Oxidations of hydrocarbons by manganese(III) tris(hexafluoroacetylacetonate)

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 X 10(-4) M-1 s(-1) at 25 degreesC 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 rates-for instance that p-methoxytoluene reacts much faster than toluene-indicate 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.