Dehydrogenative lactonization of diols with a platinum-loaded titanium oxide photocatalyst

Article abstract of DOI:10.1039/c7pp00258k

A new catalytic route for the lactonization of diols was developed by using a metal-loaded TiO₂ photocatalyst. In particular, Pt-loaded rutile TiO₂ exhibited a high photocatalytic activity with high selectivity. In addition, it was found that a heterogeneous acid catalyst can accelerate this photocatalytic lactonization.

Full text of DOI:10.1039/c7pp00258k

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Dehydrogenative lactonization of diols with platinum-loaded
titanium oxide photocatalyst†
Emiko Wada,a,b Akanksha Tyagi,a Akira Yamamoto a,c and Hisao Yoshidaa,c
Received 00th January 20xx,
Accepted 00th January 20xx
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new catalytic route for lactonization of diols was developed by Shimizu found Pt-loaded SnO2 catalyst that well promotes the
using metal-loaded TiO2 photocatalyst. Especially, Pt-loaded rutile oxidative lactonization of various diols at 453 K under solvent-
TiO2 exhibited high photocatalytic activity with high selectivity. In free conditions.12 In addition, the reaction route with metal
addition, it was found that heterogeneous acid catalyst can oxide catalysts such as tungstic acids13 or ZrO2-supported
14
accelerate this photocatalystic lactonization.
WO3 with H2O2 has been studied for the dehydrogenative
lactonization of 1,2-benzenedimethanol to form phthalide in
high yield (ca. 90%) with high selectivity. Phthalide is a
common chemical intermediate for pharmaceutical chemicals
and other fine chemicals.13–15 Although phthalide can be also
obtained through hydrogenation of phthalic anhydride by
using a heterogeneous catalyst, it requires high temperature
such as 453 K and high pressure of H2 gas.16 It has been still
studied to develop new routes for various lactonization from
Lactones, which is cyclic esters, are important chemical
intermediates or solvents in organic syntheses.1 Conventional
synthesis route for lactones is intramolecular dehydrative
esterification of hydroxycarboxylic acid by Brønsted acid.2
Recently, many research groups have reported new reaction
routes to obtain lactones, such as reactions of epoxide or
aliphatic alcohols with carbon monoxide,3 and reactions of
allenol or butadiene with carbon dioxide.4 Another promising
route is the dehydrogenative oxidation of diols, for which
many homogeneous catalysts such as Ir- or Ru-complexes have
been reported.5,6 Cu-complex with nitroxyl radicals such as
TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) efficiently
catalyzes the aerobic oxidative lactonization of diols at room
temperature under ambient conditions.7 Although Fujita et al.
diols. As
a
new methodology, we have examined
dehydrogenative lactonization of diols at a mild condition
without consuming any other reagents by using photocatalyst.
In the present study, we found that Pt-loaded rutile TiO2
photocatalyst can promote the lactonization for the
dehydrogenative lactonization of 1,2-benzenedimethanol to
phthalide (eq. 1) with high yield and selectivity, which can be
also applied to various diols for producing lactones. To our
best knowledge, this is the first report for the dehydrogenative
lactonization of diols with hydrogen evolution by
heterogeneous photocatalysis around room temperature. In
addition, it was demonstrated that heterogeneous acid
catalysts having moderately weak acid sites such as alumina
can facilitate the photocatalytic reaction further.
developed
a
reusable Ir-complex catalyst,5 homogenous
catalytic reaction system are usually considered to have
difficulties in products separation and reuse of the catalyst.
On the other hand, heterogeneous catalysts for
lactonization have been also reported. For example, Cu-based
catalysts8 and Au-loaded TiO2 catalyst9 were reported for the
lactonization of 1,4-butanediol to form γ-butyrolactone in the
gas phase. CeO2-based binary oxides were developed for the
gaseous phase lactonization of 1,6-hexanediol to ε-
caprolactone.10 As for the liquid phase lactonization, Au-
loaded hydrotalcite catalyst was active for the lactonization of
1,4-butanediol with oxygen molecules.11 Recently, Touchy and
O
OH
hν
, Ar
O
+
2 H2
(1)
OH
M/TiO2
Three kinds of TiO2 powders supplied from Catalysis
Society of Japan (JRC-TIO-8, anatase, 338 m2/g; JRC-TIO-6,
rutile, 100 m2/g; and JRC-TIO-4, anatase and rutile, 50 m2/g)
were employed as the photocatalysts, where rutile and
anatase are referred to as (R) and (A), respectively. Metal
loaded TiO2 (M/TiO2) samples were prepared by
a
conventional photodeposition method.17,18 The Pt loading
amount was 0.1 wt%. The average particle size of the loaded
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Pt nanoparticles was determined to be 2.1 nm by a CO 8(A) photocatalyst. These results suggested that the rutile TiO2
adsorption method.17 For each catalytic reaction test, 0.1 g of of large specific surface area would be efficient to produce
the M/TiO2 sample was used. Before the reaction, the sample phthalide in high yield.
was photoirradiated for 20 min by a xenon lamp (PE300BUV)
100
200
100
200
with an optical long pass filter (λ>350 nm), where the light
intensity measured at 360 nm±15 nm in wavelength was 27
mW cm–2. After an argon purge, the reaction mixture, 200 or
400 µmol of diol dissolved in 4 mL of a solvent, was introduced
into a quartz reactor (46 cm3). The reaction tests were carried
out for 1 h or more under photoirradiation. Products in gas
phase were analyzed by GC-TCD (Shimadzu, GC-8A) and those
in liquid phase were analyzed by GC-MS (Shimadzu, GCMS-
QP2020). The product amounts were determined by each
calibration curves, although only the amount of 2-
(hydroxymethyl)benzaldehyde was determined by using the
calibration curve of phthalide. As the amount of hydrogen
produced was large, it could not be determined precisely.
Figure 1 shows the time courses of the product yields in
100
80
60
40
20
0
100
80
60
40
20
0
(b)
(a)
80
160
160
80
60
120
120
60
(3)
(3)
(2)
40
80
80
40
40
20
40
20
(2)
0
0
0
0
1
2
3
0
1
2
3
Time /h
Time /h
Fig.
benzenedimethanol (
the Pt/TIO-6(R) photocatalyst (b). Products were 2-(hydroxymethyl)-
benzaldehyde ( ) and phthalide ( ). Reaction conditions: 200 µmol of
1
Time courses of products yields in lactonization of 1,2-
1
) over the Pt/TIO-8(A) photocatalyst (a) and
2
3
1,2-benzenedimethanol, 4 mL of acetonitrile, 0.1 g of the Pt/TiO2
photocatalyst. The wavelength of the irradiation light was longer
than 350 nm, and the light intensity was 27 mW cm–2 measured at
360±15 nm.
the lactonization of 1,2-benzenedimethanol (1) over the
Pt/TiO2 samples consisting of anatase or rutile TiO2
photocatalyst in an acetonitrile solution. In the initial period of
the reaction over the Pt/TIO-8(A) photocatalyst, 2-
(hydroxymethyl)-benzaldehyde (
2) was preferably formed and
Table 1. Results of the reaction tests for the dehydrogenative
lactonization of 1,2-benzenedimethanol over various metal loaded TiO2
photocatalysts in acetonitrile for 1 h. a
the yield of phthalide (3) gradually increased (Fig. 1a). Then,
phthalide became the major product after 2 h with a decrease
of 2-(hydroxymethyl)benzaldehyde yield. The yield of
phthalide reached to 60 % in 3 h. This result indicates that 2-
(hydroxymethyl)benzaldehyde is formed as an intermediate,
and phthalide is produced by further oxidation. As for the
gaseous product, only hydrogen was detected. Therefore, the
lactonization of benzenedimethanol to form phthalide
proceeds via two-step dehydrogenative oxidation (Scheme in
Table 1). However, the reaction over the Pt/TIO-8(A)
photocatalyst provided low selectivity to phthalide such as
60% after 3 h, but only 25 % after 1 h (Table 1, entry 2). The
carbon balance was also low such as 60 % after 3 h, which
means that formation of other byproducts or decomposition of
the substrate and products would also take place.
Entry Catalyst Crystal Specific Yield /µmol
Phase b surface
Phthalide,
Yield Selectivity
3
c
Phthalide Aldehyde H2
area
/ m2g-1
3
2
(%)
(%)
1
2
3
4
5
6
Pt/TIO-6 (R)
Pt/TIO-8 (A)
Pt/TIO-4 (A,R)
Pd/TIO-6 (R)
Rh/TIO-6 (R)
100
338
50
180
n.d. d
200 90
110 25
90 19
45 56
65 60
90
25
15
23
49
-
52
31
45
98
110
12
100
100
100
10
40
TIO-6
(R)
n.d.
n.d.
n.d.
-
On the other hand, the Pt/TIO-6(R) photocatalyst produced
phthalide in the yield of 90 % even at initial 1 h with high
a Reaction conditions were the same as those in Fig. 1. b(R) and (A)
c
represent rutile and anatase, respectively. Hydrogen was actually
selectivity such as 90% (Fig. 1b and Table 1, entry 1), where the detected, but the value for the obtained amount would contain large
experimental error. d not detected.
intermediate 2-(hydroxymethyl)benzaldehyde was not
detected. That is, the Pt/TIO-6(R) sample consisting of rutile
As a co-catalyst, Pd and Rh were examined on the rutile
TiO2 photocatalyst. Although the Pd/TIO-6(R) sample and the
Rh/TIO-6(R) sample also promoted the reaction (Table 1,
entries 4 and 5), the phthalide yield was less than that with the
Pt/TIO-6(R) sample. In addition, the bare rutile TiO2 sample
exhibited no activity (Table 1, entry 6). Thus, it is obvious that
the precious metal co-catalyst is necessary, and among them
Pt drastically improves the photocatalytic activity. The
deposited metal nanoparticles would enhance the electron-
hole separation as an electron receiver, decrease their
recombination, and promote the reduction of proton by the
photoexcited electron.
TiO2 showed 3.5 times higher yield of phthalide than the
Pt/TIO-8(A) sample consisting of anatase TiO2, although the
BET surface area of TIO-6(R) (100 m2/g) is smaller than that of
TIO-8(A) (338 m2/g) (Table 1, entries 1 and 2). The Pt/TIO-4(A,
R) photocatalyst consisting of both anatase and rutile phases
with low specific surface area did not afford much phthalide
(Table 1, entry 3). These results suggest that pure rutile phase
is suitable for this lactonization.
The results of the photocatalytic reaction tests with the
Pt/TiO2 samples, prepared from the calcined TIO-6(R) and TIO-
8(A) samples having lower specific surface areas, were plotted
in Fig. S1. The yield of phthalide increased with an increase of
the specific surface area, and the slope for the Pt/TIO-6(R)
photocatalysts was clearly steeper than that for the Pt/TIO-
The solvent also affected the activity and selectivity of the
photocatalyst (Table S1). Some solvents such as water,
2 | J. Name., 2012, 00, 1-3
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acetone, and THF, gave undesired byproducts such as homo- Pt/TIO-6(R) photocatalyst (Table S3, entries 3 and 4). However,
coupling products of solvent or other byproducts from the in the presence of TiNT some side reactions were also
reaction between 1,2-benzenedimethanol and solvents (Table promoted, indicating that the acid catalyst having moderately
S1, entries 2–4). When acetonitrile was used as a solvent, strong acid sites20 is not available for this purpose. In contrast,
these byproducts were not detected (Table S1, entry 1). Thus, the acid catalyst having weak acid sites such as Al2O3 can
acetonitrile was chosen as the best solvent among the tested selectively accelerate the photocatalytic reaction rate without
solvents for this lactonization reaction.
promoting the side reactions. These results suggest that the
To clarify the reason for the different activity between weak acid sites can improve the reaction to form phthalide.
anatase and rutile, some experiments were carried out (Table Separate experiment evidenced that the acid catalysts itself
2S). First, the adsorption amount of the substrate and the did not promote the reaction without photocatalyst,
product, i.e., 1,2-benzenedimethanol and phthalide, on the confirming that the acid catalysis accelerates at least one step
catalyst surface was measured in the acetonitrile in the dark. during the photocatalytic reaction, which might be the second
Both the Pt/TIO-6(R) and Pt/TIO-8(A) photocatalysts showed cyclization step. Here, it is proposed that the blended catalyst
very low and similar values (Table S2, entries 1 and 2), consisting of the Pt/TiO2 photocatalyst and the Al2O3 acid
suggesting that the adsorption property of these samples catalyst is efficient for the photocatalytically dehydrogenative
cannot explain the difference in the photocatalytic activity. lactonization. The optimization of their combination, such as
Second, the stability of the product against the successive kinds of the acid catalyst and the ratio of components, will
reaction was examined, where the Pt/TiO2 sample was further improve this reaction system. On the other hand,
photoirradiated for 1 h in the presence of 200 µmol of although the TiO2 has also acid sites,21 the yield of phthalide
phthalide in the acetonitrile solution. After the was not increased by the addition of bare TIO-6(R) sample to
photoirradiation, the amount of phthalide was not changed so the reaction mixture including the Pt/TIO-6(R) sample, but
much in the presence of the Pt/TIO-6(R) photocatalyst (Table rather decreased (Table S3, entry 2). As mentioned above, the
S2, entry 1), whereas it much decreased in the presence of the bare TIO-6(R) sample did not give phthalide under the light
Pt/TIO-8(A) sample and the Pt/TIO-4(A, R) sample (Table S2, irradiation (Table 1, entry 6). It is considered that the
entries 2 and 3). This means that the successive reaction of the additional bare TiO2 powder could rather shade the Pt/TIO-
desired product hardly takes place on the Pt loaded rutile TiO2 6(R) photocatalyst from the light to reduce the photoexcitation
photocatalyst. Therefore, it is one of the reason why the of the Pt/TIO-6(R) photocatalyst.
Pt/TIO-6(R) photocatalyst exhibited the high yield of phthalide
Here, the reaction mechanism is tentatively proposed as
with high selectivity. Since the Pt/ TIO-8(A) photocatalyst shown in Fig. S3. The photoexcitation of TiO2 photocatalyst
containing anatase TiO2 showed less selectivity even at the provides an excited electron and a hole. The hydroxy group of
initial stage of the reaction test (Fig. 1a), anatase TiO2 would 1,2-benzenedimethanol is oxidized by the hole to form the
promote not only the successive reaction of the product but radical species and proton. Then, the hydrogen radical
also some other reactions.
elimination follows to form the intermediate, 2-
The present photocatalytic lactonization consists of the (hydroxymethyl)benzaldehyde. On the other hand, the excited
two dehydrogenation steps depicted as the scheme in Table 1. electron reduces proton to form the hydrogen radical, which
The reaction did not proceed in the dark or in the absence of react with another hydrogen radical to form molecular
the photocatalyst, indicating that the lactonization proceeds hydrogen. By the second photoexcitation of the TiO2
photocatalytically. Figure S2 shows the pseudo Arrhenius plot photocatalyst, further oxidation of the hydroxy group of the 2-
for the phthalide formation with the Pt/TIO-6(R) photocatalyst, (hydroxymethyl)benzaldehyde takes place, and the formed
which was obtained from the temperature-controlled oxygen radical moiety attacks the carbonyl carbon to form the
photocatalytic reaction experiments. The apparent activation phthalide and hydrogen radical. This step might be assisted by
energy for the formation of phthalide calculated from the plot the surface acid sites when additional acid catalyst is used for
was 4.6 kJ mol−1, which would be acceptable value as a typical the reaction. Molecular hydrogen is produced again from the
activation energy for photocatalytic reactions.19 Although it hydrogen radicals. Thus, it is suggested that this photocatalytic
12
was reported in the literature
that Pt nanoparticles can lactonization is the two-photon process.
In order to expand the scope of the current reaction
function as a metal catalyst in dehydrogenative lactonization,
they did not show catalytic performance on TiO2 support. Also system, various diols were examined for lactonization with the
in the present study, the result shows that metal catalysis did Pt/TiO2 photocatalyst (Table 2). It was found that all the
not contribute to the reaction rate in the photocatalytic examined diols were transformed into the corresponding
condition. Thus, it was clarified that the rate-determining step lactones. The Pt/TIO-6(R) photocatalyst promoted all these
of the reaction is not thermally activated catalysis but reactions faster than the Pt/TIO-8(A) photocatalyst (Table 2,
photocatalysis.
Further, the addition of heterogeneous acid catalyst to the than that in the lactonization of 1,2-benzenedimethanol
reaction system was examined (Table S3). The yield of mentioned above. The lactonization of cis-1,2-
phthalide increased when the acid catalysts such as Al2O3 or cyclohexanedimethanol ( ) over the Pt/TIO-6(R) photocatalyst
protonated titanate nanotubes (TiNT)20 were introduced to the gave dominantly the corresponding lactone compounds
entries 1−8). The yields of lactones in these cases were lower
4
6
and
and
photocatalytic reaction mixture in the coexistence of the
7
((3aS,7aR)-hexahydroisobenzofuran-1(3H)-one
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(3aR,7aS)-hexahydroisobenzofuran-1(3H)-one, total 83 µmol,
In conclusion, the new synthesis route for the lactones of
42 % yield) as the stereoretentive products, and the other diols with Pt/TiO2 photocatalysts was established. The
lactone
compounds
8
and
9
((3aR,7aR)- photocatalytic lactonization proceeded via two-step
(3aS,7aS)- dehydrogenation process as a two-photon process. The Pt
hexahydroisobenzofuran-1(3H)-one
and
hexahydroisobenzofuran-1(3H)-one, total 8.0 µmol, 4.0 % loaded rutile TiO2 photocatalyst exhibited a higher yield of
yield) as the minor products of the stereoinversion reaction lactones with higher selectivity than Pt loaded anatase TiO2.
(Table 2, entry 1). The Pt/TIO-8(A) photocatalyst gave the same The blended catalyst consisting of the Pt/TiO2 photocatalyst
products with low yields (Table 2, entry 2). The Pt/TIO-6(R) and the Al2O3 acid catalyst is more efficient for the
photocatalyst gave a higher stereoretention ratio, r=11, than photocatalytic dehydrogenative lactonization than Pt/TiO2
the Pt/TIO-8(A) photocatalyst, r=5.2 (Table 2, entries 1 and 2). photocatalyst alone.
It was experimentally confirmed that the stereoinversion
products,
and 9, were formed by a photocatalytic reaction22 scholarship. A. Tyagi would like to thank JICA for providing the
and not by a catalytic keto-enol tautomerization23 or a Norrish scholarship under the IIT Hyderabad-JICA Friendship project.
I type photochemical reaction24 from the major products,
and , respectively, in the present condition since the r values
E. Wada would like to thank JSPS for the doctoral
8
7
6
Notes and references
for the Pt/TIO-6 and Pt/TIO-8 samples did not change even
after the stirring of the resulting reaction mixture in dark with
the catalyst or the photoirradiation of its filtrate without the
catalyst. It should be noted that the activity of the
photocatalytic epimerization depends on the property of the
TiO2 photocatalyst. The same tendency to retain the
stereochemical structure was observed in the reaction of
1
D. M. Paul, Essential of Organic Chemistry, 3rd ed., 2006
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trans-1,2-cyclohexanedimethanol (
but the total yields of lactone compounds decreased in
comparison with the lactonization of cis-1,2-
cyclohexanedimethanol ( ), which would be related to the
structural torsion of the lactone and . The lactonization of
5) (Table 2, entries 3 and 4),
4
Y. Inoue, Y. Sasaki and H. Hashimoto, Bull. Chem. Soc. Jan.
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8
9
5
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(
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7
8
9
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Total
46
6
+
7
8+9
1
2
3
4
5
6
7
8
Pt/TIO-6
Pt/TIO-8
Pt/TIO-6
Pt/TIO-8
4
5
6
6
–
–
9b
9b
42
17
4.0 11
3.3 5.2
20
24
3.1 21
1.9 8.1
6.8
4.5
10
Pt/TIO-6 10
Pt/TIO-8
11
13
20
Yoshida, Catal. Sci. Technol., 2017, 7, 2457.
5.0
2.5
n.d. d
18 H. Yoshida, Y. Fujimura, H. Yuzawa, J. Kumagai and T.
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Pt/TIO-6 12
Pt/TIO-8
a
Reaction conditions: 200 µmol of diol,
4 mL of acetonitrile, the other
2016,
Yoshida, Catal. Sci. Technol., 2017,
6
, 4577; A. Tyagi, A. Yamamoto, T. Kato and H.
7
conditions were the same as those in Fig. 1. b The calibration curve of starting
materials were applied to determine the yields of lactones. c r = (sum of yields
of products via stereoretention reaction) / (sum of yields of products via
stereoinversion reaction). d Not detected.
, 2616.
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4 | J. Name., 2012, 00, 1-3
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Photochemical & Photobiological Sciences
Page 6 of 6
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DOI: 10.1039/C7PP00258K
Graphical Abstract
The Pt loaded rutile TiO2 photocatalyst promotes dehydrogenative lactonization of diols.
The reaction rate is improved by addition of Al2O3. (20 words)