Copper centers

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

The mechanism is believed to be a two-step process involving reduction and oxidation of the copper center ... 

C20-0102. Blue copper proteins are blue when they contain Cu but colorless as Cu compounds. The color comes from an interaction in which a photon causes an electron to transfer from a sulfur lone pair on a cysteine iigand to the copper center. Why does this charge transfer interaction occur for Cu but not Cu+ ... 

Fee, J.A. Copper Proteins - Systems Containing the Blue Copper Center. Vol. 23,... 

Another illustration of the structural changes that may result as a consequence of alkyl substitution at the 3-position of the pyrazolyl group is provided by the structures of the dimeric copper(I) complexes [Tp]Cu 2 (36), [TpMe2]Cu 2 (36), [TpPh2]Cu 2 (37), and [TpBut]Cu 2 (37), which differ in the manner in which the tris(pyrazo-lyDhydroborato ligand bridges the two copper centers (Fig. 16). 

However, despite the rather dramatic change in coordination geometry that is observed upon comparing [TpBut Me]CuCl and [TpBut]CuCl (41), only rather minor perturbations are observed in comparing the structures of the Cud) dimers [TpBut]Cu 2 (37) and [TpBut,Me]Cu 2 (22). Thus, both the average Cu-N bond lengths and also the Cu - Cu separations in [TpBut Me]Cu 2 and [TpBut]Cu 2 are very similar. Nevertheless, although the coordination environment about each copper center is similar, the 5-methyl substituent does influence the fluxional nature of the molecule in solution. Thus, whereas [TpBut]Cu 2 is fluxional on the NMR time scale at room temperature, with a static structure that is only observed at -56°C, [TpBut Me]Cu 2 exhibits a static H NMR spectrum at room temperature. Furthermore, a static spectrum for... 

Despite numerous studies involving copper catalysts, only a few isolated copper complexes have been examined, including complexes (155)-(157). Bipyridine,397 phenanthroline,398 and pyridyli-mine cationic complexes399 all exhibit tetrahedral geometries, in which the copper center is bound to two ligands. 

Many multiple copper containing proteins (e.g., laccase, ascorbate oxidase, hemo-cyanin, tyrosinase) contain so-called type III copper centers, which is a historical name (cf. Section 5.8 for type I and type II copper) for strongly exchange-coupled Cu(II) dimers. In sharp contrast to the ease with which 5=1 spectra from copper acetate are obtained, half a century of EPR studies on biological type III copper has not produced a single triplet spectrum. Why all type III centers have thus far remained EPR silent is not understood. 

The reaction of 38 with nucleophiles gave a dinuclear species 39 in which each copper center is bonded to three C=C units, in contrast with the usual two-alkynyl-based bridge observed in many dinuclear alkynylcoppers.18 This... 

NHC ligands with a pendant group that enforces chelation have also been coordinated to copper centers. The reaction of Cu20 with pyridine fV-functionalized carbene ligand led to the formation of several compounds.91 In the case of mesityl derivatives, a dinuclear complex with a weak metal-metal interaction was isolated 60,91 whereas for the bulkier 2,6-diisopropylphenyl group, a monomeric complex was formed and characterized 61 (Figure 25).91... 

Copper olefin complexes are usually generated by the direct reaction of a Cu(l) source, the ligand, and the corresponding olefin. Copper ethylene complexes are of interest in view of their biochemical importance,98,98a-98e their applications in organic chemistry,99,99a,99b and industrial applications.100 100 Because of this, many copper alkene complexes have been reported, with different nuclearity, in compounds with one, two, or even three C=C units coordinated to a given copper center. 

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