Oxidations of Hydrocarbons by Manganese(III) Tris(hexafluoroacetylacetonate)

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Abstract
Mn(hfacac)3 is an easily prepared and reactive oxidant (hfacac = hexafluoroacetylacetonate). It forms stable solutions in benzene and methylene chloride but is rapidly reduced in acetonitrile, DMSO, acetone, and ethers. It is reduced by ferrocene to give the Mn(II) complex [Cp2Fe][Mn(hfacac)3], which has been structurally characterized. Mn(hfacac)3 also rapidly oxidizes 1-acetylferrocene, 1,1‘-diacetylferrocene, and tris(4-bromophenyl)amine. Based on an equilibrium established with tris(2,4-dibromophenyl)amine, a redox potential of 0.9 ± 0.1 V vs Cp2Fe+/0 is calculated. Mn(hfacac)3 oxidizes 9,10-dihydroanthracene (DHA) cleanly to anthracene, with a bimolecular rate constant of 6.8 × 10-4 M-1 s-1 at 25 °C in benzene solution. In the presence of small amounts of water, the manganese(II) product is isolated as cis-Mn(hfacac)2(H2O)2, which has also been structurally characterized. Mn(hfacac)3 also oxidizes xanthene to 9,9‘-bixanthene, 1,4-cyclohexadiene to benzene, and 2,4-di-tert-butylphenol to the phenol dimer. Toluene and substituted toluenes are oxidized to tolylphenylmethanes. Product analyses and relative ratesfor instance that p-methoxytoluene reacts much faster than tolueneindicate that the more electron rich substrates react by initial electron transfer to manganese. For the less electron rich substrates, such as 1,4-cyclohexadiene, a mechanism of initial hydrogen atom transfer to Mn(hfacac)3 is suggested. The ability of Mn(hfacac)3 to abstract H• is reasonable given its high redox potential and the basicity of [Mn(hfacac)3]-. In CH2Cl2 solution, oxidation of DHA is catalyzed by chloride ion.

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