Continuous flow reactions in water for the synthesis of propargylamines via a metal-free decarboxylative coupling reaction

Article abstract of DOI:10.1016/j.tetlet.2015.06.029

Abstract A range of propargylamines was synthesized via the metal-free decarboxylative coupling of alkynyl carboxylic acids with amines and paraformaldehyde in water, using a continuous flow reaction system. Aryl- and alkyl-substituted propiolic acids were found to react with secondary amines in the presence of paraformaldehyde, at 140°C in water to give the desired propargylamines in good yield.

Full text of DOI:10.1016/j.tetlet.2015.06.029

Accepted Manuscript
Continuous flow reactions in water for the synthesis of propargylamines via a
metal-free decarboxylative coupling reaction
Bongkyun Jung, Kyungho Park, Kwang Ho Song, Sunwoo Lee
PII:
S0040-4039(15)01022-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.06.029
TETL 46421
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
20 April 2015
4 June 2015
11 June 2015
Please cite this article as: Jung, B., Park, K., Song, K.H., Lee, S., Continuous flow reactions in water for the synthesis
of propargylamines via a metal-free decarboxylative coupling reaction, Tetrahedron Letters (2015), doi: http://
dx.doi.org/10.1016/j.tetlet.2015.06.029
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Tetrahedron Letters
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Continuous flow reactions in water for the synthesis of propargylamines via a metal-
free decarboxylative coupling reaction
Bongkyun Junga , Kyungho Parkb , Kwang Ho Songa,c,∗, Sunwoo Leeb,∗
aGreen School, Korea University, Seoul 136-713, Republic of Korea
bDepartment of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea
cDepartment of Chemical & Biological Engineering, Korea University, Seoul 136-713, Republic of Korea
ARTICLE INFO
ABSTRACT
Article history:
Received
A range of propargylamines was synthesized via the metal-free decarboxylative coupling of
alkynyl carboxylic acids with amines and paraformaldehyde in water, using a continuous flow
reaction system. Aryl- and alkyl-substituted propiolic acids were found to react with secondary
amines in the presence of paraformaldehyde, at 140 °C in water to give the desired
propargylamines in good yield.
Received in revised form
Accepted
Available online
2015 Elsevier Ltd. All rights reserved.
Keywords:
Flow reaction
Decarboxylative coupling
Propargylamine
Metal-free
Propiolic acid
In recent years, the development of environmentally benign
methodologies has received considerable attention in the fine
chemical industry, with novel green processes being developed
and published in increasing numbers.1 Important examples of
green reaction systems for synthetic chemistry include recyclable
catalysts2 and the use of ionic liquids3 as solvents. In addition, the
development of water-based reaction systems is of particular
importance in relation to green chemistry as the majority of waste
arises from organic solvents.4 However, although water is an
abundant and cheap solvent, it is problematic for use in reactions
requiring temperatures greater than 100 °C. Special equipment
for high-pressure use is therefore required for such reactions.
The continuous flow reactor is an economical and efficient
process system that can often be used to replace lower efficiency
batch reactors. Continuous flow reactors have preferentially been
used in organic synthesis, as a number of flow reactors can be
placed in parallel to afford the desired product on a large scale
without the requirement for large-scale reactors.5 In addition, this
system has a high surface area-to-volume ratio, thus providing
efficient heat and mass transfer.6 Moreover, the flow reaction
system displays good resistance to high solvent vapor pressures,
and can therefore be employed in reactions requiring
temperatures greater than the boiling point of the solvent.
The propargylamine moiety is an important bioactive
compound,7 and has been used as a building block for the
———
preparation of versatile heterocyclic derivatives, such as
pyrroles,8 pyrrolidines,9 pyrrolophanes,10 aminoindolizines,11 and
oxazolidinones.12 A number of preparation methods have been
reported for propargylamines,13 including the transition metal-
catalyzed A3-coupling reaction.14 This reaction is a three-
component reaction between an alkyne, an amine, and an
aldehyde, employing catalysts based on gold,15 iridium,16 zinc,17
mercury,18 nickel,19 iron,20 indium,21 and copper.22
With relation to our ongoing studies for the decarboxylative
coupling reaction of alkynyl carboxylic acids,23 we developed a
copper-catalyzed three component reaction between aldehydes,
amines, and alkynyl carboxylic acids for the preparation of
propargylamines.23s It was found that this three-component
reaction could be carried out in the absence of transition metal
catalysts when paraformaldehyde was employed as the aldehyde
source.23n In addition, the metal-free A3-coupling reaction
between
phenylpropiolic
acid,
morpholine,
and
paraformaldehyde in water at 100 °C gave the desired propargyl
amine in 97% yield.
We therefore chose to expand this green process, by
attempting to carry out the A3-coupling reaction with a variety of
amines. Disappointingly, low yields of the desired coupled
products were obtained in the majority of cases. We therefore
chose to employ the continuous flow reaction system to address
the yield issues, as this system allowed the use of
∗ Corresponding author. Tel.: +82 2 3290 3307 (K. H. Song); Tel.: +82 62 530 3385 (S. Lee)
E-mail: khsong@korea.ac.kr (K. H. Song), sunwoo@chonnam.ac.kr (S. Lee)

Scheme 1. Synthesis of propargylamines via an A3 coupling reaction.
Table 1. Optimized conditions for the solubility of the three
reagents in watera
found to result in a significant increase in yield of the desired
product. In addition, when the residence time was doubled, yields
of 89% at 120 °C and 96% at 140 °C were obtained (entries 5 and
6), with the conditions of entry 6 being chosen as the optimal
conditions for the reaction. However, when the reaction was
conducted in the pressure vessel at 140 °C for 2 h, the reaction
yield was 76% (entry 7).
Phenylpropiolic
Entry
acid
Morpholine
+
Paraformaldehyde
Solubility
1
2
3
4
5
+
Lowb
Highc
Highc
Lowb
Highc
+
+
+
+
+
aAll reagents were dissolved in water (26.0 mL). Phenylpropiolic acid (1.930
g), morpholine (1.150 g), and paraformaldehyde (0.396 g) were used in the
solubility tests. Key: + = reagent present; - reagent absent.
Table 2. Optimized reaction conditions for the continuous
flow reactora
bLow: Undissolved or precipitated solids were present.
cHigh: No undissolved or precipitated solids were observed
reaction temperatures in excess of the boiling point of the
reaction solvent. The high pressure feed in a flow reaction system
could prevent a solvent from vaporization at higher temperature.
Therefore, we herein report a metal-free synthesis of propargyl
amines in water using a continuous flow reactor system.
In order to carry out the metal-free decarboxylative coupling
reaction using a flow system with water as solvent, we first
studied the solubility of all reagents. Phenylpropiolic acid was
found to be insoluble in water, even at temperatures up to 100 °C.
However, when phenylpropiolic acid and morpholine were mixed
together in water, the solubility of both reagents increased, and a
clear solution was observed. In contrast, paraformaldehyde was
found to be soluble in water.
Based on the solubility results, two reservoirs were prepared for
carrying out the reaction. The first reservoir contained an
aqueous solution of phenylpropiolic acid and morpholine, while
the second reservoir contained an aqueous solution of
paraformaldehyde. Each reservoir was connected to the flow
channel and their flow rate was controlled by means of a syringe
pump. The two flow channels were connected to a tee-union
mixer and a mixture of the two solutions was allowed to flow
through a piece of tubing placed in an oil bath. In order to
address issues relating to high pressure within the tubing, a
backpressure regulator was attached to the end of the tubing
channel. The reaction mixtures were allowed to flow at a rate that
ensured a 1 h residence time at 100 °C, giving the desired
product in 6% yield (entry 1). In order to improve the yield, the
temperature of the system was increased (entries 2-4). This was
Entry
Temp.
(°C)
100
120
140
150
120
140
140
Residence
Time (h)b
Pressure
(bar)
1.2
2.0
4.0
Yield (%)
1
2
1
1
1
1
2
2
2
6
73
91
92
89
96
76
3
4
5.0
2.0
4.0
3.6d
5
6
7c
aReservoir A: Phenylpropiolic acid (13.2 mmol) and morpholine (13.2 mmol)
were dissolved in water to a final volume of 26 mL. Reservoir B:
Paraformaldehyde (13.2 mmol) was dissolved in water to a final volume of
26 mL. The reagent solutions in the two reservoirs were allowed to flow
through the reaction tube. Flow rate = 0.0837 mL/min = 0.0419 mmol/min =
2.5 mmol/h.
bResidence time: 1 h = 1/16 inch tube, 10 m; 2 h = 1/16 inch tube, 20 m.
cReaction was conducted in the pressure vessel at 140 °C.
dVapor pressure of reaction mixture in the vessel.

Following successful optimization of the reaction conditions,
we then chose to extend the substrate scope of the reaction. A
range of amines was tested, and the results can be seen in Table
3.24 Cyclic amines pyrrolidine (2b), piperidine (2c), and azepane
(2d) were reacted with phenylpropiolic acid and
paraformaldehyde in the flow reactor to give the corresponding
propargylamines in 89%, 96%, and 88% yield, respectively
(entries 1-3). A3-coupling reactions using acyclic amines
diallylamine (2e), and N- methylbenzylamine (2f) also gave the
desired propargylamines in good yield (entries 4 and 5). In
addition, it was found that the substrate scope could be further
expanded to vary the alkyl-substituted propiolic acids used in the
reaction. Reactions of morpholine (2a) with 2-butynoic acid (1b),
2-pentanoic acid (1c) and 2-octynoic acid (1d) gave the desired
propargylamines in good yield (entries 6 - 8). When 2-octynoic
acid (1d) was allowed to react with pyrrolidine (2b) and
paraformaldehyde, the desired propargylamine 3db was formed
in 88% yield. (entry 9). In addition, the reaction with piperidine
(2c) also showed good yield (entry 10). We found that all results
showed better yield than those of the batch reactions.23n In
contrast, all primary amines including cyclohexylamine (2g) and
aniline (2h) studied gave unsatisfactory results compared to
secondary amines (entries 11 and 12). This is comparable to the
results obtained for the reaction under batch conditions.23n
Table 3. Synthesis of propargylamine in water using the continuous flow processa
Entry
1
R1
Amine
Productb
Yield (%)c
89 [87]d
Ph
Ph
1a
1a
2b
2c
3ab
3ac
2
3
96 [91]d
88 [83]d
Ph
Ph
1a
1a
2d
2e
3ad
3ae
Ph
94 [81]d
N
4
5
6
7
Ph
Me
Et
1a
1b
1c
2f
2a
2a
3af
3ba
3ca
93 [90] d
91 [84]d
93 [81]d
8
9
n-Pent
n-Pent
1d
1d
1d
2a
2b
2c
3da
3db
3dc
89 [76]d
88 [80]d
91 [84]d
10 n-Pent
11
12
Ph
Ph
1a
1a
2g
2h
-
-
-
-
-
-
aReservoir A: Substituted propiolic acid 1 (13.2 mmol) and amine 2 (13.2 mmol) were dissolved in water to a final volume of 26 mL. Reservoir B:
Paraformaldehyde (13.2 mmol) was dissolved in water to a final volume of 26 mL. The reagent solutions in the two reservoirs were allowed to flow through the
reaction tube in an oven at 140 °C for 2 h. Flow rate = 0.0837 mL/min.
bAll compounds were characterized by comparison of their 1H and 13C NMR spectra with authentic samples and/or literature data.
cThe yield was isolated after 3 h.
dThe yield from the batch reaction.

In order to confirm the metal-free condition in the flow reaction
system, the reaction solution was subjected to ICP-MS. Based on
this data, the metal concentrations were not exceed hundreds of
ppb.
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In conclusion, we have developed an environmentally friendly
and economical continuous flow reaction system for the
preparation of propargylamines via a metal-free decarboxylative
coupling. The reaction was carried out in water in the absence
metal catalysts or other additives. The desired propargylamine
products were formed in good yield with stoichiometric
quantities of the reagents, unlike in the batch reaction where an
excess was required. Further studies on detailed mechanism are
underway in our laboratory.25 In addition, the only waste
products derived from the A3-coupling reaction reported herein
were CO2 and H2O. Finally, this continuous flow system has
enhanced heat and mass transfer characteristics and was suitable
for use at elevated temperatures beyond those of the solvent
boiling point.
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Acknowledgements
This research was supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education (NRF-
2012R1A1A2044286) and the National Research Foundation of
Korea Grant funded by the Korean Government (MEST) (2012,
University-Institute cooperation program). Spectral data were
obtained from the Korea Basic Science Institute, Gwangju
Branch.
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24. Typical experimental procedure. Aryl or alkyl substituted propiolic acid
(13.2 mmol) and amine were dissolved in water (26.0 mL) and stirred at
room temperature for 1 h. The resulting mixture was transferred to
reservoir A, which was connected to the reaction tube. Paraformaldehyde

(13.2 mmol) was then dissolved in water (26.0 mL) and stirred at 90 °C
for 1 h and cooled to room temperature. This solution was transferred to
reservoir B, which was also connected to the reaction tube. The solutions
of reagents in the two reservoirs were allowed to flow through the
reaction tube in an oven at 140 °C for 2 h. The flow rates for the two
solutions were set 0.0837 mL/min, and the rates were controlled by
means of a syringe pump. The resulting crude reaction mixture was
collected, and extracted with diethyl ether. The organic phase was then
washed according to an acid and base workup process, and dried over
anhydrous MgSO4. The desired product was then obtained by
evaporation of the organic solvent. 3aa was obtained with 96% yield.
3aa: 1H NMR (300 MHz, CDCl3) δ 7.46 – 7.42 (m, 2H), 7.36 – 7.13 (m,
3H), 3.87 – 3.65 (m, 4H), 3.50 (s, 2H), 2.78 – 2.44 (m, 4H); 13C NMR
(75 MHz, CDCl3) δ 131.4, 128.0, 127.9, 122.7, 85.3, 83.8, 66.6, 52.1,
47.83.
25. It is not clear which step is a rate-determining step in the metal free
decarboxylative coupling reaction. We failed to detect any intermediate
in this reaction pathway.

3
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A range of propargylamines was synthesized via the metal-
free decarboxylative coupling of alkynyl carboxylic acids
with amines and paraformaldehyde in water, using
Continuous flow reactions in water for the
synthesis of propagylamines via a metal-free
decarboxylative coupling reaction
a
Bongkyun Jung, Kyungho Park, Kwang Ho Song, Sunwoo Lee