Ultrasound-promoted reaction of 2-chlorobenzoic acids and aliphatic amines

Article abstract of DOI:10.1002/ejoc.200700281

An improvement to the use of DMF as a solvent for the condensation of 2-chlorobenzoic acids with aliphatic primary or secondary amines was described. A number of alkylaminobenzoic acids and dialkylaminobenzoic acids were synthesized in acceptable-to-good yield. The advantages of this procedure include readily available substrates, the use of an inexpensive copper powder without taking any precautions to exclude moisture under mild conditions and experimental ease. Furthermore, this condensation could also be achieved under nonclassical conditions by using ultrasonic irradiation at room temperature. We demonstrated that ultrasound-promoted condensation of 2-chlorobenzoic acid with aliphatic amines with the use of DMF as the solvent, especially in the case of secondary amines, affords products in high yields and reduces the reaction time to minutes. The results proved to be highly reproducible because the relevant sonochemical parameters were rigorously controlled. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700281

FULL PAPER
DOI: 10.1002/ejoc.200700281
Ultrasound-Promoted Reaction of 2-Chlorobenzoic Acids and Aliphatic Amines
[a]
[a]
[b]
[b]
Maite L. Docampo, Rolando F. Pellón,* Ana Estevez-Braun, and Angel G. Ravelo
Keywords: Ullmann condensation / Copper / Catalysis / Ultrasound / Aliphatic amines
An improvement to the use of DMF as a solvent for the con-
densation of 2-chlorobenzoic acids with aliphatic primary or
at room temperature. We demonstrated that ultrasound-pro-
moted condensation of 2-chlorobenzoic acid with aliphatic
secondary amines was described. A number of alkylamino- amines with the use of DMF as the solvent, especially in the
benzoic acids and dialkylaminobenzoic acids were synthe-
sized in acceptable-to-good yield. The advantages of this
procedure include readily available substrates, the use of an
inexpensive copper powder without taking any precautions
to exclude moisture under mild conditions and experimental
ease. Furthermore, this condensation could also be achieved
under nonclassical conditions by using ultrasonic irradiation
case of secondary amines, affords products in high yields and
reduces the reaction time to minutes. The results proved to
be highly reproducible because the relevant sonochemical
parameters were rigorously controlled.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
motion of N-methylglycine. By using -proline as a pro-
moter, the coupling reaction of aryl iodides or -bromides
with aliphatic primary amines proceeded at 60–90 °C. The
steric hindrance of secondary amines makes the coupling
reaction with aryl halides more sluggish than that of pri-
mary amines, and the coupling reaction between iodo-
benzene with morpholine or piperidine occurred at 90 °C.
When less bulky pyrrolidine was used, the coupling reaction
with aryl iodides proceeded at 65 °C.
It is well-known that the substitution of a halogen atom
onto a benzene ring does not take place easily. Classical
Ullmann chemistry along with closely related methods have
been known for a century and served well for C–N, C–S
and C–O bond formation reactions.[ These reactions re-
quire harsh prolonged heating at 200 °C or higher in the
1]
I
II
presence of stoichiometric amounts of simple Cu or Cu
salts or oxides. The discovery of the Pd-catalyzed amination
In all cases the reaction conditions were CuI (0.5 mmol),
amino acid (1 mmol), aryl halide (5 mmol), amine
Buchwald–Hartwig reaction)[ reaction was a major
2]
(
breakthrough in the chemistry of amines, which opened ac-
cess to huge numbers of previously inaccessible compounds.
Nevertheless, high costs of palladium and some problematic
functional group tolerances forced scientists to reconsider
other transition-metal catalysts, mainly nickel and copper.
A new interest in much cheaper and more practical copper-
catalyzed chemistry was brought about by the observation
that the appropriate ligand can modulate the reactivity of
(
(
7.5 mmol), K CO (10 mmol for aryl iodide) or K PO
10 mmol for aryl bromide), DMSO (3 mL) and N-methyl-
2 3 3 4
glycine or -proline, and the range of reaction time was 12
to 40 h. All reactions were carried out under a nitrogen at-
mosphere. DMF and DMSO were freshly distilled from
CaH2.
The combination of copper iodide and cesium acetate
without ligands in the presence of DMSO or DMF as the
solvent was used for the copper-mediated intermolecular
amination of aryl iodides for the synthesis of secondary
amines under an argon atmosphere. Currently, the major
restriction for this method is the low reactivity of the sec-
ondary acyclic amines, which is most likely due to steric
reasons, so the scope of amines usually includes primary
aliphatic and saturated secondary heterocycles, such as pi-
peridine, piperazine, pyrrolidine, morpholine etc.
The majority of acids investigated were either iodoben-
zoic- or bromobenzoic acids, and only a few examples in-
cluded the use of chlorobenzoic acids. Because chloroben-
zoic acids are cheaper and more readily available than
iodobenzoic- and bromobenzoic acids, we further explored
the scope of the Ullmann condensation using chlorobenzoic
the catalyst and thus enable us to achieve more effective
and more versatile catalytic systems.[
3–12]
The scope for CuI
and amino acid catalyzed reaction of aryl halides with pri-
[
14]
[
13]
mary amines was explored in a recent publication.
The
coupling reaction of electron-deficient aryl iodides with ali-
phatic primary amines occurred at 40 °C under the pro-
[
a] Departamento de Química, Centro de Química Farmacéutica,
Calle 21 y 200 Atabey, Playa. Apdo. 16042 CP 11600 Ciudad
de la Habana, Cuba
Fax: +537-2736471
[b] Instituto Universitario de Bio-Orgánica Antonio González,
Universidad de la Laguna,
Avda. Astrofisico F. Sanchez 2, 38206 La Laguna, Tenerife, Es-
paña
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 4111–4115
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4111

M. L. Docampo, R. F. Pellón, A. Estevez-Braun, A. G. Ravelo
FULL PAPER
acids. Here we present the condensation of aliphatic pri- acid (1.0 equiv.) and dipropylamine (2.0 equiv., Table 1, En-
mary or secondary amines with 2-chlorobenzoic acid deriv- try 15b) as the coupling partners with copper powder
atives in the presence of N,N-dimethylformamide as the sol- (3 wt.-%) as the catalyst.
vent under ultrasonic irradiation conditions.
Results and Discussion
In a previous publication,[15] we analyzed the influence
of some parameters on the condensation of 2-chlorobenzoic
Scheme 2. Condensation of 2-cholorobenzoic acids with aliphatic
primary or secondary amines in DMF as the solvent.
acid and substituted primary alkylamines in water as the
solvent to yield the corresponding 2-(alkylamino)benzoic
acids. It was observed that the reaction of 3 wt.-% of cop-
per, 2.0 equiv. of alkylamine per mole of 2-chlorobenzoic
acid and 1.0 equiv. of potassium carbonate afforded, after
Experimental results shown in Figures 1 and 2 demon-
strate that better yields could be obtained with 1.0 equiv. of
K CO and a reaction time of 2 h in the case of 3a (72%)
2
3
and a 4 h reaction time in the cases of 8b and 15b (61 and
0%, respectively). It is noteworthy that when we used more
than 1.0 equiv. of K CO (basic pH) the yield of 3a harshly
5
5
h, the corresponding 2-(alkylamino)benzoic acid with
2
3
yields superior to those reported in the literature with the
diminished down to 12% with the use of 4.0 equiv. of
use of other preparative procedures.[
16–18]
We also examined
K CO . Nevertheless, in the case of secondary amines (8b
2
3
the effect of ultrasonic irradiation for the synthesis of these
compounds, increasing the yields in a 20% rate after only
and 15b) an excess of K CO did not affect the reaction
2
3
yield. This indicates that in the case of primary amines it is
necessary that the reaction media has a neutral pH value.
[
15]
2
0 min (Scheme 1).
Scheme 1. Condensation of 2-cholorobenzoic acid with primary
amines in water as the solvent.
We attempted the condensation reaction of secondary
amines with 2-chlorobenzoic acid using ultrasound and
water as the solvent, but condensation did not take place
satisfactorily and salicylic acid was obtained in high yield.
It is assumed that copper ions can form chelates with 2-
Figure 1. Plot of the yield [%] obtained in the condensation of 2-
chlorobenzoic acid derivatives with a primary amine (3a), second-
ary cyclic amine (8b) or secondary acyclic amine (15b) as a function
halobenzoic acids through the carboxyl group, thereby fa- of reaction time [h].
cilitating the nucleophilic substitution.[19] These copper che-
lates are destroyed in basic reaction media, and it is then
not possible to obtain salicylic acid. We demonstrated that
if pyridine is present there is a competition between the
nucleophilic agent and the hydroxide group. So, we studied
the effect of pyridine on the Ullmann condensation and
found that pyridine acts as a ligand and reduces the reac-
[
20a,20b]
tion time.
lizes the intermediate copper complex.
These results suggest that pyridine stabi-
[20c]
It may be pos-
sible that the secondary amines have the same effect as pyr-
idine. After several trials, we found that this problem could
be solved by using DMF as the solvent. DMF reduces the
Figure 2. Plot of the yield [%] obtained in the condensation of 2-
possibility that 2-chlorobenzoic acid can be hydrolyzed chlorobenzoic acids derivatives with a primary amine (3a), second-
salicylic acid formation).[
As a first part of this work we studied the condensation
20d]
ary cyclic amine (8b) or secondary acyclic amine (15b) as a function
of K CO equivalents.
(
2
3
of 2-chlorobenzoic acid derivatives with primary and sec-
ondary amines using DMF as the solvent (Scheme 2). To
On the basis of the above results, the condensation of
find the optimal reaction conditions, different reaction different 2-chlorobenzoic acids with several primary and
times and equivalents of K CO were examined because secondary aliphatic amines was tested, and the results are
2
3
these parameters play an important role in this reaction. summarized in Table 1. For primary amines (Table 1, En-
We set out to test our proposal using 2-chlorobenzoic acid tries 1a–16a) it was found that glycine (amino acid) gave
(
2
(
1.0 equiv.) and propylamine (2.0 equiv., Table 1, Entry 3a), a better conversion relative to other alkylamines (compare
-chloro-4-fluorobenzoic acid (1.0 equiv.) and morpholine Table 1, Entries 12a–16a, and Entries 1a–11a). Presumably,
2.0 equiv., Table 1, Entry 8b) or 2-chloro-4-nitrobenzoic glycine acts as a ligand to form a copper complex, which
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Eur. J. Org. Chem. 2007, 4111–4115

Ultrasound-Promoted Reaction of 2-Chlorobenzoic Acids and Amines
FULL PAPER
Table 1. Condensation of 2-chlorobenzoic acid derivatives facilitates the coupling reaction. In general, by using DMF
(
1.0 equiv.) with primary or secondary amines (2.0 equiv.) in DMF
the yields in the condensation of primary amines were sig-
nificantly improved and the reaction time was reduced dra-
matically relative to those with water as the solvent.[
as the solvent with and without the use of ultrasound.
15]
For secondary amines (Table 1, Entries 1b–20b), we
achieved moderate coupling yields (50–71%); this can be
ascribed to the enhanced steric hindrance of the secondary
amines relative to the primary amines. In the case of sec-
ondary acyclic amines (Table 1, Entries 13b–20b), the yields
obtained were higher than (51–57%) those reported for this
[
13]
coupling. Ma
described the coupling of iodobenzene
with diethylamine, and the yield obtained was 21% in the
presence of -proline as the ligand, Cu/CuI as the catalyst
with DMSO as the solvent and a reaction time of 40 h.
They also reported the coupling of iodobenzene with dibu-
tylamine by using the same conditions and obtained only
10% yield. These results demonstrated that the steric hin-
drance of acyclic secondary amines inhibited the coupling
reaction.
To improve the yields and decrease the reaction times,
high-intensity ultrasound was employed by us in the con-
densation of 2-chlorobenzoic acids with primary and sec-
ondary amines using the Ullmann condensation reaction in
DMF as the solvent.
There are some examples concerning the use of ultra-
sound to increase both the reaction rates and the yields of
products and allowing, in many cases, milder reaction con-
[
21–26]
ditions.
High-power ultrasound has become the focus
of a new field, known as “sonochemistry”. It involves the
study of the effects of acoustic waves on chemical systems.
The cavitation produced by the passage of ultrasonic waves
through the solvent gives rise to sufficiently high tempera-
tures and pressures to clean the surface of the metal, which
hinders the formation of impurities before and during the
[
27]
reaction.
In our initial screening experiments, we chose the con-
densation of 2-chlorobenzoic acid with propylamine
(Table 1, Entry 3a), morpholine (Table 1, Entry 8b) or di-
propylamine (Table 1, Entry 15b) (2.0 equiv.) in DMF with
copper powder as the catalyst under high-intensity ultra-
sound as models to determine suitable reaction conditions.
As illustrated in Figure 3, 3a was obtained in 84% yield
after a reaction time of 25 min. When the condensation was
carried out with morpholine, 15 min were necessary to
achieve 82% yield of 8b, whereas in the case of dipropyl-
amine, 20 min were required to obtain 72% yield of 15b.
Finally, we studied the quantity of K CO to be em-
2
3
ployed in those reactions (Figure 4). It was demonstrated
that, for all cases, 1.0 equiv. of base was enough to obtain
the best yields. Despite the fact that pH control of the solu-
tion is necessary when we use classical heating conditions,
in this case the basic pH did not affect the reaction yield
under ultrasonic irradiation.
Although the results did not differ significantly at 18, 20,
3
2
0 and 35 kHz, more reproducible results were obtained at
0 kHz, and all the experiments were carried out at this
[
[
(
a] Yield of isolated and recrystallized products from EtOH/H
b] Conventional conditions, reaction time: 2 h (1a–16a) and 4 h
1b–20b). [c] Ultrasonic irradiation (20 kHz, 350 W; 25 °C), reac-
tion time: 25 (1a–16a), 15 (1b–12b) and 20 min (13b–20b).
2
O.
frequency. Higher frequencies cause higher rates of molecu-
lar motion, which results in greater power losses. If the fre-
Eur. J. Org. Chem. 2007, 4111–4115
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4113

M. L. Docampo, R. F. Pellón, A. Estevez-Braun, A. G. Ravelo
FULL PAPER
Bruker AC 250 F Spectrometer at 300 K and with DMSO as the
1
13
solvent unless otherwise stated. The H and C chemical shifts are
reported in ppm relative to residual deuterated solvent signal as an
internal reference. The coupling constants (J) are given in Hz. Mass
spectra were recorded with a Spectrometer Q-TOFII-Micromass,
Manchester Ing. Elemental analyses were carried out with a Fisons
EA 1108 CHNS-O apparatus at the Microanalytical Unit of the
Instituto de Biorgánica de la Universidad de La Laguna, Tenerife,
Spain. All reagents purchased from commercial sources were used
without further purification. The yields reported in the publication
represent an average of at least three independent runs.
Figure 3. Plot of yield [%] obtained in the condensation of 2-chlo-
robenzoic acid derivatives with a primary amine (3a), secondary
cyclic amine (8b) or secondary acyclic amine (15b) in the presence
of ultrasonic irradiation as a function of reaction time [min].
Ultrasonic Irradiation Experiments: All ultrasonic irradiation ex-
periments were carried out in
a sonochemical apparatus
SONIPRED-150. The frequency can be tuned between 18 and
3
3
5 kHz and the power can be varied up to a maximum output of
50 W. All the reactions were performed on an open flat-bottomed
glass tube (diameter: 30 mm; thickness: 1 mm; volume: 50 mL).
General Procedures for the Condensation of 2-Chlorobenzoic Acids
with Primary or Secondary Amines (1a–16a and 1b–20b)
A) Under Conventional Heating: A mixture of the 2-chlorobenzoic
acid derivative (0.04 mol), alkylamine (0.08 mol), anhydrous potas-
sium carbonate (0.02 mol) and copper powder (0.003 mol) was
heated at reflux in DMF (25 mL) for 2 h (primary amines) or 4 h
(
secondary amines). The mixture was slowly added with shaking to
HCl/H O (1:1). The solid obtained was filtered off, washed with
water and dissolved in aqueous sodium hydroxide (10%, 30 mL).
The basic solution was acidified with AcOH/H O (1:3, 40 mL) to
Figure 4. Plot of yield [%] obtained in the condensation of 2-chlo-
robenzoic acid derivatives with a primary amine (3a), secondary
cyclic amine (8b) or secondary acyclic amine (15b) in the presence
2
2 3
of ultrasonic irradiation as a function of K CO equivalents.
2
pH 5. The 2-(alkylamino)benzoic acid (1a–16a) or 2-(dialkylamino)
benzoic acid (1b–20b) crystallized, was filtered off and washed with
water. Further purification was carried out by recrystallization
quency is increased, the production of cavities in the liquid
is decreased and therefore the sonochemistry effect is less
effective.
2
from EtOH/H O.
[
28–32]
B) Under Ultrasonic Irradiation: The 2-chlorobenzoic acid deriva-
tive (0.04 mol), alkylamine (0.08 mol), anhydrous potassium car-
bonate (0.02 mol) and copper powder (0.003 mol) were added to
the flat-bottomed glass tube. The mixture was sonicated for 15–
To demonstrate the scope of the ultrasound-assisted
method, several 2-chlorobenzoic acids and different pri-
mary or secondary amines were used. The results are shown
in Table 1. Under these conditions, the yields were signifi-
cantly improved to 83–97% in primary amines (Table 1, En-
tries 1a–16a) and to 72–92% in secondary amines (Table 1,
Entries 1b–20b). The reaction time was dramatically re-
duced from hours to minutes.
25 min at 20 kHz and 300 W by using an immersion horn. The
reactions were monitored by TLC using ethyl acetate/chloroform/
acetic acid (8:6:1). After completion of the reaction, the mixture
was slowly added with shaking to HCl:H O (1:1). The solid ob-
2
tained was filtered off, washed with water and dissolved in aqueous
sodium hydroxide (10%, 30 mL). The basic solution was acidified
2
with AcOH/H O (1:3, 40 mL) to pH 5. The 2-(alkylamino)benzoic
Conclusions
acid (1a–16a) or 2-(dialkylamino) benzoic acid (1b–20b) crys-
tallized, filtered off and washed with water. Further purification
The use of DMF as the solvent for the condensation of
was carried out by recrystallization from EtOH/H
2
O.
2
-chlorobenzoic acids with aliphatic primary or secondary
The structures of the products obtained were confirmed by melting
amines affords the corresponding 2-(alkylamino)benzoic ac-
ids or 2-(dialkylamino)benzoic acids in acceptable-to-good
yields and with a simple workup procedure. Furthermore,
the use of ultrasound-promoted condensation reduces the
reaction time in all cases to minutes and improves the
yields.
1
13
point, elemental analyses, H NMR and CNMR spectroscopy
and mass spectra corresponding to those reported in the litera-
[16,18,33–36]
ture.
Supporting Information (see footnote on the first page of this arti-
cle): The experimental and spectroscopic data of 1a–16a and 1b–
20b.
Experimental Section
Acknowledgments
General: Reactions were monitored by TLC on Merck 60 F254
(
0.25 mm) plates, which were visualized by CAMAG UV–CABI-
The authors thank Miss Anais Fernández for her kindness in meas-
uring the NMR spectra. We also thank Mr. Juan Carlos Rodriguez
for his help with the literature search. Financial support by the
Project SECAB-CYTED is gratefully acknowledged.
NET II at 254 nm. Melting points were determined in open capil-
laries with a GALLENKANMP melting point apparatus and are
uncorrected. 1H and C NMR spectra were recorded with a
13
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Ultrasound-Promoted Reaction of 2-Chlorobenzoic Acids and Amines
[
FULL PAPER
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Received: March 29, 2007
Published Online: July 3, 2007
8
Eur. J. Org. Chem. 2007, 4111–4115
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