Stereoelectronic effects of the meso-substituents on the catalytic performance of iron(III) meso-tetraarylporphyrins: Pyridyl and N-methylated pyridyl groups compared to phenyl, 4-methoxyphenyl and 4-sulfonatophenyl ones

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• 作者：Saeed Zakavi ; ^{zakavi@iasbs.ac.ir} ; Tahereh Mokary Yazdeli
• 关键词：Meso-tetra(pyridyl)porphyrins ; N-methylated meso-tetra(pyridyl)porphyrins sulfonated-meso-tetra(aryl)porphyrins ; Iron porphyrins ; Oxidation of olefins ; Catalysis
• 刊名：Journal of Molecular Catalysis A: Chemical
• 出版年：2013
• 出版时间：February, 2013
• 年：2013
• 卷：367
• 期：Complete
• 页码：108-115
• 全文大小：488 K

文摘

The activity of a series of iron(III) meso-tetraarylporphyrins (aryl = phenyl, 4-methoxyphenyl, 4-sulfonatophenyl, 2-pyridyl, 3-pyridyl, 4-pyridyl and N-methyl-pyridyl counterparts) in the catalysis of oxidation reactions of olefins with tetra-n-butylammonium periodate (TBAP) or NaIO₄ in the presence of imidazole (ImH) has been studied. While the presence of ImH has no effect on the catalytic activity of the Fe-porphyrins containing pyridyl or N-methylated pyridyl substituents at the meso-positions, the catalytic performance of the other Fe-porphyrins significantly increased by the use of ImH in 5:1 molar ratio with respect to the catalyst. As was observed in the comparison of the Mn-porphyrins with pyridyl substituents, the oxidative stability of FeT(4-py)P(OAc) is remarkably higher than that of FeT(2-py)P(OAc) and FeT(3-py)P(OAc). Also, the same order of relative stability was found in the case of the N-methylated analogues. It is noteworthy that N-methylation of FeT(py)P(OAc) complexes led to the substantial decrease in their oxidative stability. Competitive oxidation of cis- and trans-stilbene, provides indirect evidence for the involvement of high valent Fe-oxo porphyrins as well as periodato iron porphyrins as the active oxidant in reactions catalyzed by the used iron porphyrins. However, the involvement of a high valent iron-oxo porphyrin species is more pronounced in the case of FeTPPS₄(OAc), FeT(2-py)P(OAc), FeT(3-py)P(OAc), FeT(4-py)P(OAc) and the N-methylated analogues. Also, an unusual preference of trans-stilbene over cis-stilbene was observed in the competitive epoxidation reaction catalyzed by FeT(3-MePy)P(OAc) and FeT(4-MePy)P(OAc) which cannot be described by the cis/trans isomerization. Furthermore, significant cis/trans isomerization observed upon oxidation of cis-stilbene in the reaction catalyzed by FeT(2-MePy)P(OAc) indicates that steric effects due to the presence of methyl groups at the ortho positions of this complex are of minor importance. On the other hand, large differences were observed in the product distribution for oxidation reactions with TBAO performed in dichloromethane and aqueous solvent. In spite of the lack of close correlation between the catalytic activity of the Fe-porphyrins and the electronic properties of the meso-substituents, in comparison with the corresponding Mn-porphyrins, there is a better correlation between the catalytic activity of the Fe-porphyrins and the electronic effects of the meso-groups.