Direct C-H bond activation of ethers and successive C-C bond formation with benzene by a bifunctional palladium-titania photocatalyst

Article abstract of DOI:10.1039/c5cy02290h

Palladium-loaded titanium oxide was found to work as a bifunctional photocatalyst for functionalization of benzene with ether upon photoirradiation, without using any special reagents. The metal-loaded TiO₂ photocatalyst activated a C-H bond of ethers and the heterogeneous Pd metal nanoparticle catalyst promoted the successive C-C bond formation between benzene and the radical species. In this reaction, benzene reacted very selectively with the α-carbon of various ethers at least in the initial stage of the reaction. Kinetic and ESR studies revealed a detailed mechanism for the reaction.

Full text of DOI:10.1039/c5cy02290h

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Direct C–H bond activation of ethers and
successive C–C bond formation with benzene by a
Cite this: DOI: 10.1039/c5cy02290h
bifunctional palladium–titania photocatalyst†
ac
Akanksha Tyagi,a Tomoya Matsumoto,a Tatsuhisa Katoab and Hisao Yoshida
*
Palladium-loaded titanium oxide was found to work as a bifunctional photocatalyst for functionalization of
benzene with ether upon photoirradiation, without using any special reagents. The metal-loaded TiO2
photocatalyst activated a C–H bond of ethers and the heterogeneous Pd metal nanoparticle catalyst pro-
moted the successive C–C bond formation between benzene and the radical species. In this reaction, ben-
zene reacted very selectively with the α-carbon of various ethers at least in the initial stage of the reaction.
Kinetic and ESR studies revealed a detailed mechanism for the reaction.
Received 30th December 2015,
Accepted 9th February 2016
DOI: 10.1039/c5cy02290h
www.rsc.org/catalysis
should require air- or moisture-free conditions, which limits
the applicability of these methods. The other is that some
1. Introduction
Activation of sp3 C–H bonds is one of the most desired reac-
tions in current chemistry because it provides an opportunity
to utilize and convert abundant and inexpensive alkanes to
valuable products.1 Especially in the view of green and sus-
tainable chemistry, the direct activation of these bonds prom-
ises efficient synthetic practices because the desired com-
pounds can be prepared in a single step instead of the
currently used multistep processes consuming extra reagents.
However, this is not an easy reaction since these bonds have
very low reactivity due to their high bond strength and attain-
ment of electronic saturation.2
kinds of harmful and reactive reagents are often required.
Hence, an innovative methodology for general and mild acti-
vation of sp3 C–H bonds has been desired to be established.
In this regard, heterogeneous photocatalysis is a promis-
ing alternative.3 In addition to the obvious advantages like
simple product separation and reusability of heterogeneous
catalysts, the use of photoenergy as the driving force would
enable us to realize unique synthetic routes. Since the photo-
excited electrons and holes themselves are very reactive, they
can simultaneously but independently induce the reductive
and oxidative reactions, respectively, thereby eliminating the
use of other harmful and reactive chemicals. Thus, the reac-
tions can be performed under mild conditions, making the
process green and sustainable.
In our previous studies, we found that metal-loaded TiO2
photocatalysts (M/TiO2, M = Pd or Pt) can activate various
types of chemicals to radical species under UV light. Based
on this concept, we have reported the photocatalytic hydroxyl-
ation4 and amination5 of aromatic rings as well as the hydra-
tion of alkenes,6 where we realized the activation of an O–H
bond in water and an N–H bond in ammonia for the forma-
tion of rather difficult O–C and N–C bonds, respectively. It
was considered that this strategy should be applicable to the
activation of sp3 C–H bonds in a similar way. To facilitate the
reaction, we focused on molecules consisting of a functional
group adjacent to the target sp3 carbon, which would result
in a decrease of the bond dissociation energy. For example,
the bond dissociation enthalpy of a C–H bond in methane is
ca. 426.8 kJ mol−1 while it becomes 401.6 kJ mol−1 in acetoni-
trile due to the adjacent cyano group.7 This methodology has
been firstly demonstrated in our previous study for the direct
cyanomethylation of the benzene ring over Pd/TiO2
Some of the earliest methods for sp3 C–H bond activation
have included the usage of strong acids or bases, but the
harsh reaction conditions would limit the substrates avail-
able for the reaction. Since then, among notable advance-
ments in this area, employing metal catalysts has become the
most popular method, where the key idea is the formation of
a new bond between the metal and the sp3 carbon.2 Even
though these methods show higher efficiency and selectivity
over the earlier methods, they all have inherent limitations.
One is the sensitive nature of the involved reagents, i.e., the
preparation, storage and usage of these metal complexes
a Graduate School of Human and Environmental Studies, Kyoto University,
Yoshida Nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan.
E-mail: yoshida.hisao.2a@kyoto-u.ac.jp
b Institute for Liberal Arts and Sciences, Kyoto University, Yoshida Nihonmatsu-
cho, Sakyo-ku, Kyoto 606-8501, Japan
c Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto
University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8520, Japan
† Electronic supplementary information (ESI) available: Effect of TiO2 structure
on the photocatalytic activity, catalytic reaction test in a flow reactor, ESR study,
thermal effects on catalysis and adsorption and XAFS analysis of a Pd/TiO2 sam-
ple. See DOI: 10.1039/c5cy02290h
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photocatalysts,8 where the adjacent cyano group would assist
the C–H bond dissociation and stabilize the generated cyano-
methyl radical to enhance the reaction efficiency for the suc-
cessive C–C bond formation with benzene.
measured in the transmission mode. The spectra were
analysed with REX 2000 software (Rigaku).
2.3. Reaction test
To expand this concept, ethers were examined in the pres-
ent study, which were expected to become α-oxyalkyl radicals
to realize the successive C–C bond formation for
functionalization of benzene. We found for the first time that
a bifunctional Pd/TiO2 photocatalyst could successfully pro-
mote the direct sp3 C–H bond activation in different ethers
and the successive sp3 C–sp2 C bond formation with benzene
to give ether-substituted benzenes (α-arylated ethers), an im-
portant class of compounds,9 with almost complete selectivity
and high yields. The yields obtained here are notable since
these kinds of photocatalytic organic transformations have
been crippled by low yields unless auxiliary chemicals or long
reaction times are used.4–6,8,10
2.3.1. Materials. All chemicals were of analytical grade and
used without further purification. The amounts of products
were determined from the GC-MS calibration curve of the au-
thentic samples procured from the industries.
2.3.2. Procedure for reaction tests. Before a photocatalytic
reaction test, the MIJx)/TiO2 sample (0.1 g) in a Pyrex glass
tube (70 mL) was subjected to pre-treatment for 1 h under
the xenon lamp light of more than 350 nm wavelength using
a long pass filter so as to clean its surface. Then, the reaction
chamber was purged with argon gas for 10 minutes followed
by the addition of reactants, benzene and ether, and stirring
under light for the desired reaction time at room tempera-
ture. After irradiation, a portion of the gaseous phase was col-
lected by means of an air-tight syringe and was analyzed
using a GC-TCD (Shimadzu, GC-8A). Then, the reaction mix-
ture was diluted with ethanol, followed by sampling of the
liquid phase using a syringe attached with a PTFE filter to
separate the MIJx)/TiO2 sample, and then analysed using a
GC-MS (Shimadzu, GCMS-QP5050A).
2. Experimental
2.1. Catalyst preparation
Various TiO2 powder samples employed for the experiments
were donated by the Catalysis Society of Japan as JRC-TiO-
8 (anatase phase, 338 m2 g−1), JRC-TiO-6 (rutile phase, 100
m2 g−1) and JRC-TiO-4 (mixture of rutile and anatase phases,
50 m2 g−1). All metal-loaded TiO2 catalysts were prepared by
the photodeposition method using the desired TiO2 powder
and an appropriate metal precursor solution of PdCl2
(Kishida, 99%), H2PtCl6·6H2O (Wako, 99.9%), RhCl3·3H2O
(Kishida, 99%) or HAuCl4·4H2O (Kishida, 99.9%). The TiO2
powder (4 g) was dispersed in ion-exchanged water (300 ml)
and was irradiated with a ceramic xenon lamp (PE300 BUV)
for 30 min. Then, methanol (100 ml) and the desired amount
of the metal precursor solution were added to the suspension
and the contents were stirred for 15 min without irradiation,
followed by 1 h of stirring under light. It was then filtered off
with suction, washed with ion-exchanged water and dried at
323 K for 12 h so as to obtain the metal-loaded TiO2 photo-
catalysts. The catalysts were referred to as MIJx)/TiO2, where
M indicates Pd, Pt, Rh or Au, and x indicates the loading
amount of the metal species in weight%.
The durability and reusability of the MIJx)/TiO2 sample was
determined using a flow reactor, the details of which are
explained in the ESI.†
2.3.3. Mechanistic studies. An ESR study was carried out in
order to validate the formation of radical species from ether in
the presence of UV light and the MIJx)/TiO2 samples. ESR mea-
surements were performed at room temperature with an
X-band spectrometer (JEOL-RE2X) by using JEOL's TE011 cavity
for the samples. PBN (N-tert-butyl-α-phenylnitrone) was chosen
as the spin trapping agent. For the ESR experiments, a suspen-
sion consisting of PBN (0.09 g), diethyl ether (DEE, 1 ml) and
the PdIJ0.1)/TiO2 catalyst (0.02 g) was prepared and a portion of
this suspension was then introduced into the ESR cavity. The
contents were then irradiated using the light of >400 nm
wavelength through a long-pass filter from a xenon lamp for
about 20 min with simultaneous ESR measurement at room
temperature. Additional information is shown in the ESI.†
Further information about the mechanism was obtained
from the results of reaction tests using isotopic compounds
such as deuterated diethyl ether, (C2D5)2O, and deuterated
benzene, C6D6, and temperature-controlled reaction tests.
2.2. Catalyst characterization
Pd K-edge XAFS (X-ray absorption fine structure) analysis was
carried out to determine the state of metal nanoparticles
loaded over TiO2 after the reaction. The spectra were
recorded at NW10A of Photon Factory at the Institute of Ma-
terials Structure Science, High Energy Accelerator Research
Organization (KEK-PF, Tsukuba, Japan) with a Si(311) double-
crystal monochromator at room temperature. For the pre-
pared catalyst samples, the spectra were recorded in the fluo-
rescence mode by using a Lytle detector filled with a krypton
(100%) flow equipped with a Ru filter (μt = 6) for the fluores-
cence and an ion chamber filled with an argon (100%) flow
for the incident X-ray. The spectra of the reference samples were
3. Results and discussion
3.1. Catalytic reaction between benzene and diethyl ether
over MIJx)/TiO2 samples
The reaction between benzene and diethyl ether (DEE, 1a)
with a PdIJ0.2)/TiO2 sample gave 1-ethoxyethylbenzene (1-EEB,
2a) as the major product through the sp2 C–sp3 C bond for-
mation between benzene and the α-position of DEE (Table 1,
entry 1), while a trace amount of 2-ethoxyethylbenzene
(2-EEB), a product of the reaction with the β-position of DEE,
was also obtained (not shown). The benzene conversion to
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(426.8 kJ mol−1 for DEE).12 Although some homocoupling
products of DEE such as 2,3-diethoxybutane and its deriva-
tives were also formed, the quantity was small. This means
that the generated α-oxyalkyl radical would preferentially re-
act with benzene before reacting with DEE or another
α-oxyalkyl radical under these conditions. It is important to
note that biphenyl, the dimer of the phenyl radical, was not
detected at all under the examined reaction conditions.
Among the gaseous products, hydrogen was the major prod-
uct while very small amounts of methane and ethane were
also detected.
Table 1 Results of the reaction tests between different benzenes and
ethers with a PdIJ0.2)/TiO2 bifunctional photocatalyst
Among the various MIJx)/TiO2 samples tested, the ones
loaded with Pd, Pt or Rh were active while the Au(0.1)/TiO2
sample was inactive for the C–C bond formation (Table 2, entries
1–4). Also, among the various TiO2 tested for this reaction,
with Pd as the co-catalyst, anatase phase TiO2 (JRC-TiO-8)
having a large specific surface area exhibited maximum activ-
ity (Table S1, ESI†). The selectivity for the α-arylated ether
(1-EEB) was always high over these metal-loaded TiO2 sam-
ples (more than 88% in Table 2 and Table S1†). The desired
product was not formed in the absence of light (Table 2, en-
try 8) or the MIJx)/TiO2 sample (Table 2, entry 9), indicating
that the reaction needed both light and the MIJx)/TiO2 sam-
ple. Moreover, when the reaction was performed with a pris-
tine TiO2 sample, only a very small amount of products was
obtained (Table 2, entry 10), highlighting the importance of
metal loading. It is notable that the PdIJx)/TiO2 samples
exhibited the highest activities for this reaction and the
highest selectivity, more than 98%, for the α-arylated ether
among the MIJx)/TiO2 samples (Table 2, entries 1, 5–7). For
the PdIJ0.1)/TiO2 sample, the amount of 1-EEB was as high as
18.8 μmol (Table 1, entry 1) which corresponds to a TON
(turnover number) of ca. 20 for 1 h per Pd atom in the system
and this value further increased with time. These results
Amount of
Reaction
time (h)
producta
Yb
(%)
Sc
(%)
Entry
Ether
Product
(μmol)
1d
2e
3f
4g
1a
1b
1c
1d
2a
2b
2c
2d1
2d2
3
3
3
1
12.9
16.8
4.1
4.1
7.9
23
30
18
1.1
99
99
99
99
a
b
GC-MS yield. Y (%): yield of the listed product = 100 × (the
amount of the product)/(the initial amount of benzene). S (%):
selectivity for the listed product among the benzene-containing prod-
ucts = 100 × (amount of the product)/(total amount of benzene-
c
d
containing products). 0.1 g of the PdIJ0.2)/TiO2 sample was used;
the wavelength of incident light was ≥350 nm; the light intensity was
56 mW cm−2 when measured at 365 20 nm wavelength using a UV
radiometer (Topcon, UVR-2, UD-36); 5 μl (56 μmol) of benzene, 2 ml
(19.2 mmol) of DEE and 0.4 μl of water were used; 1-EEB:
1-ethoxyethylbenzene; the amount of 1-EEB was determined from the
e
calibration curve of 1-EEB. 5 μl (56 μmol) of benzene, 2 ml (20
mmol) of THP and 0.4 μl of water were used; other conditions were
the same as those mentioned above; 2-PTHP: 2-phenyltetrahydro-
pyran; the amount of 2-PTHP was tentatively determined from the
f
calibration curve of 1-methoxybutylbenzene (1-MBB). 2 μl (22 μmol)
Table 2 Results of the reaction tests between benzene and DEE with
various MIJx)/TiO2 samplesa
of benzene, 1 ml (12 mmol) of THF and 0.4 μl of water were used;
the remaining conditions were the same as those mentioned above;
2-PTHF: 2-phenyltetrahydrofuran; the amount of 2-PTHF was tenta-
Productsb (μmol)
c
g
S1-EEB
(%)
tively determined from the calibration curve of 1-EEB. 100 μl (1.12
Entry
Sample
1-EEB
2-EEB
mmol) of benzene, 0.35 ml (1.19 mmol) of BME and 4 μl of water
were used; the remaining conditions were the same as those before;
BMB: butoxymethylbenzene, 1-MBB: 1-methoxybutylbenzene; the
amounts of 1-MBB and BMB were determined from the calibration
curve of 1-MBB.
1
2
3
4
5
6
7
PdIJ0.1)/TiO2
PtIJ0.1)/TiO2
RhIJ0.1)/TiO2
AuIJ0.1)/TiO2
PdIJ0.05)/TiO2
PdIJ0.2)/TiO2
PdIJ0.5)/TiO2
PdIJ0.1)/TiO2
—
18.8
1.8
1.6
0
10.8
19.2
12.8
0
0.3
0.2
0.2
0
0.2
0.4
0.1
0
98
90
88
0
98
98
99
0
the main product, 1-EEB, reached 23% for the initial 3 h.
Caronna et al.11 reported the C–C bond formation between a
reactive heteroarene and ether over a TiO2 photocatalyst with
the aid of H2O2 as an additional radical generator. In con-
trast, our system employing the Pd/TiO2 sample was free
from such an auxiliary reagent and could work under much
milder conditions to give the desired products with complete
selectivity and high yields.
8d
9e
10f
0
0.6
0
0.1
0
86
TiO2
a
Reaction conditions: 3.3 ml (31.7 mmol) of DEE, 0.7 ml (7.8 mmol)
of benzene and 0.1 g of the MIJx)/TiO2 sample were used, reaction
b
time was 1 h, λ ≥ 350 nm, and I = 40 mW cm−2 (at 365 20 nm). 1-
EEB: see Table 1, 2-EEB: 2-ethoxyethylbenzene; the amount of 2-EEB
was tentatively determined from the calibration curve of 1-EEB.
c
S1-EEB (%): selectivity for 1-EEB = 100 × (amount of 1-EEB)/(total
The high selectivity for α-substitution can be attributed to
the lower bond dissociation enthalpy of the α-C–H bonds
(397.4 kJ mol−1 for DEE) compared to their β-counterparts
d
amount of benzene-containing products). No light was supplied
e
during the reaction. No photocatalyst was used and only light was
supplied. f 0.1 g of pristine TiO2 (anatase) was used for the reaction.
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confirmed that this reaction should be a photoinduced cata-
lytic reaction.
In fact, the amount of products did not increase drasti-
cally when the reaction time increased from those indicated
in Table 1 to the longer one such as 5 h in these conditions.
This might be due to the fact that the present reactor is a
closed batch reactor which has some limitations. In a closed re-
actor, the reactants or the products including an undetectable
amount of the by-products could be adsorbed on the catalyst
surface which might cover the active sites and thus decrease
the activity. Thus, the reaction seemed to stop after a certain
period and no significant increase in product amount was ob-
served. To support this proposal, the same reaction was also
performed in a fixed-bed flow reactor as shown in the ESI.† The
results evidenced that the photocatalyst had a long life as it
maintained a constant activity for almost 10 h (Fig. S1†). This
can be explained by considering the fact that, in a flow reactor,
the reactants are being constantly removed from the catalyst
bed which does not allows the attainment of adsorption–de-
sorption equilibrium. Thus, the catalyst surface would be rela-
tively uncovered in a flow reactor than a closed batch reactor.
In fact, the catalyst was active enough to be reused for the next
cycle without any pre-treatment (Fig. S2†).
Fig. 1 ESR spectrum of the DEE radical with PBN as a spin trap.
These signals were different from those observed when
the photodissociation of PBN took place under UV light
(ESI†). Moreover, no signals were observed in the absence of
DEE, the Pd/TiO2 photocatalyst and light. Thus, it was con-
firmed that the reaction involves the oxidation of ether mole-
cules to the radical species in the presence of the PdIJ0.1)/
TiO2 photocatalyst under UV light irradiation (eqn (1)).
(1)
3.2. Catalytic reaction between benzene and other ethers over
the PdIJ0.2)/TiO2 sample
3.3.2. Study of kinetic isotope effect (inverse isotope ef-
fect). In order to get more insight about the mechanism of
this reaction, isotope experiments were performed, and the
results are shown in Table 3. For the reaction between deu-
terated benzene and DEE, the value of kH/kD was 0.92
(Table 3, entry 2) while that for the reaction between benzene
and deuterated DEE was 0.93 (Table 3, entry 3). The values of
kH/kD were not larger than unity for these reactions, which in-
dicates that the C–H bond is not cleaved during the rate-
determining step. Thus, surprisingly it can be concluded that
the C–H bond activation of both benzene and DEE would not
be the rate-determining step for this reaction.
However, it is important to consider the small inverse ki-
netic isotope effect (KIE) observed for each reaction. In order
to give the α-arylated product, the reaction must involve the
attack of the α-oxyalkyl radical on the benzene ring. This
should involve a transition state where the hybridization of
the benzene carbon would change from sp2 to sp3. This
This reaction proceeded well with other ethers too (Table 1,
entries 2–4). The reaction between benzene and THP (1b)
gave 2-phenyltetrahydropyran (2-PTHP, 2b) selectively, where
the benzene conversion to the product reached 30% for the
initial 3 h. THF (1c) also reacted with benzene selectively at
the α-position to give 2-phenyltetrahydrofuran (2-PTHF, 2c)
with high benzene conversion (18%) for 3 h. BME (1d), with
two α-carbons, gave two kinds of α-substituted products,
butoxymethylbenzene (BMB, 2d1) and 1-methoxybutylbenzene
(1-MBB, 2d2). In this case, the yield of the α-substituted prod-
ucts was not high and many by-products were observed,
which could be because the α-oxymethylene radical formed
from BME, which would produce BMB, would be less stable
and preferentially converted to other compounds before
reacting with benzene.
3.3. Mechanistic studies
3.3.1. ESR measurements. Referring to our previous
studies4–6,8,13 and considering the by-product formation, it
could be proposed that the reaction would start with the
photocatalytic radical formation from ether. Fig. 1 shows an
ESR spectrum of the formed radical species from DEE in the
presence of PBN as the spin trapping agent. The signals pres-
ent between the Mn marker (Mn2+), with the g-value of
2.0060, originate from the DEE–PBN spin adduct (eqn (S1)†).
They represent a triple doublet originating from the nitrogen
Table 3 Results of isotope experiments for the reaction between ben-
zene and DEEa
Product,
1-EEB (μmol)
b
Entry
Reactants
kH/kD
1
2
3
Benzene
Benzene-d6
Benzene
DEE
DEE
DEE-d10
7.0
7.6
7.5
—
0.92
0.93
a
Reaction conditions: 0.1 ml (1.1 mmol) of benzene, 0.4 ml (3.8
mmol) of DEE and 0.0125 g of the PdIJ0.2)/TiO2 catalyst were used,
reaction time was 1 h, λ ≥ 350 nm, and I = 40 mW cm−2 (at 365 20
(triplet) of PBN with the hyperfine coupling constant of AN
=
b
1.39 mT and the hydrogen (doublet) at the α-carbon of PBN
with AH = 0.23 mT.
nm). kH/kD was determined from the amounts of 1-EEB produced
in entries 1 and 2 for entry 2 and those in entries 1 and 3 for entry 3.
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change mainly effects the out-of-plane bending vibration of
might also be candidates for the rate-determining steps and
might be thermally accelerated. These possibilities were ex-
amined via the temperature-controlled reaction tests as de-
scribed in the ESI.†
the C–H bond which is observed at 1350 cm−1 for the sp3
C
atom and 800 cm−1 for the sp2 C atom.14 Thus, when deuter-
ated benzene was used, the change of the zero-point energies
from sp2 C to sp3 C should be considered. Quantitatively, the
difference in the zero-point energies for the C–H bonds in
the reactants and the transition state is 550 cm−1 (800 cm−1
and 1350 cm−1, respectively) while that for the C–D bonds is
403.2 cm−1 (586.8 cm−1 and 990 cm−1, respectively). Thus, be-
cause the difference in the zero-point energies at the change
from sp2 to sp3 for the C–D bonds is smaller than that for
the C–H bonds, an inverse kinetic isotope effect should be
observed. This could exactly explain what was observed for
the reaction test between deuterated benzene and DEE
(Table 3, entry 2). On the other hand, as for the inverse KIE
observed for the reaction between benzene and deuterated
DEE (Table 3, entry 3), other reasons should be considered.
As discussed above, the attack of the DEE radical on benzene
would generate a sp3 like transition state. Since D atom is
heavier than H atom, the C–D bond is stronger than the C–H
bond. This causes the C–D bond to be slightly shorter than
C–H bond. Thus, the transition state would be less sterically-
crowded in the case of D than H, because of which the reac-
tion of benzene with deuterated DEE will give an inverse KIE
(Table 3, entry 3). Thus, from the above results, it can be con-
cluded that the α-oxyalkyl radical would attack the carbon in
the benzene molecule to form the reaction intermediate hav-
ing the sp3-like carbon centre in the aromatic ring (eqn (2))
and this step is most likely to be the rate-determining step
for this reaction.
As a result, the amount of 1-EEB increased when the reac-
tion was carried out at higher temperature under light (Table
S2,† entries 1–4). However, no product was formed in the test
at higher temperature in the dark (Table S2,† entry 5), elimi-
nating the possibility that the thermal catalytic reaction with
the Pd metal catalyst proceeds independently at higher tem-
peratures. The apparent activation energy determined from
the Arrhenius plot was 35.4 kJ mol−1 (Fig. 2a). It was further
confirmed that the increase in the amount of 1-EEB at higher
temperature was not due to its increased desorption from the
catalyst surface in the adsorption–desorption equilibrium
(Table S3†). The variation of the amount of desorbed 1-EEB
with temperature gave a very gentle slope of 5.5 kJ mol−1
(Fig. 2b), which was much lower than the thermal activation
energy during the reaction under light. Thus, the increase
with temperature under light should be mainly due to
thermocatalysis promoted further at higher temperatures.
This result indicates that the rate-determining step would be
this thermal reaction between benzene and the α-oxyalkyl
radical originating from ether, which would be promoted by
the Pd catalyst, in the form of metallic Pd nanoparticles, with
the apparent thermal activation energy of 35.4 kJ mol−1 and
hence would be accelerated at higher temperatures.
3.4. State of Pd nanoparticles over TiO2
The Pd K-edge XAFS spectra revealed the state of the Pd catalyst
on TiO2. Fig. 3A shows the normalized XANES spectra of the
PdIJ0.2)/TiO2 catalyst sample and references, i.e., a metallic Pd
foil and a PdO powder sample. The spectrum of the catalyst
sample exhibited similar oscillation to metallic Pd at the higher
energy region of more than 24.4 keV although the post-edge fea-
ture was not identical to that of the metallic Pd foil. Obviously,
this was quite different from that of the PdO sample.
(2)
Fig. S3† shows the differential spectra of these XANES.
The differential spectrum of the PdIJ0.2)/TiO2 catalyst sample
exhibited a similar shape to that of the Pd metal and different
from that of PdO. The absorption edge values were evaluated
This neutral reaction intermediate would release a hydro-
gen radical to give the product (eqn (3)).
(3)
3.3.3. Study of temperature dependence. The reaction be-
tween the benzene molecule and the α-oxyalkyl radical to
form the reaction intermediate (eqn (2)) would not be a
photo-related process but a thermal process. This means that
the whole reaction might be further promoted at higher tem-
perature. If so, this might be the rate-determining step. In ad-
dition, it should be considered that other possible slow steps,
such as adsorption of reactants or desorption of products,
Fig. 2 Pseudo-Arrhenius plots for (a) temperature-controlled reactions
between benzene and DEE under light and (b) adsorption tests of 1-EEB
in the dark, with the PdIJ0.2)/TiO2 bifunctional photocatalyst. r1-EEB: for-
mation rate of 1-EEB; a1-EEB: amount of desorbed 1-EEB in the solution.
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Fig. 3 Normalized XANES spectra (A) and Fourier transforms of the k3-
weighted EXAFS (B) of the metallic Pd foil (a), the PdIJ0.2)/TiO2 sample after
the reaction test between benzene and DEE (b), and PdO powder (c). The
intensity of the FT-EXAFS of the reference samples was reduced to one third.
Fig. 4 Proposed mechanism of the reaction between benzene and ether
by a bifunctional Pd/TiO2 photocatalyst through the direct activation of
the sp3 C–H bond of ether to form the C–C bond with benzene.
from these spectra. Interestingly, the PdIJ0.2)/TiO2 catalyst sam-
ple showed a lower value (24.337 keV) than the Pd metal
(24.346 keV) and PdO (24.349 keV), implying that the Pd species
in the PdIJ0.2)/TiO2 catalyst might be electron-rich. It is known
that, when TiO2 and a precious metal nanoparticle form a semi-
conductor–metal junction, the electrons migrate to the metal
particle, which would make the metal particle electron-rich.
Thus, in the present case, the Pd nanoparticles loaded on the
surface of TiO2 might be negatively charged to some extent. In
addition, during the reaction under photoirradiation, the photo-
excited electron stored in the metallic Pd nanoparticle might
further enhance this property.
Fig. 3B shows the Fourier-transformed EXAFS spectra. The
peaks around 1.6, 2.5 and 3.0 Å correspond to the Pd–O shell in
PdO, the Pd–Pd shell in the Pd metal and the Pd–Pd shell in
PdO. Although the catalyst sample showed a noisy spectrum,
the Pd–Pd shell in the Pd metal was clearly visible, indicating
that the Pd species would be metallic. However, an additional
peak was observed at 1.6 Å. According to the curve fitting analy-
sis (Table S4†), the coordination number was 3.9 for the Pd–Pd
shell and 1.5 for the Pd–O shell. This suggests that the Pd spe-
cies on the TiO2 surface would be metallic Pd nanoparticles
connected to the TiO2 surface through oxygen atoms.
nanoparticle thermocatalyst. The reaction mechanism with the
bifunctional catalyst can be proposed as follows.
The TiO2 photocatalyst can be excited by photoirradiation
to generate the excited electron (e−) and hole (h+) (Fig. 4, i).
The hole would activate the α-C–H bond of ether to generate
the α-oxyalkyl radical and a proton as shown in eqn (1) and
(ii). Thereafter, the Pd nanoparticle would thermally catalyse
the C–C bond formation between benzene and the α-oxyalkyl
radical to form the reaction intermediate having the sp3-like
carbon centre in the aromatic ring (eqn (2) and (iii)). This
step would be the rate-determining step under these condi-
tions. The intermediate would release a hydrogen radical so
as to give the desired alpha-substituted product (eqn (3) and
(iv)). A proton would be reduced to a hydrogen radical by the
photoexcited electrons present on the Pd surface which
would couple with the previously eliminated hydrogen radical
to form hydrogen (eqn (4) and (v)).
(4)
These reaction steps are summarised in the scheme
shown in Fig. 4. It is proposed that this reaction should be a
one-photon process.
Thus, it can be proposed that the Pd species in the
PdIJ0.2)/TiO2 catalyst would be in the metallic state during the
photocatalytic reaction between benzene and DEE. This fur-
ther suggests that the Pd particles in the sample can function
as an electron-rich metal catalyst during this reaction, mak-
ing the catalyst a bifunctional photocatalyst. This electron-
rich property might be advantageous to inducing electron do-
nation to the anti-bonding π* molecular orbital of the
adsorbed benzene to activate it.
4. Conclusions
In conclusion, we successfully achieved the direct activation
of the sp3 C–H bond in various ethers followed by their suc-
cessive sp3 C–sp2 C bond formation with benzene by using a
heterogeneous bifunctional catalyst that could function as
both a photocatalyst and a palladium metal catalyst. The
photocatalyst could activate the sp3 C–H bond at the
α-carbon in various ethers to form the α-oxyalkyl radical spe-
cies and the palladium catalyst would promote the reaction
between the aromatic ring of benzene and the radical species
with the formation of the sp3 C–sp2 C bond to produce the
ether-substituted benzene, i.e., α-arylated ether.
3.5. Proposed mechanism
Based on these observations, it is clear that the Pd/TiO2 catalyst
can be recognised as a bifunctional catalyst comprising both
the TiO2 photocatalyst with the Pd co-catalyst and the Pd metal
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We hope that this new strategy for direct C–H activation
and successive C–C bond formation will be widely applicable
to other reaction systems.
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Acknowledgements
The XAFS experiments were performed under the approval of
the Photon Factory Program Advisory Committee (Proposal
No. 2014G547). This work was supported by JSPS KAKENHI
Grant Number 25105723 for Scientific Research on Innovative
Areas “Molecular Activation Directed towards Straightforward
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Catal. Sci. Technol.