Highly efficient oxidation of diphenylmethane to benzophenone employing a novel ruthenium catalyst with tert-butylhydroperoxide under mild conditions

Article abstract of DOI:10.1016/j.catcom.2013.03.031

The ruthenium complex Ru(bpbp)(pydic) (bpbp = 2,6-bis(1-phenylbenzimidazol- 2-yl), pydic = pyridine-2,6-dicarboxy acid) has been synthesized and testedin the selective oxidation of diphenylmethane tobenzophe-none utilizing tert-butylhydroperoxide as the terminal oxidant. The influence of various reaction parameters such as temperature, catalyst amount and nature of solvent on the activity and selectivity was evaluated. Diphenylmethane was converted with 94% conversion and 100% selectivity to benzophenone under the optimized reaction conditions, inwhich the turnover number (TON) of catalyst reached 94,000. Moreover, a plausible reaction mechanism through redox ruthenium species was proposed.

Full text of DOI:10.1016/j.catcom.2013.03.031

Catalysis Communications 37 (2013) 60–63
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly efficient oxidation of diphenylmethane to benzophenone
employing a novel ruthenium catalyst with tert-butylhydroperoxide
under mild conditions
Sheng-Gui Liu a, Xian-Tai Zhou b, Hong-Bing Ji b,
⁎
a
School of Chemistry Science and Technology, Development Center for New Materials Engineering & Technology in Universities of Guangdong, Zhanjiang Normal University, Zhanjiang 524048, PR China
School of Chemistry and Chemical Engineering, Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University, Guangzhou 510275, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The ruthenium complex Ru(bpbp)(pydic) (bpbp = 2,6-bis(1-phenylbenzimidazol-2-yl), pydic = pyridine-
2,6-dicarboxy acid) has been synthesized and tested in the selective oxidation of diphenylmethane to benzophe-
none utilizing tert-butylhydroperoxide as the terminal oxidant. The influence of various reaction parameters
such as temperature, catalyst amount and nature of solvent on the activity and selectivity was evaluated.
Diphenylmethane was converted with 94% conversion and 100% selectivity to benzophenone under the opti-
mized reaction conditions, in which the turnover number (TON) of catalyst reached 94,000. Moreover, a plausible
reaction mechanism through redox ruthenium species was proposed.
Received 13 December 2012
Received in revised form 24 March 2013
Accepted 25 March 2013
Available online 2 April 2013
Keywords:
Diphenylmethane
Benzophenone
Oxidation
© 2013 Elsevier B.V. All rights reserved.
Ruthenium
tert-Butylhydroperoxide
1. Introduction
Most of the above mentioned catalytic systems suffer from disadvantages
such as low efficiency, high reaction temperature, and long reaction time.
The selective oxidation of C\H bonds is one of the most challenging
transformations both in academia and industry [1–3]. As a model of ben-
zylic C\H oxyfunctionalization, the oxidation of diphenylmethane to
benzophenone is gathering much attention, as benzophenone is a key
intermediate for the synthesis of pharmaceuticals, insecticides, photo
initiators, perfumes and also ultraviolet curing agents [4–6]. The oxida-
tive process using stoichiometric quantities of oxidizing agents like
KMnO4, SeO2 and CrO3 has gradually been eliminated due to the gener-
ation of large amount of waste [7–11]. Therefore, the discovery of new
catalysts using clean oxidants such as H2O2 or tert-butylhydroperoxide
(TBHP) is gathering much attention.
So far, various transition metal-based catalysts have been intensively
investigated toward the oxidation of diphenylmethane [12–15]. Kishore
and Rodrigues reported the oxidation of diphenylmethane catalyzed by
Cu–MgAl ternary hydrotalcites at 65 °C with TBHP, which took 24 h
to achieve benzophenone with 95% yield [16]. Mesoporous vanadium
oxide was used for the homogeneous oxidation of diphenylmethane
with TBHP, but low conversion (about 40%) was observed using acetic
acid as solvent at 60 °C [17]. The catalytic activities of other heteroge-
neous catalysts such as Ag/SBA-15 [18], CrSBA-15 [19], MnO2 [20], and
chromium-exchanged zeolite (CrE-ZSM-5) [21] toward the oxidation
of diphenylmethane were examined using TBHP as a terminal oxidant.
Hydrogen peroxide is an environmental benign oxidant, which theoret-
ically generates only water as a by-product [22–24]. Bolm reported
the oxidation of saturated hydrocarbons to alcohols and ketones with
aqueous H2O2 in acetonitrile at room temperature in the presence of
iron catalysts [25]. However, compared with TBHP used in the oxidation
of diphenylmethane, lower conversion of diphenylmethane but excel-
lent selectivity toward benzophenone could be obtained by a Mn Schiff
base complex [26], nickel-coordinated organic nanotubes (Ni-ONTs)
[27] and chromium containing mesoporous MCM-41 molecular sieves
(CrMCM-41) [28] in combination with hydrogen peroxide.
Thus, the development of a catalytic system for the oxidation of C\H
bonds is highly desirable and challenging. Ruthenium complexes with
nitrogen-based ligands have been intensively investigated in order to
develop catalysts for organic oxidation processes and to simulate the
mechanism of bioorganic oxidation [29]. We reported an efficient oxida-
tion process of alcohols catalyzed by ruthenium porphyrins in the pres-
ence of molecular oxygen [30]. Nishiyama first reported the asymmetric
epoxidation by one kind of ruthenium complex based on bis(oxazolinyl)
pyridine, the ruthenium-(pyridinebisoxazoline)(pyridinedicarboxylate)
complex [Ru(pybox)(pydic)] [31]. Inspired by the efficiency of
Nishiyama's catalyst in epoxidation reactions, we adopted the introduc-
tion of 2,6-bis(1-phenylbenzimidazol-2-yl) pyridine as the counterpart
to synthesize a new catalyst with dual closed meridional stereotopes
around an active metal. With diphenylmethane as model substrate,
herein we report a highly efficient procedure for benzylic C\H bond
⁎
Corresponding author. Tel.: +86 20 84113658; fax: +86 20 84113654.
E-mail address: jihb@mail.sysu.edu.cn (H.-B. Ji).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.03.031

S.-G. Liu et al. / Catalysis Communications 37 (2013) 60–63
61
O
3 mmol) was slowly dropped in. Product samples were drawn at regu-
lar time intervals and analyzed by GC and GC–MS.
Ru(bpbp)(pydic)
TBHP,40 oC, 5h
The procedure for the large scale oxidation of diphenylmethane
to benzophenone was: diphenylmethane (10 mmol, 1.68 g) and
Ru(bpbp)(pydic) (1 × 10−3 mmol, 0.732 × 10−3 g) were added into
a reactor. The reactor containing this mixture was heated to 40 °C in
an oil bath under vigorous stirring, and then the aqueous TBHP (70%,
30 mmol) was slowly dropped in. The mixture was stirred for 5 h. After
extraction with n-hexane, the crude product was chromatographed on
silica gel (eluent: n-hexane/ethyl acetate, 1/1, v/v). Pure benzophenone
(8.5 mmol, 1.546 g) was obtained with the yield of 85% by removing
solvent.
Conv.: 94%
Select.: >99%
TON: 94000
N
N
N
N
N
Ru
O
O
3. Results and discussion
N
O
O
3.1. Effects of solvents
Ru(bpbp)(pydic)
Table 1 shows the effect of solvents on oxidation of diphenylmethane
with TBHP over Ru(bpbp)(pydic). After much experimentation on opti-
mizing solvent, it was found that the use of a less-polar solvent like tolu-
ene, benzotrifluoride and dichloromethane afforded benzophenone in
low yields (entries 1–3). High polar and protic solvents like methanol,
ethanol and water were demonstrated to be inefficient (entries 4–6).
Higher yield of benzophenone was obtained using aprotic solvents
like acetonitrile and acetone (entries 7–9). Gratifyingly, the maximum
diphenylmethane conversion of 94% was obtained with the aprotic sol-
vent ethyl acetate (entry 9). Further investigation reveals the amount
of ethyl acetate could also influence the oxidation (entry 10), presumably
due to a lower concentration of the reactant originating from the in-
creased amount of solvent.
Scheme 1. Oxidation of diphenylmethane to benzophenone catalyzed by Ru(bpbp)(pydic).
oxidation by using a novel ruthenium complex Ru(bpbp)(pydic) as cat-
alyst in the presence of TBHP (Scheme 1). This catalyst proved to be ef-
ficient with an extremely high TON (94,000) and excellent selectivity
toward benzophenone.
2. Experimental
2.1. Reagents and methods
Chemicals were of analytical grade and purchased from Aldrich
without further purification unless indicated. Solvents were of analytical
purity and used as received. Mass spectra were obtained on a Shimadzu
LCMS-2010A. Elemental analyses were carried out with an Elementar
vario EL elemental analyzer. 1H NMR was recorded on a Bruker
AVANCE 400 spectrometer (500 MHz). IR spectra were recorded on a
Bruker 550 FT-IR spectrometer. UV spectra were recorded on a
Shimadzu UV-2450 spectrophotometer.
3.2. Effects of reaction temperature
Fig. 1 shows the influence of the reaction temperature on the oxida-
tion of diphenylmethane. The conversion of diphenylmethane was little
influenced by the temperature. For example, 85% of diphenylmethane
was converted to benzophenone when the reaction was carried out
at 30 °C. When increasing the temperature to 60 °C, 95% conversion
was obtained. Higher temperature was unfavorable for the selectivity
toward benzophenone as shown in Fig. 1. When the reaction was
conducted at 50 °C or 60 °C, phenylbenzoate was observed through
Bayer–Villiger oxidation of benzophenone in the presence of TBHP
and Ru(bpbp)(pydic).
2.2. Synthesis of the ruthenium complex
The ligand 2,6-bis(1-phenylbenzimidazol-2-yl) pyridine (bpbp)
was synthesized according to previous procedures [32].
Ru(bpbp)(pydic):bpbp (444 mg, 0.96 mmol), [Ru(p-cymene)
Cl2]2 (300 mg, 0.48 mmol), pyridine-2,6-dicarboxy acid (160 mg,
0.96 mmol) and NaOH (40 mg, 1.0 mmol) were dissolved in EtOH/
H2O (2: 1, 30 mL). The whole reaction mixture was refluxed at
80 °C under N2 atmosphere. The solvent was reduced to 7 mL after
reaction for 2 h. After filtering, the dark violet precipitate was col-
lected (194 mg, 0.34 mmol, yield: 70%). Calc. for C38H24N6O4Ru: C,
62.55; H, 3.32; N, 11.52. Found: C, 62.41; H, 3.42; N, 11.45%; IR
(KBr)/cm−1: 3441, 3056, 1656, 1499, 1467, 1436, 1396, 1318,
1161, 1075, 1011, 768, 736, 697, 579, 476; 1H NMR (500 MHz,
CDCl3): δ 8.56–8.59 (d, 5H), 8.36–8.40 (d, 3H), 7.77 (s, 6H), 7.65(s,
4H) 7.03(s, 2H), 6.91–6.93 (s, 2H), 6.41–6.45 (d, 2H).
3.3. Effects of the amount of TBHP
Fig. 2 shows various amounts of the oxidant employed for the
diphenylmethane oxidation catalyzed by Ru(bpbp)(pydic). The oxi-
dation was tracked with different molar ratios of oxidant/substrate
Table 1
Oxidation of diphenylmethane catalyzed by Ru(bpbp)(pydic) with TBHP.a
Entry
Solvent
Conv. (%)b
Selectivity (%)b
1
Toluene
35
46
43
28
25
26
63
86
94
83
98
>99
96
>99
>99
>99
>99
>99
>99
>99
2
3
4
Benzotrifluoride
Dichloromethane
Methanol
2.3. Catalytic oxidation of diphenylmethane
5
Ethanol
6
Water
7
Acetone
The catalytic oxidation of diphenylmethane was carried out in a
magnetically stirred glass reaction tube fitted with a reflux condenser.
A typical procedure was as follows: diphenylmethane (1 mmol),
Ru(bpbp)(pydic) (1 × 10−4 mmol), and 0.2 mmol naphthalene (inert
internal standard) were solved in ethyl acetate (3 mL) solution. The re-
action tube containing this mixture was heated to 40 °C in an oil bath
under vigorous stirring, and then the aqueous TBHP (70% in H2O,
8
Acetonitrile
Ethyl acetate
Ethyl acetate
9
10c
a
Diphenylmethane (1 mmol), catalyst (1 × 10−4 mmol), solvent (3 mL), TBHP
(3 mmol), 40 °C, 5 h.
b
Determined by GC.
Solvent (5 mL).
c

62
S.-G. Liu et al. / Catalysis Communications 37 (2013) 60–63
100
100
80
60
40
20
0
80
60
40
20
0
30
40
50
60
0
1
2
3
4
5
Temperature/oC
Reaction time/h
Fig. 1. Effect of temperature on the conversion of diphenylmethane (
), selectivity of
Fig. 3. The oxidation of diphenylmethane with different catalyst/substrate molar ratios
over Ru(bpbp)(pydic). (■): 0, (□): 1/100,000, (●): 1/10,000, (▲): 1/1000, (Δ): 1/100,
reaction conditions: diphenylmethane (1 mmol), ethyl acetate (3 mL), TBHP (3 mmol).
benzophenone (
), reaction conditions: diphenylmethane (1 mmol), catalyst (1 × 10−
4
mmol), ethyl acetate (3 mL), TBHP (3 mmol), 5 h.
of 1, 3 and 5. As depicted in Fig. 2, the diphenylmethane conversion
increased with increasing amount of TBHP. Benzophenone with the
yield of 94% was obtained when the molar ratio oxidant/substrate
was 3. Further increasing the amount of oxidant to 5 (molar ratio
of oxidant/substrate), a little of ester product from benzophenone ox-
idation was determined. An excess amount of TBHP could promote
the Bayer–Villiger oxidation of benzophenone to phenylbenzoate.
amount of Ru(bpbp)(pydic) catalyst was 1 × 10−3 mmol, the benzo-
phenone could be obtained with an isolated yield of 85%.
3.5. Plausible mechanism
As the described above, the Ru(bpbp)(pydic)-catalyzed oxidation of
diphenylmethane with TBHP suggests the involvement of radicals. In
order to verify the free radical mechanism, a free radical inhibitor,
2,6-di-tert-butylphenol (0.2 mmol) was added. It was observed that
the running oxidation of diphenylmethane was subsequently quenched.
It is consistent with those of Kishore using M-MgAl ternary hydrotalcites
(M = Cu, Co or Fe) as catalyst [16]. Dakkach reported that Ru complexes
containing the meridional trpy (terpyridine) ligand presented the redox
3.4. Effects of the amount of catalyst
Fig. 3 shows the reaction profiles with different catalyst amounts
for the oxidation of diphenylmethane. Apparently, the reaction al-
most did not occur in the absence of catalyst. The reaction rate was
considerably enhanced when the catalyst was added to the solution,
even though the amount of catalyst was only 0.001 mol% (based on
substrate). The reaction rate increased with the increasing amount
of catalyst. As a result, 94% conversion of diphenylmethane with
100% selectivity toward benzophenone was achieved when the
amount of catalyst was 0.01 mol% (based on substrate). It is worthy
to note that the catalyst turnover number reaches 94,000.
0.5
0.4
Further increasing the amount of catalyst did not enhance the con-
version, on contrary, a slight decrease of the conversion was observed
after 2.5 h. Moreover, as a radical reaction feature, an induction peri-
od was observed clearly when the oxidation was conducted at a low
catalyst loading. The induction period for the oxidation was short-
ened with higher amount of catalyst.
initial
0.3
0.2
0.1
30 min
60 min
90 min
120 min
150 min
180 min
510 nm
Under the optimized reaction conditions, a large scale oxidation of
diphenylmethane was carried out in the presence of TBHP. When the
400
500
600
wavelength/nm
100
initial
Conv.
2.4
2.2
2.0
1.8
1.6
30 min
60 min
90 min
120 min
150 min
180 min
80
Select.
60
40
20
0
337 nm
340
352 nm
A
B
C
320
360
Oxidant/substrate (molar ratio)
A:1/1, B:3/1, C: 5/1
wavelength/nm
Fig. 2. Effect of amount of oxidant on the oxidation of diphenylmethane, reaction condi-
Fig. 4. In situ UV–vis spectra of Ru(bpbp)(pydic) in the oxidation of diphenylmethane
with TBHP.
tions: diphenylmethane (1 mmol), catalyst (1 × 10−4 mmol), ethyl acetate (3 mL), 5 h.

S.-G. Liu et al. / Catalysis Communications 37 (2013) 60–63
63
Ru2+
Ru3+
OH
Ru3+
O OH +
O OH +
O
O
+
+
+
Acknowledgments
Redox cycle
The authors thank the National Natural Science Foundation of China
(no. 21036009 and no. 21176267) and the Natural Science Foundation
of Guangdong Province (no. 10152404801000017) for providing finan-
cial support to this project.
Ru2+
O
+
H
O
O O /
+
O
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Scheme 2. The proposed mechanism for the oxidation of diphenylmethane catalyzed
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