Department of Chemistry and Biochemistry, 100 West 18th Avenue, Columbus, Ohio, USA.
Org Biomol Chem. 2013 Nov 28;11(44):7667-75. doi: 10.1039/c3ob41511b. Epub 2013 Sep 24.
We designed and prepared a spacious and gated basket of type 2 (V = 318 Å(3)) in ten synthetic steps. With the assistance of (1)H NMR spectroscopy, we found that the pyridine gates at the rim of 2 form a seam of N-H∙∙∙N hydrogen bonds, thereby adopting right- (P) and left-handed (M) helical arrangements. The recognition characteristics of the smaller basket 1 (V = 226 Å(3)) and the larger 2 for various solvents as guests were quantified by (1)H NMR spectroscopy in CD2Cl2 (61 Å(3)), CDCl3 (75 Å(3)), CFCl3 (81 Å(3)) and CCl4 (89 Å(3)); the apparent guest binding equilibria Ka were found to be inversely proportional to the affinity of bulk solvents KS for populating each host. The rate of the P/M racemization (krac, s(-1)) was, for both 1 and 2, studied in all four solvents using dynamic NMR spectroscopy. From these experiments, two isokinetic relationships (ΔS++P/M vs. ΔH++P/M) were identified with each one corresponding to a different mechanism of P/M racemization. A computational study (B3LYP/6-31+G**//PM6) of 1 and 2 in the gas phase indicates two competing racemization pathways: (a) RM1-2 describes a pivoting of a single gate followed by the rotation of the remaining two gates, while (b) RM3 depicts simultaneous (geared) rotation of all three gates. The racemization of the larger basket 2, in all four solvents (packing coefficient, PC = 0.19-0.28), conformed to one isokinetic relationship, which also coincided with the operation of the smaller basket 1 in CD2Cl2 (PC = 0.27). However, in CDCl3, CFCl3 and CCl4 (PC = 0.33-0.39), the mode of action of 1 appears to correlate with a different isokinetic relationship. Thus, we propose that the population of the basket's inner space (PC) determines the mechanism of P/M racemization. When PC < 0.3, the mechanism of operation is RM1-2, whereas, a greater packing, represented when PC > 0.3, enforces the geared RM3 mechanistic alternative.
我们设计并合成了一个具有 318 Å（3）体积的 2 型（V）的大笼，并采用 10 步合成法进行了制备。通过 1H NMR 光谱，我们发现笼的边缘吡啶门形成了 N-H∙∙∙N 氢键的缝，从而采用了右手（P）和左手（M）螺旋排列。通过在 CD2Cl2（61 Å（3））、CDCl3（75 Å（3））、CFCl3（81 Å（3））和 CCl4（89 Å（3））中使用 1H NMR 光谱定量研究了较小的笼 1（V = 226 Å（3））和较大的 2 对各种溶剂作为客体的识别特征；发现表观客体结合平衡 Ka 与每种主体填充的溶剂 KS 的亲和力成反比。使用动态 NMR 光谱在所有四种溶剂中研究了 1 和 2 的 P/M 外消旋化速率（krac，s（-1））。从这些实验中，我们确定了两种等动力学关系（ΔS++P/M 与 ΔH++P/M），它们分别对应于不同的 P/M 外消旋化机制。在气相中使用 B3LYP/6-31+G**//PM6 对 1 和 2 进行的计算研究表明，存在两种竞争的外消旋化途径：（a）RM1-2 描述了单个门的枢轴运动，然后是其余两个门的旋转，而（b）RM3 描述了所有三个门的同时（齿轮）旋转。在所有四种溶剂（堆积系数，PC = 0.19-0.28）中，较大的篮子 2 的外消旋化符合一个等动力学关系，该关系也与在 CD2Cl2 中较小的篮子 1 的操作一致（PC = 0.27）。然而，在 CDCl3、CFCl3 和 CCl4（PC = 0.33-0.39）中，1 的作用模式似乎与不同的等动力学关系相关。因此，我们提出篮子内部空间的填充（PC）决定了 P/M 外消旋化的机制。当 PC < 0.3 时，操作机制是 RM1-2，而当 PC > 0.3 时，更大的填充代表了强制齿轮 RM3 机械替代。
