Department of Chemistry & Biochemistry, Miami University, Oxford, Ohio 45056, USA.
J Am Chem Soc. 2013 May 1;135(17):6714-22. doi: 10.1021/ja4026006. Epub 2013 Apr 16.
The ortho-phenylenes are a simple class of helical oligomers and representative of the broader class of sterically congested polyphenylenes. Recent work has shown that o-phenylenes fold into well-defined helical conformations (in solution and, typically, in the solid state); however, the specific causes of this folding behavior have not been determined. Here, we report the effect of substituents on the conformational distributions of a series of o-phenylene hexamers. These experiments are complemented by dispersion-corrected DFT calculations on model oligomers (B97-D/TZV(2d,2p)). The results are consistent with a deterministic role for offset arene-arene stacking interactions on the folding behavior. On the basis of the experimental and computational results, we propose a model for o-phenylene folding with two simple rules. (1) Conformers are forbidden if they include a particular sequence of biaryl torsional states that causes excessive steric strain. These "ABA" states correspond to consecutive dihedral angles of -55°/+130°/-55° (or +55°/-130°/+55). (2) The stability of the remaining conformers is determined by offset arene-arene stacking interactions that are easily estimated as an additive function of the number of well-folded torsional states (±55°) along the backbone. For the parent, unsubstituted poly(o-phenylene), each interaction contributes roughly 0.5 kcal/mol to the helix stability (in chloroform), although their strength is sensitive to substituent effects. The behavior of the o-phenylenes as a class is discussed in the context of this model. They are analogous to α-helices, with axial aromatic stacking interactions in place of hydrogen bonding. The model predicts that the overall folding propensity should be quite sensitive to relatively small changes in the strength of the arene-arene stacking. In a broader sense, these results demonstrate that polyphenylenes may exhibit folding behavior that is amenable to simple models, and validate the use of diffusion-corrected DFT methods in predicting their three-dimensional structures.
邻联苯是一类简单的螺旋寡聚物，是更广泛的空间位阻多联苯的代表。最近的研究表明，邻联苯在溶液中（通常在固态下）折叠成明确的螺旋构象；然而，这种折叠行为的具体原因尚未确定。在这里，我们报告了一系列邻联苯六聚体的构象分布受取代基影响的情况。这些实验通过对模型低聚物（B97-D/TZV(2d,2p)）进行修正色散的 DFT 计算得到了补充。实验和计算结果一致表明，芳环-芳环堆积相互作用的偏移对折叠行为起着决定性的作用。基于实验和计算结果，我们提出了一个邻联苯折叠模型，其中包含两个简单的规则。（1）如果构象包含导致过大空间应变的特定联苯扭转状态序列，则该构象是被禁止的。这些“ABA”状态对应于连续的二面角 -55°/+130°/-55°（或+55°/-130°/+55°）。（2）其余构象的稳定性由芳环-芳环堆积相互作用决定，这些相互作用可以很容易地作为沿主链的折叠扭转状态（±55°）的数量的加和函数来估计。对于未取代的聚（邻联苯）母体，每个相互作用大约贡献 0.5 kcal/mol 到螺旋稳定性（在氯仿中），尽管它们的强度对取代基效应敏感。该模型讨论了邻联苯作为一类化合物的行为。它们类似于α-螺旋，具有轴向芳香堆积相互作用代替氢键。该模型预测，整体折叠倾向应该对芳环-芳环堆积强度的微小变化非常敏感。从更广泛的意义上讲，这些结果表明多联苯可能表现出易于用简单模型描述的折叠行为，并验证了修正色散的 DFT 方法在预测其三维结构方面的有效性。
