碘乙醇在Ni(100)表面的吸附和热分解-碳氢化合物氧化的中间产物：羟乙基和氧金属环

利用热脱附(TPD)实验和X射线光电子能谱(XPS)研究了碘乙醇在Ni(100)表面的吸附和热反应过程. 实验结果表明碘乙醇在100 K时以两种分子的形式吸附在Ni(100)的表面, 即: 以碘原子端吸附在表面或以碘原子端和羟基端同时吸附在表面. 由于两种吸附形式的分子的一致分解和吸附分子的不均匀性, 在140 K引起了较复杂的化学反应, 伴有少量的乙烯和水产生. 碘乙醇在150 K经过C—I键断裂, 有80%碘乙醇生成—O(H)CH2CH2—中间产物, 20%的碘乙醇生成羟乙基中间产物. 羟乙基在160 K的转化过程中包括两个互相竞争的化学反应: 与表面的氢原子进行还原反应生成乙醇, 或失去一个β-H原子生成表面乙烯醇. 另外, 在相同的温度下—O(H)CH2CH2—中间产物经过脱氢反应产生—OCH2CH2—氧金属环. 羟乙基和氧金属环都会发生异构, 分别在210 K和250 K生成乙醛, 这些乙醛一部分从表面脱出, 其余的部分发生分解反应产生氢气、水和一氧化碳. 在实验基础上, 进一步探讨了这种化学过程在催化中的作用和指导意义.     

六氟代戊二酮合镍/乙氧基二乙基铝催化丙烯的线性齐聚

丙烯二聚和齐聚生成高碳直链烯烃的研究已经引起人们的注意.Keim使用单组份催化剂六氟代戊二酮、1,5-环辛二烯镍络合物、Jones使用双组份催化剂β-戊二酮合镍/乙氧基二乙基铝、作者使用六氟代戊二酮合镍/三异丁基铝催化丙烯齐聚反应,都得到了直链烯烃含量较高的丙烯齐聚物.但是,上述各体系的催化剂活性相差很大.本文简要报道以甲苯为溶剂,六氟代戊二酮合镍/乙氧基二乙基铝〔简称(Hfacac)_2Ni/Et_2AlOEt〕为催化剂的丙烯齐聚反应的规律.     

铬(Ⅲ)含β-二酮混合配体配合物的合成和表征(Ⅱ)

首次合成了九种[Cr(β-dik)Cl_2L_2]型混合配体配合物。其中五种是:L为γ-甲基吡啶,β-dik分别为乙酰丙酮、苯甲酰丙酮、三氟乙酰丙酮、2-噻吩甲酰三氟丙酮、1,1,1,2,2,3,3-七氟-7,7-二甲基-4,6-辛二酮;四种是:L分别为N,N-二甲基甲酰胺,N,N-二甲基乙酰胺,β-dik分别为苯甲酰丙酮和2-噻吩甲酰三氟丙酮。并对上述配合物进行了电子光谱、红外光谱的表征工作。     

环氧丙烷和甲醇在MgO上合成1-甲氧基-2-丙醇反应机理

应用原位FT-IR详细研究了MgO上甲醇对环氧丙烷的加成反应. 结果表明在MgO催化剂上甲醇很易解离吸附形成甲氧基, 根据负碳离子机理, 吸附环氧丙烷可形成类丙烯物种. 甲氧基对类丙烯物种的加成反应遵循反-Markownikov法则, 高选择性地合成1-甲氧基-2-丙醇.     

Na2WO4—H2O2酸体系催化氧化1—甲氧基—2—丙醇的研究

研究了Na2WO4-H2O2体系在催化氧化1-甲氧基-2-丙醇为甲氧基丙酮反应中的催化活性,发现酸助剂及添加物对甲氧基丙酮的收率有较大的影响。酸助剂中,NaHSO4对活性的促进作用最好;而H2PO4^-和HPO4^2-对体系的活性不利。极性小分子甲醇、乙腈可促进1-甲氧其-2-丙醇的氧化,提高甲氧基丙酮的收率。同时还考察了甲醇量对甲氧基丙酮收率的影响。     

TiO2(110)面的弛豫结构及吸附O2的密度泛函研究

采用DFT/B3LYP方法研究了TiO2 ( 110 )的完整和氧缺陷表面的弛豫构型 ,并对O2 在氧缺陷表面的三种可能吸附构型进行了优化 ,计算了它们的吸附能、振动频率和重叠布居 .分析并预测了吸附后可能产生的物种 .本文的计算结果与XPS ,TPD和ELS等实验吻合     

碘乙醇在Ni(100)表面的化学热反应

利用热脱附(TPD)实验和X射线光电子能谱(XPS)研究了碘乙醇在Ni(100)表面的吸附和热反应过程. 实验结果表明, 碘乙醇在100 K温度下以两种分子的形式吸附在Ni(100)的表面, 既有碘原子端的吸附也有碘原子端和羟基端同时吸附在表面. 热分解反应发生在140 K, 伴有少量的乙烯和水产生. 碘乙醇在150 K经过C—I键断裂形成−O(H)CH2CH2−和羟乙基两种中间产物. 在160 K温度下−O(H)CH2CH2−脱去氢形成−OCH2CH2−氧金属环. 中间产物经过进一步分解氧化反应分别在210和250 K产生乙醛, 一部分乙醛从表面脱出, 而其余的则分解成氢气、水和CO.     

1,1-二取代炔丙醇氯代产物与4-甲基-7-羟基香豆素的异常缩合产物

报道了一类1,1-二取代炔丙醇(1)氯代产物与4-甲基-7-羟基香豆素反应的异常缩合产物. 一些实验数据表明1,1-二取代炔丙醇(1)的氯代反应中有一类未见报道的2,4-二氯-2-烯化合物(5)生成.     

氟氯烷烃与三氯化铝的反应

Park等从全氟丙烯与三氯化铝的反应产物中,分离出一系列氟氯烯烃,并指出该反应中全氟丙烯氟原子被氯原子取代的次序.CF_2ClCFClCF_2CFCl_2(1)与三氯化铝反应,仅得到CF_2ClCFClCF_2CCl_3,产率58％.我们研究了1及CFCl_2CF_2CFCl_2(2)分别与三氯化铝的反应.由1的反应得CF_     

铁铬酸锌尖晶石催化剂的表面氧种

用TPR,TPO,TPD和脉冲反应等技术以及XRD,EPR和XPS等物理方法对铁铬酸锌尖晶石的表面氧种及其与丁烯-2的反应和吸附性能进行了考察,结果表明该尖晶石表面存在α-,β-和γ-三种氧种。α-氧为表面弱结合氧,主要参与丁烯的深度氧化和使丁烯形成表面滞留物。β-氧为尖晶石结构中与Fe~(3+)配位的表面晶格氧,它参与选择氧化生成丁二烯。γ-氧为α-Fe_2O_3中的晶格氧,在所考察的条件下(270℃)基本上不参与反应。认为在铁铬酸锌的尖晶石相中的铁离子和与它配位的表面晶格氧(即β-氧)构成选择氧化的活性位。在丁烯氧化脱氢反应中通过反应条件的选择来控制表面氧种的类型和它们之间的相对密度,有可能提高活性和选择性     

Switching of alcohol oxidation mechanism on nickel surfaces by fluorine substitution

A partial switch in mechanism for the partial oxidation of alcohols on nickel surfaces can be induced by substitution of gamma-hydrogens with more electronegative fluorine atoms. While exclusive dehydrogenation to acetone via beta-hydride elimination from 2-propoxide surface species is seen with 2-propanol, some dehydration to 3,3,3-trfluoropropene is observed with 1,1,1-trifluoro-2-propanol. The latter reaction involves a gamma-hydride elimination rate-limiting step.     

Tuning of the electronic properties of a cyclopentadienylruthenium catalyst to match racemization of electron-rich and electron-deficient alcohols

The synthesis of a new series of cyclopentadienylruthenium catalysts with varying electronic properties and their application in racemization of secondary alcohols are described. These racemizations involve two key steps: 1) β-hydride elimination (dehydrogenation) and 2) re-addition of the hydride to the intermediate ketone. The results obtained confirm our previous theory that the electronic properties of the substrate determine which of these two steps is rate determining. For an electron-deficient alcohol the rate-determining step is the β-hydride elimination (dehydrogenation), whereas for an electron-rich alcohol the re-addition of the hydride becomes the rate-determining step. By matching the electronic properties of the catalyst with the electronic properties of the alcohol, we have now shown that a dramatic increase in racemization rate can be obtained. For example, electron-deficient alcohol 15 racemized 30 times faster with electron-deficient catalyst 6 than with the unmodified standard catalyst 4. The application of these protocols will extend the scope of cyclopentadienylruthenium catalysts in racemization and dynamic kinetic resolution.     

Asymmetric palladium-catalyzed hydroarylation of styrenes and dienes

Alkenes are desirable and highly versatile starting materials for organic transformations, and well-known substrates for palladium catalysis. Typically, these reactions result in the formation of a new alkene product via β-hydride elimination. In contrast to this scenario, our laboratory has been involved in the development of alkene hydro- and difunctionalization reactions, where β-hydride elimination can be controlled. We report herein the development of an asymmetric palladium-catalyzed hydroarylation, which yields diarylmethine products in up to 75% ee. Interestingly, a linear free energy relationship is observed between the steric bulk of the ligand within a certain range and the ee of the reaction.     

Mechanism of the palladium-catalyzed addition of arylboronic acids to enones: a computational study

The palladium(II)-catalyzed addition of arylboronic acids to β,β-disubstituted enones has been investigated with the BP86 density functional. The results show that the mechanism requires three steps: transmetalation, alkene insertion, and protonation. The alkene insertion is the rate-determining step. For unactivated alkenes, the Heck-type β-hydride elimination is more favored than protonation.     

Computational study on the mechanism and selectivity of C-H bond activation and dehydrogenative functionalization in the synthesis of rhazinilam

The key platinum mediated C-H bond activation and functionalization steps in the synthesis of (-)-rhazinilam (Johnson, J. A.; Li, N.; Sames, D. J. Am. Chem. Soc. 2002, 124, 6900) were investigated using the M06 and B3LYP density functional approximation methods. This computational study reveals that ethyl group dehydrogenation begins with activation of a primary C-H bond in preference to a secondary C-H bond in an insertion/methane elimination pathway. The C-H activation step is found to be reversible while the methane elimination (reductive elimination) transition state controls rate and diastereoselectivity. The chiral oxazolinyl ligand induces ethyl group selectivity through stabilizing weak interactions between its phenyl group (or cyclohexyl group) and the carboxylate group. After C-H activation and methane elimination steps, Pt-C bond functionalization occurs through β-hydride elimination to give the alkene platinum hydride complex.     

Dual-function of alcohols in gold-mediated selective coupling of amines and alcohols

Oxidative coupling of alcohols (methanol and ethanol) and dimethylamine on atomic-oxygen-activated Au(111) occurs entirely on the surface to form the corresponding amides when the alkoxy of the alcohol and the amide derived from the amine are co-adsorbed. For effective oxygen-assisted coupling the formation of the amide requires excess methanol. Mechanistic studies reveal that molecularly adsorbed methanol removes excess adsorbed atomic oxygen efficiently, precluding either secondary oxidation or oxidative dehydrogenation of dimethylamide to the imine. The adsorbed amide then can react with the aldehyde produced by β-hydride elimination from the alkoxy to form the hemiaminal, the reactive intermediate leading to coupling. The selectivity for formamide production can be increased to nearly 100?% in excess methanol.     

Impact of N-Aryl- and NHC Core-Substituents on the Coupling of Alkylzinc Nucleophiles: Is Bigger always Better?

Bulky Pd−N-heterocyclic carbene (NHC) catalysts (e. g., N-(di-2,6-(3-pentyl)phenyl), IPent) have been shown to have significantly higher reactivity in a wide variety of cross-coupling applications (i. e., C−C, C−S, C−N) than less hindered variants (e. g., N-(di-2,6-(isopropyl)phenyl), IPr). Further, chlorinating the backbone of the NHC ring sees an even greater increase in reactivity. In the cross-coupling of (hetero)aryl electrophiles to secondary alkyl nucleophiles, making the N-aryl groups larger reduces the amount of β-hydride elimination leading to alkene byproducts and chlorinating the NHC core had an even greater effect, all but eliminating alkene formation. In the present study involving the cross-coupling of primary alkyl electrophiles and nucleophiles, a sharp and surprising reversal of all of the above trends was observed. Bulkier catalysts had generally slower rate of reaction and β-hydride elimination worsened leading to extensive amounts of alkene byproducts.     

Nondissociative chain walking as a strategy in catalytic organic synthesis

“Nondissociative” chain walking is a mechanism where an alkylmetal species undergoes rapid β-hydride elimination and reinsertion to move the metal center along the alkyl chain without dissociating an alkene intermediate during the migration process. This digest summarizes the unique characteristics of the nondissociative chain walking mechanism compared to the stepwise alkene isomerization mechanism and their use in catalytic organic synthesis.     

A theoretical study of the dehydration and the dehydrogenation processes of alcohols on metal oxides using MOPAC

Using a semi-empirical molecular orbital method, PM3, and 2-propanol as an example, the dehydration and the dehydrogenation processes of alcohol on oxide catalysts were studied. The catalysts addressed here were four kinds of oxides (Al₂O₃, SiO₂, ZnO, CdO) whose reaction selectivities had been experimentally determined. The usual models consisting of a surface metal ion, several oxide ions and an isopropoxy group were used in calculations. For the dehydration, heats of formation of the models were calculated at each point of the process where the distance between a β-hydrogen of the group and a basic site (i.e. oxygen of the group or a surface oxide ion) or a metal ion was gradually shortened, or where the length of the C–O bond of the group was gradually increased. A reasonable dehydration mechanism was estimated by comparing activation energies calculated from the transitions of the heats of formation. The most probable dehydrogenation mechanism was also estimated in a similar way by gradually making an -hydrogen close to a surface oxide ion, the metal ion or a surface proton. It was concluded that the dehydration proceeds by scission of the C–O bond of the group after its oxygen was attacked by some electrophile on the surface and that the dehydrogenation proceeds by a mechanism in which an -hydrogen of the group was extracted by the metal ion.

Based on the dehydration mechanism thus deduced, alkoxy groups generated by adsorption of the primary, secondary and tertiary alcohols on SiO₂ were calculated in order to estimate the activation energies of their decompositions. In result, the order of the energies was found to be in good agreement with that of the decomposition rates experimentally determined by Kitahara. This agreement gives support to the validity of the mechanism deduced for the dehydration of alcohol.     

Rhenium-catalyzed insertion of terminal alkenes into a C(sp)-H bond and successive transfer hydrogenation

Treatment of aromatic aldimines with terminal alkenes in the presence of a rhenium catalyst, [HRe(CO)₄]_n, gives 2-alkenylbenzylamines in good to excellent yields. This reaction proceeds via the insertion of the alkene into a C-H bond at the ortho-position of the imino group of the aromatic aldimine followed by sequential β-hydride elimination from the formed alkyl rhenium intermediate and then by hydrogenation of the imino group of the aldimine.