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 ÎR,ÎS 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 ÎR,ÎS/ÎS,ÎR
= 4.2. Smaller selectivities (â1.5) were observed in reductions using borohydride reagents. Reduction of the α-imino acid by sodium dithionite is optimised under acidic conditions (pH 3.5â5.5) in a dinitrogen atmosphere, whilst the corresponding reduction by sodium borohydride is optimised under mildly alkaline conditions (pH 9â10). 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.
研究发现,
连二亚硫酸钠能将一系列与
钴(III)螯合的δ-
氨基
配体还原成相关的δ-
氨基
配体。对于双(1,2-
乙二胺)体系,产物的分布显示出对δR、δS 非对映异构体的少量立体选择性。通过还原相关的
亚胺 [(en)2Co(Vim)]2+ 来制备缬
氨络合物的选择性最为明显,其中δR,δS/δS,δR = 4.2。在使用
硼氢化试剂进行还原时,观察到较小的选择性(â1.5)。在酸性条件下(pH 值为 3.5â5.5 ),二
亚硫酸钠对δ-
亚胺酸的还原在二氮气氛中达到最佳效果,而
硼氢化钠的相应还原则在弱碱性条件下(pH 值为 9â10 )达到最佳效果。因此,这些还原条件既适用于对酸敏感的分子,也适用于对碱敏感的分子。本文提出了
亚硫酸盐还原螯合
亚胺酸的机理。