Stereoselectivity in electron-transfer reactions in chiral media

Abstract

The oxidation of [Co(edta)]²⁻ by [IrCl₆]²⁻ proceeds by both inner-sphere and outer-sphere electron-transfer pathways. In the presence of added [Co(en)₃]³⁺, the outer-sphere pathway is enhanced. When optically active [Co(en)₃]³⁺ is used, the [Co(edta)]⁻ formed is optically active, reflecting a 1.5% ΔΛ selectivity. It is proposed that the selectivity arises from preferential formation and reactivity of the ΔΛ ion pair, {[Co(edta)]²⁻,[Co(en)₃]³⁺}. Direct reaction of [Co(edta)]⁻ with [Co(en)₃]²⁺ has also been investigated in the optically active solvent, (S)-(-)-1,2-propylene carbonate. The induction is small, forming 0.75% Δ-[Co(en)₃]³⁺, consistent with the important role for hydrogen bonding in determining the precursor stereoselectivity to the exclusion of solvent.