Ortho-Hydroxylation of 4-tert-butylphenol by nonheme iron(III) complexes as a functional model reaction for tyrosine hydroxylase

Article abstract of DOI:10.1039/a606878b

Hydroxylation of 4-tert-butylphenol to 4-tert-butylcatechol is performed by a catecholatoiron(III) complex-hydroquinone-O₂ system which mimicks the roles of Fe²⁺ and Fe³⁺ in tyrosine hydroxylase; the ortho-hydroxylation of phenols by iron-oxygen active species is suggested.

Full text of DOI:10.1039/a606878b

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ortho-Hydroxylation of 4-tert-butylphenol by nonheme iron(iii) complexes as a
functional model reaction for tyrosine hydroxylase
Takuzo Funabiki,* Tomomasa Yokomizo, Shinko Suzuki and Satohiro Yoshida
Department of Molecular Engineering, Graduated School of Engineering, Kyoto University, Kyoto 606-01, Japan
Hydroxylation of 4-tert-butylphenol to 4-tert-butylcatechol
is performed by a catecholatoiron(III) complex–hydroqui-
none–O2 system which mimicks the roles of Fe2+ and Fe3+ in
tyrosine hydroxylase; the ortho-hydroxylation of phenols by
iron–oxygen active species is suggested.
substituents on catechol: hydroxylation was promoted by
electron-withdrawing substituents. Thus, the yield of 2 per
mol% Fe decreased in the order: 3,4,5,6-Cl4 (560) > 4-NO2
(410) > 4-Cl (230) > H (120) > 3,5-But2 (80) > none (i.e.
without catechol, 52), with [Fe] = [catechol] = 0.125 mmol
and [DTBHQ] = 5 mmol. The reverse order was found for the
relative rate of consumption of hydroquinones: 3,4,5,6-Cl4
(0.08) < 4-NO2 (0.24) < 4-Cl (0.65) < 3,5-But2 (0.91) < none
(2.18), indicating that electron-withdrawing substituents on
catechols, which stabilize the catecholatoiron(iii) complexes,
are efficient for the ortho-hydroxylation of phenols.
It was noticed that after consumption of hydroquinone, 2 was
converted to an unidentified polymeric product. As shown in
Fig. 1(a), this consecutive reaction of 2 was prevented by
addition of excess catechol which also, brought about an
increase in the yield of 2. This is probably because the excess
catechol effectively controls the competitive coordination of 2
Tyrosine hydroxylase (TH) is one of the pteridine-dependent
nonheme iron monooxygenases and converts tyrosine to DOPA
by hydroxylation with molecular oxygen. Very little is known
about not only the active site environment of the enzyme, but
also the role of iron in oxygen activation. TH is isolated mostly
in the form of Fe3+ [TH(Fe3+), coordinated by DOPA],1–4 but
Fe2+ [TH(Fe2+)] is suggested to be an active species.1,5–7
Scheme 11,3 shows that Fe3+ is reduced to an active TH(Fe2+) by
tetrahydropterine (BPH4)1 and coordinated by catecholamine to
give an inert species.3,4,8,9 The active species has been proposed
to be a peroxytetrahydropterin [(BPH4)–OOH] rather than an
iron–oxygen species.5,10,11
In model studies, on the other hand, results suggest that the
hydroxylation proceeds via an iron—oxygen species in the
enzymatic system. Inner-sphere transfer of a peroxo oxygen in
HOC6H4O–FeIII–OOR12 or of the OH ligand in HOC6H4O–
FeIV–OH13 to the phenolate ligands has been proposed. In the
latter case, FeII complexes such as bis(2,6-dicarboxypyri-
dine)iron(ii) and reductants such as PhNHNHPh have been
used.13 Considering that cytochrome P450-type monooxygen-
ations are catalysed by FeIII species, with donation of two
electrons and two protons in a catalytic cycle, it is probable that
the tyrosine hydroxylase-type ortho-hydroxylations are cata-
lysed by FeIII species in the presence of electron and proton
donors. In mimicking the processes of reduction of Fe3+ to Fe2+
by BPH4 and inhibition by coordination of catechol, and in
excluding the formation of the (BPH4)–OOH type active
species, we have performed hydroxylation of phenol with O2 by
iron(iii) complexes in the presence of hydroquinones as proton
and electron donors [eqn. (1)].
DOPA
TH(FeIII)
TH(FeIII)(DOPA)
BPH4
BPH2
TH(FeII)
O2 H2O
CO2H
NH2
HO
HO
CO2H
NH2
HO
BPH4
BPH2
Tyrosine
(a)
DOPA
Scheme 1
(b)
R
R
FeIII-Complex
(1)
R′
R′
OH
OH
OH
H 2O
O2
O
O
HO
OH
Catecholatoiron complexes formed in situ†14,15 were used as
the ferric complex, and 4-tert-butylphenol (ButC6H4OH 1) was
used as substrate.‡ The effect of four different hydroquinones
was studied here: 2,6-di-tert-butylhydroquinone (DTBHQ),
tert-butylhydroquinone (TBHQ), hydroquinone (HQ) and
1,4-dihydroxynaphthalene (DNH). Hydroxylation was per-
formed at 25 °C with O2 (1 atm). In a typical case, the reaction
of 1 was started by addition of a MeCN solution (7.5 cm3) of
FeCl3 (0.125 mmol) and pyridine (0.25 mmol) to 1 (25 mmol),
catechol (0.125–1.25 mmol) and hydroquinone (5.0 mmol).
4-tert-Butylcatechol 2 was formed as the sole detectable
product from 1.§ The yield of 2 was greatly dependent on the
t / h
t / h
Fig. 1 Formation of 4-tert-butylcatechol from 4-tert-butylphenol catalysed
by a tetrachlorocatecholatoiron(iii) complex under O2 (1 atm). (a) Effect of
tetrachlorocatechol concentration: [tetrachlorocatechol]:[Fe] = (5) 1:1
and (2) 10:1, FeCl3 = 0.125 mmol, pyridine = 0.25 mmol, DTBHQ = 5.0
mmol, ButC6H4OH = 25 mmol in MeCN (7.5 cm3). (b) Effect of different
hydroquinones: (2) DTBHQ, ( ~ ) DHN, (8) TBHQ and (-) HQ = 5.0
mmol, tetrachlorocatechol
pyridine = 0.25 mmol, ButC6H4OH = 25 mmol in MeCN (7.5 cm3).
= 1.25 mmol, FeCl3 = 0.125 mmol,
Chem. Commun., 1997
151

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and hydroquinone to FeIII. The end of hydroxylation does not
mean the deactivation of the active species. The reaction is
repeatedly performed by addition of hydroquinone, indicating
that the active species is maintained in the form of a
catecholatoiron(iii) complex.
tetrachlorocatechol–DTBHQ system. Since Fe2+ can be oxi-
dized to an Fe3+ species under oxygen, it is difficult to perform
the reaction only in the presence of FeII species, but the result
indicates that the FeCl3 system is much better at hydroxylation
than the FeCl2 system.
The above results indicate not only that electron transfer to
FeIII is essential for hydroxylation, but also that the presence of
FeIII species in equilibrium with FeII is important for the
catalytic hydroxylation. This is similar to the participation of
both Fe3+ and Fe2+ in the activity of tyrosine hydroxylase. The
most favourable combination of conditions studied here is the
use of the tetrachlorocatecholatoiron(iii) complex and DTBHQ,
which combine to form the most efficient active iron(ii)
species.
Hydroxylation is greatly influenced by the hydroquinones.
As shown in Fig. 1(b), the initial rate of hydroxylation is
correlated with the electron-transfer affinity of the hydro-
quinone (DHN > DTBHQ > TBHQ > HQ), as shown by the
decrease in the intensity of the characteristic band of the
catecholatoiron(iii) complexes [Fig. 2(a)]. The rapid quenching
of the reaction in spite of the initial rapid formation of 2 is
caused by the oxidation of DHN to the corresponding
ineffective quinone. Reduction of FeIII to FeII by hydroquinones
was also shown by the formation of a characteristic phenan-
throlineiron(ii) complex. In the presence of phenanthroline, the
hydroxylation does not proceed, indicating that the active
iron(ii) species is trapped by phenanthroline. The effect of the
catecholate ligand on the electron transfer shown in Fig. 2(a)
indicates that tetrachlorocatecholatoiron(iii) is less susceptible
to the electron transfer from DTBHQ than the 4-chloro-
catecholatoiron(iii) complex.
The importance of the presence of FeIII in equilibrium with
FeII suggests the mechanism shown in Scheme 2 for the
hydroxylation of phenols. Unlike reactions catalysed by FeII
complexes,13 the present results suggest that the FeIII species
[(L)Fe3+] is involved in the catalytic cycle. Support for
formation of a free radical intermediate was not found, but
addition of triphenylmethane did not affect the reaction.
Observation of characteristic LMCT bands at around 600 nm
indicated that phenols, including hydroquinones, coordinate to
Fe3+. Fig. 2(b) shows the coordination of phenol to FeIII, either
together with or replacing a catecholate ligand. The electron-
withdrawing substituents on catechol might be effective in
making the ferrous as well as ferric centre a better Lewis acid
towards the phenolic substrate. It is very likely that the ortho-
hydroxylation proceeds inside the coordination sphere, but
whether the oxygen transfer to phenol proceeds directly via a
hydroperoxoiron(iii) or oxoiron(v) species is a problem to be
solved in future.
Regardless of the importance of the iron(ii) species for
hydroxylation, the rapid formation and high yield of 2 was not
observed when FeCl2 was used in place of FeCl3 in the
(a)
(b)
(i)
(ii)
(iii)
(iv)
(v)
(iv)
(iii)
(i)
(ii)
(i)
Footnotes
(ii)
(iii)
(iv)
(v)
† Formation of three types of catecholatoiron complexes in solution,
depending on the catechols used, has been reported:13,14 [FeCl(Cat)py2]
(CatH2
=
3,5-di-tert-butylcatechol), [FeCl2(Cat)py] (CatH2
= pyro-
catechol,
4-methylcatechol, 4-chlorocatechol), [FeCl2(Cat)py2]
(CatH2 = tetrachlorocatechol).
‡Although chlorophenol is reported to be more reactive than 1, (ref. 12) we
could not detect chlorocatechol in our conditions.
§ No product other than 1 has been detected, indicating that 2 and tert-butyl-
1,2-benzoquinone 3 are converted to polymeric products.
λ / nm
λ / nm
References
Fig. 2(a) Visible spectral changes caused by addition of hydroquinones to
(- - - - -) tetrachloro- and (——) 4-chloro-catecholatoiron (iii) complexes
under argon: (i) none, (ii) HQ, (iii) TBHQ, (iv) DTBHQ and (v) DHN.
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HO
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HO
But
H+
OOH
(L)FeIII
H2O
O
But
But
(L)FeIII
HO
H+
But
But
quinone
O2
(L)FeII
O
(L)FeIII
hydroquinone
L = ligand
Scheme 2
O
H+
semiquinone
Received, 8th October 1996; Com. 6/06878B
152
Chem. Commun., 1997