Tetronics/cyclodextrin-based hydrogels as catalyst-containing media for the hydroformylation of higher olefins

Article abstract of DOI:10.1039/c6cy02070d

The rhodium-catalyzed hydroformylation of alkenes has been investigated under biphasic conditions using combinations of α-cyclodextrin (α-CD) and poloxamines (Tetronics). Thermo-responsive hydrogels containing the Rh-catalyst are formed under well-defined conditions of concentration. Hydrogels consisting of the reverse-sequential Tetronic 90R4 prove to be more effective than those containing the conventional sequential Tetronic 701. The presence of α-CD is crucial to provoke the decantation of the multiphasic system once the reaction is complete. Optimized conditions (CO/H₂ pressure, Rh-precursors, phosphanes, etc.) show that the catalytic system is especially applicable to the hydroformylation of terminal alkenes. The catalytic performance remains unchanged upon recycling as the hydrogel matrix prevents the oxidation of the phosphane.

Full text of DOI:10.1039/c6cy02070d

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Tetronics/cyclodextrin-based hydrogels as catalyst-containing
media for the hydroformylation of higher olefins
M. Chevry,a T. Vanbésien,a S. Menuel,a E. Monflier,a and F. Hapiot*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The rhodium-catalyzed hydroformylation of alkenes has been investigated in biphasic conditions using combinations of α-
cyclodextrin (α-CD) and poloxamines (Tetronics®). Thermo-responsive hydrogels containing the Rh-catalyst are formed
under well-defined conditions of concentration. Hydrogels consisting in the reverse-sequential Tetronic®90R4 prove to be
more effective than the conventional sequential Tetronics®701. The presence of α-CD is crucial to provoke the
decantation of the multiphasic system once the reaction is complete. Optimized conditions (CO/H2 pressure, Rh-
precursors, phosphanes…) show that the catalytic system is especially applicable to the hydroformylation of terminal
alkenes. The catalytic performance remain unchanged upon recycling as the hydrogel matrix prevents the oxidation of the
phosphane.
www.rsc.org/
conversion as molecular receptor, supramolecular host and
fluidifier of the aqueous/organic interface.
Introduction
Following the seminal work of O. Roelen,1 a range of
improvements have been made in transition-metal-catalyzed
hydroformylation of alkenes (Scheme 1). Various experimental
conditions have been studied,2 including single-phase
homogeneous systems,3 biphasic systems with aqueous4,5,6,7
Scheme 1 Rh-catalyzed hydroformylation of alkenes.
10,11,12,13
fluorous,8,9 or ionic liquid phases,
thermomorphic
solvent mixtures and microemulsions,14,15,16 catalyst
immobilized on a solid support,17,18,19,20 supercritical fluid-ionic
liquid
biphasic
systems,21,22,23,24,25,26,27,28
amphiphilic
nanogels,29 or continuous liquid-vapor reactors.30,31
Concurrently, we have been developing the use of
alternative reaction media capable of ensuring high catalytic
performance and reusability of the organometallic catalyst.32,33
We especially focused our attention on cyclodextrins (CDs)-
based hydrogels which are 3D networks containing large
amounts of water.34 CDs are cyclic oligosaccharides consisting
of glucopyranose units (Fig. 1). The most common ones are
CD, β-CD and -CD. They consist in 6, 7 and 8 glucose units,
respectively, linked by -D-1,4-glycosidic bonds. Although they
α-
γ
α
are water soluble, their cavity is hydrophobic and can host
guest molecules to form inclusion complexes. We recently
showed that hydrogels consisting of CDs and polyethylene
glycol (PEG) were effective in the hydroformylation of very
hydrophobic alkenes (>C12).35,36 In such catalytic system, CDs
are multifunctional entities. They are constitutive building
blocks of the hydrogels and participated in the alkene
Fig. 1 Structure of a) cyclodextrins (CDs) and b) conventional
sequential
poloxamines
and
c)
reverse-sequential
poloxamines.
Given the very good results obtained with the CD/hydrogel
combination, we have been exploring the catalytic properties
of other CD-based hydrogels in biphasic conditions. We
especially focused our interest on poloxamines (also known as
Tetronics® macromolecules). They consist of block copolymers
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based on hydrophilic poly(ethylene oxide) (PEO) and parameters such as the nature of the rhodium precursor, the
hydrophobic poly(propylene oxide) (PPO) attached on an reaction temperature or the CO/H2 pressure are also
ethylene diamine spacer. They differ from their linear discussed.
counterparts (poloxamers also known as Pluronics®
macromolecules) by their X-shaped structure. Conventional
Results and discussion
sequential Tetronics® consist of PEO blocks at the outer sphere
and PPO blocks attached to the diaminoethylene core, while
Conditions for obtaining hydrogels
reverse-sequential poloxamines show PPO blocks at the
The first set of experiments consisted in defining the range of
periphery and PEO attached to the central diamino moiety
Tetronics® concentrations in water to ensure the existence of
(Fig. 1). Their amphiphilic character can be modulated by the
a mixture of liquid phases (sol phase + organic phase) at the
number of PEO and PPO units. Depending on the PEO/PPO
reaction temperature (80 °C) and a two-phase system (gel
ratio and the length of the PEO-PPO arms, various phases with
phase + organic phase) at room temperature. The goals are
specific molecular architecture can be obtained.37,38 Combined
twofold: i) favor contacts between the organic substrate and
with CDs, Tetronics® form hydrogels through non-covalent
the catalyst at high temperature and ii) recover both the
interactions.39,40 CDs slide along the PEO/PPO polymer chains,
product and the Rh-catalyst in two different phases when
and selectively accommodate the PEO blocks. Further
cooling the system at room temperature once the reaction is
intermolecular interactions between threaded
stacked nanocylinders that aggregate into nanosized columnar
-CD domains (Fig. 2).
α-CDs form
complete. To elaborate such thermo-responsive hydrogels,
Tetronic®90R4 was first mixed in water in various proportions
at 80 °C (temperature of the studied hydroformylation
reaction). As expected, no gel could be obtained whatever the
α
concentration.
A turbid solution was observed for low
concentrated Tetronic®90R4 solutions while a demixing took
place for high concentrated Tetronic®90R4 solutions (ESI,
Table S2). On the other hand, addition of
α-CD to
Tetronic®90R4 /water mixtures gave hydrogels under well-
defined concentration and temperature ranges. Tetronic®90R4
was mixed in various proportions in aqueous saturated
solutions (870 mg α-CD (0.9 mmol) dissolved in 6 mL water).
Upon heating at 80 °C for 30 min, -CD threaded onto the
PEO-PPO chains of Tetronic®90R4 leading to Tetronic®90R4/
α-CD
α
α
-
CD polypseudoratoxanes in water (Fig. 2). The mixtures were
then cooled at 4 °C overnight and the vials were inverted.
Below 10 wt% Tetronic®90R4 (600 mg Tetronic®90R4 (0.8
mmol) in 6 mL water; tube 10, Table 1), particles flocculated in
the aqueous solutions. On the contrary, hydrogels were
observed for mixtures containing at least 600 mg
Tetronic®90R4. As expected, high proportions of
Tetronic®90R4 were required for the formation of hydrogels.54
The latter was driven by the micro-crystallization of inclusion
Fig. 2 Polypseudorotaxane aggregates of
α-CD / reverse-
sequential poloxamines (Tetronic®90R4).
complexes of
PPO blocks.55 Moreover, above 10 wt% Tetronic®90R4 in
saturated solutions of -CDs, the hydrogels showed thermo-
α-CD and PEO blocks and the micellization of the
The rich phase behavior of the resulting hydrogels prompted
researchers to investigate their applications, including enzyme,
protein and gene delivery,41,42,43,44 sustained release of
α
responsivity. Below 25 °C, they behaved as gels. Above 25 °C,
they were liquids and formed a biphasic system (ESI, Table S4).
The gel phases were recovered at room temperature for
solutions containing more than 10 wt% Tetronic®90R4 (ESI,
drugs,45,46 tissue engineering,47
P‑
glycoprotein inhibitor,48
stabilizer of metallic nanoparticles49,50 and template in the
preparation of mesoporous materials.51,52,53 However, nothing
has been described so far on the utilization of Tetronics® in
biphasic catalysis in the presence of organometallic catalytic
species. Herein we describe how the conventional sequential
Tetronics®701 (Mw 3600, 2.1 EO and 14 PO units per arm) and
the reverse-sequential Tetronic®90R4 (Mw 7200, 16 EO and 18
PO units per arm) (ESI) can retain Rh-catalysts within their 3D
networks for the hydroformylation of very hydrophobic
alkenes under biphasic conditions. We especially highlight the
benefit of using Tetronics® in combination with CDs to form
thermo-responsive hydrogels. We show that the latter clearly
ensure the reusability of the catalytic system. Other
Table S5). The gel-to-sol transition of Tetronic®90R4/α-
CD/water mixtures was observed by inverting the vials and
was more accurately monitored using a rheometer. Increasing
the temperature led to a sharp decrease in the viscosity. Fig. 3
is illustrative of the drop in viscosity around 25 °C for a mixture
containing 1250 mg Tetronic®90R4 (0.17 mmol, 25 %wt), 725
mg
limits of the system, the amount of
an aqueous solution containing 1250 mg Tetronic®90R4 (0.17
mmol) dissolved in 5 mL water. Below 600 mg -CD (0.61
α
-CD (0.75 mmol) and 5 mL water. To better define the
α
-CD was also varied from
α
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mmol),
temperature. Above 600 mg
room temperature and evolved to a sol phase above 25 °C.
a
clear solution was observed whatever the Note that the formation of hydrogels was specific to the native
α
-CD, a gel phase was formed at -CD.
α
Table 1 Optical photos of Tetronic®90R4/
α-CD/water solutions (870 mg α-CD, 6 mL water) as a function of the Tetronic®90R4
weight percentage at 20 °C. P: Precipitate; G: Gel; D: Demixing.
Tube
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
10
11
15
12
20
13
25
14
30
15
35
16
40
17
45
18
50
19
55
20
60
21
65
22
70
23
75
Weight %
Observation
P
P
P
P
P
P
P
P
P
P
G
G
G
G
G
G
G
G
G
G
G
G
G
The performance of the Tetronic®90R4-based hydrogels were
assessed in the rhodium-catalyzed hydroformylation of 1-
dodecene (model substrate). To this effect, Tetronic®90R4 (20
wt% in water) was dissolved in water with or without
α-CD
upon stirring for 20 min at 60 °C under inert atmosphere. The
solution was allowed to stand for 30 min at room temperature
before Rh(CO)2(acac) and the sodium salt of the trisulfonated
triphenylphosphane (TPPTS) were added under nitrogen. The
resulting mixture was stirred at 60 °C for another 30 min. 1-
dodecene was then added, and the mixture was stirred at 80
°C under 50 bar of CO/H2 (1:1) pressure for 1 h. Without any
phosphane, a moderate 25%-conversion was obtained (Table
2, Entry 1). However, such experiment is irrelevant as the
reaction takes place in the organic phase as suggested by the
colored upper phase of the recovered biphasic system. This is
Fig. 3 Variation of the viscosity with the temperature for a an indirect proof that the Tetronics®90R4 nitrogens are unable
mixture containing 1250 mg Tetronic®90R4 (0.17 mmol, 25 to stabilize the catalytic species within the gel. In the presence
wt%), 725 mg
α
-CD (0.75 mmol) and 5 mL water.
of TPPTS, Table 2 clearly shows that the conversions are lower
without -CD (entries 2-4), indicative of the contribution of
the Tetronic®90R4/ -CD assembly at the interfacial layer.
These results are in line with our previous results on -CD/POE
hydrogels.35,36 Moreover, without
-CD, the multiphasic
α
α
α
α
system remains emulsified once the reaction is complete and
no decantation is observed even after 12 h. Recovering the
reaction products then requires the use of an organic solvent
(diethyl ether).
a)
b)
Fig.4 a) 50 mg Tetronics®701 / 725 mg α-CD / 5 mL water; b)
1250 mg T701 / 725 mg α-CD / 5 mL water.
The conversion regularly increased with the proportion of
Tetronic®90R4, indicative of the effect of the structure of
Tetronic®90R4 at the interface between the catalyst-
containing aqueous phase and the organic substrate. Low
conversions were measured for low quantities of
Tetronic®90R4 (<20%) meaning that the flocculation of
polypseudorotaxanes in the aqueous solutions was not
appropriate to favor contacts between the Rh-catalyst and the
substrates. Conversely, high conversion values were found for
high proportions of Tetronic®90R4 (>40 %) for which a liquid-
liquid two-phase system was identified at 80 °C (Table 1). As
For example, no gel could be obtained in the presence of the
native β-CD whatever the concentration (up to 3 g (2.6 mmol)
in 5 mL water), even if β-CD is well-known to selectively
accommodate PPO blocks.56
The formation of hydrogels from
α-CD and Tetronics®701
followed the same trend. However, higher CD concentrations
were required to trigger the formation of hydrogels as
exemplified in Fig. 4. As CDs can be easily removed from the
external PEO blocks, the Tetronic®701/α-CD/water hydrogel is
less stable than the Tetronic®90R4-based hydrogel. More CDs
(35 vs 10 wt%) are then necessary to form hydrogel textures
(ESI, Table S7).
shown in Fig. 5, the α-CD/Tetronic®90R4 polypseudorotaxane
aggregates partially cover the droplets of organic substrate
resulting in a Pickering-like emulsion that favors the contacts
between 1-dodecene and the Rh-catalyst at the
Hydroformylation of 1-dodecene using Tetronic®90R4
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