The first ionic liquid-promoted three-component coupling strategy for an expeditious synthesis of β-nitrocarbonitriles/thiocyanates

Article abstract of DOI:10.1016/j.tetlet.2008.11.083

A novel and convenient three-component coupling reaction of nitromethane, aromatic aldehydes and trimethylsilyl cyanide (TMSCN) or ammonium thiocyanate has been developed for an expeditious synthesis of β-nitrocarbonitriles or β-nitrothiocyanates, respect

Full text of DOI:10.1016/j.tetlet.2008.11.083

Tetrahedron Letters 50 (2009) 640–643
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
The first ionic liquid-promoted three-component coupling strategy
for an expeditious synthesis of b-nitrocarbonitriles/thiocyanates
*
Lal Dhar S. Yadav , Ankita Rai
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:
A novel and convenient three-component coupling reaction of nitromethane, aromatic aldehydes and tri-
methylsilyl cyanide (TMSCN) or ammonium thiocyanate has been developed for an expeditious synthesis
of b-nitrocarbonitriles or b-nitrothiocyanates, respectively, via C–C and C–S bond-forming reactions. The
synthetic protocol strategically involves a one-pot sequential Henry reaction and a Michael addition effi-
ciently promoted by the same ionic liquid [bmim]OH. The main advantages of the present approach
include the use of inexpensive simple substrates and an ionic liquid as an efficient reaction promoter
for the mild synthesis in a one-pot procedure.
Received 14 September 2008
Revised 9 November 2008
Accepted 22 November 2008
Available online 27 November 2008
Keywords:
Henry reaction
Michael addition
Ionic liquids
Nitroalkanes
Carbonitriles
Thiocyanates
Ó 2008 Elsevier Ltd. All rights reserved.
Modern synthetic design demands high efficiency in terms of
minimization of synthetic steps together with maximization of
complexity.1 One of the ways to fulfil these goals is the develop-
ment and use of multicomponent reactions (MCRs) which consist
of several simultaneous bond-forming reactions and allow a highly
efficient synthesis of complex molecules starting from simple sub-
strates in a one-pot procedure.2–4 Ionic liquids (ILs) have attracted
increasing interest in organic synthesis owing to their great poten-
tial not only as environmentally benign reaction media but also as
new catalysts and reagents, and because they are also easy to
recycle.5
RCN?RCO2H, RCONH2. RCHO, RCH2NH2 and RCN4, as well as Pin-
ner and Ritter reactions) place them among the most versatile
intermediates in organic chemistry.10–23 Furthermore, organic
thiocyanates have gained considerable importance in various areas
of organosulfur chemistry24 and are useful scaffolds for the synthe-
sis of various heterocycles, some of which are associated with her-
bicidal and other important biological activity.25,26 Moreover, the
thiocyanato group occurs as an important functionality in certain
anticancer natural products formed by deglycosylation of glucosin-
olates derived from cruciferous vegetables.27,28
Considering the above points along with earlier reports on
hydrocyanation of alkenes29 and our ongoing efforts to develop
new one-pot synthetic processes,30 we report herein the first ionic
liquid [bmim]OH-promoted synthesis of hitherto unknown
b-nitrocarbonitriles 2 and b-nitrothiocyanates 3 using a three-
component coupling (3cc) strategy (Scheme 1). After some preli-
minary experimentation, it was found that b-nitrocarbonitriles 2
The importance of nitroalkanes in organic synthesis is tied up to
their propensity to undergo facile a-alkylation reactions and inter-
conversions to other important organic functional groups.6
Aliphatic nitro compounds have proven to be valuable intermedi-
ates, and are powerful synthetic tools because they facilitate the
carbon–carbon bond-forming processes.7 Most importantly, the
nitro group can be converted to a carbonyl compound using
the Nef reaction.8 An efficient nitroalkane-mediated cyclopropana-
tion is a key step in the synthesis of b-amino acid analogues of Pre-
NC
Ar
NO2
H
[bmim]OH,TMSCN
gabalin and Gabapentin that target the
a
2–d protein.9
acetonitrile, 85-90 ºC
Hydrocyanation or hydrothiocyanation of nitroalkenes affords
straightforward access to nitroalkanes bearing a nitrile or thiocya-
nate functionality of considerable synthetic utility. The diverse
transformations in which nitriles participate (for example,
81 92%
6-9 h,
2
CH3
NO2
Ar CHO
1
NCS
Ar
NO2
H
[bmim]OH,
NH4SCN
acetonitrile, 85-90 ºC
6-9 h,
85 94%
3
* Corresponding author. Tel.: +91 5322500652; fax: +91 5322460533.
E-mail address: ldsyadav@hotmail.com (L. D. S. Yadav).
Scheme 1. One-pot synthesis of b-nitrocarbonitriles 2 and b-nitrothiocyanates 3.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.083

L. D. S. Yadav, A. Rai / Tetrahedron Letters 50 (2009) 640–643
641
NC
NO2
H
NO2
NCS
Ar
NO2
H
TMSCN/ NH4SCN
[bmim]BF4
NH4OAc
step 1
Ar CHO
CH3NO2
or
Ar
Ar
1
step 2
3
2
53 58% overall yield
Scheme 2. Two-step synthesis of b-nitrocarbonitriles 2 and b-nitrothiocyanates 3.
Table 1
-H2O
Ar
O
Ar
C
H
Optimization of the catalyst for the one-pot synthesis of 2-nitro-1-(4-nitro-
H3C NO2
[bmim]OH
phenyl)ethyl thiocyanate (3d)a
4
NO2
Catalyst
1
Entry
Catalyst
Timeb (h)
Yieldc (%)
Ar
H
1
2
3
4
5
6
7
8
9
NH4OH (20 mol %)
Bu4NF (20 mol %)
9
9
9
9
9
9
9
8
6
8
36
41
48
40
43
44
83
87
94
94
O2N
CN
NH4OAc (20 mol %)
[bmim]Br (20 mol %)
[bmim]PF6 (20 mol %)
[bmim]BF4 (20 mol %)
[bmim]OH (10 mol %)
[bmim]OH (15 mol %)
[bmim]OH (20 mol %)
[bmim]OH (25 mol %)
2
Catalytic
cycle
HO Si
H2O
Ar
Ar
CN
Si
10
NO2
O2N
a
NC Si
+
For the experimental procedure, see Ref. 35.
Time for completion of the reaction at 85–90 °C as indicated by TLC.
Yield of isolated and purified product.
[bmim]
b
c
-
OH
Scheme 3. Plausible mechanism for the formation of b-nitrocarbonitriles 2.
and b-nitrothiocyanates 3 can be synthesized in moderate overall
yields (53–58%) by following a two-step process (Scheme 2).
The first step is the preparation of b-nitrostyrenes 4 by the Henry
-H2O
Ar
O
Ar
C
H
1
H3C NO2
reaction31,32
, and the second step is a hitherto unreported
[bmim]OH
Catalyst
NO2
ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate
[bmim]BF4-promoted Michael addition of trimethylsilyl cyanide
(TMSCN) or ammonium thiocyanate to b-nitrostyrenes 4 to afford
hydrocyanated and hydrothiocyanated products 2 and 3. In order
to improve the overall yields and synthesize products 2 and 3
expeditiously from nitromethane in a one-pot procedure, we
devised a novel ionic liquid [bmim]OH-promoted three-compo-
nent coupling reaction of nitromethane, aromatic aldehydes 1
and TMSCN or NH4SCN, which works well for the high yielding
4
H
Ar
SCN
O2N
3
Catalytic
cycle
NH4OH
H2O
H4N
O2N
Ar
Ar
synthesis of b-nitrocarbonitriles
2 or b-nitrothiocyanates 3
(Scheme 1). Interestingly, no formation of the corresponding cya-
nohydrin trimethyl ethers (an adduct of an aldehyde and trimeth-
ylsilyl cyanide) was observed under these conditions (Scheme 2).
We set up a series of experiments to optimize the reaction con-
ditions. Indeed, we used a very straightforward protocol and exam-
ined various basic catalysts as well as ionic liquids for the synthesis
of representative compound 3d (Tables 1 and 2). Among the cata-
lysts tested, the best result was obtained with [bmim]OH; this evi-
denced the catalytic efficacy of [bmim]OH in the reaction affording
3d in excellent yield (Table 1, entry 9). This is in accordance with
the earlier observations of Ranu et al. who introduced the basic
ionic liquid [bmim]OH for organic reactions for the first time.33
The use of other ionic liquids and basic catalysts resulted in a con-
siderable drop in yields (Table 1, entries 1–6). The optimum cata-
lyst loading for the ionic liquid [bmim]OH was found to be
20 mol % (Table 1, entry 9). It is noteworthy that a decrease in
the catalyst amount decreased the yield considerably (Table 1,
SCN
NO2
SCN NH4
+
[bmim]
-
OH
Scheme 4. Plausible mechanism of the formation of b-nitrothiocyanates 3.
substituted aldehydes were subjected to this procedure. The
results are summarized in Table 2. Both electron-donating and
electron-withdrawing substituents are tolerated to afford the
corresponding products 2 and 3 in consistently good yields. After
isolation of the product, the catalyst could be easily recycled and
reused without loss of efficiency.34,35
The 3cc approach reported herein for the envisaged synthesis of
b-nitrocarbonitriles 2 or b-nitrothiocyanates 3 was successfully
realized by stirring an equimolar mixture of an aldehyde, nitro-
methane and cyanating/thiocyanating agents, that is, TMSCN/
NH4SCN with 20 mol % of ionic liquid [bmim]OH as catalyst in
acetonitrile at 85–90 °C for the time specified in Table 2. Isolation
and purification afforded analytically pure compounds 234 and 335
in 81–94% yields. The requisite ionic liquids were prepared
employing known methods.33a,36–38 Plausible mechanisms for the
formation of 2 and 3 are depicted in Schemes 3 and 4.
entries
7 and 8). However, a catalyst loading higher than
20 mol % did not appreciably increase the yield (Table 1, entry 10).
Next, optimization of the solvent for the synthesis of 3d was
investigated and it was found that among methanol, ethanol, 1,4-
dioxane and acetonitrile, the best solvent in terms of yield was
acetonitrile. In view of the above findings, stirring the reaction
mixture in acetonitrile with catalyst [bmim]OH (20 mol %)
at 85–90 °C configures the optimized reaction condition. In order
to investigate the substrate scope of the reaction, a wide range of
In conclusion, we have demonstrated a novel, convenient and
expeditious method for the synthesis of b-nitrocarbonitriles and
b-nitrothiocyanates in excellent yields via a one-pot three-compo-

642
L. D. S. Yadav, A. Rai / Tetrahedron Letters 50 (2009) 640–643
Table 2
One-pot [bmim]OH-promoted reaction of nitromethane with aldehydes and nucleophiles to afford products 2 and 3
Entry
Aldehyde 1
Nucleophile (TMSCN/NH4SCN)
Product 2 or 3
Timea (h)
Yieldb,c (%)
NC
NO2
H
CHO
1
TMSCN
8
83
C6H5
2a
CHO
Cl
NC
NO2
H
2
3
4
TMSCN
TMSCN
TMSCN
7
8
6
89
84
92
4 ClC6H4
2b
NC
NO2
H
CHO
Cl
2 ClC6H4
2c
CHO
NC
NO2
H
4 O2NC6H4
2d
NO2
CHO
NO2
H
NC
5
6
TMSCN
TMSCN
8
9
85
81
3 O2NC6H4
2e
NO2
CHO
NC
NO2
H
4 H3COC6H4
2f
OCH3
NC
NO2
CHO
F
7
8
TMSCN
7
8
85
86
2 FC6H4
H
2g
NCS
NO2
CHO
NH4SCN
C6H5
H
3a
CHO
Cl
NCS
NO2
H
9
10
11
NH4SCN
NH4SCN
NH4SCN
6
7
6
91
87
94
4 ClC6H4
3b
NCS
NO2
H
CHO
Cl
2 ClC6H4
3c
CHO
NCS
NO2
4 O2NC6H4
H
3d
NO2
CHO
NCS
NO2
12
NH4SCN
8
88
3 O2NC6H4
H
3e
NO2

L. D. S. Yadav, A. Rai / Tetrahedron Letters 50 (2009) 640–643
643
Table 2 (continued)
Entry
Aldehyde 1
Nucleophile (TMSCN/NH4SCN)
Product 2 or 3
Timea (h)
Yieldb,c (%)
CHO
NCS
NO2
H
13
NH4SCN
NH4SCN
9
85
90
4 H3 COC6H4
3f
OCH3
NCS
NO2
H
CHO
F
14
7
2 FC6H4
3g
a
Stirring time at 85–90 °C.
b
c
Yield of isolated and purified products.
All compounds gave C, H and N analyses within 0.37% and satisfactory spectral (IR, 1H NMR, 13C NMR and EIMS) data.
27. Shahidi, F. In Sulphur Compounds in Foods; Mussinan, C. J., Keelan, M. E., Eds.;
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nent coupling of aldehydes, nitromethane and TMSCN/NH4SCN
using ionic liquid as an efficient promoter. The present protocol
for the synthesis of various functionalized nitroalkanes has high
potential for its application in organic chemistry.
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Acknowledgement
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra.
References and notes
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34. General procedure for the synthesis of b-nitrocarbonitriles 2: A mixture of an
aromatic aldehyde (1 mmol), nitromethane (1 mmol), TMSCN (1 mmol) and
[bmim]OH (20 mol %) in acetonitrile (5 ml) was stirred at 85–90 °C for an
appropriate time of 6–9 h (Table 2). After completion of the reaction (monitored
by TLC), water (10 mL) was added and the product was extracted with diethyl
ether (3 Â 15 mL). The combined extract was dried over Na2SO4, filtered,
concentrated under reduced pressure, and the crude product thus obtained was
purified by silica gel column chromatography using ethyl acetate–n-hexane
(1:9) as eluate to afford an analytically pure sample of 2. The remaining ionic
liquid was rinsed with ether (2 ml), dried under vacuum at 90 °C for 2 h to
eliminate any water trapped and reused for subsequent runs.33 Physical data of
representative compounds: Compound 2a: yellowish solid, yield 83%, mp 89 °C.
4. Orru, R. V. A.; de Greef, M. Synthesis 2003, 1471.
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IR (KBr) m .
max 3005, 2988, 2240, 1600, 1582, 1565, 1451, 753, 705 cmÀ1 1H NMR
(400 MHz; CDCl3) d: 3.92 (dd, 1H, J = 8.5, 4.7 Hz, CHCN), 4.60 (dd, 1H, J = 12.8,
4.7 Hz, CHaNO2), 4.97 (dd, 1H, J = 12.8, 8.5 Hz, CHbNO2), 7.25–7.49 (m, 5Harom).
13C NMR (100 MHz; CDCl3) d: 26.9, 77.2, 121.2, 127.3, 128.4, 129.2, 130.9. EIMS
(m/z) 176 (M+). Anal. Calcd for C9H8N2O2: C, 61.36; H, 4.58; N, 15.90. Found: C,
61.05; H, 4.72; N, 15.65.Compound 2d: Yellowish solid, yield 92%, mp 91 °C. IR
(KBr) mmax 3008, 2983, 2245, 1602, 1581, 1563, 1454, 752, 703 cmÀ1 1H NMR
.
(400 MHz; CDCl3) d: 3.98 (dd, 1H, J = 8.7, 4.8 Hz, CHCN), 4.64 (dd, 1H, J = 12.9,
4.8 Hz, CHaNO2), 4.99 (dd, 1H, J = 12.9, 8.7 Hz, CHbNO2), 7.21–7.25 (m, 2Harom),
7.33–7.46 (m, 2Harom.). 13C NMR (100 MHz; CDCl3) d: 25.7, 78.1, 120.8, 12.7,
129.1, 140.1, 148.3. EIMS (m/z) 221 (M+). Anal. Calcd for C9H7N3O4: C, 48.87; H,
3.19; N, 19.00. Found: C, 49.24; H, 3.04; N, 19.27.
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35. General procedure for the synthesis of b-nitrothiocyanates 3: The procedure
followed was the same as described above for the synthesis of 2 (Ref. 34) except
that NH4SCN (1 mmol) was used instead of TMSCN (1 mmol). The crude product
thus obtained was purified by silica gel column chromatography using ethyl
acetate–n-hexane (2:8) as eluate to afford an analytically pure sample of 3. The
remaining ionic liquid was recycled for subsequent runs as described above.33
Physical data of representative compounds: Compound 3a: pale yellow oil, yield
16. Kukushkin, V. Y.; Pombeiro, A. J. L. Inorg. Chim. Acta 2005, 358, 1.
17. Wang, M.-X. Top. Catal. 2005, 35, 117.
86%. IR (KBr) m .
max 3010, 2985, 2143, 1605, 1585, 1562, 1450, 748, 708 cmÀ1 1H
18. Brown, H. C.; Choi, Y. M.; Narasimhan, S. Synthesis 1981, 605.
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Tetrahedron 2003, 59, 5417.
23. Jayachitra, G.; Yasmeen, N.; Rao, K. S.; Ralte, S. L.; Srinivasan, R.; Singh, A. K.
NMR (400 MHz; CDCl3) d: 3.85 (dd, 1H, J = 8.4, 4.3 Hz, CHCN), 4.63 (dd, 1H,
J = 12.7, 4.3 Hz, CHaNO2), 4.96 (dd, 1H, J = 12.7, 8.4 Hz, CHbNO2), 7.25–7.49 (m,
5Harom). 13C NMR (100 MHz; CDCl3) d: 39.5, 85.7, 112.8, 126.5, 128.2, 129.6,
148.3. EIMS (m/z) 208 (M+). Anal. Calcd for C9H8N2O2S: C, 51.91; H, 3.87; N, 13.45.
Found: C, 51.72; H,3.98; N, 13.78.Compound 3d: Pale yellow oil, yield 94%. IR
(KBr) mmax 3012, 2987, 2148, 1603, 1589, 1559, 1453, 751, 710 cmÀ1 1H NMR
.
(400 MHz; CDCl3) d: 3.89 (t, 1H, J = 8.9, 4.5 Hz, CHCN), 4.66 (dd, 1H, J = 12.8,
4.5 Hz, CHaNO2), 4.81 (dd, 1H, J = 12.8, 8.95 Hz, CHbNO2), 7.19–7.23 (m, 2Harom),
7.32–7.48 (m, 2Harom). 13C NMR (100 MHz; CDCl3) d: 42.6, 84.2, 112.4, 120.7,
12.9, 128.5, 146.8. EIMS (m/z) 253 (M+). Anal. Calcd for C9H7N3O4S: C, 42.69; H,
2.79; N, 16.59. Found: C, 42.40; H, 2.98; N, 16.44.
Synth. Commun. 2003, 33, 3461.
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