Absolute asymmetric synthesis and resolution mechanism of chiral cis-bromoamminebis(ethylenediamine)cobalt(III) bromide

Abstract

The absolute asymmetric synthesis of the title complexes Lambda-(+)(D)-cis-[CoBr(NH3)(en)(2)]Br-2(1) and Delta-(-)(D)-cis-[CoBr(NH3)(en)(2)]Br-2(2)(en=1,2-ethylenediamine) as well as the improved optical resolution of cis-[CoBr(NH3)(en)(2)]Br-2.2H(2)O(3) are described in detail. The products were characterized by means of elemental analysis, TG-DTA, optical rotation, UV-Vis, and CD spectra. The distribution of enantiomeric excess(ee) of cis-[CoBr(NH3)(en)(2)]Br-2 in the absolute asymmetric synthesis was obtained by the measurements of their CD spectra. It was found that the complete resolution of rac-cis-[CoBr(NH3)(en)(2)]Br-2.2H(2)O was achieved by use of NH4(d-BCS) (ammonium d-alpha-bromocamphor-pi-sulfonate) or NH4(l-BCS) while NH4(dl-BCS) could only be partially resolved by chiral cis-[CoBr(NH3)(en)(2)]Br-2 at the same condition. It is deduced that there are notable differences in the chiral discrimination of the reciprocal resolution between cis-[CoBr(NH3)(en)(2)]Br-2 and NH4BCS. Moreover, a new "catalysis-crystal induction" mechanism in the preparation of the chiral Co(III) complex is put forward.