Reaction pathway and free energy profile for conversion of π-conjugation modes in porphyrin isomer.
Reaction pathway and free energy profile for conversion of π-conjugation modes in porphyrin isomer.
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卟啉异构体中 π-共轭模式转换的反应途径和自由能曲线。
DOI：
10.1021/jo3013379
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发表时间：
2012
期刊：
影响因子：
--
通讯作者：
Takashi Hayashi
中科院分区：
文献类型：
--
作者：
T. Matsuo;Yuji Tohi;Takashi Hayashi
Porphycene是具有18π-共轭芳香特征的卟啉结构异构体。用三氟甲基（CF（3））修饰的卟啉在骨架的外围容易地通过与质子供给还原剂的反应提供可分离的20π-共轭反芳族形式。20π共轭形式不仅可以通过溶液中的各种光谱学来表征，还可以通过X射线晶体学来表征。本文着重研究了18π共轭卟啉转化为20π共轭卟啉的自由能分布。从动力学、电化学测量和酸碱滴定的结果来看，20π共轭的CF（3）卟啉是通过一个协同的质子-电子转移（CPET）从氢醌试剂到18π共轭的形式形成的。18π共轭的CF（3）卟啉的氢原子亲和力（对于两个氢原子）计算为-490 kJ mol（-1），表明20π共轭形式的N-H键相当容易断裂。这反映了20π共轭卟啉的反芳香特性。我们认为，利用氧化还原电位和pK（a）数据进行动力学和热化学分析，不仅适用于确定π共轭大环化合物芳香/反芳香模式转化的反应途径，也适用于有机分子氧化反应的研究。
Porphycene is a structural isomer of porphyrin with 18π-conjugated aromatic character. Porphycene modified with trifluoromethyl (CF(3)) groups in the periphery of the framework readily affords the isolable 20π-conjugated antiaromatic form through a reaction with a proton-donating reductant. The 20π-conjugated form can be characterized by not only a variety of spectroscopies in solutions but also X-ray crystallography. This paper focuses on the free energy profile in the conversion of the 18π-conjugated porphycene into the 20π-conjugated form. From the results of kinetics, electrochemical measurements, and acid/base titrations, the 20π-conjugated CF(3) porphycene is formed by a concerted proton-electron transfer (CPET) from a hydroquinone reagent to the 18π-conjugated form. The hydrogen-atom affinity of the 18π-conjugated CF(3) porphycene (for two hydrogen atoms) was calculated to be -490 kJ mol(-1), indicating that the N-H bonds in the 20π-conjugated form are rather easily cleaved. This reflects the antiaromatic characteristics of the 20π-conjugated porphycene. We propose that the kinetic and thermochemical analysis using redox potentials and pK(a) data is applicable for determining the reaction pathway in conversion of aromatic/antiaromatic mode of π-conjugated macrocycles as well as popular investigations for oxidations of organic molecules.
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