Succinimide-N-sulfonic acid: A mild, efficient, and reusable catalyst for the chemoselective trimethylsilylation of alcohols and phenols

Article abstract of DOI:10.1080/10426507.2011.602377

Succinimide-N-sulfonic acid (SuSA) is easily prepared by the reaction of succinimide with chlorosulfonic acid. This reagent is able to efficiently catalyze the chemoselective trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS). All reactions were performed under mild reaction conditions, giving excellent yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426507.2011.602377

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Phosphorus, Sulfur, and Silicon and the
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Succinimide-N-Sulfonic Acid: A Mild,
Efficient, and Reusable Catalyst for the
Chemoselective Trimethylsilylation of
Alcohols and Phenols
a
a
F. Shirini & N. Ghaffari Khaligh
a
Department of Chemistry, College of Science, University of Guilan,
Rasht, 41335, Iran
Version of record first published: 31 Oct 2011
To cite this article: F. Shirini & N. Ghaffari Khaligh (2011): Succinimide-N-Sulfonic Acid: A Mild,
Efficient, and Reusable Catalyst for the Chemoselective Trimethylsilylation of Alcohols and Phenols,
Phosphorus, Sulfur, and Silicon and the Related Elements, 186:11, 2156-2165
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Phosphorus, Sulfur, and Silicon, 186:2156–2165, 2011
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Taylor & Francis Group, LLC
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ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.602377
SUCCINIMIDE-N-SULFONIC ACID: A MILD, EFFICIENT,
AND REUSABLE CATALYST FOR THE CHEMOSELECTIVE
TRIMETHYLSILYLATION OF ALCOHOLS AND PHENOLS
F. Shirini and N. Ghaffari Khaligh
Department of Chemistry, College of Science, University of Guilan, Rasht 41335,
Iran
GRAPHICAL ABSTRACT
Abstract Succinimide-N-sulfonic acid (SuSA) is easily prepared by the reaction of succin-
imide with chlorosulfonic acid. This reagent is able to efficiently catalyze the chemoselective
trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS). All reactions
were performed under mild reaction conditions, giving excellent yields.
Keywords Succinimide-N-sulfonic acid; alcohols; phenols; trimethylsilylation; hexamethyl-
disilazane
INTRODUCTION
Trimethylsilylation of organic compounds having labile hydrogen atoms find in-
1
creasing use in analytical and preparative organic chemistry. This conversion enhances the
solubility of compounds in nonpolar solvents, increases the thermal stability, and has been
extensively used to increase the volatility of compounds for gas chromatography and mass
spectrometry as well. Moreover, protection of the hydroxyl groups as their correspond-
ing trimethylsilyl ethers is of vital importance in the total synthesis of complex organic
molecules.
Generally, the formation of silyl ethers is carried out by treatment of alcohols with
silyl chlorides or silyl triflates under the influence of basic conditions. However, some of
these methods suffer from drawbacks such as lack of reactivity or the difficulty in removing
amine salts derived from the reaction of by-produced acids and co-bases during the course of
the reaction. Hexamethyldisilazane (HMDS) is a cheap, stable, and commercially available
reagent. O-silylation of alcohols using HMDS is an attractive alternative, since the only
by-product of the reaction is ammonia, which is easily removed from the reaction mixture.
However, its main drawback is its poor silylating power, which needs forceful conditions
Received 17 May 2011; accepted 29 June 2011.
We are thankful to the University of Guilan Research Council for the partial support of this work.
Address correspondence to F. Shirini, Department of Chemistry, College of Science, University of Guilan,
Rasht 41335, Iran. E-mail: shirini@guilan.ac.ir
2
156

SUCCINIMIDE-N-SULFONIC ACID: A MILD, EFFICIENT AND REUSABLE CATALYST 2157
and long reaction times. Although, a variety of catalysts have been reported for the activation
2–28
of HMDS,
most of them suffer from disadvantages such as long reaction times, forceful
conditions, low yields, low selectivity, tedious workup, and use of toxic or expensive
reagents. Consequently, a new procedure that addresses these drawbacks is desirable.
RESULTS AND DISCUSSION
Recently and in continuation of our ongoing research program on the development of
2
7–29
new methods for the protection of hydroxyl groups,
we have reported the preparation
of saccharin-N-sulfonic acid and its application for promotion of silylation of alcohols with
30
HMDS. Even though the activity of HMDS increases in the presence of this reagent,
this method is not useful for the protection of diols, acyloins, and phenols. In view of this
study, we decided to overcome these limitations by preparing succinimide-N-sulfonic acid
(SuSA) and conducting silylation reactions in the presence of this reagent.
SuSA was easily prepared by addition of an equivalent amount of cholorosulfonic
acid to succinimide (Su). The reaction is easy and clean and needs no special workup
procedure (Scheme 1).
Scheme 1
Optimization of the reaction conditions showed that the best results were obtained
when the reaction was performed in CH3CN at room temperature, while the relative ratio of
the substrate/HMDS/SuSA was 1.00/0.60/0.02, respectively. After this, different types of
alcohols were subjected to trimethylsilylation under the determined conditions (Scheme 2,
Table 1).
Scheme 2
O-silylation of benzylic alcohols, including acid-sensitive, electron-donating, or
electron-withdrawing groups, proceeds efficiently with high isolated yields (Table 2, entries
1
–16). Primary and secondary aliphatic alcohols were also efficiently converted to their
corresponding trimethylsilyl ethers under the same reaction conditions (Table 1, entries
7–24). This method was found to be useful for the protection of hindered secondary and
1
tertiary alcohols (Table 1, entries 25–28). This method is also useful for the silylation of
diols and acyloins (Table 1, entries 29–31). No elimination or rearrangement by-products
were observed at all.
In order to show the generality of the method, we used the optimized conditions for
the silylation of the structurally different phenols (Table 2). Our investigations clarified that
under the selected conditions, phenol and substituted phenols were also silylated easily and

2158
F. SHIRINI AND N. G. KHALIGH
Table 1 Trimethylsilylation of alcohols catalyzed by SuSAa,b
Time
(min)
Yield
(%)
Entry
Substrate
Product
PhCH2OTMS
2-ClC6H4CH2OTMS
2-BrC6H4CH2OTMS
2-MeC6H4CH2OTMS
2-NO2C6H4CH2OTMS
4-ClC6H4CH2OTMS
1
2
3
4
5
6
PhCH2OH
1
1
1
1
96
97
96
94
87
97
2-ClC6H4CH2OH
2-BrC6H4CH2OH
2-MeC6H4CH2OH
2-NO2C6H4CH2OH
4-ClC6H4CH2OH
30
1
1, 1, 1)c
(
(97, 95, 92)c
7
8
9
0
1
2
3
4
4-BrC6H4CH2OH
4-NO2C6H4CH2OH
4-Me2CHC6H4CH2OH
2-PhOC6H4CH2OH
3,4-Cl2C6H3CH2OH
PhCH(OH)Me
4-BrC6H4CH2OTMS
4-NO2C6H4CH2OTMS
4-Me2CHC6H4CH2OTMS
2-PhOC6H4CH2OTMS
3,4-Cl2C6H3CH2OTMS
PhCH(OTMS)Me
1
20
1
2
1
5
5
5
97
92
96
97
98
90
90
92
1
1
1
1
1
PhCH(OH)CHMe2
PhCH(OTMS)CHMe2
1
1
5
6
Ph2CHOH
Ph2CH(OTMS)
1
2
96
98
1
1
1
2
7
8
9
0
PhCH2CH2OH
PhCH2CH2CH2OH
PhCH(Me)CH2OH
PhCH2CH2OTMS
PhCH2CH2CH2OTMS
PhCH(Me)CH2OTMS
2
2
3
96
96
95
92
10
2
1
3
95
2
2
2
3
Me2CHOH
Me2CH(OTMS)
12
7
94
92
2
2
4
5
5
5
94
90
2
6
17
87
(
Continued on next page)

SUCCINIMIDE-N-SULFONIC ACID: A MILD, EFFICIENT AND REUSABLE CATALYST 2159
Table 1 Trimethylsilylation of alcohols catalyzed by SuSAa,b (Continued)
Time Yield
Entry
Substrate
Product
(min) (%)
2
7
15
87
2
8
12
91
29
30
31
PhCH(OH)CH2OH
PhCOCH(OH)Ph
PhCH(OTMS)CH2(OTMS)
PhCOCH(OTMS)Ph
5
5
4
90d
98
98
a
1
Products were characterized by their physical constants, comparison with authentic samples, and IR, H NMR,
13
and C NMR spectroscopy.
b
Isolated yields.
c
Results obtained using recycled catalyst.
d
HMDS 1.2 mmol is used.
their corresponding silyl ethers were isolated in excellent yields. We have also applied this
method for the silylation of amines and thiols. The reactions, however, did not proceed even
after a long reaction time and the starting materials were isolated intact (Table 2, entries
2
1–24).
On the basis of these results and due to the importance of the selectivity in synthetic
organic chemistry, competitive reactions were designed to evaluate the chemoselectivity
of the protocol. The results summarized in Table 3 clearly indicate that under the selected
conditions, alcohols and phenols are easily protected in the presence of amines and thiols
with very high chemoselectivity.
We have found that SuSA is a reusable catalyst, and even after four runs for silylation
of alcohols and phenols, the catalytic activity of the reagent was almost the same as
compared with that of the freshly used catalyst (Table 1, entry 6; Table 2, entry 1). The
same infrared (IR) spectrum of the reagent was obtained before and after its use in the
reactions, which demonstrates the stability of its composition.
The mechanism of the reaction is not clear, but the fast evolution of NH3 gas from
the reaction mixture (indicated by odor and litmus paper) and the reusability of the catalyst
directed us to accept the mechanism that is shown in Scheme 3 as the most probable one.
On the basis of this mechanism, SuSA reacts with HMDS to produce the reactive silylating
agent (I), which in turn reacts with the substrate to produce the requested silyl ether and
unstable complex (II). Irreversible cleavage of (II) leads to the fast evolution of NH3 and
release of SuSA, which re-enters the catalytic cycle.
In order to show the merit of the catalyst, we have compared silylation of benzhydrol
with HMDS in the presence of SuSA with some other catalysts for the similar reaction

2160
F. SHIRINI AND N. G. KHALIGH
Scheme 3
(Table 4). Higher temperature, longer reaction times, and lower yields of other methods
make our system a better choice.
In conclusion, in this article, we report the preparation of SuSA and its application
in promotion of silylation of alcohols and phenols with HMDS. Mildness of the reaction
conditions, short reaction times, excellent yields, easy workup, reusability of the catalyst,
and chemoselectivity are noteworthy advantages of this method.
EXPERIMENTAL
Chemicals were purchased from Merck, Aldrich, and Fluka Chemical and were used
without further purification. The purity determination of the products was accomplished
by thin-layer chromatography (TLC) on silica gel polygram SIL G/UV 254 plates. The
−1
mass spectra were measured under GC (70 eV) conditions. The IR spectra (ν/cm ) were
recorded as KBr pellets or films on a Perkin Elmer 781 spectrophotometer. In all the cases,
1
the H NMR spectra were recorded with a 300-MHz Bruker Avance instrument. Chemical
shifts are reported in parts per million (ppm) in CDCl3 with tetramethylsilane as an internal
13
standard. C NMR data were collected on a 75-MHz Bruker Avance instrument.
Preparation of SuSA
A 25-mL suction flask charged with a solution of succinimide (0.99 g, 10 mmol)
in dry CH2Cl2 (5 mL) was equipped with a constant pressure-dropping funnel containing
1
.165 g chlorosulfonic acid (10 mmol) and a gas inlet tube for conducting HCl gas over
an adsorbing solution, i.e., water. Chlorosulfonic acid was added dropwise over a period of
5 min while the reaction mixture was stirred slowly in an ice bath. The mixture was then
1
heated to room temperature and stirred for additional 2 h. The mixture was filtered and the
solid residue was washed with ether (two times with 5 mL) and dried under vacuum. SuSA
◦
1
was obtained as a cream-colored solid (1.74 g, 97%), mp: 83–85 C; H NMR (D2O): δ 2.61

SUCCINIMIDE-N-SULFONIC ACID: A MILD, EFFICIENT AND REUSABLE CATALYST 2161
Table 2 Trimethylsilylation of phenols catalyzed by SuSAa,b
Entry
Substrate
Product
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
PhOH
4-FC6H4OH
4-EtC6H4OH
3-MeC6H4OH
3,5-Me2C6H3OH
2,4-Me2C6H3OH
4-Me2CHC6H4OH
2-PhC6H4OH
PhOTMS
4-FC6H4OTMS
4-FC6H4OTMS
3-MeC6H4OTMS
3,5-Me2C6H3OTMS
2,4-Me2C6H3OTMS
4-Me2CHC6H4OTMS
2-PhC6H4OTMS
1 (1, 1, 1)c 98 (97, 96, 96)c
1
1
1
2
1
2
4
2
2
98
98
98
95
96
98
98
98
95
4-PhCH2C6H4OH
4-PhCH2C6H4OTMS
1
0
11
12
13
2-NH2C6H4OH
4-NH2C6H4OH
2-NH2C6H4OTMS
4-NH2C6H4OTMS
5
2
5
90
96
98
d
1
1
4
5
5
4
98d
95d
1
6
5
96d
1
1
7
8
3
94d
85e
40
1
9
15
88e
(
Continued on next page)

2162
F. SHIRINI AND N. G. KHALIGH
Table 2 Trimethylsilylation of phenols catalyzed by SuSAa,b (Continued)
Entry
Substrate
Product
Time (min)
2
Yield (%)
91
2
0
2
1
5
89
2
2
2
2
2
3
4
5
PhCH2NH2
4-MeC6H4NH2
PhCH2SH
PhCH2NHTMS
4-MeC6H4NHTMS
PhCH2STMS
120
120
120
120
0
0
0
0
4-MeC6H4SH
4-MeC6H4STMS
a
1
Products were characterized by their physical constants, comparison with authentic samples, and IR, H NMR,
13
and C NMR spectroscopy.
b
Isolated yields.
c
Results obtained using recycled catalyst.
HMDS 1.2 mmol is used.
d
e
HMDS 1.8 mmol is used.
s, 4H); H NMR (CDCl3): δ 2.75 (s, 4H), 9.18 (s, 1H); 13C NMR (CDCl3): δ 178.2, 29.5;
1
(
IR: 3500, 2960, 1770, 1700, 1360, 1295, 1195, 1050; GCMS (70 eV), m/z: 180.1[M +
+
+
1
] , 179.1[M] .
SuSA is soluble in H2O, MeOH, EtOH, CH3CN, DMF, and DMSO, while insoluble
in ether, toluene, hexane, ethyl acetate, cyclohexane, THF, and 1,4-dioxane. This reagent is
Table 3 Chemoselective trimethylsilylation reactions in the presence of SuSA
Entry
1
Substrate
Product
Time (min)
10
Conversiona
100
2
10
100
1
00
3
+
+
+
1
0
0
aBased on GC (gas chromatography).

SUCCINIMIDE-N-SULFONIC ACID: A MILD, EFFICIENT AND REUSABLE CATALYST 2163
Table 4 Comparison of the silylation of benzhydrol in the presence of SuSA with some other catalysts
Entry
Catalyst/solvent/condition
Time (min)
Yield (%)
Reference
1
2
3
4
5
6
7
8
9
LaCl3 (10 mol%)/CH2Cl2/r.t.
Al(HSO4)3 (3.5 mol%)/CH3CN/r.t.
TCCA (10 mol%)/CH2Cl2/r.t.
Mg(OTf)2 (1 mol%)/neat/r.t.
210
180
180
120
50
48
35
25
20
2
93
95
95
70
87
93
98
95
90
92
92
96
[19]
[29]
[2]
[10]
[9]
[4]
[20]
[21]
[28]
LiClO4–SiO2 (100 mg)/CH2Cl2/r.t.
◦
H3PW12O40 (1 mol%)/neat/55–60 C
Cu(OTf)2 (1 mol%)/CH3CN/r.t.
TiCl3(OTf)–SiO2 (1 mol%)/neat/r.t.
n-Bu4NBr (5 mol%)/CH3CN/r.t.
TiCl3(OTf) (1 mol%)/neat/r.t.
SaSA (10 mol%)/CH3CN/r.t.
1
1
1
0
1
2
[17]
[30]
This work
10
1
SuSA (2 mol%)/CH3CN/r.t.
also slightly soluble in CH2Cl2, CHCl3, and acetone. The pH values of SuSA show strong
acidity for this reagent in water (pKa ≈ 3.80).
Trimethylsilylation of alcohols and phenols catalyzed by SuSA
SuSA (1.8 mg, 0.02 mmol) was added to a stirred solution of the substrate (1.0 mmol)
and HMDS (97 mg, 0.6 mmol) in CH3CN (3 mL), and the mixture was stirred at room
temperature for the time mentioned in the Tables 1 and 2. After completion of the reaction
(monitored by TLC), the solvent was removed under reduced pressure and hexane (5 mL)
was added, and the catalyst was recovered by filtration and washed with n-hexane (5 mL).
The filtrate was washed with water, then dried over anhydrous MgSO4. Evaporation of
the solvent under reduced pressure gave the highly pure product without need for further
purification.
The desired pure products were characterized by comparison of their IR and NMR
3
,5,7,14,18,22–26
data as well as their boiling point with those of known compounds.
Spectroscopic data of new compounds:
1
a. Table 1, entry 20: Colorless liquid, H NMR (CDCl3): δ 7.25–7.34 (m, 5H), 3.77–3.81
(
2
d, 2H), 3.72 (t, J = 5.2 Hz, 1H), 3.63 (t, J = 5.2 Hz, 1H), 3.43 (bs, 1H, NH), 2.74 (q,
13
H), 0.14 (s, 3H), 0.12 (s, 3H), 0.10 (s, 3H); C NMR (CDCl3): δ 139.6, 128.5, 128.2,
1
27.1, 60.6, 53.5, 50.7, 1.9, 1.5, −0.49; IR: 2950, 1450, 1255, 1095, 838, 740, 693.
1
13
H NMR and C NMR results can be illustrated by the following structure for trimethyl
2-(benzylamino)ethoxy]silane:
[
1
b. Table 1, entry 31: Light yellow liquid, H NMR: δ 7.11–8.05 (m, 8H), 5.97(s, 1H), 2.32
13
(
1
s, 3H), 2.27 (s, 3H), 0.18 (s, 3H), 0.14 (s, 6H); C NMR: δ 144.8, 143.7, 138.2, 136.4,
31.0, 130.2, 129.7, 129.4, 129.2, 129.0, 127.7, 126.6, 75.8, 21.6, 21.1, 2.04, 0.11; IR:

2164
F. SHIRINI AND N. G. KHALIGH
3
010, 2950, 1690, 1680, 1670, 1600, 1506, 1253, 1180, 1120, 1100, 880, 840, 752.
1
13
H NMR and C NMR results can be illustrated by the following structure for O-
trimethylsilyl-4,4 -dimethylbenzoin:
ꢁ
1
c. Table 2, entry 18: Colorless liquid, H NMR: δ 6.70 (dd, J = 7.5 Hz, 1H), 6.52 (d, J =
3
7
0
.5 Hz, 2H), 0.28 (s, 18H), 0.26 (s, 9H); 1 C NMR: δ 148.1, 138.8, 120.4, 113.9, 0.65,
.31; IR: 2970, 2900, 1580, 1480, 1462, 1300, 1253, 1050, 910, 838.
d. Table 2, entry 19: Colorless liquid, H NMR: δ 6.02 (s, 3H), 0.25 (s, 27H); 13C NMR
1
(CDCl3): δ 156.4, 106.2, 0.18; IR (KBr): 2960, 2900, 1587, 1440, 1255, 1158, 1022,
850, 750, 688.
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