Chiral hydroperoxides as oxygen source in the catalytic stereoselective epoxidation of allylic alcohols by sandwich-type polyoxometalates: control of enantioselectivity through a metal-coordinated template

The epoxidation of allylic alcohols is shown to be efficiently and selectively catalyzed by the oxidatively resistant sandwich-type polyoxometalates, POMs, namely [WZnM(2)(ZnW(9)O(34))(2)](q)(-) [M = OV(IV), Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], with organic hydroperoxides as oxygen source. Conspicuous is the fact that the nature of the transition metal M in the central ring of polyoxometalate affects significantly the reactivity, chemoselectivity, regioselectivity, and stereoselectivity of the allylic alcohol epoxidation. For the first time, it is demonstrated that the oxovanadium(IV)-substituted POM, namely [ZnW(VO)(2)(ZnW(9)O(34))(2)](12-), is a highly chemoselective, regioselective, and also stereoselective catalyst for the clean epoxidation of allylic alcohols. A high enantioselectivity (er values up to 95:5) has been achieved with [ZnW(VO)(2)(ZnW(9)O(34))(2)](12)(-) and the sterically demanding TADOOL-derived hydroperoxide TADOOH as regenerative chiral oxygen source. Thus, a POM-catalyzed asymmetric epoxidation of excellent catalytic efficiency (up to 42 000 TON) has been made available for the development of sustainable oxidation processes. The high reactivity and selectivity of this unprecedented oxygen-transfer process are mechanistically rationalized in terms of a peroxy-type vanadium(V) template.     

Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?

This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X.     

Effect of Co-solvent on the Palladium Catalyzed Alkoxycarbonylation of Allyl Bromide in Supercritical CO_2

We have studied the palladium-catalyzed alkoxycarbonylation of allyl bromide in supercritical (sc) CO2 and found the reaction rate in sc CO2 was lower than those in some organic solvents1. The possible reason was the lower solubility of sc CO2 for the palladium catalyst. As an effective approach to improve the solubility of varieties of solutes, cosolvent such as ethanol has been widely used in the sc CO2 extraction2,3. Based on the above consideration, we chose several organic reagents…     

DETECTION OF T_g BY MODULATED DIFFERENTIAL SCANNING CALORIMETRY

The glassy transition of the polyethylene terephthalate (PET) sampleswhich have been subjected to solvent induced crystallization (SINC) was investigated bymodulated differential scanning calorimetry (MDSC) and density measurement. The dif-ferential of heat capacity signal, d C_p/dT from MDSC, was used to monitor the SINCprocess. It reveals that the T_g temperature shifts to higher value with the advancement ofSINC. When the toluene-immersing time was longer (168h), the detection of T_g becomemore difficult, because some smaller peaks emerged at the lower temperatures and theseare explained as the movement of small segments in the amorphous region. These observedresults are due to the morphology and structure introduced by the SINC process.     

Mitochondria-targeted ratiometric fluorescent probes for micropolarity and microviscosity and their applications

Two fluorescent "off-on" probes YYH1 and YYH2 were used for bioimaging mitochondrial polarity and viscosity.     

Matrix-isolation FT-IR spectra and theoretical study of dimethyl sulfate

The preferred conformations of dimethyl sulfate and their vibrational spectra were studied by matrix-isolation FT-IR spectroscopy and theoretical methods (DFT and MP2, with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Conformer GG (of C2 symmetry and exhibiting OSOC dihedral angles of 74.3 degrees ) was found to be the most stable conformer in both the gaseous phase and isolated in argon. Upon annealing of the matrix, the less stable observed conformer (GT; with C1 symmetry) quickly converts to the GG conformer, with the resulting species being embedded in a matrix-cage which corresponds to the most stable matrix-site for GG form. The highest energy TT conformer, which was assumed to be the most stable conformer in previous studies, is predicted by the calculations to have a relative energy of ca. 10 kJ mol-1 and was not observed in the spectra of the matrix-isolated compound.     

光催化降解染料ZnO催化剂的性能

光催化法是20世纪70年代发展起来的新型水处理技术，即以某些半导体材料为催化剂，利用光催化法来降解环境污染物。该方法工艺简单，成本较低，在常温下能使大多数不能或难于生物降解的有毒有机物彻底氧化分解。近年来研究最多的是TiO2，由于其带隙较宽(3．2eV)，只能吸收波长λ≤387nm的紫外光，因此研制新型光催化剂仍是重要课题。     

Efficient asymmetric synthesis of an azasugar in water

An extremely efficient asymmetric synthesis of a pyrrolidine azasugar was completed in only four steps in water, without the use of protecting groups and in 60% overall yield from a simple, achiral bis-electrophile.     

吸热型碳氢燃料在银、镧改性混合分子筛上的裂解研究

为进一步提高吸热型碳氢燃料的吸热能效，考察了吸热型碳氢燃料NNJ－150在USHY和HZSM－5混合分子筛以及银、镧改性混合分子筛催化剂上的催化裂解。结果表明，在Ag-LaUSY Ag-LaZSM-5(75:25)混合分子筛催化NNJ－150裂解反应中，低碳烯烃选择性较高，催化剂寿命较长。采用此催化剂，能较好满足吸热型碳氢燃料裂解的需要。     

我国化学纤维新品种的发展

本文紧紧围绕发展我国化纤新品种的现状和展望,提出由“产品先导型”向“技术先导型”、由“市场开发型”向“发展型”过渡的发展展望。按化纤新品种几种主要发展途径,侧重就我国化纤新品种的成就,如:合成新的高聚物,化学、物理(包括结构)改性和纱线、织物后整理等,特别对共聚、共混、多种复合技术改性等,进行了重点讨论。