Aromatic ring cleavage of 2-amino-4-tert-butylphenol by a nonheme iron(II) complex: Functional model of 2-aminophenol dioxygenases

Article abstract of DOI:10.1002/anie.201206922

Biomimetic aromatic ring cleavage: An iron(II)-2-aminophenolate complex (see picture, right) with a tetradentate ligand reacts with dioxygen to cleave the aromatic C-C bond of 2-amino-4-tert-butylphenolate to form 4-tert-butyl-2-picolinate. This complex represents a functional model of 2-aminophenol-1,6-dioxygenase (APD) and 3-hydroxyanthranilate-3,4-dioxygenase (HAD). Copyright

Full text of DOI:10.1002/anie.201206922

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DOI: 10.1002/anie.201206922
Biomimetic Model
Aromatic Ring Cleavage of 2-Amino-4-tert-butylphenol by a Nonheme
Iron(II) Complex: Functional Model of 2-Aminophenol
Dioxygenases**
Biswarup Chakraborty and Tapan Kanti Paine*
A large variety of dioxygenases, which catalyze the ring
cleavage of aromatic compounds, are found in aerobic
bacteria.[1–3] Catechol-cleaving dioxygenases are well-studied
examples in this class of enzymes.[1,4–8] Pseudomonas Pseu-
doalcaligenes, which can be found in nitrobenzene-contami-
nated soil and groundwater, is involved in the catabolism of
nitrophenol; the catabolic pathway proceeds through the
Scheme 1. Reaction catalyzed by APD and HAD.
reduction of 2-nitrophenol to 2-aminophenol.[9,10] Oxidative
À
C C bond cleavage of 2-aminophenol then forms 2-amino-
À
muconic acid semialdehyde, which spontaneously loses
a water molecule to form 2-picolinic acid. In addition, other
nitroaromatic compounds are degraded by some bacteria in
a similar pathway.[11,12] Furthermore, the metabolism of one of
the essential amino acids, tryptophan, proceeds through the
iron(II) complex that exhibits C C bond cleavage activity of
2-aminophenolate in the presence of dioxygen. Herein, we
report the synthesis, characterization, and dioxygen reactivity
of a nonheme iron(II) complex, [(6-Me3-TPA)FeII(4-tBu-
HAP)](ClO4) (1·ClO4), in which 6-Me3-TPA = tris(6-methyl-
2-pyridylmethyl)amine and 4-tBu-HAP = monoanionic 2-
À
formation of 3-hydroxyanthranilate followed by C3 C4 bond
À
cleavage to form quinolinic acid via 2-amino-3-carboxymu-
amino-4-tert-butylphenolate. The oxidative C C bond cleav-
age of 2-amino-4-tert-butylphenolate on complex 1 mimicking
the function of APD and HAD is discussed.
The iron(II)–2-aminophenolate complex (1) was isolated
from the reaction of 6-Me3-TPA ligand, iron(II) perchlorate,
and 2-amino-4-tert-butylphenol (4-tBu-H2AP) in the presence
of one equivalent of triethylamine in methanol. The yellow
solution of complex 1 in acetonitrile displays an intense
conic acid semialdehyde.[13–15]
2-Aminophenol-1,6-dioxygenase (APD),[16–18] isolated
and purified from Pseudomonas Pseudoalcaligenes, is respon-
À
sible for the C C bond cleavage of 2-aminophenols under
aerobic conditions (Scheme 1). The degradation of 3-
hydroxyanthranilate to quinolinate is catalyzed by 3-hydroxy-
anthranilate-3,4-dioxygenase (HAD) in the presence of
dioxygen (Scheme 1).[19] Structural studies show that the
active site of HAD contains an iron(II) center that is
coordinated by the “2-His-1-Glu facial triad”.[20–22] Both
APD and HAD belong to the class of nonheme iron(II)
enzymes and share functional similarity with extradiol-cleav-
ing catechol dioxygenases. A mechanism, similar to that of
extradiol-cleaving catechol dioxygenases, has been proposed
1
charge-transfer (CT) band at 404 nm. H NMR spectrum of
the complex in CDCl3 exhibits paramagnetically shifted
proton resonances in the region of À40 ppm to 60 ppm (see
Figure S1 in the Supporting Information). The NMR data
along with the magnetic moment of 5.1 mB at room temper-
ature are indicative of the high-spin nature of the iron(II)
complex. Complex 1 was further characterized by single-
crystal X-ray diffraction. Unfortunately, all attempts to grow
single crystals of 1·ClO4 were unsuccessful. However, X-ray-
quality single crystals of 1·BPh4 (see Experimental Section in
the Supporting Information) were isolated from a dichloro-
methane/methanol/diethyl ether solvent mixture at 273 K.
X-ray crystal structure of the complex cation shows a six-
coordinate iron center ligated by four nitrogen donors from
the tetradentate ligand, and one nitrogen and one oxygen
donor from the aminophenol ligand (Figure 1). The Fe–Npy
distances are in a range of 2.186(3) ꢀ to 2.305(3) ꢀ, similar to
those reported for other high-spin iron(II) complexes of the
tetradentate 6-Me3-TPA ligand.[31,32] The aminophenol ligand
binds to the metal center through N5 and O1 with Fe1–N5 and
Fe1–O1 distances of 2.282(3) and 1.934(3) ꢀ, respectively
(see Table S1 in the Supporting Information). A short Fe1–O1
distance implies a monoanionic binding of the aminopheno-
late (4-tBu-HAP). Recently, Fiedler and co-workers have
reported an iron(II)–aminophenolate complex, [(TpPh2)FeII-
À
for aromatic C C bond cleavage of 2-aminophenols
(Scheme 1).[19,23]
Despite the existence of a large number of iron complexes
with the coordinated 2-aminophenolates in different redox
states,[24–30] thus far there is no example of a biomimetic
[*] B. Chakraborty, Dr. T. K. Paine
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2A & 2B Raja S. C. Mullick Road, Jadavpur
Kolkata-700032 (India)
E-mail: ictkp@iacs.res.in
[**] T.K.P. acknowledges the DST, Government of India (Project SR/S1/
IC-51/2010) for financial support. B.C. thanks CSIR, India, for
a fellowship. The crystal-structure determination was performed at
the DST-funded National Single Crystal Diffractometer Facility in the
Department of Inorganic Chemistry, IACS.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206922.
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Chemie
Figure 1. ORTEP plot of the complex cation of 1·BPh4 with 40%
thermal ellipsoids. All hydrogen atoms except those on N5 have been
omitted for clarity. Selected bond lengths [ꢀ] and angles [deg] for
1·BPh4: Fe1–O1 1.934(3), Fe1–N1 2.305(3), Fe1–N2 2.186(3), Fe1–N3
2.290(3), Fe1–N4 2.256(3), Fe1–N5 2.282(3), C22–O1 1.320(5), C22–
C23 1.392(6), C23–C24 1.385(6), C24–C25 1.402(7), C25–C26 1.380(6),
C26–C27 1.387(6), C27–C22 1.408(6); O1-Fe1-N1 106.17(12), O1-Fe1-
N2 175.29(12), O1-Fe1-N3 102.00(13), O1-Fe1-N4 104.09(12), O1-Fe1-
N5 80.55(11), N1-Fe1-N2 77.61(12), N1-Fe1-N3 151.42(13), N1-Fe1-N4
78.57(12), N1-Fe1-N5 88.49(12), N2-Fe1-N3 74.03(12), N2-Fe1-N4
79.27(12), N2-Fe1-N5 96.88(12), N3-Fe1-N4 99.31(12), N3-Fe1-N5
91.44(12), N4-Fe1-N5 167.00(12).
(
tBuAPH)]
(TpPh2 = hydrotris(3,5-diphenylpyrazolyl)borate
and tBuAPH = monoanionic 2-amino-4,6-di-tert-butylpheno-
late), in which the coordinated aminophenolate has been
found to exhibit Fe–Namine and Fe–Ophenolate distances of
2.214(1) and 1.931(1) ꢀ, respectively.[33] The C C distances
À
Figure 2. Optical spectral changes over time during the reaction of
1·ClO4 (1 mm solution in acetonitrile, path length=0.5 cm) with
dioxygen at 298 K: a) reaction during the first 2 min, b) slow reaction
for 6 h. Inset: plot of absorbance versus time.
of the HAP ring in 1·BPh4 are found to be similar to those in
[(TpPh2)FeII(tBuAPH)]. The complex cation of 1·BPh4 has
structural similarity with the iron(II)–catecholate complex
[(6-Me3-TPA)FeII(DBCH)](ClO4) of the 6-Me3-TPA ligand
(DBCH = monoanionic 3,5-di-tert-butylcatecholate).[34] The
phenolate oxygen (O1) of 4-tBu-HAP and the amine nitrogen
(N2) of the supporting ligand occupy the axial plane of the
distorted octahedron with the O1-Fe1-N2 angle of 175.3(1)8.
The geometry of the cationic part of 1·BPh4 was optimized by
DFT calculation with the atomic coordinates obtained from
the crystal structure. The calculated bond parameters
(Table S1) of the high-spin iron(II) complex (S = 2) are in
good agreement with those obtained experimentally. A
Mulliken spin density plot of the optimized geometry (Fig-
ure S2) suggests that 94% of the total electron density is
localized on the metal center, 4% on the aminophenolate
ring, and the rest on the supporting N4 ligand (Table S2).
The iron(II)–aminophenolate complex (1·ClO4) reacts
with molecular oxygen in acetonitrile under ambient con-
ditions. A light-yellow solution rapidly turns to deep-green
within 2 min. During the reaction, the CT band at 404 nm
disappears and three new bands at 366 nm, 600 nm, and
934 nm appear rapidly (Figure 2a). A similar spectral change
has been observed in the reaction of [(6-Me3-TPA)FeII-
(DBCH)](ClO4) with dioxygen, and the origin of the intense
CT bands has been attributed to the catecholato-to-iron(III)
CT transitions.[34] In analogy to related complexes,[25,29,35] the
CT bands may be assigned to the 2-amidophenolate-to-
iron(III) CT transitions. The dark-green solution formed after
2 min shows an EPR signal at g = 4.21 typical of high-spin
iron(III) species with an S = 5/2 spin state (Figure S3).
Additionally, an isotropic signal is also observed at g = 1.99,
possibly because of the presence of a small amount of
a radical impurity in the solution. To prove the formation of
an iron(III) species in the first step, attempts were made to
independently synthesize the iron(III) complex. Unfortu-
nately, no iron(III) complex could be isolated through direct
synthesis by mixing the ligand, iron(III) salt, and 2-amino-4-
tert-butylphenol in the presence of a base. However, con-
trolled one-electron oxidation of 1·ClO4 with a stoichiometric
amount of KMnO4 results in the isolation of a dark-green
complex, 1ox·ClO4 (Experimental Section). The optical spec-
tral features and the EPR data of 1ox·ClO4 (Figure S4) bear
resemblance to those of the deep-green species observed in
the reaction of 1·ClO4 with dioxygen. Thus, the initial change
in the optical spectrum of 1·ClO4 during the reaction with
dioxygen is associated with the formation of [(6-Me3-
TPA)FeIII(4-tBu-AP)](ClO4) (1ox·ClO4).
All efforts to isolate the single crystals of 1ox·ClO4 failed.
DFT calculations were performed to optimize the geometry
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of [(6-Me3-TPA)FeIII(4-tBu-AP)]+ with S = 5/2 spin state
(Table S3). Time-dependent density functional theory
(TDDFT) calculation (Table S4) of the optimized geometry
of [(6-Me3-TPA)FeIII(4-tBu-AP)]+ with S = 5/2 spin state
exhibits three transitions in the 300–1200 nm region (Fig-
ure S5), which correlate well with the experimental data. All
these results unambiguously confirm a one-electron oxidation
of 1·ClO4 with dioxygen in the first step to form an iron(III)–
2-imidophenolate species, 1ox·ClO4. A sufficient electron
density (Table S3) is observed on 2-imidophenolate ring
(19.4%; Figure S2) in the spin density plot of [(6-Me3-
TPA)FeIII(4-tBu-AP)]+, which could facilitate further reac-
tion with dioxygen.
In the next step of the reaction with O2, the CT bands at
934 nm and 600 nm disappear immediately, and a new band
appears at 660 nm and subsequently slowly disappears over
a period of 6 h, resulting in a light-orange solution (Fig-
ure 2b). A similar spectral change is observed upon exposure
of a solution of 1ox·ClO4 in acetonitrile to dioxygen for 6 h.
During the reaction, the EPR signal at g = 1.99 slowly
disappears, leaving the high-spin iron(III) signal at g = 4.21
after 6 h (Figure S3). The ESI-MS spectrum of the oxidized
solution shows an ion signal at m/z = 566.2 with the isotope
distribution pattern calculated for [(6-Me3-TPA)Fe(4-tert-
butyl-2-picolinate)]+ (Figure 3a, and Figure S6 in the Sup-
porting Information). The formation of picolinate was further
confirmed by labeling experiments. ESI-MS of the oxidized
solution after reaction of 1·ClO4 with 18O2 displays an ion
signal at m/z = 568.2, thus suggesting the incorporation of one
18O atom into 4-tert-butyl-2-picolinate (Figure 3b and Fig-
ure S7). Furthermore, the labeling experiment with H218O and
16O2 shows about 30% incorporation of one 18O into the
product picolinate (Figure S8).
1
The H NMR spectrum of the organic product derived
from 2-amino-4-tert-butylphenolate shows four distinct and
sharp proton resonances; two doublets at 8.6 ppm (J = 5 Hz)
and 7.4 ppm (J = 5 Hz) and two singlets at 8.2 ppm and
1.36 ppm (Figure S9c). The coupling constants of the doublets
suggest that the resonances belong to aromatic protons. Of
note, a singlet at 6.7 ppm and a multiplet at 6.6 ppm are
observed in the 1H NMR spectrum of the substrate, 2-amino-
4-tert-butylphenol (Figure S9a). The position of three proton
resonances and the coupling constant of the doublets suggest
the formation of 4-tert-butyl-2-picolinate in the reaction of
1·ClO4 with dioxygen. The absence of a resonance of the
aminophenol substrate in the NMR spectrum confirms
a complete conversion of 2-amino-4-tert-butylphenol to 4-
tert-butyl-2-picolinic acid in 6 h. To further confirm the
conversion, the organic product was analyzed by GC–MS.
GC–MS of the methyl ester of the organic product shows an
ion signal at m/z = 193.2 along with expected fragmentation
patterns of methyl-4-tert-butyl-2-picolinate (Figure S10).
When the reaction is carried out in the presence of 18O2, the
ion signal at m/z = 193.2 is shifted to 195.2, and around 40%
incorporation of 18O into methyl-4-tert-butyl-2-picolinate is
observed (Figure S11). All these results confirm that the
reaction of 1·ClO4 with dioxygen leads to the formation of 4-
À
tert-butyl-2-picolinic acid through C C bond cleavage of 2-
amino-4-tert-butylphenol (Scheme 2) and functionally mimics
the reaction catalyzed by 2-aminophenol-1,6-dioxygenase
(APD) and 3-hydroxyanthranilate-3,4-dioxygenase (HAD).
Scheme 2. Reaction of the iron(II)–aminophenolate complex (1·ClO4)
with dioxygen.
À
To understand the role of the ligand in the oxidative C C
bond cleavage of aminophenolate on the model complex, an
equimolar mixture of iron(II) perchlorate, 2-amino-4-tert-
butylphenol, and triethylamine in acetonitrile was reacted
À
with dioxygen for 6 h. No C C bond cleavage of 2-amino-4-
tert-butylphenolate was observed. It is also important to
mention here that iron(II) complexes of 2-aminophenol and
2-amino-4,6-di-tert-butylphenol, generated in situ in acetoni-
À
trile, do not undergo oxidative C C bond cleavage reactions
to afford the corresponding picolinic acid.
The oxidative transformation of 2-amino-4-tert-butylphe-
nol to 4-tert-butyl-2-picolinic acid takes place through an
extradiol-cleavage pathway. Of note, the reaction of an
analogous iron(II)–catecholate complex, [(6-Me3-TPA)FeII-
Figure 3. ESI-MS (positive-ion mode in acetonitrile) of the solution
after reaction of 1·ClO4 with a) 16O2 and b) 18O2. The gray bars indicate
the corresponding computer-simulated spectra.
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(DBCH)](ClO4), with dioxygen leads to the formation of
radical species, then reacts with dioxygen to generate an
À
À
products of the intradiol-selective C C bond cleavage of the
alkylperoxo intermediate (B), leading to the C C bond
coordinated catecholate.[34] Moreover, several iron(II)–cate-
cholate complexes of tetradentate ligands have been shown to
react with dioxygen to afford mainly products of the intradiol-
selective catechol cleavage.[4] Wieghardt and co-workers have
reported the synthesis and characterization of a large number
of transition-metal complexes of 3,5-di-tert-butylaminophe-
nol- and 2-aminophenol-derived ligands.[25–28] The reaction of
substituted 2-aminophenolate with an iron(II) salt and
a tetradentate ligand (tren) in air resulted in the formation
of an iron(III)–2-iminobenzosemiquinonato radical com-
cleavage of 2-aminophenol. The resulting extradiol-cleavage
product, 2-amino-4-tert-butylmuconic acid semialdehyde,
then undergoes a cyclization reaction to form 4-tert-butyl-2-
picolinic acid (Scheme 3).
In conclusion, we have synthesized and characterized an
iron(II)–2-aminophenolate complex of a tetradentate nitro-
gen donor ligand. The biomimetic iron(II) complex (1) has
À
been shown to react with O2 to oxidatively cleave the C C
bond of 2-amino-4-tert-butylphenol to 4-tert-butyl-2-picolinic
acid. Complex 1 represents the first functional model of 2-
aminophenol dioxygenases, APD and HAD. The extradiol
cleavage of 2-amino-4-tert-butylphenol reported in this work
highlights the importance of the model complex in under-
standing the unique oxygen-dependent transformation reac-
tion of 2-aminophenol to 2-picolinic acid in a single step,
which is unprecedented in synthetic chemistry. A detailed
plex.[29] No C C bond cleavage of aminophenolate has ever
been reported to occur during the reaction of transition-metal
ions with these ligands.
For iron(II)–catecholate model complexes, the groups of
Que and Moro-oka each proposed an outer-sphere one-
electron oxidation of iron(II)–catecholate to iron(III)–cat-
echolate species.[36,37] An iron(II)–semiquinone intermediate
was proposed to react directly with dioxygen to form an
À
À
experimental and theoretical investigation on the C C bond
cleavage mechanism of substituted 2-aminophenols by bio-
mimetic iron(II) complexes is presently being pursued in our
laboratory.
À
iron(III)–alkylperoxide intermediate for C C bond cleavage
of catecholate.[38] DFT calculations on simplified enzyme–
substrate and iron(II)–catecholate model complexes also
support that the attack of dioxygen to an iron(II)–semi-
Received: August 27, 2012
Revised: October 10, 2012
Published online: November 29, 2012
À
quinone species is the key step in C C bond cleavage of
catechol.[39,40] Fiedler and co-workers recently isolated and
structurally characterized an iron(II)–2-aminophenolate com-
plex, [(TpPh2)FeII(tBuAPH)], which was converted to an
iron(II)–2-iminobenzosemiquinonato radical species upon
one-electron chemical oxidation.[33] On the basis of all these
observations, it is proposed that complex 1 reacts with
dioxygen to undergo one-electron oxidation to form an
iron(III)–2-amidophenolate species (A) in the first step. The
redox isomer of A, an iron(II)–2-iminobenzosemiquinonato
Keywords: biomimetic model · cleavage reactions ·
.
dioxygenases · enzyme models · iron
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