Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, USA.
Chemistry. 2011 Dec 16;17(51):14539-51. doi: 10.1002/chem.201102488. Epub 2011 Nov 14.
The synthesis of a new series of free-base, Ni(II) and Zn(II) 2,3,12,13-tetra(ethynyl)-5,10,15,20-tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3-diethynyl-5,20-diphenylpiceno[10,11,12,13,14,15-jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free-base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully-cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15-fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc-pVTZ(-f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol(-1) higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin-enediynes with ethene- and H-functionality substitutions at the meso-phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol(-1) in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event.
描述了一系列新的游离碱、Ni(II)和 Zn(II) 2,3,12,13-四(乙炔基)-5,10,15,20-四苯基卟啉的合成。加热时，四个乙炔基中的两个发生 Bergman 环化反应，分别以 30%、10%和痕量的产率得到单环化的 2,3-二乙炔基-5,20-二苯基菲咯啉[10,11,12,13,14,15-jklmn]卟啉。所有产物的结构均通过量子化学计算进行了研究，分离并结晶了游离碱类似物；所有化合物均表现出与理想平面结构的显著偏差。反应混合物中未分离出完全环化的双菲咯啉[20,1,2,3,4,5,10,11,12,13,14,15-fghij]卟啉。为了了解为什么只有四个乙炔基中的两个发生 Bergman 环化反应，使用 DFT 在 PWPW91/cc-pVTZ(-f)水平耦合连续溶剂化模型下检查了反应坐标。发现第二个乙炔基环化的能垒比第一个高 5.5 kcal/mol，表明存在负协同效应，第二个环化反应的速率明显较慢。还检查了偕二炔基模型卟啉-enediynes 与偕二烯和 H 官能团在间苯基环上的取代的环化反应，发现对于这些高度平面的分子，第二个环化事件的二自由基形成的能垒与第一个相似。通过在卟啉的两个吡咯环上强制施加 30°的倾斜，乙烯模型体系中二聚体的第二个能垒增加了 2 kcal/mol；这表明破坏扩展卟啉结构的π共轭是导致第二个环化反应能垒增加的原因。
