Acceptorless dehydrogenative lactonization of diols by Pt-loaded SnO2 catalysts

Article abstract of DOI:10.1039/c5ra03337c

We report herein a new heterogeneous catalytic system for dehydrogenative lactonization of various diols under solvent-free and acceptor-free conditions using 1 mol% of Pt-loaded SnO₂, providing the first successful example of acceptorless lactonization of 1,6-hexanediol to ε-caprolactone by a heterogeneous catalyst. This journal is

Full text of DOI:10.1039/c5ra03337c

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Acceptorless dehydrogenative lactonization of
diols by Pt-loaded SnO2 catalysts†
Cite this: RSC Adv., 2015, 5, 29072
a
ab
*
Abeda Sultana Touchy and Ken-ichi Shimizu
Received 24th February 2015
Accepted 18th March 2015
DOI: 10.1039/c5ra03337c
www.rsc.org/advances
We report herein a new heterogeneous catalytic system for dehy- lactonization; the solvent-free and acceptor-free lactonization of
drogenative lactonization of various diols under solvent-free and various diols was effectively catalyzed by 1 mol% of Pt-loaded
acceptor-free conditions using 1 mol% of Pt-loaded SnO2, providing SnO2 (Pt/SnO2).
the first successful example of acceptorless lactonization of 1,6-hex-
As listed in Table 1, we tested lactonization of 1,2-benzene-
dimethanol 1a under solvent-free conditions in N2 at 180 ꢀC for
36 h as a model reaction using the catalysts containing 1 mol%
(0.01 mmol) of transition metals (Pt, Ir, Re, Pd, Rh, Ru, Ag, Ni,
Cu, Co). Among various transition metal-loaded SnO2 (entries
1–10), Pt/SnO2 (entry 1) showed the highest yield (90%) of the
corresponding lactone, phthalide 2a. GCMS and TLC analyses
of the reaction mixture showed no indication of byproducts
such as products of intermolecular reactions such as oligomers.
The lactonization of 1a did not occur in the presence of 39 mg of
SnO2 itself (entry 11). Comparison of various Pt catalysts on
different support materials (SnO2, Al2O3, Nb2O5, C, SiO2, TiO2,
ZrO2, MgO, HBEA zeolite) shows that SnO2 is the best support
material (entries 1, 12–19).
Aer the reaction with Pt/SnO2 for 5 h, we carried out
quantication of the H2 evolved in the gas phase. The yields of
gas phase H2 (35%) and the lactone 2a (35%) were identical to
each other (eqn (1)), indicating that H2 was evolved quantita-
tively during the dehydrogenative lactonization. The reaction
under N2 ow did not result in higher yield than the yield under
the standard closed system.
anediol to 3-caprolactone by a heterogeneous catalyst.
Lactones are important compounds in organic synthesis and
industrial production of solvents, avors, and pharmaceutical
compounds. Dehydrogenative1–8 and oxidative9–20 lactonizations
of diols are potentially useful methods for the synthesis of
lactones. A number of homogeneous and heterogeneous cata-
lytic systems have been reported to be effective for the oxidative
lactonization of diols by oxidants, such as carbonyl
compounds,9–14 oxygen15–19 and hydrogen peroxide.20 In view of
atom-efficiency, a more preferable methodology is lactonization
of diols in the absence of an oxidant (acceptor) via an accept-
orless dehydrogenative coupling strategy.21 Homogeneous
Ru1,2,4 and Ir3,5 catalysts were reported to be effective for this
reaction, but these systems suffer from difficulties in catalyst/
product separation and catalyst reuse except for a Ir-catalyzed
system recently reported by Fujita et al.5 Although heteroge-
neous Cu catalysts are effective for the gas phase dehydrogen-
ative lactonization of 1,4-butanediol,6,7 these systems suffer
from limited scope. An exceptional example of the heteroge-
neous catalytic system for acceptorless lactonization of various
diols is reported by Kaneda et al.8 using 7.1 mol% of Cu-loaded
hydrotalcite. During our continuing studies in heterogeneous Pt
catalysts for acceptorless dehydrogenation of alcohols,22 dehy-
drogenative coupling of o-aminobenzamide with alcohols23 and
acceptorless self-coupling of primary alcohols to esters,24 we
found a new heterogeneous catalyst for the acceptorless
Table 2 shows the effect of reaction conditions on the catalytic
activity of Pt/SnO2 for the standard lactonization of 1a. The
reaction under the solvent-free conditions gave higher yield
(90%) than the yields (62–80%) in the presence of 1 mL solvents
(o-xylene, diglyme, mesitylene, n-dodecane). In the solvent-free
condition, the reaction under 1 atm N2 gave higher yield than
that under 1 atm O2 (51%). Under O2, the metallic Pt surface can
be partly covered with oxygen atoms, which inhibit the activation
ofthe diolandlowertheyield. In the solvent-freecondition under
ꢀ
N2, the reaction at lower temperature (150 C) resulted in low
aCatalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021,
Japan. E-mail: kshimizu@cat.hokudai.ac.jp; Fax: +81-11-706-9163
bElements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
Kyoto 615-8520, Japan
yield of 2a (55%). Consequently, the standard conditions was
found to be the solvent-free conditions under 1 atm N2 at 180 ꢀC
for 36 h, which gave full conversion of 1a and 90% yield of 2a.
Aer completion of the reaction, 2-propanol (10 mL) was added
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra03337c
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Table Lactonization of 1,2-benzenedimethanol (1a) by various
RSC Advances
1
catalyst was reused three times, but the yield of 2a gradually
decreased with increase in the cycle (Fig. 1). ICP-AES analysis of
the solution aer the rst cycle showed that the content of Pt in
the solution (1.6 ppm, 0.02% of Pt in the catalyst used) and that of
Sn (3.0 ppm, 0.002% of Sn in the catalyst used) were quite low.
To study the substrate scope of the present catalytic system,
the reactions of various diols were conducted (Table 3). Benzylic
diols (entries 1–4), linear (entries 5–7) and non-linear (entries
8–11) aliphatic diols were converted to the corresponding
lactones in moderate to high yields of 66–90%. GC charts of the
reaction mixture for entries 8–10 showed four to ve small
peaks due to byproducts with high boiling points, but the
amount of the byproducts were too small for their identica-
tion. From a viewpoint of biorenery, the lactonization of
1,6-hexanediol (1,6-HD) into 3-caprolactone is of particular
importance, because 1,6-HD can be produced by multistep
hydrogenation of lignocellulosic biomass such as 5-hydroxy-
methyl-2-furfural (HMF)12,13 and furfural25 and 3-caprolactone can
be converted to 3-caprolactam as an important bulk chemical.12,13
Heeres and co-workers12,13 reported the rst example of the
oxidative lactonization of 1,6-HD to 3-caprolactone using methyl
isobutyl ketone (MIBK) as oxidant and a Ru complex as homo-
geneous catalyst, but the system suffers from drawbacks such as
a need of excess amount of MIBK and difficulty in catalyst/
product separation. Obviously, acceptorless lactonization of
1,6-HD to caprolactone by a heterogeneous catalyst is preferred
from an industrial viewpoint. Our result (entry 7) gave the rst
example of acceptorless lactonization of 1,6-HD by a heteroge-
neous catalyst, giving 3-caprolactone in high yield (86%). Aer
the reactions, followed by adding 2-propanol (10 mL) to the
mixture, Pt/SnO2 was removed from the mixture and the
lactones were isolated by column chromatography, resulting in
high isolated yield (81%) of 3-caprolactone.
catalysts
Entry
Catalyst
Conv. (%)
Yield (%)
1
2
3
4
5
6
7
8
Pt/SnO2
Ir/SnO2
Ni/SnO2
Pd/SnO2
Co/SnO2
Rh/SnO2
Cu/SnO2
Ru/SnO2
Re/SnO2
Ag/SnO2
SnO2
Pt/Al2O3
Pt/Nb2O5
Pt/C
Pt/SiO2
Pt/TiO2
Pt/ZrO2
Pt/MgO
Pt/HBEA
100
15
15
10
10
5
5
4
3
2
0
9
7
6
2
2
2
5
90
9
7
6
4
3
1
1
1
0
0
3
2
1
0
0
0
0
0
9
10
11a
12
13
14
15
16
17
18
19
2
a
Catalyst amount was 39 mg.
a
Table 2 Lactonization of 1a by Pt/SnO2
Solvent
Gas
Conv. (%)
Yield (%)
No solvent
o-Xylene
Diglyme
Mesitylene
n-Dodecane
No solvent
No solvent (150 ꢀC)
N2
N2
N2
N2
N2
O2
N2
100
80
92
85
70
67
69
90
72
80
76
62
51
55
Recently, we studied the dehydrogenative esterication of
primary alcohols to esters by Pt/SnO2.24 On the basis of the
results of model reactions and the IR result that the benzalde-
hyde adsorbed on SnO2 gave the C]O stretching band at lower
wavenumber than those on g-Al2O3 and SiO2, we proposed a
possible pathway via dehydrogenation alcohol to adsorbed
aldehyde species (on Sn4+ Lewis acid site) which undergo
nucleophilic attack of another alcohol to give hemiacetal fol-
lowed by its dehydrogenation to the ester. Considering that the
similarity between this intermolecular esterication and the
a
Conditions: 1 mol% Pt/SnO2 (0.01 mmol Pt), 1 mmol 1a, 0 or 1 mL
solvent, 180 ꢀC, 36 h. Yield was determined by GC.
to the reaction mixture and catalyst was separated by centrifu-
gation. The catalyst was then washed by acetone three times,
followed by centrifugation and drying in oven (under air) at 90 ꢀC
for3h, followedbyH2-reductionat150ꢀCfor0.5 h. Therecovered
(1)
Fig. 1 Reuse of Pt/SnO2 for lactonization of 1a. Conditions are shown
in Table 1.
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Table 3 Dehydrogenative lactonization of various diols catalyzed by Pt/SnO2
Entry
1
Diol
Product
Conv. (%)
100
Yielda (%)
90 (87)
2
3
93
74
85 (81)
66
4
96
83 (78)
5
6
100
100
80 (76)
85 (82)
7
8
9
93
97
96
86 (81)
75
75
10
98
91
70
83
11
a
GC yields. Isolated yields are in parentheses.
lactonization (intermolecular esterication), it is suggested that of 1,6-HD from biomass-derived furfurals12,13,25 this method can
Sn4+ Lewis acid site is responsible for the high activity of Pt/ provide a key process in sustainable production of nylon-6 from
SnO2 for the present lactonization system.
renewable resources.
In conclusion, we developed a new heterogeneous catalyst,
Pt/SnO2, for the acceptorless dehydrogenative lactonization of
diols under solvent-free conditions. The method was effective
for the synthesis of various lactones. We gave the rst example
of the catalytic acceptorless lactonization of 1,6-HD to 3-capro-
lactone as an intermediate of 3-caprolactam, the monomer for
nylon-6. Considering recent advances in the catalytic synthesis
Experimental
Commercially available organic compounds (from Tokyo
Chemical Industry or Kanto Chemical) were used without
further purication. The GC (Shimadzu GC-2014) and GCMS
(Shimadzu GCMS-QP2010) analyses were carried out with Ultra
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ALLOY capillary column UA+-1 (Frontier Laboratories Ltd.) “Elements Strategy Initiative to Form Core Research Center”
using N2 and He as the carrier gas. and a Grant-in-Aid for Scientic Research on Innovative Areas
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Chemical Laboratory Co., Ltd.) at 500 ꢀC for 3 h. Nb2O5 was
prepared by calcination of niobic acid (CBMM) at 500 ꢀC for 3 h.
g-Al2O3 was prepared by calcination of g-AlOOH (Catapal B
Alumina purchased from Sasol) at 900 ꢀC for 3 h. ZrO2 was
Notes and references
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equipped with the same column as GC analyses as well as H
NMR and 13C NMR analyses of the isolated products. The
analysis of the gas phase product (H2) was carried out by the
mass spectrometer (BELMASS). For the catalytic tests in entries
1, 4, 5 and 7 of Table 2, we determined isolated yields of lactones
as follows. Aer the reaction, the catalyst was removed by
ltration and the reaction mixture was concentrated under
vacuum evaporator to remove the volatile compounds. Then, the
lactones were isolated by column chromatography using silica
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Acknowledgements
This work was supported by Grant-in-Aids for Scientic 25 K. Y. Chen, S. Koso, T. Kubota, Y. Nakagawa and
Research B (26289299) from MEXT (Japan), a MEXT program
K. Tomishige, ChemCatChem, 2010, 2, 547–555.
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