Aerobic oxidation of hydroquinone derivatives catalyzed by polymer-incarcerated platinum catalyst

Article abstract of DOI:10.1002/anie.200802192

(Chemical Equation Presented) It's a lock-in! A remarkably wide substrate scope of hydroquinones are oxidized to quinones in high yields in a platinum-catalyzed process with as low as 0.05 mol% catalyst. The aerobic oxidation is catalyzed by platinum nanoclusters trapped in a styrene-based polymer network (see scheme, PI Pt=polymer-incarcerated nanoclusters). The catalyst could be reused at least 13 times without any loss of catalytic activity.

Full text of DOI:10.1002/anie.200802192

Angewandte
Chemie
DOI: 10.1002/anie.200802192
Nanoclusters
Aerobic Oxidation of Hydroquinone Derivatives Catalyzed by Poly-
mer-Incarcerated Platinum Catalyst**
Hiroyuki Miyamura, Mika Shiramizu, Ryosuke Matsubara, and Shu¯ Kobayashi*
Quinones constitute an important group of substrates. Their
structure is often found in naturally occurring compounds and
is incorporated into synthetic biologically active com-
pounds.[1] They are also useful in the construction of
polycyclic molecules by Diels–Alder reactions.[1,2] The most
important property of quinones is the ease in which they
undergo redox transformations between hydroquinones and
quinones. In organic chemistry, 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) and chloranil are among the most
common oxidizing and dehydrogenating reagents.[3] In biol-
ogy, ubiquinones and plastoquinones play critical roles in
energy production based on the quinone–hydroquinone
redox reaction.[4]
presence of PI Au either did not proceed or proceeded slowly
with the aid of an added base. Herein we report the aerobic
oxidation of hydroquinones and their derivatives catalyzed by
polymer-incarcerated platinum nanoclusters (PI Pt). In this
catalytic system, a wide range of hydroquinone and catechol
derivatives, including tetrachlorohydroquinone, are smoothly
oxidized under ambient conditions with low catalyst loading
(0.05–2mol%).
PI Pt was prepared following the previously reported
procedure for PI Au (Scheme 1).[18] Different platinum load-
ing values, approximately 0.05 mmolPtgÀ1 and approximately
0.1 mmolPtgÀ1, did not result in a significant difference in
catalytic activity and the particle size, as derived from
transmission electron microscopy (TEM) images.
The oxidation of hydroquinones is one of the most direct
ways to prepare the corresponding quinones. Although
various oxidizing agents have been used for this purpose,
reduced oxidants inevitably remain as co-products in stoi-
chiometric reactions. With increasing interest in atom econ-
omy and environmental concerns, methodologies for catalytic
oxidation that use molecular oxygen are highly desirable. In
this context, catalysts such as (NO)x,[5] Pt/C or Pt/alumina,[6]
supported metalated phthalocyanine,[7] [VO(acac)2] (acac =
acetylacetonate),[8] FeIII–EDTA (EDTA = ethylenediamine-
tetraacetate),[9] CuSO4/Al2O3,[10] dinuclear Cu or Pt cata-
lysts,[11,12] (dibenzo[b,i]-1,4,8,11-tetraazacyclotetradecianato)
cobalt(II),[13] NPV6Mo6/C,[14] N,N’-bis(2’-pyridinecarboxa-
mido)-1,2-benzene]cobalt(II),[15] and copper/amine/cellulose
polymers[16] have been previously reported, although sub-
strate scope remains to be improved. In particular, the
oxidation of hydroquinones substituted with electron-with-
drawing groups (EWGs), such as chlorohydroquinone, was
either reported to be difficult or was not described, presum-
ably because of its high redox potential.[2,3]
Scheme 1. Preparation of PI Pt.
The results of the oxidation reaction on a range of
substrates under various reaction conditions are summarized
in Table 1. When tetrachlorohydroquinone was left in an O2
atmosphere in the absence of any catalyst only a negligible
Recently we have reported that styrene-based polymer-
incarcerated Au nanoclusters (PI Au) efficiently catalyze the
aerobic oxidation of hydroquinone derivatives.[17] While
various alkyl-substituted hydroquinones were smoothly oxi-
dized, reactions with EWG-substituted hydroquinones in the
Table 1: Optimization of reaction conditions.
[*] H. Miyamura, M. Shiramizu, Dr. R. Matsubara, Prof. Dr. S. Kobayashi
Department of Chemistry, School of Science and Graduate School of
Pharmaceutical Sciences, The University of Tokyo
The HFRE Division, ERATO (Japan) Science Technology Agency
(JST)
Entry
CHCl3/H2O (mL mmolsubstrateÀ1
)
Gas
Yield [%][b]
1[a]
2[c]
30/27
4
5
6
7
27/1.0
27/0
O2
O2
<1
23
9
60
>99
73
O
2
27[d]/1.0
27/1.0
27/1.0
27/1.0
O2
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
O2
Air
N2
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research from the Japan Society of the Promotion of Science (JSPS).
H.M. thanks the JSPS Fellowship for Japanese Junior Scientists.
6
[a] Without PI Pt. [b] Determined by GC analysis (internal standard:
anisole), standard curve method. [c] MgSO4 (1 equiv) was added as a
dehydrating agent. [d] THF was used instead of CHCl3.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802192.
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amount of p-chloranil was obtained (Table 1, entry 1). While
only of hydroquinones substituted with electron-donating
groups (EDGs; 1a–i) but also EWG-substituted hydroqui-
nones (1j–p). Catechols were also found to be good substrates
(2a, 2b). The reaction was very clean and selective for all
substrates, no significant by-products were detected and
complete consumption of the starting materials was observed.
Lower yields in a few cases (1b, 1g, 1n) may arise from the
inherent tendency of quinones to sublime, which could cause
a loss of products during the reaction. Analytically pure
products could be isolated by simply removing the catalyst
from the reaction mixture by filtration followed by conven-
tional phase separation and solvent removal, which simplifies
the experimental procedure. It is noted that alcohols remain
unchanged in this catalytic system (1h). p-Methoxyanisole
and p-methoxyphenol were not oxidized, and starting materi-
als were recovered.[19] These results indicate high chemo-
selectivity of PI Pt, which could be useful in further applica-
tions.
Quantitative measurement of the O2 uptake with a gas
burette confirmed 0.5 mol of O2 uptake per mole of substrate
conversion (11.8 mL of O2 for 1 mmol substrate, Scheme 2).
This finding indicates that H2O is the sole net co-product,
although H2O2 may be formed as an intermediate.[20]
The recovery and reuse of PI Pt was studied using tert-
butylhydroquinone as a substrate (Scheme 3). When the PI Pt
collected by filtration was reused after only drying, a gradual
PI Pt-catalyzed reactions in either chloroform or water alone
afforded the oxidized product only in low yield (Table 1,
entries 2and 3), homogeneous media such as THF/H 2O
resulted in an improved yield (Table 1, entry 4). Gratifyingly,
carrying out the reaction in a biphasic solvent system of
chloroform/water (27:1) led to the quantitative formation of
the desired product (Table 1, entry 5). Oxidation occurred
even under atmospheric oxygen (Table 1, entry 6), but a poor
yield was obtained when the reaction was carried out under
nitrogen (Table 1, entry 7). These observations indicate that
molecular oxygen is indispensable for the reaction and that
oxidation does not occur through H2 elimination. Inductively
coupled plasma (ICP) analysis showed that no platinum had
leached into the reaction mixture (Table 1, entry 5). To the
best of our knowledge, this is the first example of truly
efficient catalytic aerobic oxidation of electron-deficient
hydroquinone derivatives such as tetrachlorohydroquinone.
The results of aerobic oxidation catalyzed by PI Pt are
summarized in Table 2. All the hydroquinones examined
were oxidized readily (within 1–8 h) in good to excellent
yields, under additive-free conditions at room temperature.
Catalyst loading was decreased to as low as 0.05 mol% (1a),
and a maximum turnover frequency (TOF) of at least
1000 hÀ1 was observed (1a). Catalytic activity was not
inhibited by acid (1a). PI Pt catalyzed the oxidation not
Table 2: PI Pt-catalyzed aerobic oxidation of hydroquinones and catechols.
Product
Amount PI Pt
[mol% Pt]
t [h] Yield
Product
Amount PI Pt
[mol% Pt]
t
Yield
Product
Amount PI Pt
[mol% Pt]
t
Yield
[%]
[h] [%]
[h] [%]
0.1
0.05
1
5
1
>99[b]
1
0.5
3
3
99[b]
90[a]
90[b]
0.1
1
75[a]
>99[b,d]
3a
3g
3m
0.1
2
75[b]
89[a]
93[a]
1
3
2
[c]76
1
2
3
3
[b]65
3b
3c
3h
3i
3n
3o
0.2%
3.75
2
0.1
>99[a]
[c] 92
0.1
0.1
0.1
1
0.5
1
0.5
7
80[b]
2
1
1
6
3
8
88[a]
3d
3e
3j
3p
4a
3.5 >99[b]
93[b]
[c] 98
3k
1
94[b]
3
[b]99
>99[a]
3 f
3l
4b
1
[a] Yield of isolated product. [b] Yield determined by GC (internal standard: anisole), standard curve method. [c] Yield was determined by H NMR
analysis (internal standard: acetophenone). [d] HCl (1 equiv) was added.
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Keywords: hydroquinones · oxidation · platinum · polymers ·
.
supported catalysts
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decline of catalytic activity was observed,[21] although no
leaching of platinum into the reaction mixture was detected
by ICP analysis in each run. However, treatment of the
recovered catalyst with base was found to be effective in
preventing its deactivation. When the filtered catalyst was
washed with CHCl3/0.5m aqueous NaOH after each use, the
PI Pt could be reused at least 13 times to catalyze oxidation
reactions in almost quantitative yield.
In conclusion, we have developed an aerobic oxidation of
hydroquinone derivatives that is catalyzed by polymer-
incarcerated platinum nanoclusters. A remarkably wide
substrate scope has been achieved, including EWG-substi-
tuted hydroquinones, which are unprecedented substrates for
catalytic oxidation. The oxidation proceeded smoothly with as
little as 0.05 mol% PI Pt under ambient conditions, with high
selectively for the hydroquinone structure. Water is the sole
co-product and the catalyst could be reused at least 13 times
without any loss of catalytic activity. This methodology
provides not only a synthetically useful but also an environ-
mentally benign route for the preparation of quinone
derivatives.
[16] S. Muralidharan, H. Freiser, J. Mol. Catal. 1989, 50, 181.
[17] H. Miyamura, M. Shiramizu, R. Matsubara, S. Kobayashi, Chem.
Lett. 2008, 37, 360.
[18] H. Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi, Angew.
Chem. 2007, 119, 42 2 9A; ngew. Chem. Int. Ed. 2007, 46, 4151.
[19] PI Pt (1 mol%), O2 (1 atm), RT, 3 h, CHCl3 (20 mLmmolÀ1
substrate), H2O (1 mLmmolÀ1 substrate).
[20] Vigorous emission of O2 gas was observed upon treatment of an
aqueous solution of H2O2 with PI Pt.
Received: May 10, 2008
Revised: July 29, 2008
Published online: September 18, 2008
[21] When the catalyst was reused after recovery by simple filtration
and drying, the yields were > 99% (1–5th), 96% (6th), 84%
(7th), and 63% (8th).
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