Hydrogen-atom abstraction reactions by manganese(V)- and manganese(IV)-oxo porphyrin complexes in aqueous solution

Article abstract of DOI:10.1002/chem.200901362

High-valent manganese(IV or V)-oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn^V- and Mn^IV-oxo porphyrins in basic aqueous solution and investigated their reactivities in C-H bond activation of hydrocarbons. We now report that Mn^V- and Mn ^IV-oxo porphyrins are capable of activating C-H bonds of alkylaromatics, with the reactivity order of Mn^V-oxo>Mn ^IV-oxo; the reactivity of a Mn^V-oxo complex is 150 times greater than that of a Mn^IV-oxo complex in the oxidation of xanthene. The C-H bond activation of alkylaromatics by the Mn^V- and Mn ^IV-oxo porphyrins is proposed to occur through a hydrogen-atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C-H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn^IV-oxo porphyrins to Mn^V-oxo and Mn^III porphyrins is not a feasible pathway in basic aqueous solution and that Mn ^IV-oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C-H bond activation of alkylaromatics by Mn^V- and Mn^IV-oxo species proceeds through a one-electron process, in which a Mn^IV-oxo porphyrin is formed as a product in the C-H bond activation by a Mn^V-oxo porphyrin, followed by a further reaction of the Mn^IV-oxo porphyrin with substrates that results in the formation of a Mn^III porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn^V-oxo porphyrin, in which the oxidation of thioanisole by the Mn^V-oxo complex produces the starting Mn ^III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn^V-oxo species occurs by means of a two-electron oxidation process. In contrast, a Mn^IV-oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.

Full text of DOI:10.1002/chem.200901362

DOI: 10.1002/chem.200901362
Hydrogen-Atom Abstraction Reactions by Manganese(V)– and
Manganese(IV)–Oxo Porphyrin Complexes in Aqueous Solution
Chellaiah Arunkumar,[a] Yong-Min Lee,[a] Jung Yoon Lee,[a] Shunichi Fukuzumi,*[b, c] and
Wonwoo Nam*[a, c]
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Chem. Eur. J. 2009, 15, 11482 – 11489

FULL PAPER
Abstract: High-valent manganese
(IV
linear correlation between the reaction
tron process, in which a MnIV–-oxo por-
À
or V)–oxo porphyrins are considered
as reactive intermediates in the oxida-
tion of organic substrates by manga-
nese porphyrin catalysts. We have gen-
erated MnV– and MnIV–oxo porphyrins
in basic aqueous solution and investi-
rates and the C H bond dissociation
phyrin is formed as a product in the
V
À
energy (BDE) of substrates and high
kinetic isotope effect (KIE) values in
the oxidation of xanthene and dihy-
droanthracene (DHA). We have dem-
onstrated that the disproportionation
of MnIV–oxo porphyrins to MnV–oxo
and MnIII porphyrins is not a feasible
pathway in basic aqueous solution and
that MnIV–oxo porphyrins are able to
abstract hydrogen atoms from alkylaro-
C H bond activation by a Mn –oxo
porphyrin, followed by a further reac-
tion of the MnIV–oxo porphyrin with
substrates that results in the formation
of a MnIII porphyrin complex. This
result is in contrast to the oxidation of
sulfides by the MnV–oxo porphyrin, in
which the oxidation of thioanisole by
the MnV–oxo complex produces the
starting MnIII porphyrin and thioanisole
oxide. This result indicates that the oxi-
dation of sulfides by the MnV–oxo spe-
cies occurs by means of a two-electron
oxidation process. In contrast, a MnIV–
oxo porphyrin complex is not capable
of oxidizing sulfides due to a low oxi-
dizing power in basic aqueous solution.
À
gated their reactivities in C H bond
activation of hydrocarbons. We now
report that MnV– and MnIV–oxo por-
À
phyrins are capable of activating C H
bonds of alkylaromatics, with the reac-
tivity order of MnV–oxo>MnIV–oxo;
the reactivity of a MnV–oxo complex is
150 times greater than that of a MnIV–
oxo complex in the oxidation of xan-
À
matics. The C H bond activation of al-
kylaromatics by MnV– and MnIV–oxo
species proceeds through a one-elec-
À
thene. The C H bond activation of al-
Keywords: bioinorganic chemistry ·
C H activation
abstraction · manganese · porphyri-
noids · reaction mechanisms
kylaromatics by the MnV– and MnIV–
oxo porphyrins is proposed to occur
through a hydrogen-atom abstraction,
based on the observations of a good
À
· hydrogen-atom
Introduction
intermediates in the catalytic oxidation of organic
substrates.[1]
ManganeseACTHNUGRTNEUNG(III) porphyrin complexes have shown promise
AHCTUNGTRENNUNG
High-valent metal–oxo species have been implicated as the
key reactive intermediates in the catalytic cycles of dioxygen
activation by metalloenzymes. Our understanding of the en-
zymatic reactions has increased greatly by synthesizing bio-
mimetic metal–oxo complexes and investigating their spec-
troscopic and chemical properties. A notable example is the
high-valent iron(IV)–oxo complexes as chemical models of
cytochrome P450 enzymes.[1] A number of iron(IV)–oxo por-
phyrin complexes have been synthesized, characterized with
various spectroscopic techniques, and investigated in a varie-
ty of oxidation reactions, including alkane hydroxylation
and olefin epoxidation. Thus, there is no doubt that
iron(IV)–oxo porphyrin complexes are involved as reactive
as versatile catalysts in the catalytic oxidation of organic
substrates, and high-valent manganese–oxo porphyrins have
been frequently proposed as reactive intermediates in the
oxidation reactions.[2] As in iron porphyrin systems in which
two distinct high-valent iron–oxo porphyrins have been iso-
lated and characterized (i.e., an iron(IV)–oxo porphyrin and
an iron(IV)–oxo porphyrin p–cation radical),[1] two different
high-valent manganese–oxo porphyrin species, such as man-
ganese(V)– and manganese(IV)–oxo porphyrins, have been
reported in manganese porphyrin systems.[2] However, in
comparison to the spectroscopically well-characterized
iron(IV)–oxo porphyrins, only recently has the key MnV–
oxo porphyrin intermediate been characterized with various
spectroscopic techniques, such as UV/Vis, 1H NMR, reso-
nance Raman, X-ray absorption/extended X-ray absorption
fine structure (XAS/EXAFS) spectroscopy.[3] Further, while
reactivities and mechanisms of iron(IV)–oxo porphyrins
have been extensively investigated in various oxidation reac-
tions,[4] reactivities of MnV– and MnIV–oxo porphyrins in
[a] Dr. C. Arunkumar, Dr. Y.-M. Lee, J. Y. Lee, Prof. Dr. W. Nam
Department of Chemistry and Nano Science
Centre for Biomimetic Systems
Ewha Womans University, Seoul 120-750 (Korea)
Fax : (+82)2-32774441
E-mail: wwnam@ewha.ac.kr
[b] Prof. Dr. S. Fukuzumi
À
such reactions, including the C H bond activation of hydro-
Department of Material and Life Science
Graduate School of Engineering, Osaka University
SORST (Japan) Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
Fax : (+81)66879-7370
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
carbons, are less clearly understood but are the subject of
recent research including theoretical calculations.[3,5,6] Fur-
thermore, direct reactivity comparison of MnV– and MnIV–
oxo porphyrins under identical reaction conditions has
À
never been attempted in C H bond-activation reactions. In
[c] Prof. Dr. S. Fukuzumi, Prof. Dr. W. Nam
Department of Bioinspired Science
this work, we have investigated reactivities of in situ-gener-
V
IV
À
ated Mn – and Mn –oxo porphyrins in the C H bond acti-
Ewha Womans University, Seoul 120-750 (Korea)
vation of alkylaromatics in basic aqueous solution. In addi-
tion, the mechanism of the C H bond activation by Mn –
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901362.
V
À
Chem. Eur. J. 2009, 15, 11482 – 11489
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11483

W. Nam, S. Fukuzumi et al.
and MnIV–oxo porphyrins has been discussed. To the best of
our knowledge, this study reports the first direct reactivity
comparison of MnV– and MnIV–oxo porphyrins in the oxida-
tion of alkylaromatics and thioanisoles under identical reac-
tion conditions.
ure S3). When the logk2’ values were plotted against the
À
C H BDE of the substrates (xanthene, 75.5; DHA, 77; fluo-
rene, 80 kcalmolÀ1),[8] we obtained a linear correlation be-
À
tween logk2’ and the C H BDE of substrates (Figure 2).
The logk2’ values of hydride transfer from dihydronicotin-
AHCTUNGTREGaNNNU mide adenine dinucleotide (NADH) analogues, such as 10-
methyl-9,10-dihydroacridine (AcrH2) and 1-benzyl-1,4-dihy-
dronicotinamide (BNAH),[9] to 1 were also well-fitted into
the plot of logk2’ and BDE of substrates (AcrH2, 73.7;
BNAH, 67.9 kcalmolÀ1)[10] (see Figure 2; Supporting Infor-
mation, Table S1).[5f,11–13] The observation that the rate con-
Results and Discussion
A
[Mn
water-soluble manganese
ACHTUNGERTN(NUNG III) porphyrin complex,
N
ACHTUNGTRENNUNG
À
tetrafluoro-N,N,N-trimethyl-4-aniliniumyl)porphyrinato di-
anion), was treated with 2 equivalents of H2O2 and 2 equiva-
lents tert-butyl hydroperoxide to generate [MnV-
(tf4tmap)(O)2]3+ (1) and [MnIV(TF4TMAP)(O)(OH)]3+ (2),
stants decrease with the increase of the C H BDE of sub-
U
strates implicates an hydrogen-atom abstraction as the rate-
determining step for the oxidation of alkylaromatics.[14] Fur-
ther evidence for the hydrogen-atom abstraction mechanism
was obtained from measurements of kinetic isotope effect
(KIE) values in the oxidation of xanthene and DHA; KIE
values of 24(3) for xanthene and 17(3) for DHA are consis-
E
ACHTUNGTRENNUNG
respectively, in buffered H2O or buffered H2O–CH3CN (2:1)
mixture at pH 10.5 at 158C.[3c] The characterization of 1 and
À
tent with C H bond cleavage being the rate-determining
step (Figure 1c and 1d, left panels for the KIEs of xanthene
and DHA; Supporting Information, Table S1). It is worth
noting that large KIE values were often observed in the
À
C H bond activation of xanthene and DHA by high-valent
metal–oxo complexes with a low oxidizing power.[11,14c,15]
The good correlation between reaction rates and BDE of
À
2 by UV/Vis and EPR spectroscopy confirmed the forma-
tion of MnV–oxo and MnIV–oxo species, respectively (Sup-
porting Information, Figures S1 and S2 for absorption and
EPR spectra, respectively).[3,5,7] While 1 decayed to 2 slowly
(t1/2 ꢀ3.5 h), 2 was highly stable under the reaction condi-
tions (Supporting Information, Figure S1).[3c] Since the sub-
strates were not soluble in buffered H2O solution, all the re-
activity studies were carried out in a borate-buffered H2O–
CH3CN (2:1) mixture at pH 10.5 at 158C.
substrates and large KIE values in the C H bond oxidation
À
of alkylaromatics support the notion that the C H bond-ac-
tivation reactions by 1 occurs through a hydrogen-atom ab-
straction mechanism.
À
We have also investigated the reactivity of 2 in the C H
bond activation of alkylaromatics under the reaction condi-
tions of 1 (vide supra). Upon addition of substrates to a so-
lution of 2, we observed the conversion of 2 to the starting
ACTHNUTRGNEUNG
[MnIII(tf4tmap)]5+ complex at a rate much slower than that
À
The reactivity of 1 was investigated in the C H bond acti-
observed in the reactions of 1 (Figure 1b). When the sub-
strate was fluorene, 2 remained intact, indicating that 2 is
not able to oxidize fluorene due to its low oxidizing power.
Product analysis of the reaction solutions revealed that xan-
thone (43Æ8% based on the amount of 2), anthracene
(40Æ7%), and benzene (44Æ6%) were yielded as major
products in the reactions of 2 with xanthene, DHA, and cy-
clohexa-1,4-diene (CHD), respectively.[11] First-order rate
constants were determined from the kinetic data (Figure 1b,
inset), and the first-order rate constants increased linearly
with the increase of substrate concentration (Supporting In-
formation, Table S1 and Figure S4). The second-order rate
constants of 2 were determined to be much smaller than
those of 1. For example, the k2 values of 1 and 2 in the oxi-
dation of xanthene were 570 and 3.8mÀ1 sÀ1, respectively, in-
dicating that 1 is 150 times more reactive than 2 in this reac-
tion. To the best of our knowledge, these results are the first
direct reactivity comparison of MnV– and MnIV–oxo por-
À
vation of alkylaromatics with weak C H bond dissociation
energy (BDE) due to the low oxidizing power of MnV–oxo
porphyrins in basic solution.[3b,d] Upon addition of substrates
to a solution of 1, we observed the disappearance of 1, but
very interestingly, 1 was converted to 2, not to the starting
ACHTUNGTRENNUNG
[MnIII(tf4tmap)]5+ complex (Figure 1a; vide infra). However,
further reaction of 2 with substrates yielded the [MnIII-
ACHTUNGTRENNUNG
analysis of the reaction solutions revealed that xanthone
(48Æ6% based on the amount of 1 formed), anthracene
(43Æ7%), and 9-fluorenone (38Æ5%) were yielded as
major products in the oxidation of xanthene, dihydroanthra-
cene (DHA), and fluorene by 1, respectively.
À
phyrins in C H bond-activation reactions. Further, the rate
À
Pseudo-first-order fitting of the kinetic data allowed us to
determine kobs values, and the pseudo-first-order rate con-
stants increased linearly with the increase of substrate con-
centration (Supporting Information, Table S1 and Fig-
constants decreased with the increase of the C H BDE of
substrates, including alkylaromatics and NADH analogues
(vide supra; Figure 2). Furthermore, KIE values of 14(2)
and 8(1) were obtained in the oxidation of xanthene and
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Manganese–Oxo Porphyrin Reactions
FULL PAPER
Figure 2. Plot of log k2’ of 1 (red circles) and 2 (black circles) against
À
C H BDE of substrates. Second-order rate constants, k2, were deter-
mined at 158C and then adjusted for reaction stoichiometry to yield k2’
À
based on the number of equivalent target C H bonds of substrates (e.g.,
4 for DHA and CHD and 2 for BNAH, AcrH2, xanthene, and fluorene).
DHA, respectively, by 2 (Figure 1c and 1d, right panels for
the KIEs of xanthene and DHA; Supporting Information,
Table S1). Thus, based on the good correlation between re-
action rates and BDE of substrates and large KIE values in
À
the C H bond oxidation of alkylaromatics, we conclude that
À
the C H bond activation of alkylaromatics by 2 occurs
through a hydrogen-atom abstraction mechanism.
Since Newcomb and co-workers proposed that the true
active oxidant in the oxidation of substrates by MnIV–oxo
porphyrins is MnV–oxo porphyrins that were generated by a
fast disproportionation of the MnIV–oxo species in organic
solvents (Scheme 1),[4e,5d,16,17] we have examined the possibil-
ity of the disproportionation reaction of 2 through equilibri-
um in a basic aqueous solution. Based on the following ob-
servations, we conclude that the disproportionation reaction
of 2 does not occur under the present conditions. First, 1
reacts with fluorene, whereas 2 does not react with the sub-
strate due to a low reactivity. If 2 disproportionates to 1 and
Scheme 1. Proposed mechanism showing the disproportionation of MnIV
–
oxo porphyrin to MnV–oxo and MnIII porphyrins and the involvement of
the MnV– and MnIV–oxo species in C H bond-activation reactions.
À
Figure 1. a) UV/Vis spectral changes of 1 (1ꢁ10À5 m, black line) to 2
(blue line) upon addition of 10 equiv of xanthene (1ꢁ10À4 m) in borate-
buffered H2O–CH3CN (2:1) mixture at pH 10.5 at 158C. Insets show
spectral changes of Q-band (left panel) and time course of the decay of 1
monitored at 431 nm (right panel). b) UV/Vis spectral changes of 2 (1ꢁ
panel). c) Plot of kobs against the concentrations of xanthene and
[D2]xanthene for 1 (left panel) and 2 (right panel) to determine second-
order rate constants. Black and red circles indicate the oxidation of xan-
thene and [D2]xanthene, respectively. d) Plot of kobs against the concen-
trations of DHA and [D4]DHA for 1 (left panel) and 2 (right panel) to
determine second-order rate constants. Black and red circles indicate the
oxidation of DHA and [D4]DHA, respectively.
ACTHNUTRGNEUNG
10À5 m, blue line) to [MnIII(tf4tmap)]5+ (red line) upon addition of
50 equiv of xanthene (5ꢁ10À4 m) in borate-buffered H2O–CH3CN (2:1)
mixture at pH 10.5 at 158C. Insets show spectral changes of Q-band (left
panel) and time course of the decay of 2 monitored at 421 nm (blue) and
ACTHNUTRGNEUNG
the formation of [MnIII(tf4tmap)]5+ monitored at 454 nm (red) (right
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W. Nam, S. Fukuzumi et al.
[MnIII
G
We have shown above that 2 was formed as a product in
À
true active oxidant (Scheme 1, pathway B), we would ob-
serve the decay of 2 in the reaction of fluorene, because 1
reacts with fluorene and 2 is used to generate 1 by means of
the disproportionation process. However, 2 remained intact
in the reaction of fluorene, implying that there is no dispro-
the C H bond activation of alkylaromatics by 1, followed
by further reaction of 2 with substrates, which results in the
formation of the starting [MnIII
ACTHNUTRGEN(NUG tf4tmap)] complex
(Scheme 2, top). This result is contrary to the well-known
ACTHNUTRGNEUNG
portionation of 2 to 1 and [MnIII(tf4tmap)(OH)2]3+. More
evidence that supports our assertion was obtained in the oxi-
dation of substrates, such as xanthene and DHA, by 2 car-
ried out in the presence of an excess amount of [MnIII-
ACHTUNGTRENNUNG
(tf4tmap)(OH)2]3+. If 1 is the only active oxidant and there
is an equilibrium (see Scheme 1), addition of [MnIII-
(tf4tmap)(OH)2]3+ to a reaction solution of 2 shifts the equi-
ACHTUNGTRENNUNG
librium toward the inhibition of the generation of 1 and
should slow down the disappearance of 2.[5d,16] However, the
reaction rates were not affected by the presence of [MnIII-
ACHTUNGTRENNUNG
(tf4tmap)(OH)2]3+ (e.g., 1–3 equiv; Figure 3), indicating that
there is no disproportionation of
2
to 1 and [MnIII-
ACHTUNGTRENNUNG
À
able to activate the C H bonds of alkylaromatics
(Scheme 1, pathway A).
Scheme 2. MnV–oxo and MnIV–oxo porphyrins in the hydrogen-atom ab-
straction of xanthene and the oxidation of thioanisole and triphenylphos-
phine.
oxygen-rebound mechanism; hydrogen-atom abstraction
Figure 3. UV/Vis spectral changes showing the conversion of 2 (1.0ꢁ
À
from substrate C H bonds by iron(IV)–oxo porphyrin p-
ACTHNUTRGNEUNG
10À5 m, blue line) to [MnIII(tf4tmap)]5+ (red line) upon addition of
50 equiv of xanthene (5.0ꢁ10À4 m) in a borate buffered H2O–CH3CN
(2:1) mixture at pH 10.5 at 158C. Reaction a) was carried out in the ab-
cation radicals affords an iron(IV)–OH intermediate and a
caged alkyl radical, followed by a rapid transfer of the OH
group to the alkyl radical that leads to the formation of the
sence of [MnIII
ACHTUNGTRENNUNG
(tf4tmap)]5+, whereas reaction b) was carried out in the
ACHTUNGTRENNUNG
presence of [MnIII(tf4tmap)]5+ (1.5ꢁ10À5 m). Insets show spectral changes
ironACHTNUGRTNEUNG
(III) porphyrin and alcohol products (Scheme 3).[2c,d,18]
of Q-band (left panel) and time course of the decay of 2 monitored at
À
421 nm (blue) and the formation of [MnIII
454 nm (red) (right panel).
(tf4tmap)]5+ monitored at
ACHTUNGTRENNUNG
Thus, the formation of 2 as a product in the C H bond acti-
vation of alkylaromatics by 1 implies that the coupling be-
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Manganese–Oxo Porphyrin Reactions
FULL PAPER
Scheme 3. Oxygen-rebound mechanism by an iron(IV) porphyrin p-
cation radical complex.
tween 2 and alkyl radical does not occur rapidly and that
À
the C H bond activation by Mn–oxo complexes does not
follow the oxygen-rebound mechanism. Further, product
analysis of the xanthene oxidation by 1 and 2 performed in
the presence of 18O2 revealed that the xanthone product
contained oxygen derived from molecular oxygen (see Sup-
porting Information, Figure S5 for the product analysis of
xanthone with GC-MS), indicating that carbon radical
formed in the hydrogen-atom abstraction by 1 and 2 was
trapped by molecular oxygen (Scheme 2, reactions a and b).
V
À
Then, why does the C H activation by Mn –oxo species not
occur through oxygen-rebound mechanism? We propose
that the inhibition of the OH transfer from 2, which is
formed from the hydrogen-atom abstraction by 1, to a caged
alkyl radical results from a high thermal stability of the
MnIV–oxo species in basic aqueous solution, but more de-
tailed investigations are needed to understand the detailed
À
mechanism of the C H bond activation by manganese–oxo
complexes.
À
In contrast to the C H bond-activation reactions, the oxi-
dation of thioanisole by 1 produces the starting [MnIII-
ACHTUNGTRENNUNG
(TF4TMAP)(OH)2]3+ complex and thioanisole oxide
(Scheme 2, reaction c; Figure 4a), indicating that the oxida-
tion of sulfides by 1 occurs through a 2eÀ oxidation proc-
ess.[3d] Pseudo-first-order fitting of the kinetic data allowed
us to determine the kobs value to be 1.4ꢁ10À2 sÀ1 at 158C.
The pseudo-first-order rate constants increased proportion-
ally with thioanisole concentration, giving a second-order
rate constant of 1.4(2)ꢁ102 mÀ1 sÀ1 (Figure 4b). When
pseudo-first-order rate constants were determined with vari-
ous para-substituted thioanisoles and plotted against sp, a
good correlation was observed with Hammett 1 value of
À0.63 (Figure 4c).[3d] Different from 1, addition of thioani-
sole to a solution of 2 does not show any UV/Vis spectral
changes due to a low oxidizing power of 2 in the thioanisole
oxidation. However, addition of PPh3 to the solution of 2
immediately changed the UV/Vis spectrum, and the forma-
Figure 4. a) UV/Vis spectral changes of 1 (1.0ꢁ10À5 m, black line) to
ACTHNUTRGNEUNG
[MnIII(tf4tmap)]5+ (red line) upon addition of 10 equivalents of thioani-
sole (1.0ꢁ10À4 m) in a borate-buffered H2O–CH3CN (2:1) mixture at
pH 10.5 at 158C. Inset shows spectral changes of Q-band region. b) Plot
of kobs against thioanisole concentration to determine a second-order rate
constant at 158C. c) Hammett plot of log krel against sp of thioanisoles in
the reactions of 1 (1.5 mm) and para-X-substituted thioanisoles (10 equiv
to 1) at 158C.
V
À
C H bonds of alkylaromatics and that Mn –oxo is 150 times
tion of [MnII
G
more reactive than MnIV–oxo in the oxidation of xanthene.
The MnV– and MnIV–oxo species have shown a linear corre-
lation between the logarithm of the reaction rates and the
shown; Scheme 2, reaction d), indicating that the oxidation
of PPh3 by 2 occurs through a 2eÀ oxidation process. Thus,
À
À
the results discussed above suggest that the C H bond acti-
vation and oxo-transfer reactions by Mn–oxo porphyrins
C H BDE of substrates, and high KIE values were obtained
in the oxidation of xanthene and DHA by the MnV– and
occur by 1eÀ versus 2eÀ oxidation processes, respectively.
MnIV–oxo porphyrins. These results led us to conclude that
the C H bond activation of alkylaromatics by Mn – and
IV
À
MnV–oxo complexes occur through an hydrogen-atom ab-
straction mechanism. We have also demonstrated that the
disproportionation of MnIV–oxo porphyrins to MnV–oxo and
MnIII porphyrins is not a feasible pathway in basic aqueous
solutions and that MnIV–oxo porphyrins are able to abstract
Conclusions
We have shown that MnV– and MnIV–oxo porphyrins pre-
pared in basic aqueous solution are capable of activating
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11487

W. Nam, S. Fukuzumi et al.
À
ting of the kinetic data allowed us to determine kobs values. All reactions
were run at least in triplicate, and the data reported represent the aver-
age of these reactions.
hydrogen atoms from alkylaromatics. The C H bond activa-
tion by MnV– and MnIV-oxo species proceeds by means of a
one-electron process, whereas the oxidation of sulfides by a
MnV–oxo porphyrin and the oxidation of triphenylphosphine
occur through a two-electron oxidation process.
Product analysis was performed with 1 and 2 (2 mm) and substrates
(0.2m), by injecting the reaction solutions directly into GC, GC-MS, or
HPLC. Products were identified by comparing retention times and mass
patterns to those of known authentic samples. Product analysis of the re-
action solutions revealed the formation of xanthone, anthracene, ben-
zene, and 9-fluorenone as major products in the reactions of xanthene,
DHA, CHD, and fluorene, respectively. Thioanisole oxide was the only
product protected in the oxidation of thioanisole by 1. Product yields
were determined by comparison against standard curves prepared with
authentic samples and by using decane as an internal standard.
Experimental Section
Materials: All commercially available chemicals were used without fur-
ther purification unless otherwise noted. 18O2 (50% 18O-atom 18O2 and
90% 18O-atom 18O2) was purchased from ICON Services Inc. (Summit,
NJ, USA). Hydrogen peroxide (30 wt.% solution in water) and tert-butyl
hydroperoxide (tBuOOH, 70 wt.% solution in water) were obtained
from Aldrich Chemical Co. The deuterated substrate, [D4]-9,10-dihy-
droanthracene, was prepared by taking 9,10-dihydroanthracene (0.5 g,
2.7 mmol) in [D6]DMSO (3 mL) along with NaH (0.2 g, 8.1 mmol) under
an inert atmosphere.[19] After the deep red solution was stirred at room
temperature for 8 h, the reaction was quenched with D2O (5 mL). The
crude product was filtered and washed with copious amounts of H2O.
1H NMR confirmed >99% deuteration. [D2]Xanthene was prepared sim-
ilarly. 9,10-Dihydro-10-methylacridine (AcrH2) was prepared by reducing
10-methylacridinium iodide (AcrH+IÀ) with NaBH4 in methanol and pu-
rified by recrystallization from ethanol.[9,20] For the preparation of AcrH+
IÀ, acridine was treated with MeI in acetone, and then the mixture was
heated to reflux for 7 days.
Labeled oxygen experiments were carried out as follows: Oxidant
(10 mm, 5 equiv to the manganese catalyst), H2O2 or tBuOOH, was
added to a reaction solution containing [MnIII
ACHTUNTRGENNUG(tf4tmap)]ACHTUNTGNERN[UGN CF3SO3]5 (2 mm)
and xanthene (100 mm) under the atmosphere of 16O2 or 16O2/18O2 mix-
ture in buffered H2O–CH3CN (2:1) mixture at 158C. The reaction solu-
tion was stirred for 30 min. Product analysis was performed by injecting
reaction solutions directly into HPLC or by injecting product(s) isolated
by column chromatography into GC/GC-MS. Product(s) was extracted
with CH2Cl2, followed by column chromatography that was packed with
silicagel 60. Unreacted xanthene was in the first fraction, which was
eluted by 50:50% CH2Cl2–hexane. The product, xanthone, was in the
second fraction, which was eluted by 75:25% CH2Cl2–hexane. The reten-
tion time and mass pattern of the GC/GC-MS data of the isolated prod-
uct were compared with those of commercially available authentic
sample. The 16O and 18O compositions in xanthone were analyzed by the
relative abundances of m/z=195.8 for xanthone-16O and m/z=197.7 for
xanthone-18O (Supporting Information, Figure S5).
The tf4tmap porphyrin ligand, (tf4tmap=meso-tetrakis(2,3,5,6-tetra-
fluoro-N,N,N-trimethyl-4-aniliniumyl)porphyrin), and its manganese
ACHTUNGTNER(NUNG III)
porphyrin complex, [MnIII
N
ACHTUNGTRENNUNG
literature methods:[3c,21,22] The H2tf5pp porphyrin ligand (H2tf5pp=
5,10,15,20-tetrakis(pentafluorophenyl)porphyrin) was prepared using the
method reported by Lindsey et al.,[21] followed by treatment with dime-
thylamine hydrochloride to generate H2tf4dmap ligand (H2tf4dmap=
5,10,15,20-tetrakis(2,3,5,6-tetrafluoro-N,N-dimethyl-4-anilinyl)porphyrin)
Acknowledgements
The research at EWU was supported by the Korea Science and Engineer-
ing Foundation through the Creative Research Initiatives Program (to
W.N.) and KOSEF/MEST through WCU project (R31-2008-000-10010-0)
(to S.F. and W.N.), a Grant-in-Aid (No. 19205019 to S.F.), and a Global
COE program, “the Global Education and Research Centre for Bio-
Environmental Chemistry” from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (to S.F.).
in DMF at 1808C under a N2 atmosphere. [MnIII
ACTHNUTRGEN(UNG tf4dmap)]Cl was pre-
pared by heating a solution of H2tf4dmap (0.50 g, 0.47 mm) in DMF
(15 mL) solution to reflux at 1808C in the presence of excess MnIICl2
(0.87 g, 6.98 mm) under a N2 atmosphere. Subsequently, methylation was
carried out by using methyl triflouoromethane sulfonate (0.68 g, 4.14 mm)
at 608C for 12 h under a N2 atmosphere, producing [MnIII
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
sodium tetraborate decahydrate in water and the pH of the solution was
adjusted to pH 10.5 with NaOH (3.0m).
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Instrumentation: UV/Vis spectra were recorded on a Hewlett Packard
8453 spectrophotometer equipped with a circulating water bath or a Hi-
Tech Scientific SF-61 multimixing cryogenic stopped-flow instrument
equipped with a Hi-Tech Scientific KinetaScan diode array rapid scan-
ning unit. Product analysis was performed with an Agilent Technologies
6890N gas chromatograph equipped with a FID detector (GC), Thermo
Finnigan (Austin, Texas, USA) FOCUS DSQ (dual-stage quadrupole)
mass spectrometer interfaced with Finnigan FOCUS gas chromatograph
(GC-MS), and/or DIONEX Summit Pump Series P580 equipped with a
variable wavelength UV-200 detector (HPLC). Products were separated
on Waters Symmetry C18 reverse phase column (4.6ꢁ250 mm), and sam-
ples were monitored by UV Detector at a fixed wavelength of 215 nm or
254 nm. 1H NMR spectra were measured with Bruker DPX-250 spec-
trometer. EPR spectra were obtained on
ACHTUNGTRENNUNGspectrometer.
a JEOL JES-FA200
Kinetics and product analysis: Reactions for kinetics studies were fol-
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158C. [MnV(tf4tmap)(O)2]3+ (1) and [MnIV(tf4tmap)(O)(OH)]3+ (2) inter-
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
mediates were prepared by reacting [MnIII
ACHTUNTRGNENUG(tf4tmap)]ACHTUGNTRNE[NUGN CF3SO3]5 (1ꢁ
10À2 mm) with H2O2 (2 equiv) and tBuOOH (2 equiv), respectively, in a
solvent mixture of borate-buffered H2O–CH3CN (2:1) mixture at 158C.
Subsequently, appropriate amounts of substrates were added to the reac-
tion solutions. After the completion of reactions, pseudo-first-order fit-
11488
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Chem. Eur. J. 2009, 15, 11482 – 11489

Manganese–Oxo Porphyrin Reactions
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Received: May 21, 2009
Published online: October 6, 2009
Chem. Eur. J. 2009, 15, 11482 – 11489
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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