N-methylation of quinolines with CO2 and H2 catalyzed by Ru-triphos complexes

Article abstract of DOI:10.1007/s11426-017-9024-8

N-methyl-tetrahydroquinolines (MTHQs) are a kind of very useful chemicals, which can be obtained from N-methylation of amines. However, the methylation of quinolines which is a kind of highly unsaturated nitrogen-containing heterocyclic aromatic compounds has not been reported. In this work, we report the first work for the synthesis of MTHQs by methylation of quinolines using CO₂ and H₂. It was found that Ru(acac)₃-triphos [triphos: 1,1,1-tris(diphenylphosphinomethyl)ethanl] complex was very active and selective for the N-methylation reaction of quinolines, and the yield of the desired product could reach 99%.

Full text of DOI:10.1007/s11426-017-9024-8

SCIENCE CHINA
Chemistry
doi: 10.1007/s11426-017-9024-8
• ARTICLES •
· SPECIAL ISSUE · Green Chemistry
N-methylation of quinolines with CO2 and H2 catalyzed by
Ru-triphos complexes
Zhenhong He1,3, Huizhen Liu1,2*, Qingli Qian1, Lu Lu1, Weiwei Guo1,
Lujun Zhang1 & Buxing Han1,2*
1Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Thermodynamics,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
2University of Chinese Academy of Sciences, Beijing 100049, China
3College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi’an 710021, China
Received January 3, 2017; accepted February 20, 2017; published online April 27, 2017
N-methyl-tetrahydroquinolines (MTHQs) are a kind of very useful chemicals, which can be obtained from N-methylation of amines.
However, the methylation of quinolines which is a kind of highly unsaturated nitrogen-containing heterocyclic aromatic compounds
has not been reported. In this work, we report the first work for the synthesis of MTHQs by methylation of quinolines using CO2
and H2. It was found that Ru(acac)3-triphos [triphos: 1,1,1-tris(diphenylphosphinomethyl)ethanl] complex was very active and
selective for the N-methylation reaction of quinolines, and the yield of the desired product could reach 99%.
methylation, hydrogenation, quinoline derivatives, carbon dioxide, homogeneous catalysis
Citation:
He Z, Liu H, Qian Q, Lu L, Guo W, Zhang L, Han B. N-methylation of quinolines with CO2 and H2 catalyzed by Ru-triphos complexes . Sci China
Chem, 2017, 60: doi: 10.1007/s11426-017-9024-8
1 Introduction
Quinoline and its derivatives (quinolines), a kind of nitro-
gen-containing polycyclic aromatic compounds, exist in coal
and oil shale and are the by-products of petroleum refin-
ing processes [1]. N-methyl-1,2,3,4-tetrahydroquinolines
(MTHQs) are valuable chemicals in organic synthesis and
chemical industry. They are also found in many biologically
active natural products and pharmacologically relevant ther-
apeutic agents (Scheme 1) [2,3].
It has been reported that MTHQ (4a) can be synthesized
from the N-methylation of THQ (1,2,3,4-tetrahydro-quino-
line, 2a) that can be produced by selective hydrogenation of
quinolines [4]. Different C1 sources have been used in the N-
Scheme 1 Some antimalarial activity of naturally occurring alkaloids with
the structure of N-methyl-1,2,3,4-tetrahydroquinoline.
methylation reactions, such as CH3I, HCHO, HCOOH, CO2
and H2 or PhSiH3, dimethyl sulfate, MeOTf (OTf=trifluo-
romethanesulfonate) or diazomethane [5], or the reaction of
*Corresponding authors (email: liuhz@iccas.ac.cn; hanbx@iccas.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

2
He et al. Sci China Chem
quinoline with methanol [6] or CH3I and dihydropyridines
[7], as shown in Scheme 2.
RuBr3 (Ru>25%), RuI3 (Ru>20.5%), methanesulfonic
acid (98%) and paraformaldehyde (97.0%) were from
Alfa Aesar (USA). RuCl3 (99.99%), tricyclohexany-
lphosphine (97%), 1,2-bis(diphenylphosphino)benzene
(98%), 1,3-bis(diphenylphosphino)propane (98%), 1,5-
bis(diphenylphosphino)pentane (98%), bis(2-diphenylphos-
phinoethyl)phenylphosphine (97%), quinoline (99%),
6-methoxyquinoline (98%), 7-methylquinoline (98%),
CO2 is a cheap, abundant and safe carbon resource, and its
transformation into valuable chemicals and fuel has received
much attention [8]. N-methylation of amines using CO2 and
reductant such as H2 and PhSiH3 has been reported [9]. Com-
paring with PhSiH3, H2 is a cheaper and desirable reductant,
and only water is the by-product in the reaction [10]. Up
to now, both heterogeneous catalysts such as CuAlOx [11],
Pd/CuZrOx [12] and Pt-MoOx/TiO2 [13], Au nanoparticles
supported on γ-Al2O3 [14], and homogeneous catalysts
such as Ru(acac)3-triphos [15] have been developed for the
methylation of amines with CO2 and H2. Ru(acac)3-triphos
complex has been widely used in organic reactions, such
as reduction of secondary and tertiary amides to amines
[16], hydrogenation of carboxylic acids or esters to alcohols
[17], N-alkylation of amines and carboxylic acids [18], CO2
hydrogenation to methanol [19].
In this work, we found that Ru(acac)3-triphos complex
was very active and selective homogeneous catalyst for
N-methylation of quinolines with CO2 and H2, and 99% yield
of MTHQs could be obtained. This route is compared with
reported methods in Scheme 2. As far as we known, this is
the first work for the N-methylation of quinolines with CO2
and H2.
6-methylquinoline (98%),
6-chloroquinoline (97%),
3-methylquinoline (98%), 4-methylquinoline (99%) and
tetrahydrofuran (THF, 99.85%) were from J&K Chemicals
(China). Ru3(CO)12 (>98%) was from Adamas Reagent Co.,
Ltd. (China). 1,1,1-Tris(diphenylphosphinomethyl)ethanl
(97%) was from Strem Chemicals (USA). Dimethyl-
bisdiphenylphosphinoxanthene (98%) was from Energy
Chemical (China). HCOOH (98.0%) was from Sinopharm
Chemical Reagent Co., Ltd. (China). CO2 (99.99%) and H2
(99.99%) were provided by Beijing Analytical Instrument
Company (China).
Gas chromatograph (GC) analysis was carried out on an
Agilent Technologies 7890B system equipped with an HP-5
column. GC-MS characterization was conducted on an Ag-
ilent 7892B/MSD 5975C system equipped with a HP-5MS
1
column. H and 13C NMR spectra were recorded on a Bruker
Avance III HD 400 MHz NMR spectrometer (Germany) (400
MHz for 1H and 101 MHz for 13C) at ambient temperature in
CDCl3.
2 Experimental
2.2 N-methylation of quinolines with CO2 and H2
2.1 General
Typical procedures (Table 1, entry 1) were carried out as
follows: quinoline (0.129 g, 1.0 mmol), Ru(acac)3 (2.0 mg,
0.005 mmol), triphos (6 mg, 0.010 mmol), methanesulfonic
acid (MSA, 5 mg, 0.05 mmol) and dry THF (2.0 mL) were
added into a 16 mL autoclave with a Teflon inner container.
Then the reactor was sealed and purged with CO2 to remove
the air (5×8 bar). After that, 2 MPa of CO2 and 8 MPa of
H2 were charged into the reactor and the mixture was stirred
at 160 °C for 16 h. Afterwards, the reaction was quenched
by transferring it into ice-water. After it was cooled to 0 °C,
the reactor was vented slowly. The reaction mixture was an-
alyzed by gas chromatograph-mass spectrometer (GC-MS)
and GC with decane as an internal standard, or purified by
flash column chromatography on silica gel to afford the de-
sired product was characterized by 1H and 13C NMR.
All the chemicals were purchased from commercial sources
and used without further purification. Ru(acac)3 (Ru>24%),
2.3 NMR spectra of products
N-methyl-1,2,3,4-tetrahydroquinoline (4a): 1H NMR (400
MHz, CDCl3) δ 7.06 (t, J=7.8 Hz, 1H), 6.94 (d, J=7.13, 1H),
6.60 (t, J=6.7 Hz, 2H), 3.21 (t, J=5.7 Hz, 2H), 2.87 (s, 3H),
2.76 (t, J=6.32 Hz, 2H), 1.97 (m, 2H); 13C NMR (101 MHz,
CDCl3) δ 146.8, 128.8, 127.1, 122.9, 116.2, 111.0, 51.3,
39.1, 27.8, 22.5.
Scheme 2 Synthesis of N-methyl-1,2,3,4-tetrahydroquinolines.

He et al. Sci China Chem
3
Table 1 One-pot hydrogenation and N-methylation of quinoline and CO2 with H2 over various catalysts a)
b)
Yield (%)
Entry
Cat.
L
Conv. (%)
c)
2a
0
3a
0
4a
99
3
Others
1
2
Ru(acac)3
RuCl3
L9
L9
L9
L9
L9
L9
L9
L9
L1
L2
L3
L4
L5
L6
L7
L8
L10
L9
L9
L9
L9
100
60
82
94
98
100
100
99
96
95
93
93
100
58
100
96
97
78
99
83
0
1
14
1
44
46
73
86
84
99
98
95
95
89
91
86
58
73
74
63
78
99
83
–
0
3
RuBr3
32
16
12
15
0
3
4
RuI3
4
2
5
Ru3(CO)12
Pd(OAc)2
PdCl2
0
0
6
0
1
7
0
1
8
H2PtCl6
0
0
0
9
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
Ru(acac)3
0
0
1
10
11
0
0
0
0
0
4
d)
12
0
0
2
13
14
15
16
17
0
0
14
0
0
0
19
14
9
3
6
5
3
22
0
3
e) f)
18
0
0
e)
19
0
0
0
f)
20
0
0
0
f) g)
21
0
0
0
a) Reaction conditions: quinoline 1.0 mmol, Ru(acac)3 0.5 mol%, triphos 1.0%, MSA 10 mol%, THF 2 mL, 160 °C, 16 h, CO2 2 MPa, H2 8 MPa; b) GC
yield; c) others contained mainly decahydroquinoline and 5,6,7,8-tetrahydroquinoline etc; d) water (2 mL) as solvent; e) without CO2; f) without MSA; g) 1.0
mmol of 2a was the substrate.
1
6-Methoxy-1-methyl-1,2,3,4-tetrahydroquinoline (4b): H
NMR (400 MHz, CDCl3) δ=6.58 (dd, J=2.76 Hz, 1 H), 6.49
(m, 2H), 3.64 (s, 3H), 3.03 (t, J=5.65 Hz, 2H), 2.74 (s, 3H),
2.67 (t, J=6.6 Hz, 2H), 1.89 (m, 2H). 13C NMR (101 MHz,
CDCl3) δ=150.35, 140.59, 123.59, 114.08, 111.43, 111.24,
76.36, 76.04, 75.72, 54.74, 50.62, 38.86, 26.95, 21.68.
1,6-Dimethyl-1,2,3,4-tetrahydroquinoline (4c): 1H NMR
(400 MHz, CDCl3) δ 6.90 (d, J=8.06 Hz, 1H), 6.80 (s, 1H),
6.55 (d, J=8.22 Hz, 1H), 3.17 (t, J=5.6, 2H), 2.8 (s, 3H), 2.75
(t, J=6.6 Hz, 2H), 2.23 (s, 3H),1.99 (m, 2H). 13C NMR (101
MHz, CDCl3) δ=144.77, 129.62, 127.40, 125.54, 123.09,
111.43, 51.51, 39.45, 27.73, 22.68, 20.22.
J=8.42, 2H), 3.17 (t, J=5.76 Hz, 2H), 2.9 (s, 3H), 2.71 (t,
J=6.6 Hz, 2H), 2.3 (s,3H), 1.96 (m, 2H). 13C NMR (101
MHz, CDCl3) δ=146.70, 136.61, 128.77, 120.03, 117.08,
111.84, 51.45, 39.21, 27.93, 27.53, 22.73, 21.66.
1,4-Dimethyl-1,2,3,4-tetrahydroquinoline (4f): 1H NMR
(400 MHz, CDCl3) δ 7.05 (m, 2H), 6.6 (m, 2H), 3.3–3.15 (m,
3H), 2.88 (s, 3H), 2.01 (m, 1H), 1.67 (m, 2H), 1.27 (d, J=6.8
Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 128.03, 127.79,
127.07, 127.03, 116.19, 110.97, 48.30, 39.18, 31.03, 30.85,
30.06, 22.69.
6-Chloro-1-methyl-1,2,3,4-tetrahydroquinoline (4g): 1H
NMR (400 MHz, CDCl3) δ=6.99 (dd, J=2.57, 8.8 Hz, 1H),
6.89 (s, 2H), 6.47 (d, J=8.6 Hz, 1H), 3.19 (t, J=5.6 Hz,
2H), 2.85 (s, 3H), 2.71 (t, J=6.5 Hz, 3H), 1.95 (m, 2H). 13C
1,7-Dimethyl-1,2,3,4-tetrahydroquinoline (4d): 1H NMR
(400 MHz, CDCl3) δ=6.83 (d, J=7.21 Hz, 1H), 6.42 (d,

4
He et al. Sci China Chem
NMR (101 MHz, CDCl3) δ=145.31, 128.35, 126.65, 124.45,
Both 3a and 4a were detected by using Ru(acac)3 with L7
and L8 as catalyst. However, the yield of 4a was only 3%
and 5%, respectively (entries 15 and 16). Ligand L10 gave
4a with the yield of 22% (entry 17), which was much lower
than that of L9. Thus, Ru(acac)3 with L9 catalyst had highest
activity and selectivity among the catalysts that we checked,
which was possible due to their specific configurations in
Scheme 1.
The above reactions were all performed in the presence of
MSA. To check its influence on the reaction, several control
experiments were carried out. 78% yield of 2a was obtained
in the absence of MSA and CO2, which was increased to 99%
when MSA was used, indicating that the acid could promote
the hydrogenation (Table 1, entries 18 and 19). No methy-
lated product (4a) was detected in the absence of MSA for
the N-methylation of 1a and 2a (entries 20 and 21), demon-
strating that the MSA is crucial for the N-methylation, which
was similar to that of previous report [15]. The results show
that MSA simultaneously promotes the selective hydrogena-
tion of 1a and N-methylation of 2a.
The reaction conditions of direct N-methylation of 1a with
CO2 and H2 catalyzed by Ru-triphos were optimized, and the
results are given in Table 2. The conversions of 1a were all
above 95% at the reaction temperature from 120 to 180 °C
in the presence of H2. However, the yield of 4a depended
strongly on temperature. Increasing temperature from 120 to
160 °C led to an increase in yield of 4a from 57% to 99%
(Table 2, entry 1 vs. Table 1, entry 1). However, further in-
creased the temperature resulted in a lower yield of 4a, which
was probably due to the higher temperature was not favorable
120.66, 111.90, 51.12, 39.15, 27.72, 22.23.
1
Angustureine (4h): H NMR (400 MHz, CDCl3) δ 7.0–7.1
(t, 1H), 6.90–7.10 (d, 1H), 6.55–6.65 (t, 1H), 6.45–6.55 (d,
1H), 3.2 (m, 1H), 2.9 (s, 3H), 2.7–2.85 (m, 1H), 2.6–2.7 (m,
1H), 1.9 (m, 2H), 1.5–1.6 (m, 21H), 1.2–1.4 (m, 8H), 0.8–0.9
(t, 3H); 13C NMR (101 MHz, CDCl3) δ 145.4, 128.8, 127.1,
121.9, 115.2, 110.4, 59.0, 38.0, 32.1, 31.3, 25.8, 24.5, 23.6,
22.7, 14.1.
3 Results and discussion
The complexes with different ligands (L1–L10) and metals
were tested for the reaction and the results are shown in
Table 1. Ru(acac)3 with the tridentate ligand L9 was very
active and selective for the reaction and 99% yield of 4a
could be obtained (entry 1). However, the other metal species
with L9 showed very low selectivity to 4a (entries 2–8). In
addition, Ru(acac)3 was not effective for the reaction when
combined with other ligands. Most of the products were 2a
for the monophosphine ligands such as L1–L4 (entries 9–12),
indicating that the monophosphine ligands checked with
Ru(acac)3 were inactive for the methylation of 2a with CO2
and H2. The bisphosphine ligand L5 was active but afforded
low selectivity for the formation of 4a. Only hydrogenated
product 2a was detected with 86% yield. In addition to 2a,
14% yield of the over-hydrogenated products was obtained
(entry 13). The L6 gave only the product of 2a with the yield
of 58%, indicating that it was less active for the selective
hydrogenation and inactive for the methylation (entry 14).
Table 2 Optimization of reaction conditions on the coupling of selective hydrogenation and N-methylation of quinolone a)
b)
Yield (%)
Entry
T (°C)
P
/ PH
Sol.
Conv. (%)
2(MPa)
CO
2
c)
2a
39
31
48
92
28
11
0
3a
0
0
0
0
0
1
0
5
1
0
1
0
1
4a
57
65
50
0
Others
1
2
120
140
180
160
160
160
160
160
160
160
160
160
160
2/8
2/8
2/8
0/8
0.5/8
1/8
2/0
2/4
2/6
2/8
2/8
2/8
2/8
THF
THF
THF
THF
THF
THF
THF
THF
THF
cy
96
97
98
91
90
100
0
0
0
0
0
1
0
0
0
0
0
0
0
0
3
4
5
62
89
0
6
7
8
99
99
98
96
90
97
19
7
74
91
56
53
11
75
9
10
11
12
13
42
43
79
22
d)
Tol
1,4-dioxane
e)
TMB
a) Reaction conditions: quinoline 1.0 mmol, Ru(acac)3 0.5 mol%, triphos 1.0%, MSA 10 mol%, 16 h; b) GC yield; c) others contained mainly decahydro-
quinoline and 5,6,7,8-tetrahydroquinoline etc.; d) toluene; e) 1,2,4-trimethylbenzene.

He et al. Sci China Chem
5
to the stability of the catalyst. The effect of CO2 and H2
pressure on the catalytic performances was also studied.
No methylated product was detected in the absence of CO2
(Table 2, entry 4), indicating that CO2 was the source of
methyl group. When the pressure of CO2 was 0.5 MPa, 62%
yield of 4a was obtained, which was increased to 89% under
1.0 MPa of CO2 (Table 2, entries 5 vs. 6), indicating that the
higher CO2 pressure was beneficial for the reaction. Neither
2a nor 4a was detected in the absence of H2 (Table 2, entry
7), indicating that in this reaction, the hydrogenation of 1a
firstly occurred to produce 2a which was then methylated
with CO2 and H2 to 4a. The properties of the solvent also
have an effect on the reaction (Table 2, entries 10–13). Cy-
clohexane, toluene, 1,4-dioxane and 1,2,4-trimethyltoulene
gave low yields to 4a.
The dependence of the reaction time on the catalytic perfor-
mances was also studied, and the results are given in Figure 1.
When the reaction time was 4 h, 2a was produced with 80%
yield, but only 6% of 4a was detected. A prolonged reaction
time led to a decrease in 2a yield but an increase in 4a. When
the reaction time was prolonged to 8 and 12 h, the yield of
2a was decreased to 59% and 42%, while 4a was increased
to 32% and 52%, respectively. The results indicated that 2a
could be transferred into 3a which was further reduced into
4a. This hints that the N-methylation of 1a with CO2 and H2
was achieved via three steps, i.e., the selective hydrogenation
of 1a to 2a, the N-formylation of 2a to 3a, and followed by
the reduction of 3a to 4a. In the reaction, the catalyst played
at least three roles. At beginning, it catalyzed the selective
hydrogenation of 1a to 2a, which is highly important for the
whole reaction. Because the pyridine-N cannot be directly
methylated, the selective hydrogenation of pyridine unit to
piperidine is prerequisite for the formation of 4a. Then, the
catalyst catalyzed the formation of formamide, which is
recognized as the intermediate for the N-methylation of am-
ine with CO2 [12,15]. The above results suggest that this is
the rate determining step for the whole reaction.
Generally, CO2 can be reduced to HCOOH, CO or HCHO
etc., depending upon the catalyst used and reaction conditions
operated. In this work, we also investigated other C1 sources
to find out their performances in methylation of 1a, and the
results are given in Table 3. Among the C1 sources, formic
acid gave the highest yield of 4a, indicating that CO2 may
initially be transferred into HCOOH, which was then reacted
with 2a to 3a. Furthermore, the Ru catalyst can reduce the
formylamine to methylamine. The low yields of 4a obtained
from (HCHO)n and CH3OH indicated that they could also be
transferred under the given conditions but via other routes.
Having developed this reliable method for the catalytic
synthesis of 4a, we further studied the N-methylation of
substituted quinolines with CO2 and H2 to corresponding
products. The substituted quinolines with electron-donat-
ing and electron-withdrawing groups were studied and the
results are given in Table 4. The electron-donating groups,
such as CH3O– and CH3– group in the 6- and 7-positions
of quinoline, gave 4b, 4c, and 4d with the yields of 90%,
89%, and 92%, respectively (Table 4, entries 1–3). However,
longer reaction times were required to produce 4e and 4f with
the yields of 88% and 89% respectively for the CH3– in the
3- and 4- positions of quinoline (1e and 1f, Table 4, entries
4 and 5), suggesting that their activities were lower than
that of 1a. The main reason may be that the methyl group
on the pyridine unit (3- or 4-position of quinoline) hindered
the reaction. Meanwhile, 6-chloroquinoline (1g), with an
electron-withdrawing group on quinoline, gave 85% yield
of 4g after 36 h (Table 4, entry 6). The preliminary results
demonstrated that the present method has a good tolerance in
the selective hydrogenation and N-methylation of quinolines.
To our delight, (±) angustureine (4h), one of the important
alkaloids mentioned above, could be synthesized by this
method with a high yield of 86% (Table 4, entry 7).
4 Conclusions
It has been found that Ru(acac)3-triphos complex is an very
efficient catalyst for the synthesis of MTHQs from quinolines,
Table 3 Direct N-methylation of quinoline with diverse C1 sources cat-
alyzed by Ru catalyst a)
b)
Yield (%)
Conv.
(%)
Entry
C1
2a
63
78
10
89
3a
0
4a
Others c)
d)
1
2
3
4
CO
63
95
99
99
0
0
0
9
0
(HCHO)n
HCOOH
CH3OH
0
17
80
10
0
0
a) Reaction conditions were same that entry 1 in Table 1, except for the
addition of (HCHO)n, HCOOH and CH3OH as C1 sources (3 eq.); b) GC
yield; c) others contained mainly decahydroquinoline and 5,6,7,8-tetrahy-
droquinoline, etc.; d) 2 MPa of CO was added.
Figure 1 Dependence of reaction time on the catalytic performances of
N-methylation of quinoline and CO2 with H2. Reaction conditions were sim-
ilar to that of Table 1, entry 1, except for the reaction time.

6
He et al. Sci China Chem
a)
Table 4 Ru-triphos catalyzed direct N-methylation of quinolines and CO2 with H2
b)
Entry
1
Substrate
Product
Yield (%)
90
1b
1c
1d
1e
4b
4c
4d
4e
2
3
89
92
88
c)
4
c)
5
1f
4f
89
c)
6
1g
1h
4g
4h
85
86
d)
7
a) Reaction conditions are similar to that of entry 1 in Table 1; b) isolated yield; c) the reaction time was 36 h; d) Ru(acac)3 1.5 mol%, triphos 3.0 mol%.
CO2 and H2, and satisfactory yield of desired products could
be obtained from the quinolines with methyl, methoxy and
chloride groups. This route has some obvious advantages,
such as using quinolines, CO2 and H2 as the reactants. We
believe that this simple, efficient, and greener method has
potential of application.
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Acknowledgments This work was supported by National Natural Science
Foundation of China (21603235, 21373234, 21533011), Chinese Academy
of Sciences (QYZDY-SSW-SLH013) and the Recruitment Program of
Global Youth Experts of China.
Conflict of interest
The authors declare that they have no conflict of
interest.
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