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Mn(III)-Iodosylarene Porphyrins as an Active Oxidant in Oxidation Reactions: Synthesis, Characterization, and Reactivity Studies

Research paper by Mian Guo, Yong-Min Lee, Mi Sook Seo, Yong-Ju Kwon, Xiao-Xi Li, Takehiro Ohta, Won-Suk Kim, Ritimukta Sarangi, Shunichi Fukuzumi, Wonwoo Nam

Indexed on: 07 Aug '18Published on: 06 Aug '18Published in: Inorganic Chemistry



Abstract

Mn(III)-iodosylarene porphyrin adducts, [Mn(III)(ArIO)(Porp)]+, were synthesized by reacting electron-deficient Mn(III) porphyrin complexes with iodosylarene (ArIO) and characterized using various spectroscopic methods. In the epoxidation of olefins by the Mn(III)-iodosylarene porphyrin species, epoxide was formed as the sole product with high chemoselectivities and stereoselectivities. The present study reports the first example of spectroscopically well-characterized Mn(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions.Mn(III)-iodosylarene porphyrin adducts, [Mn(III)(ArIO)(Porp)]+, were synthesized by reacting electron-deficient Mn(III) porphyrin complexes with iodosylarene (ArIO) at −60 °C and characterized using various spectroscopic methods. The [Mn(III)(ArIO)(Porp)]+ species were then investigated in the epoxidation of olefins under stoichiometric conditions. In the epoxidation of olefins by the Mn(III)-iodosylarene porphyrin species, epoxide was formed as the sole product with high chemoselectivities and stereoselectivities. For example, cyclohexene oxide was formed exclusively with trace amounts of allylic oxidation products; cis- and trans-stilbenes were oxidized to the corresponding cis- and trans-stilbene oxides, respectively. In the catalytic epoxidation of cyclohexene by an electron-deficient Mn(III) porphyrin complex and sPhIO at low temperature (e.g., −60 °C), the Mn(III)-iodosylarene porphyrin species was evidenced as the active oxidant that effects the olefin epoxidation to give epoxide as the product. However, at high temperature (e.g., 0 °C) or in the case of using an electron-rich manganese(III) porphyrin catalyst, allylic oxidation products, along with cyclohexene oxide, were yielded, indicating that the active oxidant(s) was not the Mn(III)-iodosylarene adduct but probably high-valent Mn-oxo species in the catalytic reactions. We also report the conversion of the Mn(III)-iodosylarene porphyrins to high-valent Mn-oxo porphyrins under various conditions, such as at high temperature, with electron-rich porphyrin ligand, and in the presence of base (OH–). The present study reports the first example of spectroscopically well-characterized Mn(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions. Other aspects, such as one oxidant versus multiple oxidants debate, also were discussed.

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