Asymmetric hydrogenation of quinolines activated by Br?nsted acids

Article abstract of DOI:10.1016/j.tetlet.2010.04.004

Enantioselective hydrogenation of quinolines and quinoxalines catalyzed by iridium/diphosphine complex with catalytic amount of Br?nsted acid as activator was developed. In the presence of piperidine·TfOH as the activator, full conversions and up to 92% ee were obtained.

Full text of DOI:10.1016/j.tetlet.2010.04.004

Tetrahedron Letters 51 (2010) 3014–3017
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Tetrahedron Letters
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Asymmetric hydrogenation of quinolines activated by Brønsted acids
a
a,b,*
Duo-Sheng Wang , Yong-Gui Zhou
a
State Key of Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Enantioselective hydrogenation of quinolines and quinoxalines catalyzed by iridium/diphosphine com-
plex with catalytic amount of Brønsted acid as activator was developed. In the presence of piperi-
dineÁTfOH as the activator, full conversions and up to 92% ee were obtained.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 24 February 2010
Revised 30 March 2010
Accepted 2 April 2010
Available online 9 April 2010
Catalytic asymmetric hydrogenation of prochiral unsaturated
compounds, such as simple olefins, ketones, and imines, has been
intensively studied which provides a straight and versatile access
asymmetric hydrogenation of quinolines successively.10,11 It
was notable that, with these catalytic system, catalytic amount
of Brønsted acid showed positive effect. Recently, Xiao group re-
ported the Rh-catalyzed asymmetric transfer hydrogenation of
quinolines in an aqueous formate solution with high enantiose-
lectivities, the acidic buffer is crucial for the reactivity and
1
to the corresponding chiral compounds. However, the asymmetric
hydrogenation of heteroaromatic compounds such as quinolines is
much less explored until very recently due to low activity of aro-
2
12
matic compounds. Considering the importance of 1,2,3,4-tetrahy-
enantioselectivity. Zhang and co-workers developed the first
droquinolines, which are ubiquitous in natural alkaloids and have
found broad application in pharmaceutical and agrochemical syn-
thesis, the asymmetric hydrogenation of quinolines to obtain
these useful chemicals is urgently developed. In this context, two
activation strategies have been developed by us successfully
asymmetric hydrogenation of the hydrochloride salt of unpro-
tected N–H imines.
1
3
3
For the above successful hydrogenation reactions, acid is very
crucial for both the reactivity and enantioselectivity. Inspired by
these examples, we envisioned that catalytic amount of Brønsted
acid may play as activator in the asymmetric hydrogenation of
quinolines. Since the basicity of the hydrogenated products and
the quinolines is almost equivalent, the acid can recycle in the acti-
vation process (Scheme 2). During our investigation, Mashima and
co-workers reported the asymmetric hydrogenation of quinoline
(Scheme 1).
In 2003, our group reported the first example of asymmetric
hydrogenation of quinolines catalyzed by iridium/diphosphine
complex with iodine as additive to activate the catalyst.4 Thereaf-
ter, this field was flourishing with a number of effective ligands
being introduced by other groups.6 Substrate activation is the
other choice for the asymmetric hydrogenation of heteroaromatic
compounds. In 2006, we chose chloroformates as activator via
forming quinolinium and isoquinolinium salts in situ, which were
,5
,7
14
hydrochloride salts by using Ir-complexes with Difluorphos. This
result encouraged us greatly and promoted us to stick to our initial
hypothesis. Herein, we disclose an efficient Ir/diphosphine
complex-catalyzed asymmetric hydrogenation of quinolines and
8
hydrogenated smoothly. In this reaction, stoichiometric amounts
of chloroformates were employed and the activation groups were
attached covalently at the nitrogen atoms of the products, which
needed additional steps for deprotection. Therefore, searching for
a new catalytic amount of activator, which can be easily removed
after the reaction, is a complement to the substrate activation
and of significance.
A: Catalyst Activation:
N
N
I
2
Catalytic Amount
H
In 2003
In 2008, Feringa and co-workers reported the asymmetric
hydrogenation of quinolines catalyzed by iridium complexes
based on monodentate BINOL-derived phosphoramidites with
[Ir(COD)Cl] / P-P*
2
H
2
B: Substrate Activation:
1
0 mol % of piperidine hydrochloride as additive.9 Afterward,
Fan and co-workers revealed the efficiency of the phosphine-free
cationic Ru/Ts-DPEN and Ir/CF Ts-DPEN catalysts in the
N
CO
N
CO R
2
N
X
3
2
R
In 2006
2
ClCO R
Stoichiometric Amount
Scheme 1. Activation strategies for hydrogenation of quinolines.
*
Corresponding author. Tel./fax: +86 411 84379220.
E-mail address: ygzhou@dicp.ac.cn (Y.-G. Zhou).
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.004

D.-S. Wang, Y.-G. Zhou / Tetrahedron Letters 51 (2010) 3014–3017
3015
Table 2
[
Ir(COD)Cl]
2
/P-P*
a
The effect of the additives on the reactivity and stereoselectivity
N
H
H2
N
N
X
X
H
H
2
[[Ir(COD)Cl] / (R)-MeOBiPhep
N
N
H
This Work
Additive, THF, 700 psi H
2
HX
1a
2a
Catalytic Amount
Entry
Additive
Convn.b (%)
Eec (%)
Asymmetric Hydrogenation of Quinolines Activated
1
2
3
4
5
6
7
8
9
TfOH
TFA
>95
>95
>95
>95
>95
>95
>95
>95
11
79 (R)
71 (R)
74 (R)
49 (R)
53 (R)
70 (R)
75 (R)
86 (R)
7 (R)
By the Br nsted Acid
Ø
N
H
TsOHÁH
2
O
PhCO H
2
Scheme 2. Substrate activation with Brønsted acid.
Salicylic acid
HClÁquinaldine
HClÁpiperidine
TfOHÁpiperidine
AgOTf
quinoxalines with catalytic amount of Brønsted acid, piperi-
dineÁTfOH, as the activator with up to 92% ee.
Quinaldine 1a was selected as the model substrate for the con-
dition optimization. The initial reaction was carried out in toluene
10
AgOTf + TfOH
13
29 (S)
a
Conditions: 0.25 mmol of quinaldine, [Ir(COD)Cl]
2
(1 mol %), (R)-MeO–BiPhep
2
using [Ir(COD)Cl] /(R)-MeO–BiPhep and TfOH as the catalyst and
activator, respectively. To our delight, both the reactivity and
enantioselectivity were improved greatly in the presence of
(
2.2 mol %), additive (10 mol %), 3 mL of THF, rt, 16 h.
b
Determined by 1H NMR.
c
Determined by HPLC.
1
0 mol % of TfOH (Table 1, entries 1 and 2, 48–76% conversion
and 23–84% ee). This promising result encouraged us to screen
the other reaction parameters. For the solvent effect, i-PrOH and
CH Cl resulted in lower conversions. Reversed enantioselectivity
2 2
Table 3
a
The effect of ligands on the enantioselectivity
was observed for MeOH (51% ee (S)). Full conversions and high
[Ir(COD)Cl]2 / L*, TfOH
.
5 11
NC H
enantioselectivities were obtained with dioxane, THF and EtOAc
*
THF, 700 psi H , 16 h
2
N
a
N
(
entries 6–8).
H
1
2a
Considering the important impact of acid additives on activity
and enantioselectivity, a number of acid additives were evaluated
in THF (Table 2). Organic acids, such as TfOH, TFA, TsOHÁH O,
PhCO H and salicylic acid, were tested. It was shown that stronger
acids are more effective in terms of enantioselectivity, and TfOH
Convn.b (%)
Eec
(%)
Entry
Ligand
2
2
1
2
L1
L2
>95
61
86
(R)
88
1
4
gave the highest ee (entry 1, 79%). Inspired by Mashima’s report,
(
R)
1
0 mol % of quinaldineÁHCl was added as an additive, the reaction
3
4
L3
L4
>95
>95
89(R)
79
(R)
78
(R)
88
proceeded smoothly with full conversion and 70% ee (entry 6).
Piperidine hydrochloride, which was effective in Ir-catalyzed
asymmetric hydrogenation of quinolines with monodentate phos-
5
L5
L6
L3
>95
>95
>95
9
phoramidite ligand, was also investigated in our catalytic system
6
with 75% ee (entry 7). Finally, piperidine triflate was found to be
the best activator with full conversion and 86% ee (entry 8). Ques-
tion aroused as that whether the substrate was activated by acid or
the triflate anion just as a counter anion of iridium complex? To
confirm our hypothesis of acid activation, AgOTf was added instead
of TfOH, poor conversion and enantioselectivity were obtained (en-
try 9). When AgOTf was cooperated with TfOH, low reactivity was
(
R)
7d
91
(R)
35 (S)
N/A
e
8
L3
L3
O
<5
<5
O
f
9
MeO
MeO
PPh2
PPh
O
O
PPh
PPh2
O
O
PPh2
2
2
PPh
2
O
O
Table 1
L1 (R)-MeO-BiPhep
L3 (R)-SegPhos
L2 (R)-SynPhos
a
Optimization of reaction conditions
H
Cl
O
O
PPh
2
[
[Ir(COD)Cl]
2
/ (R)-MeOBiPhep
PPh
PPh
2
2
MeO
MeO
PPh
PPh
2
2
N
N
H
PPh2
Solvent, H2, TfOH (10 mol%)
1a
2a
H
Convn.b (%)
Eec (%)
L5 (S,S)-DIOP
Cl
Entry
Solvent
L4 (R)-BINAP
L6 (R)-Cl-MeO-BiPhep
1d
2
3
4
5
6
7
8
Toluene
Toluene
i-PrOH
MeOH
48
76
47
28
23 (R)
84 (R)
22 (R)
51 (S)
29 (R)
80 (R)
79 (R)
79 (R)
a
Conditions: 0.25 mmol of quinaldine, [Ir(COD)Cl] (1 mol %), Ligand (2.2 mol %),
2
piperidineÁTfOH (10 mol %), 3 mL of THF, rt, 16 h.
b
Determined by 1H NMR.
c
CH
2
Cl
2
23
Determined by HPLC.
d
Dioxane
THF
EtOAc
>95
>95
>95
With 5 mol % of piperidine?TfOH.
e
With [Rh(COD)
2
]BF
4
as metal precursor.
as metal precursor.
f
With [RuCl
2
(p-cymene)]
2
a
2
Conditions: 0.25 mmol of quinaldine, [Ir(COD)Cl] (1 mol %), (R)-MeO–BiPhep
(
2.2 mol %), TfOH (10 mol %), 3 mL of solvent, rt, 16 h.
b
Determined by 1H NMR.
c
Determined by HPLC.
observed (entry 10). The above experiments showed that it is not a
counteranion exchange process.
d
No acid was used.

3
016
D.-S. Wang, Y.-G. Zhou / Tetrahedron Letters 51 (2010) 3014–3017
Next, some commercially available chiral diphosphine ligands
quinoxalines can be hydrogenated smoothly with 57% and 65%
ee, respectively (Scheme 3).
were screened (Table 3). Enantioselectivities increased slightly
when SynPhos, SegPhos, and Cl–MeO–BiPhep were employed in-
stead of MeO–BiPhep (entries 2, 3, and 6). When BINAP and DIOP
were used, the reaction proceeded with full conversion but with
lower enantioselectivities (entries 4 and 5, 79% and 78% ee). Best
enantioselectivity was obtained with SegPhos (89% ee). When
piperidineÁTfOH was reduced to 5 mol %, the enantioselectivity in-
creased slightly (entry 7, 91% ee). The common metal precursors
In conclusion, we have developed a new strategy for iridium-
catalyzed asymmetric hydrogenation of quinolines and quinoxa-
lines by Brønsted acid-mediated substrate activation. With cata-
lytic amount of piperidine triflate (5 mol %), the reaction can
proceed smoothly with up to 92% ee. This new method provides
a supplement to the reported catalytic system with iridium-
diphosphine complex. Its application in the asymmetric hydroge-
nation of other heteroaromatic compounds is under investigation.
such as [Rh(COD)
stead of [Ir(COD)Cl]
versions and poor enantioselectivities were obtained. Thus, the
optimized conditions were [Ir(COD)Cl] /(R)-SegPhos/THF/ with
mol % of piperidineÁTfOH as an additive.
Under the optimal conditions, a variety of 2-substituted quino-
2
4 2 2
]BF and [RuCl (p-cymene)] were also tested in-
2
to ensure the scope of our strategy. Low con-
Acknowledgments
2
5
We are grateful for the financial support from National Science
Foundation of China (20872140 and 20921092), National Basic Re-
search Program (2010CB833300), and Chinese Academy of
Sciences.
lines were hydrogenated smoothly to give the desired products in
excellent yields and high enantioselectivities (Table 4, up to 92%
ee). It was found that the length of the side chain influenced both
the enantioselectivity and reactivity (entries 1–4). So iridium was
increased to 2 mol % to guarantee full conversions for all sub-
strates. It was noted that best result was obtained for quinoline
with a free hydroxyl group on the side chain (entry 5, 92% ee). 2-
Arenethyl-substituted quinolines were also hydrogenated with
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.04.004.
8
8–89% ee (entries 6 and 7). Substitution at the 6-position had
References and notes
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a
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(2 mol %), (R)-SegPhos
(
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3
Scheme 3. Asymmetric hydrogenation of quinoxalines.

D.-S. Wang, Y.-G. Zhou / Tetrahedron Letters 51 (2010) 3014–3017
3017
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