Supported N-propylsulfamic acid onto Fe3O4 magnetic nanoparticles as a reusable and efficient nanocatalyst for the protection/deprotection of hydroxyl groups and protection of aldehydes

Article abstract of DOI:10.1007/s11164-015-2239-3

N-propylsulfamic acid supported onto Fe₃O₄ magnetic nanoparticles (MNPs-PSA) as an efficient and magnetically reusable nanocatalyst has been reported for the tetrahydropyranylation/depyranylation of a wide variety of alcohols and phenols by changing the solvent medium. Also, the protection of aldehydes as acylals using Ac₂O in the presence of catalytic amount MNPs-PSA in good to high yields at room temperature under solvent-free conditions is described. After completing the reaction, the catalyst was easily separated from the reaction mixture with the assistance of an external magnetic field and reused for several consecutive runs without significant loss of their catalytic efficiency.

Full text of DOI:10.1007/s11164-015-2239-3

Res Chem Intermed
DOI 10.1007/s11164-015-2239-3
Supported N-propylsulfamic acid onto Fe3O4 magnetic
nanoparticles as a reusable and efficient nanocatalyst
for the protection/deprotection of hydroxyl groups
and protection of aldehydes
Amin Rostami1 Bahman Tahmasbi1
•
•
Fatemeh Abedi2
Received: 24 March 2015 / Accepted: 19 August 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract N-propylsulfamic acid supported onto Fe3O4 magnetic nanoparticles
(MNPs-PSA) as an efficient and magnetically reusable nanocatalyst has been
reported for the tetrahydropyranylation/depyranylation of a wide variety of alcohols
and phenols by changing the solvent medium. Also, the protection of aldehydes as
acylals using Ac2O in the presence of catalytic amount MNPs-PSA in good to high
yields at room temperature under solvent-free conditions is described. After com-
pleting the reaction, the catalyst was easily separated from the reaction mixture with
the assistance of an external magnetic field and reused for several consecutive runs
without significant loss of their catalytic efficiency.
Keywords Magnetic nanoparticles Á N-propylsulfamic acid Á Catalyst Á Hydroxyl
group Á Aldehyde Á Protection Á Deprotection
Introduction
Recycling of homogeneous catalysts is a task of great economic and environmental
importance in chemical and pharmaceutical industries [1–3]. Heterogeneous
supports allow efficient recycling by filtration; however, a substantial decrease in
the activity of the immobilized catalysts is frequently observed [4]. This can be
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2239-3)
contains supplementary material, which is available to authorized users.
&
Amin Rostami
a_rostami372@yahoo.com
1
2
Department of Chemistry, Faculty of Science, University of Kurdistan, 6617715143 Sanandaj,
Iran
Department of Chemistry, Payame Noor University, Hamedan, Iran
123

A. Rostami et al.
solved by using nanoparticles (NPs) as catalyst supports. This is because when the
size of the support materials is decreased to the nanometre scale, the surface area of
nanoparticles will increase dramatically. As a consequence, nanoparticle supports
could have higher catalyst loading capacity than many conventional support
matrixes, leading to the improved catalytic activity of the nanoparticle-supported
catalysts [5, 6]. However, the nanocatalyst supported can be separated from
products by conventional filtration or centrifugation techniques. But it is difficult,
time consuming, and expensive to separate fine particles from a mixture reaction [7–
9]. Thus, this drawback can be overcome by using magnetic nanoparticles (MNPs)
based on iron oxides as a new type of matrix, which can be rapidly isolated from the
reaction mixture using an external magnet, but they do not agglomerate after
removal of the field [10–17].
Protection/deprotection of hydroxyl groups is important transformation in
organic synthesis [18]. Tetrahydropyranylation reaction with 3,4-dihydro-2H-pyran
(DHP) is one of the most useful methods and is frequently used for the protection of
hydroxyl groups [19]. The advantage of this reaction is quick and simple protection
and deprotection [20]. In addition, tetrahydropyranyl ethers (THP-ethers) are stable
under a variety of conditions such as alkaline media, Gringard reagent, alkyl
lithiums, metal hydrides, oxidative reagents, alkylating, and acylating reagents [21].
The selective protection of carbonyl groups plays an important role in organic
synthesis as well as in the chemistry of drug design [22]. Acylal (1,1-diacetates or
germinal diacetates) formation is one of the most widely used processes for
protection of carbonyl compounds because of their stability under various reaction
conditions and their easy conversion into the parent compounds [23]. They were
used for the synthesis of dienes for Diels–Alder reactions and as a cross-linking
agent for cellulose in cotton [24, 25]. Moreover, the acylal functionality can be
converted into other functional groups by reaction with appropriate nucleophiles
[26].
Many procedures and a variety of catalysts have been reported for the protection/
deprotection of hydroxyl groups as THP-ethers and the protection of carbonyl
compounds as acylals such as SnIV(TPP)(OTf)2 [18], Al(OTf)3 [19], Ps–AlCl3 [20],
AlCl3Á6H2O [21], BEA–SO3H [22], HClO4–SiO2 [23, 25], H6P2W18O62Á24H2O
[24], InCl3/Al2O3 [26], {[K.18-Crown-6]Br3}n [27], 3,5-DNBA [28], SnIV(TNH2-
PP)(OTf)2 [29], Phospho sulfonic acid [30], P2O5/SiO2, P2O5/Al2O3 [31], TEDAMS
[32], selectfluorTM [33], sulfonated carbon [34], and TBBDA or PBBS [35].
Although these protocols represent considerable progress, the major disadvantages
of these methods include recovery and reuse of the expensive catalyst, metal
residues in the products, and the formation of large amounts of toxic waste. Hence,
there is a need to develop new catalysts that can overcome these drawbacks. In our
ongoing research program to develop new synthetic methodologies for the
protection and deprotection of organic functional groups [36–39], herein we report
MNPs-PSA as a magnetically recoverable and reusable nanocatalyst for the
protection/deprotection of alcohols and phenols as THP-ethers and the protection of
aldehydes as acylals under mild conditions.
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Supported N-propylsulfamic acid onto Fe3O4 magnetic…
Experimental
General procedure for the protection of alcohol or phenol as THP-ether
A mixture of alcohol or phenol (1 mmol), DHP (1.2–1.4 mmol), and MNPs-PSA
(5 mg, 0.95 mol%) was stirred at room temperature in dry CH2Cl2 (2 mL), and the
progress of the reaction was monitored by TLC. After completion of the reaction,
catalyst was separated by an external magnet and washed with CH2Cl2. Then, the
pure product was isolated by passing of the reaction mixture through a short column
using n-hexane and ethyl acetate (9:1) as eluent.
General procedure for the deprotection of THP-ethers into corresponding
alcohols or phenols
A mixture of THP-ethers (1 mmol) and MNPs-PSA (20 mg, 3.8 mol%) was stirred
at room temperature in CH3OH (2 mL), and the progress of the reaction was
monitored by TLC. After completion of the reaction, catalyst was separated by an
external magnet and washed with CH2Cl2. Then, the solvent was removed under
reduced pressure, and the product was purified through a short column of silica gel
to obtain the pure alcohol or phenol.
General procedure for the protection of aldehydes as acylals
A mixture of aldehyde (1 mmol), freshly distilled Ac2O (3 mmol) and MNPs-PSA
(10 mg, 1.9 mol%) was stirred at room temperature under solvent-free conditions,
and the progress of the reaction was monitored by TLC. After completion of the
reaction, catalyst was separated by an external magnet and washed with CH2Cl2
(10 mL). The organic layer was washed with saturated NaHCO3 (10 mL) and dried
over Na2SO4. Evaporation of the solvent under reduced pressure gave the pure
product.
Selected spectral data
2-(benzyloxy)tetrahydro-2H-pyran (Table 2, entry 1) 1H NMR (250 MHz,
CDCl3): d = 1.51–1.88 (m, 6H), 3.49–3.61 (m, 1H), 3.80–4.08 (m, 1H),
4.42–4.87 (m, 3H), 7.33 (s, 5H) ppm.
2-((4-methoxybenzyl)oxy)tetrahydro-2H-pyran (Table 2, entry 2) 1H NMR
(250 MHz, CDCl3): d = 1.35–1.85 (m, 6H), 3.43–4.00 (m, 5H), 4.31–4.79 (m,
3H), 6.80–6.88 (d, J = 20 Hz, 2H), 7.20–7.28 (d, J = 20 Hz, 2H) ppm.
2-((2,4-dichlorobenzyl)oxy)tetrahydro-2H-pyran (Table 2, entry 5) 1H NMR
(250 MHz, CDCl3): d = 1.50–1.74 (m, 6H), 3.42–4.60 (m, 1H), 3.74–3.92 (m,
1H), 4.20–4.88 (m, 3H), 7.12–7.48 (m, 3H) ppm.
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A. Rostami et al.
2-(benzhydryloxy)tetrahydro-2H-pyran (Table 2, entry 9) 1H NMR (250 MHz,
CDCl3): d = 1.59–2.07 (m, 6H), 3.51–370 (m, 1H), 3.78–4.07 (m, 1H), 4.78 (s,
1H), 5.93 (s, 1H), 7.43 (s, 10H) ppm.
2-(4-methoxyphenoxy)tetrahydro-2H-pyran (Table 2, entry 17) 1H NMR
(250 MHz, CDCl3): d = 1.62–1.85 (m, 6H), 3.51–3.93 (m, 5H), 5.27 (s, 1H),
6.74–7.03 (m, 4H) ppm.
p-tolylmethylene diacetate (Table 6, entry 4) 1H NMR (250 MHz, CDCl3):
d = 1.79 (s, 6H), 2.05 (s, 3H), 6.85–7.13 (m, 4H), 7.32 (s, 1H) ppm.
(4-fluorophenyl)methylene diacetate (Table 6, entry 6) 1H NMR (250 MHz,
CDCl3): d = 2.09 (s, 6H), 7.00–7.12 (m, 2H), 7.44–7.54 (m, 2H), 7.63 (s, 1H) ppm.
(2-chlorophenyl)methylene diacetate (Table 6, entry 7) 1H NMR (250 MHz,
CDCl3): d = 2.11 (s, 6H), 7.40–7.51 (m, 4H), 7.85 (s, 1H) ppm.
(4-chlorophenyl)methylene diacetate (Table 6, entry 8) 1H NMR (250 MHz,
CDCl3): d = 2.03 (s, 6H), 7.12–7.38 (m, 4H), 7.55 (s, 1H) ppm.
(2-nitrophenyl)methylene diacetate (Table 6, entry 13) 1H NMR (250 MHz,
CDCl3): d = 2.11 (s, 6H), 7.51–7.60 (m, 1H), 7.66–7.74 (m, 2H), 7.93–8.06 (m,
1H), 8.18 (s, 1H) ppm.
Results and discussion
The catalytic application of MNPs-PSA in the protection and deprotection
of hydroxyl groups as THP-ethers
MNPs-PSA was initially synthesized and characterized according to our very
recently reported protocol [39]. The loading of the sulfamic acid function in MNPs
was calculated from ion exchange pH-analysis and back-titration of the catalyst,
which confirmed a loading of approximately 1.9 mmol g-1. We have tested the
catalytic activity of MNPs-PSA for the protection of alcohols and phenols as
corresponding THP-ethers using DHP at room temperature in CH2Cl2 and their
deprotection to the corresponding parent compounds was also investigated in
CH3OH at room temperature (Scheme 1).
In order to optimize the tetrahydropyranylation reaction conditions, we examined
the protection of benzyl alcohol as a model compound using DHP (1.2 mmol) in the
presence of different amounts of catalyst in various solvents at room temperature
Scheme 1 MNPs-PSA catalyzed the protection and deprotection of alcohols and phenols as THP-ethers
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Supported N-propylsulfamic acid onto Fe3O4 magnetic…
Table 1 Optimization of the reaction conditions for the protection of benzyl alcohol as a model com-
pound at room temperature
Entry
Solvent
Catalyst (mg)
Time (min)
Conversion (%)
1
2
CH3CN
CH3CN
n-Hexane
EtOAc
20
10
20
20
20
20
15
10
5
20
60
100
100
100
100
100
100
100
100
100
Trace
3
110
60
4
5
Solvent-free
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
30
6
\5
\5
\5
8
7
8
9
10
0
300
The bold letters represent the most effective reaction conditions
(Table 1). As shown in Table 1, 5 mg of MNPs-PSA in CH2Cl2 was found to be
ideal for the complete protection of benzyl alcohol (Table 1, entry 9).
The generality of this approach has been demonstrated by a facile tetrahydropy-
ranylation of primary and secondary benzylic (Table 2, entries 1–9), allylic (Table 2,
entry 10), cyclic (Table 2, entries 11–13), and linear primary Table 2, entries 14–15)
alcohols and various phenols (Table 2, entries 16–21) using DHP in the presence of
MNPs-PSA as catalyst in CH2Cl2 at room temperature. The products were obtained in
good to high yields in short reaction times under mild reaction conditions.
Subsequently, we found that THP-ethers can be converted to the corresponding
alcohols and phenols in good to excellent yields, using the same catalyst, but using a
different molar ratio in MeOH at room temperature (Scheme 1). In order to optimize
the depyranylation reaction conditions, we examined the deprotection of 2-(benzy-
loxy)tetrahydro-2H-pyran as a model compound in various solvents such as EtOH,
CH3OH, H2O, CH3CN, and THF and in the presence of different amounts of MNPs-
PSA (Table 3). The best results were obtained using 20 mg of MNPs-PSA in CH3OH.
The various THP-ethers were successfully deprotected using 20 mg of catalyst in
CH3OH in high to excellent yields in a short reaction time at room temperature
(Table 4).
The catalytic application of MNPs-PSA in the protection aldehyde as acylal
using Ac2O
In the other effort, we tested the catalytic activity of MNPs-PSA in the protection of
aldehydes as acylals using Ac2O at room temperature under solvent-free conditions
(Scheme 2).
123

A. Rostami et al.
Table 2 MNPs-PSA (5 mg, 0.95 mol%) catalyzed protection of alcohols or phenols (1 mmol) using
DHP (1.4 mmol) as THP-ethers in CH2Cl2 at room temperature
Entry
1
Alcohol or phenol
Product
Time (min)
8
Isolated yield (%)a
98b
2
3
4
5
6
7
30
200
25
8
97b
94b
95
94
98
7
8
200
60
93
93
9
300
91
10
11
350
180
66
70
12
13
150
180
80
60
14
15
20
95
85
160
16
17
60
50
95c
96c
18
19
20
40
35
80
93c
98c
86c
21
50
94c
a
b
c
All products were characterized by comparison of their spectral (1H-NMR, IR spectroscopy) [19]
1.2 mmol of DHP was used
DHP was added dropwise over a period of 30 min
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Supported N-propylsulfamic acid onto Fe3O4 magnetic…
Table 3 Optimization of the reaction conditions for the deprotection of 2-(benzyloxy)tetrahydro-2H-
pyran (1 mmol) as a model compound at room temperature
Entry
Solvent
MNPs-PSA (mg)
Time (min)
Conversion (%)
a
1
2
3
4
5
6
7
8
a
H2O
20
20
20
20
30
20
10
0
360
360
360
360
55
–
a
–
EtOH
a
–
CH3CN
THF
a
–
CH3OH
CH3OH
CH3OH
CH3OH
100
70
100
200
360
100
Trace
The reaction was not completed
In order to optimize the reaction conditions, we examined the protection of
benzaldehyde as a model compound using different amounts of Ac2O and catalyst in
various solvents (Table 5). As shown in Table 5, Ac2O (3 mmol) in the presence of
a catalytic amount of MNPs-PSA (10 mg) at room temperature under solvent-free
conditions was found to be the ideal reaction conditions for the complete protection
of benzaldehyde. Also, inferior results were obtained in CH2Cl2, n-hexane and
CH3CN as solvent.
In order to generalize the scope of the reaction, a series of aromatic aldehydes
was subjected to the protection reaction under the optimized reaction conditions,
and the results are presented in Table 6. The acylals were obtained in good to
high yields (Table 6, entries 1–14). It should be mentioned that the protection of
a, b-unsaturated and aliphatic and aldehydes require longer reaction time, high
amount of catalyst, and elevated temperature (Table 6, entries 15–17).
The suggested mechanism of this reaction is shown in Scheme 3.
Catalyst recovery and reuse
For practical purposes, the ability to recycle the catalyst easily is highly desirable.
To investigate this issue, the recyclability of the catalyst was examined for the
protection of benzyl alcohol as THP-ether and benzaldehyde as acylal. After the
completion of the reaction, the catalyst was easily and rapidly separated from the
product using an external magnet and decantation of the reaction solution (Fig. 1).
The remaining magnetic nanocatalyst was washed with CH2Cl2 to remove residual
product. Then, the reaction vessel was charged with fresh starting material before
the next reaction. The catalyst can be recycled up to 10 runs without any significant
loss of activity in both cases (Fig. 2). In addition, one of the attractive features of
123

A. Rostami et al.
Table 4 MNPs-PSA (20 mg, 3.8 mol%) catalyzed deprotection of THP-ethers (1 mmol) as their cor-
responding alcohols or phenols in CH3OH at room temperature
Isolated yield
Entry
1
THP-ether
Product
Time (min)
70
(%)
98
2
3
4
5
40
120
80
96
94
95
97
95
6
7
180
160
90
90
8
9
70
60
85
88
10
11
130
160
90
95
12
13
210
91
14
15
16
110
150
120
96
92
91
17
18
19
20
21
115
60
89
90
91
87
88
110
50
240
22
130
92
123

Supported N-propylsulfamic acid onto Fe3O4 magnetic…
MNPs-PSA (Cat.)
Solvent-Free, r.t.
RCH(OAc)2
RCHO
+
Ac2O
Scheme 2 MNPs-PSA catalyzed the protection of aldehydes as acylals using Ac2O
Table 5 Optimization of the reaction conditions for the protection of benzaldehyde using Ac2O as acylal
Entry
Solvent
Catalyst (mg)
Ac2O (mmol)
Time (min)
Conversion (%)
a
1
2
3
4
5
6
7
8
a
CH3CN
10
10
10
10
5
3
240
240
135
30
–
a
–
CH2Cl2
3
n-Hexane
3
100
100
100
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
3
3
90
a
–
5
2.5
2
420
420
420
a
–
5
a
–
5
1.5
The reaction was not completed
this novel catalyst system is the rapid (within 5 s) and efficient separation of the
catalyst (100 %) by using an appropriate external magnet, which minimizes the loss
of catalyst during separation.
The efficiency of catalyst
The efficiency of this nanocatalyst is demonstrated by comparing our result on the
tetrahydropyranilation of benzyl alcohol with data from other catalysts (Table 7).
The advantages or the characteristic aspects of this catalyst in comparison with
other previously reported catalysts are: this catalyst can be rapidly and simply
separated from reaction media owing to its magnetical properties, while keeping
high reaction yield and low reaction time. In addition, the MNPs-PSA catalyst is
stable, not moisture sensitive, nontoxic, and no special efforts are required for the
preparation.
Conclusions
In summary, we have developed an efficient, simple, and mild process for the
tetrahydropyranylation/depyranylation of a wide range of hydroxyl groups and the
protection of aldehydes as acylals in the presence of MNPs-PSA as a magnetic
123

A. Rostami et al.
Table 6 MNPs-PSA (10 mg, 1.9 mol%) catalyzed protection of aldehydes (1 mmol) as acylals using
Ac2O (3 mmol) under solvent-free conditions at room temperature
Entry
1
RCHO
Product
CH(OAc)2
Time (min)
30
Yield (%)a
90
CHO
CHO
CH(OAc)2
2
3
240
180
50b
75b
MeO
MeO
MeO
MeO
CHO
CHO
CH(OAc)2
CH(OAc)2
OMe
OMe
4
5
83
70
80
Me
Me
OH
OH
CHO
CH(OAc)2
380
Br
CHO
Br
CH(OAc)2
CH(OAc)2
6
7
220
80
96
66
F
F
CHO
Cl
Cl
CHO
CH(OAc)2
8
9
85
70
Cl
Cl
CHO
CH(OAc)2
395
60c
Cl
Cl
Cl
Cl
CHO
CH(OAc)2
10
180
53
Cl
Cl
Cl
Cl
CHO
CH(OAc)2
11
12
120
120
90
75
Br
CHO
Br
CH(OAc)2
Br
Br
CHO
CH(OAc)2
13
14
270
120
85
71
NO2
CHO
NO2
CH(OAc)2
CHO
CH(OAc)2
CH(OAc)2
15
360
50d
16
17
125
150
84d
81d
CHO
a
All products were characterized by comparison of their spectral (1H-NMR, IR spectroscopy) with those
of authentic samples [30]
b
20 mg of MNPs-PSA and 5 mmol of Ac2O were used
c
5 mmol of Ac2O was used
d
Reaction condition: aldehyde (1 mmol), MNPs-PSA (25 mg), Ac2O (5 mmol) at 35 °C under solvent-Free
123

Supported N-propylsulfamic acid onto Fe3O4 magnetic…
Scheme 3 Proposed mechanism for the protection of aldehydes as acylals using Ac2O catalyzed by
MNPs-PSA
Fig. 1 Image showing MNPs-PSA can be separated by applied magnetic field. A reaction mixture in the
absence (left) or presence of a magnetic field (right)
nanocatalyst in good to high yields at room temperature. The notable advantages of
MNPs-PSA as a catalyst are: (1) The preparation of MNPs-PSA is simple from
easily available compounds; (2) The separation of MNPs-PSA is rapid, simple, and
efficient by using an appropriate external magnet, which minimizes the loss of
catalyst during separation; (3) MNPs-PSA is reused for several times (10 runs) with
little loss of activity; these significant features make it a promising material for
practical and large-scale applications.
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A. Rostami et al.
Fig. 2 The recycling experiment of MNPs-PSA in protection of benzaldehyde using Ac2O at room
temperature for 30 min (red lines) and protection of benzyl alcohol using DHP at room temperature for
8 min (green lines). (Color figure online)
Table 7 Comparison of the activity of MNPs-PSA with the various catalysts in the tetrahydropyrani-
lation of benzyl alcohol using DHP
Entry
Catalyst
Time (min)
Yield (%)a
References
1
2
3
4
5
6
7
a
Ps–AlCl3
42
20
97
93
97
94
98
96
98
[20]
{[K.18-Crown-6]Br3}n
3,5-DNBA
[27]
120
12
[28]
TEDAMS
[32]
SnIV(TNH2PP)(OTf)2
Sulfonated carbon
MNPs-PSA
180
120
8
[29]
[34]
This work
Isolated yields
Acknowledgments We are grateful to the University of Kurdistan Research Councils for partial support
of this work. We are also thankful to Dr. A. Ghorbani-Choghamarani, Department of Chemistry, Faculty
of Science, Ilam University, for his valuable suggestions.
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