TBAI/TBHP-Mediated Cascade Cyclization toward Sulfonylated Indeno[1,2-c]quinolines

Article abstract of DOI:10.1021/acs.orglett.7b03246

Treatment of ortho-amino-substituted aryldiyne derivatives with sulfonyl hydrazides in the presence of tetrabutylammonium iodide (TBAI) and tert-butyl hydroperoxide (TBHP) led to a cascade cyclization reaction to yield sulfonylated indeno[1,2-c]quinolines

Full text of DOI:10.1021/acs.orglett.7b03246

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
TBAI/TBHP-Mediated Cascade Cyclization toward Sulfonylated
Indeno[1,2‑c]quinolines
Jatuporn Meesin,† Manat Pohmakotr,† Vichai Reutrakul,† Darunee Soorukram,† Pawaret Leowanawat,†
Saowanit Saithong,‡ and Chutima Kuhakarn*,†
†Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH−CIC), Faculty of Science, Mahidol
University, Rama 6 Road, Bangkok 10400, Thailand
‡Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH−CIC), Faculty of Science, Prince of
Songkla University, Hat Yai, Songkla 90112, Thailand
S
* Supporting Information
ABSTRACT: Treatment of ortho-amino-substituted aryldiyne
derivatives with sulfonyl hydrazides in the presence of
tetrabutylammonium iodide (TBAI) and tert-butyl hydro-
peroxide (TBHP) led to a cascade cyclization reaction to yield
sulfonylated indeno[1,2-c]quinolines in moderate to good
yields. The features of the methodology include metal-free
reaction, the ease of reagent handling, and a broad functional group tolerance.
Scheme 1. Cascade Cyclization Reaction
here are increasingly high demands for an atom-
Teconomical synthesis and straightforward strategy for
the construction of highly functionalized carbocyclic and
heterocyclic compounds due to their unique physical properties
and their applications in several fields.1 One of the attractive
synthetic strategies that serves these needs is a cascade or
tandem cyclization reaction.2 Through this strategy, com-
pounds with high molecular complexity, including function-
alized polycyclic and heterocyclic skeletons, can be constructed
in highly efficient way.3 Numerous approaches have been
continually developed to access highly functionalized nitrogen-
containing tetracyclic molecules of chemical and biological
importance via tandem reactions.4 Despite their significance,
available synthetic routes to access indene-fused quinoline
analogues are rare.5 Therefore, there have been tremendous
efforts to search for novel and facile methods for accessing
these scaffolds.
Sulfone-containing molecules exhibit unique and diverse
ranges of behaviors including both chemical and biological
features. For instance, the sulfonyl group was found to be a key
structural motif in a wide variety of bioactive compounds,6 drug
molecules,7 and versatile synthetic intermediates.8 As a
consequence, novel and efficient methods for the installation
of a sulfonyl motif into organic frameworks to access
sulfonylated compounds have received much attention.
Recently, Schipper and co-workers reported a copper(II)
mediated nucleophilic addition/cascade cyclization reaction of
aryl 1,6-diynes with sulfinate salts to allow for the synthesis of
sulfonylated tetracyclic compounds [Scheme 1, (a)].9 The
reaction was proposed to proceed through a rare ionic
mechanism through nucleophilic addition of a sulfinate salt to
a Cu(II) activated diyne leading to the formation of two new
C−C bonds and a C−S bond formation.
More recently, Jiang and co-workers described visible-light
photocatalytic bicyclization of 1,7-enynes with sulfinic acids
using eosin Y as a catalyst and the reaction proceeded through a
radical process [Scheme 1, (b)].10 The reaction is metal-free
and provided a wide range of sulfone-containing benzo[a]-
fluoren-5-ones. According to these elegant examples as well as
our recent report on the chemistry of 2-alkynyl-N,N-dialkylani-
lines,11 we describe herein tetrabutylammonium iodide
(TBAI)/tert-butyl hydroperoxide (TBHP)-mediated the syn-
thesis of sulfonylated indeno[1,2-c]quinolines from ortho-
amino-substituted aryldiyne derivatives 1 using sulfonyl
hydrazides 2 as the sulfur source [Scheme 1, (c)]. The reaction
Received: October 19, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03246
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
is metal-free and facile, involving the formation of C−S, C−C,
and C−N bonds via a cascade strategy and can accommodate a
wide range of substrate scopes.
mL, 0.083 M) (Table 1, entry 11). Finally, it is worth noting
that the reaction did not occur in the absence of TBAI (Table
1, entry 12).
After achieving the optimal reaction conditions (Table 1,
entry 11), we next evaluated the scope and the generality of the
reaction. First, the reactions of 1a with a collection of sulfonyl
hydrazides were explored and the results are summarized in
Scheme 2. Under the optimized reaction conditions,
Exploratory studies were carried out employing N,N-
dimethyl-2-((2-(phenylethynyl)phenyl)ethynyl)aniline (1a)
and toluenesulfonyl hydrazide (2a) as benchmarking substrates
to screen for optimized reaction conditions. On the basis of our
previous report, we first exposed 1a (0.25 mmol) and 2a (5
equiv) to I2 (1.2 equiv), TBHP (6 equiv) in EtOAc at refluxing
temperature for 2 h.11 Unfortunately, although 1a was
consumed, the crude products showed multiple spots (TLC
analysis). Extensive investigations were carried out to search for
optimized reactions and the results are summarized in Table 1.
Scheme 2. Scope of Sulfonyl Hydrazide 2
a
Table 1. Optimization of Reaction Conditions
b
entry
TBAI (equiv)
oxidant
solvent
yield (%)
arenesulfonyl hydrazides bearing electron-donating substituent
groups on the phenyl ring including p-CH3 and p-CH3O readily
underwent the reaction and gave the corresponding sulfony-
lated indeno[1,2-c]quinolines 3a−3c in good yields (62−80%
yields). The electronic nature of the substituents imposed
significant effects on the chemical yields, and in the case of
electron-withdrawing substituted arenesulfonyl hydrazides
including p-Br, p-Cl, p-NO2, and m-F, the corresponding
products 3d−3g were obtained in low to moderate yields (18−
51% yields). Sterically demanding 2,4-dimethylbenzenesulfonyl
hydrazide was successfully engaged in the reaction and
smoothly gave the desired product 3h in 46% yield.
Unfortunately, alkanesulfonyl hydrazides were somewhat
incompatible under the reaction conditions. While ethanesul-
fonyl hydrazide provided product 3i in low yield (24% yield),
methanesulfonyl hydrazide failed to yield the desired product;
trace amount of 3j was detected (TLC analysis).
c
1
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.0
2.0
1.5
−
TBHP
TBHP
TBHP
TBHP
EtOAc
THF
46
48
60
74
63
18
0
c
2
c
3
H2O
4
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
d
5
TBHP
6
H2O2
7
DTBP
TBPB
TBHP
TBHP
TBHP
TBHP
8
28
69
64
80
−
9
10
11
12
e
MeOH
MeOH
e
a
Reaction conditions: 1a (0.25 mmol), 2a (2 equiv), 70% TBHP in
H2O (3.0 equiv) in solvent (2 mL), reflux, open air, 2 h. Isolated
yields. Reaction was carried out at 80 °C. TBHP in decane (5.5 M).
MeOH (3 mL) was employed.
b
c
d
e
Next we set out to evaluate the scope of substrates 1 with
different substituted patterns and electronic properties on the
aniline moiety (R2) and the arylethynyl moiety (R1) toward the
sulfonylated indeno[1,2-c]quinoline formation (Scheme 3). A
broad range of 1 readily reacted with toluenesulfonyl hydrazide
to give access to the corresponding sulfonylated indeno[1,2-
c]quinolines 3 with structural diversity in acceptable yields
(37−89% yields). As depicted in Scheme 3, a collection of
substituents on the aniline moiety (R2), including methyl,
bromo, chloro and fluoro groups were well tolerated leading to
the desired products 3k−3o in good to excellent yields (60−
89% yields). It is worth mentioning here that in cases where
products 3 were isolated in poor yields, the reactions gave
complex crude mixtures (TLC analysis). Notably, the
corresponding C3-sulfonylated indole products were not
observed, and in some cases, the starting aryldiynes were
recovered in the range of 15−25% yields.11
Electronic characteristics and substituted patterns of
substituents on the arylethynyl moiety (R1) did not impede
the reaction leading to the desired sulfonylated indeno[1,2-
c]quinolines 3p−3x in moderate to good yields (57−79%
yields). Notably, sterically encumbered α-naphthyl- and 2-
thienyl-substituted sulfonylated indeno[1,2-c]quinolines 3y and
3z were also successfully prepared in 44% and 37% yields,
respectively. Finally, alkylethynyl substrate readily reacted to
Gratifyingly, the cascade reaction was achieved when molecular
iodine was replaced by TBAI. Thus, when 1a (0.25 mmol) and
2a (2 equiv) were treated with TBAI (1.5 equiv) and TBHP
(70% in H2O, 3 equiv) and the reaction mixture was stirred in
ethyl acetate (EtOAc, 2 mL, 0.125 M) at refluxing temperature
for 2 h, the reaction readily took place and the desired
tetracyclic indeno[1,2-c]quinoline-3-sulfonylindole 3a was
obtained in moderate yield (46% yield) (Table 1, entry 1).
The product 3a was identified and characterized by means of
spectroscopic techniques and its structure was further
confirmed by single-crystal X-ray crystallography (see Support-
ing Information). Under similar reaction conditions, the
reaction performed in either THF or water resulted in slightly
better yields (Table 1, entries 2 and 3). The reaction proceeded
more efficiently and delivered 74% yield of 3a when the
reaction was carried out in MeOH at refluxing temperature
(Table 1, entry 4). Next, different types of oxidants including
5.5 M TBHP in decane, 30% aqueous hydrogen peroxide, di-
tert-butyl peroxide (DTBP), and tert-butyl peroxybenzoate
(TBPB), were screened but the results were less satisfactory
(Table 1, entries 5−8). A decreased or increased stoichiometry
of TBAI did not appreciably effect to the reaction efficiency
(Table 1, entries 9 and 10). The best yield (80% yield) of 3a
was obtained when the reaction was performed using MeOH (3
B
DOI: 10.1021/acs.orglett.7b03246
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
is proposed as described in Scheme 5. First, TBAI interacts with
TBHP to generate radical species which then react with
Scheme 3. Scope of N,N-Dimethyl-2-((2-
(arylethynyl)phenyl)ethynyl)aniline Derivatives 1
Scheme 5. Plausible Mechanism
sulfonyl hydrazide 2 to generate a sulfonyl radical with the
extrusion of nitrogen gas. Next, the sulfonyl radical
regioselectively inserts to an alkynyl moiety of 1 producing a
vinyl radical intermediate A. Subsequent 5-exo-dig cyclization
leads to a vinyl radical intermediate B. Radical-initiated
cyclization can proceed into two pathways. In path A, a vinyl
carbon radical and an electron of nitrogen atom recombines
forming a C−N bond leading to a radical cation intermediate
C.13 Alternatively, in path B, nucleophilic addition by the
nitrogen lone pair onto the vinyl radical carbon of B leads to a
radical-zwitterion intermediate D.14 Both C and D can readily
be oxidized by TBHP generating a quinolinium intermediate E.
Next, iodide ion promotes dealkylation to finally yield
sulfonylated indeno[1,2-c]quinolines 3.
yield alkyl-substituted sulfonylated indeno[1,2-c]quinoline 3a′
although in low yield (38% yield).
In order to shed some light on the reaction mechanism
toward an access of sulfonylated indeno[1,2-c]quinoline
formation from ortho-amino-substituted aryldiyne derivatives
1, a series of control experiments were carried out (Scheme 4).
Scheme 4. Control Experiments
In conclusion, we have disclosed a novel and concise
synthetic route for the construction of sulfonylated indeno[1,2-
c]quinolines via a cascade annulation of sulfonyl hydrazide and
ortho-amino-substituted aryldiyne derivatives. The reaction is
smoothly promoted by TBAI and aqueous TBHP as a primary
oxidant under metal-free conditions. Our approach involves
concomitant production of C−S, C−C, and C−N bonds
leading to two new rings in a single step. A wide range of
functionalized substrates are proved to be tolerant and
compatible under the reaction conditions providing the desired
products in moderate to good yields. Evaluation of biological
activities of the synthesized sulfony-containing indeno[1,2-
c]quinoline derivatives is ongoing in our laboratory.
ASSOCIATED CONTENT
■
Consumption of 1a was observed when it was exposed to the
standard reaction conditions but in the absence of sulfonyl
hydrazide [Scheme 4, (a)]. Product 3a was not obtained when
TBHP was excluded from the reaction [Scheme 4, (b)]. This
result highlighted the important role of TBHP in the present
conversion. Next, to validate whether 1a reacted with 2a in the
present reaction via a radical cascade process, radical trapping
experiments were also conducted by employing TEMPO
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl and BHT (2,6-di-tert-
butyl-4-methylphenol) as radical scavengers [Scheme 4, (c)].
The reaction was suppressed by both TEMPO and BHT
affording 3a in a trace amount and low yield, respectively. This
observation indicated the possibility of a radical mechanism.
On the basis of our preliminary mechanistic investigation and
the previously reported work,12 a possible reaction mechanism
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03246.
Experimental details and characterization data (PDF)
Accession Codes
CCDC 1579071 contains 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.
C
DOI: 10.1021/acs.orglett.7b03246
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chutima.kon@mahidol.ac.th.
ORCID
Chutima Kuhakarn: 0000-0003-4638-4356
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Thailand Research Fund (BRG5850012 and
IRN58W0005), the Center of Excellence for Innovation in
Chemistry (PERCH−CIC), the Office of the Higher Education
Commission, Mahidol University under the National Research
Universities Initiative, PICS6663 ISMA (France/Thailand), and
the Franco−Thai Cooperation Program in Higher Education
and Research (PHC Siam 2017) for financial support. The
Institute for the Promotion of Teaching Science and
Technology and Science Achievement Scholarship of Thailand
(SAST) for financial support through student scholarship to
J.M. are also gratefully acknowledged.
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DOI: 10.1021/acs.orglett.7b03246
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