Sulfonic acid-functionalized ordered nanoporous Na+- montmorillonite (SANM): A novel, efficient and recyclable catalyst for the chemoselective N-Boc protection of amines in solventless media

Article abstract of DOI:10.1016/j.catcom.2011.03.030

Sulfonic acid-functionalized ordered nanoporous Na⁺- montmorillonite (SANM) was used as an efficient catalyst for N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free conditions at room temperature. Various aliphatic, aromatic, heterocyclic amines and aminols were protected as their corresponding mono-carbamates in excellent yields and short reaction times. No competitive side reactions such as isocyanate, urea, and N,N-di-Boc formation were observed. The reported method is mild, chemoselective and has the advantages such as heterogeneous catalysis, low cost and the recyclability of the catalyst.

Full text of DOI:10.1016/j.catcom.2011.03.030

Catalysis Communications 12 (2011) 1088–1094
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Sulfonic acid-functionalized ordered nanoporous Na+-montmorillonite (SANM): A
novel, efficient and recyclable catalyst for the chemoselective N-Boc protection of
amines in solventless media
⁎
Farhad Shirini , Manouchehr Mamaghani, Seyyed Vahid Atghia
Department of Chemistry, College of Science, University of Guilan, Rasht 41335, P.O. Box 1914, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Sulfonic acid-functionalized ordered nanoporous Na+-montmorillonite (SANM) was used as an efficient
catalyst for N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free
conditions at room temperature. Various aliphatic, aromatic, heterocyclic amines and aminols were protected
as their corresponding mono-carbamates in excellent yields and short reaction times. No competitive side
reactions such as isocyanate, urea, and N,N-di-Boc formation were observed. The reported method is mild,
chemoselective and has the advantages such as heterogeneous catalysis, low cost and the recyclability of the
catalyst.
Article history:
Received 2 January 2011
Received in revised form 15 March 2011
Accepted 19 March 2011
Available online 26 March 2011
Keywords:
Amines
di-tert-butyl dicarbonate (Boc2O)
N-Boc protection
Na+-montmorillonite
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, there has been considerable growth of interest in
the catalysis of organic reactions by solid acid catalysts. Solid catalysts
Protection and deprotection of organic functional groups play an
essential role in the multistep organic synthesis [1,2]. The presence of
an amino group in various biologically active compounds makes its
protection a necessity during their synthesis. The tert-butoxycarbonyl
(Boc) group is one of the most useful functionalities for the protection
of amines and amine derivatives, owing to its stability towards
nucleophiles in strong basic conditions and because of its easy
removal [1,2].
Various reagents and procedures have been employed over the
years to introduce this group using Boc2O, and the examples of them
are nucleophilic basic catalysts such as DMAP [3,4], NH2OH [5] or
K2CO3[1] and, Lewis acid catalysts like iodine [6], yttria–zirconia [7],
ZrCl4[8], Zn(ClO4)2·6H2O [9], LiClO4[10], montmorillonite K10 or KSF
[11] and La(NO3)3[12]. These procedures, although effective, have
various drawbacks such as requirement of long reaction times, use of
solvent, potential hazards (e.g., perchlorates are strong oxidizers and
explosive in nature [13,14] and ZrCl4 is highly moisture sensitive,
decomposes on storing and liberates corrosive HCl fumes), difficulty
in preparation of catalyst (e.g., preparation of yttria–zirconia involves
use of sulfuric acid at 500 °C) and formation of side-products such as
isocyanates [15,16], poly-acylated products [17], urea [18] and N,N-di-
Boc derivatives [18,19].
provide numerous opportunities for recovering and recycling cata-
lysts from reaction environments. These features can lead to improved
processing steps, better process economics, and environmentally
friendly industrial manufacturing. Silica materials with ordered
structure are the best candidate to be used as solid acid catalysts.
Clays and zeolites as silica nanostructure materials have been widely
used in organic transformations as solid acid catalysts, because they
have many advantages such as ease of handling, non-corrosiveness,
low cost and ease of regeneration. One of the best-known examples of
zeolites is MCM-41, which is a structurally well-ordered mesoporous
material with a narrow pore size distribution between 1.5 and 10 nm,
depending on the surfactant cation and a very high surface area up to
1500 m2 g−1. It has been proven that Si-MCM-41 lacks Brönsted acid
sites and exhibits only weak hydrogen-bonded type sites. An
additional possibility to develop acidic solids is the modification of
the surface of suitable support materials, as the chemical function-
alities of these materials can be uniformly modified by covalent
anchoring of different organic moieties [20]. Also owing to their
Brönsted and Lewis acidities, clays, both in their natural and ion
exchanged forms are used as efficient catalysts for various organic
transformations. However traditional heterogeneous catalysts are
rather limited in the nature of their active sites and thus the scope of
reactions that they can accomplish [21], for example montmorillonite
K-10 only have H+ sites in its galleries and reactants must be entered
into galleries for the reaction [22] and other clays should change their
gallery's cations with active cations, that required hard procedure for
preparation and long time [23]. These require the simple surface
⁎
Corresponding author. Tel./fax: +98 131 3233262.
E-mail address: shirini@guilan.ac.ir (F. Shirini).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.03.030

F. Shirini et al. / Catalysis Communications 12 (2011) 1088–1094
1089
100
modification of these solids to increase their active sites for better
interaction with reactants.
90
80
70
60
2. Experimental
Na-MMT
SANM
2.1. Materials
Chemicals were purchased from Southern Clay Products, Fluka,
Merck, and Aldrich chemical companies. All yields refer to the isolated
products. Products were characterized by their physical constants and
comparison with authentic samples. The purity determination of the
substrate and reaction monitoring were accompanied by TLC on silica-
gel polygram SILG/UV 254 plates.
25
100
175
250
325
400
475
550
Temperature (°C)
2.2. Instrumention
Fig. 1. TGA curves of Na+–MMT and SANM.
Thermogravimetric analyses (TGA) were conducted by using a
TGA PYRIS 1 thermoanalyzer instrument. Samples were heated from
25 to 600 °C at ramp 10 °C/min under N2 atmosphere. Wide-angle X-
ray diffraction (XRD) measurements were performed at room
temperature on a Siemens D-500 X-ray diffractometer (Germany),
using Ni-filtered Co-Kα radiation (λ=0.15418 nm). Elemental
analyses of the samples were performed on Elemental Vario EL III
instrument. The IR (KBr) spectra were recorded on Shimadzu
Corporation 200-91-527 instrument. The pristine clay and its
sulfonated derivative dried at 100 °C in oven before recording the
spectrum. N2 physisorption at liquid nitrogen temperature using a
SHIBATA APP.SA-1100 surface area analyzer and standard multipoint
BET analysis methods were carried out. Samples were degassed in
flowing N2 for 2 h at 250 °C before N2 physisorption measurements
were obtained.
morillonite before and after sulfonation are shown in Fig. 2. It should
be noted that sodium montmorillonite and sulfonated sodium
montmorillonite were dried at 100 °C before analyses. The basal
spacing for the Na+–MMT is 9.8°A with 2θ=9°. As shown by the XRD
patterns, the basal spacing of the sulfonic anchored sodium
montmorillonite is obviously bigger than that of Na+–MMT
(12.9°A), indicating that the sulfonic acid moiety has been interca-
lated into montmorillonite interlayer spaces. This increase in the basal
spacing can lead to the better interaction between the reactants in the
acid catalysis reactions, comparing to the montmorillonites K-10 and
KSF [22].
2.4.3. IR analysis
The infrared spectra of Na+–MMT and Na–MMT–SO3H are
presented in Fig. 3. In the case of Na+–MMT, the peak at 3623 cm−1
attributed to the OH units bonded to the aluminum and/or
magnesium in Na+–MMT. The peaks at ca. 3433 and 1640 cm−1
correspond to the \OH stretching vibration of the adsorbed water
and the bands at 1040 and 914 cm−1 can be collectively attributed to
Si\O stretching vibrations [25].
The spectrum of sulfonated clay displays almost the same pattern as
that of pristine clay, but the intensity of free \OH peak at 3623 cm−1 is
reduced in modified clay, that it is due to free ―OH sulfonation. Also the
band between 900 and 1300 cm−1 that centered in 1040 cm−1 is
flattened in sulfonated clay which can be attributed to the integration of
the additional S\O bands with Si\O stretching bonds.
2.3. Catalyst preparation
A 500 mL suction flask charged with 2.5 g Na+-montmorillonite
(Southern Clay Products) and 10 mL CHCl3, was equipped with a
constant pressure dropping funnel containing chlorosulfonic acid
(0.50 g, 9 mmol) and a gas inlet tube for conducting HCl gas into water
as adsorbing solution. Chlorosulfonic acid was added drop wise over a
period of 30 min while the reaction mixture was stirred slowly in an
ice bath (0 °C). After addition was completed, the mixture was stirred
for additional 30 min to remove all HCl. Then, the mixture was filtered
and the solid residue washed with methanol (20 mL) and dried at
room temperature to obtain SANM as white powder (2.58 g).
2.4. Catalyst characterizations
2.4.4. Elemental analysis
Because the pristine clay (sodium montmorillonite) doesn’t have
any sulfur content, the elemental analysis was used for the
determination of the amount of sulfur content of SANM. The results
2.4.1. Thermal analysis
Thermogravimetric analysis (TGA) was performed for character-
ization of SANM in comparison with sodium montmorillonite. Fig. 1
provides the TGA curve of pristine clay (Na+–MMT) and catalyst. The
TGA curve of Na+–MMT displays a weight loss below 100 °C which is
corresponding to the loss of the physically adsorbed water, also there
is a slight weight loss (ca. 8 wt.%) in the range of 100–600 °C, possibly
corresponding to dehydroxylation of montmorillonite.
12.9 °A
For SANM there is a small mass loss (ca. 1 wt.%) in the range of 25–
240 °C, attributed to the loss of the bonded H2O within the gallery
similar to Na+–MMT. However a greater mass loss which started from
240 °C can be attributed to the decomposition of −SO3H group
anchored to the clay surface. These show that Na+–MMT–SO3H has a
greater thermal stability (until 240 °C) than Na+–MMT [24].
9.8 °A
SANM
Na-MMT
2.4.2. Powder X-ray diffraction
4
5
6
7
8
9
10
In the reaction between sodium montmorillonite and chlorosul-
fonic acid, the expansion of the montmorillonite layers was
demonstrated by X-ray diffraction. The XRD patterns of the mont-
2
θ
(degree)
Fig. 2. XRD patterns of Na+–MMT and SANM.

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F. Shirini et al. / Catalysis Communications 12 (2011) 1088–1094
Fig. 3. IR spectra of Na+–MMT (a) and sulfonated Na+–MMT (b).
Scheme 1. Preparation of SANM.
obtained showed that 1 mg of sulfonated sodium montmorillonite
consists 0.32% of sulfur. On the basis of the sulfur moiety it can be
concluded that the amount of H+ sites is 0.42 mol g−1 of SANM.
The residue contained only the catalyst and was kept for recovery. The
filtrate was washed with water (3×20 mL) and brine (2×20 mL) and
dried over anhydrous MgSO4, then solvent was distillated off under
vacuum to yield the highly pure N-Boc derivative. 1H NMR, 13C NMR
and IR spectra were consistent with the assigned structures and by
comparison with those reported in the literature [6–12].
2.4.5. Surface area and pore distribution measurements
N2 adsorption measurements, which have been a powerful tool for
nano- or mesoporous material characterization, were performed to
obtain more insights into the modified porous nano-silica materials.
The textural properties of Na-montmorillonite were substantially
maintained over sulfonic acid functionalization. A slight decrease in
surface area was observed for Na-montmorillonite and SANM from 53
to 40 m2/g, respectively. This suggests that sulfonic acid may be well
confined in the pores of the mesoporous clay, Na-montmorillonite,
and indicates an ordered mesoporosity of the support even after the
intercalation of sulfonic acid moieties [26].
3. Results and discussion
Protection of amines with di-tert-butyl dicarbonate under base-
catalyzed condensation leads to the formation of N-tert-Boc protected
amines, CO2 and t-BuO−. The liberated t-BuO−undergoes proton
exchange with the N-tert-Boc products to form isocyanates, which can
further react with the starting amine to form urea derivatives. In the
2.5. General procedure for synthesis of N-tert-butylcarbamates
An amine (1 mmol) was added to a magnetically stirred mixture of
SANM (5 mg) and di-tert-butyl dicarbonate (0.24 g, 1.1 mmol) at
room temperature. The mixture was stirred until completion of
reaction (TLC or GC), then diluted with EtOAc (10 mL) and filtered.
Scheme 2. N-Boc protection of amines.

F. Shirini et al. / Catalysis Communications 12 (2011) 1088–1094
1091
Table 1
Boc protection of amines in the presence of SANM under solvent-free conditions.a
Entry
Substrate
Product
Time
(min)
Conv.
(%)
Yielda
(%)
NH2
NH2
NHBoc
NHBoc
1
2
1
1
100
100
96
91
OH
OH
Ph
N
Ph
N
3
4
5
1
100
100
93
88
H
Boc
NHBoc
NH2
NH2
HO
HO
5
6
5
100
100
95
95
NHBoc
20
NH2
NH2
NHBoc
NHBoc
Br
Cl
Br
Cl
7
8
9
63
15
7
100
100
100
94
97
90
NH2
NHBoc
M
e
Me
Et
NH2
Et
NHBoc
MeO
NHBoc
MeO
NH2
10
11
22
20
100
100
96
94
Me
Me
NH2
NHBoc
NH2
OH
NHBoc
OH
12
13
14
180
3 h
100
100
100
95
94
HS
NH2
HS
NHBoc
NH2
NHBoc
8.1 h
97b
N
S
N
S
15
10
100
98
NH2
NHBoc
OH
OBoc
16
17
6 h
0
0
Nrc
Nrc
OH
OBoc
6 h
Cl
Cl
18
19
20
6 h
6 h
6 h
0
0
0
Nrc
Nrc
Nrc
OH
OH
OBoc
OBoc
Me
SH
Me
SBoc
Products were characterized by their physical constants, comparison with authentic samples and IR and NMR spectroscopy.
a
Isolated yield.
1 mL of CH3CN is added to reaction mixture.
The starting material remained unchanged.
b
c

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F. Shirini et al. / Catalysis Communications 12 (2011) 1088–1094
parallel pathway, in the presence of t-BuO−, N-tert-Boc protected
amines react with (Boc)2O to form N,N-di-Boc derivatives [11]. These
side reactions may be eliminated if the in situ generation of t-BuO−
can be avoided. In some examples of Lewis acid-catalyzed re-
actions [7–10], the side products such as isocyanates, N,N-di-Boc
and urea derivatives were not formed. These observations are
attributed to the coordination of the catalyst with the carbonyl
groups of (Boc)2O which activates them for the nucleophilic attack of
Boc protection with (Boc)2O under the selected conditions. The
results are summarized in Table 1.
Aliphatic amines (Table 1; entries 1 and 4) gave the corresponding
N-Boc products in 88–96% yields in 1–5 min. Aromatic amines having
various substituents such as Br, Cl, Me, Et, OMe, OH and SH groups
were converted to their N-tert-Boc derivatives efficiently (entries 5–
13). Amino groups attached to naphtalene (entry 14) and aromatic
heterocycle (entry 15) afforded the desired products in quantitative
yields. No isocyanate or urea formation was observed at all (IR). Under
the selected conditions N-Boc protection of alcohols, phenols and
thiols was not successful and the starting material was recovered
unchanged after 6 h (Table 1, entries 16–20).
The selectivity of a method determines the importance of its
application in organic reactions. Therefore, the chemoselectivity of this
method was also investigated and the results are reported in Table 2.
After performing one reaction under the conditions described in
Table 1, the catalyst was recovered by filtration, washed with acetone
(10 mL), and dried at 100 °C then reused for a consecutive run under
the same reaction conditions. Thus, after the first run, which gave the
corresponding carbamate from benzyl amine in 100% conversion, and
after recovery, the catalyst was subjected to a second Boc protection
reaction from which it also gave the benzyl carbamate in 100%
conversion; the average time for 25 consecutive runs was 1.88 min
and 100% conversion for all, which clearly demonstrates the practical
recyclability of this catalyst (Fig. 4). This reusability demonstrates the
high stability and turnover of solid silica based sulfonic acid (SANM)
under the conditions employed. It should be noted that, the
recyclability test was stopped after 25 runs.
amine producing the desired products and t-BuOH instead of t-BuO−
.
Hence, electrophilic strategy provided a better method for N-tert-
butoxycarbonylation of amines.
Recently, preparation of new catalysts for the organic reactions, by
modification of the hydroxyl groups of various hetero and homoge-
nous compounds by sulfonic moiety, became an important part of our
research program [27–30].
In continuation of these studies, herein, we wish to report the
preparation of sulfonic acid-functionalized ordered nanoporous Na+
-
montmorillonite (SANM) and its application, as a new, efficient and
recyclable catalyst, in the Boc protection of amines (Schemes 1 and 2).
SANM can be easily prepared by the drop wise addition of chloro-
sulfonic acid to a mixture of Na–MMT in CHCl3 at 0 °C. It is important
to note that, this reaction is easy and clean without any extra work-up
procedure due to immediate emission of HCl gas from the reaction
vessel. This white heterogeneous, nonhygroscopic solid acid is very
stable under reaction conditions.
In order to optimize the reaction conditions, we conducted the N-
Boc protection of benzyl amine (1 mmol) with Boc2O (1.1 mmol) in
the presence of SANM (10 mg) with stirring at ambient temperature
without solvent, which is completed immediately. Reduction of the
amount of SANM to 5 mg/mmol of amine did not affect the result
of the reaction. To illustrate the efficiency of SANM in these reac-
tions, protection of benzyl amine, was also carried out in the presence
of Na+ montmorillonite. The reaction was not completed after 1 h.
Obviously, the sulfonation of Na–MMT is important for the reaction.
After optimization of the reaction condition various aliphatic,
aromatic and heterocyclic amines and aminols were subjected to N-
The possible mechanism for the N-Boc protection of various
amines in the presence of SANM as a promoter is shown in Scheme 3.
On the basis of this mechanism, SANM catalyzes the reaction by the
electrophilic activation of (Boc)2O to form a zwitterionic species,
making the carbonyl group susceptible to nucleophilic attack by the
amine. Successive elimination of CO2 and tert-BuOH results in the
formation of N-Boc derivatives and regenerates SANM in the reaction
mixture [31].
Table 2
Chemoselective N-Boc protection.
Entry
1
Substrate (s)
Product (GC yield)
Time (min)
1
NHBoc
HO
100
NH2
HO
Ph
5
1
OH
OH
2
3
Ph
N
N
H
Boc
100
OH
OBoc
NH2
NH2
NHBoc
+
+
100
100
100
100
100
0
0
0
0
0
1
18
1
4
5
+
+
+
+
+
+
+
+
OH
NHBoc
OBoc
OBoc
NH2
NH2
OH
NHBoc
NHBoc
6
7
NH2
NH2
NHBoc
NHBoc
2
NH2
NHBoc

F. Shirini et al. / Catalysis Communications 12 (2011) 1088–1094
1093
Time (min)
Conversion (%)
Table 3
100
90
80
70
60
50
40
30
20
10
0
Compared performance of various catalysts in the N-Boc protection of aniline.
Entry
Catalyst
Catalyst load
Time
Solvent
Yield
(%)
(min)
1
2
3
4
5
SANM [this work]
Yttria–zirconia [7]
Zn(ClO4)2.6H2O [9]
[TMG][Ac] [31]
Sulfonic acid functionalized
silica [32]
5 mg
5
Neat
96
90
92
98
83
18.6 g
28 mg
18 mg
20 mg
14 h
12 h
5
CH3CN
CH2Cl2
Neat
45
CH2Cl2
6
7
8
9
10
11
12
13
14
15
16
Iodine [33]
Yb(OTf)3[33]
FeCl3[33]
Uncatalyzed [33]
Silica gel [33]
H3PW12O40[34]
Thioglycoluril [35]
Thiourea [36]
Sulfamic acid [37]
Sulfamic acid [37]
HClO4–SiO2[38]
10 mol%
10 mol%
10 mol%
–
30
30
30
48 h
18 h
8
8
40
5
Neat
Neat
H2O
Neat
Neat
CH2Cl2
EtOH
Toluene
H2O
95
89
95
60
69
85
95
95
99
98
100
50 mg
15 mg
10 mol%
7.6 mg
5 mol%
5 mol%
50 mg
1
3
5
7
9
11 13 15 17 19 21 23 25
Run
5
5
Neat
Neat
Fig. 4. Recyclability of SANM for the N-Boc protection of benzyl amine.
To illustrate the efficiency of the present method, Table 3
compares our result in N-Boc protection of aniline with the same
result reported by the relevant reagents in the literature.
protocol is highly chemoselective offering potential in different
applications. The methodology also has several other advantages
such as: (i) high reaction rates and excellent yields (ii) no side
reactions (iii) ease of preparation and handling of the catalyst
(iv) cost efficiency and effective reusability of the catalyst (v) use of
inexpensive catalyst with lower loading; and (vi) simple experi-
mental procedure and solvent-free conditions. Further work to
explore this novel catalyst in other organic transformations is in
progress.
4. Conclusion
In conclusion, we have developed a simple, efficient and
chemoselective protocol for the N-tert-butoxycarbonylation of
various amines using SANM as a novel heterogeneous catalyst. The
Scheme 3. Proposed mechanism.

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F. Shirini et al. / Catalysis Communications 12 (2011) 1088–1094
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