Synthesis of multi-functionalized quinolines and 1,2-dihydroquinolines through FeCl3-mediated reactions of carbonyl compounds with 2-vinylanilines

Article abstract of DOI:10.1002/jccs.201800001

A facile and efficient approach toward the synthesis of functionalized quinolines and 1,2-dihydroquinolines from carbonyl compounds and 2-vinylanilines is described. The protocol utilizes the nonhazardous and less expensive FeCl₃ as catalyst wi

Full text of DOI:10.1002/jccs.201800001

Received: 1 January 2018
Revised: 6 March 2018
Accepted: 12 March 2018
DOI: 10.1002/jccs.201800001
A R T I C L E
Synthesis of multi-functionalized quinolines and 1,2-
dihydroquinolines through FeCl3-mediated reactions of carbonyl
compounds with 2-vinylanilines
Sha Liu | Gaoqiang Li | Feng Xu
Key Laboratory of Macromolecular Science of
A facile and efficient approach toward the synthesis of functionalized quinolines
Shaanxi Province, School of Chemistry and
Chemical Engineering, Shaanxi Normal
and 1,2-dihydroquinolines from carbonyl compounds and 2-vinylanilines is
described. The protocol utilizes the nonhazardous and less expensive FeCl3 as cat-
alyst with wide functional group tolerance and avoiding heavy metal impurities in
the products.
University, Xi'an, China
Correspondence
Gaoqiang Li and Feng Xu, Key Laboratory of
Macromolecular Science of Shaanxi Province,
School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an, Shaanxi
710062, China.
KEYWORDS
annulations, carbonyl compounds, iron, quinolines, vinylanilines
Email: gqli@snnu.edu.cn; fengxu@snnu.edu.cn
Funding information
National Natural Science Foundation of China,
Grant/Award Numbers: 21172138, 21302117,
21572123; Fundamental Research Funds for the
Central Universities, Grant/Award Numbers:
GK201603047, GK201601003
arylamines and 1,3-dicarbonyl compounds.[19–21] Neverthe-
1
| INTRODUCTION
less, the majority of these methods suffer from drawbacks
such as strong acidic or basic conditions, high reaction tem-
peratures, long reaction times, excessive oxidants, low
regioselectivities, and so on. In recent decades, significant
progress has been made in the development of transition-
metal-catalyzed methods such those using iridium,[22]
rhodium,[23–26] ruthenium,[27–32] palladium,[33–36] nickel,[37]
gold,[38–40] silver,[41–45] copper,[46–53] and iron[54–56]
because of their functional group tolerance, stero- and
regioselectivity, and excellent yields under mild reaction
conditions. Although these procedures provide efficient
routes for assembling the quinoline skeleton, most of them
use expensive and often toxic metal catalysts, which often
generate heavy metal impurities in the product and limit
their large-scale applications. Therefore, the development of
novel, simple, and expeditious approaches for the rapid con-
struction of functionalized quinolones, especially with non-
hazardous and less expensive metals, is still highly
desirable.
Quinoline derivatives are a well-known class of nitrogen-
containing heterocyclic compounds which are important
synthetic targets for the pharmaceutical and agrochemical
industries as well as in academic laboratories because of
their rich chemistry and numerous applications. They have
been found to possess a wide range of biological properties
such as antibacterial, antimalarial, anti-inflammatory, anti-
fungal, antiasthmatic activity.[1–7] Moreover, they have been
widely applied as valuable building blocks for the prepara-
tion of various functionalized materials, sensors, and
dyes.[8–12] Because of their interesting biological activities
and potential in materials science, a number of efficient
methods for the synthesis of functionalized quinolines have
been well documented over the years. The classical methods
for the preparation of quinolines include the Skraup
(or Doebner–von Miller) synthesis from anilines and
α,β-unsaturated carbonyl compounds in the presence of an
oxidizing agent,[13–15] the Friedländer annulations from
unstable ortho-aminobenzaldehydes and carbonyl com-
pounds bearing a reactive α-methylene group under acidic
or basic conditions,[16–18] and Combes reactions from
More recently, Brønsted acids-mediated reactions such
those using as pTSA,[57] camphorsulfonic acid,[58] and
HOTf[59,60] have been successfully used to access quinoline
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J. Chin. Chem. Soc. 2018;1–5.
http://www.jccs.wiley-vch.de
1

2
LIU ET AL.
TABLE 1 Optimization of the reaction conditionsa
derivatives. The direct use of Brønsted acids indeed may
overcome the problem of the heavy metal impurities. How-
ever, the strong Brønsted acids may also lead to weak func-
tional group tolerance and narrow the substrate scope.
Among the numerous transition-metal catalysts, the iron
salts, which are generally thought of as green catalysts, are
common Lewis acids with very low cost and sometimes
amazing catalytic ability. Developing iron-salt-mediated
reactions for the construction of functionalized quinolines
may simultaneously solve the problems of both heavy metal
impurities and weak functional group tolerance. Here, we
report FeCl3-mediated reactions of aldehydes or ketones
with 2-vinylanilines for rapid-access, multi-functionalized
2,4-disubstituted and 2,3-disubstituted quinolines as well as
2,2,4-trisubstituted 1,2-dihydroquinolines.
CHO
Fe salt
+
solvent, Temp.
NH2
N
1a
2a
3a
Entry
1
Fe salt
FeCl3
Solvent
None
Temp. (ꢀC)
Yield (%)b
80
41
85
53
71
94
54
74
68
-
2
FeCl3
CH2Cl2
1,4-Dioxane
THF
Reflux
80
3
FeCl3
4
FeCl3
Reflux
80
5
FeCl3
FeCl3
Toluene
CH3CN
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
6
80
7
FeBr3
80
8
Fe(OTf )3
Fe2(SO4)3
FeCl2
80
2
| RESULTS AND DISCUSSION
9
80
10
11
12
13
14c
15d
80
63
Trace
32
93
56
95
Our investigation began with the reaction of 2-(1-phenylvi-
nyl)aniline 1a and benzaldehyde 2a ꢀin the presence of FeCl3
under solvent-free condition at 80 C. To our delight, the
target quinoline 3a was smoothly isolated in 41% yield
(Table 1, entry 1). Encouraged by this initial result, we fur-
ther examined the effects of common organic solvents
(Table 1, entries 2–6). As shown in Table 1, the usual sol-
vents, such as CH2Cl2, 1,4-dioxane, THF, toluene, and
CH3CN, were applied to promote the model reaction. When
Fe(acac)2
FeCl3
80
r.t.
FeCl3
Reflux
80
FeCl3
FeCl3
80
a
Rection conditions: 1a (0.4 mmol), 2a (0.8 mmol, 2 equiv), and Fe salt
(10 mol%) were mixed in the solvent under air and stirred at the indicated tem-
perature until the reaction was complete (TLC analysis).
b
c
d
Isolated yield.
ꢀ
the reaction was performed in toluene at 80 C, the highest
5 mol% FeCl3 was used.
15 mol% FeCl3 was used.
yield of 94% was obtained (Table 1, entry 5). Furthermore,
different iron salts were next evaluated. Fe(III) salts such as
FeBr3 and Fe(OTf)3 only gave moderate yield and
Fe2(SO4)3 was completely ineffective in this transformation.
The Fe(II) salt, FeCl2, also successfully afforded the desired
quninoline 3a, albeit with moderate yield (Table 1, entry
10). However, when Fe(acac)2 was used to promote the
transformation, only traces of quinoline 3a was detected
(TLC analysis). Next, we examined the influence of the
reaction temperature on the reaction efficiency. When the
reaction temperature was reduced to room temperature, the
yield dropped sharply to 32%. When the reaction mixture
was heated at reflux, 93% yield of quinoline 3a was
groups smoothly afforded the corresponding 2,4-diaryl
substituted quinolines in high yields. It can be concluded
that the electronic properties of the substituents on the alde-
hydes have only a relatively small effect on the reaction per-
formance (entry 2 vs. entry 3). 2-Arylquinolines generally
have better pharmacological activity because of their poor
oxidizability.[61,62] Furthermore, 2-arylquinolines also pro-
vide convenient precursors to chiral dihydro- and tetrahy-
droquinolines, which are important biologically active
structures.[63–65] Therefore, the current strategy is very
meaningful to design and rapidly construct 2-arylquinolines.
Ortho-substituted, meta-substituted, or para-substituted ben-
zyaldehydes all showed good reactivity in this transforma-
tion. In general, steric effects play an important role in the
reactivity of the aldehydes. For example, 3-methyl benzal-
dehyde only gave the desired annualtion product in 27%
yield. However, 4-methyl benzaldehyde produced quinoline
3d in 51% yield. It was interesting that the more sterically
demanding ortho-substituted benzyaldehydes with halo sub-
stituents and 1-naphthaldehyde still successfully afforded
the desired quinolines in good to excellent yields. Func-
tional groups such as methyl, methoxy, trifluoromethyl,
cyano, fluoro, chloro, and bromo are well tolerated in
ꢀ
obtained. Obviously, 80 C was the best reaction tempera-
ture for the current transformation. In addition, the catalyst
loading was also investigated. When the loading of FeCl3
was reduced from 10 to 5 mol%, only unsatisfactory yield
of compound 3a was achieved (56% yield, Table 1, entry
14). Further increase of the FeCl3 loading to 15 mol%
afforded the desired quinoline 3a in 95% yield.
Having identified the optimal reaction conditions
(Table 1, entry 5), we next investigated the generality of the
transformation. First, the reactions of various aldehydes
with 2-(1-phenylvinyl)aniline 1a were carried out, and the
results are summarized in Table 2. Aromatic aldehydes
bearing either electron-withdrawing or electron-donating

LIU ET AL.
3
TABLE 2 Examples of 2,4-disubstituted and 2,3-disubstituted quinoline
aniline 1u at the optimal reaction condition, the correspond-
ing 2-phenyl-4-thienylquinoline 3u was smoothly achieved
in 63% yield. Next, 2-(1-alkylvinyl)anilines were used to
react with benzaldehyde to evaluate the substrate scope.
When 2-(1-methylethenyl)aniline 1v was subjected to the
reaction system with benzaldehyde, the desired annulations
product 3v was generated in low yield (22%). Surprisingly,
when the structurally similar 2-(1-methylenepentyl)benzena-
mine 1w was used to carry out the transformation, an accept-
able yield of the quinoline 3w was achieved (66% yield). We
further investigated the reactivity of 2-(2-phenylethenyl)ben-
zenamine 1x with 4-chlorobenzaldehyde at the same reaction
condition, which has a phenyl substituent on the other carbon
of the carbon–carbon double bond. To our delight, sterically
demanding 2,3-diaryl-substituted quinoline 3x was still suc-
cessfully obtained in satisfactory yield (51%).
To further explore the synthetic utility of this method, a
survey of the reaction of 2-vinylanilines 1 with acetophe-
none derivatives 4a–g at the above-mentioned optimized
reaction condition to prepare 1,2-dihydroquinolines was
then carried out, and the results were depicted in Table 3.
The reactions of 2-(1-phenylethenyl)aniline and acetophe-
none and its derivatives with substitutions such as 4-MeO,
4-Me and 2,6-di-OMe all successfully produced the corre-
sponding 2,2,4-trisubstituted 1,2-dihydroquinolines in
acceptable yields (more than 50% yield). Even the sterically
hindered 2,6-dimethoxy-acetophenone still smoothly
afforded the desired 1,2-dihydroquinoline 5e in up to 58%
yield. 1-Acetonaphthone and 2-acetonaphthone were also
good candidates for 1,2-dihydroquinoline synthesis, and
derivative synthesisa
R1
R1
N
R2
O
H
R2
R3
FeCl3 (10 mol%)
Toluene, 80oC
+
R3
NH2
1
3
2
Entry
1
R1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
R2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
R3
T (hr)
3
Yield (%)b
94
Ph
3
3a
3b
3c
3d
3e
3f
2
4-OMe-Ph
4-CF3-Ph
4-Me-Ph
4-CN-Ph
4-F-Ph
12
3
87
3
90
4
12
8
51
5
68
6
3
74
7
4-Cl-Ph
4-Br-Ph
3-OMe-Ph
3-F-Ph
8
3g
3h
3i
74
8
8
55
9
12
8
53
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
3j
71
3-Cl-Ph
3-CF3-Ph
3-Me-Ph
2-F-Ph
8
3k
3l
60
8
73
12
8
3m
3n
3o
3p
3q
3r
3s
3t
27
57
2,4-di-Cl-Ph
2-Br-Ph
2-CF3-Ph
1-Naphthyl
2-Thienyl
2-Furyl
Ph
3
87
8
71
3
83
3
84
3
92
12
8
34
2-Thienyl
Me
nBu
3u
3v
3w
3x
63
Ph
12
12
8
22
Ph
66
TABLE 3 Examples of 2,2,4-trisubstituted 1,2-dihydroquinoline
derivative synthesisa
H
4-Cl-Ph
51
a
All the reactions were carried out by employing 2-aminostyrene 1(0.4 mmol),
aldehyde 2 (0.8 mmol), and FeCl3 (10 mol%) in toluene at 80 ꢀC for the time
indicated.
R1
R1
O
FeCl3 (10 mol%)
Toluene, 80oC
+
R3
b
Isolated yield.
R2
R3
NH2
N
H
R2
1
4
5
current protocol. It is noteworthy that the synthesis of
halogen-substituted quinolines has great significance
because the halogen gives an excellent lever for further conve-
nient structural elaborations by means of simple coupling
strategies or the like. Next, we turned our attention to the
examination of heteroaromatic aldehydes. 2-Formylthiophene
reacted efficiently with 2-(1-phenylvinyl)aniline 1a to afford
the desired 2-thienyl-4-phenylquinoline 3s in very high yield
(92%). Nevertheless, when furaldehyde was used to carry out
the annulation reaction with compound 1a, only 34% yield of
the corresponding quinolinone 3t was successfully isolated,
which was probably due to the instability of the furan ring
under such reaction conditions.
Entry
R1
Ph
Ph
Ph
Ph
Ph
Ph
R2
R3
T (hr)
5
Yield (%)b
1
Ph
Me
Me
Me
Me
Me
Me
Me
12
10
8
5a
5b
5c
5d
5e
5f
5g
5h
5i
50
68
74
66
58
50
50
72
0
2
4-OMe-Ph
4-Me-Ph
2-Naphthyl
2,6-di-OMe-Ph
1-Naphtyl
Ph
3
4
10
14
12
12
5
5
6
7
2-Thienyl
8
Ph
Ph
Ph
Spirocyclohexyl
nPr
9
10c
nPr
Ph
48
12
Ph
5j
0
a
All the reactions were carried out by employing 2-aminostyrene 1(0.4 mmol),
ketone 4 (0.8 mmol), and FeCl3 (10 mol%) in toluene at 80 ꢀC for the time
indicated.
To further study the wide substrate scope, various 2-
aminostyrenes with different kinds of substituents located in
carbon–carbon double bond were next examined. When 2-
(1-phenylvinyl)aniline 1a was replaced by 2-(1-thienylvinyl)
b
c
Isolated yield.
Benzophenone was used.

4
LIU ET AL.
gave the desired products in 50 and 66% yields, respec-
tively. When 2-(1-phenylethenyl)aniline was replaced by 2-
(1-thienylethneyl)aniline to perform the annulation with
acetophenone, the corresponding 1,2-dihydroquinoline 5g
was isolated in 50% yield. No decrease of reactivity was
observed compared with 2-(1-phenylethenyl)aniline. In
addition, cyclohexanone 4h and 4-heptanone 4i were also
selected as candidates to evaluate the generality of this pro-
tocol. As shown, cyclohexanone was an excellent substrate
for current cyclization process, affording the desired dihy-
droquinoline 5h in 72% yield within 5 hr. However, no
cyclized dihydroquinoline product was observed for 4-
heptanone even though prolonging the reaction time to
48 hr, which may be due to the poor eletrophilicity of 4-
heptanone. When the sterically demanding benzophenone
was next used to carry out the transformation, unfortunately,
no desired 1,2-dihydroquinoline was formed.
following features: (a) iron-catalyzed process, (b) mild reac-
tion conditions without expensive transition-metal catalysts
and additional oxidant, (c) wide functional group tolerance,
(d) avoidance of toxic and heavy metal impurities. All these
make it a good choice for use in the pharmaceutical and
chemical industries.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (21572123, 21172138, and 21302117)
and the Fundamental Research Funds for the Central Uni-
versities (GK201601003 and GK201603047).
ORCID
Gaoqiang Li
http://orcid.org/0000-0002-6841-3763
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ꢀ
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4
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