Oxygenative cleavage of catechols including protocatechuic acid with molecular oxygen in water catalysed by water-soluble non-heme iron(III) complexes in relevance to catechol dioxygenases

Article abstract of DOI:10.1039/b110098j

Catechol dioxygenase model oxygenations have been performed for the first time in water by using water-soluble nonheme iron(III) complexes, enabling the oxygenation of protocatechuic acid and other catechols.

Full text of DOI:10.1039/b110098j

Oxygenative cleavage of catechols including protocatechuic acid with
molecular oxygen in water catalysed by water-soluble non-heme iron(III
complexes in relevance to catechol dioxygenases
)
Takuzo Funabiki,* Daisuke Sugio, Nobuhiko Inui, Matsutaka Maeda and Yutaka Hitomi
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501,
Japan. E-mail: funabiki@scl.kyoto-u.ac.jp; Fax: +81-75-753-5925; Tel: +81-75-753-5925
Received (in Cambridge, UK) 6th November 2001, Accepted 14th January 2002
First published as an Advance Article on the web 1st February 2002
Catechol dioxygenase model oxygenations have been per-
formed for the first time in water by using water-soluble
nonheme iron(III) complexes, enabling the oxygenation of
protocatechuic acid and other catechols.
cleavage product 3a (398% yield after 14 h, based on [Fe]) at pH
3.4.§ 2a was not detected in aqueous conditions. At higher pH,
the reaction stopped at an earlier stage to give a lesser yield of
3a accompanied with 4a (214% yield, 3a +4a = 84+16, at pH
8.0). We expected that PBSA having two SO32 groups forms a
more active complex than BPSA by forming an iron center with
higher Lewis acidity, but as shown in Fig. 1A the promoting
effect of the increasing number of SO32 was not significant and
the higher yield of product was obtained rather by the BPSA
system.
4-Chlorocatechol (1b) was selectively converted to 5,
indicating that only 4b is initially formed and dehydro-
chlorinated.8 This result is very different from the case in
acetonitrile, in which both 3b and 5 are products in the catalytic
oxygenation by Fe(TPA)Cl.8 The yields of 5 at 50 °C and pH
6.0 were 33% (2 h) and 110% (6 h) in the BPSA system and
59% (2 h) and 75% (6 h) in the PBSA system, indicating that the
former gives higher yields than the latter. 2b was not detected
as an intermediate.
No model complex in organic solvents has oxygenated
protocatechuic acid (1c), but we here found that 1c is selectively
and catalytically oxygenated to give only 3c in aqueous
solution. As shown in Fig. 1B, the reaction is very slow and the
promoting effect of the SO32 group is significant: the yields of
3c are 59% (BPSA) and 123% (PBSA) after 36 h, at 50 °C and
pH 6.0. A small amount of 2c was detected only in the early
stage (7% at 2 h), suggesting the oxygenation proceeds by the
stepwise oxygen insertion as proposed.4,7 However, different
from the reactions in acetonitrile, the yields of 2a ~ 2c are too
low to support the formation of free 2 as a precursor of lactonic
acid.3,9
Catechol dioxygenases play key roles in the metabolism of
various aromatic compounds, converting aromatics to aliphatics
with insertion of molecular oxygen between a C–C bond of a
benzene ring. Since the first example of cleavage of 3,5-di-tert-
butylcatechol by a simple nonheme iron complex,1–3 various
types of functional models have been developed.4–7 These
model reactions have been performed exclusively in organic
solvents such as acetonitrile. Because of growing interest in the
development of catalytic reactions in aqueous solutions, we
here synthesized water-soluble tripodal ligands, with which we
first successfully performed the intradiol cleavage of catechols
via water-soluble catecholatoiron(III) complexes. Catechols
examined here are 4-tert-butylcatechol (1a), 4-chlorocatechol
(1b) and protocatechuic acid (1c), which are fairly soluble in
water. Ic is a typical substrate in the enzymatic oxygenations,
but has been hard to oxygenate by model complexes. Here we
first show that not only 1c but also other catechols are
catalytically oxygenated by water-soluble iron complexes in
water (Scheme 1) and that the reactivity and selectivity are
greatly dependent on both catechol and ligand.
The water-soluble ligands used are bis(2-pyridylmethyl)((4-
sulfo-2-pyridyl)methyl)amine (BPSA) and (2-pyridylmethyl-
)bis((4-sulfo-2-pyridyl)methyl)amine (PBSA).† Oxygenation
was started by mixing aqueous buffer solutions of catechols and
of the complexes prepared in situ ([FeCl3]+[ligand] = 1+2 ~ 3)
under 1 atm O2 at pH 3.4 (glycine buffer) or 6.0 (MES buffer:
2-morpholinoethanesulfonic acid–NaOH) (the solutions keep
homogeneous at pH 3.4 ~ 7.0).‡
It is noteworthy that the type of cyclization to form lactonic
acid is greatly dependent on the catechol substituent. It is known
Fig. 1 shows the effects of pH and the ligand in the
oxygenation of 1a ([Fe]+[ligand]+[1a] = 1+2+4) at 25 °C. We
found that 4-tert-butylcatechol is a good substrate which is
readily oxygenated in aqueous media without being accom-
panied by miscellaneous reactions such as polymerization. 1a
was quantitatively and selectively converted to the intradiol
Fig. 1 Time course of the oxygenations of 1a at 25 °C and 1c at 50 °C with
O2, catalysed by the water-soluble Fe(III) complexes. Yields of 3a or 3c
[mol%] are based on the amount of Fe used. (A) Effect of pH in the
oxygenation of 1a (8 mM) in the BPSA system ([FeCl3]+[BPSA]+[1a] =
1+2+4) (solid line) and comparison with the PBSA system (broken line): (B)
Effect of the ligand at pH 6.0 in the oxygenation of 1c (12 mM )
([FeCl3]+[ligand]+[1c]) = 1+3+3).
Scheme 1
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CHEM. COMMUN., 2002, 412–413
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that the cyclization of muconic acid is catalysed by muconate
cycloisomerase in the case of the enzyme,10 but in the model
systems free muconic acids as precursors of lactonic acid have
not been detected. To determine if free muconic acid is involved
in controlling the type of cyclization, we have studied whether
3-chloro-cis,cis-muconic acid (6) is converted to 5 under the
reaction conditions in a similar way to muconate cycloisomer-
ase II.10,11 Interestingly, we found that 6 gave mainly 3b (86%)
rather than 5 (14%) at pH < 6.0 and no cyclization was
observed at pH > 8.0. This indicates that 6 is not a precursor of
5 in the model system and suggests that the cyclization of
muconate ligands takes place before elimination of free
muconic acids. The coordination mode of muconate ligand may
affect the type of cyclization, controlling the selectivity.
Formation of catecholatoiron complexes as reactive inter-
mediates was shown by the characteristic LMCT bands3,12,13
and by the diminishment of their peak intensities under oxygen,
though the complexes have not been isolated. The complexes of
1a exhibited bands at 694 nm (BPSA) and 701 nm (PBSA)
while the higher energy bands were not clear. Complexes of 1b
exhibited peaks at 416 and 734 nm (BPSA) and 417 and 738 nm
(PBSA) (Fig. 2A), and those of 1c at 504, 753 nm (BPSA) and
543, 784 nm (PBSA) (Fig. 2B). The spectra of the complexes
prepared in situ in aqueous media are less clear-cut than those in
organic solvents. This is because the water-soluble catechols
form catecholatoiron complexes in water even without addition
of ligands,14 exhibiting bands at 404 and 757 nm (1a), 423 and
725 nm (1b), 566 nm (1c), and can be in equilibrium with
catecholate complexes with BPSA or PBSA ligands even in the
presence of excess of these ligands. Oxygenations of catechols
proceed no doubt preferentially via catecholatoiron complexes
with BPSA or PBSA ligands because the complexes without
these ligands are stable under oxygen and give no oxygenated
products.
12450327), the Japan Society for Promotion of Science for
grant in Bilateral Program, and S. Ogo and Y. Watanabe at the
Institute for Molecular Science for measuring the ESI mass
spectra.
Notes and references
† The sodium salt of BPSA was synthesized by the reaction of ((4-chloro-
2-pyriryl)methyl)bis(2-pyridylmethyl)amine with sodium bisulfate in
water–methanol in the 79% yield: 1H NMR (400 MHz, 25 °C, D2O) d 3.94
(s, 4H), 3.95 (s, 2H), 7.25(m, 2H), 7.48 d, 1H, J 5.37 Hz), 7.51 (d, 2H, J 7.81
Hz), 7.68 (s, 1H), 7.73 (m 2H), 8.35 (d, 2H, J 4.89 Hz), 8.45 (d, 1H, J 5.37
Hz); 13C NMR (100 MHz, 25 °C, D2O) d 60.5, 60.7, 118.4, 119.8, 123.1,
124.4, 138.3, 147.4, 149.0, 151.2, 157.1, 159.7; ESI-MS: m/z 393.2
([C18H17N4(SO3Na)H]+), 415.2 ([C18H17N4(SO3Na)Na]+). The sodium salt
of PBSA was synthesized by the reaction of bis((4-chloro-2-pyridyl)me-
thyl)(2-pyridiylmethyl)amine with sodium bisulfate in water–methanol in
64% yield: 1H NMR (400 MHz, 25 °C, D2O) d 3.95 (s, 2H), 4.03 (s, 4H),
7.21 (m, 1H), 7.47 (d, 1H, J 7.38 Hz), 7.49 (d, 2H, J 5.38 Hz), 7.61 (s, 2H),
7.70 (m, 1H), 8.32 (d, 1H, J 5.38 Hz), 8.51 (d, 2H, J 5.38 Hz); 13C NMR
(100 MHz, 25 °C, D2O) d 60.0, 60.1, 117.3, 118.8, 121.7, 123.3, 136.5,
146.6, 148.1, 150.0, 156.2, 158.4; ESI-MS: m/z 448.8 ([C18H17N4(SO-
3)(SO3H)]2), 470.8 ([C18H17N4(SO3)(SO3Na)]2).
‡ Oxygenations were performed in a 20 cm3 cylindrical flask at 25 or 50 °C
and under 1 atm O2. In a typical case, reactions were started by addition of
1 cm3 buffer solution of 1 (0.008 mmol) to 1 cm3 buffer solution containing
FeCl3 (0.004 mmol) and ligand (0.008 mmol). Product extraction and
analysis were performed as reported previously.15
§ Oxygenated products from 1b were identified and quantitatively analyzed
as reported before.8 The product from 1a was identified as follows. 4-tert-
butyl-5-carboxymethyl-2-furanone (3a): 1H NMR (400 MHz, 25 °C,
CDCl3) d 1.26 (s, 9H), 2.56 (dd, 1H, J 9.27, 16.60 Hz), 3.11 (dd, 1H, J 2.93,
16.60 Hz), 5.42 (ddd, 1H, J 1.46, 2.93, 9.27 Hz), 5.90 (d, 1H, J 1.46 Hz); 13
C
NMR (100 MHz, 25 °C, CDCl3) d 29.4, 33.6, 38.3, 79.2, 116.3, 172.4,
173.6, 179.3; DI-MS; m/z 198. 5-tert-butyl-5-carboxylmethyl-2-furanone
(4a): 1H NMR (400 MHz, 25 °C, CDCl3) d 1.02, 2.91, 3.00, 6.17, 7.53; 13
C
The present results reveal that water-soluble iron complexes
oxygenate catechols in a different mode from those in organic
media and achieve oxygenation of protocatechuic acid. As seen
from the selective oxygenation of 4-tert-butylcatechol, water
was found effective in preventing miscellaneous radical
reactions of catechols, enabling the use of catechols other than
3,5-di-tert-butylcatechol which are not substrates in the enzy-
matic system. Since enzymatic reactions proceed in aqueous
media, this new model system will give the more direct
information about oxygenation mechanisms than those in
organic solvents. In addition, this system holds out the
possibility of oxygenatively decomposing water-soluble aro-
matic pollutants in the environment.
NMR (100 MHz, 25 oC, CDCl3) d 25.3, 30.9, 37.8, 92.3, 122.6, 156.5,
172.4, 174.1; DI-MS: m/z 198. The product from 1c was identified as
4-carboxyl-5-carboxymethyl-2-furanone (3c): 1H NMR (400 MHz, 25 °C,
CD3CN) d 2.65(dd, 1H, J 16.6, 8.1 Hz), 3.14 (1H, J 16.6, 2.9 Hz), 5.47 (ddd,
1H, J 8.1, 2.9, 2.0 Hz), 6.32 (d, 2H, J 2.0 Hz); 13 C NMR (100 MHz, 25 °C,
CD3CN) 37.1, 79.4, 128.0, 157.3, 162.2, 170.0, 171.5; DI-MS; m/z 186.
4-carboxyloxacyclohepta-3,5-diene-2,7-dione (2c) was not isolated, but
detected by 1H NMR (400 MHz, 25 °C, CDCl3) d 6.51 (d, 1H, J 15.6 Hz),
6.65 (s, 1H), 8.05 (dd, 1H, J 13.5, 1.3 Hz).
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Fig. 2 Absorption spectra of catecholate complexes with water-soluble
ligands. (A) Complexes of 1b at pH 3.4 ([FeCl3]+[ligand]+[1b] = 1+2+4).
(B) Complexes of 1c at pH 6.0.
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