Copper-Catalyzed Cascade Cyclization of Arylsulfonylhydrazones Derived from ortho-Alkynyl Arylketones: Regioselective Synthesis of Functionalized Cinnolines

Article abstract of DOI:10.1021/acs.orglett.9b03134

A novel copper-catalyzed cascade reaction of arylsulfonylhydrazones derived from ortho-alkynyl arylketones was accomplished. This reaction provides concise access to diversified cinnolines in good yields. The mechanistic investigations have disclosed involvement of the key alkynyl amination, 1,4-aryl migration, desulfonylation, and diazo radical cyclization cascade in the transformation.

Full text of DOI:10.1021/acs.orglett.9b03134

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Cascade Cyclization of Arylsulfonylhydrazones
Derived from ortho-Alkynyl Arylketones: Regioselective Synthesis of
Functionalized Cinnolines
Biao Yao,† Tao Miao,*,† Wei Wei,† Pinhua Li,† and Lei Wang*,†,‡
†Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P. R. China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Shanghai 200032, P. R. China
S
* Supporting Information
ABSTRACT: A novel copper-catalyzed cascade reaction of arylsulfonylhydrazones derived from ortho-alkynyl arylketones was
accomplished. This reaction provides concise access to diversified cinnolines in good yields. The mechanistic investigations have
disclosed involvement of the key alkynyl amination, 1,4-aryl migration, desulfonylation, and diazo radical cyclization cascade in
the transformation.
Scheme 1. Design of Radical Aryl Migration−Cyclization
adical cascade cyclization is a powerful tool for rapidly
building intricate molecular architectures in a chemo-
Cascade Reactions
R
selective and atom-economical manner.1 Over the past few
decades, several efforts were focused on the radical addition−
cyclization cascades, where free radical species adds to the
double or triple bond followed by intramolecular cyclization to
form cyclic molecules.2 Recently, radical cascade cyclization
involving aryl migration has become a highly attractive strategy
for the synthesis of valuable compounds in an efficient way,
which are otherwise not available through conventional
methods.3 Nevado and co-workers have developed a novel
method for the synthesis of functionalized oxindoles from a
variety of radicals and N-(arylsulfonyl)acrylamides through
radical addition of double bond/aryl migration/desulfonylation
cascade cyclization.4 The Nevado group furthermore docu-
mented other elegant examples of the multistep radical cascade
reaction of well-designed ortho-vinyl- or ortho-alkynyl-sub-
stituted N-(arylsulfonyl)arylamides and generated function-
alized indanes and indolo[2,1-a]isoquinolines in high stereo-
selectivity by unconventional bond cleavage and construction
reactions (Scheme 1a).5 However, this radical cascade
cyclization involving aryl migration is rare and is still an
unsolved challenge largely because of the unavailability of
appropriate reagents and the difficulty of identifying suitable
reaction systems that would work nicely in each step of the
reaction while maintaining control of the reaction selectivity.
Herein, we report a copper-catalyzed cascade cyclization of
readily available arylsulfonylhydrazones derived from ortho-
alkynyl arylketones under mild condition. The reaction
provides an effective and practical avenue to functionalized
cinnolines in a regioselective fashion (Scheme 1b).
anticancer, antibacterial, antifungal, antihypertensive, and
antiulcer activities,6 as well as electrochemical and luminescent
properties.7 Accordingly, much attention from both synthetic
and medicinal chemists has been paid to the chemistry of
cinnolines, and a number of studies including classical
Widman−Stoermer route were reported for their preparation.8
Although great achievements have been made, the develop-
Cinnolines are important heterocycles comprising challeng-
ing framework and unique biological activities, including
Received: September 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03134
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Organic Letters
Letter
ment of a novel atom/step-economical and efficient protocol
from readily available starting materials to access the cinnoline
framework with a diverse substitution pattern is highly
desirable.
Initially, N-tosylhydrazone derived from ortho-alkynyl
acetophenone was chosen as the reaction substrate to optimize
the reaction conditions (Table 1). When the reaction of 1a was
quently investigated, and they were found to be less effective
than TBHP for this reaction (Table 1, entries 5−11). Different
bases were also screened. Na2CO3 and Cs2CO3 were found to
be efficient, while t-BuOK, Et3N, and pyridine diminished the
efficiency (Table 1, entries 12−16). It is worth noting that the
Cu catalyst, TBHP, and K2CO3 are indispensable for this
tandem process (Table 1, entries 17−19). Furthermore, when
the amount of oxidant, base, or the reaction time was reduced,
the yield of 2a was also decreased (Table 1, entries 20−22).
When the reaction was carried out for a prolonged time or at
elevated temperature, product 2a was obtained with 84% and
81% yield, respectively (Table 1, entries 23−24).
a
Table 1. Optimization of the Reaction Conditions
Having optimized the reaction conditions, we explored the
scope of this transformation. First, arylsulfonyl substrates with
an electron-donating or electron-withdrawing group at the
para-position of the aromatic ring that is directly bound to the
two nitrogen atoms produced the corresponding cinnolines
2a−2h in 70−86% yields (Scheme 2). Notably, this reaction
b
entry
catalyst
CuCl2
CuBr2
Cu(OAc)2
CuI
oxidant
base
yield (%)
1
2
3
4
5
6
7
8
TBHP
TBHP
TBHP
TBHP
CHP
TBPB
DTBP
DCP
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
t-BuOK
Et3N
pyridine
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
63
68
86
0
40
0
0
0
0
0
a
Scheme 2. Scope of Arylsulfonyl Substrates
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
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
9
H2O2
O2
10
11
12
13
14
15
16
17
18
19
20
Na2S2O8
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
0
75
80
57
50
38
0
c
d
0
e
29
67
61
60
84
81
f
g
21
22
23
24
h
i
j
a
Reaction conditions: 1a (0.25 mmol), catalyst (5 mol %), oxidant (3
equiv), base (1 equiv), DCE/CH3OH (3:1, 4 mL), N2, 40 °C, 5 h.
TBHP = tert-butyl hydrogenperoxide (70% in water), CHP = cumyl
hydroperoxide, TBPB = tert-butyl peroxybenzoate, DTBP = di-tert-
b
butyl peroxide, DCP = dicumyl peroxide. Yield of isolated product.
a
c
d
e
f
Reaction conditions: 1 (0.25 mmol), Cu(OAc)2 (5 mol %), TBHP
Without catalyst. Without oxidant. Without base. 2.0 equiv of
g
h
i
(3 equiv), K2CO3 (1 equiv), DCE/CH3OH (3:1, 4 mL), N2, 40 °C, 5
h; isolated yield based on 1. Gram-scale reaction (4.0 mmol 1a) was
performed.
TBHP was used. 0.5 equiv of K2CO3 was used. 40 °C for 3 h. 40
b
j
°C for 8 h. 60 °C for 5 h.
carried out in the presence of 5 mol % CuCl2, three equivalents
of tert-butyl hydrogenperoxide (TBHP, 70% in water) and one
equivalent of K2CO3 in a mixed-solvent of 1,2-dichloroethane
(DCE) and CH3OH (3:1) at 40 °C for 5 h, the formation of
cinnoline 2a was obtained in 63% yield (Table 1, entry1). The
structure of 2a was confirmed by X-ray crystal analysis.
Reaction in the presence of CuBr2 was also examined, and a
comparable result was obtained (Table 1, entry 2). To our
delight, the addition of Cu(OAc)2 instead of CuCl2
dramatically improved the yield of 2a up to 86% (Table 1,
entry 3). However, CuI was not a suitable catalyst, and no
reaction was observed (Table 1, entry 4). Several other
oxidants, such as CHP (cumyl hydroperoxide), TBPB (tert-
butyl peroxybenzoate), DTBP (di-tert-butyl peroxide), DCP
(dicumyl peroxide), H2O2, O2, and Na2S2O8 were subse-
shows complete regioselectivity, as 1,4-aryl migration takes
place exclusively through the carbon atom bound to the SO2
group in the reaction substrates. The bulkier 2-naphthyl-
arylsulfonyl group was also well tolerated in this trans-
formation, and cinnoline derivative 2i was observed with
good yield. When the substituent was placed at the meta-
position of the sulfonyl group, the reaction led to a mixture of
regioisomers (4:3 ratio of 2j:2j′) because of the second C−N
bond formation occurring at both the para- and ortho-positions
of the chlorine group. Further, the reaction was scaled up to
4.0 mmol for the practical application, giving 2a in 78% yield
(1.06 g).
The influence of the substituted group (R2) at the alkyne
moiety was also explored, and the results are summarized in
B
DOI: 10.1021/acs.orglett.9b03134
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Organic Letters
Letter
Scheme 3. Aromatic alkynes containing substituents, such as
Me, OCH3, F, Cl, and Br at the para-, meta-, or ortho-position
Scheme 4. Scope of 1 with Substituents at Aromatic Ketone
Moiety
a
a
Scheme 3. Scope of 1 with Substituents at Alkyne Moiety
a
Reaction conditions: 1 (0.25 mmol), Cu(OAc)2 (5 mol %), TBHP
(3 equiv), K2CO3 (1 equiv), DCE/CH3OH (3:1, 4 mL), N2, 40 °C, 5
h; isolated yield based on 1.
annulation reaction. It is well-known that cinnolines containing
the pyridine motifs are found in the core structures of many
bioactive molecules.6 We thus investigated the synthesis of
these compounds by using N-tosylhydrazones derived from 3-
acetylpyridine as substrates. To our delight, the reactions led to
the formation of products 2ai and 2aj in 72% and 61% yield,
respectively (Scheme 5).
a
Reaction conditions: 1 (0.25 mmol), Cu(OAc)2 (5 mol %), TBHP
(3.0 equiv), K2CO3 (1.0 equiv), DCE/CH3OH (3:1, 4 mL), N2, 40
°C, 5 h; isolated yield based on 1.
of the phenyl ring underwent the reaction efficiently to provide
the desired cinnoline derivatives (2k−2r) in 70−82% yields. It
should be noted that the Cl and Br substituents on the phenyl
ring were well tolerated, which can facilitate further
modifications at the halogenated positions. Furthermore,
heterocyclic alkynes that are usually sensitive to oxidative
conditions, such as 2-thiophene alkyne, was also well tolerated
in this protocol and gave cinnoline 2s in 60% yield.
Particularly, cyclopropyl alkyne could also be used in this
transformation under the optimized conditions, providing the
functionalized product 2t in 73% yield. Other aliphatic alkynes
including n-hexyl and n-heptyl groups worked well in this
domino reaction, affording products 2u and 2v in 71% and
75% yield, respectively. However, when a trimethylsilyl
substituted alkyne was used as the substrate under the present
conditions, the trimethylsilyl group was removed by hydrolysis
and gave the product 2w with 51% yield.
To further investigate the scope of the reaction, the
reactivity of different N-tosylhydrazones derived from aromatic
ketones were explored, as described in Scheme 4. Substituted
acetophenones with substituents including Me, OCH3, F, Cl,
and CF3 could tolerate the catalytic conditions well, generating
the corresponding products 2x−2ae in moderate to good
yields. Importantly, the method was shown to be useful in the
construction of substituted cinnolines containing biologically
relevant moieties such as F and CF3, as in the case of 2z, 2ac,
and 2ae.9 Furthermore, N-tosylhydrazone derived from
acetylnaphthalene successfully engaged in this radical cycliza-
tion cascade, generating the product 2af in 50% yield. In
addition, when R3 was either an ethyl or cyclopropyl group, the
desired cinnolines 2ag and 2ah were obtained in 67% yield and
65% yield, respectively.
Scheme 5. Synthesis of the Analogues of Bioactive
Cinnolines
For understanding the mechanism of tandem reaction,
several control experiments were conducted (Scheme 6). First,
an isotope labeling experiment was carried out by using TBHP
(in decane) and H218O. To our surprise, the 16O-labeled
product 2a was obtained in 80% yield. However, when 2.0
equiv of Na18OH was added, 2a was isolated in 21% yield, and
18O-labeled 2a was the major product. These isotopic labeling
experiments supported that the oxygen in the products comes
from −OH, which was generated from TBHP via a redox
reaction (Scheme 6a). Subsequently, a kinetic isotope effect
(KIE = 1.2) was observed in the competing intermolecular
reaction of 1b and [D5]-1b. This isotopic effect confirmed that
the cleavage of the C(sp2)−H on the arylsulfonyl group should
not be involved in the rate-determining step (Scheme 6b).
Furthermore, when 1a was subjected to the reaction with the
addition of 2 equiv of a common free-radical trapping reagent,
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or BHT (bu-
tylhydroxytoluene), the reaction was significantly inhibited,
implying that a radical process was involved. In the former
reaction, the TEMPO radical adducts (I and IV), carbocation
II, and intermediate III were detected by HPLC-HRMS
analysis (Scheme 6c), suggesting that the reaction might occur
Finally, to demonstrate the synthetic potential of the
procedure, we briefly explored the application of the oxidative
C
DOI: 10.1021/acs.orglett.9b03134
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Letter
vinyl-copper complex B. Subsequently, the homolysis of
complex B afforded vinyl radical C and copper(I), which
could be oxidized to regenerate copper(II) for the next
catalytic cycle in the presence of TBHP. A 5-ipso cyclization
then took place on the sulfonyl aromatic ring, delivering
spirocyclic intermediate D, which underwent a rapid rear-
omatization with concomitant desulfonylation to produce the
key aminyl radical E, constructing the new C−C bond. The
generated aminyl nitrogen-centered radical E could resonate
with C-centered radical species F, which was oxidized by
TBHP to form the benzylic cation II. Hydrolysis of cation II
led to intermediate III in the presence of −OH. Subsequently,
the oxidation of intermediate III by t-BuO• onto the OH
group afforded O-centered radical G, and successive N−C
bond homolysis furnished diazo radical H with a carbonyl
group, followed by an intramolecular cyclization with the
aromatic ring, delivering aryl radical J. Then, J was oxidized to
generate cationic intermediate K, which yielded the final
product cinnoline 2a by the base-assisted deprotonation.
In conclusion, we have developed a highly efficient cascade
cyclization reaction of easily available arylsulfonylhydrazones
derived from ortho-alkynyl arylketones to afford synthetically
and medicinally important cinnolines in good yields. In this
transformation, a new C(sp2)−C(sp2) bond, two C(sp2)−N
bonds, and a new ring are formed in one pot through a copper-
catalyzed cascade alkynyl amination/1,4-aryl migration/
desulfonylation/cyclization under mild conditions. Mechanistic
studies suggest that the key intermediates, including vinyl and
diazo radicals, and a carbocation were involved during the
reaction. Furthermore, this powerful strategy has been
successfully applied to construct analogs of bioactive cinno-
lines.
Scheme 6. Mechanistic Studies
through the formation of vinyl and diazo radicals and a
carbocation in situ. The above results implied that a cascade
radical/carbocation cyclization process triggered by the
copper-catalyzed alkynyl amination was involved in this
transformation.
Based on the control experiments and previous literature,4,10
a plausible pathway was proposed (Scheme 7). First, the
intramolecular aminocupration of 1a occurred upon alkyne
activation by a copper(II) intermediate A, which originated
from copper catalyst via removal of acetic acid anion, to form
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03134.
Scheme 7. Proposed Mechanistic Pathway
Full experimental details and characterization data for all
products (PDF)
Accession Codes
CCDC 1906726−1906727 contain the supplementary crys-
tallographic 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 Authors
*E-mail: taomiao@chnu.edu.cn
*E-mail: leiwang88@hotmail.com
ORCID
Pinhua Li: 0000-0002-8528-8087
Lei Wang: 0000-0001-6580-7671
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.9b03134
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding support of National
Science Foundation of China (21772062, 21602072) and the
National Science Foundation of Anhui Education Department
(KJ2019ZD66 and KJ2016A643).
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