Cu-Catalyzed Tandem Aerobic Oxidative Cyclization for the Synthesis of 3,3′-Bipyrroles from the Homopropargylic Amines

Article abstract of DOI:10.1021/acs.orglett.8b02201

A Cu-catalyzed method for the synthesis of 3,3′-bipyrroles from homopropargylic amines through tandem aerobic oxidative cyclization involving the formation of C-C bond has been developed. The features of this reaction are a small number of Cu catalysis and simple starting substrates. Moreover, this procedure exhibits good functional group tolerance and a series of 3,3′-bipyrroles derivatives are obtained in moderate to good yields.

Full text of DOI:10.1021/acs.orglett.8b02201

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cu-Catalyzed Tandem Aerobic Oxidative Cyclization for the
Synthesis of 3,3′-Bipyrroles from the Homopropargylic Amines
Zhenjie Qi,† Yong Jiang,‡ Bingxiang Yuan,† Yanning Niu,§ and Rulong Yan*,†
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Gansu,
China
‡School of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing, China
§Department of Teaching and Research, Nanjing Forestry University, Huaian, China
S
* Supporting Information
ABSTRACT: A Cu-catalyzed method for the synthesis of 3,3′-
bipyrroles from homopropargylic amines through tandem
aerobic oxidative cyclization involving the formation of C−C
bond has been developed. The features of this reaction are a
small number of Cu catalysis and simple starting substrates.
Moreover, this procedure exhibits good functional group
tolerance and a series of 3,3′-bipyrroles derivatives are obtained
in moderate to good yields.
Scheme 1. Synthesis of Substituted 3,3′-Bipyrroles
ubstituted pyrroles, as privileged heterocyclic compounds,
1
Sare found in many natural products,
pharmaceutical
agents,2 and materials science.3 Furthermore, pyrrole moieties
have also emerged as important functional groups in
biologically active compounds.4 Accordingly, the construction
of the pyrrole scaffolds has attracted intensive research interest
and many synthetic methods have been developed.5 Never-
theless, most known methods are effective for the synthesis of a
single pyrrole ring.6 Efficient synthetic strategies for the
generation of substituted 3,3′-bipyrroles are very rare.
Bipyrrole and their derivatives are widespread in natural
products,7 materials science,8 and medicinal chemistry.9 Until
now, the approach for the synthesis the substituted 3,3′-
bipyrroles has been very limited. Traditional methods for the
synthesis of substituted 3,3′-bipyrroles focus on the function-
alization of the preconstructed pyrrole nucleus, as reported by
the Uno group (Scheme 1).7c,10 The group of Hua also has
developed a straightforward method for the synthesis of 3,3′-
bipyrroles by oxidative C−H homocoupling of 1,2,5-
trisubstituted pyrroles in the presence of FeCl3.10e Hence,
research on the synthesis of 3,3′-bipyrroles based on
construction of pyrrole scaffolds is even less. Gleiter’s group
has reported that Pd-catalyzed rearrangement of 1,6-
diisopropyl-1,6-diazacyclodeca-3,8-diyne to N,N′-diisopropyl-
3,3′-bispyrrole in a one-pot reaction involving the formation of
a C−C bond.11
The group of Jaisankar disclosed that 3,3′-bipyrroles could
be synthesized from diaroylacetylene, the appropriate 1,3-
dicarbonyls, and ammonium acetate through double Michael
addition reaction.12 Although these impressive advances have
been achieved based on construction of pyrrole scaffolds, the
synthesis of 3,3′-bipyrroles from simple substrates still remains
a great challenge. Herein, our group reports a Cu-catalyzed
method for the synthesis of 3,3′-bipyrroles from the simple
homopropargylic amines via C−C and C−N bond formations.
Initially, investigation commenced with the reaction of N-(3-
phenylprop-2-yn-1-yl)aniline (1a) and CuCl (10 mol %) in
toluene at 110 °C under a nitrogen atmosphere. The 1,1′,2,2′-
Received: July 14, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02201
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Letter
a
tetraphenyl-1H,1′H-3,3′-bipyrrole (2a) was formed in 31%
yield (Table S1, entry 1). The structure of 2a was determined
by X-ray crystallographic analysis. After extensive screening of
different reaction parameters (see Supporting Information
(SI), Table S1), the optimum conditions were identified to be
CuCl (5 mol %) in DMSO at 110 °C under an air atmosphere
(entry 13).
Scheme 3. Scope of Aminoalkynes
With the optimized reaction conditions established, the
universality and scope were subsequently investigated, and the
results are illustrated in Scheme 2. To our delight,
a
Scheme 2. Scope of Aminoalkynes
a
Reaction conditions: 1a (0.3 mmol), CuCl (5 mol %), DMSO (2
mL), 110 °C, air.
yields of R1 substituents with electron-donating groups were
slightly higher than with the electron-withdrawing groups. As
good results were obtained with the R1 substituents as an aryl
group, we turned our attention to R1 substituents as an alkyl
group. As shown in Scheme 3, the substrates with alkyl groups
performed well in this process and the desired products were
afforded in good yields (4l−4p). However, the substrate 3q
cannot work and no desired product 4q was detected.
Moreover, the homopropargylic amines with two different
functional groups on the phenyl ring of R1 and R2 were also
suitable for this transformation, generating the desired
products in good yields (4r, 4s).
a
Reaction conditions: 1a (0.3 mmol), CuCl (5 mol %), DMSO (2
mL), 110 °C, air.
homopropargylic amines with different groups on benzene
rings of R2 substituents performed well and the desired
products were isolated in ideal yields (2b−2k). In general,
homopropargylic amines with an electron-donating group on
benzene rings showed higher reactivity than those with an
electron-withdrawing group (2b−2i). These results may be
caused by the increased reactivity of the alkynyl group in
homopropargylic amines with electron-donating groups.
Naphthalen-1-yl homopropargylic amine was also a suitable
substrate for this process, giving the desire product 2l in 52%
yield. Moreover, the standard conditions were also tolerated
with heteroarene homopropargylic amines such as quinolyl and
thienyl and the corresponding products were obtained in 45%
and 51% yields, respectively.
More challenging substrates were also evaluated in this
reaction, and the results are illustrated in Scheme 4. When N-
Scheme 4. Scope of 3,3′-Bipyrroles
The substrates 1o and 1p with H and alkyl groups cannot
work in this method, and no desired products were detected.
Furthermore, the scope of the substrate investigation was
enlarged to substituents R1 of homopropargylic amines, and
the results are shown in Scheme 3. Under the optimized
reaction conditions, the substrates with different groups on the
benzene ring of R1 substituents were compatible with this
transformation and generated the desired products in
moderate yields (4b−4k). Similar to R2 substituents, the
(1,4-diphenylbut-3-yn-1-yl)aniline 5a and N-(1-(4-chloro-
phenyl)-4-phenylbut-3-yn-1-yl)aniline 5b were subjected to
the standard conditions, the expected products 6a and 6b were
isolated in 51% and 60% yields, respectively.
In order to obtain further insight into this reaction, several
control experiments were carried out in Scheme 5. First, the
radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy
B
DOI: 10.1021/acs.orglett.8b02201
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Scheme 5. Control Experiments
Scheme 6. Proposed Mechanism
intermediate 14. Finally, the desired product 2a is afforded
through oxidation catalyzed by CuCl.
In conclusion, we have developed a direct protocol to
synthesize 3,3′-bipyrroles from aminoalkynes with CuCl
catalysis. Broad group tolerance makes this method useful to
construct such unique scaffolds from simple aminoalkynes, and
the desired 3,3′-bipyrroles are afforded in ideal yields.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02201.
(TEMPO) and 2,6-di-tert-butyl-4-methyl-phenol (BHT) were
employed for this reaction. The reactions were significantly
inhibited when 2.0 equiv of BHT or TEMPO were used. The
results demonstrate that this transformation should be a radical
pathway under the standard conditions. Then the substrate 1-
phenyl-1H-pyrrole was used to trap the radical intermediate.
Unfortunately, no useful radical intermediate was detected.
Further study disclosed that the compound 7 was isolated in
22% yield under standard reaction conditions after 15 min and
disappeared as the experiment proceeded. Then compound 7
was used as the substrate under the optimized conditions, and
the desired product 2a was afforded in 71% yield. Consistent
with our expectation, the intermediate 7 was subject to the
optimized conditions with TEMPO (2.0 equiv) and no desired
product was formed. Based on the above results, compound 7
should be the intermediate for this reaction. Moreover, the
HRMS (High Resolution Mass Spectrometry) measurement
was also performed with compound 7 proceeding under
standard conditions after 15 min. To our delight, the important
peaks at m/z = 222.1275, 457.2266, and 439.2162 correspond-
ing to C16H16N [M + H]+, C32H29N2O [M + H]+, C32H27N2
[M + H]+ were observed (Figure S1, SI). These results
indicate that the intermediates 10, 13, and 14 possibly exist in
this transformation.
1
Experimental methods, X-ray data for 2a, and H and
13C NMR spectra of all compounds (PDF)
Accession Codes
CCDC 1843793 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yanrl@lzu.edu.cn.
ORCID
Zhenjie Qi: 0000-0002-0784-3188
Yong Jiang: 0000-0002-7300-9984
Rulong Yan: 0000-0002-0450-3065
Notes
The authors declare no competing financial interest.
On the basis of the above results, a plausible mechanism is
proposed as shown in Scheme 6. The intermediate 7 is
generated from the substrate 1a through first the coordination
of CuCl and then nucleophilic attraction of H2O under the
standard conditions. Then radical intermediate 8 is formed
through oxidation. Meanwhile, the intermediate 9, which is
obtained from 7 through intramolecular cyclization, gives the
cyclic compound 10 with hydrogen ion elimination. Radical
addition between intermediate 8 and 10 produces the radical
intermediate 11. Then 12 is generated from 11, giving
compound 13 via oxidative aromatization with Cu-catalysis.
Intermolecular cyclization occurs with 13 to form the
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21672086), the Gansu Province Science
Foundation for Youths (1606RJYA260), and the Fundamental
Research Funds for the Central Universities (lzujbky-2018-81).
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