Copper complexes bonding functions

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

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>. 

Various methodologies for catalytic direct arylations via C—H bond activation employing transition metals other than palladium have been developed in recent years. In particular, rhodium- and ruthenium-based complexes have enabled the development of promising protocols for catalytic direct arylations. Whilst rhodium catalysts were found broadly applicable to the direct aryiation of both arenes, as well as heteroarenes, ruthenium-catalyzed chelation-assisted C—H bond function-ahzations could be used for the conversion of a variety of attractive organic electrophiles. In addition, inexpensive copper and iron salts have recently been shown as economically attractive alternatives to previously developed more expensive catalysts. Given the economically and environmentally benign features of selective C—H bond functionalizations, the development of further valuable protocols is expected in this rapidly evolving research area. 

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