Generation of an optically active octahedral cobalt complex by a chiral autocatalysis

Abstract Optically active cis-[CoBr(NH 3 )(en) 2 ]Br 2 could be readily obtained by the reaction of optically inactive [Co(H 2 O) 2 {(OH) 2 Co(en) 2 } 2 ](SO 4 ) 2 with ammonium bromide in a water suspension. The optically active cis-[CoBr(NH 3 )(en) 2 ]Br 2 was not readily racemized in its aqueous solution, indicating it not to be produced by total spontaneous resolution but by asymmetric synthesis. This synthesis was exclusively observed when the weight ratio of[Co(H 2 O) 2 {(OH) 2 Co(en) 2 } 2 ](SO 4 ) 2 to water was above 0.31. Since the crystal of each enantiometer of cis-[CoBr(NH 3 )(en) 2 ]Br 2 introduced into the reaction system lead to the preferential production of the same enantiomer, chiral autocatalysis was shown to be performed during the reaction. In the concentrated suspension, crystallization of cis-[CoBr(NH 3 )(en) 2 ]Br 2 that was produced by the reaction would be started during the relatively early stage of the reaction. If several crystals to be initially produced are enantiomerically unequivalent by statistical fluctuation, it may cause the spontaneous generation of the enantiomeric predominance.