Rare earth reaction medium

The catalyst choice was based on several unique properties of the rare-earth Lewis acids (1) ready availability, (2) a large number of triflates with varied Lewis acidity, (3) compatibility with amine nucleophiles and (4) potential for reactions in aqueous medium. 

Because of the extensive amount of waste generated in traditional Friedel-Crafts reactions, it is not surprising that this reaction has been studied in RTIL. Early examples included the use of catalytic chloroaluminate ionic liquids. However, the moisture sensitivity of such systems was a drawback. Therefore, water-stable rare-earth Lewis acids, such as Sc(CF3S03)3, have come to be used for these reactions.The same Lewis acid has also been used to catalyse Diels-Alder reactions in RTILs.Interestingly, in this example, the RTIL not only provided a means for recycling the catalyst but also accelerated the rate and improved selectivity. It has also been demonstrated that a moisture stable, Lewis acidic, catalytic ionic liquid could be prepared from choline chloride and zinc dichloride, and that this was an excellent medium for the Diels-Alder reaction. Yields of 90% or more were achieved in reaction times of between 8 min and 5h for a range of dienes and dienophiles. 

The selectivity of most methods can be increased by proper selection of the pH of the analytical medium. For reagents of the R-OH type there exists a relationship between colour reactions and hydrolytic reactions of certain elements. In strongly acid solutions, colour reactions proceed with those elements that have easily hydrolyzable cations, such as Zr, Hf, Th, U(rV) and Ti. In moderately acidic solutions the reactions also proceed with Fe(IlI), Al, and U(IV) in weakly acid and neutral solutions, with rare-earth elements, Fe(II), Cu, Mn, and in alkaline solutions, with Ca, Sr, and Mg. Easily hydrolyzable species react in more acidic solutions, whereas other ions react only in less acidic ones. 

In strongly acid solutions (1-10 M HCl) Arsenazo III reacts only with Th, Zr, Hf and U(rV). The molar absorptivities, e, of the complexes with these metals are about 10. At pH 1-4 Arsenazo III reacts with U(VI), Sc, Fe(ni), Bi, and rare earths. The sensitivity of the colour reactions is lower in this case ( -510 ). The use of Arsenazo III in strongly acid medium overcomes difficulties connected with the hydrolysis of some multivalent metals (c.g., Zr, Th, U). In the determination of these metals the high acidity enhances the selectivity of the reagent. 

Many heterogeneously catalyzed reactions are performed using metal particles dispersed on oxide supports, commonly on alumina or silica but also many others, including transition metal and rare earth metal oxides. Metal clusters have interesting reactions with or at the surfaces of such oxide materials. It has recently been shown that the oxide surface is actually a good medium for the synthesis of... 

As mentioned above, rare earth triflates, Cu(OTf)2, and AgOTf were foimd to act as Lewis acids in aqueous media. It would be of great importance to know which factors are key to the success of organic reactions in water. To address this issue, 1-15 metal chlorides, perchlorates, and triflates were screened in the aldol reaction of benzaldehyde with the silyl enol ether inwater-THF(l 9) (Scheme 3.13). This screening revealed that not only Sc(III), Y(lll), and Ln(III) but also Fe(II), Cu(II), Zn(II), Cd(II), and Pb(II) worked as Lewis acids in this medium to afford the desired aldol adducts in high yields. 

The solvent extraction reaction chemistry in specific medium, extractants, pH, diluents, and in synergistic systems are discussed in relation to the transfer of the rare earth extractable complex from the aqueous phase to the organic phase. 

Vapor-solid reactions are carried out in sealed evacuated Vycor tubes. These are designed to maintain physical separation of liquid Se and solid M to preclude a rapid violent reaction. Se vapor is distilled over M filings at temperatures slowly increasing up to 950°C over a period of 4 to 50 h. The temperature is then held there for an additional14to 150 h. Miller etal. [1,2], Miller, Himes [4], also see [3]. Forth preparation of M2S 3 with M = Sc, Y, Tb, Dy, Ho,Tm, Yb, the rare earth metals were placed in an AI2O3 boat to prevent reaction with the quartz ampule used as an envelope. In the case of Sc, the reaction was accelerated with I2 as a transport medium, Dismukes, White [5]. The direct synthesis was also used by Guittard et al. [6, 7], Muir [8], Kleber et al. [9], and Klemm, Koczy [16]. MI3 is used as flux for M = Ce, Pr, Nd byTakeshitaet al. [10]. 

With aqueous solutions in pressurised cells, the temperature can be varied in a very broad range. Many fundamental electrochemical data have been obtained in this medium. Thermodynamic quantities such as activity coefficients of ions [252], equilibrium double-layer capacity [261], zeta potential of metals [233], potential-pH diagrams [206] or properties of the palladium-hydrogen electrode were determined [262]. Exotic systems, e.g. the solvation of rare earth atoms in liquid gallium [234], have been studied. Main research interests in subcritical aqueous solution were focused on the kinetics, reaction mechanism and transport properties. 

Water is the most abundant molecule on Earth and the universal solvent in which the chemistry of the life processes mostly occur. Nevertheless, the modern synthetic chemist rarely uses, or even considers, water as a medium in which to perform organic reactions. 

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