Stereochemistry copper complexes

Scheme 61).22 149 150 152 229 230 The methylene-bridged bis(oxazoline)s (70) can afford either neutral copper complexes of the semicorrin type (72) or cationic copper complexes (73). Cyclopro-panation with copper complexes of type (70) shows similar stereochemistry to that with the corresponding copper semicorrin complexes.229-231 Alternatively, bis(oxazoline) ligand (71), bearing... 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

A number of the above complexes may be alternatively considered as macrocyclic ligands or compartmental ligands, but as the emphasis has been primarily in terms of the local copper(I) stereochemistry and the polynuclear nature of the complexes, they have been included above. As there is no crystallographic data on biological copper(I) systems, this section will have to await the further refinement of the structure of Panutirus Interruptus hemocyanin.353... 

While the range of copper(II) complexes involving Cu—Br bonds (Table 81c) is more limited than those for Cu—Cl bonds (Table 81b), the structures tend to be analogous, with short Cu—Br distances of 2.4 A and long distances of ca. 3.0 A. Simple iodine complexes of the copper(II) ion are unknown, in view of the ready reduction of copper(II) to copper(I) with the liberation of iodine, nevertheless, copper-iodine bonds are formed if the copper(II) ion is stabilized by complex formation, as in [Cu(bipy)2I]I (420),1299 the structure of which involved the first example of five-coordinate copper(II) stereochemistry.1300 Semi-coordinate iodide... 

Recently, density functional calculations were performed to determine the nature and stereochemistry of the olefin insertion into the Cu-B bond of (NHC)Cu boryl complexes (NHC = iV-heterocyclic carbene). The theoretical calculations confirm that the mechanism of insertion involves a nucleophilic attack of the boryl ligand on the coordinated olefin. Furthermore, the hyperconjugation of Cu-C (bond angles, which was also experimentally confirmed by the X-ray diffraction studies of these boryl-copper complexes <2007OM2824>. 

The different oxidation states of copper ions not only favor distinct ligands but distinct stereochemistries of the copper-complexes as well. The conformational geometry of a complex depends on the orbital geometry of the central ion [17]. Sterical hindrances excluded, Cu+-ligands favor a tetrahedral conformation, while those of Cu2+ favor a square planar conformation (Table 4) [17]. In the copper-binding sites of proteins, ligands and conformations may differ greatly from those normally preferred. 

There are some resolved structures for this type of compound, for example, l//-pyrrolo[l,2-c]imidazole (96) <89H(29)1137>, and the 3,5-dioxo-2,3-dihydro-5//-pyrrolo[l,2-c]imidazole (97) <9lJOC858>. Carbene rhodium(I) (98), or the similar cobalt(I) complexes have been elucidated by x-ray analysis, and a very interesting study of stereochemistry of these compounds has been made <83JOM(250)C9, 85JOM(296)173>. The copper complex (99) of 3-oxo-l-thioxo-2,3-dihydro-l//-pyrrolo[l,2-c]imidazole has been studied <84ZN(B)586>. 

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