Ligand-Assisted Removal

Anions can promote hydrolysis of complex cations by producing ion pairs of enhanced reactivity (see 2.178). Usually however, ligands accelerate the removal of a coordinated ligand by entering the metal coordination sphere with it and thereby labilizing it towards hydrolysis. We have already seen the effect of coordinated OH on the enhanced labilities of Fe(III) and Cr(III). Dissociative mechanisms and considerable acceleration are promoted by CFlj, CN , SOj and other groups on inert Cr(III), Co(III) and Pt(IV) complexes. Nitrate ions, for example, reduce the half-life for replacement of water in Cr(H20)g by dmso from —380 h to 10 s  

In some cases, the unidentate ligand is liberated at the end of the reaction. Usually, however, the ligand is found in both the reactant and the product. The effect has been most systematically examined for Ni(II). Coordinated NHj and polyamines have the largest accelerating influence. The rate acceleration induced by macrocycles resides primarily in reduced AFTI values (by 15-26 kJ mol ). The 6- and 5-coordination of solvated tetramethylcyclam complexes is controlled by the conformation at the 4 N-centers, 2 and 3. These complexes exchange by and 4 mechanisms, respectively, as indicated by positive and negative values (Table 4.9). Also Sec. 4.9. 

There are linear correlations between log k (formation) and certain properties of the ligand (number of nitrogen atoms or electron-donor constant ). The enhanced rate resides largely 

We have been concerned in this section with the formation of adducts and reactions of the type (S, and Sy representing different ligand entities)  

We are interested in the effect of L, compared with, on the rate constant (A-,) for the process. Ternary complex formation depicted in (4.41) has been actively studied, to a large extent because of the biological implications of the results.  

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As already remarked, in compounds wer-IrX3 (PMe2Ph)3, the most labile ligand is the group X (X = H, Me, halogen) trans to a phosphine. The Ag+-assisted removal of chloride in (I) yields [IrCl2(H20)(PMe2Ph)3]+ (XII) where the H20 is very weakly bound (Ir—O 2.189 A, compare Ir(H20) + 2.041 A) and readily replaced. 

Take the fractions containing the ligand and remove the water on a rotary evaporator to yield a white powder of the hydrochloride salt. To assist in drying, add a small amount of methanol and rotary evaporate to dryness again. Dry the solid finally in a vacuum desiccator (3 h, 0.05 mmHg) to yield the hydrochloride salt which melts at high temperature with decomposition. 

Notwithstanding the advance attained, directing group-assisted C(sp )-H bond functionalization have to deal with the appended auxiliary first introduction then final removal process. The recent contributions on bifunctional ligand-assisted... 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Metal ions can assist in the dissociation (hydrolysis) of complexes containing multidentate ligands. The metal ion may not necessarily complex with the detached ligand, for example, in the metal-assisted acid-catalyzed aquation of Cr(C204)3. Ref. 112. Usually however the metal ion removes and complexes the ligand as in... 

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