Hydrocarbon oxidation by Bis-mu-oxo manganese dimers: electron transfer, hydride transfer, and hydrogen atom transfer mechanisms.
Described here are oxidations of alkylaromatic compounds by dimanganese mu-oxo and mu-hydroxo dimers [(phen)(2)Mn(IV)(mu-O)(2)Mn(IV)(phen)(2)](4+) ([Mn(2)(O)(2)](4+)), [(phen)(2)Mn(IV)(mu-O)(2)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(2)](3+)), and [(phen)(2)Mn(III)(mu-O)(mu-OH)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(OH)](3+)). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn(2)(O)(2)](3+) to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)(2)Mn(III)(mu-OH)(2)Mn(II)(phen)(2)](3+) ([Mn(2)(OH)(2)](3+)). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn(2)(O)(OH)](3+) as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+). [Mn(2)(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 [Mn(2)(O)(2)](3+). The oxidation of toluene by [Mn(2)(O)(2)](4+), however, is not inhibited by [Mn(2)(O)(2)](3+). Oxidation of a mixture of C(6)H(5)CH(3) and C(6)H(5)CD(3) 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 [Mn(2)(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 pK(a) 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 [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+) are consistent with their 79 and 75 kcal mol(-)(1) affinities for H(*). In the oxidation of p-methoxytoluene by [Mn(2)(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.
Larsen
Anna S
AS
Department of Chemistry, University of Washington, Campus Box 351700, Seattle 98195-1700, USA.
Wang
Kun
K
Lockwood
Mark A
MA
Rice
Gordon L
GL
Won
Tae-Jin
TJ
Lovell
Scott
S
Sadílek
Martin
M
Turecek
Frantisek
F
Mayer
James M
JM
D000873:Anthracenes / Q000737:chemistry; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004583:Electrons; D005449:Fluorenes / Q000737:chemistry; D006841:Hydrocarbons, Aromatic / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D007700:Kinetics; D008345:Manganese / Q000737:chemistry*; D017895:Manganese Compounds / Q000737:chemistry; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry; D011135:Polysaccharides, Bacterial / Q000737:chemistry
D000873:Anthracenes; D005449:Fluorenes; D006841:Hydrocarbons, Aromatic; D017895:Manganese Compounds; D009942:Organometallic Compounds; D016877:Oxidants; D010087:Oxides; D011135:Polysaccharides, Bacterial; C041509:fluorene; D008345:Manganese; C027424:manganese oxide; D006859:Hydrogen; C002563:xanthan gum