TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 1825–1828
Ionic liquid-phase asymmetric catalytic hydrogenation: hydrogen
concentration effects on enantioselectivity
Alexsandro Berger, Roberto F. de Souza, Marcelo R. Delgado and Jairton Dupont*
Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Av. Bento Gon c¸ alves,
9500 Porto Alegre 91501-970 RS, Brazil
Received 19 March 2001; accepted 23 July 2001
Abstract—Molecular hydrogen is almost four times more soluble in the ionic liquid 1-n-butyl-3-methylimidazolium tetra-
fluoroborate (BMI·BF ) than in its hexafluorophosphate (BMI·PF ) analogue at the same pressure. The Henry coefficient
4
6
−
3
−1
−1
−4
−1
−1
solubility constant for the solution BMI·BF /H is K=3.0×10 mol L atm and 8.8×10 mol L atm for BMI·PF /H , at
4
2
6
2
room temperature. The asymmetric hydrogenation of (Z)-a-acetamido cinnamic acid and kinetic resolution of (±)-methyl-3-
hydroxy-2-methylenebutanoate by (−)-1,2-bis((2R,5R)-2,5-diethylphospholano)benzene(cyclooctadiene)rhodium(I) trifluoro-
methanesulfonate and dichloro[(S)-(−)-2,2%-bis(di-p-tolylphosphino)-1,1%-binaphthyl]ruthenium(II) complexes immobilised in
BMI·PF and BMI·BF were investigated. Remarkable effects in the conversion and enantioselectivity of these reactions were
6
4
observed as a function of molecular hydrogen concentration in the ionic phase rather than pressure in the gas phase. © 2001
Elsevier Science Ltd. All rights reserved.
1
. Introduction
neous asymmetric hydrogenations the solution concen-
tration of hydrogen, rather than the pressure in the
gas phase, is the kinetic variable that must be consid-
ered when evaluating the enantioselectivity data
obtained under different gas–liquid mass transfer con-
Molten salts based on the 1-alkyl-3-methylimidazolium
cation exhibit a relatively wide electrochemically stable
window, a broad range of room temperature liquid
compositions, negligible vapour pressure, and excellent
chemical and thermal stabilities. These materials have
been used as the mobile phase in organometallic
biphasic catalytic reactions ranging from Ziegler–
Natta-type processes to hydroformylations.
ever, in only a few cases, asymmetric organometallic
catalytic reactions have been carried out in ionic liq-
10
ditions.
Herein, we report our results for the determination of
the solubilities of molecular hydrogen in these imida-
zolium ionic liquids and the influence of the hydrogen
concentration over the asymmetric hydrogenation of
1
–3
How-
(
Z)-a-acetamido cinnamic acid as well as the asym-
4
–9
metric kinetic resolution of (±)-methyl-3-hydroxy-2-
methylenebutanoate by Rh(I)- and Ru(II)-catalyst
uids.
tion processes.
hydrogenation of atropic acid by Ru-BINAP
BINAP=2,2%-bis(diphenylphosphino)-1,1%-binaphthyl)
These studies essentially comprise hydrogena-
4,5,9
In this respect, it was found that the
precursors immobilised in BMI·PF and BMI·BF4
6
11
ionic liquids.
(
complex immobilised in 1-n-butyl-3-methylimida-
zolium tetrafluoroborate (BMI·BF4) furnishes 5e.e.
essentially independent of the hydrogen-pressure. In
contrast, in the reduction of tiglic acid by the same
catalyst dissolved in 1-n-butyl-3-methylimidazolium
2. Results and discussion
2
.1. Hydrogen solubility in 1-n-butyl-3-methylimida-
zolium ionic liquids
hexafluorophosphate (BMI·PF ) ionic liquid, the e.e. is
6
9
hydrogen-pressure dependent. These apparently confl-
icting results are probably related to a different hydro-
gen solubility in the two ionic liquids. Moreover, it is
well known that in various homogenous and heteroge-
The gas–liquid mass transfer coefficients and molecu-
lar hydrogen solubilities in the ionic liquids have been
determined using a known procedure (see Section 4).
12
The Henry coefficient solubility constant for the solu-
−
3
−1
−1
tion BMI·BF /H is K=3.0×10 mol L atm and
4
2
−1
−
4
−1
*
0
Corresponding author.
8.8×10 mol L atm for BMI·PF /H , at room tem-
6 2
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