Catalysis/catalysts aqueous media

First of all, given the well recognised promoting effects of Lewis-acids and of aqueous solvents on Diels-Alder reactions, we wanted to know if these two effects could be combined. If this would be possible, dramatic improvements of rate and endo-exo selectivity were envisaged Studies on the Diels-Alder reaction of a dienophile, specifically designed for this purpose are described in Chapter 2. It is demonstrated that Lewis-acid catalysis in an aqueous medium is indeed feasible and, as anticipated, can result in impressive enhancements of both rate and endo-exo selectivity. However, the influences of the Lewis-acid catalyst and the aqueous medium are not fully additive. It seems as if water diminishes the catalytic potential of Lewis acids just as coordination of a Lewis acid diminishes the beneficial effects of water. Still, overall, the rate of the catalysed reaction... 

To a certain extent the expression multicomponent catalysts is an arbitrary one. There is no doubt that the pure chemical elements and pure chemical compounds have to be called single component catalysts. It is, however, questionable whether a material such as steel should be classified as a single component system or as a multicomponent system. Some of the multicomponent catalysts, for instance, the iron-alumina catalyst consist of two separate solid phases but it would be misleading to accept the presence of more than one phase as the decisive criterion for multicomponent catalysts. The more than additive catalytic action of Cu-ions and Fe-ions in an homogeneous aqueous medium represents obviously a case of multicomponent catalysis, although it occurs in a single-phase system. As to solid multicomponent catalysts, they usually consist of more than one single phase, but there are exceptions to this rule, such as in cases in which mixed crystals or solid solutions are formed from the components. 

As mentioned earlier (Section 1.5) another example of novel catalysis in an aqueous medium is the use of lanthanide triflates as water-tolerant Lewis acid catalysts for a variety of organic transformations in water [39]. 

A discussion of catalysis would not be complete without a comparison of the catalysts we normally encounter in the laboratory or in industry with those that occur naturally (i.e., enzymes). Enzymes are proteins that are either soluble in the aqueous medium of the cell or attached to a cellular membrane. Soluble enzymes resemble homogeneous transition metal catalysts in ways other than their solubility characteristics. Enzymes have at least one region that serves as... 

The use of water as a suitable medium for catalysis has received much attention in recent years [1]. The increasing interest in this field stems from obvious economic and safety considerations. From an industrial point of view, an aqueous medium translates into waste reduction costs as well as the potential recovery of the catalyst via a biphasic process. The latter process is the foundation of the Ruhr-chemie/Rhone-Poulenc hydroformylation of alkenes, where, in 1998, it was reported to produce approximately 10% of the world s C4—C5 aldehyde capacity [2]. Furthermore, replacing flammable, carcinogenic, and explosive organic solvents with water leads to a safer working environment (cf. Section 5.2). 

Nevertheless, the use of in catalysis has been far less successful than that of organic peroxides, such as TBHP and peracids [2, 6-8]. The reasons for this are worth mentioning here. Most important organic substrates and aqueous HjOj are mutually insoluble. Water and polar solvents, which are most suited to HjOj, compete with the latter for active sites, poisoning the catalyst. Water-sensitive products, such as the epoxides, can be decomposed, at the reaction conditions, by the aqueous medium [5]. 

V. Loddo, G. Marco, C. Marto, L. Palmisano,V. Rives, A. Sclafania, Preparation and characterisation of Ti02 (anatase) supported on Ti02 (rutile) catalysts employed for 4-nitrophenol photodegradation in aqueous medium and comparison with Ti02 (anatase) supported on AI2O3 , Applied Catalysis B Environmental, 20, 29-45, (1999). 

The reaction of triphase catalysis is carried out in a three-phase liquid (organic)-solid (catalyst)-liquid (aqueous) medium. In general, the reaction mechanism of the triphase... 

The aza-Diels-Alder reaction of imines with diene of Danishefsky is an important route to 2,3-dihydro-4-pyridones. A number of Lewis acids have been used to catalyze the reaction in organic solvents. In water the reaction was realized by acid catalysis via iminium salts or by Bronsted acids. The montmorillonite K-10 catalyzed this cycloaddition in water or in aqueous acetonitrile in excellent yield. Recently Kobayashi has performed the reaction in water at room temperature under neutral conditions in two (imine - - diene) or three (aldehyde -b amine -b diene) component versions by using sodium triflate as catalyst. Imine intermediates from the indium-mediated reaction, in aqueous medium at 50° C, between aromatic nitro compounds and 2,3-dihydrofuran undergo aza-Diels-Alder cycloadditions to give tetrahydroquinoline derivatives in good overall yields. ... 

An efficient coordination between Lewis acid and dienophile ensures an efficient catalysis provided that the coordination of adduct to the catalyst is not very strong. Water hampers the complexation because it is a hard solvent and interferes with the interactions between a hard Lewis acid and hard sites of the dienophile. This is the reason why the catalytic effect can be more marked in organic media than in aqueous media. By combining the aqueous medium and the Lewis acid catalyst, the Diels-Alder reaction can be markedly accelerated but it is the water that contributes much more to the rate enhancement because of the enforced hydrophobic interactions and hydrogen-bonding interactions. 

Okawara and coworkers (Ref. 10) first attempted to use phase transfer catalysis to modify a finely dispersed poly(vinyl chloride) powder in aqueous medium using nucleophiles such as azide, dithiocarbamate, or thiophenoxide ions in the presence of tetrabutyl ammonium salts. While a maximum conversion of 10% was obtained with the first two nucleophiles, thiophenoxide afforded a 30% conversion. Although the authors indicate that the reaction took place only at the surface of the polymer particles, the fairly high conversion obtained with thiophenoxide might suggest otherwise. A second report from the same laboratory (Ref. 11) focuses on reactions of poly(vinyl chloride) solutions with sodium azide in the presence of various catalysts. As expected, the reaction is strongly catalyzed by cationic surfactants such as dimethyl distearyl ammonium chloride or tetrabutyl ammonium chloride which both afford essentially complete conversion to the azido polymer. In contrast, tetrabutyl ammonium iodide is totally ineffective. 

Homogeneous catalysts and the reaction components of these catalyst-catalyzed reactions remain in the same phase. Thus, the homogeneous catalysis requires both the catalyst and the reactants or the reaction components to be present in the same phase. For example, the net reaction between acetone and bromine in an acidic aqueous medium is expressed as the following ... 

Most studies of ORR catalysis by metalloporphyrins have been carried out using water-insoluble catalysts absorbed on a graphite electrode in contact with aqueous solution. In a limited number of cases, four other approaches have been used catalysts imbedded in an inert film (i.e., Nafion or lipid) on the electrode surface self-assembled monolayers of catalysts catalysts in aqueous or mixed organic/aqueous solutions in contact with an electrode and catalysis in mixed aqueous/organic medium using... 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

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