Ligand substitution reactions copper

The reactions of copper(II) complexes to be considered are of two types (i) ligand substitution reactions and (ii) redox reactions, i.e. electron transfer involving a change in oxidation state, copper(I)/(II)/(III). 

For a study on variation of product stereochemistry with ligand substitution in copper catalyzed reactions see Evans, D. A. Johnson, J. S. Burgey, C. S. Campos, K. R. Tetrahedron Lett. 1999, 40, 2879-2882. 

Bis(thiosemicarbazones) have been made that display powerful activity against established solid tumours in rodents, and that are dependent upon the presence of copper in the host. The N2S2 metal coordination site in these ligands reacts with Cu to form highly stable complexes. Thus, ligand substitution reactions are unfavorable. As the number of alkyl groups increases, the redox potential declines as much as 100-200 mV, until it is out of range for... 

Molecular motion based on the electrochemical-induced ligand substitution reaction has been reported in a copper(I) rotaxane (Figure 37). This rotaxane is composed of a 2,9-diphenyl-l,10-phenanthroline-containing cyclic poly ether ring, a tetraphenylmethane-stoppered molecular string that contains another 2,9-disubstitnted phenanthroline chelate... 

The ligands in a complex can be exchanged, wholly or partially, for other ligands. This is a type of substitution reaction. It happens if the new complex formed is more stable than the original complex. The complexes of copper(II) ions can be used to show ligand substitution reactions. 

It may be concluded from die different examples sliown here tiiat die enantio-selective copper-catalyzed allylic substitution reaction needs ftirdier improvemetiL High enantioselectivities can be obtained if diirality is present in tiie leaving group of die substrate, but widi external diiral ligands, enantioselectivities in excess of 9096 ee have only been obtained in one system, limited to die introduction of die sterically hindered neopeatyl group. 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

Tanner et al. (58) investigated the asymmetric aziridination of styrene using bis(aziridines) such as 85. Low induction is observed with these ligands, Eq. 64. A significant electronic effect was noted with the p-fluoro-phenyl substituted bis(az-iridine) 85c (59). A binaphthyl-derived diamine was used as a ligand for the copper-catalyzed aziridination of dihydronaphthalene (81). The product was formed in 21% ee and 40% yield, Eq. 65. Other structurally related ligands proved to be less selective in this reaction. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

The moderate ees obtained with the copper arenethiolate ligands discussed above prompted a search for new chiral ligands for use in asymmetric allylic substitution reactions. The binaphthol-derived phosphoramidite ligand 32, used successfully by Feringa et al. in copper-catalyzed 1,4-addition reactions [37], was accordingly tested in the reaction between 21 and n-BuMgl. 

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