Ultrasound-assisted aza-Michael reaction in water: A green procedure

Article abstract of DOI:10.1016/j.ultsonch.2011.11.009

The conjugate addition of amines to conjugated alkenes (commonly known as aza-Michael reaction) constitutes a key step for the synthesis of various complex natural products, antibiotics, α-amino alcohols and chiral auxiliaries. Ultrasound-induced addition of several amines to α, β-unsaturated ketones, esters and nitriles has been carried out very efficiently in water as well as under solvent-free conditions. No catalysts or solid supports have been used in this method. Remarkable enhancement of reaction rate has been observed in water under ultrasound-induced method. This environmentally benign procedure has provided clean formation of the products with better selectivity.

Full text of DOI:10.1016/j.ultsonch.2011.11.009

Ultrasonics Sonochemistry 19 (2012) 969–973
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Ultrasound-assisted aza-Michael reaction in water: A green procedure
⇑
Debasish Bandyopadhyay, Sanghamitra Mukherjee, Luis C. Turrubiartes, Bimal K. Banik
Department of Chemistry, The University of Texas-Pan American, 1201 West University Drive, Edinburg, TX 78539, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2011
Received in revised form 3 October 2011
Accepted 13 November 2011
Available online 4 December 2011
The conjugate addition of amines to conjugated alkenes (commonly known as aza-Michael reaction) con-
stitutes a key step for the synthesis of various complex natural products, antibiotics,
a-amino alcohols
and chiral auxiliaries. Ultrasound-induced addition of several amines to , b-unsaturated ketones, esters
a
and nitriles has been carried out very efficiently in water as well as under solvent-free conditions. No cat-
alysts or solid supports have been used in this method. Remarkable enhancement of reaction rate has
been observed in water under ultrasound-induced method. This environmentally benign procedure has
provided clean formation of the products with better selectivity.
Keywords:
Aza-Michael
Ultrasound
Amine
Ó 2011 Published by Elsevier B.V.
Enone
Green chemistry
1. Introduction
Ultrasound-induced synthesis of organic molecules is a power-
ful green synthetic approach [22,23]. Over the past decade, protec-
Nitrogenous heterocycles and their derivatives have broad
application in synthetic, natural product, materials, and biological
chemistry. Consequently, syntheses of these types of molecules
are subjects of considerable interest [1]. Conjugate addition reac-
tions of nitrogen-centered heterocyclic nucleophiles to electron-
deficient olefins serve as a powerful preparative method in the area
of heterocyclic chemistry. The aza-Michael addition of nucleo-
tion of the environment and waste prevention has been
increasingly emphasized by researchers from both academia and
industry [24]. For this reason the elimination or reduction of vola-
tile solvents in organic synthesis is an important goal. These types
of processes can contribute significantly in green chemistry
[25,26]. A recent trend of synthetic organic chemistry indicates
that ultrasound can be used as an important tool to achieve a num-
ber of chemical reactions in high yield and within a shorter reac-
tion time [27]. In this context development of solvent-free
synthetic methods or the replacement of hazardous solvents with
environmentally benign solvents has become an important and
popular research topic in recent years. We have developed ultra-
sound-assisted aza-Michael reaction without using any catalysts
or solid supports. We have tested this new method with different
solvents as well as under solvent-free conditions. High yields are
obtained in water and also under solvent-free conditions. How-
ever, the reaction in water proceeds much faster than organic sol-
vents or solvent-free conditions (Scheme 1).
philes to a, b-unsaturated ketones, esters and nitriles is established
as a powerful tool for carbon–nitrogen bond formation. This reac-
tion is widely reported as a key step for the synthesis of kinase
inhibitor [2], alkaloids [3], lactams [4], amino acid [5], aziridine
[6], and b-aminophosphonates [7]. Therefore, a number of new cat-
alysts, supports, methods and solvents are reported for this reac-
tion. Some of which includes ZrCl4 [5], cinchona alkaloid [8],
hexafluoroisopropyl (HFIP) alcohol [9], silica gel supported over
perchloric acid (HClO4–SiO2) [10], LaCl3 [11], triethylammonium
acetate (TEAA) [12], alkaline Al2O3 [13], polystyrenesulfonic acid
[14], amberlyst-15 [15], silica gel [16], p-toluenesulfonic acid un-
der high pressure [17], ionic liquid [18], organocatalyst [19], en-
zyme [20] and tetrabutylammonium bromide [21]. These
methods have extended the scope of this reaction. However they
have limitations in some of the following areas: costly catalysts,
low yield, long reaction time, difficult product isolation procedure
and use of toxic metal catalysts as well as hazardous solvents.
2. Experimental
2.1. General: apparatus and analysis
Melting points were determined in a Fisher Scientific electro-
chemical Mel-Temp⁄ manual melting point apparatus (Model
1001) equipped with a 300 °C thermometer. Elemental analyses
(C, H, N) were conducted using the Perkin–Elmer 2400 series II ele-
⇑
Corresponding author. Tel.: +1 956 6658741; fax: +1 956 3845006.
E-mail address: banik@utpa.edu (B.K. Banik).
1350-4177/$ - see front matter Ó 2011 Published by Elsevier B.V.
doi:10.1016/j.ultsonch.2011.11.009

970
D. Bandyopadhyay et al. / Ultrasonics Sonochemistry 19 (2012) 969–973
Table 1
Effect of solvents on aza-Michael reaction of piperidine (1 mL) and methyl acrylate
(1 mL) in 1 mL solvent under ultrasound.
Entry Solvent
Time
(min)
Yield
(%)a
1
2
3
4
5
6
7
8
Tetrahydrofuran
Ethanol
Methanol
Dichloromethane
Acetone
Acetonitrile
Water
60
60
60
60
60
60
5
45
62
57
55
48
71
98
96
Scheme 1.
Water (without ultrasound, stirring at room
30
temperature)
No solvent
mental analyzer, their results were found to be in good agreement
( 0.2%) with the calculated values for C, H, N. Sonication was per-
formed in B5510-DTH, Branson ultrasonic cleaner (Model-5510,
frequency of 42 kHz and an output power of 135 W) with digital
timer, heater, temperature monitor & degas. Inside tank dimen-
sions were 11.5 in. ꢀ 9.5 in. ꢀ 6 in. (length ꢀ width ꢀ height) with
a fluid capacity of 2.5 gallons. FT-IR spectra were measured on a
Bruker IFS 55 Equinox FT-IR spectrophotometer as KBr discs. 1H
NMR (300 MHz) and 13C NMR (75.4 MHz) spectra were obtained
at room temperature with JEOL Eclipse-300 equipment using
TMS as internal standard and CDCl3 as solvent. Analytical grade
chemicals (Sigma–Aldrich Corporation) were used throughout the
project. Deionized water was used for the preparation of all aque-
ous solutions.
9
15
93
a
Isolated yield.
126.29 (4C), 127.81 (4C), 127.98 (2C), 129.39 (4C), 129.97 (4C),
135.10 (2C), 136.22 (2C), 138.52 (2C), 197.91 (2C). Anal. Calcd for
43H50N2O2: C, 82.39; H, 8.04; N, 4.47. Experimental: C, 82.27; H,
C
7.89; N, 4.52.
3. Results and discussion
Development of an efficient and simple aza-Michael procedure
in water without any catalyst or support is timely and highly chal-
lenging. To evaluate the effect of solvents piperidine (1 mmol) and
methyl acrylate (1 mmol) in 1 mL of solvent were used as a probe
(Table 1). Moderate yields of the products were obtained when tet-
rahydrofuran, ethanol, methanol, dichloromethane, acetone or ace-
tonitrile were used as solvents under ultrasound irradiation for
60 min (entries 1–6, Table 1). The desired product was isolated in
98% yield within 5 min in presence of water (entry 7, Table 1).
The same reaction in water produced 96% yield of the Michael ad-
duct in 30 min when it was performed at room temperature (entry
8, Table 1). Solvent-free condition gave 93% yield of the Michael
adduct in 15 min (entry 9, Table 1). Six- and three-fold rate accel-
erations were observed under ultrasound and solvent-free condi-
tions in comparison with reactions that were conducted in water
at room temperature.
2.2. General procedure for the aza-Michael reaction
Michael acceptor (1 mmol) and amine (1 mmol) in water (1 mL)
was irradiated at room temperature in a B5510-DTH (Branson
ultrasonic cleaner; Model-5510, frequency 42 kHz with an output
power of 135 W), as specified in Table 2. After completion of the
reaction, it was extracted with diethyl ether (2 ꢀ 5 mL), washed
with brine solution (10 mL) and dried over Na2SO4. The extract
was then concentrated and the crude product was purified using
flash chromatography (silica gel, 30% EtOAc/70% hexane) to afford
pure compound. Products obtained from entries (1–15) are already
reported (Table 2) and are easily identified by comparison of their
spectroscopic data.
Our ultrasound-induced reaction (Scheme 1) in water has been
tested with several amines and unsaturated ketones, unsaturated
nitrile an unsaturated ester. The results are spectacular (Table 2).
All theproducts were identified by their NMR, IR and elementalanal-
ysis data. All the ultrasound-assisted reactions in water were very
rapid compared to reactions in organic solvents under identical con-
ditions. No catalysts were necessary to complete the reaction. In a
typical experiment, piperidine (1 mmol) and methyl acrylate
(1 mmol) in 1 mL water were irradiated using ultrasound to obtain
the corresponding methyl 3-(piperidine-1-yl)propanoate in 98%
yield (entry 1) and the reaction was completed within 5 min.
Encouraged by this result, diverse amines were tested with various
Michael acceptors. All the reactions were completed within 10 min
to give the desired products in excellent yields (86–99%), with high
regio and chemo selectivity. No side products were formed. In gen-
eral, the nucleophilic addition of amines to carbonyl compounds de-
pends on steric effects of the partners used in the reactions. In
contrast, primary, secondary (cyclic, heterocyclic and acyclic), ben-
zylic as well as aromatic amines produce products in excellent yield
in the present study. This method suggests that it is not necessary to
use large excess of corrosive acid, catalytic amounts of Lewis acids or
solid acidic surfaces in Michael reaction of amines with unsaturated
ketones, esters and nitriles. Primary amines produced mono addi-
tion products (entries 10, 11, 13, 14, and 15) selectively and no bis
addition products were formed. The reaction between piperidine
with methyl acrylate (entry 1) in solvent-free condition gave 93%
2.3. Spectral data for the new compounds (entries 16 and 17, Table 2)
2.3.1. 3,30-(Piperazine-1,4-diyl)bis(1,3-diphenylpropan-1-one) (entry
16, Table 2)
Yellowish-brown amorphous solid. Mp. 126–128 °C; IR (KBr
disc.
m
cmꢁ1): 3124, 1652 1594, 1476, 1260, 1072, 971, 861, 735,
718, 673; 1H NMR d (ppm): 2.73 (m, 8H, piperizine methylene),
2.98 (m, 4H, methylene), 4.38 (m, 2H, methine), 7.27–7.40 (m,
10H, Ar–H), 7.59–8.01 (m, 10H, Ar–H); 13C NMR d (ppm): 50.91
(4C), 69.12 (2C), 71.32 (2C), 126.91 (2C), 127.81 (4C), 127.92
(4C), 128.74 (4C), 128.11 (4C), 135.12 (2C), 137.10 (2C), 139.43
(2C), 201.11 (2C). Anal. Calcd for C34H34N2O2: C, 81.24; H, 6.82;
N, 5.57. Experimental: C, 81.03; H, 6.71; N, 5.51.
2.3.2. 3,30-(4,40-(Propane-1,3-diyl)bis(piperidine-4,1-diyl))bis(1,3-
diphenylpropan-1-one) (entry 17, Table 2)
Brownish-white amorphous solid. Mp. 141–144 °C; IR (KBr disc.
m
cmꢁ1): 3138, 1659, 1588, 1461, 1265, 1071, 975, 863, 723, 675;
1H NMR d (ppm): 1.21–1.35 (m, 10H, aliphatic H), 1.49–1.61 (m,
6H, aliphatic H), 2.39–2.56 (m, 8H, aliphatic H), 2.83 (m, 2H, meth-
ylene), 3.06 (m, 2H, methylene), 4.49 (m, 2H, methine), 7.27–7.31
(m, 6H, Ar–H), 7.42 (m, 4H, Ar–H), 7.54–7.65 (m, 6H, Ar–H),
7.98–8.01 (m, 4H, Ar–H); 13C NMR d (ppm): 19.21 (2C), 24.27
(1C), 26.01 (4C), 36.71 (2C), 48.79 (4C), 71.89 (2C), 73.05 (2C),

D. Bandyopadhyay et al. / Ultrasonics Sonochemistry 19 (2012) 969–973
971
Table 2
Ultrasound-assisted aza-Michael reaction following Scheme 1.
Entry
1
Amine
Enone
Product
Time (min)
5
Yield (%)a
98
Refs.
[28a]
2
3
(1a)
5
5
96
95
[28a]
(2a)
(2b)
[28b]
4
5
(1b)
(1b)
5
5
91
95
[28b]
[28c]
6
(1a)
10
92
[28d]
7
8
9
(2a)
(2b)
(2d)
5
5
93
90
93
[28d]
[28d]
[28c]
(1c)
(1c)
10
10
11
nBuNH2 (1e)
(1e)
(2a)
(2b)
5
5
99
95
[28a]
[28a]
12
(2a)
(2e)
(2b)
5
97
[28a]
13
14
5
5
92
95
[28b]
[28b]
15
16
5
93
87
[28b]
–
10
–
17
(2f)
10
86
a
Isolated yield.

972
D. Bandyopadhyay et al. / Ultrasonics Sonochemistry 19 (2012) 969–973
(d) S. Chandrasekhar, R.V.N.S. Murali, C.R. Reddy, Enantioselective synthesis of
(ꢁ)-lasubine II, Tetrahedron Lett. 50 (2009) 5686–5688.
yield of the Michael adduct in 15 min whereas 98% yield of the same
Michael adduct was obtained in 5 min. It is interesting to note that
this reaction produced products with primary aromatic amines
and electron-deficient alkenes (entries 13 and 14).
[4] (a) S. Comesse, M. Sanselme, A. Daïch, New and expeditious tandem sequence
aza-Michael/intramolecular nucleophilic substitution route to substituted
c-
lactams: synthesis of the tricyclic core of ( )-martinellines, J. Org. Chem. 73
(2008) 5566–5569;
The presence of water accelerated the reaction probably through
hydrogen bond formation with the carbonyl group and this in-
creased the electrophilic character at the b-carbon of the unsatu-
rated compounds [29]. As a result, nucleophilic attack by the
amine increased significantly. On the other hand, hydrogen bond
formation between the oxygen atom of water and the hydrogen-
atom of the amine increased the nucleophilic power of the N-atom
of the amine. The introduction of ultrasound (i.e., sound energy
with frequencies in the range 15 kHz–1 MHz) into liquid reaction
mixtures is known to cause a variety of chemical transformations.
Ultrasonic irradiation of liquid reaction mixtures induces electro
hydraulic cavitations by which the radii of preexisting gas cavities
in the liquid oscillate in a periodically changing pressure. These
oscillations eventually become unstable, forcing violent implosion
of the gas bubbles. The rapid implosion of a gaseous cavity is
accompanied by adiabatic heating of the vapor phase of the bubble,
yielding localized and transient high temperatures and pressures.
Thus, the apparent chemical effects in liquid reaction media are
either direct or indirect consequences of these extreme conditions
[30]. Water has a high dielectric constant with a permanent dipole
moment, which allows the coupling between the oscillating electric
field and the molecular tumbling to occur with high efficient heat-
ing. Therefore, water acts as a pseudo-organic solvent at elevated
temperature. Isolation of products is also facilitated due to the de-
creased solubility of organic materials upon post reaction cooling.
On the other hand, organic reactions in water without using harm-
ful organic solvents is also one of the current focuses because water
is abundant, nontoxic and environment-friendly compared with
any organic solvents.
A. Samarat, J.B. Kraïem, T.B. Ayed, H. Amri, An efficient synthetic route to
functionalized d-lactams, Tetrahedron 64 (2008) 9540–9543.
[5] K. Damera, K.L. Reddy, G.V.M. Sharma, An efficient ZrCl4 catalyzed aza-Michael
addition reaction: synthesis of C-linked carbo b3-amino acids, Lett. Org. Chem.
6 (2009) 151–155.
[6] S. Sternativo, F. Marini, F.D. Verme, A. Calandriello, L. Testaferri, M. Tiecco, One-
pot synthesis of aziridines from vinyl selenones and variously functionalized
primary amines, Tetrahedron 66 (2010) 6851–6857.
[7] E.V. Matveeva, P.V. Petrovskii, I.L. Odinets, Efficient synthesis of racemic b-
aminophosphonates via aza-Michael reaction in water, Tetrahedron Lett. 49
(2008) 6129–6133.
[8] D. Perdicchia, K.A. Jørgensen, Asymmetric aza-Michael reactions catalyzed by
cinchona alkaloids, J. Org. Chem. 72 (2007) 3565–3568.
[9] K. De, J. Legros, B. Crousse, D. ele Bonnet-Delpon, Solvent-promoted and -
controlled aza-Michael reaction with aromatic amines, J. Org. Chem. 74 (2009)
6260–6265.
[10] C. Mukherjee, A.K. Misra, Aza-Michael addition of amines to activated alkenes
catalyzed by silica supported perchloric acid under a solvent-free condition,
Lett. Org. Chem. 4 (2007) 54–59.
[11] A.V. Narsaiah, Lanthanum trichloride (LaCl3): an efficient catalyst for conjugate
addition of amines to electron-deficient olefins, Lett. Org. Chem. 4 (2007) 462–
464.
[12] A.K. Verma, P. Attri, V. Chopra, R.K. Tiwari, R. Chandra, Triethylammonium
acetate (TEAA):
a recyclable inexpensive ionic liquid promotes the
chemoselective aza- and thia-Michael reactions, Monatsh. Chem. 139 (2008)
1041–1047.
[13] X. Ai, X. Wang, J.-m. Liu, Z.-m. Ge, T.-m. Cheng, R.-t. Li, An effective aza-Michael
addition of aromatic amines to electron-deficient alkenes in alkaline Al2O3,
Tetrahedron 66 (2010) 5373–5377.
[14] V. Polshettiwar, R.S. Varma, Tandem bis-aza-Michael addition reaction of
amines in aqueous medium promoted by polystyrenesulfonic acid,
Tetrahedron Lett. 48 (2007) 8735–8738.
[15] A.P. Esteves, M.E. Silva, L.M. Rodrigues, A.M.F. Oliveira-Campos, R. Hrdina, Aza-
Michael reactions with vinyl sulfones and Amberlyst-15 as catalyst,
Tetrahedron Lett. 48 (2007) 9040–9043.
[16] L. You, S. Feng, R. An, X. Wang, D. Bai, Silica gel accelerated aza-Michael
addition of amines to
5147–5149.
a,b-unsaturated amides, Tetrahedron Lett. 49 (2008)
[17] S. Azad, T. Kobayashi, K. Nakano, Y. Ichikawa, H. Kotsuki, Efficient Brønsted
acid-catalyzed aza-Michael reaction of amides and ureas with ,b-
a
4. Conclusion
unsaturated enones under high-pressure conditions, Tetrahedron Lett. 50
(2009) 48–50.
In conclusion, ultrasound-induced ecofriendly aza-Michael
reaction is reported. In this method no metallic, enzymatic or cor-
rosive catalysts or solid supports are used. The present procedure
has notable advantages that include simple operation procedure,
environmentally benign reaction conditions, faster reactions and
high yields of products. The method as reported herein will find
applications in other areas of research.
[18] A.-G. Ying, L. Liu, G.-F. Wu, G. Chen, X.-Z. Chen, W.-D. Ye, Aza-Michael addition
of aliphatic or aromatic amines to a,b-unsaturated compounds catalyzed by a
DBU-derived ionic liquid under solvent-free conditions, Tetrahedron Lett. 50
(2009) 1653–1657.
[19] (a) J. Lv, H. Wu, Y. Wang, Organocatalytic enantioselective aza-michael
additions of N-heterocycles to
(2010) 2073–2083;
a,b-unsaturated enones, Eur. J. Org. Chem. 11
(b) X. Liu, Y. Lu, Bifunctional thiourea-promoted cascade aza-Michael–Henry-
dehydration reactions: asymmetric preparation of 3-nitro-1,2-
dihydroquinolines, Org. Biomol. Chem. 8 (2010) 4063–4065;
(c) S.R. Roy, A.K. Chakraborti, Supramolecular assemblies in ionic liquid
catalysis for aza-Michael reaction, Org. Lett. 12 (2010) 3866–3869.
[20] K.P. Dhake, P.J. Tambade, R.S. Singhal, B.M. Bhanage, Promiscuous
Candida antarctica lipase B-catalyzed synthesis of b-amino esters via
aza-Michael addition of amines to acrylates, Tetrahedron Lett. 51 (2010)
4455–4458.
Acknowledgement
We gratefully acknowledge the funding support from National
Cancer Institute (NIH/NCI-P20, Grant #5P20CA138022-02).
[21] G. Imanzadeh, F. Ahmadi, M. Zamanloo, Y. Mansoori, Tetrabutylammonium
bromide media aza-Michael addition of 1,2,3,6-tetrahydrophthalimide to
symmetrical fumaric esters and acrylic esters under solvent-free conditions,
Molecules 15 (2010) 7353–7362.
[22] H. Xu, W.-M. Liao, H.-F. Li, A mild and efficient ultrasound-assisted synthesis
of diaryl ethers without any catalyst, Ultrason. Sonochem. 14 (2007) 779–
782.
[23] K.P. Guzen, A.S. Guarezemini, A.T.G. Orfao, R. Cella, C.M.P. Pereiraa, H.A.
Stefani, Eco-friendly synthesis of imines by ultrasound irradiation,
Tetrahedron Lett. 48 (2007) 1845–1848.
References
[1] A.R. Katritzky, A.F. Pozharskii, Handbook of Heterocyclic Chemistry, second ed.,
Pergamon, Oxford, 2002.
[2] Q. Lin, D. Meloni, Y. Pan, M. Xia, J. Rodgers, S. Shepard, M. Li, L. Galya, B. Metcalf,
T.-Y. Yue, P. Liu, J. Zhou, Enantioselective synthesis of janus kinase inhibitor
INCB018424 via an organocatalytic aza-Michael reaction, Org. Lett. 11 (2009)
1999–2002.
[3] (a) S. Fustero, S. Monteagudo, M. Sánchez-Roselló, S. Flores, P. Barrio, C. del
Pozo, N-Sulfinyl amines as a nitrogen source in the asymmetric intramolecular
aza-Michael reaction: total synthesis of (ꢁ)-pinidinol, Chem. Eur. J. 16 (2010)
9835–9845;
[24] P.T. Anastas, J.C. Warner, Green Chemistry Theory and Practice, Oxford
University Press, Oxford, UK, 1998.
[25] V. Polshettiwar, R.S. Varma, Aqueous microwave chemistry:
a clean and
green synthetic tool for rapid drug discovery, Chem. Soc. Rev. 37 (2008) 1546–
(b) S.-W. Liu, H.-C. Hsu, C.-H. Chang, H.-H.G. Tsai, D.-R. Hou, Asymmetric
synthesis of (ꢁ)-lentiginosine by double aza-Michael reaction, Eur. J. Org.
Chem. (2010) 4771–4773;
1557.
[26] K. Tanaka, F. Toda, Solvent-free organic synthesis, Chem. Rev. 100 (2000)
1025–1074.
(c) S. Fustero, D. Jiménez, J. Moscardó, S. Catalán, C. del Pozo, Enantioselective
organocatalytic intramolecular aza-Michael reaction: a concise synthesis of
(+)-sedamine, (+)-allosedamine, and (+)-coniine, Org. Lett. 9 (2007) 5283–
5286;
[27] V. Bejan, C. Moldoveanu, I.I. Mangalagiu, Ultrasound assisted reactions of
steroid analogous of anticipated biological activities, Ultrason. Sonochem. 16
(2009) 312–315.

D. Bandyopadhyay et al. / Ultrasonics Sonochemistry 19 (2012) 969–973
973
[28] (a) M.K. Chaudhuri, S. Hussain, M.L. Kantam, B. Neelima, Boric acid: a novel
and safe catalyst for aza-Michael reactions in water, Tetrahedron Lett. 46
(2005) 8329–8331;
(d) N. Srivastava, B.K. Banik, Bismuth nitrate-catalyzed versatile Michael
reactions, J. Org. Chem. 68 (2003) 2109–2114.
[29] B.C. Ranu, S. Banerjee, Significant rate acceleration of the aza-Michael reaction
in water, Tetrahedron Lett. 48 (2007) 141–143.
(b) N. Azizi, M.R. Saidi, LiClO4 Accelerated Michael addition of amines to
a, b-
unsaturated olefins under solvent-free conditions, Tetrahedron 60 (2004) 383–387;
(c) J. Cabral, P. Laszlo, L. Mahe, M.-T. Montaufier, S.L. Randriamahefa, Catalysis
of the specific Michael addition: the example of acrylate acceptors,
Tetrahedron Lett. 30 (1989) 3969–3972;
[30] M.R. Hoffmann, I. Hua, R. Hochemer, Application of ultrasonic irradiation for
the degradation of chemical contaminants in water, Ultrason. Sonochem. 3
(1996) S163–S172.