Highly efficient enantio-selective hydrogenation of methyl acetoacetate over an improved tartaric acid-modified raney nickel catalyst

Article abstract of DOI:10.1246/cl.2006.910

Using methanol as the reaction medium (an appropriate amount of sodium bromide was directly added to it), the highly efficient enantio-selective hydrogenation of methyl acetoacetate (MAA) was achieved over a tartaric acid-modified Raney nickel catalyst. 85% ee was attained under the comparatively mild conditions (0.6 MPa, 60°C, 1 h). Copyright

Full text of DOI:10.1246/cl.2006.910

910
Chemistry Letters Vol.35, No.8 (2006)
Highly Efficient Enantio-selective Hydrogenation of Methyl Acetoacetate over
an Improved Tartaric Acid-modified Raney Nickel Catalyst
Hangning Chen, Rong Li, Huanling Wang, Liang Yin, Fushan Wang, and Jiantai Maꢀ
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China
(Received April 24, 2006; CL-060489; E-mail: majiantai@lzu.edu.cn)
Using methanol as the reaction medium (an appropriate
ifying solution (94 ꢁC, 250 mL, cTA ¼ 0:2 mol/L) and modified
amount of sodium bromide was directly added to it), the highly
efficient enantio-selective hydrogenation of methyl acetoace-
tate (MAA) was achieved over a tartaric acid-modified Raney
nickel catalyst. 85% ee was attained under the comparatively
mild conditions (0.6 MPa, 60 ꢁC, 1 h).
twice. pH of the modifying solution was adjusted to the required
value using a potentiometric titration with 1 mol/L NaOH. After
that, the catalyst was successively washed 5 times with 250 mL
of distilled water and 100 mL of methanol. In this step, no NaBr
was added into the modifying solution. Finally, the modified
catalyst (0.2 g) prepared in this manner was quickly introduced
into 4 mL of reaction solvent, in which an appropriate amount
of NaBr was directly added, then introduced 2 mL of MAA
(Acros 99+%). The hydrogenation reaction was carried out at
60 or 100 ꢁC. A stainless steel autoclave (100 mL) with a mag-
netic stirrer was used for high-pressure hydrogenation reactions.
The initial hydrogen pressure was 0.6–9 MPa and decreased as
reaction progressed. In order to obtain 100% conversion of
the substrate, all the reactions were carried out until no further
consumption of hydrogen was observed.
Increased attention has focused on the heterogeneous enan-
tio-selective catalysts, because they can be separated and reused
easily. One of the few heterogeneous catalytic systems identified
for asymmetric hydrogenation with synthetically useful enantio-
selectivities is the tartaric acid-modified Raney nickel (TA-
MRNi), which can hydrogenate various ꢀ-ketoesters (80–98%
ee)1–3 and 2-alkanones (72–85% ee)4,5 with high optical yields.
In the traditional method, the TA-MRNi catalyst was pre-
pared by soaking the Raney nickel catalyst in a hot slight acidic
aqueous solution of tartaric acid and NaBr, based on the idea that
the non-enantio-differentiating (n.e.d) sites could be removed at
least in part by acid-corrosion and the rest could be deactivated
by poisoning with the NaBr adsorbed.6 Harada et al. demonstrat-
ed that inorganic salts in the modification solution increased the
optical yield for the hydrogenation over the modified Raney
nickel but retarted the hydrogenation rate.7
In the 1990’s, Tai et al. proposed the ultrasonic-irradiation
method for washing Raney nickel. The improved TA-MRNi-U
catalyst showed high enantio-selectivity and hydrogenation
activity in the hydrogenation of a series of ꢀ-ketoesters and
1,3-diketones. 86% ee was achieved in the hydrogenation of
MAA.2 Recently, Osawa and co-workers applied in situ modifi-
cation (l-(+)-tartaric acid and NaBr were directly added to the
reaction media) of fine Ni powder and reduced Ni catalysts to
the enantio-selective hydrogenation of MAA.8–10 Appling the re-
duced Ni, a high optical yield of 89% was attained. However,
those aforementioned results were achieved under high initial
hydrogen pressure (9–10 MPa) and long reaction time as those
of the previous reports, which revealed the low hydrogenation
activity of the TA-MRNi catalyst.
The optical yield was determined polarimetrically by means
of the optical activity measurement of the product in its concen-
trated state. Digital polarimeter Perkin-Elmer polarimeter-341
(U.S.A.) was used for measurements. The optical yield (ee)
was then calculated in accordance with the following equation:
[a]20 product
D
e.e. ¼ oy (%) ¼
ꢃ 100
ꢂ22:95
Reaction rate as well as enantio-selectivity was clearly de-
ˇ
´
pendent on the type of reaction medium. Kukula and Cerveny
had tested that the reaction performed in methanol was 5 times
faster than in THF, but the enantio-selectivity was significantly
lower. When using methanol as the reaction medium, the highest
ee of 37.3% was attained.13 Therefore, in order to obtain a high
optical yield, the aprotic semi-polar solvents were always used
as the reaction medium. In contrast with previous investiga-
tions,2,13,14 from Table 1, it could be seen that the hydrogenation
activity and the enantio-selectivity were as follows, THF <
BuOH < EtOH < MeOH. It showed that high reaction rate
and enantio-selectivity could easily be achieved when methanol
served as the reaction medium.
Modifying pH had a considerable effect on the optical yield
The aim of our present investigation was to enhance the
enantio-selective efficiency of the TA-MRNi catalytic system
and reduce the demand of reaction conditions, based on the
idea of directly adding NaBr to the hydrogenation solvent as a
‘‘promoter.’’11
Table 1. Dependence of enantio-selectivity and reaction time
on different types of solventa
Entry
Solvent
Reaction time/h
ee/%
1
2
3
4
THF
BuOH
EtOH
MeOH
3
2
1
0.4
24
64
71
75
In the present experiment, W-4 type of Raney Ni was freshly
prepared with the conventional method.12 The obtained Raney
Ni (usually 3g) was washed 5 times with 250 mL of distilled
water, and then with a so-called pre-modification, 200 mL of
1 wt % aqueous solution of l-(+)-tartaric acid (Acros 99+%).
This washing procedure was carried out with stirring, using a
magnetic stirrer for a period of 10 min. The catalyst was again
washed with 250 mL of distilled water, and introduced to a mod-
aModifying conditions: cTA ¼ 0:2 mol/L, pH ¼ 4:0, T ¼
94 ꢁC, t ¼ 60 min; Reaction conditions: solvent (4 mL),
MAA (2 mL), catalyst (0.2 g), NaBr (10 mg), H2 pressure
(6 MPa), 60 ꢁC.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.8 (2006)
911
Table 2. Enantio-selective hydrogenation of MAA under dif-
ferent reaction conditionsa
90
80
70
60
50
40
30
Entry
Catalyst
T/ꢁC H2/MPa tReact/h ee/%
1
2
3
4
5
6
7
8
9
TA-MRNi-Ub
TA-MRNi
TA-MRNi
TA-MRNi
TA-MRNi
TA-MRNi
TA-MRNi
60
60
60
60
60
100
100
6
9
6
2
0.6
6
2
9–10
9–10
0.4
0.3
0.4
0.8
1
0.3
0.6
5
81
77
83
84
85
75
75
80
86
2
3
4
5
6
7
8
TA-NaBr-MRNic 100
Modifying pH
TA-MRNi-Ud
100
2.5
Figure 1. Dependence of the enantio-selectivity on modifying
pH. Modifying conditions: cTA ¼ 0:2 mol/L, T ¼ 94 ꢁC, t ¼
60 min; Reaction conditions: methanol (4 mL), MAA (2 mL),
catalyst (0.2 g), NaBr (10 mg), H2 pressure (6 MPa), 60 ꢁC.
aModifying conditions: cTA ¼ 0:2 mol/L, pH ¼ 4:4, T ¼
94 ꢁC, t ¼ 60 min; Reaction conditions: MAA (2 mL), meth-
anol (4 mL), catalyst (0.2 g), NaBr (15 mg). bThe ultrasonic-
irradiated Raney nickel was used. cRef. 14, modifying condi-
tion: cTA ¼ 1 wt %, cNaBr ¼ 10 wt %, pH ¼ 3:2; Reaction
solvent: methyl propionate. dRef. 2, the modifying conditions
and reaction solvent were the same as Ref. 14, except that the
ultrasonic-irradiated Raney nickel was used.
90
80
70
60
50
40
30
20
10
0.4 h
enantio-selectivity and hydrogenation rate were prominently
affected by the reaction temperatue. In addition, it could be seen
that the enantio-selectivity and the hydrogenation activity had
not been significantly improved by employing the ultrasonic-
irradiated Raney nickel. In our research, the high enantio-selec-
tivity of 85% was achieved in the hydrogenation of MAA under
the much less demanding conditions (0.6 MPa, 60 ꢁC, 1 h),
which strongly suggested the highly enantio-selective hydroge-
nation efficiency of the improved TA-MRNi/methanol, NaBr
catalytic system.
2 h
0
5
10 15 20 25 30
The amount of NaBr /mg
Figure 2. Dependence of enantio-selectivity on the amount
of NaBr. Modifying conditions: cTA ¼ 0:2 mol/L, pH ¼ 4:4,
T ¼ 94 ꢁC, t ¼ 60 min; Reaction conditions: methanol (4 mL),
MAA (2 mL), catalyst (0.2 g), H2 pressure (6 MPa), 60 ꢁC.
References
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3
4
5
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and 20 mg. These results indicated that Brꢂ could selectively de-
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pressure between 0.6 and 6 MPa, while the reaction time was
obviously dependent on the pressure. It also showed that the
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Published on the web (Advance View) July 8, 2006; doi:10.1246/cl.2006.910