Pd-catalyzed C-H bond activation of benzene in the CO2-expanded solvent

Article abstract of DOI:10.1016/j.catcom.2009.12.019

In the CO₂-expanded trifluoroacetic acid, the catalytic activation and functionalization of the C-H bond in benzene to synthesize biphenyl and phenol was investigated by using palladium catalyst and dioxygen. It was found that an extra 40% improvement of catalytic efficiency could be achieved by charging 30 atm of CO₂. Meanwhile, the presence of the small amount of water would apparently promote the catalytic oxidation. The solvent and substrate isotope effect were also investigated to provide the mechanistic information.

Full text of DOI:10.1016/j.catcom.2009.12.019

Catalysis Communications 11 (2010) 560–562
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Pd-catalyzed C–H bond activation of benzene in the CO -expanded solvent
2
a,b
a
a
a,*
Ping Liang , Hui Xiong , Huajun Guo , Guochuan Yin
a
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Analytical and Testing Center, Huazhong University of Science and Technology, Wuhan 430074, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 October 2009
Received in revised form 10 December 2009
Accepted 11 December 2009
Available online 21 December 2009
In the CO -expanded trifluoroacetic acid, the catalytic activation and functionalization of the C–H bond in
benzene to synthesize biphenyl and phenol was investigated by using palladium catalyst and dioxygen. It
was found that an extra 40% improvement of catalytic efficiency could be achieved by charging 30 atm of
2
CO . Meanwhile, the presence of the small amount of water would apparently promote the catalytic oxi-
2
dation. The solvent and substrate isotope effect were also investigated to provide the mechanistic
information.
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
C–H bond activation
Benzene
Palladium
CO -expanded solvent
2
1
. Introduction
2
there has been no reported example to introduce the CO -ex-
panded solvent system to the transition metal catalyzed electro-
philic oxidation, a selective and widely preferred process in
Catalytic activation and functionalization of the C–H bond in
the robust substrates like benzene has been a long challenge in
chemistry [1]. Due to the substantial barriers in the direct catalytic
synthesis, their products like phenol and biphenyl are generally
obtained through the indirect routes [2]. Recently, the palladium-
catalyzed direct synthesis of biphenyl and phenol from benzene
2
organic synthesis. In the present work, the CO -expanded liquid
medium has been first applied in the Pd(II)-catalyzed C–H bond
activation in benzene to synthesize biphenyl and phenol through
an electrophilic pathway, in which the enhanced oxygen solubility
and mass transportation provide a new approach for accelerating
the regeneration of the active Pd(II) species.
2 2 8
with oxygen or K S O oxidant has been investigated indepen-
dently by different groups [3–6]. In order to accelerate the regen-
eration of the active Pd(II) species from the reduced Pd(0) by
oxygen, versatile co-catalysts including simple metal ions and het-
eropolyacids were widely tested. However, due to the low solubil-
ity of oxygen in the reaction medium and the relatively low
oxidation rate of Pd(0) by oxygen or other co-catalysts, the regen-
erations of the Pd(II) species were not always satisfied.
Recently, a CO -expanded solvent system was introduced and
2
applied in oxidation of substituted phenols and toluene [7,8]. In
both cases, a radical chain process dominates the reactions. In
2
. Experimental section
2.1. General procedure for catalytic C–H activation
In a general oxidation reaction, Pd(OAc)
2 2
(0.05 mmol), Cu(OAc)
(
0.2 mmol), and benzene (0.5 mL) were added in trifluoroacetic
acid/water (5:1, 6 mL) in a glass tube equipped with a glass lid with
several holes for gas diffusion. The glass tube was put into a 50 mL
stainless autoclave, then, the autoclave was charged with CO
40 atm) first, then oxygen (20 atm). The charged autoclave was
2
comparison with the routine organic solvents, the CO
solvent has demonstrated its remarkable advantages including
1) high oxygen solubility and mass transportation of reactants in
liquid phase, (2) CO tunable polarity of the reaction medium facil-
itating versatile requirements of reactions, (3) enhanced protection
of fire and explosion because of the large existence of inert CO
and (4) replacement of organic solvent by expanding CO repre-
senting a green process. To our knowledge, previous to this work,
2
-expanded
(
stirred at 84 °C in an oil bath (temperature 95 °C) for 16 h. The
product analysis was performed by H NMR with an internal stan-
dard (cyclohexene). The doublet at d = ꢀ7.6 ppm was selected for
quantitative analysis of biphenyl, and the doublet of doublets at
d = 6.7–6.8 ppm were used for quantitative analysis of phenol.
(
1
2
2
,
2
2
2.2. Catalytic C–H activation of benzene using D O as water source
The reaction medium was prepared by adding deuterated water
2
O, 0.5 mL) into trifluoroacetic anhydride (3.67 mL) slowly to
*
Corresponding author. Tel.: +86 27 87543732; fax: +86 27 87543632.
E-mail address: gyin@mail.hust.edu.cn (G. Yin).
(D
1
566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.12.019

P. Liang et al. / Catalysis Communications 11 (2010) 560–562
561
generate the deuterium labeled trifluoroacetic acid (TFA-d) sol-
vent, then Pd(OAc) (0.05 mmol), Cu(OAc) (0.2 mmol), and ben-
zene (0.5 mL) were added in the resulting solvent in a glass tube
equipped with a glass lid with several holes for gas diffusion. The
glass tube was put into a 50 mL stainless autoclave, and the auto-
Table 2
The influence of water content in the CO
2
-expanded solvent.a
Phenol (mmol)
2
2
Run
H O (mL)
2
Biphenyl (mmol)
Total yield (%)b
1
2
3
4
5
Non
0.25
0.5
1.0
2.0
0.19
0.25
0.30
0.47
0.33
0.17
0.13
0.11
0.07
0.10
10.1
11.2
16.5
17.9
13.5
clave was charged with CO
2
(40 atm) first, then oxygen (20 atm).
The charged autoclave was stirred at 84 °C in an oil bath (temper-
1
ature 95 °C) for 16 h. The product analysis was performed by
H
a
NMR with an internal standard sample.
Conditions: TFA (5 mL), Pd(OAc)
0.5 mL), CO (40 atm), O (20 atm), 84 °C, 16 h.
Based on benzene added.
2
(0.05 mmol), Cu(OAc)
2
(0.2 mmol), benzene
(
2
2
b
3
. Results and discussion
0
.47 mmol by adding 1 mL of water to the total 5 mL of TFA sol-
The catalytic activation and functionalization of the C–H bond
in benzene using CO -expanded solvent was generally investigated
with simple palladium(II) acetate catalyst and oxygen as the termi-
nal oxidant. To enhance the electrophilic attack capability of the
Pd(II) species, trifluoroacetic acid (TFA) was selected as the organic
vent, and 17.9% of the total yield could be achieved, even that
the yield of phenol dropped from 0.17 mmol to 0.07 mmol. When
2
2
.0 mL of water were added in 5 mL TFA, yield of biphenyl dropped
to 0.33 mmol with the total yield of 13.5%, while the yield of phe-
nol slightly increased up to 0.1 mmol. In that case, it became a bi-
phasic system upon the addition of benzene, which apparently
disturbed the reaction. Thus, a water content tuning the ratio of
biphenyl/phenol products was observed for this Pd/Cu catalyzed
C–H bond activation. Such a phenomenon is very rare in the elec-
trophilic C–H activation process, since it is generally believed that
the presence of water would block the attack of the late transition
metal ions like Pd(II) on the unfunctional substrates [12,13]. A
plausible explanation for the role of water would be that the pres-
ence of the reasonable amount of water could promote the disso-
ciation of TFA to increase the proton concentration which plays
the role in re-oxidation of Pd(0) to the active Pd(II) by dioxygen.
composition of this CO
summarized in Table 1. At 84 °C, using Pd(OAc)
charged 40 atm of CO and 20 atm of oxygen, it provided
.16 mmol of biphenyl, 0.047 mmol of phenol products, and the
2
-expanded medium. The reaction data are
2
catalyst with
2
0
corresponding total yield was 6.5% based on benzene substrate.
At the end of the reaction, a palladium black precipitate was gen-
erally observed, indicating the deactivation of catalyst. In order
to accelerate the regeneration of the active Pd(II) species, Cu(OAc)
2
was selected as the co-catalyst, because it had already demon-
strated its efficient capability in re-oxidizing Pd(0) to the Pd(II)
species in Wacker process [9]. Indeed, addition of the Cu(II) ion
could improve the yield of phenol (0.17 mmol) substantially and
the total yield went up to 10.1% from 6.5%. An alternative choice
to prevent palladium black formation is to add certain ligands to
the Pd(II) ion [10]. However, the selected ligand, (+)-sparteine,
did not prevent the aggregation of the Pd(0) atom, otherwise,
bipyridine (bpy) or 1,10-phenannthroline (phen) caused complete
loss of catalytic activity although the aggregation was inhibited. In
the partial oxidation of methane to methanol, trifluoroacetic anhy-
dride (TFAA) was introduced to convert methanol into methyl ester
for preventing its further oxidation [11]. At the early consideration
of avoiding further oxidation of phenol and removing the water
Fig. 1 displays the influence of CO
activity. Charging of CO to generate a CO
remarkably improved the catalytic efficiency. For examples, with-
out CO charged, the yield of biphenyl and phenol were only
.34 mmol and 0.05 mmol, respectively, with the total yield of
2.9%, even that 1 mL of water have been added to accelerate the
were charged, 0.48 mmol of biphe-
2
pressure on the catalytic
2
2
-expanded medium
2
0
1
reaction. When 30 atm of CO
2
nyl and 0.063 mmol of phenol could be achieved with the total
yield of 18.1%, indicating an extra 40% improvement of catalytic
activity by simply charging CO
the CO -expanded TFA is not currently available because of the
equipment limit, a similar system, that is, the CO -expanded acetic
2
. Although the phase behavior of
2
generated during the reaction, little TFAA were added for esterifi-
1
2
cation of phenol product. Indeed, no phenol was detected by
H
2
acid, has demonstrated improved dioxygen solubility and CO -ex-
panded behavior [7]. Thus, it can be expected that the increased
NMR in the reaction mixtures as expected, while the yield of biphe-
nyl remained unchanged. However, only minor ester products
1
were monitored by H NMR. Because the considerably high yield
of phenol (0.17 mmol) was obtained in the absence of TFAA, the
addition of TFAA had apparently compressed the phenol formation.
After realizing the negative effect of TFAA in phenol formation,
biphenyl
phenol
0.6
total yield
2
the influence of water content in the CO -expanded solvent was
1
8
investigated and the results are listed in Table 2. Obviously, the
presence of reasonable amount of water would accelerate the cat-
alytic reaction. The yield of biphenyl increased from 0.19 mmol to
0.5
0.4
16
14
12
10
0.3
Table 1
-expanded medium.a
2
The results for catalytic C–H activation in benzene in the CO
Total yield (%)c
0.2
0.1
Run
Additives
–
Biphenyl (mmol)
Phenol (mmol)
1
2
3
0.16
0.19
0.19
0.16
Non
Non
0.047
0.17
Non
0.098
Non
6.5
10.1
6.7
7.4
Non
Non
Cu(OAc)
Cu(OAc)
2
b
2
4
5
6
(+)-Sparteine
bpy
phen
0.0
0
10
20
30
40
50
60
Non
CO pressure (atm)
2
a
Conditions: Pd(OAc)
2
(0.05 mmol), Cu(OAc)
2
(0.2 mmol), or organic ligand
(20 atm), 84 °C, 16 h.
(
0.1 mmol), TFA (5 mL), benzene (0.5 mL), CO
2
(40 atm), O
2
Fig. 1. The influence of CO
2
pressure on the catalytic activity. Conditions: TFA
5 mL), water (1 mL), Pd(OAc) (0.05 mmol), Cu(OAc) (0.2 mmol), benzene
0.5 mL), O (20 atm), 84 °C, 16 h.
b
c
TFA (4 mL), TFAA (1 mL).
Based on benzene added.
(
(
2
2
2

5
62
P. Liang et al. / Catalysis Communications 11 (2010) 560–562
Table 3
Proton isotopic effect in the palladium catalyzed C–H bond activation in benzene.
OH
HOAc
a
H
2
O
HOAc
II
II
+
Pd(0)
Pd(0)
+
Pd (OAc)
2
Pd (OAc)
Run
TFAA (mL)
Proton source
Biphenyl (mmol)
Phenol (mmol)
1
2
3
4
3.86
3.67
4.42
4.38
H
2
O, 0.5 mL
O, 0.5 mL
O, 1.5 mL
O, 1.5 mL
0.16
0.16
0.27
0.28
0.17
0.065
0.13
D
2
H
2
D
2
0.027
HOAc
a
Conditions: Pd(OAc)
2
(0.05 mmol), Cu(OAc)
2
(0.2 mmol), benzene (0.5 mL), CO
2
PdII
+
(
40 atm), O (20 atm), 84 °C, 16 h.
2
Scheme 1. Proposed mechanism for Pd-catalyzed functionalization of benzene to
generate biphenyl and phenol.
oxygen concentration and improved mass transportation by charg-
ing a moderate CO pressure would accelerate the catalyst regen-
eration here. However, the solvent polarity would decrease with
the charging of CO , causing the negative effect on catalyst regen-
eration and C–H bond activation. Thus, the over-charged CO pres-
2
yield whereas it promotes the biphenyl formation, the Ph–
Pd(II)(OAc) intermediate prefers activating the second benzene to
generate the Ph–Pd(II)–Ph intermediate rather than producing
phenol in the presence of water.
In summary, this work demonstrated a moderately charged CO
pressure would substantially improve the Pd-catalyzed C–H bond
activation, and the presence of water would accelerate the oxida-
2
2
sure led to the dropping of the total catalytic efficiency, even that
the yield of phenol kept slightly improved as shown in Fig. 1. Obvi-
2
ously, only a moderate pressure of CO
catalytic efficiency.
2
is essential to improve the
In order to further investigate the role of water in the formation
of biphenyl and phenol, solvent isotope labeling experiments were
conducted by using D O as the proton source (Table 3). Although
2
the addition of water could improve biphenyl yield as shown in Ta-
ble 2, one may see that the formation of biphenyl is not influenced
by the proton source, indicating that water may not be involved in
the rate determining step of biphenyl formation; while a normal
solvent isotope effect is observed for phenol formation. In Table 3,
replacing the normal water by water-d apparently delays phenol
2
tion process. The novel application of the CO -expanded solvent
in this work provides a new approach to accelerate those transition
metal mediated electrophilic oxidations, and meanwhile, to reduce
the usage of the toxic organic solvents for green chemistry.
Acknowledgments
Supported by the new faculty fund from Huazhong University
of Science and Technology is deeply appreciated. Product analysis
formation (yield of 0.17 mmol for H
run 1 and 2, kH2 =kD2
¼ 2:6; 0:13mmol for H
for D O in run 3 and 4, k =k
transfer was involved in the transition state of phenol formation
as well as those in the other reactions [14], and apparently, the sol-
vent isotope effect is water content tunable. In addition, no incor-
poration of deuterium into benzene substrate was observed by 2H
2
O vs 0.065 mmol for D
O vs 0.027 mmol
¼ 4:8), suggesting that proton
2
O in
1
by H NMR and GC–MS were performed in Analytical and Testing
O
O
2
Center, Huazhong University of Science and Technology.
2
H
2
O
2
D O
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1
1