Chelate ligand modification

A. Secondary Effects Through Chelate Ligand Modification... 

Furthermore, Pd-catalyzed aminations can be sequentially coupled with alkene insertion and amination. Wolfe and Lira [102] have established a transformation involving two different sequential metal-catalyzed reactions that lead to AT-aryl-2-benzylindolines 125 in moderate to excellent yields upon formation of two C - N bonds and one C - C bond in a one-pot process (Scheme 45). Interestingly, the selective installation of two different aryl groups in this sequence can be accomplished by in situ modification of the Pd catalyst system Pd-126 upon addition of the chelating ligand dpephos prior to addition of the second aryl bromide (Scheme 46). The selectively substituted indoline derivatives 127 were isolated in good to excellent yields. 

Hybrid, chelate, or molecularly modified precursor routes also utilize alkoxide compounds for the B-site species. Unlike true sol-gel processes, these routes rely on molecular modification of alkoxide compounds through reactions with other reagents, namely chelating ligands, such as acetic acid, acetylacetone, or amine compounds. Despite this difference, chelate processes stiU share several common attributes with methoxyethanol-based sol-gel processes, most importantly, the formation of oligomeric species during solution synthesis. 

The K C bonding mode includes most of the reported work on ylides. However, modifications of the structure of the ylide could be advantageous, in particular the introduction of additional donor atoms to form chelate ligands. The combination of the pure n-donor properties of the ylide with those of the auxiliary donor atom could be used for tuning the steric and electronic properties of ylide complexes. There are reports of useful C,P- and C,C-chelates, which will be detailed here. 

Danish workers have synthesized several polydentate ligands derived from (44a) that illustrate the sensitivity of the spin transition to small ligand modifications and indicate that the chelate effect may be as important as conjugation in the ligand. Thus, with (44j n = 2,3 and R = = H) low-spin... 

The solubilities of chelates with palladium, rhodium, lead and copper have been measured with static and dynamic spectroscopic methods at different temperatures and different pressures. The influence of different ligands, ligand modifications and of the metal ions coordination sphere on the solubility were studied and compared to the influence of pressure and temperature of the supercritical carbon dioxide (scCOa). In a series of C2, C4, Ce, Cg copper dithiocarbamates a maximum solubility was measured with the butyl substituents. 

Fig. 9. Ligand modifications of increasing chelate ring size and resulting redox potentials.

Many workers have proposed chelating ligands based on polyfvinylimidazole) or polyfvinylimidazole) decaiboxylate groups. It has been suggested that die loading rates for base metals, Cu(D), Ni(H), and Co(H) are enhanced with this type of structural modification. 

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