Biphasic hydroformylation in ionic liquids: Interaction between phosphane ligands and imidazolium triflate, toward an asymmetric process

Article abstract of DOI:10.1039/b716015a

Biphasic hydroformylation of dec-1-ene and styrene, at commercially competitive rates, can be successfully performed in imidazolium triflate ionic liquids; the ionic liquid network forms 'inclusion complexes' with the phosphane ligands used to modify the rhodium catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716015a

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Biphasic hydroformylation in ionic liquids: interaction between
phosphane ligands and imidazolium triflate, toward an asymmetric
process{
Lo¨ıc Leclercq, Isabelle Suisse and Francine Agbossou-Niedercorn*
Received (in Cambridge, UK) 17th October 2007, Accepted 27th November 2007
First published as an Advance Article on the web 6th December 2007
DOI: 10.1039/b716015a
rate and selectivity of rhodium catalysed hydroformylation of dec-
1-ene and styrene under biphasic conditions (Fig. 1). We show
that, with a careful choice of the substituents on the imidazolium
cation, quite relevant activities can be attained as well as some
enantioselectivity. We also show how the interactions existing
between the IL and the solutes influence the overall properties of
the catalytic system.
Biphasic hydroformylation of dec-1-ene and styrene, at
commercially competitive rates, can be successfully performed
in imidazolium triflate ionic liquids; the ionic liquid network
forms ‘inclusion complexes’ with the phosphane ligands used to
modify the rhodium catalyst.
Biphasic homogeneous catalytic systems can be considered as the
most efficient ecological reaction media to allow proper recovery
and recycling of soluble catalysts.1 Fluorous-biphasic, aqueous-
biphasic, supercritical, and ionic liquids (ILs, which are salts that
melt below 100 uC)2 media have been developed for that purpose.
Currently, only an aqueous biphasic system is in industrial use for
the hydroformylation of propene and 1-butene catalysed by a
rhodium complex of the sodium salt of trisulfonated triphenylpho-
sphine (TPPTS). Due to the low solubility of higher alkenes in
water, the process is limited to light olefins. Several solutions have
been reported to enhance the solubility of higher olefins in an
aqueous medium viz. the use of co-solvents,3 of modified
cyclodextrins,4 and of surfactants.5 Unfortunately, the selectivity
towards the linear aldehyde product decreased even though the
rates were higher. In addition, the formation of stable emulsions
and leaching of the catalyst were sometimes observed.3,5
As mentioned by many authors, purity of the ILs is essential for
a catalytic process in order to avoid undesirable modifications of
the active species.9 Thus, we have used triflate based ILs either
commercially available or prepared according to our reported
procedure which avoids a route via intermediate imidazolium
chlorides.10 As
a control experiment, we carried out the
hydroformylation of dec-1-ene in the presence of Rh(CO)2(acac)
in [BMIM][TfO]{ without a phosphane ligand (unmodified
catalyst). A high activity (1315 h21) was observed along with a
low regioselectivity (n/iso = 0.5). These values are typical of ligand
free rhodium catalysed hydroformylations11 and close to the
results reported by Mehnert.9 We next performed the hydro-
formylation of dec-1-ene in different ILs.{ The rhodium catalysts
were prepared from Rh(CO)2(acac) and three phosphorus ligands
(TPP, TPPMS, TPPTS) (Table 1). The initial turnover frequencies
(TOF0) were generally high. Interestingly, the TOF0 measured for
catalysts obtained from Rh(CO)2(acac) and TPP increased linearly
with the length of the alkyl substituent on the imidazolium cation
(entries 1, 3, 5, 8, and 10). This can be related to the decrease of the
overall organisation of the ILs with the increase of the imidazolium
alkyl length. The resulting reduction of hydrogen bonding and
enhancement of the tensioactivity12 within the ILs are beneficial to
the efficiency of the catalytic system. Concomitantly, the solubility
of the alkene and CO/H2 rises with the increase of the length of the
alkyl side chain on the imidazolium cation.13 The impact of the
ionic liquid on the rate of the hydroformylation strongly suggests
that the reaction is occurring in the ionic liquid phase.
Organic–IL systems can constitute interesting alternatives to
aqueous biphasic media. Indeed, rhodium catalysed hydroformy-
lation has been performed in organic–IL systems and the catalytic
results are dependent upon the combination of cations, anions,
and phosphorus ligands. For example, 1-butyl-3-methylimidazo-
lium hexafluorophosphate [BMIM][PF6] has been applied in the
hydroformylation of light olefins catalysed by a rhodium complex
of triphenylphosphine (TPP) or TPPTS which is retained in the IL
phase.6 For higher olefins, the phase distribution of the
components of the catalytic system and the phase in which the
catalytic reaction is occurring are central issues for successful
processes.7 Also, the exact solvation patterns of solutes, including
the catalytic species, still need to be assessed in neat ILs or in the
key domain of the interface, even if recent molecular simulations
provide some information.8
As part of our studies on catalytic reactions in ILs, we report
here on the influence of various imidazolium triflate salts on the
Universite´ des Sciences et Technologies de Lille 1, CNRS Unite´ de
Catalyse et de Chimie du Solide UMR 8181, ENSC(CHIMIE), BT C7,
BP 90108, 59652, Villeneuve d’Ascq Cedex, France.
E-mail: francine.agbossou@ensc-lille.fr; Fax: +33 3 20 47 05 99;
Tel: +33 3 20 43 49 27
{ Electronic supplementary information (ESI) available: Experimental
data, titration and T-ROESY. See DOI: 10.1039/b716015a
Fig. 1 Hydroformylation of dec-1-ene and styrene in IL-biphasic
systems.
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Chem. Commun., 2008, 311–313 | 311

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Table 1 Hydroformylation of dec-1-ene in imidazolium triflate ionic
liquidsa
Conv. Sel.
Phosphane (%)b (%)b n/isob (h21 c
TOF0
)
Entry Cation
1
2
3
4
5
6
7
8
9
[EMIM]
[EMIM]
[BMIM]
[BMIM]
[MBMIM] TPP
[MBMIM] TPPMS
[MBMIM] TPPTS
[HMIM]
[HMIM]
[OMIM]
[OMIM]
TPP
58
62
88
89
97
98
99
100
97
99
98
85
82
89
89
87
90
88
90
86
72
66
2.8
2.1
2.8
2.2
2.7
2.4
1.8
2.7
1.4
2.7
1.2
400
760
530
960
700
925
1300
910
TPPTS
TPP
TPPTS
TPP
TPPTS
TPP
TPPTS
1045
1260
1225
10
11
a
Fig. 2 31P NMR titration profile for addition of [MBMIM][TfO] to
Reaction conditions: Rh(CO)2(acac)
= 0.02 mmol, ligand =
0.105 mmol; IL = 4 mL, n-undecane = 2.03 mmol, dec-1-ene =
phosphanes in [D8]THF at 300 K.
b
20.60 mmol, CO/H2 (1 : 1) = 50 bar, 80 uC, 1500 rpm. GC
(Chrompack CP-92001, CPSil-5CB (25 m 6 0.12 mm)) after 6 h.
Initial turnover frequency (mole of alkene converted per mole of
rhodium per h).
c
To substantiate our assumptions, 2D NMR experiments have
been performed with the aim of detecting spatial interactions
between the sulfonated ligand TPPMS and the pure
[MBMIM][TfO]. T-ROESY data, which are anticipated to be
helpful for our purpose,17 have been acquired on a coaxial tube
containing a mixture of TPPMS and [MBMIM][TfO] (16% m/m)
placed in an NMR tube charged with D2O. The existence of cross-
peaks between the sulfonated and unsubstituted aromatic protons
of the ligand and the protons located on the imidazolium ring fully
proves the interactions and gives information on the arrangement
between the solute and the IL (Fig. 3). The weak cross-peaks
observed between the [MBMIM] protons H(2), H(4), H(5),
N–CH3, H(9), and H(10) of the imidazolium cation and those of
the sulfonated phenyl residue of the TPPMS suggest that the
TPPMS exchanges the sodium for an imidazolium cation in the
mixture. On the other side, the non-sulfonated phenyl residue
interacts strongly with H(4), H(5), N–CH3, H(6), H(9), and H(10).
This NMR pattern establishes the existence of p-stacking
interactions between the two aromatic rings (imidazolium and
We next replaced the TPP phosphane by the sulfonated
derivatives TPPMS and TPPTS. The TOF0 increased but the
n/iso ratio diminished significantly (entries 5–7). The n/iso ratio and
the reaction rate tend to shift to the values of the control
experiment mentioned above, typically observed for unmodified
rhodium catalysts.11 The equilibrium existing in the reaction
medium generating the active species selective for either the linear
or the branched aldehyde product is influenced by the interactions
between the IL and the phosphine ligand. In other words, the
stronger the interactions between the IL and the phosphane, the
less ‘mobile’ the phosphane in the IL medium. This slowed motion
probably limits the formation of the HRh(CO)L2 species selective
for the linear product formation. The role that an ionic liquid can
play in the formation of active species in transition-metal catalysed
hydrogenations has been stressed.14 It is clear here that the
hydroformylation results are dependent on both the mobile phase
and the phosphine on the catalyst. It can be suggested that in the
presence of sulfonated phosphines, the active catalytic species is
essentially in the ionic liquid phase, whereas in the presence of TPP
the hydroformylation can take place in the ionic liquid phase and
in the organic phase.15
The imidazolium salt can be considered as a molecular receptor
through hydrogen bonding and p-stacking interactions with the
phosphane ligands. It’s easy to deduce that, in the case of neutral
TPP, the properties of the catalyst are preserved with respect to
those obtained in a pure organic medium. In contrast, the catalytic
properties displayed in the presence of the ionic phosphanes
TPPMS and TPPTS can be altered in the ILs. To confirm these
supramolecular interactions, we performed an NMR study.
First, we carried out 31P NMR titration experiments on
phosphane ligands in 1-(2-methylbutyl)-3-methylimidazolium tri-
flate ([MBMIM][TfO]) (Fig. 2). The 31P NMR spectra were
recorded in [D8]THF solutions containing identical concentrations
of TPP or TPPMS while varying the concentration of the IL
[MBMIM][TfO].16 We could deduce a 1 : 1 complex between
[MBMIM][TfO] and the two phosphanes. We propose that, for
the sulfonated phosphane, the IL is principally an anion receptor
Fig. 3 Partial contour plot of T-ROESY spectrum of mixture containing
TPPMS and MBMIM 16% (m/m), 300 K, external lock: D2O. The
structure deduced for the MBMIM–TPPMS complex is also presented.
2
…
through C–H
X hydrogen bonding and, for the neutral TPP,
other supramolecular links like p-stacking interactions take place.
312 | Chem. Commun., 2008, 311–313
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Table 2 Rh–TPPTS catalysed hydroformylation of styrene in
[MBMIM][TfO]a
higher olefins. The reaction proceeds at commercially competitive
rates with good separation ability. The decrease observed in the
linear to branched aldehydes ratio for dec-1-ene and the drop in
activity is due to ‘inclusion complexes’ formation between
imidazolium cations and ionic phosphane ligands. Such complexes
were also responsible for the asymmetric induction observed
during styrene hydroformylation. The combination of an achiral
organometallic catalyst with a chiral IL acting as both the reaction
medium and the chiral inducer is a way to induce stereoselectivity
during a catalytic process.18d The IL is interacting with species
present during catalysis providing elements valuable for further
developments. These results support the idea that the supramo-
lecular assistance by the imidazolium salts is a phenomenon of
general importance even if complex. This approach seems
particularly suitable when ionic species intervene in the catalytic
process. Due to the current interest in the development of the
preparation of chiral ILs, fruitful developments are expected for
asymmetric catalytic applications.
Conv. Sel.
Entry Chirality T (uC) (%)b (%)b n/isob (h21 c
TOF0
)
ee
(%)d
1
2
3
4
a
¡
S
S
80
80
50
45
98
97
98
98
96
98
97
98
0.2
0.2
0.1
0.1
1610
rac
4
6
12
1560
1095
655
S
Reaction conditions: Rh(CO)2(acac)
= 0.02 mmol, ligand =
0.105 mmol; [MBMIM][TfO] = 4 mL, n-undecane = 2.03 mmol,
styrene = 20.60 mmol, CO/H2 (1 : 1) = 50 bar, 80 uC, 1500 rpm.
b
GC (Chrompack CP-92001, CPSil-5CB (25 m 6 0.12 mm)) after
6 h. Initial turnover frequency (mole of alkene converted per
c
d
mole of rhodium per h). GC (Chrompack CP-93001, Chirasil-Dex
(25 m 6 0.32 mm) (R) enantiomer).
phenyl). Wipff has also deduced from simulations that the cations
[BMIM]+ are p-stacked with the phenyl moieties of TPPMS
ligands and form hydrogen bonds with the sulfonate groups.8 For
small dilutions of the pure mixture (up to 25% m/m in [D8]THF),
similar cross-peaks remain between the unsubstituted phenyl
group of TPPMS and the MBMIM cation. Nevertheless, the
contacts with the sulfonated phenyl group are lost. For higher
dilutions (over 50% m/m in [D8]THF), all contacts are lost; only
anion exchange remains.
This work was supported by the CNRS and the Ministe`re de la
Recherche (grant to L. L.). We are most grateful to Dr Fre´de´ric
Hapiot for T-ROESY experiments.
Notes and references
Finally, we wondered whether a chiral ionic liquid (CIL)18
would induce stereoselectivity in the hydroformylation of styrene.
The results are reported in Table 2.
{ Cations: [EMIM] 1-ethyl-, [BMIM] 1-butyl-, [MBMIM] 1-(2-methyl-
butyl)-, [HMIM] 1-hexyl-, [OMIM] 1-octyl-3-methyl-imidazolium.
1 B. Cornils, J. Mol. Catal. A: Chem., 1999, 143, 1.
2 T. Welton, Chem. Rev., 1999, 99, 2071.
Interestingly, a reaction carried out in the optically pure IL (S)-
[MBMIM][TfO] proceeded with some enantioselectivity (4% ee)
(entry 2). Thus, the strong interactions, i.e. hydrogen bonding and
p-stacking existing between the TPPTS anion and the chiral
imidazolium cation, are creating a chiral environment around the
rhodium active centre immobilised in the IL. The latter is thus able
to transmit chiral information. However, the solvation of the other
solutes of the catalytic mixture by chiral imidazolium cations can
probably also play a role in the transmission of the chiral
information.
3 P. Purwanto and H. Delmas, Catal. Today, 1995, 24, 135.
4 E. Monflier, H. Bricout, F. Hapiot, S. Tilloy, A. Aghmiz and
A. M. Masdeu-Bulto, Adv. Synth. Catal., 2004, 346, 425.
5 C. Yang, X. Y. Bi and Z. S. Mao, J. Mol. Catal. A: Chem., 2002, 187,
35.
6 Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem., Int. Ed. Engl.,
1995, 34, 2698.
7 J. Dupont, R. F. de Souza and P. A. Z. Suarez, Chem. Rev., 2002, 102,
3667.
8 N. Sieffert and G. Wipff, J. Phys. Chem. B, 2007, 111, 4951.
9 C. P. Mehnert, R. A. Cook, N. C. Dispenziere and E. J. Mozeleski,
Polyhedron, 2004, 23, 2679.
Interaction of the metal centre with the IL might provide altered
catalytic species.19 The results of the control experiment carried out
in the IL and in the absence of any external ligand (see above)
suggest we can rule out the participation of, for example,
imidazolylidene-rhodium species in the catalytic process. We could
increase the enantioselectivity up to 12% ee while decreasing the
temperature from 80 to 45 uC (entries 2–4). Although it is a
challenging task to induce enantioselectivity during a hydrofor-
mylation reaction, here, a quite ‘simple’ chiral IL is inducing
selectivity in the hydroformylation of styrene. The imidazolium salt
can be considered a chiral molecular receptor through hydrogen
bonding and p-stacking interactions for the achiral catalyst.
Recycling experiments of the Rh–TPPMS catalyst in
[MBMIM][TfO] were performed by decantation of products.
After washing with hexane and drying, the mixture containing the
rhodium complex was charged with dec-1-ene and H2/CO and
placed in identical reaction conditions. The activity and selectivity
of the catalyst were the same after five consecutive recyclings. This
is indicative of the stability of the catalyst in the IL medium and of
its recycling potential.
10 L. Leclercq, I. Suisse, G. Nowogrocki and F. Agbossou-Niedercorn,
Green Chem., 2007, 9, 1097.
11 B. Cornils, in New Synthesis with Carbon Monoxide, ed. J. Falbe,
Springer, Berlin, 1980, p. 1.
12 M. G. Freire, P. J. Carvalho, A. M. Fernandes, I. M. Marrucho,
A. J. Queimada and J. A. P. Coutinho, J. Colloid Interface Sci., 2007,
314, 621.
13 J. Dupont and P. A. E. Suarez, Phys. Chem. Chem. Phys., 2006, 8, 2441.
14 (a) P. A. Z. Duarez, J. E. L. Dullius, S. Einloft, R. F. de Souza and
J. Dupont, Inorg. Chim. Acta, 1997, 255, 207; (b) C. Daguenet and
P. J. Dyson, Organometallics, 2006, 25, 5811.
15 J. Dupont, S. M. Silva and R. F. de Souza, Catal. Lett., 2001,
77, 131.
16 (a) Z. Wang, Q. Wang, Y. Zhang and W. Bao, Tetrahedron Lett., 2005,
46, 4657; (b) P. Bonhoˆte, A.-P. Dias, N. Papageorgiou,
K. Lalyanasundaram and M. Gra¨tzel, Inorg. Chem., 1996, 35, 1168.
17 J. Dupont, J. Braz. Chem. Soc., 2004, 15, 341.
18 (a) C. Baudequin, D. Bregeon, J. Levillain, F. Guillen, J. C. Plaquevent
and A.-C. Gaumont, Tetrahedron: Asymmetry, 2005, 16, 3921; (b)
L. C. Bronco, P. M. P. Gois, N. M. T. Lourenc¸o, V. B. Kurteva and
C. A. M. Afonso, Chem. Commun., 2006, 2371; (c) P. S. Schulz,
N. Mu¨ller, A. Bo¨smann and P. Wasserscheid, Angew. Chem., Int. Ed.,
2007, 46, 1293; (d) M. Schmitkamp, D. Chen, W. Leitner,
J. Klankermayer and G. Francio, Chem. Commun., 2007, 4012.
19 C. J. Mathews, P. J. Smith, T. Welton, A. J. P. White and D. J. Williams,
Organometallics, 2001, 20, 3848.
In conclusion, we have demonstrated that imidazolium triflate
salts can be used in an IL-biphasic hydroformylation process of
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Chem. Commun., 2008, 311–313 | 313