Aromatic hydroxylation of anthracene derivatives by a chromium(III)-superoxo complex via proton-coupled electron transfer

Article abstract of DOI:10.1039/c9cc03245b

The chemistry of metal-superoxo intermediates started being unveiled in oxidation reactions by enzymes and their synthetic model compounds. However, aromatic hydroxylation reactions by the metal-superoxo species are yet to be demonstrated. In this study, we report for the first time that the hydroxylation of aromatic compounds such as anthracene and its derivatives by a mononuclear nonheme Cr(iii)-superoxo complex, [(Cl)(TMC)Cr^III(O₂)]⁺ (1), occurs in the presence of triflic acid (HOTf) via the rate-determining proton-coupled electron transfer (PCET) from anthracene to 1, followed by a fast further oxidation to give anthraquinone. The rate constants of electron transfer from anthracene derivatives to 1 in the presence of HOTf are well analyzed in light of the Marcus theory of electron transfer.

Full text of DOI:10.1039/c9cc03245b

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Aromatic Hydroxylation of Anthracene Derivatives by a
Chromium(III)-Superoxo Complex via Proton-Coupled
Electron Transfer†
Received 00th October 2016,
Accepted 00th January 20xx
a
a
a
ab
Tarali Devi, Yong-Min Lee, Wonwoo Nam,* and Shunichi Fukuzumi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The chemistry of metal-superoxo intermediates started being
unveiled in oxidation reactions by enzymes and their synthetic
model compounds. However, aromatic hydroxylation reactions
by the metal-superoxo species have yet to be demonstrated. In this
study, we report for the first time that the hydroxylation of
aromatic compounds such as anthracene and its derivatives by a
mononuclear
nonheme
Cr(III)-superoxo
complex,
III
+
[
(
(
(Cl)(TMC)Cr (O
HOTf) via the rate-determining proton-coupled electron transfer
PCET) from anthracene to 1, followed by a fast further oxidation
2
)] (1), occurs in the presence of triflic acid
to give anthraquinone. The rate constants of electron transfer
from anthracene derivatives to 1 in the presence of HOTf are well
analyzed in light of the Marcus theory of electron transfer.
Scheme 1 Schematic representation of oxidation of anthracene by
Metal-superoxo species have been proposed as the foremost
intermediate generated during dioxygen activation process by the
[(Cl)(TMC)CrIII(O )]
+
(1) in the presence of triflic acid (HOTf).
2
isolation and characterization of a number of metal-superoxo
intermediates have been achieved in model systems9 and their
reactivity studies have been examined towards electrophilic hydrogen
atom transfer (HAT), oxygen atom transfer (OAT), hydride transfer
and nucleophilic aldehyde deformylation reactions.10 It has also been
reported very recently that chemical properties of the metal-superoxo
species can be tuned by the presence of acid.11 Although advances
have been made in understanding reactivities of metal-superoxo
species, their chemical properties still need to be fully explored and
yet to be understood in broad aspects.
Aromatic hydroxylation has been described as one of the most
challenging chemical processes mediated by many metalloenzymes,
including cytochromes P450 and nonheme iron mono- and
dioxygenases for biotransformations of pharmaceuticals and other
xenobiotics, and high-valent oxoiron(IV) species have been invoked
as active oxidants in the aromatic hydroxylation reactions.12 In
biomimetic studies, high-valent metal-oxo species have shown
reactivities in the aromatic hydroxylation reactions, and their
mechanistic studies have also been investigated through a combined
experimental and theoretical approach.13 However, to the best of our
knowledge, no report is available for a metal-superoxo species
capable of hydroxylating aromatic compounds in both heme and
nonheme systems.
reduced metal centres in both heme and nonheme metalloenzymatic
systems.1-3 The intermediates have also been invoked as active
oxidants in the C-H bond activation of organic substrates by nonheme
enzymes containing iron (e.g., isopenicillin N synthase, myo-inositol
oxygenase, 2-hydroxyethylphosphonate dioxygenase) and copper
4
(
e.g., dopamine β-monooxygenase, tyramine β-monooxygenase). In
succession to metal-superoxo species, metal-(hydro)peroxo and
metal-oxo species are also proposed to be generated during substrate
oxygenation via dioxygen activation in natural and model systems.5
-7
From the last few decades the high-valent metal-oxo species have
been intensively investigated in a variety of oxidation reactions in
8
heme and nonheme systems. However, efforts have also been made
by the bioinorganic and biomimetic communities recently to explore
the chemistry of metal-superoxo intermediates. Thus, successful
a
Department of Chemistry and Nano Science, Ewha Womans University, Seoul
03760, Korea. E-mail: wwnam@ewha.ac.kr, fukuzumi@chem.eng.osak-u.ac.jp
b
Faculty of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502,
Japan.
†
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, Tables S1 and S2, and Fig. S1-S10. See DOI: 10.1039/x0xx00000x
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Fig. 1 UV-vis spectral changes observed in the 6-electron oxidation of Fig. 2 Dependence of k on [HOTf] for the oxidation of anthracene (0.50
ox
anthracene (0.50 mM) by 1 (1.0 mM) in the presence of HOTf (1.0 mM) mM) by 1 (0.50 mM) in the presence of HOTf (0–10 mM) at 233 K.
2
in MeCN at 233 K. The inset shows the time profile monitored at 550 Inset shows the plot of kox vs. [HOTf] .
nm due to the decay of 1.
1
6
When the product obtained in the oxidation of anthracene by 1- O
[(Cl)(TMC)Cr ( O )] ) in the presence of HOTf in MeCN at 233 K
2
was compared with that obtained in the oxidation of anthracene by 1-
2
In
biomimetic
studies,
a
Cr(III)-superoxo
complex,
III 16
+
(
III
)]+ (1; TMC = 1,4,8,11-tetramethyl-1,4,8,11-
[
(Cl)(TMC)Cr (O
2
tetraazacyclotetradecane), was successfully synthesized and
characterized spectroscopically and structurally. In addition, its
reactivities towards hydrogen atom transfer (HAT), oxygen atom
transfer (OAT) and hydride transfer reactions were reported as well.1
Very recently, it was also shown that the reactivity of 1 was
remarkably enhanced towards OAT reactions in the presence of triflic
acid (HOTf).11 Such a remarkable enhancement of reaction rate in
OAT reaction by 1 in the presence of HOTf was explained due to the
much more positive shift of one-electron reduction potential of 1 (Ered
18O
([(Cl)(TMC)Cr ( O
III 18
)] ) in the presence of HOTf, peak at m/z =
+
2
2
1
6
16
2
2
08 for anthraquinone- O, O in GC-MS spectrum shifted to m/z =
1
8
18
12 for anthraquinone- O, O (Fig. S4 for GC-MS, ESI†), indicating
4
that two oxygen atoms in anthraquinone originated from 1. In addition,
product analysis by CSI-MS and EPR measurements revealed that
III
3+
chromium(III) complex, [Cr (TMC)] (2), was produced as the final
inorganic product (Fig. S5, ESI†).
The kinetic measurements of the anthracene oxidation by 1 were
performed by following the decay of absorbance at 550 nm due to 1
in the presence of HOTf (2.5 mM) in MeCN at 233 K. The decay rate
of 1 in the oxidation of anthracene by 1 is given by eqn (1). According
=
1.12 V vs. SCE) in the presence of HOTf (2.5 mM). As our ongoing
efforts to elucidate detailed mechanistic insights of 1 into oxidation
reactions, we have performed the reactivity study of 1 towards
hydroxylation of aromatic compounds in the presence of HOTf. 1,
which was reported as a three-electron oxidant for the oxidation of
NADH analogue,14c can perform six-electron oxidation of anthracene
to produce anthraquinone in the presence of HOTf under
stoichiometric conditions in acetonitrile (MeCN) at 233 K. The
mechanism of the proton-assisted hydroxylation of anthracene and its
derivatives by 1 is clarified by comparison with the proton-coupled
electron-transfer (PCET) reaction of 1 with one-electron donors in
light of the Marcus theory of electron transfer. Thus, we believe that
the present study provides a new scope to expand the reactivity of
metal-superoxo species with HOTf as a hydroxylating agent of
aromatic compounds.
Notably, the oxidation of anthracene by 1 does not occur in the
absence of acid (i.e., HOTf) in MeCN at 233 K (Fig. S1, ESI†).
However, anthracene was fully oxidized (6-electron oxidation) by two
equivalents of 1 (3-electron oxidant) in the presence of two
equivalents of HOTf as shown in Fig. 1 (see also Scheme 1). The
stoichiometry of the reaction was confirmed by UV-vis spectral
titration experiments following the decay of absorption band at λmax
50 nm due to 1 upon addition of anthracene in the presence of HOTf
in MeCN at 233 K (Fig S2, ESI†).
The product analysis after the reaction was performed by H-NMR
and GC-MS measurements, resulting that anthraquinone as six-
electron oxidized product of anthracene was produced (Fig. S3 for
NMR and Fig. S4 for GC-MS, ESI†). The yield of anthraquinone was
found to be ~100% as analyzed with H-NMR using cyclohexane as
an internal reference. The source of oxygen in the anthraquinone
product was confirmed by 18O-labeling experiment.
to the stoichiometry of the six-electron oxidation of anthracene by 1
III
(
Scheme 1), the initial concentrations of 1. ([Cr (O
2
)]
0
) and
anthracene ([An]
0
) are given by eqns (2) and (3), respectively. Then,
eqn (1) is rewritten by eqn (4).
III
III
–d[Cr (O )]/dt = k [Cr (O )][An]
2
ox
2
(1)
(2)
(3)
III
III
)] + [CrIII]
[
Cr (O
2
)]
0
= [Cr (O
2
III
[An] = [An] + 2[Cr ]
0
III
III
III
III
–
d[Cr (O
2
)]/dt = kox[Cr (O
2
)]([An]
0
– 2[Cr (O
2
)]
0
+ 2[Cr (O
2
)])
(4)
Solving the differential eqn (4) with eqns (2) and (3) gives eqn (5)
log[x/(C + 2x)] = –(C/2.3)koxt + log[x /(C + 2x )] (5)
, C = [An] , x /(C
0
0
III
III
III
where x = [Cr (O
2x ) = 1. Thus, plot of log[x/(C + 2x)] vs. t gives a straight line that
passes through zero with slope = –(C/2.3)kox, where C is –x . The kox
2
)], x
0
= [Cr (O
2
)]
0
0
– 2[Cr (O
2
)]
0
0
+
0
0
=
value of the oxidation of anthracene by 1 in the presence of HOTf (2.5
mM) in MeCN at 233 K was determined by using eqn (5) to be 41(4)
M–1 s–1 (Fig. S6 and Table S1, ESI†).
When anthracene was replaced by deuterated anthracene
anthracene-d10), no kinetic isotope effect (KIE) was observed (Fig.
S6, ESI†), suggesting that no C-H bond cleavage is involved in the
rate-determining step. The reactivity of 1 towards oxidation of
anthracene was further investigated varying HOTf concentration (Fig.
S7, ESI†). The kox value of the oxidation of anthracene by 1 increased
5
1
(
1
2
| J. Name., 2016, 00, 1-3
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Fig. 3 Driving force (–∆Get) dependence of rate constants (log ket or log kox
)
of electron transfer from ferrocene derivatives (black circles; (1) 1,1′-
dimethylferrocene, (2) ferrocene, (3) bromoferrocene, (4) 1,1′-
dibromoferrocene) and from anthracene derivatives (red circles; (5) 9,10-
dimethylanthracene, (6) 9-methylanthracene, (7) anthracene (8) 9-
bromoanthracene and (9) 9-anthracenecarboxaldehyde) to 1 in the presence
of HOTf (2.5 mM) at 233 K. The red and black lines are Marcus lines
calculated with λ values of 1.53 and 2.32 eV, respectively.
proton
transfer
from
9,10-dihydroxyanthracene
to
III
)]3+ to finally produce 2, accompanied by the
[Cr (TMC)(O
2
H
2
elimination of water.
The kinetic measurements for the oxidation of 9-substituted
anthracene derivatives (XAn), such as 9-methylanthracene, 9-
bromoanthracene and 9-anthracenecarboxaldehyde, were also
examined in the presence of HOTf (2.5 mM) in MeCN at 233 K.
According to the stoichiometry of the four-electron oxidation of XAn
by 1 (3-electron oxidant), the kinetic equations are derived to
determine the second-order rate constant (kox) (see Experimental
Section in ESI†; Determination of kox value for the four-electron
oxidation of substrate by 3-electron oxidant). The kox values of the
four-electron oxidation of XAn by 1 were determined by using eqn
Scheme 2. Plausible reaction mechanism.
(
S5) in ESI† (see also Fig. S10 and Table S2, ESI†).
The kinetic measurements for the oxidation of 9,10-
with an increase in HOTf concentration (Fig. S8 and Table S1, ESI†),
exhibiting second-order dependence with respect to HOTf
concentration as shown in Fig. 2 [eqn (6)].15
dimethylanthracene were also performed in the presence of HOTf (2.5
mM) in MeCN at 233 K. The second-order rate constants (kox) for the
two-electron oxidation of 9,10-dimethylanthracene by 1 (3-electron
oxidant) was determined by using eqn (S10) in ESI† (see
Experimental Section in ESI†; Determination of kox value for the two-
electron oxidation of substrate by 3-electron oxidant; Fig. S11 and
Table S2, ESI†).
The rate constants of proton assisted hydroxylation of anthracene
and its derivatives by 1 is then compared with the rate constants of
outer-sphere electron-transfer reactions as explained by the Marcus
theory of electron transfer [eqn (7)],18
k
ox = k
2
[HOTf]2
(6)
Based on the results described above, the plausible mechanism of
aromatic hydroxylation of anthracene by 1 in the presence of HOTf in
MeCN at 233 K is shown in Scheme 2, where six-electron oxidation
of anthracene by 1 in the presence of HOTf proceeds through the rate
determining initial electron transfer from anthracene to 1 to produce
III
3+
2 2
anthracene radical cation and [Cr (TMC)(O H )] . Although the
intermediates after the rate-determining step (r. d. s.) cannot be
observed, the initial electron-transfer step may be followed by further
oxidation of anthracene radical cation to generate 9-
2
k
et = Zexp[–(/4)(1 + ΔGet/) /(k
B
T)]
(7)
IV
2+
hydroxyanthracene and [Cr (TMC)(O)] with the elimination of two
protons. 9-Hydroxyanthracene is known to be tautomerized to
anthrone,16 which is further oxidized by an electron transfer to the
where Z is the collision frequency which is normally taken as 1 × 1011
M–1 s–1, which corresponds to k
TK/h (k is the Boltzmann constant,
B
B
–
1 18
T is the absolute temperature, K (= ~0.020 M ) is the formation
constant of the precursor complex, and h is the Planck constant), λ is
the reorganization energy of electron transfer and ΔGet is the free
energy change of electron transfer. The driving force (–∆Get) values
were determined from the difference between the one-electron
oxidation potentials (Eox) of electron donors and the one-electron
generated [Cr (TMC)(O)]2+ in the presence of two protons to produce
IV
III
9
-hydroxyanthracene radical cation and a [Cr (TMC)]3+ species.17
Further electron transfer takes place from 9-hydroxyanthracene
radical cation to another molecule of 1 in the presence of water to
III
3+
2 2
generate 9,10-dihydroxyanthracene and [Cr (TMC)(O H )] . In the
final step, anthraquinone is produced by stepwise electron transfer and
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reduction potential of 1 (Ered = 1.12 V vs. SCE) in the presence of
HOTf (2.5 mM), as given by eqn (8), where e is the elementary charge.
Karlin, Isr. J. Chem., 2016, 56, 738; (d) E. G. KovaVlieewvaAratinclde OJn. liDne.
Lipscomb, Nat. Chem. Biol., 2008, 4, 186; (e) J. M. Bollinger, Jr. and
DOI: 10.1039/C9CC03245B
C. Krebs, Curr. Opin. Chem. Biol., 2007, 11, 151.
5
(a) C.-C. Wang, H.-C. Chang, Y.-C. Lai, H. Fang, C.-C. Li, H.-K. Hsu,
Z.-Y. Li, T.-S. Lin, T.-S. Kuo, F. Neese, S. Ye, Y.-W. Chiang, M.-L.
Tsai, W.-F. Liaw and W.-Z. Lee, J. Am. Chem. Soc., 2016, 138, 14186;
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Kitagawa and H. Masuda, Chem. Commun., 2006, 4428; (c) Y. M. Kim,
K.-B. Cho, J. Cho, B. Wang, C. Li, S. Shaik and W. Nam, J. Am. Chem.
Soc., 2013, 135, 8838.
–
∆Get = e(Ered – Eox
)
(8)
The driving force dependence of logarithm of the rate constants of
outer-sphere electron transfer (ket) from one-electron donors (i.e.,
ferrocene derivatives) to 1 in the presence of HOTf (2.5 mM) in
MeCN at 233 K is shown in Figure 3 (black circles), where the log ket
values11 are plotted against the –ΔGet values. The driving force
dependence of ket is well fitted by the black line in Figure 3 using eqn
6
7
8
(a) J.-G. Liu, Y. Shimizu, T. Ohta and Y. Naruta, J. Am. Chem. Soc.,
2
010, 132, 3672; (b) R. Cao, C. Saracini, J. W. Ginsbach, M. T.
Kieber-Emmons, M. A. Siegler, E. I. Solomon, S. Fukuzumi and K. D.
Karlin, J. Am. Chem. Soc., 2016, 138, 7055.
(
7) with the λ value of 2.32 eV.11 The rate constants of electron
transfer from anthracene derivatives (kox) to 1 are also plotted against
ΔGet values in Figure 3 (red circles), where the driving force
(a) D. Kumar, W. Thiel and S. P. de Visser, J. Am. Chem. Soc., 2011,
133, 3869; (b) Y.-M. Lee, S. Hong, Y. Morimoto, W. Shin, S.
Fukuzumi and W. Nam, J. Am. Chem. Soc., 2010, 132, 10668; (c) J.
Cho, J. Woo and W. Nam, J. Am. Chem. Soc., 2012, 134 ,11112.
(a) M. Guo, T. Corona, K. Ray and W. Nam, ACS Cent. Sci., 2019, 5,
–
dependence of kox is fitted with somewhat smaller reorganization
energy of λ = 1.53 eV (red line in Fig. 3). The smaller λ value for the
case of anthracene derivatives in the Marcus plot compared with that
of outer-sphere electron transfer from one-electron donors (i.e.,
ferrocene derivatives) to 1 may be accounted by much faster electron
1
3; (b) H. B. Gray and J. R. Winkler, Acc. Chem. Res., 2018, 51, 1850;
(c) S. Fukuzumi, T. Kojima, Y.-M. Lee and W. Nam, Coord. Chem.
Rev., 2017, 333, 44; (d) M. Puri and L. Que, Jr., Acc. Chem. Res., 2015,
48, 2443.
(a) C.-W. Chiang, S. T. Kleespies, H. D. Stout, K. K. Meier, P.-Y. Li,
E. L. Bominaar, L. Que, Jr., E. Münck and W.-Z. Lee, J. Am. Chem.
Soc., 2014, 136, 10846; (b) S. Hong, K. D. Sutherlin, J. Park, E. Kwon,
M. A. Siegler, E. I. Solomon and W. Nam, Nat. Commun., 2014, 5,
exchange
between
9,10-dimethylanthracene
and
9,10-
8
–1 –1 18a
dimethylanthracene radical cation (5.0  10 M s ) compared to
that between ferrocene and ferrocenium cation (5.3  10 M s ) in
MeCN
9
6
–1 –1
1
8b
and small bond reorganization energy of anthracene
1
2c
derivatives, which are delocalized π-compounds.
5
440; (c) A. Kunishita, M. Kubo, H. Sugimoto, T. Ogura, K. Sato, T.
In conclusion, aromatic hydroxylation of anthracene by 1 has been
made possible in the presence of HOTf to produce anthraquinone,
which is the fully oxidized (six-electron oxidation) product of
anthracene. The reaction proceeds via the rate-determining PCET
from anthracene to 1, followed by the subsequent fast oxidation
reactions all the way to anthraquinone. In addition, no KIE (i.e., KIE
Takui and S. Itoh, J. Am. Chem. Soc., 2009, 131, 2788; (d) M. T.
Kieber-Emmons, J. Annaraj, M. S. Seo, K. M. Van Heuvelen, T. Tosha,
T. Kitagawa, T. C. Brunold, W. Nam and C. G. Riordan, J. Am. Chem.
Soc., 2006, 128, 14230.
0 (a) M. Sankaralingam, Y.-M. Lee, W. Nam and S. Fukuzumi, Coord.
Chem. Rev., 2018, 365, 41; (b) A. D. Ure and A. R. McDonald, Synlett,
1
1
2
015, 26, 2060.
1 T. Devi, Y.-M. Lee, W. Nam and S. Fukuzumi, J. Am. Chem. Soc.,
018, 140, 8372.
derivatives to 1 were evaluated in comparison with the outer-sphere 12 (a) C. M. Bathelt, L. Ridder, A. J. Mulholland and J. N. Harvey, J. Am.
=
1) also suggests no involvement of C-H bond cleavage in the rate-
determining step, which is PCET. The rates of PCET from anthracene
2
electron-transfer reactions from one-electron donors to 1 in the
presence of HOTf in light of the Marcus theory of electron transfer.
This study provides valuable mechanistic insight into the
hydroxylation of aromatic substrates by metal-superoxo species via
the rate-limiting PCET process.
Chem. Soc., 2003, 125, 15004; (b) M. Asaka and H. Fujii, J. Am. Chem.
Soc., 2016, 138, 8048; (c) N. Sharma, J. Jung, Y.-M. Lee, M. S. Seo,
W. Nam and S. Fukuzumi, Chem. Eur. J., 2017, 23, 7125.
1
1
3 (a) S. P. de Visser, K. Oh, A.-R. Han and W. Nam, Inorg. Chem., 2007,
4
6, 4632; (b) D. Kumar, G. N. Sastry and S. P. de Visser, J. Phys.
Chem. B, 2012, 116, 718.
This work was supported by a SENTAN project from Japan Sci-
ence and Technology Agency (JST) to S.F. and JSPS KAKENHI (No.
4 (a) J. Cho, J. Woo and W. Nam, J. Am. Chem. Soc., 2010, 132, 5958;
(b) J. Cho, J. Woo and W. Nam, J. Am. Chem. Soc., 2012, 134, 11112;
(c) T. Devi, Y.-M. Lee, J. Jung, M. Sankaralingam, W. Nam and S.
Fukuzumi, Angew. Chem. Int. Ed., 2017, 129, 3564.
1
6H02268 to S.F.) from MEXT, Japan, and the NRF of Korea through
CRI (NRF-2012R1A3A2048842 to W.N), GRL (NRF-2010-00353 to
W.N.), and Basic Science Research Program
2017R1D1A1B03029982 to Y.-M.L. and 2017R1D1A1B03032615
to S.F.).
1
5 (a) J. Park, Y. Morimoto, Y.-M. Lee, W. Nam and S. Fukuzumi, J. Am.
Chem. Soc., 2012, 134, 3903; (b) Y. Morimoto, H. Kotani, J. Park, Y.-
M. Lee, W. Nam and S. Fukuzumi, J. Am. Chem. Soc., 2011, 133, 403.
6 H.-G. Korth and P. Mulder, J. Org. Chem., 2013, 78, 7674.
7 It should be noted that, in the oxidation of anthracene by
(
1
1
IV
2+
References
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occurred (see Fig. S9, ESI†), indicating that Cr(III)-superoxo complex
1
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DOI: 10.1039/C9CC03245B
Aromatic hydroxylation of anthracene by a mononuclear nonheme
Cr(III)-superoxo complex proceeds via the rate-determining
proton-coupled electron transfer, followed by fast further
oxidation to anthraquinone.
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