Ring opening of epoxides by using cross-linked poly(4-vinylpyridine)-supported thiocyanate in the presence of polymer-supported sulfuric acid under solvent-free conditions

Article abstract of DOI:10.1080/10426507.2014.887079

Cross-linked poly(4-vinylpyridine)-supported thiocyanate ion, [P₄-VP]SCN, is introduced as recyclable polymeric reagent for the efficient and regioselective conversion of epoxides to thiocyanohydrins in the presence of cross-linked poly(4-vinylpyridine)-supported sulfuric acid, [P4_-VP]H₂SO₄, as a solid proton source and as catalyst under solvent-free conditions. The ring-opening reaction of epoxides take place very fast under solvent-free conditions and the regioselective thiocyanohydrins are prepared in high yields at 65°C. The present method offers advantages such as short reaction time, simple reaction work-up, and the polymeric reagents can be regenerated and reused several times without significant loss of their activity.

Full text of DOI:10.1080/10426507.2014.887079

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Phosphorus, Sulfur, and Silicon and the Related
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Ring opening of epoxides by using cross-linked poly (4-
vinylpyridine) supported thiocyanate in the presence
of polymer-supported sulfuric acid under solvent-free
conditions
Mohammad Ali, Karimi Zarchia & Ali Tarabsaza
a Department of Chemistry, College of Science, Yazd University, P. O. Box 89195-74, Yazd,
Iran
Accepted author version posted online: 30 Apr 2014.
To cite this article: Mohammad Ali, Karimi Zarchi & Ali Tarabsaz (2014): Ring opening of epoxides by using cross-linked poly
(4- vinylpyridine) supported thiocyanate in the presence of polymer-supported sulfuric acid under solvent-free conditions,
Phosphorus, Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2014.887079
To link to this article: http://dx.doi.org/10.1080/10426507.2014.887079
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RING OPENING OF EPOXIDES BY USING CROSS-LINKED POLY (4-
VINYLPYRIDINE) SUPPORTED THIOCYANATE IN THE PRESENCE OF
POLYMER-SUPPORTED SULFURIC ACID UNDER SOLVENT-FREE CONDITIONS
Mohammad Ali, Karimi Zarchi and Ali Tarabsaz
Department of Chemistry, College of Science, Yazd University, P. O. Box 89195-74, Yazd, Iran.
Corresponding author; Tel: +98-351-8211670-9; FAX: +98-351-8210644. E-mail address: makarimi@yazd.ac.ir
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Abstract
Cross-linked poly (4-vinylpyridine) supported thiocyanate ion, [P4-VP]SCN is introduced as
recyclable polymeric reagent for the efficient and regioselective conversion of epoxides to
thiocyanohydrins in the presence of cross-linked poly (4-vinylpyridine) supported sulfuric acid,
[P4-VP]H2SO4, as a solid proton source and as catalyst under solvent-free conditions. The ring
opening reaction of epoxides take place very fast under solvent-free conditions and the
regioselective thiocyanohydrins are prepared in high yields at 65 °C. The present method offers
advantages such as short reaction time, simple reaction work-up, and the polymeric reagents can
be regenerated and reused several times without significant loss of their activity.
Keywords: Ring opening of epoxide, Thiocyanohydrin, Polymer-supported reagent, Solvent-
free conditions, Regioselectivity.
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INTRODUCTION
The ring opening reaction of epoxides with nucleophiles is a very useful approach in organic
synthesis for preparation of functionalized oxygenated compounds.1-3
Nucleophilic addition to epoxides is an easy step for preparation of several 1, 2-disubstituted
products such as β-hydroxythiocyanates. The thiocyanate functional group will be readily
transformed into the other sulfur-bearing functionalities.4-12
The ring opening of epoxides with thiocyanates is a widely used method for the preparation of
thiiranes. The formation of thiiranes from oxiranes and thiocyanates has been explained by the
intermediacy of the corresponding β-hydroxythiocyanate that have not been isolated due to rapid
conversion into the corresponding thiiranes. 13-23 However, several methods are reported for the
preparation of thiocyanohydrins. 24-41 They are limited to specific epoxides and are not applicable
as versatile reagents for the preparation of β-hydroxythiocyanates and some of these methods
suffer from disadvantages such as long reaction times.32,33, low regioselectivity. 37,38,40-45, using
of organic solvents 27,29,32,33,39, using phase-transfer catalysts 32,35, using of expensive
catalysts29,33,34,37,39, low yields 27, or involve high temperature reaction conditions.27, 35 Therefore,
it seems that there is still a need for development of novel methods that proceed under mild and
eco-friendly conditions.
Solid acids have many advantages such as simplicity in handling, decreased reactor and plant
corrosion problems, and more environmentally safe disposal in different chemical processes. On
the other hand, any reduction in the amount of strong inorganic acids needed and any
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simplification in handling procedures are required for risk reduction and economic advantage.41
In addition there is current research and general interest in heterogeneous systems because such
systems have importance in industry and in developing technologies.42 Recently Borah et al.
reported the synthesis, characterization and application of poly(4-vinylpyridine)-supported
Brønsted acid as reusable catalyst for acetylation reaction.43 In this study, the use of this solid
acidic resin as a new proton source for ring opening of epoxides using cross-linked poly (4-
vinylpyridine supported thiocyanate ion, [P4-VP]SCN, was studied.
Solid phase synthesis, in particular, polymer supported nucleophiles, has been widely used in
organic synthesis.23, 43-61
A literature search shows that there are a few reports based on polymer-supported thiocyanate
ion .23, 51,52,62,63 Very recently Cainelli et al. reported a polymeric reagent for synthesis of alkyl
thiocyanates.62 Also Hodge et al. reported that Amberlite A-26 converted a few alkyl halides to
the corresponding alkyl thiocyanates (seven examples) under the same conditions of the last
report.63 Tamami and Kiasat reported that epoxides were converted to their corresponding
episulfides using Amberlite IRA-400 supported thiocyanate.23 We have recently reported an
efficient method for preparation of cross-linked poly(4-vinylpyridine) supported thiocyanate ion
[P4-VP]SCN, and used for synthesis of alkyl thiocyanates from alkyl halides,51 synthesis of aryl
thiocyanates via diazotization-thiocyanation of arylamines.52 To the best of our knowledge, there
are no reports on polymer-supported thiocyanate ion for thiocyanation of epoxide rings. In
continuation of our studies on the development of application of [P4-VP]SCN in organic
synthesis51,52, in this report, we wish to disclose a simple, convenient and efficient method for
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synthesis of thiocyanohydrins by using [P4-VP]SCN/[P4-VP]H2SO4 under solvent-free
conditions.
RESULTS AND DISCUSION
In connection with our organic program to develop environmentally friendly methods using
polymer-supported reagents (50-61), herein we wish to report an extremely convenient and
efficient method for preparation of different β-hydroxythiocyanates from regioselective ring
opening of epoxides by using cross-linked poly (4-vinylpyridine) supported thiocyanate ion [P4-
VP]SCN, in the presence of cross-linked poly (4-vinylpyridinie) supported sulfuric acid, [P4-
VP]H2SO4, as solid proton source and as new polymeric phase transfer catalyst under solvent-
free conditions at 65 °C. [P4-VP]H2SO4 is easily prepared by the reaction of poly (4-
vinylpyridine) cross-linked with 2% divinyl benzene, [P4-VP] 2% DVB with mixture of diethyl
ether and H2SO4 and [P4-VP]SCN is easily prepared by ion exchange reaction between cross-
linked poly (N-methyl-4-vinylpyridinium) iodide with an excess amount of potassium
thiocyanate in water (Scheme 1).
The polymeric reagent is used in a single step reaction; its main advantage over non-polymeric
reagents is its insolubility in the reaction medium and consequently its easier work-up by a
simple filtration. The reactions can be driven to completion using excess amounts of reagent
without the fear of excess of reagent separating from the products. One of the most disadvantage
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of the polymeric reagents is their high costly but, in the present method, polymeric reagents can
be regenerated and reused several times without significant loss of their activity as given in
Scheme 1.
[P4-VP]SCN can be used as a mild and efficient polymeric reagent for regioselective ring
opening of epoxides in the presence of a solid polymeric acid, [P4-VP]H2SO4 and consequently
the corresponding β-hydroxythiocyanates are prepared (Scheme 2).
In order to increase the yields of β-thiocyanohydrins optimization of the reaction conditions was
accomplished. Phenylglycidyl ether (1mmol) in the presence of [P4-VP]H2SO4 was chosen as a
model substrate and was converted to the corresponding β-hydroxythiocyanates in different
solvents such as carbon tetrachloride, chloroform, acetonitrile, dichloromethane, ethanol, and
H2O at different temperature and under solvent-free conditions. Also, we have investigated a
number of different reaction conditions for synthesis of β-hydroxythiocyanates and the results
are summarized in Table 1. It was observed that, a molar ratio of 1/2.8/3.4 of phenylglycidyl
ether/[P4-VP]H2SO4/[P4-VP]SCN under solvent-free conditions at 65 °C were the best to
achieve the highest yield of the product as indicated in Table 1 (entry 16).
We then applied these conditions in ring opening of different epoxides to prepare the
corresponding thiocyanohydrins and the results are given in Table 2. This new, simple method
can be successfully applied for the synthesis of a wide range of β-hydroxythiocyanates starting
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from the corresponding epoxides. A variety of epoxides such as styrene oxide, allylglycidylether,
cyclohexene epoxide, 1,2-epoxy-3-(4-nitrophenoxy) propane, glycidyl tosylate, N-(2,3-
epoxypropyl) phthalimide, and epichlorohydrin have been converted to corresponding β-
hydroxythiocyanates using [P4-VP]SCN/[P4-VP]H2SO4 under solvent-free conditions at 65 °C
(Table 2).It is important to note that these polymeric reagents were stable and could be stored for
a long time (months) without losing their activity and could be readily used for conversion of
epoxides to the corresponding thiocyanohydrins. Also, these polymeric reagents could be
regenerated and reused for several times (Scheme 1, entries 3-6 in Table 2).
The role of [P4-VP]H2SO4 is not only to activate the epoxide but to promote ring opening
reaction as phase-transfer catalyst. Generally under basic condition the nucleophile slowly
attacks the less substituted β-carbon through an expected SN2 mechanism and under acidic
condition the reaction is much faster likely due to the protonation of the oxygen of the epoxide
ring. In the present study, the cleavage of epoxides took place with high regioselectivity.
Epoxides afforded the products of opening at the benzylic position or partially stable carbocation
intermediates (more substituted carbon) through an SN1-like mechanism. Although recently, we
observed that, 2-arylepoxides afforded the products of opening at the benzylic position through
an SN1-like mechanism, while 2-alkylepoxides gave the products formed by cleavage at the less
substituted carbon through an SN2 borderline mechanism when [P4-VP]HCl and [P4-VP]HBr
were used for synthesis of β-halohydrin products.61
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1
The β-hydroxythiocyanate products were characterized by FT-IR; and H- and 13C-NMR
spectroscopy and physical properties were compared with literature values of known compounds.
In this respect, the appearance a strong band in the region 2150–2156 cm-1 for stretching
vibration of the –SCN and another strong band in the region 3340-3477 cm-1 for stretching
vibration of the O-H group indicate the formation of corresponding β-hydroxythiocyanate
products.
Also in Figure 1, the FT-IR spectra of [P4-VP] 2% DVB, [P4-VP]I and [P4-VP]SCN (a, b and c
respectively) and in Figure 2, the FT-IR spectra of [P4-VP] 2% DVB, and [P4-VP]H2SO4 (a and b
respectively) are compared. The pyridine pendant groups exhibit a strong C=C stretching band at
1
1
1597 cm− and C=N stretching absorption at 1412 cm− . When the pyridine ring is quaternized
1
with methyl iodide, these peaks shift towards higher wave numbers (1636 cm− for C=C and
1
1445 cm− for C=N). Also, in the spectrum of [P4-VP] 2% DVB, there is no absorption in the
region N-H stretching (spectrum (a) in Figures 1 and 2), but in the spectrum of [P4-VP]H2SO4 (b
1
in Figure 2) the corresponding N-H stretching vibration is observed at 3398cm‒ and the C=C and
1
1
C=N stretching peaks shift towards higher wave numbers (1634 cm− for C=C and 1501 cm− for
C=N) .
In Table 3, other reported methods for nucleophilic ring opening of phenylglycidyl ether for
preparation of thiocyanated product are compared with the present method. As Table 3 reveals,
the reaction time of the present method is shorter than all of the previously reported methods.
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This can probably be attributed to the high local concentration of thiocyanate ion species inside
the pores of the polymer. Although some of the previously reported methods such as entries 3
and 9-11, have high yields and short reaction times, the present method offers advantages such
as mild reaction conditions, simple reaction work-up, and the polymeric reagents can be
regenerated and reused several times. On the other hand, all of the above methods have needed
refluxing in acetonitrile especially entries 10 and 11, and reactions must be performed under
nitrogen gas. This is not in agreement with green chemistry while, in the present method, the
reactions were performed under solvent-free conditions.
CONCLUSIONS
In summary, we describe a novel, eco-friendly, and efficient protocol for the synthesis of
thiocyanohydrins by the regioselective ring opening of epoxides with [P4-VP]SCN in the
presence of [P4-VP]H2SO4 as an inexpensive and green polymeric catalyst. This method suffer
several advantages including mild reaction conditions, high conversions, high regioselectivity,
short reaction time, clean reaction profiles, and high isolated yields which make it a useful and
attractive process for the synthesis of thiocyanohydrins.
EXPERIMENTAL
Materials and instruments
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The chemicals were either prepared in our laboratory or were purchased from Fluka (Buchs,
Switzerland), Aldrich (Milwaukee, WI), or and Merck (Germany) chemical companies. Poly (4-
vinylpyridine) cross-linked with 2% divinyl benzene (DVB), [P4-VP] 2% DVB; (white powder,
100–200 mesh) was purchased from Fluka. [P4-VP]I and [P4-VP]I were synthesized according
to our reported procedures.50 The progress of the reaction was monitored by thin-layer
chromatography (TLC) with silica gel PolyGram SIL G/UV 254 plates. All products were
characterized by comparison of their melting points, Germany, Fourier transform infrared (FT-
1
IR) spectroscopy, and H NMR spectral data with those of known samples, and all yields refer
to the isolated pure products. The FTIR spectra were obtained with a Bruker Equinox (model
55), Germany, and the NMR spectra were recorded on a Bruker AC 400 Avance DPX
spectrophotometer, Germany at 400 MHz in CDCl3 solution.
Preparation of [P4-VP]SCN
[P4-VP]SCN, was prepared as our previously reported method50 by ion exchange reaction
1
1
between [P4-VP]I and KSCN (aq.), the I− ion was released in the solution and SCN− ion was
1
supported on the polymer. After filtration, the amount of the I− ion (that was exchanged with
1
1
SCN− ion) was determined by potentiometric titration of the filtrates with a 0.1 mol L− aqueous
1
solution of silver nitrate and, consequently, the capacity of SCN− ion on the polymer was
1
obtained (3.4 mmol g− of the polymer).
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Preparation of [P4-VP]H2SO4
To a solution of 0.6 mL of concentrated sulfuric acid (16 M) in distilled water (10 mL), [P4-VP]
2% DVB (1 g) was added, and stirred for 24 h. The mixture was filtered and washed with
distilled water until the filtrate gave a negative test for H2SO4. Then it was dried in vacuum at 50
°C for 5 h to give 2.2 g of [P4-VP]H2SO4. The amount of acid group present in the polymeric
chain was calculated based on the weight of supported polymer salt obtained and the weight of
poly(4-vinylpyridine) used. The capacity of the polymer was determined by potentiometric
titration of the unreacted sulfuric acid by poly(4-vinylpyridine) used, with a 0.1 M solution of
sodium hydroxide. The obtained capacity of polymer was 5.56 mmol/g of the polymer.
General procedure for conversion of epoxides using [P4-VP]SCN/[P4-VP]H2SO4 under
solvent-free conditions
In a mortar, a mixture of an epoxide (1mmol), [P4-VP] SCN (0.50 g, 1.7 mmol) and [P4-
VP]H2SO4 (0.50 g, 2.8 mmol) were pulverized for 3-5 min. The mixture was stirred at 65 °C in a
water bath for the appropriate time as indicated in Table 2. The progress of the reaction was
monitored by TLC using n-hexane/ethyl acetate (80:20) as eluent. After completion of the
reaction, the mixture was poured in of CHCl3 (10 mL). The suspension was filtered, and was
washed with CHCl3 (3×5 mL). The combined organic layer was dried over anhydrous CaCl2, and
was filtered. The solvent was evaporated to obtain β-hydroxythiocyanates in good to high
isolated yields (80-95%). When further purification is needed, preparative TLC or column
chromatography on silica gel provides highly pure products.
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Preparation of 2-thiocyanato-3-phenoxypropanol from phenylglycidyl ether by using [P4-
VP]SCN/[P4-VP]H2SO4 under solvent-free conditions as a typical procedure
In a mortar, a mixture of phenylglycidyl ether (1mmol) [P4-VP] SCN (1.00 g, 3.4 mmol) and [P4-
VP]H2SO4 (0.5 g, 2.8 mmol) were pulverized for 3-5 min. The mixture was stirred at 65 °C in a
water bath for 15 min. The progress of the reaction was monitored by TLC using n-hexane/ethyl
acetate (80:20) as eluent. After completion of the reaction, the mixture was poured into CHCl3
(10 mL). Then, the suspension was filtered, and was washed with CHCl3 (3×5 mL). The
combined organic layer was dried over anhydrous CaCl2, and was filtered. The solvent was
evaporated to obtain 2-hydroxy-3-phenoxypropylthiocyanate in high isolated yield (198 mg,
95%).
Regeneration of [P4-VP]SCN
The spent cream-colored polymer (1.00 g) was added to a solution of methyl iodide (20 mmol) in
acetonitrile (10 mL), and the mixture was slowly stirred for 24 h at room temperature. The
polymer was filtered and added to a 3 M aqueous solution of potassium thiocyanate (40 mL), and
was slowly stirred for 24 h. The mixture was filtered and washed with distilled water (3 × 8 mL),
and dried under vacuum in the presence of P2O5 at 40 °C overnight. The activity of the
regenerated polymer was the same as the original form. The regenerated polymer reused several
times, without significant loss of their activity(Table 2, entries 3–6).
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61. Karimi Zarchi, M. A.; Tarabsaz, A. J. Polym. Res. 2013, 20, 208-216.
62. Cainelli, G.; Manescalchi, F.; Panunzio, M. Synthesis 1979, 1979, 141-144.
63. Harrison, C. R.; Hodge, P. Synthesis 1980, 299-301.
64. Sharghi, H.; Nejad, A. H.; Nasseri, M. A. New J. Chem. 2004, 28, 946-951.
65. Sharghi, H.; Nejad, A. H. Phosphorus, Sulfur, Silicon Relat. Elem. 2003, 79, 2297-2305.
66. Sharghi, H.; Nasseri, M. A. Phosphorus, Sulfur, Silicon Relat. Elem. 2003, 178, 1353-1359.
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Table 1: Optimization of the reaction conditions for ring opening of model substrate
(phenylglycidyl ether, 1mmol)
[P4-VP]SCN
Yield a
)%(
Catalyst
Temp
(°C)
entry
Solvent
Time (h)
(mmol of
SCN- ion)
(mmol of H2SO4)
-
1
2
CCl4
CHCl3
-
65
65
65
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
1.7
3.4
3.4
8
-
8
-
-
3
CH3CN
-
8
4
CH2Cl2
-
8
-
-
5
C2H5OH
H2O b
-
65
65
rt
8
-
6
-
8
7
C2H5OH
H2O b
[P4-VP]H2SO4 (2.8)
[P4-VP]H2SO4 (2.8)
[P4-VP]H2SO4 (2.8)
[P4-VP]H2SO4 (2.8)
[P4-VP]H2SO4 (2.8)
-
8
35
50
78
87
85
-
8
rt
8
12
9
CH2Cl2
45
45
45
45
rt
10
11
12
13
14
15
16
C2H5OH
H2O b
2.5
1
Solvent-Free
Solvent-Free
Solvent-Free
Solvent-Free
Solvent-Free
5
[P4-VP]H2SO4 (2.8)
[P4-VP]H2SO4 (2.8)
[P4-VP]H2SO4 (1.4)
[P4-VP]H2SO4 (2.8)
2
40
80
85
95
65
65
65
30 min
30 min
15 min
a Isolated yields;
b In water as solvent, H2O was reacted as nucleophile and 1,2-diol product was obtained
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Table 2: Ring opening of epoxides using [P4-VP]SCN/[P4-VP]H2SO4 under solvent-free
conditions at 65 °C
Product
Time
(min)
Yield
(%)a
Entry
Epoxide
1
2
15
15
95
95
3 a
15
92
4 a
5 a
15
20
88
82
6 a
7
20
15
80
90
8
9
15
15
92
92
10
25
85
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11
12
25
25
80
80
a
b
Isolated yields; The entries 3-6, refer to the use of the [P4-VP]SCN that is recycled first,
second, third and fourth time, respectively, under identical conditions.
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Table 3: Comparison of different methods for synthesis of 3-phenoxy-1-
hydroxypropylthiocyanates
Entry Reaction conditions
Time
(min)
Isolated
yield (%)
Ref.
1
2
[P4-VP]SCN/[P4-VP]H2SO4, solvent-free, 65 °C
15
60
20
15
90
150
60
50
10
25
22
45
95
96
96
88
90
80
83
80
91
96
96
95
A
PEGa-SO3H/NH4SCN/CH2Cl2/rt.
T(4-OHP)Pb/NH4SCN/CH3CN/reflux
MPTCc/NH4SCN/H2O
28
29
30
32
33
34
37
40
64
65
66
3
4
5
PTCd/NH4SCN/CH3CN/rt
6
Selectfluor c/NH4 SCN/CH3CN/rt.
PEGa-SO3H/NH4SCN/H2O/rt.
7
8
DDQf/NH4SCN/CH3CN/reflux
BABMBg/NH4SCN/CH3CN/reflux
COIIT(4-OHP)Ph/NH4SCN/CH3CN/reflux/N2
[P(TPP)Cl2]Cli/NH4SCN/CH3CN/reflux/N2
PPIj/NH4SCN/CH3CN/reflux
9
10
11
12
A: Present method (Table 2, entry 2); a PEG: Poly (ethylene glycol)-bound sulphonic acid; b T(4-
c
OHP)P: 5,10,15,20-Tetrakis (4-nitrophenyl) porphyrin; MPTC: Multi-site phase-transfer
catalyst;
d
e
PTC: poly [N-(2-aminoethylacrylamido] trimethyl ammonium chloride; Selectfluor: 1-
f
Chloromethyl-4-fluoro-1, 4-diazoniabicyclo [2.2.2] octane bis(tetrafluoroborate); DDQ: 2,3-
g
Dichloro-5, 6-dicyano-1, 4-benzoquinone;
BABMB: bis [2-(o-aminophenoxy) methyl]-4-
bromo-1-methoxybenzene; h COII T (4-OHP) P: Metalloporphyrins; [P(TPP)Cl2]Cl: Dichloro
i
j
(5,10,15,20) tetraphenylporphyrine) phosphorus (V) chloride;
imidazolidine.
PPI: 2-Phenyl-2-(2-pyridyl)
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Scheme 1: Preparation stages of [P4-VP]SCN, and [P4-VP]H2SO4, the plausible reaction
pathway of ring opening of epoxides and regeneration of the polymeric reagent.
Scheme 2: Synthesis of thiocyanohydrins using [P4-VP]SCN/[P4-VP]H2SO4 under solvent-free
conditions at 65 °C.
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Figure 1: The FT-IR spectra of [P4-VP] 2% DVB, [P4-VP]I and [P4-VP]SCN (a, b and c
respectively)
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Figure 2: The FT-IR spectra of [P4-VP] 2% DVB, and [P4-VP]H2SO4 (a and b respectively)
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