Highly enantioselective hydrogenation of new 2-functionalized quinoline derivatives

Article abstract of DOI:10.1016/j.tetlet.2012.06.114

The asymmetric hydrogenation of a new series of 2-functionalized quinolines has been developed in the presence of in situ generated catalysts obtained from [Ir(cod)Cl]₂/(R)-bisphosphine/I₂ combinations. The enantioselectivity levels

Full text of DOI:10.1016/j.tetlet.2012.06.114

Tetrahedron Letters 53 (2012) 4747–4750
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Tetrahedron Letters
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Highly enantioselective hydrogenation of new 2-functionalized
quinoline derivatives
Anna M. Maj a,c, Isabelle Suisse a,b,c, Catherine Méliet a,c, Christophe Hardouin e,
a,c,d,
⇑
Francine Agbossou-Niedercorn
a
Université Lille Nord de France, F-59000 Lille, France
Université Lille1 Sciences et Technologies, F-59655 Villeneuve d’Ascq, France
ENSCL, CCM-CCCF, F-59652 Villeneuve d’Ascq, France
CNRS, UCCS UMR 8181, F-59655 Villeneuve d’Ascq, France
Oril Industrie, 13, rue Auguste Desgenétais, 76210 Bolbec, France
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
The asymmetric hydrogenation of a new series of 2-functionalized quinolines has been developed in the
Received 23 May 2012
Revised 25 June 2012
Accepted 26 June 2012
Available online 2 July 2012
2 2
presence of in situ generated catalysts obtained from [Ir(cod)Cl] /(R)-bisphosphine/I combinations. The
enantioselectivity levels were as high as 84–94% ee for the synthesis of 1,2,3,4-tetrahydroquinolines.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Asymmetric catalysis
Hydrogenation
Quinolines
Iridium
Optically active substituted tetrahydroquinolines constitute the
principal structural unit of many natural alkaloids which display a
wide range of physiological activities.1 In addition, they are very
useful synthetic intermediates for the preparation of biologically
active compounds for pharmaceutical, agrochemical, and fine
O
O
N
CH3
N
CH3
2
chemical industries. (À)-Angustureine and (À)-Galipinine (Fig. 1)
(
-)-Angustureine
(
-)-Galipinine
which show, for example, antiplasmodial and cytotoxic activities,
respectively, are two members of the array of chiral 2-substituted
tetrahydroquinolines of interest.2 The catalytic asymmetric
hydrogenation of quinolines provides a very convenient and
straightforward access to non-racemic substituted tetrahydroquin-
Figure 1. Selected bioactive quinoline derivatives isolated from Galipea officinalis.
,3
4
–8
olines.
Successful asymmetric hydrogenation of quinoline deriv-
Cs CO3
2
atives with iridium catalysts modified by chiral diphosphines and
5
i-PrOH or
n-PrOH
assisted by iodine has been demonstrated. Other efficient chiral
N
COOC H7
3
N
COOCH3
6
7
iridium as well as ruthenium hydrogenation catalysts and orga-
1
3
or
4
8
nometallic and organic based transfer hydrogenation processes
have also been reported. Regardless of the recent progresses, highly
enantioselective hydrogenation of various functionalized quino-
lines still remains a challenge. Previous reports have focused gener-
ally on aryl, alkyl, and benzyl 2-substituted quinolines. However, a
PBr3
N
N
CH OH
CH Cl
2 2
2
N
6
CH Br
2
5
6
HN(Boc)2
K2CO3
NBoc2
CH Br
2
N
⇑
Corresponding author. Tel.: +33 (0)320674927; fax: +33 (0)320674488.
E-mail addresses: anna.maj@ensc-lille.fr (A.M. Maj), isabelle.suisse@ensc-lille.fr
7
(
I. Suisse), meliet@ensc-lille.fr (C. Méliet), christophe.hardouin@fr.netgrs.com (C.
Hardouin), agbossou@ensc-lille.fr (F. Agbossou-Niedercorn).
Scheme 1. Synthesis of the new substrates.
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.114

4
748
A. M. Maj et al. / Tetrahedron Letters 53 (2012) 4747–4750
larger substrates scope is desirable in order to increase the promise
applications of this methodology.
methyl quinoline 2-carboxylate 1. The n-propyl quinoline 2-car-
boxylate 3 and isopropyl quinoline 2-carboxylate 4 esters were ob-
tained by transesterification of methyl ester 1 in the presence of a
catalytic amount of cesium carbonate in the appropriate alcohol.
As already described, quinolin-2-yl methanol 5 was obtained by
classical reduction of methyl ester 1 in the presence of sodium
Following our interest in the asymmetric hydrogenation of aro-
9
matic heterocycles, we sought to examine new 2-substituted quin-
olines which would present a high synthetic potential for further
elaboration into valuable molecules. For this purpose, we explored
5
a
2
-substituted quinolines bearing esters (1–4), hydroxymethyl (5),
borohydride. Substitution of the hydroxy moiety by a bromide
in the presence of PBr led to 2-(bromomethyl)quinoline 6. The
bromomethyl (6), and protected amine (7) groups which represent
especially convenient functionalities for organic synthesis.
The syntheses of the new 2-functionalized quinolines are
shown in Scheme 1. They have been performed from commercial
3
synthesis of the amino protected quinoline 7 has been performed
from 2-(bromomethyl)quinoline 6 by reaction with di-tert-butyli-
mino-dicarboxylate in the presence of potassium carbonate afford-
ing the N,N-diBoc-quinolin-2ylmethanamine 7.
For our preliminary hydrogenation experiments, we chose
methyl quinoline 2-carboxylate 1 as the substrate and varied the
chiral ligand under the standard reaction conditions (toluene as
Table 1
a
Asymmetric hydrogenation of quinoline-2-carboxylates 1-4
5a,5l
2 2
the solvent, I as the additive, 50 bar H , 20 °C) (Table 1).
[
Ir(cod)Cl]2
Among the large array of ligands employed for screening, members
of MeO-Biphep family L1–L5, Synphos L6, Difluorphos L7, and P-
Phos L8 and L9 furnished the most promising results (Fig. 2, Table
chiral ligand
*
O
O
N
H
R
N
R
I2
O
O
toluene, rt
1). The reactions proceeded smoothly with high conversions. The
1
-4
8-11
highest enantioselectivities of 72–74% ee were achieved by apply-
ing ligands L7 and L8 (entries 8 and 9). The transfer of the chiral
information is related to the dihedral angle of the biaryl and the
stereoelectronic properties of the P-substituents of the ligands.
b
c
c
Entry
Substrate
Ligand
Conv. (%)
Yield (%)
ee (%)
d
1
R = Me, 1
L1
L1
L2
L3
L4
L5
L6
L7
L8
L8
L9
L7
L8
L7
L7
L7
100
100
98
75
99
88
88
97
70
99
91
98
95
82
89
87
94
94
66
53
49
38
21
50
25
74
72
72
55
94
88
85
90
75
d,e
2
5
g
3
4
5
6
As already highlighted by Chan, the catalysts bearing P-Phos li-
gand L8 could be prepared under air without pre-degassing and
drying of the solvent, it displayed similar activity and enantioselec-
tivity as if prepared under inert atmosphere (entry 10 vs 9). How-
ever, an erosion of the enantioselectivity was observed when the
catalyst bearing L1 was prepared under air (entry 2 vs 1).5
Next, we examined the hydrogenation of three other esters 2–4
with bulkier groups in the presence of Difluorphos L7 and P-Phos
L8 ligands (entries 12–16). The selectivity of hydrogenation of
ethyl ester 2 increased significantly compared to results obtained
with the methyl congener 1 (from 74 to 94% ee in the presence
of L7) (entries 8 and 12). Nonetheless, a further increase of the
bulkiness of the ester from ethyl to propyl groups resulted in a sub-
stantial erosion of the enantioselectivity. Actually, the hydrogena-
tion of n- and i-propyl esters 3 and 4 proceeded with 90% and 75%
ee, respectively (entries 15 and 16).
97
d
7
100
100
88
94
95
99
99
100
100
100
8
g,5l
9
e
10
11
12
13
14
15
16
R = Et, 2
e
100
100
100
R = n-Pr, 3
R = i-Pr, 4
a
All reactions were performed on
H2 = 50 bar; 20 °C; toluene 7 mL; 17 h.
2
a 1 mmol scale; S/Ir/L/I = 100/1/1.1/5;
P
b
Conversion determined by GC and RMN.
Yield determined by HPLC analysis on Chiralcel OJ-H column (hexane/i-PrOH
0:30, 1 mL/min).
c
7
d
It is important to mention that for methyl (8) and ethyl (9) ester
hydrogenation products, we observe the slow dehydrogenative
3
0 h.
e
The catalyst was prepared under air.
Cl
O
MeO
MeO
PAr2
PAr2
MeO
MeO
PPh2 MeO
PPh2 MeO
PR2
PR2
O
O
PPh2
PPh2
Cl
O
Ar = Ph, (R)-MeO-Biphep L1
Ar = 3,4,5-(MeO) C H , L2
L3
R = iPr, L4
(R)-Synphos, L6
O
3
6
2
R =
,
L5
OMe
O
O
F
F
N
N
O
O
O
O
PPh2
PPh2
MeO
MeO
PAr2
PAr2
PPh2
PPh2
F
F
O
O
OMe
(R)-Difluorphos, L7
Ar = Ph, (R)-P-Phos, L8
Ar = 3,5-(Me) C H , (R)-Xyl-P-Phos, L9
(R)-Segphos, L10
2
6
3
Figure 2. Bisphosphine ligands used.

A. M. Maj et al. / Tetrahedron Letters 53 (2012) 4747–4750
4749
Table 2
down as 77% yield was obtained within 17 h (entry 5). It is not
straightforward to observe a likely harmful effect on the stability
of the catalyst especially in the presence of isopropanol as the yield
a
Asymmetric hydrogenation of 5
[
Ir(cod)Cl]2
5
a,5e
chiral ligand
and ee remained high.
*
OH
OH
N
We then explored the hydrogenation of the 2-bromomethyl
substituted quinoline 6 (Table 3). Beside the expected hydrogena-
tion product 13, we observed the formation of the 2-methyl-
I2
N
H
1
solvent
2
5
1
,2,3,4-tetrahydroquinoline 14 resulting from bromide cleavage.
b
c
c
Entry
1
Ligand
Solvent
Conv. (%)
Yield (%)
ee (%)
The result exhibiting the best compromise between yield and
enantioselectivity into 13 was obtained in the presence of ligand
L8 (entry 2). The reaction provided 13 in 100% yield and with
L7
Toluene
Toluene
4
100
100
97
77
100
99
4
100
100
97
77
100
99
nd
83
75
84
81
70
70
78
80
d
2
3
4
Toluene/MeOH
Toluene/i-PrOH
Toluene/i-PrOH
Toluene/i-PrOH
Toluene/i-PrOH
Toluene/i-PrOH
Toluene/i-PrOH
8
1% of ee. The constant higher ee obtained for 14 suggests that
e
the cleavage of the bromide is occurring most probably prior to
hydrogenation of the quinoline heterocycle. Indeed, the results
are close to those obtained for the hydrogenation of 2-methyl
5
f
6
L9
L2
L8
L10
7
f
8
97
100
97
100
5k
substituted quinoline. When using the corresponding 2-chloro-
f
9
ethyl substituted quinoline as a substrate, we observed a system-
atic cleavage of the chloride and could not find conditions
providing selectively the hydrogenated chloroethyl product.
Finally, we concentrated on the nitrogen substituted quinoline
7 which could be of interest for preparing chiral aminoquinolines
(Table 4). The N,N-diBoc protected substrate 7 was easily synthe-
sized from the bromo substituted quinoline 6 (Scheme 1) and
provides a direct access to the corresponding amino tetrahydro-
quinoline after hydrogenation and deprotection. The hydrogena-
tions were carried out under our standard conditions with
different ligands. Thus, the hydrogenation of 7 could be performed
a
All reactions were performed on
2
a 1 mmol scale; S/Ir/L/I = 100/1/1.1/5;
P
H2 = 50 bar; 20 °C; 17 h; solvent 7 mL, toluene/MeOH or toluene/i-PrOH = 8/1.
b
Conversion determined by GC and RMN.
Yield determined by HPLC analysis on a Chiralcel OD column (hexane/i-PrOH
c
9
0:10, 1 mL/min).
d
2
1
3
0 °C, 10 days;
0 °C, 17 h;
0 °C; 17 h
e
f
aromatization of the nitrogen heterocycle leading back to sub-
strates 1 and 2 when the products were stored in solution under
the light.
1
0
Afterward, we considered the asymmetric hydrogenation of 2-
hydroxymethyl quinoline 5 under our standard reaction conditions
Table 4
a
Asymmetric hydrogenation of 7
(Table 2).
Due to the low solubility of 5, the hydrogenation in neat toluene
[Ir(cod)Cl]2
was sluggish and required about 10 days to go to completion. The
hydrogenation product was however obtained with 83% ee (entry 2
vs 1). The addition of methanol allowed improving the solubility of
chiral ligand
*
N
R
N
H
2
R
I2
R = CH NBoc2
7
toluene
15
2
R = CH NBoc
5
and consequently the efficiency of the hydrogenation into 12 (en-
2
try 3). Thus, when varying the ratio of toluene and methanol, we
found an optimal combination of 8:1. Even if the reaction could
go to completion within 17 h, the selectivity into 12 dropped from
b
c
c
Entry
Substrate
Ligand
Conv. (%)
Yield (%)
ee (%)
1
2
3
4
7
L7
L8
L10
100
100
100
100
100
100
100
100
83
85
87
91
8
3 to 75% ee compared to results obtained in neat toluene (entry 3
vs 2). The substitution of MeOH by iPrOH allowed an increase of
the enantioselectivity from 75% to 84% ee (entry 4). This result is
the best among all attempts while varying further the chiral ligand
d
a
All reactions were performed on
H2 = 50 bar; 20 °C; toluene 7 mL; 17 h.
Conversion determined by GC and RMN.
2
a 1 mmol scale; S/Ir/L/I = 100/1/1.1/5;
P
(
entries 6–9 vs 4). The hydrogenation of 5 has been reported only
b
once by Zhou in neat iPrOH but with a lower enantioselectivity
c
Yield determined by HPLC analysis: Chiralcel OD column (hexane/i-PrOH 98:2,
0.2 mL/min).
5
a
(
75% ee). A lowering of the reaction temperature to 10 °C had a
d
0
°C, 80 bar.
slight influence on the enantioselectivity and the reaction slowed
Table 3
a
Asymmetric hydrogenation of 6
[Ir(cod)Cl]2
chiral ligand
*
Br
+
*
Br
I2
N
N
H
14
N
6
H
solvent
13
b
c
c
c
c
Entry
Ligand
Conv. (%)
Yield 13 (%)
Yield 14 (%)
ee 13 (%)
ee 14 (%)
d
1
L7
L8
100
100
100
100
65
100
55
30
—
45
56
69
81
45
79
90
—
82
84
2
3
e
4
L10
44
a
b
c
All reactions were performed on a 1 mmol scale; S/Ir/L/I
Conversion determined by GC and RMN.
Yield determined by HPLC analysis on Chiralcel OD column (hexane/i-PrOH 98:2, 1.5 mL/min).
2
= 100/1/1.1/5; PH2 = 30 bar; 20 °C; 17 h; toluene/i-PrOH 8:1; 7 mL.
d
e
5
2
0 bar H
0 bar H
2
.
.
2

4
750
A. M. Maj et al. / Tetrahedron Letters 53 (2012) 4747–4750
in an excellent yield (100%) and with a high enantioselectivity of
7% with Segphos L10 (entry 3). By lowering temperature to 0 °C,
the ee could be increased up to 91% with always a complete con-
5. Examples of Iridium catalyzed hydrogenation of quinolines assisted by I2: (a)
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8
2
2
003, 125, 10536–10537; (b) Lu, S.-M.; Han, X.-W.; Zhou, Y.-G. Adv. Synth. Catal.
004, 346, 909–912; (c) Reetz, M. T.; Li, X. Chem. Commun. 2006, 2159–2160;
version (entry 4).
In conclusion we have developed the highly enantioselective
asymmetric hydrogenation of a range of 2-functionalized quino-
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and bromide substituents. This methodology provides an access to
synthetically useful chiral tetrahydroquinolines with excellent
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3
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