Efficient oxidation of hydroquinone and alcohols by tailor-made solid polyaniline catalyst

Article abstract of DOI:10.1016/j.tetlet.2008.11.062

We described the efficient and convenient catalytic oxidation of hydroquinone and primary/secondary alcohol compounds into benzoquinone and aldehyde/ketone using various oxidation states of polyaniline. We analyzed the oxidation catalyzing capabilities of the pernigraniline base polymer by designing a tailor-made catalyst to be used as a powerful oxidant, and it was easily recovered and regenerated for recycling.

Full text of DOI:10.1016/j.tetlet.2008.11.062

Tetrahedron Letters 50 (2009) 559–561
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient oxidation of hydroquinone and alcohols by tailor-made
solid polyaniline catalyst
b
a
a
a,
Chan Woo Lee a, , Sung Ho Jin , Koo Sik Yoon , Han Mo Jeong , Ki-Whan Chi
*
*
a Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea
b Department of Chemistry Education, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We described the efficient and convenient catalytic oxidation of hydroquinone and primary/secondary
alcohol compounds into benzoquinone and aldehyde/ketone using various oxidation states of polyani-
line. We analyzed the oxidation catalyzing capabilities of the pernigraniline base polymer by designing
a tailor-made catalyst to be used as a powerful oxidant, and it was easily recovered and regenerated
for recycling.
Received 23 September 2008
Revised 17 November 2008
Accepted 18 November 2008
Available online 21 November 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Polyaniline
Pernigraniline base
Hydroquinone
Alcohol
Oxidation
The catalyzed oxidation of hydroquinone (HQ)/phenol is being
extensively investigated with the dual objectives of removing phe-
nolic waste from industrial effluent and achieving regio-selective
oxidation of useful products. The latter process often involves a
milder reaction condition. The oxidation of hydroquinone to
benzoquinone (Q) is a very important physiological reaction and
copper metal is found to be the most efficient catalyst for this reac-
tion.1,2 A general method for the preparation of quinones is the oxi-
dation of HQ and its derivatives.3 A number of oxidants, for
example, mineral acids,4 Fermy’s salt,5 lead tetraacetate,6 cerium
ammonium nitrate,7 and other transition metal ions,8 have been
employed, but most of these suffer from drawbacks such as
homogenous reaction conditions and the generation of toxic
wastes. Thus, new catalytic protocols need to be developed to ful-
fill the demands of green chemistry. For the 1:1 HQ–Q complex, in
the presence of light, an electron can be donated from the HQ to
the Q, forming a charge-transfer complex with a characteristic pur-
ple color.9
sion polymerization (SSDP) method results in an insoluble solid
catalyst for the dehydrogenative oxidation of primary amines
and primary alcohols.13–16 The structure of PANI consists of a
repeating unit of benzenoid and quinoid, which is similar to the
structure of quinone derivatives. Accordingly, if the quinoid moiety
of PANI was introduced into a hydroquinone or alcohol system, it is
expected that HQ and primary/secondary alcohol compounds
would be readily oxidized by suitable attractions such as a hydro-
gen bond and
p–p interaction between the PANI and these com-
pounds. In a continuation of our ongoing work, we focused on
the simple and efficient oxidation using tailor-made solid PANI
HQ
O
H
O
H
O
H
N
N
N
N
Among polymers, polyaniline (PANI) is well known to have var-
ious oxidation states such as leucoemeraldine base (LB, fully re-
duced form), emeraldine base (EB, half reduced form),
pernigraniline base (PB, fully oxidized form), which permits the
construction of a reversible redox cycle for the potential catalytic
reactions.10–12 Recently, we demonstrated that the synthesis of
nano-porous PANI emeraldine salt (ES) by self-stabilized disper-
Oxidation
PB
LB
H
N
H
N
H
N
H
N
O
O
O
ketone
Q
* Corresponding authors. Fax: +82 052 257 2348 (C.W.L.).
E-mail address: cw5113@ulsan.ac.kr (C. W. Lee).
Scheme 1. Design for the oxidation of HQ or secondary alcohol using a tailor-made
solid PANI catalyst.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.062

560
C. W. Lee et al. / Tetrahedron Letters 50 (2009) 559–561
catalyst, which had never been used for the oxidation of HQ or pri-
mary/secondary alcohols (Scheme 1). These reactions are, to the
best of our knowledge, the first example of the direct redox reac-
tions system being catalyzed by the various oxidation states of
PANI. In addition, the key strengths of solid-phase organic synthe-
sis include the relatively easy work-up, purification, and recycling:
simple filtration that separates the reagents, starting materials, and
solvents from the desired products.
tion. Accordingly, the PB polymer is a powerful oxidant than ES
polymer, which does not have a quinoid structure.
To study the dehydrogenative oxidation mechanism of HQ, we
proposed a hydrogen atom abstraction mechanism by hydrogen
bonding interaction (Scheme 2). Initially, hydrogen atom abstrac-
tion from HQ to the quinoid moiety (QM) of PB occurs to give
the protonated quinoid moiety (QMH) radicals and phenoxy radi-
cal species. The QMH changes into the corresponding resonance
structure by a single electron transfer to neighboring nitrogen. Fi-
nally, the resulting nitrogen radical of QMH abstracts another H
atom of HQ to give Q.
Punniyamurthy and co-workers reported about EB-supported
metal complex-catalyzed aerobic oxidation of alcohols to alde-
hydes and ketones.17 In a previous study, we also reported that pri-
mary alcohols are successfully oxidized by ES catalyst.15 However,
secondary alcohols did not oxidize to the corresponding ketone
compounds. To improve the synthetic method for the oxidation
The PANI was prepared by the SSDP method with a honey-comb
like nano-structure. The PB, ES, EB, and LB were prepared according
to previous studies.13,14 Initially, we attempted the oxidation of HQ
(0.11 g, 1 mmol) with the PB (0.2 g) in the presence of dioxane
(20 mL) at room temperature for 30 min after sonication for
5 min to produce the desired Q product as shown in Table 1 (entry
1). The colorless solution changed to purple due to the charge-
transfer complexes associated with the formation of Q. The resul-
tant product was exclusively and rapidly obtained by a simple fil-
tration and then confirmed by comparison with an authentic
sample. To obtain further information regarding the redox behav-
ior, we characterized the oxidation state of PANI after transition
into the LB structure by IR spectroscopy. Thus, the characteristic
reduced peak at 1495 cmÀ1 of the resulting PANI was identical to
that of the synthesized LB form.15 From the oxygen bubbling and
catalytic amount of PB (50 mg) at 80 °C for 6 h, the reaction rate
was slowly progressed and the yield of Q was still retained (entry
2). This finding means that the oxidation process was activated by
the catalytic pathway. We think that the amount of HQ in the early
stage of oxidation was in kinetically predominant competition
with the reduction process of PB to give the LB form, which was re-
tarded by the oxidation process of HQ. With the oxygen bubbling,
the LB polymer was readily oxidized to the PB form for the catalytic
pathway. The choice of a solvent is also important. The reaction in
a polar aprotic solvent such as dioxane gave an exclusive yield,
while the reaction in a polar protic solvent such as ethanol resulted
in lower yield (compare the entries 1 and 3). This shows that the
hydrogen bonding between HQ and PANI is hindered by a protic
solvent.
HQ
O
H
O
H
O
H
O
H
N
N
N
N
H atom abstraction
PB
H
N
N
N
N
N
QMH
HO
O
H
N
N
N
HO
O
H atom abstraction
To investigate another potential catalyst, the ES, EB doped with
HCl was used for the oxidation of HQ. The reaction result (entry 4)
shows that HQ can be oxidized by ES, but the efficiency was infe-
rior to that of the PB (compare the entry 1). The efficiency was im-
proved under the combinations of oxygen bubbling and at high
temperature conditions (entries 5 and 6). These results show that
not only the PB but also the ES polymer which have radical cation
structure referred to as the polaron band that can be used as an
oxidant because this radical species can participate in oxidation
process by hydrogen atom abstraction pathway. As expected, the
LB and EB forms under dioxane solvent did not oxidize to HQ (en-
try 7). These results imply that PANI should be in oxidized form in
order to oxidize HQ; the fully oxidized form, PB can oxidize effec-
tively, however reduced form such as LB and EB does not. In addi-
tion, we think, quinoid moiety of PB is designed to oxidize HQ as a
tailor-made catalyst by hydrogen bonding donor-acceptor interac-
H
N
H
N
H
H
N
N
LB
O
O
Scheme 2. The proposed oxidation mechanism of HQ.
Table 2
Oxidation of alcohols by various oxidation states of PANIa
Entry
1
Substrate
PANI
PB
Yield (%)
82
CH OH
2
2
3
4
CH2OH
ES
PB
ES
38
78
—
Table 1
Cinnamyl alcohol
Oxidation of HQ according to the various oxidation states of PANI
Entry
Catalyst
O2
Time (h)
Solvent
Temp. (°C)
Yield (%)
OH
1
2
3
4
5
6
7
PBa
Â
o
Â
Â
o
o
Â
0.5
6
0.5
1
2
6
Dioxane
Dioxane
EtOH
Dioxane
Dioxane
Dioxane
Dioxane
rt
80
rt
P95
P90
63
47
68
PBb
5
6
PB
PB
PB
71
54
61
OH
PBa
ESa
rt
ESa
80
80
80
OH
ESb
56
—
EB or LB
12
7
2-Butanol
a
a
The reactant (1 mmol) and catalyst (200 mg) were used.
The reactant (1 mmol) and catalyst (50 mg) were used.
The alcohol (1 mmol) and oxidant (50 mg) were heated at 80 °C for 6 h under
b
oxygen bubbling in dioxane.

C. W. Lee et al. / Tetrahedron Letters 50 (2009) 559–561
561
Table 3
of HQ and primary/secondary alcohols to Q or aldehyde/ketone,
and it can be easily recovered and regenerated. The advantages
of this environmentally benign and safe protocol include a simple
reaction setup and chemo-selectivity. We are currently working on
the reaction mechanism and the extension of the biological oxida-
tion reaction for various glucose and heterocyclic compounds.
Recycling of the PB for the oxidation of HQa
Cycle
PB
Conversion (%)
Yield (%)
1
2
3
Fresh
93
5
95
87
Trace
92
Recoveredb
Regeneratedb
a
Reaction condition: HQ (1 mmol), dioxane (30 mL), and the PB (200 mg) at
room temperature.
Acknowledgments
b
Recycled PB: after oxidation reaction, PB was separated from the reaction
mixture by filtration, and the isolated PB was oxidized by APS.
S.H. Jin was supported by the Korea Science and Engineering
Foundation (KOSEF) grant funded by the Korean government
(MOST) (No. M10600000157-06J0000-15710). This letter was ded-
icated in memory of retirement of Prof. Jeong Soo Ahn (University
of Ulsan).
of secondary alcohols, we examined the various primary/secondary
alcohols such as 4-methyl cyclohexanol, 2-butanol, and isopropyl
alcohols using a PB catalyst (Table 2). Interestingly, all of the pri-
mary/secondary alcohols oxidized to give the corresponding alde-
hyde/ketone compounds at a moderate yield. In the ES polymer,
the corresponding product of the primary alcohol is poorly pro-
duced (entry 2) and no reaction is observed in the oxidation of sec-
ondary alcohols (entry 4).15 However, when catalytic amount of
the PB (50 mg) was used with oxygen bubbling at 80 °C for 6 h,
the yield of the benzaldehyde was increased drastically (entry 1)
and secondary alcohols were efficiently oxidized (entries 5 and
6). As in the oxidation of HQ, these results show that oxidized form
of PANI, PB, is a more powerful oxidant for secondary alcohols than
ES. As expected, the oxidation of alcohols did not occur with fully
reduced LB form of PANI.
References and notes
1. Saladino, R.; Neri, V.; Mincione, E.; Filippone, P. Tetrahedron 2002, 58, 8493.
2. Jakubiak, A.; Owsik, A.; Kolarz, B. React. Funct. Polym. 2005, 65, 161.
3. Dudfield, P. J.. In Comprehensive Organic Synthesis; Trost, B. M., Flemming, J. M.,
Eds.; Pergamon: Oxford, 1991; Vol. 7, p 345.
4. Brown. S. W.; Br. Patent Appl. 1991, 913, 323.
5. Cherkaoni, O.; Nebois, P.; Filloin, H.; Domard, M.; Fenet, B. Tetrahedron 1995,
52, 9499.
6. Hansen, D. W., Jr.; Pappo, R.; Garland, R. B. J. Org. Chem. 1988, 53, 4244.
7. Brimble, M. A.; Duncalf, L. J.; Phythiom, S. J. J. Chem. Soc., Perkin Trans. 1 1997,
1399.
8. Barton, D. H. R.; Finet, J. P.; Tomas, M. Tetrahedron 1988, 44, 6397.
9. Regeimbal, J.; Gleiter, S.; Trumpower, B. L.; Yu, C. A.; Diwakar, M.; Ballou, D. P.;
Bardwell, J. C. A. Proc. Natl. Acad. Sci. U.S.A 2003, 100, 13779.
10. Cao, Y.; Heeger, A. J. Synth. Met. 1992, 52, 193.
The PB can be recycled without loss of activity or selectivity. As
shown in Table 3, the yield of Q is very low when PB used for oxi-
dation is used again without further treatment compared with that
by fresh one (Table 3). However, when the recovered PB was regen-
erated by oxidation with oxidant such as ammonium persulfate
(APS), the yield was drastically increased.
11. Cao, Y.; Heeger, A. J. Phys. Rev. B 1994, 49, 5988.
12. MacDiamid, A. G.; Epstein, A. J. Synth. Met. 1994, 35, 231.
13. Lee, K. H.; Cho, S. U.; Park, S. H.; Heeger, A. J.; Lee, C. W.; Lee, S. H. Nature 2006,
441, 65.
14. Lee, S. H.; Lee, D. H.; Lee, C. W. Adv. Funct. Mater. 2005, 15, 1495.
15. Chi, K.-W.; Hwang, H. Y.; Lee, C. W. Synth. Met. 2008, in press.
16. Kim, J. M.; Han, D. K.; Lee, C. W.; Kim, S. H.; Gong, M. S.; Ahn, K. D. Bull. Korean
Chem. Soc. 1998, 19, 611.
In conclusion, we have found that fully oxidized form of PANI,
PB, is an effective solid oxidant for the dehydrogenative oxidation
17. Reddy, S. R. D. S.; Punniyamurthy, T. Tetrahedron Lett. 2004, 45, 3561.