3
60
Chemistry Letters Vol.37, No.3 (2008)
Polymer Incarcerated Gold Catalyzed Aerobic Oxidation
of Hydroquinones and Their Derivatives
Ã
Hiroyuki Miyamura, Mika Shiramizu, Ryosuke Matsubara, and Shu 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),
Hongo, Bunkyo-ku, Tokyo 113-0033
(
Received December 11, 2007; CL-071373; E-mail: shu kobayashi@chem.s.u-tokyo.ac.jp)
Polymer-incarcerated gold (PI Au) cluster catalysts mediat-
Table 1. Aerobic oxidation of methylhydroquinone (2a) to tol-
uquinone (2d) in the presence of PI Au (1 mol %) under O2
ed aerobic oxidation of hydroquinones and catechols to quinones
very efficiently under mild conditions. The characteristic role of
water in the reaction system was also observed.
Solvent
mL/mmol substrate)
Conversion Selectivity Yield
/%a
Entry
T
t/h
/%a,b
/%a
(
1
2
3
4
BTF (4.0)
BTF (4.0)/H2O(4.0)
rt 12
rt 12
12
73
25
51
3
37
Quinones and their derivatives are an important class of
compounds not only in organic synthesis but also in biochemis-
Toluene(4.0)/H2O(4.0) rt 12
CH2Cl2(4.0)/H2O(4.0) rt 12
CH Cl (7.6)/H O(0.4) rt 5.5
82
85
29
24
24
20
1
d
try. In organic chemistry, they are useful as oxidizing re-
agents and starting materials for synthesis of polycyclic com-
5
98
96
93
94
93
2
2
2
1,2
c,d
6
7
CH2Cl2(7.6)/H2O(0.4) rt 5.5
>99
>99
>99
88
pounds1
,3,8c,9
Moreover, the redox properties of quinones are im-
d
c
CH2Cl2(22.8)/H2O(1.2) rt
CH2Cl2(22.8)/H2O(1.2) rt
1
1
6
6
6
1
>99
>99
67
>99
>99
57
8
portant for living cells, where ubiquinones (coenzymes Q) act as
biochemical oxidizing agents to mediate the electron-transfer
9
CH2Cl2(4.0)/H2O(4.0)
CH2Cl2(7.6)/H2O(0.4)
CH2Cl2(8.0)
0
0
0
1
10
11
91
13
91
83
13
processes involved in energy production. In plant tissues the
1
>99
>99
plastquinone performs a similar function in photosynthesis.
The most direct way to obtain quinone derivatives is oxida-
tion of the corresponding hydroquinones. Whilst several metal-
1
2d CHCl3(7.6)/H2O(0.4) rt
98
98
a
c
Determined by GC analysis. bSelectivity = yield of 2d/conversion.
Atmospheric air was used instead of O2. No leaching of Au to the reaction
d
4
5
based oxidizing reagents, such as silver oxide, silver carbonate,
6
mixture was observed by ICP analysis.
7
potassium nitrosodisulfonate, chromium oxidants, hypervalent
8
9
iodine, and cerium ammonium nitrate (CAN), have been de-
veloped, these reagents usually require stoichiometric amounts
of metal oxidants, and thus a large amount of waste is formed.
In this context, oxidation using molecular oxygen or hydrogen-
peroxide11 catalyzed by easily separable heterogeneous catalysts
is ideal from the viewpoint of green chemistry, atom economy,
combinatorial synthesis, etc.
conversions (Entries 2–4). The selectivity was improved when
ꢀ
the reaction was carried out at 0 C or the volume and the ratio
of the organic solvent was increased (Entries 5–7, 9, and 10),
although the reaction was sluggish in the absence of water
(Entries 1 and 11). Similar results were obtained when atmo-
spheric air was used instead of oxygen (Entry 8). Further optimi-
zation has revealed that the conditions shown in Entry 12
were the best, where excellent conversion and selectivity have
been achieved. It is noted that no leaching of Au to the reaction
mixture was observed by Inductive Coupled Plasma (ICP)
analysis (<0:04%) (Entries 5–7 and 12).
With the optimal conditions in hand, several hydroquinone
derivatives and catechols were oxidized by PI Au under atmo-
spheric oxygen or air. Hydroquinone (1a) was oxidized smooth-
ly to give 1,4-quinone in good yield even in the presence of
0.5 mol % of the catalyst or under atmospheric air (Table 2,
Entries 1 and 2). Hydroquinones with alkyl substituents 2–6a
were also oxidized to afford the corresponding quinone deriva-
tives in excellent yields (Entries 3–7). Phenylhydroquinone
(7a) and naphthoquinone (8c) were also converted to the oxi-
dized products under the same conditions (Entries 8 and 9).
While oxidation of electron poor bromohydroquinone (9a) was
slow, the reaction was accelerated by addition of a base, afford-
ing bromoquinone in moderate yield (Entry 10). Oxidation of
catechols 10b and 11b was also accelerated by a base, providing
the corresponding o-quinones in high yields (Entries 11 and 12).
It should be noted that in most cases analytically pure products
were obtained in excellent yields by simple phase separation
without further purification.
1
0
Recently, we reported that gold nanocluster catalysts immo-
bilized on polystyrene-based polymers with cross-linking
moieties, PI Au, were highly effective for aerobic oxidation of
alcohols to ketones and aldehydes in the presence of water and
a base.12 Since bases accelerated the alcohol oxidation reactions
significantly in this process, we proposed that the ꢀ-hydrogens
of alcohols were abstracted as protons by bases. On the basis
of this assumption, we envisioned that acidic hydroquinones
might be potentially reactive substrates for gold-catalyzed
aerobic oxidation. Here, we report aerobic oxidation of
hydroquinones to quinones catalyzed by PI Au under very mild
conditions, and the characteristic role of water in this reaction
is also described. To the best of our knowledge, this is the first
example of aerobic oxidation of hydroquinones to quinones
1
4
1
3
facilitated by gold catalysts.
PI Au, which was prepared according to the reported meth-
od,12 was used in the aerobic oxidation of methylhydroquinone
(2a) to toluquinone (2d) under atmospheric oxygen. When ben-
zotrifluoride (BTF) was used as a solvent, the desired product
was obtained in low conversion and low selectivity (Table 1,
Entry 1). Addition of water dramatically increased conversion
and selectivity (Entry 2). Screening of organic solvents revealed
that dichloromethane is a suitable solvent for achieving high
Although the extensive mechanistic study has not yet been
Copyright Ó 2008 The Chemical Society of Japan
Miyamura, Hiroyuki
