Organocatalytic oxidative dehydrogenation of dihydroarenes by dioxygen using 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ) and NaNO2

Article abstract of DOI:10.3390/molecules13123236

The oxidative dehydrogenation of dihydroarenes catalyzed by 2,3-dichloro-5,6-dicyano-benzoquinone(DDQ) and NaNO₂ with dioxygen is reported. The combination of DDQ and NaNO₂ showed high efficiency and high selectivity, compared with other benzoquinones and anthraquinones, e.g., >99% conversion of 9,10-dihydroanthracene with 99% selectivity for anthracene can be obtained at 120 °C under 1.3 MPa O₂ for 8 h. Excellent results were achieved in the oxidative dehydrogenation of variety of dihydroarenes.

Full text of DOI:10.3390/molecules13123236

Molecules 2008, 13, 3236-3245; DOI: 10.3390/molecules13123236
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Organocatalytic Oxidative Dehydrogenation of Dihydroarenes
by Dioxygen Using 2,3-Dichloro-5,6-dicyano-benzoquinone
(DDQ) and NaNO2
Wei Zhang 1, 2, Hong Ma 1, Lipeng Zhou 3, Zhiqiang Sun 1, Zhongtian Du 1, Hong Miao 1 and Jie
Xu 1,*
1
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, P. R. China; E-mail: zhangwei@dicp.ac.cn (W. Z), mahong@dicp.ac.cn
(H. M.), szq@dicp.ac.cn (Z-Q. S.), duzhongtian@dicp.ac.cn (D-Z. T.), miaohong@dicp.ac.cn (H.
M.)
2
Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China
3
Department of Chemistry, Institute of Catalysis, Zhengzhou University, Zhengzhou 450001, P. R.
China; E-mail: zhoulipeng@zzu.edu.cn (L-P. Z.)
* Author to whom correspondence should be addressed. E-mail: xujie@dicp.ac.cn
Received: 5 November 2008; in revised form: 10 December 2008 / Accepted: 12 December 2008 /
Published: 18 December 2008
Abstract: The oxidative dehydrogenation of dihydroarenes catalyzed by 2,3-dichloro-5,6-
dicyano-benzoquinone(DDQ) and NaNO2 with dioxygen is reported. The combination of
DDQ and NaNO2 showed high efficiency and high selectivity, compared with other
benzoquinones and anthraquinones, e.g., >99% conversion of 9,10-dihydroanthracene with
99% selectivity for anthracene can be obtained at 120 °C under 1.3 MPa O2 for 8 h.
Excellent results were achieved in the oxidative dehydrogenation of variety of
dihydroarenes.
Keywords: Oxidative dehydrogenation; DDQ; Dioxygen; Redox couples.

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Introduction
The dehydrogenation of hydrocarbons to various alkenes and aromatic olefins is an important
project in modern chemical manufacture and scientific research [1, 2]. A very attractive approach is
oxidative dehydrogenation. Oxidants such as dioxygen, halogens, sulfur compounds, etc., could accept
hydrogen and thus the thermodynamics of the dehydrogenation process shifts to the right side. These
exothermic oxidation reactions also provide necessary process heat to compensate for the endothermic
dehydrogenation and thus dehydrogenation can be successfully operated under moderate conditions [3].
On the other hand, the selectivity is difficult to control in the presence of oxidants, and byproducts are
thus generated. Recently, noble-metal catalysts such as Ru(OH)x/Al2O3 and Ru(TMP)(O)2 have been
found to readily catalyze oxidative dehydrogenation reactions with high selectivity [4, 5], but
unfortunately the need for noble metals makes such a dehydrogenation process uneconomic. There is
strong incentive for developing new cheaper oxidative dehydrogenation catalysts which
simultaneously employ dioxygen as oxidizing agent.
High-potential quinones such as DDQ are usually used as hydrogen acceptors in many types of
reactions such as dehydrogenation of hydrocarbons and benzylic alcohols, oxidation of alcohols and
allylic ethers, and direct cross-dehydrogenative-coupling reactions, etc [6-9]. Unfortunately, quinones
are often used with stoichiometric amount because they are converted into quinols after accepting
hydrogen and can not be recycled to the initial state. In order to solve the above problem, efforts have
been made to regenerate the quinones. Several reagents such as Mn(OAc)3, HClO4, HIO4, and HNO3
showed efficiency in converting quinols to quinones [10, 11]. But in most cases these reagents were
still required in stoichiometric or in excess amounts.
In a previous study at our laboratory, anthraquinones and NHPI were coupled to form an efficient
organocatalytic system in the aerobic oxidation of hydrocarbons under moderate reaction conditions.
The redox transformation between anthraquinones and anthraquinols can be cycled by NHPI/PINO
[12-15]. Recently, we have designed a novel DDQ/NaNO2 catalytic system for dehydrogenation of
9,10-dihydroanthracene (Scheme 1) [16]. In the present work, benzoquinones and anthraquinones
combined with NaNO2 were studied in terms of activity and selectivity. Further, the detailed operation
conditions including catalyst loading, reaction time, and temperature for DDQ/NaNO2 were optimized.
Moreover, its application in the oxidative dehydrogenation of variety of dihydroarenes was
investigated. At last the redox catalytic cycle was proposed. The redox couples quinone/quinol and
NO2/NO were expected to create a catalytic cycle in the presence of O2 in the dehydrogenation. This
novel organocatalytic system offers new thoughts for design of highly selective catalyst for oxidative
dehydrogenation of aromatic hydrocarbons.
Scheme 1. Catalytic oxidative dehydrogenation of 9,10-dihydroanthracene to anthracene.
O2
DDQ/NaNO2

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Results and Discussion
Comparison of different benzoquinones and anthraquinones combined with NaNO2
The effect of a variety of benzoquinones and anthraquinones (Figure 1) including BQ, TCQ, DDQ,
AQ, CAQ, and TCAQ combined with NaNO2 in the oxidative dehydrogenation of 9,10-
dihydroanthracene was explored (Figure 2).
Figure 1. The studied benzoquinones and anthraquinones.
O
O
O
Cl
Cl
Cl
Cl
Cl
Cl
CN
CN
O
O
O
BQ
TCQ
DDQ
O
O
Cl O Cl
Cl
O
AQ
O
CAQ
Cl O Cl
TCAQ
BQ: benzoquinone; TCQ: 2,3,5,6-tetrachloro-1,4-benzoquinone; AQ: anthra-
quinone; CAQ: 2-chloroanthraquinone; TCAQ: 1,4,5,8-tetrachloroanthraquinone
Figure 2. The oxidative dehydrogenation of 9,10-dihydroanthracene with different
a
quinones and NaNO2 .
Conv.
ATC
AQ
100
80
60
40
20
0
BQ
TCQ DDQ AQ CAQ TCAQ
Quinones
a Reaction conditions: 10 mL toluene, 0.5 mmol 9,10-dihydroanthracene, 0.05 mmol quinone, 0.05
mmol NaNO2, 120 °C, 1.3 MPa O2, 8 h. When using AQ/NaNO2, the amount of by-product AQ is
the difference between the GC measurement value and the loading amount in AQ/NaNO2. ATC:
anthracene

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The results showed that both benzoquinones and anthraquinones exhibited considerable activity. It
was notable that DDQ/NaNO2 showed high catalytic efficiency. The conversion of 9,10-
dihydroanthracene was >99% and selectivity to anthracene was 99%. Besides DDQ/NaNO2,
TCQ/NaNO2 also showed good results. The selectivity to anthracene was 96% with 47% conversion.
Comparably, BQ/NaNO2 was low active but high selective. The conversion only reached 15% and the
selectivity to 9,10-dihydroanthracene was >99%. Traces of AQ were detected as the sole by-product
by GC-MS measurement. The activity of benzoquinones varied following the order: DDQ>TCQ>BQ.
These activity results coincide with the reduction potential order of benzoquinones [17-19]. The
electron-withdrawing group can increase the reduction potential and thus the dehydrogenation ability
increases. In case of different anthraquinones (AQ, CAQ, and TCAQ), the results were not
satisfactory, as both anthracene and considerable amounts of AQ were produced. The occurred
oxidative side-reaction suggested anthraquinones have catalytic ability in catalyzing aerobic oxidation
of 9,10-dihydroanthracene to AQ. On the basis of these results, it could be concluded that DDQ
combined with NaNO2 was the highest active and selective among the quinones investigated.
Optimization of the catalyst loading amounts, reaction time, and temperature forDDQ/NaNO2
Performing as an excellent oxidative dehydrogenation catalyst, DDQ combined with NaNO2 was
further studied under detailed conditions. The results of oxidative dehydrogenation with different
catalyst loading amount are listed in Table 1. The loading amounts of DDQ and NaNO2 were equal
and increased simultaneously from 1 mol% to 10 mol%. Obviously, the conversion is low at lower
catalyst loading (1 mol%, 2 mol%, and 3 mol%), and then increased to >99% at 5 mol% and 10 mol%.
So 5% DDQ and 5% NaNO2 are selected as the optimum loading amounts. Furthermore, we examined
the efficiency of air. With 5% DDQ and 5% NaNO2, it only gave 64% conversion and >99%
selectivity to anthracene under 1.3 MPa air. By increasing the catalyst loading amount to 10% DDQ
and 10% NaNO2, the conversion was increased to >99% with >99% selectivity to anthracene, similar
to that under 1.3 MPa O2. The results indicated that oxidative dehydrogenation can perform under air,
but the efficiency is lower than that under O2.
Table 1. Effect of catalyst loading amount on the oxidative dehydrogenation of 9,10-
dihydroanthracene to anthracene a.
Catalyst loading
(mol%)
Products and selectivity
(%)
Entry
Conversion (%)
DDQ
NaNO2
ATC
>99
>99
>99
99
99
>99
>99
AQ
-
-
1
2
3
4
5
1
2
3
5
10
5
1
2
3
5
10
5
19
35
56
>99
>99
64
-
1
1
-
6 b
7 b
10
10
>99
-
a Under the same reaction conditions as described in Figure 2; b 1.3 MPa air.

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Table 2. Effect of reaction time on the catalytic oxidative dehydrogenation of 9,10-
dihydroanthracene to anthracene a.
Catalyst loading
Products and
selectivity(%)
Time Conv.
(mol%)
NaNO2
Entry
(h)
(%)
DDQ
ATC
AQ
1
2
3
4
5
5
5
5
5
5
0
0
0
5
5
5
5
0
5
0
0
1
3
5
8
5
5
5
5
42
47
54
>99
10
8
>99
>99
>99
99
>99
>99
>99
-
-
-
-
1
-
-
-
-
6
7
3
n.d.
8 b
a Under the same reaction conditions as described in Figure 2; b under N2 atmosphere.
In order to further study the process, the influence of reaction time on the catalytic performance of
DDQ/NaNO2 was investigated. As illustrated in Table 2, the conversion of 9,10-dihydroanthracene
increased very rapidly in the initial 1 h (entry 1). When the reaction time was up to 8 h, the conversion
of 9,10-dihydroanthracene reached >99%. In the whole reaction process, the selectivity for anthracene
was extremely high, especially in the initial 1-5 h, and no oxidative by-products were detected (entries
1-3). To identify the characteristics of each components of DDQ/NaNO2 and function of O2, further
study was carried. The reactions were run for 5 h, avoiding the disturbance of further slight oxidation
occurring at long reaction times. It was observed that 54% of 9,10-dihydroanthracene was
dehydrogenated with DDQ/NaNO2 (entry 3). When 5 mol% DDQ was used individually, the
conversion was decreased dramatically from 54% to 10% (entry 5). It suggested NaNO2 can promote
DDQ dehydrogenate 9,10-dihydroanthracene. Only using 5 mol% NaNO2, dehydrogenation reaction
can proceed to a limited extent with 8% conversion (entry 6). Moreover, without DDQ/NaNO2, only a
small amount of anthracene was produced (entry 7). This indicated that 9,10-dihydroanthracene is
difficult to dehydrogenate with NaNO2 or O2. In this system, NaNO2 readily decomposes to NO. NO
can be rapidly oxidized to NO2 in the presence of O2, and the generated NO2 can oxidize 2,3-dichloro-
5,6-dicyano-1,4-hydroquinone (DDQ-H2) to DDQ. Similarly, the quick oxidation of NO to NO2 by O2
was observed in aerobic oxidation of alcohols using TEMPO/Br2/NaNO2 catalyst [20], and also found
in CH4 oxidation with Pd/C/NaNO2/BQ [21]. It cannot proceed without the organocatalytic system in
the absence of O2 (entry 8). This proved the indispensability of O2.
The effect of temperature on the rate of reaction was examined with DDQ (5 mol%) and NaNO2 (5
o
o
mol%) for 8 h (Figure 3). It can be seen that elevating the reaction temperature from 60 C to 120 C
o
o
barely affected the selectivity to anthracene, which remained at high values (>99% at 60 C-100 C,
o
and 99% at 120 C). On the other hand, the conversion of 9,10-dihydroanthracene was significantly
increased when enhancing the reaction temperature. At 120 oC, the conversion of 9,10-
dihydroanthracene achieved >99%. The results indicated that the optimized temperature for oxidative
dehydrogenation with DDQ/NaNO2 was 120 oC.

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Figure 3. Effect of reaction temperature on the oxidative dehydrogenation of 9,10-
dihydroanthracene a.
100
80
60
40
20
0
Conv.
ATC
AQ
60
80
100
o
120
Temperature ( C)
a
0.025 mmol quinone, 0.025 mmol NaNO2; other conditions were the same as
described in Figure 2.
Oxidative dehydrogenation of variety of dihydroarenes with DDQ/NaNO2
To explore the application scope of this organocatalytic system, oxidative dehydrogenation of
variety of dihydroarenes including 9,10-dihydroanthracene, 1,4-cyclohexadiene, acenaphthene, 9,10-
dihydrophenanthrene, 1,2-dihydronaphthalene, and iminodibenzyl by DDQ/NaNO2 were investigated
(Table 3).
a
Table 3. Oxidative dehydrogenation of different dihydroarenes with DDQ/NaNO2 .
Product and
Conversion
Entry Substrates
selectivity/yield
(%)
(%)
1
>99
99/99
2
3
>99
91
>99/99
>99/91
4
5
77
68
>99/77
>99/68
N
6b
32
N
H
H
74/24
a
Reaction conditions: 0.5 mmol substrate; other conditions were the same as
described in Figure 2; b the main by-product cannot be confirmed.

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This organocatalytic system exhibited high selectivity. In the cases of 9,10-dihydroanthracene, 1,4-
cyclohexadiene, and 1,2-dihydronaphthalene, yields to anthracene, benzene, naphthalene were 99%,
99%, and 91%, respectively. For other substrates, moderate yields (around 70%) were achieved under
the same reaction conditions (entries 4, 5). When iminodibenzyl was reacted, it only gave 32%
conversion with 74% selectivity to iminostilbene. It is worthwhile to note that Thummel et al. had
examined the DDQ-induced dehydrogenation rates in the aromatization of 1,4-dihydro-
benzocycloalkenes and 1,4-dihydronaphthocycloalkenes. They provided evidence to support that a
positively charged intermediate was involved in an initial rate-limiting hydride transfer to DDQ. The
intermediate carbonium ion would be destabilized by electron-withdrawing groups and stabilized by
the electron-donating groups, which causes different hydride losing rates [22]. Here the phenomenon
that iminodibenzyl showed low activity was consisting with their findings. It may probably because
the electron-withdrawing effect of a N atom destabilized the intermediate carbonium ion, and thus the
activity of iminodibenzyl was reduced.
The proposed catalytic cycle
Based on the above results, the functions of the DDQ and NaNO2 are as follows. (1) DDQ can
dehydrogenate 9,10-dihydroanthracene to anthracene. (2) NaNO2 is suggested to act as a convenient
nitrogen oxide source, and to readily decompose to NO. NO will rapidly react with O2 to form NO2.
The released NO2 can abstract hydrogen from 9,10-dihydro-anthracene to produce anthracene. Since it
is difficult to dehydrogenate 9,10-dihydroanthracene with NO2, the reaction is kept at low conversion.
In the absence of O2, NO can not be oxidized to NO2, and thus dehydrogenation can not occur. (3) O2
may be responsible for the recovery of NO2 from NO, and NO2 will abstract hydrogen from DDQ-H2
to generate DDQ, and thus the dehydrogenation by DDQ can proceed. In our previous work, the
suggestion was proven by two additional experiments [16]. When 0.5 mmol DDQ and 0.5 mmol 9,10-
o
dihydroanthracene were stirred at 120 C for 8 h without O2, it was found that 99% of 9,10-
dihydroanthracene was converted to anthracene and considerable amounts of DDQ-H2 were detected
by HPLC measurements. This demonstrated that DDQ can stoichiometrically dehydrogenate 9,10-
dihydroanthracene to anthracene. Then 0.5 mmol NaNO2 and 0.5 mmol DDQ-H2 were mixed and
stirred under 1.3 MPa O2, DDQ-H2 was fully oxidized to DDQ. It revealed that in the presence of O2,
NO can be oxidized to NO2, and NO2 can readily oxidize DDQ-H2 to DDQ.
A tentative mechanism for the catalytic cycle is proposed in Scheme 2. The substrate 9,10-
dihydroanthracene is readily dehydrogenated by DDQ to produce anthracene. After abstracting
hydrogen, DDQ is reduced to DDQ-H2. DDQ-H2 is re-oxidized to DDQ again by NO2 generated in-
situ, and NO is produced. In the presence of O2, NO is readily oxidized to NO2. Thus the
dehydrogenation can proceed. The net reaction is: 9,10-dihydroanthracene reacted with O2 to form
anthracene and H2O.

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Scheme 2. The proposed catalytic cycle of DDQ/NaNO2 for oxidative dehydrogenation of
9,10-dihydroanthracene to anthracene.
O
O
H2
O2
Cl
Cl
CN
CN
NO
O
OH
Cl
Cl
CN
CN
NO2
OH
Conclusions
In conclusion, a quinone-based organocatalytic system was developed for the oxidative
dehydrogenation of 9,10-dihydroanthracene with dioxygen under mild conditions. Compared with
other quinones, DDQ combined with NaNO2 showed high efficiency. It gave >99% conversion of
9,10-dihydroanthracene with 99% selectivity for anthracene at 120 °C under 1.3 MPa O2 for 8 h. The
organocatalytic system was also efficient for the oxidative dehydrogenation of variety of
dihydroarenes. Two redox couples – DDQ/DDQ-H2 and NO2/NO – were combined to create a
catalytic cycle in the presence of O2. DDQ/NaNO2 is highly effective and economical. It may be one
of the substitutions for the conventional oxidative dehydrogenation catalyst.
Experimental
General
BQ (99%), TCQ (99%), DDQ(98%), AQ (99%), CAQ (99%), TCAQ (99%), 9,10-dihydro-
anthracene (98%), anthracene (99%), 1,4-cyclohexadiene (97%), acenaphthene (99%), 9,10-dihydro-
phenanthrene (95%), 1,2-dihydronaphthalene (>98%), and iminodibenzyl (97%) were purchased from
J&K Chemical Ltd. NaNO2 (99%) and toluene (99.5%) were purchased from Tianjin Kermel
Chemical Reagent Development Center.
Oxidation reaction and products analysis
The typical catalytic reaction was performed in a 50 mL stainless steel autoclave equipped with a
magnetic stirrer. Toluene (10 mL), 9,10-dihydroanthracene (0.5 mmol), quinine (0.05 mmol), and
NaNO2 (0.05 mmol) were placed in the autoclave. After the desired temperature was reached, O2 was
pressurized (ca. 1.3 MPa) into the reactor and the pressure was kept constant by supplying dioxygen
during the reaction. The oxidation products were identified by Agilent 6890N GC/5973 MS detector
and quantitated by Agilent 4890D GC equipped with FID detector.

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Acknowledgements
We gratefully thank National Natural Science Foundation of China (20603038, 20672111) for the
financial support of this work.
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