Oxidation of 3,5-di-tert-butylcatechol in the presence of V-polyoxometalate

Article abstract of DOI:10.2478/s11696-011-0119-x

The oxidase and dioxygenase reactions of 3,5-di-tert-butylcatechol (DTBC, I ) in the presence of V-polyoxometalate were studied. It was found that the addition of a Lewis base quenched the V-polyoxometalate-catalysed catechol dioxygenase reaction and catalysed the oxidase reaction selectively. The existence of V-polyoxometalate accelerates the autoxidation rate of I as demonstrated by the rate measurements. ESR and UV-VIS spectra showed that the Lewis base destroyed the dioxygenation reaction catalyst as formed and restrained its regeneration by suppressing the coordination of catechol radical to vanadium. The by-products of the dioxygenation and oxidation reactions are H₂O and H₂O₂, respectively.

Full text of DOI:10.2478/s11696-011-0119-x

Chemical Papers 66 (3) 211–215 (2012)
DOI: 10.2478/s11696-011-0119-x
ORIGINAL PAPER
Oxidation of 3,5-di-
-butylcatechol in the presence
of V-polyoxometalate
aXue-Feng Hu*, a,bLei Wu
aKey Laboratory of Coastal Environmental Processes, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences,
Yantai 264003, China
bGraduate University of Chinese Academy of Sciences, Beijing 100049, China
Received 29 August 2011; Revised 28 October 2011; Accepted 28 October 2011
The oxidase and dioxygenase reactions of 3,5-di-tert-butylcatechol (DTBC, I ) in the presence
of V-polyoxometalate were studied. It was found that the addition of a Lewis base quenched the
V-polyoxometalate-catalysed catechol dioxygenase reaction and catalysed the oxidase reaction selec-
tively. The existence of V-polyoxometalate accelerates the autoxidation rate of I as demonstrated
by the rate measurements. ESR and UV-VIS spectra showed that the Lewis base destroyed the
dioxygenation reaction catalyst as formed and restrained its regeneration by suppressing the co-
ordination of catechol radical to vanadium. The by-products of the dioxygenation and oxidation
reactions are H2O and H2O2, respectively.
c
ꢀ 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: 3,5-di-tert-butylcatechol, catechol oxidase, base-catalysed, V-polyoxometalate-enhanced
Introduction
of catechol by activating oxygen. They found that
the vanadium-based catechol dioxygenation was ini-
tiated via the oxidase product: quinone. The induc-
tion period which preceded the dioxygenation reac-
tion was the time for the formation of quinone and
then the real dioxygenase catalyst (Yin et al., 2005).
The oxidase-product-induced catalysis was thought to
be general and had been verified by independent ex-
amples of both oxidative and reductive catalysis (Ha-
gen et al., 2005; Groves et al., 1996). Research into
the oxidase reaction mechanism and the relationship
between the oxidase reaction and dioxygenation reac-
tion are not only beneficial in elucidating the mecha-
nism of the underlying biological catecholase reaction
but also helpful in designing new O2-activating metal
complexes catalysts.
The oxidation of organic substrates and degrada-
tion of aromatic organic pollutants using molecular
oxygen are of great interest, from both the economic
and environmental perspectives. Metal-containing re-
dox enzymes are ubiquitous in nature and many can
perform such functions under mild conditions (Sigel &
Pyle, 2007; Cowan, 1998; Ragsdale & Kumar, 1996).
Catechol oxidase and catechol dioxygenases are two
important enzymes in these investigations. Oxyge-
nases catalyse the incorporation of oxygen atoms from
molecular oxygen into the substrate, while oxidases
use dioxygen as an electron acceptor. Many functional
model complexes for metalloenzymes have been stud-
ied extensively in order to elucidate the reaction mech-
anisms involved and to help design biomimetic cata-
lyst systems for various reactions requiring activation
of O2 (Lin et al., 2001; Cox & Que, 1988; Koval et
al., 2006; Gao & Xu, 2006). Morris et al. (2009) re-
ported the vanadium-catalysed dioxygenase reaction
Oxidation of 3,5-di-tert-butylcatechol (DTBC, I )
to 3,5-di-tert-butyl-1,2-benzoquinone (DTBQ, II ) by
O2 is known to take place in the presence of a base. It
was reported that dioximatomanganese(II) and dioxi-
matoiron(II) complexes accelerated the triethylamine
*Corresponding author, e-mail: xfhu@yic.ac.cn

212
X. F. Hu, L. Wu/Chemical Papers 66 (3) 211–215 (2012)
Fig. 1. Dioxygenase reaction (A) and oxidase reaction (B) of I. Reaction conditions: i) I (0.1 M), 1,2-dichloroethane, O2 (0.1 MPa),
[(C4H9)4N]3[V10O28H3] (10−5 M), 65◦C; ii) I (0.1 M), 1,2-dichloroethane, O2 (0.1 MPa), [(C4H9)4N]3[V10O28H3] (10−5
M), TEA (3 × 10−4 M), 65◦C; for all compounds: R = tert-butyl.
(TEA)-catalysed oxidation of I to II, although these
complexes alone have no or very low oxidation activ-
ity (Szigyártó et al., 2006; May et al., 2006). The gen-
eral pattern for the mechanism is the formation of
a ternary catalyst-O2-substrate intermediate, which
is the rate-determining step: this undergoes H-atom
transfer, generating a semiquinone anionic radical. V-
polyoxometalate displays dioxygenase activity in the
absence of a base after an induction period. However,
the V-polyoxometalate-enhanced base-catalysed oxi-
dase reaction has not been reported. ESR is often used
to determine a change in the valence state of the metal
complex intermediate in order to elucidate the elec-
tron transfer pathway of the oxidation of an organic
compound.
In this paper, we report that: (i) after an induc-
tion period, quinone, the oxidase product, instead of
the dioxygenase product, is formed immediately as
the major product following addition of the Lewis
base into the reaction system of vanadium/I; (ii) V-
polyoxometalate accelerates the base-catalysed cate-
chol oxidase reaction rate; (iii) the by-product of the
oxidase reaction is H2O2.
at room temperature under vacuum and the residues
were dissolved in CDCl3 and then analysed by GC,
1H and 13C NMR spectroscopy. The NMR spectra
were recorded on a Bruker AVANCE 400 spectrom-
eter and compared with the published data (Weiner
& Finke, 1999). GC analyses were performed on a HI-
TACHI G-3900 GC spectrometer equipped with a FID
detector and a SGE BP-5 capillary column (30 m ×
0.25 mm) with the following temperature programme:
initial temperature, 180◦C; heating rate, 5◦C min−1
;
final temperature, 220◦C (final time, 20 min). The IR
spectra (in KBr disc) were obtained on a TENSOR 27
FTIR spectrometer. UV-VIS spectra were recorded on
a Lambda Bio 20 spectrophotometer. The H2O2 test
was performed as follows: H2O2 was extracted three
times from the reaction solution using deionised wa-
ter and then detected by the spectrophotometric DPD
method (Bader et al., 1988).
Results and discussion
When present alone in a solution, [(C4H9)4N]3[V10
O28H3] catalyses neither the catechol oxidase reac-
tion nor the catechol dioxygenase reaction immedi-
ately. However, four dioxygenation products together
with ca. 17 % of II (the net mass balance of the
five products was ∼ 95 %) were obtained after an
hour’s induction period and a further six hours of
reaction, as shown in Fig. 1. When the Lewis base,
TEA, was added to the vanadium/I solution, the ox-
idase reaction proceeded immediately and was com-
pleted after three hours with high selectivity without
any discernible induction period. To clarify whether
the oxidase reaction of catechol in the presence of
the base occurs only in the induction period of the
catechol dioxygenation reaction, namely prior to for-
mation of the dioxygenase catalyst, TEA was added
into the polyoxometalate-catalysed oxygenation reac-
tion solution of I after the induction period and it
was found that unreacted catechol was also autoxi-
dised to II with high selectivity. The induction pe-
riod in the vanadium/I reaction system is the time in
Experimental
Compounds I and II were obtained from Acros and
Fluka, respectively. Horseradish peroxidase (POD),
which was used to assess the quantity of H2O2, was
obtained from the Huamei Biologic Engineering Co.
and N,N-dialkyl-p-phenylenediamine (DPD) was ob-
tained from Merck. [(C4H9)4N]3[V10O28H3] were pre-
pared by a reported method, and the identity of the
polyoxometalate was confirmed by IR spectroscopy by
comparison with published data (Day et al., 1987). All
other chemicals and solvents were of analytical grade
and were used as received.
Separation and purification of the main products
were accomplished by column chromatography, first
using CHCl3 as an eluent, and the acidic product
was obtained by elution with methanol. The result-
ing five main fractions were evaporated to dryness

X. F. Hu, L. Wu/Chemical Papers 66 (3) 211–215 (2012)
213
which the autoxidation-initiated dioxygenation cata-
lyst is formed (Yin et al., 2005); the lack of the in-
duction period in the presence of the base is kinetic-
substantiated evidence that the vanadium complexes
formed in the initial stage are the real oxidase catalyst.
This suggests that the addition of the base not only
restrains the formation of real dioxygenation catalyst
but also destroys or deactivates the real dioxygenation
catalyst formed.
On the one hand, the Lewis base quenches the
V-polyoxometalate-catalysed catechol dioxygenase re-
action and catalyses the oxidase reaction. On the
other hand, the existence of V-polyoxometalate af-
fects the autoxidation rate of the base-catalysed au-
toxidation of I. The autoxidation product II shows
maximum absorption at 404 nm. The effects of V-
polyoxometalate on the autoxidation of I are illus-
trated by the time-dependent UV-VIS spectra at
404 nm. It could be argued that the surface oxy-
gen atoms of V-polyoxovanadate showed basicity and
served as a bonding site for protons in some cases.
However, the V-polyoxometalate could not catalyse
the autoxidation of I in the absence of a Lewis base
during the induction period. The V-polyoxometalate
did not show the base-catalysis properties. The solu-
tion value of V-polyoxometalate for the first protona-
tion constant pKa (µ3-O) is 5.8 (Henry, 2002) and the
pKa of I is 10.05 (Jovanovic et al., 1995). It would
be difficult for V-polyoxometalate to acquire a proton
from I in 1,2-dichloroethane solution. This indicates
that TEA is the base catalyst for the base-catalysed
autoxidation reaction. When V-polyoxometalate was
added to the base-catalysed autoxidation system of
I, remarkably, the autoxidation rate increased imme-
diately, as shown in Fig. 2. This indicates that the
V-polyoxometalate and a Lewis base can catalyse the
catechol autoxidation reaction simultaneously. To the
best of our knowledge, this is the first report that the
V-polyoxometalate-catalysed catechol dioxygenase re-
action was restrained but the oxidase reaction rate of
catechol was significantly accelerated in the presence
of a Lewis base.
Fig. 2. Effect of V-containing polyoxometalate on the oxi-
dation rate of I by monitoring the increase in ab-
sorbance at a wavelength of 404 nm of the prod-
uct II: in the presence (1) and in the absence (2)
of [(C4H9)4N]3[V10O28H3]). The added base: (A)
pyridine, (B) triethylamine. [(C4H9)4N]3[V10O28H3]
(10−5M), I (0.02 M), pyridine and triethylamine (3 ×
10−4 M), 25◦C.
Fig. 3. ESR spectrum of dioxygenation reaction (1) and au-
toxidation reaction (2).
No ESR signal was observed in the solution of the
dioxygenation reaction during the induction period
because no V(IV) compounds and semiquinone radi-
cals were formed. After the induction period, a 10-line
ESR spectrum centred at g = 2.004 was observed dur-
ing the dioxygenation reaction, as shown in Fig. 3, this
should be a signal of semiquinone radical complexed
with vanadium. The 10-line spectrum was assigned
to V(I )2(II ) by Morris et al. (2009). The ESR spec-
trum in the presence of TEA showed the characteris-
tic signal of 3,5-di-tert-butyl-1,2-benzosemiquinonate
anionic radical intermediate (DTBSQ) as shown in
Fig. 3 (Szigyártó et al., 2006). This indicates that
the semiquinone radical is not bound to the vanadium
complex but exists in a free state. The oxidase reac-
tion solution in the presence of TEA does not display
the V(IV) signal either in fluid or in frozen solution
(77 K), although V(IV) showed a marked eight-line
signal in the presence of TEA even at room tempera-
ture (Branca et al., 1990). The electron transfer from
I to O2 to form semiquinone may occur via the hydro-
gen bond, vanadium or via both. At present, we have
no direct ESR evidence that vanadium is involved. We
speculate that the hydrogen bond may play an impor-
tant role in the electron transfer from I to O2 in the
presence of a base.
Fig. 4 shows the ESR signal changes during the
oxidation of I upon the addition of different concen-
trations of TEA. When the concentration of TEA was
lower than that of vanadium, the addition of TEA sig-
nificantly reduced the strength of the 10-line ESR sig-
nal of the vanadium complex. However, the ESR signal

214
X. F. Hu, L. Wu/Chemical Papers 66 (3) 211–215 (2012)
Fig. 5. Effect of oxygen on the vanadium–catechol complex
spectrum: catalyst and I, in air (1); catalyst and I, de-
oxygen (2); catalyst, I, and pyridine, in air (3).
Fig. 4. ESR changes during dioxygenation after the addi-
tion of different concentrations of base, [V10O28H3]3−
(5 × 10−4 M), I (0.011 M); concentration of TEA: 0.014
M (1), 0.0058 M (2), 0.0014 M (3), no TEA (4).
changed from the 10-line spectrum to the semiquinone
radical signal after the concentration of TEA exceeded
that of vanadium. This means that the addition of a
sufficient amount of TEA destroys the catalyst formed
in the dioxygenation reaction and restrains its regen-
eration through suppressing coordination of the cate-
chol radical to the vanadium centre but remaining in
a free state. In this manner, the vanadium complex
and base co-catalyse the oxidase reaction of I.
The ESI-MS as a semi-quantitative method is ca-
pable of identifying the main species in a solution. Sig-
nificantly, two common major peaks (DTBC− of m/z
221, and V(DTBC)−3 of m/z 711) are detected in the
mass spectra in the presence and absence of a base. It
is worth noting that the ratio of DTBC−/V(DTBC)3−
in the presence of the base is lower than that in the
absence of the base. The results show that I and vana-
dium form the V(DTBC)−3 complex first, irrespective
of the presence or absence of the base, but the addition
of the base facilitates the formation of a ligand anion
and then facilitates the formation of the V(DTBC)3−
complex.
Under the dioxygenation reaction conditions,
V(DTBC)−3 can be converted to [VO(DTBSQ)
(DTBC)]2 after an induction period when [Na(CH3
OH)2]2[V(DTBC)3 ·4CH3OH is used as the pre-cata-
lyst (Yin & Finke, 2005). So it may be supposed that
V(DTBC)−3 , formed when [(C4H9)4N]3[V10O28H3] is
used as the pre-catalyst, initiates the dioxygenation
reaction after the induction period. No induction is
observed in the oxidation reaction in the presence
of the base, which means that V(DTBC)−3 and the
base co-catalyse the oxidation of DTBC2−. The ex-
periments show that [(C4H9)4N]3[V10O28H3] first pro-
duces V(DTBC)−3 , and then it performs dioxygenation
after an induction period in the absence of the base,
and oxidation reactions immediately in the presence
of the base, respectively.
We also determined the effect of oxygen on the
vanadium–catechol complex spectrum using a UV-VIS
spectrometer as shown in Fig. 5. The absorption spec-
trum shifted towards lower energy when the complex
was exposed to oxygen and shifted back to the original
absorption band after deoxygenation. This indicates
that oxygen forms weak bonds with the vanadium–
catechol complex: the binding of oxygen causes the
vanadium–catechol complex spectrum to shift and the
binding of oxygen is weak enough for it to be released
by deoxygenating using an inert gas. When pyridine
was added to the reaction solution, oxygen had no
effect on the absorption spectrum. Interestingly, the
absorption spectrum of the vanadium–catechol com-
plex in the presence of pyridine is similar to that of
the vanadium–catechol complex under deoxygenation
conditions. We deduced that the base occupied the
binding site of oxygen in the vanadium–catechol com-
plex so that it restrained the formation of the real
dioxygenation catalyst, and further prevented the for-
mation of dioxygenation products.
Only a small quantity (< 2 × 10−6 M) of H2O2
was detected in the V-polyoxometalate-catalysed cat-
echol dioxygenase reaction. However, in the presence
of TEA, the formation of H2O2 in the oxidase reaction
of I was detected. This indicates that H2O2 is a by-
product of the oxidase reaction. The small amount of
H2O2 generated, concomitant with the formation of
II product in the V-polyoxometalate-catalysed cate-
chol dioxygenase reaction, is rapidly consumed by the
vanadium catalyst (Yin et al., 2005), hence H2O2 was
difficult to detect. However, I was oxidised to II with
a high selectivity and reaction rate in the presence of
TEA, which led to the formation of a larger quantity
of H2O2. The formation rate of H2O2 is higher than
the consumption rate of H2O2 in the presence of TEA,
hence H2O2 accumulated at the initial stage of the re-
action. The formation rate of H2O2 decreased with the

X. F. Hu, L. Wu/Chemical Papers 66 (3) 211–215 (2012)
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oxidation of I but the consumption rate remained al-
most the same; then the H2O2 concentration began to
decrease, as shown in Fig. 6. TEA is known to react
with H2O2 to give Et3NO and H2O. However, just
the catalytic amount of TEA was added during the
autoxidation reaction in the research presented in this
paper. TEA would have been consumed prior to the
completion of the autoxidation reaction had it reacted
with the H2O2 formed so as to give Et3NO. Hence, we
believe that the H2O2 decomposition was caused by
V-polyoxometalate not TEA.
Conclusions
In the presence of a base, the dioxygenase reaction
changes to the oxidase reaction without an induction
period. ESR signals show that the semiquinone radical
binds to vanadium in the dioxygenation reaction but
exists in a free state in the presence of the base. V-
polyoxometalate can enhance the base-catalysed cat-
echol autoxidation reaction.
Acknowledgements. The generous financial support pro-
vided by the National Science Foundation of China (No.
20807036, No. 41076040) and the Yantai Science & Technol-
ogy Bureau (Project 2010160) is gratefully acknowledged.
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