Copper-mediated cross-coupling-cyclization-oxidation: A one-pot reaction to construct polysubstituted pyrroles

Article abstract of DOI:10.1039/c3cc48728h

A novel and efficient procedure for the synthesis of polysubstituted pyrroles has been developed in this work. The polysubsituted pyrroles were synthesized directly from terminal alkenes, amines and β-keto esters through cross-coupling-cyclization-oxidation in the presence of a catalytic amount of cuprous chloride. This method provides a one-pot synthesis route from terminal alkenes to polysubstituted pyrroles for the first time and opens a new area in cuprous catalysis. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc48728h

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Copper-mediated cross-coupling–cyclization–
oxidation: a one-pot reaction to construct
polysubstituted pyrroles†
Cite this:DOI: 10.1039/c3cc48728h
Received 15th November 2013,
Accepted 16th January 2014
Pei Liu,‡ Jin-ling Liu,‡ Heng-shan Wang, Ying-ming Pan,* Hong Liang and
Zhen-Feng Chen*
DOI: 10.1039/c3cc48728h
www.rsc.org/chemcomm
A novel and efficient procedure for the synthesis of polysubstituted as it provides a direct and reliable approach to synthesize a variety of
pyrroles has been developed in this work. The polysubsituted valuable products.10 In contrast, transition-metal-catalyzed C–H func-
pyrroles were synthesized directly from terminal alkenes, amines tionalization of terminal alkenes is relatively rare because the C(sp2)–
and b-keto esters through cross-coupling–cyclization–oxidation in H bond of simple alkenes is less reactive toward transition metals
the presence of a catalytic amount of cuprous chloride. This than alkynes. Therefore, an efficient and atom-economic synthesis of
method provides a one-pot synthesis route from terminal alkenes multifunctional heterocycles via direct C–H functionalization–cycliza-
to polysubstituted pyrroles for the first time and opens a new area tion of terminal alkenes is very challenging but attractive. As a result
in cuprous catalysis.
of development on the transition-metal-catalyzed C–H functionaliza-
tion of alkenes in our group,11 herein, we report, for the first time, an
Pyrroles are privileged structures that have been found in numerous efficient and straightforward protocol for synthesis of pyrroles with
biologically active compounds such as natural products, pharmaceu- terminal alkenes under mild conditions. This protocol is also novel
ticals, and agrochemicals.1,2 This key heterocyclic core has been for the copper catalyzed sequential reaction.
widely used in both organic synthesis and materials science.3 In this
To optimize the reaction conditions for the three-component
regard, there are many reports for the synthesis of pyrroles,4 such as cross-coupling–cyclization–oxidation process, a variety of catalysts
the classical Knorr reaction,5 Hantzsch reaction,6 and Paal–Knorr and solvents were screened with styrene 1a, aniline 2a and ethyl
condensation reaction.7 Another type of synthesis procedures with acetoacetate 3a as model substrates (Table 1). Initially, the reaction of
metal-catalyzed reactions have also been developed.8 However, some 1a (0.75 mmol), 2a (0.5 mmol), and 3a (0.5 mmol) in the presence of
of these methods require tedious workup, harsh reaction conditions 10 mol% CuCl in DMSO at 80 1C for 24 h gave the substituted
and long reaction times. Others involve multistep synthetic opera- pyrroles 4aaa in 78% yield (Table 1, entry 1). In addition, in the
tions or result in low yields. Therefore, a straightforward, convenient, presence of other catalysts such as BiCl3, ZnCl2, InCl3, FeCl3, PdCl2,
and highly regioselective route to synthesize pyrroles using basic RhCl3 or RuCl3, most of the starting material 1a was recovered
chemical materials is highly attractive. To the best of our knowledge, (Table 1, entries 2–8). The reaction with AuBr3 as a catalyst yielded
the construction of pyrroles from simple terminal alkenes has not no target product 4aaa at all (Table 1, entry 9). More catalysts were
been reported.
also screened but none of them gave higher yields than CuCl (see
Novel reactions that can selectively functionalize carbon–hydrogen ESI†). Solvent plays a key role in this reaction too. Compared to the
bonds are of intense interest to the chemical community because reaction in DMSO, the reactions in DCE and 1,4-dioxane produced
they offer new strategic approaches for synthesis chemistry.9 Since much lower pyrrole yields under the same conditions (Table 1,
the last decade, transition-metal-catalyzed C–H functionalization of entries 10 and 11 vs. entry 1). Other solvents including PhCl, CH3NO3,
terminal alkynes has become an important type of organic reaction, CH3COOH, PhCH3 and DMF are not desired (Table 1, entries 12–16).
Hence, the optimized reaction conditions for this Cu-mediated cross-
coupling–cyclization–oxidation process are 10 mol% CuCl in DMSO
Key Laboratory for the Chemistry and Molecular Engineering of Medicinal
at 80 1C with a reaction time of 24 h.
Resources (Ministry of Education of China), School of Chemistry & Chemical
Engineering of Guangxi Normal University, Guilin 541004, People’s Republic of
China. E-mail: panym2013@hotmail.com, chenzfubc@yahoo.com;
Fax: +86-773-5803930; Tel: +86-773-5846279
With the optimized reaction conditions in hand, various terminal
alkenes 1 and amines 2 were reacted with different b-keto esters 3 to
produce the corresponding pyrrole products 4 (Table 2). The reaction
was readily extended to a variety of aryl-substituted terminal alkenes.
Catalyzed by CuCl, almost all the reactions with aryl-substituted
terminal alkenes produced a high yield of pyrrole under the
† Electronic supplementary information (ESI) available. CCDC 972540. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc48728h
‡ These authors contributed equally to this work.
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Table 1 Optimization of reaction conditionsa
Table 2 Synthesis of polysubstituted pyrroles 4 catalyzed by CuCla,b
Entry
Catalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
CuCl
BiCl3
ZnCl2
InCl3
FeCl3
PdCl2
RuCl3
RhCl3
AuBr3
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DCE
1,4-Dioxane
PhCl
CH3NO3
CH3COOH
PhCH3
DMF
78
0
0
0
0
0
0
0
0
45
36
0
0
0
0
0
9
10
11
12
13
14
15
16
a
Reaction conditions: styrene 1a (0.75 mmol), aniline 2a (0.5 mmol),
ethyl acetoacetate 3a (0.5 mmol), catalyst (10 mol% to 2a), solvent
b
(4.0 mL), 80 1C, 24 h. Isolated yield of the pure product based on 2a.
optimized conditions. Terminal aryl alkene 1b with an electron-
donating group at the aryl ring (Ar = 4-MeOC6H4) is reactive and
yielded product 4baa in 85% yield (Table 2, entry 4baa). Substrates 1c
and 1d possessing an electron-withdrawing group (Ar = 4-BrC6H4,
4-FC6H4) at the benzene ring are also reactive and afforded the
desired products 4caa and 4daa in 72% and 70% yields, respectively
(Table 2, entries 4caa and 4daa). Obviously, electron-rich terminal
alkenes yield desired products in higher yields than electron-poor
terminal alkenes do. It is striking that steric effects had no obvious
effect on this sequential reaction. Regardless of the substitution
pattern of the aryl ring (ortho or para), the aryl olefins used in the
reactions gave the corresponding pyrrole products in 81–83% yields
(Table 2, entries 4eaa and 4faa). Additionally, terminal alkenes
containing a naphthalene moiety can also be employed to obtain
the pyrrole scaffold in high yield (Table 2, entry 4gaa). Unfortunately,
when the acrylate esters (e.g. ethyl acrylate) and internal alkenes
(e.g. trans-stilbene) were used, the reactions failed to afford the
desired products. Substrate 2b possessing an electron-donating
group (R1 = 4-MeC6H4) on the benzene ring produced the desired
product 4aba in 73% yield. Substrates 2c and 2d, with electron-
withdrawing groups (R1 = 4-BrC6H4, 4-FC6H4) on the benzene ring,
a
Reaction conditions: terminal alkenes 1a (0.75 mmol), aromatic
amines 2a (0.5 mmol), b-keto esters 3a (0.5 mmol), CuCl (10 mol% to
2a), solvent (4.0 mL), 80 1C, 24 h. Isolated yield of the pure product
based on 2a. The reactions were carried out at 100 1C, for 36 h.
b
c
produced the desired products 4aca and 4ada in 84% and 86% (Table 2, entries 4aab–4aaf). Crystallized 4aab from ethanol is a single
yields, respectively (Table 2, entries 4aca and 4ada). It was noticed crystal and its molecular structure was confirmed by X-ray analysis
that functional groups such as fluoro, bromo, methyl and methoxy (Fig. 1). The reaction of 2-methoxyethyl acetoacetate 3g with 1a and
were tolerated under the reaction conditions. When the N-phenyl 2a produced pyrrole 4aag in 74% yield (Table 2, entry 4aag). In
groups were replaced with N-alkyl groups, the desired pyrroles could addition, the corresponding pyrroles with different substituents on
be obtained in high yields at higher reaction temperature with a the 2-position were successfully synthesized in high yields (Table 2,
longer reaction time (Table 2, entries 4bea and 4cea).
entries 4aah and 4bah). This efficient and modular one-pot construc-
Various substituted b-keto esters 3 were also found as suitable tion of pyrrole scaffolds from three simple and commercially avail-
reaction partners with terminal alkenes 1 and aromatic amines 2 in able starting materials complements the Hantsch-type pyrrole
this reaction. The ester groups of 3, including methyl, iso-propyl, iso- synthesis and extends its applications.
butyl, tert-butyl and benzyl ester, reacted with styrene 1a and aniline
An extended example is b-enamino ester which underwent sub-
2a to afford the corresponding pyrrole products in high yields sequent cross-coupling–cyclization–oxidation with terminal alkene
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Scheme 4 Possible reaction mechanism.
in the synthesis of pyrazoles starting from b-imino esters and nitriles,
where b-imino esters acts as nucleophiles that attack the nitrile.13
In summary, we have developed an efficient approach to the
synthesis of polysubstituted pyrroles via a copper-catalyzed three-
component reaction of terminal alkenes, amines and b-keto esters
under aerobic conditions. Due to the easy availability of the starting
materials and potential applications of products, this method is
highly prospective in organic synthesis and medicinal chemistry.
This protocol also represents an extremely simple, efficient, and
atom-economic way to construct the substituted pyrroles in good
yield with high selectivity. Thus it complements the method for the
rapid formation of multifunctional heterocycles.
Fig. 1 X-ray crystal structure of 4aab.
Scheme 1 Synthesis of pyrrole from b-enamino ester and styrene.
We acknowledge the project 973 (2011CB512005), Ministry of
Education of China (IRT1225), the National Natural Science Founda-
tion of China (21362002, 41206077 and 81260472), Guangxi Natural
Science Foundation of China (2012GXNSFAA053027 and
2011GXNSFD018010), Guangxi Scientific Research and Technology
Development Program (No. 1355004-3), and Bagui Scholar Program
of Guangxi for financial support.
Scheme 2 Cross-coupling–cyclization–oxidation of styrene 1a, aniline 2a
and ethyl 2-methylacetoacetate 3i. Reaction conditions: 1a (0.75 mmol), 2a
(0.5 mmol), 3i (0.5 mmol), CuCl (10 mol%) in 4 mL of DMSO at 80 1C for 24 h.
1a. The reaction yielded the desired pyrrole product 4aaa in 82% yield
(Scheme 1).
However, the reaction of ethyl 2-methylacetoacetate 3i with
1a and 2a under the optimized conditions failed to afford the
desired product (Scheme 2).
To explore the possible reaction pathway, isotope deuterium-
labeled styrene 1a-d was used to react with aniline 2a and ethyl
acetoacetate 3a. The substituted pyrrole 4aaa-d was obtained in
81% yield. Over 96% of deuterium was incorporated in the
product (Scheme 3).
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