Catalytic ring-opening allylation of cyclic acetals with allylsilanes using silica-alumina

Article abstract of DOI:10.1039/c005033d

Ring-opening allylation of cyclic acetals has required stoichiometric amounts of Lewis acids. In this paper, catalytic ring-opening allylation in the presence of silica-alumina is reported. This ring-opening allylation showed high atom economy. Consecutiv

Full text of DOI:10.1039/c005033d

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www.rsc.org/greenchem | Green Chemistry
Catalytic ring-opening allylation of cyclic acetals with allylsilanes using
silica-alumina†
Ken Motokura, Hirokazu Yoneda, Akimitsu Miyaji and Toshihide Baba*
Received 13th April 2010, Accepted 25th June 2010
First published as an Advance Article on the web 21st July 2010
DOI: 10.1039/c005033d
Ring-opening allylation of cyclic acetals has required stoi-
chiometric amounts of Lewis acids. In this paper, catalytic
ring-opening allylation in the presence of silica-alumina is
reported. This ring-opening allylation showed high atom
economy. Consecutive allylation/hydrolysis using silica-
Scheme 2 Catalytic consecutive ring-opening allylation/hydrolysis
alumina afforded homoallyloxyalcohols in good to high
yields.
using acid.
homoallyloxysilylether, occurs catalytically. It is also well known
that the hydrolysis of silylether is promoted by Brønsted acids.16
In the catalytic consecutive ring-opening allylation/hydrolysis,
silanol is expected to be the only by-product.
Introduction
Homoallyloxyalcohol is an important building block for the
synthesis of organic compounds such as amino sugars1 because
it has allyl, hydroxyl, and ether functional groups. Consecutive
ring-opening allylation/hydrolysis affords the homoallyloxyal-
cohol (Scheme 1). Ring-opening allylation of cyclic acetals has
been demonstrated in the presence of stoichiometric amounts
of Lewis acid reagents, such as TiCl4,2–11 AlCl3,6 SnCl4,6 Ti(Oi-
Pr)4,7–11 BF3·Et2O12,13 and TMSOTf.13 Recently, Spafford and
co-workers reported using a Bi(OTf)3 catalyst for ring-opening
allylation in the presence of acid anhydrates.14 The acid anhy-
drate reacts with a “Lewis acid-ring-opened acetal complex”,
regenerating the Bi catalyst and producing an ester instead of
the corresponding homoallyloxyalcohols.
Results and discussion
Ring-opening allylations of 2-phenyl-1,3-dioxolane (1) with
allyltrimethylsilane (2), using various Brønsted acids, are sum-
marized in Table 1. Silica-alumina showed high activity and
selectivity, affording an 83% yield of homoallyloxysilylether
(3) (entry 1).17 A moderate yield of 3 was obtained with H+-
beta zeolite (entry 2). H+-montmorillonite displayed a high
conversion of acetal 1; however, selectivity toward desired
allylated product 3 was much lower than that of the silica-
alumina, owing to the formation of by-product 4 (entries 3 and
4). Amberlyst and Nafion also showed low selectivity (entries
5 and 6). Other zeolites we tested were found to be less active
(entries 7–10). Silica with a mesoporous structure (FSM-16)
can promote the allylation reaction to afford an 18% yield of
3 (entry 11). The selectivity of product 3 was very low with
triflic acid (entry 12). The desired reaction scarcely proceeded
with other homogeneous Brønsted acids, such as H2SO4 and
p-toluenesulfonic acid (entries 13 and 14).
Scheme 1 Consecutive ring-opening allylation/hydrolysis using a
stoichiometric amount of Lewis acid.
To optimize the reaction conditions, the ring-opening allyla-
tion of 1 was carried out using silica-alumina in various solvents,
as shown in Table 2. The reaction proceeded smoothly with non-
polar solvents such as toluene and heptane (entries 1 and 2), but
the product 3 was hardly formed when using polar solvents with
high dielectric constant values (entries 4–6).
During the allylation of 1, silica-alumina was filtrated at
40% conversion of 1. No ◦allylation reaction occurred when the
filtrate was treated at 60 C for 26 h. This result suggests that
the active site for the ring-opening allylation is on the silica-
alumina surface. In order to estimate the active species, the
ring-opening allylation of 1 was carried out using silica-alumina
treated with a NaCl/NaOH solution (Na-silica-alumina) and
amorphous SiO2 (Table 1, entries 16 and 17). The allylation
did not occur when using Na-silica-alumina instead of silica-
alumina. This result demonstrated that an acid site on the silica-
alumina promotes the allylation reaction. Silica-alumina has
We have attempted the catalytic synthesis of homoallyloxyal-
cohols from cyclic acetals, allylsilanes and water via ring-
opening allylation/hydrolysis using a Brønsted acid catalyst in
a single reactor (Scheme 2). Because Brønsted acid catalysts
have been reported in the deacetalization of cyclic acetals,15
it can be said that Brønsted acids activate cyclic acetals as
electrophiles. If the interaction between a Brønsted acid site
and ring-opened acetal is weaker than that for the Lewis
acids, it is possible that the ring-opening allylation, affording
Interdisciplinary Graduate School of Science and Engineering,
Department of Environmental Chemistry and Engineering, Tokyo
Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama,
226-8502, Japan. E-mail: tbaba@chemenv.titech.ac.jp
† Electronic supplementary information (ESI) available: Detailed
reaction procedures and product identification data. See DOI:
10.1039/c005033d
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Table 1 Ring-opening allylation of 2-phenyl-1,3-dioxolane (1) with allyltrimethylsilane (2) using acids a
Conversion (%) b
Yield (%) b
Entry
Acid
Amount/g
t/h
1
2
3 c
4 d
1
2
3
4
5
6
7
8
Silica-alumina
H+-Beta
0.05
0.05
0.05
0.01
0.05
0.05
0.15
0.15
0.15
0.15
0.05
—
—
—
—
0.05
0.05
8
27
1
1.5
1
3
27
27
27
27
27
1
27
27
27
27
27
95
75
>99
53
58
52
42
24
10
24
25
79
11
27
18
11
4
94
86
>99
58
76
63
56
52
44
33
45
95
41
71
43
40
37
83
43
38
13
22
22
8
7
3
H+-Montmorillonite
H+-Montmorillonite
Amberlyst
48
23
29
18
<1
1
3
<1
2
35
<1
2
<1
<1
<1
Nafion
H+-Mordenite
H+-USY
1
9
H+-L
<1
<1
18
8
2
<1
1
10
11
12
13
14
15
16
17
H+-ZSM-5
FSM-16
CF3SO3H e
p-TsOH·H2O f
g
H2SO4
AlCl3
h
Na-Silica-alumina
Amorphous SiO2
1
<1
a Reaction conditions: 1 (3.7 mmol), 2 (4.4 mmol), toluene (3.0 mL), 60 ◦C, Ar atmosphere. b Determined by GC using internal standard. c Yield
based on acetal 1. d Yield = [4 (mmol) ¥ 2/1 (mmol)] ¥ 100. e CF3SO3H (0.18 mmol). f p-TsOH·H2O (0.26 mmol). g H2SO4 (0.18 mmol). h AlCl3
(0.18 mmol).
Table 2 Silica-alumina-catalyzed ring-opening allylation of 2-phenyl-
1,3-dioxolane (1) with allyltrimethylsilane (2) using various solvents a
In Table 1, almost all zeolites showed low catalytic performance
due to restricted pore size. On the other hand, a large by-
product (4) was obtained in the case of H+-montmorillonite,
Amberlyst and Nafion, due to high accessibility to the acid
site. Production of large molecules such as polyarene has been
reported using these solid acids.20 Accessibility to the Brønsted
acid site on silica-alumina is expected to be suitable for the
selective production of homoallyloxysilylether (3).
To reveal the reaction pathway, ring-opening allylation of 1
using two differently substituted allylsilanes, methallyltrimethyl-
silane and allyltriethylsilane, was carried out. The results are
shown in Scheme 3. Allylated products having methallyl-SiMe3
or allyl-SiEt3 groups formed more preferentially than allyl-
SiMe3 or methallyl-SiEt3. This result indicates that at least part
and possibly all of the reaction proceeds via intermolecular
silyl transfer. Stoichiometric amounts of Lewis acid reagent are
reported in literature to be necessary for the ring-opening ally-
lation of acetals.2–13 Bi(OTf)3 acts as a catalyst for ring-opening
allylation, but the reaction scarcely proceeds unless an acid anhy-
dride is used as a trapping reagent for the Lewis acid-ring-opened
acetal complex.14 The Lewis acid-ring-opened acetal complex
Conversion (%) b
Yield (%) b
Entry
Solvent
1
2
3 c
4 d
1
2
3
4
5
6
Toluene
95
96
90
15
36
<1
94
92
93
51
29
33
81
76
76
3
<1
<1
7
n-Heptane
1,4-Dioxane
Acetonitrile
2-Propanol
DMF
12
13
1
<1
<1
a Reaction conditions: 1 (3.7 mmol), 2 (4.4 mmol), silica-alumina (0.05
g), solvent (3.0 mL), 60 ◦C, Ar atmosphere. b Determined by GC using
internal standard. c Yield based on acetal 1. d Yield = [4 (mmol) ¥ 2/1
(mmol)] ¥ 100.
Brønsted acid sites due to the [Si–O(H+)–Al] moiety and the
weakly acidic Si–OH group.18 The product yield was less than
1% in the case of amorphous SiO2 having a Si–OH group (Table
1, entry 17), so the Brønsted acid site due to the [Si–O(H+)–Al]
moiety is more likely to be the main active site promoting the
ring-opening allylation.19 The amount of acid sites, including
the active Brønsted acid sites in silica-alumina, was estimated to
be 0.154 mmol g-1 by NH3-TPD analysis. This clearly indicates
that the Brønsted acid site acts as a catalytically active species
for the ring-opening allylation.
Catalytic activity and selectivity for the allylation may depend
on the accessibility of the Brønsted acid site on the solid surface.
Scheme 3 Ring-opening allylation of 1 using two differently substituted
allylsilanes.
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Scheme 5 Hydrolysis of 3 with and without silica-alumina.
References
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Scheme 4 Catalytic synthesis of homoallyloxyalcohols using silica-
alumina.
is thus expected to be stable under anhydrous conditions. On
the other hand, in our heterogeneous Brønsted acid-catalyzed
reaction system (Table 1), additives such as acid anhydrides were
not required. This suggests that the interaction between the solid
Brønsted acid and the ring-opened acetal is weaker than that of
the homogeneous acids. Effective exchange between H+ on the
ring-opened acetal and the SiMe3 species occurs on the solid
surface after the nucleophilic addition of the allylsilane.
To obtain homoallyloxyalcohol from homoallyloxysilylether
(3), water and acetone were added to the reactor following
the ring-opening allylation of 1 with 2 using silica-alumina.
Acetone acted as a good co-solvent for the hydrolysis. The
hydrolysis was carried out at 25 ◦C to avoid side-reactions
and the resulting reaction mixture was stirred for 23 h. The
corresponding homoallyloxyalcohol (5) was obtained with a
77% yield, based on the acetal (1) used (Scheme 4, R–Ph).21
After the hydrolysis of silyl ether product 3, silanol and siloxane
formed as by-products. As shown in Scheme 4, this consecutive
allylation/hydrolysis could be extended to other acetals, giving
good yields of the corresponding homoallyloxyalcohols. 2-Aryl-
and alkyl-1,3-dioxolanes behaved as substrates in the presence
of silica-alumina. Simple 1,3-dioxolane (R–H) also reacted with
allylsilane and H2O, to afford 2-(homoallyloxy)ethanol with a
68% yield. The reaction of a ketal, such as 2,2-dimethyl-1,3-
dioxolane, scarcely proceeded under identical conditions.
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of 3 barely occurred when no silica-alumina catalyst was used
(Scheme 5). This result indicates that silica-alumina promotes
the hydrolysis.
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17 The reaction scarcely proceeded with 1-hexene instead of al-
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1, entry 1 except for the alkene substrate.
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21 After the hydrolysis reaction, reuse of the silica-alumina catalyst was
examined. The used silica-alumina was separated from the reaction
mixture, then calcined at 500 ◦C. The catalytic activity of the used
catalyst for ring-opening allylation decreased compared with fresh
silica-alumina (yield of 3: 83% for 8 h (fresh), 52% for 8 h (reused)).
However, after the prolonged reaction time, a satisfactory yield of 3
was obtained (73% for 20 h). Hydrolysis of 3 also proceeded with the
used catalyst to afford 5 in 70% yield based on 1.
Summary
In summary, silica-alumina was found to be a catalyst for the
synthesis of homoallyloxyalcohols from cyclic acetal, allylsilane,
and water. Surface Brønsted acid sites acted as the catalytically
active species for the ring-opening allylation of various cyclic
acetals. Hydrolysis of silylethers was also promoted by silica-
alumina. A detailed reaction mechanism and a study of a larger
scope of substrates are underway.
This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1373–1375 | 1375
©