In Operando Analysis of Diffusion in Porous Metal-Organic Framework Catalysts

The potential to exert atomistic control over the structure of site-isolated catalyst sites, as well as the topology and chemical environment of interstitial pore spaces, has inspired efforts to apply porous metal-organic frameworks (MOFs) as catalysts for fine chemical synthesis. In analogy to enzyme-catalyzed reactions, MOF catalysts have been proposed as platforms in which substrate confinement could be used to achieve chemo- and stereoselectivities that are orthogonal to solution-phase catalysts. In order to leverage the tunable pore topology of MOFs to impact catalyst selectivity, catalysis must proceed at interstitial catalyst sites, rather than at solvent-exposed interfacial sites. This Minireview addresses challenges inherent to interstitial MOF catalysis by 1) describing the diffusional processes available to sorbates in porous materials, 2) discussing critical factors that impact the diffusion rate of substrates in porous materials, and 3) presenting in operando experimental strategies to assess the relative rates of substrate diffusion and catalyst turnover in MOF catalysis. It is anticipated that the continued development of in operando tools to evaluate substrate diffusion in porous catalysts will advance the application of these materials in fine chemical synthesis.     

Auto‐Tandem Catalysis: A Single Catalyst Activating Mechanistically Distinct Reactions in a Single Reactor

Domino reactions have received great attention as efficient synthetic methodologies for the construction of structurally complex molecules from simple materials in a single operation. Catalysts in domino reactions have also been well studied. In these reactions, a catalyst activates the substrate(s) only once, and the structure of the product is delineated at that time. Recently, the new concept of “tandem catalysis” in domino reactions, in which catalyst(s) sequentially activate more than two mechanistically distinct reactions, has been proposed. Tandem catalysis is categorized into three subclasses: orthogonal‐, auto‐, and assisted‐tandem catalyses. Auto‐tandem catalysis is defined as a process in which one catalyst promotes more than two fundamentally different reactions in a single reactor. An overview of recent and significant achievements in auto‐tandem catalysis is presented in this paper.     

多级孔芳香骨架材料的合成及固载硫脲催化剂的研究

合成了一种多级孔芳香骨架材料(PAF-70); 使用由氨基修饰过的单体, 应用该合成策略得到了同样具有窄分布介孔的含有氨基活性位点的PAF材料, 并通过硫脲单体与其氨基活性位点的反应, 将硫脲基团引入PAF-70材料中, 获得了含有硫脲催化位点的材料(PAF-70-thiourea). 氮气吸附-脱附测试结果显示, PAF-70存在孔径分布较窄的介孔, 介孔孔径为3.8 nm, 与模拟计算值(约3.7 nm)吻合. 热重分析结果表明, PAF-70具有很高的热稳定性. PAF-70在大多数溶剂中可以稳定存在, 具有良好的化学稳定性. 将PAF-70-thiourea作为催化剂, 应用在N-溴代琥珀酰亚胺(NBS)氧化醇类的反应中, 其表现出较高的催化活性、 较高的稳定性和广泛的底物适用性. 与含有相同硫脲催化位点的金属有机框架(MOF)材料(IRMOF-3-thiourea)作为催化剂对比, 进一步证实PAFs材料非常适合作为催化有机反应的固载平台.     

Urea-Functionalized Fe4L6 Cages for Supramolecular Gold Catalyst Encapsulation to Control Substrate Activation Modes

The excellent catalytic performances of enzymes in terms of activity and selectivity are an inspiration for synthetic chemists and this has resulted in the development of synthetic containers for supramolecular catalysis. In such containers the local environment and pre-organization of catalysts and substrates leads to control of the activity and selectivity of the catalyst. Herein we report a supramolecular strategy to encapsulate single catalysts in a urea-functionalized Fe₄L₆ cage, which can co-encapsulate a functionalized urea substrate through hydrogen bonding. Distinguished selectivity is obtained, imposed by the cage as site isolation only allows catalysis through π activation of the substrate and as a result the selectivity is independent of catalyst concentration. The encapsulated catalyst is more active than the free analogue, an effect that can be ascribed to transitionstate stabilization rather than substrate pre-organization, as revealed by the MM kinetic data. The simple strategy reported here is expected to be of general use in many reactions, for which the catalyst can be functionalized with a sulfonate group required for encapsulation.     

Thermodynamic and kinetic stability of synthetic multifunctional rigid-rod beta-barrel pores: evidence for supramolecular catalysis

The lessons learned from p-octiphenyl beta-barrel pores are applied to the rational design of synthetic multifunctional pore 1 that is unstable but inert, two characteristics proposed to be ideal for practical applications. Nonlinear dependence on monomer concentration provided direct evidence that pore 1 is tetrameric (n = 4.0), unstable, and "invisible," i.e., incompatible with structural studies by conventional methods. The long lifetime of high-conductance single pores in planar bilayers demonstrated that rigid-rod beta-barrel 1 is inert and large (d approximately 12 A). Multifunctionality of rigid-rod beta-barrel 1 was confirmed by adaptable blockage of pore host 1 with representative guests in planar (8-hydroxy-1,3,6-pyrenetrisulfonate, KD = 190 microM, n = 4.9) and spherical bilayers (poly-L-glutamate, KD < or = 105 nM, n = 1.0; adenosine triphosphate, KD = 240 microM, n = 2.0) and saturation kinetics for the esterolysis of a representative substrate (8-acetoxy-1,3,6-pyrenetrisulfonate, KM = 0.6 microM). The thermodynamic instability of rigid-rod beta-barrel 1 provided unprecedented access to experimental evidence for supramolecular catalysis (n = 3.7). Comparison of the obtained kcat = 0.03 min(-1) with the kcat approximately 0.18 min(-1) for stable analogues gave a global KD approximately 39 microM3 for supramolecular catalyst 1 with a monomer/barrel ratio approximately 20 under experimental conditions. The demonstrated "invisibility" of supramolecular multifunctionality identified molecular modeling as an attractive method to secure otherwise elusive insights into structure. The first molecular mechanics modeling (MacroModel, MMFF94) of multifunctional rigid-rod beta-barrel pore hosts 1 with internal 1,3,6-pyrenetrisulfonate guests is reported.     

Transient Substrate‐Induced Catalyst Formation in a Dynamic Molecular Network

In biology enzyme concentrations are continuously regulated, yet for synthetic catalytic systems such regulatory mechanisms are underdeveloped. We now report how a substrate of a chemical reaction induces the formation of its own catalyst from a dynamic molecular network. After complete conversion of the substrate, the network disassembles the catalyst. These results open up new opportunities for controlling catalysis in synthetic chemical systems.     

The esterolytic activity of poly(1-methyl-4- and -5-vinylimidazole) in water.

The water solubility of poly(1-Me-5-VIm) has made it possible to achieve phenomenal rate enhancements and to gain even greater insight into the mechanism of catalysis by polymeric imidazoles. The poly(1-Me-5-VIm)-catalyzed hydrolysis of S12- exhibited saturation in excess catalyst nad in excess substrate. Inhibition of the poly(1-Me-5-Vim) catalyzed hydrolysis of Sn- type substrates by analog inhibitors was also observed. The saturation apparently did not follow a simple Michaelis-Menten mechanism; however, the results could be rationalized by analogy to certain enzymatic systems. Multisite enzymes have long been known to display kinetic patterns different from that exhibited by enzymes with only one active site, i.e. such phenomena as sigmoidal rate vs [S] plots. These phenomena may arise entirely as a result of the multisite nature of the enzyme. Consequently, a synthetic macromolecular catalyst with multiple sites might also be expected to display such characteristics. The poly(1-Me-5-VIm)-catalyzed hydrolysis of S12- is apparently the first synthetic system in which such phenomena have been observed. The intermediacy of an apolar polymer substrate complex for the poly(1-Me-5-VIm)-catalyzed hydrolysis of S12- in water was given support by studies of the effect of temperature on the rate of hydrolysis. Activation parameters were determined for catalysis by 1,5-DMIm and by poly(1-Me-5-VIm). These results showed that the rate enhancement exhibited by the polymer was due entirely to a favorable entropy term.     

Remote desymmetrization at near-nanometer group separation catalyzed by a miniaturized enzyme mimic

The chirality of biological receptors often requires syntheses of therapeutic compounds in single enantiomer form. The field of asymmetric catalysis addresses enantioselective synthesis with chiral catalysts. Chemical differentiation of sites within molecules that are separated in space by long distances presents special challenges to chiral catalysts. As the distance between enantiotopic sites increases within a substrate, so too may the requirements for size and complexity for the catalyst. The extreme of catalyst complexity could be defined by macromolecular enzymes and their amazing capacity to effect stereospecific reactions over long distances between reactive sites and enzyme-substrate contacts. We report here a synthetic, miniaturized enzyme mimic that catalyzes a desymmetrization reaction over a very long distance.     

Oxidation of 2-acetoxytoluene with ozone in acetic anhydride

Kinetics and mechanism of the reaction of ozone with 2-acetoxytoluene in acetic anhydride in the presence of sulfuric acid were studied. It was shown that the prevailing reaction route under these conditions is ozonolysis (89%), and the selectivity of the oxidation of the substrate by the methyl group is no higher 9%. However, in the presence of manganese(II) sulfate as a catalyst, the selectivity increases to 76%. The major reaction products were 2-acetoxybenzyl acetate (59%) and 2-acetoxybenzylidene diacetate (17%). In the presence of a manganese bromide catalyst, the oxidation depth increases, and the major reaction product is already 2-acetoxybenzylidene (66%), while the yield of 2-acetoxybenzyl acetate is 15%. The mechanism of the redox catalysis is considered, that explains the experimental results.     

Asymmetric Phase‐Transfer‐Catalyzed Intramolecular N‐Alkylation of Indoles and Pyrroles: A Combined Experimental and Theoretical Investigation

Asymmetric phase‐transfer catalysis (PTC) has risen to prominence over the last decade as a straightforward synthetic methodology for the preparation of pharmacologically active compounds in enantiomerically pure form. However, the complex interplay of weak nonbonded interactions (between catalyst and substrate) that could account for the stereoselection in these processes is still unclear, with tentative pictorial mechanistic representations usually proposed. Here we present a full account dealing with the enantioselective phase‐transfer‐catalyzed intramolecular aza‐Michael reaction (IMAMR) of indolyl esters, as a valuable synthetic tool to obtain added‐value compounds, such as dihydro‐pyrazinoindolinones. A combined computational and experimental investigation has been carried out to elucidate the key mechanistic aspects of this process.     

Quantitative analysis of the effect of salt concentration on enzymatic catalysis.

Like pH, salt concentration can have a dramatic effect on enzymatic catalysis. Here, a general equation is derived for the quantitative analysis of salt-rate profiles: k(cat)/K(M) = (k(cat)/K(M))(MAX)/[1+([Na+]/K[Na+])(n')], where (k(cat)/K(M))(MAX) is the physical limit of k(cat)/K(M), K(Na+) is the salt concentration at which k(cat)/K(M) = (k(cat)/K(M))(MAX)/2, and -n' is the slope of the linear region in a plot of log(k(cat)/K(M)) versus log [Na+]. The value of n' is of special utility, as it reflects the contribution of Coulombic interactions to the uniform binding of the bound states. This equation was used to analyze salt effects on catalysis by ribonuclease A (RNase A), which is a cationic enzyme that catalyzes the cleavage of an anionic substrate, RNA, with k(cat)/K(M) values that can exceed 10(9) M(-1) s(-1). Lys7, Arg10, and Lys66 comprise enzymic subsites that are remote from the active site. Replacing Lys7, Arg10, and Lys66 with alanine decreases the charge on the enzyme as well as the value of n'. Likewise, decreasing the number of phosphoryl groups in the substrate decreases the value of n'. Replacing Lys41, a key active-site residue, with arginine creates a catalyst that is limited by the chemical conversion of substrate to product. This change increases the value of n', as expected for a catalyst that is more sensitive to changes in the binding of the chemical transition state. Hence, the quantitative analysis of salt-rate profiles can provide valuable insight into the role of Coulombic interactions in enzymatic catalysis.     

Highly Efficient and Stereoselective Thioallylation of Alkynes: Possible Gold Redox Catalysis with No Need for a Strong Oxidant

Stereoselective thioallylation of alkynes under possible gold redox catalysis was accomplished with high efficiency (as low as 0.1 % catalyst loading, up to 99 % yield) and broad substrate scope (various alkynes, inter‐ and intramolecular fashion). The gold(I) catalyst acts as both a π‐acid for alkyne activation and a redox catalyst for Au^I/III coupling, whereas the sulfonium cation generated in situ functions as a mild oxidant. This novel methodology provides an exciting system for gold redox catalysis without the need for a strong oxidant.     

Recycling of homogeneous hydrogenation catalysts by dialysis coupled catalysis

Although transition-metal complexes are very attractive as homogeneous catalysts in fine chemistry, their high prices often limit their applications. A means to recycle those catalysts would solve this problem and would simultaneously facilitate the downstream purification of the product. This is now realized in a new concept in which homogeneous catalysis is coupled to dialysis. The advantages of homogeneous catalysis (off-the-shelf catalysts, high activities and selectivities) are thus combined with those of heterogeneous catalysis (easy catalyst separation from product solution, reuse of catalyst, and possibility for continuous operation). Since the heart of the process is the membrane, self-prepared membranes were preferred as they allow a better control and understanding of the separation characteristics. Rhodamine B was used as a probe molecule to define the working conditions of the membrane. The concept is proven to work for two relevant chiral reactions: a hydrogenation with Ru-BINAP and a hydrogen transfer reaction with Ru-TsDPEN [BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine); TsDPEN= tosyl-N,N'-diphenyl-1,2-ethanediamine].     

Thermoregulated Liquid/Liquid Biphasic Catalysis and Its Application

A brief overview of our recent research results of thermoregulated liquid/liquid biphasic catalysis is presented. Emphasis is given to the general principles of thermoregulated phase-transfer catalysis (TRPTC) and thermoregulated phase-separable catalysis (TPSC). In addition, the applications of TRPTC and TPSC in biphasic catalysis are also discussed. The introduction of TRPTC to biphasic system is free from the shortcomings of classical aqueous/organic two-phase catalysis, in which the application scope is restrained by the water solubility of the substrate. Meanwhile, TPSC provides a very simple and reliable way to deal with the separation of catalyst in homogeneous catalysis. The common advantages of TRPTC and TPSC are characterized by homogeneous catalysis coupled with convenient biphasic separation.     

水蒸气气氛煤中温催化气化动力学研究

以碳酸钾为催化剂,用热天平的等温热重法研究了四种不同变质程度煤焦常压下水蒸气催化气化反应动力学。在加和不加碳酸钾条件下,测定了温度为700~850℃煤焦的化学反应控制条件下的碳转化率与时间的关系。碳酸钾催化剂的加入对变质程度越高煤的气化催化作用越大。加碳酸钾的碳转化率与时间的关系用混合模型和修正随机孔模型可以良好的拟合关联,均相模型关联较差。利用修正随机孔模型拟合关联出了四种煤焦催化水蒸气气化反应的活化能和指前因子,活化能为90.317~167.861kJ/mol,指前因子和活化能之间具有补偿效应。     

Cover Picture

The cover picture shows a representation of supramolecular cluster catalysis, which is commonly regarded as a field lying at the interface of homogeneous and heterogenous catalysis. Homogeneous catalysis is epitomized by Halpern's elucidation of the molecular details of the enantioselective hydrogenation of prochiral alkenes (top flask) catalyzed by a rhodium diphosphane complex. The three‐way catalyst is the emblem of heterogeneous catalysis (left flask). The central structure depicts a cluster capable of hydrogenating aromatic substrates under mild biphasic conditions. Most interestingly, the mechanism relies solely on hydrophobic interactions between the catalyst and the substrate. Such weak contacts are reminiscent of enzymatic catalysis as exemplified by triterpene cyclases, which convert squalenes into steroid precursors (right flask). The system described thus lies at the interface of enzymatic, homogeneous, and heterogeneous catalysis. Further details are reported by Süss‐Fink et al. on p. 99 ff.     

The adequacy of the observed kinetic order in catalyst and the differential selectivity patterns to the hypothesis of the cooperative mechanism of catalysis of the Suzuki—Miyaura reaction

The generally accepted mechanism of the Suzuki—Miyaura reaction suggests a sequential activation of the substrate (aryl halide) and the reagent (arylboronic acid) by a palladium catalyst with the formation of unsymmetric biaryl as a result of a single turnover of the catalytic cycle, i.e., it is linear from the kinetic point of view. At the same time, the use of an unconventional kinetic approach based on the analysis of the differential selectivity of the reaction, rather than the regularities of catalytic activity, indicates the inadequacy of the linear mechanism, that is consistent with the hypothesis of a nonlinear (the so-called cooperative) mechanism of catalysis, in which the product is formed as a result of the substrate and reagent activation by two different palladium-containing intermediates in two parallel catalytic cycles. The experimentally observed low kinetic orders of the Suzuki—Miyaura reaction with respect to the concentration of the palladium catalyst precursor under the ligand-free conditions of catalysis are also consistent with the cooperative mechanism and can be due to the changes in the relative amount of the catalyst in two parallel catalytic cycles and/or to the process of catalyst deactivation.

     

Homogeneous chemical catalysis of electrochemical reactions: Competition with catalyst consumption

Chemical catalysis as opposed to redox catalysis involves an electrophilic-nucleophilic reaction between the substrate and the reduced form of the catalyst instead of an electron transfer reaction between these two species. The adduct thus formed then decomposes regenerating the catalyst. Decomposition may, however, occur along a competing pathway leading to a side-product with progressive consumption of the catalyst. The kinetics of the competition is analyzed in the framework of polarography, rotating disc electrode voltammetry and controlled potential coulometry. Two main cases are considered corresponding respectively to a one- and a two-electron catalysis. The effect of the initial concentration on the shifting of the system from one case to the other is discussed. This is finally compared to what occurs when redox analysis replaces chemical catalysis in the competition with the catalyst consumption.     

SEM研究PET核孔膜的光接枝聚合

以PET核孔膜为基材 ,二苯甲酮为引发剂 ,采用光接枝方法实现了丙烯酸和丙烯酰胺在核孔膜上的接枝 ,用扫描电镜 (SEM)直接观察了接枝前后膜的表面形貌 ,考察了不同因素对于接枝位置和接枝效果的影响 .发现膜材料本身特性和接枝反应条件对接枝位置和接枝效果有较大影响 .通过光接枝能够实现膜孔的封盖、缩小、填堵等不同的效果 .采用正侧涂布法反应 ,标准直孔 ,特别是小孔径膜 (0 4 μm) ,不利于孔内的接枝 ,接枝主要在膜的表面 ,从而产生孔封盖效应 .双锥形的非标准直孔 ,由于孔壁的受光性好 ,容易发生孔壁上的接枝从而被填充 .大孔径的膜 (5 μm) ,需要加入交联剂才能在孔壁上形成厚的接枝层 .提出了一种新的反应方法 背侧吸附法 ,反应液依靠毛细作用由膜的底部吸入膜孔 ,膜的向光侧表面不存在反应液 ,接枝只发生在膜孔内 ,从而得到很好的填孔效果 .     

Triphase Catalysis [New synthetic methods (27)]

Triphase catalysis (TC) has recently been introduced as a unique form of heterogeneous catalysis in which the catalyst and each of a pair of reactants are located in separate phases. Based on this concept, new synthetic methods have been developed for aqueous phase–organic phase reactions using a solid phase catalyst. Although it is only at an early stage of development, TC shows considerable potential for practical use. Our mechanistic understanding of these highly complex catalytic systems is at present very limited and detailed examination will be required before their relationship to phase-transfer, micellar, and interfacial catalysis becomes clear.