Phosphonium ionic liquid-catalyzed michael addition of mercaptans to,-Unsaturated ketones

Article abstract of DOI:10.1080/00397910903221050

A clean and efficient Michael addition reaction on chalcones using phosphonium ionic liquid catalyst (PhosIL-Cl) is described. The method provides several advantages, such as simple workup, environmental friendliness, mild conditions, and excellent yields

Full text of DOI:10.1080/00397910903221050

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Phosphonium Ionic Liquid–Catalyzed
Michael Addition of Mercaptans to α,β-
Unsaturated Ketones
Swapnil R. Sarda a , Wamanrao N. Jadhav b , Amit S. Shete b ,
Kiran B. Dhopte b , Sachin M. Sadawarte c , Prashant J. Gadge c &
Rajendra P. Pawar d
a Department of Chemistry, J. E. S. College, Jalna, India
b Organic Chemistry Synthesis Laboratory, Dnyanopasak College,
Parbhani, India
c Department of Biotechnology, MGM College of CSIT, Parbhani, India
d Department of Chemistry, Deogiri College, Aurangabad, India
Published online: 17 Jun 2010.
To cite this article: Swapnil R. Sarda , Wamanrao N. Jadhav , Amit S. Shete , Kiran B. Dhopte , Sachin
M. Sadawarte , Prashant J. Gadge & Rajendra P. Pawar (2010) Phosphonium Ionic Liquid–Catalyzed
Michael Addition of Mercaptans to α,β-Unsaturated Ketones, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:14, 2178-2184,
DOI: 10.1080/00397910903221050
To link to this article: http://dx.doi.org/10.1080/00397910903221050
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Synthetic Communications1, 40: 2178–2184, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903221050
PHOSPHONIUM IONIC LIQUID–CATALYZED MICHAEL
ADDITION OF MERCAPTANS TO a,b-UNSATURATED
KETONES
Swapnil R. Sarda,1 Wamanrao N. Jadhav,2 Amit S. Shete,2
Kiran B. Dhopte,2 Sachin M. Sadawarte,3 Prashant J. Gadge,3
and Rajendra P. Pawar4
1Department of Chemistry, J. E. S. College, Jalna, India
2Organic Chemistry Synthesis Laboratory, Dnyanopasak College, Parbhani,
India
3Department of Biotechnology, MGM College of CSIT, Parbhani, India
4Department of Chemistry, Deogiri College, Aurangabad, India
A clean and efficient Michael addition reaction on chalcones using phosphonium ionic liquid
catalyst (PhosIL-Cl) is described. The method provides several advantages, such as simple
workup, environmental friendliness, mild conditions, and excellent yields. In addition, the
ionic liquid was chosen as a green solvent, recovered, and reused several times in subsequent
reactions.
Keywords: Chalcones; Michael addition; phosphonium ionic liquids; thiol
INTRODUCTION
Environmental pressure to reduce waste and reuse materials has driven studies
in green chemistry. Ionic liquids (ILs) have recently emerged as an alternative to
organic solvents in traditional chemical reactions, mainly because of their stability,
insolubility in organic solvents, low volatility, and easy recyclability.[1] These advan-
tages render the use of ILs more ecofriendly than the organic solvents. Phosphonium
ILs have attracted growing interest in the past few years because of their negligible
vapor pressure, high thermal capacity, and wide liquid range.
Michael addition is one of the most important carbon–carbon and carbon–
heteroatom bond-forming reaction in organic synthesis.[2] The addition of thiols to
electron-deficient alkenes is a very useful process for making carbon–sulfur bonds.
1,4-Addition of thiols to a,b-unsaturated carbonyl compounds to form carbon–
sulfur bonds constitutes a key reaction in various biosynthetic processes as well as
in organic synthesis.
Received May 7, 2009.
Address correspondence to Rajendra Pundlikrao Pawar, Department of Chemistry, Deogiri
College, Aurangabad, India. E-mail: rppawar@yahoo.com
2178

PHOSPHONIUM IONIC LIQUID–CATALYZED MICHAEL ADDITION
2179
Scheme 1. Synthesis of 3-substituted thio-1,3-diphenyl propan-1-one.
Numerous methods have been reported in the literature regarding the
1,4-addition of thiols to electron-deficient olefins activated by different bases.[3] These
reactions were also investigated using Lewis bases[4] and different Lewis acids[5] such
as FeCl3, InCl3, InBr3, Bi(NO3)3, Hf(OTf)3, Bi(OTf)3, Yb(Otf)3, and Cu(BF4)2. Alter-
natively, modified methods had been used for these reactions, using materials and
techniques such as ILs,[6] L-proline,[7] iodine,[8] transition metals, p-TsOH–KSF,
microwaves,[9] ultrasound, heteropoly acids, NH4Cl,[10] fluorapatite,[11] water,[12]
KF=Al2O3,[13] natural phosphate, organocatalysis,[14] MgO, thiourea,[15] and NaH-
SO4 ꢀ SiO2.[16] However, many of these methods suffer from harsh reaction con-
ditions, toxic reagents, prolonged reaction times, poor yields, and low selectivity.
In many cases, the catalyst and excess reagents are not recoverable. With the goal
of avoiding the typical disadvantages resulting from the presence of such catalysts,
a large number of alternative procedures have been developed in the past few years.
Although several modifications have been made to counter these problems, there is a
need to develop better strategies for the Michael addition reaction, which is achieved
using phosphonium ILs as catalysts.
In continuation of our work in IL-catalyzed reaction,[17] herein we describe a
simple and convenient method for the Michael addition of mercaptans to a,b-
unsaturated ketones using tetradecyl (trihexyl) phosphonium chloride ionic liquid
(PhosIL-Cl) in excellent yields and short reaction time at room temperature
(Scheme 1).
RESULTS AND DISCUSSION
Phosphonium-based ILs are highly basic[18] and have the generic formula
[PR3R0] X, where both R and R0 are alkyl groups and X is a halide.[19] A large radius
and polarizable lone pair make them more nucleophilic. The structural formula of
PhosIL-Cl is shown in Fig. 1.
Figure 1. Structure of tetradecyl-(trihexyl)-phosphonium chloride ionic liquid [PhosIL-Cl].

2180
S. R. SARDA ET AL.
Table 1. Michael addition of thiol on chalcones using phosphonium ionic liquids
Products
Chalcones
Thiol
Time (h)
Mp (ꢁC)
Yield (%)a
3a
2.5
111–112
90
3b
3c
2.5
3
64–67
Oil
85
88
3d
3e
3f
2.5
3
105–106
123–124
107–108
90
85
88
3
3g
3.5
121–122
80
3h
3i
2.5
2.5
3
Oil
90
88
85
88
98–100
160–162
116–117
3j
3k
3
3l
3.5
128–130
82
aIsolated and unoptimized yield.

PHOSPHONIUM IONIC LIQUID–CATALYZED MICHAEL ADDITION
2181
Table 2. Performance of recycled phosphonium ionic liquids
Yield (%) of PhosIL-Cl
Entry
Products
Recycle 1
Recycle 2
Recycle 3
1
2
3a
3b
97
95
96
95
96
94
Phosphonium salts are more thermally stable than ammonium salts.
Phosphonium-based ILs have greater viscosities than the ammonium counterparts,
at room temperature. However, on heating at 70–100 ꢁC, their viscosities generally
decrease, and the addition of reactants or catalysts can also further reduce the
viscosity. An important difference between imidazolium and phosphonium ILs is
the acidic protons present in the former. Compared to phosphonium cations,
imidazolium cations are not entirely inert and interact with solutes either through
hydrogen bonding or through the aromatic nature of ring system. Tetralkylphospho-
nium salts do not have such acidic protons or aromatic rings; consequently, they
have less potential for interaction with solutes. Recently, phosphonium ILs were
used[20] in the degradation of phenol, esterification, Wittig reaction, Heck reactions,
and Suzuki cross-coupling reactions.
In a model condensation reaction, thiol and chalcone in tetradecyl (trihexyl)
phosphonium chloride IL were stirred at room temperature. After completion of
reaction as monitored by thin-layer chromatography (TLC), the usual workup
afforded pure products (Table 1). The reaction proceeded cleanly at room tempera-
tures. After completion of the reaction, addition of water and hexane to the reaction
mixture resulted in the formation of a three-phase system, with the organic layer at
the top, IL in the middle, and aqueous layer at the bottom. The products were iso-
lated from the organic layer by extraction=decantation. Isolation of product was
easy because phosphonium ILs have remarkable solvent properties. Phosphonium
IL was recovered and reused several times in subsequent reactions with negligible
change in its efficiency (Table 2).
EXPERIMENTAL
Materials and Methods
Melting points were measured in open glass capillaries on a Perfit Electro-
thermal melting-point apparatus and are uncorrected. 1H NMR spectra were
recorded at room temperature on a 200-MHz Varian Inova Spectrometer in CDCl3
using tetramethylsilane (TMS) as internal standard. Infrared (IR) spectra (using KBr
pellets) were obtained from a Varian 640 Fourier transform (FT)–IR instrument.
The reactions were monitored on TLC using precoated plates (silica gel on alumi-
num, Merck). Column chromatography was performed using Acme silica gel
(100–200 mesh). All reagents were obtained from commercial sources and used
without further purification. Solvents for chromatography were distilled before
use. The products were also characterized by comparison of their melting points with
literature values.

2182
S. R. SARDA ET AL.
General Experimental Procedure
A mixture of chalcone (1 mmol) and thiol (1 mmol) in 2 ml PhosIL-Cl was
stirred at room temperature for an appropriate time. After completion of the reac-
tion as monitored by TLC, water and hexane (5 þ 5 ml) were added to the reaction
mixture, which resulted in the formation of a three-phase system: organic layer on
the top, IL in the middle, and aqueous layer at the bottom. The obtain product
(3a–l) was isolated from the organic layer by successive extractions (3 ꢂ 10 ml) using
hexane=ethyl acetate (9.5:0.5). The IL was washed with water and hexane, dried, and
reused several times to carry out the same experiment. The crude product was
purified by column chromatography using hexane=ethyl acetate (8:2) as an eluent
and characterized by comparison of IR, 1H NMR, and melting point with literature
values (Table 1).
Spectral Data of Selected Compounds
1,3-Diphenyl-3-phenylsulfenylpropan-1-one (3a). IR (KBr) n: 1680 cmꢃ1
;
1H NMR (300 MHz CDCl3) d: 3.85 (H, dd, CH2), 4.05 (H, dd, CH2), 5.20 (H, t,
CH), 7.20–7.80 (13H, m, ArH), 7.9 (2H, d, ArH); MS (m=z): 318 (Mþ): 205, 109.
3-(4-Chlorophenyl)-1-phenyl-3-phenylsulfenylpropan-1-one
(3b). IR
1
(KBr) n: 1730 cmꢃ1; H NMR (300 MHz CDCl3) d: 3.65 (2H, m CH2, m), 4.97
(H, t, CH), 7.15–7.55 (12H, m, ArH), 7.9 (2H, d, ArH); MS (m=z): 353 (Mþ):
241.81, 109.
CONCLUSION
In summary, a simple and efficient process has been reported using recyclable
phosphonium IL. The selectivity of reaction makes this method more attractive and
useful than the present methodologies.
ACKNOWLEDGMENTS
The authors are thankful to Dr. P. L. More, Dnyanopasak College, Parbhani,
and Dr. R. S. Agrawal, J. E. S. College, Jalna, for encouragement during the process
of carrying out this work. The authors are also thankful to the University Grants
Commission for funding under the Minor Research Project program.
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