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

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.