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

In order to apply boronic acid-saccharide interactions to the chiroselective synthesis of Delta- and Lambda-[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 Delta vs, Lambda 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 Delta- or Lambda-[Co-III(bpy)](3+). The Delta-isomer was obtained in excess from most D-saccharides but the h-isomer was also obtained from D-fructose and D-fucose. At 4 degrees C, the largest e.e. for bpydb was attained with D-glucose (47% e.e.; Delta excess). Under the same reaction conditions the bpymb + D-glucose system gave 16% e.e. (Delta excess). The e.e. of the bpydb + D-glucose system increased with lowering the reaction temperature and at -25 degrees 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 Delta vs. A equilibrium can be shifted in either direction by the selection of saccharide enantiomers.