A R T I C L E S
Fukuzumi et al.
analogs to (TMP)MnIV(O); the disproportionation of (TMP)-
MnIV(O) to (TMP)MnIIIX and [(TMP)MnV(O)]+ turned out to be
the rate-determining step, and the second-order rate constants were
determined in the hydride-transfer reactions. In contrast to the
hydride-transfer reactions, rates of electron transfer from a series
of ferrocene derivatives to (TMP)MnIV(O) obeyed pseudofirst-order
kinetics. The kinetic comparison between the hydride-transfer and
electron-transfer reactions of (TMP)MnIV(O) enables us to distin-
guish two different reaction pathways occurring by one common
oxidant: Hydride transfer from NADH analogs to (TMP)MnIV(O)
occurs by the formation of [(TMP)MnV(O)]+ via the dispropor-
tionation pathway, whereas direct electron transfer from ferrocene
derivatives to (TMP)MnIV(O) occurs in the electron transfer
reaction. In addition, driving force dependence of rate constants
of electron transfer from a series of ferrocene derivatives to
(TMP)MnIV(O) is analyzed in light of the Marcus theory of electron
transfer,14 leading us to determine the reorganization energy of
electron transfer of (TMP)MnIV(O). The latter result provides an
excellent opportunity to compare the reactivities in hydride-transfer
reactions and the reorganization energies in electron-transfer
reactions by a manganese(IV)-oxo porphyrin and nonheme
iron(IV)-oxo complexes.15 The investigated compounds in this
study are presented in Chart 1.
Scheme 1
in nature. However, Newcomb and co-workers reported recently
that Mn(IV)-oxo complexes are capable of oxygenating Ph3P,
Ph3N, and cis-stilbene in organic solvents.12,13 In the study,
(Porp)MnIV(O) has been shown to react with substrates to give
[(Porp)MnIII]+ and oxygenated products, but the formation of
(Porp)MnII species was not observed.12 On the basis of the
inverted reactivity of (Porp)MnIV(O) bearing different porphyrin
ligand, the authors proposed that [(Porp)MnV(O)]+, produced
by the disproportionation of (Porp)MnIV(O), is the actual oxidant
responsible for the oxidation of organic substrates (Scheme 1,
pathway B).12 However, the second-order kinetics for the
disproportionation of (Porp)MnIV(O) to produce [(Porp)MnIII]+
and [(Porp)MnV(O)]+ have yet to be directly observed, although
the disproportionation rate constants for (TPP)MnIV(O) (TPP
) 5,10,15,20-tetraphenylporphyrin) and (TPFPP)MnIV(O) (TPF-
PP ) 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin) were
estimated to be 3 × 109 and 1 × 106 M-1 s-1, respectively.12
Further, there are two possible pathways for the reactions of
(Porp)MnIV(O) with substrates: the direct oxidation of substrates
by (Porp)MnIV(O) (Scheme 1, pathway A) and the oxidation of
substrates by [(Porp)MnV(O)]+ formed from (Porp)MnIV(O) via
the disproportionation equilibrium (Scheme 1, pathway B).12,13
Thus, the reaction pathway may vary depending on the
substrates and/or reaction types, and it is of interest to understand
factors that control the reaction pathways of (Porp)MnIV(O) in
oxidation reactions.
Experimental Section
Materials. Commercially available reagents, 10-methylacridone,
acridine, methyl iodide (MeI), NaBH4, 5,10,15,20-tetrakis(2,4,6-
trimethylphenyl)porphyrin (TMP), and ferrocene (Tokyo Chemical
Industry Co., Ltd.), dimethylferrocene (Aldrich Chemical Co.),
1-bromoferrocene (Alfa Aesar GmbH & Co. KG), and LiAlD4 and
NaBD4 (CIL, Inc.), were the best available purity and used without
further purification unless otherwise noted. Acetonitrile (MeCN)
and ether were dried according to the literature procedures and
distilled under Ar prior to use.16 m-Chloroperbenzoic acid (m-
CPBA) was purified by washing with phosphate buffer (pH 7.4)
followed by water and then dried under reduced pressure.17 9,10-
Dihydro-10-methylacridine (AcrH2) was prepared from 10-methyl-
acridinium iodide (AcrH+I-) by reduction with NaBH4 in methanol
and purified by recrystallization from ethanol.18 AcrH+I- was
prepared by the reaction of acridine with methyl iodide in acetone
and was converted to the perchlorate salt (AcrH+ClO4-) by the
addition of magnesium perchlorate to the iodide salt (AcrH+I-) and
purified by recrystallization from methanol.18 The dideuterated
compound, [9,9′-2H2]-10-methylacridine (AcrD2), was prepared
from 10-methylacridone by reduction with LiAlD4 in ether.18
9-Phenyl-9,10-dihydro-10-methylacridine (AcrHPh) and 9-methyl-
9,10-dihydro-10-methylacridine (AcrHMe) were prepared by the
reduction of AcrH+I- with the corresponding Grignard reagents
(PhMgBr or MeMgBr).19 9-Phenyl-10-methylacridinium perchlorate
(AcrR+ClO4-: R ) Me and Ph) was prepared by the reaction of
10-methylacridone in dichloromethane with the corresponding
Grignard reagents (RMgX) and purified by recrystallization from
ethanol-diethyl ether.20 (TMP)MnIII(Cl) was prepared by adding
We report herein the first direct evidence for the dispropor-
tionation of (TMP)MnIV(O) [TMP ) 5,10,15,20-tetrakis(2,4,6-
trimethylphenyl)porphyrin] to [(TMP)MnIII]+ and [(TMP)-
MnV(O)]+ that is the true oxidant in hydride transfer from a
series of dihydronicotinamide adenine dinucleotide (NADH)
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