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Bryant JR, Taves JE, Mayer JM: Oxidations of hydrocarbons by manganese (III) tris (hexafluoroacetylacetonate). Inorg Chem. 2002 May 20;41(10):2769-76.
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 [Cp (2) Fe][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 Cp (2) Fe (+/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 degrees C in benzene solution. In the presence of small amounts of water, the manganese (II) product is isolated as cis-Mn (hfacac)(2)(H (2) O)(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 CH (2) Cl (2) solution, oxidation of DHA is catalyzed by chloride ion. |
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