Poly(4-vinylpyridine) catalyzed chemoselective O-TMS protection of alcohols and phenols and N-Boc protection of amines

Article abstract of DOI:10.1007/s13738-011-0060-5

Poly(4-vinylpyridine) (PVP) acts as an efficient and reusable catalyst for the selective and efficient trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS) and N-tert-butoxycarbonylation of amines with (Boc)₂O. All reactions were performed under mild conditions in good to high yields. Iranian Chemical Society 2012.

Full text of DOI:10.1007/s13738-011-0060-5

J IRAN CHEM SOC (2012) 9:495–502
DOI 10.1007/s13738-011-0060-5
ORIGINAL PAPER
Poly(4-vinylpyridine) catalyzed chemoselective O-TMS protection
of alcohols and phenols and N-Boc protection of amines
•
Farhad Shirini Nader Ghaffari Khaligh
Received: 22 July 2011 / Accepted: 2 December 2011 / Published online: 5 January 2012
Ó Iranian Chemical Society 2012
Abstract Poly(4-vinylpyridine) (PVP) acts as an efficient
and reusable catalyst for the selective and efficient trim-
ethylsilylation of alcohols and phenols with hexamethyl-
disilazane (HMDS) and N-tert-butoxycarbonylation of
amines with (Boc)2O. All reactions were performed under
mild conditions in good to high yields.
ammonia, which is easily removed from the reaction
mixture. However, the low silylating power is considered
as the main drawback for the application of HMDS.
Therefore, an appropriate catalyst should be used for
activation of this reagent.
Because of its applicability in peptide and nucleoside
syntheses as well as heterocyclic chemistry and also due to
great stability of the N-Boc group in the course of various
base-catalyzed nucleophilic substitutions and catalytic
hydrogenation reactions, the N-tetr-butyloxycarbonyl pro-
tection (Boc) has attracted much attention of many organic
chemists. Di-tert-butyldicarbonate [(Boc)2O], as a stable
and commercially available reagent, is widely used for this
purpose. Many methods have been reported to improve the
rate of the N-Boc protection with (Boc)2O using basic or
acidic catalysts [6–10]. Although these procedures provide
an improvement, most of them suffer from disadvantages
such as long reaction times, tedious work-up, the use of
corrosive or moisture sensitive reagents, formation of side-
products during base-catalyzed reactions, difficulty in the
preparation of the reagent, and using excess amounts of the
reagent in the case of Lewis acid catalyzed reactions.
Keywords Poly(4-vinylpyridine) Á Alcohols Á Phenols Á
Amines Á Protection
Introduction
Trimethylsilylation is one of the most important and pop-
ular methods for protecting the alcoholic and phenolic
hydroxyl groups during a multi-step synthesis [1, 2].
Generally, the formation of silyl ethers carried out by
treatment of alcohols with silyl chlorides or silyl triflates in
the presence of stoichiometric amounts of a base such as
4-(N,N-dimethylamino) pyridine [3], Li2S [4], and some-
times a nonionic super base catalyst [5]. However, some of
these methods suffer from drawbacks such as lack of
reactivity or difficulty in removing amine salts derived
from the reaction of by-produced acids and co-bases during
the course of the reaction.
Experimental
Hexamethyldisilazane (HMDS) is an inexpensive and
commercially available reagent used for the silylation of
alcohols and phenols. Its handling does not need special
precautions, and work-up of the reaction mixture is not
time consuming because the by-product of the reaction is
General
Chemicals were purchased from Merck, Aldrich and Fluka
Chemical Companies and used without further purification.
The purity determination of the products was accomplished
by TLC on silica gel polygram SIL G/UV 254 plates. The
MS were measured under GC (70 eV) conditions. The IR
spectra were recorded on a Perkin Elmer 781 Spectro-
F. Shirini (&) Á N. G. Khaligh
Department of Chemistry, College of Science,
University of Guilan, 41335 Rasht, Islamic Republic of Iran
e-mail: shirini@guilan.ac.ir
1
photometer. In all the cases, the H NMR spectra were
123

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J IRAN CHEM SOC (2012) 9:495–502
(s, 3H), 2.32 (s, 3H), 5.97 (s, 1H), 7.11–7.45 (d,d,d,
6H), 7.85–8.05 (d,d,d, 2H); 13C NMR (CDCl3,
75 MHz): d 0.11, 2.04, 21.1, 21.6, 75.8, 126.6,
127.7, 129.0, 129.2, 129.4 129.7, 130.2, 131.0,
136.4, 138.2, 143.7, 144.8.
recorded with Bruker Avance 300 MHz instrument.
Chemical shifts are reported in parts per million in DMSO
and/or CDCl3 with tetramethylsilane as an internal stan-
dard. 13C NMR data were collected on Bruker Avance
75 MHz instrument.
(d) Table 1, entry 23: Colorless liquid; IR (KBr): m 2970,
2900, 1580, 1480, 1462, 1300, 1253, 1050, 910,
General procedure for the silylation of alcohols
and phenols
838 cm-1 1H NMR (CDCl3, 300 MHz): d 0.26
.
(s, 9H), 0.28 (s, 18H), 6.52 (d, J = 7.5 Hz, 2H),
6.70 (dd, J = 7.5 Hz, 1H); 13C NMR (CDCl3,
75 MHz): d 0.31, 0.65, 113.9, 120.4, 138.8, 148.1.
(e) Table 1, entry 24: Colorless liquid; IR (KBr): m 2960,
2900, 1587, 1440, 1255, 1158, 1022, 850, 750,
A mixture of the substrate (1 mmol), HMDS (0.6 mmol)
and PVP(10 mg) in acetonitrile (2 mL) was stirred at room
temperature and the reaction progress was monitored by
TLC. Upon completion, the solvent was removed under
reduced pressure, then n-hexane was (5 mL) added and the
catalyst was separated by filtration, washed with water,
then acetone, and dried and reused. Evaporation of the
solvent from the filtrate gave almost pure product(s). Fur-
ther purification proceeded by bulb to bulb distillation
under reduced pressure or recrystallization to afford pure
silyl ether.
688 cm-1 1H NMR (CDCl3, 300 MHz): d 0.25
.
(s, 27H), 6.02 (2, 3H); 13C NMR (CDCl3, 75 MHz):
d 0.18, 106.2, 156.4.
(f) Table 3, entry 10: White solid, m.p. 99–101 °C; IR
(KBr): m 3310, 3300, 3110, 3090, 2990, 1773, 1710,
1650, 1540, 1446, 1278, 1250, 1222, 1150, 1112,
1080, 1045 cm-1 1H NMR (DMSO, 300 MHz):
.
d 1.63 (s, 9H), 7.31–7.52 (m, 4H), 11.7(s, 1H). 13C
NMR (DMSO, 75 MHz): d 28.3, 87.8, 113.4, 123.5,
125.1, 127.9, 133.8, 147.7, 161.5, 167.5.
General procedure for the N-Boc protection of amines
(g) Table 3, entry 12: Out-white solid, m.p. 57–59 °C;
IR (KBr): m 3320, 2910, 2860, 1680, 1520, 1450,
A mixture of the substrate (1 mmol), (Boc)2O (1 mol) and
poly(4-vinylpyridine) (20 mg) was stirred at room tem-
perature for the appropriate time (Table 1). The progress of
the reaction was monitored by TLC. After completion of
the reaction, the mixture was washed with ethyl acetate, the
catalyst was separated by filtration, washed with water,
then acetone, and dried and reused. Evaporation of the
solvent followed by column chromatography (silica gel)
eluting with ethyl acetate-petroleum ether (2:8) gave the
desired product in good to high yields.
1
1360, 1317, 1248, 1170, 1010, 875 cm-1; H NMR
(DMSO, 300 MHz): d 1.32–1.39 (m, 10H), 1.42–1.57
(m, 8H), 1.70–1.73 (m, 2H), 3.34 (bs, 1H), 6.72
(d, J = 7.7 Hz, 1H). 13C NMR (DMSO, 75 MHz):
d 24.1, 28.2, 28.7, 35.0, 51.6, 77.6, 155.1.
(h) Table 3, entry 13: Yellow solid, m.p. 55–57 °C; IR
(KBr): m 3380, 2990, 2940, 2840, 2820, 1680, 1585,
1515, 1458, 1360, 1325, 1290, 1260, 1230, 1162,
1
1140, 1020, 990, 842, 808 cm-1. H NMR (DMSO,
300 MHz) d : 1.36 (s, 9H), 2.60 (t, J = 7.0 Hz, 2H),
3.09 (t, J = 7.0 Hz, 2H), 3.70 (s, 3H), 3.73 (s, 3H),
6.68 (d, J = 8.25 Hz, 1H), 6.76 (s, 1H), 6.84 (d, J =
8.25 Hz, 2H). 13C NMR (DMSO, 75 MHz) d : 28.69,
35.49, 42.12, 55.73, 77.89, 112,26, 112.83, 120.83,
132.29, 147.58, 149.00, 155.95.
Spectral data for new compounds:
(a) Table 1, entry 7: Colorless liquid; IR (KBr): m 2950,
2900, 1590, 1436, 1255, 1105, 1080, 868, 840 cm-1
.
1H NMR (CDCl3, 300 MHz): d 0.14 (s, 3H), 0.19
(s, 3H), 0.21 (s, 3H), 4.72 (s, 2H), 7.15–7.48 (m, 3H),
8.04 (d, J = 8.04, 3H); 13C NMR (DMSO, 75 MHz): d
-0.5, 1.5, 1.9, 66.4, 127.2, 128.4, 139.6, 149.9, 164.6.
(b) Table 1, entry 9: Colorless liquid.; IR (KBr): m 2950,
1450,1255, 1095, 838,740,693 cm-1.1HNMR(CDCl3,
300 MHz): d 0.11(s, 3H), 0.13(s, 3H), 0.14(s, 3H), 2.74
(q, 2H), 3.43 (b, 1H), 3.63 (t, J = 5.2 Hz, 1H), 3.72
(t, J = 5.2 Hz, 1H), 377–3.81 (d, 2H), 7.25–7.34
(m, 5H); 13C NMR (CDCl3, 75 MHz): d 0.49, 1.5, 1.9,
50.7, 53.5, 60.6, 127.1, 128.2, 128.5, 139.6.
(i) Table 3, entry 15: Colorless solid, m.p. 50–52 °C IR
(neat): 3490, 3300, 2994, 2930, 1696, 1680, 1540,
1
1520, 1362, 1300, 1250, 1170, 1080, 1040 cm-1. H
NMR (DMSO, 300 MHz) d = 1.12 (s, 6H), 1.35
(s, 9H), 3.28 (s, 2H), 4.72 (bs, 1H), 6.11 (s, 1H). 13C
NMR (DMSO, 75 MHz) d = 23.98, 28.72, 53.51,
68.18, 77.67, 154.86.
(j) Table 3, entry 18: Colorless oil; IR (neat) m 2990,
2840, 1678, 1520, 1450, 1360, 1272, 1238, 1170,
1050, 898 cm-1
.
1H NMR (DMSO, 300 MHz):
(c) Table 1, entry 15: Light yellow liquid; IR (KBr):
m 3010, 2950, 1690, 1680, 1670, 1600, 1506, 1253,
d 1.36–1.57 (m, 15H), 1.44 (s, 9H), 3.29–3.34
(m, 4H); 13C NMR (DMSO, 75 MHz): d 24.3, 25.5,
28.2, 46.6, 77.6, 157.3.
1180, 1120, 1100, 880, 840, 752 cm-1 1H NMR
.
(CDCl3, 300 MHz): d 0.14 (s, 6H), 0.18 (s, 3H), 2.27
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497
Table 1 Trimethylsilylation of alcohols and phenols
Entry
Substrate
Product
Time (min)
Yield (%)a
1
2
3
4
5
6
PhCH2OH
PhCH2OTMS
15
1
96
98
98
97
62
93
3-MeOC6H4CH2OH
2-MeC6H4CH2OH
4-ClC6H4CH2OH
3-NO2C6H4CH2OH
3-MeOC6H4CH2OTMS
2-MeC6H4CH2OTMS
4-ClC6H4CH2OTMS
3-NO2C6H4CH2OTMS
5
4
120
10
OH
OTMS
7
8
10
5
87
98
N
N
OH
OH
OTMS
OTMS
9
7
96
OH
OTMS
N
N
H
H
10
11
12
PhCH(OH)Me
Ph2CHOH
PhCH(OTMS)Me
Ph2CHOTMS
10
50
10
75
98
98
OH
OTMS
OTMS
13
120
70
OH
14
15
PhCH2C(OH)Me2
PhCH2C(OTMS)Me2
120
4
67
96
TMSO
HO
Me
Me
Me
Me
O
O
OTMS
16
17
18
15
1
90b
99
OH
OH
OTMS
OH
OH
OTMS
20
97
OTMS
Me
Me
Me
OH
Me
19
30
98
OTMS
NH2
NH2
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Table 1 continued
Entry
20
Substrate
Product
Time (min)
5
Yield (%)a
81
OH
OTMS
21
5
97
OH
OH
OTMS
22
23
5
98b
99c
OTMS
OH
OH
OTMS
OTMS
35
OH
OTMS
OH
OTMS
24
5
99c
OH
OTMS
HO
OH
TMSO
OTMS
25
26
27
360
360
360
0
0
0
SH
STMS
NHTMS
NHTMS
NH2
NH2
28
29
PhCH2OH ? PhSH
4-ClC6H4CH2OH ? PhNH2
PhCH2OTMS ? PhSTMS
20
4
100d ? 0e
100d ? 0e
4-ClC6H4CH2OTMS ? PhNHTMS
1
The structures of the products were confirmed by comparison of their spectral data (IR, H NMR, 13C NMR) with those of known sample
a
Isolated yield
b
1.2 mmol of HMDS was used
c
1.8 mmol of HMDS was used
d
Conversion
(k) Table 3, entry 19: Colorless oil; IR (neat) m 2996,
2890, 1680, 1525, 1458, 1250 1178, 1015, 883 cm-1
Results and discussion
.
1H NMR (DMSO, 300 MHz): d 1.32–1.39 (m, 9H),
1.42–1.57 (m, 4H), 1.70–1.73 (m, 4H), 2.91–2.93 (m,
2H), 3.34 (m, 2H); 13C NMR (DMSO, 75 MHz): d
24.1, 28.2, 35.0, 52.6, 78.6, 157.4.
Very recently, Ghorbani-Choghamarani et al. [11] reported
the preparation of poly(4-vinylpyridinium tribromide)
and its application in the promotion of the silylation of
alcohols and phenols with HMDS. In continuation of our
ongoing research program on the development of new
methods for the protection of the hydroxyl group [12–15],
we have found that poly(4-vinylpyridine), as a cheap
and commercially available reagent, is efficiently able
to catalyze the above-mentioned reactions by itself
(Scheme 1).
(l) Table 3, entry 20: Colorless oil; IR (neat) m 2900,
2860, 1678, 1520, 1448, 1362, 1317, 1272 1160, 1020,
1
900 cm-1. H NMR (CDCl3, 300 MHz): d 1.06–1.21
(m, 2H), 1.36 (s, 9H), 1.58–1.71 (m, 8H), 3.10 (s, 3H),
3.50 (s, 1H); 13C NMR (DMSO, 75 MHz): d 24.9,
27.9, 28.6, 31.0, 32.4, 54.4, 77.9, 156.9.
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J IRAN CHEM SOC (2012) 9:495–502
499
corresponding silyl ethers are isolated in good to high
yields (Table 1, entries 17–24). Because of the stability of
amines and thiols under the selected conditions (Table 1,
entries 25–27), the selective silylation of alcohols in the
presence of the above-mentioned substrates was investi-
gated (Table 1, entries 9, 19, 28, 29).
HMDS / PVP
ROH
ROSiMe3
CH3CN, r.t.
R=alky, aryl
Scheme 1 Trimethylsilylation of alcohols and phenols
In order to show the merit of this method, Table 2
compares the results obtained from the silylation of benz-
hydrol by our method with some of those reported in the
literature [11, 16–21].
Scheme 1
Optimization of the reaction conditions showed that the
best results were obtained when the reaction of the sub-
strate (1 mmol) with HMDS (0.6 mmol) was performed in
CH3CN (2 mL) at room temperature using 10 mg of PVP.
After optimization of the reaction conditions, different
types of alcohols were subjected to trimethylsilylation
using this method
Although the actual role of PVP is not clear, the
mechanism shown in Scheme 2 is selected as the most
probable one.
Scheme 2
As shown in Table 1, primary benzylic alcohols
(including electron releasing or withdrawing groups) and
aliphatic alcohols were trimethylsilylated with excellent
yields (Table 1, entries 1–9). The protection of benzylic
and aliphatic secondary alcohols was satisfactory subject as
well (Table 1, entries 10–12). Interestingly, 1-adamantanol
and 2-methyl-1-phenyl-2-propanol, as models of the hin-
dered tertiary alcohols, were converted to the correspond-
ing trimethylsilyl ethers under the same reaction conditions
(Table 1, entries 13, 14). This method was found to be
useful for trimethylsilylation of diols and acyloins
(Table 1, entries 15, 16). No elimination and rearrange-
ment of by-products were observed at all. Phenols also
undergo silylation easily using this method, and their
This reagent was then used for the protection of amines as
their N-tert-butoxycarbamates with (Boc)2O under similar
conditions. N-Boc protection of aniline was chosen as the
probe reaction. However, the reaction was not as efficient as
the O-silylation of alcohols under the pre-described condi-
tions. Therefore, the reaction was performed in the absence
of solvent at room temperature using 20 mg of PVP. In this
condition, aniline was N-Boc protected for 2 min to furnish
the product with excellent yield (95%) (Scheme 3).
Scheme 3
Then the protocol was followed using a series of aliphatic,
aromatic and heteroaromatic amines and aminols for
Table 2 Comparison of the results obtained from the protection of benzhydrol using HMDS in the presence of PVP and some of the other
catalysts
Entry
Catalyst/solvent/condition
Time (min)
Isolated yield (%)
Reference
1
2
3
4
5
6
7
8
Poly(vinylpyridinium tribromide) (7 mg)/CH3CN/r.t.
LaCl3(10 mol%)/CH2Cl2/r.t.
10
210
180
180
120
50
96
93
95
95
70
87
93
98
11
16
Al(HSO4)3 (3.5 mol%)/CH3CN/r.t.
TCCA(10 mol%)/CH2Cl2/r.t.
17
18
Mg(OTf)2 (1 mol%)/neat/r.t.
19
LiClO4-SiO2 (100 mg)/CH2Cl2/r.t.
H3PW12O40 (1 mol%)/neat/55–60 °C
PVP (10 mg)/CH3CN/r.t.
20
48
21
50
This work
Scheme 2 Proposed
mechanism of the silylation of
alcohols and phenols
(Me3Si)2NH
ROH
[PVP-SiMe3] [(Me3Si)NH]
[PVP-SiMe3] [NH2]
N
PVP
PVP=
;
ROSiMe3
*
n
*
ROSiMe3 +NH3
ROH
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J IRAN CHEM SOC (2012) 9:495–502
generalization and the results are shown in Table 3.
Regardless of the nature of the substituent attached to
them, all of the above-mentioned substrates produced
excellent yields in a very short reaction time. No
(Boc)2O (1 equv.)/ PVP (20 mg)
Solvent-free, r.t., 2 min., 95 %
PhNH2
PhNH-Boc
Scheme 3 N-Boc protection of amines
Table 3 N-Boc protection of amines catalyzed by PVP
Entry
Substrate
Product
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
PhNH2
PhNH-Boc
2
15
95
94
93
96
95
92
87
96
2-HOC6H4NH2
4-HSC6H4NH2
3-ClC6H4NH2
4-BrC6H4NH2
4-MeC6H4NH2
3-MeOC6H4NH2
2-HOC6H4NH-Boc
4-HSC6H4NH-Boc
3-ClC6H4NH-Boc
4-BrC6H4NH-Boc
4-MeC6H4NH-Boc
3-MeOC6H4NH-Boc
27
47
40
20
105
25
NH2
NH-Boc
9
2
96
84
H
N
H
N
NH2
NH2
NH-Boc
N
S
N
S
10
15
NH-Boc
N
N
11
12
1
2
96
92
NH2
NH-Boc
NH2
NH-Boc
13
1
92
NH2
NH-Boc
MeO
MeO
OMe
OMe
14
15
1
2
83
86
NH2
NH2
NH-Boc
NH-Boc
HO
HO
HO
HO
16
17
2
2
92
92
OH
OH
N
H
N
Boc
H
N
Boc
N
N
N
18
19
4
6
87
91
NH
N-Boc
NH
NH
20
21
14
1
84
96
NHMe
N(Boc)Me
PhCH2NH2
PhCH2NH-Boc
1
Products were characterized by their physical constants, comparison with authentic samples and IR and H NMR spectroscopy. Isolated yields
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J IRAN CHEM SOC (2012) 9:495–502
501
Table 4 Comparison of the results obtained from the protection of aniline using (Boc)2O in the presence of PVP and some of the other catalysts
Entry
Catalyst
Time (h)
Solvent
Isolate yield
Reference
1
2
3
4
5
6
7
FeCl3
1
Neat
89
90
92
75
83
60
95
6
Yttria-zirconia
Zn(ClO4)2.6H2O
b-Cyclodextrin
Sulfonic acid-functionalized silica
Uncatalyzed
14
Acetonitrile
Dichloromethane
Water
7
12
8
2.5
0.75
48
9
Dichloromethane
Neat
10
6
PVP
2 (min)
Neat
Present method
competitive side reactions leading to formation of isocya-
nates, urea, and N,N-di-Boc derivatives were observed at
all. Alcohols, phenols, and thiols remain intact under the
same reaction conditions. Therefore, the method can be
useful for the chemoselective N-tert-butoxycarbonylation
of amines in the presence of alcohols, phenols, and thiols
(Table 3, entries 2, 3, 14–16).
protection of aniline with the best results from the literature
[6–10].
Also the mechanism of the reaction is not clear, but
based on the literature [22], our observations and obtained
results, the mechanism which is shown in Scheme 4 is
proposed for the clarification of the catalytic role of PVP in
the reported method. This mechanism shows that PVP
undergoes nucleophilic attack on (Boc)2O and generates
the tetrahedral intermediate I. Elimination of CO2 from I
leads to the formation of II that can further react with
amine to form the requested product, tert-BuOH and PVP,
which reenters the catalytic cycle.
To show the efficiency of the described method in
comparison with procedures previously reported in the
literature, in Table 4 we compared our results of the N-Boc
O
N
O
Scheme 4
Me3CO
*
O
OCMe3
(Boc)2O
From the context of green approach, the reusability of the
catalyst was tested in the silylation of 4-chlorobenzyl alcohol
and phenol, and N-Boc protection of aniline and cyclohex-
ylamine. After performing one reaction under the conditions
described in Tables 1 and 3, the catalyst was recovered by
filtration, washed with water, then acetone and dried, then
reused for a consecutive run under the same reaction con-
ditions. Thus, after the first run, which gave the requested
products from the above mentioned substrates in 100%
conversion, and after recovery, the catalyst was subjected to
a second protection reaction from which it also gave the
corresponding silyl ethers or cabamates in 100% conversion;
the average time for 8 consecutive runs was 4, 1, 2.25, and
3.5 min, respectively, in 100% conversion for all, which
CO2
N
*
n
(I)
OCMe3
O
*
n
*
N
OCMe3
(PVP)
R1
N R2
H
CMe3
O
*
n
*
O
*
(II)
R1R2N-Boc
+ Me3COH
N
OCMe3
R1R2NH
*
n
Scheme 4 Proposed mechanism of the N-Boc protection of amines
Table 5 Recyclability of PVP in the silylation of 4-chlorobenzyl alcohol and phenol, and N-Boc protection of aniline and cyclohexylamine
Product
Run
1
(min/%)
2
(min/%)
3
(min/%)
4
(min/%)
5
(min/%)
6
(min/%)
7
(min/%)
8
(min/%)
4-ClC6H4CH2OTMS
PhOTMS
4/97
1/99
2/95
1/96
4/97
1/99
2/95
1/93
4/97
1/98
2/95
3/90
4/97
1/98
2/95
3/90
4/97
1/98
2/94
4/87
4/96
1/96
2/92
5/87
4/96
1/96
3/90
5/85
4/96
1/95
3/90
6/85
PhNH-Boc
NH-Boc
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502
J IRAN CHEM SOC (2012) 9:495–502
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In conclusion, in this study we have developed a mild,
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protection of amines in the presence of poly(4-vinylpyri-
dine). The significant advantages of this methodology are
cheapness, availability, reusability and nontoxicity of the
reagent, relatively short reaction times, high yields of the
products, selectivity and easy and clean work-up. These
advantages make the present method useful for large-scale
operations.
Acknowledgments We are thankful to the University of Guilan
Research Council for the partial support of this work.
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