Oxidant-free lactonization of diols using a hydrotalcite-supported copper catalyst

Article abstract of DOI:10.3987/COM-09-11814

We newly synthesized well-crystallized hydrotalcite-supported copper nanoparticles, denoted as Cu/HT(c), which acted as a highly efficient heterogeneous catalyst for oxidant-free lactonization of various diols under liquid-phase conditions. The Cu/HT(c) catalyst could be recovered by simple filtration and reused without the significant loss of its activity and selectivity.

Full text of DOI:10.3987/COM-09-11814

HETEROCYCLES, Vol. 80, No. 2, 2010
855
HETEROCYCLES, Vol. 80, No. 2, 2010, pp. 855 - 861. © The Japan Institute of Heterocyclic Chemistry
Received, 7th August, 2009, Accepted, 18th September, 2009, Published online, 18th September, 2009
DOI: 10.3987/COM-09-11814
OXIDANT-FREE
LACTONIZATION
OF
DIOLS
USING
A
HYDROTALCITE-SUPPORTED COPPER CATALYST
Yusuke Mikami,a Kaori Ebata,a Takato Mitsudome,a Tomoo Mizugaki,a
Koichiro Jitsukawa,a and Kiyotomi Kanedaa,b,*
a Department of Materials Engineering Science, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan. E-mail: kaneda@cheng.es.osaka-u.ac.jp;
Fax: +81 6-6850-6260; Tel: +81 6-6850-6260
b Research Center for Solar Energy Chemistry, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Abstract – We newly synthesized well-crystallized hydrotalcite-supported
copper nanoparticles, denoted as Cu/HT(c), which acted as a highly efficient
heterogeneous catalyst for oxidant-free lactonization of various diols under
liquid-phase conditions. The Cu/HT(c) catalyst could be recovered by simple
filtration and reused without the significant loss of its activity and selectivity.
Lactones are important as components of flavors, medicines, and solvents, and as synthetic intermediates
for herbicides and rubbers.1 Among many approaches to the synthesis of lactones,2 the oxidant-free
catalytic lactonization of diols is one of the most attractive protocols because of easy availability of diols,
high atom efficiency, and production of only nonpolluting molecular hydrogen as a coproduct (Scheme
1).3 In particular, supported Cu-particle catalysts are economically attractive for the oxidant-free
lactonization of diols.4 However, to date, such catalysts require high temperatures and the addition of
hazardous chromium compounds to prevent the aggregation of Cu particles.
O
HO
OH
O
catalyst
+ 2H2
Scheme 1. Oxidant-free catalytic lactonization of diols

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HETEROCYCLES, Vol. 80, No. 2, 2010
We have designed efficient heterogeneous catalysts based on the properties of hydrotalcites (HT;
Mg6Al2(OH)16CO3･nH2O).5 Very recently, we found that Cu nanoparticles supported on HT (Cu/HT)
showed high catalytic activity for the oxidant-free dehydrogenation of various alcohols into the
corresponding carbonyl compounds.6 The Cu/HT catalyst was much more active than other
heterogeneous Cu catalysts because Cu nanoparticles were highly stable on HT and did not aggregate into
inactive Cu species.
To further investigate the catalytic potential of Cu/HT, herein we newly synthesized well-crystallized
hydrotalcite-supported Cu nanoparticles, denoted as Cu/HT(c). These nanoparticles act as a highly
efficient heterogeneous catalyst for the oxidant-free lactonization of various diols under liquid-phase
conditions. The Cu/HT(c) catalyst overcomes problems previously reported for many Cu catalyst systems,
such as the need for additional chromium compounds, high temperatures, and high catalyst loadings.4 The
catalyst can be removed from the reaction mixture by simple filtration and reused without the loss of its
activity and selectivity.
To synthesize Cu/HT(c), we upgraded the Cu/HT previously prepared6 by subsequent hydrothermal
treatment of HT.7 The specifics are as follows: 70 mL of a mixed salt solution containing MgCl2 (21
mmol) and AlCl3 (7.0 mmol) was added quickly (within 5 s) to 280 mL of an aqueous NaOH solution
(150 mM) under vigorous stirring, followed by stirring for 30 min under Ar atmosphere and subsequent
separation by centrifugation. The resulting slurry was washed with deionized water (150 mL), dispersed
in 100 mL of deionized water, and transferred to a Teflon-lined stainless-steel autoclave, which was
placed in a preheated oven and treated hydrothermally at 100 °C for 16 h. The suspension was dried by
evaporation to afford well-crystallized white solid HT(c) (1.2 g). The XRD measurement (Figure 1)
showed the peak intensity of (003) to be 2.5 times greater than that reported previously for HT. Crystallite
sizes of HT(c) and HT calculated with Scherrer’s equation were 19 and 16 nm, respectively. The obtained
HT(c) (1.0 g) was added to 50 mL of an aqueous solution of Cu (II) trifluoromethanesulfonate (0.80
mmol). The mixture was stirred for 2 min, adjusted to a pH of 8.0 by the addition of an aqueous solution
of ammonia (25%), further stirred for 1 h at 25 °C, and filtered, and the solid was washed with deionized
water and dried. The resulting pale blue powder, HT(c)-supported Cu (II) species CuII/HT(c), was dried in
vacuo at 180 °C and reduced with 1 atm of H2 to afford well-crystallized Cu/HT(c) (4.5 wt%). XRD
measurement (Figure 1) shows the peak intensity of (003) to be 2.0 times greater than that reported
previously for Cu/HT. TEM analysis (Figure 2) shows the mean Cu-nanoparticle diameter to be 6.1 nm
(standard deviation 2.4 nm), indicating the formation of nanoparticles of uniform size. The size of these
nanoparticles is smaller than those of our previous Cu/HT (mean diameter 7.5 nm, standard deviation 4.3
nm).
We applied the Cu/HT(c) catalyst to the model reaction of the lactonization of 1,4-butanediol (1). Under

HETEROCYCLES, Vol. 80, No. 2, 2010
857
(B)
(A)
(iv)
40
(iii)
(ii)
30
20
10
0
(i)
0
2
4
6
8
10 12 14
0
10
20
30
40
50
60
70
10 nm
2θ (degrees)
Particle size (nm)
Figure 1. XRD patterns: (i) HT(c); Figure 2. (A) TEM image of CuHT(c); (B) Size distribution
diagram for the Cu nanoparticles of Cu/HT(c)
(ii) HT; (iii) Cu/HT(c); (iv) Cu/HT
Ar atmosphere, 1 (1.0 mmol) was added to mesitylene (5.0 mL) in the presence of Cu/HT(c) (0.10 g) and
the mixture was stirred at 150 °C.8 After 3 h, 1 was completely converted to give -butyrolactone (2) in
>99% yield together with two equivalents of molecular hydrogen; other products from 1 such as
tetrahydrofuran, n-butanol, and -hydroxybutyraldehyde were not formed (Table 1, entry 1). After
removal of the Cu/HT(c) catalyst from the reaction mixture at 50% conversion of 1, further stirring of the
filtrate under similar conditions did not yield any products, and negligible amounts of Cu species (0.08%
of the Cu species initially present) were detected in the filtrate by inductively coupled plasma (ICP)
analysis. These results clearly show that lactonization occurs on the surface of Cu/HT(c).
Among the various supports containing Cu nanoparticles that we examined, Cu/HT(c) showed the highest
catalytic activity (>99% yield of 2), and Cu/SiO2 showed excellent activity (95% yield of 2) (Table 1,
entries 1 and 6). In contrast, Cu/Al2O3, Cu/TiO2, Cu/MgO, and Cu/HAP afforded much lower yields of 2
(Table 1, entries 8–11). Interestingly, the present Cu/HT(c) catalyst showed significantly higher activity
than our previous Cu/HT; the latter, prepared by coprecipitation, afforded 64% yield of 2 (Table 1, entry
1 vs. entry 7). The difference in performance between Cu/HT(c) and Cu/HT may be due to the former’s
smaller, uniform particle size, as discussed above. The solvent was also found to affect yield significantly.
Mesitylene afforded excellent yield of 2, whereas DMA and DMSO did not (Table 1, entries 1, 4, and 5).
The scope of Cu/HT(c) for the lactonization of a wide range of diols is shown in Table 2. Various diols
were oxidized to afford the corresponding lactones in high yields; aliphatic diols and a diol containing an
aromatic group were sufficiently converted to lactones (Table 2, entries 1–8). 1,4-Pentanediol selectively
yielded -valerolactone (Table 2, entry 2),9 indicating that a primary hydroxyl function of 1,4-pentanediol

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HETEROCYCLES, Vol. 80, No. 2, 2010
Table 1. Optimization of reaction conditions for the lactonization of 1 using various Cu catalystsa
O
Cu catalysts (Cu:0.071 mmol)
O
+ 2H2
OH
HO
solvent, 150 oC, Ar flow, 3 h
1
2
Conv. (%)b
Yield (%)b
Solvent
Cu catalyst
Entry
1
2c
3d
4
mesitylene
mesitylene
mesitylene
DMA
>99
>99
95
42
0
>99
>99
95
42
0
Cu/HT(c)
Cu/HT(c)
Cu/HT(c)
Cu/HT(c)
Cu/HT(c)
Cu/SiO2
Cu/HTe
5
DMSO
6
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
96
64
99
59
48
31
95
64
63f
59
46
31
7
Cu/Al2O3
Cu/TiO2
Cu/MgO
Cu/HAP
8
9
10
11
a Reaction conditions: Cu catalyst (Cu: 0.071 mmol), 1,4-butanediol (1.0 mmol), solvent (5.0
mL). b Determined by GC and LC using an internal standard technique. c Reuse 1. d Reuse 2.
e HT was synthesized by coprecipitation. f Tetrahydrofuran was formed as a byproduct.
preferentially oxidizes to hydroxyaldehyde, which is in equilibrium with alactol, and subsequent
oxidation of the lactol yields the corresponding -valerolactone. Furthermore, the Cu/HT(c) catalyst
effectively catalyzes reaction of a nitrogen-containing diol of N-methyldiethanolamine to afford the
corresponding product 4-methyl-2-morpholinone in quantitative yield (Table 2, entry 9).
The Cu/HT(c) catalyst was applicable successfully for the scale-up conditions. Dehydrogenation of 1 (3.6
g) at a 40-mmol scale afforded 95% isolated yield of 2 (3.3 g) in which the turnover number (TON)
reached 531. The value of TON is higher than those reported for other heterogeneous Cu catalysts such as
Cu–Zn–Ru (TON = 107),4d CuO–Cr2O3–BaO (TON = 38),4i and CuCr2O4 (TON = 47 and 60).4j,4k
Moreover, the Cu/HT(c) catalyst proved to be recyclable for dehydrogenation of 1, with similar reaction
rates (Table 1, entries 2 and 3). TEM analysis shows that the average Cu-nanoparticle diameter for the
reused catalyst is similar to that for the fresh catalyst, and there is no apparent aggregation, suggesting
good durability for recycling.

HETEROCYCLES, Vol. 80, No. 2, 2010
859
Table 2. Lactonization of various diols catalyzed by Cu/HT(c)a
O
HO
OH
O
Cu/HT(c) (Cu: 0.071 mmol)
mesitylene, 150 oC, Ar flow
+ 2H2
1.0 mmol
Diol
Entry
1
Product
Time (h)
3
Conv. (%)b
>99
Yield (%)b
>99 (97)
O
O
OH
HO
O
O
5
4
>99
>99
78c
OH
OH
2
3
HO
HO
O
O
O
>99 (98)
: = 1:1
O
O
O
OH
4
5
6
2
1
>99
>99
85
99 (99)
80d
OH
O
O
OH
OH
O
O
HO
OH
10
83
O
O
HO
OH
10
8
99
77e
7
8
O
O
HO
HO
OH
OH
>99
>99 (96)
O
O
N
9
12
99
98 (93)
N
a Reaction conditions: Cu/HT(c) (0.10 g, Cu: 0.071 mmol), diol (1.0 mmol), mesitylene (5.0 mL).
b Determined by GC and LC using an internal standard technique; values in parenthese are the
isolated products. c 5-hydroxy-2-pentanone and 2-methyl-4,5-dihydrofuran were obtained in 22%
yield. d 2-methylbenzenemethanol and 2-methylbenzaldehyde were obtained in 20% yield.
e 5-hexen-2-one and 6-methyl-3,4-dihydro-2H-pyran were obtained in 22% yield.

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HETEROCYCLES, Vol. 80, No. 2, 2010
In conclusion, we found that Cu nanoparticles supported on well-crystallized HT (Cu/HT(c)) are highly
active heterogeneous catalysts for the lactonization of diols under liquid-phase conditions. The Cu/HT(c)
catalyst exhibits high activity and wide applicability for various diols without the need of oxidants or
chromium compounds. The catalyst is reusable, is highly resistant to aggregation of Cu particles, and is
applicable to scale-up.
ACKNOWLEDGEMENTS
This investigation was supported by a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan. This work was also supported by a
Grant-in-Aid for Scientific Research on Priority Areas (No.18065016, "Chemistry of Concerto
Catalysis") from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Some
experiments were carried out using the facilities at the Research Center for Ultrahigh Voltage Electron
Microscopy, Osaka University. One of the authors Y.M. thanks the JSPS Research Fellowships for
Young Scientists. Y. M. express his special thanks for The Global COE (center of excellence) Program
“Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University.
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under Ar flow for 3 h. The solid catalyst was removed by centrifugation and the yield was
determined by GC analysis with naphthalene as an internal standard. The amount of generated
hydrogen is measured by gas burette attached to the reactor.
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