Br?nsted acid-catalyzed metal- and solvent-free quinoline synthesis from: N -alkyl anilines and alkynes or alkenes

Article abstract of DOI:10.1039/c7gc03175k

Br?nsted acid-catalyzed cyclization reactions of N-alkyl anilines with alkynes or alkenes in the presence of oxygen gas as an oxidant under metal- and solvent-free conditions are described. Various quinoline derivatives are obtained in satisfactory to exc

Full text of DOI:10.1039/c7gc03175k

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Brønsted Acid-Catalyzed Metal- and Solvent-Free Quinoline
Synthesis from N-alkyl Anilines and Alkynes or Alkenes
Waqar Ahmed,a† Sheng Zhang,a† Xiaoqiang Yu,a Yoshinori Yamamoto,a,c and Ming Baoa,b,
Received 00th January 20xx,
Accepted 00th January 20xx
*
Brønsted acid-catalyzed cyclization reactions of N-alkyl anilines with alkynes or alkenes in the presence of oxygen gas as an
oxidant under metal- and solvent-free conditions are described. Various quinolinne derivatives are obtained in satisfactory
to excellent yields. Different groups, such as methyl, fluoro, chloro, bromo, methoxy, and ester linked on benzene rings,
are tolerated under optimized reaction conditions.
DOI: 10.1039/x0xx00000x
www.rsc.org/
(a) Previous work: classical name reactions
Skraup-Doebner-Von Miller
R2
R3
Introduction
Reaction
Quinoline motif has attracted considerable interest due to its
O
applications1
in
photoelectric
antimalarial,
materials2
and
antibacterial,
Combes-Conrad-Limpach
Reaction
NH2
R2
R3
N
R2
potent
pharmaceuticals3,
R1
R1
+
including
O
O
O
antileishmanial, anti-inflammatory, and insecticidal agent.4 Many
methods have been established for the conversion of anilines to
various substituted quinolines in the presence of different reaction
partners since the late 1800s. Various classical name reactions are
identified for quinoline synthesis; these reactions include Skraup,
Doebner–von Miller, Combes, Conrad–Limpach, Povarov,
Friedländer, and Pfitzinger reactions (Scheme 1a).5 These methods
either use adverse conditions, such as high temperature of
approximately 200 °C, or hazardous reagents in stoichiometric
amount, which adversely affect the environment-friendly chemistry
and atom economy. Moreover, some methods rely on reagents for
quinoline synthesis that needs additional steps.
R3
Povarov Reaction
Ar
+
O
N
R3
NH2
O
R1
R1
Friedländer Reaction
R4
R3
R2
N
R2
R4
O
R3
R4
H
N
Pfitzinger Reaction
R1
R1
O
O
COOH
(b) Previous work: transition metal catalysis
Recently, transition metal-catalyzed coupling reactions have been
used as potent tools for the synthesis of heterocyclic compounds,
especially quinolines6. Fe, Cu, In, Bi, Pd, and Ag salts have been used
as catalysts for quinoline synthesis. Furthermore, a transition-
metal-catalyzed quinoline synthesis uses N-alkyl anilines and
N
Ar
H
metal-cat. (Fe, Cu, In)
oxidant, solvent
N
Ar
N
Ar
R2
R1
(CDC)
R1
R1
R2
(c) This work: Brønsted acid catalysis
arylacetylenes
or
arylethylenes
through
direct
cross-
N
R2
H
O
N
acid-cat. (TfOH), O2
neat
dehydrogenative coupling (CDC) of C–H bond to construct new C–C
bond (Scheme 1b).7 Given its high atom economy and environment-
friendly nature, CDC reaction has frequently been used to construct
quinoline motif. Liu8 and Mancheño9 groups independently
reported iron-catalyzed synthesis of quinolines using either N-
benzylanilines or N-aryl glycine esters and arylacetylenes or
N
R2
R3
R2
R1
(CDC)
R1
R3
R1
> 50 examples
metal-, solvent-free with cheaper and green oxidant
Scheme 1. Various approaches for the synthesis of quinolines.
arylethylenes as reaction partners in the presence of di-
tertiary-butyl peroxide or TEMPO oxoammonium salt as an
oxidant in stoichiometric amount through direct CDC method.
Liu and coworkers recently reported about copper-catalyzed
synthesis of quinolines via CDC reaction of N-aryl glycine esters
with olefins in the presence of either K2S2O8 (in stoichiometric
amount) or N-hydroxyphthalimide combined with O2 as
oxidants.10 Jia and Wang revealed that the CDC reaction of N-
aryl glycine esters with olefins or arylacetylene for synthesis of
quinolines can be promoted in the presence of catalytic InCl3
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116023, China. Fax: +86-0411-84986181; Tel: +86-0411-84986180; E-mail:
mingbao@dlut.edu.cn.
b School of Petroleum and Chemical Engineering, Dalian University of Technology,
Panjin 124221, China.
c WPI-AIMR (WPI-Advanced Institute for Materials Research), Tohoku University,
Sendai 980-8577, Japan and Research Organization of Science and Technology,
Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.
†These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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and tris(4-bromophenyl)ammonium hexachloroantimonate
(TBPA+•) using O2 as the terminal oxidant.11 Quinoline
synthesis via transition-metal-catalyzed CDC reaction
commonly requires the use of costly inorganic or organic
oxidants. Notably, Jia and coworkers recently succeeded on
the metal-free CDC reaction of N-benzylanilines with
arylethylenes for quinoline synthesis in the presence of
catalytic radical cation salt, namely, TBPA+• using O2 as the
oxidant.12
Scheme 2. Synthesis of Quinolines from N-alkyl anilines and
Alkynesa
Although these existing methods can provide easy access to
quinoline derivatives, the development of remarkably efficient,
environment-friendly, and economic organic synthetic process
is still desirable. In the course of our research on the
development of efficient methods for heterocyclic synthesis13,
we found that quinolines can readily be obtained from the
Brønsted acid-catalyzed metal- and solvent-free CDC reaction
of N-alkyl anilines with arylacetylenes or arylethylenes using O2
as the oxidant (Scheme 1c). Results are reported in this paper.
Results and discussion
Table 1. Optimization of Reaction Conditionsa
entry
1
acid cat.
TFA
solvent
DCE
O2
time (h)
15
yield (%)b
48
O2 in air
O2 in air
O2 in air
2
PTSA
CH3SO3H
BzOH
H2SO4
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
none
DCE
15
15
15
15
45
3
DCE
40
4
DCE
O
2 in air
O2 in air
2 in air
O2 in air
NRc
trace
57
5
DCE
6
DCE
O
15
15
15
15
15
15
15
24
24
24
24
24
7
toluene
1,4-dioxane
DMF
43
8
O
O
2 in air
2 in air
54
9
43
NRc
10
11
12
13
14
15
16
17
CH3CN
DCE
O2 in air
O
2 balloon
62
none
O2 balloon
77
none
O
2 balloon
2 balloon
83
none
O
76d
71e
NRc
NRc
none
O2 balloon
N2
none
none
O2 balloon
a
Reaction conditions: N-benzyl aniline (1a, 0.2 mmol, 36.7 mg), phenylacetylene (2a, 1.0
mmol, 102.1 mg), Brønsted acid (15 mol%), and solvent (2.0 mL), 120 °C. Isolated yield. c
No reaction; the starting materials were recovered. d The reaction occurred at 110 °C. e The
reaction was performed in the presence of 10 mol% TfOH.
b
Reaction conditions: N-alkyl aniline (
µL, 0.03 mmol, 15 mol%) at 120 °C for 24 h under O2 atmosphere
isolated materials.
1
, 0.2 mmol), alkyne (
2
, 1.0 mmol), and TfOH (2.7
.
aAll yields are of
N-Benzyl aniline (1a) cyclization with phenylacetylene (2a) was
selected as a model to optimize the reaction conditions.
Results are shown in Table 1. Brønsted acid catalysts, including
trifluoroacetic acid (TFA), p-toluene sulfonic acid (PTSA),
methanesulfonic acid, benzoic acid (BzOH) acid, sulphuric acid
(H2SO4) and trifluoromethanesulfonic acid (TfOH), were
initially screened in 1,2-dichloroethane (DCE) under air at
120 °C for 15 h. A relatively high yield (57%) of quinoline 3a
was obtained when TfOH was used as the catalyst (entry 6 vs.
entries 1–5). The solvent was subsequently screened, but the
3a yield could not be improved (entry 6 vs. entries 7–10).
Additionally, an increased 3a yield (62%) was observed when
the model reaction was performed under pure oxygen
atmosphere (O2 balloon) in DCE for 15 h (entry 11 vs. entry 6).
The further increased yield was observed under solvent-free
conditions (entry 12, 77%). The highest 3a yield was finally
obtained when the model reaction was treated for prolonged
time (entry 13, 24 h, 83%). The 3a yield decreased with the
reduced reaction temperature and acid catalyst loading
(entries 14 and 15). No reaction was observed in the absence
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of oxygen gas or acid catalyst (entries 16 and 17). Therefore, proceeded well to produce the corresponding quinoline
we performed the subsequent reactions of the N-alkyl anilines products 3b and 3c in good yields (84% and 81%, respectively).
with arylacetylenes or arylethylenes in the presence of TfOH as Good to excellent yields were obtained in the reactions of N-
a catalyst at 120 °C under solvent-free conditions for 24 h. benzyl-4-methylanilines 1d
During optimization, starting materials were recovered along Me, NO2, or Cl) on ortho-, meta-, or para-position of benzyl
with product in case of low yield (table 1). group 1d 1k 80%–90%). The reactions of N-(3,4-
–1k bearing a substituent (F, Br,
(
– ,
On the basis of the optimized reaction conditions, we dichlorobenzyl)-4-methylaniline (1l), 4-methyl-N-(naphthalen-
explored the scope and limitation of this type of cyclization 1-ylmethyl)aniline (1m), N-((5-bromothiophen-2-yl)methyl)-4-
reaction. Results are summarized in Scheme 2. Initially, alkyne methylaniline (1n), and ethyl 2-(p-tolylamino)acetate (1o) also
2a was used as a reaction partner to investigate the scope of proceeded well and produced quinoline products 3l–3o in
N-alkyl anilines. As described in Table 1, the desired product 3a 78%–92% yields. Moreover, the reaction of N-benzylanilines
1p
aniline ring also produced corresponding quinoline products
3p 3r in 81%–85% yields. Afterward, the reactions of N-
benzyl-4-methylaniline (1c) with various arylacetylenes 2b 2k
were examined to explore the scope of alkyne substrates.
Quinolines 3s 3β were obtained in 68%–86% yields. These
–1r having substituent (Cl, Br, and MeO) at para-position of
Scheme 3. Synthesis of Quinolines from N-substituted
Arylamines and Alkenesa
–
–
–
results indicated that different groups, such as methyl, fluoro,
chloro, bromo, methoxy, nitro, and ester linked on benzene
rings, were tolerated under the optimized reaction conditions.
The notably maintained Br and Cl atoms in the structures of
products should make the products considerably useful in
organic transformation.
We found that arylethylenes, instead of arylacetylenes, can
be used for quinoline synthesis under the optimized reaction
conditions. Results are summarized in Scheme 3. The similar
good results were obtained, as shown in Scheme 2. Quinoline
products 3a–3y and 3α
good yields (61%–89%).
–
3
δ
were obtained in satisfactory to
O
H
TfOH (15 mol%)
120 °C, O2, 24 h
N
N
(1)
(2)
(3)
1a
5, 59%
N
Ph
Ph
O
N
TfOH (15 mol%)
+
+
Ph
Ph
120 °C, O2, 24 h
Ph
, 87%
5
3a
3a
N
O
N
TfOH (15 mol%)
120 °C, N2, 24 h
Ph
, 81%
5
Scheme 4. Control experiments.
Control experiments were conducted to gain insights into
the mechanism of this type of cyclization reaction (Scheme 4).
An imine-N-oxide
aniline substrate 1a was treated solely under the optimized
reaction conditions (Eq. 1). Subsequently, imine-N-oxide was
5 (59%) was obtained when the N-benzyl
5
used as a starting material to react with phenylacetylene under
the standard conditions; 87% of quinoline 3a was obtained (Eq.
2). The quinoline 3a was also obtained in good yield (81%)
when the imine-N-oxide
5 was treated with phenylacetylene
under N2 atmosphere (Eq. 3). These results suggested that the
current cyclization reaction might involve imine-N-oxide
intermediate.
Reaction conditions: N-substituted arylamine (
TfOH (2.7 µL, 0.03 mmol, 15 mol%) at 120 °C for 24 h under O2 atmosphere
of isolated materials.
1
, 0.2 mmol), alkene (
4
, 1.0 mmol), and
.
aAll yields are
was obtained in 83% yield. The reactions of N-(4-fluorobenzyl)-
2- methylaniline 1b and N-benzyl-4-methylaniline (1c)
(
)
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spectrometer (400 MHz for 1H, 100 MHz for 13C) or a Varian
Inova-500 spectrometer (500 MHz for 1H, 125 MHz for 13C);
CDCl3 and TMS were used as solvent and internal standard,
respectively. The chemical shifts are reported in ppm
downfield (δ) from TMS, the coupling constants J are given in
Hz. The peak patterns are indicated as follows: s, singlet; d,
doublet; t, triplet; q, quartet; m, multiplet. High-resolution
mass spectra were recorded on either
a Q-TOF mass
spectrometry or a GC-TOF mass spectrometry. TLC was used
with SiO2 (silica gel 60 F254, Merck) as stationary phase. Spots
were viewed under UV light. Flash column chromatography
was performed with SiO2 (80 mesh) as stationary phase.
General Procedure for the Synthesis of Quinolines 3
N-substituted arylamines 1a–1r (0.20 mmol), 2a
phenylacetylenes (110 µL, 1 mmol, 5 equiv.), or styrenes (115
µL, 1 mmol, 5 equiv.) were added to a clean, oven-dried
reactor and stirred for 1 min. Triflouromethanesulfonic acid
(2.7 µL, 0.03 mmol, 15 mol%) was then added. Oxygen was
purged directly from the cylinder such that the environment
was completely saturated with it for 24 h at 120 °C. Afterward,
the reaction mixture was purified by flash column
chromatography on silica gel (eluent: petroleum ether/DCM =
Scheme 5. Proposed mechanism.
On the basis of our experimental outcomes and previous
reports,8,14 a plausible catalytic cycle is proposed to account
for the present Brønsted acid-catalyzed cyclization reaction
(Scheme 5). N-Benzylaniline substrate 1a was oxidized in the
presence of O2 and acid to produce imine-N-oxide
5
5. The
imine-N-oxide subsequently underwent Diels–Alder-type
8:2) to yield quinoline 3.
cyclization reaction in the presence of phenylacetylene or
styrene as a dienophile at heating conditions to generate an
Procedure for synthesis of Imine-oxide 5
imine-N-oxide intermediate
a proton from TfOH to form intermediate
rearomatization reaction to produce intermediate
Protonation of N-OH in intermediate occurred to generate
Dehydration and aromatization of
finally occurred to yield quinoline product 3a
A
. The intermediate
A abstracted
B, which underwent
1a was added into an oven-dried 100 mL round bottom flask,
(366.4 mg, 2.0 mmol). Triflouromethanesulfonic acid (27.0 µL,
0.3 mmol) was also added to the above flask after heating,
until 1a was melted. Oxygen was purged directly from the
cylinder at 120 °C for 24 h. The acid catalyst and water
generated were evaporated on rotary. The crude mixture was
dissolved in diethyl ether:DCM mixture (9.5:0.5), washed two
times with K2CO3 solution and subsequently with distilled
water, and dried over anhydrous Na2SO4. The solvent was
evaporated, which resulted in yellow feathery solid. This solid
was washed with ice-cold diethyl ether, which yielded white
crystals of titled compound.
C
.
C
intermediate
intermediate
D.
D
(generated from phenylacetylene) or an intermediate
(generated from styrene) and regenerated acid catalyst TfOH.
The intermediate subsequently underwent dehydrogenation
reaction in the presence of O2 gas at heating conditions to
E
E
generate the target product 3a
.
Conclusions
A convenient and efficient method for quinoline synthesis was
developed using simple and readily available N-alkyl anilines
and arylacetylenes or arylethylenes as the starting materials.
The Brønsted acid-catalyzed metal- and solvent-free
cyclization proceeded well under O2 atmosphere to furnish
quinolines in satisfactory to excellent yields. The widespread
availability of the starting materials, cheap and environment-
friendly oxidant, and experimental simplicity make the present
methodology considerably useful in organic synthesis.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Nos. 21372035, 21573032, and 21602026) for their
financial support. This work was also supported by the China
Postdoctoral Science Foundation (No. 2016M590226).
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
Experimental
General Information
The reactions were performed in clean, oven-dried reactors
fitted with air-tight stoppers. All the chemicals were used
without further purification. The solvents were used after
drying according to the given procedure. 1H and 13C NMR
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Graphical Abstract
Brønsted Acid-Catalyzed Metal- and Solvent-Free Quinoline Synthesis from N-alkyl
Anilines and Alkynes or Alkenes
Waqar Ahmed,a† Sheng Zhang,a† Xiaoqiang Yu,a Yoshinori Yamamoto,a,c,d and Ming Baoa,b,
*
A convenient and efficient method for quinoline synthesis was developed using simple and readily
available starting materials under green conditions.