Gauche-conformations, copper complexes

Among several chiral cyclic and acyclic diamines, (R,R)-cyclohexane-l,2-diamine-derived salen ligand (which can adopt the gauche conformation) was most effective in providing high enantioselectivity [38]. Further, the introduction of substituents at the 3,4, 5 and 6 positions on the aromatic ring of catalyst 39c was not advantageous, and resulted in low enantioselectivity [32,37,39]. The metal ions from first-row transition metals - particularly copper(II) and cobalt(II) - that could form square-planar complexes, produced catalytically active complexes for the asymmetric alkylation of amino ester enolates [38]. 

Figure 8.2 Gauche- and anti-conformations of a copper(salen) complex.

Figure 7-65 shows the X-ray structure of the complex [7,16-bis(ferrocenylmethyl)-l,4,10,13-tetrathia-7,16-diaza-cyclooctadecane]copper(i), [CuL ] and that of the free ligand L [186]. The copper(i) centre assumes a distorted tetrahedral geometry. With respect to the macrocyclic conformation of the free ligand, which possesses two gauche and two anti C-S-C-C bonds and two anti N-C-C-S bonds, complexation with copper (i) maintains the two gauche and two anti C-S-C-C bonds, but the two N-C-C-S bonds become gauche. 

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