Catalytic asymmetric hydrogenation of quinoline carbocycles: Unusual chemoselectivity in the hydrogenation of quinolines

Article abstract of DOI:10.1039/c5cc01971k

The reduction of quinolines selectively took place on their carbocyclic rings to give 5,6,7,8-tetrahydroquinolines, when the hydrogenation was conducted in the presence of a Ru(η³-methallyl)₂(cod)-PhTRAP catalyst. The chiral ruthenium catalyst converted 8-substituted quinolines into chiral 5,6,7,8-tetrahydroquinolines with up to 91:9 er. This journal is

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DOI: 10.1039/C5CC01971K
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Catalytic Asymmetric Hydrogenation of Quinoline
Carbocycles: Unusual Chemoselectivity in the
Hydrogenation of Quinolines
Cite this: DOI: 10.1039/x0xx00000x
Ryoichi Kuwano,*,a,b Ryuhei Ikedaa and Kazuki Hirasadaa
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
substrate.13
hydrogenation of quinoline carbocycles to yield optically active
5,6,7,8-tetrahydroquinolines. PhTRAP–ruthenium catalyst
allows the hydrogenation of various 8-substituted quinolines to
give the corresponding tetrahydroquinolines with good
enantioselectivities.
We have developed highly enantioselective hydrogenations
of various heteroarenes with a chiral catalyst, 1–ruthenium
complex.14 In the course of our study on the asymmetric
hydrogenation, the hydrogenation of 2-phenylquinoline (2a)
was attempted by using Ru(η3-methallyl)2(cod)–1–Et3N
catalyst (Scheme 1). To our surprise, the dearomatization of 2a
exclusively took place on its carbocycle to afford 5,6,7,8-
tetrahydroquinoline 3a in 97% yield. No formation of 4a was
Here, we report
a
catalytic asymmetric
The reduction of quinolines selectively took place on their
carbocyclic rings to give 5,6,7,8-tetrahydroquinolines, when
the hydrogenation was conducted in the presence of Ru(η3-
methallyl)2(cod)–PhTRAP catalyst. The chiral ruthenium
catalyst converted 8-substituted quinolines into chiral 5,6,7,8-
tetrahydroquinolines with up to 91:9 er.
Catalytic asymmetric hydrogenation of heteroarene or arene is
an attractive approach for creating a chiral center on 5- or 6-
membered ring.1 The potential usefulness has stimulated many
chemists to develop chiral catalyst for the asymmetric reduction
of heteroarenes during the last decade. Nowadays, asymmetric
catalysis allows various heteroarenes to be converted to the
fully or partly saturated chiral heterocycles with high
detected in the reaction.
The unusual chemoselectivity
enantiomeric excesses.
As compared to heteroarenes,
stimulated us to develop the catalytic asymmetric
hydrogenation of quinoline carbocycles.
carbocyclic arenes have been unexplored as the substrates of
the catalytic asymmetric hydrogenation, because they are
highly stabilized with their aromaticity.2,3 Overcoming the
difficulty in breaking the aromaticity, Glorius successfully
developed the asymmetric hydrogenation of the carbocycles in
6-alkyl-2,3-diphenylquinoxalines, which were converted to the
corresponding 5,6,7,8-tetrahydroquinoxalines with up to 94:6
er.4 Subsequently, we reported that substituted naphthalenes
were hydrogenated with high enantioselectivities through the
chiral catalyst, which is composed of ruthenium and trans-
chelating chiral bisphosphine ligand, PhTRAP (1).5,6
Ru(η3-methallyl)2(cod) (1.0%)
1 (1.1%), Et3N (10%)
H2 (5.0 MPa), EtOAc
N
2a
Ph
N
3a
97% yield
Ph
Ph
80°C, 24 h
H
Ph2P
Me
Fe
Fe
N
H
Quinoline is the most studied substrate for the catalytic
asymmetric hydrogenation of arenes.7–9 Commonly, its pyridine
ring was exclusively reduced to give optically active 1,2,3,4-
tetrahydroquinoline, even when a chiral ruthenium complex
was used as the catalyst.10 Anomalistically, the carbocycle of
quinoline is known to be selectively reduced with hydrogen in
4a
Me
PPh2
not detected
H
(S,S)-(R,R)-PhTRAP (1)
Scheme)1%%Ruthenium<catalyzed%hydrogenation%of%2a)
11
the presence of achiral PtO2 or Chaudret’s catalyst.12 The
The hydrogenation of quinoline-6-carboxylate 2b was
carried out under the reaction condition indicated in Scheme 1
(Table 1, entry 1). The substrate 2b was completely consumed
within 24 h, but its pyridine moiety was selectively reduced to
stereoselective hydrogenation of quinoline carbocycles has
been developed by using the platinum catalyst, but the reaction
requires
a stoichiometric chiral auxiliary to modify the
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give 1,2,3,4-tetrahydroquinoline 4b as the major product.
A
accompanied by formation of 1,2,3,4-tetrahydroquinoline 4.
small amount of 5,6,7,8-tetrahydroquinoline 3b was obtained The substituent on 5-, 6-, or 7-position might somewhat hinder
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from the reaction. The enantiomeric ratio of 3b was only 66:34. the formation of 3. The enantiomeric ratios of 3f–3h were
The molar ratio of 3b to 4b and the enantiopurity of 3b scarcely moderate.
Meanwhile, the hydDrOoIg: e10n.a10ti3o9n/C5CoCf019718K-
varied in the absence of Et3N (entry 2). These results suggest methoxyquinoline (2i) took place on its carbocycle without the
that the trialkylamine might be insufficient in basicity for the reduction of its heterocycle (entry 6). The position of the
desired chemoselective hydrogenation. Various bases were methoxy group affected the enantioselectivity as well as the
evaluated for the reaction of 2b (entries 3–6). The use of a chemoselectivity. The hydrogenation product 3i was obtained
guanidine or amidine base, which is more basic than Et3N,15 with higher enantiomeric ratio than 3f–3h. The ruthenium
facilitated the hydrogenation of the carbocycle (entries 3 and 4). catalyst cleaved the benzylic C–O bond to form 3’ in 2-
The chemoselectivity was reversed when DBU was used in propanol. The undesired hydrogenolysis was completely
place of Et3N. Furthermore, alkali metal carbonates were suppressed by conducting the reaction in an aprotic solvent,
favorable for the desired hydrogenation (entries 5 and 6). The such as ethyl acetate (entry 7).
Furthermore, the
percentage of 3b rose with the increasing polarity of solvent enantioselectivity was scarcely affected by the reaction
(entries 7–11). The trans-chelating property of 16a may be temperature (entry 8). The catalyst loading can be reduced to
crucial for the selective reduction of the carbocyclic arenes. 0.5% without loss of enantioselectivity (entry 9).
The
The pyridine ring of 2b was selectively reduced when the chemoselectivity completely inverted in the hydrogenation of
hydrogenation was conducted with common bidentate 7,8-disubstituted quinoline 2j, which exclusively gave the
bisphosphines, which chelate to a transition-metal in a cis achiral 1,2,3,4-tetrahydroquinoline 4j in low yield (entry 10).
manner.16
In contrast to the chemoselectivity, the
Table 2 Hydrogenation of methoxyquinolinesa
stereoselectivity was scarcely affected by the base additive and
the solvent. Enantiomeric ratio of the hydrogenation product
remarkably increased when 6-alkylquinoline 2c was used as the
substrate, but the molar ratio of 3c to 4c was ca. 1:1 (entry 12).
The isopropyl group of 2c may sterically obstruct the
interaction between the catalyst and the carbocycle.
[Ru] (2.0%)
1 (2.2%)
5
R
R
R
K2CO3 (20%)
H2 (5.0 MPa)
24 h
N
N
N
H
N
8
3’
2
4
3
Table 1 Hydrogenation of 6-substituted quinolinesa
2d: R = 3-MeO
2e: R = 4-MeO
2f: R = 5-MeO
2g: R = 6-MeO
2h: R = 7-MeO
2i: R = 8-MeO
[Ru] (1.0%)
1 (1.1%)
base (10%)
R
R
R
H2 (5.0 MPa)
80°C, 24 h
2j: R = 8-MeO-7-Ph
N
N
N
H
2
4
3
3:3’:4b
100:0:0
100:0:0
52:40:8
75:0:25
86:0:14
86:14:0
100:0:0
100:0:0
100:0:0
0:0:24
Yield/%c
Er (3)d
–
2b: R = MeO2C
2c: R = iPr
Entry
1
2
3
4
5
6
7
2
Solvent
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
EtOAc
EtOAc
EtOAc
EtOAc
Temp/°C
80
2d
2e
2f
2g
2h
2i
2i
2i
2i
2j
91
99
40
79
84
79
90
80
94
–
80
80
80
80
80
80
60
60
–
Entry
1
2
Base
Et3N
Solvent
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
toluene
THF
i-PrOH
i-PrOH
MeOH
i-PrOH
3:4b
7:93
Er (3)c
66:34
66:34
67:33
67:33
67:33
65:35
66:34
70:30
67:33
67:33
60:40
81:19
71:29
79:21
68:32
90:10
91:9
91:9
91:9
–
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2c
2
3
4
5
6
7
8
9
–
7:93
TMGd
DBU
30:70
72:28
88:12
93:7e
8:92
64:36
93:7f
85:15
86:14
47:53h
K2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
DBU
8
9e
10f
60
a [Ru] = Ru(η3-methallyl)2(cod). b Determined by the 1H NMR analysis of
reaction mixture. The 1H NMR analysis indicated full conversion of 2 unless
otherwise noted. c Isolated yields. d Determined by HPLC analysis. e With
0.5% catalyst loading. f 24% conversion.
10
11
12g
K2CO3
DBU
a [Ru] = Ru(η3-methallyl)2(cod). b Determined by the 1H NMR analysis of
reaction mixture. The 1H NMR analysis indicated full conversion of 2 in all
entries. c Determined by HPLC analysis. d TMG = 1,1,3,3-
Tetramethylguanidine. e 3b was isolated in 86% yield. f A small amount of
isopropyl ester was formed. g For 48 h. h 3c and 4c were isolated in 33% and
46% yield, respectively.
As shown in Table 3, the 1–ruthenium catalyst converted
various 8-substituted quinolines to the corresponding chiral
5,6,7,8-tetrahydroquinolines with good enantiomeric ratios. As
with methoxyquinoline 2i, protected 8-hydroxyquinoline 2j was
hydrogenated to 3j with 90:10 er in high yield (entry 1). The
reduction of the aryl-substituted quinolines 2k–2m also
selectively took place on their carbocycles to give the desired
chiral products 3k–3m with 86:14 er (entries 2–4). The
enantiomeric ratio was scarcely affected by the electronic
property of the para-substituent in 2l or 2m. The ortho-
substituent of 2n disturbed the stereoselectivity as well as the
chemoselectivity (entry 5). For the hydrogenation of 8-
alkylquinolines, [RuCl(p-cymene)(1)]Cl exhibited higher
To investigate the effect of the position of substituent on
quinoline core, we conducted the hydrogenations of a series of
methoxyquinolines in 2-propanol with the 1–ruthenium catalyst
(Table 2). Quinolines 2d and 2e, which have a methoxy group
on the pyridine ring, were exclusively converted to achiral 3d
and 3e, respectively (entries 1 and 2).
Also 5,6,7,8-
tetrahydroquinolines 3f–3h are preferentially formed in the
hydrogenations of 2f–2h, which have a methoxy group on the
carbocyclic ring (entries 3–5). The reactions, however, were
enantioselectivity
than
in-situ-generated
Ru(η3-
methallyl)2(cod)–1 catalyst (entries 6 and 7). The preformed
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catalyst transformed 2o and 2p into 3o and 3p with 89:11 and 1,4-addition of H2 to the C5–C8 1,3-diene moiety, respectively.
91:9 er, respectively (entries 7 and 8).
In these pathways, the dearomatization of the carbocycle is
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accompanied by the chiral induction.
To confirm the
Table 3 Hydrogenation of 8-substituted quinolinesa
DOI: 10.1039/C5CC01971K
possibility of path A, the hydrogenation of 5,6-
dihydroquinoline 5k was carried out under the optimized
condition for the asymmetric hydrogenation of 2 (eqn 2). The
hydrogenation product 3k was obtained with low enantiomeric
ratio. The observed stereoselectivity rules out path A.
[Ru] (2.0%), 1 (2.2%)
K2CO3 (20%)
N
N
N
H
4j–4p
H2 (5.0 MPa)
iPrOH, 60°C, 24 h
R
R
R
2j–2p
3j–3p
Ru(η3-methallyl)2(cod) (2.0%)
1 (2.2%), K2CO3 (20%)
Entry
1e
2f
3
4
R (2)
TIPSOCH2O (2j)
Ph (2k)
4-MeOC6H4 (2l)
4-CF3C6H4 (2m)
2-MeC6H4 (2n)
Me (2o)
3:4b
100:0
96:4
100:0
100:0
71:29
96:4g
94:6g
94:6
Yield/%c
Er (3)d
(2)
N
N
97
87
94
88
56
75
71
86
90:10
86:14
86:14
86:14
70:30
88:12
89:11
91:9
H2 (5.0 MPa)
iPrOH, 60°C, 1 h
5k
3k
62% yield, er = 44:56
Ph
Ph
5
To further investigate the pathway of the hydrogenation, the
deuterations of 2i and 2k were carried out with the 1–ruthenium
catalyst. The use of D2 induced the reduction of the pyridine
ring as well as significantly decreased the reaction rate.16
Although D2 scarcely reacted with 2k, substrate 2i was
deuterated to give 3i-d in 15% yield (by 1H NMR) (eqn 3). The
deuteration of 2i was accompanied with the formations of 3’-d
and 4i-d. In 3i-d, four deuterium atoms were incorporated at
each of the 5-, 6-, 7-, and 8-positions with all cis
stereochemistry. The stereochemistry may also rule out path A
if the initial step does not proceed with a high degree of
6
7h
8h
Me (2o)
cHex (2p)
a [Ru] = Ru(η3-methallyl)2(cod). bDetermined by the 1H NMR analysis of
reaction mixture. The 1H NMR analysis indicated full conversion of 2 in all
entries. c Isolated yields. d Determined by HPLC analysis. e In EtOAc. f At
h
40 °C for 48 h. g Determined by the GC analysis of reaction mixture.
[Ru(p-cymene)(1)]Cl and DBU were used in place of Ru(η3-
methallyl)2(cod)–1 and K2CO3, respectively.
The hydrogenation product 3j was treated with TBAF to
give 8-hydroxy-5,6,7,8-tetrahydroquinoline 3q with little loss
of the enantiopurity (eqn 1).17 The optically active alcohol 3q
is useful as a chiral building block for preparing chiral
catalysts18 or various 8-substituted tetrahydroquinolines.19 The
absolute configuration of 3q was assigned to be R with the sign
of its optical rotation.
enantioface discrimination.
Furthermore, the observed
deuterium distribution suggests that the present hydrogenation
of quinoline carbocycles involves no migration of the C–C
double bond in dihydroquinoline intermediate 6 or 7.
Ru(η3-methallyl)2(cod) (2.0%)
1 (2.2%), K2CO3 (20%)
N
D2 (1.0 MPa)
TBAF
THF, rt, 5 h
iPrOH, 60°C, 24 h
2i
OMe
(1)
N
N
90% D
H
D
>80% D
TIPSOCH2O
OH
D5
D3
D
H
3j
3q
16% D
36% D
H
90% yield, er = 90:10 (R)
!
(3)
90% D
D
N
N
N
H
As shown in Scheme 2, three pathways can be speculated
for the present ruthenium-catalyzed hydrogenation of quinoline
carbocycles. In path A, the C5–C6 double bond is first
saturated with H2 to dearomatize the carbocycle, and then the
remaining C–C double bond in the resulting intermediate 5 is
enantioselectively hydrogenated with the chiral ruthenium
catalyst to give the optically active product 3. Path B or C
starts from the hydrogenation of the C7–C8 double bond or the
3’-d
D
OMe
91% D
3i-d
4i-d
OMe
6% yield
15% yield, er = 89:11
16% yield
In conclusion, the PhTRAP–ruthenium complex allows the
hydrogenation of quinolines 2 to selectively produce 5,6,7,8-
tetrahydroqunolines 3. The unusual chemoselectivity may be
caused by the trans-chelating property of the chiral ligand.
Various 8-substituted quinolines were converted to the
corresponding products 3 with good enantiomeric ratio (up to
91:9). Additionally, some experimental results suggested that
the aromaticity-breaking step is accompanied by the chiral
induction in the present asymmetric hydrogenation of quinoline
carbocycles, while the chiral induction took place in the
reduction of alkenyl ether intermediate in the asymmetric
hydrogenation of naphthalenes reported by us.5 Further
mechanistic studies are in progress.
N
path A
H2
H2
5
R
R
R
5
H2
H2
N
N
N
path B
8
This work was partly supported by ACT-C (JST). We
thank the Cooperative Research Program of “Network Joint
Research Center for Material and Devices” for HRMS
measurement.
R
R
2
3
6
H2
H2
path C
N
7
Notes and references
Scheme2 2! ! Three! possible! pathways! for! the! hydrogenation! of! quinoline!
carbocycles.!
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a
Department of Chemistry, Graduate School of Sciences, and
International Research Center for Molecular Systems (IRCMS), Kyushu
University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
b JST ACT-C, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
2012, 134, 2442; (m) X.-F. Cai, R.-N. Guo, M.-W. Chen, L. Shi and
Y.-G. Zhou, Chem. Eur. J., 2014, 20, 7245.
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DOI: 10.1039/C5CC01971K
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iridium and ruthenium, see: (a) C. Wang, C. Li, X. Wu, A. Pettman
and J. Xiao, Angew. Chem. Int. Ed., 2009, 48, 6524; (b) X.-F. Tu and
L.-Z. Gong, Angew. Chem. Int. Ed., 2012, 51, 11346; (c) X.-F. Cai,
W.-X. Huang, Z.-P. Chen and Y.-G. Zhou, Chem. Commun., 2014,
50, 9588.
†
Electronic Supplementary Information (ESI) available: Experimental
details and analytical data. See DOI: 10.1039/c000000x/
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