Studies on the Lanthanum‐Induced Pinacol Coupling of Aromatic Aldehydes and Ketones in Aqueous Media

The pinacol coupling of aromatic aldehydes and ketones was carried out in 16–71% yield using La–36%HAc–CH₂Cl₂ system with stirring at rt in 4 h. The reactions in the same system gave pinacols in 12–91% yield under ultrasound irradiation at rt in 2 h.     

Highly Enantioselective Aldol Reactions between Acetaldehyde and Activated Acyclic Ketones Catalyzed by Chiral Primary Amines

Highly enantioselective cross‐aldol reactions between acetaldehyde and activated acyclic ketones are reported for the first time. Various acyclic ketones, such as saturated and unsaturated keto esters, reacted with acetaldehyde in the presence of a chiral primary amine and a Brønsted acid to afford optically enriched tertiary alcohols in good yields and with excellent enantioselectivities. Trifluoromethyl ketones were tolerable under the reaction conditions, thereby affording the trifluoromethyl carbinol in good‐to‐excellent yields and enantioselectivities. Structural modification of the chiral amines from the same chiral source switched the stereoselectivity of the products. The utility of aldol chemistry was demonstrated in the brief synthesis of functionally enriched δ‐lactones. Theoretical calculations on the transition‐state structure indicated that the protonated tertiary amine could effectively activate the carbonyl group of a keto ester to promote the addition process through hydrogen‐bonding interaction and, simultaneously, provide an appropriate attacking pattern for the approach of the keto ester to the enamine, which is formed from acetaldehyde and the chiral catalyst, on a particular face, resulting in high enantioselectivity.     

Near‐Infrared‐Emitting CdxHg1−xSe Nanorods Fabricated by Ion Exchange in an Aqueous Medium

Whereas CdSe nanorods that are grown in organic solution have a hexagonal wurtzite structure, which is the limiting case for exchange, HgSe is more commonly encountered as a cubic zinc blende system. An exchange process was performed at room temperature and at atmospheric pressure in an aqueous environment after phase transfer of the original CdSe nanorods, which reinforced the tendency for the endpoint of HgSe to be cubic. Consequently, we observed that under ambient conditions, the exchange process terminated with an average composition of only Cd_0.9Hg_0.1Se. Following the changes during the process by optical spectroscopy and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), we observed that the Hg²⁺ ions diffused into the rods to a point limited by the formation of stacking faults due to the different lattice structures of the two limiting cases of zinc blende and wurtzite. HAADF‐STEM and energy dispersive spectroscopy analyses also confirmed that the Hg substitution did not occur uniformly throughout the individual nanorods, as Hg‐poor and Hg‐rich regions coexist around the stacking faults. The formation of near‐infrared‐emitting alloyed Cd_xHg_1?xSe nanorods in an aqueous medium highlights the subtle dependence of the ion‐exchange process on the differences in the crystal structures of the two endpoint lattices.     

A Robust Palladium(II)–Porphyrin Complex as Catalyst for Visible Light Induced Oxidative C?H Functionalization

A series of palladium(II)–porphyrin complexes that display dual emissions with lifetimes up to 437 μs have been synthesized. Among the four complexes, PdF₂₀TPP is an efficient and robust catalyst for photoinduced oxidative C? H functionalization by using oxygen as terminal oxidant. α‐Functionalized tertiary amines were obtained in good to excellent yields by light irradiation (λ>400 nm) of a mixture of PdF₂₀TPP , tertiary amine, and nucleophile (cyanide, nitromethane, dimethyl malonate, diethyl phosphite, and acetone) under aerobic conditions. Four examples of intramolecular cyclized amine compounds could be similarly prepared. Comparison of the UV‐visible absorption spectra before and after the photochemical reaction revealed that PdF₂₀TPP was highly robust (>95 % recovery). The practical application of PdF₂₀TPP has been revealed by the photochemical reactions performed by using a low catalyst loading (0.01 mol %) and on a 10 mmol scale. The PdF₂₀TPP catalyst could sensitize photoinduced oxidation of sulfides to sulfoxides in excellent yields. Mechanistic studies revealed that the photocatalysis proceeded by singlet‐oxygen oxidation.     

基于PMAC的碳素电极螺纹加工车床数控化改造

针对普通车床加工大直径、大牙形碳素电极螺纹存在的精度和效率问题,根据车床加工锥螺纹的工作原理,采用旋风铣削的方式,构建基于PMAC的开放式数控系统,将CW61160普通车床改造为数控碳素电极螺纹加工专用机床,改造后的车床能完成程序解释、计算插补、伺服控制等任务.通过实际生产检验,车床改造部分满足国家数控重型卧式车床精度标准要求.     

人参茎中水溶性多糖的研究——酸性杂多糖S—3的纯化与结构测定

从人参(Panax ginseng C.A.Mey)茎中提取的水溶性多糖经酸性乙醇分级和氯化钙分级得酸性杂多糖 S—3,S—3主要由半乳糖醛酸构成。果胶酶解、高碘酸氧化和 Smith 降解等实验表明 S—3的主链由α(1→4)连接的半乳糖醛酸构成。     

A molecular mechanics (MM2) study of Furan,Thiophene, and related compounds

The molecular mechanics calculations reported earlier for nitrogen heterocycles have now been extended to include the title compounds, and related molecules. It is in general possible to calculate these structures with an accuracy that compares favorably with experiment.     

GI/G/1系统等待时间分布的数值近似法

求解排队系统的等待时间分布对于系统规划及性能分析具有重要意义 ,在排队系统 (GI/G/1)中这一问题通常难以得到显式的理论解。从该问题的 Wiener- Hopf积分方程出发 ,利用排队系统的固有特征将问题转化为一个线性方程组 ,并讨论了使用迭代法求解该方程组的收敛性和复杂度。文中给出了几种系统模型下的数值实验数据 ,并与已有方法进行了比较 ,结果表明 :该方法在不同模型、不同负载下均能给出精确的计算结果 ,实验中通过合理选择计算参数可将误差控制在 0 .0 5 %以内。该方法易于实现、计算效率高 ,具有较好的实用性。     

零售业上市公司现金持有水平研究

现金是企业生存发展的"血液",它与企业的经营发展息息相关,在行业竞争的大环境下,发挥现金持有的竞争优势可以提升企业的市场竞争力.持有现金的机会成本和短缺成本都会影响企业的价值,因此确定合理的现金持有水平尤为重要.本文以我国A股零售行业85家上市公司2010～2019年的数据作为研究对象,研究了零售行业的现金持有水平的影响因素,包括财务特征和公司治理两个角度,并提出了对策与建议.