N-Propargylation of secondary amines directly using calcium carbide as an acetylene source

Article abstract of DOI:10.3184/174751917X14949427622099

A one-pot N-propargylation of secondary amines has been achieved by heating the amine with formaldehyde and calcium carbide in DMSO in the presence of CuCl as a catalyst. Fifteen examples of propargylic tertiary amines, 12 of which are novel, were efficiently prepared in yields of 65-84%. The advantages of the method are broad substrate scope and a simple work-up procedure.

Full text of DOI:10.3184/174751917X14949427622099

JOURNAL OF CHEMICAL RESEARCH 2017
VOL. 41 JUNE, 341–345
RESEARCH PAPER 341
N-Propargylation of secondary amines directly using calcium carbide as an
acetylene source
Rugang Fu and Zheng Li*
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, P.R. China
A one-pot N-propargylation of secondary amines has been achieved by heating the amine with formaldehyde and calcium carbide in
DMSO in the presence of CuCl as a catalyst. Fifteen examples of propargylic tertiary amines, 12 of which are novel, were efficiently
prepared in yields of 65–84%. The advantages of the method are broad substrate scope and a simple work-up procedure.
Keywords: N-propargylation, secondary amine, calcium carbide, acetylene source, propargylic tertiary amine
Results and discussion
Propargylation is an efficient method to synthesise important
organic intermediates and fine chemicals bearing propargyl
groups. Many reagents have been utilised for the propargylation
of organic compounds, such as the propargylation of
aromatic compounds1 or indoles2,3 with propargyl alcohol; the
propargylation of polyfluoroarenes with propargyl phosphate;4
the propargylation of aldehydes and ketones with propargyl
boronate,5−7 allenylboronate,8 potassium allenyltrifluoroborate9
or allenyltrichlorosilane;10 the S-propargylation of thiols with
propargylic carbonate;11 the propargylation of amines with
propargyl halides;12 the propargylation of N-sulfonylketimines
with allenylboronate;13 and the propargylation of glyoxylic
oxime ether with propargyl mesylate.14
The N-propargylation of compounds bearing nitrogen atoms
is also a very important reaction. Zhu and Hu15 reported an
example of N-propargylation of indoline using a propargylic
ester. Martinaranda et al.16,17 and Rao and co-workers18 reported
the N-propargylation of imidazoles using propargyl bromide.
Curran and co-workers19 reported the N-propargylation
of 2-pyridones using propargyl bromide. Eycken and co-
workers20 reported the N-propargylation of secondary amines
with propiolic acids. However, most of the reagents for
N-propargylation are rather expensive. Therefore, it is necessary
to develop a convenient method for N-propargylation using
cheaper reagents.
Calcium carbide, CaC2, is a stable and inexpensive solid that
is commercially available on the ton scale, and is extensively
used as a raw material to produce acetylene by hydrolysis.
Several recent reports have revealed that acetylene can be
generated in situ by using calcium carbide directly as an easy-
to-handle and efficient source for many types of chemical
transformation (reviewed by Ananikov and co-workers21).
Our work is a modification of a method of N-propargylation
of secondary amines using calcium carbide as the source of
the acetylene reported by Zhang and co-workers.22−24 In those
reports, CuI was used as a catalyst, the solvent was MeCN and
the main substrates were aromatic aldehydes.
Initially, the reaction of tetrahydroisoquinoline (1a), calcium
carbide and paraformaldehyde was selected as a model reaction
to examine the feasibility of N-propargylation under various
conditions, including the use of different catalysts and solvents
at certain temperatures for an appropriate time (Scheme 1,
Table 1). It was found that catalysts played an important role
in the reaction. The reaction did not take place in the absence
of a catalyst (entry 1). Copper (II) bromide and chloride as
catalysts were tested, but only a trace amount of the product
N-propargyl tetrahydroisoquinoline (2a) was observed (entries
2 and 3). However, cuprous halides, such as cuprous iodide,
cuprous bromide and cuprous chloride, were efficient catalysts,
and the reaction gave the N-propargylation product 2a in good
to high yield (entries 4−6). Among them, the best yield was
obtained by using cuprous chloride as a catalyst (entry 6). In
addition, the selection of the solvent was also important. The
reaction did not occur in EtOH, 1,4-dioxane and PhMe because
of the poor solubility of calcium carbide. In contrast, DMSO,
DMF and MeCN were practicable solvents, and the reaction in
Table 1 The effect of reaction conditions (catalyst, solvent, time,
temperature) on the yield of 2a (Scheme 1)a
Entry Catalyst Solvent
Time (h) Temperature (°C) Yield (%)b
1
2
3
4
5
6
7
8
–
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
MeCN
DMSO
DMSO
DMSO
DMSO
4
4
4
4
4
4
4
4
12
12
12
4
100
100
100
100
100
100
100
100
50
0
Trace
Trace
62
65
81
33
Trace
32
CuBr2
CuCl2
CuI
CuBr
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
9
10
11
12c
80
66
120
100
73
0
aReaction conditions: a stirred mixture of tetrahydroisoquinoline 1a (1 mmol), (CH2O)n (3.0
mmol), CaC2 (2.5 mmol), H2O (3 mmol) and catalyst (0.1 mmol) in solvent (3 mL) was
heated at a certain temperature under N2 for an appropriate time.
bIsolated yield.
In this paper, we report the convenient N-propargylation of
a variety of secondary amines by reacting formaldehyde and
calcium carbide as an acetylene source in the presence of CuCl
in DMSO to synthesise propargylic tertiary amines by a one-
pot procedure.
cWithout water.
Ca
Catalyst
Solvent
N
NH
O)n
(CH2
C
C
2a
1a
Scheme 1
* Correspondent. E-mail: lizheng@nwnu.edu.cn

342 JOURNAL OF CHEMICAL RESEARCH 2017
a
Table 2 Yields of propargylic tertiary amines 2a–o (Scheme 2)
Entry
Amine
Aldehyde
Product
Yield (%)b
81
NH
NH
N
N
1
(CH2O)n
2a
2b
2
CH3CH2CHO
69
NH
3
4
CH3CH2CH2CHO
(CH2O)n
71
83
N
2c
NH
N
2d
H
N
N
5
(CH2O)n
84
2e
O
O
6
7
(CH2O)n
(CH2O)n
78
80
NH
N
2f
HN
N
N
NH
2g
8
(CH2O)n
72
N
N
N
HN
2h
N
9
(CH2O)n
(CH2O)n
79
82
N
N
NH
NH
2i
N
10
2j
N
N
H
11
12
13
(CH2O)n
(CH2O)n
(CH2O)n
77
73
71
2k
2l
N
N
N
H
N
H
2m
N
NH
14
15
(CH2O)n
(CH2O)n
81
65
N
2n
NH
N
N
2o
aReaction conditions: a stirred mixture of a secondary amine (1 mmol), an aldehyde (3.0 mmol), CaC2 (2.5 mmol), H2O (3 mmol) and CuCl (0.1 mmol) in DMSO (3 mL) was heated at 100 °C
under N2 for 4 h.
bIsolated yield.

JOURNAL OF CHEMICAL RESEARCH 2017 343
these solvents gave 2a in diminishing yields (entries 6−8). The
most effective solvent was DMSO, giving a yield of 81% (entry
6). Furthermore, it was found that 100 °C was an appropriate
temperature for the reaction. Lower and higher temperatures did
not significantly improve the yield even for prolonged reaction
times (entries 9−11). In addition, the presence of 3 equiv. of water
was essential, as the reaction without water gave no product
(entry 12).
equivalent of secondary amine. For the acyclic secondary
amines, especially benzyl secondary amines, the corresponding
reactions proceeded smoothly, and the N-propargylation products
2k–m were obtained in high yield (entries 11−13). However,
when diisopropylamine and dibutylamine were substrates, the
corresponding N,N-dialkyl-but-2-yne-1,4-diamines 2n and 2o
were the major products (entries 14 and 15), which implied that
the reaction did not end as N-propargylation products due to their
small steric hindrance. Note that reactions using heterocyclic
secondary amines bearing conjugate structures, such as indole,
carbazole and benzotriazole, were unfruitful because of the low
nucleophilicity of their NH groups. In addition, primary amines,
such as aniline, benzyl amine and butyl amine, did not participate
in a similar reaction either.
A plausible mechanism is proposed for the N-propargylation
of secondary amines using calcium carbide as an acetylene
source (Scheme 3). Calcium carbide first reacts with water to
give ethynylcalcium hydroxide A as an intermediate, which can
further react with cuprous chloride to afford cuprous acetylide
intermediate B. Nucleophilic intermediate B attacks the
iminium ion C generated in situ from secondary amine 1 and
paraformaldehyde to give the corresponding propargylamine
intermediate D which can be readily converted into the
corresponding final product propargyl tertiary amines 2a–h and
2k–m in the presence of water. In some cases, propargyl tertiary
amines could further combine with cuprous chloride to form
intermediate E. E further reacts with iminium ion C to give but-
2-yne-1,4-diamines 2i, 2j, 2n and 2o.
Using the optimised conditions, a variety of secondary
amines were N-propargylated (Scheme 2) and the results are
shown in Table 2. The reactions worked well for a wide range of
substrates, including heterocyclic and acyclic amines, affording
the desired products in good to high yield. Three-component
reactions of tetrahydroisoquinoline 1a and calcium carbide
with other aliphatic aldehydes such as propanal and n-butanal
gave N-propargyltetrahydroisoquinolines 2b and 2c in high
yield (entries 2 and 3). This indicated that the three-component
reactions could probably be extended to longer chain aldehydes.
We then carried out the N-propargylation of various secondary
amines with paraformaldehyde (entries 4−15). Reactions of
five heterocyclic amines gave N-propargylation products 2d–h
in high yield (entries 4−8). The diamine piperazine giving a
bis derivative 2g. However, the reactions of pyrrolidine and
piperidine mainly gave, respectively, 1,4-di(pyrrolidin-1-yl)
but-2-yne 2i (entry 9) and 1,4-di(piperidin-1-yl)but-2-yne 2j
(entry 10). These results meant that the N-propargylation
intermediates produced in situ were sufficiently reactive for the
terminal alkynyl groups to undergo further reaction with another
R1
R1
R3
Ca
trace H
DMSO, 100 oC
CuCl,
2O
NH
+
+
N
R3
CHO
R2
C
C
R2
2a-o
Scheme 2
Ca
H
2O
[Cu]
C
C Cu
C
CH
(HO)Ca
(HO)Ca
C
C
A
B
R1
R2
H
H
R1
O)n
(CH2
N
H
N
R2
1
C
[Cu]
R2
N
R2
N
(HO)Ca
H
2O
R1
R1
D
2a-h, 2k-m
Ca(OH)2
[Cu]
H
H
R1
R2
R2
N
R2
C
N
R2
Cu
N
R1
R1
N
R1
2i,
2n, 2o
2j,
E
[Cu]
Scheme 3

344 JOURNAL OF CHEMICAL RESEARCH 2017
J = 2.3 Hz, 1H), 2.03–1.99 (m, 2H); 13C NMR (150 MHz, CDCl3): δ
144.6, 129.1, 126.9, 124.0, 117.5, 112.0, 79.7, 71.5, 49.2, 40.7, 27.7, 22.4.
Anal. calcd for C12H13N: C, 84.17; H, 7.65; N, 8.18; found: C, 84.24; H,
7.62; N, 8.14%.
In conclusion, we have successfully developed a highly
effective three-component reaction of a secondary amine, an
aldehyde and calcium carbide to synthesise propargyl tertiary
amines in a one-pot reaction. This cost-efficient procedure can be
easily employed for the preparation of important propargylamine
building blocks using standard laboratory equipment. Moreover,
these reactions confirm the potential of using calcium carbide
as an acetylene replacement in organic synthesis. This easy-to-
handle protocol will help contribute to the effort to use calcium
carbide as a sustainable and cost-efficient carbon source in
modern organic synthesis.
4-(Prop-2-yn-1-yl)morpholine (2f): Bright yellow liquid (lit.25
1
colourless oil; lit.26 colourless solid); H NMR (600 MHz, CDCl3):
δ 3.71–3.63 (m, 4H), 3.23 (d, J = 2.3 Hz, 2H), 2.50 (bs, 4H), 2.23 (t,
J = 2.3 Hz, 1H); 13C NMR (150 MHz, CDCl3): δ 78.3, 73.4, 66.7, 52.1,
47.1. Anal. calcd for C7H11NO: C, 67.17; H, 8.86; N, 11.19; found: C,
67.08; H, 8.89; N, 11.23%.
1
1,4-Di(prop-2-yn-1-yl)piperazine (2g): Colourless liquid; H NMR
(600 MHz, CDCl3): δ 3.30 (d, J = 2.4 Hz, 4H), 2.64 (bs, 8H), 2.24 (t,
J = 2.4 Hz, 2H); 13C NMR (150 MHz, CDCl3): δ 78.6, 73.3, 51.6, 46.7.
Anal. calcd for C10H14N2: C, 74.03; H, 8.70; N, 17.27; found: C, 73.98;
H, 8.68; N, 17.34%.
Experimental
All starting materials were commercially available and used as
received. Calcium carbide was purchased from Aldrich (purity: 80%).
Unless otherwise noted, the solvents and reagents were reagent grade
and used without any further purification. Column chromatographic
separations were carried out on a flash chromatographic system using
silica gel and petroleum ether (60−90 °C)/ethyl acetate as the eluent.
For thin layer chromatography (TLC), silica gel plates precoated with
GF-254 were used. 1H NMR and 13C NMR spectra were recorded on a
Mercury-600 BB or 400 BB instrument using CDCl3 as the solvent and
Me4Si as the internal standard. Elemental analyses were performed on
a Vario El Elemental Analysis instrument.
1-Benzhydryl-4-(prop-2-yn-1-yl)piperazine (2h): White solid; m.p.
1
97–99 °C (petroleum ether/EtOAc); H NMR (400 MHz, CDCl3): δ
7.44–7.38 (m, 4H), 7.28–7.22 (m, 4H), 7.18–7.13 (m, 2H), 4.22 (s, 1H),
3.29 (d, J = 2.4 Hz, 2H), 2.59 (m, 5H), 2.45 (bs, 3H), 2.24 (t, J = 2.4 Hz,
1H); 13C NMR (100 MHz, CDCl3): δ 142.8, 128.5, 128.0, 127.0, 79.0,
76.2, 73.2, 52.1, 51.8, 46.8. Anal. calcd for C20H22N2: C, 82.72; H, 7.64;
N, 9.65; found: C, 82.77; H, 7.62; N, 9.61%.
1
1,4-Di(pyrrolidin-1-yl)but-2-yne (2i): Yellow liquid; H NMR (600
MHz, CDCl3): δ 3.41 (s, 4H), 2.60 (t, J = 6.0 Hz, 8H), 1.78 (m, 8H);
13C NMR (150 MHz, CDCl3): δ 79.8, 52.5, 43.2, 23.8. Anal. calcd for
C12H20N2: C, 74.95; H, 10.48; N, 14.57; found: C, 74.96; H, 10.44; N,
14.60%.
Preparation of propargyl tertiary amines (2a–o); general procedure
A mixture of secondary amine (1.0 mmol), calcium carbide (0.20 g,
2.5 mmol), aldehyde (3 mmol), cuprous chloride (0.01 g, 0.1 mmol)
and water (0.05 mL, 3 mmol) in DMSO (3 mL) was stirred at 100 °C
under N2 for 4 h. The reaction was monitored by TLC. After reaction
completion, the resulting mixture was filtered to remove the solid, and
the liquor was extracted with ethyl acetate (3 × 10 mL) and washed
with saturated brine (3 × 10 mL). The resulting organic phase was
dried over anhydrous sodium sulfate and concentrated under reduced
pressure. The residue was purified by column chromatography using
petroleum ether and ethyl acetate (v/v 10:1) as the eluent.
2-(Prop-2-yn-1-yl)-1,2,3,4-tetrahydroisoquinoline (2a): Brown
liquid; 1H NMR (600 MHz, CDCl3): δ 7.16–7.11 (m, 3H), 7.05–7.02 (m,
1H), 3.77 (s, 2H), 3.51 (d, J = 2.4 Hz, 2H), 2.95 (t, J = 6.0 Hz, 2H),
2.84 (t, J = 6.0 Hz, 2H), 2.28 (t, J = 2.4 Hz, 1H); 13C NMR (150 MHz,
CDCl3): δ 134.5, 133.7, 128.6, 126.6, 126.1, 125.6, 78.7, 73.3, 54.29,
49.7, 46.7, 29.2. Anal. calcd for C12H13N: C, 84.17; H, 7.65; N, 8.18;
found: C, 84.11; H, 7.68; N, 8.21%.
2-(Pent-1-yn-3-yl)-1,2,3,4-tetrahydroisoquinoline (2b): Brown
liquid; 1H NMR (600 MHz, CDCl3): δ 7.16–7.12 (m, 3H), 7.08–7.05
(m, 1H), 3.87 (d, J = 14.6 Hz, 1H), 3.73 (d, J = 14.6 Hz, 1H), 3.48–3.45
(m, 1H), 2.96–2.93 (m, 3H), 2.75–2.71 (m, 1H), 2.32 (d, J = 2.1 Hz,
1H), 1.84–1.79 (m, 2H), 1.09 (t, J = 7.4 Hz, 3H); 13C NMR (150 MHz,
CDCl3): δ 135.1, 134.4, 128.6, 126.7, 126.0, 125.6, 81.3, 73.4, 58.9, 51.8,
47.2, 29.6, 26.7, 11.2. Anal. calcd for C14H17N: C, 84.37; H, 8.60; N,
7.03; found: C, 84.42; H, 8.58; N, 7.00%.
1
1,4-Di(piperidin-1-yl)but-2-yne (2j): Yellow liquid; H NMR (600
MHz, CDCl3): δ 3.29 (s, 4H), 2.52–2.48 (m, 8H), 1.67–1.57 (m, 8H),
1.42–1.41 (m, 4H); 13C NMR (150 MHz, CDCl3): δ 80.0, 53.3, 47.9,
25.8, 23.9. Anal. calcd for C14H24N2: C,76.31; H, 10.98; N, 12.71; found:
C, 76.41; H, 10.93; N, 12.66%.
N-Benzyl-N-methylprop-2-yn-1-amine (2k): Colourless liquid (lit.26
1
colourless liquid); H NMR (400 MHz, CDCl3): δ 7.39–7.28 (m, 5H),
3.58 (s, 2H), 3.31 (d, J = 2.4 Hz, 2H), 2.35 (s, 3H), 2.28 (t, J = 2.4 Hz,
1H); 13C NMR (100 MHz, CDCl3): δ 138.3, 129.2, 128.3, 127.3, 78.6,
73.4, 60.0, 44.9, 41.8. Anal. calcd for C11H13N: C, 82.97; H, 8.23; N,
8.80; found: C, 82.90; H, 8.26; N, 8.84%.
N-Benzyl-N-ethylprop-2-yn-1-amine (2l): Colourless liquid;
1H NMR (400 MHz, CDCl3): δ 7.28–7.15 (m, 5H), 3.54 (s, 2H), 3.25
(d, J = 2.4 Hz, 2H), 2.52 (q, J = 7.2 Hz, 2H), 2.13 (t, J = 2.4 Hz, 1H),
1.03 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 138.8, 129.2,
128.3, 127.1, 78.6, 73.1, 57.6, 47.4, 40.9, 12.9. Anal. calcd for C12H15N:
C, 83.19; H, 8.73; N, 8.08; found: C, 83.24; H, 8.70; N, 8.06%.
N,N-Dibenzylprop-2-yn-1-amine (2m): Yellow liquid (lit.26
colourless solid; lit.27 yellow crystals, m.p. 42 °C); 1H NMR (400 MHz,
CDCl3): δ 7.40 (d, J = 7.2 Hz, 4H), 7.34–7.30 (m, 4H), 7.26–7.22 (m,
2H), 3.69 (s, 4H), 3.26 (d, J = 2.3 Hz, 2H), 2.28 (t, J = 2.3 Hz, 1H);
13C NMR (150 MHz, CDCl3): δ 138.8, 129.0, 128.3, 127.2, 78.5, 73.4,
57.4, 41.2. Anal. calcd for C17H17N: C, 86.77; H, 7.28; N, 5.95; found: C,
86.73; H, 7.30; N, 5.97%.
N1,N1,N4,N4-Tetraisopropylbut-2-yne-1,4-diamine (2n): Yellow
2-(Hex-1-yn-3-yl)-1,2,3,4-tetrahydroisoquinoline
(2c):
Brown
1
liquid; H NMR (600 MHz, CDCl3): δ 3.40 (s, 4H), 3.19–3.15 (m,
liquid; 1H NMR (600 MHz, CDCl3): δ 7.13–7.11 (m, 3H), 7.06–7.04 (m,
1H), 3.86 (d, J = 14.6 Hz, 1H), 3.71 (d, J = 14.6 Hz, 1H), 3.56–3.54
(m, 1H), 2.99–2.86 (m, 3H), 2.73–2.69 (m, 1H), 2.31–2.28 (m, 1H),
1.81–1.71 (m, 2H), 1.62–1.46 (m, 2H), 0.98 (t, J = 7.8 Hz, 3H);13C NMR
(150 MHz, CDCl3): δ 135.1, 134.3, 128.6, 126.7, 125.9, 125.5, 81.4,
73.3, 56.8, 51.8, 47.2, 35.5, 29.6, 19.8, 13.8. Anal. calcd for C15H19N: C,
84.46; H, 8.98; N, 6.57; found: C, 84.41; H, 9.01; N, 6.59%.
4H), 1.08 (d, J = 6.6 Hz, 24H); 13C NMR (150 MHz, CDCl3): δ 81.9,
48.2, 34.3, 20.6. Anal. calcd for C16H32N2: C, 76.13; H, 12.78; N, 11.10;
found: C, 76.22; H, 12.73; N, 11.05%.
N1,N1,N4,N4-Tetrabutylbut-2-yne-1,4-diamine
(2o):
Colourless
liquid; 1H NMR (600 MHz, CDCl3): δ 3.40 (s, 4H), 2.43 (t, J = 7.2 Hz,
8H), 1.44–1.38 (m, 8H), 1.34–1.26 (m, 8H), 0.90 (t, J = 7.8 Hz, 12H);
13C NMR (150 MHz, CDCl3): δ 79.3, 53.6, 41.9, 29.7, 20.7, 14.0; Anal.
calcd for C19H38N2: C, 77.48; H, 13.01; N, 9.51; found: C, 77.56; H,
12.96; N, 9.48%.
2-(Prop-2-yn-1-yl)isoindoline (2d): Brown liquid; 1H NMR (400
MHz, CDCl3): δ 7.19 (s, 4H), 4.06 (s, 4H), 3.62 (d, J = 2.4 Hz, 2H), 2.29
(t, J = 2.4 Hz, 1H);13C NMR (150 MHz, CDCl3): δ 139.8, 126.8, 122.3,
79.1, 73.9, 57.1, 42.7. Anal. calcd for C11H11N: C, 84.04; H, 7.05; N, 8.91;
found: C, 84.08; H, 7.04; N, 8.88%.
Acknowledgments
The authors thank the National Natural Science Foundation
of China (21462038, 21362034) and Key Laboratory of Eco-
Environment-Related Polymer Materials for Ministry of
Education for the financial support of this work.
1-(Prop-2-yn-1-yl)-1,2,3,4-tetrahydroquinoline (2e): Yellow liquid;
1H NMR (600 MHz, CDCl3): δ 7.09 (t, J = 8.5 Hz, 1H), 6.98 (d,
J = 7.3 Hz, 1H), 6.73 (d, J = 8.2 Hz, 1H), 6.67 (t, J = 7.3 Hz, 1H), 4.02
(d, J = 2.3 Hz, 2H), 3.31–3.27 (m, 2H), 2.77 (t, J = 6.5 Hz, 2H), 2.14 (t,

JOURNAL OF CHEMICAL RESEARCH 2017 345
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stl.publisher.ingentaconnect.com/content/stl/jcr/supp-data
Received 15 April 2017; accepted 30 April 2017
Paper 1704708
https://doi.org/10.3184/174751917X14949427622099
Published online: 18 May 2017
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