Ionic liquid [Hmim]HSO4-promoted one-pot oxidative conjugate addition of sulfur-centred nucleophiles to Baylis-Hillman adducts

Article abstract of DOI:10.1016/j.tetlet.2008.03.016

The first example of the one-pot oxidative conjugate addition of sulfur-centred nucleophiles to Baylis-Hillman adducts is reported. The reaction involves oxidation of Baylis-Hillman adducts with NaNO₃ in the Br?nsted acidic ionic liquid [Hmim]H

Full text of DOI:10.1016/j.tetlet.2008.03.016

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3142–3146
Ionic liquid [Hmim]HSO4-promoted one-pot oxidative
conjugate addition of sulfur-centred nucleophiles
to Baylis–Hillman adducts
Lal Dhar S. Yadav *, Vishnu P. Srivastava, Rajesh Patel
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
Received 5 February 2008; revised 2 March 2008; accepted 5 March 2008
Available online 8 March 2008
Abstract
The first example of the one-pot oxidative conjugate addition of sulfur-centred nucleophiles to Baylis–Hillman adducts is reported.
The reaction involves oxidation of Baylis–Hillman adducts with NaNO3 in the Brønsted acidic ionic liquid [Hmim]HSO4 to give [E]-a-
cyanocinnamaldehydes followed by conjugate hydrothiocyanation/hydrosulfenylation with NH4SCN/PhSH to afford the corresponding
b-thiocyanato (or b-phenylsulfenyl)-a-cyanohydrocinnamaldehydes diastereoselectively in 76–89% yields in a one-pot procedure. After
isolation of the product, the ionic liquid [Hmim]HSO4 could be easily recycled for further use.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Baylis–Hillman adducts; Conjugate addition; Oxidation; Ionic liquids; Cinnamaldehydes; Stereoselective synthesis
The formation of carbon–sulfur bond constitutes a key
reaction in biosynthetic processes as well as in organic syn-
thesis.1 The conjugate addition of sulfur-centred nucleo-
philes to a,b-unsaturated carbonyl compounds is one of
the most important methods for the synthesis of b-sulfur
functionalized carbonyl compounds. Sulfur functionalities,
viz. thiocyanate and thioether, are well known in various
areas of organo-sulfur chemistry and are of considerable
importance from both chemical and biological viewpoints.
The thioether structural fragment is an important
molecular tool as bioisosteric replacements in rational drug
design.2 Organic thiocyanates, representing a masked mer-
capto group, are useful scaffolds for various organic trans-
formations especially in the synthesis of heterocycles such
as thiazoles, thiazines and as biocides.3 The thiocyanato
group is a prominent functional motif found in certain
anticancer natural products formed by the deglycosylation
of glucosinolates derived from cruciferous vegetables.4
N
N
H
SPh
SCN
CN
OH
CN
O
HSO4
NaNO3, 80 oC, 1-2 h
O
O
NH4SCN or PhSH
rt, 2-3 h
or
CN
CN
3
1
4
2
Scheme 1. Ionic liquid-promoted oxidative conjugate addition to BH adducts.
*
Corresponding author. Tel.: +91 5322500652; fax: +91 5322460533.
E-mail address: ldsyadav@hotmail.com (L. D. S. Yadav).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.016

L. D. S. Yadav et al. / Tetrahedron Letters 49 (2008) 3142–3146
3143
Table 1
One-pot oxidative conjugate addition of sulfur-centred nucleophiles to BH adducts
SCN
SPh
OH
1. [Hmim]HSO4, NaNO3, 80 oC
O
O
or
2. NH4SCN or PhSH, rt
CN
CN
CN
4
3
1
Entry
BH adduct
Nucleophile
NH4SCN
Product
SCN
Reaction time (h)
Yielda,b (%)
OH
3a
4
80
O
O
CN
OH
CN
SPh
4a
3b
PhSH
3.5
3
81
84
CN
OH
CN
SCN
O
O
NH4SCN
CN
OH
CN
SPh
4b
3c
4c
3d
4d
PhSH
3.5
3
85
87
89
84
86
CN
OH
CN
SCN
O
NH4SCN
PhSH
CN
OH
CN
SPh
MeO
MeO
O
3
CN
CN
MeO
Br
MeO
Br
OH
SCN
O
O
NH4SCN
PhSH
5
CN
OH
CN
SPh
Br
Br
4
CN
OH
CN
SCN
O
3e
4e
3f
NH4SCN
PhSH
4
86
88
76
79
CN
OH
CN
SPh
Cl
Cl
O
3.5
5
CN
OH
CN
SCN
Cl
Cl
O
NH4SCN
PhSH
CN
OH
CN
SPh
O2N
O2N
O
4f
4.5
CN
CN
O2N
O2N
a
Yield refers to pure products after column chromatography.
All compounds gave C, H and N analyses within 0.35% and satisfactory spectral (IR, 1H NMR, 13C NMR and EIMS) data.
b

3144
L. D. S. Yadav et al. / Tetrahedron Letters 49 (2008) 3142–3146
Furthermore, b-thiocyanated and b-sulfenylated carbonyl
compounds are an important class of synthons for a wide
variety of compounds and thus, enhance the versatility of
molecules containing sulfur functionalities.5 Hence, the
development of a simple, convenient and efficient method-
ology for their synthesis is highly desirable. Herein, Baylis–
Hillman chemistry has been applied for this purpose.
The Baylis–Hillman reaction is an efficient carbon–
carbon bond forming reaction yielding densely functional-
ized molecules,6 thereby providing handles for further
manipulation in a multitude of synthetic organic transfor-
mations.7 The utility of Baylis–Hillman (BH) adducts, par-
ticularly in the form of their modified version (e.g., acetyl
derivatives) as carbon electrophiles, and their derivatization
with various nucleophilic reagents including thiocyanates,
alkyl or aryl thiolates providing a wide range of potential
synthetic intermediates and precursors for the synthesis of
a variety of biologically active natural and unnatural
products has been well documented in the literature.8–10
Unmodified BH adducts have been employed as
Michael acceptors to produce functionalized aldol prod-
ucts with oxygen, sulfur, nitrogen and carbon-centred
nucleophiles.11 Recently, oxidative conjugate addition of
carbon-centred nucleophiles to BH adducts has been
reported.12 However, there has been no report on oxidative
conjugate addition of sulfur-centred nucleophiles to BH
adducts.
cinnamaldehydes 2. The formation of 3 and 4 was highly
diastereoselective in favour of the syn isomers. The
diastereomeric ratios in the crude isolates were determined
by 1H NMR spectroscopy to note any inadvertent
alteration of these ratios during subsequent purification.
As determined by 1H NMR spectroscopy, the crude
isolates of 3 and 4 were found to be diastereomeric
mixtures containing 91–95% and 93–96% of the syn isomer,
respectively.
It is worth mentioning here that the synthesis of [E]-a-
cyanocinnamaldehydes, representing an important class
of synthons for the synthesis of various biologically active
and heterocyclic molecules,17 has been achieved previously
from BH adducts in two steps, that is, an aqueous sulfuric
acid mediated isomerization of the BH adducts to [E]-a-
cyanocinnamyl alcohols and subsequent oxidation with
PCC to afford 2.18 The present method advantageously
provides an easy access to [E]-a-cyanocinnamaldehydes 2
from BH adducts with sole E-stereoselectivity in a one-
pot reaction using the NaNO3-[Hmim]HSO4 reagent–sol-
vent system (Table 2).19 The E-stereoselectivity of mole-
cules 2 was assigned on the basis of 1H NMR spectra
and literature precedent.18
We also attempted the oxidative conjugate hydrothio-
cyanation/hydrosulfenylation using NaNO3–[Hmim]NO3
as well as NaNO3–[Hmim]H2PO4 separately under the
same reaction conditions (Table 1). The reaction was
unsuccessful in the former case indicating the need for an
acidic hydrogen which is absent in [Hmim]NO3 to catalyze
the oxidation of the BH adducts into [E]-a-cyanocinnam-
aldehydes 2 (Scheme 2). However, the reaction proceeded
with the NaNO3–[Hmim]H2PO4 system but relatively low
yields of 3 and 4 (28–39%) were obtained. This is probably
due to the lower Brønsted acidity associated with [H2PO4].
Thus [Hmim]HSO4 plays a dual role, that is, as an acid cat-
alyst and solvent for both oxidation and hydrothiocyana-
tion/hydrosulfenylation.
Ionic liquids (ILs) have gained considerable interest as
environmentally benign reaction media, catalysts and
reagents, and are easy to recycle.13 Recently, Brønsted
acidic ionic liquids have been deemed as promising alterna-
tives for acid catalyzed reactions and play a dual solvent–
catalyst role in a variety of reactions including esterifica-
tion of carboxylic acids, protection of alcohols and
carbonyl groups, oxidation of alcohols, alcohol dehydro-
dimerization, pinocol/benzopinocol rearrangement, in
Mannich reactions and cleavage of ethers.14
One-pot sequential multi-step reactions are of increasing
academic, economical and ecological interest because they
address fundamental principles of synthetic efficiency and
reaction design. The continued interest of synthetic chem-
ists in the BH reaction and our ongoing efforts to devise
one-pot protocols involving conjugate addition15 have
encouraged us to develop the present Brønsted acidic ionic
liquid-promoted oxidative sulfur functionalization of BH
adducts (Scheme 1), opens a new aspect of their synthetic
utility.
After isolation of products 2–4, the ionic liquid
[Hmim]HSO4 could be recycled for four times up to 71%
recovery and reused without any loss of efficiency.16,19
Table 2
Synthesis of [E]-a-cyanocinnamaldehydes 2 from BH adducts 1
O
OH
[Hmim]HSO4
NaNO3, 80 oC
R
R
CN
CN
The present one-pot procedure for oxidative hydrothio-
cyanation/hydrosulfenylation involves stirring an equi-
molar mixture of NaNO3 and BH adduct 1 in 1-
methylimidazolinium hydrogen sulfate [Hmim]HSO4 at
80 °C for 1–2 h followed by the addition of 1.1 equiv of
NH4 SCN or PhSH and stirring at rt for a further 2–3 h
to afford the corresponding hitherto unknown b-thiocya-
nated product 3 in 76–87% yields or b-sulfenylated com-
pounds 4 in 79–89% yields (Table 1).16 The process
proceeds through the formation of isolable [E]-a-cyano-
1
2
Producta
R
Time (h)
Yieldb (%)
2a
2b
2c
2d
2e
2f
a
Ph
1.5
1.3
1
1.7
1.2
2
82
80
84
77
79
75
4-CH3C6H4
4-CH3OC6H4
3-BrC6H4
4-ClC6H4
4-NO2C6H4
2a, 2b and 2e are known compounds.18
Yields of isolated and purified products.
b

L. D. S. Yadav et al. / Tetrahedron Letters 49 (2008) 3142–3146
3145
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O
OH
CN
OH2
CN
O N O
-H2O
[Hmim]HSO4
NaNO3
1
O
NO2
O
H
+
N
O
NO
-HNO2
O
CN
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H2O
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CN
3
4
Scheme 2. A plausible mechanism for the formation of 3 and 4.
The requisite BH adducts and Brønsted acidic ionic liquids
were prepared employing the known methods.20,21
In summary, we have reported the first example of the
one-pot oxidative conjugate addition of sulfur-centred
nucleophiles to BH adducts using the NaNO3–[Hmim]-
HSO4 system to afford b-thiocyanato (or b-phenylsulfen-
yl)-a-cyanohydrocinnamaldehydes, diastereoselectively.
Furthermore, application of the same reagent–solvent sys-
tem for the one-pot isomerization–oxidation of BH
adducts to afford [E]-a-cyanocinnamaldehydes has also
been demonstrated, which is advantageous over the exist-
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We sincerely thank SAIF, CDRI, Lucknow, for provid-
ing microanalyses and spectra. One of us (R.P.) is grateful
to the CSIR, New Delhi, for the award of a Junior
Research Fellowship.
References and notes
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5855–5857; (b) Leblanc, B. L.; Jursic, B. C. Synth. Commun. 1998, 28,
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M.; You, M.; Gerhaeusers, C.; Pezzuto, J. M.; Moon, R. C.;
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16. General procedure for the synthesis of b-thiocyanato-a-cyanohyd-
rocinnamaldehydes (3) and b-phenylsulfenyl-a-cyanohydrocinnam-
aldehydes (4): A mixture of BH adduct 1 (1 mmol) and NaNO3
(1 mmol) was stirred in 1 mL of [Hmim]HSO4 at 80 °C for 1–2 h
(Table 2). The reaction mixture was cooled to rt, and NH4SCN or
5. Bakuzis, P.; Bakuzis, M. L. F. J. Org. Chem. 1981, 46, 235–239.
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3146
L. D. S. Yadav et al. / Tetrahedron Letters 49 (2008) 3142–3146
PhSH (1.1 mmol) was added. The mixture was further stirred at rt for
17. (a) Montgomery, J. A.; Niwas, S.; Rose, J. D.; Secrist, J. A., III;
Babu, Y. S.; Bugg, C. E.; Erion, M. D.; Guida, W. C.; Ealick, S. E. J.
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18. Basavaiah, D.; Kumaragurubaran, N.; Padmaja, K. Synlett 1999,
1630–1632.
19. General procedure for the synthesis of [E]-a-cyanocinnamaldehydes (2):
A stirred solution of BH adduct 1 (1 mmol) and NaNO3 (1 mmol) in
1 mL of [Hmim]HSO4 was heated at 80 °C for 1–2 h (Table 2). The
reaction progress was monitored by TLC. Upon completion, the
reaction mixture was cooled to rt and extracted with ether/ethyl
acetate (3 Â 10 mL). The combined organic phase was dried over
MgSO4, filtered and evaporated under reduced pressure. The resulting
crude product was purified by silica gel column chromatography
using a gradient mixture of hexane/ethyl acetate (9:1) as eluent to give
the pure aldehydes 2. The remaining ionic liquid was recycled for
subsequent runs as described above using H2SO4 (1 mmol).16 Physical
data of a representative compound. Compound 2c: White solid, yield
84%, mp 112–113 °C. IR (KBr) mmax 3078, 2221, 2740, 2809, 1686,
1602, 1576, 1508, 1464, 833 cmÀ1. 1H NMR (400 MHz; CDCl3/TMS):
2–3 h. After completion of the reaction (monitored by TLC), the
product was extracted with ether/ethyl acetate (3 Â 10 mL). The
combined extract was dried over MgSO4, filtered, concentrated under
reduced pressure and purified by silica gel column chromatography
(hexane/ethyl acetate 9.5:0.5) to afford the desired product (3) or (4).
After isolation of the product, the remaining ionic liquid was washed
with diethyl ether (2 Â 10 mL) to remove any organic impurity, then
H2SO4 (2.1 mmol in the case of compound 3 and 1 mmol in the case
of 4) was added, the mixture was stirred at 80 °C for 1 h, cooled to
about À5 °C in an ice-salt bath. The precipitated solid was filtered out
and the filtrate was dried under vacuum to afford the IL [Hmim]HSO4
which was used in subsequent runs. Physical data of representative
compounds. Compound 3a: Yellowish solid, yield 80%, mp 102–
103 °C. IR (KBr) mmax 3022, 2241, 2116, 1731, 1603, 1579, 1456, 764,
710 cmÀ1 1H NMR (400 MHz; CDCl3/TMS) d: 3.66 (dd, 1H,
.
J = 3.9 Hz, J = 2.1 Hz, 2-H), 4.82 (d, 1H, J = 3.9 Hz, 3-H), 7.11–
7.33 (m, 5Harom), 9.54 (d, 1H, J = 2.1 Hz, CHO). 13C NMR
(100 MHz; CDCl3/TMS): d 36.6, 50.9, 112.3, 116.1, 126.9, 128.6,
129.2, 140.9, 197.8. EIMS (m/z): 216 (M+). Anal. Calcd for
C
11H8N2OS: C, 61.09; H, 3.73; N, 12.95. Found: C, 60.82; H, 4.08;
d 3.87 (s, 3H, OMe), 6.88 (d, 2Harom, J = 8.6 Hz), 7.36 (d, 2Harom,
N, 12.83.
J = 8.6 Hz), 7.96 (s, 1H, CHPh), 9.63 (s, 1H, CHO). 13C NMR
(100 MHz; CDCl3/TMS): d 55.4, 112.2, 113.6, 114.8, 126.9, 128.1,
158.5, 160.1, 187.1. EIMS (m/z): 187 (M+). Anal. Calcd for
Compound 4f: Yellowish solid, yield 79%, mp 122–123 °C. IR (KBr)
mmax 3055, 2932, 2245, 1728, 1586, 1520, 1345, 760, 714. 1H NMR
(400 MHz; CDCl3/TMS): d 3.71 (dd, 1H, J = 3.9 Hz, J = 2.1 Hz,
2-H), 4.69 (d, 1H, J = 3.9 Hz, 3-H), 7.24–8.27 (m, 9Harom), 9.56 (d,
1H, J = 2.1 Hz, CHO). 13C NMR (100 MHz; CDCl3/TMS): d 30.7,
51.3, 117.2, 121.6, 124.9, 127.4, 129.2, 129.8, 135.8, 148.2, 149.4,
198.2. EIMS (m/z): 312 (M+). Anal. Calcd for C16H12N2O3S: C,
61.53; H, 3.87; N, 8.97. Found: C, 61.69; H, 3.98; N, 8.62.
C
11H9NO2: C, 70.58; H, 4.85; N, 7.48. Found: C, 70.79; H, 4.56; N,
7.26.
´
20. Mehdi, H.; Bodor, A.; Lantos, D.; Horvath, I. T.; de Vas, D. E.;
Binnemans, K. J. Org. Chem. 2007, 72, 1517–1524.
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