A novel Pt/C-catalyzed transfer hydrogenation reaction of p-benzoquinone to produce p-hydroquinone using cyclohexanone as an unexpectedly effective hydrogen source

Article abstract of DOI:10.1002/aoc.4505

A new method for the production of p-hydroquinone via a Pt/C-catalyzed reduction of p-benzoquinone is developed. Different from the conventional transfer hydrogenation reactions that usually use secondary alcohols such as isopropanol as the hydrogen source, in this work, it is unexpectedly found that cyclohexanone is a more effective hydrogen source than secondary alcohols, even cyclohexanol. This reaction affords acceptable yields of p-hydroquinone with very high turnover number (1109) of the Pt/C catalyst. A mechanism of this interesting reaction is proposed on the basis of the results of a series of control experiments, GC–MS analysis as well as dynamic studies.

Full text of DOI:10.1002/aoc.4505

Received: 6 March 2018
DOI: 10.1002/aoc.4505
Revised: 12 June 2018
Accepted: 12 June 2018
F U L L P A P E R
A novel Pt/C‐catalyzed transfer hydrogenation reaction
of p‐benzoquinone to produce p‐hydroquinone using
cyclohexanone as an unexpectedly effective hydrogen source
1
,2
1,3
3
1
1
Fang Wang | Lin Xu | Cheng Sun | Lei Yu
| Qing Xu
1
Guangling College, School of Chemistry
A new method for the production of p‐hydroquinone via a Pt/C‐catalyzed
reduction of p‐benzoquinone is developed. Different from the conventional
transfer hydrogenation reactions that usually use secondary alcohols such as
isopropanol as the hydrogen source, in this work, it is unexpectedly found that
cyclohexanone is a more effective hydrogen source than secondary alcohols,
even cyclohexanol. This reaction affords acceptable yields of p‐hydroquinone
with very high turnover number (1109) of the Pt/C catalyst. A mechanism of
this interesting reaction is proposed on the basis of the results of a series of
control experiments, GC–MS analysis as well as dynamic studies.
and Chemical Engineering, Yangzhou
University, Yangzhou, Jiangsu 225009,
China
2
Yangzhou Polytechnology Institute,
Yangzhou, Jiangsu 225127, China
3
Jiangsu Yangnong Chemical Group Co.
Ltd, Yangzhou, Jiangsu 225009, China
Correspondence
Lei Yu and Qing Xu, Guangling College,
School of Chemistry and Chemical
Engineering, Yangzhou University,
Yangzhou, Jiangsu 225009, China.
Email: yulei@yzu.edu.cn; xuqing@yzu.
edu.cn
KEYWORDS
cyclohexanone, hydroquinone, p‐benzoquinone, platinum, transfer hydrogenation
Funding information
Top‐notch Academic Programs Project of
Jiangsu Higher Education Institutions;
Nature Science Foundation of Guangling
College, Grant/Award Number: No.
ZKZD17005; Priority Academic Program
Development of Jiangsu Higher Education
Institutions; NNSFC, Grant/Award Num-
ber: 21202141
1
| INTRODUCTION
methods that can avoid the generation of solid waste or
those can be performed under much safer conditions
Hydroquinone (HQ) is a significant industrial intermedi-
ate being abundantly and extensively used in the prepara-
tion of electrode materials, black‐and‐white developing
agents, anthraquinone and azo dyes, rubber antioxidants,
are of great value from the industrial production
viewpoint.
Transfer hydrogenation reactions using certain
organic molecules as hydrogen sources have also
attracted much attention of chemists, and have been
comprehensively employed in both academia and indus-
try, attributed to the high efficiencies of many hydrogen
[1]
stabilizers, etc. In industry, it is conventionally pro-
duced by reduction of p‐benzoquinone (BQ) with iron
[2]
powder or gaseous hydrogen as the reductant. How-
ever, the former method inevitably leads to the genera-
tion of large amounts of iron mud, a solid waste of
which it is difficult to dispose; whereas the latter method
suffers from potential dangers due to the use of flamma-
ble and explosive gaseous hydrogen. Therefore, new
[
3,4]
donors and transfer hydrogenation reactions.
Many
transfer hydrogenation reactions such as coal liquefaction
with tetralin can be performed under mild and safe con-
[
5]
ditions, and at industrial‐level production. During our
continuing works in developing new catalytic methods
Appl Organometal Chem. 2018;e4505.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
1 of 6
https://doi.org/10.1002/aoc.4505

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WANG ET AL.
[
6]
of potential application in industry, we developed a Pt/
C‐catalyzed method for transfer hydrogenation of mesityl
oxide to methyl isobutyl ketone using isopropanol as the
cyclohexanone/BQ = 9.0 and 0.055 mol% of Pt/C, as we
initially employed, should be the preferable reaction con-
ditions (Table 2, entry 2 versus entries 1 and 3). Reducing
and enhancing the amount of Pt catalyst both led to
decreased HQ yield (Table 2, entry 2 versus entries 4
and 5). HQ began to be generated at a temperature higher
than 60 °C (Table 2, entries 7 versus 6). Although the
reaction at 80 °C afforded higher HQ selectivity than that
at 142 °C, the BQ conversion ratio was low (Table 2,
entry 7). It was found that 100 °C was the most favorable
reaction temperature, and BQ was fully converted to pro-
duce HQ in increased yield (Table 2, entries 8 versus 9). It
should also be pointed out that, beside the cyclohexanone
dehydrogenation product cyclohex‐2‐en‐1‐one (1),
[7]
hydrogen source. More recently, Pt/C was also found
to be a good catalyst for transfer hydrogenation of BQ to
produce HQ. However, interestingly, different from
[7,8]
known methods,
in this novel Pt/C‐catalyzed reaction,
cyclohexanone was unexpectedly found to be a much
more effective hydrogen donor than the commonly used
secondary alcohols. Herein, we report the details of this
work.
2
| RESULTS AND DISCUSSION
4‐(cyclohexa‐1,5‐dien‐1‐yloxy) phenol (2) and (1,1′‐bi
The Pt‐catalyzed reduction reaction of BQ with
isopropanol as the hydrogen donor was initially tested,
but unfortunately only 5.1% of BQ was converted, while
HQ, the desired product, was not detected (Table 1,
entry 1). Experiments with isooctanol as an alternative
hydrogen source were then conducted, but still no HQ
was generated, even at elevated reaction temperature
(cyclohexan))‐3‐ene‐2,2′,5‐trione (3) were also observed
in GC–MS analysis as the byproducts of the reaction
(Figure 1). Thus, further efforts were made to enhance
the selectivity and yield of HQ through inhibiting the
generation of the byproducts.
a
TABLE 2 Condition optimizations
(Table 1, entries 2 and 3). We were glad to find that the
reaction with cyclohexanol led to almost full conversion
of BQ, and HQ was generated in 18.8% yield (Table 1,
entry 4). Surprisingly, the reaction with cyclohexanone,
which has rarely been employed as hydrogen source,
afforded even higher HQ yield (32.8%) owning to the
increased reaction selectivity (Table 1, entry 5). The excel-
lent performances of cyclohexanone over other regular
hydrogen donors may be attributed to a completely new
reaction mechanism (vide infra).
Cyc/
BQ
Pt
(%)
Conv.
(%)
Sel.
(%)
Yield
(%)
Entry
t (°C)
1
2
3
4
5
6
7
8
9
4.5
9.0
15.0
9.0
9.0
9.0
9.0
9.0
9.0
0.055
0.055
0.055
0.025
0.11
142
142
142
142
142
60
97.6
100
32.4
32.8
33.6
30.4
30.8
0
31.6
32.8
33.6
30.0
30.8
0
100
98.7
100
Parallel experiments were then performed to further
optimize the reaction conditions. First, considering the
full conversion of BQ and the cost of the reaction, using
0.055
0.055
0.055
0.055
15.4
52.7
100
80
42.2
36.6
36.4
22.2
36.6
36.4
TABLE 1 Transfer hydrogenation of BQ with various hydrogen
100
120
a
donors
100
a
50 mmol of BQ was employed. Abbreviations: Cyc/BQ = molar ratio of
cyclohexanone versus BQ, Pt = Pt molar ratio on the basis of BQ amount,
conv. = BQ conversion, sel. = HQ selectivity, yield = HQ yield; these results
were obtained using HPLC and calculated through the external curve.
Hydrogen
Entry donor
t
Conv.
Sel.
(%)
Yield
(%)
(°C) (%)
1
2
3
4
5
Isopropanol
Isooctanol
80
80
5.1
0
0
0
0
6.1
16.8
99.3
Isooctanol
142
142
142
0
0
Cyclohexanol
Cyclohexanone
18.9
32.8
18.8
32.8
100
a
50 mmol of BQ, 450 mmol of hydrogen donor and 540 mg of Pt/C (contains
1
wt% of Pt) were employed. Abbreviations: conv. = BQ conversion, sel. =
HQ selectivity, yield = HQ yield; these results were obtained using HPLC
and calculated through the external curve.
FIGURE 1 Observed byproducts of the reaction (detected using
GC–MS)

WANG ET AL.
3 of 6
a
The reaction was then optimized by tuning the pH
3
TABLE 4 Screening of amount of (n‐Bu) N
values of the solution with HOAc or (n‐Bu) N. It was found
that both acid and base conditions facilitated the process to
3
(
(%)
n‐Bu)
3
N/BQ
Conv.
(%)
Sel.
(%)
Yield
(%)
Entry
enhance the BQ conversion (Table 3, entries 1 and 2 versus
1
2
3
4
5
6
7
1.7
82.3
99.8
100
41.4
61.1
60.5
60.3
58.2
56.1
52.5
34.1
61
3
; 4 versus 3). Among these conditions, the weak alkaline
3.3
8.3
condition with pH = 7.5 was preferable, affording almost
full conversion of BQ and enhanced HQ selectivity and
yield at 55.8 and 55.7% respectively (Table 3, entries 4 ver-
sus 1–3). Other bases, including both inorganic and
organic bases, were also tested to adjust the reaction pH,
60.5
60.2
58
16.6
33.3
66.6
99.8
99.6
99.5
99.4
55.8
52.2
and (n‐Bu) N, as we initially employed, was found to be
3
100
the favorable one (Table 3, entries 4 versus 5–9). The ligand
effect of organonitrogen bases may affect the catalytic
activity of Pt and lead to different results even at the same
pH value (Table 3, entries 4–9).
a
Reaction was performed under the optimized conditions described in
Table 3, entry 4 (except for amount of base). Abbreviations: conv. = BQ con-
version, sel. = HQ selectivity, yield = HQ yield; these results were obtained
using HPLC and calculated through the external curve.
Reactions with various amounts of (n‐Bu) N were con-
3
ducted (Table 4, entries 1–7). With 1.7 mol% of (n‐Bu) N,
3
HQ was afforded in 34.1% yield (41.4% selectivity), while
a
TABLE 5 Comparison of catalytic metals
8
2.3% of BQ was converted within 2 h (Table 4, entry 1).
Doubling the amount of base led to almost full conversion
99.8%) of BQ and the selectivity of HQ was enhanced to
1.1% (Table 4, entry 2). The reaction was hardly improved
by increasing the amount of base, and 3.3 mol% of
n‐Bu) N was finally screened to be the most favorable
Conv.
(%)
Sel.
(%)
Yield
(%)
Entry Catalyst
TON
(
6
1
2
3
1% Pt/C (0.055 mol%) 99.8
61.1
36.1
53.6
61
1109
3.9
CuCl
Raney nickel
18.4 mol%)
5% Pd/C (0.5 mol%) 99.6
2
(8 mol%)
86.9
31.4
53.4
(
3
99.7
2.9
amount of base (Table 4, entry 2 versus entries 3–7).
Furthermore, the reactions with Pt, Cu, Ni and Pd cat-
alysts were simultaneously performed for comparison
(
4
59.2
59
118
a
(Table 5, entries 1–4). With the same weight (540 mg) of
Reaction was performed under the optimized conditions described in
Table 3, entry 4 (except for the catalyst). Abbreviations: conv. = BQ conver-
sion; sel. = HQ selectivity; yield = HQ yield; TON = turnover number of the
metal catalyst; these results were obtained using HPLC and calculated
through the external curve.
Pt/C, CuCl , Raney nickel and Pd/C as catalyst, the trans-
2
fer hydrogenation reactions led to BQ conversions of 99.8,
8
6.9, 99.7and 99.6% and HQ selectivities of 61.1, 36.1, 53.6
a
TABLE 3 Screenings of additives for the reaction
and 59.2, respectively (Table 5, entries 1–4). Among the
four catalysts, Pt/C was undoubtedly the most active
one, affording an overwhelmingly high reaction turnover
number (TON = 1109) over the other three catalysts
Conv.
(%)
Sel.
(%)
Yield
(%)
(
Table 5, entry 1 versus entries 2–4). Scanning electron
Entry
pH
Additive
microscopy images of the Pt/C catalyst (given in
supporting information) showed that the materials pos-
sessed nanoscale structures, which provided large specific
surface area and anchored Pt in the materials as the cat-
alytically active reaction sites for the hydrogenation
1
2
3
4
5
6
7
8
9
2.5
4.5
6.0
7.5
7.5
7.5
7.5
7.5
7.5
HOAc
HOAc
HOAc
80.6
69.2
55.4
99.9
65.0
58.3
63.9
99.8
99.9
49.6
46.2
41.0
55.8
41.3
36.1
46.1
53.1
54.9
40.0
32.0
22.7
55.7
26.8
21.0
29.5
53.0
54.8
(n‐Bu)
NaHCO
NH ⋅H
(i‐C
Pyridine
3‐Picoline
3
N
[
9]
reactions.
3
The mechanisms of this interesting transfer hydroge-
nation reaction were our next concern. In order to get
more information for a mechanism study, a control
experiment under the optimized reaction conditions
described in Table 3 but without base was performed
3
2
O
8
H
17
)
3
N
(
Scheme 1). It was found that the reaction was obviously
a
50 mmol of BQ, 450 mmol of cyclohexanone and 540 mg of Pt/C (contains
retarded (Scheme 1 versus Table 3, entry 4) and the BQ
conversion fell to 35.1% at 2 h, while both HQ selectivity
and yield decreased as well (Scheme 1).
1
wt% of Pt) were employed. Abbreviations: conv. = BQ conversion, sel. =
HQ selectivity, yield = HQ yield; these results were obtained using HPLC
and calculated through the external curve.

4
of 6
WANG ET AL.
SCHEME 1 Control experiment
Dynamic studies were performed to further disclose
the role of the base in the process. In the presence of
(
(
n‐Bu) N, BQ was completely converted within 2 h
3
Figure 2; pH = 7.5); whereas, in the absence of (n‐Bu) N,
3
it took 6 h for the same reaction to complete the full con-
version of BQ (Figure 2; pH = 6.2). Comparison of the
experimental results in Scheme 1 with those in Table 3
(entry 4) as well as the dynamic study results illustrated
in Figure 2 showed that base was crucial for the reaction
to convert BQ quickly.
FIGURE 3 Comparison of HQ selectivity in parallel reactions
with/without base
Moreover, besides BQ conversion, addition of base
could also enhance the HQ selectivity of the reaction.
As shown in Figure 3, the HQ selectivity of the reaction
with base quickly reached its peak within 2 h (Figure 3;
pH = 7.5), while it took a longer time for the reaction
without base to afford the highest HQ selectivity
(Figure 3; pH = 6.2). Therefore, considering the fact that
the reaction without base required prolonged reaction
time and led to decreased HQ selectivity, it was supposed
that base might not only accelerate but also direct the
reaction steps during the processes.
A series of parallel experiments were then conducted
at 30–110 °C under the optimized conditions (pH = 7.5;
Table 3, entry 4) to investigate the effect of the reaction
temperature. As shown in Figure 4, although BQ could
be completely converted at low temperatures such as
3
0 °C, the selectivity of the product could be enhanced
by elevating the reaction temperature. This phenomenon
FIGURE 4 Effect of reaction temperature
suggested that this reaction might involve multiple steps
or directions and the higher reaction temperature facili-
tated the reduction of BQ to HQ.
Based on above experimental results as well as litera-
[
10–12]
ture reports,
Scheme 2). The first step of the reaction may occur with
the conversion of cyclohexanone into its enol form 4
equation (1) in Scheme 2). This step can be facilitated
a plausible mechanism is proposed
(
(
by both a base (Figures 2 and 3) and an acid (Table 3,
entries 12 versus 3), but clearly the base is more prefera-
ble (Table 3, entries 4 versus 1 and 2). Attack of the base
to the enol proton destroyed the O─ H bond and its elec-
tron pair shifted to the C─ O to form a carbonyl, while
the π‐electrons in the enol C═ C transferred to Pt to form
[10]
FIGURE 2 Comparison of BQ conversion in parallel reactions
the C─ Pt bond and generated the intermediate 5.
The
with/without base
generation of intermediate 5 could be confirmed by the

WANG ET AL.
5 of 6
SCHEME 2 Possible mechanism of the
transfer hydrogenation reaction
observed byproduct 3 (Figure 1), which was produced via
the Michael addition of 5 with BQ (equation (2) in
4 | EXPERIMENTAL PROCEDURES
[11]
Scheme 2).
The dehydrogenation of 5 via β‐hydrogen
provided the key species 6 with Pt─ H
4.1 | General Methods
[12]
elimination
Reagents were purchased from suppliers with purities of
more than 98% and were directly used as received.
Solvents were analytically pure (AR) and directly used
without any special treatment. HPLC analysis was per-
formed using an Agilent HPLC 1260 instrument with an
Agilent Eclipse XDB‐C18 chromatographic column
bonds as well as the cyclohexenone byproduct 1. This step
is confirmed by the observation of 1 in GC–MS spectra of
the reaction mixtures (Figure 1). The byproduct 1 might
further react with the product HQ to give the byproduct
2
(equation (3) in Scheme 2), which was also observed
in the GC–MS spectra (Figure 1). Formation of the
byproducts 2 and 3 (equations (2) and (3) in Scheme 2)
may account for the high conversion ratio of BQ but mod-
erate selectivity for HQ in the reaction; consequently, a
moderate reaction temperature (such as 100 °C) is
required to inhibit the side reactions and ensure the
highest yield of HQ (Figure 4). Finally, reduction of BQ
by 6 might afford the target product HQ and regenerate
the active Pt/C catalyst.
(
4.6 × 250 mm × 5 μm) using chromatographically pure
MeCN as the mobile phase solvent. GC–MS analysis
was performed with a Thermo Fisher Trance 1300‐ISQ
LT instrument.
4
.2 | Experimental Details for Transfer
Hydrogenation Reaction of BQ to Produce
HQ (Typical Example for the Reaction in
Table 3, Entry 4)
3
| CONCLUSIONS
Amounts of 50 mmol of BQ, 450 mmol of cyclohexanone,
1
.65 mmol of (n‐Bu) N (3.3%) and 540 mg of Pt/C (1 wt%
3
We report an efficient Pt/C‐catalyzed transfer hydrogena-
tion reaction of BQ to produce HQ, which is a very
important industrial intermediate. Different from the pre-
viously reported transfer hydrogenation reactions, this
method employs cyclohexanone as the hydrogen source
instead of the classical secondary alcohols such as
isopropanol. The starting material BQ was almost fully
converted to produce the desired HQ in acceptable yield.
Investigations of the mechanisms and applications of this
novel reaction are ongoing in our laboratory.
Pt) were initially added into a 100 ml round‐bottom flask.
The mixture was heated at 100 °C for 2 h and sent for
HPLC analysis. The detailed data tables for the figures
in the text are given in the supporting information.
ACKNOWLEDGMENTS
The work was financially supported by NNSFC
(21202141), Priority Academic Program Development of
Jiangsu Higher Education Institutions, Top‐notch

6
of 6
WANG ET AL.
Academic Programs Project of Jiangsu Higher Education
Institutions and Nature Science Foundation of Guangling
College (no. ZKZD17005). We thank the testing center of
Yangzhou University for assistances in analysis. We
thank Changzhou Chen for assistances in experiments.
Zhang, X. Zhang, Y. Ding, Q. Xu, Cat. Sci. Technol. 2016, 6,
1804.
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How to cite this article: Wang F, Xu L, Sun C,
Yu L, Xu Q. A novel Pt/C‐catalyzed transfer
hydrogenation reaction of p‐benzoquinone to
produce p‐hydroquinone using cyclohexanone as
an unexpectedly effective hydrogen source. Appl
Organometal Chem. 2018;e4505. https://doi.org/
10.1002/aoc.4505
3
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