Alkyltriflate-triggered annulation of arylisothiocyanates and alkynes leading to multiply substituted quinolines through domino electrophilic activation

Article abstract of DOI:10.1021/ol500221u

The reaction of arylisothiocyanate, alkyltriflate, and alkynes leads to variously substituted quinolines in high yields. The reaction undergoes alkyltriflate-triggered domino electrophilic activation and avoids the use of a transition-metal catalyst. A variety of functional groups are tolerated in the quinoline ring.

Full text of DOI:10.1021/ol500221u

Letter
pubs.acs.org/OrgLett
Alkyltriflate-Triggered Annulation of Arylisothiocyanates and
Alkynes Leading to Multiply Substituted Quinolines through Domino
Electrophilic Activation
Peng Zhao,† Xiaoyu Yan,† Hang Yin,† and Chanjuan Xi*,†,‡
†Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, China
‡State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: The reaction of arylisothiocyanate, alkyltriflate, and alkynes leads to
variously substituted quinolines in high yields. The reaction undergoes alkyltriflate-
triggered domino electrophilic activation and avoids the use of a transition-metal
catalyst. A variety of functional groups are tolerated in the quinoline ring.
uinoline scaffolds are found in a wide variety of
quinolines. The transformation consisted of a cascade reaction
of the arylisothiocyanate 1 with alkyltriflate 2 to form alkylthio-
substituted carbenium ion 3, which followed the reaction with
alkyne 4 to form intermediate 5, and subsequent electrophilic
annulation to give quinoline 6, as the schematic representation
indicates in Scheme 2.
1
Q pharmacologically and biologically active molecules.
They have also been employed as ligands for the preparation of
OLED luminescent complexes2 and organocatalysts for
asymmetry synthesis.3 Owing to the important application of
quinolines, their synthesis had been extensively studied since
the first discovery.4,5 By far the most prevalent strategies for
constructing quinoline rings are the classic annulation reactions
Scheme 2. Proposed Mechanism
such as Friedlander,6 Combes,7 and Conrad−Limpach−Knorr
̈
syntheses.8 These named reactions generally proceed via
condensation, Michael addition, or nucleophilic substitution,
which usually lack tolerated functional groups. Recently, there
are also many new metal-catalyzed protocols for preparing
quinolines.9 From the viewpoint of sustainable chemistry, the
development of a new metal-free pathway toward quinoline is
more attractive especially for the drug screen. Consequently, we
sought to develop a convenient and diversifiable route under
metal-free conditions. Herein, we report alkyltriflate-mediated
domino electrophilic activation of arylisothiocyanates and
alkynes to afford multiply substituted quinolines (Scheme 1).
In the preliminary experiment, we used phenyl isothio-
cyanate 1a (0.6 mmol), methyl triflate 2a (0.6 mmol), and
diphenylacetylene 4a (0.5 mmol) as starting material in 1,2-
dichloroethane (DCE) for 12 h at 60 °C, affording 2-
(methylthio)-3,4-diphenylquinoline 6aaa in 20% yield (Table
1, entry 1). When the reaction temperature was increased to
100 and 130 °C, the yield of the product increased to 60% and
67%, respectively (entries 2−3). Then, different solvents were
screened such as THF, CH3CN, dimethyl formamide (DMF),
dimethyl sulfoxide (DMSO), dichlomethane (CH2Cl2), chloro-
form (CHCl3), tetrachloromethane (CCl4), and n-hexane
(entries 4−11). DCE was found to be the superior solvent
for this reaction (entry 3). When the ratio of the three
components was converted to 1.2:1.5:1, the yield of 6aaa
increased to 80% (entry 12). The optimal ratio of three
Scheme 1. Alkyltriflate-Mediated Reaction of
Arylisothiocyanates and Alkynes To Afford Quinolines
Alkyltriflate is often used as the alkylation reagent at a
heteroatom such as nitrogen, oxygen, and sulfur, to induce an
electrophilic center for carbon−carbon or carbon−heteroatom
bond formation.10 Inspired by this chemistry and part of our
ongoing project on the formation of heterocycles,11 we
envisioned that the reaction of alkyltriflates, arylisothiocyanates,
and alkynes would provide a general approach to substituted
Received: December 23, 2013
Published: February 11, 2014
© 2014 American Chemical Society
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Letter
Table 1. Optimization of the Reaction Conditions for
Formation of 6aaa
Scheme 3. Synthesis of Diphenyl-Substituted Quinolines
a
a
from Variously Substituted Phenylisothiocyanates
b
entry temp [°C] time [h]
solvent
ratio [1a:2a:4a] yield [%]
1
60
100
130
100
130
130
130
100
100
130
130
130
130
130
130
130
130
110
120
130
130
130
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
1
DCE
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.2:1
1.2:1.5:1
1.2:2:1
20
2
DCE
60
3
DCE
67
4
THF
NR
NR
NR
NR
52
5
CH3CN
DMF
DMSO
CH2Cl2
CHCl3
CCl4
6
7
8
9
28
10
11
12
13
14
15
16
17
18
19
20
21
22
10
n-hexane
DCE
28
80
DCE
87
DCE
1.2:2.5:1
1.2:3:1
88
a
The yields are of isolated products.
DCE
87
DCE
1.5:2.5:1
1.5:3:1
88
a
Scheme 4. Synthesis of Quinolines from Various Substrates
DCE
94 (80)
81
DCE
1.5:3:1
DCE
1.5:3:1
86
DCE
1.5:3:1
31
3
DCE
1.5:3:1
65
6
DCE
1.5:3:1
75
a
The reaction was performed with 0.5 mmol of diphenylacetylene in
b
sealed tube. The yield was evaluated by 1H NMR with CH2Br2 as the
internal standard. Isolated yield was given in parentheses.
compounds was 1.5:3:1, in which the product 6aaa was
afforded in 94% yield (entry 17). When the reaction
temperature was decreased to 120 and 110 °C, the desired
product was formed in 86% and 81% yield, respectively (entries
18−19). Furthermore, the effect of reaction time was also
examined; the yield of product was increased with the extended
reaction time (entries 20−22). On the basis of the above
results, the optimal conditions are shown in entry 17.
Under the optimized conditions, a study on the substrate
scope was carried out. First, a variety of arylisothiocyanates 1
were used in the reaction with methyl triflate 2a and
diphenylacetylene 4a to synthesize quinolines with a range of
substituents of arylisothiocyanate, including 4-methyl, 4-
methoxyl, 4-bromo, 4-trifluoromethyl, and 2-methyl groups.
Some results are summarized in Scheme 3. In all cases, the
reaction proceeded smoothly and the desired products were
formed in satisfactory yields (Scheme 3, 6aaa−6faa). The
structure of 6caa was further confirmed by single-crystal X-ray
diffraction (see the Supporting Information). When 3-methoxy-
phenyl isothiocyanate 1g was used in this reaction, the products
were obtained as two isomers (6gaa = 24%; 6g′aa = 48%). It is
noteworthy that naphthalene nucleus substrates such as 1-
naphthyl isothiocyanate 1h and 2-naphthyl isothiocyanate 1i
were employed to afford the corresponding product in 63% and
65% yields, respectively (6haa and 6iaa).
a
The yields are of isolated products.
conditions. This synthetic method gives quinolines with high
regioselectivity when unsymmetric alkynes were used. The
reaction of 1-phenyl-1-propyne 4b or 1-phenyl-1-butyne 4c
afforded quinolones 6aab or 6aac in high yields with a phenyl
group at the 4-position and an alkyl group at the 3-position.
The structure of 6aac was demonstrably confirmed by HMBC
spectra analysis (see the Supporting Information). When 1-
bromo-2-phenylacetylene 4d and 1-chloro-2-phenylacetylene
4e were used as substituents, the exclusive product was formed
in good yields (6aad and 6aae). To our delight, crystals of 6aad
suitable for X-ray analysis were obtained (see the Supporting
Information). It clearly shows that the product 6aad had a
phenyl group in the 4-position and a bromo group at the 3-
Second, a range of alkynes were handled with arylisothio-
cyanates and alkyltriflates to synthesize quinolines with a variety
of substituents at the 3- and 4-positions. As shown in Scheme 4,
the reaction of terminal or internal alkynes bearing aryl, alkyl,
halogen, and ester groups proceeded smoothly under optimized
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Letter
position. Furthermore, utilization of ethyl phenylpropiolate 4f
also afforded desired quinoline 6abf in 52% yield with a phenyl
group at the 4-position and an ester group at the 3-position.
Terminal alkynes with an aryl group also gave the
corresponding product in high yields with an aryl group at
the 4-position (6aag and 6aah). Their structures were
confirmed by HMBC NMR spectra analysis. The high
regioselectivity for unsymmetric alkynes may be attributed to
the cation stabilizing nature of the aryl group for intermediate
5. It is noteworthy that alkyl acetylenes were transformed into
the corresponding quinolines in low yield (6aaj and 6hai).
Besides methyl triflate, other triflates such as ethyl triflate 2b
and 3-chloropropanyl triflate 2c could also be used in this
reaction and gave the desired product 6aba and 6aca in 73%
and 55% yield, respectively. Notably, treatment of the reaction
with HOTf led to the formation of a complex inseparable
mixture. It is unlike the HOTf-induced transformation of 2-
alkynylphenyl isothiocyanates.12
Scheme 7. Transformation of Thiomethyl Group to Other
Substituents
2-(methylthio) group could be smoothly replaced by alkyl/aryl
groups by treatment with either a methyl or phenyl Grignard
reagent in the presence of bis(triphenylphosphino)nickel
dichloride to obtain 2-methyl- and 2-phenylquinolines 12 and
13 in 54% and 66% yield,14 respectively. Moreover, 6aab could
be oxidized by m-CPBA generating the 3-methyl-2-(methyl-
sulfonyl)-4-phenylquinoline.15 The 2-methylsulfonyl group on
the quinoline ring could not only be easily substituted by
morpholine and aniline producing 2-aminoquinoline 14 and 15
in 67% and 72% yield,15 respectively, but also be easily replaced
by ethanol generating the 2-ethyoxylquinoline 16 in 72% yield.
As illustrated in Scheme 2, the reaction of arylisothiocyanate
with alkyltriflate is envisioned to give the adduct 3. To verify
the adduct 3 as an intermediate, we carried out the reaction of
phenyl isothiocyanate 1a and methyl triflate 2a. Monitoring of
this adduct by HRMS spectroscopy revealed the formation of
the intermediate 3 (see Supporting Information). Introduction
of diphenylacetylene 4a to this mixture led to formation of
product 6aaa (50% NMR yield). This observation provides
compelling evidence that 3 is a viable intermediate under the
reaction conditions.
This new method provides opportunities for the construction
of polycyclic quinolines which were exemplified in Scheme 5.
Scheme 5. Synthesis of Polycyclic Quinolines
Two polycyclic quinolines (7 and 8) were smoothly prepared
from 1a and ω-alkyn-1-yl-triflate 9 via a sequential
intermolecular and intramolecular electrophilic activation
transformation.
As shown in Scheme 6, a further application of our method is
in the synthesis of benzo[i]phenanthridines that are useful
In summary, a concise, metal-free, and one-pot method has
been developed for the facile synthesis of functionalized
quinolines from readily available arylisothiocyanates, alkyl-
triflates, and alkynes. This method has been found to be
generally useful for the preparation of a variety of substituted
quinolines. The reaction demonstrates excellent reactivity, good
functional group tolerance, complete regioselectivity, and high
yields. The synthetic utilities were further displayed in
convenient syntheses of polycyclic quinolines and benzo[i]-
phenanthridines.
Scheme 6. Synthesis of Benzo[i]phenanthridine
ASSOCIATED CONTENT
* Supporting Information
■
compounds in organic photochromics.13 Compound 10 was
prepared from readily available 1a, 2a, and 1,4-diphenylbuta-
1,3-diyne 4k in one pot. The structure of product 10 was also
confirmed by single-crystal X-ray diffraction (see Supporting
Information).
In this reaction, there is a thioalkoxyl group at the 2-position
of the quinoline ring, and there are many methods that can be
used to convert the C−S bond into C−H, C−C, C−N, and C−
O bonds. Scheme 7 shows examples for the transformation of
the thiomethyl group to other substituents on the quinoline
ring to extend the diversity. 3-Methyl-2-(methylthio)-4-phenyl-
quinoline 6aab can be transformed to the sulfur-free quinoline
11 by treatment with Raney-Ni in refluxing ethanol.14 Also the
S
Experimental procedures, full characterization including 1H and
13C NMR data for all new compounds, copies of spectra for all
compounds, and the X-ray structure of product and X-ray data
(CIF) for 6caa, 6aad, and 10. These materials are available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: cjxi@tsinghua.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Key Basic Research
Program of China (973 program) (2012CB933402) and
National Natural Science Foundation of China (21032004
and 21272132).
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