A highly efficient and extensively reusable "dip catalyst" based on a silver-nanoparticle-embedded polymer thin film

Achieving a harmonious combination of the efficiency of homogeneous catalysts with the reusability of heterogeneous catalysts is a fundamental and challenging problem. Metal nanoparticles in a suitable matrix offer a potential solution. However an ideal design is yet to be realized, because the critical requirements of facile access to the catalyst, its durability, and ease of retrieval and reuse are difficult to reconcile. We report herein a multilayer free‐standing thin‐film catalyst based on silver nanoparticles, generated in situ inside poly(vinyl alcohol) by using a facile protocol, which shows excellent efficiency and extensive reusability in the prototypical reaction, the reduction of 4‐nitrophenol by sodium borohydride. The “dip catalyst” film, which can start/stop the reaction instantaneously by mere insertion/removal, is used 30 times leading to a total turnover number (TON) of ≈3390, which is unprecedented for this reaction. The efficiency of the catalyst is reduced only marginally at the end of these runs, promising further extended usage. The unique advantage of convenient catalyst monitoring is illustrated by the periodic spectroscopic and microscopic examinations of the thin film, which revealed the basis of its durability. The demonstrated potential of metal‐nanoparticle‐embedded polymer thin films, coupled with their versatility and ease of fabrication, promises extensive applications in chemical catalysis.     

"Designer acids": combined acid catalysis for asymmetric synthesis

Lewis and Brønsted acids can be utilized as more‐effective tools for chemical reactions by sophisticated engineering (“designer acids”). The ultimate goal of such “designer acids” is to form a combination of acids with higher reactivity, selectivity, and versatility than the individual acid catalysts. One possible way to take advantage of such abilities may be to apply a “combined acids system” to the catalyst design. The concept of combined acids, which can be classified into Brønsted acid assisted Lewis acid (BLA), Lewis acid assisted Lewis acid (LLA), Lewis acid assisted Brønsted acid (LBA), and Brønsted acid assisted Brønsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more‐organized structures that allow an effective asymmetric environment.     

铜铝尖晶石“缓释催化”*

催化化学是物理化学中的重要内容,有着较强的实践意义。催化剂寿命是催化剂的主要指标之一,关系工业催化的经济成本。常用的负载型催化剂常由于反应温度高导致活性金属聚集长大而失活,采取增加载体表面积、提高负载量、增强金属/载体的相互作用等方法可以在一定程度上提高催化剂的稳定性和寿命,却无法从根本上阻止活性金属聚集长大的热力学趋势。近年来笔者将活性铜组分均匀分散到氧化铝体相中形成尖晶石型缓释催化剂,从而有效增加了催化剂的稳定性,有望充实物理化学中关于催化化学的内容。     

Superbeads: immobilization in "sweet" chemistry

Enzymatic oligosaccharide synthesis using recombinant glycosyltransferases is able to overcome the difficulties associated with chemical methods. Nonetheless, sugar nucleotide regeneration cycles are necessary for the glycosylation. The multistep enzyme reaction can be efficiently carried out on superbeads that are prepared by immobilizing multienzyme mixtures on bead support through fused binding domains.     

Rapid ketone transfer hydrogenation by employing simple, in situ prepared iridium(I) precatalysts supported by "non-N--H" P,N ligands

The catalytic utility in ketone transfer hydrogenation (TH) of the preformed complexes [Ir(cod)(kappa(2)-2-NMe(2)-3-PiPr(2)-indene)](+)X(-) ([2 a](+)X(-); X: PF(6), BF(4), and OTf; cod: eta(4)-1,5-cyclooctadiene; OTf: trifluoromethanesulfonate), [Ir(cod)(kappa(2)-1-PiPr(2)-2-NMe(2)-indene)](+)OTf(-) ([2 b](+)OTf(-)), [Ir(cod)(kappa(2)-2-NMe(2)-3-PiPr(2)-indenide)] (3), and [Ir(cod)(kappa(2)-o-tBu(2)P-C(6)H(4)-NMe(2))](+)PF(6) (-) ([4](+)PF(6) (-)), as well as of related mixtures prepared from [{IrCl(cod)}(2)] and various P,N-substituted indene or phenylene ligands, was examined. Whereas [2 a](+)X(-), [2 b](+)OTf(-), 3, and related in situ prepared Ir catalysts derived from P,N-indenes proved to be generally effective in mediating the reduction of acetophenone to 1-phenylethanol in basic iPrOH at reflux (0.1 mol % Ir; 81-99 % conversion) in a preliminary catalytic survey, the structurally related Ir catalysts prepared from (o-R(2)P-C(6)H(4))NMe(2) (R: Ph, iPr, or tBu) were observed to outperform the corresponding P,N-indene ligands under similar conditions. In the course of such studies, it was observed that alteration of the substituents at the donor fragments of the supporting P,N ligand had a pronounced influence on the catalytic performance of the derived catalysts, with ligands featuring bulky dialkylphosphino donors proving to be the most effective. Notably, the crystallographically characterized complex [4](+)PF(6) (-), either preformed or prepared in situ from a mixture of [{IrCl(cod)}(2)], NaPF(6), and (o-tBu(2)P-C(6)H(4))NMe(2), proved to be highly effective in mediating the catalytic transfer hydrogenation (TH) of ketones in basic iPrOH, with near quantitative conversions for a range of alkyl and/or aryl ketones and with very high turnover-frequency values (up to 230 000 h(-1) at >50 % conversion); this thereby enabled the use of Ir loadings ranging from 0.1 to 0.004 mol %. Catalyst mixtures prepared from [{IrCl(cod)}(2)], NaPF(6), and the chiral (alphaS,alphaS)-1,1'-bis[alpha-(dimethylamino)benzyl]-(R,R)-2,2'-bis(dicyclohexylphosphino)ferrocene (Cy-Mandyphos) ligand proved capable of mediating the asymmetric TH of aryl alkyl ketones, including that of the hindered substrate 2,2-dimethylpropiophenone with an efficiency (0.5 mol % Ir; 95 % conversion, 95 % ee) not documented previously in TH chemistry.     

Similarities and differences between the "relativistic" triad gold, platinum, and mercury in catalysis

Relativistic effects in the valence shell of the elements reach a maximum in the triad Pt-Au-Hg and determine their catalytic activity in organic reactions. In this Review we examine the catalytic activity of Pt, Au, and Hg compounds for some representative reactions, and discuss the respective benefits and disadvantages along with other relevant properties, such as toxicity, price, and availability. For the reactions considered, gold catalysts are generally more active than mercury or platinum catalysts.     

d-Mannitol-derived novel chiral thioureas: Synthesis and application in asymmetric Henry reactions

Five novel thioureas have been obtained through multi-step reactions from d-Mannitol as starting material and applied as catalysts in the asymmetric Henry reaction. Using catalyst 7a, (1S,2R)-2-nitro-1-phenylpropan-1-ol containing two chiral centers was obtained in high yield and with high selectivity (up to 95% yield, 87% ee, 91:9 dr). This catalyst also retained activity in the presence of water, affording a up to 93% yield, 88% ee, and 94:6 dr.     

"Nonsolvent" applications of ionic liquids in biotransformations and organocatalysis

The application of room-temperature ionic liquids (RTILs) as (co)solvents and/or reagents is well documented. However, RTILS also have "nonsolvent" applications in biotransformations and organocatalysis. Examples are the anchoring of substrates to RTILs; ionic-liquid-coated enzymes (ILCE) and enzyme-IL colyophilization; the construction of biocatalytic ternary reaction systems; the combination of enzymes, RTILs, membranes, and (bio)electrochemistry; and ionic-liquid-supported organocatalysts. These strategies provide more robust, more efficient, and more enantioselective bio- and organocatalysts with many practical applications. As shown herein, RTILs offer a wide range of promising alternatives to conventional chemistry.     

生物化学中酶促"不可逆"反应及其相关问题辨析

在生物化学教学和研究中,一些酶促反应被归类为不可逆反应。然而,此处的"不可逆"与物理化学中的"不可逆"有所不同,这一表述本身常导致学生的困惑。不仅如此,"不可逆"的实际含义及其衍生的观念可能进一步使学生产生很多对基本生化反应过程的严重误解。本文以物理化学所定义的"不可逆"为准,重新讨论生物化学中所指"不可逆"的准确含义,并举例讨论了包括"速控步""限速酶""高能磷酸键"在内的若干因生物化学中"不可逆"的不准确定义造成的认知误区。     

“手性分子博物馆”之旅

This article describes a dream of Little R visiting the 'Chiral Molecule Museum'. It introduces the structure of chiral molecules, the experiment to demonstrate the existence of chiral molecules-experiments on the separation of optical isomers of sodium ammonium tartrate by Pasteur, the two enantiomers of limonene; chiral drug thalidomide, and asymmetric catalysis.