咪唑与单线态氧(1O2)1,2-环加成反应的理论研究

采用较新的半经验分子轨道方法Austin Model 1(简称AM1方法), 辅以Berny梯度优化方法, 对单线态氧(~1O_2)与咪唑的1,2-环加成反应,进行了理论研究。计算获得实验尚未检测到的4,5-二氧环丁烷(4,5-dioxetane)的结构, 并在反应势能面上找到单重态双自由基中间体及通过该中间体的两步反应的过渡态。通过对过渡态的结构特征、虚振动方向以及对反应过程的电荷分布情况、轨道相互作用等的分析, 说明该反应是经由单重态双自由基中间体的分步反应。两步反应的活化势垒分别为39.2 kJ·mol~(-1)和150.5 kJ·mol~(-1)。     

合成环氧乙烷新途径的从头算研究

在MP4/6-31G*//RHF/6-31G*理论水平上，对单态甲撑插入甲醛中碳氧双链生成环氧乙烷的反应(CH2_(~1A)＋CH_2O→C_2H_4O)进行了从头算研究。发现其反应历程由两步组成：1) 反应物沿反应坐标接近，体系能量单调下降，生成平而松散环状的分子复合物(MC)；2) 分子复合物沿反应坐标经由一过渡态(TS)重排为产物环氧乙烷，此步的势垒只有36.99 kJ·mol~(-1)。进而计算了该反应的热力学函数和动力学性质，并进行了讨论。当设法将甲醛引入产生活性中间体CH_2(~1A)的体系中，该反应有可能成为在室温下制备环氧乙烷的非催化途径。     

MgO活化甲烷碳氢键的密度泛函研究

在B3LYP/6-311 + G（2d, 2p）水平上计算了MgO + CH_4 → Mg+CH_3OH反应的 单态势能曲线。结果发现MgO和CH_4发生相互作用,首先形成两种类型的分子-分子 复合物（MgOCH_4和OMgCH_4）;分子-分子复合物OMgCH_4能发生进一步转化,即 MgO插入到CH_4的C-H键中,产生中间体HOMgCH_3,此中间体在本反应中是能量上最 稳定的构型;它还有可能进一步发生反应,产生原子-分子复合物MgCH_3OH,但其 活化能太高,为299.8kJ·mol~(-1),是整个反应的速率控制步骤;最后一步是 MgCH_3OH放出CH_3OH分子,整个反应放热146.1 kJ·mol~(-1)。     

HNCO热解为CO2和HNCNH的反应机理理论研究

利用从头算ＲＨＦ／３－２１Ｇ方法研究了ＨＮＣＯ二聚后生成ＨＮＣＮＨ和ＣＯ２的反应机理。计算表明,该反应是分步反应,由反应物经第一过渡态生成四元环中间体,再经过第二过渡态分解为产物,与实验得到的结论一致。反应的第一步是速度控制步骤,计算得到的活化位垒为１７２．５５ｋＪ·ｍｏｌ＾－１,与实验上测得的１７６．４０±１６．３０ｋＪ·ｍｏｌ＾－１相吻合。反应的第二位垒为８３．６８ｋＪ·ｍｏｌ＾－１,在实验条     

Stereoselective Synthesis of 2-(4-Hydroxyphenyl)-3-hydroxymethyl-1,4-benzodioxane-6-aldehyde—The Key Intermediate of Sinaiticin

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双马来酰亚胺树脂固化过程的红外光谱分析

利用傅里叶变换红外光谱法研究双马来酰亚胺树脂固化过程中的结构变化,认为双马来酰亚胺树脂固化反应分两进行,第一步是低温下的“ENE”反应,第二步是高温下的Diels-Alder反应,树脂第一步固化反应达到一定程度后,要进一步提高固化反应程度,必须提高温度才能使第二步固化反应发生。     

Mechanistic information from low-temperature rapid-scan and NMR measurements on the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex

A detailed kinetic study of the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex was undertaken in dichloromethane over the temperature range of -90 to +10 degrees C by stopped-flow techniques. Time-resolved UV-vis monitoring of the reaction allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. The second-order rate constant for the protonation step was determined to be 15200 +/- 400 M(-1) s(-1) at -78 degrees C, and the corresponding activation parameters are DeltaH = 15.2 +/- 0.6 kJ mol(-1) and DeltaS = -85 +/- 3 J mol(-1) K(-1), which are in agreement with the addition of a proton that results in the formation of the platinum(IV) hydrido complex. The kinetics of the second, methane-releasing reaction step do not show an acid dependence, and the MeCN concentration also does not significantly affect the reaction rate. The activation parameters for the second reaction step were found to be DeltaH = 75 +/- 1 kJ mol(-1), DeltaS = +38 +/- 5 J mol(-1) K(-1), and DeltaV = +18 +/- 1 cm(3) mol(-1), strongly suggesting a dissociative character of the rate-determining step for the reductive elimination reaction. The spectroscopic and kinetic observations were correlated with NMR data and assisted the elucidation of the underlying reaction mechanism.     

Evidence for the formation of a mo-h intermediate in the catalytic cycle of formate dehydrogenase

DFT/BP86/TZVP and DFT/B3LYP/TZVP have been used to investigate systematically the reaction pathways associated with the H-transfer step, which is the rate-determining step of the reaction HCOO(-) ? CO(2) + H(+) + 2e(-), as catalyzed by metalloenzyme formate dehydrogenase (FDH). Actually, the energetics associated with the transfer from formate to all H (proton or hydride) acceptors that are present within the FDH active site have been sampled. This study points to a viable intimate mechanism in which the metal center mediates H transfer from formate to the final acceptor, i.e. a selenocysteine residue. The Mo-based reaction pathway, consisting of a β-H elimination to metal with concerted decarboxylation, turned out to be favored over previously proposed routes in which proton transfer occurs directly from HCOO(-) to selenocysteine. The proposed reaction pathway is reminiscent of the key step of metal-based catalysis of the water-gas shift reaction.     

Potentiometric determination of p-chloranil and the kinetic study of its alkaline hydrolysis, using a chloranilate selective electrode

Two Ion-Selective Electrodes (ISEs), one commercial chloride ISE and one home-made chloranilate (Chl(2-)) ISE, were applied to study the alkaline hydrolysis of p-chloranil. This reaction proceeds through a very fast first step producing monochloranilate (Chl(-)) and chloride ions followed by a slow, rate determining step, producing chloranilate (Chl(2-)) and chloride ion. Kinetic equations specific for this reaction scheme, were derived to calculate reaction rate constants from E-t curves. The potentiometric selectivity coefficient K(pot) = 2.23 x 10(4) was determined for the response of the chloranilate electrode to Chl(-) anion and used for the kinetic calculations. The reaction rate constant for the slow step of the hydrolysis was found to be 0.227 M(-1) . sec(-1) at 25 degrees and the activation energy 1.56 x 10(4) Cal/mol. The total potential change following the completion of the first fast step is proportional to the log C of chloranil. Concentrations of chloranil 5-500 ppm can be determined with a precision of about 2%. This potentiometric method was evaluated for the determination of chloranil in fungicide preparations (Spergon) and the results obtained were compared with those of the official iodometric method. Good agreement was found.     

The mechanism of formamide hydrolysis in water from ab initio calculations and simulations

The neutral hydrolysis of formamide in water is a suitable reference to quantify the efficiency of proteolytic enzymes. However, experimental data for this reaction has only very recently been obtained and the kinetic constant determined experimentally is significantly higher than that predicted by previous theoretical estimations. In this work, we have investigated in detail the possible mechanisms of this reaction. Several solvent models have been considered that represent a considerable improvement on those used in previous studies. Density functional and ab initio calculations have been carried out on a system which explicitly includes the first solvation shell of the formamide molecule. Its interaction with the bulk has been treated with the aid of a dielectric continuum model. Molecular dynamics simulations at the combined density functional/molecular mechanics level have been carried out in parallel to better understand the structure of the reaction intermediates in aqueous solution. Overall, the most favored mechanism predicted by our study involves two reaction steps. In the first step, the carbonyl group of the formamide molecule is hydrated to form a diol intermediate. The corresponding transition structure involves two water molecules. From this intermediate, a water-assisted proton transfer occurs from one of the hydroxy groups to the amino group. This reaction step may lead either to the formation of a new reaction intermediate with a marked zwitterionic character or to dissociation of the system into ammonia and formic acid. The zwitterionic intermediate dissociates quite easily but its lifetime is not negligible and it could play a role in the hydrolysis of substituted amides or peptides. The predicted pseudo-first-order kinetic constant for the rate-limiting step (the first step) of the hydrolysis reaction at 25 degrees C (3.9x10(-10) s(-1)) is in excellent agreement with experimental data (1.1x10(-10) s(-1)).     

GeX2(X=F,Cl)与甲醛环加成反应机理的理论研究

The mechanisms of the cycloaddition reaction of singlet GeX2(X=F,Cl) with formaldehyde was studied employing the HF/6-311+G theory. The electron-correlation corrections have been further considered by the fourth-order Muller-Plesset perturbation theory (MP4SDTQ/6-311+G). The results show that this reaction proceeds in two steps: ① Difluorogemylene and formaldehyde form an intermediate complex, which is a barrierless exothermal reaction; ② the intermediate complex isomerizes to form the product, which is a rate-control step in the whole reaction. In the second step, the calculated barrier heights are 216.7 and 196.4 kJ/mol before and after considering electron-correlation effects. Compared with that of the cycloaddition reaction of difluorosilylene with formaldehyde, the cycloaddition reaction of difluorogemylene with formaldehyde is relatively slow, whereas the cycloaddition reaction of dichlorogemylene with formaldehyde can be comparable in speed.     

Enantioselective total synthesis of macrolide antitumor agent (-)-lasonolide A

[structure: see text] An enantioselective total synthesis of (-)-lasonolide A is described. The upper tetrahydropyran ring was constructed stereoselectively by an intramolecular 1,3-dipolar cycloaddition reaction. The bicyclic isooxazoline led to the tetrahydropyran ring as well as the quaternary stereocenter present in the molecule. The lower tetrahydropyran ring was assembled by a catalytic asymmetric hetero-Diels-Alder reaction as the key step. Three stereocenters were enantioselectively installed in this single step reaction.     

An efficient route to 6-(het)aryl-2-methyl-2,3-dihydro-1H-pyridin-4-ones as potential nAChRs ligands

A new efficient pathway to synthesise 6-(het)aryl-2-methyl-2,3-dihydro-1H-pyridin-4-ones is described. This reaction sequence involved, as a key step, a Suzuki cross-coupling reaction between various boronic acids and an 6-iodo-2,3-dihydropyridin-4-one. A final deprotecting step furnished the attempted products.     

Mechanistic analysis of reductive nitrosylation on water-soluble cobalt(III)-porphyrins

The reactions of NO and/or NO2- with three water-soluble cobalt porphyrins [Co(III)(P)(H2O)2]n, where P = TPPS, TCPP, and TMPyP, were studied in detail. At pH < 3, the reaction with NO proceeds through a single reaction step. From the kinetic data and activation parameters, the [Co(III)(P)(NO)(H2O)]n complex is proposed to be the primary product of the reaction with NO. This complex reacts further with a second NO molecule through an inner-sphere electron-transfer reaction to generate the final product, [Co(III)(P)(NO-)](n-1). At pH > 3, although a single reaction step is also observed, a systematic study as a function of the NO and NO2- concentrations revealed that two reaction steps are operative. In the first, NO2- and NO compete to substitute coordinated water in [Co(III)(P)(H2O)2]n to yield [Co(III)(P)(NO)(H2O)]n and [Co(III)(P)(NO2-)(H2O)](n-1) as the primary reaction products. Only the nitrite complex could be detected and no final product formation was observed during the reaction. It is proposed that [Co(III)(P)(NO)(H2O)]n rapidly reacts with NO2- to form the nitrite complex, which in the second reaction step reacts with another NO molecule to generate the final product through an inner-sphere electron-transfer reaction. The reported results are relevant for the interaction of vitamin B(12a) with NO and NO2-.     

Synthesis of substituted decalones by diels-alder reaction or by sequential michael reaction - which one is more selective?

Acetylcyclohexene is transformed into decalones by a two step Michael reaction with high stereo- and regioselectivity, while the Diels - Alder reaction in this case shows poor selectivity.     

A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI)

The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.     

硅烯与甲醛环加成反应的理论研究

本文用RHF/6-31G^*解析梯度方法研究了单重态硅烯与甲醛环加成反应的机理,用二级微扰方法对各构型的能量进行了相关能校正,并用统计热力学方法和过渡态理论计算了该反应在不同温度下的热力学函数的变化和动力学性质。结果表明,此反应历程由两步组成:1)硅烯与甲醛生成一中间配合物,是一无势垒的放热反应,2)中间配合物异构化为产物,此步势垒经零点能校正后只有51.4kJ·mol^-^1(MP2/6-31G^*//6-31G^*);从热力学和动力学的综合角度考虑,该反应在300～400K温度下进行为宜,如此,反应既有较大的自发趋势和平衡常数,又具有较快的反应速率。     

Reactions of substituted (methylthio)benzylidene Meldrum's acids with secondary alicyclic amines in aqueous DMSO. Evidence for rate-limiting proton transfer

The replacement of the methylthio group of substituted methylthiobenzylidene Meldrum's acids (2-SMe-Z) by secondary alicyclic amines occurs by a three-step mechanism. The first step is a nucleophilic attachment of the amine to 2-SMe-Z to form a zwitterionic intermediate T(+/-)(A); the second step involves deprotonation of T(+/-)(A) to form T(-)(A); while the third step represents general acid-catalyzed conversion of T(-)(A) to products. At high amine and/or high KOH concentration nucleophilic attachment is rate limiting. At low amine and low KOH concentration the reaction follows a rate law that is characteristic for general base catalysis which, in principle, is consistent with either rate-limiting deprotonation of T(+/-)(A) or rate-limiting conversion of T(-)(A) to products. A detailed structure-reactivity analysis indicates that for the reactions with piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine it is deprotonation of T(+/-)(A) that is rate limiting, while for the reaction with piperidine, conversion of T(-)(A) to products is rate limiting.     

Mechanism of the axial ligand substitution reactions on the head-to-tail alpha-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex with olefins

Reactions of the head-to-tail alpha-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex having equivalent two platinum atoms, Pt(N(3)O), with p-styrenesulfonate and 4-penten-1-ol were studied kinetically. Under the pseudo first-order reaction conditions in which the concentration of the Pt(III) dinuclear complex is much smaller than that of olefin, a consecutive basically four-step reaction was observed for the reaction with p-styrenesulfonate, but for the reaction with 4-penten-1-ol, the reaction was three step. The olefin pi-coordinates to one of the two equivalent Pt atoms in the first step (step 1), followed by the second pi-coordination of another olefin molecule to the other Pt atom (step 2). In the next step (step 3), the nucleophilic attack of water to the first pi-coordinated olefin initiates its pi-sigma bond conversion on the Pt atom, and the second pi-bonding olefin molecule on the other Pt atom is released. Finally, dissociation of the alkyl group on the Pt(N(3)O) and reduction of the Pt(III) dinuclear complex to the Pt(II) dinuclear complex occur (step 4). The first water substitution with olefin (step 1) consists of two paths, the reaction of the diaqua dimer complex (path a) and the reaction of the aquahydroxo dimer complex (path b), whereas the second substitution (step 2) proceeds through three reaction paths: the normal path of the direct substitution of H(2)O (path c), the path of the coordinated OH(-) substitution (path d), and the path via the coordinatively unsaturated five-coordinate intermediate (path e). The reaction with p-styrenesulfonate proceeds through paths c, d, and e, whereas the reaction with 4-penten-1-ol proceeds through paths c and d. The third step (step 3) for the reaction with p-styrenesulfonate involves the coordinatively unsaturated intermediate, but that for the 4-pentene reaction does not. The reactivities of the HH dimer and HT dimer with olefins are compared and discussed.     

An aza-Achmatowicz approach toward the hydroxylated piperidine alkaloids (+/-)-azimic acid and (+/-)-deoxocassine

[reaction: see text] The synthesis of several cis-2,3,6-trisubstituted piperidines has been developed employing the aza-Achmatowicz oxidation as the key reaction step. Its usage is illustrated by the facile synthesis of the piperidin-3-ol alkaloids (+/-)-deoxocassine and (+/-)-azimic acid.