Cu-Catalyzed Denitrogenative Transannulation of 3-Aminoindazoles to Assemble 1-Aminoisoquinolines and 3-Aminobenzothiophenes

Article abstract of DOI:10.1021/acs.orglett.9b02288

We disclose a novel Cu-catalyzed denitrogenative transannulation of 3-aminoindazoles to afford diverse functionalized 3-aminobenzothiophenes and 1-aminoisoquinolines, in which denitrogenative transannulation of 3-aminoindazoles is reported for the first t

Full text of DOI:10.1021/acs.orglett.9b02288

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Cu-Catalyzed Denitrogenative Transannulation of 3‑Aminoindazoles
To Assemble 1‑Aminoisoquinolines and 3‑Aminobenzothiophenes
†
†
‡
,†,‡,§
Yao Zhou, Ya Wang, Yixian Lou, and Qiuling Song*
†Institute of Next Generation Matter Transformation, College of Materials Science & Engineering at Huaqiao University, 668 Jimei
Boulevard, Xiamen, Fujian 361021, China
‡
Collaborative Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology,
Hangzhou, Zhejiang 310000, China
§
Fujian University Key Laboratory of Molecule Synthesis and Function Discovery, College of Chemistry, Fuzhou University,
Fuzhou, Fujian 350108, China
*
S Supporting Information
ABSTRACT: We disclose a novel Cu-catalyzed denitrogenative
transannulation of 3-aminoindazoles to afford diverse functionalized
3
-aminobenzothiophenes and 1-aminoisoquinolines, in which deni-
trogenative transannulation of 3-aminoindazoles is reported for the first
time. This transformation proceeds via an “extrude-and-sew” strategy
with an unprecedented radical reactivity of 3-aminoindazoles.
he development of new strategies for successive multiple
bond cleavage and multiple bond formation to forge
complex molecules is an important goal in organic chemistry.
displaying high atom economy and providing an efficacious
T
and direct approach to access intricate frameworks, it is still a
1
powerful tool to acquire complicated compounds. Another
Recently, a variety of “cut-and-sew” reactions have been
alternative tactic involves cleavage of two bonds and extrusion of
1
developed by organic chemists as the “cut-and-sew” tactics
a small molecule (such as CO, N , CO , etc.) from the parent
2 2
could rapidly construct complex molecules by incorporating two
skeletons and reorganizing bond connections. This “cut-and-
sew” strategy typically involves oxidative addition of transition
metals into strained C−C bonds followed by 2π unit insertion
Scheme 1a). Although most excellent “cut-and-sew” trans-
formations design and prepare special strained substrates,
substrates, followed by introduction of a new group from
another reactant, named “extrude-and-sew” transformations
2
(Scheme 1b). 3-Aminobenzothiophene and 1-aminoisoquino-
line derivatives are widely found as privileged core structural
3
,4
motifs in many bioactive compounds. Meanwhile, they also
serve as versatile building blocks for the generation of
(
5
,6
isoquinoline and benzothiophene derivatives. In this letter,
we demonstrate a novel “extrude-and-sew” transformation of 3-
aminoindazoles to assemble the valuable 3-aminobenzothio-
phene and 1-aminoisoquinoline derivatives.
Scheme 1. “Cut-and-Sew” Transformations and “Extrude-
and-Sew” Transformations
7
8
Since the research groups of Gevorgyan, Fokin, and
9
Murakami reported the pioneering studies on denitrogenation
reaction of triazoles to generate highly electrophilic α-imino
metal carbenoid species, during the past decades, myriads of
work based on the transition-metal-catalyzed denitrogenative
transannulation of triazoles have been reported for the synthesis
10,11
of azo-heterocyclic compounds.
In addition to the triazoles,
the denitrogenative transannulation of benzotriazinones was
12
also developed, in which diverse isoquinolone derivatives were
produced via regioselective insertion of benzotriazinones with
various unsaturated compounds. In 2018, Murakami disclosed a
Rh-catalyzed enantioselective denitrogenative transannulation
of 1H-tetrazoles with styrenes for the construction of 3,5-diaryl-
Received: July 2, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02288
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
3a
2
-pyrazolines.
In the same year, Chattopadhyay demon-
several copper catalysts. Disappointingly, only marginal
improvements were obtained when other Cu salts, such as
strated an excellent Ir-catalyzed intramolecular transannulation
1
3b
of tetrazoles.
Very recently, Chattopadhyay’s group also
CuCl , CuI, and CuBr , were used in place of Cu(OAc) (Table
2
2
2
disclosed the Fe(II)-mediated intermolecular denitrogenative
transannulation of 1,2,3,4-tetrazoles, overriding traditional click
1, entries 11−13). To enhance the reaction yield, we also
inspected a series of bases (Table 1, entries 14−19). Organic
bases, such as DBU, just lead to a stagnant reaction. The yield of
1
3c
chemistry. Although the denitrogenative transannulations of
triazoles, benzotriazinones, and tetrazoles have been previously
studied, the denitrogenative transannulation of 3-amino-
indazoles has never been reported so far. As part of our ongoing
3
aa was significantly increased to 81% when Cs CO was
2 3
employed as the base (Table 1, entry 19). The attempt to
increase the amount of Cs CO resulted in a lower yield of 3aa
2
3
14
(Table 1, entry 20). The model reaction failed to deliver higher
yields whether we increased or decreased the temperature
interest in exploring new reactions of 3-aminoindazoles,
herein, we describe a novel Cu-catalyzed denitrogenative
transannulation of 3-aminoindazoles to afford diverse function-
alized 3-aminobenzothiophenes and 1-aminoisoquinolines
(
entries 21 and 22).
Having identified the optimal reaction conditions, the scope
of this Cu-catalyzed oxidative transannulation of 3-amino-
indazoles to afford 1-aminoisoquinoline derivatives was
evaluated (Scheme 2). A variety of functionalized enamines
(Scheme 1c).
To experimentally verify our hypothesis, we selected the
commercially available 3-amino-1H-indazole (1a) as a model
substrate to react with (Z)-ethyl 3-amino-3-phenyl acrylate
Scheme 2. Substrate Scope with Respect to Enamines*
(2a). At the outset, when the reaction was performed in the
presence of Cu(OAc) and tert-butyl peroxybenzoate (TBPB) in
2
CH CN at 60 °C using K CO as the base, desired product ethyl
3
2
3
1
-amino-3-phenylisoquinoline-4-carboxylate (3aa) was ob-
tained in 47% yield (Table 1, entry 1). Subsequently, the
model reaction was subjected to various oxidants to investigate
the effect of the oxidants (Table 1, entries 2−10). Among the
oxidants tested, tert-butyl hydroperoxide (TBHP) demonstra-
ted the optimal efficiency (Table 1, entry 3). We then screened
Table 1. Optimization of the Reaction Conditions
a
b
entry
[Cu]
oxidant
base
yield (%) of 3aa
1
2
3
4
5
6
7
8
9
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuCl2
TBPB
DTBP
TBHP
O2
K CO
47
27
61
31
36
2
3
3
3
3
3
3
3
3
3
3
3
3
3
K CO
2
K CO
2
c
K CO
2
K S O
K CO
2
2
8
2
Na S O
K CO
25
2
2
8
2
*
LPO
PIFA
IBX
PhIO
K CO
<10
trace
22
43
18
56
43
trace
10
58
All reactions unless otherwise stated were carried out with 1 (0.3
2
mmol), 2 (0.2 mmol), Cu(OAc) (20 mol %), CsCO (2 equiv), and
TBHP (2 equiv) in MeCN (1.0 mL) heated at 60 °C for 20 h. 1 (0.2
mmol), 2 (0.4 mmol).
K CO
2
3
2
a
K CO
2
10
11
12
13
14
15
16
17
18
19
20
21
22
K CO
2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
K CO
2
CuI
CuBr2
K CO
2
K CO
2
were surveyed initially. The electronic effects of the substituents
on the aromatic rings of the β-enamine esters had no significant
effect on the reaction yields, and the corresponding 1-
aminoisoquinolines (3aa−3ah) were obtained in good to high
yields. The structure of 3ae was unambiguously confirmed by X-
ray crystallographic analysis. Heteroaromatic β-enamine esters
such as 2i−2k were tolerable under the standard conditions, as
well, delivering the targeted products 3ai−3ak in 55, 70, and
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DBU
NaOAc
NaOH
Li CO
22
2
3
K PO
65
81
3
4
d
Cs CO
2
3
3
3
3
e
f
Cs CO
2
62
73
60
Cs CO
2
8
1% yield, respectively. In addition to the aromatic β-enamine
g
Cs CO
2
esters, the alkyl-substituted β-enamine esters 2l−2s were also
good candidates in this denitrogenative transannulation,
affording the expected 1-aminoisoquinolines 3al−3as in
moderate to good yields. (Z)-3-Amino-N-phenylbut-2-enamide
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), [Cu] (20 mol
), oxidant (2 equiv), base (2 equiv), under air, 60 °C, 20 h. GC
yield. When O balloon was used for 24 h, the yield of 3aa was 58%.
Isolated yield. Cs CO (3 equiv). 80 °C. RT; LPO = lauroyl
peroxide; PIFA = [bis(trifluoroacetoxy)iodo]benzene; IBX = 2-
b
%
c
2
d
e
f
g
2
t could be engaged in this reaction, as well, and product 3at was
2
3
achieved in 60% yield. Enamines bearing ketone and nitrile were
iodylbenzoic acid.
also found to be good “sew” partners, providing the desired 1-
B
DOI: 10.1021/acs.orglett.9b02288
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
aminoisoquinolines 3au and 3av in 67 and 78% yields,
respectively.
Scheme 4. Substrate Scope for the Synthesis 3-
Aminobenzothiophenes
a
Subsequently, the scope with regard to 3-aminoindazoles was
inspected (Scheme 3). Different substituted 3-aminoindazoles
Scheme 3. Substrate Scope with Respect to 3-
Aminoindazoles*
a
All reactions unless otherwise stated were carried out with 1 (1.5
equiv), 4 (0.3 mmol), Cu(OAc) (20 mol %), NaOH (3 equiv), and
2
b
TBHP (2 equiv) in MeCN (1.0 mL) heated at 80 °C for 22 h. 2
*All reactions unless otherwise stated were carried out with 1 (1.5
equiv of K CO was used.
2
3
equiv), 2 (0.2 mmol), Cu(OAc) (20 mol %), Cs CO (2 equiv), and
TBHP (2 equiv) in MeCN (1.0 mL) heated at 60 °C for 20 h. 1 (2
equiv).
2
2
3
a
corresponding 3-aminobenzothiophenes (5ba−5ia) were
achieved in moderate to good yields.
The denitrogenative transannulation of 3-aminoindazoles can
be readily scaled up without a loss of efficiency (Scheme 5). The
could be successfully transformed into the targeted 1-amino-
isoquinolines under this novel Cu-catalyzed denitrogenative
transannulation of 3-aminoindazoles. 3-Aminoindazole deriva-
tives that bear either electron-donating or electron-withdrawing
groups on the aromatic rings were very compatible, rendering
the targeted products (3ba−3ea, 3fc, 3gb) in 46−79% yields.
Borneo-(−)-derived 3-aminoindazole was proven to be a good
donor in this denitrogenative transannulation, enabling the
production of 3ha in 57% yield. Halo-substituted 3-amino-
indazoles, such as fluoro, chloro, and bromo, could work
smoothly under the optimal conditions, furnishing the desired
products (3ia, 3ja, 3kc) in 61, 65, and 59% yield, respectively.
Inspired by the successful preparation of 1-aminoisoquino-
lines through this Cu-catalyzed denitrogenative transannulation
of 3-aminoindazoles, we then moved our focus to explore other
reagents as the “sew” partners. The mercaptoacetates aroused
our attention. If the mercaptoacetates could be transformed
under this “extrude-and-sew” reaction, the valuable 3-amino-
benzothiophenes would be acquired. To verify our hypothesis,
the reaction of 3-amino-1H-indazole (1a) and methyl 2-
mercaptoacetate (4a) was carried out. The expected product
Scheme 5. Larger Synthesis and Synthetic Applications
product 1-amino-3-phenylisoquinoline-4-carboxylate (3aa)
could be gained in 72% yield when 1a (7.5 mmol) and 2a (5
mmol) were exposed to the standard conditions. The products
of this denitrogenative transannulation reaction could be
conveniently derivatized. An extended fused ring system 6
could be constructed via Rh-catalyzed dual C−H activation with
3
-aminobenzothiophene 5aa was generated indeed with good
yield when the reaction was performed at 80 °C using NaOH as
the base. As shown in Scheme 4, in addition to methyl 2-
mercaptoacetate, the ethyl, 2-ethylhexyl, and butyl 2-mercap-
toacetates were good “sew” partners in this transformation, as
well, delivering the targeted products (5ab−5ad) in 68, 62, and
5d
1,2-diphenylethyne. In addition, the copper-catalyzed domino
reaction between 5ca and 2-bromopyridine could produce a
good yield of 6H-benzo[4,5]thieno[3,2-d]pyrido[1,2-a]-
pyrimidin-6-one 7 (Scheme 5), which is an analogue of
15
6
6% yield, respectively. Next, we evaluated this denitrogenative
transannulation reaction by using methyl 2-mercaptoacetate
4a) in conjunction with a round of 3-aminoindazoles, and
benzo[b]thiophenes having antimicrobial activity.
To gain a deeper understanding of these transformations, we
conducted several control experiments. When unsubstituted, 3-
(
C
DOI: 10.1021/acs.orglett.9b02288
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
iodo, and 3-carboxylic acid indazoles were used as substrates, no
ring-opening products were observed (Scheme 6a), which
Scheme 7. Proposed Reaction Mechanism
Scheme 6. Primary Mechanism Studies
abstracts a hydrogen atom from compound A, which
successively undergoes extrusion of one molecular nitrogen to
generate the radical intermediate B. Addition of the aryl radical
B to the enamine 2 provides the radical intermediate C, which is
subjected to the oxidation by the Cu(III) species and
deprotonation of the in situ generated carbocation species,
giving compound D. In the meantime, the Cu(II) species is
regenerated to continue the catalytic cycle. Compound D goes
through the intramolecular nucleophilic attack to render
compound E. Finally, tautomerism of compound E furnishes
the desired product 3.
In summary, we have successfully developed a novel Cu-
catalyzed denitrogenative transannulation of 3-aminoindazoles
for the construction of 1-aminoisoquinolines and 3-amino-
benzothiophenes. Diverse functionalized 1-aminoisoquinoline
and 3-aminobenzothiophene derivatives were obtained in good
yields via an “extrude-and-sew” strategy with wide substrate
scope. Compared to previous work for denitrogenative trans-
annulation, most of these excellent transformations undergo the
ionic mechanism. However, our current denitrogenative trans-
annulation of 3-aminoindazoles proceeds via a radical pathway.
This protocol is readily scaled up without loss of its efficiency.
Further applications of this novel denitrogenative trans-
annulation of 3-aminoindazoles are underway in our group.
suggested that the NH group installed on the C3 position of
2
indazoles is indispensable for this ring opening of the indazole
ring. When 3-amino-1H-indazole (1a) was submitted to the
reaction conditions without the “sew” partners mercaptoace-
tates or enamines, a trace amount of benzonitrile was detected
by GC-MS (Scheme 6b). No product was observed when the
reaction was performed without Cu salt, oxidant, or base
ASSOCIATED CONTENT
Supporting Information
■
*
S
(
Scheme 6c), which suggested that the Cu salt, oxidant, and base
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b02288.
were all required for this transformation. When the reaction was
carried out in the absence of Cs CO , compound 8 was isolated,
2
3
General experimental procedures and spectroscopic data
for the corresponding products (PDF)
the structure of which was unequivocally confirmed by X-ray
crystal analysis. When 8 was used as starting material to expose
under the basic conditions, 96% yield of 3aa was acquired, which
manifested that 8 was a key intermediate in this transformation
Accession Codes
CCDC 1831421, 1842359, and 1844187 contain the supple-
mentary 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.
(
Schemes 6d and 6e). When 3 equiv of the common radical
probes TEMPO and BHT were added to the reactions under the
identified conditions, a trace amount of 3aa was observed
(
Scheme 6f). In addition, compound 9 and radical dimer
compound 10 were detected by HRMS when 1,1-diphenyl-
ethylene and 3-aminoindazole 1a were exposed under the
conditions without base, indicating that a radical process might
be involved in this denitrogenative transannulation reaction
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qsong@hqu.edu.cn.
ORCID
(
Scheme 6g).
On the basis of these experiment results, a possible reaction
mechanism for this Cu-catalyzed denitrogenative transannula-
tion of 3-aminoindazoles is depicted in Scheme 7. First, 3-
aminoindazole 1 suffers from oxidation to afford compound A
with the assistance of oxidant. Meanwhile, the low-valent Cu(II)
species donates an electron to TBHP to form the Cu(III) species
Yao Zhou: 0000-0002-3500-7355
Qiuling Song: 0000-0002-9836-8860
Notes
16
and the tert-butoxyl radical. Then, the tert-butoxyl radical
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.9b02288
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
10) For a review on denitrogenative transannulation of triazoles, see:
ACKNOWLEDGMENTS
■
(
a) Chattopadhyay, B.; Gevorgyan, V. Angew. Chem., Int. Ed. 2012, 51,
Financial support from the National Natural Science Founda-
tion (21772046) and the Natural Science Foundation of Fujian
Province (2016J01064) is gratefully acknowledged. We also
thank the Instrumental Analysis Center of Huaqiao University
for analysis support.
8
1
62. (b) Gulevich, A. V.; Gevorgyan, V. Angew. Chem., Int. Ed. 2013, 52,
371. (c) Davies, H. M. L.; Alford, J. S. Chem. Soc. Rev. 2014, 43, 5151.
(d) Jiang, Y.; Sun, R.; Tang, X.-Y.; Shi, M. Chem. - Eur. J. 2016, 22,
17910.
(11) For some selected examples on denitrogenative transannulation
of triazoles, see: (a) Ma, X.; Pan, S.; Wang, H.; Chen, W. Org. Lett.
2
014, 16, 4554. (b) Yadagiri, D.; Chaitanya, M.; Reddy, A. C. S.;
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DOI: 10.1021/acs.orglett.9b02288
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