In order to apply boronic acid-saccharide interactions to the chiroselective synthesis of Δ- and Λ-[CoIII(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 [CoII(bpyba)3]4--saccharide complexes, which were oxidized to the substitution-inactive [CoIII(bpyba)3]3--saccharide complexes. In this stage, the Δ vs. Λ ratio was fixed. The complexes were converted to [CoIII(bpy)]3+ by treatment with AgNO3 and the e.e. was determined by comparison with authentic Δ- or Λ-[CoIII(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.
|Number of pages||8|
|Journal||Journal of the Chemical Society. Perkin Transactions 2|
|Publication status||Published - 1998 Oct|
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