Nickel-Catalyzed Cyclization Strategy for the Synthesis of Pyrroloquinolines, Indoloquinolines, and Indoloisoquinolines

Article abstract of DOI:10.1021/acs.orglett.0c01055

An inexpensive and benchtop stable Ni-catalyst/Zn system for the synthesis of pyrrolo/indoloquinolines and indolo[2,1-a]isoquinolines is explored. This platform provides a one-pot entry for the preparation of various pyrrolo and indoloquinolines/isoquinolines, which involves successive C-C and C-N bond formation, respectively. In addition, we have also performed the preliminary photophysical studies for the synthesized compounds.

Full text of DOI:10.1021/acs.orglett.0c01055

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Letter
Nickel-Catalyzed Cyclization Strategy for the Synthesis of
Pyrroloquinolines, Indoloquinolines, and Indoloisoquinolines
Sampath Thavaselvan and Kanniyappan Parthasarathy*
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ABSTRACT: An inexpensive and benchtop stable Ni-catalyst/Zn system for the synthesis of pyrrolo/indoloquinolines and
indolo[2,1-a]isoquinolines is explored. This platform provides a one-pot entry for the preparation of various pyrrolo and
indoloquinolines/isoquinolines, which involves successive C−C and C−N bond formation, respectively. In addition, we have also
performed the preliminary photophysical studies for the synthesized compounds.
Scheme 1. Metal-Catalyzed Synthesis of Pyrrolo/
indoloquinolines
yrrolo/indoloquinolines are an important class of hetero-
Pcycles widely found in many bioactive naturally occurring
compounds and pharmaceuticals.1 In addition, these com-
pounds shows interesting photophysical properties, with regard
to applications such as semiconductors,2a,b dyes,2c and dye-
sensitized solar cells (DSSCs).2d
Despite their great significance, the synthesis of pyrroloqui-
nolines and indoloquinolines were rarely explored, because of
the requirement of multistep and restricted availability of
starting materials.2a,3 However, a metal catalyst provides a
great entry for the synthesis of various heterocyclic
compounds. So far, only a few metal-catalyzed syntheses of
pyrrolo/indoloquinolines are known. Thus, an example of
pyrroloquinolines by copper-catalyzed Ullmann coupling,
followed by condensation of aldehydes has been reported in
2010 (Scheme 1a).4 Lautens group’s sequentially utilized
intermolecular and intramolecular reactions for the con-
struction of pyrrolo/indoloquinolines by using Pd(OAc)2/
PPh3/norbornadiene/Cs2CO3 mediated C−H annulations
followed by in situ retro-Diels−Alder reaction.5a A palla-
dium-catalyzed domino reaction of 1-(2-iodophenyl)-1H-
pyrrole with bromoalkyl-aryl alkynes (Scheme 1b)5b and
efficient Pd-catalyzed tandem reaction of gem-dibromoolefins
with organoboronic acids are also known to provide pyrrolo/
indoloquinolines frameworks (Scheme 1c).5c Later on, Dong
and Chen et al. reported a rhodium-catalyzed dual C−H
activation reaction of 2-substituted imidazoles with alkynes to
form aza-fused quinolines.6 Recently, the group of Lan and
You co-workers reported facile access to various indolo[1,2-
a]quinolines by a rhodium-catalyzed C−H activation with
indolyl aldehydes and alkynes (Scheme 1d).7
Received: March 23, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01055
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Among the above existing methodologies, the most of them
required costly metal complex and tedious reaction conditions.
Nowadays, the development of inexpensive, environmentally
benign alternative metal catalysts is most sought in organic
synthesis. In this regard, to our knowledge, so far, Ni/Zn
combination of C−H annulations is not explored. Herein, we
wish to report a first Ni/Zn system for construction of pyrrolo/
indoloquinolines and indolo[2,1-a]isoquinolines through C−H
and C−N annulations, respectively (Scheme 1e). Note that
Cheng’s group developed a series of Ni/Zn-system for easy
access to various carbocyclic and heterocyclic compounds.8
The reaction of 1-(2-iodophenyl)-1H-pyrrole (1a, 0.30
mmol) with diphenyl acetylene (2a, 0.60 mmol) in the
presence of NiBr2(dppe)/Zn in toluene at 130 °C for 16 h
gave 4,5-diphenylpyrrolo[1,2-a]quinoline 3a in 90% isolated
optimization studies, we found that the 1a (0.30 mmol), 2a
(0.60 mmol), NiBr2(dppe) (10 mol %), and Zn (3 equiv), in
toluene at 130 °C for 16 h, is the most appropriate condition
for the formation of 3a in 90% isolated yield (Table 1, entry
10). Until now, the best solvent has been toluene. Other
solvents such as tetrahydrofuran (THF), 1,4-dioxane, 1,2-
dichloroethane (1,2-DCE), and dimethylformamide (DMF)
were ineffective for this catalytic reaction (Table 1, entries 11−
15).
Under the optimized reaction conditions, 4-substituted 2-
iodoarylpyrroles 1b−1d with 2a were tested; those results are
summarized in Scheme 2. Thus, electron-donating 4-methyl-2-
iodophenylpyrrole (1b) underwent annulations effectively with
2a to give 4,5-diphenylpyrrolo[1,2-a]quinolines (3b) in 87%
1
yield. The product 3a was confirmed by its H and 13C NMR
Scheme 2. Results of Annulation of 1-(2-iodophenyl)-1H
ab
analyses, as well as MS analysis.
pyrroles with Alkynes
To examine the feasibility of annulations reaction, we began
our optimization study of 1-(2-iodophenyl)-1H-pyrrole 1a
with diphenyl acetylene 2a. When the starting materials 1a and
2a was treated in the presence of Zn alone in acetonitrile at 90
°C for 16 h, desired compound 3a was not formed (Table 1,
a b
,
Table 1. Optimization Studies
b
Zn
(x equiv)
temperature, yield
entry
catalyst
solvent
T (°C)
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
−
NiBr2
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
3
3
3
3
3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
toluene
toluene
THF
90
90
90
90
90
−
−
12
26
20
31
45
52
60
NiBr2/PPh3
NiBr2/dppe
NiBr2(PPh3)2
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
NiBr2(dppe)
90
110
110
110
130
110
110
110
130
130
90
trace
trace
−
−
trace
1,4-dioxane
1,2-DCE
xylenes
3
3
DMF
a
Reactions were conducted using 1a (0.30 mmol), 2a (0.60 mmol),
catalyst (10 mol %), and Zn (x equiv), in solvent (2 mL) at 130 °C
b
for 16 h. Isolated yield.
entry 1). Then, the combination of NiBr2/Zn did not give the
desired product (Table 1, entry 2). Upon employing NiBr2/
PPh3 and NiBr2/dppe system with Zn powder, we started to
observe the product 3a in 12% and 26% yields, respectively
(Table 1, entries 3 and 4). The NiBr2(PPh3)2 and NiBr2(dppe)
complexes were also effective for the cyclization reaction
(Table 1, entries 5 and 6). Furthermore, increasing the
reaction temperature as well as Zn equivalent increased the
yield of product (Table 1, entries 7 and 8). Instead of CH3CN,
the use of toluene solvent improves the product yield 3a in
60% (Table 1, entry 9). To our delight, after several
a
Reaction conditions: 1 (0.30 mmol), 2 (0.60 mmol), and Zn (3
b
equiv), in toluene (2 mL) at 130−150 °C for 16 h. Isolated yield.
B
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a b
,
yield. Similarly, 4-fluoro-2-iodophenylpyrrole (1c) and 4-
chloro-2-iodophenylpyrrole (1d) are also reacted nicely with
2a to provide products 3c and 3d in 85% and 83% yields,
respectively. In addition to 2a, substituted alkynes (2b−2f)
were investigated for the cyclization reactions. Thus, alkynes
having both electron-rich and electron-deficient groups reacted
nicely with 1a to furnish expected cyclized products 3e−3h in
good yields. Similarly, di(2-thienyl)acetylene 2f with 1a gave
4,5-di(thiophen-2-yl)pyrrolo[1,2-a]quinoline 3i in 77% yield.
Unfortunately, the 3,4-disubstituted pyrrole 1e with 2a failed
to give a cyclized product under standard reaction conditions
(130 °C −150 °C), because of the steric (or) electronic effect
of ester and methyl groups on the 3,4-position of pyrrole 1e.
However, we recovered both of the starting materials, along
with partial debromination of 3,4-disubstituted pyrrole 1e.
To evaluate the scope of the present reaction, treatment of
1-(2-bromoaryl)-1H-indole 1f reacted well with 2a in the
presence of NiBr2(dppe) (10 mol %), Zn (3 equiv), toluene at
150 °C for 16 h to give 5,6-diphenylindolo[1,2-a]quinoline 3j
in 67% yield (Scheme 2 bottom). Under similar reaction
conditions, various internal alkynes 2b, 2c, 2e, 2f effectively
reacted with 1f to afford the corresponding 5,6-diary-
lindoloquinolines 3k−3n in 71%−69% yields (Scheme 2). In
addition to 1f, substituted indoles (1g and 1h) were also tested
for this cyclization reaction. Thus, the 1-(2-bromophenyl)-5-
methoxy-1H-indole (1g) nicely reacted with alkyne (2b) to
give cyclized product 3o in 58% yield. However, 1-(2-
bromophenyl)-5-chloro-1H-indole (1h) unfortunately failed
to give an expected product 3p with alkynes (2a or 2b) under
standard reaction conditions.
Scheme 3. Results of Annulations of 4 with Alkynes
Furthermore, the scope of the present Ni/Zn annulations
reaction can also be applied to 2-(2-bromophenyl)-1H-indoles
with alkynes and the results are summarized in Scheme 3. The
reaction of 2-(2-bromophenyl)-1H-indole (4a) with dipheny-
lacetylene (2a), NiBr2(dppe) (10 mol %), and Zn (3 equiv), in
toluene at 130 °C for 16 h afforded 5,6-diphenylindolo[2,1-
a]isoquinoline 5a in 83% yield.
a
Reaction conditions: 4 (0.30 mmol), 2 (0.60 mmol), and Zn (3
b
equiv), in toluene (2 mL) at 130 °C for 16 h. Isolated yield.
regioisomeric products 5i and 5i′ in 56% combined yield with
a 60:40 ratio. However, under similar reaction conditions,
phenylacetylene with 4a failed to give expected product, rather
we could observe only a complex mixture on the TLC. In
addition to 4a, 2-(2-bromophenyl)-5-methyl-1H-indole (4b),
2-(2-bromophenyl)-5-chloro-1H-indole (4c) and 2-(2-bromo-
phenyl)-5-fluoro-1H-indole (4d) worked well with 2a to give
products 5j−5l in good yields. Similarly, the regioisomeric 2-
(2-bromophenyl)-5,6-dimethyl-1H-indole (4e)/2-(2-bromo-
phenyl)-4,5-dimethyl-1H-indole (4e′) (8:2 ratio) effectively
reacted with 2a to give isomeric products 5m and 5m′ in 60%
yield with 8:2 ratio, respectively (Scheme 3).
The efficiency of the cyclization reaction was demonstrated
through the gram-scale preparation of 5a (Scheme 4). Thus,
the reaction of 2-(2-bromophenyl)-1H-indole 4a (0.5 g, 1.83
mmol) with diphenylacetylene 2a (0.65g, 3.67 mmol),
NiBr2(dppe) (10 mol %), and Zn (3 equiv), in toluene at
130 °C for 16 h afforded product 5a in 78% yield.
Note that, so far, a few examples of Ni/Zn-catalyzed C−C/
C−N bond formation have been explored for the synthesis of
heterocyclic compounds.8,9 The ruthenium- and rhodium-
catalyzed C−H/N-H activation for the construction of
indolo[2,1-a]isoquinolines has been reported.10 However,
very recently, Chatani and co-workers reported a Ni-catalyzed
synthesis of 5,6-disubstituted indolo[2,1-a]isoquinolines via
oxidative C−H/N−H coupling of 2-phenylindoles with
alkynes.11 Thus, the development of efficient alternative
catalysts/approaches for the synthesis of useful 5,6-
diarylindolo[2,1-a]isoquinolines is still desirable. Under similar
reaction conditions, other internal alkynes (2b, 2c, 2d, and 2f)
nicely participated with 4a to give corresponding annulated
products 5b−5e in 60%−75% yields. Interestingly, oct-4-yne
(2g) also reacted with 4a to afford the expected compound 5f
in 80% yield. To understand the regioselectivity of this
reaction, we performed using the unsymmetrical alkynes as
substrates for the reaction with 4a. For example, 1-phenyl-1-
propyne (2h) underwent cyclization with 4a at 130 °C for 16 h
to afford two regioisomers 5g and 5g′ (80:20 ratio) in 72%
combined yield (Scheme 3). Similarly, the reaction between 4a
and 1-phenyl-1-butyne (2i), to afford product 5h in 78% yield,
Furthermore, the synthetic transformations of 5a are also
explored. Thus, the treatment of 5,6-diphenylindolo[2,1-
a]isoquinoline 5a with ethyl acrylate in the presence of
Pd(OAc)2/Cu(OAc)2 in a mixture of solvent (DMF/DMSO)
at 70 °C for 18 h gave Heck coupling product 6a in 79%
isolated yield.12 Next, the 1,6-conjugate addition reaction of
a
5a with p-quinone methide in the presence of ZnBr2 (10
mol %) in DCE at room temperature for 1 h gave the
corresponding triarylmethane 7a in 91% yield (Scheme 4).12b
In addition, we have also performed preliminary photophysical
studies for the synthesized pyrrolo/indoloquinoline and
1
along with a trace of isomer 5h′ (observed in H NMR).
Furthermore, the regiochemistry of product 5h was confirmed
by single-crystal X-ray diffraction. Encouraged by these results,
the terminal alkyne 1-hexyne (2j) reacted with 4a to give
C
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formation of a five-membered azanickelacycle V with release
of HX. Next, the coordinative insertions of the alkyne 2a into
the azanickelacycle V to give intermediates VI and VI′.
Reductive elimination of intermediates VI and VI′ affords
cyclized product 5a and regenerate Ni(0) species for the next
cycle.
Scheme 4. Gram Scale Reaction of 4a with 2a and Synthetic
Applications
In conclusion, a new method for the construction of
pyrrolo/indoloquinolines and indolo[2,1-a]isoquinolines by
air-stable nickel catalyst via C−C and C−N bond connection,
respectively, in one pot have been developed. This protocol
provides an opportunity for the efficient synthesis of various
heterocycles from simply prepared starting materials. In this
transformation, which involves an activation of the C−H or
N−H bond through nickel, coordinative insertion was followed
by a reductive elimination approach. In addition, we have also
performed preliminary photophysical studies for the synthe-
sized compounds. Further studies toward investigation on the
reaction mechanism and the application of this methodology
on the organic materials are underway.
indolo[2,1-a]isoquinoline compounds (see the Supporting
Information for details).
On the basis of previous metal-catalyzed annulation
reactions,8,13 we proposed the following Ni-catalyzed reaction
mechanism in Scheme 5A. First, the catalytic cycle initiated by
ASSOCIATED CONTENT
* Supporting Information
■
sı
Scheme 5. Plausible Mechanism
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01055.
General experimental procedure, photophysical studies,
and characterization details (PDF)
Accession Codes
CCDC 1955044 and 1985686 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Kanniyappan Parthasarathy − Department of Organic
Chemistry, University of Madras, Tamil Nadu, Chennai 600
025, India; orcid.org/0000-0002-6496-9625;
Email: kparthasarathy@unom.ac.in
Author
Sampath Thavaselvan − Department of Organic Chemistry,
University of Madras, Tamil Nadu, Chennai 600 025, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01055
Notes
Zn metal, which reduces Ni(II)-complex to active Ni(0)-
complex.8 Then, 1-(2-iodophenyl)-1H-pyrrole 1a undergo
oxidative addition to Ni(0) species, followed by pyrrole
C(2)−H activation to a give five-membered nickelacycle II.
Next, coordinative insertion of alkyne 2a into the intermediate
II generated a seven-membered nickelacycles III (or) III′.
Subsequent reductive elimination of III (or) III′ furnished
product 3a and the regeneration of the active nickel(0) species.
Furthermore, we also proposed a possible mechanism for the
present C−N bond formation reaction in Ni-catalysis (Scheme
5B). The first step likely starts with the reduction of Ni(II) to
Ni(0) species with the aid of zinc metal. Oxidative addition of
2-(2-bromophenyl)-1H-indole 4a to Ni(0) leads to the
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.P. gratefully acknowledge the financial support of the DST-
SERB (YSS/2014/000566) New Delhi, India, and we also
thank DST-FIST for NMR and HRMS facilities. We thank
SAIF, IIT Madras, for X-ray crystallography analysis.
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