Meerwein-Ponndorf-Verley-type reduction over a metal-free TiO2 photocatalyst in alcohol: Chemoselective hydrogenation of chlorobenzaldehyde to chlorobenzyl alcohol

Article abstract of DOI:10.1246/cl.160476

p-Chlorobenzaldehyde was almost quantitatively and chemoselectively reduced to p-chlorobenzyl alcohol in alcoholic suspensions of a metal-free titanium(IV) oxide photocatalyst under hydrogen-free conditions. Since alcohols act as electron donors and hydrogen sources, this reaction can be regarded as a heterogeneous Meerwein-Ponndorf-Verley-type reduction over a photocatalyst. Excellent chemoselectivity was also observed in the intermolecular competitive reduction of benzaldehyde (BA) and chlorobenzene (CB), i.e., only BA was reduced to benzyl alcohol, with CB not being reduced.

Full text of DOI:10.1246/cl.160476

Received: May 12, 2016 | Accepted: May 26, 2016 | Web Released: June 3, 2016
CL-160476
Meerwein­Ponndorf­Verley-type Reduction over a Metal-free TiO Photocatalyst in Alcohol:
2
Chemoselective Hydrogenation of Chlorobenzaldehyde to Chlorobenzyl Alcohol
Makoto Fukui, Atsuhiro Tanaka, Keiji Hashimoto, and Hiroshi Kominami*
Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University,
Kowakae, Higashiosaka, Osaka 575-8502
(
E-mail: hiro@apch.kindai.ac.jp)
p-Chlorobenzaldehyde was almost quantitatively and che-
moselectively reduced to p-chlorobenzyl alcohol in alcoholic
suspensions of a metal-free titanium(IV) oxide photocatalyst
under hydrogen-free conditions. Since alcohols act as electron
donors and hydrogen sources, this reaction can be regarded as a
heterogeneous Meerwein­Ponndorf­Verley-type reduction over
a photocatalyst. Excellent chemoselectivity was also observed in
the intermolecular competitive reduction of benzaldehyde (BA)
and chlorobenzene (CB), i.e., only BA was reduced to benzyl
alcohol, with CB not being reduced.
Scheme 1. Meerwein­Ponndorf­Verley (MPV)-type reduction
of CBAD in an alcoholic suspension of TiO photocatalyst.
2
3) alcohols are inexpensive and have moderate boiling points and
are thus easy to handle; 4) ethanol produced from biomass has
attracted much attention as a chemical and energy source.
The Meerwein­Ponndorf­Verley (MPV) reduction means
the reduction of ketones and aldehydes to their corresponding
alcohols in the presence of a sacrificial alcohol. The beauty of
the MPV reduction lies in its high chemoselectivity toward
carbonyl groups. The MPV reduction has traditionally been
carried out using homogeneous catalysts such as aluminum and
titanium alkoxides, and over the past two decades, an increasing
number of reports on heterogeneous catalysts for MPV reduction
Keywords: TiO2 photocatalyst
|
Chemoselective hydrogenation
|
Meerwein–Ponndorf–Verley-type reduction
When titanium(IV) oxide (TiO2) is photoirradiated, photo-
generated electrons and positive holes are formed in the
conduction band and valence band, and they induce reduction
and oxidation, respectively. Since titanium is an abundant
9
have been published. However, heterogeneous photocatalytic
MPV-type reduction has not been reported except by Kohtani
et al., who focused on the mechanism of the photocatalytic
element and TiO is chemically stable and not toxic for humans
2
1
0
and the environment, efforts have been made to use TiO2 as a
photocatalyst for environmentally friendly material transforma-
hydrogenation of acetophenones over TiO2.
In this study, we examined the photocatalytic MPV-type
reduction (hydrogenation) of benzaldehyde having a reducible
functional group, i.e., chloro group, in an alcoholic suspension
of metal-free TiO2 under a hydrogen (H2)-free condition and
found that the p-chlorobenzaldehyde (CBAD) was chemo-
selectively reduced (hydrogenated) to the corresponding alcohol,
p-chlorobenzyl alcohol (CBAO), in this very simple catalyst
system (Scheme 1).
1­4
tions.
Due to the strong oxidation ability of holes in the
valence band of TiO2, highly selective photocatalytic oxidation
is generally difficult. Photocatalytic reduction utilizing photo-
generated electrons has not been extensively investigated
because it is thought that photogenerated electrons are not
applicable to photocatalytic reduction due to their potential of ca.
¹
0.3 V vs. SHE. Recently, we reported that a TiO2 photocatalyst
5
can be used for the reduction of nitrobenzenes to aminobenzenes
and that nitrobenzenes having reducible functional groups were
chemoselectively reduced to the corresponding aminobenzenes.
Bare TiO powder (TIO-8, supplied by the Catalysis Society
2
of Japan as Japan Reference Catalysts, 50 mg) was suspended in
6
3
ethanol (5 cm , Wako Pure Chemical Industries, Osaka) con-
An electron donor is needed to continuously obtain a reduced
product because the photocatalytic reaction consists of reductive
and oxidative reactions. In the photocatalytic reduction of
taining CBAD (50 ¯mol, Wako Pure Chemical Industries) in a
test tube. The tube was sealed with a rubber septum and then
photoirradiated at a wavelength >300 nm using a high-pressure
mercury arc (400 W, Koike Precision Instruments) under argon
(Ar) with magnetic stirring at 298 K. After the reaction, the gas
phase (hydrogen and carbon dioxide) was analyzed using gas
chromatographs (Shimadzu, GC-8A equipped with MS-5A and
Porapak Q columns). After the suspension had been filtered to
remove the particles, the amounts of CBAD and the product(s)
were determined using a gas chromatograph (Shimadzu, GC-
2025 equipped with a DB-1 column).
7
nitrobenzenes, various electron donors can be used. Since
most nitrobenzenes are soluble in alcohols, various alcohols,
especially methanol, are often used as the electron donor as well
as the solvent of the photocatalytic reaction. Some advantages of
the use of alcohols as electron donors in photocatalytic reactions
7
can be pointed out: 1) oxidation of alcohols simultaneously
produces the corresponding carbonyl compounds, which means
utilization of positive holes in oxidation; 2) production of
a valuable oxidized compound greatly increases the atom
Figure 1 shows the time courses of the amounts of CBAD
remaining and CBAO formed in the photocatalytic reduction
8
efficiency (eq 1) of the reaction;
of CBAD in an ethanolic suspension of TiO . The amount of
2
Atom efficiency (%)
CBAD monotonously decreased with photoirradiation time and
CBAD was almost completely consumed after 10 min, while
CBAO was obtained almost quantitatively (>99% yield). The
high yield of CBAO indicates that hydrogenation of the benzene
ðmolecular weight of the desired productÞ
¼
ðsum total of molecular weights of all substances producedÞ
ꢀ
100
ð1Þ
Chem. Lett. 2016, 45, 985–987 | doi:10.1246/cl.160476
© 2016 The Chemical Society of Japan | 985

Table 1. Effects of various reaction conditions on photo-
catalytic chemoselective reduction of CBAD to CBAO in
a
alcoholic suspensions of TiO2
TiO2b
mg
Light Time Gas
source /min phase
Yield
/%
Entries
Alcoholc
/
1
2
3
4
5
50
50
®
®
50
50
50
50
EtOH Hg arc
10
10
10
10
10
10
10
20
Ar
Ar
Ar
Ar
Air
Ar
Ar
Ar
b
>99
®
EtOH
EtOH
(Dark)
(Dark)
®
®
EtOH Hg arc
EtOH Hg arc
EtOH Hg arc
EtOH Hg arc
2-PrOH Hg arc
70
d
6
7
>99
>99
87, (96)
e
f
8
a
CBAD; 50 ¯mol, Reaction temperature: 298 K. JRC-TIO-8.
Alcohol: 5 cm . Second use. Third use. Acetone yield based
on consumption of CBAD.
Figure 1. Time courses of CBAD remaining, CBAO formed,
and material balance of CBAD and CBAO in an ethanolic
suspension of TiO2 under irradiation of UV light.
c
3
d
e
f
ring, elimination of the chloro group, or re-oxidation of the
hydroxy group of CBAO did not occur in the present system.
The use of a large amount of ethanol as solvent greatly shifted
the equilibrium to the product side and almost completely
suppressed the reverse reaction. It is notable that there is no
report on the photocatalytic chemoselective reduction of an
aldehyde group having a chloro group. To evaluate the
selectivity of the reaction and side-reactions occurring under
the present conditions, an indicator, i.e., material balance (MB),
was calculated by eq 2:
and unidentified by-products were formed in the reaction mixture
(Entry 5). In the presence of O , photogenerated electrons were
2
captured by O2 to give the active oxygen species, and these
species probably induced the degradation of CBAD and/or the
product(s). Thus, since active oxygen species decompose various
organic compounds, almost all cases of photocatalytic reduction
are carried out under deaerated conditions. To examine the
durability of the TiO2 photocatalyst in this reaction system, TiO2
was used repeatedly. After reaction for 10 min, TiO particles
2
were recovered by simple filtration from the reaction mixture
and were reused. As summarized in Entries 6 and 7, TiO2
photocatalysts were reusable at least twice without notable loss
of activity. Since quantitative analysis of acetaldehyde as an
oxidation product of ethanol was difficult, 2-propanol was used
in place of ethanol (Entry 8). From quantitative analysis of the
oxidized product, i.e., acetone, it can be concluded that the MPV-
type reaction as shown in Scheme 1, i.e., redox reaction between
nðCBADÞ þ nðCBAOÞ
MB ¼
ð2Þ
n0ðCBADÞ
where n(CBAD) and n(CBAO) are the amounts of CBAD and
CBAO during the photocatalytic reaction, respectively, and
n0(CBAD) is the amount of CBAD before the photocatalytic
reaction. As is also shown in Figure 1, the value of MB was
almost unity during the reaction, indicating that no other
intermediates were produced. Other reduced products such as H2
were not formed, indicating that the photogenerated electrons
CBAD and 2-propanol, occurred over the TiO photocatalyst.
2
The results for 2-propanol show that various alcohols are
applicable for the reduction of CBAD to CBAO.
(
in other words, ethanol) were selectively used for the reduction
The high chemoselectivity for the photocatalytic intramo-
lecular chemoselective reduction of CBAD was further inves-
tigated in the intermolecular competitive reduction of benzalde-
hyde (BA) and chlorobenzene (CB), and the results are shown in
Figure 2. As expected, BA (48 ¯mol) was reduced to give benzyl
alcohol in 93% yield, while CB (48 ¯mol) was not reduced at all.
These results clearly demonstrate that the photocatalytic system
showed complete chemoselectivity for the carbonyl group in the
presence of inter- and intramolecular chloro groups.
An action spectrum is a strong tool for determining whether
an observed reaction occurs via a photoinduced process or
a thermocatalytic process. To obtain the apparent quantum
efficiency (AQE) in this reaction system, a multiwavelength
irradiation monochromator (MM-3, Bunkoukeiki Co., Ltd.) was
used and the light intensity was determined using a spectror-
adiometer (USR-45D, Ushio Inc.). The values of AQE at each
centered wavelength of light were calculated from the ratio of
twice the amount of CBAO formed and the amount of photons
irradiated using eq 3:
of CBAD. No H formation in the present reaction system is
attributed to the use of bare (metal-free) TiO2. Of course, no H2
was added as reducing gas. This H2-free system is safe and
2
attractive because there is no need to use H as a reducing
2
reagent for CBAD and to remove H2 from the reaction system.
Effects of various reaction conditions on the photocatalytic
reduction of CBAD to CBAO were investigated, and the results
are summarized in Table 1. Entry 1 is the result at 10-min
photoirradiation in Figure 1. As shown in Entries 2 and 3,
photoirradiation and TiO were indispensable for the formation
2
of CBAO, indicating that CBAD was reduced to CBAO by TiO2
photocatalysis. The results of Entries 2 and 3 also indicate that
neither thermocatalytic reaction over TiO2 nor thermal reactions
of CBAD occurred at 298 K. In the absence of TiO under UV
2
light irradiation (Entry 4), an unidentified by-product(s) was
observed, but no CBAO was formed, indicating that UV light
from the Hg arc induced a photochemical side-reaction of
CBAD. The large difference in the results of Entries 1 and 4, i.e.,
presence and absence of TiO2 under UV irradiation, indicates that
the photochemical side-reaction was drastically suppressed by
TiO2. The presence of oxygen (O2) decreased the yield of CBAO
2
ꢀ the amount of alcohol formed
amount of incident photons
AQE ¼
ꢀ 100
ð3Þ
986 | Chem. Lett. 2016, 45, 985–987 | doi:10.1246/cl.160476
© 2016 The Chemical Society of Japan

Table 2. Effect of type of TiO2 sample on photocatalytic
chemoselective reduction of CBAD to CBAO in ethanolic
suspensions
5
4
3
2
1
0
0
0
0
0
0
a
TiO2b
JRC-TIO-)
SBET
¹1 d
Yield
/%
MB
/%
Phasec
2
(
/m g
72
1
2
3
4
5
6
7
8
9
A
A
R
A, (R)
R, (A)
R
A
A
A
A
A, (R)
A
A
R
®
3
32
47
5
33
48
64
52
47
18
42
38
48
3
14
92
71
89
90
66
97
98
>99
90
18
48
54
2.6
100
270
313
290
311
97
0
5
10 15 20 25
Irradiation time / min
Figure 2. Intermolecular competitive reduction of benzalde-
hyde and chlorobenzene in an ethanolic suspension of TiO2
under irradiation of UV light.
1
0
1
1
71
94
85
73
1
1
2
3
290
59
93
e
MT-150A
a
CBAD: 50 ¯mol, TiO2: 50 mg, Ethanol: 5 cm , Reaction time:
b
5
min. JRC-TIO series registered at the Catalysis Society of
c
Japan as Japan Reference Catalysts. Minor form in biphase
samples is shown in parenthesis. Determined by BET method.
Supplied from Tayca.
d
e
reaction can be regarded as a heterogeneous Meerwein­
Ponndorf­Verley-type reduction over a photocatalyst. Environ-
mentally friendly material transformations can be achieved by
the present photocatalytic process under metal-, additive-, and
hydrogen-free conditions, with only substrates being dissolved
in alcohols.
Figure 3. Absorption spectrum (right axis) and action spec-
trum of TiO2 in the reduction of CBAD to CBAO in an ethanolic
suspension of TiO2 (50 ¯mol, left axis).
This work was partly supported by JSPS KAKENHI Grant
Numbers 26289307, 26630415, and 15K18269. This work was
also supported by MEXT-Supported Program for the Strategic
Research Foundation at Private Universities 2014­2018, subsidy
from MEXT and Kindai University.
As shown in Figure 3, AQE was in agreement with the
absorption spectrum of TiO . Therefore, it can be concluded that
2
the reduction of CBAD in an ethanolic suspension was induced
by photoabsorption of TiO2. As is also shown in Figure 3, the
AQE reached 17% at 350 nm, indicating that the photocatalytic
hydrogenation of CBAD proceeded with high efficiency of
photon utilization as well as good chemical aspects, such as
selectivity and MB.
To evaluate the effects of the physical properties of TiO2,
representative commercial TiO2 samples, which were registered
at the Catalysis Society of Japan as Japan Reference Catalysts
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Chem. Lett. 2016, 45, 985–987 | doi:10.1246/cl.160476
© 2016 The Chemical Society of Japan | 987