Facile reduction of coordinated α-imino acids to amino acids by dithionite and borohydride

Abstract

Sodium dithionite was found to reduce a range of α-iminoacidato ligands, chelated to cobalt(III), to the related α-aminoacidato species. For the bis(1,2-ethanediamine) system, the distribution of the products showed a small stereoselectivity towards the ΛR,ΔS diastereoisomers. The most significant selectivity was exhibited in the preparation of the valinato complex, via reduction of the related imine, [(en)₂Co(Vim)]²⁺, where ΛR,ΔS/ΛS,ΔR = 4.2. Smaller selectivities (≈1.5) were observed in reductions using borohydride reagents. Reduction of the α-imino acid by sodium dithionite is optimised under acidic conditions (pH 3.5–5.5) in a dinitrogen atmosphere, whilst the corresponding reduction by sodium borohydride is optimised under mildly alkaline conditions (pH 9–10). Reduction conditions are therefore available to suit molecules with either acid or base sensitive functional groups. A mechanism for the reduction of the chelated imino acids by dithionite is proposed.