Synergistic catalysis

Heteropoly acids can be synergistically combined with phase-transfer catalysis in the so-called Ishii-Venturello chemistry for oxidation reactions such as oxidation of alcohols, allyl alcohols, alkenes, alkynes, P-unsaturated acids, vic-diols, phenol, and amines with hydrogen peroxide (Mizuno et al., 1994). Recent examples include the epoxidations of alkyl undecylenates (Yadav and Satoskar, 1997) and. styrene (Yadav and Pujari, 2000). 

From the fundamental knowledge concerning the interfacial complexation mechanism obtained from the kinetic studies on chelate extraction, ion-association extraction, and synergistic extraction, one can design the interfacial catalysis. The main strategy is to raise the concentration of the reactant or intermediate at the interface. 

This excellent regiocontrol was exploited by subjecting terminal alkenes and hydroxyalkynoates to ruthenium catalysis conditions to afford butenolides and pentenolides (Equation (23)).36 The Alder-ene reaction occurs preferentially to form the G-G bond at the alpha-carbon of the alkynoate. The unusually high regioselectivity is attributed to a synergistic effect derived from an enhanced coordination of the hydroxyl group to the ruthenium. 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

Homogeneous Chemical Catalysis of the Reduction of Carbon Dioxide. Synergistic Effect of Bronsted and Lewis Acids... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

Figure 5 Survey of possible synergistic promotion effects occurring in Co-based Fischer-Tropsch catalysis (A) water-gas shift reaction, (B) hydrogenationjdehydrogenation reactions and (C) H2S adsorption...

Studies of catalytic asymmetric Mukaiyama aldol reactions were initiated in the early 1990s. Until recently, however, there have been few reports of direct catalytic asymmetric aldol reactions [1]. Several groups have reported metallic and non-metallic catalysts for direct aldol reactions. In general, a metallic catalysis involves a synergistic function of the Bronsted basic and the Lewis acidic moieties in the catalyst (Scheme 2). The Bronsted basic moiety abstracts an a-pro-ton of the ketone to generate an enolate (6), and the Lewis acidic moiety activates the aldehyde (3). 

Scheme 11.1 also summarizes other impressive examples of the performance of the CBS method [1-8]. Several excellent reviews on the CBS method have appeared recently [1, 2], and no detailed discussion of the development of the process or substrate scope shall be presented in this review. Please note, however, that the oxazaborolidine-catalyzed borane reduction of ketones is a prime example of bi-functional catalysis [2, 9] - as shown in Scheme 11.2, the current mechanistic picture involves simultaneous binding of both the ketone and the borane to the Lewis-acidic (boron) and Lewis-basic (nitrogen) sites of the catalyst A. In the resulting ternary complex B, the reaction partners are synergistically activated toward hydride transfer. 

Li, J., L. Zheng, L. Li, G. Shi, Y. Xian, and L. Jin. 2007. Photoelectro-synergistic catalysis combined with a FLA system application on determination of chemical oxygen demand. Talanta 72 1752-1756. 

Transformation of the 5-oxopentanals 981 to 6-substituted tetrahydropyran-2-ones 982 can be achieved by synergistic catalysis using samarium diiodide and 2-propanethiol in excellent yield (Equation 382) <19990L1989>. 

Overall, the quantum mechanical approach to uncovering the sources of enzyme catalysis is one of building up understanding by incrementally adding on models of portions of the enzyme environment (for example, a reaction field or an explicit model of an active site residue) in order to discover how they affect the activation parameters and detailed mechanism of the reaction in question, and whether these components of the enzymatic surroundings produce additive or synergistic effects when combined. 

Firsova et al. (122) reported that the room temperature Mossbauer spectrum of supported tin molybdate, which had been aged in vacuo at 723 K, showed the presence of tetravalent tin. Only after exposure to oxygen at 473 K did the sample act as an adsorbent for propylene. It then gave a Mossbauer spectrum that showed the reduction of the tetravalent tin to the divalent state. Reduction without exposure to oxygen was achieved at 673 K but supported tin in the absence of molybdenum was not reduced. The results were interpreted in terms of the proposals (123) for the synergistic oxidation-reduction during catalysis. 

Catalysed substitution reactions of an unusual kind are collected together in this section. In each case, the catalysis of the reaction by a homogeneous entity is assisted by the surface of a solid. The resulting reinforcement of catalytic effects is frequently described as synergistic. The homogeneous and heterogeneous catalysts quite often possess a species in common, for example Ag+ ions and solid Agl, and many of the homogeneously catalysed reactions exhibit autocatalysis as a result. 

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