bioactive compounds. The direct coupling of allylic C−H bonds and commonly available amines is a major synthetic challenge. An allylic C−H amination of 1,4-dienes has been accomplished by palladium catalysis. With aromatic amines, branch-selective allylic aminations are favored to generate thermodynamically unstable Z-allylamines. In addition, more basic aliphatic cyclic amines can also engage in the reaction, but linear dienyl allylic amines are the major products.     

Thermodynamic and Kinetic Studies on the SiH + XH_3 (X=N, P) Reactions

Silicon chemistry has been attracted more attention because of its applications to the production of thin silicon films and the etching of silicon wafers in micro-electronics1. Silylidyne (SiH), which plays an important role in plasma chemical vapor deposition (CVD) processes, has been investigated in experimental research2-5. The reaction mechanism of SiH insertion reaction with small molecules such as XH3 (X=N, P) was recently studied by means of M豯ler-Plesset perturbation theory6.…     

Density functional theory study of the regio‐ and stereoselectivity of diels–alder reactions of 5‐Aryl‐2‐pyrones

A theoretical study of the mechanism and regio‐ and stereoselectivity of Diels–Alder reactions of 5‐aryl‐2‐pyrones (Ar = Ph, 4‐(MeS)‐Ph) with substituted alkenes (CHZ = CH₂, Z = COMe, OAc) is performed at the B3LYP/6‐31G(d) level. The analysis of the relevant stationary points of the potential energy surface and intrinsic reaction coordinate calculations show that these cycloadditions are undergoing through asynchronous concerted mechanisms yielding to the formation of the 5‐endo isomers as the major cycloadducts. The calculation of activation and reaction energies indicates that the 5‐endo cycloadducts are favored both kinetically and thermodynamically. The obtained results are in good agreement with experimental outcomes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Experimental and computational studies of trialkylaluminum and alkylaluminum chloride reactions with silica

Reactions of trimethylaluminum, triethylaluminum, and diethylaluminum chloride and ethylaluminum dichloride with silica gel have been studied experimentally by infrared spectroscopy and elemental analysis. The silica gel was subjected to different pretreatments to alter surface functionalities prior to reaction. In all cases the extent of surface modification reaction follows the trend unmodified > 600 degrees C pretreated > hexamethyldisilazane (HMDZ) pretreated > 600 degrees C/HMDZ pretreated. All of the aluminum compounds studied completely react non-hydrogen-bonded silanols, while also reacting with hydrogen-bonded silanols and siloxanes. Primarily monomeric surface species result from the surface modification reaction. Ethylaluminum chlorides preferentially react with silanols through cleavage of the Al-C bond rather than the Al-Cl bond. Singly bonded Si(s)-O-AlCl(2) surface species are readily synthesized by reaction of ethylaluminum dichloride with HMDZ-pretreated silica gel. Bridged bonded (Si(s)-O)(2)-AlCl surface species are readily synthesized by reaction of diethylaluminum chloride with HMDZ-pretreated silica gel. Computational ab initio studies of the cluster Si(4)O(6)(OH)(4) as a model to study the reaction of monomeric and dimeric methylaluminum dichloride with a silica silanol are also described. Comparison of the potential energy surface (PES) of monomer and dimer indicates that the energetics favor monomer reaction, consistent with experimental results. The energy cost in the dimer reaction is primarily from cleavage of a bridged Al-Cl bond upon adsorption. This does not occur when the monomer adsorbs. A comparison of the PES for the two reaction pathways resulting from cleavage of either an Al-Cl or Al-C bond indicates that while the former reaction is slightly kinetically favored (E(a) = 23.1 kJ/mol for Al-Cl bond cleavage versus E(a) = 31.1 kJ/mol for Al-C bond cleavage), the latter is strongly thermodynamically favored with an overall free energy difference between the two reaction pathways of 135 kJ/mol favorable to Al-C bond cleavage. These reactions are thermodynamically controlled.     

Nonselective bromination-selective debromination strategy: selective bromination of unsymmetrical ketones on singly activated carbon against doubly activated carbon

[reaction: see text] We have found a new synthetic method for the preparation of the alpha-bromoketones that are brominated in the less activated terminal position of unsymmetrical ketones. Brominations in short reaction times (kinetically controlled) provided internally brominated compounds as a major product. However, brominations in longer reaction times (thermodynamically controlled) gave more of the terminally brominated compound through the reversible reaction by Br(2) and produced hydrogen bromide. Several brominated compounds at the terminal position were successfully prepared through the new synthetic route.