Substituent effects on the catalytic activity of a series of manganese meso-tetra(aryl)porphyrins: (2-, 3-, 4)-Pyridyl, 4-sulfonatophenyl and 3-sulfonato-4-methoxyphenyl groups compared to phenyl and 4-methoxyphenyl ones

Catalytic activity and oxidative stability of a series of Mn-porphyrins (Mn-pors) with pyridyl (2-, 3- or 4-), 4-sulfonatophenyl, 3-sulfonato-4-methoxyphenyl, phenyl and 4-methoxyphenyl in oxidation of olefins with tetra-n-butylammonium periodate has been investigated in a comparative study. While the complexes with pyridyl substituents, MnT(py)P(OAc) are generally more stable than those with the other aryl groups toward oxidative degradation in reaction conditions, their catalytic activity is usually lower than these Mn-pors. MnT(4-py)P(OAc) was found to be unusually more stable than MnT(2- or 3-py)P(OAc). The order of catalytic performance of the used Mn-pors and their oxidative stability are less correlated with the electronic properties of the meso substituents. On the basis of indirect evidence obtained in competition oxidation of cis- and trans-stilbene, the involvement of a six coordinate (IO4)Mn(III)(Porphyrin)(ImH) and a high valent Mn-oxo species has been proposed. However, in the case of MnT(py)P(OAc), the latter seems to be more dominant than the former.

Oxidation of olefins with tetra-n-butylammonium periodate catalyzed by the Mn(III) complexes of porphyrins baring 2-, 3- or 4-pyridyl substituents at the meso-positions shows the significant influence of the position of the heteroatom on the catalytic activity of the Mn-porphyrins. Interestingly, the catalytic performance of these Mn-porphyrins is usually lower than that of Mn-porphyrins with phenyl, 4-OCH3phenyl and 4-SO3phenyl.Figure optionsDownload high-quality image (155 K)Download as PowerPoint slideHighlights
► Low catalytic activity of the Mn-porphyrins with meso pyridyl substituents.
► Significant effect due to the position of the heteroatom of pyridyl groups.
► Low correlation between the catalytic activity and the electronic effects.