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Zeolite Molecular Sieve 4Å: A ReusableCatalyst for Fast and Efficient Conversionof Epoxides to Thiiranes with ThioureaRonak Eisavi a , Behzad Zeynizadeh a & Mohammad Mehdi Baradarania

a Department of Chemistry, Faculty of Science , Urmia University ,Urmia, Iran

To cite this article: Ronak Eisavi , Behzad Zeynizadeh & Mohammad Mehdi Baradarani (2011) ZeoliteMolecular Sieve 4Å: A Reusable Catalyst for Fast and Efficient Conversion of Epoxides to Thiiraneswith Thiourea, Phosphorus, Sulfur, and Silicon and the Related Elements, 186:9, 1902-1909, DOI:10.1080/10426507.2010.550267

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Phosphorus, Sulfur, and Silicon, 186:1902–1909, 2011Copyright C© Taylor & Francis Group, LLCISSN: 1042-6507 print / 1563-5325 onlineDOI: 10.1080/10426507.2010.550267

ZEOLITE MOLECULAR SIEVE 4A: A REUSABLE CATALYSTFOR FAST AND EFFICIENT CONVERSION OF EPOXIDESTO THIIRANES WITH THIOUREA

Ronak Eisavi, Behzad Zeynizadeh,and Mohammad Mehdi BaradaraniDepartment of Chemistry, Faculty of Science, Urmia University, Urmia, Iran

GRAPHICAL ABSTRACT

Abstract Various epoxides are readily converted to their corresponding thiiranes bythiourea/zeolite molecular sieve 4Å system in refluxing MeOH. All reactions were carried outwithin 12–25 min to give thiiranes in 80%–99% yields. The catalyst saves its catalytic activityfor several times at this transformation. Stereospecific conversion of (R)-(+)-styrene oxide to(S)-(+)-styrene episulfide was achieved by this combination system.

Keywords Epoxide; thiirane; thiourea; stereospecific; zeolite molecular sieve 4Å

INTRODUCTION

Thiiranes (episulfides) are the simplest sulfur heterocycles and occur in nature,mostly in the plants.1 These three-member heterocyclic compounds have found wideapplication in pharmaceutical, pesticide, herbicide, and polymer industries.2 Moreover,they played a pivotal role as versatile building blocks in asymmetric synthesis.3 A lit-erature review shows that thiiranes are prepared by various methods; however, the moststraightforward synthesis is the conversion of epoxides to thiiranes by an oxygen–sulfurexchange reaction. This protocol has been achieved frequently by a combination of sul-fur transfer agents with Lewis acids, inorganic/organic solid supports, and other promot-ers. Thiourea,4 dimethylthioforamide,5 inorganic thiocyanates,6 polymer-supported thio-cyanates,7 and silica-supported KSCN8 or (NH2)2CS9 are the reagents that have beenused for the conversion of epoxides to thiiranes. The combination systems of differ-ent Lewis acids or reagents such as RuCl3,10 InBr3,11 BiCl3,12 Bi(TFA)3,13 Bi(OTf)3,13

TiO2,14 TiO(TFA)2,15 TiCl3(OTf),15 Mg(HSO4)2,16 Al(DS)3·3H2O,17 SbCl3,18 LiBF4,19

Received 20 December 2010; accepted 19 January 2011.The financial support of this work was gratefully acknowledged by the Research Council of Urmia University.Address correspondence to Behzad Zeynizadeh, Department of Chemistry, Faculty of Science, Urmia

University, Urmia, Iran. E-mail: bzeynizadeh@gmail.com

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ZEOLITE MOLECULAR SIEVE 4Å 1903

LiClO4,20 SiO2-HBF4,21 I2,22 oxalic acid,23 montmorillonite K-10,24 2,4,6-trichloro-1,3,5-triazine,25 (NH4)8[CeW10O36].20H2O,26 CAN,27 Sn(TTP)(OTf)2,28 Sn(TT P)(BF4)2,29

[bmim]PF6,30 β-cyclodextrin,31 poly(4-vinylpyridine)-supported Ce(OTf)4,32 polystyrene-supported AlCl3,33 polymeric cosolvents,34 etidronic acid,35 and microwave irradiation36

with thiourea or NH4SCN have also been reported for this transformation. Moreover,solvent-free conversion of epoxides to thiiranes was achieved recently by thiourea/NH4Cl37

and Dowex-50WX8-supported thiourea38 systems.Although a vast variety of reagents or methods have been reported for the preparation

of thiiranes from epoxides, however, most of these protocols suffer from some disadvan-tages. Therefore, the development and introduction of convenient methods that use cheap,reusable, and commercially available reagents is practically concerned and it is still indemand. So, we wish to introduce thiourea/zeolite molecular sieve (ZMS) 4Å system asan efficient and eco-friendly protocol for conversion of structurally different epoxides tothiiranes in refluxing MeOH (Scheme 1).

Scheme 1

RESULTS AND DISCUSSION

ZMSs are crystalline and highly porous materials that belong to the class of hydratedaluminosilicates.39 These micropores are of molecular sizes and give zeolites, adsorp-tion, catalytic,40 and ion exchange properties41 of paramount importance in the chem-ical industries. Moreover, interest is growing on the study of new zeolite applicationsrelated to process intensification,42 green chemistry,43 hybrid materials,44 medicine,45 ani-mal food uses,46 optical- and electrical-based applications,47 multifunctional fabrics,48 andnanotechnology.49

Thiourea has been one of the most widely used sulfurating agents for the conversionof epoxides to thiiranes; however, low reactivity, extended reaction times, low yield ofproducts, and, in some cases, desulfuration of the resulting thiiranes to olefins50 are themajor disadvantages of using thiourea as a sulfurating agent. Along the outlined strategies,a literature review shows that transformation of epoxides to thiiranes with thiourea in thepresence of ZMS 4Å has not been investigated yet. Therefore, in the course of our studies ingreen protocols for the preparation of thiiranes from epoxides37,38 and in order to evaluatethe influence of ZMS 4Å on this transformation, we decided to study the capability ofthiourea/ZMS 4Å system for the conversion of epoxides to thirranes.

The optimization experiments showed that the reaction of styrene oxide (1 mmol)with thiourea (2 mmol) in the presence of ZMS 4Å (0.1 g) was carried out successfully inrefluxing MeOH. The epoxide was converted to the corresponding thiirane in 98% yieldwithin 15 min. The effect of other solvents such as CH3CN, THF, acetone, n-hexane,and EtOH was also investigated for the typical experiment. The results showed that therate enhancement and yield of styrene episulfide in MeOH were higher than the othersolvents (Table 1). It is notable that in the absence of the molecular sieve, thiourea was

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1904 R. EISAVI ET AL.

Table 1 Optimization of reaction conditions for conversion of styrene oxide to styrene episulfide withthiourea/zeolite molecular sieve 4Å (ZMS-4) systema

Reaction component molar ratio ZMS-4 (g) Solvent Condition Time (h) Conversion (%)b

Epoxide/thiourea (1:2) 0.1 n-hexane Reflux 240 50Epoxide/thiourea (1:2) 0.1 Acetone Reflux 300 55Epoxide/thiourea (1:2) 0.1 CH3CN Reflux 180 75Epoxide/thiourea (1:2) 0.1 THF Reflux 180 70Epoxide/thiourea (1:2) 0.1 EtOH Reflux 30 90Epoxide/thiourea (1:2) 0.05 MeOH Reflux 30 100Epoxide/thiourea (1:2) 0.1 MeOH Reflux 15 100Epoxide/thiourea (1:2) 0.1 MeOH r.t. 45 100Epoxide/thiourea (1:2) 0.2 MeOH Reflux 10 100Epoxide/thiourea (1:2) – MeOH Reflux 90 80

aAll reactions were carried out with 1 mmol of styrene oxide. bConversions less than 100% were determinedon the basis of recovered epoxide.

less efficient in the conversion of styrene oxide to styrene episulfide in refluxing MeOH(Table 1). The capability of this synthetic protocol was further investigated by the reactionof activated, deactivated, and cyclic epoxides with thiourea/ZMS 4Å system under theoptimized conditions. Table 2 shows the general trend and versatility of this syntheticmethod. As seen, all reactions were carried out successfully within 12–25 min to givethiiranes in 80%–99% yields. Moreover, the stereochemistry of this synthetic method wasstudied by the reaction of optically pure (R)-(+)-styrene oxide with thiourea/ZMS 4Åsystem at room temperature. (S)-(+)-Styrene episulfide was obtained in 95% yield and90% optical purity. The optical purity of the product was measured by comparing theobtained specific rotation ([α]25

D = +39.5 ◦, n-heptane) with the reported rotation ([α]25D =

−15.7◦, n-heptane, 35.8% ee) for (R)-(−)-styrene episulfide51 (Scheme 2).

Scheme 2

The green aspect of this synthetic method was studied by recovery of molecular sieve4Å from the reaction mixture and then reusing it for the conversion of styrene oxide tostyrene episulfide. The results showed that the regenerated molecular sieve saves its catalyticactivity for several times in the titled conversion. In addition, the efficiency of thiourea/ZMS4Å was highlighted by comparing some of our results with those of achieved by thiourea/120◦C,4 thiourea/SiO2,9 thiourea/NH4Cl,37 and thiourea/Dowex-50WX838 systems (Table 3).A case study shows that the present protocol is more efficient than the reported methods.

In summary, we have shown that structurally different epoxides are easily and effi-ciently converted to their corresponding thiiranes with thiourea/ZMS 4Å system in refluxingMeOH. Stereospecific conversion of (R)-(+)-styrene oxide to (S)-(+)-styrene episulfidewas achieved perfectly by this combination system. So, we think that the cheapness andreusability of molecular sieve 4Å, short reaction times, and high yields of the productsafford this synthetic method as a green and useful addition to the present methodologies.

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ZEOLITE MOLECULAR SIEVE 4Å 1905

Table 2 Conversion of epoxides to thiiranes with thiourea/zeolite molecular sieve 4Å systema

Molar ratioepoxide/ Time Yield

Epoxide Thiirane thiourea (min) (%)b Ref.

1:2 15 98 37

1:2 17 95 35

1:2 13 96 35

1:2 15 92 37

1:2 12 81 35

1:2 25 94 37

1:2 18 95 37

1:2 17 99 37

1:2 18 99 37

1:2 20 96 30

1:2 15 98 30

1:2 15 98 30

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Table 2 Conversion of epoxides to thiiranes with thiourea/zeolite molecular sieve 4Å systema (Contonued)

Molar ratioepoxide/ Time Yield

Epoxide Thiirane thiourea (min) (%)b Ref.

1:2 20 97 37

1:2 15 97 37

1:2 15 95 37

1:2 16 93 30

1:2 25 80 30

1:2 16 92 30

aAll reactions were carried out in the presence of ZMS 4Å (0.1 g, per 1 mmol of epoxide) in refluxing MeOH.bIsolated yields.

EXPERIMENTAL

General

All reagents and substrates were purchased from commercial sources with the bestquality and were used without further purification. IR and 1H/13C NMR spectra wererecorded on Thermo Nicolet Nexus 670 FT-IR and 300 MHz Bruker Avance spectrometers,respectively. The products were characterized by their spectra data and comparison withthe reported data in literature. Thin layer chromatography (TLC) was applied for thepurity determination of substrates, products, and reaction monitoring over silica gel 60 F254

aluminium sheet.

Conversion of Epoxides to Thiiranes with Thiourea/ZMS 4A System:

General Procedure

In a round-bottomed flask (10 mL) equipped with a magnetic stirrer and condenser,a solution of the epoxide (1 mmol) and thiourea (0.152 g, 2 mmol) in MeOH (3 mL)was prepared. Molecular sieve 4Å (0.1 g) was then added to the resulting solution andthe reaction mixture was stirred magnetically under reflux condition for 12–25 min. Theprogress of the reaction was monitored by TLC. After completion of the reaction, MeOHwas evaporated and diethyl ether (5 mL) was added to the reaction mixture followed bystirring for 5 min. The mixture was filtered and the organic solvent was evaporated to give

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Tabl

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Com

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thiir

anes

with

thio

urea

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rdi

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a

Thi

oure

a/Z

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4bT

hiou

rea/

120

◦ C4

Thi

oure

a/Si

O2

9T

hiou

rea/

NH

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rea/

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aT

ime

Yie

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Silic

aT

ime

Yie

ldT

hiou

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4C

lT

ime

Yie

ldT

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ldE

poxi

de(m

mol

)(g

)(m

in)

(%)

(mm

ol)

(min

)(%

)(m

mol

)(g

)(m

in)

(%)

(mm

ol)

(g)

(min

)(%

)(m

mol

)(g

)(m

in)

(%)

20.

115

982

1565

22.

880

952

0.5

3095

20.

560

93

20.

118

992

1584

22.

812

095

20.

575

942

0.5

2091

20.

115

972

6077

22.

840

922

0.5

1593

20.

530

96

20.

120

972

6080

22.

845

932

0.5

3096

20.

520

95

20.

115

922

2592

22.

819

092

20.

540

652

0.5

6085

a All

reac

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1908 R. EISAVI ET AL.

crude thiirane for further purification by a short-column chromatography over silica gel(80%–99% yield) (Table 2).

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