Mild and efficient procedure for michael addition of n-heterocycles to,-unsaturated compounds using anhydrous K3PO4 as catalyst

Article abstract of DOI:10.1080/00397910903029867

Imidazole, 1,2,4-triazole, indole, and benzotriazole undergo conjugate additions with,α,β-unsaturated carbonyl compounds in the presence of anhydrous potassium phosphate at ambient temperature to afford the corresponding Michael adducts in excellent yields.

Full text of DOI:10.1080/00397910903029867

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Mild and Efficient Procedure for
Michael Addition of N-Heterocycles
to α,β-Unsaturated Compounds Using
Anhydrous K3PO4 as Catalyst
Xueling Hou a , Helimay Hemit a , Jianping Yong a , Lifei Nie a & Haji
Akber Aisa a
a Key Laboratory of Chemistry of Plant Resources of Arid Area,
Xinjiang Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Urumqi, China
Version of record first published: 04 Mar 2010.
To cite this article: Xueling Hou, Helimay Hemit, Jianping Yong, Lifei Nie & Haji Akber Aisa (2010):
Mild and Efficient Procedure for Michael Addition of N-Heterocycles to α,β-Unsaturated Compounds
Using Anhydrous K3PO4 as Catalyst, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 40:7, 973-979
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Synthetic Communications1, 40: 973–979, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903029867
MILD AND EFFICIENT PROCEDURE FOR MICHAEL
ADDITION OF N-HETEROCYCLES TO
a,b-UNSATURATED COMPOUNDS USING
ANHYDROUS K3PO4 AS CATALYST
Xueling Hou, Helimay Hemit, Jianping Yong, Lifei Nie, and
Haji Akber Aisa
Key Laboratory of Chemistry of Plant Resources of Arid Area, Xinjiang
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Urumqi, China
Imidazole, 1,2,4-triazole, indole, and benzotriazole undergo conjugate additions with
a,b-unsaturated carbonyl compounds in the presence of anhydrous potassium phosphate
at ambient temperature to afford the corresponding Michael adducts in excellent yields.
Keywords: Anhydrous potassium phosphate; N-heterocycle; imidazole; Michael addition
N-Substituted imidazoles and their derivatives, obtained through alkylation
reactions[1] or Michael additions,[2] are of interest in pharmaceutical chemistry
because of their pharmacodynamic properties. The Michael reaction, which is a
conjugate addition reaction of nucleophiles to unsaturated carbonyl compounds,
requires basic conditions[3] or acidic catalysts.[4] Recently, several Lewis acids were
found to catalyze the conjugate addition of heterocyclic compounds to electron-
deficient olefins, such as BiNO3,[5] InX3,[6] AuCl3,[7] SmI3,[8] CeCl3,[9] and other
metal salts of trifluoromethanesulfonate such as Bi(III), Yb(III), Hf(IV), Sc(III),
Ga(III), and Cu(II) have also been successfully used.[10] Moreover, the use of metal
complex (such as Sc, Au, Cu, Zn)[11] and enzyme[12] as the catalyst have also been
reported.
However, there are only a few reports about the base-catalyzed conjugated
addition of heterocyclic compounds to a,b-unsaturated carbonyl compounds, even
though basic clays,[13] KF=Al2O3,[14] and guanidine[15] were reported. Unfortunately,
under not only acidic conditions, but also basic ones, many of these catalytic proce-
dures require long reaction times (several days), rigorous reaction conditions, or
highly dangerous chemicals. Thus, development of a fast and facile protocol that
could be performed at ambient temperature for the Michael addition of
Received February 7, 2009.
Address correspondence to Xueling Hou, Key Laboratory of Chemistry of Plant Resources of Arid
Area, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 40-1 Beijing
South Road, Urumqi 830011, China. E-mail: xlhou@ms.xjb.ac.cn
973

974
X. HOU ET AL.
N-heterocycles to a,b-unsaturated compounds becomes particularly fascinating and
remains a great challenge.
Professor Li’s group has reported a simple one-pot procedure for the
preparation of a different kind of b-electron-withdrawing group substituted ethyl
dithiocarbamate by the Michael addition of an amine and carbon disulfide to
electrophilic alkenes in the presence of anhydrous potassium phosphate under mild
conditions in good yields.[16] Although heterocyclic compounds were not involved,
we reasoned that the imidazole may work well using our method considering the
specific structure of imidazole. Herein, we report the result of our efforts in this
direction.
First, we selected the reaction of imidazole with methyl acrylate in the presence
of anhydrous potassium phosphate as a model. In the initial studies of the screening
of the volume of the catalyst, 100, 50, 25, and 10 mol% K3PO4 were screened. The
reactions were quenched after 3 h. Their separated yields are 98, 97, and 86%,
respectively, which shows that 25 mol% K3PO4 was the most suitable because of
good yield and short reaction time. Different solvents [dimethylformamide
(DMF), tetrahydrofuran (THF), acetone, CH2Cl2, MeCN] and reaction tempera-
tures (0 ꢀC to 40 ꢀC) were examined. The reaction results were evaluated qualitatively
by thin-layer chromatography (TLC). Rapid conversion was observed when the
reaction was carried out in CH3CN (97% yield) or DMF (96% yield). Poor conver-
sion (17%) was obtained in CH2Cl2 when 50 mol% K3PO4 was used as catalyst.
Moderate yield (65%) was achieved in THF. The reaction temperature has little
influence on the reaction yield. It was revealed that the optimal reaction conditions
were using 25 mol% K3PO4 as catalyst and CH3CN as solvent at room temperature.
The desired compound, 3-imidazol-1-yl-propionic acid methyl ester (3a), was
obtained under the optimized reaction conditions after 3 h in 97% yield (Scheme 1).
With the effective catalyst system in hand, the conjugate addition of imidazole
to other a,b-unsaturated compounds was investigated (Scheme 2). As shown in
Table 1, the catalytic system is suitable for a variety of a,b-unsaturated compounds
including a,b-unsaturated esters, a,b-unsaturated amide, and a,b-unsaturated
nitriles. The reaction of all a,b-unsaturated esters were completed within 3 h in good
to excellent yields except for compound 3d, which has methyl methacrylate as the
substrate (entry 4). Even after 24 h, the conjugate addition of imidazole to methyl
methacrylate still has a lesser isolated yield (47%), perhaps because of the steric effect
of the methyl group in methyl methacrylate, which affected the attack of imidazole
to the ester. Acrylamide and acrylonitrile (entries 5 and 6) were used as Michael
acceptors and gave products 3e and 3f within 3 h in 98% and 99% yields, respectively.
To check the scope of the N-heterocyclic compounds in this reaction, we
extended this K3PO4 catalyst system to the conjugate addition of 1,2,4-1H-triazole
(4), indole (5), and 1H-benzotriazole (6) to methyl acrylate (Scheme 3). As expected,
Scheme 1. The optimized reaction condition.

ANHYDROUS K3PO4-CATALYZED MICHAEL ADDITION
975
Scheme 2. The conjugate addition of imidazole to various a,b-unsaturated compounds.
Table 1. Conjugate addition of imidazole 1 to a,b-unsaturated carbonyl compounds 3a–3f
Entry
1
2
Product
Yield (%)a
Reaction time (h)
3a
97
3
2
3
4
3b
3c
3d
95
98
47
2.5
3
24
5
6
3e
3f
98
99
2
3
aIsolated yield by silica-gel column chromatography.
Scheme 3. The conjugate addition of other N-heterocyclic compounds with methyl acrylate.

976
X. HOU ET AL.
Table 2. Attempts on other heterocycles in the presence of anhydrous K3PO4
Entry
Heterocycle
Product
Yield (%)a
Reaction time (h)
1
2
3
4
5
Imidazole
1-H-1,2,4-Triazole
Indole
3a
7
97
92
68
67
21
3
3
8
24
6
1-H-Benzotriazole
1-H-Benzotriazole
9a
9b
6
aIsolated yield by silica-gel column chromatography.
the K3PO4-catalyzed conjugate addition reaction of 1,2,4-1H-triazole (4) to methyl
acrylate was very fast and afforded 3-[1,2,4]triazole-1-yl-propionic acid methyl ester
(7) in 92% isolated yield (Table 2). The reaction of indole with methyl acrylate was
very slow, and 3-indole-1-yl-propionic acid methyl ester (8) was obtained in 68%
yield after 2 days. Further research on the reaction of 1H-benzotriazole with methyl
acrylate gave a mixture of 3-benzotriazole-1-yl-propionic acid methyl ester (9a) and
3-benzotriazole-2-yl-propionic acid methyl ester (9b) in 67% and 21% yields, respect-
ively (Table 2, entry 4). The formation of 9b may be because of the resonant struc-
ture of 1H-benzotriazole under the basic conditions. Changing the solvent from
CH3CN to DMF or increasing the reaction temperature cannot improve the ratio
of 9a and 9b.
In conclusion, we have demonstrated the first use of anhydrous K3PO4 as
catalyst for the Michael addition of N-heterocycles to a series of a,b-unsaturated
compounds. The catalyst is cheaper, milder, and more efficient. The reactions were
carried out at room temperature to afford the desired products in good yields in
short reaction times. This strategy is quite general, and it works with a broad range
of N-heterocycles, including imidazole, triazole, indole, and benzotriazole.
Melting points were measured on a Buchi B-540 apparatus and were uncor-
¨
1
rected. H NMR spectra were recorded on a Varian Inova-400 spectrometer, using
CDCl3 or DMSO-d6 as solvent and tetramethylsilane (TMS) as an internal standard.
Electrospray ionization (ESI) mass spectra were recorded using a Waters ZQ4000=
2690 LC–MS spectrometer (solvent: methanol; positive mode). Thin-layer chromato-
graphy (TLC) was carried out on precoated GF254 silica-gel plates. Column
chromatography was performed using G60 H silica gel. Anhydrous potassium
phosphate was obtained from dehydration of K3PO4 ꢁ 3H2O. The other reagents
and solvents were of commercial quality and used without further purification.
GENERAL SYNTHETIC PROCEDURE
N-Heterocycles (1 mmol) and a,b-unsaturated carbonyl compounds (1.2 mmol)
were added to a 10-mL flask containing anhydrous K3PO4 (0.25 mmol), and the
mixture was stirred at room temperature for a period of time. The reaction mixture
was filtered, and the filtrate was evaporated under reduced pressure. The residue was
purified by flash-column chromatography (silica gel, petroleum ether–ethyl acetate)
to obtain the corresponding Michael adducts.
All compounds can be stored at rt for several months without decomposition.

ANHYDROUS K3PO4-CATALYZED MICHAEL ADDITION
977
3-Imidazole-1-yl-propionic Acid Methyl Ester (3a)
1
Yellow oil. H NMR (400 MHz, CDCl3): d ¼ 2.750 (t, 2H, J ¼ 6.8 Hz), 3.660
(s, 3H), 4.237 (t, 2H, J ¼ 6.6 Hz), 6.900 (s, 1H), 7.008 (s, 1H), 7.475 (s, 1H). ESI-MS:
m=z ¼ 155 (M þ 1).
3-Imidazole-1-yl-propionic Acid Ethyl Ester (3b)
1
Yellow oil. H NMR (400 MHz, CDCl3): d ¼ 1.245 (t, 3H, J ¼ 7.6 Hz), 2.769
(t, 2H, J ¼ 6.2 Hz), 4.162 (d, d, 2H, J ¼ 7.2 Hz, 14.4 Hz), 4.271 (t, 2H, J ¼ 6.6 Hz),
6.935 (s, 1H), 7.050 (s, 1H), 7.511 (s, 1H). ESI-MS: m=z ¼ 169 (M þ 1).
3-Imidazole-1-yl-propionic Acid t-Butyl Ester (3c)
Yellow oil. 1H NMR (400 MHz, CDCl3): d ¼ 1.423 (s, 9H), 2.680 (t, 2H,
J ¼ 6.8 Hz), 4.220 (t, 2H, J ¼ 6.8 Hz), 6.930 (t, 1H, J ¼ 1.2 Hz), 7.041 (t, 1H,
J ¼ 1.1 Hz), 7.499 (s, 1H). ESI-MS: m=z ¼ 197 (M þ 1).
3-Imidazole-1-yl-butyric Acid Methyl Ester (3d)
1
Yellow oil. H NMR (400 MHz, CDCl3): d ¼ 1.184 (d, 3H), 2.875 (m, 1H),
3.667 (s, 3H), 4.007 (m, 1H), 4.238 (m, 1H), 6.878 (d, 1H, J ¼ 0.8 Hz), 7.263
(s, 1H), 7.448 (s, 1H). ESI-MS: m=z ¼ 169 (M þ 1).
3-Imidazole-1-yl-propionamide (3e)
1
White solid; mp 139–140 ꢀC. H NMR (400 MHz, DMSO): d ¼ 2.524 (t, 2H,
J ¼ 6.8 Hz), 4.151 (t, 2H, J ¼ 6.8 Hz), 6.846 (t, 1H, J ¼ 1.2 Hz), 6.922 (s, 1H), 7.108
(t, 1H, J ¼ 1.2 Hz), 7.377 (s, 1H), 7.558 (s, 1H). ESI-MS: m=z ¼ 140 (M þ 1).
3-Imidazole-1-yl-propionitrile (3f)
1
Yellow oil. H NMR (400 MHz, CDCl3): d ¼ 2.814 (t, 2H, J ¼ 6.8 Hz), 4.268
(t, 2H, J ¼ 6.8 Hz), 7.018 (s, 1H), 7.119 (s, 1H), 7.566 (s, 1H). ESI-MS: m=z ¼ 122
(M þ 1).
3-[1,2,4]-Triazole-1-yl-propionic Acid Methyl Ester (7)
1
Yellow oil. H NMR (400 MHz, CDCl3): d ¼ 2.908 (t, 2H, J ¼ 6.4 Hz), 3.671
(s, 3H), 4.463 (t, 2H, J ¼ 6.4 Hz), 7.915 (s, 1H), 8.131 (s, 1H). ESI-MS: m=z ¼ 156
(M þ 1).
3-Indole-1-yl-propionic Acid Methyl Ester (8)
1
Yellow oil. H NMR (400 MHz, CDCl3): d ¼ 2.802 (t, 2H, J ¼ 6.8 Hz), 3.640
(s, 3H), 4.430 (t, 2H, J ¼ 6.8 Hz), 6.477 (m, 1H), 7.084 (t, 1H, J ¼ 1.2 Hz), 7.207
(m, 1H), 7.319 (m, 1H), 7.623 (m, 1H). ESI-MS: m=z ¼ 204 (M þ 1).

978
X. HOU ET AL.
3-Benzotriazole-1-yl-propionic Acid Methyl Ester (9a)
Colorless oil. 1H NMR (400 MHz, CDCl3): d ¼ 3.102 (t, 2H, J ¼ 6.8 Hz), 3.647
(s, 3H), 4.902 (t, 2H, J ¼ 6.8 Hz), 7.353 (m, 1H), 7.492 (m, 1H), 7.612 (m, 1H), 8.043
(m, 1H). ESI-MS: m=z ¼ 206 (M þ 1).
3-Benzotriazole-2-yl-propionic Acid Methyl Ester (9b)
White solid, mp 82–83 ꢀC. 1H NMR (400 MHz, CDCl3): d ¼ 3.198 (t, 2H,
J ¼ 6.8 Hz), 3.727 (s, 3H), 5.043 (t, 2H, J ¼ 6.8 Hz), 7.379 (d, d, 2H, J ¼ 13.6,
6.8 Hz), 7.861 (d, d, 2H, J ¼ 13.6, 6.8 Hz). ESI-MS: m=z ¼ 206 (M þ 1).
ACKNOWLEDGMENTS
We thank the West Light Foundation of the Chinese Academy of Sciences
(CAS, O729361401) for the financial support. Research was also supported by the
CAS=State Administration of Foreign Experts Affairs (SAFEA) International
Partnership Program for Creative Research Teams.
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