Cobalt(III)-catalyzed synthesis of pyrroles from enamides and alkynes

Article abstract of DOI:10.1039/c6ra01992g

An efficient and regioselective cobalt-catalyzed synthesis of multi-substituted pyrroles is reported by the use of readily available enamides and alkynes. The success of the process relies on the employment of a catalytic amount of Cp?Co(CO)I₂ together with CuO as the oxidant.

Full text of DOI:10.1039/c6ra01992g

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Cobalt(III)-Catalyzed Synthesis of Pyrroles from
Enamides and Alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
Wenlong Yu,a Wei Zhang,a Yue Liu,a Yougui Zhou,a Zhanxiang Liu,a Yuhong Zhanga,b,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Despite these distinguished advances made in the
An efficient and regioselective cobalt-catalyzed synthesis of multi-
substituted pyrroles is reported by the use of readily available
enamides and alkynes. The success of the process relies on the
employment of a catalytic amount of Cp*Co(CO)I2 together with
CuO as the oxidant.
development of new catalytic methods for the synthesis of
pyrroles, there is still a primary demand for alternative
approaches that involve cheap and environmentally friendly
catalysts. In fact, the development of less expensive and
environmentally more benign catalysts has become one of the
major goals of organic synthesis. In this respect, cobalt-
catalyzed reactions offer attractive prospects in terms of
sustainable chemistry, and has been extensively investigated
as an alternative catalyst in the field of direct C-H
functionalizations.8 Pioneering progress has been made by
Murahashi,9 Kisch,10 Nakamura,11 Kanai,12 Daugulis,13 and later
advanced by the groups of Glorius,14 Yoshikai,15 Ackermann,16
Ellman,17 Song,18 Chang,19 Ge,20 and others.21 We recently
reported an efficient and practical cobalt-catalyzed synthesis
of pyrrolidinones from aliphatic amides and terminal alkynes.22
In course of our continuing interest in direct functionalization
of C-H bonds of enamides,23 we report herein a cobalt-
catalyzed new method for the construction of C-C and C-N
bonds simultaneously, which features synthesis of multi-
substituted pyrroles in high yields from readily available
enamides24 and alkynes. The success of the process relies on
the employment of a catalytic amount of Cp*Co(CO)I2 together
with CuO as the oxidant.
Pyrroles are important five-membered heterocycles and can
be found as key structural units in biologically active natural
products and diverse therapeutic agents.1 They are also served
as useful skeletons in many functional materials.2
Consequently, tremendous efforts have been devoted to the
approaches of this privileged heterocyclic core,3 and transition
metal-catalyzed reactions based on C-H cleavage take the
prominent roles in the recent advances of pyrrole synthesis.4
Fagnou5a and Glorius5b first reported the facile access of
pyrroles independently from the annulation of alkynes with
enamides by rhodium catalysis. In these transformations, the
reaction performed without the prefunctionalization of the
substrates, and the starting materials are low cost and ready
availability. Later, this chemistry is successfully extended to
the ruthenium catalysts by Wang,6a Ackermann,6b and Liu,6c
and both N-acetyl substituted and N-unsubstituted pyrroles
was accessed by slight modification of the reaction conditions.
Very recently, Loh’s and Guan’s research groups, respectively,
developed the palladium-catalyzed new method for the
synthesis of pyrroles using enamides and alkynes as the
starting materials.7a,7b
For initial optimization of the reaction conditions and the
identification of the best cobalt source, N-(1-
phenylvinyl)acetamide (1a) with 1,2-diphenylethyne (2a) were
chosen as model substrates (Table 1). It was found that the
previous reaction conditions for the cyclization of aliphatic
amides and alkynes22 failed to give the product (Table 1, entry
1). To our delight, treatment of 1a (1.0 equiv) with 2a (1.5
equiv) in the presence of 10 mol % of Cp*Co(CO)I2 and 2.2
equiv of Cu(OAc)2 in CF3CH2OH at 100 °C for 8 h gave the
desired N-acetyl substituted pyrrole 3aa in 30% yield (Table 1,
entry 2). The change of oxidant as silver salts showed poor
effect on the promotion of the yield (Table 1, entries 3-4). It
was found that a large amount of acetophenone, which was
produced by the decomposition of enamide, was detected
when Cu(OAc)2 and silver salts were used as the oxidants. In
aDepartment of Chemistry, Zhejiang University, Hangzhou 310027, China. E-mail:
yhzhang@zju.edu.cn
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
730000, China
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1. Optimization of the Reaction Conditionsa
the transformation displayed high functional group tolerance and
proved to be reliable methodology for preparation of
a
multisubstituted pyrroles. Both the electron-rich and electron-
deficient enamides showed good reactivity to give the
corresponding pyrroles in good yields (3aa-3ga). Naphthylenamides
and 2-benzofuranyl enamide participated in the reaction smoothly
to give the desired pyrroles in high yields (3ha-3ja). Thienylenamide
underwent the reaction to give the thienylsubstituted pyrrole 3ka.
Electron-deficient enamide showed good reactivity to give the
corresponding pyrrole 3la in good yield. Alkyl substituted enamide
afforded relatively lower yield (3ma) due to the heavy
decomposition. A variety of functional groups, including alkyl,
methoxyl, phenyl, fluoro, chloro, bromo, and ester, were well
tolerated in the reaction, which may allow the further construction
of complex molecules. The unstable N-vinylacetamide 3n carried
out the reaction to provide 2,3-substituted pyrrole 3na but in low
yield.
entry
1c
2
Additive 1
Na2CO3
AgOTf
AgOTf
AgOTf
AgOTf
AgOTs
AgBF4
AgOTFA
AgSbF6
AgOTf
AgOTf
-
Additive 2
TBAI
Oxidant
Ag2CO3
Cu(OAc)2
AgOAc
Ag2CO3
CuO
Yieldb (%)
NR
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
NaOAc
LiOAc
KOAc
-
30
3
22
4
16
5
91
Scheme 1. Scope of Enamides in the Synthesis of
Substituted Pyrrolesa,b
6
CuO
45
7
CuO
NR
8
CuO
30
9
CuO
40
10
11
12
13
CuO
79
CuO
64
CuO
Trace
Trace
AgOTf
CuO
a Reactions were carried out by using 1a (0.1 mmol), 2a (0.15 mmol),
Cp*Co(CO)I2 (10 mol %), oxidant (2.2 equiv), additive 1 (20 mol %),
additive 2 (20 mol %), CF3CH2OH (1.0 mL), 100 °C, air, 8 h. b Isolated
yield. cCo(OAc)2·4H2O (10 mol %), Ag2CO3 (4 equiv), TBAI (3 equiv),
Na2CO3 (3 equiv), pyridine (2 equiv), PhCF3 (1.0 mL), 100 °C. TBAI =
Tetrabutylammonium Iodide.
contrast, CuO was compatible with the substrates and showed an
obvious improvement of the reactivity to give the pyrrole product
3aa in 91% yield (Table 1, entry 5). Among the several silver(I) salts
tested as iodide abstractor from precatalyst Cp*Co(CO)I2,12e AgOTf
showed the highest reactivity, albeit AgBF4 was ineffective to the
reaction (Table 1, entries 5-9). Furthermore, the counterion of
acetate salts influenced the reaction significantly. For example,
LiOAc and NaOAc promoted the reaction, but KOAc showed the
best effect to give 3aa in excellent yield (Table 1, entries 5, 10-11).
The reaction in the absence of either KOAc or AgOTf gave a trace
3aa product, indicating that both AgOTf and KOAc were essential to
generate the active species of cationic catalytic Co(III) in situ (Table
1, entry 12-13).12e After screening the solvents, CF3CH2OH gave the
best yields (see the Supporting Information for details).
aReactions were carried out by enamides 1 (0.1 mmol), diphenyl-
acetylene 2a (0.15 mmol), Cp*Co(CO)I2 (10 mol %), AgOTf (20 mol
%), KOAc (20 mol %), CuO (2.2 equiv) and CF3CH2OH (1.0 mL) at 100
°C in air for 8 h. b Isolated yield. cAir was replaced by N2.
We next examined the substrate scope of various enamides under
the optimized reaction conditions as shown in Scheme 1. In general,
The reactivity of a range of functionally diverse alkynes was
investigated as shown in Scheme 2. Symmetrical diaryl internal
2 | J. Name., 2012, 00, 1-3
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aReactions were carried out by using 1a (0.1 mmol), 2 (0.15 mmol),
Cp*Co(CO)I2 (10 mol %), AgOTf (20 mol %), KOAc (20 mol %), CuO
(2.2 equiv), CF3CH2OH (1.0 mL), 100 °C, air, 8 h. bIsolated yield.
alkynes with either electron-donating or electron-withdrawing
groups participated in the reaction smoothly to afford pyrrole 3ab-
3af in moderate to excellent yields. Notably, 1,2-di(thiophen-2-
yl)ethyne was tolerated to give the thiophene containinge pyrrole
3ag in good yield. Comparing with the palladium catalytic system, 7a,
This procedure could be scaled up with equal ease to a 5 mmol
7b
this cobalt catalytic system presented excellent regioselectivity. scale and afforded the product in 66% yield. (Scheme 3)
For example, when aryl alkyl-disubstituted alkynes were used as the
substrates, the C-C bond formation always took place at the alkyl- Scheme 3. Gram-Scale Reaction of 1aa,b
substituted carbon of alkynes and the C-N bond generated between
the nitrogen and aryl-substituted carbon of alkynes. Other
regioisomers were not observed in these reactions. The
transformation was sensitive to the hindrance of the alkynes. The
steric alkyl aryl alkynes showed the less reactivity to give the
corresponding pyrroles in lower yields (3ak, 3al). Interestingly, a Reactions were carried out by using 1a (5 mmol), 2a (7.5 mmol),
trimethyl(phenylethynyl)silane participated in the reaction but Cp*Co(CO)I2 (10 mol %), AgOTf (20 mol %), KOAc (20 mol %), CuO
finally gave the TMS-cleavage pyrrole 3ai in 60% yield. Less (2.2 equiv), CF3CH2OH (5.0 mL), 100 °C, air, 8 h. b Isolated yield.
reactivity of ethyl 3-phenylpropiolate was observed and the pyrrole
12e
Based on the observed results and previous reports,
a
3aj was obtained in low yield. The aliphatic internal alkyne was
plausible catalytic cycle is proposed as shown in Scheme 4. Iodide
abstraction from precatalyst Cp*Co(CO)I2 by AgOTf and ligand
exchange generates the catalyst A, which undergoes metalation at
the vinyl C-H with the assistance of amide group to give the
cobaltacyclic complex B. The subsequent migratory insertion of
alkynes into cobaltacycle B forms the intermediate C, which
undergoes the reductive elimination to give the pyrrole product and
liberate Co(I) complex. The oxidation of Co(I) by CuO regenerates
Co(III) catalyst to fulfill the catalytic cycle.
inactive.
Scheme 2. Scope of Alkyne in the Synthesis of
Multisubstitueted Pyrroles a,b
Scheme 4. Proposed Mechanism
In conclusion, we have developed an efficient cobalt-catalyzed
new method for the synthesis of multi-substituted pyrroles from
readily available enamides and alkynes. In this transformation,
carbon-carbon and carbon-nitrogen bonds are simultaneously
constructed and a range of synthetically useful alkynes were
tolerated to give the diverse pyrroles in moderate to high yields.
Comparing with the palladium catalytic system, this cobalt catalytic
system presented excellent regioselectivity. Further studies to
explore cobalt-catalyzed C-H functionalization to construct the
heterocycles are ongoing in our laboratory.
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176-179. (c) K. Murugan and S. T. Liu, Tetrahedron Lett, 2013, 54,
2608-2611.
Acknowledgements
Funding from Natural Science Foundation of China (No.
21272205, 21472165), National Basic Research Program of
China (No. 2011CB936003), and the Program for Zhejiang
Leading Team of S&T Innovation (2011R50007) is
acknowledged.
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