Main-Chain Optically Active Riboflavin Polymer for Asymmetric Catalysis and Its Vapochromic Behavior
Main-Chain Optically Active Riboflavin Polymer for Asymmetric Catalysis and Its Vapochromic Behavior
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DOI：
10.1021/ja306159t
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发表时间：
2012-09-12
影响因子：
15
通讯作者：
Yashima, Eiji
中科院分区：
文献类型：
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作者：
Iida, Hiroki;Iwahana, Soichiro;Yashima, Eiji
以天然核黄素(维生素B-2)为原料，分三步合成了一种以核黄素为主链的新型光学活性聚合物(Poly1)。Poly-1的核黄素残基被转化为5-乙基核黄素阳离子(得到Poly2)，这些阳离子可以通过羟化/脱羟基反应可逆地转化为相应的4a-羟基核黄素(得到Poly2OH)。这种可逆的结构变化伴随着可见的颜色变化以及吸收光谱和圆二色谱(CD)的显著变化。Poly-2的核Overhauser效应谱(NOESY)和圆二色谱(CD)显示了一种超分子扭曲的螺旋结构，通过分子间核黄素单元的面对面堆积获得了过量的单手性，证明了链间核黄素单元之间的NOE相关性和在黄素生色团区域诱导的强烈的Cotton效应。聚-2在核糖基的辅助下，通过诱导超分子螺旋手性转移，以非对映选择性的方式进行4a-位的羟基化反应，形成83：17的聚-2OH非对映异构体混合物。Poly-2的非对映选择性显著高于相应的单体模型(64.5：35.5)，表明在堆叠的Poly-2主链中诱导的超分子手性导致手性放大。光学活性的聚-2分子筛有效地催化了硫化物与过氧化氢的不对称有机催化氧化反应，生成了光学活性亚硫醚，其对映体过剩(Ee)高达60%，其对映体选择性高于单体对映体(30%ee)。此外，当暴露于伯胺和仲胺时，聚-2由于在薄膜中形成4a-胺加合物而表现出独特的高速汽致变色行为。
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-ethyl-riboflavinium 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.