Assembly of polycyclic N-heterocycles: Via copper-catalyzed cycloamination of indolylquinones and aromatic amines

Article abstract of DOI:10.1039/d1ob00666e

The copper-catalyzed cycloamination of indolylquinones and various (hetero)aromatic amines under ligand-free conditions for the synthesis of polycyclic N-heterocycles has been developed. This method allows facile access to polycyclic N-heterocycles with the tolerance of chloride, bromide, amino, thio, etc. groups in moderate to high yields (60-89%).

Full text of DOI:10.1039/d1ob00666e

Organic &
Biomolecular Chemistry
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Assembly of polycyclic N-heterocycles via copper-
catalyzed cycloamination of indolylquinones and
aromatic amines†
Cite this: Org. Biomol. Chem., 2021,
19, 4593
Yu Dong, *a Ting Mei,a Ji-Xian Ye,a Xiang-Long Chen,a Hui Jiang,a Bo Chang,a
Zhi-Fan Wang, a Zhi-Chuan Shi,b Zhong-Hui Li*a and Bing He*a
Received 6th April 2021,
Accepted 1st May 2021
The copper-catalyzed cycloamination of indolylquinones and various (hetero)aromatic amines under
ligand-free conditions for the synthesis of polycyclic N-heterocycles has been developed. This method
allows facile access to polycyclic N-heterocycles with the tolerance of chloride, bromide, amino, thio,
etc. groups in moderate to high yields (60–89%).
DOI: 10.1039/d1ob00666e
rsc.li/obc
tion,13 and thermal or Lewis acid catalyzed Diels–Alder reac-
Introduction
tion followed by aromatization.14 Despite their merits, many
The nitrogen-containing heterocycle nucleus is an important existing synthetic approaches have several shortcomings.
structural scaffold frequently found in drugs, functional However, some of them rely heavily on the use of expensive
materials and natural products.1 Among nitrogen-containing transition metal catalysts, and cannot avoid harsh conditions,
heterocycles, polycyclic N-heterocycles are widespread in multistep synthesis, poor yield of the proposed product and
nature and exhibit several interesting biological activities.2 The the preparation of the prerequisite functional groups.
representative compounds include the quinone-fused polycyc-
Transition-metal-catalyzed C–H amination is a step-econ-
lic N-heterocycles pixantrone (experimental antineoplastic omical and straightforward synthetic methodology to form aro-
drug with fewer toxic effects on cardiac tissue)3 and ascidide- matic C–N bonds.15 In particular, transition-metal catalyzed
min (possessing prominent cytotoxic properties)4 and the cross-dehydrogenative coupling (CDC) amination is highly
indole-fused carbazole rebeccamycin (topoisomerase I inhibi- desirable owing to aromatic amines being utilized as reac-
tor),5 and the naphtho[a]carbazole (a potential candidate for tants.16 Recently, we discovered that polycyclic N-heterocyclic
cancer treatment)6 (Fig. 1). On the other hand, polycyclic complexes are synthesized by cobalt-catalyzed cycloamination
N-heterocycles exhibit interesting properties for application as reaction between indolylquinones and aromatic amines and
fluorescent probes7 and optoelectronic materials.8
Pentacyclic complexes bearing a quinone skeleton have
been used for metal ion recognition and have also been proven
to be suitable receptors for the colorimetric sensing of certain
anions.9 Owing to their various applications, numerous
methods for the synthesis of annellated polycyclic complexes
bearing a quinone skeleton have been developed. The repre-
sentative approaches usually include the classical Friedel–
Crafts reactions,10 Lewis acid catalyzed intramolecular cycliza-
tion with a Lewis acid as a catalyst,11 introduction of the 9,10-
carbonyl functions by the oxidation of the corresponding
hydrocarbon,12 transition-metal-catalyzed oxidative cycliza-
aCollege of Chemistry and Life Science, Sichuan Provincial Key Laboratory for
Structural Optimization and Application of Functional Molecules, Chengdu Normal
University, Chengdu, 611130, P. R. China. E-mail: ssdy1990@126.com
bSouthwest Minzu University, Chengdu 610041, P. R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00666e
Fig. 1 A representative polycyclic N-heterocyclic complex.
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(2 mL), at 120 °C, for 16 h (Table 1, entry 1), the desired
product 3aa was obtained in 87% isolated yield. Other metal
salts including CuBr, CuI, CuBr2, Cu(OAc)2, CuSO4·5H2O,
CuCl2·2H2O and Cu(OTf)2 did not provide better results for
the cycloamination (entries 2–6, see the ESI†). Subsequently,
the variation of t-BuOK to t-BuONa, KOH, CH3ONa, K2CO3,
Et3N, or DMAP did not show any improvement (entries 7–9,
see the ESI†). The reaction performed under air atmosphere in
the absence of a catalyst or a base afforded very low yield of
the products or no product (entries 14 or 15), which showed
that the catalyst or base played a pivotal role in obtaining the
desired product. The use of DMF as a solvent was crucial, as
the reaction gave poor results in other solvents such as DMAC,
NMP, DMSO, CH3NO2, DCE or CH3CN (Table 1, entries 10–13,
see the ESI†). It should be noted that increasing or decreasing
the reaction temperature gave slightly inferior results, indicat-
ing that the transformation was sensitive to temperature (see
the ESI†).
With the optimized reaction conditions in hand, sub-
sequently, the scope of the anilines was investigated (Table 2).
A variety of substituted anilines were compatible with the Cu-
catalyzed cycloamination, regardless of the electronic pro-
perties of the substrates (3ab–3ai). It is noteworthy that the
valuable groups (NH2, SH, F, Cl, and Br) could be readily toler-
ated, which provides an opportunity for further elaboration.
Although ortho-monosubstituted aromatic amines are steri-
cally hindered even for cyclization, the ortho-substituted ani-
lines underwent reactions successfully to give the corres-
ponding aminated products (3aj–3am). For disubstituted ani-
lines, the reaction was also found to proceed smoothly (3an–
3ap). In particular, naphthylamine and aminopyrene were
amenable under our reaction conditions and provided the
expected cyclization products 3aq and 3ar. Even heteroanilines
are well tolerated in this reaction. The use of aminopyridine
provides moderate yields of the desired product (3as). The
strongly coordinating groups (pyridine), which were employed
as reagents for direct C–H functionalization, were fully toler-
ated with high chemoselectivity and regioselectivity.
Unfortunately, no desired product can be obtained when ali-
phatic amines are used.
Scheme 1 Copper-catalyzed cycloamination for the synthesis of poly-
cyclic N-heterocycles.
t-BuOK mediated oxidative coupling amination of 1,4-naphtho-
quinone and related 3-indolylnaphthoquinones with amines.17
Further studies revealed that commercially available CuCl is
very efficient for Cu-catalyzed cycloamination reaction between
indolylquinones and aromatic amines (Scheme 1). It is note-
worthy that unactivated indolylquinones and simple aromatic
amines were employed as the starting materials with good
functional group tolerance. Herein, we wish to disclose our
results.
Results and discussion
We selected the reaction of readily available indolylnaphtho-
quinone (1a) and aniline (2a) as the model reaction for opti-
mizing the CDC aromatic amination conditions (see Table 1,
as well as Tables S1–S5 in the ESI†). To our delight, under the
reaction of CuCl (10 mol%), t-BuOK (2.0 equiv.), and DMF
Table 1 Optimization of the reaction conditionsa
Entry
[Cat.]
Base
Solvent
Yieldb
Next, a series of substituted indolylnaphthoquinones was
tested for the cycloamination sequence (Table 3). As summar-
ized in Table 3, the reaction was compatible with a variety of
indole moieties (1a–1i) bearing electron-donating and elec-
tron-withdrawing substituents to produce the desired polycyc-
lic N-heterocyclic products (3aa–3ia) in moderate to good
yields (62–87%). Notably, the gram-scale synthesis afforded
1.50 g of 3ba in 83% yield. To our delight, the reaction also
showed good compatibility with a wide range of valuable func-
tional groups such as fluoro (3ha), chloro (3fa), and bromo
(3ga) groups. Tolerance to the halogen atoms was noteworthy
since they have been frequently used for further modifications.
N-Methylindoles and N-benzylindole naphthoquinone can
smoothly react to give the corresponding products in good
yields (3aa and 3ba). Unfortunately, no desired product can be
obtained when 1c is used as the substrate. Moreover, we were
1
2
3
4
5
6
7
8
CuCl
CuBr
CuI
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuONa
KOH
DMAP
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
No
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMAC
DMSO
CH3NO2
NMP
DMF
DMF
87
43
61
49
42
45
60
51
NR
52
31
NR
NR
15
NR
CuBr2
Cu(OAc)2
Cu(OTf)2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
No
9
10
11
12
13
14
15
CuCl
a Reaction conditions: 1a (0.3 mmol), 2a (0.6 mmol, 2.0 equiv.), [cat.]
(10 mol%), base (0.6 mmol, 2.0 equiv.), solvent (2.0 mL), 16 h, air,
120 °C. b Isolated yield. DMF = N,N-dimethylformamide; DMAC = N,N-
dimethylacetamide; NMP = N-methylpyrrolidone; DMSO = dimethyl
sulfoxide; DMAP = 4-dimethylaminopyridine; N.R. = no reaction.
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Table 2 Scope for anilinesa
Table 3 Scope with respect to indolylquinones with anilinea
a Reaction conditions: 1 (0.3 mmol), 2a (0.6 mmol), CuCl (10 mol%),
t-BuOK (2.0 equiv.), DMF (2.0 mL), 16 h, air, 120 °C. Isolated yield. N.
R. = no reaction. b In a 5 mmol scale.
pleased to find that the position of the substituent on the
indole moiety showed no obvious influence on the reaction
outcome, and substituents at C4-(3da), C5-(3ea–3ga), C6-(3ha)
or C7-(3ia) were all well tolerated in the reaction. 1,4-
Anthraquinone-substituted indole 1j was also employed,
affording the corresponding product 3ja in 83% yield.
Control experiments were conducted to clarify the cycloami-
nation reaction pathway (Scheme 2). The studies revealed that
the radical scavenger TEMPO (2,2,6,6-tetramethyl-1-piperidiny-
loxy) did not inhibit the reaction under standard conditions,
ruling out the radical mechanism (Scheme 2a). At 40 °C for
2 h, the reaction of model compounds 1a and 2a generated the
coupling product 4a in a yield of 63% (Scheme 2b). The reac-
tion of model compounds 1a and 2a generated the coupling
a Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol), CuCl (10 mol%),
t-BuOK (2.0 equiv.), DMF (2.0 mL), 16 h, air, 120 °C. Isolated yield.
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Finally, the intermediate B underwent reductive elimination
producing the desired product 3aa and regenerating Cu(III)
with the oxidant (O2).19
Conclusions
In conclusion, we have successfully demonstrated Cu(I)-cata-
lyzed cycloamination reaction between indolylquinones and
various (hetero)aromatic amines to accomplish the polycyclic
N-heterocyclic molecules. The significant aspects of our work
allow modest functional group tolerance, including electron-
donating and electron-withdrawing groups, which were compa-
tible under the current methodology.
Conflicts of interest
Scheme 2 Control experiments.
There are no conflicts to declare.
product 4a without CuCl in a yield of 68% (Scheme 2c). The
studies revealed the Michael addition process of the aniline to
the quinone in the presence of a base at first. Under the stan-
dard conditions, the reaction of compound 4a generated the
product 3aa in a yield of 81% (Scheme 2d). We surmised that
the coupling product 4a should be a key intermediate in this
reaction. Without CuCl, the reaction of compound 4a gener-
ated the product 3aa in just 12% yield (Scheme 2e). The reac-
tion of compound 4a under N2 delivered only a trace amount
of the product (Scheme 2f). The studies demonstrated the
importance of CuCl and O2.
Acknowledgements
We are grateful for the financial support from the Foundation
of Chengdu Normal University Talent Introduction Research
Funding (2021YJRC202020), Applied Basic Research Project of
Sichuan Provincial Science and Technology Department
(2018JY0262) and the National Nature Science Foundation of
China (grant number 21703020).
On the basis of this and previous reports, a possible reac-
tion mechanism was proposed (Scheme 3). Initially, the
Michael addition of indolylnaphthoquinone (1a) and aniline
(2a) in the presence of a base gave the intermediate A, which
was immediately oxidized to intermediate 4a by O2 or the oxi-
dative naphthoquinone.18 On the other hand, a copper salt
was oxidized to the Cu(III) species in the presence of O2, which
then reacted with intermediate 4a giving the Cu(III) species B.
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