Cu(OAc)2-Mediated Cascade Annulation of Diarylalkyne Sulfonamides through Dual C-N Bond Formation: Synthesis of 5,10-Dihydroindolo[3,2-b]indoles

Article abstract of DOI:10.1021/acs.orglett.6b01343

An unusual cascade reaction featuring annulation of diarylalkyne sulfonamides to form 5,10-dihydroindolo[3,2-b]indoles has been realized with Cu(OAc)₂ as the sole oxidant. This unprecedented process encompasses two sequential C-N bond formation

Full text of DOI:10.1021/acs.orglett.6b01343

Letter
pubs.acs.org/OrgLett
Cu(OAc)2‑Mediated Cascade Annulation of Diarylalkyne
Sulfonamides through Dual C−N Bond Formation: Synthesis of 5,10-
Dihydroindolo[3,2‑b]indoles
Junchao Yu,† Daisy Zhang-Negrerie,† and Yunfei Du*,†,‡
†School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, P. R. China
‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China
S
* Supporting Information
ABSTRACT: An unusual cascade reaction featuring annulation of
diarylalkyne sulfonamides to form 5,10-dihydroindolo[3,2-b]indoles has
been realized with Cu(OAc)2 as the sole oxidant. This unprecedented
process encompasses two sequential C−N bond formations, allowing for
an efficient synthesis of the biologically important indoloindole derivatives.
extended heteroacene also bearing a pyrrolo[3,2-b]pyrrole core,
has been widely adopted as a building block for preparing
various high-spin organic polymers6 and OLED polymers.7
Several synthetic approaches have been developed for the
formation of 5,10-dihydroindolo[3,2-b]indoles, with most of
them requiring the formation of a C−N bond as the key step.8
Among the first syntheses of 5,10-dihydroindolo[3,2-b]indole
reported were the reduction of o,o-dinitrobenzil with tin(II)
chloride under acidic conditions (Scheme 2, path a)9 and that
of 2-(o-nitrophenyl)indoles with P(OEt)3 (Scheme 2, path
b).10 The indoloindole skeleton can also be assembled through
Fischer condensation of indolones with hydrazine derivatives
(Scheme 2, path c)11 or through the reduction of 6,12-
dichlorodibenzo[b,f ][1,5]-diazocines in the presence of excess
cene- and heteroacene-based materials have found wide
Aapplications in organic field-effect transistors (OFET),
organic light-emitting diodes (OLED), and organic photo-
voltaic cell designs due to the extensively delocalized π-
electrons in the fused aromatic structures.1 Besides, the
heteroacenes are expected to exhibit potential biological
activities due to the existence of the heterocyclic motif.2 In
heteroacenes, the type and positions of the hetero atoms can
not only significantly improve the stability of the materials1b but
also effectively fine-tune the electronic properties and molecular
packing of the single crystal. For these reasons, developing new
heterotetracenes has received much attention in the past several
years.
Our literature research offered a few reports on the synthesis
of heteroacenes bearing a pyrrolo[3,2-b]pyrrole core, although
their OFET properties have been claimed in recent reviews.3
Pyrrolo[3,2-b]pyrrole has been generally identified as the most
efficient electron donor among 10π-electron systems,4 although
it is unstable and very difficult to synthesize.5 On the other
hand, 5,10-dihydroindolo[3,2-b]indole (VII also known as
dibenzopyrolo[3,2-b]pyrrole, DBPP; Scheme 1), a well-known
Scheme 2. Synthetic Strategies toward the Construction of
DBPP Skeleton
Scheme 1. Representative Heteroacene Compounds in
OFET
Received: May 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01343
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Letter
zinc under acidic conditions (Scheme 2, path d).12 Recently,
Pd-catalyzed cross-couplings between 2,3-dibromo-1-methyl-
1H-indole, m-bromophenyl-boronic acid, and amines with
subsequent cyclization provided a two-step strategy to access
this privileged molecule (Scheme 2, path e).13 Although these
strategies all have their respective merits in the construction of
the indoloindole skeleton, we believe developing additional
methods, those being more convenient, efficient, and/or
economical, is still in high demand. In this letter, we report
an unusual reaction featuring a cascade reaction consisting of
double cyclization of diarylalkyne sulfonamides in forming 5,10-
dihydroindolo[3,2-b]indoles (Scheme 2, path f).
Currently, cascade reactions, especially those performed in
one pot during which multistep reactions proceed in a single
manipulation without isolation or purification of any of the
intermediates, have received much attention due to the obvious
reasons of high efficiency and convenience during the assembly
of complex molecular structures.14,15 Such reactions has been
adopted and proven to be powerful synthetic means for the
conversion of internal alkynes into bioactive polycyclic
heterocycles.16 Literature survey on intramolecular trans-
addition of nitrogens across triple bonds showed only two
examples: (1) Pd(II)/I(III)-based intramolecular annulation of
internal alkyne through successive C−N bond formations
leading to the formation of annelated indoles;16a (2) Pd(II)-
catalyzed intramolecular diamination of diarylacetylene accom-
panied by a C(sp3)−N bond cleavage, affording the bioactive
indoloisoquinolinones.16b To the best of our knowledge, there
has not yet been any report on direct addition of nitrogen
atoms across the diarylalkyne triple bond leading to a fused
polycyclic indoloindole derivative.
Owing to the many promising applications of indoloindoles
in material science, we aimed to develop a new method for
efficient syntheses of various functionalized 5,10-dihydroindolo-
[3,2-b]indoles from diarylalkyne compounds. Built upon our
previous work, we first explored the feasibility of hypervalent
iodine reagent, PIDA as the oxidant in the presence of BF3·
Et2O in carrying out the postulated transformation, but
unfortunately, no desired product was obtained (Table 1,
entry 1). The reaction did not occur either when other
hypervalent iodine reagents of PIFA and PhIO were applied,
even at elevated temperatures (not shown). We also carried out
several experiments under standard conditions formulated in
relevant literatures,16 but no desired product was obtained in
any of the cases (Table 1, entries 2 and 3).
a
Table 1. Optimization of Reaction Conditions
b
entry
oxidant
PIDA
solvent
t (°C)
yield (%)
cd
e
,
1
2
3
4
5
6
7
8
9
DCE
80
n.d.
f
PIDA
DMF
DMSO
MeCN
DMF
AcOH
toluene
DCE
120
80
n.d.
n.d.
56
74
62
68
65
82
73
64
72
75
g
Pd(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuCl2
80
80
100
110
120
120
140
120
120
120
DMF
DMF
DMF
DMF
DMF
10
11
12
13
Cu(OTf)2
Cu(OAc)2
h
a
Reaction conditions: all reactions were carried out with 1a (0.4
mmol), oxidant (0.88 mmol) in solvent (4.0 mL) for 2 h, unless
otherwise stated. Isolated yield. The reaction was conducted at rt to
80 °C. The reaction was conducted using 0.4 mmol of BF3·Et2O. No
desired product. Pd(OAc)2 (20 mmol %) was added. The reaction
was conducted using 0.04 mmol of TBAI and 0.8 mmol of AcOH.
b
c
d
e
f
g
h
PPh3 (10 mmol %) was added.
catalytic conditions are not applicable to this transformation
(see Supporting Information (SI) for details).
Under optimized conditions (Table 1, entry 9), a series of
diarylalkyne sulfonamides 1 were subjected to these conditions
to investigate the scope of this newly established method. The
results are listed in Scheme 3. We first evaluated the electronic
effect of substituent R1 (Scheme 3, 2a−g). It was found that
substrates with a singly substituted aryl moiety improved the
yield, with electron-donating groups (MeO, Me, di-Me) slightly
more favored than electron-withdrawing groups (F, Cl, Br). In
sharp contrast, substrates containing disubstituted aryl groups
all resulted in lower yields than the unsubstituted 1a, except for
the cases with the two substituents being −Me and −OMe,
which gave the same yield (Scheme 3, 2h−m).
Substrates containing the R3 substituent rather than the tosyl
group, including phenylsulfonyl and p-Cl-phenylsulfonyl, gave
similar yields as that of 1a (Scheme 3, 2n−p). However, no
desired product was obtained with R3 being an acetyl or tert-
butyloxy carbonyl group (not shown), probably due to the less
electron-withdrawing ability of the acetyl or tert-butyloxy
carbonyl groups. These results implied that the sulfonyl
group was essential for the transformation, acting as the
protecting group for the N−H moiety as well as a strong
electron-withdrawing group to enhance the nucleophilicity in
this reaction.
After many trials to identify an effective oxidation system, we
came across the first satisfactory yield of 56% in a system
consisting of Cu(OAc)217 as the oxidant and MeCN as solvent
(Table 1, entry 4). The solvent screening study showed that
DMF remained as the most appropriate solvent, in comparison
to AcOH, MeCN, toluene, and DCE (Table 1, entries 4−8).
To our delight, increasing the temperature to 120 °C shortened
the reaction time and improved the yield to 82% (Table 1,
entry 9). Further increasing the temperature to 140 °C only
slightly lowered the yield (Table 1, entry 10). Using other types
The remarkable easy-to-handle nature of the process allows
this one-pot protocol to be suitable for gram-scale production.
Product 2a (1 g) was successfully achieved in the one-pot
fashion under the standard conditions in 80% yield. The
structure of 2i was unambiguously established by X-ray
crystallography (see SI for details).
18
of copper oxidants such as CuCl2 and Cu(OTf)2 resulted in
either sluggish reactions or no conversion at all (Table 1,
entries 11 and 12). We further conducted the reaction with
triphenylphosphine added as ligand, but no improvement of
yield was observed (Table 1, entry 13). Besides, we explored a
series of conditions to probe the possibility of using a catalytic
amount of Cu(OAc)2 in combination with a stoichiometric
amount of terminal oxidant. However, the results indicated that
The mechanism of the reaction is depicted in Scheme 4.19−22
The overall process consists of two steps. In step 1, Cu(OAc)2
acted as a catalyst,19 first, through ligation of the acetylene
accompanied by the loss of the acetate ion (1a → 4 → 5), then
B
DOI: 10.1021/acs.orglett.6b01343
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
when the amount of Cu(OAc)2 was decreased to only 0.2
equiv, the reaction of 1a afforded the indole 3, isolated and its
structure verified, in a high yield of 86%, with an additional 10%
of the doubly cyclized product 2a (Scheme 5).
Scheme 3. Oxidative Cyclization of Diaryalkynes 1 to
Indoloindole Derivatives 2
a
Scheme 5. Mechanistic Studies
Step 2 of the mechanism20 started with the formation of
intermediate 8 due to H-abstraction at the amine moiety by the
acetoxy radical, generated from Cu(OAc)2. Due to the
instability of the Cu-OAc moiety as well as the high-
temperature conditions, the cis-aminocupration of the Cu-
OAc moiety to a C−C double bond via intramolecular
migratory insertion21,22 led to intermediate 9. Subsequent
departure of CuOAc resulted in the benzyl radical 10, which
was oxidized to the iminium 11 by a second acetoxy radical,
released again from a Cu(OAc)2 molecule while forming the
acetate ion. Deprotonation of 11 by the acetated anion led to
the title product 2a.
The radical pathway in step 2 was well supported by the
observations that when intermediate 3 was subjected to the
standard conditions in the presence of 2.2 equiv of TEMPO,
the yield of 2a was found to be only 28%. Second, the
requirement of two Cu(OAc)2 molecules for the reactions in
step 2 as portrayed in the mechanism was also consistent with
our experimental result: when only 1.1 equiv of Cu(OAc)2 was
used to react with intermediate 3 under standard conditions,
incomplete conversion to 2a was observed even after a more
prolonged reaction time.
a
Reaction conditions: all reactions were carried out with 1 (0.4 mmol),
oxidant (0.88 mmol) in DMF (4.0 mL) at 120 °C unless otherwise
stated. Isolated yield. The structure of 2i was confirmed by X-ray
crystallography.
b
c
Scheme 4. Proposed Mechanistic Pathway
In summary, we have discovered a novel cascade reaction
which provides a novel approach for the construction of 5,10-
dihydroindolo[3,2-b]indoles, namely, through Cu(OAc)2-medi-
ated cascade annulation of internal diarylalkyne sulfonamides.22
The new methodology not only carries desirable benefits such
as good functional group tolerance and ligand-free conditions
but also supports gram-scale synthesis of the products in good
yields. The proposed mechanism describes this unprecedented
process as consisting of two sequential cyclizations with two
C−N bond formations during which Cu(OAc)2 serves as a
catalyst during the first step and an oxidant during the second
step.
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.6b01343.
intramolecular nucleophilic substitution by the nitrogen moiety
followed by deprontonation of 6 by the acetate ion (5 → 6 →
7), and finally exchange of ligands leading to intermediate 3,
and, at the same time, regeneration of the catalyst.
The proposal of the cascade reaction consisting of two
distinct steps is confirmed by the following control experiment:
Experimental procedures, data of compounds character-
ization (PDF)
X-ray data for 2i (CIF)
C
DOI: 10.1021/acs.orglett.6b01343
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Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: duyunfeier@tju.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Y.D. acknowledges the National Natural Science Foundation of
China (#21472136), Tianjin Research Program of Application
Foundation and Advanced Technology (#15JCZDJC32900),
and the National Basic Research Project (#2015CB856500) for
financial support.
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DOI: 10.1021/acs.orglett.6b01343
Org. Lett. XXXX, XXX, XXX−XXX