Halides ligand substitution

Ligand Substitutions in Alkyl Halides Ligand substitutions in alkyl halides,... 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Subsequently, these catalysts were evaluated in the enantioselective desymmetri-sation of achiral trienes, and three distinct trends in catalyst selectivity were found. Firstly, catalysts 56a-b with two phenyl moieties on the backbone of the A -heterocycle exhibited higher enantioselectivity than those with a fused cyclohexyl group as the backbone 55a-b. Secondly, mono-ort/io-substituted aryl side chains induced greater enantioselectivity than symmetrical mesityl wing tips. Thirdly, changing the halide ligands from Cl to I" increased the enantioselectivity. As a result, catalyst 56b turned out to be the most effective. For example, 56b in the presence of Nal was able to promote the desymmetrisation of 57 to give chiral dihydrofuran 58 in up to 82% conversion and 90% ee (Scheme 3.3). 

Another general procedure to prepare gold(II) complexes consists of substitution reactions on gold(II) derivatives. Halide ligands can be substituted by neutral donor ligands2025-2032 such as... 

R. J. Puddephatt No. Nobody has prepared such complexes and the synthesis is not trivial. Substitution of halide ligands in octahedral platinum(IV) derivatives is typically very slow, and a better route (suggested by J. K. Kochi) might involve oxidation of platinum(II) metallacyclobutanes with peroxides. It would certainly be worthwhile to attempt this synthesis in view of the promise of enhanced reactivity. 

Fig. 2. Ligand substitution as a prodrug strategy for metallochem-otherapeutics (a) general scheme of prodrug activation by ligand substitution hydrolysis of a metal—halide bond is a typical activation pathway of metal-based anticancer drugs, as exemplified by the activation of cisplatin (b) and a ruthenium—arene complex (c).

As a simple example of isovalent replacement of hydride by halide ligands, let us first consider the set of monohalide substitutions on tetrahedral OsH4 and pyramidal... 

Prolonged standing in a THF solution results in its decomposition as outlined above however, by brief warming in THF followed by slow cooling, single crystals of the compound have been obtained. Its molecular stereochemistry is meridional in the solid state.9 Substitution reactions of the THF ligand by phosphine and halide ligands have been described.9,15... 

Since these substitution reactions follow a two-term rate law, it is clear that solvent effects are very significant. Poorly coordinating solvents are benzene, carbon tetrachloride and sterically hindered alcohols and strongly coordinating solvents are water, lower alcohols, DMF, DMSO, acetonitrile and nitromethane. The first-order rate constants are greater in DMSO than in water. Since the majority of precursor platinum complexes used in synthetic and mechanistic studies are halo complexes, the replacement of halide ligands by solvent and the reversibility of this reaction are important features of platinum halide chemistry. 

Although direct complex formation is observed kinetically (stopped flow) and spectrophotometrically, where X = Br or Cl, the reaction with I results in an oxidation of the halide. The reactions are rapid and there is the question of inner- or outer-sphere electron transfer, for the [14]aneN4 complex. However, further studies (140) using ligand substituted (dimethyl) complexes reveal that for the rac-Me2[14]aneN4 isomer, two processes are observed, k = 2.9 x 104 M-1 sec-1 and a subsequent redox step, krci = 5.5 x 103 M-1 sec-1, both of which are iodide dependent. The mechanism proposed involves the formation of an octahedral complex which further reacts with a second mole of I- in the redox step ... 

Because of their increased acidity, it is possible for phenols to effect total substitution of halide ligands in cases where alcohols cause only partial substitution. Hence, although WC16 reacts with ethanol to give the Wv complex [WCl3(OEt)2]2,35 phenol in benzene gives W(OPh)6 in excellent yields.4,36 Similarly, the phenoxides of Ti,37 Nb and Ta38 have been obtained. In cases where only partial replacement by phenol occurs, the use of substituents that increase the phenolic acidity can sometimes aid the reaction, for example as shown in equation (7).39,4(1... 

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