Catalytic asymmetric hydrogenation in a room temperature ionic liquid using chiral Rh-complex of ionic liquid grafted 1,4-bisphosphine ligand

Article abstract of DOI:10.1039/b309304b

Introduction of imidazolium ionic liquid pattern to the catalyst not only avoided catalyst leaching but also increased the stability of catalyst in ionic liquid, and thus, the Rh-complex of 1,4-bisphosphine bearing two imidazolium salt moieties was succes

Full text of DOI:10.1039/b309304b

Catalytic asymmetric hydrogenation in a room temperature ionic liquid
using chiral Rh-complex of ionic liquid grafted 1,4-bisphosphine ligand†
Sang-gi Lee,*a Yong Jian Zhang,ab Jing Yu Piao,a Hyeon Yoon,c Choong Eui Song,a Jung Hoon Choib
and Jongki Hongc
a Life Sciences Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul
130-650, Korea. E-mail: sanggi@kist.re.kr
b Department of Chemistry, Hanyang University, Seoul 133-791, Korea
c
Hazardous Substance Research Team, Korea Basic Science Institute, Seoul 136-701, Korea
Received (in Cambridge, UK) 6th August 2003, Accepted 5th September 2003
First published as an Advance Article on the web 23rd September 2003
Introduction of imidazolium ionic liquid pattern to the
catalyst not only avoided catalyst leaching but also increased
the stability of catalyst in ionic liquid, and thus, the Rh-
complex of 1,4-bisphosphine bearing two imidazolium salt
moieties was successfully immobilized in an ionic liquid and
reused several times for the hydrogenation of an enamide
without significant loss of catalytic efficiency.
To facilitate the separation and subsequent reuse of chiral
catalysts, methods for immobilizing homogeneous catalysts
have been pursued for decades.1 One of the promising
approaches is the use of two-phase systems, in which the phase
of preference of the catalyst differs from that of the substrate,
allowing facilitation of catalyst recovery from products by
phase-separation.2 In this context, much attention has recently
been devoted to room temperature ionic liquids (ILs), in
particular, consisting of 1-butyl-3-methylimidazolium (bmim)
cations and their counter anions.3 In many cases, the catalysts
can be easily immobilized in ILs, and thus, separated by simple
phase-separation and recycled. In spite of their high potential as
vehicles for catalyst immobilization, relatively few examples on
the utilization of ILs in asymmetric catalytic hydrogenation
have been reported.4–7 The chiral Ru complexes of BINAP or
BINAP analogs were efficiently immobilized in ILs and reused
several times in the hydrogenation of olefins and ketones
without significant loss of catalytic efficiencies.5,6 In contrast to
Ru-complexes, the catalytic activity of air-sensitive chiral Rh-
bisphosphine catalysts such as the Rh-(R,R)-Me-DuPhos com-
plex was largely decreased after first run.7 Therefore, the
problems associated with leaching and/or stability of the
reactive chiral Rh-complex in ILs still remained to be solved.
Recently it has been clearly demonstrated that attachment of
imidazolium salts to achiral Rh-8and Ru-catalysts9 increased the
preferential solubility of the catalysts to IL, and the catalysts
were successfully reused without significant loss of catalytic
efficiency. During our ongoing efforts on the utilization of ionic
liquids for catalysis,10 we decided to apply this ionic tag
strategy to Rh-catalyzed asymmetric hydrogenation in ILs.
Herein we report ionic liquid grafted chiral Rh-complexes of
1,4-bisphosphine ligand 1 having two 1,2-dimethylimidazolium
salt tags resembling closely the IL reaction medium and its
catalytic efficiency and reusability in asymmetric hydro-
genation of an enamide in an IL.
diol was mesylated, then reacted with potassium diphenylphos-
phide to give the bisphosphine 5. All initial attempts to obtain
the imidazolium salt from 5 were unsuccessful due presumably
to P-methylation, and provided complicated products. Even-
tually, it was found that the use of the Rh-complex of
bisphosphine 5 could protect the P atoms during N-methylation
of imidazole moieties with Me3O+BF4 to give 1.13 Thus,
2
bisphosphine 5 was first mixed with [Rh(cod)2][BF4] in
methylene chloride for 30 min, and then treatment with
trimethyloxonium tetrafluoroborate at room temperature for 2 h
afforded the Rh-complex 1. In 31P NMR, the singlet P signal
(210.1 ppm) of 5 was shifted down field (32.6 ppm, d, JRh–P
=
139.2 Hz) indicating the formation of the C2-symmetric Rh-
complex 1.
The catalytic efficiencies of 1 and Me-BDPMI 2 (for
comparison), which exhibited excellent catalytic efficiency in
organic solvent,12 were examined in [bmim][SbF6]/iPrOH two-
phase system for the hydrogenation of an enamide (Table 1).‡
As expected, the ionic liquid grafted Rh-complex 1 was
successfully immobilized in the IL, which can be reused three
times without any loss of catalytic efficiency (entries 1 ~ 3). In
the fourth run, the catalytic activity was slightly decreased
The route for the synthesis of the chiral Rh-complex 1 is
illustrated in Scheme 1.† The 2-methylated imidazolium moiety
has been chosen because of the lability of the C(2)-methine
proton in imidazolium salt toward transition metals.11 The
bisphosphine 5, a precursor for 1, can easily be prepared from 3
which is obtained from tartaric acid as described previously.12a
The N,NA-dialkylation of 3 with 1-bromo-4-chlorobutane fol-
lowed by reaction with 2-methylimidazole afforded bis(imida-
zole) 4. After deprotection of the O-benzyl groups, the resulting
Scheme 1 Synthesis of 1. (i) (a) NaH/THF/Br(CH2)4Cl, reflux, 12 h (91%),
(b) NaH/DMF, 2-methylimidazole, rt, 40 h (90%). (ii) (a) Pd(OH)2/
cyclohexene/EtOH, reflux, 10 h (85%), (b) MsCl/Et3N, rt, 4 h (94%), (c)
KPPh2/THF–DMF(v/v = 10/1), rt, 8 h (48%). (iii) [Rh(cod)2]BF4/MC , rt,
30 min then Me3+OBF42 rt, 2 h.
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b3/b309304b/
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CHEM. COMMUN., 2003, 2624–2625
This journal is © The Royal Society of Chemistry 2003

Table 1 Rh-Catalyzed asymmetric hydrogenation of N-acetylphenylethena-
mine using 1 and Me-BDPMI 2 in [bmim][SbF6]/iPrOH two-phase solvent
systemsa
the volatiles, an ionic liquid, [bmim][SbF6](1 mL) and a solution of
substrate (50 mg, 3.1 3 1021 mmol) in iPrOH (2 mL) were added. The
mixture was hydrogenated under 1 atm. H2 at rt for 1 h. To determine the
conversion and enantioselectivity, the iPrOH layer was separated, and
subjected without any purification into GC equipped with CP-Chirasil-Dex-
CB chiral column and 1H NMR. For catalyst recycling, a degassed solution
of enamide in iPrOH was added again to the ionic liquid layer remaining in
the reaction vessel.
§ We also examined the catalytic activity of 2 in other ionic liquids such as
[bmim][PF6] (1st run: 100% conv., 2nd run: 100% conv., 3rd run: 78% conv.,
4th run: 51% conv.), [bmim][BF4] (1st run: 100% conv. 2nd run: 80% conv.,
3rd run: 50% conv. 4th run: 20% conv.).
Entry
Cat
1
Run
Time (h) Conv.(%)b %eec
1
2
3
4
5
6
7
8
9
1
2
3
4
4
1
2
3
4
4
1
1
1
100
100
100
82
97.0
96.6
96.2
95.4
95.4
95.8
95.1
94.2
91.4
88.0
95.6
1
1
1 (a) C. E. Song and S.-g. Lee, Chem. Rev., 2002, 102, 3495; (b) Q.-H.
Fan, Y. M. Li and A. S. C. Chan, Chem. Rev., 2002, 102, 3385.
2 (a) W. A. Hermann and C. W. Kohpainter, Angew. Chem., Int. Ed. Engl.,
1993, 32, 1524; (b) Aqueous-Phase Organometallic Catalysis, ed. B.
Cornils, W. A. Hermann, Wiley-VCH, Weinheim, 1998; (c) B. E.
Hanson, in Chiral Catalyst Immobilization and Recycling, ed. D. E. De
Vos, I. F. J. Vankelecom, P. A. Jacobs, Wiley-VCH, Weinheim, 2000,
p. 8196.
3 (a) P. Wasserscheid and W. Keim, Angew. Chem. Int. Ed., 2000, 39,
3772; (b) T. Welton, Chem. Rev., 1999, 99, 2071; (c) K. R. Seddon, J.
Chem. Tech. Biotechnol., 1997, 68, 351; (d) J. Dupont, R. F. de Souza
and P. A. Z. Suarez, Chem. Rev., 2002, 102, 3667.
8
1
1
1
100
100
100
78
2
2
1
51
10
11d
12
1
85
100
a Cat :substrate = 0.01 :1; [bmim][SbF6]/iPrOH = 1/2 (v/v); Reaction
temperature: 20 C; H2 Pressure: 1 atm. b Determined by NMR and GC.
o
c Determined by chiral GC using CP-Chirasil Dex CB column. d Reaction
carried out in the presence of 0.5 mol% of 2.
4 Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem., Int. Ed. Engl.,
1995, 34, 2698.
5 (a) A. L. Monterio, F. K. Zim, R. F. de Souza and J. Dupont,
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M. R. Delgado and J. Dupont, Tetrahedron: Asymmetry, 2001, 12,
1825.
6 (a) R. A. Brown, P. Pollet, E. McKoon, C. A. Eckert, C. L. Liotta and
P. G. Jessop, J. Am. Chem. Soc., 2001, 123, 1254; (b) P. G. Jessop, R.
R. Stanly, R. A. Brown, C. A. Eckert, C. L. Liotta, T. T. Ngo and P.
Pollet, Green Chem., 2003, 5, 123; (c) H. L. Ngo, A. Hu and W. Lin,
Chem. Commun., 2003, 1912.
(entry 4), but the reaction was completed when the reaction time
was prolonged to 8 h (entry 5). Moreover, the enantioselectivity
was also not much decreased. However, the catalytic efficiency
of Rh-Me-BDPMI complex 2 dropped significantly after two
cycles. Thus, the conversions and enantioselectivities were
decreased in the third (entry 8) and fourth (entry 9) runs. The
reactions did not complete even with prolonged reaction time
(entry 10).§
In ICP-AES analysis of the iPrOH layer separated from the
first run (entry 1) with 1, no Rh ( < 1 ppm) and phosphorus ( < 3
ppm) were found within the detection limits. Whereas 2% of Rh
and 6% of phosphorus atom were leached out to the iPrOH layer
during the phase-separation of the first run (entry 6). These
results clearly indicate that attachment of an imidazolium ionic
tag could increase the preferential solubility to ILs. However,
catalyst leaching is not the only reason for the decreased
catalytic activity of catalyst 2 upon recycling – the complete
conversion of the reaction carried out in the presence of 0.5
mol% of 2 (entry 11) supports this.
Taken all together, it could be concluded that an introduction
of imidazolim salt moieties on the ligand backbone not only
avoids the problem of catalyst leaching from the IL layer but
also increases the catalyst stability. Studies on the imidazolium
ionic tag strategy to increase the reusability and stability of the
other chiral catalysts in environmentally benign ionic liquids
will surely be continued.
7 S. Guernik, A. Wolfson, M. Herskowitz, N. Greenspoon and S. Geresh,
Chem. Commun., 2001, 2314.
8 (a) J. Sirieix, M. Ossberger, B. Betzemeier and P. Knochel, Synlett,
2000, 1613; (b) C. C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt
and P. Wasserscheid, Organometallics, 2000, 19, 3818; (c) F. Favre, H.
Olivier-Bourbigou, D. Commereuc and L. Saussine, Chem. Commun.,
2001, 1360; (d) P. Wasserscheid, H. Waffenschmidt, P. Machnitzki, K.
W. Kottsieper and O. Stelzer, Chem. Commun., 2001, 451; (e) R. P. J.
Bronger, S. M. Silva, P. C. J. Kamer and P. W. N. M. van Leeuwen,
Chem. Commun., 2002, 3044.
9 During preparation of this manuscript, ionic liquid supported Ru-
carbene complex has been reported. See (a) Q. Yao and Y. Zhang,
Angew. Chem. Int. Ed., 2003, 42, 3395; (b) N. Audic, H. Clavier, M.
Mauduit and J.-C. Guillemin, J. Am. Chem. Soc., 2003, 125, 9248.
10 (a) S.-g. Lee, J. H. Park, J. Kang and J. K. Lee, Chem. Commun., 2001,
1698; (b) C. E. Song, W. H. Shim, E. J. Rho, S.-g. Lee and J. H. Choi,
Chem. Commun., 2001, 1122; (c) C. E. Song, D.-u. Jung, E. J. Rho, S.-g.
Lee and D. Y. Chi, Chem. Commun., 2002, 3038; (d) S.-g. Lee and J. H.
Park, Bull. Korean. Chem. Soc., 2002, 23, 1367; (e) S.-g. Lee and J. H.
Park, J. Mol. Cat. A: Chem., 2003, 33, 2301.
This work was supported by NCCP, KIST, NRL program
from MOST and CMDS at KAIST
11 D. Bourissou, O. Guerret, F. P. Gabbaï and G. Bertrand, Chem. Rev.,
2000, 100, 39.
12 (a) S.-g. Lee, Y. J. Zhang, C. E. Song, J. K. Lee and J. H. Choi, Angew.
Chem. Int. Ed., 2002, 41, 847; (b) S.-g. Lee and Y. J. Zhang, Org. Lett.,
2002, 4, 2429.
13 The protection of phosphine group by a catalytically active Rh metal has
been utilized in the synthesis of a Rh-complex of tetrahydroxy
bisphosphine: See, J. Holz, D. Heller, R. Stürmer and A. Börner,
Tetrahedron Lett., 1999, 40, 7059.
Notes and references
‡ A mixture of 5 (2.8 mg, 3.7 3 1023 mmol) and added [Rh(cod)2]BF4 (1.3
mg, 3.1 3 1023 mmol) in methylene chloride (1 mL) was stirred for 30 min
at rt, and then, trimethyloxonium tetrafluoroborate (1.1 mg, 7.4 3 1023
mmol) was added and the mixture was stirred for 2 h. After evaporation of
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