322 Du et al.
Asian J. Chem.
= 1 (4.63), D2O (2.82). And the activity and enantioselectivity
were also decreased as this trend, the conversion decreased
from 70 to 12 % and the enantioselectivity decreased from 29
to 10 %. Although, compared to methanol (or CD3OD), water
is a stronger hydrogen bond donor, it seems that the methanol
might be easier to form hydrogen bond with the carbonyl oxygen
of ethyl pyruvate. The negative effect of water was probably
due to rapid hydration of the activated ketone, as reported by
Baiker et al.22 that even 0.8 vol % water in the system could
lead to approximate 1 % ethyl pyruvate hydrated (Fig. 2b).
The hydrogenation of C-OH bond is much slower compared
to ketone23 and the hydration would hinder the coordination
of α-carbonyl with iridium complex, so the presence of trace
water caused the activity and the enantioselectivity to decrease.
When acetic acid was used as solvent, though the hydrogen
bond could be formed between the acetic acid and the ethyl
pyruvate, the NH2 group of the ligand could be protonated
significantly by acetic acid, which was unfavorable for the
coordination of ligand with iridium complex. So under our
reaction conditions, the hydrogen bond between the α-carbonyl
group of ethyl pyruvate and the solvent could be formed easily
in MeOH (or CD3OD), the α-carbonyl was activated by the
hydrogen bond and could be hydrogenated more easily. There-
fore, the highest conversion and enantioselectivity of ethyl
pyruvate were observed in methanol.
selectivity was 39 % for iridium catalysts). NMR experiments
have revealed that the hydrogen bond between the solvent and
the α-carbonyl group of ethyl pyruvate facilitated this hydro-
genation reaction.
ACKNOWLEDGEMENTS
Financial support of the National Science Talents Fund
for Research Training and ResearchAbility Improvement project
(J1103315) was gratefully acknowledged.
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TABLE-3
PEAK AREAS AND AREA RATIOS FOR
THE TWO PARTS OF Ha AND Hb
-OCH2-
Haa
1.60 0.40
CH3CO-
Hba
0.42
Solvent
Hab
Hab/Haa
4.00
Hbb
2.57
Hbb/Hba
6.12
CD3OD
CD3OD: D2O
= 6:1
CD3OD: D2O
= 3:1
1.57 0.40
3.93
3.02
2.51
2.57
0.47
0.54
5.34
4.76
1.51 0.50
CD3OD: D2O
= 1:1
1.46 0.55
1.43 0.59
2.65
2.42
2.41
2.20
0.52
0.78
4.63
2.82
D2O
athe integral area for the protons of non-hydrogen bonding part
bthe integral area for the protons of hydrogen bonding part
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Conclusion
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In conclusion, we have demonstrated a highly active (1R,
2R)-DPEN modified [Ir(COD)Cl]2 for the enantioselective
hydrogenation of ethyl pyruvate, 29 % ee was obtained by
our catalysis system (up to date, the best value of enantio-