Effect of imidazole on biomimetic cyclohexane oxidation by first-, second-, and third-generation manganese porphyrins using PhIO and PhI(OAc)2 as oxidants

• Study of the axial coordination equilibrium between imidazole and Mn-porphyrins.
• Stability of the third-generation catalysts toward oxidative destruction by PhI(OAc)2.
• The substituents at meso-aryl groups influence the catalytic activity.

In this work, spectrophotometric titrations of first- (MnIIITPPCl), second- (MnIIIAPTPPCl and MnIIIT4CMPPCl), and third- (MnIIIBr9APTPPCl and MnIIIBr8T4CMPPCl) generation manganese(III) porphyrins ([MnIIIP]+) were carried out in order to investigate the axial coordination equilibrium between imidazole (Im) and these metalloporphyrin complexes. Cyclohexane oxidation by PhIO or PhI(OAc)2 catalyzed by the aforementioned [MnIIIP]+, in the presence of various [MnIIIP]+/Im molar ratios, was investigated as a means to study the contribution that the penta-, [MnIIIP(Im)]+, or hexacoordinate, [MnIIIP(Im)2]+, species at equilibrium may exert into catalyst efficiency and oxidative stability. The computational program SQUAD was used to analyze the spectrophotometric data and calculate the equilibrium constants used by program HySS to generate the species distribution curves for the various [MnIIIP]+/Im systems. In general, higher catalytic efficiency in the PhIO systems was achieved with the use of imidazole ranging from 1:0.5 to 1:5 [MnIIIP]+/Im ratio, depending on the nature of [MnIIIP]+. The catalytic systems with PhI(OAc)2 as oxidant were more sensitive to Im addition, and optimum yields were achieved with lower [MnIIIP]+/Im ratio (up to 1:1). It is noteworthy that the presence of imidazole reduced the usual instability of the third-generation catalyst MnIIIBr8T4CMPPCl toward oxidative destruction by PhI(OAc)2, but did not exert such a protective effect in the PhIO oxidations.

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