Mechanistic Studies on Covalent Assemblies of Metal-Mediated Hemi-Aminal Ethers.
Mechanistic Studies on Covalent Assemblies of Metal-Mediated Hemi-Aminal Ethers.
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DOI：
10.1039/c4sc02495h
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发表时间：
2015-01-01
期刊：
影响因子：
8.4
通讯作者：
Anslyn EV
中科院分区：
文献类型：
--
作者：
Jo HH;Edupuganti R;You L;Dalby KN;Anslyn EV
最近报道了在包含手性醇的四组分组装中使用可逆共价键合，提供了一种通过圆二色光谱法测定醇对映体过量的方法。最近报道了在包含手性醇的四组分组装中使用可逆共价键合，提供了一种通过圆二色光谱法测定醇对映体过量的方法。实验探究了该组装体的机理，该组装体由 2-甲酰基吡啶 (2-PA)、二吡啶胺 (DPA)、Zn(ii) 和醇组成，可产生类 tren 配体的锌配合物。研究重点是半缩醛胺 (1) 转化为半缩醛胺醚 (3) 的转化机制，从而掺入第四种成分。研究发现，需要分子筛和 3 至 4 当量的醇来驱动 1 至 3 的转化。尝试通过添加强路易斯酸来分离该反应中的中间体，结果发现了五元环吡啶鎓盐 (5)，但在暴露于 Zn(ii) 和醇时，组装过程中得到了不同的产物。这被解释为支持亚胺物种的中介作用。动力学研究表明，在大量过量的醇中，1 到 3 的转化是零级的，这支持了在与醇反应之前中间体的速率决定形成。此外，1 和 3 相互转化的速率常数大小相似，支持正向和反向反应具有相似的速率决定步骤 (rds) 的观点。哈米特图显示 rds 涉及负电荷的产生（解释为正电荷的损失），支持了 Zn(ii) 从组装体中解络生成 1 和 3 的脱辅基形式是速率决定因素的观点。将各个机理结论结合起来，创建 1 和 3 相互转化的定性反应坐标图。
The use of reversible covalent bonding in a four-component assembly incorporating chiral alcohols was recently reported to give a method for determining the enantiomeric excess of the alcohols via CD spectroscopy. The use of reversible covalent bonding in a four-component assembly incorporating chiral alcohols was recently reported to give a method for determining the enantiomeric excess of the alcohols via CD spectroscopy. Experiments that probe the mechanism of this assembly, which consists of 2-formylpyridine (2-PA), dipicolylamine (DPA), Zn(ii) and alcohols to yield zinc complexes of tren-like ligands, are presented. The studies focus upon the mechanism of conversion of a hemi-aminal (1) to a hemi-aminal ether (3), thereby incorporating the fourth component. It was found that molecular sieves along with 3 to 4 equivalents of alcohol are required to drive the conversion of 1 to 3. Attempts to isolate an intermediate in this reaction via addition of strong Lewis acids led to the discovery of a five-membered ring pyridinium salt (5), but upon exposure to Zn(ii) and alcohols gave different products to the assembly. This was interpreted to support the intermediacy of an iminium species. Kinetic studies reveal that the conversion of 1 to 3 is zero-order in alcohol in large excesses of alcohol, supporting rate-determining formation of an intermediate prior to reaction with alcohol. Further, the magnitudes of the rate constants for interconversion of 1 and 3 are similar, supporting the notion that there are similar rate-determining steps (rds) for the forward and reverse reactions. Hammett plots show that the rds involves creation of a negative charge (interpreted as the loss of positive charge), supporting the notion that the decomplexation of Zn(ii) from the assemblies to generate apo-forms of 1 and 3 is rate-determining. The individual mechanistic conclusions are combined to create a qualitative reaction coordinate diagram for the interconversion of 1 and 3.