In nonaqueous media free-base porphyrins are easily protonated by strong proton donors to form the diprotonated porphyrins. The two-electron reduction of these species leads to phlorin cation species. To investigate the electroreduction products that follow the formation of these species, we evaluate the effect of increasing amounts of HClO on the electroreduction mechanism of the phlorin cation of -tetraphenyl porphyrin. We demonstrate that the phlorin cation undergoes a reversible protonation reaction to form a porphodimethene, which is further reduced to yield the chlorin-phlorin cation, a 4-electron reduced phlorin macrocycle with a hydrogenated pyrrolic C = C double bond. This undergoes further protonation and a 2-electron reduction to form a phlorin macrocycle, the bacteriochlorin-phlorin cation, wherein two pyrrole rings are hydrogenated. All these reduction processes occur at similar potential values, yet we managed to selectively form the 2-, 4- or 6- electron reduced product by a strict control of the number of protons in the medium. To the best of our knowledge, this is the first report of the electrochemical reduction of C = C bonds of pyrrolic rings. Thus, a new pathway for the electrochemical reduction of porphyrins is proposed.

中文翻译：

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苯甲腈强酸性介质中内消旋四苯酚阳离子的电还原机理

在非水介质中，游离碱卟啉很容易被强质子供体质子化，形成二质子化的卟啉。这些物质的双电子还原产生根皮苷阳离子物质。为了研究这些物质形成后的电还原产物，我们评估了 HClO 用量的增加对四苯基卟啉的卟啉阳离子电还原机制的影响。我们证明根卟啉阳离子经历可逆质子化反应形成二卟啉，进一步还原生成二氢卟酚-根卟啉阳离子，这是一种具有氢化吡咯 C = C 双键的 4 电子还原根皮苷大环。它经历进一步的质子化和2电子还原，形成根皮素大环，即菌绿素-根皮苷阳离子，其中两个吡咯环被氢化。所有这些还原过程都在相似的电势值下发生，但我们通过严格控制介质中质子的数量，设法选择性地形成 2、4 或 6 电子还原产物。据我们所知，这是吡咯环C=C键电化学还原的第一份报告。因此，提出了卟啉电化学还原的新途径。