Photocatalytic chemoselective reduction of epoxides to alkenes along with formation of ketones in alcoholic suspensions of silver-loaded titanium(IV) oxide at room temperature without the use of reducing gases

Article abstract of DOI:10.1039/c3cc49340g

(2,3-Epoxypropyl)benzene was chemoselectively reduced to allylbenzene along with formation of ketones in alcoholic suspensions of a silver-loaded titanium(iv) oxide photocatalyst at room temperature under atmospheric pressure without the use of reducing gases, and various epoxides were also reduced to the corresponding alkenes. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc49340g

Showcasing research from Dr H. Kominami’s Surface Design
Chemistry Laboratory/Applied Chemistry, Kinki University,
Higashiosaka, Osaka, Japan.
As featured in:
Photocatalytic chemoselective reduction of epoxides to alkenes
along with formation of ketones in alcoholic suspensions of
silver-loaded titanium(IV) oxide at room temperature without the
use of reducing gases
Epoxides such as (2,3-epoxypropyl)benzene were
chemoselectively reduced to alkenes over silver-loaded TiO2
photocatalyst at room temperature without hydrogenation of the
C═C double bond to the C–C single bond even in the presence
of an H2 by-product.
See Hiroshi Kominami et al.,
Chem. Commun., 2014, 50, 4558.
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Photocatalytic chemoselective reduction of
epoxides to alkenes along with formation of
ketones in alcoholic suspensions of silver-loaded
titanium(IV) oxide at room temperature without
the use of reducing gases†
Cite this: Chem. Commun., 2014,
50, 4558
Received 9th December 2013,
Accepted 6th March 2014
DOI: 10.1039/c3cc49340g
Hiroshi Kominami,* Satoshi Yamamoto, Kazuya Imamura, Atsuhiro Tanaka and
Keiji Hashimoto
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(2,3-Epoxypropyl)benzene was chemoselectively reduced to allyl- UV absorber because it is inexpensive and non-toxic to humans
benzene along with formation of ketones in alcoholic suspensions and the environment. Since the photocatalytic reaction satisfies
of a silver-loaded titanium(IV) oxide photocatalyst at room tempera- almost all of the 12 proposed requirements for green chemistry,5
ture under atmospheric pressure without the use of reducing gases, organic synthesis of various compounds using photocatalysts
and various epoxides were also reduced to the corresponding alkenes. has recently been studied by many researchers,6 and the number
of papers on photocatalytic reduction of organic compounds by
Deoxygenation of epoxides is important for transformation from using photogenerated electrons has recently been increasing.7
an epoxy group to a CQC double bond as a deprotection Our research group has recently reported that 3-nitrostyrene was
reaction.1 However, conventional deoxygenation requires stoi- chemoselectively reduced to 3-aminostyrene without reduction
chiometric reagents such as phosphines, silanes, iodides, and of the CQC double bond to 3-ethylaniline in a suspension of a
heavy metals2 and gives a large amount of undesirable waste, TiO2 photocatalyst in the presence of hole scavengers at room
although many efforts have been devoted to improving reaction temperature under metal-free and reducing gas-free condi-
conditions and minimizing the use of toxic reagents by using tions.8a Subsequently, our group reported that 3-aminostyrene
catalytic systems.3 Therefore, a ‘‘green’’ method for deoxygena- and acetone were simultaneously produced almost stoichio-
tion of epoxides to alkenes is required. Recently, excellent metrically as the reduced and oxidized products, respectively,
catalysts for deoxygenation of epoxides, i.e., gold (Au) and silver when 2-propanol was used as a solvent also acting as a hole
(Ag) supported on hydrotalcite (HT), were reported.4 The Au–HT scavenger.8b In this study, we examined a new photocatalytic
and Ag–HT catalysts exhibited high selectivity for alkenes free redox system, i.e., reductive deoxygenation of epoxides and
from subsequent hydrogenation to alkanes. However, even in the oxidation of alcohols, and found that epoxides were chemoselec-
series of Au–HT and Ag–HT catalysts, temperatures higher than tively converted to the corresponding alkenes without reduction
333 K and/or reducing gases such as carbon monoxide and of the CQC double bond together with formation of the corres-
hydrogen (H2) are essential. Longer reaction time and/or severe ponding ketones over Ag-loaded TiO2 at room temperature
conditions are required for deoxygenation of benzylic epoxide, without the use of reducing gases.
i.e., (2,3-epoxypropyl)benzene (EPB) to allylbenzene (ALB). There-
fore, a new catalytic system working at room temperature with- is shown in Fig. S1(a) (ESI†). Silver particles were observed in
out the use of reducing gases is keenly desired. the image, indicating that Ag nanoparticles were deposited on
A transmission electron microscope (TEM) image of Ag–TiO2
When titanium(IV) oxide (TiO2) is irradiated by UV light, the TiO2 surface. The average diameter of Ag particles in the
charge separation occurs and thus-formed electrons (eꢀ) in the sample was determined to be 2.4 nm (Fig. S1(b), ESI†).
conduction band and holes (h+) in the valence band cause reduction
Fig. 1 shows time courses of the amounts of EPB remaining
and oxidation, respectively. TiO2 has been used for a long time and ALB and acetone formed in a 2-propanol suspension of
as an indispensable inorganic material such as a pigment and a Ag–TiO2 under deaerated conditions. The amount of H2, i.e.,
another reduced product, is also plotted in Fig. 1. Just after
photoirradiation, the amount of EPB monotonously decreased,
while ALB and acetone were formed as the reduction product
of EPB and the oxidation product of 2-propanol, respectively.
Department of Applied Chemistry, Faculty of Science and Engineering,
Kinki University, Kowakae, Higashiosaka, Osaka 577-8502, Japan.
E-mail: hiro@apch.kindai.ac.jp
After 60 min, EPB was almost completely consumed and ALB
was obtained almost quantitatively (99% yield) after 90 min.
† Electronic supplementary information (ESI) available: Experimental procedure,
Fig. S1 and S2. See DOI: 10.1039/c3cc49340g
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Fig. 2 Effects of metal co-catalysts (1.0 wt%) on the yields of ALB and H2
produced in photocatalytic reductive deoxygenation of EPB in 2-propanol
suspensions for 30 min.
Fig. 1 Time courses of the amounts of EPB, ALB, H2 and acetone in a 2-
propanol suspension of 1.5 wt% Ag–TiO2 photocatalysts under deaerated
conditions.
confirmed that no reaction occurred. These results indicate that
H2 in the gas phase does not contribute to the reduction of EPB.
Fig. 2 shows effects of metal co-catalysts (1.0 wt%) on yields of
ALB and H2 produced in photocatalytic reductive deoxygenation of
EPB in 2-propanol suspensions for 30 min. Bare TiO2 produced a
small amount of ALB, while Ag–TiO2 predominantly produced ALB,
as shown in Fig. 1. When gold (Au) and copper (Cu) were used as the
co-catalysts, only a small amount of ALB was produced and a large
amount of H2 was evolved as the reduction product. Palladium (Pd)
and platinum (Pt) had no effect on reduction of EPB to ALB under
the present conditions, i.e., only dehydrogenation of 2-propanol
(eqn (3)) occurred over Pd–TiO2 and Pt–TiO2. The values of material
balance of EPB and ALB were close to unity except for Pd–TiO2, while
the value of Pd–TiO2 was less than 0.1, indicating that another
side reaction consuming EPB and/or ALB occurred over Pd–TiO2.
The results shown in Fig. 2 clearly indicate that Ag was the sole
co-catalyst effective for photocatalytic reductive deoxygenation of
EPB to ALB in 2-propanol suspensions.
Expected reaction processes of photocatalytic reductive
deoxygenation of EPB to ALB over Ag are shown in Scheme 1:
(1) surface H species are formed on Ag by reduction of protons by
photogenerated electrons under irradiation of UV light, while 2-
propanol is oxidized by holes (oxidation process not shown in
Scheme 1), (2) EPB is reduced by the surface H species with the aid
of Ag as a co-catalyst, resulting in formation of ALB and water and
in regeneration of the photocatalyst, and (3) H2 formation also
occurs as a result of elimination of the surface H species. Reaction
rates of (2) and (3) are important to determine the selectivity and
yield of ALB in this reaction. In the case of Ag–TiO2, the reaction
rate of (2) is larger than that of (3) probably because of strong
interaction between Ag and oxygen of EPB and larger activation
energy for H2 formation. Less production of ALB over other
The reaction, i.e., reductive deoxygenation of EPB to ALB along
with oxidation of 2-propanol to acetone, is shown in eqn (1).
(1)
We also examined two blank reactions in the absence of
Ag–TiO2 (photochemical reaction) and in the absence of light
(thermal reaction) and confirmed that no reaction occurred in either
case. The yield of ALB was much larger than the amount of Ag
(7.0 mmol) loaded on the TiO2 surface. These results indicate that the
observed reaction under the present conditions was photocatalytic,
i.e., TiO2 and light contributed to the formation of ALB and acetone,
and Ag worked as a co-catalyst. From the ratio of the amount of ALB
and the amount of photons irradiated (eqn (2)), apparent quantum
efficiency (AQE) was calculated to be 13% in the experiment using a
UV light-emitting diode, indicating that deoxygenation of EPB to
ALB occurred effectively over Ag–TiO2.
2 ꢁ amount of ALB
AQEð%Þ ¼
ꢁ 100
(2)
number of incident photons
(3)
As shown in Fig. 1, H2 was also formed, indicating that
reduction of protons (H+) by photogenerated electrons also
occurred. Production of H2 along with formation of acetone
can be regarded as dehydrogenation of 2-propanol (eqn (3)).
When bare (Ag-free) TiO2 was used for dehydrogenation of
2-propanol (in the absence of EPB), H2 formation was negligible,
indicating that Ag loaded on TiO2 acted as sites for H2 evolution
(H+ reduction) as well as ALB formation (EPB reduction).
As shown in Fig. 1, dehydrogenation of 2-propanol (eqn (3))
occurred under excessive photoirradiation after complete con-
sumption of EPB. Simultaneous formation of ALB and H2 and
preservation of ALB after complete consumption of EPB indicate
that Ag–TiO2 had no activity in hydrogenation of ALB even in the
presence of H2. This is an important and excellent performance of
the Ag–TiO2 photocatalyst to maintain a high yield of ALB. We
examined another reaction, i.e., thermal reaction of EPB in the
presence of Ag–TiO2 under H2 (1 atm) in the dark at 298 K, and
Scheme 1 Expected reaction processes of photocatalytic reductive
deoxygenation of EPB to ALB over Ag–TiO2.
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Table 1 Photocatalytic deoxygenation of epoxides to the corresponding
epoxides other than p-conjugated epoxides such as styrene
oxide (entry 3). Ag–TiO2 was effective for deoxygenation of the
epoxide of an acyclic compound (entry 8), though longer reaction
time was required. In the deoxygenation of epoxides of alicyclic
compounds, less selectivity was obtained (entries 9 and 10). Since
these compounds are photosensitive, the selectivity may be
improved by the use of a gentle light source such as black light.
In summary, a new photocatalytic chemoselective redox system,
i.e., reductive deoxygenation of epoxides to the corresponding
alkenes and oxidation of alcohols to the corresponding ketones,
was examined at room temperature without the use of reducing
gases. Alkenes and ketones were simultaneously produced over
silver-loaded TiO2, and less reactive (2,3-epoxypropyl)benzene was
almost quantitatively reduced to allylbenzene without reduction of
the CQC double bond to propylbenzene even in the presence of an
H2 by-product. Chemoselective reduction of styrene oxides to
styrenes was achieved even over bare TiO2. Since alcohols other
than 2-propanol can be used as solvents and hole scavengers
(electron donors), various combinations of epoxides and alcohols
can be used. The results obtained in this study provide a wide
possibility of photocatalytic redox reaction including chemoselec-
tive production under gentle conditions such as room temperature
and atmospheric pressure without the use of reducing gases.
This work was partly supported by a Grant-in-Aid for Scientific
Research (No. 23560935) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT) of Japan. H. K. is grateful
for financial support from the Faculty of Science and Engineering,
Kinki University. K. I. and A. T. appreciate the Japan Society for
the Promotion of Science (JSPS) for a Research Fellowship for
young scientists.
alkenes in alcoholic suspension of Ag(1.5)–TiO2 at 298 Ka
Entry
1
Epoxides
Alkenes
t/h
1.5
1.5
0.5
1.5
Yieldb/%
Sel.b/%
99
99
96
1
2c
99
3d
31
4e
99
99
5
1
1
96
85
97
85
6d
7
8
3
5
85
89
90
99
9
3
3
11
26
11
59
10
a
Reaction conditions: Ag–TiO2 (50 mg), substrate (50 mmol), 2-propanol
(5 cm3), room temperature, and under Ar. Determined by GC using
b
Notes and references
1 (a) E. J. Corey and W. G. Su, J. Am. Chem. Soc., 1987, 109, 7534;
(b) G. A. Kraus and P. J. Thomas, J. Org. Chem., 1988, 53, 1395;
(c) W. S. Johnson, M. S. Plummer, S. P. Reddy and W. R. Bartlett,
J. Am. Chem. Soc., 1993, 115, 515.
c
d
an internal standard. Second use. Photocatalyst (bare TiO2).
e
2-Pentanol instead of 2-propanol.
photocatalysts may be explained by weaker or no interaction
with oxygen and a smaller activation energy for H2 formation
over the other metals.
2 R. C. Larock, Comprehensive Organic Transformations, Wiley,
New York, 1999, p. 272, and references therein.
3 (a) Z. Zhu and J. H. Espenson, J. Mol. Catal. A, 1995, 103, 87;
(b) K. P. Gable and E. C. Brown, Synlett, 2003, 2243; (c) J. B. Arterburn,
M. Liu and M. C. Perry, Helv. Chim. Acta, 2002, 85, 3225; (d) T. Itoh,
T. Nagano, M. Sato and M. Hirobe, Tetrahedron Lett., 1989, 30, 6387;
(e) H. Isobe and B. P. Branchaud, Tetrahedron Lett., 1999, 40, 8747.
4 (a) Y. Mikami, A. Noujima, T. Mitsudome, T. Mizugaki, K. Jitsukawa
and K. Kaneda, Tetrahedron Lett., 2010, 51, 5466; (b) T. Mitsudome,
A. Noujima, Y. Mikami, T. Mizugaki, K. Jitsukawa and K. Kaneda,
Angew. Chem., Int. Ed., 2010, 49, 5545; (c) T. Mitsudome, A. Noujima,
Y. Mikami, T. Mizugaki, K. Jitsukawa and K. Kaneda, Chem. – Eur. J.,
2010, 16, 11818; (d) A. Noujima, T. Mitsudome, T. Mizugaki,
K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2011, 50, 2986;
(e) T. Mitsudome, Y. Mikami, M. Matoba, T. Mizugaki, K. Jitsukawa
and K. Kaneda, Angew. Chem., Int. Ed., 2012, 51, 136.
5 P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, 1998.
6 M. A. Fox and M. T. Dulay, Chem. Rev., 1993, 93, 341.
7 (a) G. Palmisano, V. Augugliaro, M. Pagliaro and L. Palmisano, Chem.
Commun., 2007, 3425; (b) G. Palmisano, E. Garcia-Lopez, G. Marci,
V. Loddo, S. Yurdakal, V. Augugliaro and L. Palmisano, Chem.
Commun., 2010, 46, 7074.
8 (a) K. Imamura, K. Hashimoto and H. Kominami, Chem. Commun.,
2012, 48, 4356; (b) K. Imamura, T. Yoshikawa, K. Hashimoto and
H. Kominami, Appl. Catal., B, 2013, 134–135, 193.
Applicability of the Ag–TiO2 photocatalyst for a chemoselec-
tive redox system was investigated, and the results are summar-
ized in Table 1. Upon the re-use in deoxygenation of EPB to ALB,
Ag–TiO2 exhibited almost the same activity as that in the first use
(entries 1 and 2). Styrene oxides were reduced to the corres-
ponding styrenes selectively with high yields (entries 5 and 7) as
well as the deoxygenation of EPB (entry 1). When 2-pentanol was
used in place of 2-propanol, ALB was obtained almost quantita-
tively along with formation of 2-pentanone (entry 4, Fig. S2, ESI†),
indicating that various alcohols can be used for the present
chemoselective redox system. We also found that deoxygenation
of styrene oxide occurred even over bare TiO2 (entry 6), although
the performance was slightly lower than that of Ag–TiO2 (entry 5).
The use of metal-free TiO2 has another advantage that only
deoxygenation of epoxides occurs, i.e., eqn (3) does not occur
because H2 evolution requires co-catalysts such as Pt and Ag.
However, bare TiO2 did not show activities in deoxygenation of
4560 | Chem. Commun., 2014, 50, 4558--4560
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