Ligands imine from oxidation

In aqueous solution Cu1 is frequently observed to disproportionate to Cu11 and copper metal, and imines are in most cases the minority species when amines and carbonyl compounds are mixed in water [27]. When imines and copper] I) are present in the same solution, however, this pattern of stability reverses. Imines are excellent ligands for Cu1, stabilizing the metal in this oxidation state, and metal coordination can prevent imines from hydrolyzing. We were thus able to prepare complex 1 from the precursors shown at left in Scheme 1.1 [24]. 

Like BINOL, salicylaldehyde imines have become very important in asymmetric catalysis and a variety of polydentate ligands prepared from chiral monoamines and diamines are employed in oxidation reactions, carbenoid reactions and Lewis acid catalyzed reactions. As in the previous section, this section emphasizes the effect of the phenol moiety on the asymmetric catalysis. An imine derived from a chiral 1-phenethylamine and salicylaldehyde was employed in the copper catalyzed asymmetric cyclopropanation by Nozaki, Noyori and coworkers in 1966, which is the first example of the asymmetric catalysis in a homogeneous system . Salicylaldehyde imines with ethylenediamine (salen) have been studied extensively by Jacobsen and Katsuki and their coworkers since 1990 in asymmetric catalysis. Jacobsen and coworkers employed the ligands prepared from chiral 1,2-diamines and Katsuki and coworkers sophisticated ligands possess chirality not only at the diamine moiety but also at the 3,3 -positions. 

By using combinations of hydrogenation and dehydrogenation reactions it has been possible to obtain nickel derivatives of the Curtis macrocycle containing from zero to four imine groups (Curtis, 1968 1974). Related reactions in the presence of a variety of other central metal ions have been described. The electrochemical oxidation of the Cu(ii) complex of the reduced Curtis ligand proceeds initially via a two-electron step to yield the monoimine complex (296) (Olson Vasilevskis, 1971). 

An interesting approach to rhenium bioconjugate formation that has been developed but apparently has yet to be fully evaluated from the standpoint of applicability and stability is the use of chelating and nonchelating phosphine imine and phosphine oxide ligands [110]. For... 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Curtis-type macrocyclic complexes are sufficiently stable to undergo a variety of oxidation and reduction processes to yield a range of complexes containing from zero to four imine linkages (Scheme 10).40 66 84 85 96 The oxidative dehydrogenation process is metal-ion dependent, as illustrated by the alternative ligand structure developed from the iron(II) complex (equation 14) 97 9S... 

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