A boronic acid–diol interaction is useful for chiroselective transcription of the sugar structure to the Δ- versus Λ-[CoIII(bpy)3]3+ ratio
作者:Toshihisa Mizuno、Masayuki Takeuchi、Itaru Hamachi、Kazuaki Nakashima、Seiji Shinkai
DOI:10.1039/a803382j
日期:——
In order to apply boronic acidâsaccharide interactions to the chiroselective synthesis of Î- and Î-[CoIII(bpy)3]3+ saccharide-binding ligands, 2,2â²-bipyridine-4-boronic acid (bpymb) and 2,2â²-bipyridine-4,4â²-diboronic acid (bpydb) were newly synthesized. It was shown that most D-saccharides form cyclic 1â¶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 bpydbâD-saccharide complexes with Co(OAc)2 gave the substitution-active [CoII(bpyba)3]4ââsaccharide complexes, which were oxidized to the substitution-inactive [CoIII(bpyba)3]3ââsaccharide complexes. In this stage, the Î vs. Î 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 Î- or Î-[CoIII(bpy)]3+. The Î-isomer was obtained in excess from most D-saccharides but the Î-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.; Î excess). Under the same reaction conditions the bpymb + D-glucose system gave 16% e.e. (Î excess). The e.e. of the bpydb + D-glucose system increased with lowering the reaction temperature and at â25 °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(2,2â²-bipyridine)âmetal complexes. Furthermore, the Î vs. Î equilibrium can be shifted in either direction by the selection of saccharide enantiomers.
为了将硼酸-糖类相互作用应用于对映选择性合成Δ-和Λ-[CoIII(bpy)3]3+糖结合配体,合成了2,2′-联吡啶-4-硼酸(bpymb)和2,2′-联吡啶-4,4′-二硼酸(bpydb)。结果表明,大多数D-糖类与bpydb形成环状1:1复合物,从而生成具有圆二色性(CD-active)的物质。正的激子耦合带表明两个吡啶环以顺时针方向扭转((R)-手性)。相比之下,bpymb未能生成这样的CD活性物质。将bpydb-D-糖类复合物与Co(OAc)2处理,得到具有取代活性的[CoII(bpyba)3]4––糖类复合物,这些复合物进一步被氧化为不活跃的[CoIII(bpyba)3]3––糖类复合物。在这一阶段,Δ与Λ的比例被固定。通过与AgNO3处理,将这些复合物转化为[CoIII(bpy)]3+,并通过与正品Δ或Λ-[CoIII(bpy)]3+的比较确定其对映体过量(e.e.)。从大多数D-糖类中得到的Δ-异构体过量,而从D-果糖和D-岩藻糖也能获得Λ-异构体。在4°C时,bpydb与D-葡萄糖反应得到的最大e.e.为47%(Δ过量)。在相同反应条件下,bpymb + D-葡萄糖系统的e.e.为16%(Δ过量)。在降低反应温度时,bpydb + D-葡萄糖系统的e.e.增加,在-25°C时达到79% e.e.。以上结果清楚地表明,糖模板合成是一种制备手性三(2,2′-联吡啶)-金属复合物的新概念。此外,通过选择糖的对映异构体,可以使Δ与Λ的平衡向任一方向偏移。