The first one-pot oxidative 1,2-acetoxysulfenylation and 1,2-disulfenylation of Baylis-Hillman alcohols in an ionic liquid

Article abstract of DOI:10.1016/j.tetlet.2009.04.030

The first example of tandem oxidation and 1,2-acetoxysulfenylation/1,2-disulfenylation of Baylis-Hillman (BH) alcohols to afford 1,2-acetoxysulfides/1,2-dithioethers is reported. The reaction involves oxidation of BH alcohols with IBX in [bmim]Br to give

Full text of DOI:10.1016/j.tetlet.2009.04.030

Tetrahedron Letters 50 (2009) 3801–3804
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The first one-pot oxidative 1,2-acetoxysulfenylation and 1,2-disulfenylation
of Baylis–Hillman alcohols in an ionic liquid
*
Lal Dhar S. Yadav , Chhama Awasthi
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first example of tandem oxidation and 1,2-acetoxysulfenylation/1,2-disulfenylation of Baylis–Hillman
(BH) alcohols to afford 1,2-acetoxysulfides/1,2-dithioethers is reported. The reaction involves oxidation of
BH alcohols with IBX in [bmim]Br to give b-ketomethylene compounds in situ followed by CuI-imidazole-
catalyzed 1,2-acetoxysulfenylation with an organodisulfide and acetic acid under air to afford vicinal
acetoxysulfides in excellent yields with complete regioselectivity. In the absence of the Cu(I) catalyst,
1,2-disulfenylation takes place to give vicinal dithioethers in 81–90% yields.
Received 7 March 2009
Revised 1 April 2009
Accepted 3 April 2009
Available online 14 April 2009
Keywords:
Baylis–Hillman adducts
Ionic liquids
Ó 2009 Elsevier Ltd. All rights reserved.
Hypervalent iodine
Oxidation
Sulfides
Regioselective synthesis
Improving the efficiency of organic synthesis including
minimizing the energy, cost, and chemical waste is the major part
of current research area in organic chemistry. In this regard one-
pot sequential multistep reactions are of increasing academic, eco-
nomical, and ecological interest for the synthesis of new chemical
entities with diverse functionalities.1 Introduction of sulfur func-
tionality into unsaturated organic molecules is an important pro-
cess for the drug development and other fine chemical
synthesis.2 Transition metal-catalyzed introduction of a sulfide
group to an unsaturated C–C bond is an important procedure in
synthetic organic chemistry.3
1,2-Hydroxysulfides are versatile building blocks for the syn-
thesis of higher functionalized organic molecules,4 namely, thioke-
tones,5 allyl alcohols,6 cyclic sulfides,7 benzothiazepines8, and
benzoxathiepines.9 They exhibit great synthetic utility in the field
of pharmaceuticals10 and natural products,11 particularly for the
synthesis of leukotrienes (LTs) such as LTC4 and LTD4. These com-
pounds have been synthesized by ring opening of epoxides,12 and
thiol-oxygen co-oxidation (TOCO) reactions of olefins,13 but the
methods have various disadvantages such as poor regioselectivity,
lower yields, and undesirable side products. However, methods for
a transition metal-catalyzed preparation of 1,2-hydroxysulfides
using olefin and disulfide are very limited,14 which has been the
main driving force for the present investigation.
The addition of disulfides to olefins is a simple and reliable way
for the synthesis of various sulfur-containing compounds. Previous
reports on this type of reactions revealed the utility of mild acid
catalysts, either in a stoichiometric or a catalytic amount, which
includes BF3ÁOMe2,15 PhIO–TfOH,16 and GaCl3
or a catalytic
17
amount of transition metal complexes such as [CpRuCl(Cod)].18
Recent times have witnessed a considerable upsurge of interest
in the application of Baylis–Hillman (BH) adducts in organic syn-
thesis.19 The three chemospecific groups in BH adducts could be
appropriately tailored to generate an array of chemically and phar-
maceutically important molecules.20 Similarly, among hypervalent
iodine reagents,21 2-iodoxybenzoic acid (IBX) has become
a
reagent of choice due to its easy handling, ready availability, toler-
ance to moisture,22 mild reaction conditions, zero toxic waste gen-
eration, and selective oxidation of alcohols.23 Ionic liquids (ILs)
have recently gained recognition as possible alternative solvents
in various chemical processes because of their many fascinating
properties. Although ILs are still significantly greener than volatile
organic solvents, there are environmental issues with their biode-
gradability and toxicity, hence recently considerable attention has
been paid in this regard.24 Moreover, ILs are simple, easy to recycle,
inexpensive to prepare, and their properties can be fine tuned by
changing the anion or the alkyl group attached to cation.24
Prompted by the above reports and pursuing our work on the
development of new one-pot synthetic methodologies,25 especially
using BH adducts,25c–e we devised a novel one-pot oxidative 1,2-
acetoxysulfenylation/disulfenylation of BH adducts to form 1,2-
acetoxysulfides/1,2-dithioethers in excellent yields. Our initial
* Corresponding author. Tel.: +91 5322500652; fax: +91 5322460533.
E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.030

3802
Lal Dhar S. Yadav, C. Awasthi / Tetrahedron Letters 50 (2009) 3801–3804
Table 1
Table 2
Optimization of Cu-catalyst for the synthesis of 1,2-acetoxysulfide 4a (R1 = H, R2 = Ph)
One-pot oxidative synthesis of 1,2-acetoxysulfides 4 (Scheme 1)
Entry
Cu-catalyst
Catalyst loading (mol %)
Reaction timea (h)
Yieldb (%)
Product 4
R1
Organic disulfide 3
Reaction timea (h)
Yieldb,c (%)
1
2
3
4
5
6
7
8
CuBr
CuI
CuI
CuI
CuCl
CuCl2
CuOAc
Cu(OAc)2
5
2
5
10
5
5
10
7
7
48
80
90
90
7
10
11
10
a
b
c
d
e
f
H
OMe
Cl
NO2
H
OMe
Cl
PhS–SPh
PhS–SPh
PhS–SPh
PhS–SPh
n-C3H7S–SC3H7-n
n-C3H7S–SC3H7-n
n-C3H7S–SC3H7-n
n-C3H7S–SC3H7-n
7
7
8
8.5
8.5
8
8.5
9
90
93
89
86
87
89
85
83
7
10
10
10
10
5
5
g
h
NO2
a
a
Stirring time at 50 °C (Scheme 1).26
Yield of isolated and purified product 4a.
Stirring time at 50 °C.
Yield of pure product 4 after column chromatography.
All compounds gave C, H, and N analyses within 0.37% and satisfactory spec-
b
b
c
tral (IR, 1H NMR, 13C NMR, and EIMS) data.
efforts to probe the desired oxidative acetoxysulfenylation of BH
alcohols 1 were focused on the reactivity of combined CuI/imidaz-
ole/(R2S)2 system for transfer of the equivalent of RS+ to double
bond, which is the active species for sulfenylation. The reaction
was performed in open air and it did not take place under nitrogen
atmosphere, that is, oxygen is necessary for the present reaction
conditions.
In order to optimize Cu-catalyst, a brief study of several readily
available copper salts was carried out. Different copper-catalysts
such as CuCl, CuCl2, CuI, CuOAc, Cu(OAc)2, and CuBr were investi-
gated in the presence of imidazole. Among these CuCl, CuCl2,
CuOAc, and Cu(OAc)2 gave no appreciable effect on the reaction
as they afforded only poor yields (7–10%) of product 4a (Table 1,
entries 5–8). However, CuBr was found to be active, but the yield
of 4a was moderate (Table 1, entry 1). CuI gave the best results
(Table 1, entries 2–4) among all the copper salts. Furthermore,
for optimization of catalyst loading we carried out the reaction
by using 2, 5, and 10 mol % of CuI and the excellent yield of 4a
was obtained with 5 mol %. No improvement in the yield was
noticed on increasing the catalyst loading from 5 mol % to
10 mol % (Table 1, entries 3 and 4). Hence, we utilized 5 mol % of
CuI/imidazole catalyst system in our experiments. Interestingly,
in the absence of copper-catalyst, 1,2-dithioethers 5 were exclu-
sively obtained instead of 1,2-acetoxysulfides 4.
of small amount of water (0.5 mL) and did not give satisfactory
yields of 4. Thus, the present optimized procedure involves stirring
of BH alcohols 1 with IBX in [bmim]Br at rt for 1–2 h followed by
the addition of 5 mol % CuI-imidazole, AcOH, and organic disulfide
3 in the same reaction vessel and stirring at 50 °C for 7–9 h to
afford the corresponding hitherto unknown 1,2-acetoxysulfides 4
in 83–93% yields (Table 2) with complete regioselectivity.26
A plausible mechanism for the formation of 1,2-acetoxysulfide
4 is depicted in Scheme 2. The CuI-catalyst 6 reacts with disulfide
3 to furnish the sulfonium species 7 which transfers the electro-
phile R2S+ to BH olefin 2 leading to the final product 4 via three-
membered cyclic sulfonium ion 8. The intermediate R2SCu(I)Ln
9
formed in the reaction cannot sulfenylate 2 as such, but it is recy-
cled to disulfide 3 under acidic conditions in air to establish the
sulfenylation cycle.14b Thus, both the organosulfide groups of
disulfide are utilized in the 1,2-acetoxysulfenylation (Scheme 2).
After a successful attempt at the synthesis of 1,2-acetoxysul-
fides 4, we tried one-pot oxidative disulfenylation of BH alcohols
1 using IBX followed by organic disulfides 3 in [bmim]Br (Scheme
1). The reaction worked well in the absence of the Cu-imidazole
catalyst system. Probably, the acidic nature of [bmim]Br promotes
the reaction (Scheme 3, structure 10). The procedure involves the
oxidation of BH alcohol 1 with IBX in [bmim]Br followed by the
addition of organic disulfide 3 in the same reaction vessel and stir-
ring at rt for 10–12 h to afford dithioether 5 in excellent yields
(Table 3).27 When we used acetic acid with organic disulfide 3 in
the reaction, only dithioether 5 was obtained instead of 1,2-acet-
oxysulfide 4. The reason behind it could be the more nucleophilic-
ity of R2SÀ than AcOÀ. It shows the importance of copper-catalyst
in 1,2-acetoxysulfenylation of 2, where R2SÀ is readily converted
into R2SSR2 3 in the presence of air and AcOÀ is available as the
only effective nucleophile to afford 4 (Scheme 2).
The presence of a ligand is prerequisite for the success of the
present reaction, thus several amine ligands were investigated,
and it was found that the bulky ligand DABCO was less effective
than
L-proline or imidazole.
However, imidazole was more effective than
L-proline, therefore,
we opted imidazole as the ligand in the present experiments.26
Next, the effect of ILs was examined. We tested the present
reaction in six different ILs, 1-butyl-3-methylimidazolium (bmim)
tetrafluoroborate ([bmim]BF4), [bmim]PF6, [bmim]Br, butylpyridi-
nium (bpy) tetrafluoroborate (bpyBF4), bpyPF6, bpyBr. Among
these ILs, [bmim]Br dissolved IBX at rt and gave the best result
in the one-pot oxidative 1,2-acetoxysulfenylation and 1,2-disulfe-
nylation leading to 4 and 5 (Scheme 1). The other five ILs tested
did not dissolve IBX even when heated to 80 °C in the presence
A plausible mechanistic pathway for the formation of dithio-
ethers 5 is depicted in Scheme 3. Due to the acidity of C-2 proton
of imidazolium cation,28 its hydrogen bond interaction with the
sulfur of disulfide 3 increases electrophilicity of the disulfide
(structure 10). This facilitates the transfer of R2S+ species to the
CuI-imidazole
O
SR2
CN
R2SSR2
3
OH
O
R1
OAc
AcOH, air
50 oC, 7-9 h
4
CN
CN
IBX
O
[bmim]Br, rt, 1-2 h
SR2
CN
R1
R1
R2SSR2
1
2
3
rt, 10-12 h
R1
SR2
5
Scheme 1. One-pot oxidative 1,2-acetoxysulfenylation and 1,2-disulfenylation of BH alcohols 1.

Lal Dhar S. Yadav, C. Awasthi / Tetrahedron Letters 50 (2009) 3801–3804
3803
O
O
CN
S R2
CN
R1
R1
2
8
AcOH
[bmim]Br
rt, 1-2 h
IBX
O
OH
SR2
CN
CN
R1
OAc
R2S SR2
R2SCu(I)Ln
9
R1
4
1
Cu
H, O2 (air)
7
(Ln = imidazole)
Cu(I)Ln
3
6
(R2S)2
3
Scheme 2. Plausible mechanism for the synthesis of 1,2-acetoxysulfides 4 from BH alcohols 1.
O
CN
Br
Br
N
N
N
N
S
R1
2
R2SSR2
-R2S
H
3
S
R2
R2
10
O
O
O
SR2
CN
CN
S R2
CN
S R2
R1
SR2
R1
5
R1
8
SR2
Scheme 3. Plausible mechanism for the formation of 5.
BH olefin 2 to afford the desired product 5 through cyclic sulfo-
nium ion 8.
organosulfide groups of disulfide. The present methodology opens
up a new aspect of the synthetic utility of BH adducts.
The requisite BH alcohols and ILs were prepared by employing
known methods.29–31 After isolation of products 4 and 5, the IL
[bmim]Br could be recycled for four times with up to 76% recovery
and reused without any loss of efficiency.26,27
In summary, we have documented, for the first time, a one-pot
oxidative 1,2-acetoxysulfenylation and 1,2-disulfenylation reac-
tions as a novel entry to vicinal acetoxysulfides/dithioethers
directly from BH alcohols in the IL [bmim]Br. This reaction regiose-
lectively affords the product and enables the use of both
Acknowledgment
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.04.030.
References and notes
Table 3
One-pot oxidative synthesis of 1,2-dithioethers 5 (Scheme 1)
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Product 5
R1
Organic disulfide 3
Reaction time (h)a
Yieldb,c (%)
a
b
c
d
e
f
H
OMe
Cl
NO2
H
OMe
Cl
PhS–SPh
PhS–SPh
PhS–SPh
PhS–SPh
n-C3H7S–SC3H7-n
n-C3H7S–SC3H7-n
n-C3H7S–SC3H7-n
n-C3H7S–SC3H7-n
11
10
10.5
12
11.5
11
88
90
87
85
86
87
83
81
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158 °C. IR (KBr) mmax 3055, 2992, 2858, 2242, 1743, 1698, 1583, 1451, 760,
697 cmÀ1 1H NMR (400 MHz; CDCl3/TMS): d 2.08 (s, 3H, OCOCH3), 4.72 (d, 1H,
.
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122.3, 124.8, 126.0, 127.5, 128.6, 133.6, 137.7, 139.4, 174.3, 198.9. EIMS (m/z):
325 (M+). Anal. Calcd for C18H15NO3S: C, 66.50; H, 4.65; N, 4.30. Found: C,
66.80; H, 5.02; N, 3.98. Compound 4e: yellowish solid, yield 87%, mp 120–
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.
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2005, 46, 7507; (h) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002,
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698 cmÀ1 1H NMR (400 MHz; CDCl3/TMS): d 3.33 (d, 1H, J = 12.2 Hz, b-Ha),
.
3.79 (d, 1H, J = 12.2 Hz, b-Hb), 7.09–7.66 (m, 13Harom), 7.99 (t, 2H, J = 7.6 Hz,
Harom). 13C NMR (100 MHz; CDCl3/TMS): d 38.5, 62.1, 116.9, 123.3, 123.7, 124.8,
125.2, 126.0, 126.6, 127.5, 128.6, 129.3, 133.7, 137.8, 138.9, 198.8. C22H17NOS2:
C, 70.45; H, 4.56; N, 3.73. Found: C, 70.07; H, 4.28; N, 3.91. Compound 5e:
yellowish solid, yield 86%, mp 133–135 °C. IR (KBr) mmax 3051, 2988, 2856,
2238, 1697, 1580, 1451, 758, 697 cmÀ1 1H NMR (400 MHz; CDCl3/TMS): d 0.83
.
(t, 3H, J = 7.1 Hz, CH3), 0.89 (t, 3H, J = 7.1 Hz, CH3), 1.38–1.43 (m, 4H, 2 Â CH2),
2.39 (t, 2H, J = 7.2 Hz, CH2), 2.47 (t, 2H, J = 7.2 Hz, CH2), 3.18 (d, 1H, J = 12.2 Hz,
b-Ha), 3.41 (d, 1H, J = 12.2 Hz, b-Hb), 7.52 (t, 1H, J = 7.7 Hz, Harom), 7.63 (t, 2H,
J = 7.4 Hz, Harom), 7.96 (t, 2H, J = 7.6 Hz, Harom). 13C NMR (100 MHz; CDCl3/TMS)
d: 15.1, 15.4, 22.4, 23.1, 32.0, 33.3, 35.8, 51.4, 116.8, 127.9, 128.5, 133.4, 138.8,
198.7. EIMS (m/z): 307 (M+). Anal. Calcd for C16H21NOS2: C, 62.57; H, 6.89; N,
4.56. Found: C, 62.95; H, 6.54; N, 4.25.
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26. General procedure for the synthesis of 1,2-acetoxysulfides 4:
A mixture of
[bmim]Br (3 mL), water (0.5 mL), and IBX (1 mmol) was stirred at rt for 5–
10 min followed by addition of BH alcohol 1 (1 mmol) and stirring at rt for 1–
2 h. After complete oxidation (monitored by TLC), CuI (0.05 mmol), imidazole
(0.05 mmol), organodisulfide 3, (0.5 mmol) and acetic acid (0.7 mL) were
added and the reaction mixture was stirred for a further 7–9 h at 50 °C under
air (Table 2). Then, it was diluted with saturated NaHCO3 solution (10 mL) and
extracted with ether (3 Â 10 mL). The combined organic layers were washed
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