Ligand assistance

For L=NH3 (1) and L=Pr2NH (3), the isotherms are of type II as expected for non-porous materials [27]. Sample 2 shows a significant uptake at 0.6

narrow particle-size distribution which results in a more regular packing with interparticle pores of size similar to that of the particles [27]. The latter shows that the ligand-assisted synthesis does not only allow one to affect the total surface area and particle size, but also the size distribution which is an important tool for tailoring the particle properties. 

Rossi, L.M., Nangoi, I.M. and Costa, N.J.S. (2009) Ligand-assisted preparation of palladium supported nanopartides a step toward size control. Inorganic Chemistry, 48 (11), 4640-4642. 

Despite the extensive utility of osmylation reactions, the mechanism remains controversial - generally focusing on either a concerted [3 + 2] cycloaddition to directly yield the cyclic osmate ester or a two-step mechanism where [2 + 2] cycloaddition forms a 4-membered osmaoxetane which then undergoes a ligand-assisted ring expansion to yield the cyclic osmate ester216 (Scheme 21). 

This problem may, however, be a special case because the oxidation of pteridine alkenes in the presence of sensitive substituents such as alkylthio was shown to be possible using ligand-assisted catalysis with osmium tetraoxide or DMDO (Scheme 10). In the case of pteridine 65, a clean series of transformations to afford the pterin 66 was possible <20030BC664, 2005PTR53004/04>. However, if oxidation was carried out under moist conditions, the corresponding 2-oxopteridine (lumazine) was always obtained as illustrated by the conversion of 67 into the epoxide 68. 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

For attack of F on acrylic acid, first a hydrogen bonded complex is formed which then proceeds to the transition state and then to a stable carbanion. The methyl in the methacrylic acid reduces stabilization of the carbanion as predicted. Subsequent studies using ammonia as the nucleophile indicated that attack proceeded by a rate-determining intramolecular proton transfer from the nucleophile to the ligand, assisted by a discrete water molecule that acts as a catalyst17. They predicted that acrolein underwent 1,4-addition, acrylic acid either 1,2- or 1,4-addition and acrylonitrile 1,2-addition. 

Recently, Taillefer et al. reported an Fe/Cu cooperative catalysis in the assembly of N-aryl heterocycles by C—N bond formation [90]. Similarly, Wakharkar and coworkers described the N-arylation of various amines with aryl halides in the presence of Cu—Fe hydrotalcite [91]. Interestingly, Correa and Bolm developed a novel and promising ligand-assisted iron-catalyzed N-arylation of nitrogen nucleophiles without any Cu co-catalysts (Scheme 6.19) [92]. Differently substituted aryl iodides and bromides react with various amides and N-heterocycles. The new catalyst system consists of a mixture of inexpensive FeCl3 and N,N -dimethylethylenediamine (dmeda). Clearly, this research established a useful starting point for numerous future applications of iron-catalyzed arylation reactions. 

In the course of the development of CAs, the question of the stability of Gd3+ chelates in the serum has often been raised. Besides, the possibility of the metal and ligand-assisted dissociation of Gd3+ complexes, which was mentioned above, the formation of precipitates of Gd(P04) and Gd2(C03)3 was additionally assumed to be possible [2]. The concentration of phosphate (0.38 mM) and particularly carbonate ions (24.5 mM) in the plasma is relatively high [21], while the solubilities of Gd(P04) and Gd2(C03)3 are very low. Many experimental data and also the calculations carried out by Cacheris et al. indicate the stability of the clinically used CAs in the serum [17,22]. However, Magerstadt et al. observed a decrease in the concentration of Gd(DTPA)2- in the serum at 37°C, while the concentration of the more inert Gd(DOTA)- remained practically unchanged. They assumed the formation of a precipitate of GdP04 [35]. 

The role of alkali metal cations in the [ RuCL(p-cymene) 2l-pseudo-dipepLide-catalysed enantioselective transfer hydrogenation of ketones with propan-2-ol has been examined. Lithium salts were shown to increase the enantioselectivity of the reaction when 2-PrONa or 2-PrOK was used as the base. An alternative reaction mechanism for the pseudo-dipeptide-based systems, in which the alkali metal cation is an important player in the ligand-assisted hydrogen-transfer step, has been proposed.370... 

It is worthwhile to analyze why co-existing soft ligands assist low oxidation numbers. If we want to make a copper(I) compound, it is very difficult to try the aqua ion, the fluoride or the anhydrous sulphate because they disproportionate to the metallic element and a higher oxidation state, here Cu(II). However, as seen in Eq. (7) it is easier to make the ammonia complex Cu(NH3)2 under anaerobic conditions, and even easier to make copper(I) complexes of pyridine and of conjugated bidentate ligands such as 2,2 -dipyridyl and 1.10-phenanthroline. The experimental problems are reversed in the case of iodides and cyanides, where it is easy to precipitate Cul or CuCN or to prepare solutions in an excess of the ligand containing Cul J,... 

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