Asymmetric transfer hydrogenation of prochiral ketones catalyzed by aminosulfonamide-ruthenium complexes in ionic liquid

Article abstract of DOI:10.2478/s11532-010-0134-8

Chiral aminosulfonamides containing imidazolium group were used as ligands for the ruthenium(II)-catalyzed asymmetric transfer hydrogenation of prochiral ketones in ionic liquid, affording good to excellent conversions and enantiomeric excesses. The catalytic system could be easily recovered and reused several times. Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-010-0134-8

Cent. Eur. J. Chem. • 9(1) • 2011 • 175-179
DOI: 10.2478/s11532-010-0134-8
Central European Journal of Chemistry
Asymmetric transfer hydrogenation of prochiral
ketones catalyzed by aminosulfonamide-
ruthenium complexes in ionic liquid
Research Article
*
Zhongqiang Zhou , Yong Sun, Aiqing Zhang
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, College of Chemistry and Materials
Science, South-Central University for Nationalities, Wuhan 430074, China
Received 18 September 2010; Accepted 30 October 2010
Abstract:Chiral aminosulfonamides containing imidazolium group were used as ligands for the ruthenium(II)-catalyzed asymmetric transfer
hydrogenation of prochiral ketones in ionic liquid, affording good to excellent conversions and enantiomeric excesses. The catalytic
system could be easily recovered and reused several times.
Keywords: Asymmetric transfer hydrogenation • Aminosulfonamide • Ionic liquid • Ketones
©
Versita Sp. z o.o.
environmentally responsible strategy. The combination
of the “green solvents” and the “recyclable catalytic
system” would provide an ideal chemical process for
environmentally friendly asymmetric synthesis. We
recently communicated that the recyclable ruthenium
catalysts prepared from ionic chiral aminosulfonamide
ligands 1a and 1b (Fig. 1) with [RuCl (p-cymene)]
were used in the asymmetric transfer hydrogenation of
prochiral ketones in water [3]. As an extension of this
work, herein we describe the recyclable catalytic system
generated in situ from ruthenium complex and ionic chiral
aminosulfonamide ligands 1 to catalyze the asymmetric
transfer hydrogenation of ketones in ionic liquid.
1
. Introduction
Asymmetric transfer hydrogenation (ATH) of prochiral
ketones has emerged as a very valuable synthetic tool to
obtain optically pure secondary alcohols because of its
operational simplicity, the easy availability of hydrogen
sources, lower cost and safety [1]. The ruthenium
complexes containing arene and N-(p-toluenesulfonyl)-
2
2
1,2-diphenylethylenediamine(TsDPEN)liganddeveloped
by Noyori and co-workers are efficient catalysts for
the asymmetric transfer hydrogenation of ketones [2].
Recently, a number of researchers have shown that
catalytic ATH of ketones can be carried out efficiently
using monosulfonylated-1,2-diphenylethylenediamines
as ligands with sodium formate as hydrogen source
in aqueous solution, 2-propanol and a formic acid/
triethylamine azeotrope as hydrogen donor as well as
2
. Experimental Procedure
solvent [3-38]. However, to the best of our knowledge, Melting points were determined on a melting point
there are only two reports using monosulfonylated- apparatus and were uncorrected. Specific rotations were
1
,2-diphenylethylenediamines as ligands for the ATH recorded in a WZZ-3 digital polarimeter. IR spectra were
1
with ionic liquids as solvents [39,40]. Ionic liquids have obtained on a Nexus 470 FTIR spectrophotometer. H
attracted increasing interest in recent years because NMR spectra were recorded on a Bruker Avance
of their properties of nonvolatility, nonflammability, 400 with TMS as internal standard. The conversions
thermal stability, controlled miscibility, low toxicity and were measured by GC-MS on a Agilent 5973N (HP-5ms
reusability. The use of ionic liquids as a solvent is a more capillary column). The chemicals used in this work were
*
E-mail: zhou-zq@hotmail.com
1
75

Asymmetric transfer hydrogenation of prochiral ketones catalyzed
by aminosulfonamide-ruthenium complexes in ionic liquid
O
Ph
Ph
HN
Ph
Ph
O
O
O
N
N
H N HN S
3
H N HN S
NH
N
CH3
3
2
CF CO
3 2
O
O
N
CH3
2
CF CO
3
2
1a
1b
Figure 1. Chiral aminosulfonamides 1a and 1b
purchased from the Alfa Aesar and Sinopharm Chemcial temperature 250°C, detector temperature 250°C,
Reagent Co., Ltd. Chiral aminosulfonamides 1a and 1b column temperature 150°C. t 7.45 min for (S), 8.08 min
R
were prepared from (R,R)-1,2-diphenylethylenediamine for (R).
by following the procedure we previously reported [3].
(S)-2-Chloro-1-(p-methylphenyl)ethanol.
Ionic liquid [bmim][PF ] was prepared according to The GC conditions were: inlet pressure 96kPa, flow rate
6
-1
literature procedures [41].
1 mL min , injector temperature 250°C, detector
temperature 250°C, column temperature 150°C.
tR 9.78 min for (S), 10.83 min for (R).
2
.1. General procedure for asymmetric
transfer hydrogenation in ionic liquid
(
R)-1-(p-Chlorophenyl)ethanol. The GC conditions
-1
were: inlet pressure 62 kPa, flow rate 0.6 mL min ,
(R,R)-1 (0.012 mmol) and [RuCl (p-cymene)] (3.1 mg, injector temperature 250°C, detector temperature
2
2
0
.005 mmol) were dissolved in [bmim][PF ] (1 mL). The 250°C, column temperature 140°C. t 15.30 min for (R),
6
R
resulting solution was stirred at 40°C for 1 h under an 17.34 min for (S).
argon atmosphere. Formic acid/triethylamine azeotrope
(R)-1-(p-Bromophenyl)ethanol. The GC conditions
0.5 mL) and acetophenone (1.0 mmol) were then were: inlet pressure 93 kPa, flow rate 1 mL min ,
-1
(
introduced. The mixture was stirred at 40°C for a certain injector temperature 250°C, detector temperature
period of time. After cooling to room temperature, the 250°C, column temperature 140°C. t 16.41 min for (R),
R
organic compounds were extracted with hexane-ether 18.86 min for (S).
(
1:1, v/v, 3×5 mL) using a syringe. The conversion
(R)-1-(p-Methylphenyl)ethanol. The GC conditions
-1
was determined by GC-MS analysis. The enantiomeric were: inlet pressure 58 kPa, flow rate 0.6 mL min ,
excess was determined by GC (Agilent 6890N) equipped injector temperature 250°C, detector temperature
with a chiral column (Chirasil-Dex CP 7502). Absolute 250°C, column temperature 130°C. t 11.55 min for (R),
R
configuration was assigned by comparison of the sign 12.77 min for (S).
of the specific rotation with that reported [42,43]. Formic
acid (1.0 mmol) and acetophenone (1.0 mmol) were were: inlet pressure 96 kPa, flow rate 1 mL min , injector
(R)-1-(p-Methoxyphenyl)ethanol. The GC conditions
-1
added into the residual ionic liquid solution for a new temperature 250 °C, detector temperature 250°C,
reaction cycle.
column temperature 150°C. t 6.8 min for (R), 7.1 min
R
for (S).
(
S)-2-Chloro-1-(p-chlorophenyl)ethanol. The GC
2
.2. Chiral GC analyses of chiral aromatic
alcohols
conditions were: inlet pressure 110 kPa, flow rate
1.2 mL min , injector temperature 250°C, detector
R)-1-Phenylethanol. The GC conditions were: temperature 250°C, column temperature 150°C. tR
inlet pressure 87 kPa, flow rate 1 mL min , injector 18.51 min for (S), 21.41 min for (R).
temperature 250°C, detector temperature 250°C,
(R)-1-(m-Bromophenyl)ethanol. The GC conditions
column temperature 120°C. t 8.67 min for (R), 9.73 min were: inlet pressure 96 kPa, flow rate 1 mL min ,
-1
(
-1
-1
R
for (S).
injector temperature 250°C, detector temperature
(
R)-1-Phenylpropanol. The GC conditions were: 250°C, column temperature 150°C. t 9.67 min for (R),
R
-1
inlet pressure 58 kPa, flow rate 0.6 mL min , injector 10.45 min for (S).
temperature 250°C, detector temperature 250°C,
(R)-1-(m-Methoxyphenyl)ethanol. The GC conditions
column temperature 130°C. tR 13.08 min for (R), were: inlet pressure 81 kPa, flow rate 0.8 mL min ,
3.79 min for (S).
injector temperature 250°C, detector temperature
S)-2-Chloro-1-phenylethanol. The GC conditions 250°C, column temperature 150°C. t 8.04 min for (R),
-1
1
(
R
-1
were: inlet pressure 96 kPa, flow rate 1 mL min , injector 8.49 min for (S).
176

Z. Zhou, Y. Sun, A. Zhang
(
R)-1-(o-Methylphenyl)ethanol. The GC conditions (S/C) ratio of 100:1. Acetophenone was converted into
-1
were: inlet pressure 60 kPa, flow rate 0.6 mL min , (R)-1- phenylethanol in 97% ee with 100% conversion in
injector temperature 250°C, detector temperature 8husing[bmim][PF ]assolventwhenaminosulfonamide
6
250°C, column temperature 130°C. t 14.79 min for (R), (R,R)-1a was used as the ligand (entry 1, Table 1). By
R
1
7.87 min for (S).
using (R,R)-1b, (R)-1-phenylethanol was obtained
in 100% conversion with 86% ee in 8 h (entry 2,
Table 1).This is in stark contrast to the observation made
with HCOONa in water, under which acetophenone
was fully converted into (R)-1-phenylethanol in 2 h with
3
. Results and Discussion
Ionic liquids are receiving growing attention as a means comparable enantioselectivity [3]. ATH with HCOONa
to immobilize catalysts. Catalysts that are soluble in ionic as the hydrogen source in water has shown faster
liquids may be recycled together with the ionic liquid, rates than ATH with formic acid/triethylamine azeotrope
after extraction with the immiscible solvents used for as the hydrogen source in ionic liquid. The hydrogen
product separation. Asymmetric transfer hydrogenation source plays an important role [25]. The reaction rate
of various aromatic ketones has been examined using was affected by pH values and HCOOH/NEt3 molar
1
a and 1b as ligands in neat water using sodium formate ratios. For example, , lowering the HCOOH/NEt molar
3
as the hydrogen source [3]. Considering ionic chiral ratios slightly lowered the reaction rate (entries 3 and
aminosulfonamides 1 are insoluble in most common 4, Table 1). The enantioselectivity varied with the pH
organic solvents but are highly soluble in water and ionic value as well, lowering the HCOOH/NEt molar ratios
3
liquids, theATH of ketones was carried out in ionic liquid. lowered the enantioselectivity dramatically from 98%ee
On the basis of Ohta’s work [39], ionic liquid 1-butyl-3- to 84%ee (molar ratio HCOOH/NEt =1.2:1) and 81%ee
3
methylimidazolium hexafluorophosphate [bmim][PF6] (molar ratio HCOOH/NEt =0.7:1) when the reactions
3
was chosen for the immobilization of the catalysts.
were complete.
The ruthenium catalysts were prepared in situ by
Encouraged by the results mentioned above, the
reacting aminosulfonamides (R,R)-1 with [RuCl (p- asymmetric transfer hydrogenation of various kinds
2
cymene)] inionicliquidsunderargonatmosphereat40°C of aromatic ketones using Ru-1a as a catalyst was
2
for 1 h, and the reduction was started by introducing the investigated in [bmim][PF ]. As shown in Table 1, the
6
formicacid/triethylamineazeotrope(molarratioHCOOH/ Ru-1a catalyst delivered high conversions for most of
NEt =2.5:1) and acetophenone with a substrate/catalyst the ketones in 8 h reaction time and in most cases the
3
a
Table 1. Asymmetric transfer hydrogenation of prochiral ketones in [bmim][PF6]
O
OH
[
RuCl (p-cymene)] / 1
Y
Y
2
2
X
X
HCOOH-NEt , [bmim][PF ]
3
6
Entry
X
Y
Ligand
Time (h)
Conversion (%)
ee (%)
Configuration
1
2
H
H
H
H
H
H
H
H
H
CH3
Cl
Cl
H
H
H
H
Cl
H
1a
1b
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
8
8
9
9
23
8
8
8
8
8
48
8
9
9
48
70
100
100
100
100
84
100
100
100
100
81
97
86
84
81
97
86
96
95
96
94
74
94
97
97
81
83
R
R
R
R
R
S
S
R
R
R
R
S
R
R
R
R
b
3
c
4
5
6
7
8
9
H
p-CH3
p-Cl
p-Br
p-CH3
p-CH O
p-Cl
m-Br
m-CH O
o-CH3
o-CH3
1
0
11
91
3
12
13
14
15
16
100
100
100
54
H
H
H
3
77
a
Unless otherwise indicated, reactions were carried out using 1 mmol of ketone, 0.5 mL of formic acid/triethylamine azeotrope, and a S/C ratio of 100 in
1
mL of [bmim][PF ] under argon atmosphere at 40°C.
6
b
The HCOOH/NEt molar ratio was 1.2:1.
3
c
The HCOOH/NEt molar ratio was 0.7:1.
3
1
77

Asymmetric transfer hydrogenation of prochiral ketones catalyzed
by aminosulfonamide-ruthenium complexes in ionic liquid
a
Table 2. Catalyst recycling in asymmetric transfer hydrogenation of acetophenone in [bmim][PF6]
Run
Ligand
Time (h)
Conversion (%)
ee (%)
Configuration
1
2
3
1a
1a
1a
8
22
80
100
100
100
97
96
94
R
R
R
a
Reactions were carried out using 1 mmol of acetophenone, 0.5 mL of formic acid/triethylamine azeotrope, and a S/C ratio of 100 in 1 mL of [bmim]
[
PF ] under argon atmosphere at 40 °C in the first run. Since the second run, formic acid (1.0 mmol) and acetophenone (1.0 mmol) were added in every
6
recycling run.
enantioselectivitiesweregoodtoexcellent,witheevalues containing the catalyst was recycled and reused
reaching up to 97%. Exceptions were encountered for for the next reaction by recharging formic acid and
aryl ketones bearing strongly electron-donating groups. acetophenone under the same conditions. Although
The p-Me, p-OMe and o-Me substituted acetophenones catalyst recycling led to a quick loss of catalytic activity,
were much less active. For instance, the reduction of a very high conversion (100%) was obtained after a
p-methoxyacetophenone led to a 91% conversion in prolonged time of 80 h. The catalytic system could be
74% ee in 48 h (entry 11, Table 1), the reduction of easily recovered and reused three times with a slight
o-methylacetophenone led to a 77% conversion in 83% loss of enantioselectivity. The activity of the catalyst
ee in 70h (entry 16, Table 1). Steric effects might have showed a remarkable drop on the recycled application.
also contributed in the case of o-methylacetophenone. The reason is probably due to some decomposition of
m-Methoxyacetophenone displayed faster rate than the catalyst.
p-methoxyacetophenone. The results found in this
study are consistent with those previously reported
[
8]. As for the asymmetric transfer hydrogenation of 4. Conclusions
propiophenone, excellent enantioselectivity (97%
ee) and good reactivity (84% conversion after 23 h) In summary, chiral aminosulfonamides containing
were observed (entry 5, Table 1). The present results an imidazolium group were used as ligands for
reveal that ionic chiral aminosulfonamide 1a and 1b the ruthenium(II)-catalyzed asymmetric transfer
were efficient ligands for the ruthenium(II)- catalyzed hydrogenation of prochiral ketones in ionic liquid,
asymmetric transfer hydrogenation of prochiral ketones affording good to excellent conversions and
in water [3] as well as ionic liquid.
enantiomeric excesses. The catalytic system could be
From the viewpoint of green chemistry it is highly easily recovered and reused several times. Studies to
desirable that the catalyst can be recovered and reused. further optimize the reaction conditions and to apply
Recovery and reusability of the catalytic system were chiral aminosulfonamides 1 to other asymmetric catalytic
tested by carrying out consecutive cycles in [bmim] reactions are underway.
[
PF ] using formic acid/triethylamine azeotrope as the
6
hydrogen source and acetophenone as a standard
substrate (Table 2). An attractive feature of the present
catalytic system lies in the fact that the catalyst can be
readily removed from the product by addition of a low
polarity solvent. After the completion of the reaction, the
catalytic system was easily recovered after extraction of
the reduced products with ether. The residual solution
Acknowledgements
Financial support of this work by the Natural Science
Foundation of Hubei Province (2007ABA291) is
gratefully acknowledged.
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