Heterogeneously catalyzed efficient hydration of alkynes to ketones by tin-tungsten mixed oxides

Article abstract of DOI:10.1002/chem.201002761

The Sn-W mixed oxide prepared by calcination of the Sn-W mixed hydroxide precursor with a Sn/W molar ratio of 2:1 at 800 °C (SnW2-800) acts as an efficient heterogeneous catalyst for the hydration of alkynes. Structurally diverse terminal and internal alkynes, including aromatic, aliphatic, and double-bond-containing ones, can be converted into the corresponding ketones in moderate to high yields. The catalytic activity of SnW2-800 is much higher than those of previously reported heterogeneous catalysts and commonly utilized acid catalysts. The observed catalysis was truly heterogeneous, and the retrieved catalyst can be reused at least three times with retention of its high catalytic performance. The reaction rate for the SnW2-800-catalyzed hydration was decreased by addition of 2,6-lutidine and the hydration hardly proceeded in the presence of an equimolar amount of this compound with respect to that of the Bransted acid sites in SnW2-800.

Full text of DOI:10.1002/chem.201002761

FULL PAPER
DOI: 10.1002/chem.201002761
Heterogeneously Catalyzed Efficient Hydration of Alkynes to Ketones by
Tin–Tungsten Mixed Oxides
Xiongjie Jin, Takamichi Oishi, Kazuya Yamaguchi, and Noritaka Mizuno*[a]
Abstract: The Sn–W mixed oxide pre-
pared by calcination of the Sn–W
mixed hydroxide precursor with a Sn/
W molar ratio of 2:1 at 8008C (SnW2-
800) acts as an efficient heterogeneous
catalyst for the hydration of alkynes.
Structurally diverse terminal and inter-
nal alkynes, including aromatic, ali-
phatic, and double-bond-containing
ones, can be converted into the corre-
sponding ketones in moderate to high
yields. The catalytic activity of SnW2-
800 is much higher than those of previ-
ously reported heterogeneous catalysts
and commonly utilized acid catalysts.
The observed catalysis was truly heter-
ogeneous, and the retrieved catalyst
can be reused at least three times with
retention of its high catalytic perfor-
mance. The reaction rate for the
SnW2-800-catalyzed hydration was de-
creased by addition of 2,6-lutidine and
the hydration hardly proceeded in the
presence of an equimolar amount of
this compound with respect to that of
the Brønsted acid sites in SnW2-800.
Therefore, the present hydration is
mainly promoted by the Brønsted acid
sites in SnW2-800.
Keywords: alkynes · heterogeneous
catalysis · hydration · ketones ·
tin–tungsten mixed oxides
Introduction
plexes (ketones for Markovnikov addition; aldehydes for
anti-Markovnikov addition), these systems have shortcom-
ings, particularly the recovery and reuse of (expensive) cata-
lysts and/or the indispensable use of acidic reagents as co-
catalysts, which can cause problem due to product contami-
nation.
The hydration of alkynes is one of the most important and
attractive C O bond forming reactions because alkynes
[1]
À
are abundant and water is an inexpensive and environmen-
tally friendly reagent. The hydration of alkynes possesses a
long history and has classically been carried out in the pres-
ence of acidic reagents.[1] However, the acid-mediated hy-
dration generally requires large amounts of an acidic re-
agent (typically ꢀ100 mol%) and/or highly toxic additives,
such as mercury(II) oxide as well as other mercury(II)
salts.[1] Several efficient catalytic hydration procedures with
transition-metal salts or complexes have recently been de-
veloped.[1,2] In particular, Nolan and co-workers have very
recently reported some efficient homogeneous hydration
systems with [(IPr)AuCl]/AgSbF6 (IPr=1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene), which show high catalytic ac-
tivity at parts per million catalyst loadings.[2a] Although vari-
ous kinds of alkyne can be converted into the corresponding
carbonyl compounds with transition-metal salts or com-
The development of easily recoverable and recyclable, by
filtration or centrifugation, heterogeneous catalysts can
solve the problems of the homogeneous systems and has re-
ceived particular research interest by organic synthetic
chemists.[3] To the best of our knowledge, there are only a
few reports of the hydration of alkynes with mercury-free[4]
heterogeneous catalysts, for example, metal-cation-exchange
acidic resins (M-resins; M=CuII, PdII, or RuIII),[5] AuI-con-
taining mesoporous silica (AuI-MS),[6] and polystyrene-sup-
ported sulfonic acid (PS-SO3H).[7] The catalytic activity of
M-resins is low and very long reaction times are required to
attain high yields (typically >100 h with a 2 mol% catalyst
loading).[5] Although various terminal alkynes, including aro-
matic and aliphatic ones, can be hydrated to the correspond-
ing ketones with AuI-MS (not reported for internal alkynes),
homogeneous H2SO4 is required as
a
co-catalyst
(10 mol%).[6] Although the system with PS-SO3H, reported
by Kobayashi and co-workers, is the first example of hetero-
geneous hydration with a catalytic amount of a Brønsted
acid (H+: 10 mol%), the catalytic activity is low and the re-
ported applicability is limited to reactive 4-ethynyltoluene.[7]
[a] X. Jin, T. Oishi, 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
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Table 1. Properties of the Sn–W oxide catalysts.
Therefore, efficient, widely applicable (to terminal as well as
internal alkynes) heterogeneous hydration systems without
any additives are currently unknown, and their development
is a challenging subject.
Catalyst
Sn/W Calcination BET[a]
No. of
Brønsted
ratio
T [8C]
G
]
acidic sites[b] acid/Lewis
C
]
acid sites[c]
SnW2-400
SnW2-600
SnW1-800
SnW1.5-800
SnW2-800
SnW5-800
SnW10-800
SnW2-1000
2
2
1
1.5
2
5
400
600
800
800
800
800
800
1000
143
118
35.8
77.4
73.6
68.8
49.4
29.5
290
189
73
124
134
78
43:57
54:46
58:42
66:34
73:27
Quite recently, we reported that the Sn–W mixed oxide
prepared by calcination of a Sn–W hydroxide precursor with
a Sn/W molar ratio of 2:1 at 8008C (SnW2-800) acts as an
À
effective and reusable heterogeneous acid catalyst for C C
[d]
–
–
bond-forming reactions, for example, in the cyclization of
citronellal, the Diels–Alder reaction, and the cyanosilylation
of carbonyl compounds with trimethylsilyl cyanide.[8] As
shown for the cyclization of citronellal, the protonation of
the carbonyl oxygen takes place initially, followed by cycli-
zation via a carbocation-type transition state.[8] We have
now discovered that SnW2-800 can act as an efficient heter-
ogeneous catalyst for the hydration of various alkynes
[Eqs. (1) and (2)]. The catalysis was truly heterogeneous
and the retrieved Sn–W oxide catalyst can be reused at least
three times without an appreciable loss of its high catalytic
performance. Herein, we focus on the synthetic scope of the
Sn–W oxide catalyzed hydration. In addition, a possible re-
action mechanism is mentioned.
[d]
10
2
55
27
[d]
–
[a] From the N2 adsorption isotherm. [b] The total number of acidic sites
was determined by NH3–TPD (TPD=temperature-programmed desorp-
tion) measurements. The values contain ꢁ10% experimental errors.
[c] The ratios were determined by IR spectra of pyridine adsorbed on the
Sn–W oxides. [d] IR spectra of pyridine adsorbed on these Sn–W oxides
could not be measured because of low transmittance (nearly 0%).
Results and Discussion
The effect of catalysts, solvents, and water: Eight different
Sn–W mixed oxide catalysts were prepared by calcination of
hydroxide precursors with different Sn/W ratios (see the Ex-
perimental Section). The Sn–W oxide with a Sn/W molar
ratio of x prepared by calcination at T8C is designated as
SnWx-T. The properties of SnWx-T catalysts are summar-
ized in Table 1. The catalytic activity of these Sn–W oxides
for the hydration of ethynylbenzene (1a) to acetophenone
(2a) is largely dependent on the calcination temperature
and the Sn/W molar ratio (Figure 1a). The reaction rate
with SnW2-T increased with an increase in T, reached a
maximum at T=8008C, and then decreased. The reaction
hardly proceeded with SnW2-1000, probably owing to the
very small number of acidic sites (Table 1). The reaction
rate of SnWx-800 increased with an increase in x, reached a
maximum at x=2, and then decreased (Figure 1b). SnW2-
800 showed the highest catalytic activity for the hydration of
1a to 2a. The catalytic activity of Sn–W oxides for the previ-
Figure 1. The hydration of 1a to 2a with various Sn–W oxides: a) the
effect of calcination temperature (Sn–W hydroxide with a Sn/W molar
ratio of 2:1 calcined at T [8C], SnW2-T) and b) the Sn/W molar ratio
(Sn–W hydroxides with Sn/W molar ratio of x calcined at 8008C, SnWx-
800). Reaction conditions: catalyst (50 mg), 1a (0.5 mmol), water
(2.5 mmol), cyclooctane (2 mL), 1008C, 15 min. Yields were determined
by GC using diphenyl as an internal standard.
terizations that strong Brønsted acid sites are generated on
the aggregated polytungstate species in SnW2-800,[8] which
can efficiently catalyze these transformations. Thus, the hy-
dration reactions were hereafter carried out with SnW2-800.
Among the solvents examined, nonpolar and nonaromatic
solvents, such as cyclooctane and n-octane, selectively gave
the hydration products (ꢀ96% selectivity; Table 2, entries 1
and 2). Although, toluene gave 2a in 80% yield, the Frie-
del–Crafts-type vinylation of toluene with 1a occurred to
À
ously reported C C bond-forming reactions was also depen-
dent on the calcination temperature and Sn/W molar ratio,
and SnW2-800 showed the highest catalytic activity for these
reactions as well.[8] It has been revealed by various charac-
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Tin–Tungsten Mixed Oxides
FULL PAPER
Table 2. Optimization of reaction conditions for the hydration of 1a to
2a.[a]
Table 3. Hydration of 1a to 2a with various catalysts.[a]
Entry
Water/1a
Solvent
Conversion
Yield
of 1a [%]
of 2a [%]
Entry
Catalyst
Conversion of 1a [%]
Yield of 2a [%]
1
2
5:1
5:1
5:1
5:1
5:1
5:1
5:1
1:1
2:1
cyclooctane
n-octane
toluene
98
49
>99
<1
<1
1
<1
98
97
94
49
1
2
3
4
5
6
7
8
9
10
11[d]
12[d]
13[d]
14[d]
15
SnW2-800
SnO2
WO3
H2WO4
SnO2 +WO3
80
<1
<1
1
4
22
6
33
29
6
<1
1
79
n.d.
n.d.
n.d.
n.d.
22
6
12
27
1
n.d.
n.d.
n.d.
n.d.
n.d.
3[b]
4
80[c]
n.d.
n.d.
1
ethanol
5
6
7
8
acetonitrile
1,4-dioxane
N,N-dimethylformanide
cyclooctane
cyclooctane
cyclooctane
cyclooctane
cyclooctane
[b]
Sn–W hydroxide[c]
H-mordenite
HY
n.d.
90
92
38
n.d.
n.d.
9
SO42À/ZrO2
Nafion
10
11
12
7:1
10:1
20:1
40
<1
<1
Amberlyst-15
H2SO4
p-TsOH
H3PW12O40
none
[a] Reaction conditions: SnW2-800 (50 mg), 1a (0.5 mmol), water (0.5–
10 mmol), cyclooctane (2 mL), 1008C, 30 min. Yields were determined by
GC using biphenyl as an internal standard. n.d.=not detected.
[b] 20 min. [c] The Friedel–Crafts-type vinylation of toluene with 1a pro-
ceeded to some extent (ꢁ18%).
2
3
<1
[a] Reaction conditions: catalyst (50 mg), 1a (0.5 mmol), water
(2.5 mmol), cyclooctane (2 mL), 1008C, 15 min. Yields were determined
by GC using biphenyl as an internal standard. n.d.=not detected. [b] A
mixture of SnO2 (25 mg) and WO3 (25 mg). [c] The precursor of Sn–W
oxide. [d] Catalyst (H+: 1.3 mol% with respect to 1a). The quantity was
the same as that of the acid sites in the Sn–W oxide used in entry 1.
some extent (ꢁ18%; Table 2, entry 3). Protic and polar sol-
vents, such as ethanol, acetonitrile, 1,4-dioxane, and N,N-di-
methylformamide, were poor solvents for these reactions,
likely owing to strong coordination to the active sites
(Table 2, entries 4–7). The reaction rate for the hydration of
1a was almost independent of the amount of water used, up
to five equivalents, with respect to 1a (Table 2, entries 1, 8,
and 9). However, the hydration was strongly inhibited by
the addition of a large amount of water (ꢀ10 equiv with re-
spect to 1a; Table 2, entries 11 and 12).
tries 7–11). No hydration proceeded in the presence of cata-
lytic amounts (H+: 1.3 mol%) of H2SO4, p-toluenesulfonic
acid (p-TsOH), or H3PW12O40 in cyclooctane (Table 3, en-
tries 12–14). These acid-mediated hydration reactions have
typically been carried out in alcohols or cyclic ethers.[1]
However, even if the hydration reactions of 1a were carried
out in ethanol at reflux or 1,4-dioxane at 1008C by using
catalytic amounts (H+: 1.3 mol%) of H2SO4, p-TsOH, or
H3PW12O40, the corresponding ketone (2a) was hardly ob-
tained.
Next, various acid catalysts were applied to the hydration
of 1a to 2a. SnW2-800 showed a higher catalytic activity for
the hydration of 1a than any of the various other catalysts
examined; a 79% yield of 2a was selectively obtained in
15 min by using 50 mg of the catalyst (number of acidic
sites: only 1.3 mol% with respect to 1a; Table 3, entry 1). In
this case, the turnover frequency (TOF) reached 246 hÀ1
(based on the number of acidic sites). This value is much
higher than those of previously reported mercury-free heter-
Scope of the Sn–W oxide catalyzed hydration: The scope of
the SnW2-800-catalyzed hydration of alkynes was examined
next. Various structurally diverse terminal and internal al-
kynes, including aromatic, aliphatic, and double-bond-con-
taining ones, can be converted into the corresponding ke-
tones in moderate to high yields. The ketone products can
easily be isolated by simple column chromatography on
silica gel (see the Experimental Section), and the isolated
yields are reported in Table 4 (see the values in parenthe-
ses). For all of the terminal alkynes in Table 4, ketones
(Markovnikov addition) were obtained exclusively, with no
formation of the corresponding aldehydes (anti-Markovni-
kov addition). The hydration of ethynylbenzene derivatives
containing electron-donating or electron-withdrawing sub-
stituents (1a–k) efficiently proceeded to afford the corre-
sponding acetophenone derivatives in moderate to high
yields (Table 4, entries 1–11). The reaction rates for the
ethynylbenzene derivatives with electron-donating substitu-
ents were larger than those with electron-withdrawing ones,
suggesting an electrophilic nature of the SnW2-800-cata-
lyzed hydration (vide infra). Reactive functional groups,
ogeneous catalysts (M-resins, 0.12–0.36 hÀ1 [5]
;
AuI-MS, 4.1–
6.7 hÀ1 [6] and PS-SO3H, 0.17 hÀ1).[7] Notably, the amount of
;
catalyst can be considerably reduced; if the hydration of 1a
(0.5 mmol) was carried out at 1008C with 5 mg of SnW2-800
(0.13 mol% acidic sites and 9.8 mass% with respect to 1a),
an 88% yield of 2a was obtained in 6 h (in this case, water
(0.1 mmol) was added to the reaction mixture every
30 min).
No hydration of 1a occurred in the absence of a catalyst
(Table 3, entry 15). Neither were SnO2, WO3, H2WO4, or a
physical mixture of SnO2 and WO3 effective for the hydra-
tion (Table 3, entries 2–5). The catalytic activity of SnW2-
800 was higher than that of its Sn–W hydroxide precursor
(Table 3, entry 6). Notably, the catalytic activity of SnW2-
800 was much higher than those of commonly utilized heter-
ogeneous acid catalysts, such as zeolites (H-mordenite and
HY), SO42À/ZrO2, Nafion, and Amberlyst-15 (Table 3, en-
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N. Mizuno et al.
Table 4. Hydration of various alkynes.[a]
tion with the p-electrons of the
aromatic ring. Not only aromat-
ic alkynes, but also aliphatic ter-
minal (1o–r) and internal (1s
and t) ones can be hydrated to
Entry
Substrate
Time
[h]
Product
Yield
[%]
1[b]
2
1a
1b
0.5
0.3
2a
2b
92 (91)
the
corresponding
ketones
>99 (93)
(Table 4, entries 15–20). In the
case of enyne 1r, no hydration
of the double bond occurred
during the reaction (Table 4,
entry 18).
3
4
1c
1d
2.5
0.5
2c
2d
93 (91)
>99 (88)
In the case of the SnW2-800-
catalyzed transformations of
propargylic alcohols, the main
products were different from
those for the terminal alkynes
in Table 4. For 1-ethynylcyclo-
hexenol (1u), dehydration pro-
ceeded initially, followed by hy-
dration to give 2r as the main
5
6
1e
1 f
0.6
0.5
2e
2 f
95 (92)
98 (95)
7
1g
2
2g
87 (80)
8
1h
1i
1.5
1
2h
2i
97 (91)
88
9[c,d]
10
product
[Rupe
rearrange-
ment;[9] Eq. (3)]. The transfor-
mation of 1-phenyl-2-propyn-1-
1j
1k
1l
0.3
24
6
2j
2k
2l
94 (72)
43
11[c,d]
12[c,d]
À
ol (1v), which contains no C H
bonds at the b-position, mainly
>99 (83)
gave
trans-cinnamaldehyde
(2v), likely through the forma-
tion of an allenyl cation, fol-
lowed by hydration [Meyer–
Schuster
Eq. (4)].
13
1m
8
2m
94 (79)
14
1n
1o
1p
1q
1r
2
10.5
2.5
4
2n
2o
2p
2q
2r
2o
2s
2t
93 (85)
88
rearrangement;[9]
15[d,e]
16[d,e]
17[d,e]
18[f]
To verify whether the ob-
served catalysis is truly hetero-
geneous, the SnW2-800 catalyst
was removed by hot filtration
and the reaction was carried
out with the filtrate under the
same conditions. The hydration
was completely stopped by re-
moval of the catalyst (Figure 2).
Furthermore, it was confirmed
by inductively coupled plasma
atomic emission spectroscopic
(ICP-AES) analysis that Sn and
90 (84)
76
1
>99
19[c,d,e]
20[c,d,e]
1s
1t
17
12
80 [g]
82 (78)
[a] Reaction conditions: SnW2-800 (50 mg), alkyne (0.5 mmol), water (2.5 mmol), cyclooctane (2 mL), 1008C.
Yields were determined by GC using biphenyl as an internal standard. The values in parentheses are the iso-
lated yields. [b] Water (1 mmol). [c] SnW2-800 (100 mg). [d] 1208C. [e] Water (1.5 mmol). [f] Toluene (2 mL).
[g] A mixture of 2o (41% yield) and 2s (39% yield).
such as amino and nitro groups, remained intact (Table 4,
entries 10 and 11). The hydration of 1,4-diethynylbenzene
(1l) exclusively gave 1,4-diacetylbenzene (2l; Table 4,
entry 12). The hydration of internal, aromatic alkynes (1m
and n) proceeded efficiently to give the corresponding ke-
tones (Table 4, entries 13 and 14), although these reactions
required longer reaction times in comparison with those of
terminal ones. Notably, methylphenylacetylene (1n) gave
propiophenone (2n) exclusively, without the formation of 1-
phenyl-2-propanone (Table 4, entry 14). This is likely due to
the stabilization of a transient vinyl cation through conjuga-
W species are barely present in the filtrate (Sn: <0.0013%;
W: <0.0015%). These results show that the nature of the
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Tin–Tungsten Mixed Oxides
FULL PAPER
It has been reported that 2-bromopropiophenone ethylene
acetal (3a) can be converted into 2-bromopropiophenone
(4a), 2-bromoethyl 2-phenylpropanoate, or 5-methyl-6-
phenyl-2,3-dihydro-1,4-dioxin through catalysis by Brønsted
acids, hard Lewis acids, or soft Lewis acids, respectively.[12]
In the presence of SnW2-800, the reaction of 3a gave 4a ex-
clusively and the products of Lewis acid catalysis were not
formed [Eq. (5)], which also indicates that SnW2-800 is
acting as a Brønsted acid catalyst.[8]
Figure 2. The effect of removal of the SnW2-800 catalyst (verification of
heterogeneous catalysis). The reaction conditions were the same as those
described in Table 2. The arrow indicates the removal of the catalyst by
filtration.
Next, the SnW2-800-catalyzed competitive hydration reac-
tions of para- and meta-substituted ethynylbenzene deriva-
tives were carried out. The reactivity order for ethynylben-
zene derivatives is as follows: p-OCH3 (kX/kH =85.7)>p-
CH3 (5.9)>m-CH3 (1.4)>p-H (1.0)>p-Cl (0.44)>m-Cl
(0.11)>p-CF3 (0.067). Figure 4 shows the relationship be-
observed catalysis is truly heterogeneous.[10] After the reac-
tion was completed, the catalyst can be retrieved by simple
filtration. The SnW2-800 catalyst can then be reused at least
three times with no appreciable loss of its high catalytic per-
formance; for the hydration of 1a with the retrieved SnW2-
800 catalyst, under the conditions described in Table 4, 92,
90, and 88% yields of 2a were obtained for the 1st, 2nd,
and 3rd reuse experiments, respectively.
tween logACTHNUTRGNEUNG
(kX/kH) and the Brown–Okamoto s+ constant for
Mechanistic studies: To clarify the nature of the active sites
of SnW2-800 for the hydration, the reaction of 1a was car-
ried out in the presence of 2,6-lutidine. It is known that 2,6-
lutidine selectively interacts with Brønsted acid sites and
cannot interact with Lewis acid sites due to the steric hin-
drance caused by its methyl groups.[11] As shown in Figure 3,
the hydration of 1a was almost completely inhibited by the
presence of an equimolar amount of 2,6-lutidine with re-
spect to that of the Brønsted acid sites. Therefore, the hy-
dration is mainly promoted by the Brønsted acid sites in the
SnW2-800 catalyst.
Figure 4. A Hammett plot for the competitive hydration of para- and
meta-substituted ethynylbenzene derivatives (X=p-OCH3, p-CH3, m-
CH3, p-H, p-Cl, m-Cl, and p-CF3): a logACTHNUTRGNEU(NG kX/kH) versus Brown–Okamoto
s+ plot. Reaction conditions: SnW2-800 (100 mg), ethynylbenzene
(0.5 mmol), substituted ethynylbenzene (0.5 mmol), water (1 mmol), cy-
clooctane (2 mL), 1008C. Slope (Hammett 1+ value)=À2.31 (R2 =0.99).
each compound.[13] A linear relationship is observed and the
slope (Hammettꢁs 1+ value) is À2.31.[14] This large, negative
value suggests an electrophilic nature of the hydration, in
which the hydration likely proceeds via a positively charged
transition state, with the positive charge on the a-carbon
atom adjacent to the phenyl ring, that is, a vinyl cation.[15]
The correlation with the Brown–Okamoto s+ constants
shows a better fit than that with more commonly used s
constants, in particular for electron-donating substituents,
such as p-OCH3 and p-CH3, which also suggests that the for-
mation of a positively charged transition state is involved in
the hydration reaction pathway.[15]
Figure 3. The dependence of the reaction rate on the amount of 2,6-luti-
dine added to the SnW2-800-catalyzed hydration of 1a. The bar in the x
axis indicates the number of Brønsted acid sites in the SnW2-800 catalyst
(indicated with an experimental error). Reaction conditions: SnW2-800
(15 mg), 1a (0.5 mmol), water (0.5 mmol), 2,6-lutidine, cyclooctane
(2 mL), 1008C.
If 1a was heated with SnW2-800 in p-xylene (in the ab-
sence of water), the Friedel–Crafts-type vinylation proceed-
Chem. Eur. J. 2011, 17, 1261 – 1267
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1265

N. Mizuno et al.
(5 Torr) was introduced into the cell, and the disk was kept at 1008C for
30 min. After the disk was evacuated at 1508C for 1 h, the spectrum was
recorded at room temperature. The amount of pyridine adsorbed on
Lewis and Brønsted acid sites was determined by using the following in-
tegrated molar extinction coefficients; 2.22 and 1.67 cmmmolÀ1 for Lewis
ed to afford 1,4-dimethyl-2-(1-phenylethenyl)benzene (5a)
[Eq. (6)].[16] This also supports the formation, by the SnW2-
800 catalyst, of a vinyl cation from an alkyne.
(1451 cmÀ1) and Brønsted acid sites (1538 cmÀ1), respectively.[18]
2À
Reagents: H-mordenite (JRC-Z-HM15, SiO2/Al2O3 =14.9) and SO4
/
ZrO2 (JRC-SZ-1) were supplied by the Catalysis Society of Japan. HY
(CBV400, SiO2/Al2O3 =5.1) was supplied by Zeolist. Nafion (Nafionꢂ
NR-50) and Amberlyst-15 were purchased from Wako and ORGANO,
respectively. Substrates and solvents were commercially obtained from
TCI, Wako, or Aldrich (reagent grade) and purified prior to use.[19]
Therefore, this SnW2-800-catalyzed hydration likely pro-
ceeds through the following mechanism. Initially, protona-
tion of an alkyne takes place to generate a vinyl cation.
Then, nucleophilic attack of water[17] on the transiently
formed vinyl cation proceeds to give a vinylic alcohol, fol-
lowed by tautomerization to form the corresponding ketone
as the final product.
Preparation of Sn–W mixed oxides: The Sn–W mixed oxide catalysts
were prepared according to the following procedure.[8] First, the Sn–W
mixed-hydroxide precursors were prepared by the following co-precipita-
tion method.[20] The preparation of the Sn–W hydroxide with a Sn/W
molar ratio of 2:1 is described here as a typical example. Na2WO4·2H2O
(2.47 g, 7.5 mmol) was dissolved in deionized water (15 mL), followed by
addition, in a single step, of SnCl4·5H2O (5.26 g, 15 mmol). After stirring
the solution for 1 h at room temperature (ꢁ22–238C), more deionized
water (60 mL) was added to the reaction mixture, and the colorless solu-
tion gradually became a white slurry. After stirring for 24 h at room tem-
perature, the resulting white precipitate of Sn–W hydroxide was filtered
off, washed with a large amount of deionized water (ꢁ2.0 L), and dried
in vacuo to afford the Sn–W hydroxide precursor as a white powder
(4.5 g). The quantities of Sn and W were 38.3 and 33.6 wt%, respectively.
Elemental analysis indicated 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 hydroxide with different Sn/W molar ratios (Sn/W=1:1,
1.5:1, 2:1, 5:1, and 10:1) were successfully prepared by changing the
molar ratio of the starting metal solutions. By calcination of the corre-
sponding hydroxide precursor at different temperatures (400–10008C) for
3 h under an air atmosphere, eight different Sn–W oxide catalysts were
prepared. The properties of the Sn–W oxide catalysts are summarized in
Table 1.
Conclusion
The Sn–W mixed oxides, especially SnW2-800, act as effi-
cient heterogeneous catalysts for the hydration of structural-
ly diverse alkynes. Various terminal and internal alkynes, in-
cluding aromatic, aliphatic, and double-bond-containing
ones, can be converted into their corresponding ketones in
moderate to high yields. However, with propargylic alcohols,
Rupe rearrangement or Meyer–Schuster rearrangement
occur instead. The catalytic activity of SnW2-800 is much
higher than those of previously reported heterogeneous cat-
alysts, including M-resins, AuI-MS, and PS-SO3H, as well as
commonly utilized acid catalysts. The catalysis is truly heter-
ogeneous and the SnW2-800 can be reused. This perfor-
mance raises the prospect of using this type of simple
mixed-oxide catalyst for various laboratory-scale organic
syntheses, as well as practical acid-catalyzed reactions.
Catalytic alkyne hydration: In this paper, the Sn–W oxide prepared by
the calcination of the hydroxide precursor with a Sn/W molar ratio of 2:1
at 8008C (SnW2-800) was generally used in the reactions. The catalytic
reactions were carried out as follows. Into a Pyrex-glass screw-cap vial
(volume: ꢁ20 mL) were successively placed the Sn–W oxide catalyst
(50–100 mg), an alkyne (0.5 mmol), water (1–20 equiv with respect to the
alkyne), and cyclooctane (2 mL). A Teflon-coated magnetic stir bar was
added and the reaction mixture was vigorously stirred at 100–1208C.
After the reaction was completed, the catalyst was removed by filtration.
Then, an internal standard (biphenyl, 15.4 mg) was added to the filtrate
and the mixture was analyzed by GC. For isolation of the products (ke-
tones), the internal standard was not added and the crude filtrate was di-
rectly subjected to column chromatography on silica gel (Silica Gel 60N
(63–210 mm), Kanto Chemical, 2.5 cm, IDꢃ20 cm length) by initially
using only n-hexane to elute cyclooctane and the alkyne and then by
using n-hexane/diethyl ether (1:1 v/v) to elute the product. The isolated
products were identified by comparison of their mass and 1H and
13C NMR spectra with those of authentic samples. The retrieved catalyst
was washed with acetone, and dried in vacuo prior to being reused.
Experimental Section
Instruments: GC analyses were performed on a Shimadzu GC-2014 with
an FID detector equipped with a DB-WAX ETR or Rtx-200 capillary
column. Mass spectra were recorded on a Shimadzu GCMS-QP2010
equipped with a TC-5HT capillary column at an ionization voltage of
70 eV. Liquid-state NMR spectra were recorded on a JEOL JNM-EX-
270. 1H and 13C NMR spectra were measured at 270 and 67.8 MHz, re-
spectively. The ICP-AES analyses were performed with a Shimadzu
ICPS-8100. The BET surface area was measured on a Micromeritics
ASAP 2010 and calculated from the N2 adsorption isotherm with the
BET equation. The NH3-TPD profile was measured on a BEL Japan
Multitask TPD with a quadrupole mass spectrometer. The sample was
pretreated (evacuated at 3008C for 1 h) and NH3 (20 Torr at 1008C for
10 min) was adsorbed. After excess NH3 was removed, He (the carrier
gas) was flowed into the cell (50 mLminÀ1). After the baseline stabilized,
Acknowledgements
This work was supported in part by the Global COE Program (Chemistry
Innovation through Cooperation of Science and Engineering), the Japan
Chemical Innovation Institute (JCII), and Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science, and
Technology.
the temperature was linearly increased to 8008C at a rate of 108C minÀ1
.
The amount of desorbed NH3 was quantified by a mass spectrometer by
using the m/z 16 fragment. The IR spectrum of pyridine adsorbed on Sn–
W oxides was measured on a Jasco FT/IR-460 plus spectrometer. The
sample (ꢁ50 mg) was pressed into a disk with a diameter of 20 mm. The
disk was evacuated in the in situ IR cell at 3008C for 1 h. Then, pyridine
1266
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Chem. Eur. J. 2011, 17, 1261 – 1267

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Received: September 27, 2010
Published online: December 1, 2010
Chem. Eur. J. 2011, 17, 1261 – 1267
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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