Ethylenediamine complexes, substituent effect

There is abundant evidence that steric hindrance to coordination will decrease the formation constant of any complex formed and may even inhibit complex formation altogether. Numerical data are available for transition metals and substituted ethylenediamines,27 or substituted oxines.-3 for 1,10-phenanthroline and its 2- and 2,9-methyl-substituted derivatives28 and for other systems.29 Since the introduction of a 2-methyl substituent into oxine diminishes 0 while increasing pK2, it is not surprising that the effective concentration [Al(2-MeOxine)3] may not be great enough to cause precipitation, although this is achieved in non-aqueous media such as ethanol. 

Katsuki and co-workers have examined AE with Mn(salen) catalysts 12a, b bearing stereogenic centers on the 3- and 3 -positions of the salen ligand [63, 64]. The importance of these substituents was dramatically demonstrated with complex 12a. Even with an achiral ethylenediamine backbone, catalyst 12a effected the epoxidation of frans-stilbene in 61% ee (Scheme 2) [65]. Further optimization of the 3,3 -substituents led to the discovery of the more elaborate catalyst 12b, which incorporates axial dissymmetry into the aromatic portion of... 

The relative importance of steric and ICB effects in nickel(n) complex formation has been investigated further by Turan, who used iViV-dimethylethylenediamine (iViV-diMeen), iV -dimethylethylenediamine (iVAT -diMeen), and iVAW -trimethyl-ethylenediamine (triMeen) as incoming ligands. The results (Table 1) are compared with those for other polyamines and use is made in the analysis of the organic substituent function Es, defined as log kjk, where k and k are rate constants representing, respectively, the substituted and reference ligands. It is pointed out... 

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