Remarkable Solvent, Porphyrin Ligand, and Substrate Effects on Participation of Multiple Active Oxidants in Manganese(III) Porphyrin Catalyzed Oxidation Reactions

Abstract

The participation of multiple active oxidants generated from the reactions of two manganese(III) porphyrin complexes containing electron-withdrawing and -donating substituents with peroxyphenylacetic acid (PPAA) as a mechanistic probe was studied by carrying out catalytic oxidations of cyclohexene, 1-octene, and ethylbenzene in various solvent systems, namely, toluene, CH(2) Cl(2) , CH(3) CN, and H(2) O/CH(3) CN (1:4). With an increase in the concentration of the easy-to-oxidize substrate cyclohexene in the presence of [(TMP)MnCl] (1a) with electron-donating substituents, the ratio of heterolysis to homolysis increased gradually in all solvent systems, suggesting that [(TMP)Mn-OOC(O)R] species 2a is the major active species. When the substrate was changed from the easy-to-oxidize one (cyclohexene) to difficult-to-oxidize ones (1-octene and ethylbenzene), the ratio of heterolysis to homolysis increased a little or did not change. [(F(20) TPP)Mn-OOC(O)R] species 2b generated from the reaction of [(F(20) TPP)MnCl] (1b) with electron-withdrawing substituents and PPAA also gradually becomes involved in olefin epoxidation (although to a much lesser degree than with [(TMP)Mn-OOR] 2a) depending on the concentration of the easy-to-oxidize substrate cyclohexene in all aprotic solvent systems except for CH(3) CN, whereas Mn(V)=O species is the major active oxidant in the protic solvent system. With difficult-to-oxidize substrates, the ratio of heterolysis to homolysis did not vary except for 1-octene in toluene, indicating that a Mn(V)=O intermediate generated from the heterolytic cleavage of 2b becomes a major reactive species. We also studied the competitive epoxidations of cis-2-octene and trans-2-octene with two manganese(III) porphyrin complexes by meta-chloroperbenzoic acid (MCPBA) in various solvents under catalytic reaction conditions. The ratios of cis- to trans-2-octene oxide formed in the reactions of MCPBA varied depending on the substrate concentration, further supporting the contention that the reactions of manganese porphyrin complexes with peracids generate multiple reactive oxidizing intermediates.

Solvent effects on catalytic activity of manganese porphyrins with cationic, anionic and uncharged meso substituents: Indirect evidence on the nature of active oxidant species

“…It is noteworthy that in manganese and iron porphyrin catalyzed oxidation of olefins with TBAP in dichloromethane, low valent periodato manganese(III) and iron(III) porphyrin species were found as the dominant active oxidant species. Furthermore, the formation of multiple active oxidants was evident for both heme and non‐heme manganese catalysts in reaction with various terminal oxidants [3e, f, 5b–f, 6]. In a recent work,[1e] we have reported the catalytic activity of a series of water soluble and insoluble iron(III) and Mn(III) meso‐tetra(aryl)porphyrins in the oxidation of alkenes with sodium periodate in water.…”

“…In this study, the effects of solvent on the product distribution and total conversion were used to provide indirect evidence on the nature of the active oxidant involved in the oxidation reactions. It is noteworthy that due to the high reactivity of high valent manganese oxo porphyrin intermediates at room temperature, most of these species have been characterized by low temperature UV–vis spectroscopy or competitive oxidation of the cis ‐ and trans isomers of sterically hindered olefins such as stilbene and octene [3c–f, 5b–e, 6a]. Furthermore, due to the environmental problems associated with the use of halogenated solvents, oxidation of olefins in environmentally preferable solvents is of great interest in the field of biomimetic catalysis.…”

The first manganese N-confused porphyrins catalyzed oxidation of alkene

“…Significant effect of solvent on the catalytic activity of manganese complexes of porphyrins and corroles have been reported recently [18,42]. To determine the effect of solvent on the catalytic oxidation by 3a, the reaction was carried out in different solvents (Table 1, Entry 5-10).…”

“…The metal complexes of N-confused porphyrins have been demonstrated to be potent catalysts in cyclopropanation of styrene with high trans-selectivity [12,13] and oxygen atom transfer reactions [14]. While numerous manganese complexes of porphyrins [15][16][17][18], corroles [19][20][21][22][23], phthalocyanines [24] and related macrocycles [25] have been extensively studied as catalysts in the oxidation of organic substrates, to the best of our knowledge, no report was found for manganese N-confused porphyrins catalyzed oxidation of organic substrates so far. Herein, we wish to report the synthesis and spectroscopic characterization of various manganese(III) N-methyl N-confused tetraarylporphyrins MnNCTArPs (Scheme 1 3a-3f), and their catalytic activity for styrene oxidation.…”

Porphyrins functionalized by quaternary pyridinium units

“…From among a great number of reports concerning manganese porphyrins [74][75][76][77], some examples will be presented here. Manganese porphyrins are of interest due to their properties as oxidation catalysts.…”