Mechanism for the Homolytic Cleavage of Alkyl Hydroperoxides by the Manganese(III) Dimer MnIII₂(2-OHsalpn)₂

The oxidation of Mn III 2 (2-OHsalpn) 2, 1 (2-OHsalpn = 1,3-bis(salicylideneamino)-2-propanol), with tert -butyl hydroperoxide was studied in organic media. A one-electron reaction occurs resulting in initial formation of the oxidized Mn III Mn IV (2-OHsalpn) 2 OH species 2 and a tert -butoxy radical. The Mn III Mn IV (2-OHsalpn) 2 OH complex can be readily protonated to yield Mn III Mn IV (2-OHsalpn) 2 + . Upon addition of excess tert -butyl hydroperoxide, singlet dioxygen is evolved in an exothermic reaction indicative of radical-induced disproportionation of the hydroperoxide. In the presence of a radical scavenger, the oxygen evolution is strongly attenuated. This system was observed to oxygenate cyclohexene to yield 2-cyclohexen-1-one, 2-cyclohexen-1-ol, and traces of cyclohexene oxide. Cumene is oxygenated to 2-phenyl-2-propanol and acetophenone, and cyclohexane is oxygenated to cyclohexanone and cyclohexanol. However, 18 O-labeling experiments show that the oxygen in the products results exclusively from reactions with aerobic dioxygen and not from the tert -butyl hydroperoxide oxidant. These results indicate that oxygenation occurs by radical-initiated aerobic autoxidation and not via oxo transfer from a high-valent manganese oxo species. 2 was studied by UV−vis, IR, NMR, EXAFS, XANES, and EPR spectroscopies, which support the assignment of a dinuclear manganese(III/IV) terminal hydroxo structure. 2 was also formed by the direct reaction of Mn III Mn IV (2-OHsalpn) 2 + with OH -, further supporting the description of 2 as having a terminal hydroxide ligand. The kinetics of its formation from 1 and tert -butyl hydroperoxide were examined and found to be first-order in peroxide. Titration experiments confirm that 1 equiv of peroxide is required to convert 1 to 2, supporting the conclusion that the peroxide undergoes a one-electron reduction. This system demonstrates that oxy radicals formed from homolytic one-electron cleavage of a peroxide by a manganese(III) complex can give small amounts of olefin oxidation species (i.e. epoxides) normally considered evidence for high-valent manganese oxo-transfer chemistry. It therefore underscores the need to test any system for which high-valent metal oxo transfer is proposed to occur using rigorous oxygen-labeling experiments to determine the source of oxygen.