Preparation of substituted pyrroles via Ru(II)-catalyzed coupling of alkynes with enamides

Article abstract of DOI:10.1016/j.tetlet.2013.03.013

An efficient ruthenium-catalyzed oxidative cyclization of enamides with alkynes in water or dimethoxyethane leading to the pyrrole derivatives is reported. Typically, a mixture of ethyl 2-(acetylamino)acrylate, diphenylacetylene, [Ru(p-cymene)Cl₂] (2 mol %), KPF₆, and Cu(OAc)₂ in water was heated under refluxing conditions providing the N-acetylated pyrrole product in 95% yield.

Full text of DOI:10.1016/j.tetlet.2013.03.013

Tetrahedron Letters xxx (2013) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Preparation of substituted pyrroles via Ru(II)-catalyzed coupling of alkynes
with enamides
⇑
Kaliyappan Murugan, Shiuh-Tzung Liu
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient ruthenium-catalyzed oxidative cyclization of enamides with alkynes in water or dimethoxy-
ethane leading to the pyrrole derivatives is reported. Typically, a mixture of ethyl 2-(acetylamino)acry-
late, diphenylacetylene, [Ru(p-cymene)Cl2] (2 mol %), KPF6, and Cu(OAc)2 in water was heated under
refluxing conditions providing the N-acetylated pyrrole product in 95% yield.
Received 3 January 2013
Revised 22 February 2013
Accepted 5 March 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Pyrrole
Enamide
Coupling reaction
Alkyne
Ruthenium
Pyrrole scaffold is an important core structure in many pharma-
ceuticals,1 natural products,2 and functional materials.3 Hence, the
development of synthesis of pyrrole derivatives has driven atten-
tion to chemists. After pioneering Knorr-pyrrole synthesis,4 a vast
number of methods have been developed over the years such as
cycloaddition,5 metal catalyzed cyclization,6 multicomponent and
tandem reactions.7 Among them, the transition metal catalyzed
C–H bond functionalization strategy for the pyrrole synthesis
ought to be a more atom-economical approach and thus receives
much attention.
In recent days the utility of enamides as precursors appears,
which causes C–C and C–N bond forming reactions leading to the
pyrrole ring synthesis (Scheme 1).8 Glorius9 and Fagnou10 have
independently reported Rh(III) catalyzed oxidative coupling of ena-
mides with internal alkynes, they synthesized a number of substi-
tuted pyrroles in good to moderate yields. However, the methods
described above required expansive rhodium complexes as the cat-
alysts. The use of cheaper, environmentally benign alternative cat-
alysts is still desirable in organic transformation. Very recently we
have reported Ru(II) catalyzed synthesis of N-acetyl enamides.11
Inspired by aforementioned reports, our endeavor begins to find
the utility of Ru(II) catalyzed reaction of enamides with unactivat-
ed alkynes. In this work, we wish to report an efficient synthesis of
substituted pyrroles by oxidative cyclization of N-acetyl enamides
with various symmetrical and unsymmetrical alkynes in the pres-
ence of [Ru(p-cymene)Cl2], KPF6, Cu(OAc)2ꢀH2O as oxidant and
DME as solvent, notably water also as a compatible solvent for
many substrates.12
We began our study on the coupling of ethyl pyruvate derived
N-acetylenamide 1a with 1,2-diphenylacetylene (Tolan) (2a) (Eq.
(1)). In a typical experiment, a mixture of enamide 1a, alkyne 2a,
[Ru(p-cymene)Cl2] (3 mol %, based on 1a), KPF6 (20 mol %), and
Cu(OAc)2ꢀH2O (1.0 equiv) in a solvent was heated at 100 °C for
12 h. The reaction resulted in the formation of the substituted pyr-
role 3a in 75% isolated yield (Table 1, entry 1). The product was
confirmed by NMR and mass analysis. Without KPF6 or
Cu(OAc)2ꢀH2O, the reaction rate was sluggish, indicating the
requirement of KPF6 and stoichiometric amount of oxidant for a
better performance.
Ph
Ph
NHAc
EtO
EtO
Ph
ð1Þ
N
Ac
O
O
Ph
1a
2a
3a
Further solvent screening, acetonitrile appeared to be unsuit-
able for the catalysis, but in the case of acetone, the product forma-
tion was moderate only at elevated temperature (Table 1, entries
2–4 and 26). Chloro solvents (DCE and CHCl3) and DME performed
well and yielded the desired pyrrole in high yields (Table 1, entries
5–8). Surprisingly, water was compatible with this reaction and the
desired product was obtained in high yield 95% at 2 mol % catalyst
loading (entry 9), notably the reaction rate drastically increased as
compared to the organic solvents. Lowering the catalyst loading
down to 1 mol % (Table 1, entry 10), the product conversion
⇑
Corresponding author. Tel.: +8862 3366 1661; fax: +8862 3366 8671.
E-mail address: stliu@ntu.edu.tw (S.-T. Liu).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.013
Please cite this article in press as: Murugan, K.; Liu, S.-T. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.03.013

2
K. Murugan, S.-T. Liu / Tetrahedron Letters xxx (2013) xxx–xxx
R1
R1
smoothly and yielded the annulated product 3f almost in quantita-
tive yield. These results indicate that there is no reactivity differ-
ence between electron poor and rich alkynes in oxidative
annulation reactions.
NHAc
R
[M]
R
R1
R1
R1
R1
N
Ac
R1
R1
Further, the scope of the reaction extended to symmetrically
substituted biaryl and heteroaryl alkynes, which were reacted
smoothly with 1a and furnished the desired products 3g and 3h
in 81% and 99% yields, respectively. Aliphatic alkynes such as 3-
hexyne and 4-octyne reacted similarly to aromatic alkynes and
yielded the corresponding pyrrole derivatives 3i and 3j in high
yields. Compared to the previous report,12 this condition exhibits
higher reactivity for biaryl, heteroaryl, and aliphatic alkynes.
Regioselective synthesis of organic molecules is a challenging
task. Under the optimized conditions described above, we also ex-
tended to test the regioselectivity with unsymmetrical alkynes
(2k–m), (Scheme 3). N-Acetyl enamide 1a underwent oxidative
annulation with 1-phenyl-1-propyne (2k) affording coupling prod-
uct 3k in high yield 97% with excellent level of regioselectivity, the
bulky group reside proximal to amide nitrogen. This selectivity
pattern is quite similar with the previous reports by other’s
works.10,12 Similarly 1-phenyl-1-pentyne (2l) and 1-(3-phenyl-
prop-2-ynyloxy)benzene (2m) also regioselectively reacted with
enamide 1a exclusively providing annulated products 3l and 3m
in 92% and 90%, respectively, with high regioselectivity.
The scope of ruthenium(II) catalyzed oxidative annulation reac-
tion was further examined with various aryl-substituted enamides
with 1,2-diphenylacetylene (2a) (Scheme 4). Thus, phenyl, o-meth-
ylphenyl, and 2-naphthyl substituted N-acetyl enamides under-
went oxidative annulation with alkyne 2a under the optimized
condition, providing corresponding annulated products 3n–p in
good to moderated yields, but these substrates required little high-
er catalyst loading for better performance. Furthermore, we exam-
ined the cyclization of 1,3-diyene with enamide under the
O
NHAc
[M]
MeO2C
N
Ac
MeO
Scheme 1. Preparation of pyrroles via oxidative coupling of enamides with alkynes.
lowered even with the increase of the reaction time. In these
screens, we found that the catalyst loading at 2 mol % level showed
an optimal condition for the reaction in either water or DME. The
turn over frequency for producing 3a in water medium reaches
23.5 [(mol of product)/(mol of catalyst)ꢀh], which is superior to
the other reported data.12 In addition, several oxidants such as
K2S2O8, oxone, and AgOAc were examined (Table 1, entries 11–
25), but in lower yields or inactive for this reaction.
Under the optimized reaction conditions, several substituted al-
kynes 2a–j reacted efficiently with N-acetyl enamide 1a to give the
corresponding substituted pyrrole derivatives 3a–j in high yields
(Scheme 2). Thus, halogen (chloro and bromo) substituted biaryl
alkynes reacted with enamide 1a smoothly and yielded the corre-
sponding pyrrole derivatives 3b and 3c, respectively, in quantita-
tive yields, but in the case of water medium the product yield
was moderate due to the lower solubility. To improve the reactiv-
ity in water medium we tried various conditions like changing the
additive and using phase transfer catalyst, finally all attempts were
unsuccessful. With electron rich alkynes the oxidative coupling
proceeded smoothly and afforded the corresponding annulated
products 3d and 3e in 87% and 99% yields, respectively, in the or-
ganic medium. Similarly, electron deficient alkyne reacted
Table 1
Optimization of reaction conditionsa
Entry
[Ru(p-cymene)Cl2] (mol %)
Oxidant (equiv)
Solvent
Temp (°C)
Time (h)
Yieldb
%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
3
3
3
3
3
3
3
2
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
—
2
—
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Zn(OAc)2 (1.0)
Cu(OAc)2 (0.5)
K2S2O8 (1.0)
Oxone (1.0)
AgOAc (1.0)
Zn(OAc)2 (1.0)
Cu(OAc)2 (0.5)
K2S2O8 (1.0)
Oxone (1.0)
AgOAc (1.0)
Zn(OAc)2 (1.0)
Cu(OAC)2 (0.5)
K2S2O8 (1.0)
Oxone (1.0)
AgOAc (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
—
t-AmOH
Acetone
Acetone
MeCN
CHCl3
DCE
DME
DME
H2O
H2O
H2O
H2O
H2O
H2O
H2O
DME
DME
DME
100
rt
60
100
90
12
12
12
12
6
12
12
12
2
75
NR
50
NR
92
90
97
97
95
85
30
70
NR
NR
10
110
110
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
110
110
110
6
20
2
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
25c
72c
20c
10c
10c
10c
NR
NR
NR
5c
DME
DME
MeCN
MeCN
MeCN
MeCN
MeCN
Acetone
DME
80c
NR
Trace
NR
DME
DME
—
a
b
c
Reaction conditions: 1a (1.0 equiv), 2a (1.1 equiv), KPF6 (20 mol %), and [Ru(p-cymene)Cl2] in solvent, DCE = 1,2-dichloroethane, DME = dimethoxyethane.
Isolated yields.
Yield determined by NMR using mesitylene as the internal standard, NR = no reaction.
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K. Murugan, S.-T. Liu / Tetrahedron Letters xxx (2013) xxx–xxx
3
Ph
R
Ph
R
[Ru(p-cymene)Cl2]2 (2 mol %)
KPF6 (20 mol %)
Cu(OAc)2 H2O (1 equiv)
Solvent
NHAc
EtO
Ph
EtO
R
N
DME
O
O
1a
EtO
EtO
Ac
3a-j
Cl
R
ð2Þ
Ph
N
Ac
110 °C
+
N
Ac
2a-j
1a
O
Ph
O
Br
Ph
2n
3q, 6 h, 80%, 2:1
In summary, we have reported an efficient method for the prep-
EtO
EtO
aration of pyrroles via the ruthenium catalyzed oxidative annula-
tion of enamide with alkyne. In particular, the reaction can be
carried out in the aqueous medium. Furthermore, the optimal con-
dition developed in this work requires less amount of [Ru(p-cym-
ene)Cl2] as catalyst (at the level of 2 mol %), which makes this
method useful for synthetic application.
EtO
N
N
Br
N
Cl
Ac
O
Ac
O
O
Ac
3c
3a
3b
H2O: 6 h, 62%
DME: 6 h, 99%
H2O: 4 h, 60%
DME: 4 h, 99%
H2O: 2 h, 95%
DME:12 h, 97%
OMe
COOEt
Acknowledgment
EtO
EtO
EtO
N
N
Ac
N
Ac
COOEt
OMe
O
Ac
3d
We thank the National Science Council for the financial support
(NSC-100-2113-M002-001-MY3).
O
O
3f
3e
H2O: 6 h, 58%
DME:12 h, 99%
H2O: 6 h, 65%
DME:10 h, 87%
H2O: 6 h, 50%
DME: 6 h, 99%
Supplementary data
S
S
Supplementary data (experimental procedure and spectral data
of all pyrrole products) associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.03.
013.
EtO
EtO
N
N
O
Ac
Ac
O
3i
EtO
3h
N
Ac
DME:15 h, 90%
O
DME: 6 h, 99%
3g
EtO
N
References and notes
DME:12 h, 81%
O
Ac
DME:12 h, 97%
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Ph
[Ru(p-cymene)Cl2]2 (5 mol %)
KPF6 (20 mol %)
Cu(OAc)2 H2O (1 equiv)
NHAc
R
N
2a
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product formed, but the regioselectivity is poor and obtained 2:1
regioisomeric product in 80% yield (Eq. (2)).
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4
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Please cite this article in press as: Murugan, K.; Liu, S.-T. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.03.013