Cyclodextrin-phosphane possessing a guest-tunable conformation for aqueous rhodium-catalyzed hydroformylation

Article abstract of DOI:10.1039/c1cc16326d

The inclusion of a guest inside the cavity of a new water-soluble cyclodextrin-phosphane allows controlling the natural conformation of this ligand leading to an inversion of the regioselectivity during aqueous hydroformylation reaction.

Full text of DOI:10.1039/c1cc16326d

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COMMUNICATION
Cyclodextrin–phosphane possessing a guest-tunable conformation for
aqueous rhodium-catalyzed hydroformylationw
abc
abc
Diem Ngan Tran,abc Francois-Xavier Legrand,abc Stephane Menuel, Herve Bricout,
´
¸
´
´
abc
Sebastien Tilloy and Eric Monflier*abc
Received 11th October 2011, Accepted 10th November 2011
DOI: 10.1039/c1cc16326d
The inclusion of a guest inside the cavity of a new water-soluble
cyclodextrin–phosphane allows controlling the natural confor-
mation of this ligand leading to an inversion of the regioselectivity
during aqueous hydroformylation reaction.
In the field of the design of new ligands for organometallic
catalysis, an alternative approach has been recently highlighted
to generate various ligands from supramolecular interactions.1 As
pioneering examples, the teams of Reek and Breit independently
reported the elaboration of bidentate ligands by supramolecular
interactions. Reek et al. developed a strategy based on the
coordination ability2 while Breit et al. focused their attention on
blocks built by hydrogen bonding interactions.3 This approach
has then emerged as a powerful and flexible tool to build new
catalytic systems in organic medium.4 In aqueous medium, the
formation of inclusion complexes between a water-soluble host
and a guest by hydrophobic interactions has recently appeared as
a smart way to access to supramolecular bidentate ligands for
aqueous catalysis. More precisely, a triaryl-phosphane can be
included in a mono-amino-b-cyclodextrin to build a water-soluble
supramolecular bidentate P,N-ligand.5 Associated to a platinum
precursor, this ligand allowed performing hydrogenation reaction
in aqueous medium. In this communication, we report that
hydrophobic interactions can also be used to control the
conformation of a water-soluble ligand. More precisely, we have
found that the geometry of a CD modified by a diphenyl-
phosphane group (Scheme 1; 1) can be tuned by the inclusion
of a guest inside the CD cavity. This supramolecular control
allowed orienting the regioselectivity during the hydroformylation
reaction catalyzed by rhodium complexes based on 1.
Scheme 1 Structure of 1.
reaction (procedure in ESIw). To obtain a highly water-soluble
compound, we have used a platform based on a randomly
methylated CD (RameCD) since RameCDs are among the
more water-soluble CDs.8 The solubility of this CD phosphane
1
in water was equal to 15 mM at 20 1C. Two-dimensional H
homonuclear NMR experiments based on the T-ROESY
sequence were performed to evidence potential spatial proxi-
mities between the different protons of 1, since CDs possessing
a pendant group can exhibit a self-inclusion phenomenon
(Fig. 1).9 Only slight dipolar contacts were observed between
the phenyl protons and the CD inner protons. Nevertheless, it
was impossible to exactly distinguish the different protons due
to the overlapped resonances. Indeed, 1 is not a well defined
compound but a mixture of partially methylated CDs. How-
ever, it is possible to propose a schematic representation for 1
where the primary face of the CD is capped by one of the two
phenyl groups. As this molecule possesses well-identified
hydrophobic and hydrophilic parts, the behaviour of this
CD phosphane was also investigated by surface tension measure-
ments. The evolution of the surface tension (g) versus the
concentration of 1 in water is presented in Fig. 2. The surface
tension gradually decreased until a break in the slope of the curve
but no plateau was observed. This breaking point in the surface
tension profile was identified as a critical aggregation concen-
tration (CAC) and was equal to 11 mM. The formation of
aggregates could be due to the association of the hydrophobic
parts of 1 (i.e. phenyl groups). Indeed, in the case of Rame-b-CD,
the decrease in surface tension was largely lower compared to 1
and only a hydrotropic behavior was observed. These results
indirectly showed that the penetration of the phosphane moiety
is not deep since the hydrophobic parts can aggregate.
The CD phosphane 1 was synthesized from borane-protected
diphenylpropynylphosphane6 and randomly methylated
mono-6-azido-6-deoxy-b-CD7 by Cu(I)-catalyzed azide–alkyne
cycloaddition reaction followed by a borane-deprotection
a Univ Lille Nord de France, F-59000 Lille, France
b
´
UArtois, Unite de Catalyse et de Chimie du Solide (UCCS),
Rue Jean Souvraz, SP 18, F-62307 Lens, France.
E-mail: eric.monflier@univ-artois.fr; Fax: +33 3 21 79 17 55;
Tel: +33 3 21 79 17 72
c CNRS, UMR 8181, France
w Electronic supplementary information (ESI) available: Synthesis,
analytical data and NMR or MALDI spectra. See DOI: 10.1039/
c1cc16326d
c
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Chem. Commun., 2012, 48, 753–755 753

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Fig. 3 2D TROESY NMR spectrum of a 1 : 1 mixture of AdCO2Na/1
in D2O (25 1C, 6 mM/2 mM) and schematic representation of this
inclusion complex.
Fig. 1 2D TROESY NMR spectrum in D2O (25 1C, 1.2 mM) and
schematic representation of 1.
comparison, the values obtained with b-CD and Trime-b-CD
(a permethylated b-CD) are equal to 51 000 MÀ1 and 850 MÀ1
,
respectively. This last result showed that although a capping
phenomenon was observed, the CD cavity is able to deeply
include a guest.
The catalytic behaviour of the combination between a
rhodium species and 1 was investigated in hydroformylation
reaction (Table 1). The experiments were performed with
methyl 4-pentenoate (a water-soluble substrate). Note that
NMR experiments showed that this substrate is not included
in the CD cavity (see ESIw). When 1 was used as a ligand, total
conversion was reached after 6 h with a linear to branched
aldehyde ratio (l/b) equal to 40/60 (Table 1, entry 1). In order
to determine if the catalytic activity and selectivity can be
controlled by supramolecular interaction, the hydroformylation
reaction was performed in the presence of ACNa (3 equivalents
compared to 1). At the chosen concentration, more than 99%
Fig. 2 Surface tension curve of aqueous solutions of 1 and Rame-b-CD
at 25 1C.
To estimate inclusion capacity of 1, attempts were made to
include an external host in the cavity of 1. 1-Adamantanecarboxy-
late sodium salt (ACNa) was selected because of its high
affinity for the b-CD cavity.10 The NMR continuous variation
method (Job’s plot) clearly showed that 1 formed a 1 : 1
inclusion complex with ACNa (see ESIw). 2D T-ROESY
NMR experiments performed on a solution containing 1 and
ACNa showed intense correlation peaks between the adamantyl
protons and internal CD protons proving again the formation of
inclusion complexes (Fig. 3). In addition, not only no cross peak
was observed between the aromatic and adamantyl protons but
also the correlation between the phenyl and internal CD protons
observed in the case of 1 alone has disappeared (Fig. 1). These
observations unambiguously proved that ACNa is able to fully
expulse the included phenyl group. The schematic representation
of this inclusion complex is depicted in Fig. 3. The association
constant for this 1 : 1 inclusion complex was then determined by
isothermal titration calorimetry (ITC). The value was found
to be very high, equal to 46 600 MÀ1 (see ESIw). Indeed, for
Table 1 Rhodium-catalyzed hydroformylation of methyl 4-pentenoatea
Entry
Ligand
Time/h
Guest
Cb (%)
Sc (%)
l/bd
1
2
3
4
5
6
1
1
1
6
6
6
2
2
2
(À)
99
99
98
98
98
98
99
99
99
99
99
99
40/60
64/36
65/35
64/36
64/36
64/36
ACNa
SD
TPPTS
TPPTS
TPPTS
(À)
ACNa
SD
a
Experimental conditions: Rh(acac)(CO)2 (1.94 Â 10À3 mmol), ligand
(7.76 Â 10À3 mmol), H2O (6 mL), substrate (0.97 mmol), ACNa or SD
(23.3Â 10À3 mmol); 1500 rpm, T: 80 1C, P(CO/H2: 1/1) = 50 bar.
b
c
d
Olefin conversion. Aldehydes selectivity. Ratio of linear to
branched aldehyde products.
c
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754 Chem. Commun., 2012, 48, 753–755

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gratefully acknowledged for generous gifts of cyclodextrins.
We thank Prof. D. Landy (ULCO, Dunkerque, France) for
association constant determination.
Notes and references
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Scheme 2 Schematic representation of the mechanism leading to
linear or branched products.
of the CD cavities are occupied by ACNa (see ESIw for calcula-
tion). The conversion and aldehydes selectivity are not different
from the experiments performed without ACNa (compare entries
1 and 2). However, the regioselectivity was inverted moving from
a l/b distribution of 40/60 to 64/36. In the presence of ACNa, the
linear product becomes preponderant. In order to ensure that
this interesting behaviour was not specific to the nature of
ACNa, two types of control experiments were performed. At
first, a supplementary experiment was performed with sodium
dodecanoate (SD) as guest. SD forms an inclusion complex with
1 and it is also able to displace the included phenyl group
(see ESIw). Expectedly, the l/b ratio was also inverted (compare
entries 1 and 3). Other experiments were performed with trisulfo-
nated triphenylphosphane (TPPTS) as ligand instead of 1. The
first was performed without a guest, the second with ACNa and
the third with SD (Table 1; entries 4–6). In all cases, no change
was observed in activity and selectivities. These three last results
indicated that the increase in the l/b ratio was not due to the
simple presence of an additional compound. This increase has
been attributed to a modification of the conformation of 1 in the
presence of a guest (Scheme 2). As an increase in steric crowding
around the Rh metal center is known to promote the formation
of a linear product, these results suggest that the 1/guest ligand is
bulkier than 1.11 So, 1 or 1/guest possesses a specific bulkiness
leading preferentially to branched or linear products.
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To resume, we have successfully prepared a valuable water-
soluble CD-phosphane for aqueous organometallic catalysis.
Indeed, rhodium-catalyzed hydroformylation reaction can be
performed in water by using this CD-phosphane as ligand.
The conformation of this ligand is controllable by inclusion of
a guest leading to a variation of the regioselectivity during
hydroformylation reaction. By varying the nature of the guest,
it will be possible to access to a library of new cyclodextrin–based
supramolecular ligands with specific properties.
6 R. J. Detz, S. A. Heras, R. de Gelder, P. W. N. M. van Leeuwen,
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This work was supported by the Centre National de la
Recherche Scientifique (CNRS). Dr D. N. Tran is grateful to
the Agence Nationale de la Recherche for financial support
(ANR CP2D 2007 CATAQ). Dr F. X. Legrand is grateful to
the Ministere de l’Education et de la Recherche (2007-2010) and
9 (a) M. T. Reetz, J. Rudolph and R. Goddard, Can. J. Chem., 2001,
79, 1806; (b) C. Machut-Binkowski, F. X. Legrand, N. Azaroual,
S. Tilloy and E. Monflier, Chem.–Eur. J., 2010, 16, 10195.
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11 P. W. N. M. van Leeuwen and C. Claver, Rhodium Catalyzed
Hydroformylation, Kluwer Academic Publishers, Dordrecht,
2000.
the Reseau de Recherche (RDR2 – Chimie eco-compatible –
´ ´
2008) for financial support. Roquette Freres (Lestrem, France) is
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 753–755 755