Synthesis of substituted 2-alkylquinolines by visible-light photoredox catalysis

Article abstract of DOI:10.1039/c9ob02259g

The condensation of anilines and alkenyl ethers has been demonstrated by employing visible-light photoredox catalysis. The resulting method enables the synthesis of substituted 2-alkylquinolines under mild and simple conditions with good substrate scope and high yields.

Full text of DOI:10.1039/c9ob02259g

Organic &
Biomolecular Chemistry
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Synthesis of substituted 2-alkylquinolines by
visible-light photoredox catalysis†
Yousheng Mei,a Jie Liu,*a Lei Wang a,b and Pinhua Li
Cite this: Org. Biomol. Chem., 2020,
18, 86
a
*
Received 18th October 2019,
Accepted 23rd November 2019
The condensation of anilines and alkenyl ethers has been demonstrated by employing visible-light photo-
redox catalysis. The resulting method enables the synthesis of substituted 2-alkylquinolines under mild
and simple conditions with good substrate scope and high yields.
DOI: 10.1039/c9ob02259g
rsc.li/obc
and unsaturated carbonyl compounds or olefins. Alkenyl
ethers in the synthesis of 2-alkylquinolines have limited appli-
Introduction
The synthesis of quinoline derivatives has been extensively cations and, to the best of our knowledge, there are only four
developed for more than 100 years, because of their “privi- related literature studies.12 New synthesis routes to 2-alkylqui-
leged” structures that are widely featured in many natural pro- nolines in one step under mild conditions are still desired and
ducts,1 important intermediates for asymmetric synthesis2 and would have major impacts on the availability of 2-alkylquinolines.
pharmaceuticals.3 2-Alkylquinolines, for example, are associ-
Visible-light-initiated organic reactions have demonstrated
ated with a wide range of biomedical properties, such as anti- their synthetic utility and attracted growing interest over the
bacterial, antitumor, and antimalarial, and are used as anti- past decade. By activation with visible light, a photocatalyst
HIV agents.4 As a consequence of their proven bioactivity, the can convert the photoenergy into chemical energy to activate a
synthesis of substituted quinolines has been a subject of great range of organic substrates via single-electron transfer (SET).13
focus in medical and organic chemistry over the years. A variety of organic transformations through a single-electron-
Traditionally, there are two distinctive methods to prepare qui- transfer process have been elegantly carried out under
noline derivatives, one is the classic transformation under irradiation of visible light with the assistance of photocatalysts.
metal-free conditions, such as the Skraup reaction, Doebner– Molecular oxygen, an ideal, clean and inexpensive source, is
Von Miller reaction, Friedländer reaction, Conrad–Limpach often used in many visible light mediated reactions, one of the
reaction and so on.5 The other is the transition-metal-catalyzed reasons is that it can act as oxidases14 or oxygenases15 and
reaction, with metal catalysts including rhodium,6 ruthenium,7 afford active superoxide radical anions. As part of our ongoing
palladium,8 cobalt,9 nickel10 and other metal complexes.11 project on the study of molecular oxygen as oxidases under
Cheng and co-workers reported the nickel-catalyzed cyclization visible light conditions,16 we describe herein a new route to con-
of 2-iodoanilines with alkynyl aryl ketones to give 2,4-di- struct 2-alkylquinoline frameworks from anilines and alkenyl
substituted quinolones, which mainly can be employed for the ethers using 2,4,6-triphenylpyranium tetrafluoride (TPT) as the
synthesis of 2-aryl-4-substituted quinolines.10 In 2016, Jiang photocatalyst and oxygen in air as oxidases under blue LED
and co-workers reported a palladium-catalyzed aerobic oxi- irradiation at room temperature (Scheme 1).
dative approach to afford 2-arylquinolines from allylbenzenes
with aniline.8a These reactions can be employed for the syn-
thesis of various quinoline skeletons, but only a few of them
Results and discussion
are for the construction of 2-alkylquinolines. Generically,
2-alkylquinolines are synthesized from functionalized anilines
Initially, we examined the model reaction of methyl 4-amino-3-
methylbenzoate (1a) with ethoxyethene (2a) in DMF by using
2,4,6-triphenylpyryliumfluoroborate (TPT, 4 mol%) as the
photoredox catalyst at room temperature under irradiation of 3
aKey Laboratory of Green and Precise Synthetic Chemistry and Applications,
Ministry of Education; Department of chemistry, Huaibei Normal University,
Huaibei, Anhui 235000, P. R. China. E-mail: jieliu_1024@163.com,
pphuali@126.com
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Synthetic strategies for the synthesis of 2-alkylquinolines
c9ob02259g
from anilines and alkenyl ethers.
86 | Org. Biomol. Chem., 2020, 18, 86–92
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W blue LED (450–455 nm) in air for 16 h. It was pleasing to
find that the reaction does indeed proceed and afford the
desired product 3a in 61% yield (Table 1, entry 1). While in the
absence of TPT, no reaction was observed (Table 1, entry 2).
The structure of 3a was characterized by using HRMS and
NMR spectra. Inspired by this result, various solvents were
screened. No desired product was observed while the reactions
were carried out in EtOH and THF (Table 1, entries 3 and 4).
Gratifyingly, an improved yield (88%) of 3a was obtained when
the reaction was conducted in CH3CN (Table 1, entry 5). Other
solvents including CHCl3, 1,2-dichloroethane (DCE), 1,4-
dioxane and PhCl gave inferior results and led to the for-
mation of 3a in 14–45% yields (Table 1, entries 6–9). However,
Ru(bpy)3Cl2, fac-Ir(ppy)3 and organic dyes (including methyl-
ene blue, Mes-Acr+ClO4−, rose bengal, and eosin Y) as photo-
catalysts exhibited no efficiency to the model reaction (Table 1,
entries 10–15). The ratio of 1a to 2a, as well as the reaction
time was optimized, and the results are presented in Table 1
(entries 16 and 17). When air was replaced by O2 and N2, 87%
and 0% of the desired product were formed respectively (Table 1,
entry 18), suggesting that O2 in air is essential for the reaction.
With the optimized conditions in hand, we next tested the
versatility of this system for the construction of functionalized
alkyl quinolines. Various o-toluidines and several other alkenyl
ethers were investigated and the results are summarized in
Scheme 2. This reaction was amenable to a wide range of
o-toluidines. Their derivatives containing electron-withdrawing
groups, such as Cl, Br, I, COOEt, CN and NO2 at the para-posi-
Scheme 2 The scope of substrates [reaction conditions: 1 (0.20 mmol),
2 (0.60 mmol), TPT (0.008 mmol, 4 mol%) in CH3CN (2.0 mL) at room
temperature in air under 3 W blue LED (450–455 nm) for 16 h; isolated
yield of the product after column chromatography].
Table 1 Optimization of the reaction conditionsa
tion of o-toluidine rings, generated the corresponding 6-substi-
tuted-2,8-dimethylquinolines in yields ranging from 57% to
81% (Scheme 2, 3b–3g). The electron-donating groups
(p-chlorophenyl, p-trifluoromethylphenyl, m-methoxyphenyl
and Me at the para-position of o-toluidine rings) could also be
tolerated, giving the desired products (Scheme 2, 3h–3k) in
52–71% yields. Notably, o-toluidines with electron-withdrawing
groups on the benzene rings generated higher yields of the
products than those having benzene rings with electron-donat-
ing groups due to their electronic effect. After investigating the
para-position of o-toluidine rings, the meta-position of the
o-toluidine rings was further tested under the optimal con-
ditions. 3-Substituted and 5-substituted o-toluidines did not
affect the efficiency of the reaction, affording the desired pro-
ducts (Scheme 2, 3l–3x) in moderate yields. The position of the
substituents on the o-toluidine rings has no significant influ-
ence on their reactivity. We next evaluated the feasibility of
various alkenyl ethers in this transformation. It was found that
terminal olefins, such as, 2-methyl-1-(vinyloxy)propane,
2-methyl-2-(vinyloxy)propane, 1-(vinyloxy)butane and 1-(vinyl-
oxy)propane all gave the same product 3a with moderate
Entry
Photocatalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
TPT
—
TPT
TPT
TPT
TPT
TPT
TPT
DMF
DMF
EtOH
THF
61
0
0
0
88
40
45
14
35
0
0
0
0
0
0
CH3CN
CHCl3
DCE
1,4-Dioxane
PhCl
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
9
TPT
10
11
12
13
14
15
16
17
18
Ru(bpy)3Cl2
fac-Ir(ppy)3
Methylene blue
Mes-Acr+ClO4
−
Rose bengal
Eosin Y
TPT
TPT
TPT
42c, 61d, 89e
56f, 85g
87h,0i
a Reaction conditions: Methyl 4-amino-3-methylbenzoate (1a,
0.20 mmol), ethoxyethene (2a, 0.60 mmol), and the photocatalyst
(0.008 mmol, 4 mol%) in the solvent (2.0 mL) at room temperature in
air under 3 W blue LED (450–455 nm) for 16 h. b Isolated yield. 2a
c
(1.0 equiv.). 2a (2.0 equiv.). 2a (4.0 equiv.). f 6 h. g 24 h. h Oxygen
d
e
balloon instead of air. i Under N2.
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ESR spectra are presented in Fig. S2 of the ESI.† There was no
signal when DMPO was added into a solution of 2a and TPT in
air-saturated acetonitrile in the absence of light irradiation
(Fig. S2a†). But irradiation of the air-saturated acetonitrile
solution of 2a and TPT in air with blue LED (450–455 nm,
3 W) resulted in the formation of a strong characteristic signal
•−
of the O2 adduct with DMPO (Fig. S2b†). When the reaction
•−
time was prolonged, a stronger characteristic signal of O2 was
Scheme 3 The scope of substrates [reaction conditions: 1 (0.20 mmol),
2a (0.60 mmol), TPT (0.008 mmol, 4 mol%) in CH3CN (2.0 mL) at room
temperature in air under 3 W blue LED (450–455 nm) for 16 h; isolated
yield of the product after column chromatography].
•−
observed (Fig. S2c and S2d†), indicating the formation of O2
in the reaction. Finally, fluorescence quenching experiments
were also carried out to check for an energy transfer process
between TPT and ethoxyethene under visible-light irradiation.
As shown in ESI,† the emission intensity of excited TPT gradu-
ally decreased with the increasing of the concentration of ethox-
yethene (2a). The result indicated that the single-electron trans-
fer process actually occurred between TPT and ethoxyethene
(2a) under visible-light irradiation.
yields. It was gratifying to find that non-terminal olefins
including (E)-1-ethoxyprop-1-ene and (E)-1-ethoxybut-1-ene
could also react with diverse 4-substituted o-toluidines to
give the corresponding 6-substituted-2,8-dimethylquinolines
(Scheme 2, 3ab–3ec) in 61–76% yields. However, 4-ethoxy-2-
methyl-1,2,3,4-tetrahydroquinolines (Scheme 3, 4a and 4b)
were obtained in 24% and 39% yields when the reaction of
aniline and p-chloroaniline with ethoxyethene (2a) was carried
out under standard conditions. It has been shown that the
ortho-methyl substitution of aniline is necessary. Surprisingly,
when the ortho-methyl was replaced with ortho-trifluoromethyl,
ortho-ethyl and ortho-methoxyl, the corresponding 4-ethoxy-2-
methyl-1,2,3,4-tetrahydroquinolines (Scheme 3, 4c–4d) were
also obtained in 32–50% yields. Until now, the role of the
ortho-methyl group is unclear, and more studies to identify the
role of the o-methyl are currently underway in our laboratory.
To further investigate the reaction mechanism of this trans-
formation, some control experiments were performed, shown
in Scheme 4. When a known radical scavenger, such as 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO, 1.5 equiv.), was added to
the reaction of methyl 4-amino-3-methylbenzoate (1a) with
ethoxyethene (2a) under standard conditions, the reaction was
inhibited completely. When a radical scavenger, 1,1-diphenyl-
ethylene was added to the reaction of 1a and 2a, intermediate
IV was surprisingly stabilized by 1,1-diphenylethylene and
compound 5 was observed by HRMS analysis (see the ESI† for
details). These results suggested that a radical process was
possibly involved in the reaction. Additionally, in order to
understand the role of oxygen and the species of oxygen
involved in the present reaction, 5,5-dimethyl-pyrroline-N-
Although the exact mechanism of the reaction is not clear,
the possible mechanistic pathway based on current mechanis-
tic investigations is shown in Scheme 5. Firstly, the photo-
catalyst (PC, TPT) was excited by the irradiation of blue LED to
its excited state PC*, which then undergoes the single electron
transfer (SET) process with ethoxyethane to generate the
highly electrophilic radical cation I and PC•−. The generated
radical cation I reacts with o-toluidine 1a to generate radical
cation II, which undergoes rearrangement to afford radical
cation III. Simultaneously, the oxidation of PC•− by O2 in air
regenerated PC for the next run and produced a superoxide
radical anion (O2•−), which was confirmed by electron spin
resonance (ESR). The radical cation III was subjected to depro-
tonation to convert radical IV along with the formation of
•OOH. Cation V was formed by the oxidation of radical IV by
•OOH along with the formation of −OOH. Subsequently,
cation V attacks another 2a molecule, leading to the formation
of intermediate VI, then VII was formed by the electrophilic
substitution reaction. Finally, an aromatization process was
used to generate the final product 3a by the elimination of
ethanol with the assistance of the hydrogen proton.
•−
oxide (DMPO) was employed to capture O2 and the resulting
Scheme 4 The control experiment.
88 | Org. Biomol. Chem., 2020, 18, 86–92
Scheme 5 Plausible mechanism for the reaction.
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Ethyl 2,8-dimethylquinoline-6-carboxylate (3e). White solid.
1H NMR (400 MHz, CDCl3) δ: 8.34 (s, 1H), 8.10 (s, 1H), 8.05 (d,
J = 8.4 Hz, 1H), 7.29 (d, J = 8.4 Hz, 1H), 4.43 (q, J = 7.2 Hz, 2H),
Experimental section
General remarks
All H NMR and 13C NMR spectra were recorded on a Bruker 2.81 (s, 3H), 2.76 (s, 3H), 1.44 (t, J = 7.2 Hz, 3H); 13C NMR
FT-NMR spectrometer (400 MHz or 600 MHz and 100 MHz or (100 MHz, CDCl3) δ: 166.5, 160.2, 148.8, 137.3, 136.9, 128.7,
150 MHz, respectively). All chemical shifts are given as δ value 128.5, 126.9, 125.4, 122.2, 61.0, 25.7, 17.8, 14.3. HRMS (ESI)
(ppm) with reference to tetramethylsilane (TMS) as an internal [M + H]+ calcd for C14H16NO2: 230.1176, found: 230.1178.
1
standard. The peak patterns are indicated as follows: s, singlet;
2,8-Dimethylquinoline-6-carbonitrile (3f). White solid.
d, doublet; t, triplet; m, multiplet; q, quartet. The coupling 1H NMR (400 MHz, CDCl3) δ: 8.03 (d, J = 8.4 Hz, 1H), 8.00 (s,
constants, J, are reported in hertz (Hz). High resolution mass 1H), 7.64 (s, 1H), 7.38 (d, J = 8.4 Hz, 1H), 2.79 (s, 3H), 2.78 (s,
spectroscopy data of the products were collected on Agilent 3H); 13C NMR (100 MHz, CDCl3) δ: 161.3 148.1, 138.8, 136.4,
Technologies 6540 UHD Accurate-Mass Q-TOF LC/MS (ESI).
All reactions were setup in air. All chemicals were [M + H]+ calcd for C12H11N2: 183.0917, found: 183.0915.
purchased commercially and used without further purifi-
2,8-Dimethyl-6-nitroquinoline (3g). Yellow solid. 1H NMR
cation. Products were purified by flash chromatography on (400 MHz, CDCl3) δ: 8.57–8.56 (m, 1H), 8.29 (s, 1H), 8.17 (d, J =
131.7, 129.7, 125.5, 123.2, 119.0, 108.8, 25.8, 17.7. HRMS (ESI)
200–300 mesh silica gels, SiO2.
8.4 Hz, 1H), 7.42 (d, J = 8.4 Hz, 1H), 2.86 (s, 3H), 2.81 (s, 3H);
13C NMR (100 MHz, CDCl3) δ: 162.0, 149.2, 144.4, 139.4, 137.9,
125.0, 123.5, 122.4, 122.1, 25.9, 18.1. HRMS (ESI) [M + H]+
calcd for C11H11N2O2: 203.0815, found: 203.0815.
Typical procedure for the synthesis of substituted
2-alkylquinolines by visible-light photoredox catalysis
A 5 mL oven-dried reaction vessel equipped with a magnetic
6-(4-Chlorophenyl)-2,8-dimethylquinoline (3h). White solid.
stirrer was charged with methyl 4-amino-3-methylbenzoate 1H NMR (400 MHz, CDCl3) δ: 8.03 (d, J = 8.4 Hz, 1H), 7.75–7.74
(1a, 0.20 mmol), ethoxyethene (2a, 0.60 mmol), TPT (4 mol%) (m, 2H), 7.63–7.61 (m, 2H), 7.45–7.42 (m, 2H), 7.29 (d, J =
and acetonitrile (2.0 mL). The reaction vessel was exposed to 8.4 Hz, 1H), 2.85 (s, 3H), 2.77 (s, 3H); 13C NMR (100 MHz,
blue LED (450–455 nm, 3 W) irradiation at room temperature CDCl3) δ: 158.2, 146.5, 139.3, 137.2, 136.6, 136.4, 133.5,
in air with stirring for 16 h. After completion of the reaction, 129.0, 128.8, 128.5, 126.5, 123.2, 122.2, 25.7, 18.1. HRMS (ESI)
the mixture was concentrated to yield the crude product, [M + H]+ calcd for C17H15ClN: 268.0888, found: 268.0892.
which was further purified by flash chromatography (silica gel,
petroleum ether/ethyl acetate = 100 : 1) to give the desired White solid. H NMR (400 MHz, CDCl3) δ: 8.00 (d, J = 8.4 Hz,
2,8-Dimethyl-6-(4-(trifluoromethyl)phenyl)quinoline
(3i).
1
product 3a.
1H), 7.75 (s, 2H), 7.73 (s, 2H), 7.70 (s, 1H), 7.67 (s, 1H), 7.27 (d,
J = 8.4 Hz, 1H), 2.85 (s, 3H), 2.75 (s, 3H); 13C NMR (100 MHz,
CDCl3) δ: 158.5, 146.7, 144.3, 137.4, 136.4, 136.3, 129.4 (q, J =
Characterization data for the products
Methyl 2,8-dimethylquinoline-6-carboxylate (3a). White 32.3 Hz), 128.7, 127.5, 126.4, 125.7 (q, J = 3.7 Hz), 125.0 (q, J =
1
solid. H NMR (400 MHz, CDCl3) δ: 8.30 (s, 1H), 8.07 (s, 1H), 270.0 Hz), 123.8, 122.2, 25.6, 18.1. HRMS (ESI) [M + H]+ calcd
8.01 (d, J = 8.4 Hz, 1H), 7.26 (d, J = 8.4 Hz, 1H), 3.95 (s, 3H), for C18H15F3N: 302.1151, found: 302.1154.
2.79 (s, 3H), 2.74 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 166.9,
6-(3-Methoxyphenyl)-2,8-dimethylquinoline
(3j).
White
160.3, 148.7, 137.2, 136.9, 128.6, 128.5, 126.4, 125.3, 122.2, solid. 1H NMR (400 MHz, CDCl3) δ: 8.02 (d, J = 8.4 Hz, 1H),
52.1, 25.7, 17.8. HRMS (ESI) [M + H]+ calcd for C13H14NO2: 7.77 (s, 2H), 7.38 (t, J = 7.6 Hz, 1H), 7.29–7.26 (m, 2H),
216.1019, found: 216.1016.
7.24–7.22 (m, 1H), 6.93–6.90 (m, 1H) 3.88 (s, 3H), 2.85 (s, 3H)
6-Chloro-2,8-dimethylquinoline (3b). White solid. 1H NMR 2.76 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 160.0, 157.9, 146.5,
(400 MHz, CDCl3) δ: 7.88 (d, J = 8.4 Hz, 1H), 7.57–7.56 (m, 1H), 142.3, 137.7, 136.9, 136.4, 129.8, 129.1, 126.5, 123.3, 122.0,
7.46 (s, 1H), 7.26 (d, J = 8.4 Hz, 1H), 2.76 (s, 3H), 2.73 (s, 3H); 119.8, 113.1, 112.8, 55.3, 25.6, 18.1. HRMS (ESI) [M + H]+ calcd
13C NMR (100 MHz, CDCl3) δ: 158.1, 145.4, 138.9, 135.3, 130.6, for C18H18NO: 264.1383, found: 264.1383.
130.0, 127.0, 124.0, 122.5, 25.5, 17.7. HRMS (ESI) [M + H]+
calcd for C11H11ClN: 192.0575, found: 192.0577.
2,6,8-Trimethylquinoline (3k). White solid. 1H NMR
(400 MHz, CDCl3) δ: 7.90 (d, J = 8.4 Hz, 1H), 7.36 (s, 2H), 7.22
6-Bromo-2,8-dimethylquinoline (3c). White solid. 1H NMR (d, J = 8.4 Hz, 1H), 2.76 (s, 3H), 2.73 (s, 3H), 2.46 (s, 3H).
(400 MHz, CDCl3) δ: 7.89 (d, J = 8.4 Hz, 1H), 7.75 (s, 1H), 7.60 13C NMR (100 MHz, CDCl3) δ: 156.9, 145.6, 136.1, 135.6,
(s, 1H), 7.28–7.26 (m, 1H), 2.76 (s, 3H), 2.73 (s, 3H); 13C NMR 134.8, 131.8, 126.4, 124.3, 121.6, 25.5, 21.4, 17.8. HRMS (ESI)
(100 MHz, CDCl3) δ: 158.3, 145.7, 139.1, 135.2, 132.5, 127.5, [M + H]+ calcd for C12H14N: 172.1121, found: 172.1123.
127.4, 122.5, 118.9, 25.6, 17.6. HRMS (ESI) [M + H]+ calcd for
C11H11BrN: 236.0069, found: 236.0071.
5-Chloro-2,8-dimethylquinoline (3l). White solid. 1H NMR
(400 MHz, CDCl3) δ: 8.33 (d, J = 8.8 Hz, 1H), 7.38–7.33 (m, 2H),
6-Iodo-2,8-dimethylquinoline (3d). White solid. 1H NMR 7.27 (d, J = 8.8 Hz, 1H), 2.720–2.718 (m, 6H); 13C NMR
(400 MHz, CDCl3) δ: 7.97–7.96 (m, 1H), 7.84 (d, J = 8.4 Hz, 1H), (100 MHz, CDCl3) δ: 158.4, 147.4, 135.9, 132.8, 128.9, 128.5,
7.76 (s, 1H), 7.24 (d, J = 8.4 Hz, 1H), 2.73 (s, 3H), 2.72 (s, 3H); 125.1, 124.3, 122.2, 25.3, 17.7. HRMS (ESI) [M + H]+ calcd for
13C NMR (100 MHz, CDCl3) δ: 158.5, 146.0, 138.9, 137.7, 134.9, C11H11ClN: 192.0575, found: 192.0578.
134.2, 128.0, 122.3, 90.7, 25.6, 17.4. HRMS (ESI) [M + H]+ calcd
for C11H11IN: 283.9931, found: 283.9933.
5-Bromo-2,8-dimethylquinoline (3m). White solid. 1H NMR
(400 MHz, CDCl3) δ: 8.35 (d, J = 8.4 Hz, 1H), 7.60 (d, J = 7.6 Hz,
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1H), 7.36–7.32 (m, 2H), 2.76 (s, 3H), 2.73 (s, 3H). 13C NMR 1H), 7.40 (d, J = 8.8 Hz, 1H), 7.23 (d, J = 8.4 Hz, 1H), 2.89 (s,
(100 MHz, CDCl3) δ: 158.6, 147.6, 136.7, 135.5, 129.6, 128.9, 3H), 2.72 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 158.7, 147.3,
125.6, 122.7, 119.0, 25.3, 17.8. HRMS (ESI) [M + H]+ calcd for 136.4, 136.1, 129.6, 125.9, 125.6, 125.2, 121.6, 25.6, 17.4.
C11H11BrN: 236.0069, found: 236.0068.
5-Iodo-2,8-dimethylquinoline (3n). White solid. 1H NMR 236.0072.
HRMS (ESI) [M + H]+ calcd for C11H11BrN: 236.0069, found:
(400 MHz, CDCl3) δ: 8.19 (d, J = 8.4 Hz, 1H), 7.89 (d, J = 7.2 Hz,
7-Iodo-2,8-dimethylquinoline (3v). White solid. 1H NMR
1H), 7.29 (d, J = 8.4 Hz, 1H), 7.22 (d, J = 7.6 Hz, 1H), 2.76 (s, (400 MHz, CDCl3) δ: 7.92 (d, J = 8.4 Hz, 1H), 7.82 (d, J = 8.8 Hz,
3H), 2.74 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 158.7, 147.4, 1H), 7.29–7.24 (m, 2H), 2.97 (s, 3H), 2.73 (s, 3H); 13C NMR
140.3, 137.8, 136.3, 130.5, 128.1, 123.2, 95.1, 25.2, 17.8. HRMS (100 MHz, CDCl3) δ: 158.5, 146.6, 140.5, 136.1, 135.5, 126.3,
(ESI) [M + H]+ calcd for C11H11IN: 283.9931, found: 283.9930.
125.8, 121.9, 102.6, 25.6, 23.1. HRMS (ESI) [M + H]+ calcd for
Methyl 2,8-dimethylquinoline-5-carboxylate (3o). White C11H11IN: 283.9931, found: 283.9928.
solid. 1H NMR (400 MHz, CDCl3) δ: 9.22 (d, J = 8.8 Hz, 1H),
Methyl 2,8-dimethylquinoline-7-carboxylate (3w). White
8.09 (d, J = 7.2 Hz, 1H), 7.52 (d, J = 7.6 Hz, 1H), 7.36 (d, J = solid. 1H NMR (400 MHz, CDCl3) δ: 7.96 (d, J = 8.4 Hz, 1H),
9.2 Hz, 1H), 3.96 (s, 3H), 2.83 (s, 3H), 2.74 (s, 3H); 13C NMR 7.82 (d, J = 8.4 Hz, 1H), 7.58 (d, J = 8.8 Hz, 1H), 7.29 (d, J = 8.4
(100 MHz, CDCl3) δ: 167.3, 157.9, 146.8, 143.0, 134.3, Hz, 1H), 3.95 (s, 3H), 3.07 (s, 3H), 2.75 (s, 3H); 13C NMR
129.6, 128.0, 125.1, 124.2, 123.0, 52.0, 25.4, 18.7. (100 MHz, CDCl3) δ: 168.7, 158.5, 146.8, 139.7, 135.9,
HRMS (ESI) [M + H]+ calcd for C13H14NO2: 216.1019, found: 130.2, 127.7, 125.6, 124.9, 122.9, 52.0, 25.6, 14.5. HRMS (ESI)
216.1020.
[M + H]+ calcd for C13H14NO2: 216.1019, found: 216.1018.
Ethyl 2,8-dimethylquinoline-5-carboxylate (3p). White solid.
2,7,8-Trimethylquinoline (3x). White solid. 1H NMR
1H NMR (400 MHz, CDCl3) δ: 9.20 (d, J = 8.8 Hz, 1H), 8.06 (d, (400 MHz, CDCl3) δ: 7.96 (d, J = 8.4 Hz, 1H), 7.51 (d, J = 8.0 Hz,
J = 7.6 Hz, 1H), 7.46 (d, J = 7.6 Hz, 1H), 7.31 (d, J = 9.2 Hz, 1H), 1H), 7.29 (d, J = 8.0 Hz, 1H), 7.20 (d, J = 8.0 Hz, 1H), 2.76
4.42 (q, J = 7.2 Hz, 2H), 2.80 (s, 3H), 2.71 (s, 3H), 1.42 (t, J = (s, 3H), 2.75 (s, 3H), 2.50 (s, 3H); 13C NMR (100 MHz, CDCl3)
7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ: 166.7, 157.7, 146.7, δ: 157.7, 146.9, 137.0, 136.1, 133.8, 128.3, 124.7, 124.4, 120.6,
142.7, 134.1, 129.4, 127.9, 125.0, 124.5, 122.7, 60.7, 25.2, 18.6, 25.7, 20.7, 13.1. HRMS (ESI) [M + H]+ calcd for C12H14N:
14.2. HRMS (ESI) [M + H]+ calcd for C14H16NO2: 230.1176, 172.1121, found: 172.1119.
found: 230.1177.
Methyl 2-ethyl-3,8-dimethylquinoline-6-carboxylate (3ab).
2,8-Dimethylquinoline-5-carbonitrile (3q). White solid. White solid. 1H NMR (400 MHz, CDCl3) δ: 8.251–8.247 (m,
1H NMR (400 MHz, CDCl3) δ: 8.36 (d, J = 8.4 Hz, 1H), 7.78 (d, 1H), 8.012–8.010 (m, 1H), 7.77 (s, 1H), 3.95 (s, 3H), 2.95 (q, J =
J = 7.2 Hz, 1H), 7.55 (d, J = 7.6 Hz, 1H), 7.45 (d, J = 8.8 Hz, 1H), 7.6 Hz, 2H), 2.79 (s, 3H), 2.42 (s, 3H), 1.41 (t, J = 7.2 Hz, 3H);
2.84 (s, 3H), 2.79 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 159.7, 13C NMR (100 MHz, CDCl3) δ: 167.2, 163.9, 147.4, 137.1, 136.4,
146.2, 143.8, 133.3, 131.6, 128.6, 125.9, 123.8, 117.3, 107.6, 129.9, 127.8, 127.5, 126.4, 126.1, 52.0, 29.3, 18.8, 17.7, 11.7.
25.5, 18.4. HRMS (ESI) [M + H]+ calcd for C12H11N2: 183.0917, HRMS (ESI) [M + H]+ calcd for C15H18NO2: 244.1332, found:
found: 183.0918.
244.1333.
2,6,8-Trimethylquinoline (3r). White solid. 1H NMR
Methyl 3-ethyl-8-methyl-2-propylquinoline-6-carboxylate (3ac).
1
(400 MHz, CDCl3) δ: 8.14 (d, J = 8.8 Hz, 1H), 7.38 (d, J = 7.2 Hz, White solid. H NMR (400 MHz, CDCl3) δ: 8.33–8.32 (m, 1H),
1H), 7.25 (d, J = 8.8 Hz, 1H), 7.16 (d, J = 7.2 Hz, 1H), 2.76 8.04–8.03 (m, 1H), 7.86 (s, 1H), 3.96 (s, 3H), 2.96 (t, J =
(s, 3H), 2.74 (s, 3H), 2.59 (s, 3H); 13C NMR (100 MHz, CDCl3) 7.2 Hz, 2H), 2.84–2.79 (m, 5H), 1.99–1.90 (m, 2H), 1.33 (t,
δ: 157.2, 147.1, 134.4, 132.6, 131.9, 129.0, 125.7, 125.6, 121.0, J = 7.2 Hz, 3H), 1.08 (t, J = 7.2 Hz, 3H); 13C NMR
25.5, 18.4, 17.9. HRMS (ESI) [M + H]+ calcd for C12H14N: (100 MHz, CDCl3) δ: 167.3, 162.8, 147.3, 137.1, 135.9, 134.7,
172.1121, found: 172.1119.
128.1, 127.5, 126.5, 126.2, 52.1, 37.5, 25.0, 21.5, 17.7, 14.2,
7-Fluoro-2,8-dimethylquinoline (3s). White solid. 1H NMR 14.1. HRMS (ESI) [M + H]+ calcd for C17H22NO2: 272.1645,
(400 MHz, CDCl3) δ: 7.94 (d, J = 8.4 Hz, 1H), 7.56–7.52 (m, 1H), found: 272.1651.
7.23–7.18 (m, 2H), 2.73 (s, 3H), 2.68 (d, J = 2.4 Hz, 3H);
6-Chloro-3-ethyl-8-methyl-2-propylquinoline (3bc). White
13C NMR (100 MHz, CDCl3) δ: 160.8 (d, J = 243.4 Hz), 158.8, solid. H NMR (600 MHz, CDCl3) δ: 7.64 (s, 1H), 7.492–7.488
147.8 (d, J = 9.8 Hz), 136.1, 126.1 (d, J = 10.4 Hz), 123.3, 120.6 (m, 1H), 7.37–7.36 (m, 1H), 2.90 (t, J = 7.2 Hz, 2H), 2.78–2.75
(d, J = 10.0 Hz), 120.5 (d, J = 15.5 Hz), 115.5 (d, J = 26.7 Hz), (m, 2H), 2.73 (s, 3H), 1.94–1.88 (m, 2H), 1.29 (t, J = 7.2 Hz, 3H),
25.6, 8.8 (d, J = 4.3 Hz). HRMS (ESI) [M + H]+ calcd for 1.06 (t, J = 7.2 Hz, 3H); 13C NMR (150 MHz, CDCl3) δ: 160.4,
1
C11H11FN: 176.0870, found: 176.0874.
143.7, 138.9, 136.0, 132.6, 130.4, 128.7, 127.7, 123.3, 37.3, 25.0,
7-Chloro-2,8-dimethylquinoline (3t). White solid. 1H NMR 21.6, 17.6, 14.2, 14.1. HRMS (ESI) [M + H]+ calcd for
(400 MHz, CDCl3) δ: 7.89 (d, J = 8.4 Hz, 1H), 7.42–7.37 (m, 2H), C15H19ClN: 248.1201, found: 248.1203.
7.19 (d, J = 8.4 Hz, 1H), 2.84 (s, 3H), 2.71 (s, 3H); 13C NMR
(100 MHz, CDCl3) δ: 158.7, 147.3, 136.0, 134.6, 134.2, 126.7, solid. H NMR (600 MHz, CDCl3) δ: 7.65 (s, 1H), 7.60 (s, 1H),
125.7, 124.8, 121.4, 25.5, 14.2. HRMS (ESI) [M + H]+ calcd for 7.483–7.481 (m, 1H), 2.88 (t, J = 7.8 Hz, 2H), 2.76–2.72 (m, 5H),
6-Bromo-3-ethyl-8-methyl-2-propylquinoline (3cc). White
1
C11H11ClN: 192.0575, found: 192.0576.
1.92–1.88 (m, 2H), 1.27 (t, J = 7.2 Hz, 3H), 1.07–1.04 (m, 3H);
7-Bromo-2,8-dimethylquinoline (3u). White solid. 1H NMR 13C NMR (150 MHz, CDCl3) δ: 160.5, 143.9, 139.0, 136.0, 132.4,
(400 MHz, CDCl3) δ: 7.91 (d, J = 8.4 Hz, 1H), 7.56 (d, J = 8.8 Hz, 131.2, 128.2, 126.7, 118.7, 37.3, 25.0, 21.5, 17.5, 14.2, 14.1.
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HRMS (ESI) [M + H]+ calcd for C15H19BrN: 292.0695, found:
292.0697.
Conclusions
Ethyl 3-ethyl-8-methyl-2-propylquinoline-6-carboxylate (3ec). In summary, we have developed a convenient method for the
White solid. 1H NMR (400 MHz, CDCl3) δ: 8.34 (s, 1H), 8.05 (s, direct synthesis of 8-methyl-2-alkylquinolines from o-tolui-
1H), 7.88 (s, 1H), 4.43 (q, J = 7.2 Hz, 2H), 2.96 (t, J = 7.2 Hz, dines and alkenyl ethers through using visible-light photo-
2H), 2.85–2.79 (m, 5H), 1.99–1.90 (m, 2H), 1.44 (t, J = 7.2 Hz, redox catalysis under mild conditions. This method provides a
3H), 1.33 (t, J = 7.6 Hz, 3H), 1.08 (t, J = 7.6 Hz, 3H); 13C NMR new strategy for the construction of quinoline derivatives with
(100 MHz, CDCl3) δ: 166.8, 162.7, 147.2, 137.0, 135.8, 134.7, excellent functional group compatibility, providing rapid
128.0, 127.5, 126.8, 126.2, 61.0, 37.5, 25.0, 21.5, 17.8, 14.4, access to biological quinoline motifs. A plausible reaction
14.2, 14.1. HRMS (ESI) [M + H]+ calcd for C18H24NO2: mechanism through the electronic transfer pathway has been
286.1802, found: 286.1801.
proposed and further studies are currently underway in our
4-Ethoxy-2-methyl-1,2,3,4-tetrahydroquinoline (4a). Yellow laboratory.
1
liquid. H NMR (600 MHz, CDCl3) δ: 7.34 (d, J = 7.8 Hz, 1H),
7.01–6.99 (m, 1H), 6.69–6.66 (m, 1H), 6.46–6.45 (m, 1H),
4.67–4.64 (m, 1H), 3.73–3.68 (m, 2H), 3.59–3.55 (m, 1H), Conflicts of interest
3.54–3.50 (m, 1H), 2.24–2.21 (m, 1H), 1.66 (q, J = 10.8 Hz, 1H),
There are no conflicts to declare.
1.28 (t, J = 7.2 Hz, 3H), 1.24 (d, J = 6.6 Hz, 3H); 13C NMR
(150 MHz, CDCl3) δ: 144.5, 128.1, 127.3, 122.5, 117.4, 113.8,
73.7, 63.3, 46.3, 36.0, 22.5, 15.7. HRMS (ESI) [M + H]+ calcd for
C12H18NO: 192.1383, found: 192.1381.
Acknowledgements
6-Chloro-4-ethoxy-2-methyl-1,2,3,4-tetrahydroquinoline (4b).
Yellow liquid. H NMR (600 MHz, CDCl3) δ: 7.43–7.42 (m, 1H),
We gratefully acknowledge the National Natural Science
1
Foundation of China (21901081 and 21772062) and the Young
Talent Key Project of Anhui Province (170808J02) for financial
support.
7.07–7.05 (m, 1H), 6.31 (d, J = 8.4 Hz, 1H), 4.57–4.55 (m, 1H),
3.75–3.69 (m, 1H), 3.67 (s, 1H), 3.58–3.53 (m, 1H), 3.50–3.47
(m, 1H), 2.24–2.20 (m, 1H), 1.58 (q, J = 10.8 Hz, 1H), 1.28 (t, J =
7.2 Hz, 3H), 1.23 (d, J = 6.0 Hz, 3H); 13C NMR (150 MHz,
CDCl3) δ: 143.4, 130.7, 129.8, 124.5, 115.3, 108.9, 73.3, 63.8,
46.1, 35.5, 22.3, 15.6. HRMS (ESI) [M + H]+ calcd for
C12H17ClNO: 226.0993, found: 226.0995.
6-Chloro-4-ethoxy-2-methyl-8-(trifluoromethyl)-1,2,3,4-tetra-
hydroquinoline (4c). Yellow liquid. 1H NMR (600 MHz, CDCl3)
δ: 7.38–7.37 (m, 1H), 7.29–7.27 (m, 1H), 4.63 (t, J = 5.6 Hz, 1H),
4.51 (d, J = 7.2 Hz, 1H), 3.73–3.70 (m, 1H), 3.69–3.62 (m, 2H),
1.88–1.86 (m, 2H), 1.24–1.22 (m, 3H), 1.21–1.19 (m, 3H);
13C NMR (150 MHz, CDCl3) δ: 143.6, 132.8, 126.6 (q, J =
5.9 Hz), 124.3 (q, J = 270.8 Hz), 114.4 (q, J = 29.7 Hz), 113.7,
100.7, 61.8, 61.5, 45.6, 40.8, 20.5, 15.3. HRMS (ESI) [M + H]+
calcd for C13H16ClF3NO: 294.0867, found: 294.0868.
6-Bromo-4-ethoxy-8-ethyl-2-methyl-1,2,3,4-tetrahydroquino-
line (4d). Yellow liquid. 1H NMR (600 MHz, CDCl3)
δ: 7.35–7.34 (m, 1H), 7.02–7.01 (m, 1H), 4.60–4.58 (m, 1H),
3.72–3.69 (m, 1H), 3.58–3.54 (m, 1H), 3.53–3.49 (m, 2H), 2.36
(q, J = 7.8 Hz, 2H), 2.24–2.21 (m, 1H), 1.63–1.57 (m, 1H),
1.29–1.27 (m, 6H), 1.20 (t, J = 7.2 Hz, 3H); 13C NMR (150 MHz,
CDCl3) δ: 140.9, 129.3, 128.4, 127.4, 124.1, 109.0, 73.6, 63.7,
46.1, 35.4, 23.3, 22.6, 15.6, 12.4. HRMS (ESI) [M + H]+ calcd for
C14H21BrNO: 298.0801, found: 298.0803.
Notes and references
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1
Yellow liquid. H NMR (600 MHz, CDCl3) δ: 7.01–6.99 (m, 1H),
6.65–6.64 (m, 2H), 4.72–4.69 (m, 1H), 4.11 (s, 1H), 3.81 (s, 3H),
3.72–3.67 (m, 1H), 3.58–3.54 (m, 1H), 3.53–3.48 (m, 1H),
2.24–2.20 (m, 1H), 1.70 (q, J = 10.8 Hz, 1H), 1.28–1.26 (m, 6H);
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HRMS (ESI) [M + H]+ calcd for C13H20NO: 222.1489, found:
222.1491.
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Organic & Biomolecular Chemistry
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