One-pot thioetherification of aryl halides using thiourea and alkyl bromides catalyzed by copper(I) iodide free from foul-smelling thiols in wet polyethylene glycol (PEG 200)

Article abstract of DOI:10.1002/adsc.200900671

In this article, we have developed a new protocol for the thioarylation of structurally diverse alkyl bromides such as benzyl, cinnamyl, n-octyl, cyclohexyl, cyclopentyl, and tert-butyl bromides with aryl iodides, bromides and an activated chloride using thiourea catalyzed by copper(I) iodide in wet polyethylene glycol (PEG 200) as an eco-friendly medium in the presence of potassium carbonate at 80 and 100°C under an inert atmosphere. The process is free from foul-smelling thiols which makes this method more practical for the thioetherification of aryl halides. Another important feature of this method is the variety of alkyl bromides which are commercially available for the in situ generation of thiolate ions with respect to the existing protocols in which the less commercially available thiols are directly used for the preparation of arylthio ethers.

Full text of DOI:10.1002/adsc.200900671

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DOI: 10.1002/adsc.200900671
One-Pot Thioetherification of Aryl Halides Using Thiourea and
Alkyl Bromides Catalyzed by Copper(I) Iodide Free from Foul-
Smelling Thiols in Wet Polyethylene Glycol (PEG 200)
Habib Firouzabadi,a,* Nasser Iranpoor,a,* and Mohammad Gholinejada
a
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
Fax : (+98)-711-228-0926; phone: (+98)-711-228-4822; e-mail: firouzabadi@chem.susc.ac.ir or iranpoor@chem.susc.ac.ir
Received: September 28, 2009; Revised: November 8, 2009; Published online: December 28, 2009
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: In this article, we have developed a new
protocol for the thioarylation of structurally diverse
alkyl bromides such as benzyl, cinnamyl, n-octyl, cy-
clohexyl, cyclopentyl, and tert-butyl bromides with
aryl iodides, bromides and an activated chloride
using thiourea catalyzed by copper(I) iodide in wet
polyethylene glycol (PEG 200) as an eco-friendly
medium in the presence of potassium carbonate at
80 and 1008C under an inert atmosphere. The pro-
cess is free from foul-smelling thiols which makes
this method more practical for the thioetherification
of aryl halides. Another important feature of this
method is the variety of alkyl bromides which are
commercially available for the in situ generation of
thiolate ions with respect to the existing protocols
in which the less commercially available thiols are
directly used for the preparation of arylthio ethers.
tion of carbon-heteroatom bonds which has been
under rigorous consideration in recent years.[3] This
strategy has been broadly utilized in organic synthesis
and materials science.[4] The preparation of thioethers
by the reaction of aryl halides with thiols is important
and has been under consideration and widely studied
in recent years using palladium,[5] nickel,[6] cobalt,[7]
copper,[8] indium[9] and iron[10] salts as catalysts for
this purpose. Even though some of these studies have
led to the opening of new and efficient protocols
under more eco-friendly conditions, the direct use of
volatile and foul-smelling thiols is still a main draw-
back, which could lead to strict environmental, and
safety problems and thus place a ceiling on any scale-
up operation. In addition, the majority of the report-
ed reactions proceed in harmful organic solvents their
disposal of which is the nowadays a major problem
for the chemical industry. Moreover, organic solvents
are expensive, toxic, flammable and not recyclable.
However, polyethylene glycols (PEGs) and their de-
rivatives, possessing negligible vapor pressure, are
known to be inexpensive, thermally stable, recovera-
ble, non-toxic compounds which serve as suitable
media for environmentally friendly and safe chemical
reactions.[11]
Keywords: alkyl bromides; aryl halides; polyethy-
lene glycol; thioarylation; thioetherification; thiour-
eas
Aryl and aryl alkyl thioethers are important com-
In recent years, we have paid attention to organic
pounds which constitute the backbone of many bio- reactions conducted in eco-friendly media such as
logically and pharmaceutically active compounds. In water,[12] polyethylene glycol[13] and ionic liquids.[14] As
À
fact, a number of drugs which are employed for Alz- a part of our interest in C S bond forming reactions,
heimerꢀs disease, Parkinsonꢀs disease, diabetes, as well very recently we have reported two novel protocols
À
as immune and inflammatory diseases carry aryl sul- for C S bond formation by one-pot Michael addition
fide moieties in their backbone structure.[1]
reactions via an odorless process involving the in situ
Exploration of new protocols for C S bond genera- generation of S-alkylisothiouronium salts in water[12b]
tion, which can lead to the discovery of eco-friendly, or in wet polyethylene glycol[13]. After reporting these
cheap and more efficient synthetic protocols for the protocols, we became interested to study the possibili-
preparation of industrial and biologically active ty for the thioetherification of aryl halides via a pro-
organo-sulfur compounds has attracted a great deal of cess free from the foul smell of thiols. To the best of
attention in recent years.[2] Cross-coupling reactions our knowledge, utilization of non-thiolic precursors
conducted in the presence of transition metal catalysts for the one-pot thioetherification of aryl halides via
are one of the most powerful tactics for the genera- transition metal-catalyzed reactions has not yet been
À
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119

Habib Firouzabadi et al.
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(0.05 mmol), potassium carbonate (4.0 mmol) in
water (0.05 mL,) and 3 mL of polyethylene glycol
(PEG 200) at 808C under an inert atmosphere was
studied. The reaction proceeded smoothly and after
24 h the desired phenyl benzyl thioether was isolated
in 80% yield. Then these optimized conditions were
applied for the reaction of structurally different bro-
mides such as benzyl, cinnamyl, n-octyl, cyclohexyl,
cyclopentyl, and tert-butyl bromides with aryl iodides,
bromides and an activated chloride with thiourea cat-
Scheme 1. Copper(I) mediated one-pot thioetherification of
aryl halides using thiourea and alkyl bromides in wet poly-
ethylene glycol (PEG 200).
reported in the literature. Now in this communication, alyzed by CuI in wet polyethylene glycol (PEG 200)
we describe an efficient, one-pot, odorless process for in the presence of K2CO3 at 808C and 1008C produc-
thioetherification of different aryl halides (I, Br, Cl) ing the corresponding aryl alkyl thioethers as present-
in wet polyethylene glycol (PEG 200) using thiourea ed in Table 1.
and alkyl bromides under ligand-free conditions cata-
lyzed by copper(I) iodide using K2CO3 as a base at 80 large-scale operation, we have scaled-up the reaction
or 100 8C (Scheme 1). of 1-bromo-4-nitrobenzene to the 10-mmol, 2.02-gram
In order to show the features of the reaction for
To optimize the reaction conditions, first the reac- scale with 11 mmol of benzyl bromide under similar
tion of benzyl bromide (1.1 mmol), iodobenzene optimized reaction conditions as mentioned in the
(1 mmol), thiourea (1.2 mmol) in the presence of CuI preceding paragraph. The reaction proceeded well
Table 1. Thioetherification of different aryl halides using thiourea, alkyl bromides catalyzed by CuI in wet PEG 200.
Entry
1
ArX
RX
Product
Time/Temp. [8C]
Yield [%][a]
80
1a
2a
3a
3b
3c
3d
3a
3c
3d
24/80
2
3
4
5
6
7
1b
1c
1d
1e
1f
1g
2a
2a
2a
2a
2a
2a
24/80
65
24/80
80
12/80
90
24/100
24/100
12/100
80
75
87 (86)[b]
8
9
1h
1k
2a
2a
3e
3d
12/100
12/100
88
78
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One-Pot Thioetherification of Aryl Halides Using Thiourea and Alkyl Bromides
Table 1. (Continued)
Entry
ArX
RX
Product
Time/Temp. [8C]
Yield [%][a]
89
10
1d
2b
3f
3f
24/80
11
1g
2b
12/100
85
12
13
14
1b
1c
1d
2c
2c
2c
3g
3h
3h
24/100
24/80
12/80
68
78
90
15
16
17
18
19
20
21
22
23
24
1g
1b
1d
1g
1h
1d
1g
1k
1h
1g
2c
2d
2d
2d
2d
2e
2e
2e
2e
2f
3h
3j
12/100
24/100
24/80
86
70
88
86
90
90
85
73
87
62
3k
3k
3l
12/100
12/100
12/80
3m
3m
3m
3n
3o
12/100
24/100
12/100
30/80
25
1h
2f
3o
48/80
51
[a]
Reactions were performed with 1 mmol ArX, 5 mol% CuI (for ArI) or 10 mol% (for ArBr and ArCl), 1.1 mmol RBr,
1.2 mmol thiourea, 0.3 mL H2O, 3 mL PEG 200 and 4 mmol K2CO3 under an inert atmosphere.
In parenthesis the yield of the reaction performed with 10 mmol of 1-bromo-4-nitrobenzene, 10 mol% CuI, 11 mmol
benzyl bromide, 12 mmol thiourea, 3 mL H2O, 30 mL PEG 200 and 40 mmol K2CO3 under an inert atmosphere is pre-
sented.
[b]
and 86% of the desired thioether was isolated this reaction proceeds by the in situ generation of an
(Table 1, entry 7). S-alkylisothiouronium salt which is hydrolyzed in the
We have also proposed a mechanism for this reac- reaction mixture to produce a thiolate moiety and
tion which is presented by Scheme 2. We believe that urea.[12b,13] In this step, urea is hydrolyzed in situ to
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Habib Firouzabadi et al.
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(Table 1). The reaction mixture was cooled to room temper-
ature and extracted with EtOAc. Evaporation of the solvent
and purification by flash column chromatography on silica
gel (EtOAc/n-hexane) gave the desired aryl alkyl thioethers;
yield: 65–90%.
1
3a: H NMR (250 MHz, CDCl3): d=7.22–7.7.12 (m, 10H),
3.49 (s, 2H); 13C NMR (62.9 MHz, CDCl3): d=137.7, 136.9,
129.8, 129.4, 128.8, 127.7, 127.5, 126.6, 43.5; anal. calcd. for
C13H12S: C 77.96, H 6.04, S 16.01; found: C 77.89, H 6.06, S
16.06%.
3b: 1H NMR (250 MHz, CDCl3): d=7.17–7.08 (m, 7H),
6.70–6.66 (m, 2H), 3.88 (s, 2H), 3.62 (s, 3H); 13C NMR
(62.9 MHz, CDCl3): d=158.1, 137.0, 132.9, 127.8, 127.3,
125.9, 125.0, 113.3, 54.2, 40.1; anal. calcd. for C14H14OS: C
73.01, H 6.13, S 13.92; found: C 72.96, H 6.11, S 13.95%.
1
3c: H NMR (250 MHz, CDCl3): d=7.30–7.10 (m, 10H),
6.98–6.95 (m, 2H), 4.13 (s, 2H), 2.21 (s,3H); 13C NMR
(62.9 MHz, CDCl3): d=136.7, 135.4, 131.4, 129.6, 128.5,
127.9, 127.4, 126.0, 125.9, 38.7, 20.0; anal. calcd. for C14H14S:
C 78.46, H 6.58, S 14.96; found: C 78.49, H 6.59, S 14.92%.
3d: 1H NMR (250 MHz, CDCl3): d=8.02–7.97 (m, 2H),
7.32–7.17 (m, 7H), 4.16 (s, 2H); 13C NMR (62.9 MHz,
CDCl3): d=147.2, 145.1, 135.4, 129.7, 128.8, 128.7, 127.8,
126.5, 123.9, 36.9; anal. calcd. for C13H11NO2S: C 63.66, H
Scheme 2. A proposed mechanism for thia-arylation of aryl
halides using alkyl bromides and thiourea catalyzed by CuI.
produce carbon dioxide gas and ammonia as the final
products of the hydrolysis reaction.[15] The generated 4.52, N 5.71, S 13.07; found: C 63.61, H 4.48, N 5.77, S
13.10%.
thiolate ion which is a synthetic equivalent of thiol
and an odorless moiety may react by the oxidative ad-
dition reaction followed by an elimination reaction
with the aryl organocupper intermediate which has
been formed by the reaction of aryl halides with CuI.
This proposed mechanism is a fairly accurate presen-
tation of what may happen during the progress of the
reaction, based on common analogies and considera-
tions.[16] The proposed pathway is shown in Scheme 2.
In conclusion, in this communication, we have re-
ported a novel protocol for thia-arylation reactions
using a process which is free from foul-smelling thiols.
This makes the method more easy and practical for
the thioetherification of aryl halides. Another impor-
tant feature of this method is the variety of commer-
cially available alkyl bromides which makes the syn-
thesis much easier than that using the corresponding
thiols. By this protocol the preparation of structurally
diverse alkyl aryl thioethers becomes more practical
and easier than available protocols in which thiols are
directly used for the preparation of alkyl aryl thioeth-
ers.
1
3e: H NMR (250 MHz, CDCl3): d=8.9 (s, 1H), 8.49 (s,
2H), 7.29–7.12 (m, 5H), 4.02 (s, 2H); 13C NMR (62.9 MHz,
CDCl3): d=157.2, 155.4, 134.9, 127.8, 127.7, 126.7, 37.9;
anal. calcd. for C11H10N2S: C 65.32, H 4.98, N 13.85, S 15.85;
found: C 65.26, H 4.99, N 13.88, S 15.87%.
3f: 1H NMR (250 MHz, CDCl3): d=8.05–8.01 (d, J=
9.0 Hz, 2H), 7.60–7.22 (m, 7H), 6.58–6.03 (m, 2H), 3.81–
3.78 (d, J=6.5 Hz, 2H); 13C NMR (62.9 MHz, CDCl3): d=
152.8, 134.0, 132.3, 131.5, 129.1, 128.6, 126.9, 126.3, 123.9,
123.0, 33.9; anal. calcd. for C15H13NO2S: C 66.40, H 4.83, N
5.16, S 11.82; found: C 66.46, H 4.79, N 5.20, S 11.81%.
3g: 1H NMR (250 MHz, CDCl3): d=7.32–7.26 (m, 2H),
6.77–6.71 (m, 2H), 3.71 (s, 3H), 2.82 (m, 1H), 1.84–1.18 (m,
10H); 13C NMR (62.9 MHz, CDCl3): d=158.2, 134.5, 123.9,
115.3, 113.2, 54.2, 46.8, 32.3, 28.6, 25.0, 24.7; anal. calcd. for
C13H17OS: C 70.23, H 8.16, S 14.42; found: C 70.16, H 8.22,
S 14.45%.
3h :1H NMR (250 MHz, CDCl3): d=7.25–7.20 (m, 2H),
7.04–6.97 (m, 2H), 2.94–2.93
ACHTUNGTNER(NUNG m, 1H), 2.23 (s, 3H), 1.86–1.85
(m, 2H), 1.68–1.66
AHCTUNGTRENNUNG
(62.9 MHz, CDCl3): d=136.8, 132.7, 131.2, 131.0, 129.5, 47.0,
33.3, 26.1, 25.7, 21.0; MS: m/e=206 [M+]; anal. calcd. for
C13H18S: C 75.68, H 8.79, S 15.54; found: C 75.64, H 8.85, S
15.52%.
3i: 1H NMR (250 MHz, CDCl3): d=8.05–7.99 (m, 2H),
7.29–7.23 (m, 2H),3.32–3.24 (m, 1H), 1.99–1.95 (m, 2H),
1.74–1.70 (m, 2H), 1.70–1.27 (m, 6H); 13C NMR (62.9 MHz,
CDCl3): d=145.9, 144.0, 126.5, 122.8, 43.7, 31.8, 24.8, 24.5;
anal. calcd. for C12H15NO2S: C 60.73, H 6.37, N 5.90, S
13.51; found: C 60.76, H 6.32, N 5.95, S 13.47%.
Experimental Section
General Procedure for Thioetherification of Aryl
Iodides
3j: 1H NMR (250 MHz, CDCl3): d=7.32–7.26 (m, 2H),
6.79–6.73 (m, 2H), 3.73 (s, 3H), 3.37–3.32 (m, 1H), 1.90–
1.86 (m, 2H), 1.70–1.68 (m, 2H), 1.56–1.46 (m, 4H);
13C NMR (62.9 MHz, CDCl3): d=158.9, 138.1, 134.1, 116.3,
114.3, 55.2, 47.9, 33.4, 30.5, 24.6; anal. calcd. for C12H16OS:
C 69.19, H 7.74, S 15.39; found: C 69.17, H 7.78, S 15.37.
A three-necked flask was purged with argon gas and then
was charged with CuI (10 mg, 0.05 mmol), potassium car-
bonate (552 mg, 4.0 mmol), thiourea (91 mg, 1.2 mmol), aryl
iodied (1 mmol), alkyl bromide (1.1 mmol), water
(0.05 mL,), PEG (3 mL) and the mixture was magnetically
stirred and heated at 808C for the appropriate reaction time
122
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One-Pot Thioetherification of Aryl Halides Using Thiourea and Alkyl Bromides
3k: 1H NMR (250 MHz, CDCl3): d=8.06–8.01 (m, 2H),
7.28–7.23 (m,2H), 3.70–3.65 (m, 1H), 2.13–2.10 (m, 2H),
1.74–1.58 (m, 6H); 13C NMR (62.9 MHz, CDCl3): d=147.8,
145.8, 125.4, 122.8, 43.3, 32.3, 23.9; MS: m/e=223 [M+];
anal. calcd. for C11H13NO2S: C 59.17, H 5.87, N 6.27, S
14.36; found: C 59.22, H 5.86, N 6.26, S 14.38%.
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1
3l: H NMR (250 MHz, CDCl3): d=8.96 (s, 1H), 8.63 (s,
2H), 3.59–3.48 (m, 1H), 1.75–1.70 (m, 2H), 1.61–1.51 (m,
6H); 13C NMR (62.9 MHz, CDCl3): d=157.2, 155.7, 133.4,
45.7, 33.7, 24.6; MS: m/e=180 [M+]; anal. calcd. for
C9H12N2S: C 59.97, H 6.71, N 15.54, S 17.78; found: C 59.95,
H 6.68, N 15.61, S 17.77%.
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1
3m: H NMR (250 MHz, CDCl3): d=8.03–7.96 (m, 2H),
7.23–7.16 (m, 2H), 2.89 (t, J=7.5 Hz, 2H), 1.64–1.58 (m,
2H), 1.35–1.17 (m, 10H), 0.80–0.75 (m, 3H); 13C NMR
(62.9 MHz, CDCl3): d=148.2, 144.7, 125.8, 123.8, 31.8, 31.7,
29.1, 29.0, 28.8, 28.4, 22.6, 14.0; MS: m/e=267 [M+]; anal.
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found: C 62.85, H 7.94; N 5.29, S 11.94%.
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3n: H NMR (250 MHz, CDCl3): d=9.28 (s, 1H), 8.6 (s,
2H), 2.88 (t, J=7.5 Hz, 2H), 1.61–1.53 (m, 2H), 1.35–1.20
(m, 10H), 0.81–0.77 (m, 3H); 13C NMR (62.9 MHz, CDCl3):
d=156.1, 155.5, 33.4, 31.6, 29.0, 28.9, 28.5, 22.5, 14.0; MS:
m/e=224 [M+]; anal. calcd. for C12H20N2S: C 64.24, H 8.98,
N 12.49, S 14.29; found: C 64.17, H 8.97, N 12.56, S 14.32%.
3o: 1H NMR (250 MHz, CDCl3): d=8.10–8.06 (d, J=
8.80 Hz, 2H), 7.60–7.57 (d, J=7.75 Hz, 2H), 1.28 (s, 9H);
13C NMR (62.9 MHz, CDCl3): d=147.6, 142.2, 136.8, 123.2,
47.5, 31.0; anal. calcd. for C10H13NO2S: C 56.85, H 6.20, N
6.63, S 15.17; found: C 56.90, H 6.18, N 6.58, S 15.23%.
Acknowledgements
We gratefully acknowledge the support of this work by the
Shiraz University Research Council and TWAS Chapter of
Iran based at ISMO. We are also grateful to Mr. H. Sajedian
Fard for NMR spectra.
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