The hydroamination of alkenes with sulfonamides catalyzed by the recyclable silica gel supported triflic acid

Article abstract of DOI:10.1016/j.tetlet.2011.09.025

The vast applications of triflic acid (TfOH) in catalysis are severely limited by its corrosive and fuming properties. Immobilization of TfOH on silica gel well solves these problems and affords efficient recovery and reusability of TfOH. Two types of supported TfOH, the prepared silica gel supported TfOH and the in situ silica gel adsorbed TfOH, both exhibit good catalytic activity and reusability in the hydroamination of alkene with sulfonamide. The in situ silica gel adsorbed catalyst has been used for 5 runs with maintained reactivities and yields, which are superior to the performance of the prepared silica gel supported TfOH. For a series of alkenes and various sulfonamides, the heterogeneous hydroamination reactions catalyzed by both types of silica gel supported TfOH to afford similar moderate to excellent yields.

Full text of DOI:10.1016/j.tetlet.2011.09.025

Tetrahedron Letters 52 (2011) 6113–6117
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Tetrahedron Letters
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The hydroamination of alkenes with sulfonamides catalyzed by
the recyclable silica gel supported triflic acid
Pei Nian Liu a,b, , Fei Xia a, Zheng Le Zhao a, Qing Wei Wang a, Yu Jie Ren c
⇑
a Shanghai Key Laboratory of Functional Materials Chemistry and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong
Road, Shanghai 200237, China
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
c School of Chemistry and Environmental Engineering, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 June 2011
Revised 29 August 2011
Accepted 6 September 2011
Available online 10 September 2011
The vast applications of triflic acid (TfOH) in catalysis are severely limited by its corrosive and fuming
properties. Immobilization of TfOH on silica gel well solves these problems and affords efficient recovery
and reusability of TfOH. Two types of supported TfOH, the prepared silica gel supported TfOH and the
in situ silica gel adsorbed TfOH, both exhibit good catalytic activity and reusability in the hydroamination
of alkene with sulfonamide. The in situ silica gel adsorbed catalyst has been used for 5 runs with main-
tained reactivities and yields, which are superior to the performance of the prepared silica gel supported
TfOH. For a series of alkenes and various sulfonamides, the heterogeneous hydroamination reactions cat-
alyzed by both types of silica gel supported TfOH to afford similar moderate to excellent yields.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Hydroamination
Triflic acid
Supported catalyst
Alkene
Sulfonamide
Introduction
through simple filtration. Moreover, the applications of the sup-
ported catalysts in the continuous flow-type processes are poten-
The synthesis of amines from the simple chemicals, such as al-
kenes is a very important area in modern organic chemistry, due to
the extraordinary importance of the amine compounds in the syn-
thesis of natural products, fine chemicals and drugs.1 The intermo-
lecular hydroamination of alkenes with sulfonamides has attracted
considerable interest in recent years because of the atom-economy
feature2 and the easy transformation of the hydroamination prod-
ucts to amines via N-desulfonylation.3 Recently, platinum(II),4
gold(I),5 copper(II),6 and other metal salts7 have been explored as
effective catalysts for the hydroamination of alkenes with sulfona-
mides. Moreover, NBS8 and a series of Brønsted acids, such as
[PhNH3][B(C6F5)4],9 triflic acid,10 H-montmorillonite,11 heteropoly
acids12, and SO3H-functionalized ionic liquids13 have also been de-
scribed to be effective catalysts for the intermolecular hydroamin-
ation of alkenes with sulfonamides. Although prominent progress
has been achieved in the homogeneous hydroamination of alkenes
with sulfonamides, the heterogeneous reactions catalyzed by the
reusable supported catalysts still remain to explore.14
tially allowed, which are important in the industry. Triflic acid
(TfOH), namely super acid,16 is perhaps the most versatile Brønsted
acid catalyst in a vast stretch of organic reactions,17 but its applica-
tions are severely limited by the corrosive and fuming properties
which cause problems in handling, transportation, and disposal.
Recently, we have prepared the silica gel supported TfOH which
solves the corrosive and fuming problems of TfOH. Although the
silica gel supported TfOH has been successfully applied to the addi-
tion of b-dicarbonyl compounds to alcohols and alkenes,18 the
preparation procedures of the silica gel supported TfOH are still te-
dious and inevitably consume the solvent and energy. In a succes-
sive endeavor to explore more effective methods to immobilize
TfOH and the further applications of the silica gel supported TfOH,
we herein report the immobilization of TfOH through two different
approaches and their applications in catalyzing the heterogeneous
hydroamination of alkenes with sulfonamides.
Results and discussion
With the demanding for ‘green chemistry’, the immobilization
of the homogeneous catalysts to the solid supporters such as silica
gel has attracted great interest,15 because easy separation of the
products and efficient recovery of the catalysts can be achieved
The hydroamination of 1a with 2a catalyzed by the prepared
TfOH–SiO2
As the first stage of this study, the silica gel supported TfOH
(TfOH–SiO2) was prepared through adsorption of TfOH onto silica
⇑
Corresponding author. Tel.: +86 21 64250552; fax: +86 21 64252758.
E-mail address: liupn@ecust.edu.cn (P.N. Liu).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.025

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P. N. Liu et al. / Tetrahedron Letters 52 (2011) 6113–6117
gel.18 The prepared TfOH–SiO2 was then used as the catalyst for the
hydroamination of norbornylene (1a) with p-toluenesulfonamide
(2a). It can be seen from Table 1 that the prepared TfOH–SiO2
can not catalyze the reactions of 1a and 2a in n-heptane, THF,
methanol, acetonitrile, and nitromethane at 70 °C (Table 1, entries
1–5). As dioxane, ethyl acetate, and 1,2-dichloroethane (DCE) were
used as the solvents, better reaction results were obtained and
even 96% yield was observed in chloroform (Table 1, entries 6–
9). When toluene was used as the solvent for the hydroamination
of 1a with 2a, we were delighted to obtained very good 98% yield
after 4 h at 70 °C. To expel the possibility that the reaction was cat-
alyzed by the leached TfOH, we stirred TfOH–SiO2 in toluene at
70 °C for 4 h, then filtered out the solid catalyst at 70 °C, and added
1a and 2a to the resulting solution and stirred at 70 °C for 4 h, but
no product was detected. The result proves that the hydroamina-
tion of 1a with 2a is a heterogeneous reaction catalyzed by the
supported TfOH rather than the homogeneous reaction catalyzed
by the leached TfOH. Moreover, the prepared TfOH–SiO2 was
recovered readily and reused for 3 runs with almost maintained
reactivities and yields, but it lost the catalytic ability in the 4th
run (Table 1, entries 10–13). To investigate the catalytic reactivities
of the fresh and recovered TfOH–SiO2 in the reactions of 1a and 2a,
the conversions of the consecutive reactions at different times
were monitored by HPLC. The results showed that the recovered
TfOH–SiO2 in the 2nd and 3rd runs possessed the same or even
better catalytic reactivities in comparison with the fresh TfOH–
SiO2. Why did the reactivity of the 4th reaction drop sharply?
Firstly, we observed that the color of the reaction mixture in the
3rd run turned to deep brown from light yellow, indicating the
decomposition of TfOH. Moreover, in the consecutive reactions,
through the elemental analysis of F and S in TfOH–SiO2 after each
reaction, we demonstrated that 5–8% of TfOH leaches out from
TfOH–SiO2 in one cycle of reaction.18 So, we propose that the
remarkable decrease of reactivity in the 4th run is deduced by
the decomposition and the leaching of TfOH. If the ratio of 1a–2a
was changed from 2:1 to 1:1 or 1:2, worse reaction results were
observed (Table 1, entries 14, 15). Note that the reaction of 1a
and 2a did not proceed at room temperature (Table 1, entry 16).
Moreover, when silica gel without TfOH was used as the catalyst,
the reaction of 1a and 2a did not proceed, illustrating that silica
gel only acted as the supporter of TfOH and did not catalyze the
reaction. At last, the homogeneous reaction of 1a and 2a with 5%
TfOH as the catalyst was carried out and 93% yield was obtained
(Table 1, entry 17). This control experiment reveals that the heter-
ogeneous reaction with silica gel as the adsorbent is a little supe-
rior to the homogeneous reaction (98% vs 93% yield).
The hydroamination of 1a with 2a catalyzed by the in situ silica
gel adsorbed TfOH
Although the immobilization of TfOH on the silica gel well
solves the problems caused by the corrosive and fuming properties
of TfOH, the preparation procedures for the silica gel supported tri-
flic acid still consume much solvent and energy. If the recovery and
reusability of TfOH could be achieved by directly adding silica gel
as the adsorbent of TfOH, the procedures to prepare the silica gel
supported TfOH would be avoided and TfOH could be recovered
and reused much more conveniently. So, we attempted to adsorb
TfOH onto the silica gel by directly adding the silica gel to the tol-
uene solution containing TfOH. When silica gel (200–300 mesh,
0.1 g) was added to the toluene solution (2 mL) containing TfOH
(4.4 lL, 0.05 mmol), the foggy solution became clear immediately,
indicating that TfOH was adsorbed by the silica gel. The resulting
mixture was stirred for 30 min at room temperature and 1a
(2 mmol) and 2a (1 mmol) were added. The hydroamination of
1a with 2a at 70 °C gave excellent 95% yield after 4 h and the cat-
alyst was readily recovered by simple filtration. In the subsequent
reaction of 1a with 2a, we were delighted to find that the in situ
silica gel adsorbed TfOH can be used for 4 runs with maintained
reactivity and yields (entries 1, 2, Table 2). It is well known that sil-
ica gel contains lots of hydroxyl groups which can bind water
firmly via hydrogen bonds. On the other hand, TfOH is miscible
with water. So, with the hope to reduce the leaching of TfOH from
silica gel in the hydrophobic solvent, thus to improve the reusabil-
ity of the silica gel adsorbed TfOH, we added a small amount of
water to the in situ silica gel adsorbed TfOH. It can be seen from
Table 2 that this strategy does work and the best reusability of 5
Table 1
a
The hydroamination of 1a with 2a catalyzed by the prepared TfOH–SiO2
O
S
H
O
S
5 mol% TfOH-SiO2
70 oC, Solvent
N
+
NH2
O
O
Table 2
1a
3a
2a
The hydroamination of 1a with 2a catalyzed by the in situ silica gel adsorbed TfOHa
O
S
H
O
S
TfOH, Solvent
Silica gel, H2O
N
+
NH2
Entry
Run
Solvent
1a:2a
Yieldb (%)
O
O
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
2
3
4
1
1
1
1
n-Heptane
THF
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
1:1
1:2
2:1
2:1
0
0
0
5
1a
3a
2a
Methanol
Acetonitrile
Nitromethane
Dioxane
Ethyl acetate
DCE
Chloroform
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Entry
TfOH (mol %)
H2O: silica gel
Solvent
Run
Yieldb (%)
5
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
5
5
3
3
5
0
0
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DCE
1–4
5
1–4
5
1–5
6
1–4
5
1–3
4
1–3
4
93–95
8
92–95
17
92–97
24
92–96
12
92–95
0
93–94
9
23
59
91
96
98
97
93
4
1:10
1:10
1:5
1:5
3:10
3:10
1:5
1:5
1:5
1:5
-
9
10
11
12
13
14
15
16c
17d
9
58
49
0
10
11
12
13c
DCE
Toluene
Toluene
Toluene
Toluene
Toluene
93
1
91
a
a
Reagents and conditions: 1a (2.0 mmol), 2a (1.0 mmol), TfOH–SiO2 (0.10 g,
Reagents and conditions: 1a (2.0 mmol), 2a (1.0 mmol), TfOH (4.4 lL,
containing 0.05 mmol TfOH), solvent (2 mL), 70 °C, 4 h unless noted.
0.05 mmol), silica gel (0.1 g), solvent (2 mL), 70 °C, 4 h unless noted.
b
b
Isolated yields based on the sulfonamide.
This reaction was carried out at 25 °C for 28 h.
The reaction was carried out with liquid TfOH (5 mol %) as the catalyst.
Isolated yields based on 2a.
c
c
The reaction was carried out with liquid TfOH (4.4
(0.02 mL) as the catalyst.
lL, 0.05 mmol) and H2O
d

P. N. Liu et al. / Tetrahedron Letters 52 (2011) 6113–6117
6115
Table 3
The hydroamination of alkenes with sulfonamides catalyzed by both types of silica gel supported TfOH
O
S
H
5 mol% HOTf-SiO2 (Method A) Or
O
N
H O)
2
5 mol% HOTf-SiO2(
(Method B)
R3
+
H2N
S
O
R1
R2
R3
Toluene, 85 o
C
R1
R2
O
Entry
1d
Alkene
1a
Sulfonamide
Product
Yielda (Method A)b
Yielda (Method B)c
O
O
S
H
H
S
NH2
N
N
95
95
93
O
O
2a
3a
O
O
S
S
NH2
NH2
2
72
O
O
1a
2b
2c
3b
O
O
S
H
H
N
N
S
3
4
5
95
88
74
95
94
84
O
O
1a
3c
O
O
S
Cl
NH2
S
Cl
O
O
1a
1b
3d
2d
2a
O
S
O
H
NH2
N
S
O
O
3e
O
S
O
S
H
H
H
NH2
NH2
N
N
N
6
88
43
1b
O
O
3f
O
2b
O
S
S
O
7
8
9
71
86
93
65
30
87
O
1b
1b
3g
O
2c
O
S
Cl
NH2
S
Cl
O
O
3h
O
2d
2a
O
S
H
NH2
N
N
S
O
O
1c
1c
3i
O
S
O
S
H
NH2
NH2
10
88
82
O
O
2b
3j
O
O
S
H
H
N
N
S
11
12
13
74
91
67
70
86
73
O
O
1c
1c
2c
3k
O
O
S
Cl
NH2
S
Cl
O
O
3l
2d
2a
O
S
O
S
H
NH2
N
O
O
3m
1d
1d
O
S
O
S
H
NH2
N
14
65
64
O
O
2b
3n
(continued on next page)

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P. N. Liu et al. / Tetrahedron Letters 52 (2011) 6113–6117
Table 3 (continued)
Entry
Alkene
Sulfonamide
Product
Yielda (Method A)b
Yielda (Method B)c
O
O
S
H
H
S
NH2
N
N
15
72
68
O
O
2c
1d
1d
3o
O
O
S
Cl
NH2
S
Cl
16
57
70
O
O
3p
2d
O
S
O
S
H
N
NH2
17
18
19f
91
72
89
94
MeO
MeO
MeO
O
O
1e
1e
3q
2b
O
S
O
S
H
N
Cl
NH2
Cl
85
MeO
O
O
2d
O
3r
O
NH2
N
N. P.e
H
1f
2e
3s
a
Isolated yield based on the sulfonamide.
b
Reagents and conditions of method A: alkene (4 mmol), sulfonamide (1 mmol), prepared TfOH–SiO2 (0.100 g, containing 0.05 mmol TfOH), toluene (2 mL), 85 °C, 20 h
unless noted.
c
Reaction conditions of method B: alkene (4 mmol), sulfonamide (1 mmol), TfOH (4.4
noted.
lL, 0.05 mmol), silica gel (0.1 g), H2O (20 lL), toluene (2 mL), 85 °C, 20 h unless
d
Alkene: 2 mmol, reaction time: 4 h, temperature: 70 °C.
N. P. means no product.
TfOH–SiO2 (0.200 g, containing 0.10 mmol TfOH) was used.
e
f
runs with maintained reactivity and yields is achieved when
appropriate amount of water (20 L, silica gel:H2O = 5:1) is added
in most cases the reactions were carried out at 85 °C for 20 h with
4:1 molar ratio (alkene/sulfonamide) to achieve high yields. With
the prepared TfOH–SiO2 or the in situ adsorbed TfOH–SiO2(H2O)
as the catalysts, the reactions of norbornene 1a with sulfonamides
2a, 2c, and 2d all afforded the exo-products with excellent 88–95%
yields. But in the hydroamination of 1a with ortho-methyl substi-
tuted toluenesulfonamide 2b, in situ adsorbed TfOH–SiO2(H2O)
demonstrated better results in comparison with the prepared
TfOH–SiO2 (93% vs 72% yields, entry 2, Table 3). For the hydroam-
ination of cylcopentene 1b with various sulfonamides 2a–d, the
products 3e–h were obtained in good or moderate yields with
the prepared TfOH–SiO2 as the catalyst (Table 3, entries 5–8).
When in situ adsorbed TfOH–SiO2(H2O) was used as the catalyst,
products 3e and 3g were obtained but the reactions of 1b with
2b and 2d only gave rather low yields (Table 3, entries 5–8). As
cyclohexene 1c was used to react with sulfonamides 2a–d, better
results were observed in good to excellent yields with two types
of silica gel supported TfOH as the catalysts (Table 3, entries 9–
12). When cyclooctene 1d with bigger ring was used to react with
sulfonamides 2a–d, the reactions exhibited similar lower yields
with both catalysts, probably because of the less stability of the
cyclooctyl cation19 (Table 3, entries 13–16). The methoxyl substi-
tution on aromatic rings is typically considered unstable under
strong Brønsted acid conditions in nucleophilic addition reactions.
The previous homogeneous reactions employing 5% TfOH led to the
decomposition of allylanisole at 85 °C.10a The heterogeneous reac-
tions of allylanisole 1e with sulfonamides 2b and 2d were carried
out and good yields were still obtained with both catalysts, demon-
strating the tolerance of functional group in the heterogeneous
reactions with the silica gel supported TfOH as the catalysts (Ta-
ble 3, entries 17 and 18). Note that in these reactions, anti-Mark-
ovnikov’s rule products were not observed. When allylanisole 1e
reacted with sulfonamides 2a and 2c, the reactions also proceeded
in moderate yields but the pure products could not be isolated be-
l
to the first batch of reaction mixture (entries 3–8, Table 2).
Through testing the catalytic ability of the hot solution filtered
the solid catalyst, we also confirmed that the hydroamination of
1a with 2a did be a heterogeneous reaction catalyzed by the sup-
ported TfOH rather than the homogeneous reaction catalyzed by
the leached TfOH. The conversions in every consecutive reactions
were also monitored by HPLC and similar catalytic reactivities
were observed in different batches of the in situ silica gel adsorbed
TfOH, which were comparable with the prepared TfOH–SiO2 cata-
lyst. Moreover, the color change of the reaction mixture from light
yellow to deep brown was also observed in the 5th reaction, so we
similarly attribute the reactivity drop in the 6th reaction mixture
to the decomposition and the leaching of TfOH. When 1,2-dichloro-
ethane (DCE) was used as the solvent, the hydroamination of 1a
with 2a afforded similar yield and reactivity, but the reusability
of the catalyst decreased to 3 runs (entries 9 and 10, Table 2).
When the catalyst loading was decreased to 3 mol %, the hydroam-
ination also proceeded smoothly but the reusability of the catalyst
decreased as well (entries 11 and 12, Table 2). It is noteworthy that
without adding the silica gel as the adsorbent, the reaction of 1a
and 2a with 5 mol % liquid TfOH as the catalyst only gave 91% yield
in the presence of H2O (entry 13, Table 2), indicating that the addi-
tion of silica gel is beneficial for the catalytic reaction.
The hydroamination of various alkenes with a series of
sulfonamides catalyzed by both types of TfOH–SiO2
Subsequently, the hydroamination reactions of a series of al-
kenes with various sulfonamides were examined using the pre-
pared TfOH–SiO2 (Method A) or the in situ adsorbed TfOH–
SiO2(H2O) (Method B) as the catalysts (Table 3). For most sub-
strates, the reactions under 70 °C could not afford high yields, thus

P. N. Liu et al. / Tetrahedron Letters 52 (2011) 6113–6117
6117
cause of the existing inseparable rearrangement products. In an ef-
References and notes
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with amide 2e with the prepared TfOH–SiO2 as the catalyst at en-
hanced catalyst loading (10 mol %), and the product 3s was ob-
tained successfully in 89% yield (Table 3, entry 19). Note that
with in situ adsorbed TfOH–SiO2(H2O) as the catalyst, this reaction
did not proceed. The proposed main reason is that the catalytic
ability of TfOH in H2O might be lowered due to the strong binding
with H2O and that was not strong enough to catalyze the reaction
of the styrene and amide with comparatively low reactivity.
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Conclusions
For the first time, the recovery and recycling of TfOH, a versatile
Brønsted acid catalyst, has been realized through directly adding
silica gel as the adsorbent to the catalytic reaction mixture. The
prepared TfOH–SiO2 and the in situ adsorbed TfOH–SiO2(H2O) have
both been successfully applied as the recyclable catalysts for the
hydroamination of alkenes with sulfonamides. The in situ adsorb-
ing of TfOH on silica gel not only avoids the procedure to prepare
TfOH–SiO2, but also demonstrates better reusability than the pre-
pared TfOH–SiO2. For a series of alkenes and various sulfonamides,
the hydroamination reactions afforded moderate to excellent
yields. Our methods have provided the environmentally friendly
protocols possessing the potential for applications in industry.
Moreover, the new protocol of in situ adsorbing TfOH on silica
gel would open a door to directly achieve the efficient recovery
and reusability of various Brønsted acid catalysts and Lewis acid
catalysts through simply adding the wet silica gel as the adsorbent
to the catalytic reaction mixtures.
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This work was supported financially by the National Natural
Science Foundation of China (project Nos. 20902020, 21172069),
the Innovation Program of Shanghai Municipal Education Commis-
sion (Project No. 12ZZ050) and the Fundamental Research Funds
for the Central Universities.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.09.025.
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