Classical and green cyclohexane oxidation catalyzed by manganese porphyrins: Ethanol as solvent and axial ligand

Exploring manganese pyridylporphyrin isomers for cyclohexane oxidation: First‐generation catalysts are better than third‐generation ones

“…[39,74,88] In our system, we suggest that the presence of imidazole (even in low concentrations) and the pyridyl group attached at the meso-position of the porphyrinic macrocycle could compete by the axial positions of the Mn(III) ion, inactivating the catalyst. This behavior was verified by Moreira Meireles and da Silva Martins [9] in systems with imidazole and ethanol acting simultaneously as axial ligands.…”

“…Several catalysts are studied for cyclohexane oxidation, [3][4][5][6][7] among which iron-and manganese-porphyrins (FeP and MnP, respectively) can be highlighted. [8][9][10][11] Pioneering work in this area was developed by Groves et al, [12] who promoted the reaction using iodosylbenzene (PhIO) as an oxidant, dichloromethane as the solvent, and a FeP as the catalyst (mesotetraphenyliron(III) chloride, [Fe(TPP)Cl]). It affords moderate catalytic results (8% cyclohexanol) once the metalloporphyrin structure is easily destroyed under the oxidizing conditions of the catalytic reaction.…”

“…[18] These present higher catalytic activity in comparison to the first-generation analogs, due to the steric hindrance promoted by the substituents and/or increased reactivity of the high-valence active species because of the presence of the electron-withdrawing groups. [9,19] The substitution of the hydrogen atoms by electron-withdrawing and/or bulky groups at the β-pyrrole positions of the porphyrinic macrocycle resulted in the third-generation catalysts. [18] It was expected that this modification would lead to more efficient catalysts for alkane oxidations than the first-and second-generation ones.…”

“…In this way, the use of cocatalysts, such as imidazole or other nitrogenated bases, has been demonstrated in the literature to improve the efficiency of the oxidation reactions mediated by metalloporphyrins. [8,9,74,85] These species can interact with one of the free active sites of the metallic center, destabilizing the high-valence active species [Mn(V)OP] and favoring the oxygen atom transfer to the substrate. [85] In the same way, water can also be used as a cocatalyst, as described in the literature.…”

“…PhIO is the classical oxidant employed in oxidations mediated by synthetic metalloporphyrins. [9,12,75] It has the advantage of generating [Mn(V)OP] species in the reactional medium, although it is potentially explosive, insoluble in most organic solvents, and unstable toward the spontaneous decomposition. [68,92] When compared to systems with PhI(OAc) 2 (Figure 5), we verified lower product total yields with PhIO except for isomer 6.…”