A Tin-tungsten mixed oxide as an efficient heterogeneous catalyst for C-C Bond Bond-Forming reactions

Article abstract of DOI:10.1002/chem.200802536

The tin-tungsten mixed oxide prepared by the calcination of the tin-tungsten hydroxide precursor with a Sn/W molar ratio of 2 at 800 °C (SnW2-800) acts as an effective and reusable solid catalyst for C-C bond-forming reactions, such as the cyclization of citronellal, the Diels-Alder reaction, and the cyanosilylation of carbonyl compounds with trimethylsilyl cyanide (TMSCN). Various kinds of structurally diverse aliphatic, aromatic, and unsaturated, heteroatom-containing substrates could be converted into the desired products in high to excellent yields. The observed catalyses for these reactions were truly heterogeneous and the recovered catalyst could be reused several times without an appreciable loss of its high catalytic performance. The Bronsted acid sites generated on the aggregated polytungstate species on SnW2-800 likely play an important role in the C-C bond-forming reactions.

Full text of DOI:10.1002/chem.200802536

FULL PAPER
DOI: 10.1002/chem.200802536
A Tin–Tungsten Mixed Oxide as an Efficient Heterogeneous Catalyst for
À
C C Bond-Forming Reactions
Yoshiyuki Ogasawara,[a] Sayaka Uchida,[a, b] Kazuya Yamaguchi,[a, b] and
Noritaka Mizuno*[a, b]
Abstract: The tin–tungsten mixed
oxide prepared by the calcination of
the tin–tungsten hydroxide precursor
with a Sn/W molar ratio of 2 at 8008C
(SnW2–800) acts as an effective and re-
cyanide (TMSCN). Various kinds of
structurally diverse aliphatic, aromatic,
and unsaturated, heteroatom-contain-
ing substrates could be converted into
the desired products in high to excel-
lent yields. The observed catalyses for
these reactions were truly heterogene-
ous and the recovered catalyst could be
reused several times without an appre-
ciable loss of its high catalytic perfor-
mance. The Brønsted acid sites gener-
ated on the aggregated polytungstate
species on SnW2–800 likely play an im-
À
usable solid catalyst for C C bond-
forming reactions, such as the cycliza-
tion of citronellal, the Diels–Alder re-
action, and the cyanosilylation of car-
bonyl compounds with trimethylsilyl
Keywords: cyanosilylation · cycliza-
tion · Diels–Alder reactions · heter-
ogeneous catalysis · tin · tungsten
À
portant role in the C C bond-forming
reactions.
À
Introduction
tion, which allows the formation of a C C bond and the
protection of a hydroxyl function.[4] Cyanohydrin trimethyl-
silyl ethers produced by the cyanosilylation can be further
transformed into a wide range of important synthetic inter-
mediates such as a-hydroxy acids, a-amino acids, and b-
amino alcohols.[4]
À
C C bond-forming reactions are of great importance in or-
ganic syntheses and often involve the activation of carbonyl
compounds by acid catalysts.[1] For example, the cyclization
of citronellal is an important acid-catalyzed reaction, and
isopulegol formed by the cyclization has been used as an in-
termediate of fragrances and (À)-menthol.[2] The associated
Diels–Alder reaction has also been widely used in natural
product syntheses because the reaction can regio- and ste-
reoselectively proceed to give the corresponding six-mem-
bered ring compounds with up to four stereocenters.[3] From
the standpoint of green chemistry, the Diels–Alder reaction
is ideal because its atom economy is theoretically 100%.[3]
The cyanosilylation of carbonyl compounds with trimethyl-
silyl cyanide (TMSCN) is also a very important transforma-
Traditionally, mineral acids such as H2SO4 and HF, and
Lewis acids such as AlCl3, ZnBr2, and BF3·etherate have
been used to promote the above-mentioned transformations
(sometimes used in stoichiometric amounts), which makes
the workup procedures troublesome and results in the pro-
duction of vast amounts of environmentally-unfriendly inor-
ganic salts as wastes. Therefore, these transformations have
stimulated considerable interest, and many efficient catalysts
have been developed to date.[5–7] However, most of them are
homogeneous systems and there are only a few widely
usable and highly active heterogeneous catalysts[5–7] in spite
of the significant advantages from the standpoint of catalyst
recovery and recycling.[8] In many heterogeneous systems,
the mass of the catalysts used is much larger than that of
substrates. This is certainly not desirable and therefore, the
reduction of the mass of the catalysts is also very impor-
tant.[8]
[a] Y. Ogasawara, Dr. S. Uchida, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7–3–1 Hongo, Bunkyo-ku, Tokyo 113–8656 (Japan)
Fax : (+81)3-5841-7220
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
[b] Dr. S. Uchida, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency
We have very recently reported that tin–tungsten mixed
hydroxides (Sn–W hydroxides) prepared by a simple co-pre-
cipitation method efficiently catalyze the dehydration of var-
ious aldoximes to the corresponding nitriles and the one-pot
synthesis of nitriles from the corresponding aldehydes and
4–1–8 Honcho, Kawaguchi, Saitama, 332–0012 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802536.
Chem. Eur. J. 2009, 15, 4343 – 4349
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4343

hydroxylamine.[9] During the course of our investigations,
we found that the tin–tungsten mixed oxides (Sn–W oxides)
prepared by the calcination of the Sn–W hydroxide with a
Sn/W molar ratio of 2 at 8008C (SnW2–800) acts as an effec-
tive and reusable solid catalyst for the cyclization of citro-
nellal, the Diels–Alder reaction, and the cyanosilylation of
carbonyl compounds with TMSCN. In contrast, the uncal-
cined Sn–W hydroxide precursor did not show any catalytic
activity for these transformations (entry 17, Table 1).
Herein, we report the scope of the present SnW2–800-cata-
SnO2 (entry 14, Table 1), WO3 (entry 15, Table 1), or a mix-
ture of SnO2 and WO3 (entry 16, Table 1).
À
lyzed reaction of three kinds of C C bond-forming reac-
tions, namely, the cyclization of citronellal, the Diels–Alder
reaction, and the cyanosilylation of carbonyl compounds
with TMSCN, and also discuss the active site of SnW2–800.
The catalyst precursor of the Sn–W hydroxide was not
active for the present cyclization (entry 17, Table 1). The
catalytic activity of SnW2–800 was much higher than those
of acid-type zeolites such as H-MOR and H-Y (entries 18
and 19, Table 1). The observed catalysis for the SnW2–800-
catalyzed cyclization was truly heterogeneous,[10,11] and the
SnW2–800 catalyst could be reused at least three times with-
out appreciable loss of its high catalytic performance (en-
tries 8–10, Table 1).
Until now, various kinds of heterogeneous catalysts such
as cation-exchanged montmorillonites, acid-type zeolites,
metal-substituted zeolites, mixed oxides, and supported
metal salts have been reported to be active for the cycliza-
tion of 1.[5] However, these systems require high reaction
temperatures and/or large amounts of catalysts to attain
high yields of the corresponding cyclized products (see
Table S2 in the Supporting Information). In contrast, the
present SnW2–800-catalyzed cyclization proceeded smoothly
at 208C with a small amount of
Results and Discussion
À
Catalytic C C bond-forming reactions: We prepared ten
kinds of Sn–W oxides by the calcination of the hydroxide
precursors (see Experimental Section and Table S1 in the
Supporting Information). The Sn–W oxide with the Sn/W
o
molar ratio of x calcined at T C is designated as SnWx–T.
Among the SnWx–T catalysts, SnW2–800 showed the high-
est catalytic activity for the cyclization of (R)-(+)-citronellal
(1) (entry 7, Table 1). The present SnW2–800 also showed
high performance for the cyclization of 3-methylcitronellal
(3) [Eq. (1)]. The reaction hardly proceeded in the absence
of the catalyst (entry 20, Table 1), or in the presence of
catalyst (10 mg). In addition,
when the cyclization was car-
Table 1. Cyclization of 1 with various catalysts.[a]
ried out with SnW2–800
(30 mg) at 808C, the reaction
was complete within only
Entry
Catalyst
Conv.
[%]
Select.
[%]
Isomer ratio
2a:2b:2c:2d
R
1 min, and the reaction rate
based on the catalyst mass
(33 mmolminÀ1 gÀ1) was higher
[mmolminÀ1 gÀ1
]
1
2
3
4
5
6
7
SnW1.5–400
SnW2–400
SnW1.5–600
SnW2–600
SnW1–800
SnW1.5–800
SnW2–800
SnW2–800
SnW2–800
SnW2–800
SnW5–800
SnW10–800
SnW2–1000
SnO2
12
12
25
46
28
86
95
87
93
92
69
30
4
93
95
94
94
89
90
93
93
92
92
90
91
92
–
–
–
–
94
84
–
59:35:4:2
59:35:4:2
57:37:4:2
59:37:3:1
60:36:3:1
58:38:3:1
58:38:3:1
58:38:3:1
58:38:3:1
58:38:3:1
58:38:3:1
59:37:3:1
62:35:3:<1
–
0.53
0.26
0.72
1.6
0.74
6.9
8.7
9.2
8.8
7.5
3.7
1.1
0.024
–
than
those
(0.022–
22 mmolminÀ1 gÀ1) of previous-
ly reported heterogeneous cat-
alysts (see Table S2 in the Sup-
porting Information).
8[b]
9[b]
10[b]
11
12
13
14
15
16[c]
17[d]
18
19
20
The cyclization of 1 showed
a complex selectivity pattern,
since 1 was converted into four
different diastereoisomers 2a–
2d (see Table 1). In the pres-
ence of Lewis acids, high dia-
stereoselectivities in favor of
2a (>70%) have been ob-
served.[5] With Brønsted acids,
the diastereoselectivities are
typically lower (ca. 50–70%)
than those with Lewis acids.[5]
In the present cyclization with
Sn–W oxides, the diastereose-
nr
nr
nr
nr
6
WO3
–
–
–
–
–
–
SnO2 + WO3
Sn–W hydroxide
H-MOR
H-Y
none
61:35:3:1
67:29:3:1
–
0.094
0.10
–
8
nr
[a] Reaction conditions: 1 (1 mmol), catalyst (10 mg), dichloromethane (5 mL), 208C, 1 h. Conversion and se-
lectivity were determined by GC analysis. nr=no reaction. [b] These experiments used a recycled catalyst; 1st
reuse (entry 8), 2nd reuse (entry 9), and 3rd reuse (entry 10). [c] A mixture of SnO2 (5 mg) and WO3 (5 mg).
[d] Sn/W=2.
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Chem. Eur. J. 2009, 15, 4343 – 4349

À
Sn–W Mixed Oxide Catalyst for C C Bond-Forming Reactions
FULL PAPER
lectivities in favor of 2a were 58–62% (Table 1) and these
values were in good agreement with those obtained with
Brønsted acid catalysts.[5]
S3 in the Supporting Information).[6] The present Diels–
Alder reaction was also heterogeneously catalyzed and the
SnW2–800 catalyst could be reused with retention of its high
catalytic performance (entries 2–4, 11, and 12, Table 2).
The applicability of SnW2–800 to the cyanosilylation of
carbonyl compounds with TMSCN was also examined.[12]
The SnW2–800 catalyst was applicable to a wide range of
structurally diverse aldehydes and ketones (Table 3). The
catalytic activity of SnW2–800 was higher than those of pre-
viously reported solid catalysts such as H-Y, H-ZSM-5,
MCM-41, Er-MCM-41, SiO2-Al2O3, Fe-montmorillonite,
MgO, and CaO except for Al-MCM-41 (see Table S4 in the
Supporting Information).[7] Under the present conditions, no
desilylation products, that is, cyanohydrins, were detected.
All benzylic aldehydes (8a–8 f) and a ketone (8g) were con-
verted into the corresponding aromatic cyanohydrin trime-
thylsilyl ethers in almost quantitative yields (entries 1–10,
Table 3). The reaction rates
The high performance of SnW2–800 as a heterogeneous
catalyst was further demonstrated for the Diels–Alder reac-
tion.[12] In the presence of SnW2–800, various combinations
of dienes (5a–5d) and dienophiles (6a–6e) were converted
into the corresponding Diels–Alder adducts in high to excel-
lent yields (Table 2). The reactions of cyclic dienes (5b–5d)
with dienophiles mainly gave endo adducts (entries 7–14,
Table 2). The reaction rate based on the catalyst mass for
the SnW2–800-catalyzed reaction of 5a with 6a reached up
to 140 mmolhÀ1 gÀ1. This value was the highest among those
(2.1–107 mmolhÀ1 gÀ1) for previously reported heterogene-
ous catalysts including metal oxides, Fe-montmorillonite,
H3PW12O40/SiO2, Zn-Al-MCM-41, Ru-hydroxyapatite, Al-
MCM-41, Cu-montmorillonite, and Ca-V-apatite (see Table
did not change much with re-
spect to electronic and steric
Table 2. Diels–Alder reaction catalyzed by SnW2–800.[a]
Entry
Diene
Deinophile
Product
Time [h]
Yield [%][b]
variations due to the substitu-
ents on the aromatic rings. An
acid-sensitive piperonal (8 f)
selectively produced the corre-
sponding cyanohydrin trime-
thylsilyl ether without decom-
position of the methylenedioxy
ring (entry 9, Table 3). Not
only aromatic carbonyl com-
pounds but also aliphatic
linear ones (8h and 8i) were
selectively converted into the
corresponding cyanohydrin tri-
methylsilyl ethers in excellent
yields (entries 11 and 12,
Table 3). Sterically hindered
cyclic ketones (8j–8l) also re-
acted with TMSCN to give the
corresponding products in high
yields (entries 13–15, Table 3).
The present SnW2–800-cata-
lyzed system was also applica-
ble to the nitrogen-containing
aldehyde of 3-pyridinecarbox-
yaldehyde (8m), affording the
corresponding silyl ether in
high yield, although a longer
reaction time was requied
(entry 20, Table 3). An a,b-un-
saturated aldehyde of trans-
cinnamaldehyde (8n) was con-
verted into the corresponding
1,2-adduct without the geomet-
rical isomerization (entry 21,
Table 3). 2-Cyclohexen-1-one
(9o) gave the corresponding
1,2- (9o) and 1,4-adducts (9o’)
1[c]
1
1
1
1
97[e]
97[e]
95[e]
96[e]
2[c,d]
3[c,d]
4[c,d]
5a
6a
7a
5[f]
5a
5a
5b
6b
6c
6b
7b
7c
7d
3
2
4
82 (9)
6
90 (64)
7[f]
82[g] (11[g]
)
8
9
5b
6c
7e
3
3
86[g]
5c
5c
5c
6c
6d
6e
7 f
7g
7h
97[g]
10
0.5
0.5
0.5
96[h] (11[i])
91[h]
11[d]
12[d]
87[h]
13
14
0.3
0.5
95[j] (7[k]
)
5d
6c
7i
90[g] (78[g]
)
[a] Reaction conditions: diene (3 mmol), dienophile (2 mmol), SnW2–800 (100 mg), dichloromethane (6 mL),
208C. [b] Isolated yields based on dienophiles. The endo/exo ratios were determined by H NMR spectroscopy.
1
Values in parentheses are yields obtained without catalyst under the same reaction conditions (blank experi-
ments). [c] Diene (1.5 mmol), dienophile (1 mmol), Sn–W oxide (50 mg), dichloromethane (3 mL). [d] These
experiments used a recycled catalyst; 1st reuse (entries 2 and 11), 2nd reuse (entries 3 and 12), and 3rd reuse
(entry 4). [e] Determined by GC, based on dienophile. [f] Diene (2.4 mmol), dienophile (2 mmol). [g] endo
>99%. [h] endo/exo=78/22. [i] endo/exo=80/20. [j] endo/exo=90/10. [k] endo/exo=86/14.
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N. Mizuno et al.
Table 3. Cyanosilylation catalyzed by SnW2–800.[a]
À
Active sites for the C C bond-
Entry
Substrate
Product
Time [h]
0.5
Yield [%][b]
99 (78)
forming reactions: As men-
tioned above (Table 1), the cat-
alytic activities of the Sn–W
oxides for the cyclization of 1
were highly dependent on the
calcination temperatures and
the Sn/W molar ratios. The re-
action rates of SnW2–T in-
creased with an increase in T,
reached the maximum at T=
8008C, and then decreased,[13]
and the reaction rates of
SnWx–800 increased with a de-
crease in x, reached the maxi-
mum at x=2, and then de-
1
8a
8b
9a
9b
2
0.5
99
3
0.5
0.5
0.5
0.5
99(99)
99
99
4[c]
5[c]
6[c]
8c
9c
99
7
8
9
8d
8e
8 f
9d
9e
9 f
0.5
0.5
0.5
98 (83)
98 (95)
99 (99)
creased.
Accordingly,
the
SnW2–800 catalyst showed the
highest catalytic activity for
the cyclization. The catalytic
activities of the Sn–W oxides
for the Diels–Alder reaction
and cyanosilylation were also
dependent on the calcination
temperatures and the Sn/W
molar ratios, and SnW2–800
showed the highest catalytic
activities for these reactions in
the same order as that for the
cyclization (see Tables S5 and
S6 in the Supporting Informa-
tion).
To clarify the active sites of
Sn–W oxides for the cycliza-
tion, the reaction of 1 in the
presence of 2,6-lutidine was
carried out. It is well known
that 2,6-lutidine can selectively
interact with Brønsted acid
sites and cannot interact with
Lewis acid sites due to the
steric hindrance.[14] Figure 1
shows the dependence of the
reaction rates of SnW2–800
and SnW2–400 on the amounts
of 2,6-lutidine added. The reac-
tions with SnW2–800 and
SnW2–400 were almost com-
pletely inhibited by the pres-
ence of equimolar amounts of
2,6-lutidine with respect to
those of the Brønsted acid sites
10
11
8g
8h
9g
9h
0.5
0.5
95 (90)
99
12
13
8i
8j
9i
9j
1
1
99
99 (98)
14
8k
8l
9k
9l
2
95 (93)
15
1
1
1
1
1
99 (99)
99
99
97
94
16[c]
17[c]
18[c]
19[c]
20
21
8m
8n
9m
9n
24
1
85 (80)
99 (99)
9o
9oꢀ
98
22
8o
1
(9o/9oꢀ=77/23)
[a] Reaction conditions: Substrate (1 mmol), TMSCN (4 mmol), SnW2–800 (50 mg), 1,2-dichloroethane
(0.5 mL), room temperature (ca. 22–238C). [b] Determined by GC or 1H NMR spectroscopy. Values in paren-
theses are isolated yields under 5 mmol conditions for ketones (fivefold scale-up) or 10 mmol conditions for al-
dehydes (tenfold scale-up). [c] These experiments used a recycled catalyst; 1st reuse (entries 4 and 16), 2nd
reuse (entries 5 and 17), 3rd reuse (entries 6 and 18), and 4th reuse (entry 19). The initial rates for the recycle
runs were almost the same as that for the run using fresh catalyst.
in the ratio of 77/23 (entry 22, Table 3). This selectivity was
different from those (9o: >99%) with solid bases such as
MgO and CaO.[7b,c] The SnW2–800 catalyst could be reused
without an appreciable loss of its high performance (en-
tries 4–6, and 16–19, Table 3).
(97 and 124 mmolgÀ1, respectively). Upon the addition of
2,6-lutidine (30 mmolgÀ1), the reaction rates of SnW2–800
significantly decreased by 93%, whereas those of SnW2–400
gradually decreased by 40%.[15] All these results show that
the present cyclization is mainly promoted by the Brønsted
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Chem. Eur. J. 2009, 15, 4343 – 4349

À
Sn–W Mixed Oxide Catalyst for C C Bond-Forming Reactions
FULL PAPER
&
Figure 1. The dependence of the reaction rates of SnW2–800 ( ) and
~
SnW2–400 ( ) on the amounts of 2,6-lutidine added for the cyclization of
1. (Inset: The dependence of the relative rates on the amounts of 2,6-luti-
dine added. The rates in the absence of 2,6-lutidine were taken as unity).
acid sites on Sn–W oxides, and that SnW2–800 possesses
strong Brønsted acid sites. The SnW2–800-catalyzed Diels–
Alder reaction of 5a with 6a, and the cyanosilylation of 8i
were carried out in the presence of 2,6-lutidine. The reaction
rates decreased with an increase in the amounts of 2,6-luti-
dine used, and these reactions were completely inhibited by
the presence of an equimolar amount of 2,6-lutidine with re-
spect to that of Brønsted acid sites (97 mmolgÀ1). Therefore,
the Diels–Alder reaction and the cyanosilylation are also
promoted by the Brønsted acid sites on SnW2–800.
Figure 2. W–LI edge XANES spectra of a) Sn–W hydroxide (Sn/W=2),
b) SnW2–400, c) SnW2–600, d) SnW2–800, e) SnW2–1000, and f) WO3.
It has been reported that 2-bromopropiophenone ethylene
acetal (10) can be converted into 2-bromopropiophenone
(11a), 2-bromoethyl-2-phenylpropanoate (11b), and 5-
methyl-6-phenyl-2,3-dihydro-1,4-dioxin (11c) catalyzed by
Brønsted acids, hard Lewis acids, and soft Lewis acids, re-
spectively.[16] In the presence of SnW2–800, the reaction of
10 gave 11a selectively, and the products due to the Lewis
acid catalysis were not formed [Eq. (2)]. This result also
leads to the identification of SnW2–800 as a Brønsted acid
catalyst.
Figure 3. Raman spectra of a) Sn–W hydroxide (Sn/W=2), b) SnW2–400,
c) SnW2–600, d) SnW2–800, and e) SnW2–1000. The asterisk indicates
the band of KNO3 added as an internal standard.
The X-ray absorption near-edge spectra (W–LI edge
XANES) of Sn–W hydroxides and oxides showed that they
for W-O-W (750–900 cmÀ1,
Figure 4) and Sn-O-Sn spe-
cies
(600–700 cmÀ1,
Figure 4) were observed,
showing the presence of
polytungstate species, likely
isopolytungstate clusters.[17]
The XRD patterns of the
have similar distorted WO6 octahedral environments
(Figure 2). This suggests that the marked difference in the
acidity of Sn–W oxides does not result from changes in their
local structures.
In the Raman and IR spectra of the Sn–W hydroxide pre-
cursor (Sn/W=2), the stretching mode for W=O species
(958 cmÀ1, Figure 3 and 4) and overlapped stretching modes
Sn–W hydroxide showed broad signals around d=3.32, 2.66,
and 1.75 ꢁ, and the signal positions almost agreed with
those of SnO2 with the rutile structure (Figure 5).[18] All
these results suggest that a SnO2-like phase is dominantly
formed in the Sn–W hydroxide precursor and that poly-
tungstate species are highly dispersed on SnO2 and/or incor-
porated into the SnO2 matrix.
Chem. Eur. J. 2009, 15, 4343 – 4349
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4347

N. Mizuno et al.
polytungstate species because the electron density on these
aggregated species can be delocalized.[19]
Conclusion
The SnW2–800 catalyst was an effective heterogeneous cata-
lyst for the cyclization of citronellal, the Diels–Alder reac-
tion, and the cyanosilylation. The performance raises the
prospect of using this type of simple mixed-oxide catalyst
for various organic syntheses and industrial acid-catalyzed
reactions.
Experimental Section
Instruments: GC analyses were performed on a Shimadzu GC-2014 gas
chromatograph with a FID detector equipped with a TC-1 or TC-WAX
capillary column. Mass spectra were recorded on a Shimadzu GCMS-
QP2010 spectrometer equipped with a TC-5HT capillary column at an
ionization voltage of 70 eV. Liquid-state NMR spectra were recorded on
Figure 4. IR spectra of a) Sn–W hydroxide (Sn/W=2), b) SnW2–400, c)
SnW2–600, d) SnW2–800, and e) SnW2–1000.
1
a JEOL JNM-EX-270 spectrometer. H and 13C NMR spectra were mea-
sured at 270 and 67.8 MHz, respectively. The ICP-AES analyses were
performed with a Shimadzu ICPS-8100 spectrometer. The BET surface
area was measured on a micromeritics ASAP 2010 analyser and calculat-
ed from the N2 adsorption isotherm with the BET equation. The XRD
patterns were recorded on a Rigaku Multiflex diffractometer (CuKa, l=
1.5405 ꢁ, 40 kV–50 mA). The Raman spectra were measured on a Jasco
NRS-1000 spectrometer equipped with
a CCD detector (cooled to
À708C). The samples were analyzed under ambient conditions. The
532 nm line of the laser was used as the excitation source. The laser
power at the sample was 2 mW. The IR spectra of the Sn–W oxides were
measured on a Jasco FT/IR-460 plus. The X-ray absorption spectra were
recorded at the BL9C beamline of PF at KEK, Japan (proposal No.
2007G096).
Reagents: H-MOR (JRC-Z-HM15, SiO2/Al2O3 =14.9) and H-Y
(CBV400, SiO2/Al2O3 =5.1) were supplied from the Catalysis Society of
Japan and Zeolist, respectively. Substrates (except for 3 and 10) and sol-
vents were commercially obtained from Tokyo Chemical Industry or
Kanto Chemical (reagent grade) and purified prior to the use.[20] 3-Meth-
ylcitronellal (3) was synthesized by the reaction of citral with cuprate.[21]
2-Bromopropiophenone ethylene acetal (10) was synthesized by the reac-
tion of 2-bromopropiophenone (11a) with ethyleneglycol.[16]
Figure 5. XRD patterns of a) Sn–W hydroxide (Sn/W=2), b) SnW2–400,
c) SnW2–600, d) SnW2–800, and e) SnW2–1000.
Preparation of Sn–W oxides: The Sn–W oxide catalysts were prepared ac-
cording to the following procedure. First, the Sn–W hydroxide precursors
were prepared by the following co-precipitation method.[9] The prepara-
tion of the Sn–W hydroxide with a Sn/W molar ratio of 2 is given as a
typical example.[9] Na2WO4·2H2O (2.47 g, 7.5 mmol) was dissolved in de-
ionized water (15 mL), followed by the addition of SnCl4·5H2O (5.26 g,
15 mmol) in a single portion. After the solution had been stirred for 1 h
at room temperature (ca. 22–238C), deionized water (60 mL) was added
to the solution in a single step and the colorless solution gradually
became a white slurry. After the white slurry had been stirred for 24 h at
room temperature, the resulting white precipitate of the Sn–W hydroxide
was filtered off, washed with a large amount of deionized water (ca.
2.0 L), and dried in vacuo to afford the Sn–W hydroxide as a white
powder (4.5 g). The contents of Sn and W were 38.3 and 33.6 wt%, re-
spectively (the Sn/W molar ratio was 1.8). The elemental analysis indicat-
ed that the Sn/W molar ratio of the hydroxide was in good agreement
with that of the starting metal solution. Five kinds of Sn–W hydroxides
with different Sn/W molar ratios (Sn/W=1, 1.5, 2, 5, and 10) were suc-
cessfully prepared by changing the molar ratios of the starting metal solu-
tions. By calcinating the corresponding hydroxide precursors at different
temperatures (400–10008C) for 3 h under an air atmosphere, ten kinds of
Sn–W oxide catalysts were prepared.
The XRD patterns of SnW2–400 and SnW2–600 were in-
trinsically identical to that of the hydroxide precursor
(Figure 5). The intensities of the stretching mode for W=O
species of SnW2–400 and SnW2–600 were very similar to
that of the hydroxide precursor (Figure 3). Upon increasing
the calcination temperature to 8008C, weak signals (ca.
10% of tungstate species) assignable to the triclinic WO3
phase were observed in the XRD pattern (Figure 5), and the
intensity of the signals associated with W=O species de-
creased in the Raman spectrum (Figure 3). Therefore, the
polytungstate species likely start to aggregate at 8008C.[19]
Finally, most of the tungstate species are converted into
WO3 crystallites at 10008C, as observed in the Raman spec-
trum and the XRD pattern (Figure 3 and Figure 5). In the
present Sn–W oxides (in particular SnW2–800), the strong
Brønsted acid sites would be generated on the aggregated
4348
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Chem. Eur. J. 2009, 15, 4343 – 4349

À
Sn–W Mixed Oxide Catalyst for C C Bond-Forming Reactions
FULL PAPER
Procedures for catalytic reactions: The catalytic reactions were carried
out as follows. The catalyst was pretreated at 3008C for 1 h under re-
duced pressure (<10À2 Torr). After the pretreatment, the temperature
was lowered to room temperature, Ar gas was introduced to the system,
and then the substrate and solvent were introduced. The reaction was pe-
riodically monitored by GC analysis. After the reaction was complete,
the catalyst was separated by filtration, washed with solvent, dried in
vacuo, and calcined at 5008C prior to being reused. The products were
identified by the comparison of mass and NMR spectra with those of au-
thentic samples.
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Acknowledgements
This work was supported in part by the Global COE Program (Chemistry
Innovation through Cooperation of Science and Engineering), the Core
Research for Evolutional Science and Technology (CREST) program of
the Japan Science and Technology Agency (JST), and Grants-in-Aid for
Scientific Researches from Ministry of Education, Culture, Sports, Sci-
ence and Technology.
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Received: December 4, 2008
Published online: March 13, 2009
Chem. Eur. J. 2009, 15, 4343 – 4349
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4349