Stereoselective Effects in Reactions of Metal Complexes III. Asymmetric hydrogenation of 5, 7, 7, 12, 14, 14-hexamethyl-1, 4, 8, 11-tetraazacyclotetradeca-4, 11-diene-nickel (II)

The catalytic hydrogenation in acidic solution (pH ～ 2) of the title compound

1 In order to represent this and the related compounds by meaningful abbreviations, we shall adopt the numerotation system proposed in the literature 8] 12]. The complete abbreviation of the title compound is Ni(5, 7, 7, 12, 14, 14-Me₆-14]-4, 11-diene-1, 4, 8, 11-N₄)]²⁺. As in the present work the 14-membered ring system with six methyl groups remains unchanged, we shall use Ni(4, 11-dieneN₄)]²⁺ and Ni(4, 11-aneN₄)]²⁺ and Ni(4, 11-aneN₄)]²⁺ for the complex with the unsaturated and saturated ligand, respectively.

Ni(4, 11-dieneN₄)]²⁺ (I) has been studied. The reaction yields only C-meso- 5, 7, 7, 12, 14, 14-hexa-methyl-1, 4, 8, 11-tetraaza-cyclotetradecane-nickel (II) (C-meso-Ni(4, 11-aneN₄)]²⁺), when meso-Ni(4, 11-dieneN₄)]²⁺ is the starting material. Rac-Ni(4, 11-dieneN₄)]²⁺ yields the unstable α-C-rac-Ni(4, 11-aneN₄)]²⁺. When optically active Ni(4, 11-dieneN₄)]²⁺ is reduced, optically active α-Ni(4, 11-aneN₄)]²⁺ is obtained, which in neutral or basic solution shows mutarotation due to conversion into optically active β-Ni(4, 11-aneN₄)]²⁺ no racemization is observed. Reaction with cyanide ions yields the optically active free tetramine ligand. The reaction mechanism of this asymmetric synthesis is discussed.