Aromatic Expanded Isophlorins: Stable 30π Annulene Analogues with Diverse Structural Features

Synthesis and structural diversity of novel aromatic expanded isophlorins are described. Expanded isophlorins are higher analogues of 20π isophlorin; 30π isophlorins are the simplest examples of expanded isophlorins. They are synthesized from easy to make precursors. Owing to the sp2 carbons along the conjugated network, they represent higher analogues of annulenes which are not realized until date. In contrast to the parent isophlorin 2, expanded isophlorins 7−9 and 11−13 are aromatic (4n + 2)π systems and also differ in their optical properties and structural features. They exhibit ring inverted structures and the number of ring inversions is dependent on the nature of the heteroatoms present in the core of the macrocycle. This was confirmed by both 1H NMR spectroscopy and single crystal-ray diffraction analysis. The upfield chemical shifts for the protons of the inverted rings confirmed the diatropic ring current effects as expected for (4n + 2)π aromatic systems. Solid state analysis revealed a planar conformation for 7 and near planar conformations for other macrocycles. They also show strong intermolecular interactions through C(sp2)−H···F−C(sp2) hydrogen bonds.

本研究详述了新型芳香性扩环异卟啉（expanded isophlorins）的合成方法与结构多样性。扩环异卟啉是20π异卟啉（isophlorin）的高阶同系物，其中30π异卟啉是扩环异卟啉中最简单的实例。该类化合物可由易于制备的前体合成。由于其共轭骨架中含有sp²杂化碳原子，该类化合物代表了迄今尚未被成功合成的轮烯（annulenes）高阶同系物。与母体异卟啉2相比，扩环异卟啉7−9与11−13均属于芳香性(4n+2)π体系，且在光学性质与结构特征上存在显著差异。该类化合物呈现环反转结构，且环反转的数量取决于大环核心所含杂原子的种类。该结论通过氢核磁共振（¹H NMR）光谱学与单晶X射线衍射分析得到了验证。反转环上质子的高场化学位移，印证了(4n+2)π芳香体系所预期的抗磁环电流效应。固态分析结果显示，化合物7呈现平面构象，其余大环则呈近平面构象。该类化合物还可通过C(sp²)−H···F−C(sp²)氢键形成较强的分子间相互作用。