721-54-0Relevant articles and documents
Hydrocarbon oxidation by bis-μ-oxo manganese dimers: Electron transfer, hydride transfer, and hydrogen atom transfer mechanisms
Larsen, Anna S.,Wang, Kun,Lockwood, Mark A.,Rice, Gordon L.,Won, Tae-Jin,Lovell, Scott,Sadilek, Martin,Turecek, Frantisek,Mayer, James M.
, p. 10112 - 10123 (2007/10/03)
Described here are oxidations of alkylaromatic compounds by dimanganese μ-xo and μ-hydroxo dimers [(phen)2MnIV (μ-O)2MnIV(phen)2]4+ ([Mn2(O)2]4+), [(phen)2MnIV (μ-O)2MnIII(phen)2]3+ ([Mn2(O)2]3+), and [(phen)2MnIII (μ-O)(μ-OH)MnIII(phen)2]3+ ([Mn2(O)(OH)]3+). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn2(O)2]3+ to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)2MnIII (μ-OH)2Mn(phen)2]3+ ([Mn2(OH)2]3+). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn2(O)(OH)]3+ as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn2(O)2]3+ and [Mn2(O)(OH)]3+. [Mn2(O)2]4+ is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn2(O)2]3+. The oxidation of toluene by [Mn2(O)2]4+, however, is not inhibited by [Mn2(O)2]3+. Oxidation of a mixture of C6H5CH3 and C6H5CD3 shows a kinetic isotope effect of 4.3 ± 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn2(O)2]4+. Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e-, H?, and H- transfers have been determined from redox potential and pKa measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn2(O)2]3+ and [Mn2(O)(OH)]3+ are consistent with their 79 and 75 kcal mol-1 affinities for H?. In the oxidation of p-methoxytoluene by [Mn2(O)2]4+, hydride transfer is thermochemically 24 kcal mol-1 more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.
