Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford OX1 3TA, United Kingdom.
J Am Chem Soc. 2020 Jul 29;142(30):13219-13226. doi: 10.1021/jacs.0c06269. Epub 2020 Jul 17.
The link between allosteric cooperativity and template-directed synthesis has been investigated by studying complexes in which two oligopyridine ligands bind inside a zinc porphyrin nanoring in a stacked arrangement. The binding of a 6-porphyrin nanoring to two tridentate ligands (with -triazine or benzene cores) occurs with high negative allosteric cooperativity (α ≈ 10-10). Formation constants for 1:1 and 1:2 complexes were determined by UV-vis-NIR denaturation titration, using pyridine as a competing ligand, and cooperativity factors were confirmed by NMR spectroscopy. The rate constants for formation of the 1:1 and 1:2 complexes are approximately equal, and the negative cooperativity can be attributed to faster dissociation of the 1:2 complex. These tridentate ligands are not effective templates for directing the synthesis of the 6-porphyrin nanoring, in keeping with their negative cooperativity of binding. In contrast, the binding of a 12-porphyrin nanoring to two hexadentate ligands occurs with high positive allosteric cooperativity (α > 40), and the ligand is an effective Vernier template for directing the synthesis of the 12-porphyrin nanoring. This stacked Vernier template approach creates the product in an open circular conformation, which is advantageous for preparing macrocycles that do not easily adopt a figure-of-eight geometry.
已经通过研究两种寡吡啶配体在堆叠排列方式下在锌卟啉纳米环内结合的复合物，研究了变构协同作用与模板导向合成之间的关系。具有 -三嗪或苯核的三联吡啶配体与 6-卟啉纳米环的结合具有很高的负变构协同作用（α≈10-10）。通过使用吡啶作为竞争配体的紫外可见近红外变性滴定法确定了 1:1 和 1:2 复合物的形成常数，并通过 NMR 光谱证实了协同因子。1:1 和 1:2 复合物的形成速率常数大致相等，负协同作用归因于 1:2 复合物更快的解离。这些三联吡啶配体不是指导 6-卟啉纳米环合成的有效模板，这与它们的结合负变构协同作用一致。相比之下，12-卟啉纳米环与两个六齿配体的结合具有很高的正变构协同作用（α>40），并且该配体是指导 12-卟啉纳米环合成的有效 Vernier 模板。这种堆叠 Vernier 模板方法以开环构象产生产物，这有利于制备不易采用 8 字形几何形状的大环。
