Hydrogenation of quinolines using a recyclable phosphine-free chiral cationic ruthenium catalyst: Enhancement of catalyst stability and selectivity in an ionic liquid

Article abstract of DOI:10.1002/anie.200802237

(Chemical Equation Presented) Liquid assets: The catalyst (S,S)-1 exhibits an unprecedented reactivity and excellent enantioselectivity for the title reaction when it is carried out in neat ionic liquid (see scheme; BMIM=1-n-butyl-3-methylimidazolium, Tf=trifluoromethanesulfonyl, Ts=4-toluenesulfonyl). The ionic liquid facilitates the catalyst recycling and enhances its stability and selectivity.

Full text of DOI:10.1002/anie.200802237

Communications
DOI: 10.1002/anie.200802237
Asymmetric Catalysis
Hydrogenation of Quinolines Using a Recyclable Phosphine-Free
Chiral Cationic Ruthenium Catalyst: Enhancement of Catalyst
Stability and Selectivity in an Ionic Liquid**
Haifeng Zhou, Zhiwei Li, Zhijian Wang, Tianli Wang, Lijin Xu, Yanmei He, Qing-Hua Fan,*
Jie Pan, Lianquan Gu, and Albert S. C. Chan*
Dedicated to Professor Zhi-Tang Huang on the occasion of his 80th birthday
Room-temperature ionic liquids (RTILs) have recently
received a great deal of attention as alternative reaction
However, all catalysts for such reactions have at least one
phosphine ligand around the metal center and are often air
sensitive. From the viewpoints of both scientific interest and
[1]
[7]
media. Numerous catalytic reactions have proven feasible in
a variety of ionic liquids, with many reactions displaying
enhanced reactivities and selectivities, and some of which
practical application, it is highly desirable to develop recy-
clable and phosphine-free chiral catalysts for the highly
enantioselective hydrogenation of quinolines. Few examples
of phosphine-free homogeneous catalysts capable of activat-
[
2]
were not possible in common organic solvents. Furthermore,
RTILs have served as a promising means to immobilize a
catalyst, therefore facilitating product isolation and offering
an opportunity to reuse the catalyst. However, the use of
[8]
ing molecular hydrogen have been reported. Recently,
6
Noyori and co-workers reported that chiral h -arene/N-
[
1f,g,2f,3]
II
RTILs in asymmetric catalytic reactions is still limited.
tosylethylenediamine–Ru complexes (which are known as
The recycling and reuse of chiral catalysts in ionic liquids have
often been problematic because of the instability and/or
leaching of the catalysts. From a practical standpoint, devel-
opment of highly effective and recyclable catalysts in ionic
liquids for use in asymmetric hydrogenation remains a
challenge: in particular for heteroaromatic substrates which
are difficult to hydrogenate.
excellent catalysts for asymmetric transfer hydrogenation, for
example Ru/Ts-dpen) can be used for the asymmetric hydro-
genation of prochiral ketones under slightly acidic condi-
[
9]
tions. Inspired by this important breakthrough and follow-
ing our continued pursuit of developing effective and
environmentally benign catalyst systems for asymmetric
[10]
hydrogenations, herein we report a practical and efficient
Although a variety of chiral Rh, Ru, and Ir complexes
have been efficient and enantioselective reagents for the
hydrogenation of prochiral olefins, ketones, and imines,
catalyst system of Ru/Ts-dpen in [BMIM]PF (BMIM = 1-n-
butyl-3-methylimidazolium) for the enantioselective hydro-
genation of quinolines (Scheme 1).
6
[
4]
most of these catalysts failed to give satisfactory results in
the asymmetric hydrogenation of heteroaromatic com-
[
5]
pounds. A few successful examples for the asymmetric
[6]
hydrogenation of quinolines have recently been reported.
[
*] Dr. H. Zhou, Z. Li, Z. Wang, T. Wang, Y. He, Prof. Q.-H. Fan, J. Pan
Beijing National Laboratory for Molecular Sciences
Center for Chemical Biology, Institute of Chemistry
Chinese Academy of Sciences, Beijing 100190 (China)
E-mail: fanqh@iccas.ac.cn
Prof. L. Xu, Prof. A. S. C. Chan
Institute of Molecular Technology for Drug Discovery and Synthesis
and Department of Applied Biology & Chemical Technology
The Hong Kong Polytechnic University, Hong Kong (China)
E-mail: bcachan@polyu.edu.hk
Scheme 1. Asymmetric hydrogenation of quinoline 2a catalyzed by Ru/
Ts-dpen catalysts. Ts-dpen=N-(p-toluenesulfonyl)-1,2-diphenylethylene-
diamine.
Dr. H. Zhou, Prof. L. Gu
School of Pharmaceutical Science, Sun Yat-Sen University
Guangzhou, 510275 (China)
We have found that quinoline substrates could be
efficiently hydrogenated to give 1,2,3,4-tetrahydroquinolines
with excellent enantioselectivities (up to 99% ee) in neat
ionic liquid without the need for additives. Furthermore, the
catalyst, which was highly stable in ionic liquid and main-
tained the same activity even after exposure to air for 30 days,
could be easily recycled.
[
**] Financial support from the National Natural Science Foundation of
China (grant no. 20325209, 20418002 and 20532010), the Major
State Basic Research Development Program of China (grant no.
2005CCA06600), the Chinese Academy of Sciences, the National
Natural Science Foundation/Hong Kong Research Grants Council
Joint Research Scheme (N_PolyU506/04), and Hong Kong UGC
Areas of Excellence Scheme (AOE/P-10/01) is greatly appreciated.
Considering that methanol has been successfully used in
the asymmetric hydrogenation of ketones by Noyori and co-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802237.
[
9]
workers, we first examined the asymmetric hydrogenation
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Angewandte
Chemie
of 2-methylquinoline (2a) catalyzed by (S,S)-1a in methanol.
High enantioselectivity and conversion were obtained at
room temperature under 50 atmospheres of hydrogen with a
substrate/catalyst ratio of 100:1 (Table 1, entry 1). To our
knowledge, this is the first example of a highly enantioselec-
tive hydrogenation of heteroaromatic compounds catalyzed
by phosphine-free transition-metal complexes.
and enantioselectivity both maintained up until the sixth run
(Table 2, runs 1–6), and a slight decrease in reactivity was
observed thereafter (Table 2, runs 7 and 8). No signals
1
associated with the IL were detected in H NMR spectra of
[
a]
Table 2: Reuse of (S,S)-1a in the asymmetric hydrogenation of 2a.
[
b]
[c]
[d]
Run
Conversion [%]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
100
100
100
100
100
100
88
91
97
96
97
96
95
84
80
99
99
99
98
98
98
98
97
Table 1: Asymmetric hydrogenation of 2a catalyzed by (S,S)-1a or (S,S)-
[
a]
1
b.
[
b]
[c]
Entry Cat. Solvent
Reaction conditions Conv. [%]
ee [%]
H [atm]
2
T [8C]
[
d]
[d]
1
2
3
4
5
6
7
8
1a MeOH
1a [BMIM]PF6
1b [BMIM]PF6
50
50
50
50
20
80
50
50
25
25
25
25
25
25
50
25
100(100)
100 (30)
<5
<5
54
100
100
100
94(96)
[
d]
[d]
99(À50)
n.d.
n.d.
98
82
[
e]
f]
1
1a
1a
1a
[BMIM]PF6
[BMIM]PF6
[BMIM]PF6
[a] Reaction conditions: see Table 1, entry 2. [b] Determined by H NMR
spectroscopy. [c] Yield of isolated product. [d] Determined by HPLC on a
chiral stationary phase.
99
98
99
1a [BMIM]PF6
1a [BMIM]PF6
[
[
a] Reaction conditions: 2a (0.20 mmol) in [BMIM]PF6 or MeOH
1.0 mL), (S,S)-1 (1.0 mol%), 258C, 15–24 h. [b] Determined by
the extracted crude product. Inductively coupled plasma
(ICP) spectroscopy further showed that no significant leach-
ing of Ru occurred during the extraction of organic products.
From the ICP experiments, we estimated that less than 0.4%
(
1
H NMR spectroscopy. [c] Determined by HPLC on a chiral stationary
phase. The predominant product has the S configuration. [d] Data in
brackets were obtained by using acetophenone as the substrate under
+
À
of the Ru catalyst had leached from the [BMIM]PF into the
6
otherwise identical reaction conditions. [e] BnMe N Cl (5.0 mol%) was
3
hexane extract. These results further demonstrated the high
stability of the Ru/Ts-dpen catalyst in an ionic liquid.
Next, we examined a variety of substituted quinoline
derivatives in asymmetric hydrogenation reactions that were
added. [f] The catalyst was stored in [BMIM]PF in air for 30 days after
the first run and then used for the reaction. n.d.=not determined.
6
When methanol was replaced by neat [BMIM]PF , the
catalyzed by (S,S)-1a in [BMIM]PF (Table 3). In general, all
6
6
hydrogenation of 2a proceeded smoothly to give the partially
reduced product 3a in quantitative yield and with an even
higher enantioselectivity (94% vs. 99% ee, compare Table 1,
entry 1 vs. 2). This result was of particular interest because the
hydrogenation of acetophenone was found to be sluggish in
2-alkyl-substituted quinolines were hydrogenated with good
yields and remarkable enantioselectivities (up to 99% ee;
Table 3, entries 1–13). The reaction was found to be insensi-
tive to the length of the alkyl side chain as well as the
existence of free hydroxy groups on the side chain (Table 3,
entries 1–10). Substitution at the 6-position had no obvious
effect on either yield or enantioselectivity (Table 3,
entries 11–14). For example, the hydrogenation of 2k pro-
ceeded smoothly in good yield and with much higher
enantioselectivity than that obtained with the Ir/MeO-
biphep catalyst (99% (Table 3, entry 11 ) vs. 84% ee;
[
BMIM]PF , and afforded the alcohol in a much lower
6
enantioselectivity with the opposite configuration (96% vs.
[
11]
À50% ee, compare Table 1, entry 1 vs. 2). Notably, catalysts
S,S)-1b (with chloride as the counterion) or (S,S)-1a (with
(
+
À
BnMe N Cl as the additive) showed no reactivity in
3
[
BMIM]PF (Table 1, entries 3 and 4). The enantioselectivity
6
[
6a]
of this reaction was insensitive to the H pressure and the
biphep = 2,2’-bis(phosphanyl)biphenyl). Interestingly, sub-
strate 2n bearing an acetyl group at the 6-position could be
selectively hydrogenated to tetrahydroquinoline 3n in good
yield with 96% ee (Table 3, entry 14). Hydrogenation of 2n
under high pressure (100 atm) gave only slightly lower yield
and enantioselectivity. These results indicated that the hydro-
genation with (S,S)-1a in ionic liquid was selective for C=N
2
temperature (Table 1, entries 5–7). A low conversion was
obtained under relatively lower pressure (Table 1, entry 5).
Even after exposing the catalyst solution to air for 30 days, the
reused catalyst gave almost the same activity and enantiose-
[
12]
lectivity (Table 1, entry 8). However, under identical reac-
tion conditions, catalyst (S,S)-1a in methanol decomposed
within one week. The dramatic enhancement of the catalyst
stability by an ionic liquid was probably a result of the
[
15]
(quinoline) over C=O bonds. Significantly, the enantiose-
lectivities for the hydrogenation of these substrates are
among the highest reported to date.
solvation effect of [BMIM]PF on the cationic Ru catalyst and
6
[
13,14]
the very low solubility of oxygen in the ionic liquid.
To further investigate whether the catalytic pathway
follows the same concerted mechanism proposed for the
The recyclability of the Ru catalyst (S,S)-1a in
[
9b]
[
BMIM]PF was then examined by using 2a as the model
reduction of ketones, we carried out the hydrogenation of
2a with a stoichiometric amount of hydride complex (R,R)-4
(Table 4). To our surprise, no reaction between (R,R)-4 and
2a occurred, even with an excess of the hydride (5 equiv) after
12 h (Table 4, entry 1). Instead, the protonated species of 2a
reacted smoothly with the hydride (R,R)-4 to give the reduced
6
substrate. Upon completion of the reaction, the reduced
product was easily separated by extraction with n-hexane. The
ionic liquid phase was recharged with 2a and again subjected
to hydrogenation under the same reaction conditions. The Ru
catalyst was reused eight times, with retention of reactivity
Angew. Chem. Int. Ed. 2008, 47, 8464 –8467
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Communications
Table 3: Enantioselective hydrogenation of quinoline derivatives using (S,S)- Table 4: Stoichiometric reaction between 2a and hydride complex (R,R)-
[
a]
1
a.
4 (where X=H) in [BMIM]PF6.
[
a]
[b]
Entry Reactants
Additive
none
Product Conv. [%] ee [%]
1
2
3
2a+
R,R)-4
(5 equiv)
2a·TfOH+
(R,R)-4
(5 equiv)
2a·TfOH+
(R,R)-4
no
–
–
(
reaction
1
2
[b]
[c]
Entry Substrate
R
R
Product Yield [%] ee [%]
none
3a
3a
48
95
99
99
1
2
3
2a
2b
2c
H
H
H
Me
Et
3a
3b
3c
96
95
95
99
[
[
[
d]
d]
d]
(99)
98
3e·TfOH
(1 equiv)
(98)
nPr
99
(5 equiv)
(98)
1
[
e]
[a] Determined by H NMR spectroscopy. [b] Determined by HPLC on a
chiral stationary phase.
4
5
2d
2e
H
H
nBu
n-pentyl
3d
3e
95
97
98
98
6
7
2 f
H
H
3 f
96
96
97
97
2g
3g
susceptible to 1,2-hydride transfer, to give the final product 3a
[
19]
and 1a. Recently, Ru-catalyzed ionic hydrogenation
of
8
9
2h
2i
H
H
3h
3i
96
92
98
99
iminium cations has been proposed by Norton and co-
[8d]
workers where the iminium ions were reduced through a
+
À
stepwise H /H transfer process.
In conclusion, we have demonstrated the first phosphine-
free cationic Ru/Ts-dpen catalyst to exhibit unprecedented
reactivity and high enantioselectivity in the hydrogenation of
quinolines in neat ionic liquid. The use of ionic liquid not only
facilitates the recyclability, but also enhances the stability and
selectivity of the catalyst. Detailed mechanistic study and
further exploration of this catalyst for the hydrogenation of
other heteroaromatic compounds are in progress.
1
0
2j
H
3j
93
99
1
1
1
1
1
2
3
4
2k
2l
2m
2n
MeO
Me
F
Me
Me
Me
3k
3l
3m
3n
91
94
92
87 (81)
99
99
98
96
[
e]
[f]
CH CO Me
3
[f]
(95)
[a] Reaction conditions: substrate (0.20 mmol) in [BMIM]PF6 (1.0 mL),
(S,S)-1a (1.0 mol%), H2 (50 atm), 258C, 15–24 h. [b] Yield of isolated
product. [c] Determined by HPLC on a chiral stationary phase. Products 3a–
c, 3e–h, and 3k–m have the S configuration, whereas 3d, 3i,j, and 3n have
the R configuration. [d] Data in brackets were obtained by employing the
Experimental Section
A typical procedure for the asymmetric hydrogenation of quinolines
reused catalyst which was stored in [BMIM]PF in air for 30 days before the
6
(Table 1, entry 2) and the catalyst recycling: A 50 mL glass-lined
second run. [e] (R,R)-1a was used. [f] Data in bracket were obtained under
stainless-steel reactor equipped with a magnetic stirrer bar was
1
00 atmospheres of H2.
product in 48% yield with identical enantioselectivity to
that of the catalytic reaction (Table 4, entry 2). The
reaction mixture retained a yellow color throughout,
indicating the absence of the 16-electron amide complex
(
purple). Furthermore, in the presence of a tetrahydro-
quinoline salt proton source (1 equiv; which was
generated by mixing 3e with TfOH), the protonated
2
a reacted with the hydride (R,R)-4 to give the reduced
product in 95% yield and 99% ee (Table 4, entry 3).
On the basis of these observations, we speculate that
the hydrogenation of quinoline occurs by a different
mechanism to that of the hydrogenation of ketone in
methanol. A proposed ionic catalytic pathway is shown
in Scheme 2. In this mechanism, dihydrogen is first
coordinated to the cationic 1a and the complex formed
is then deprotonated by 2a to generate the active
[16]
monohydride complex 4 and the activated 2a.
A
subsequent 1,4-hydride transfer affords the enamine 5
and the regenerated catalyst 1a. Similarly, 5 is activated
Scheme 2. Proposed mechanism for the hydrogenation of 2a catalyzed by 1a
in an ionic liquid. The substituents on the arene and ethylenediamine ligands
are omitted for clarity.
[
17,18]
by protonation to form an iminium cation,
which is
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Angewandte
Chemie
charged with Ru catalyst ((S,S)-1a, 1.5 mg, 0.002 mmol) and 2-
Kashiwabara, M. Ohsumi, H. Kusano, J. Am. Chem. Soc. 2008,
methylquinoline (2a; 28.6 mg, 0.20 mmol) in [BMIM]PF (1.0 mL).
130, 808.
6
The autoclave was closed, and the final pressure of the hydrogen gas
was adjusted to 50 atmospheres after purging the autoclave with
hydrogen gas several times. The reaction mixture was stirred at room
temperature for a predetermined period of time. After the autoclave
was depressurized, the hydrogenated product was extracted with n-
hexane or diethyl ether (6 ꢀ 1 mL) before being passed through a
short column of silica gel. The residual ionic liquid phase containing
the catalyst was recycled and reused for the next reaction by
recharging of the substrates under the same reaction conditions.
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1
The conversion of the reaction was based on H NMR spectra, and
the ee value was determined by HPLC on a chiral stationary phase
(91% yield, 99% ee).
2008, 120, 771; Angew. Chem. Int. Ed. 2008, 47, 759.
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Received: May 13, 2008
Published online: September 18, 2008
Keywords: asymmetric catalysis · hydrogenation · ionic liquids ·
.
quinolines
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2n under otherwise identical reaction conditions, similar chemo-
selectivity and enantioselectivity was observed but a slightly
lower conversion into 3n was obtained (72% yield, 96% ee, and
24% starting material).
[
4] a) R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley,
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[17] As expected, hydrogenation of 3-methylquinoline catalyzed by
(S,S)-1a in ionic liquid gave a racemic mixture of products. See
the Supporting Information for details.
[
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