Silicon-based thiourea-mediated and microwave-assisted thio-Michael addition under solvent-free reaction conditions

Article abstract of DOI:10.2478/s11696-011-0052-z

Silicon-based thiourea (SiliaBond Thiourea) (Si-THU), a heterogeneous catalyst, has been applied to the highly selective C-S bond formation via Michael addition of thiols to a,β-unsaturated carbonyl compounds under solvent-free conditions at 55-60°C. The

Full text of DOI:10.2478/s11696-011-0052-z

Chemical Papers 65 (5) 707–713 (2011)
DOI: 10.2478/s11696-011-0052-z
ORIGINAL PAPER
Silicon-based thiourea-mediated and microwave-assisted
thio-Michael addition under solvent-free reaction conditions
a
b
b
aKamalakannan Prabakaran, Machindra Gund, Tae Kyu Kim, Euh Duck
Jeong, Chae Young Oh, a,bFazlur-Rahman Nawaz Khan*, Jong Sung Jin*
b
b
aOrganic & Medicinal Chemistry Research Laboratory, Organic Chemistry Division, School of Advanced Sciences,
VIT University, Vellore, Tamil Nadu 632 014, India
bDivision of High Technology Materials Research, Busan Centre, Korea Basic Science Institute,
Busan 618 230, Republic of Korea
Received 4 February 2011; Revised 28 March 2011; Accepted 29 March 2011
Silicon-based thiourea (SiliaBondꢀ Thiourea) (Si-THU), a heterogeneous catalyst, has been ap-
plied to the highly selective C—S bond formation via Michael addition of thiols to α,β-unsaturated
carbonyl compounds under solvent-free conditions at 55–60◦C. The thio-Michael addition prod-
ucts were obtained in an excellent yield under optimised conditions. This methodology involving a
metal-free as well as a metal scavenger catalyst has been found to be an alternative method for the
thio-Michael addition reaction.
c
ꢀ 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: SiliaBondꢀ Thiourea, solvent-free Michael reaction, thioethers
Introduction
these methods combines good yield with high selectiv-
ity. In the present article, an efficient catalytic system
is demonstrated which offers quantitative yield, purity,
and also selectivity.
Organosulphur compounds such as thioethers are
versatile intermediates for synthetic transformations
(Kondo & Mitsudo, 2000) to biologically active com-
pounds and are recognised as stimulants of the central
nervous system (Jacobson, 2009). Several procedures
for the synthesis of thioethers are known in the lit-
erature, including: alkylation of thiols (Salvatore et
al., 2005), thiohydrolysis of imidothiolates (Sudalai
et al., 2000), oxidative sulphuration of benzyl halides
(Kanagasabapathy et al., 2001), transesterification of
dithioesters with thiols (Thang et al., 1999), thio-
Michael addition reaction (Barahman & Pershang,
2006), exchange reaction of bis(thiocarbonyl) disul-
phides with azo compounds (Perrier & Tokolpuckdee,
2005), and condensation of thiols and alcohols (Falck
et al., 1999). However, all of these methods suffer from
a nucleophilic regiocontrol problem, they require ei-
ther a very high temperature or the presence of specific
structures on the heterocyclic ring. Similarly, none of
The thio-Michael addition reactions have emerged
as one of the most powerful tools for the C—S bond
formation (Amini et al., 2006). In recent years, several
modifications have been made including various cat-
alysts, solvent-free conditions, etc. (Wight & Davis,
2002; Barahman & Pershang, 2006). Many catalysts
were used in thio-Michael reactions including ionic liq-
uids, amino acids, alumina, iodine, InBr3, HClO4—
SiO2 montmorillonite clays, and silica nanoparticles
(Ranu & Dey, 2004; Kumar & Akanksha, 2007; Firouz-
abadi et al., 2006; Chu et al., 2005; Bandini et al.,
2002; Khatik et al., 2007; Banerjee et al., 2010). How-
ever, these catalysts have been shown to have draw-
backs viz. some ionic liquids liberate HF during re-
cycling and also the disposability of these liquids lim-
its their application. Moreover, some catalysts showed
longer reaction times and high reaction temperatures,
*Corresponding author, e-mail: jsjin@kbsi.re.kr; nawaz f@yahoo.co.in

708
K. Prabakaran et al./Chemical Papers 65 (5) 707–713 (2011)
S
and 3, Tables 1–5) in good yields. The use of a solvent-
free reaction in combination with heterogeneous and
reusable catalysts represents one of the more power-
ful green chemical technology procedures. Si-THU is
widely used industrially as a metals scavenger cata-
lyst, catalyst support and reagent (Robinson & Sny-
der, 1955).
NH
Si
HN
ꢀ
Fig. 1. SiliaBond Thiourea.
were expensive, provided moderate yields, and re-
quired expensive metal precursors. Here we report on
the multi-purpose catalyst SiliaBondꢀ Thiourea (Si-
THU) (Fig. 1) in the solvent-free microwave assisted
thio-Michael reaction.
In order to overcome the drawbacks noted above,
in the present study of thio-Michael addition reaction,
a great deal of effort has been directed towards suc-
cessful exploration of an efficient catalytic system with
high selectivity in the synthesis of thioethers (Figs. 2
Experimental
The Si-THU required for the present investigation
was obtained from SiliCycle Inc. (USA) and used af-
ter oven-drying. The required chemicals for the study
were obtained from Aldrich (India) and are used with-
out further purification. All reactions were carried out
in resealable tubes, in a high-purity nitrogen atmo-
sphere. The progress of the reactions was monitored
by flexible TLC plates of silica gel 60 (Thomas Scien-
CH3
S
CH3
i
R-SH
R
O
O
IIIa−i
Ia−i
IIa
O
O
SH
N N
S
IIIa
S
CH3
N N
CH3
N
SH
S
N
CH3
N
N
CH3
IIIf
If
Ia
O
O
N
N
CH3
CH3
N
S
SH
N
SH
S
S
IIIg
Ib
Ig
IIIb
O
O
SH
S
CH3
SH
S
N
CH3
N
i-Pr
i-Pr
IIIh
Ic
IIIc
Ih
O
N
N
N
N
O
N
S
N
S
N
N
CH3
HS
H3C
S
SH
N
N
S
CH3
CH3
IIIi
Ii
IIId
Id
N
O
N
N
N
S
CH3
SH
IIIe
Ie
Fig. 2. Synthesis of thio-Michael products III from thiols, I using methylvinyl ketone, IIa. i) Si-THU (30 mass %), microwave (300
W), 60◦C, 15 min.

K. Prabakaran et al./Chemical Papers 65 (5) 707–713 (2011)
709
O
O
i
R-SH
n
R
n
S
Id–f,j,k
IIb,c
n = 1, 0
n = 0,1
IIIj−o
n = 1, 0
n = 0,1
O
O
N N
O
N
S
S
N
H
F3C
N
S
F3C
N
SH
SH
S
Ij
IIIo
IIIm
IIIl
O
N N
O
N N
N
N
N
H
S
N
S
CH3
IIIk
IIIn
Ik
O
N
S
S
O
O
IIb
IIIj
IIc
Fig. 3. Synthesis of thio-Michael products, III from thiols, I using cycloalk-2-en-ones, IIb, IIc. i) Si-THU (30 mass %), microwave
(300 W), 60◦C, 15 min.
tific, India). Melting points were measured on Elchem
Microprocessor (BVN instruments, India) based DT
apparatus in open capillary tubes. 1H NMR and
13C NMR spectra of compounds in CDCl3 were ob-
tained using a Bruker Spectrospin Avance DPX400
Ultrashield 400 MHz spectrometer (India) and chem-
ical shifts were reported in δ relative to tetramethyl-
silane (TMS) as an internal standard. IR spectra
were obtained on a Nucon Infrared spectrophotometer
(KBr disc) (India). LC–MS analyses were performed
with LCMS-Agilent-1100 series (India). Ion Trap Mass
spectra were recorded on ABI QSTAR XL ESI-TOF
Mass spectrometer (Sciex Model) (India) operating at
70 eV using fast atom bombardment technique.
Table 1. Effect of catalyst on the thio-Michael reactiona of Ia
and IIa
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
L-proline
L-proline ester
L-glutamic acid
NR
NR
NR
60
50
70
Silica gel (60–120 mesh)
Aluminium oxide (neutral)
Aluminium oxide (basic)
Si-THU
89
73
Si-Thiol
a) Catalyst (50 mass, %), microwave (300 W), 60◦C, 15 min.
NR – no reaction.
General procedure of synthesis of thio ethers
using various enones (IIIa–IIIo)
evaporated under vacuum on a rotary evaporator to
afford the desired product. The compounds were puri-
fied by column chromatography using neutral alumina
eluted with petroleum ether/EtOAc (ϕr = 9 : 1). The
products III obtained were characterised by 1H-NMR,
Thiol, I (1.0 eq) and α,β-unsaturated carbonyl
compound, II (1.2 eq) Si-THU (30 mass %) were
mixed and microwave-irradiated at 60◦C (300 W) for
15 min. After completion of the reaction, dichloromethane 13C-NMR, LC-MS, and FTIR spectral techniques.
(10 mL) was added to the mixture and the mix-
ture was filtered to remove Si-THU. The mixture was
washed with an aqueous solution of NaOH (10 %, 10
mL) and extracted twice with dichloromethane (2 ×
10 mL). The organic layer was separated and dried
over anhydrous Na2SO4 and filtered. The filtrate was
Results and discussion
As part of our ongoing interest in the use of hetero-
geneous catalysts (Prabakaran & Khan, 2010; Khan et
al., 2009a, 2009b, 2010), we have studied thiol-Michael

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K. Prabakaran et al./Chemical Papers 65 (5) 707–713 (2011)
Table 2. Optimisation of Si-THU catalyst concentration in the
thio-Michael reactiona of Ie, IIa, and Id, IIb
reaction in the synthesis of thioethers using Si-THU
as a multi-purpose, heterogeneous and reusable cat-
alyst under solvent-free conditions. The thio-Michael
reaction was carried out simply by mixing thiols I,
enones II, and the heterogeneous catalyst Si-THU (30
mass %) in the absence of solvent followed by irradia-
tion in a microwave device set at 300 W for 15 min at
60◦C. The reaction afforded the desired Michael prod-
ucts III in good yields (Figs. 2 and 3, Tables 1–5).
Optimisation of reaction conditions was carried out
by choosing thio-Michael reaction of Ia with IIa as a
model reaction (Fig. 2, Tables 1 and 4). Optimisation
of the reaction conditions included the examination
of different heterogeneous catalysts as shown in Ta-
ble 1 (Entries 1–8). The Si-THU catalyst was found
to be superior to the other catalysts tested affording
the desired product in good yield (Table 1, entry 7).
Other catalysts such as aluminium oxide (basic) and
SiliaBondꢀ Thiol (Si-Thiol) produced moderate yields
(Table 1, entries 6 and 8) whereas amino acid type cat-
alysts, L-proline, L-proline ester, L-glutamic acid (Ta-
ble 1, entries 1–3) etc. did not participate in the reac-
tion at all and the remaining catalysts were found to
be least effective.
Catalyst
mass %
Time
min
Yield
%
Entry
Ketone
Product
1
2
3
4
5
6
7
8
9
IIa
IIa
IIa
IIa
IIa
IIa
IIb
IIb
IIb
IIb
none
10
20
30
40
50
10
20
30
120
45
25
15
15
15
60
30
15
15
40
62
73
87
88
89
45
69
84
83
IIId
IIId
IIId
IIId
IIId
IIId
IIIj
IIIj
IIIj
IIIj
10
40
a) Microwave (300 W), 60◦C, 15 min.
Table 3. Reusability of Si-THU catalyst and its efficiency
Entry
Number of cycles
Yield (%)
1
2
3
4
5
1
2
3
4
5
89
86
83
78
72
The influence of the amount of catalyst was also in-
vestigated, using the reactions of Ie and IIa, Id and IIb
(Table 2). The result indicated that without catalyst
the reaction was longer with lower yields (Table 2, en-
try 1). An increase in the amount of catalyst increased
the yield and the reaction time decreased substantially
(Table 2, entries 2–10). The optimum amount of cat-
alyst in the reaction was found to be 30 mass %. The
reusability of the catalyst was investigated (Table 3)
by examining successive runs of the reactions using the
recycled catalyst, i.e., catalyst recovered by filtration
from the reaction mixture, washed with ethyl acetate,
and dried. Then it was utilised in the second run of the
reaction. The result showed that the efficiency of the
catalyst remained unaltered up to three cycles with
higher product yields (Table 3, entries 1–3) and a fur-
ther two cycles gave moderate yields (Table 3, entries
4 and 5). Under the optimised conditions, various thio-
Michael products were synthesised and the results are
reported in Tables 4 and 5. The Michael products,
IIIa–IIIo obtained were of high yield (75–90 %) and
purity. The purified products were characterised by
Table 4. Michael reactiona of thiols I and methylvinyl ketone
IIa
Entry
Thiol
Product
Yieldb(%)
1
2
3
4
5
6
7
8
9
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
Ii
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
IIIg
IIIh
IIIi
89
76
86
78
87
78
75
77
75
a) Si-THU (30 mass %), microwave (300 W), 60◦C, 15 min; b)
isolated yield.
1
different spectral techniques including H NMR, 13C
Table 5. Michael reactiona of thiols I and cycloalk-2-en-ones
NMR, IR, and MS techniques (Table 6).
Entry
Thiol
Ketone
Product
Yieldb(%)
Conclusions
1
2
3
4
5
6
Id
Ie
Ij
Ib
If
IIb
IIb
IIb
IIc
IIc
IIc
IIIj
IIIk
IIIl
IIIm
IIIn
IIIo
84
79
82
90
82
84
In the present study, the formation of C—S bond
via Michael addition of thiols to α,β-unsaturated ke-
tones under solvent-free, microwave-assisted condi-
tions was successfully carried out using Si-THU as an
efficient catalyst. The yields and purity of the prod-
ucts were high and the optimisation procedure of the
reaction was very simple and not time-consuming. We
Ik
a) Si-THU (30 mass %), microwave (300 W), 60◦C, 15 min; b)
isolated yield.

K. Prabakaran et al./Chemical Papers 65 (5) 707–713 (2011)
Table 6. Spectral data of newly prepared compounds
711
Compound
Spectral data
IIIa
IR, ν˜/cm−1: 3409, 3056, 2922, 2852, 1711, 1588, 1553, 1492, 1450, 1437, 1402, 1355, 1307, 1264, 1151, 988, 848, 782,
763, 690, 651, 569
1H NMR (CDCl3), δ: 8.17–8.15 (d, J = 8.5 Hz, 3H), 7.84–7.82 (d, J = 5.8 Hz, 2H), 7.70–7.65 (t, J = 6.9 Hz, 1H),
7.56–7.49 (m, 3H), 7.44–7.42 (t, J = 6.0 Hz, 1H), 3.73–3.69 (t, J = 6.8 Hz, 2H), 3.11–3.08 (t, J = 6.8 Hz, 2H), 2.18
(s, 3H)
13C NMR (CDCl3), δ: 207.2, 158.5, 149.3, 139.2, 136.4, 130.6, 128.7, 128.5, 128.3, 127.7, 127.5, 126.8, 126.5, 124.8,
124.4, 112.8, 43.5, 30.1, 23.5
MS, m/z: 308.2 (M+ + 1)
IIIb
IIIc
IIId
IIIe
IIIf
IR, ν˜/cm−1: 3474, 3408, 2991, 2938, 1706, 1487, 1422, 1362, 1312, 1289, 1224, 1154, 1033, 989, 897, 739, 538
1H NMR (CDCl3), δ: 4.22–4.18 (t, J = 7.8 Hz, 2H), 3.95–3.91 (t, J = 6.1 Hz, 2H), 3.28–3.24 (t, J = 8.0 Hz, 2H),
3.00–2.97 (t, J = 6.1 Hz, 2H), 2.19 (s, 3H)
13C NMR (CDCl3), δ: 206.9, 196.8, 58.5, 44.0, 40.6, 30.0, 27.9
MS, m/z: 190.0 (M+ + 1)
IR, ν˜/cm−1: 3411, 3070, 3017, 2959, 2929, 2870, 1713, 1495, 1407, 1359, 1155, 1096, 1050, 1013, 826, 540
1H NMR (CDCl3), δ: 7.30–7.27 (t, J = 8.1 Hz, 2H), 7.17–7.15 (d, J = 8.2 Hz, 2H), 3.12–3.08 (t, J = 7.4 Hz, 2H),
2.92–2.85 (q, 1H), 2.77–2.73 (t, J = 7.3 Hz, 2H), 2.15 (s, 3H), 1.25–1.23 (d, J = 6.9 Hz, 6H)
13C NMR (CDCl3), δ: 206.7, 147.5, 132.1, 130.3, 127.1, 43.2, 33.6, 30.0, 28.1, 23.8
MS, m/z: 223.20 (M+ + 1)
IR, ν˜/cm−1: 3468, 3060, 2922, 1709, 1456, 1425, 1358, 1236, 1157, 1071, 993, 754, 724, 665
1H NMR (CDCl3), δ: 7.87–7.85 (d, J = 7.6 Hz, 1H), 7.77–7.75 (d, J = 8.0 Hz, 1H), 7.44–7.40 (t, J = 7.4 Hz, 1H),
7.32–7.28 (t, J = 8.0 Hz, 1H), 3.57–3.53 (t, J = 6.7 Hz, 2H), 3.09–3.06 (t, J = 6.7 Hz, 2H), 2.2 (s, 3H)
13C NMR (CDCl3), δ: 206.3, 166.5, 153.0, 135.2, 126.0, 124.2, 121.4, 121.0, 43.2, 29.9, 26.8
MS, m/z: 238.0 (M+ + 1)
IR, ν˜/cm−1: 3392, 3039, 2903, 1707, 1562, 1548, 1405, 1378, 1342, 1188, 1156, 954, 899, 809, 739, 627, 575
1H NMR (CDCl3), δ: 8.50–8.49 (d, J = 4.8 Hz, 2H), 6.97–6.95 (t, J = 4.8 Hz, 1H), 3.34–3.30 (t, J = 6.8 Hz, 2H),
2.95–2.92 (t, J = 7.2 Hz, 2H), 2.17(s, 3H)
13C NMR (CDCl3), δ: 206.8, 172.1, 157.2, 116.4, 43.3, 30.0, 24.6
MS, m/z: 183.4 (M+ + 1)
IR, ν˜/cm−1: 3480, 3129, 3057, 2944, 1707, 1544, 1419, 1352, 1221, 1197, 1166, 1108, 1058, 998, 849, 778, 744, 674,
632, 575
1H NMR (CDCl3), δ: 7.74 (s, 1H), 4.46–4.42 (t, J = 8.0 Hz, 2H), 3.57 (s, 3H), 3.04–3.00 (t, J = 6.8 Hz, 2H), 2.20
(s, 3H)
13C NMR (CDCl3), δ: 205.3, 166.4, 139.3, 44.4, 40.9, 32.6, 30.0
MS, m/z: 186.4 (M+ + 1)
IIIg
IIIh
IR, ν˜/cm−1: 3411, 3045, 2994, 2934, 1709, 1577, 1555, 1453, 1413, 1358, 1281, 1152, 1122, 1043, 1009, 985, 757, 719,
617, 575
1H NMR (CDCl3), δ: 8.42–8.40 (d, J = 5.2 Hz, 1H), 7.48–7.44 (td, J = 7.6 Hz, J = 1.6 Hz, 1H), 7.16–7.14 (d, J =
8.0 Hz, 1H), 6.99–6.95 (t, J = 4.8 Hz, 1H), 3.38–3.35 (t, J = 6.8 Hz, 2H), 2.93–2.90 (t, J = 6.8 Hz, 2H), 2.17 (s, 3H)
13C NMR (CDCl3), δ: 207.1, 158.5, 149.4, 135.8, 122.3, 119.3, 43.7, 30.0, 23.6
MS, m/z: 182.4 (M+ + 1)
IR, ν˜/cm−1: 3385, 3035, 2898, 2832, 1708, 1622, 1574, 1538, 1478, 1407, 1359, 1216, 1161, 1108, 1027, 984, 803, 705,
575
1H NMR (CDCl3), δ: 8.40–8.38 (dd, J = 6.2 Hz, J = 1.60 Hz, 2H), 7.11–7.09 (dd, J = 6.2 Hz, J = 1.60 Hz, 2H),
3.22–3.18 (t, J = 7.1 Hz, 2H), 2.86–2.83 (t, J = 7.1 Hz, 2H), 2.19 (s, 3H)
13C NMR (CDCl3), δ: 205.7, 149.3, 124.8, 120.6, 42.1, 30.0, 24.1
MS, m/z: 182.4 (M+ + 1)
IIIi
IIIj
IIIk
IR, ν˜/cm−1: 3585, 3000, 2948, 2901, 2774, 1712, 1453, 1410, 1355, 1310, 1167, 1105, 1004, 925, 851, 786, 740, 680,
575, 531
1H NMR (CDCl3), δ: 4.54–4.51 (t, J = 7.0 Hz, 2H), 3.89 (s, 3H), 3.15–3.12 (t, J = 7.08 Hz, 2H), 2.22 (s, 3H)
13C NMR (CDCl3), δ: 204.3, 164.1, 43.1, 40.2, 34.6, 29.9
MS, m/z: 187.0 (M+ + 1)
IR, ν˜/cm−1: 3071, 2957, 1696, 1583, 1459, 1403, 1346, 1302, 1250, 1217, 1124, 1052, 998, 886, 731, 629, 573
1H NMR (CDCl3), δ: 7.59–7.57 (d, J = 7.8 Hz, 1H), 7.52–7.50 (d, J = 7.4 Hz, 1H), 7.42–7.38 (t, J = 7.2 Hz, 1H),
7.33–7.29 (dt, J = 7.8 Hz, J = 0.8 Hz, 1H), 3.24–3.17 (t, J = 14.0 Hz, 1H), 2.75–1.98 (m, 8H)
13C NMR (CDCl3), δ: 206.6, 139.7, 126.4, 124.5, 121.8, 113.0, 55.4, 42.9, 40.6, 26.9, 26.0, 22.0
MS, m/z: 264.4 (M+ + 1)
IR, ν˜/cm−1: 3406, 3114, 3032, 2940, 2865, 1706, 1561, 1545, 1446, 1421, 1375, 1313, 1187, 1060, 969, 802, 772, 748,
626
1H NMR (CDCl3), δ: 8.51–8.50 (d, J = 4.8 Hz, 2H), 6.99–6.97 (t, J = 4.8 Hz, 1H), 4.22–4.12 (m, 1H), 2.97–1.86
(m, 8H)
13C NMR (CDCl3), δ: 208.6, 171.3, 157.3, 116.7, 47.4, 42.5, 40.9, 30.8, 24.4
MS, m/z: 208.4 (M+ + 1)

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K. Prabakaran et al./Chemical Papers 65 (5) 707–713 (2011)
Table 6. (continued)
Compound
Spectral data
IIIl
IR, ν˜/cm−1: 3060, 2945, 2870, 1710, 1595, 1555, 1472, 1377, 1320, 1220, 1165, 1110, 1072, 1007, 970, 937, 831, 791,
746
1H NMR (CDCl3), δ: 8.70 (s, 1H), 7.70–7.67 (dd, J = 8.4 Hz, J = 1.8 Hz, 1H), 7.26–7.24 (d, J = 8.4 Hz, 1H),
4.35–4.29 (m, 1H), 2.91–2.87 (dd, J = 14.4 Hz, J = 4.6 Hz, 1H), 2.60–1.87 (m, 8H)
13C NMR (CDCl3), δ: 199.8, 162.7, 150.7, 132.8, 129.9, 122.4, 41.9, 40.9, 38.1, 30.9, 25.6, 22.7
MS, m/z: 276.0 (M+ + 1)
IIIm
IIIn
IIIo
IR, ν˜/cm−1: 2935, 1735, 1694, 1471, 1425, 1301, 1223, 1131, 1038, 992, 866, 754, 674, 592
1H NMR (CDCl3), δ: 5.60–5.56 (m, 1H), 4.06–3.99 (m, 2H), 3.35–3.31 (t, J = 7.6 Hz, 2H), 2.71–1.98 (m, 6H)
13C NMR (CDCl3), δ: 213.4, 197.6, 55.0, 51.6, 40.5, 37.7, 27.6, 26.3
MS, m/z: 202.2 (M+ + 1)
IR, ν˜/cm−1: 3452, 3126, 2919, 1735, 1540, 1468, 1417, 1345, 1222, 1146, 1057, 997, 895, 865, 793, 726, 639, 568, 534
1H NMR (CDCl3), δ: 7.78 (s, 1H), 5.60–5.56 (t, J = 6.8 Hz, 1H), 3.60 (s, 3H), 2.68–2.29 (m, 6H)
13C NMR (CDCl3), δ: 214.8, 166.2, 139.5, 55.6, 42.9, 36.5, 32.5, 28.3
MS, m/z: 198.0 (M+ + 1)
IR, ν˜/cm−1: 3136, 3045, 2917, 1739, 1536, 1427, 1394, 1346, 1241, 1137, 1008, 983, 892, 800, 643, 563
1H NMR (CDCl3), δ: 7.83 (s, 1 H), 5.65–5.61 (t, J = 6.8 Hz, 1H), 2.75–2.23 (m, 6H)
13C NMR (CDCl3), δ: 214.7, 165.6, 136.4, 55.3, 42.8, 36.5, 28.4
MS, m/z: 184.0 (M+ + 1)
conclude that an efficient catalytic system has been
demonstrated in the thio-Michael reaction which af-
forded selectivity as well as high yield and purity.
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249–251. DOI: 10.1016/j.molcata.2006.03.043.
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to the design of ligands for G protein-coupled receptors
(GPCRs). Bioconjugate Chemistry, 20, 1816–1835. DOI:
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Khan, F. N., Manivel, P., Prabakaran, K., Hathwar, V. R.,
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Acknowledgements. We gratefully acknowledge financial
support from the Department of Science and Technology, Gov-
ernment of India (Grant No. SR/FTP/CS-99/2006). The au-
thors are grateful to the VIT University management for their
generous support and facilities. We also wish to thank the
SAIF, IIT Madras, Chennai for providing the NMR and MS
facility.
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