Synthesis of BINAP ligands with imidazole tags for highly enantioselective Ru-catalyzed asymmetric hydrogenation of β-keto esters in ionic liquid systems

Article abstract of DOI:10.1016/j.molcata.2013.03.015

The imidazole-tagged BINAP ligands were synthesized and used for Ru-catalyzed asymmetric hydrogenation of β-keto esters in ionic liquid (IL) systems. The Ru-BINAP catalysts with the imidazolium tags show high catalytic activity and enantioselectivity, which closely parallel the performance of unmodified BINAP. The catalyst recycling experiments using [bmim]Tf ₂N/MeOH system demonstrated that introducing imidazolium moieties to the BINAP backbone can effectively enhance the affinity of the Ru-catalysts to the IL, reduce Ru leaching and improve catalysts stability, and after several cycles no significant loss of activity and enantioselectivity was observed.

Full text of DOI:10.1016/j.molcata.2013.03.015

Journal of Molecular Catalysis A: Chemical 374–375 (2013) 22–26
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis of BINAP ligands with imidazole tags for highly enantioselective
Ru-catalyzed asymmetric hydrogenation of ␤-keto esters in ionic liquid systems
∗
Xin Jin , Fang-fang Kong, Zhi-qiang Yang, Fei-fei Cui
Department of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The imidazole-tagged BINAP ligands were synthesized and used for Ru-catalyzed asymmetric hydro-
genation of ␤-keto esters in ionic liquid (IL) systems. The Ru-BINAP catalysts with the imidazolium tags
show high catalytic activity and enantioselectivity, which closely parallel the performance of unmodified
BINAP. The catalyst recycling experiments using [bmim]Tf2N/MeOH system demonstrated that introduc-
ing imidazolium moieties to the BINAP backbone can effectively enhance the affinity of the Ru-catalysts
to the IL, reduce Ru leaching and improve catalysts stability, and after several cycles no significant loss
of activity and enantioselectivity was observed.
Received 25 October 2012
Received in revised form 16 March 2013
Accepted 17 March 2013
Available online xxx
Keywords:
Asymmetric hydrogenation
Catalyst recycling
Ionic liquids
©
2013 Elsevier B.V. All rights reserved.
Ionic tags
Ruthenium
1
. Introduction
product from the ILs may risk the loss of chiral catalyst to the
product phase. Alternatively, introducing ionic tags to the phos-
phine ligand has been found as an effective way to enhance affinity
between the chiral catalyst and the ILs [20–24]. For example,
attaching polar groups such as phosphonic acid [25,26] or ammo-
nium [27] to the BINAP backbone has been reported to effectively
inhibit catalyst loss and enhance catalyst stability in the ILs.
To date, BINAP ligand modified with imidazole or imidazolium
group has not been reported. Herein we report the synthesis of
Chiral phosphine ligands are an important constituent of chiral
transition metal catalysts. The pivotal problem in asymmetric catal-
ysis involves the preparation of superior chiral ligands that complex
with transition metals to form catalysts with high activity and
high stereoselectivity [1]. To date thousands of chiral phosphine
ligands have been synthesized, among which the C -symmetric
2
BINAP ligand has exhibited strong chiral recognition ability and
has been widely used in various catalytic asymmetric reactions [2].
For instance, Ru-BINAP is well recognized as a highly efficient cat-
alyst for the asymmetric hydrogenation of various functionalized
olefins and ketones, such as ␣,␤-unsaturated carboxylic acids [3],
allylic and homoallylic alcohols [4], enamides [5], ␤-keto esters [6],
ꢀ
a BINAP derivative bearing two imidazole tags (5,5 -diimidazole
BINAP, Scheme 1) and its application in Ru-catalyzed asymmetric
hydrogenation of ␤-keto esters in imidazole-based IL systems.
␤
-hydroxy or ␤-amino ketones [7], and simple ketones [8]. Nev-
2. Experimental
ertheless, despite such success, the industrial application of BINAP
has remained very limited due to problems in recycling the very
expensive chiral catalyst [9].
2.1. General details
In recent years, liquid–liquid biphasic catalytic systems have
received extensive attention where ionic liquids (ILs) are used as
the carrier for recyclable catalyst [10–12]. The chiral Ru-BINAP
complexes immobilized in ILs have been used in the hydrogenation
of olefins and ketones to give high activity and enantioselectiv-
ity [13–19]. However, because BINAP is a weakly polar molecule
that has poor affinity to ILs, the extraction of hydrogenation
All other reagents and ionic liquids (ILs) were obtained commer-
cially. The ILs contained <0.2 wt% water and <150 ppm halide, and
no precipitate formed upon addition of aqueous AgNO . All reac-
3
tions were carried out under an argon atmosphere using standard
Schlenk techniques. The solvents and reagents used were rigor-
ously deoxygenated prior to use. The hydrogenation reactions were
performed in a home-made 60 mL stainless steel autoclave. The
conversions and enantioselectivity were determined by GC on a
lipodex A (25Ym × 0.25 mm) chiral column. NMR spectra were
recorded on Bruker 500YMB instrument. High resolution mass
spectra (HRMS) were obtained using a Q-Tof Ultima Global mass
∗ Corresponding author. Tel.: +86 532 840 22719; fax: +86 532 840 22719.
E-mail address: jinx1971@163.com (X. Jin).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.03.015

X. Jin et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 22–26
23
N
ı = 30.0, 29.3; HRMS (Q-Tof MS, ES+): m/z = 721.2175, calcd. for
+
C47H35O N P [M+H] : 721.2168.
2
2 2
N
ꢀ
2
.4. Preparation and characterization of (S)-5,5 -diimidazole
BINAP (4)
PPh2
PPh2
The synthesis and purification of 4 followed the published
method [28]. A mixture of 2 (0.2 g, 0.26 mmol) and phenylsi-
PPh2
PPh2
◦
lane (1.5 mL, 12.2 mmol) was heated to 130 C. Three portions of
trichlorosilane (3 × 0.4 mL) were added by syringe after 1 h, 3 h, and
1
5 h and the reaction was stirred for 2 h. The mixture was cooled
and the volatiles were removed under reduced pressure to afford
a white solid. The solid was washed with degassed cyclohexane,
filtered and evaporated. The 4 was obtained in quantitative yield.
N
N
imidazole-tagged BINAP
20
1
[
˛] D: −90.4 (c 0.5, DMF); H NMR (500.0 MHz, CDCl ): ı = 6.91
3
(S)-BINAP
(d, J = 8.5 Hz, 2H), 6.97 (t, J = 7.6 Hz, 2H), 6.99–7.25 (m, 20H), 7.36
(
7
d, J = 7.0 Hz, 2H), 7.40 (s, 2H), 7.47 (s, 2H), 7.53 (d, J = 8.5 Hz, 2H),
1
3
.58 (d, J = 8.5 Hz, 2H), 8.23 (s, 2H); C NMR (125.7 MHz, CDCl ):
3
Scheme 1. Structure of the original and the functionalized BINAP ligands.
ı = 121.8, 124.6, 125.2, 128.4, 128.5, 128.6, 129.1, 129.3, 132.3,
32.7, 132.8, 132.9, 133.8, 134.3, 134.4, 134.5, 135.7, 136.4, 138.7,
1
3
1
spectrometry. The data of Ru loss were measured by ICP-AES on a
IRIS Istrepid II XSP apparatus.
144.0; P NMR (202.4 MHz, CDCl3): ı = 13.1; HRMS (Q-Tof MS,
ES+): m/z = 755.2497, calcd. for C50H37N4P2 [M+H] : 755.2488.
+
ꢀ
2.5. Preparation and characterization of (S)-5-imidazole BINAP
2
.2. Preparation and characterization of (S)-5,5 -dibromo
(
5)
BINAPO (1)
The 5 was synthesized as described process for 4. [˛]20D: −127.1
The synthesis and purification followed the published procedure
1
1
(c 0.5, DMF); H NMR (500.0 MHz, CDCl ): ı = 6.65 (d, J = 8.0 Hz, 1H),
6
(
[
28]. H NMR (500.0 MHz, CDCl ): ı = 6.61 (dd, J = 8.5, 7.5 Hz, 2H),
3
3
.90 (t, J = 7.5 Hz, 1H), 6.90–7.25 (m, 22H), 7.34–7.42 (m, 3H), 7.45
dd, J = 8.5, 3.0 Hz, 1H), 7.51 (s, 1H), 7.59 (dd, J = 8.5, 2.0 Hz, 1H),
7.69 (s, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.95 (d, J = 8.5 Hz, 1H), 8.75
6
2
2
1
1
.71 (d, J = 8.5 Hz, 2H), 7.23–7.41 (m, 16H), 7.54 (dd, J = 11.5, 8.5 Hz,
H), 7.64 (d, J = 7.5 Hz, 2H), 7.67–7.71 (m, 4H), 8.30 (dd, J = 8.5,
.0 Hz, 2H); 13C NMR (125.7 MHz, CDCl ): ı = 122.8, 126.0, 126.7,
3
1
3
(
s, 1H); C NMR (125.7 MHz, CDCl ): 118.5, 120.2, 120.8, 123.4,
26.8, 127.9, 128.2, 129.4, 131.2, 131.4, 131.9, 132.0, 132.3, 132.4,
3
3
1
124.8, 126.2, 126.7, 127.9, 128.1, 128.2, 128.3, 128.8, 130.4, 131.1,
132.7, 132.9, 133.0, 133.3, 133.6, 134.2, 134.3, 134.5, 135.7, 135.9,
32.5, 133.5, 134.4, 142.7; P NMR (202.4 MHz, CDCl ): ı = 28.6;
3
+
ESI-MS (+): [M+H] = 813.1.
3
1
1
36.6, 136.9, 139.6, 143.2, 145.8; P NMR (202.4 MHz, CDCl ):
3
ı = −13.7, −14.2; HRMS (Q-Tof MS, ES+): m/z = 689.2277, calcd. for
ꢀ
2
.3. Preparation and characterization of (S)-5,5 -diimidazole
+
C
4
H
35
N P [M+H] : 689.2270.
7
2 2
BINAPO (2) and (S)-5-imidazole BINAPO (3)
ꢀ
2.6. Preparation of 6, 7 and their corresponding Ru(II) dibromide
complexes 6-Ru and 7-Ru in situ [29]
The (S)-5,5 -diimidazole BINAPO (2) was synthesized by a amino
acid promoted Cu-catalyzed C–N bond formation. A mixture of
1
(3.6 g, 4.4 mmol), imidazole (6.1 g, 89.0 mmol), K CO3 (2.5 g,
2
Under an argon atmosphere, imidazole-tagged BINAP ligands
4 or 5 (0.0031 mmol) were dissolved in 0.5 mL of degassed CH2Cl2.
Then 0.9 ␮L (0.0062 mmol) or 0.45 ␮L (0.0031 mmol) of an aqueous
HBr solution (40%) was added for ligand 4 or for ligand 5, respec-
1
8.0 mmol), CuI (0.35 g, 1.8 mmol), and l-proline (0.42 g, 3.6 mmol)
◦
in 70 mL of DMSO was stirred at 140 C for 48 h under an argon
atmosphere. The cooled mixture was filtered off, and ethyl acetate
was added to the filtrate. The organic layer was washed with
brine, dried over Na SO , and concentrated in vacuum. The residual
solid was purified by a silica gel column chromatography (elu-
◦
tively. The solution was allowed to stir for 30 min at 25 C after
2
4
which the CH Cl2 was removed. To the residue were added 1.0 mg
2
of ruthenium precursor Ru[(2-methylallyl)2 (COD)] (0.0031 mmol)
and 0.5 mL of degassed acetone. Next, 0.9 ␮L of an aqueous HBr
solution (40%) was added. The catalyst solution was allowed to
stir for 30 min after which the acetone was removed to give the
catalysts 6-Ru or 7-Ru. The catalysts 6-Ru or 7-Ru were used imme-
diately for the hydrogenation of ␤-keto esters.
ent: EtOAc/EtOH/H O = 15/2/1) to afford the pure 2 as white solid
2
(
1.7 g, 49.1%) and 3 as a byproduct (0.78 g). The characterization
2
5
1
of 2: [˛] D: −187.3 (c 0.5, DMF); H NMR (500.0 MHz, CDCl ):
3
ı = 6.97 (dd, J = 8.5, 7.5 Hz, 2H), 7.02 (d, J = 8.5 Hz, 2H), 7.22–7.45
(m, 22H), 7.50 (dd, J = 11.5, 9.0 Hz, 2H), 7.62–7.69 (m, 6H), 7.82 (s,
13
2
1
1
H); C NMR (125.7 MHz, CDCl ): ı = 121.8, 121.9, 125.2, 128.0,
28.1, 128.2, 129.5, 129.7, 130.2, 130.7, 131.5, 131.8, 132.0, 132.4,
32.9, 133.8, 134.0, 134.2, 138.4, 143.0;
3
3
1
2.7. General procedure for the homogeneous Ru-catalysed
asymmetric hydrogenation of ˇ-keto esters in methanol
P NMR (202.4 MHz,
CDCl ): ı = 29.1; HRMS (Q-Tof MS, ES+): m/z = 787.2393, calcd.
for C50H37O N P [M+H] : 787.2386; The characterization of 3:
3
+
2
4 2
25
1
Typically, the preceding catalysts 6-Ru (0.0031 mmol) or 7-Ru
[
(
˛] D: −198.7 (c 0.5, DMF); H NMR (500.0 MHz, CDCl ): ı = 6.77
3
(0.0031 mmol) were dissolved in 2 mL of degassed methanol under
d, J = 8.5 Hz, 1H), 6.86 (t, J = 8.0 Hz, 1H), 6.90 (dd, J = 8.5, 7.5 Hz,
an argon atmosphere. The ␤-keto esters (substrate/Ru = 1000) were
added to the solution of catalyst. Next, the reaction mixture was
transferred to a 60 mL autoclave. The hydrogenation was carried
out under 4 MPa pressure of hydrogen at 60 C for 20 h. The con-
versions and ee value were determined by GC on a lipodex A
1
H), 7.04 (d, J = 8.5 Hz, 1H), 7.20–7.50 (m, 22H), 7.60–7.72 (m, 5H),
.80 (s, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.90 (dd, J = 8.5, 2.0 Hz, 1H);
7
13
C NMR (125.7 MHz, CDCl ): ı = 121.5, 121.8, 124.9, 126.0, 126.8,
3
◦
1
1
1
27.3, 127.6, 127.9, 128.0, 128.1, 128.4, 129.5, 129.7, 130.1, 131.2,
31.3, 131.9, 132.0, 132.3, 132.4, 132.6, 133.3, 133.5, 133.7, 133.9,
3
1
(25m × 0.25 mm) chiral capillary column.
34.0, 134.3, 134.5, 142.3, 143.7; P NMR (202.4 MHz, CDCl ):
3

2
4
X. Jin et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 22–26
Table 1
ꢀ
introducing the imidazolium groups to the 5,5 -positions of BINAP
did not incur negative electronic or steric effects. In comparison,
the activity and enantioselectivity of 7-Ru decreased slightly.
In the following experiments, we investigated how different ILs
in the catalyst system affected the catalytic activity and enantio-
selectivity of 6- and 7-Ru using methyl acetoacetate (MAA) as the
standard substrate (Table 2). As expected, in the pure ILs including
[bmim]PF , [bmim]BF and [bmim]Tf N, the ligands 6, 7 and BINAP
Ru-catalyzed asymmetric hydrogenation of ␤-keto esters with catalysts 6-Ru and
a
7
-Ru.
O
O
OH
O
Catalyst
MeOH
+
H2
R1
OR2
R1
OR2
Substrate
6-Ru
Conver. (Ee) (%)
7-Ru
Conver. (Ee) (%)
(S)-BINAP-Ru
Conver. (Ee) (%)
6
4
2
all exhibited no activity. Although the low solubility of H in ILs may
2
O
O
O
reduce catalytic activity [14,18,30,31], we believe the primary rea-
son for catalyst deactivation is due to the inhibition of ILs to the
catalytic hydrogenation reaction. Dyson and co-workers [32] have
found that for catalytic reactions in ILs involving transition metal
complex, if halide dissociation is a prerequisite for forming homo-
geneous active species, then catalyst deactivation may occur due to
the low salvation enthalpy of halide in imidazolium-based ILs that
consequently results in the difficult dissociation of the halide from
the precatalysts. In the hydrogenation of ␤-keto esters catalyzed by
Ru(II) dibromide complexes, the bromide dissociation from the Ru
complex is a vital step for forming the catalytically active species.
Thus the suppression of activity may have resulted from the diffi-
cult dissociation of bromide in ILs. Similar observations have been
reported by other researchers [17–19,30,33]. In addition, normal
solvents can participate in the catalytic cycle of transition metal
catalyzed reactions and thus effectively enhance the catalytic per-
formance [17,18,33,34]. However, when ILs were used as the sole
reaction medium, the lack of solvent involvement may severely
impede the catalytic activity and enantioselectivity of asymmetric
hydrogenation.
1
1
1
1
00 (99)
00 (98)
00 (99)
00 (94)
100 (98)
81 (95)
96 (97)
100 (91)
100 (99)
OMe
O
100 (99)
100 (99)
OEt
O
O
O
Et
OEt
OEt
O
100 (95)
Ph
a
◦
Ru[(2-methylallyl)2(COD)] precursor 1.0 mg, p(H2) = 4 MPa, T = 60 C, t = 20 h, lig-
ands/Ru = 1, substrate/Ru = 1000, 2 mL methanol.
2
.8. General procedure for Ru-catalysed asymmetric
hydrogenation of MAA in IL/MeOH systems and for catalyst
recycling
Typically, the preceding catalysts 6-Ru (0.0031 mmol) or 7-Ru
0.0031 mmol) were dissolved in IL/MeOH (IL: 1 g, MeOH: 1 mL)
under an argon atmosphere. The MAA (36 mg, 0.31 mmol) were
added to the solution of catalyst. Next, the reaction mixture was
transferred to a 60 mL autoclave. The hydrogenation was carried
To solve these problems, we investigated the effects of methanol
as cosolvent on the catalytic activity and enantioselectivityof asym-
metric hydrogenation of MAC (Table 2, entries 4–9). As expected, in
the ILs/MeOH systems, the catalysts 6-Ru, 7-Ru and BINAP-Ru gave
comparable activity and enantioselectivity as to their reactions in
(
◦
out under 4 MPa pressure of hydrogen at 60 C for 20 h. After releas-
methanol. The [bmim]Tf N/MeOH system was found to be slightly
2
ing the hydrogen, the reaction mixture was transferred to a Schlenk
tube under an argon atmosphere. Next the MeOH was removed
under reduced pressure and ether was added to extract the prod-
uct (1 mL × 2). The top solvent layer was removed by a syringe for
GC analysis, and the new substrate and MeOH was added to the IL
for next catalyst recycling.
better than [bmim]BF /MeOH. This may be attributed to (1) par-
4
ticipation of methanol as a temporary ligand and a proton donor
in the catalytic cycle [34], (2) adding methanol that might facili-
tate the solvation of the dissociated bromide [32], and (3) higher
H2 solubility in ILs/MeOH systems than in sole IL medium.
Finally, we evaluated the recycling and reuse of the chiral cata-
lyst 6-Ru and the reference catalyst BINAP-Ru for the asymmetric
hydrogenation of MAA (Table 3). The hydrogenation was carried out
3
. Results and discussion
in the homogeneous [bmim]Tf N/MeOH system. After the reaction
2
As shown in Scheme 2, in order to synthesize imidazole-
completed, methanol was removed under reduced pressure and the
methyl hydroxybutyrate (MHB) was extracted using diethyl ether,
and new substrate was then replenished to carry out the new round
of catalyzed hydrogenation. It can be seen from the results in Table 3
that the activity and enantioselectivity of BINAP-Ru decreased dra-
matically after the first cycle, at which point 4.63% Ru was leached
into the ether phase as indicated by ICP-AES analysis. In contrast,
the catalyst 6-Ru remained highly active in all four cycles and its
enantioselectivity decreased slowly only after the second cycle, and
the Ru loss was only 0.27%.
ꢀ
tagged BINAP ligand, we used the optically active (S)-5,5 -dibromo
ꢀ
BINAP dioxide ((S)-5,5 -dibromo BINAPO, 1) as the starting mate-
rial, which was obtained by a regioselective bromination of
BINAPO according to the method of Lemaire and co-workers [28].
ꢀ
The imidazole groups were introduced to the 5,5 -positions of
BINAPO through a l-proline promoted Cu-catalyzed C N bond
formation. During this reaction, minor amount of debrominated
monosubstituted product 5-imidazole BINAPO (3) was obtained
ꢀ
in addition to the major product 5,5 -diimidazole BINAPO (2). The
reduction of the phosphine oxide [28] was achieved using a mix-
ture of PhSiH3 and HSiCl3 to quantitatively obtain 5,5 -diimidazole
According to results of catalyst recycling, in the BINAP-
ꢀ
Ru/[bmim]Tf N/MeOH system the catalyst tends to transfer into
2
BINAP (4) and 5-imidazole BINAP (5). To evaluate the catalysis per-
formance of imidazolium-tagged BINAP ligands, the HBr salts (6 and
the product phase due to the weak polarity of BINAP molecule, and
thus leading to loss and oxidation of Ru-catalyst, which are the
main reasons that account for the dramatic loss of catalytic activ-
ity and enantioselectivity in the subsequent catalytic cycles. After
attaching two imidazolium groups to BINAP, the catalyst 6-Ru then
possessed better solubility in and stronger affinity to imidazolium-
based ILs due to their structural similarity, which greatly reduced
Ru-catalyst loss, stabilized the catalyst and improved the catalyst
recyclability. In spite of this, it is worth noting that ee values start to
drop obviously after 3 recycles (to 89%), which indicates that slow
7) of 4 and 5 as well as their corresponding Ru(II) dibromide com-
plexes were prepared in situ [29] to give the cationic Ru-catalysts.
Using BINAP-Ru as the reference, the catalysts 6-Ru and 7-
Ru were used in the hydrogenation of several ␤-keto esters in
methanol (Table 1). It can be seen that at a substrate/Ru molar ratio
◦
of 1000, 60 C temperature and 4 MPa hydrogen pressure, 6-Ru gave
comparably high activity and enantioselectivity as to (S)-BINAP-Ru
for all tested substrates. The results indicate that symmetrically

X. Jin et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 22–26
25
Scheme 2. Synthesis of imidazole- and imidazolium-tagged BINAP ligands.
Table 2
Performance of ligands 6 and 7 in Ru-catalyzed asymmetric hydrogenation of MAA in different reaction medium
a
O
O
OH
O
catalyst
Reaction medium
+
H2
OCH3
OCH3
Entry
Catalyst
Reaction medium
Conver. (%)
Ee (%)
1
2
3
4
5
6
7
8
9
6-Ru or 7-Ru or(S)-BINAP-Ru
6-Ru or 7-Ru or(S)-BINAP-Ru
6-Ru or 7-Ru or(S)-BINAP-Ru
6-Ru
6-Ru
7-Ru
7-Ru
(S)-BINAP-Ru
(S)-BINAP-Ru
[bmim]PF6
[bmim]BF4
[bmim]Tf2N
[bmim]BF4/MeOH
[bmim]Tf2N/MeOH
[bmim]BF4/MeOH
[bmim]Tf2N/MeOH
[bmim]BF4/MeOH
[bmim]Tf2N/MeOH
–
–
–
100
100
100
100
100
100
–
–
–
97
98
96
97
98
98
a
◦
Ru[(2-methylallyl)2(COD)] precursor 1.0 mg, p(H2) = 4 MPa, T = 60 C, t = 20 h, ligands/Ru = 1, substrate/Ru = 1000; entries 1–3: 1 g IL; entries 4–9: 1 g IL and 1 mL MeOH.
Table 3
Recycling and reuse of 6-Ru and (S)-BINAP-Ru in asymmetric hydrogenation of MAA in [bmim]Tf2N/MeOH system
a
O
O
OH
O
Catalyst
+
H
2
[
bmim]Tf N/MeOH
2
OCH3
OCH3
Run
6-Ru
(S)-BINAP-Ru
Conversion (SMHBb) (%)
Ee (%)
Conversion (SMHBb) (%)
Ee (%)
1
2
3
4
100 (100)c
100 (100)
100 (100)
100 (100)
98
96
92
89
100 (100)d
95 (76)
87 (52)
–
98
86
65
–
a
b
c
◦
Ru[(2-methylallyl)2(COD)] precursor 1.0 mg, p(H2) = 4 MPa, T = 60 C, t = 20 h, ligands/Ru = 1, substrate/Ru = 100, 1 g IL, 1 mL MeOH.
Selectivity to methyl hydroxybutyrate (MHB) due to formation of the corresponding ketal and hemiketal.
0
.27% of the Ru leached into ether extract.
.63% of the Ru leached into ether extract.
d
4

2
6
X. Jin et al. / Journal of Molecular Catalysis A: Chemical 374–375 (2013) 22–26
catalyst oxidation still occurred during the catalyst recycling due
to some contamination with traces of air [31,35].
[4] H. Takaya, T. Ohta, N. Sayo, H. Kumobayashi, S. Akutagawa, S. Inoue, I. Kasahara,
R. Noyori, J. Am. Chem. Soc. 109 (1987) 1596–1597.
[
5] R. Noyori, M. Ohta, Y. Hsiao, M. Kitamura, T. Ohta, H. Takaya, J. Am. Chem. Soc.
08 (1986) 7117–7119.
1
4
. Conclusion
[6] R. Noyori, T. Ohkuma, M. Kitamura, H. Takaya, N. Sayo, H. Kumobayashi, S.
Akutagawa, J. Am. Chem. Soc. 109 (1987) 5856–5858.
[
7] K. Mashima, K. Kusano, N. Sato, Y. Matsumura, K. Nozaki, H. Kumobayashi, N.
Sayo, Y. Hori, T. Ishizaki, J. Org. Chem. 59 (1994) 3064–3076.
ꢀ
In conclusion, using (S)-5,5 -dibromo BINAPO as the starting
ꢀ
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