Metal Cocatalyst Directing Photocatalytic Acetonylation of Toluene via Dehydrogenative Cross-Coupling with Acetone

Article abstract of DOI:10.1007/s10562-019-02923-3

Abstract: A heterogeneous metal-loaded titanium oxide photocatalyst provided an efficient route to bring out direct dehydrogenative cross-coupling between toluene and acetone without consuming any additional oxidizing agent. The nature of the metal nanoparticle cocatalyst deposited on TiO₂ photocatalyst dictated the product selectivity for the cross-coupling. Pd nanoparticles on TiO₂ photocatalyst allowed a C–C bond formation between the aromatic ring of toluene and acetone to give 1-(o-tolyl)propan-2-one (1a1) with high regioselectivity, while Pt nanoparticles on TiO₂ photocatalyst promoted the cross-coupling between the methyl group of toluene and acetone to give 4-phenylbutan-2-one (1b) as the acetonylated product. These results demonstrated that the selection of the metal cocatalyst on TiO₂ photocatalyst could determine which C–H bonds in toluene, aromatic or aliphatic, can react with acetone. Two kinds of reaction mechanisms were proposed for the photocatalytic dehydrogenative cross-coupling reaction, depending on the property of the metal nanoparticles, i.e., only Pd nanoparticles can catalyze the reaction between aromatic ring and the acetonyl radical species. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02923-3

Catalysis Letters
https://doi.org/10.1007/s10562-019-02923-3
Metal Cocatalyst Directing Photocatalytic Acetonylation of Toluene
via Dehydrogenative Cross‑Coupling with Acetone
Akanksha Tyagi1 · Tomoya Matsumoto2 · Akira Yamamoto1,2 · Tatsuhisa Kato2,3 · Hisao Yoshida1,2
Received: 18 May 2019 / Accepted: 29 July 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A heterogeneous metal-loaded titanium oxide photocatalyst provided an efcient route to bring out direct dehydrogenative
cross-coupling between toluene and acetone without consuming any additional oxidizing agent. The nature of the metal nano-
particle cocatalyst deposited on TiO2 photocatalyst dictated the product selectivity for the cross-coupling. Pd nanoparticles
on TiO2 photocatalyst allowed a C–C bond formation between the aromatic ring of toluene and acetone to give 1-(o-tolyl)
propan-2-one (1a1) with high regioselectivity, while Pt nanoparticles on TiO2 photocatalyst promoted the cross-coupling
between the methyl group of toluene and acetone to give 4-phenylbutan-2-one (1b) as the acetonylated product. These results
demonstrated that the selection of the metal cocatalyst on TiO2 photocatalyst could determine which C–H bonds in toluene,
aromatic or aliphatic, can react with acetone. Two kinds of reaction mechanisms were proposed for the photocatalytic dehy-
drogenative cross-coupling reaction, depending on the property of the metal nanoparticles, i.e., only Pd nanoparticles can
catalyze the reaction between aromatic ring and the acetonyl radical species.
Graphic Abstract
Keywords Titanium oxide · Photocatalysis · C–C cross-coupling · Reaction mechanism · Dehydrogenative cross-couplings
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02923-3) contains
supplementary material, which is available to authorized users.
3
* Hisao Yoshida
Institute for Liberal Arts and Sciences (ILAS), Kyoto
University, Yoshida Nihonmatsu-cho, Sakyo-ku,
Kyoto 606-8501, Japan
yoshida.hisao.2a@kyoto-u.ac.jp
1
Elements Strategy Initiative for Catalysts and Batteries
(ESICB), Kyoto University, Kyotodaigaku-Katsura,
Nishikyo-ku, Kyoto 615-8520, Japan
2
Graduate School of Human and Environmental Studies,
Kyoto University, Yoshida Nihonmatsu-cho, Sakyo-ku,
Kyoto 606-8501, Japan
Vol.:(0123456789)
1 3

A. Tyagi et al.
1 Introduction
Pd/TiO2
H2
+
O
TiO2 photocatalysis has been well examined for the mitiga-
tion of various energy and environmental issues like hydro-
gen production for renewable energy by water splitting [1–3],
CO2 reduction [4–6], and air and water purifcation by the
degradation of organic pollutants [7, 8]. The selective organic
reactions like C–C couplings, on the contrary, are still infancy
[9, 10]. The photoirradiation of TiO2 generates electrons and
holes in the conduction and valence bands that can oxidize and
reduce diferent organic molecules or ions to form the corre-
sponding radical species. So far, many reports have shown that
a careful selection of the substrates [11–13], controlling the
morphology of the photocatalyst [14–16], or the deposition of
cocatalysts on the photocatalyst [17–19] can notably improve
the selectivity in these TiO2 catalyzed organic reactions [20].
Particularly, cocatalysts, which are usually responsible for the
charge separation on TiO2, can signifcantly afect the selectiv-
ity of the photocatalytic reactions [21–24].
1a1
UV light
365 nm
O
Selectivity(%)= 89; Regioselectivity(%)= 77
+
O
Pt/TiO2
1
acetone
toluene (
)
H2
+
1b
Selectivity(%)= 96
Scheme 1 Direct photocatalytic DCC between toluene and acetone
with diferent M/TiO2 samples. The reaction conditions were same as
those mentioned in Table 1
2 Experimental Section
2.1 Catalyst Preparation
The metal-loaded titanium oxide photocatalysts (M(x)/
TiO2, M: Pd, Pt, Au, x: loading amount in wt%) were
prepared by a photodeposition method. The TiO2 sam-
ple was obtained from Catalysis Society of Japan (JRC-
TIO-8, 338 m2g−1). 4 g of this sample was dispersed in
300 mL ion-exchanged water and stirred under the light
irradiation from a xenon lamp (PE300BUV) for 30 min.
Then, 100 mL methanol and desired volume of the metal
precursor solution (PdCl2 for Pd, H2PtCl6·6H2O for Pt,
and HAuCl4·4H2O for Au) were added and the resultant
suspension was stirred in the dark for 15 min and under
the light irradiation for 1 h. The suspension was fltered,
washed with ion-exchanged water, and dried at 326 K for
12 h, which provided a M(x)/TiO2 (M=Pd, Pt, or Au) pho-
tocatalyst. The Rh- and Ru-loaded TiO2 samples [Rh(0.5)/
TiO2 and Ru(0.5)/TiO2] were prepared by an impregnation
method. 4 g of the JRC TIO-8 sample was dispersed in
100 mL ion-exchanged water and stirred at room tempera-
ture for 10 min. Then, the desired volume of the metal
precursor solution (RhCl3·6H2O for Rh and RuCl3·3H2O
for Ru) was added and the suspension was heated at 363 K
to evaporate the water to dryness. After drying for 12 h
at 326 K in an electric oven, the resultant powder was
calcined at 723 K for 1 h to get the M(0.5)/TiO2 (M = Rh
or Ru) samples.
In the present work, we report the selective photocatalytic
activation of the C–H bonds in arenes by a heterogeneous
metal-loaded TiO2 photocatalyst (M/TiO2) to carry out the
acetonylation of toluene via a direct dehydrogenative cross-
coupling (DCC) with acetone, where a C–H bond of toluene
and that of acetone are cleaved to make a new C–C bond with
the concomitant release of hydrogen gas without consuming
any additional oxidizing agent. The products are arylacetones,
a common precursor to medicinal compounds like ampheta-
mine [25]. Thus, our study provides a simple and sustainable
methodology over the commonly used methods [26–28] to
prepare arylacetones. In this study, it was found that the nature
of the metal nanoparticles loaded on TiO2 photocatalyst sig-
nifcantly afected the product selectivity. The Pd-loaded TiO2
sample (Pd/TiO2) promoted the DCC between the aromatic
ring of toluene and acetone to give ortho and para substituted
products namely, 1-(o-tolyl)propan-2-one (1a1) and 1-(p-tolyl)
propan-2-one (1a2) majorly. The Pt/TiO2 sample, in contrast,
promoted the cross-coupling between the methyl groups of
both toluene and acetone to give 4-phenylbutan-2-one (1b)
(Scheme 1). A systematic study revealed that the formation of
the two types of products, i.e., aromatic substituted and methyl
substituted, follows diferent mechanistic routes that require
the diferent abilities of the deposited metal cocatalysts. Thus,
by choosing the suitable metal cocatalyst, we achieved the
selective activation of C–H bonds in toluene and consequently
the DCC with acetone.
2.2 Photocatalytic Activity Test
All chemicals were of analytical grade and used with-
out any further purification; toluene (Kishida Chemi-
cals), aniline (Kishida Chemicals), o-toluidine (Wako
Pure Chemicals), benzylamine (Nacalai Tesque), acetone
(Nacalai Tesque), acetophenone (Wako Pure Chemicals),
1 3

Metal Cocatalyst Directing Photocatalytic Acetonylation of Toluene via Dehydrogenative…
1-(o-tolyl)propan-2-one (Aldrich), 1-(p-tolyl)propan-2-one
(Alfa Aesar), toluene-d8 (Aldrich), and acetone-d6 (TCI
Chemicals). The photocatalytic reaction tests were done
in a Pyrex test tube (70 mL) as a closed reactor, under
argon atmosphere. Typical conditions are mentioned here.
At frst, the photocatalyst (0.1 g) was irradiated under the
light from the xenon lamp to clean up its surface. Then,
the reactants (4.0 mL in total) were added and the test
tube was sealed with a silicon septum and purged with a
fow of argon gas for 10 min and stirred under the light
irradiation for 3 h (the wavelength of the incident light was
365 20 nm, the irradiation area was 4.95 cm2, and the
intensity was 180 mW cm−2). After the photoirradiation,
a part of the gas phase (0.5 mL) was taken in a gas-tight
syringe and analyzed by GC-TCD (Shimadzu, GC-8A).
The reaction mixture was filtered by using a syringe
equipped with a PTFE flter, diluted with a solvent, includ-
ing an internal standard (n-decane), and then analyzed by
GC–MS (QP5050A).
amount was minimized in the suitable reaction conditions.
Also detected was 4-hydroxy-4-methylpentan-2-one as an
aldol condensation product, which can be catalyzed by the
acidic property of TiO2 surface. Although hydrogen was
detected in the gas phase, the amount of hydrogen and the
DCC products was not balanced, which is probably due to
side reactions [29–32].
The reaction done with the bare TiO2 sample gave only
1b and the aromatic DCC did not proceed (Table 1, entry
1). Among the diferent metal cocatalysts screened on the
TiO2 photocatalyst, the Pd and Pt cocatalysts gave the largest
amount of the DCC products (Table 1, entries 2–6). The Au-
and Ru-loaded TiO2 samples showed low activity while the
Rh-loaded sample was inactive for these reactions. The prod-
ucts were not obtained in the reaction carried out in the dark
or without the TiO2 photocatalyst under the light irradiation,
supporting that the DCC products were photocatalytically
obtained (Table 1, entries 7 and 8, respectively).
Interestingly, the selectivity to the DCC products varied
with the metal cocatalyst deposited on the TiO2 photocata-
lyst. The DCC products should be divided into two: 1a1
and 1a2 were the products of the cross-coupling between
the aromatic ring of toluene and acetone, while 1b was that
with the methyl group of toluene. The selectivity S (%) to
aromatic DCC products is shown in Table 1. Among the two
aromatic DCC products (1a1 and 1a2), the regioselectiv-
ity to 1a1, defned as R (%) and also listed, was high in all
reactions (R≥75%). Methyl group is an electron donating
group which has an induction efect (+I) at ortho and para
positions. This efect is stronger at ortho position than para,
resulting in higher regioselectivity to 1a1 over 1a2. While
3 Results and Discussion
3.1 Photocatalytic Reaction
Table 1 shows the results of the screening of the photocata-
lysts for the dehydrogenative acetonylation of toluene with
acetone. The reaction gave three products, 1-(o-tolyl)propan-
2-one (1a1), 1-(p-tolyl)propan-2-one (1a2), and 4-phenylb-
utan-2-one (1b). The reaction also gave some homo-cou-
pling products from toluene like 1,2-diphenylethane, whose
Table 1 Direct photocatalytic
DCC between toluene
and acetone with diferent
photocatalysts
Entry
Photocatalyst
Products (μmol)
S (%)a
R (%)b
1a1
1a2
Sum of 1a1+1a2
1b
1
2
3
4
TiO2
0.00
1.23
0.15
0.06
0.00
0.00
0.00
0.00
8.04
0.00
0.36
0.03
0.00
0.00
0.00
0.00
0.00
2.69
0.00
1.59
0.18
0.06
0.00
0.00
0.00
0.00
10.73
0.53
0.19
1.19
0.60
0.29
0.00
0.00
0.00
0.74
0
–
Pd(0.5)/TiO2
Pt(0.5)/TiO2
Au(0.5)/TiO2
Ru(0.5)/TiO2
Rh(0.5)/TiO2
Pd(0.5)/TiO2
None
89
13
9
0
–
–
–
94
77
82
100
–
–
–
5
6
7c
8d
9
–
75
Pd(5.0)/TiO2
Reaction conditions: 0.1 mL (0.94 mmol) of toluene, 3.9 mL (52.6 mmol) of acetone, 0.1 g of M(x)/TiO2
photocatalyst were used for a 3 h reaction. Wavelength of the irradiated light: λ=365 20 nm, light inten-
sity I=180 mW cm−2 (measured at 365 20 nm)
aSelectivity (%) to aromatic DCC products: S (%)=100×[sum of the amounts of 1a1 and 1a2 (μmol)/total
amount of acetonylated products (μmol)]
bRegioselectivity (%) to 1a1: R (%)=100×[amount of 1a1 (μmol)/sum of 1a1 and 1a2 (μmol)]
cThe reaction was carried out in the dark
dThe reaction was carried out without any photocatalyst under the light irradiation
1 3

A. Tyagi et al.
the Pd cocatalyst promoted the DCC reaction between the
aromatic ring and acetone with high selectivity (S = 89%)
and high regioselectivity (R = 77%) to give 1a1 (Table 1,
entry 2), other metals like Pt promoted the DCC reaction at
the methyl group of toluene to give 1b with high selectiv-
ity (S=13%) (Table 1, entry 3). These results indicate that
the metal nanoparticles on the TiO2 photocatalyst play an
important role in the photocatalytic reaction that governs
the product selectivity. The amount of Pd on the TiO2 pho-
tocatalyst was optimized to improve the yield of the DCC
products (Fig. S1). The Pd(5.0)/TiO2 photocatalyst gave the
largest yield of the aromatic DCC products (1a1 and 1a2),
which is also listed in Table 1, entry 9.
The observed spectrum was simulated by using EasySpin
[35] (Fig. 1b). Analysis revealed that the observed spectrum
couldn’t be assigned to a single radical species. It consisted
of three sets of a triple-doublet arising from the N and the
α-H of DMPO-acetone radical adduct (Eqs. 1 and 2). Fur-
ther, the spectrum changed with time (data not shown).
Based on these results, we conclude that the radical species
derived from acetone, trapped by DMPO, might be unstable
and thus, have diferent spatial orientations, which would
give individual signals in the ESR spectrum. Due to this
complexity, the observed spectrum couldn’t be assigned to
a particular conformation. However, these results prove that,
at least, the acetone molecule is oxidized by the photogen-
erated holes on the Pd/TiO2 photocatalyst (Eq. 1).
3.2 Mechanistic Studies
On the other hand, the observation of 1,2-diphenylethane
in the photocatalytic reaction supported the formation of the
benzyl radicals from toluene by the oxidation of its methyl
group. Thus, the photogenerated holes on TiO2 could oxidize
both toluene and acetone to radical species.
3.2.1 Radical Formation
Based on our previous studies on the photocatalytic C–C
cross-coupling reactions, [29, 31, 33, 34] the present photo-
catalytic DCC between toluene and acetone is also consid-
ered to involve the hole oxidation of the organic molecules
to produce their corresponding radical species and protons.
In the present study, the formation of acetonyl radical during
the photoirradiation of the Pd/TiO2 sample was confrmed
by the ESR measurements (see Supplementary Information
for details). Photoirradiation of a suspension of acetone and
a spin trap reagent (DMPO) with the Pd/TiO2 photocata-
lyst gave some ESR signals (Fig. 1a). The signals were not
obtained without the photocatalyst, acetone, or light irra-
diation, suggesting that the radical species are produced
from acetone upon its oxidation by the photogenerated holes
on the TiO2 photocatalyst.
Pd/TiO2
O
O
(1)
(2)
+
+
H
h
h
ν
O
O
N
+
N
O
O
3.2.2 Kinetic Experiments
Next, kinetic experiments were performed to get insight into
the reaction mechanism. The photocatalytic reaction tests
were done with deuterated acetone (acetone-d6) and deuter-
ated toluene (toluene-d8).
Table 2 shows the results of the reaction test carried out
with the Pt(0.5)/TiO2 photocatalyst giving 1b as the main
product. Compared to the reaction done without any deuter-
ated reagent (Table 2, entry 1), the amount of 1b was signif-
cantly decreased in the reaction carried out with deuterated
Table 2 Isotopic experiments for the direct photocatalytic DCC
between toluene and acetone with the Pt(0.5)/TiO2 photocatalyst
Entry
Reactants
1b (μmol)
kH/kaD
1
2
3
Toluene
Toluene-d8
Toluene
Acetone
Acetone
Acetone-d6
0.60
0.18
0.47
–
3.3
1.3
All reaction conditions were the same as those shown in Table 1
Fig. 1 Experimental (a) and simulated (b) ESR spectra of acetone-
DMPO solution with Pd/TiO2 photocatalyst under the light irradia-
tion. Pd loading amount was 0.75 wt%
akH/kD =[amount of 1b in the reaction without any isotopic reactant
(μmol)]/[amount of 1b with a isotopic reactant (μmol)]
1 3

Metal Cocatalyst Directing Photocatalytic Acetonylation of Toluene via Dehydrogenative…
toluene and the kH/kD ratio was clearly larger than unity,
indicating a normal kinetic isotope efect (KIE) (Table 2,
entry 2). This result suggests that the cleavage of the ben-
zylic C–H bond of toluene to give benzyl radical would be
the rate determining step (RDS) for the formation of 1b in
this condition. When the reaction was carried out with deu-
terated acetone, the reaction rate slightly (20%) reduced and
the value of kH/kD was a little larger than unity (Table 2,
entries 1 and 3). This proposed that the cleavage of the C–H
bond of acetone to give acetonyl radical would be the RDS
for the 1b formation in this condition. Thus, it is suggested
that both toluene and acetone are activated to form the cor-
responding radical species and the RDS can be varied when
the hydrogen was replaced with deuterium.
the structure of the 1-(o-tolyl)propan-2-one (1a1) suggest
that the RDS in the production of 1a1 would be the cleavage
of the C–H bond of the aromatic ring in toluene. However,
the literature has shown that the phenyl radical of toluene
is unstable and rapidly converts to benzyl radical [36], i.e.,
the direct fssion of the aromatic C–H bond cannot give 1a1
as a product. Thus, to form 1a1, toluene should not be oxi-
dized to phenyl radical and rather remain a molecule. This
requires that the photocatalytically generated acetonyl radi-
cal attacks the toluene molecule. This will lead to a radical
transition state which will convert to 1a1 by the elimination
of a hydrogen radical, belonging to toluene. So, the aromatic
C–H bond of toluene would be broken from the transition
state rather than the toluene molecule. As indicated by the
isotopic experiments, this elimination of a hydrogen radical
from toluene might be the RDS for the formation of 1a1.
Table 3 shows the results obtained with the Pd(5.0)/TiO2
photocatalyst that gave 1a1 as the major product. The reac-
tion done with deuterated toluene (Table 3, entry 2) gave a
small amount of 1a1, contrary to the reaction done without
any isotopic compound (Table 3, entry 1). The value of kH/kD
was more than unity, corresponding to the normal KIE. The
amount of 1a1 remained almost unchanged with the replace-
ment of acetone with deuterated acetone (Table 3, entries 1
and 3). Thus, the value of kH/kD was unity. These results and
3.2.3 Temperature‑Controlled Photocatalytic Reaction
Tests
Several studies have shown that Pd nanoparticles are cru-
cial for the photocatalytic aromatic substitution reactions
by carbon centered radicals like acetonitrile [29, 33] and
ether. [34, 37] In these reactions, Pd was proposed to have a
catalytic role which involves the activation of the aromatic
ring to facilitate its reaction with the photogenerated radical
species. Besides, Pt nanoparticles are recognized as an ef-
cient electron receiver on TiO2 photocatalyst to increase the
photocatalytic activity. [38, 39] So, to elucidate the diferent
role of Pd and Pt nanoparticles in the formation of 1a1 and
1b, respectively, we studied the efect of temperature on the
photocatalytic DCC between toluene and acetone with the
Pd/TiO2 and Pt/TiO2 samples at diferent temperatures. The
details are mentioned in the Supplementary Information.
Table 3 Isotopic experiments for the direct photocatalytic DCC
between toluene and acetone with the Pd(5.0)/TiO2 photocatalyst
a
Entry
Reactants
1a1 (μmol)
kH/kD
1
2
3
Toluene
Toluene-d8
Toluene
Acetone
Acetone
Acetone-d6
2.53
0.98
2.57
–
2.6
1.0
All reaction conditions were the same as those shown in Table 1
akH/kD =[amount of 1a1 in the reaction without any isotopic reactant
(μmol)]/[amount of 1a1 with a isotopic reactant (μmol)]
T × 10-3 / K-1
3
A
3.1
3.2
3.3
3.4
3.5
2
0
-0.5
-1
(a)
B
(b)
1
0
(b)
(a)
-1
-1.5
-2
-2
3.15
3.2
3.25
3.3
3.35
3.4
T × 10-3 / K-1
-2.5
Fig. 2 Pseudo Arrhenius plots of the temperature-controlled photocatalytic DCC between toluene and acetone with the Pd(5.0)/TiO2 photocata-
lyst (A) and the Pt(0.5)/TiO2 photocatalyst (B), to give 1a1 (a) and 1b (b)
1 3

A. Tyagi et al.
O
For the reaction carried out with the Pd(5.0)/TiO2 sample,
the amount of 1a1 steadily increased with temperature while
that of 1b varied only slightly (Fig. 2A, and Table S1, entries
1–3). The apparent activation energies, Ea, calculated from
the pseudo Arrhenius plot of this data, were 73 kJ mol−1
for 1a1 and 10 kJ mol−1 for 1b (Fig. 2A). No products
were formed in the reaction carried out in the dark at high
temperature (Table S1, entry 4). Similar experiments were
carried out with the Pt(0.5)/TiO2 sample (Table S1, entries
5–7). The amount of both products did not vary much with
temperature and the Ea were 10 and 8 kJ mol−1 for both 1a1
and 1b, respectively (Fig. 2B).
Pd
TiO2
H2
(i)
(iv)
1a1
e
h
TiO2
Pd
O
Radical addition-elimination
mechanism
O
H
(ii)
Pd
H
TiO2
(iii)
O
O
H
e
The comparable and small Ea for 1b with the two samples
(Fig. 2A-b and B-b) indicates that, irrespective of the nature
of the metal cocatalyst on TiO2, the formation of 1b pro-
ceeds essentially without thermal energy in a similar way but
with photoenergy. Thus, it would be a purely photocatalytic
process. The slight increase in the formation rate of 1b with
temperature might be due to some non-catalytic processes
like electron migration, electron transfer to metal nanoparti-
cles, and desorption of products from the surface [40].
The Ea for 1a1, on the other hand, was quite diferent on
the two samples (Fig. 2A-a and B-a). Especially, the value
with the Pd/TiO2 photocatalyst was quite high for a usual
photocatalytic process. This suggests the involvement of a
certain non-photocatalytic step in this reaction. The forma-
tion of 1a1, as discussed before, is proposed to follow a radi-
cal addition–elimination mechanism involving the reaction
between acetonyl radical and toluene molecule, catalyzed by
the Pd nanoparticles. This step might be further promoted
at higher temperature, which will increase the yield of 1a1
(Fig. 2A-a). The small Ea obtained for 1a1 with the Pt/TiO2
sample indicates, such catalysis is not possible by Pt nano-
particles, which is why the yield of 1a1 is low with the Pt/
TiO2 sample. Thus, the Pd nanoparticles were evidenced
to play a catalytic role in the photocatalytic acetonylation
of toluene to give 1a1, where the Pd nanoparticles would
absorb and activate the aromatic ring of toluene to react with
the acetonyl radical. In contrast, the Pt nanoparticles would
function as not a catalyst but an electron receiver to promote
the formation of benzyl and acetonyl radicals to give 1b.
h
Pd
TiO2
H
Fig. 3 Proposed reaction mechanism for the direct photocatalytic
DCC between toluene and acetone to give 1a1 by the Pd/TiO2 pho-
tocatalyst
electron moves to Pd nanoparticles. (ii) The hole can oxi-
dize an acetone molecule to generate the acetonyl radical
species and proton (H+). The electron on the Pd nano-
particles can reduce the proton to hydrogen radical (H·).
(iii) The acetonyl radical species can attack the toluene
molecule, assisted by the catalysis of Pd nanoparticles, to
generate a radical transition state. This is a radical addi-
tion–elimination mechanism [29, 33, 34], which is the
most important step to change the product selectivity.
Here, the acetonyl radical species preferably attacks the
ortho position. (iv) Elimination of a hydrogen radical from
this transition state gives the acetonylated product 1a1.
The two hydrogen radicals can combine to give hydrogen
gas. This is a one-photon process. A similar attack of the
acetonyl radical at the para position in toluene will give
1a2. Although the mechanism for the 1a1 and 1a2 forma-
tion on the Pt/TiO2 photocatalyst has not clarifed because
of the low yields, the diferent activation energy suggested
that it takes place via a diferent mechanism from the Pd/
TiO2 photocatalyst with a slow rate.
Figure 4 shows the mechanism for the formation of 4-phe-
nylbutan-2-one (1b) by the Pt/TiO2 photocatalyst. (i) Pho-
toexcitation of the TiO2 photocatalyst generates electrons
and holes in its conduction and valence band, respectively.
The electrons move to the Pt nanoparticles leaving the holes
on TiO2. (ii) The holes can oxidize both methyl group of
toluene and acetone to generate their corresponding radical
species and protons. The protons can be reduced to hydro-
gen radicals by the electrons on the Pt surface. (iii) The two
organic radical species can combine to give the acetonylated
product 1b, and (iv) the two hydrogen radicals can combine
3.2.4 Proposed Reaction Mechanism
Based on these observations, we have proposed the fol-
lowing two mechanisms for the direct photocatalytic DCC
between toluene (1) and acetone. At frst the mechanism
for the formation of 1-(o-tolyl)propan-2-one (1a1) by the
Pd/TiO2 photocatalyst is described (Fig. 3). (i) The pho-
toexcitation of the TiO2 photocatalyst generates electron
and hole. The hole moves to the TiO2 surface while the
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Metal Cocatalyst Directing Photocatalytic Acetonylation of Toluene via Dehydrogenative…
Pt
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4 Conclusions
In this report, we have accomplished the direct photocatalytic
DCC between toluene and acetone to give acetonylated com-
pounds. The nature of the metal cocatalysts deposited on the
TiO2 surface signifcantly afected the selectivity of the DCC
products. Pd promoted the DCC between the aromatic ring
of toluene and acetone with high selectivity (94%) via the
radical addition–elimination mechanism, which was attrib-
uted to the catalytic property of the Pd nanoparticles for the
reaction between aromatic ring and organic radical species
in the C–C couplings, in other words, the Pd nanoparticles
would adsorb and activate the aromatic ring of toluene to react
with the acetonyl radical. Pt nanoparticles, on the contrary,
promoted the DCC between the methyl group of toluene and
acetone with high selectivity (87%) via the radical–radical cou-
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Acknowledgements The present project was fnancially supported by
Core Research for Evolutional Science and Technology, Japan Science
and Technology Agency (CREST, JST; JPMJCR1541).
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