Promoter ligands

Hydrogenation conditions were optimized by addition of triphenylphosphine as promoter ligand, and the hydrogenation degree of SBS was up to 89%. Hy-... 

Naramura, M., et al., c-Cbl and Cbl-b regulate T cell responsiveness by promoting ligand-induced TCR down-modulation. Nat Immunol, 2002,... 

Mittag T, et al. Retinol modulates site-specific mobility of 118. apo-cellular retinol-binding protein to promote ligand binding. 

Treatment of P-hydroxy ketones with tetramethylammonium triacetoxyborohy-dride [Me4NHB(OAc)3] complements the chelation approach described above by affording the corresponding 1,3-trans diols. Acetic acid-promoted ligand exchange provides an alkoxydiacetoxyborohydride intermediate in which the proximal ketone is stereoselectively reduced by intramolecular hydride transfer. Alkyl substituents in the a-position do not diminish the 1,3-asymmetric induction. 

Another problem in aqueous catalysis is the very low solubility of organic substrates in water. One way to solve this problem is the use of a promoter ligand another solution seems to be supported aqueous phase catalysis. 

Other methods of promoting ligand dissociation include protonation ... 

A few examples of vinylic to aryl rhodium migration have also been observed. One such example was discovered independently by two groups almost at the same time [77,78], In this reaction, aryl propargyl alcohols undergo a rhodium-catalyzed isomerization to furnish indanone products (Table 12). The reaction proceeds by a fairly complicated mechanism shown in Scheme 19. Thus, after base-promoted ligand... 

Furthermore, steric bulk increase around the metal center has been proven useful to promote ligand-centered radical reactivity. Several examples of ligand-centered reactivity in metalloporphyrin systems have been previously discussed. The (TMP)RhCO (TMP = tetramesitylporphyrin) couples through the CO ligand to form the diketone dimer.90... 

Catalysis of 1-octene hydroformylation in biphasic systems by [Rh(cod)Cl]2-TPPTS can be dramatically enhanced by addition of lipophilic phosphines, e.g., Ph3P. The rate enhancement by the promoter ligands is believed to be the result of an increasing local concentration of the catalytic species at the interface [57]. Mixed-ligand complexes HRh(CO)(TPPTS)3 x(Ph3P)x prepared separately from HRh(CO)(TPPTS) and Ph3P are likely not to be catalyst precursors. 

Several concepts have been suggested to increases the rates in aqueous-phase catalytic conversion of higher substrates such as addition of conventional surfactants [3, 5] (cf. Sections 4.5 and 6.1.5), counter (inverse)-phase transfer catalysis using /3-cyclodextrins [6] (cf. Section 4.6.1), addition of promoter ligands, e.g. PPh3 [7], or co-solvents (cf. Section 4.3). However, addition of foreign compounds militates against the facile catalyst separation and purification of the products and increase the costs as well. 

However, it can also be argued that the TPP simply enhances the solubility in the organic phase of the mixed rhodium complexes that are formed, just as rhodium complexes with TPPDS or TPPMS instead of TPPTS do. In addition, it has to be considered that the promoter ligand TPP will stay in the crude aldehyde mixture after phase separation and will have to be separated by a distillation step. 

Ligand mixtures with tailored activities (for example TPP as a promoter ligand and additive to TPPTS [25]) are attractive for scientific work and also enable particular effects to be achieved. Mixtures might be controlled in academic work. However, for industrial use it must be ascertained that all catalyst components can be recirculated simultaneously and in the same way with little expense, which can virtually never be ensured in the case of complicated mixtures. The same applies to the use of soluble polymers (both as phase separation agents and as ligands cf. Section 7.6 and [26]). 

Many catalysts are metals, metal oxides or other simple salts, or metal complexes. For example, formation of platinum(IV) complexes involving ligand substitution is an extremely slow process, due to the kinetic inertness of this oxidation state. However, the addition of small amounts of a platinum(II) complex to the reaction mixture leads to excellent catalysis of the reaction, assigned to mixed oxidation state bridged intermediates that promote ligand transfer. 

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