A boronic acid–diol interaction is useful for chiroselective transcription of the sugar structure to the Δ- versus Λ-[CoIII(bpy)3]3+ ratio

Abstract

In order to apply boronic acid–saccharide interactions to the chiroselective synthesis of Δ- and Λ-[Co^III(bpy)₃]³⁺ saccharide-binding ligands, 2,2′-bipyridine-4-boronic acid (bpymb) and 2,2′-bipyridine-4,4′-diboronic acid (bpydb) were newly synthesized. It was shown that most D-saccharides form cyclic 1∶1 complexes with bpydb to afford the CD-active species. The positive exciton coupling band implies that two pyridine rings are twisted in a clockwise direction ((R)-chirality). In contrast, such a CD-active species was not yielded from bpymb. The treatment of the bpydb–D-saccharide complexes with Co(OAc)₂ gave the substitution-active [Co^II(bpyba)₃]^4––saccharide complexes, which were oxidized to the substitution-inactive [Co^III(bpyba)₃]^3––saccharide complexes. In this stage, the Δ vs. Λ ratio was fixed. The complexes were converted to [Co^III(bpy)]³⁺ by treatment with AgNO₃ and the e.e. was determined by comparison with authentic Δ- or Λ-[Co^III(bpy)]³⁺. The Δ-isomer was obtained in excess from most D-saccharides but the Λ-isomer was also obtained from D-fructose and D-fucose. At 4 °C, the largest e.e. for bpydb was attained with D-glucose (47% e.e.; Δ excess). Under the same reaction conditions the bpymb + D-glucose system gave 16% e.e. (Δ excess). The e.e. of the bpydb + D-glucose system increased with lowering the reaction temperature and at –25 °C it reached 79% e.e. The foregoing results clearly establish that the saccharide-templated synthesis is useful as a new concept for the preparation of chiral tris(2,2′-bipyridine)–metal complexes. Furthermore, the Δ vs. Λ equilibrium can be shifted in either direction by the selection of saccharide enantiomers.