含1,2二取代的苯并咪唑配体的Zn(Ⅱ)络合物上化学固定CO_2转化为环状碳酸酯（英文）

合成了一种新的Zn(Ⅱ)配合物ZnCl_2(L1)_2](1)(L1为2-(2-噻吩)-1-(2-噻吩甲基)-1H-苯并咪唑),并采用NMR和IR光谱、元素分析、ESI-HRMS光谱测定和热重分析等对它进行了表征,其分子结构也由单晶X射线衍射确定.络合物1含有单核四面体Zn(Ⅱ)单元,即所谓的锁定的几何结构,这源自分子中存在弱的分子间S···p和p-p配体间相互作用.通过简易的合成路线即可制得苯并咪唑配体及其与Zn(Ⅱ)配合物.采用CO_2与环氧化物耦合生成环状碳酸酯反应考察了1的催化活性,以及反应条件的影响.该配合物在无溶剂条件下可高效催化多种环氧化物的转化,具有较好的转化率,TONs和TOFs.     

Estimation of the Diffusion Coefficient from Crossings

In a previous work we proposed a kernel method for estimating the value of a state‐dependent diffusion coefficient σ(x) from discrete time observations. We propose an estimator of the value σ(x), based on the squared length of the transitions where a crossing past the level x takes place and we investigate its limit properties. Using the Poisson equation and other limit theorems for discrete martingales, we prove consistency and asymptotic mixed normality of this estimator. This revised version was published online in June 2006 with corrections to the Cover Date.     

Synthesis,Structure, and Antimalarial Activity of Some Enantiomerically Pure,cis-fused cyclopenteno-1,2,4-trioxanes

Two pairs of enantiomerically pure cis-fused cyclopenteno-1,2,4-trioxanes ( 7 , ent- 7 and 8 , ent- 8 ) are prepared (Schemes 1–3). Their identities are established by dye-sensitized photo-oxygenation of ent- 7 and 8 , ent- 8 to the allylichydroperxides, reduction to the corresponding alcohols, and conversion to the (1S)-camphanates (Scheme 4), the structures of which are determined by X-ray analysis. The dynamic properties of ent- 7 are investigated by NMR spectroscopy and PM3 calculations. Evidence for an easily accessible twist-boat conformation is obtained. The in vitro and in vivo antimalarial activities of 7 , ent- 7,8 , and ent- 8 as well as those of the racemic mixtures are evaluated against Plasmodium falciparum, P. berghei, and P. yoelii. No correlation is observed between configuration and activity. Racemates and pure enantiomers have commensurate activities. The mode of action on the intraerythrocytic parasite is rationalized in terms of close docking by the twist-boat conformer of the trioxane on the surface of a molecule of heme, single-electron transfer to the O? O σ* orbital, and scission to the acetal radical which then irreversibly isomerizes to a C-centered radical, the ultimate lethal agent (Scheme 5).     

Direct electrochemical reduction of a bromo-propargyloxy ester at vitreous carbon cathodes in dimethylformamide

Cyclic voltammograms for the reduction of ethyl 2-bromo-3-(3^′,4^′-dimethoxyphenyl)-3-(propargyloxy)propanoate (1) at glassy carbon electrodes in dimethylformamide containing tetraalkylammonium salts exhibit three prominent waves corresponding to cleavage of the carbon–bromine bond and to subsequent reduction of ethyl trans-3-(3^′,4^′-dimethoxyphenyl)-prop-2-enoate (4). Controlled-potential electrolyses of 1 at potentials corresponding to reduction of the carbon–bromine bond afford 4 as the major product with an average yield of 56%. In the presence of a proton donor (1,1,1,3,3,3-hexafluoro-2-propanol), the quantity of 4 decreases slightly, and 2-(3^′,4^′-dimethoxyphenyl)-3-(ethoxycarbonyl)-4-methyl-2,5-dihydrofuran (3) is obtained in moderate amount (26%). We propose a mechanistic scheme whereby the major products are formed via a combination of one- and two-electron processes.     

Cis/trans stereochemical effects in the negative chemical ionization/OH− mass spectra of strained-ring azabicycloalkanes using MIKE and CA/MIKE spectrometry

Stereospecific decomposition reactions of isomeric (cis and trans) deprotonated molecules from azabicycloalkane derivatives as azetidinols generated under negative chemical ionization (NCI)/OH^? have been examined using mass-analysed ion kinetic energy (MIKE) and collisional activation (CA)/MIKE spectra. These measurements together with the ones obtained on specifically labelled compounds enabled us to determine the origin of the stereochemical effects. The results indicate that the hydroxylic proton constitutes the preferential (?90%) site for the deprotonation process. Subsequent fragmentations of the deprotonated species observed in the second field-free region of a reversed geometry instrument are affected by the stereochemistry of the hydroxylic group. The isomer with the hydroxyl group in the cis position relative to the hydrogen at the ring junction mainly loses H₂O, while the trans isomer eliminates CH₃˙, both processes occurring with high specificity. Labelling studies indicate that two major pathways exist for the elimination of H₂O from the cis isomer and the loss of CH₃˙ from the trans isomer. The course of the reaction is determined by the ability of the stereoisomers to transfer a proton during the first decomposition step. When the size of the lactam ring is increased from a five-membered ring to a six- or seven-membered ring, these stereochemical effects tend to become less pronounced.     

Molecular interconversion behaviour in comprehensive two-dimensional gas chromatography

Comprehensive two-dimensional gas chromatography (GC x GC) is shown to provide information on dynamic molecular behaviour (interconversion), with the interconversion process occurring on both columns in the coupled-column experiment. The experiment requires suitable adjustment of both experimental conditions and relative dimensions of each of the columns. In this case, a longer column than normally employed in GC x GC allows sufficient retention duration on the second column, which permits the typical plateau-shape recognised for the interconversion process to be observed. The extent of interconversion depends on prevailing temperature, retention time, and the phase type. Polyethylene glycol-based phases were found to result in high interconversion kinetics, although terephthalic acid-terminated polyethylene glycol had a lesser extent of interconversion. Much less interconversion was seen for phenyl-methylpolysiloxane and cyclodextrin phases. This suggests that for the oximes, interconversion largely occurs in the stationary phase. Examples of different extents of interconversion in both dimensions are shown, including peak coalescence on the first column with little interconversion on the second column.     

The reaction of S-nitroso-N-acetyl-D,L-penicillamine (SNAP) with the angiotensin converting enzyme inhibitor, captopril--mechanism of transnitrosation

Kinetic studies involving the use of both stopped-flow and diode array spectrophotometers, show that the reaction between SNAP and captopril in the presence of the metal ion sequestering agent, EDTA, occurs in two well-defined stages. The first stage is a fast reaction while the second stage is slow. The first stage has been postulated to be transnitrosation, and the second stage involves the decay of the newly formed RSNO to effect nitric oxide (NO) release. Both stages are found to be dependent on captopril and H+ concentration. The rates of the transnitrosation increased drastically with increasing pH in the first stage, signifying that the deprotonated form of captopril is the more reactive species. In the case of the second stage the variation in pH showed an increase in rate up to pH 8 after which the rate remained unchanged. Both stages were clearly distinguishable and easily monitored separately. Transnitrosation is a reversible reaction with the tendency for the equilibrium to break down at high thiol concentration. Second-order rate constants were calculated based on the following derived expressions: -d[SNAP]/dt=k(f)((K(SHCapSH)[CapSH](t))/(K(SHCapSH)+[H+]))[SNAP]. k(f) is the second-order rate constant for the forward reaction of the reversible transnitrosation process. At 37 degrees C, k(f)= 785 +/- 14 M(-1) s(-1), activation parameters [Delta]H(f)++= 49 +/- 2 kJ mol(-1), (Delta)S(f)++=-32 +/- 2 J K(-1) mol(-1). The activation parameters demonstrate the associative nature of the transnitrosation mechanism. The second stage has been found to be very complex, as a variety of nitrogen products form as predicted before. However, the following expression was derived from the initial kinetic data: rate =k1K[SNOCap][CapS-]/(K[CapS-]+ 1) to give k1= 13.3 +/- 0.4 x 10(-4) s(-1) and K= 5.59 +/- 0.53 x 10(4) M(-1), at 37 degrees C, where k1 is the first-order rate constant for the decay of the intermediate formed during the reaction between SNOCap and the remaining excess CapSH present at the end of the first stage reaction. Activation parameters are (Delta)H1++= 37 +/- 1 kJ mol(-1), (Delta)S1++=-181 +/- 44 J K(-1) mol(-1).     

Intramolecular Oxygen Transfer on the Ozonolysis of a Diphenylcyclopentene Derivative

The ozonolysis of cis-3,4a,7,7a-tetrahydro-3,3-dimethyl-6,7a-diphenylcyclopenta[1,2,4]trioxine ( 1 ) in CH₂Cl₂ at ?78° gave the secondary endo ozonide 2 (43% yield) and an acetal 3 (27% yield) derived from O-insertion at the ortho position of the C(7a) phenyl substituent. Both structures were elucidated by X-ray. Repetition of the ozonolysis in MeOH/CH₂Cl₂20:3 at ?78° also gave the same two products in 12 and 15% yields, repectively, together with the hemiperacetal 4 (54% yield) formally derived from the secondary ozonide by addition of MeOH.     

Structural Effects on the Intrinsic Basicities of alpha,beta-Unsaturated Lactones and Ketones

The proton affinities of 2(5H)-furanone, 1 (836 kJ/mol), 5,6-dihydro-2H-pyran-2-one, 2 (862 kJ/mol), cyclopentenone, 3 (857 kJ/mol), and cyclohexenone, 4 (863 kJ/mol), have been measured by Fourier transform ion cyclotron resonance techniques. A comparison is made with (reexamined) data concerning saturated cyclic and unsaturated aliphatic analogs. Three general observations are made. First, the basicity is found to increase with the size of the ring. Second, unsaturated lactones are more basic than their corresponding aliphatic unsaturated esters. Third, unsaturated and saturated lactones have almost identical gas-phase basicities, while unsaturated and saturated lactones have almost identical gas-phase basicities, while unsaturated cyclic ketones are more basic than their saturated analogs. All these experimental findings have been rationalized by means of ab initio calculations up to the G2(MP2,SVP) level. The basicity trends along the series are the result of two main factors: the different hybridization pattern of the carbonyl carbon as the size of the ring changes and, in the case of lactones, the nonbonding interaction between the proton attached to the carbonyl group and the ether-like oxygen which contributes to the enhanced stability of the protonated form. For unsaturated ketones the C=C double bond participates fully in the change in charge distribution induced by the protonation, while for unsaturated lactones the existence of an oxygen atom within the ring impedes this shift of the electron density.     

A priori assessment of the stereoelectronic profile of phosphines and phosphites

This research has demonstrated the utility of a rigorously calibrated, molecular mechanics/semiempirical quantum mechanical protocol for developing stereoelectronic (Tolman) maps for phosphine ligands. A computational analysis of alkyl and aryl phosphines in common usage suggests that these ligands are quite similar stereoelectronically. A noticeable gap in the Tolman map for common phosphines is observed for large, electron-poor phosphines. Several candidates meeting these criteria were identified, the most promising of which is P(t-C(4)F(9))(3). Phosphines in which the phosphorus participates in a ring, which comprise a very small subset of reported phosphines, have very interesting stereoelectronic properties, particularly those in which the ligating phosphorus is part of a three-membered ring. In terms of steric properties, the symmetric deformation coordinate proposed by Orpen and co-workers on the basis of crystallographic studies is calculated with sufficient accuracy using PM3(tm) to allow good confidence in predictions of novel phosphines. For quantification of the electronic properties of phosphines, we analyzed changes in the CO stretching frequency upon changing the ancillary phosphine ligands.