Ultrasound promoted the synthesis of N-propargylic β-enaminones

Article abstract of DOI:10.1016/j.ultsonch.2011.06.011

The synthesis of 14 novel N-propargylic β-enaminones from the reaction of β-alkoxy vinyltrihalomethyl[carboxyethyl] ketones [R ³C(O)CHC(R¹)OMe, where R³ = CF₃, CCl₃, CO₂Et and R¹ = Me, Et, Pr, Bu, i-Pent, CH₂CH₂CO₂Me] with propargyl amines [R ²NHCH₂CCH, where R² = Pr, PhCH₂] is reported. Yields, solvents and reaction times needed for reaction completion, by microwave irradiation (MW), conventional thermal heating (TH) and under ultrasound irradiation (US) are compared. The best results were obtained under US irradiation in good to excellent yields (70-93%).

Full text of DOI:10.1016/j.ultsonch.2011.06.011

Ultrasonics Sonochemistry 19 (2012) 227–231
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Ultrasound promoted the synthesis of N-propargylic b-enaminones
b
a
c
a
Marcos A.P. Martins a, , Marcelo Rossatto , Liziê D.T. Prola , Lucas Pizzuti , Dayse N. Moreira ,
⇑
Patrick T. Campos a, Clarissa P. Frizzo a, Nilo Zanatta a, Helio G. Bonacorso a
a NUQUIMHE, Departamento de Química, Universidade Federal de Santa Maria, 97105 900 Santa Maria, RS, Brazil
b Instituto Federal de Educação, Ciência e Tecnologia Farroupilha, 98280 000 Panambi, RS, Brazil
c Universidade Federal da Grande Dourados, Faculdade de Ciências Exatas e Tecnológicas, 79804 970 Dourados, MS, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of 14 novel N-propargylic b-enaminones from the reaction of b-alkoxy vinyltrihalometh-
yl[carboxyethyl] ketones [R3C(O)CH@C(R1)OMe, where R3 = CF3, CCl3, CO2Et and R1 = Me, Et, Pr, Bu, i-Pent,
CH2CH2CO2Me] with propargyl amines [R2NHCH2C„CH, where R2 = Pr, PhCH2] is reported. Yields, sol-
vents and reaction times needed for reaction completion, by microwave irradiation (MW), conventional
thermal heating (TH) and under ultrasound irradiation (US) are compared. The best results were obtained
under US irradiation in good to excellent yields (70–93%).
Received 16 September 2010
Received in revised form 18 May 2011
Accepted 16 June 2011
Available online 2 July 2011
Keywords:
Enones
Ó 2011 Elsevier B.V. All rights reserved.
Enaminones
Propargyl amines
N-propargylic b-enaminones
1. Introduction
method to construct trihalomethylated compounds is to use
halogen-containing building blocks as starting reactants [19,20].
b-Enaminones are useful synthetic intermediates due to the
presence of the ambident nucleophilic character of the enamine
moiety and the ambident electrophilic character of the enone moi-
ety, and their utilization in organic synthesis is currently a subject
of great interest [1]. These compounds have attracted much
attention due to the fact that they are important synthons for the
synthesis of many biologically active compounds such as dopamine
auto-receptor agonists [2], acetylcholinesterase inhibitors [3] and
anticonvulsants [4]. They are also useful intermediates for the prep-
aration of several aminoacids [5], aminols [6], peptides and alkaloids
[7]. In addition, they have been employed as synthetic building
blocks of a wide variety of heterocycles [8], as well as in the obtain-
ment of other synthetic intermediates [9].
The most well-known and exploited route to synthesize
b-enaminones involves the direct condensation of b-dicarbonyl
compounds with amines in refluxing aromatic hydrocarbon, with
azeotropic removal of water [10]. Some improved procedures have
been reported for the reaction of b-dicarbonyl compounds with
amines using Lewis acid as catalysts [11], ionic liquids [12], and
others [13–18].
b-Enamino trihalomethyl ketones are commonly prepared from
b-alkoxyvinyl trihalomethyl ketones [20] or acetylenes [21]. One
of the most effective methods used to synthesize b-enamino triha-
lomethyl ketones is the amination reaction of b-alkoxyvinyl tri-
fluoro[chloro]methyl ketones [22]. Nevertheless, there are always
some drawbacks with these procedures, such as long reaction times
[23] high temperatures [12a], use of costly catalysts [11] and high
catalyst loading [18,11]. Although the synthesis of b-enaminones
has been very widely explored, few papers have demonstrated the
use of secondary amines, since they are less reactive than primary
amines, or the employment of environmentally friendly technolo-
gies [20]. Therefore, the development of new starting materials with
good efficiency, convenient procedures, and delivery of better yields
is of great interest.
Alkynes are extremely important building blocks in organic syn-
thesis [25], as the richness of this functional group renders it extre-
mely versatile as a synthon for a diverse array of modern synthetic
methods, and the introduction of this group into b-enaminones is
of great interest. The synthesis of N-propargylic b-enaminones has
been reported to be carried out using a number of methods, includ-
ing: the Sonogashira cross-coupling of terminal alkynes with acyl
chlorides [26], the reaction of 2,2-difluoro-4-alkylamino-1,3,2-diox-
aborinane and propargyl amine [27], the use of b-keto esters with
triethyl orthoformate and acetic anhydride followed by the addition
of propargyl amine, affording the corresponding ethyl nicotinoylac-
rylate [28], N-alkylation of b-enaminones with 3-bromopropyne
It is well-known that the introduction of trihalomethyl groups
into heterocyclic compounds may have a significant influence on
their biological and physical properties [19a]. The most convenient
⇑
Corresponding author. Tel.: +55 55 3220 8756; fax: +55 55 3220 8031.
E-mail address: mmartins@base.ufsm.br (M.A.P. Martins).
1350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2011.06.011

228
M.A.P. Martins et al. / Ultrasonics Sonochemistry 19 (2012) 227–231
[29], condensation of amines with 1,3-dicarbonyl compounds [30],
and waste minimization compared with traditional methods [33a].
Furthermore, among alternative media, water is a very benign sol-
vent for organic transformation and it offers green chemistry bene-
fits and expedites the synthesis of diverse heterocycles [34].
Considering our interesting in new halogen- and alkynyl-containing
building blocks, herein we describe an efficient and mild approach to
the synthesis of new N-propargylic b-enaminones from the reaction
between b-alkoxy vinyltrihalomethyl[carboxyethyl]ketones and
propargyl amines using environmentally benign methods.
gold-catalyzed reaction [24], and the employment of
rated ketones [31].
a,b-unsatu-
In the last few years, the development of synthetic protocols
employing ultrasound irradiation has led to an epoch-making
change in organic reactions and has enabled the activation of poorly
reactive substrates [32]. The notable features of the ultrasound ap-
proach are enhanced reaction rates, formation of purer products in
high yields, easier manipulation and improved energy conservation
Table 1
Reaction conditions of enone 1a with propargyl amine 4.
O
H
Me
O
Me
OMe
i
N
H
+
F3C
N
F3C
H
1a
4
6a
Entry
Amine
Solvent
Methoda
Time
T (°C)
Product
Yield (%)b
1
2c
3
4
4
4
4
4
4
4
4
4
4
4
MeCN
MeCN
MeCN
EtOH
EtOH
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
[BMIM][BF4]
US
MW
TH
20 min
20 min
24 h
70–74
74
reflux
70–72
72
66
44
r.t.
50
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
90
73
90
70
75
90
89
50
60
58
37
4
US
20 min
20 min
20 min
20 min
20 min
20 min
20 min
20 min
5c
6
MW
US/IL
US/IL
TH/IL
TH/IL
TH/IL
MW/IL
7d
8
9
10
73
74
11e
a
b
c
US: Ultrasound irradiation; MW: Microwave irradiation; TH: thermal heating; IL: ionic liquid.
Yield of the isolated product.
A mixture of the desired product and unidentified byproducts was observed.
IL as co-solvent: 4 mL of IL and 6 mL of MeCN.
d
e
A mixture of the product and the starting material, in 50% of each one, was identified by NMR 1H.
Table 2
Conditions for the synthesis of compounds 6–10.
O
R1
O
R1
R2
i
N
H
+
R3
N
R3
OMe
H
R2
H
1a-f, 2a, 3a
4,5
6a-f, 7a,c-f, 8a, 9a,10a
i: MeCN, US, 70-74°C, 20 min (70-93%)
Reactants
Enone
Product
Yielda (%)
R3
R1
Amine
R2
1a
1b
1c
1d
1e
1f
1a
1c
1d
1e
1f
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CCl3
CCl3
CO2Et
Me
Et
Pr
Bu
4
4
4
4
4
4
5
5
5
5
5
4
5
5
Pr
Pr
Pr
Pr
Pr
Pr
Bn
Bn
Bn
Bn
Bn
Pr
6a
6b
6c
6d
6e
6f
7a
7c
7d
7e
7f
90
85
90
93
90
91
87
86
85
89
84
90
75
70
i-Pent
CH2CH2CO2Me
Me
Pr
Bu
i-Pent
CH2CH2CO2Me
Me
Me
Me
2a
2a
3a
8a
9a
10a
Bn
Bn
a
Yield of the isolated product.

M.A.P. Martins et al. / Ultrasonics Sonochemistry 19 (2012) 227–231
229
and ultrasound irradiation) for 15 min. However, the starting
material was not totally converted into the desired product.
For comparison, the reaction of N-propyl-N-prop-2-yn-1-amine
4 and 1,1,1-trifluoro-4-methoxy-3-penten-2-one 1a was also car-
ried out under conventional thermal heating, using refluxing ace-
2. Results and discussion
The starting materials, b-alkoxyvinyl ketones 1–3, were synthe-
sized from the acylation reaction of the respective enol ether or
acetal with trifluoroacetic anhydride, trichloroacetyl chloride or
ethyl oxalyl chloride [19]. Propargyl amines were prepared from
the reaction of prop-2-yn-1-yl methanesulfonate with primary
amines. Prop-2-yn-1-yl methanesulfonate was synthesized accord-
ing to the methodology described in the literature [35].
We began our study investigating the reaction of enone 1a with
N-propyl-N-prop-2-yn-1-amine 4, in equimolar amounts of the
reactants. An extensive reaction optimization was performed in or-
der to find the best condition to synthesize the N-propargylic b-
enaminones (Table 1). Initially, the synthesis of 6a was evaluated
using three different methodologies: ultrasound irradiation (US),
microwave irradiation (MW) and conventional thermal heating
(TH) during 20 min. Acetonitrile, ethanol and the ionic liquid
[BMIM][BF4] were selected as solvents. A 95–100% conversion
product found was in all methodologies, using the same reaction
time. The choice of acetonitrile as solvent was based on the prod-
uct yield. In an attempt to decrease the reaction time, the above
reaction was performed using these same methodologies (conven-
tional thermal heating with molecular solvents or ionic liquid;
microwave irradiation with molecular solvents or ionic liquid
tonitrile.
A longer time (24 h) was necessary to obtain the
product 6a with total conversion and yields similar to the US
methodology.
In order to evaluate the scope and limitations of this reaction,
we extended it to other enones and amines. All the reactions were
carried out in acetonitrile with equimolar amounts of the reactants
1–3 with 4,5 employing ultrasound irradiation during 20 min, in
accordance with the previous optimization (Table 2). N-Propargylic
b-enaminones 6–10 were obtained in good to excellent yields, and
all them were fully characterized with 1H and 13C NMR and GC–MS
(see Supplementary material).
The single-crystal X-ray diffraction patterns of N-Propargylic
b-enaminones 6–10 were not obtained, since all products were
isolated as oils. However we obtained a crystal structure of a
very similar compound, where R3 = CF3 and R2 = Ph, and the
E-configuration proposed to compounds 6–10 was confirmed. Crystal
structure of this compound obtained using ORTEP [36] (Fig. 1).
In all cases, some common features were found for the 1H and
13C NMR chemical shifts, which are described as follows. The 1H
NMR spectra of N-propargylic b-enamino trihalomethyl [ethoxy]
ketones presented two signals related to the coupling between
H6 and H7 as one of the main characteristics for these compounds
(Fig. 2). Hydrogen H6 was a doublet, while H8 was observed as a
triplet, both with a coupling constant of 4J = 2 Hz. Vinyl hydrogen
(H3) showed a chemical shift in the range of 5.30–5.54 ppm
(CF3), 5.80–5.98 (CCl3) and 5.97–6.14 (CO2Et) in line with the R3
substituent present in the enaminone. The 13C NMR spectra pre-
sented characteristic signals for C-6 in the range of 39.7–
40.5 ppm, C-8 at 73.5–74.3 ppm, C-7 at 75.9–77.2 ppm and C-3
at 85.9–88.4 ppm. Carbon C-1 presented a signal at 164 ppm
(CO2Et), 99.2–99.4 ppm (CCl3) and 117.4–117.9 ppm (CF3). Carbon
C-2 presented a chemical shift at the range of 174.6–175.8 ppm
when R3 = CF3, 179.1–180.7 ppm, when R3 = CCl3 and 177.8, when
R3 = CO2Et. The signal of C1 (CF3) was observed as a quartet due
to the coupling between the carbon- and the fluorine atoms, with
a coupling constant of 1J = 289 Hz. This behavior was also observed
for C2 (C@O), though the coupling constant was of 2J = 33 Hz.
Thus, based on previous works [37], the following sequence of
steps appears to afford a satisfactory explanation for the mecha-
nism of formation of the N-propargylic b-enaminones (Scheme 1).
This reaction involves the initial attack of the amine nitrogen atom
on the enone b-carbon with a charge delocalization to the carbonyl
group (type Michael-addition) and a subsequent elimination of the
alcohol molecule to furnish the desired products. There are many
works in the literature about Michael-addition reactions under
ultrasonic conditions [38,39]. However none of them give a specific
reason why US should facilitate this particular reaction. Therefore,
we consider that the most probably cause of the increased effi-
ciency using ultrasound irradiation is due to the cavitation phe-
nomena. During cavitation, the collapse of bubbles induces
mechanical stress that can be transmitted to a target bond [33b].
Fig. 1. A view of an N-propargylic b-enaminone with atomic labeling. Displacement
ellipsoids are drawn at the 50% probability level. H atoms are represented by circles
of arbitrary radii.
4
J
= 2 Hz
1
O
2
H
R
4
5
7
3
6
N
R
8
3
1
2
R
H
Fig. 2. Typical coupling constant of N-propargylic b-enamino trihalomethyl
[ethoxy]ketones.
O
R1
O
R1
O
OMe
R3
OMe
H
R3
N
R3
R2
N
R1
R2
H
-MeOH
R2
H
N
H
H
Scheme 1.

230
M.A.P. Martins et al. / Ultrasonics Sonochemistry 19 (2012) 227–231
4.3. Synthesis of N-propargylic b-enaminones 6–10
4.3.1. Method I (TH)
Propargyl amines
4 (1 mmol) were slowly (drop-by-drop)
added to a round-bottomed flask containing enone 1a (1 mmol)
and acetonitrile (2 mL), under magnetic stirring. After the addition,
the mixture was kept under stirring at room temperature during
12 h. The solvent was removed in a rotatory evaporator and the
products were purified by chromatographic column, using deacti-
vated silica and a mixture of ethyl acetate/hexane (1:10) as the
eluent.
4.3.2. Method II (TH/IL)
Fig. 3. Heating profile for US irradiation using MeCN as solvent for the synthesis
of 6a.
Propargyl amines
4 (1 mmol) were slowly (drop-by-drop)
added to a round-bottomed flask containing enone 1a (1 mmol)
and [BMIM][BF4] (1 mmol), under magnetic stirring. After the addi-
tion, the mixture was kept under stirring at 73 °C during 20 min.
Dichloromethane (5 mL) was added and the product was washed
with water (3 Â 5 mL). The solvent was removed in a rotatory
evaporator and the products were purified by chromatographic
column, using deactivated silica and a mixture of ethyl acetate/
hexane (1:10) as the eluent.
3. Conclusion
A concise and general synthetic route toward a series of N-prop-
argylic b-enaminones from the reaction of b-alkoxy vinil trihalom-
ethyl [carboxyethyl] ketones with propargyl amines using
ultrasound irradiation has been successfully developed.
4.3.3. Method III (MW)
A 10 mL microwave vessel equipped with a standard cap (com-
mercially furnished with vessel by CEM Discover) was filled with
the propargyl amines 4 (1 mmol), enone 1a (1 mmol) and 2 mL
of the solvent (see Table 1). After the vessel was sealed, the sample
was irradiated for the time and temperature indicated in Table 1,
which was plotted in Synergies Version 3.5.9 software applying a
maximum level of irradiation of 200 W and a maximum level of
internal vessel pressure of 250 psi. The irradiation power was in
a range from 1 to 95 W for reactions at 70–73 °C, without simulta-
neous cooling. The reaction mixture was subsequently cooled to
50 °C by compressed air. The reaction mixture was isolated with
dichloromethane (5 mL) and washed with water (3 Â 5 mL). The
dichloromethane was evaporated under vacuum and the products
were removed in a rotatory evaporator and purified by chromato-
graphic column, using deactivated silica and a mixture of ethyl ace-
tate/hexane (1:10) as the eluent.
4. Experimental
Unless otherwise indicated, all common reactants and solvents
were used as obtained from commercial suppliers without further
purifications. 1H and 13C NMR spectra were recorded on a Bruker
DPX 400 (1H at 400.13 MHz and 13C at 100.62 MHz) in 5 ppm sam-
ple tubes at 298 K (digital resolution 0.01 ppm) in CDCl3/TMS
solutions. Mass spectra were registered in a HP 5973 MSD con-
nected to a HP 6890 GC and interfaced by a Pentium PC. The GC
was equipped with a split-splitless injector, autosampler, cross-
linked to a HP-5 capillary column (30 m 0.32 mm of internal diam-
eter), and helium was used as the carrier gas. IR spectra were
recorded on a Tensor 27 – Bruker spectrometer with KBr pellets
in the 4000–400 cmÀ1 region. Reactions were performed using a
CEM Discover (300 W) microwave mono Mode for Synthesis con-
trolled by Synergis Version 3.5.9 software. The irradiation power
was established at a maximum level of 200 W, the internal vessel
pressure at a maximum level of 250 psi. The reactions in US were
carried out with a tapered microtip probe (6.5 mm) connected to
a 500 Watt Sonics Vibra-cell ultrasonic processor equipped with
integrated temperature control. The device operates at 20 kHz of
frequency and the amplitude was set to 20% of the maximum
power output.
4.3.4. Method IV (MW/IL)
A 10 mL microwave vessel equipped with a standard cap (com-
mercially furnished with vessel by CEM Discover) was filled with
the propargyl amines
4 (1 mmol), enone 1a (1 mmol) and
[BMIM][BF4] (1 mmol) (see Table 1). After the vessel was sealed,
the sample was irradiated for the time and temperature indicated
in Table 1, which was plotted in Synergies Version 3.5.9 software
applying a maximum level of irradiation of 200 W and a maximum
level of internal vessel pressure of 250 psi. The irradiation power
was in a range from 1 to 95 W for reactions at 70–73 °C, without
simultaneous cooling. The reaction mixture was subsequently
cooled to 50 °C by compressed air. The reaction mixture was iso-
lated with dichloromethane (5 mL) and washed with water
(3 Â 5 mL). The dichloromethane was evaporated under vacuum
and the products were removed in a rotatory evaporator and puri-
fied by chromatographic column, using deactivated silica and a
mixture of ethyl acetate/hexane (1:10) as the eluent.
4.1. Synthesis of prop-2-yn-1-yl methanesulfonate
Prop-2-yn-1-yl methanesulfonate was synthesized according to
the methodology described in the literature [35].
4.2. Synthesis of propargyl amines 4,5
To a stirred solution of amine (propylamine, benzylamine,
100 mmol) in THF (40 mL) at room temperature, prop-2-yn-1-yl
methanesulfonate (50 mmol) was added. The mixture was stirred
for 4 h at room temperature. Sodium bicarbonate solution
(3 Â 50 mL) was added and the products (4,5) were extracted with
diethyl ether (3 Â 30 mL). The solvent was evaporated in a rotary
evaporator and the products 4,5 were purified by distillation under
reduced pressure.
4.3.5. Method V (US)
Equimolar amounts (1 mmol) of both propargyl amines 4 and
enone 1a (1 mmol) in 10 mL of acetonitrile or IL were mixed and
the reaction mixture was irradiated with ultrasound irradiation at
70–74 °C for 20 min (Fig. 3). The solvent was removed in a rotatory
evaporator and the products were purified by chromatographic

M.A.P. Martins et al. / Ultrasonics Sonochemistry 19 (2012) 227–231
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The authors thank the Conselho Nacional de Desenvolvimento
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