Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.
J Am Chem Soc. 2012 Sep 12;134(36):15103-13. doi: 10.1021/ja306159t. Epub 2012 Aug 29.
A novel optically active polymer consisting of riboflavin units as the main chain (poly-1) was prepared from naturally occurring riboflavin (vitamin B(2)) in three steps. The riboflavin residues of poly-1 were converted to 5-ethylriboflavinium cations (giving poly-2), which could be reversibly transformed into the corresponding 4a-hydroxyriboflavins (giving poly-2OH) through hydroxylation/dehydroxylation reactions. This reversible structural change was accompanied by a visible color change along with significant changes in the absorption and circular dichroism (CD) spectra. The nuclear Overhauser effect spectroscopy (NOESY) and CD spectra of poly-2 revealed a supramolecularly twisted helical structure with excess one-handedness through face-to-face stacking of the intermolecular riboflavinium units, as evidenced by the apparent NOE correlations between the interstrand riboflavin units and intense Cotton effects induced in the flavinium chromophore regions. The hydroxylation of poly-2 at the 4a-position proceeded in a diastereoselective fashion via chirality transfer from the induced supramolecular helical chirality assisted by the ribityl pendants, resulting in a 83:17 diastereomeric mixture of poly-2OH. The diastereoselectivity of poly-2 was remarkably higher than that of the corresponding monomeric model (64.5:35.5), indicating amplification of the chirality resulting from the supramolecular chirality induced in the stacked poly-2 backbones. The optically active poly-2 efficiently catalyzed the asymmetric organocatalytic oxidation of sulfides with hydrogen peroxide, yielding optically active sulfoxides with up to 60% enantiomeric excess (ee), whose enantioselectivity was higher than that catalyzed by the monomeric counterpart (30% ee). In addition, upon exposure to primary and secondary amines, poly-2 exhibited unique high-speed vapochromic behavior arising from the formation of 4a-amine adducts in the film.
一种新型的手性聚合物，由核黄素单元作为主链（聚-1），通过三步法从天然核黄素（维生素 B2）制备。聚-1 的核黄素残基转化为 5-乙基核黄素鎓阳离子（得到聚-2），通过羟化/脱氢反应可以可逆地转化为相应的 4a-羟基核黄素（得到聚-2OH）。这种可逆的结构变化伴随着可见颜色的变化，同时吸收和圆二色性（CD）光谱也发生了显著变化。聚-2 的核 Overhauser 效应光谱（NOESY）和 CD 光谱表明，通过分子间核黄素鎓单元的面对面堆积，形成了超分子扭曲的螺旋结构，具有额外的单螺旋手性，这可以通过链间核黄素单元之间的明显 NOE 相关以及黄素生色团区域中强烈的Cotton 效应得到证明。聚-2 在 4a-位的羟化反应具有立体选择性，这是通过手性诱导的超分子螺旋手性的手性转移来实现的，这种手性转移得到了核糖侧链的辅助，导致聚-2OH 的非对映异构体混合物的比例为 83:17。聚-2 的非对映选择性明显高于相应的单体模型（64.5:35.5），表明超分子诱导的堆积聚-2 骨架中的螺旋手性得到了放大。手性聚合物聚-2 能够有效地催化过氧化氢不对称有机催化氧化硫醚反应，生成对映体过量高达 60%的光学活性亚砜，其对映选择性高于单体模型（30%ee）。此外，聚-2 在暴露于伯胺和仲胺时，在薄膜中形成 4a-胺加合物，表现出独特的高速蒸气变色行为。
