Application of 1,4-bis(3-methylimidazolium-1-yl)butane ditribromide [bMImB]·(Br3)2 ionic liquid reagent for selective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1039/c5ra06182b

1,4-Bis(3-methylimidazolium-1-yl)butane ditribromide [bMImB]·(Br₃)₂ as an efficient reagent is used for the selective oxidation of dialkyl and alkyl aryl sulfides to the corresponding sulfoxides in high yields under mild conditions. The products can easily be isolated by just washing the highly water soluble 1,4-bis(3-methylimidazolium-1-yl)butane ditribromide [bMImB]·(Br₃)₂. The spent reagent can be recovered several times and reused without any significant loss.

Full text of DOI:10.1039/c5ra06182b

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Application of 1,4-bis(3-methylimidazolium-1-yl)
butane ditribromide [bMImB]$(Br3)2 ionic liquid
reagent for selective oxidation of sulfides to
sulfoxides†
Cite this: RSC Adv., 2015, 5, 70265
Abbas Amini Manesh, a Fereshteh Hosseini Eshbala,a Saba Hemmatiab and Hojat Veisia
*
1,4-Bis(3-methylimidazolium-1-yl)butane ditribromide [bMImB]$(Br3)2 as an efficient reagent is used for the
selective oxidation of dialkyl and alkyl aryl sulfides to the corresponding sulfoxides in high yields under mild
conditions. The products can easily be isolated by just washing the highly water soluble 1,4-bis(3-
methylimidazolium-1-yl)butane ditribromide [bMImB]$(Br3)2. The spent reagent can be recovered several
times and reused without any significant loss.
Received 7th April 2015
Accepted 30th July 2015
DOI: 10.1039/c5ra06182b
www.rsc.org/advances
Owing to their importance as intermediates in organic sulfoxides.15 Extension of the Br2 oxidation methodology to
synthesis1 and their key role in enzyme activation,2 organosulfur aromatic suldes resulted in the competitive aromatic electro-
compounds, such as sulfoxides are much sought aer. The philic substitution of bromine along with electrophilic attack
oxidation of suldes is the most straightforward method for the on sulfur, thereby affording unwanted products along with low
synthesis of sulfoxides. Consequently, the selective oxidation of yields of the target molecules. Several tribromides have been
suldes to sulfoxides has been a challenge for many years and is reported, that are, cetyltrimethylammonium tribromide
a useful transformation in organic chemistry.3 The results (CTMATB),16 1,8-diazabicyclo[5,4,0]-tetrabutylammonium tri-
prompted us to verify the possibility of the achievement of a new bromide,17 pyridine hydrobromide perbromide,18 DABCO–
procedure for the oxidation of suldes. Since the rst reported bromine,19 1,3-di-n-butylimidazolium tribromide[BBIm]Br3,20
synthesis of sulfoxides by Maercker in 1865, a number of and
1-butyl-3-methylpyridinium
tribromide([BMPy]Br3).21
methods have been developed for the transformation of suldes Sodium metaperiodate, NaIO4, might be a good choice for the
to sulfoxides,4 and therein many reagents are available for the oxidation of suldes to sulfoxides,22 but it is prohibitively
oxidation of suldes.5–9
expensive. Furthermore, the preparation and recycling of the
Unfortunately, most of the methods use hazardous, toxic reagent are both very difficult. In view of the above, a protocol is
reagents10 or complex reaction procedures,11 accompanied with required for the selective oxidation of suldes to sulfoxides in a
overoxidation to sulfone or other unwanted side products.12,13 clean and cost effective manner under mild conditions. Most
Furthermore, that types of reactions in which the catalyst is relevantly, vanadium bromoperoxidase (VBrPO)23 oxidizes
inherently dissolved in organic phase, require the separation of suldes to sulfoxides in the presence of hydrogen peroxide
the expensive catalyst from the product, and consequently, a wherein the tribromide (Br3ꢀ) formed in situ has been suggested
chromatographic procedure for the recovery of the catalyst is to be responsible for the sulde oxidation.
invariably needed.14
Development of environmentally benign and efficient
Organic tribromide reagents (OTBs) are preferable as processes with simple work-up and high purity of the products
oxidants, molecular bromine might serve as a good alternative, with high yields is currently receiving considerable attention.
but its use has been restricted because of being expensive, toxic Recently, some ionic liquid tribromides (IL-Br3ꢀ) were repor-
and environmentally unfriendly. Also, it contaminates the main ted.24 In this context, tribromide ionic liquid plays an important
product with the formation of side products such as sulfonic role in the process of obtaining sulfoxide derivatives with the
acids, sulnic acids, bromo substituted suldes and ultimate goal of hazard-free and waste-free efficient synthesis of
sulfoxides that are valuable intermediates for the construction
of various ne chemicals as useful building blocks in asym-
metric synthesis.25 In recent years, there has been considerable
interest in the developing green chemistry26 for organic
synthesis due to environmental demand and sustainability.
There are various kinds of alkyl imidazolium salts which are
commercially available as room temperature ionic liquids
aDepartment of Chemistry, Payame Noor University, Tehran, Iran. E-mail:
a_aminima@yahoo.com; Fax: +98 81 32542400
bResearch Center of Oils and Fats, Kermanshah University of Medical Sciences,
Kermanshah, Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra06182b
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Scheme 1
(RTILs). (RTILs) have received much attention due to their more nucleophilic than alkyl aryl suldes (entries 2–8). The
unique properties, such as negligible volatility, non- effect of an alkyl group makes the sulfur atom more nucleo-
inammability, thermal stability, ease of recyclability, broad philic and hence comparatively more reactive under the present
liquid temperature ranges and the ability to dissolve a wide experimental conditions. Under these conditions functional
range of inorganic and organic compounds.27,28 The combina- groups such as hydroxyl and acetate remain unaffected (entries
tion of the alkylimidazolium cation with the tribromide anion 6–7 and 13) (Scheme 2).
should therefore lead to RTIL bromine analog.
In order to test the ability of optimized system in the
In continuation of our interest in oxidation of suldes,29 oxidation of selenide compounds, we have used methyl phenyl
herein, [bMImB]$(Br3)2 that previously synthesized (Scheme 1), selenide under same conditions. We observed that no product
and used for bromination,30 we have used for the selective was formed aer 24 h.
oxidation of suldes to sulfoxides in very good yields under
mild conditions.
Against the backdrop of the reagent acting as a brominating
agent33 for aromatics and olens as well as a deprotecting
[bMImB]$(Br3)2 was easily prepared by treating 1-methylim- agent34,35 for dithioacetals and thiols, and as a protecting/
idazole (2 equiv.) with 1,4-dibromobutane (1 equiv.) at room deprotecting agent for carbonyl-1,3-oxathiolane interconver-
temperature. The resultant 1,4-bis(3-methylimidazolium-1-yl) sion, it is believed that the active species in these reactions is
butane di(bromide) [bMImB]$(Br)2 as light brown crystals was molecular bromine generated in situ, which in turn acts as an
treated with molecular bromine. Then, the orange precipitate of electrophile. Importantly, the amount of the active species can
1,4-bis(3-methyl
imidazolium-1-yl)butane
ditribromide be tuned by regulating the amount of reagent, an operation that
[bMImB]$(Br3)2 was ltered (96% yield) and recrystallized in is rather difficult in the direct use of liquid bromine or a
acetonitrile to obtain large crystals. The compound was char- bromine solution. An electrophilic attack of bromine on sulfur
acterized by spectral and analytical data. This crystalline then leads to the intermediate [A], which is then hydrolyzed by
compound is stable for several months at room temperature water to give the corresponding sulfoxide as shown in Scheme 3.
without loss of its activity.
An interesting feature of this reagent is that it can be
To test the oxidation efficiency of this reagent, 1.2 mmol of regenerated at the end of the reaction by the extraction with
the reagent 1,4-bis(3-methylimidazolium-1-yl)butane ditri- water and can be used several times without losing its activity.
bromide [bMImB]$(Br3)2 was added to 1.0 mmol of suldes in a The aqueous phase containing highly water-soluble IL was
mixed acetonitrile (2 mL)–water (1 mL). As indicated in Table 1, easily concentrated in vacuo to recycle [bMImB]$(Br)2, which
the reactions are completed within 5–20 minutes in high yields. then could be used to regenerate [bMImB]$(Br3)2 with bromine
This procedure works successfully for a number of substrates. (Scheme 4). Regeneration and reusability are two signicant
All the reactions were quenched by adding water. This caused features of ionic liquids.36 The effectiveness of ionic liquid, their
precipitation of the products, which were readily extracted and recyclability and regeneration processes were observed for the
then dried. The reaction is generalized through entries 1–14 as oxidation of methyl phenyl sulde and found to be effective up
shown in Table 1.
to 6 cycles (Fig. 1).
The results summarized in Table 1 explains the faster and
For showing the practical applicability of the catalyst, we
easier oxidation of dialkyl suldes (entries 9, 11–14), which are have scaled up the reaction of methyl phenyl sulde up to 10
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a
Table 1 Oxidation of sulfides to sulfoxides using [bMImB]$(Br3)2
Entry
Substrate
Product
Yield (%)
Time (min)
Ref.
31
1
88
18
2
95
18
16
3
4
97
99
16
15
16
16
5
6
7
90
88
87
16
17
19
28
28
28
8
9
86
94
20
10
28
16
10
11
90
93
20
5
16
16
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Table 1 (Contd.)
Entry
12
Substrate
Product
Yield (%)
89
Time (min)
6
Ref.
28
13
HOC2H4SCH3
CH3SCH3
91
98
4
4
32
16
14
a
Reaction conditions: 1.2 mmol [bMImB]$(Br3)2, 1 mmol substrate, 3 mL solvent at 25 ꢁC.
(Br3)2. Melting point: 132 ꢁC. UV (CH3CN) 267 nm. IR (KBr) 3158,
3136, 3086, 2966, 2856, 1635, 1595, 1570, 1556, 1464, 1400, 1162,
1103, 837, 742, 617 cmꢀ1. 1H NMR (D2O) (ppm) d: 8.83 (s, 2H, CH
imidazolium); 7.55 (m, 4H, CH imidazolium); 4.33 (t, 4H, NCH2);
3.96 (s, 6H, NCH3); 1.99 (m, 4H, CH2CH2). 13C NMR (CDCl3) (ppm)
d: 123.7, 123.6 and 122.0 (CH imidazolium); 48.6 (NCH2); 35.8
(NCH3); 26.0 (CH2). Anal. calcd for C12H20Br6N4: C, 20.60; H, 2.88;
N, 8.01; Br, 68.51. Found: C, 20.10; H, 2.19; N, 7.65; Br, 67.87%.
Scheme 2
mmol with 12 mmol [bMImB]$(Br3)2 under the optimized
reaction conditions. Results indicated that reaction performed
effectively and desired product obtained in 97% isolated yield.
In summary, we have developed an efficient and versatile
method for the oxidation of suldes to sulfoxides using 1,4-bis(3-
methylimidazolium-1-yl)butane ditribromide, based on the oxida-
tion of bromide by a peroxometal intermediate. The byproduct
[bMImB]$(Br)2 can be recycled to give [bMImB]$(Br3)2, thereby
rendering the protocol economic. The mild reaction conditions,
simple experimental procedure, rapid conversion, good yields, and
reusability of the reagent are notable advantages of the method.
General procedure for the oxidation of suldes to sulfoxides
Methyl phenyl sulde (1 mmol) in a mixed acetonitrile (2 mL)–
water (1 mL) solvent reacted with [bMImB]$(Br3)2 (1.2 mmol)
under stirring at room temperature for 15 min. The reaction was
monitored by TLC and no overoxidation detected. On comple-
tion of the reaction, acetonitrile was removed under reduced
pressure and water added. The product was extracted with ethyl
acetate, dried over Na2SO4 and then evaporated to dryness,
while the aqueous layer was retained for recovery of
[bMImB]$(Br)2. Methyl phenyl sulfoxide was isolated in 99%
yield. The aqueous layer containing [bMImB]$(Br)2 was
concentrated by evaporation until a crystallin compound
obtained. The [bMImB]$(Br)2 thus recovered and was recycled to
[bMImB]$(Br3)2 following the described procedure.
Experimental
Preparation of 1,4-bis(3-methylimidazolium-1-yl)butane
ditribromide [bMImB]$(Br3)2
In a fume cupboard, molecular bromine (1.0 mL, 2.0 mmol) was
added dropwise over 10 min to 1,4-bis (3-methylimidazolium-1-yl)
butane dibromide (0.380 g, 1.0 mmol) under stirring and cooling
Regeneration of ionic liquid
in an ice-bath affording a deep orange solid [bMImB]$(Br3)2 ionic Aer the addition of water into the reaction mixture, the water
liquid, and stirring was continued for 2 h. The orange precipitate soluble ionic liquid 1,4-bis(3-methylimidazolium-1-yl)butane
was recrystallized from acetonitrile to yield 1.77 g (96% yield) of dibromide [bMImB]$(Br)2 was recovered by evaporation of the
1,4-bis (3-methylimidazolium-1-yl)-butane ditribromide [bMImB]$ aqueous solution and washing with diethyl ether in each case.
Scheme 3 Proposed mechanism for the oxidation of sulfide to the corresponding sulfoxide with [bMImB]$(Br3)2.
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Scheme 4
Entry 9 (dibenzyl sulfoxide). Mp 132–134 ꢁC. 1H NMR
(CDCl3): 3.81 (s, 2H), 3.92 (s, 2H), 7.10–7.50 (m, 10H).
Entry 10 (diphenyl sulfoxide). 1H NMR (CDCl3): 7.65–7.63
(m, 4H), 7.47–7.43 (m, 6H).
Entry 11 (diallyl sulfoxide). Oil. 1H NMR (CDCl3): 3.57 (d, J ¼
7.1 Hz, 4H), 5.24–5.38 (m, 4H), 5.72–5.96 (m, 2H).
Entry 12 (dibutyl sulfoxide). Mp 34–35 ꢁC. 1H NMR (CDCl3):
0.94 (t, J ¼ 7.2), 1.40–1.47 (m), 1.59–1.63 (m), 2.52–2.55 (t).
1
Entry 13 (2-(methylsulnyl)ethanol). H NMR (CDCl3): 1.94
(s, 3H), 2.50 (t, 2H), 3.48 (s, 1H), 3.55 (t, 2H).
Entry 14 ((methylsulnyl)methane). 1H NMR (CDCl3): 2.62
(s, 6H).
Fig. 1 Recyclability of the catalyst.
The recovered ionic liquid [bMImB]$(Br)2 was dried at 40 ꢁC for
12 h, and treated with molecular bromine (2 mmol) under
stirring and cooling in an ice-bath for 2 h to regenerate 1,4-
Acknowledgements
The authors thank Payame Noor University (PNU) for support-
ing this work.
bis(3-methylimidazolium-1-yl)butane
ditribromide
[bMImB]$(Br3)2 ionic liquid and reused for the subsequent
reaction without loss of activity.
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