CuBr/FeCl3 catalysis: A novel and efficient method for the preparation of new aryl (Iminomethyl)propargyl ether derivatives via C-H activation of aryl propargyl ethers

Article abstract of DOI:10.1002/hlca.201100072

CuBr/FeCl₃ Catalysis as a novel and efficient method has been developed for the preparation of new aryl propargyl imine ether derivatives via C-H activation of aryl propargyl ethers, followed by reaction with imines generated from aldehydes and amines. Copyright

Full text of DOI:10.1002/hlca.201100072

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Helvetica Chimica Acta – Vol. 94 (2011)
CuBr/FeCl3 Catalysis: a Novel and Efficient Method for the Preparation of
New Aryl (Iminomethyl)propargyl Ether Derivatives via CꢀH Activation of
Aryl Propargyl Ethers
by Gautham Santhosh Kumar, Chavva Kurumurthy, Pillalamarri Sambasiva Rao, Banda Veeraswamy,
Pamulaparthi Shanthan Rao, and Banda Narsaiah*
Fluoro Organic Division, Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500607, India
(phone: þ 91-40-27193630; fax: þ 91-40-27160387; e-mail: narsaiah_1984@yahoo.co.in)
CuBr/FeCl3 Catalysis as a novel and efficient method has been developed for the preparation of new
aryl propargyl imine ether derivatives via CꢀH activation of aryl propargyl ethers, followed by reaction
with imines generated from aldehydes and amines.
Introduction. – The modern trend in synthetic organic chemistry is directed to
multicomponent reactions (MCRs) because of their added advantage over linear
reactions in synthesis of highly complex and medicinally important compounds [1].
Notably, the propargylamine (¼ prop-2-yn-1-ylamine) derivatives are considered as
attractive targets for their functionality and derivatization. A3-Coupling of alkyne,
aldehyde, and amine is one of the synthetic ways widely used to prepare propargyl-
amine derivatives. It is mainly prepared by CꢀH activation of alkyne with metal,
followed by reaction with imine generated in situ from aldehydes and amines. In former
reports, A3-couplings were catalyzed by Au in H2O [2], CuBr in H2O [3], Zn(OTf)2/
R3N [4], Au nanoparticles [5], etc. Enantioselective synthesis of propargylamine
derivatives was also accomplished by using CuBr [6]. Similarly, propargylamine
derivatives were prepared by Cu/Ru-catalyzed coupling of alkynes with imines [7][8].
The same concept was further exploited by Roy et al. [9] to prepare (iminomethyl)-
propargyl ether derivatives by coupling of propargyl glycoside ethers with different
aldehydes and amines using a Cu/Ru catalyst. Based on the supporting supplementary
data of [9], the products are found to be ꢀglycoside propargyl imine etherꢁ derivatives.
However, the authors claim the products as ꢀglycoside propargyl amine ether
derivativesꢁ. Some of the propargylimine derivatives such as 1-azabut-1-en-3-ynes
were prepared from arylsulfonyloxime [10], imidoyl chlorides [11][12], nitrones [13],
alkynyl imines [14], and arylacetylene. To the best of our knowledge, except [9], no
report was available for the preparation of aryl (iminomethyl)propargyl ether
derivatives, and thus attracted our attention. As a sequel to our endeavor in developing
new catalysts and methodologies, we report here the development of a simple, efficient
and environmentally benign approach for new aryl (iminomethyl)propargyl ether
derivatives using CuBr/FeCl3 as catalyst.
Aryl (Iminomethyl)propargyl Ethers. – Aryl propargyl ethers 1 were prepared
under Claisen conditions (K2CO3, propargyl bromide, acetone). The multicomponent
ꢂ 2011 Verlag Helvetica Chimica Acta AG, Zꢃrich

Helvetica Chimica Acta – Vol. 94 (2011)
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reaction of aryl aldehydes 2, aryl amines 3, and propargyl ethers 1 in presence of CuBr/
FeCl3 catalyst gave the new aryl (iminomethyl)propargyl ether derivatives 4 (Table 4).
The synthetic sequence involves formation of imine at 608, followed by reaction with
activated alkyne at 1208. To test the versatility of the reaction, diverse substituted aryl
aldehydes and aryl amines were reacted with the activated alkyne leading to the
corresponding products in high yields. Further, the described reaction was also
attempted with heptanal, however, with no success. Aliphatic aldimines are less stable
and show low reactivity towards carbon nucleophiles [3]. It is noteworthy to mention
that the reaction without the catalytically active couple, i.e., only with CuBr or FeCl3,
did not lead to the formation of product. Moreover, the ether 1a, on reaction with
different aldehydes and aniline using CuBr/FeCl3 couple at room temperature or at
reflux temperature in MeCN, gave also no reaction. At high temperature, i.e., 1508,
ether 1 was cleaved to phenol. In the Table, the products formed in each reaction are
compiled.
Table. Synthesis of Aryl (Iminomethyl)propargyl Ether Derivatives
Entry
1
R1
2
R2
3
R3
4
Yield [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
H
H
H
H
H
H
H
H
H
4-Me
–
2a
2b
2c
2d
2e
2f
2a
2a
2a
2a
2a
2g
2a
2e
Ph
3a
3a
3a
3a
3a
3a
3b
3c
3d
3a
3a
3a
3a
3a
Ph
Ph
Ph
Ph
Ph
Ph
4a
4b
4c
4d
4e
4f
4g
4h
4i
78
72
75
70
70
75
78
75
75
72
78
4-ClꢀC6H4
4-BrꢀC6H4
4-MeOꢀC6H4
4-MeꢀC6H4
4-FꢀC6H4
Ph
Ph
Ph
Ph
Ph
4-MeꢀC6H4
4-FꢀC6H4
9
1a
2-FꢀC6H4
10
11
12
13
14
1a
Ph
Ph
Ph
Ph
Ph
4j
4k
1ba)
1a
H
–
–
Hexyl
Ph
4-MeꢀC6H4
No reaction
4l
4m
1cb)
1cb)
42
40
a
b
)
.
)
.
A possible mechanism is proposed which involves the activation of the terminal
CꢀH bond of the alkyne by FeCl3, possibly generated in situ by reaction with FeIII and
the activation of the imine with copper. The iron-activated intermediate immediately

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Helvetica Chimica Acta – Vol. 94 (2011)
reacts with the activated imine to yield the nucleophilic addition product of the aryl
(iminomethyl)propargyl ether derivative. It seems to be quite unstable in the reaction
medium used, and thus reductive elimination of two H-atoms takes place resulting in
the product, i.e., the aryl (iminomethyl)propargyl ether derivative. The proposed
pathway for imine addition via CꢀH activation is outlined in the Scheme.
Scheme. Proposed Mechanism for Imine Addition via CꢀH Activation
In conclusion, we have developed an efficient way for the synthesis of new aryl
(iminomethyl)propargyl ether derivatives by CuBr/FeCl3-catalyzed A3-coupling of aryl
propargyl ethers, aldehydes, and amines. The reaction was established with diverse
aldehydes and amines for general application.
Authors are grateful to Dr. J. S. Yadav, Director, IICT for his constant encouragement, and G. S. K.,
C. K., P. S. R. , and B. V. thank the Council of Scientific and Industrial Research (CSIR), India, for the
financial support in the form of JRF fellowship.
Experimental Part
General. All of the reactions were monitored by TLC on precoated silica gel 60 F254; spots were
visualized with UV light. Column chromatography (CC): Merck silica gel (60 – 120 mesh). M.p.: Casia-
Siamia (VMP-AM) melting-point apparatus; uncorrected. IR Spectra: Perkin-Elmer FT-IR 240-C
spectrophotometer; KBr discs; in cmꢀ1. 1H- and 13C-NMR spectra: Bruker AV (300 and 75 MHz, resp.),
in CDCl3, TMS as internal standard; d in ppm, J in Hz. Electron impact (EI) and chemical ionization
(CI) MS: VG 7070 H instrument; at 70 eV; m/z. CHN Analyses: Vario EL analyzer.

Helvetica Chimica Acta – Vol. 94 (2011)
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Typical Procedure for the A3-Coupling Reaction. A mixture of aldehyde (1 mmol) and arylamine
(1.1 mmol) was stirred at 608 for 1 h. Then, aryl propargyl ether (1.1 mmol) in MeCN (2 ml) was added,
followed by CuBr (30 mol-%) and FeCl3 (3 mol-%) in a sealed tube. It was externally heated at 110 –
1208 for 1 – 2 h, and after completion of the reaction, the mixture was diluted with CH2Cl2 (15 ml) and
washed with H2O (30 ml). The org. layer was separated, dried (Na2SO4), and concentrated under
vacuum. The residue thus obtained was purified by CC (hexane/AcOEt 3 :1).
N-(4-Phenoxy-1-phenylbut-2-yn-1-ylidene)benzenamine (4a; Table, Entry 1). M.p. 120 – 1228. IR:
1
2154 (ꢀC ꢁ Cꢀ), 1601 (C¼N), 1245 (CꢀOꢀC). H-NMR: 5.53 (s, CH2O); 6.95 – 7.04 (m, 3 arom. H);
7.26 – 7.32 (m, 2 arom. H); 7.44 – 7.56 (m, 4 arom. H); 7.71 (t, J ¼ 8.3, 1 arom. H); 7.98 (t, J ¼ 8.3, 2 arom.
H); 8.14 – 8.21 (m, 3 arom. H). 13C-NMR: 66.9; 74.3; 79.1; 114.8; 117.2; 121.4; 122.6; 124.8 126.4; 127.5;
.
127.7; 128.7; 129.1; 129.3; 129.4; 129.6; 130.4; 139.4; 142.5; 148.2; 157.1; 158.3. MS: 311 (Mþ ), 218 ([M ꢀ
.
OPh]þ ). Anal. calc. for C22H17NO (311.38): C 84.86, H 5.50, N 4.50; found: C 84.57, H 5.42, N 4.41.
N-[1-(4-Chlorophenyl)-4-phenoxybut-2-yn-1-ylidene]benzenamine (4b; Table, Entry 2). M.p. 108 –
1108. IR: 2164 (ꢀC ꢁ Cꢀ), 1600 (C¼N), 1240 (CꢀOꢀC). 1H-NMR: 5.53 (s, CH2O); 6.94 – 7.05 (m, 3
arom. H); 7.28 – 7.32 (m, 2 arom. H); 7.45 (t, J ¼ 7.6, 1 arom. H); 7.72 (d, J ¼ 8.3, 2 arom. H); 7.78 (t, J ¼
6.8, 1 arom. H); 7.83 (t, J ¼ 7.6, 2 arom. H); 8.12 (d, J ¼ 8.3, 1 arom. H); 8.21 (d, J ¼ 8.3, 1 arom. H). MS:
.
.
345 (Mþ ), 252 ([M ꢀ OPh]þ ). Anal. calc. for C22H16ClNO (345.82): C 76.41, H 4.66, N 4.05; found: C
76.04, H 4.60, N 3.93.
N-[1-(4-Bromophenyl)-4-phenoxybut-2-yn-1-ylidene]benzenamine (4c; Table, Entry 3). M.p. 103 –
1058. IR: 2163 (ꢀC ꢁ Cꢀ), 1603 (C¼N), 1255 (CꢀOꢀC). 1H-NMR: 5.52 (s, CH2O); 6.98 – 7.09 (m, 3
arom. H); 7.28 – 7.31 (m, 2 arom. H); 7.54 (t, J ¼ 7.6, 1 arom. H); 7.62 (d, J ¼ 8.3, 2 arom. H); 7.73 (t, J ¼
6.8, 1 arom. H); 7.96 (t, J ¼ 7.6, 2 arom. H); 8.23 (d, J ¼ 8.3, 2 arom. H); 8.31 (d, J ¼ 8.3, 1 arom. H). MS:
.
.
390 (Mþ ), 297 ([M ꢀ OPh]þ ). Anal. calc. for C22H16BrNO (390.27): C 67.71, H 4.13, N 3.59; found: C
68.10, H 4.04, N 3.47.
N-[1-(4-Methoxyphenyl)-4-phenoxybut-2-yn-1-ylidene]benzenamine (4d; Table, Entry 4). M.p. 106 –
1088. IR: 2153 (ꢀC ꢁ Cꢀ), 1601 (C¼N), 1264 (CꢀOꢀC). 1H-NMR: 2.61 (s, MeO); 5.53 (s, CH2O); 6.95 (t,
J ¼ 6.7, 1 arom. H); 7.01 – 7.12 (m, 2 arom. H); 7.25 – 7.30 (m, 2 arom. H); 7.41 (t, J ¼ 6.7, 1 arom. H); 7.48 –
7.53 (m, 2 arom. H); 7.56 (d, J ¼ 8.7, 1 arom. H); 7.81 (d, J ¼ 8.7, 2 arom. H); 8.08 (d, J ¼ 8.7, 1 arom. H);
.
.
8.15 (d, J ¼ 8.7, 2 arom. H). MS: 341 (Mþ ), 248 ([M ꢀ OPh]þ ). Anal. calc. for C23H19NO2 (341.4): C
80.92, H 5.61, N 4.10; found: C 80.76, H 5.52, N 3.98.
N-[1-(4-Methylphenyl)-4-phenoxybut-2-yn-1-ylidene]benzenamine (4e; Table, Entry 5). M.p. 103 –
1058. IR: 2142 (ꢀC ꢁ Cꢀ), 1608 (C¼N), 1252 (CꢀOꢀC). 1H-NMR: 2.46 (s, Me); 5.57 (s, CH2O);
6.91 – 7.12 (m, 3 arom. H); 7.29 – 7.34 (m, 4 arom. H); 7.54 (t, J ¼ 7.6, 1 arom. H); 7.77 (t, J ¼ 7.6, 1 arom.
.
H); 7.92 (d, J ¼ 8.3, 2 arom. H); 8.08 (d, J ¼ 8.3, 2 arom. H); 8.23 (d, J ¼ 9.1, 1 arom. H). MS: 325 (Mþ ),
.
232 ([M ꢀ OPh]þ ). Anal. calc. for C23H19NO (325.4): C 84.89, H 5.89, N 4.30; found: C 85.04, H 5.80, N
4.28.
N-[1-(4-Fluorophenyl)-4-phenoxybut-2-yn-1-ylidene]benzenamine (4f; Table, Entry 6). M.p. 102 –
1048. IR: 2158 (ꢀC ꢁ Cꢀ), 1603 (C¼N), 1252 (CꢀOꢀC). 1H-NMR: 5.54 (s, CH2O); 6.98 – 7.23 (m, 4
arom. H); 7.31 – 7.46 (m, 2 arom. H); 7.58 (d, J ¼ 7.6, 1 arom. H); 7.66 (d, J ¼ 8.3, 2 arom. H); 7.74 (t, J ¼
6.8, 1 arom. H); 7.92 (d, J ¼ 8.3, 1 arom. H); 8.03 (d, J ¼ 8.3, 2 arom. H); 8.21 (d, J ¼ 8.3, 1 arom. H).
.
.
MS: 329 (Mþ ), 236 ([M ꢀ OPh]þ ). Anal. calc. for C22H16FNO (329.37): C 80.23, H 4.90, N 4.25; found:
C 80.39, H 4.81, N 4.14.
4-Methyl-N-(4-phenoxy-1-phenylbut-2-yn-1-ylidene)benzenamine (4g; Table, Entry 7). M.p. 101 –
1
1038. IR: 2162 (ꢀC ꢁ Cꢀ), 1605 (C¼N), 1265 (CꢀOꢀC). H-NMR: 2.54 (s, Me) 5.56 (s, CH2O); 6.93
(t, J ¼ 7.5, 1 arom. H); 7.09 (d, J ¼ 8.4, 2 arom. H); 7.18 – 7.32 (m, 2 arom. H); 7.43 (t, J ¼ 7.5, 1 arom. H);
7.42 – 7.56 (m, 3 arom. H); 7.78 (d, J ¼ 8.4, 2 arom. H); 8.19 (d, J ¼ 8.4, 1 arom. H); 8.26 (d, J ¼ 7.5, 2
.
.
arom. H). MS: 325 (Mþ ), 232 ([M ꢀ OPh]þ ). Anal. calc. for C23H19NO (325.4): C 84.89, H 5.89, N 4.30;
found: C 85.01, H 5.81, N 4.18.
4-Fluoro-N-(4-phenoxy-1-phenylbut-2-yn-1-ylidene)benzenamine (4h; Table, Entry 8). M.p. 105 –
1
1078. IR: 2155 (ꢀC ꢁ Cꢀ), 1600 (C¼N), 1262 (CꢀOꢀC). H-NMR: 5.52 (s, CH2O); 6.96 (t, J ¼ 7.5, 1
arom. H); 7.09 (s, J ¼8.4, 1 arom. H); 7.11 – 7.31 (m, 2 arom. H); 7.43 (t, J ¼ 7.5, 1 arom. H); 7.48 – 7.54 (m,
4 arom. H); 7.82 (d, J ¼ 8.4, 2 arom. H); 8.18 (d, J ¼ 8.4, 1 arom. H); 8.25 (d, J ¼ 7.5, 2 arom. H). MS: 329

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Helvetica Chimica Acta – Vol. 94 (2011)
.
.
(Mþ ), 236 ([M ꢀ OPh]þ ). Anal. calc. for C22H16FNO (329.37): C 80.23, H 4.90, N 4.25; found: C 80.37,
H 4.81, N 4.14.
2-Fluoro-N-(4-phenoxy-1-phenylbut-2-yn-1-ylidene)benzenamine (4i; Table, Entry 9). M.p. 108 –
1
1108. IR: 2153 (ꢀC ꢁ Cꢀ), 1600 (C¼N), 1258 (CꢀOꢀC). H-NMR: 5.53 (s, CH2O); 6.95 (t, J ¼ 7.5, 1
arom. H); 7.06 (d, J ¼ 8.4, 2 arom. H); 7.18 – 7.30 (m, 2 arom. H); 7.43 (t, J ¼ 7.5, 1 arom. H); 7.48 – 7.52
(m, 3 arom. H); 7.88 (d, J ¼ 7.5, 2 arom. H); 8.18 (d, J ¼ 8.4, 1 arom. H); 8.29 (d, J ¼ 7.5, 2 arom. H). MS:
.
.
329 (Mþ ), 236 ([M ꢀ OPh]þ ). Anal. calc. for C22H16FNO (329.37): C 80.23, H 4.90, N 4.25; found: C
80.07, H 4.82, N 4.13.
N-[4-(4-Methylphenoxy)-1-phenylbut-2-yn-1-ylidene]benzenamine (4j; Table, Entry 10). M.p. 106 –
1088. IR: 2156 (ꢀC ꢁ Cꢀ), 1608 (C¼N), 1252 (CꢀOꢀC). 1H-NMR: 2.56 (s, Me); 5.53 (s, CH2O);
6.92 – 7.20 (m, 3 arom. H); 7.29 – 7.31 (m, 2 arom. H); 7.42 (t, J ¼ 7.6, 1 arom. H); 7.49 – 7.60 (m, 3 arom.
.
H); 7.74 (d, J ¼ 8.3, 2 arom. H); 8.12 (d, J ¼ 8.3, 2 arom. H); 8.28 (d, J ¼ 7.6, 1 arom. H). MS: 325 (Mþ ),
.
218 ([M ꢀ OPh]þ ). Anal. calc. for C23H19NO (325.4): C 84.89, H 5.89, N 4.30; found: C 84.73, H 5.80, N
4.18.
N-[4-(Naphthalen-2-yloxy)-1-phenylbut-2-yn-1-ylidene]benzenamine (4k; Table, Entry 11). M.p.
1
130 – 1328. IR: 2151 (ꢀC ꢁ Cꢀ), 1607 (C¼N), 1242 (CꢀOꢀC). H-NMR: 5.68 (s, CH2O); 7.10 – 7.29 (m,
3 arom. H); 7.32 – 7.49 (m, 2 arom. H); 7.52 – 7.67 (m, 2 arom. H); 7.69 (t, J ¼ 7.9, 1 arom. H); 7.76 – 7.96
(m, 4 arom. H); 8.11 (t, J ¼ 8.8, 2 arom. H); 8.21 – 8.34 (m, 2 arom. H); 8.38 (d, J ¼ 8.8, 1 arom. H).
13C-NMR : 67.1; 74.6; 78.4; 107.4; 117.5; 118.9; 122.9; 124.2; 125.0; 126.7; 127.0; 127.6; 127.7; 127.8; 129.0;
.
129.1; 129.5; 129.7; 129.9; 130.7; 134.5; 139.6; 142.6; 148.4; 156.4; 157.4. MS: 361 (Mþ ), 218 ([M ꢀ
.
OPh]þ ). Anal. calc. for C26H19NO (361.44): C 86.40, H 5.30, N 3.88; found: C 86.24, H 5.21, N 3.76.
6-Phenyl-2-{[4-phenyl-4-(phenylimino)but-2-yn-1-yl]oxy}-4-(trifluoromethyl)-3-pyridinecarbonitrile
(4l; Table, Entry 13). M.p. 230 – 2368. IR: 2153 (ꢀC ꢁ Cꢀ), 2224 (ꢀC ꢁ N), 1605 (C¼N), 1245 (CꢀOꢀC).
1H-NMR: 6.01 (s, CH2O); 7.31 (d, J ¼ 7.4, 2 arom. H); 7.39 – 7.46 (m, 4 arom. H); 7.54 (d, J ¼ 7.4, 2 arom.
H); 7.61 (t, J ¼ 7.4, 1 arom. H); 7.69 (s, 1 arom. H); 7.78 (d, J ¼ 7.4, 2 arom. H); 8.12 (m, 2 arom. H); 8.36
.
.
(d, J ¼ 7.4, 2 arom. H). MS: 481 (Mþ ), 218 ([M ꢀ OPh]þ ). Anal. calc. for C29H18F3N3O (481.47): C
72.34, H 3.77, N 8.73; found: C 72.44, H 3.69, N 8.61.
2-{[4-(4-Methylphenyl)-4-(phenylimino)but-2-yn-1-yl]oxy}-6-phenyl-4-(trifluoromethyl)-3-pyridine-
carbonitrile (4m; Table, Entry 14). M.p. 241 – 2468. IR: 2151 (ꢀC ꢁ Cꢀ), 2224 (ꢀC ꢁ N), 1607 (C¼N),
1
1242 (CꢀOꢀC). H-NMR: 2.46 (s, Me); 6.09 (s, CH2O); 7.32 (d, J ¼ 7.4, 2 arom. H); 7.42 – 7.50 (m, 4
arom. H); 7.56 (d, J ¼ 7.4, 2 arom. H); 7.69 (s, 1 arom. H); 7.77 (d, J ¼ 7.4, 2 arom. H); 8.15 (t, J ¼ 7.6, 1
.
.
arom. H); 8.20 – 8.32 (m, 3 arom. H). MS: 495 (Mþ ), 232 ([M ꢀ OPh]þ ). Anal. calc. for C30H20F3N3O
(495.49): C 72.72, H 4.07, N 8.48; found: C 72.80, H 4.01, N 8.36.
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Received February 17, 2011