Synergetic effect of randomly methylated β-cyclodextrin and a supramolecular hydrogel in Rh-catalyzed hydroformylation of higher olefins

Article abstract of DOI:10.1021/cs5004883

A significant improvement in Rh-catalyzed hydroformylation of very hydrophobic alkenes was achieved using a biphasic catalytic system consisting of a substrate-containing organic phase and a catalyst-containing hydrogel phase [consisting of poly(ethylene glycol) 20000 (PEG20000) and α-cyclodextrin (α-CD)]. The catalytic performance of the Pickering emulsion that resulted from the formation of α-CD/PEG20000 crystallites at the oil droplet surface proved to be greatly dependent upon the presence of additives. We showed that controlled uploads of randomly methylated β-cyclodextrin (RAME-β-CD) within the supramolecular hydrogel could positively affect both the catalytic activity and chemoselectivity of the hydroformylation reaction. Conversely, no Pickering emulsion could be observed using excess RAME-β-CD, resulting in the subsequent degradation of the catalytic performance. Optical microscopy and optical fluorescence microscopy supported the catalytic results and allowed us to explain the role of RAME-β-CD. Indeed, controlled uploads of RAME-β-CD prevented the saturation of the oil droplet surface. RAME-β-CD acted as a fluidifier of the Pickering emulsion and accelerated the dynamics of exchange between the substrate-containing organic phase and the catalyst-containing hydrogel phase. Morever, RAME-β-CD acted as a receptor that participated in the conversion of the alkene by supramolecular means.

Full text of DOI:10.1021/cs5004883

Letter
pubs.acs.org/acscatalysis
Synergetic Effect of Randomly Methylated β‑Cyclodextrin and a
Supramolecular Hydrogel in Rh-Catalyzed Hydroformylation of
Higher Olefins
Jonathan Potier, Stephane Menuel, Eric Monflier, and Frederic Hapiot*
́
́
́
Universite
́
́ ́
d’Artois, CNRS UMR 8181, Unite de Catalyse et de Chimie du Solide-UCCS UArtois, Faculte des Sciences Jean Perrin,
rue Jean Souvraz, SP18, 62307 Lens Cedex, France
S
* Supporting Information
ABSTRACT: A significant improvement in Rh-catalyzed
hydroformylation of very hydrophobic alkenes was achieved
using a biphasic catalytic system consisting of a substrate-
containing organic phase and a catalyst-containing hydrogel
phase [consisting of poly(ethylene glycol) 20000 (PEG20000)
and α-cyclodextrin (α-CD)]. The catalytic performance of the
Pickering emulsion that resulted from the formation of α-CD/
PEG20000 crystallites at the oil droplet surface proved to be
greatly dependent upon the presence of additives. We showed
that controlled uploads of randomly methylated β-cyclodextrin
(RAME-β-CD) within the supramolecular hydrogel could
positively affect both the catalytic activity and chemoselectivity of the hydroformylation reaction. Conversely, no Pickering
emulsion could be observed using excess RAME-β-CD, resulting in the subsequent degradation of the catalytic performance.
Optical microscopy and optical fluorescence microscopy supported the catalytic results and allowed us to explain the role of
RAME-β-CD. Indeed, controlled uploads of RAME-β-CD prevented the saturation of the oil droplet surface. RAME-β-CD acted
as a fluidifier of the Pickering emulsion and accelerated the dynamics of exchange between the substrate-containing organic phase
and the catalyst-containing hydrogel phase. Morever, RAME-β-CD acted as a receptor that participated in the conversion of the
alkene by supramolecular means.
KEYWORDS: hydrogel, cyclodextrins, Pickering emulsion, biphasic catalysis, hydroformylation
oncurrently to their applications in biology,1,2 medicine,3,4
materials,5−7 and sensing,8−11 supramolecular hydrogels
alkenes. The existence of Pickering emulsions between the
organic phase and the hydrogel compartment significantly
improved the catalytic activity at the aqueous/organic interface.
As such, very hydrophobic alkenes could be converted into
aldehydes through hydroformylation of the terminal CC
double bonds under CO/H2 pressure (50 bar), the water-
soluble Rh catalyst being immobilized in the α-CD/PEG
hydrogel phase. However, the catalytic activity regularly
decreased with time as the Pickering emulsions became too
stable with time. Actually, after 4 h, the organic droplet surface
was saturated with α-CD/PEG crystallites and the conversion
leveled off. To overcome the issue, we implemented successive
depressurization/pressurization cycles to break the stable
Pickering emulsions and recover dynamics of exchange at the
aqueous/organic interface. For each cycle, 40−45% of the
remaining substrate was sequentially converted. Full conversion
could thus be obtained even for very hydrophobic alkenes
(C12−C18). Although effective, the depressurization/pressur-
C
have emerged as a promising class of hydrophilic media for
catalysis. Their main advantage lies in their easy preparation by
physical binding between two complementary compounds
through noncovalent interactions.12−14 Moreover, the thermor-
esponsive properties of supramolecular hydrogels were of
particular interest for catalytic applications. For example, their
ability to deswell and swell reversibly to concentrate substrates
and catalysts within the hydrogel matrix greatly accelerates the
reaction rate.15,16 Our group showed that supramolecular
hydrogels proved to be especially effective as templates for in
situ RuNPs syntheses and their subsequent use in hydro-
genation reactions.17 A hydrogel-based system capable of
continuous self-monitoring and self-regulating behavior was
also investigated, yielding very interesting results in four
exothermic catalytic reactions.18
Recently, we demonstrated that supramolecular hydrogels
consisting of poly(ethylene glycol) (PEG) and α-cyclodextrin
(α-CD) could be advantageously used in the sol phase to
convert organic substrates under aqueous biphasic conditions.19
The α-CD/PEG combination led to nanosized columnar α-CD
domains (crystallites) that formed Pickering emulsions
(particle-stabilized emulsions) in the presence of hydrophobic
Received: April 15, 2014
Revised: June 11, 2014
© XXXX American Chemical Society
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Letter
ization technique was quite cumbersome and required a step-
by-step procedure.
the hydrogel. Additionally, the chemoselectivity of the
hydroformylation reaction was also positively affected by the
H1/RAME-β-CD couple. From 64% without RAME-β-CD, the
proportion of aldehydes increased to 77% in the presence of
RAME-β-CD, an increase probably related to the well-known
protecting ability of the CD cavity toward the terminal CC
bond (limitation of side isomerization reactions).22 Note that
the regioselectivity was not significantly altered by the presence
of RAME-β-CD in the hydrogel (Supporting Information). The
values of the linear/branched ratio obtained using the H1/
RAME-β-CD mixtures were between those measured for the
separated components, suggesting that the equilibria between
the catalytic Rh species were not modified during the catalytic
cycle. In this regard, the organic phase was recovered at room
temperature once the reaction was complete to quantify any
traces of Rh. No trace of Rh could be detected by ICP, thus
demonstrating that the Rh catalyst was immobilized in the
hydrogel phase. Other proof of the absence of Rh in the organic
phase was given by the following experiment. After a reaction
time of 3 h, an H1/RAME-β-CD catalytic system was cooled,
and the organic phase was collected under nitrogen. Once an
aliquot had been withdrawn for analysis, water (6 mL) and
fresh 1-hexadecene (140 equiv with regard to Rh) were added
to the organic phase. After reaction for an additional 3 h at 80
°C under 50 bar of CO/H2, no conversion could be measured,
indicative of the absence of Rh catalysts in the organic phase.23
To gain insight into how the system proceeded, the amount
of RAME-β-CD was varied from 5 to 40 equiv with regard to
the Rh catalyst. Five hydroformylation reactions were
conducted. The results were translated into a conversion
spread (excess conversion value obtained with the H1/RAME-
β-CD couple over the value measured with RAME-β-CD) as a
function of the amount of RAME-β-CD (Figure 2). An optimal
Herein, we report on a more practical strategy that makes use
of a randomly methylated β-cyclodextrin (RAME-β-CD) to
fully hydroformylate hydrophobic alkenes with a very short
reaction time in one go. We showed that addition of RAME-β-
CD prevented the saturation of the oil droplet surface, resulting
in a linear conversion of alkenes over time.
The catalytic system used in this study consisted of a
rhodium precursor [Rh(CO)2(acac)] and a water-soluble
phosphane [TPPTS (sodium salt of the trisulfonated
triphenylphosphane)] that stabilized the Rh catalyst in a
supramolecular hydrogel (H1) [1/1 mixture of poly(ethylene
glycol) with a molecular weight of 20000 (PEG20000) and α-
CD].
The hydroformylation reactions were conducted under 50
bar of CO/H2 at 80 °C. In Figure 1 are represented five
Figure 1. Effect of additives on the Rh-catalyzed hydroformylation of
1-hexadecene. Conditions: 1/5/140 Rh/TPPTS/1-hexadecene mix-
ture, 80 °C, 50 bar of CO/H2.
different kinetic curves relative to the hydroformylation of 1-
hexadecene. Three of them describe the kinetic profiles of the
constitutive components [hydrogel H1, randomly methylated
α-cyclodextrin (RAME-α-CD), and RAME-β-CD]. Each of
them led to a moderate conversion within 6 h (from 40 to
70%). The other two kinetic profiles reflected the conversion
variations observed when a mixture of H1 and RAME-α-CD
(red curve) or RAME-β-CD (blue curve) was used as an
aqueous medium. Ten equivalents of randomly methylated
CDs with regard to the Rh catalyst was dissolved in H1.20
When mixed with H1, RAME-α-CD did not alter the catalytic
performance. Indeed, its availability at the aqueous/organic
interface was significantly reduced because of its natural
tendency to thread onto the PEG20000 chains (formation of
polypseudorotaxanes).21 Conversely, it clearly appeared that
the H1/RAME-β-CD mixture led to a catalytic performance
that was far better than that of the separated components or
that of the H1/RAME-α-CD mixture. Indeed, an almost linear
conversion variation was observed, and 100% 1-hexadecene was
converted within 1.5 h. This constituted the best conversion
ever for this substrate under aqueous biphasic conditions. Upon
comparison to the catalytic performance obtained with the
separated components, a synergetic effect resulted from the
Pickering emulsion and the presence of RAME-β-CD within
Figure 2. Variation of the conversion spread (excess conversion value
obtained with the H1/RAME-β-CD couple over the value measured
with RAME-β-CD) as a function of the amount of RAME-β-CD (with
regard to the Rh catalyst) in the Rh-catalyzed hydroformylation of 1-
hexadecene. Conditions: 1/5/140 Rh/TPPTS/1-hexadecene mixture,
80 °C, 50 bar of CO/H2, t = 1 h.
conversion spread of 36% was obtained for 15 equiv of RAME-
β-CD with regard to the Rh catalyst. Above that value, the
spread drastically decreased, reaching a value of 2% when 40
equiv of RAME-β-CD was added to the hydrogel. Thus, excess
RAME-β-CD significantly altered the catalytic performance of
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Letter
the system. More information about the catalytic system was
obtained by optical microscopy measurements.
Using 15 equiv of RAME-β-CD with regard to the Rh
catalyst, a well-dispersed oil in water (O/W) emulsion was
observed at 80 °C (Figure 3a). With 20 equiv of RAME-β-CD,
Figure 3. Optical microscopy performed at 80 °C of mixtures
containing 1-hexadecene (1.2 mL), H1 (6 mL), and (a) 15, (b) 20,
and (c) 40 equiv of RAME-β-CD with regard to Rh.
Figure 4. Optical fluorescence microscopy of mixtures containing 1-
hexadecene (1.2 mL), H1 (6 mL), and (a) 15 or (b) 40 equiv of
RAME-β-CD with regard to Rh prepared at 80 °C and observed at 20
°C. DiI (a and b) and DiO (c and d) were used to stain the α-CD/
PEG crystallites.
there were fewer oil droplets that had larger diameters (Figure
3b), indicative of a smaller aqueous/organic interface. With 40
equiv of RAME-β-CD (Figure 3c), no oil droplet could be
detected, suggesting that the Pickering emulsion was broken. In
that case, the organic and aqueous phases coexisted separately.
The extent of the aqueous/organic interface logically appeared
to be directly connected to the catalytic performance. The
higher the extent of the aqueous/organic interface, the better
the conversion. Additionally, RAME-β-CD retained its ability to
recognize 1-hexadecene and participated in its conversion by
supramolecular means. Conversely, the same catalytic experi-
ment conducted with an H1/RAME-γ-CD combination led to
a poor conversion of 16%, an aldehyde selectivity of 64%, and a
linear/branched aldehyde ratio of 2.7. These results were very
similar to those observed using H1 alone (22% conversion) or
using an H1/RAME-α-CD combination (20% conversion).
The conversion was far from that obtained using an H1/
RAME-β-CD combination (68% conversion within 1 h).
However, 1-hexadecene could be fully converted within 6 h
using the H1/RAME-γ-CD combination, suggesting that
RAME-γ-CD still acted as a fluidifier but not as a molecular
receptor. We do believe that the lower affinity between the
RAME-γ-CD cavity and the alkenes was responsible for this
catalytic result. Indeed, contrary to RAME-β-CD whose cavity
could accommodate the substrate, RAME-γ-CD was too wide
to properly recognize 1-hexadecene, resulting in lower
association constants with the substrate.
CD/PEG crystallites as none of them could be detected by
optical fluorescence microcopy in that case (Figure 4b,d).
These observations from the catalytic results and the optical
microscopies echoed recent results we obtained on CD-based
catalytic micellar systems. Indeed, we showed that the addition
of RAME-β-CD in an aqueous medium could have a beneficial
impact on the catalytic performances of phosphane-based
aggregates in the Rh-catalyzed hydroformylation reaction24 and
in the Pd-catalyzed cleavage of allyl carbonates (Tsuji−Trost
reaction).25 The exchanges between the hydrophobic substrate-
containing aggregate core and the catalyst-containing aqueous
phase were then greatly favored, resulting in an improvement in
the catalytic performance. In the study presented here, RAME-
β-CD played a similar role and accelerated the dynamics of
exchange in the Pickering emulsion, i.e., between the organic
phase and the catalyst-containing hydrogel. In fact, RAME-β-
CD adsorbed at the aqueous/organic interface and inserted
itself between the α-CD/PEG crystallites to prevent the
saturation phenomenon of the oil droplet surface observed in
our previous work.19 Accordingly, while asymptotic kinetic
profiles were obtained without RAME-β-CD, the conversion
variation became linear in its presence (Figure 1). The reaction
could be performed in one go and no longer required a step-by-
step procedure.
Given the aforementioned results obtained with 1-
hexadecane as the substrate, the study was extended to other
relevant substrates. First, the length of the substrate alkyl chain
was varied. Within only 1 h, 100% 1-dodecene and 88% 1-
tetradecene were already converted (Table 1, runs 1 and 2,
respectively). Logically, the conversion regularly decreased
when the substrate hydrophobicity increased. However, even
half of the very hydrophobic 1-octadecene was hydroformylated
under the same experimental conditions (Table 1, run 4). Note
that the regular decrease in conversion observed when an
increase in the length of the alkyl chain indirectly confirmed the
absence of catalytic species in the organic phase.23 Similar
catalytic activities would have been observed otherwise.
Increasing the length of the substrate alkyl chain also affected
the aldehyde selectivity because of a poor fit between the
substrate and the RAME-β-CD cavity (Supporting Informa-
tion). Interestingly, biobased substrates could also be hydro-
As demonstrated throughout our previous study,8 the
existence of an extended interface resulted from the presence
of a Pickering emulsion. To confirm that Pickering emulsions
were still active during the catalytic process, optical
fluorescence microscopy was conducted at 80 °C on two
samples containing the substrate (1.2 mL), hydrogel H1 (6
mL), and 15 or 40 equiv of RAME-β-CD with regard to Rh (3
mg). DiI or DiO (2% by weight) was added to the mixture to
stain the α-CD/PEG crystallites. The sample that contained 1-
hexadecene, H1, and 15 equiv of RAME-β-CD produced
crystallites that covered the oil droplet surface (Figure 4a,c).
Accordingly, the presence of 15 equiv of RAME-β-CD probably
altered the Pickering emulsion (as revealed by the improve-
ment in the catalytic performance) but did not hamper the
formation of α-CD/PEG crystallites on the oil droplet surface.
Conversely, excess RAME-β-CD prevented the formation of α-
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Letter
Notes
Table 1. Rh-Catalyzed Hydroformylation of Higher Olefins
Using Pickering Emulsions in the Presence of RAME-β-CD
a
The authors declare no competing financial interest.
b
c
run
substrate
1-dodecene
conversion (%) selectivity (%) l/b
ACKNOWLEDGMENTS
■
1
2
3
4
5
100
88
94
83
77
65
98
55
1.8
1.9
2.0
1.7
1.7
1-tetradecene
Roquette Frer
̀
es (Lestrem, France) is gratefully acknowledged
1-hexadecene
68
for generous gifts of cyclodextrins. We also thank Dr. Jean-
Francois Blach and Anne-Marie Lenfant for use of microscopic
facilities.
1-octadecene
46
̧
methyl 10-undecenoate
methyl oleate
100
25
d
6
−
a
Catalytic conditions: substrate (1.63 mmol), Rh(CO)2(acac) (3 mg,
0.012 mmol), TPPTS (33 mg, 0.058 mmol), α-CD (600 mg, 0.61
mmol), PEG (0.36 mg, monomer/α-CD stoichiometric ratio), RAME-
β-CD (228 mg, 0.18 mmol), 6 mL of H2O, 80 °C, 50 bar of CO/H2, t
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ASSOCIATED CONTENT
■
S
* Supporting Information
Preparation of the hydrogels, details of catalytic experiments,
and results of optical microscopy and optical fluorescence
microscopy. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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(23) As suggested by one of the referees, the existence of HRh(CO)3
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aldehyde selectivity. Regardless, we found the Rh species were totally
recovered at room temperature.
■
Corresponding Author
*Phone: +33 3 21 79 17 73. Fax: +33 3 21 79 17 55. E-mail:
frederic.hapiot@univ-artois.fr.
Funding
́
J.P. is grateful to the Region Nord-Pas-Calais and the Centre
National de la Recherche Scientifique (CNRS) for financial
support (2010−2013).
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