Copper(I)-catalyzed decarboxylative coupling of propiolic acids with secondary amines and aldehydes

Article abstract of DOI:10.1002/ejoc.201402338

An efficient microwave-assisted synthesis of tertiary propargylamines has been developed by copper(I)-catalyzed decarboxylative A³-coupling reaction of an alkynylcarboxylic acid with a secondary amine and an aldehyde. A wide range of diversely substituted propargylamines could be synthesized in high yields. The optimal reaction conditions are also effective for 3-alkylpropiolic acids and for terminal propiolic acid. Copyright

Full text of DOI:10.1002/ejoc.201402338

FULL PAPER
DOI: 10.1002/ejoc.201402338
Copper(I)-Catalyzed Decarboxylative Coupling of Propiolic Acids with
Secondary Amines and Aldehydes
Denis S. Ermolat’ev,[a] Huangdi Feng,[a,b] Gonghua Song,[c] and Erik V. Van der Eycken*[a]
Keywords: C–C coupling / Microwave chemistry / Multicomponent reactions / Synthetic methods / Propargylation
An efficient microwave-assisted synthesis of tertiary prop-
argylamines has been developed by copper(I)-catalyzed de-
carboxylative A3-coupling reaction of an alkynylcarboxylic
acid with a secondary amine and an aldehyde. A wide range
of diversely substituted propargylamines could be synthe-
sized in high yields. The optimal reaction conditions are also
effective for 3-alkylpropiolic acids and for terminal propiolic
acid.
Introduction
Multicomponent reactions (MCRs) have attracted much
attention in the framework of combinatorial chemistry ow-
ing to their synthetic efficiency and procedural simplicity.[1]
These reactions constitute a valuable tool for the creation of
large libraries of structurally related, drug-like compounds,
which enable rapid lead identification and lead optimization
in drug discovery. MCRs provide a viable synthetic strategy
to access complex structures from rather simple starting
materials through one-pot methodology, and in particular
exhibit high atom economy and selectivity.[2] A typical ex-
ample of such a process is the three-component coupling of
an aldehyde, an amine and an alkyne (A3-coupling) through
C–H activation to afford propargylamines.[3–7]
Several propargylamines have been shown to be highly
potent and irreversible selective monoamine oxidase type-B
inhibitors[8] (Figure 1). These compounds have been used
for the treatment of neuropsychiatric disorders such as
Alzheimer’s and Parkinson’s disease.
Propargylamines are generally used in organic synthesis
as precursors and versatile building blocks for the prepara-
tion of nitrogen-containing heterocyclic compounds such as
pyrrolidines,[9] pyrroles,[10] oxazolidinones,[11] aminoindoliz-
ines[12] and 2-aminoimidazoles.[13] They also act as key in-
Figure 1. Propargylamine inhibitors of type-B monoamine oxidase.
termediates[14] for the construction of biologically active
compounds like isosteres, β-lactams, oxotremorine sub-
strates, conformationally restricted peptides, and thera-
peutic drug molecules.[15] Traditionally, propargylamines
are synthesized by nucleophilic attack of lithium acetylides
or Grignard reagents to imines.[16]
In recent years, enormous progress has been made to ex-
pand the scope of the direct addition of alkynes to carbon–
nitrogen double bonds either from prepared imines or from
aldehydes and amines in a one-pot procedure with various
transition-metal catalysts through C–H activation of ter-
minal alkynes. These include AgI salts,[17] AuI/AuII salts,[18]
AuIII–salen complexes,[19] CuI salts,[20] Hg2Cl2,[21] and Cu/
Ru dimetallic systems[22] under homogeneous conditions.
Recently, AuIII[23] and AgI[24] in ionic liquids were used to
catalyze A3-coupling reactions. Also, more sophisticated al-
ternative energy sources like microwave[7] and ultrasonic[25]
irradiation have been used in the presence of CuI salt.
Despite its great potential, CuI-catalyzed decarboxylative
strategies are rarely used for the formation of C–C bonds
through A3-coupling reaction.[26] Recently, the group of
Sunwoo Lee reported a metal-free decarboxylative A3-cou-
pling reaction between a propiolic acid, a secondary amine
and paraformaldehyde.[27] There is a limited number of lit-
erature examples in which alkynes with a short alkyl chain
are employed in the A3-coupling reaction. To the best of
our knowledge, only one example of a terminal propargyl-
[a] Laboratory for Organic & Microwave-Assisted Chemistry
(LOMAC), Department of Chemistry, Katholieke Universiteit
Leuven,
Celestijnenlaan 200F, 3001 Leuven, Belgium
E-mail: erik.vandereycken@chem.kuleuven.be
http://chem.kuleuven.be/en/research/mds/lomac/
[b] The Chinese National Compound Library, Shanghai Institute
of Materia Medica, Chinese Academy of Sciences,
3728 Jinke Road, Zhangjiang High-Tech Park, Shanghai
201203, China
[c] Shanghai Key Laboratory of Chemical Biology, East China
University of Science and Technology,
130 Meilong Road, Shanghai 200237, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402338.
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Copper(I)-Catalyzed Decarboxylative Coupling of Propiolic Acids
amine was reported that uses trimethylsilylacetylene.[28] In Table 1. Optimization of the reaction conditions for decarbox-
ylative A3-coupling.[a]
this regard, our group recently discovered that a CuI-cata-
lyzed decarboxylative coupling reaction could be used ef-
ficiently to obtain oxazolidin-2-ones from a propiolic acid,
a primary amine and an aldehyde in a one-pot fashion
(Scheme 1).[29] Herein we report a broadly applicable micro-
wave-assisted CuI-catalyzed decarboxylative coupling reac-
tion of a propiolic acid with a secondary amine and an
aldehyde to synthesize diversely substituted tertiary prop-
argylamines (Scheme 1).
Entry
Catalyst [mol-%]
Time
[min]
Temperature
[°C]
Yield
[%][b]
1
2
20% CuBr
20% CuCl
15
15
15
15
15
15
15
15
15
15
15
15
40
6 h
15
100
100
100
100
100
100
100
100
100
100
80
74
72
34
82
77
52
73
35
63
50
80
73
76
79[d]
0
3
4
5
6
7
8
9
10
11
12
13
14
15
20% CuOAc
20% CuI
20% Cu(MeCN)4PF6
20% CuOTf
20% Cu(OTf)2
20% CuTC[c]
10% CuI
5% CuI
20% CuI
20% CuI
20% CuI
20% CuI
no catalyst
120
100
100
100
[a] Reactions were performed on a 1 mmol scale with isovaler-
aldehyde, dibenzylamine and phenylpropiolic acid (1:1.2:1.5) in tol-
uene (1 mL) under microwave irradiation at 100 W maximum
power. [b] Isolated yields based on isovaleraldehyde. [c] CuTC =
copper(I) thiophene-2-carboxylate. [d] Conventional heating.
Scheme 1.
valeraldehyde and 3-phenylpropiolic acid were examined
with different secondary amines. The corresponding cou-
Results and Discussion
Initially, we studied the coupling of isovaleraldehyde (1), pling products were obtained in high yields (Table 2, En-
dibenzylamine (2) and 3-phenylpropiolic acid (3) in toluene tries 8–17). Only trace amounts of by-products, which re-
at 100 °C in the presence of CuBr (20 mol-%) under micro- sulted from the Michael addition of amine to propiolic
wave irradiation (Table 1). To our delight, the desired prod- acid, were observed by 1H NMR spectroscopy and GC–MS
uct 4{1} was obtained in 74% yield within 15 min (Table 1, of the crude reaction mixtures.
Entry 1). To improve the yield, various copper(I) and cop-
Subsequently, our investigations were focused on the use
per(II) salts were examined (Table 1, Entries 2–8), which of various 3-substituted propiolic acids 3. When 3-alkylpro-
were shown to be effective for similar reactions.[30] Among piolic acids were used, the corresponding products 4 were
them, CuI provided the highest yield (Table 1, Entry 4). obtained in good yields (Table 2, Entries 18–25).
When the reactions were performed with lower catalyst
Surprisingly, the use of 3-methylpropiolic acid in this de-
loadings, decreased yields were obtained (Table 1, Entries 9 carboxylative procedure represents an elegant alternative
and 10). Different temperatures were also tested, but did for the application of gaseous and explosive propyne in a
not improve the yields (Table 1, Entries 11 and 12). Re- classical A3-coupling reaction (Table 2, Entry 18; R4 = Me).
markably, Cu(OTf)2 was slightly less efficient than CuI Similarly, 3-triisopropylsilylpropiolic acid was also appli-
(Table 1, Entry 7).[31] Finally, when the reaction was con- cable (Table 2, Entry 25). The best yields were obtained
ducted by using conventional heating under the same condi- when 3-aryl-substituted propiolic acids were used (Table 2,
tions, the desired propargylamine 4{1} was obtained in a Entries 26–28). Encouraged by these results, we then tried
yield of 79% after an extended reaction time of 6 h (Table 1, to combine various aldehydes 1 and amines 2 with propiolic
Entry 14). Remarkably, no product was observed in the ab- acids 3 in this coupling process. We were pleased to find
sence of catalyst (Table 1, Entry 15).
that most of the combinations led to the formation of the
To investigate the scope of this decarboxylative A3-cou- desired products in good to excellent yields (Table 2, En-
pling reaction, various aldehydes 1 were examined under tries 29–34). Finally, the use of the terminal propiolic acid
the above optimized conditions (Table 1, Entry 4).[31] For (Table 2, Entries 35–39; R4 = H) afforded the correspond-
aliphatic aldehydes, the reaction afforded the corresponding ing coupling products in 43–78% yields.
products 4 in moderate to good yields (Table 2, Entries 1–
A tentative mechanism for the CuI-catalyzed one-pot
6). The use of aromatic 1-naphthaldehyde gave the product three-component decarboxylative coupling reaction is pro-
in decreased yield (Table 2, Entry 7). The reactions of iso- posed in Scheme 2.[32] The CuI catalyst reacts with the alk-
Eur. J. Org. Chem. 2014, 5346–5350
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. S. Ermolat’ev, H. Feng, G. Song, E. V. Van der Eycken
FULL PAPER
Table 2. Scope of the decarboxylative A3-coupling reaction.[a]
Entry
Aldehyde 1
Amine 2
R4[b]
Yield [%][c]
1
2
3
4
5
6
7
8
isobutyraldehyde
butyraldehyde
valeraldehyde
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
morpholine
piperidine
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
45
65
63
64
61
57
25
89
88
94
82
86
90
91
80
93
92
78
85
88
78
76
72
79
74
95
90
92
96
91
70
86
84
91
72
68
74
64
78
hexanal
cyclohexanecarbaldehyde
3-phenylpropionaldehyde
1-naphthaldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
isovaleraldehyde
cyclohexanecarbaldehyde
cyclohexanecarbaldehyde
3-phenylpropionaldehyde
3-phenylpropionaldehyde
valeraldehyde
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
4-methylpiperidine
1,4-dioxa-8-azapirodecane
1-benzylpiperazine
diethylamine
dibutylamine
dihexylamine
N-methylbenylamine
N-allylmethylamine
dibenzylamine
Me
Et
Pr
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
dibenzylamine
iPr
tBu
amyl
hexyl
TIPS
2-naphthyl
PMP
4-tolyl
Ph
dibenzylamine
dibenzylamine
4-methylpiperidine
N-allylmethylamine
morpholine
4-methylpiperidine
4-methylpiperidine
piperidine
tetrahydroisoquinoline
tetrahydroisoquinoline
dibenzylamine
N-methylbenzylamine
N-methylbenzylamine
Ph
Ph
Ph
Ph
Ph
H
H
H
isobutyraldehyde
valeraldehyde
isovaleraldehyde
isobutyraldehyde
benzaldehyde
H
H
isovaleraldehyde
[a] A mixture of aldehyde (1 mmol), amine (1.2 mmol), alkyne (1.5 mmol), CuI (20 mol-%) in toluene (1 mL) was irradiated at a maximum
temperature of 100 °C and a maximum power of 100 W for 15 min. [b] TIPS = triisopropylsilyl; PMP = p-methoxyphenyl. [c] Isolated
yields based on aldehydes are reported.
ynylcarboxylic acid 1 to form the copper carboxylate A,
which undergoes decarboxylation at elevated temperature
to provide the reactive alkynylcopper species B. The copper
acetylide B attacks the in situ formed iminium salt C to
result in the formation of the propargylamine 4 and regen-
eration of the CuI catalyst.
To prove the mechanism we carried out a comparative
study by using 3,3-dimethylbut-1-yne (5) as an alkyne
equivalent of 4,4-dimethylpent-2-ynoic acid under micro-
wave irradiation (Table 3). By applying similar reaction con-
ditions as for the decarboxylative A3-coupling reaction, the
desired product 4{23} was obtained in only 52% isolated
yield (Table 3, Entry 2). Comparable yields were achieved
Scheme 2. Tentative mechanism for the decarboxylative A3-cou-
pling reaction.
only after 35 min at 100 °C (Table 3, Entry 4) or after
15 min at 120 °C (Table 3, Entry 7).
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Copper(I)-Catalyzed Decarboxylative Coupling of Propiolic Acids
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15
25
35
45
15
15
100
100
100
100
100
110
120
43
52
64
77
82
74
78
[a] Reactions were performed on a 1 mmol scale with isovaler-
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Supporting Information (see footnote on the first page of this arti-
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of propargylamines 4.
[18]
Acknowledgments
[19]
[20]
The authors wish to thank the Fund for Scientific Research – Flan-
ders, Belgium (F. W. O.), the Research Fund of the Katholieke Uni-
versiteit Leuven, and the Industrial Research Fund of the Kathol-
ieke Universiteit Leuven for financial support to the laboratory.
D. S. E. is grateful to the F. W. O. for a postdoctoral fellowship.
H. D. F. thanks the China Scholarship Council for financial sup-
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D. S. Ermolat’ev, H. Feng, G. Song, E. V. Van der Eycken
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Received: March 31, 2014
Published Online: July 22, 2014
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