Impact of cyclodextrins on the behavior of amphiphilic ligands in aqueous organometallic catalysis

Article abstract of DOI:10.3762/bjoc.8.167

In this study, we showed that the addition of randomly modified β-cyclodextrin (RAME-β-CD) in aqueous medium could have a beneficial impact on the catalytic performances of phosphane-based aggregates in the Pd-catalyzed cleavage of allyl carbonates (Tsuji-Trost reaction). The RAME-β-CD/phosphane supramolecular interactions helped explain the catalytic results. The presence of RAME-β-CD in the aqueous compartment improved the phosphane-based aggregate dynamics. The exchanges between the hydrophobic substrate-containing aggregate core and the catalyst-containing aqueous phase were then greatly favored, resulting in an increase in the catalytic performances.

Full text of DOI:10.3762/bjoc.8.167

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Impact of cyclodextrins
on the behavior of amphiphilic ligands
in aqueous organometallic catalysis
Hervé Bricout1, Estelle Léonard2, Christophe Len2, David Landy3,
Frédéric Hapiot*1 and Eric Monflier*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1479–1484.
1Université Lille Nord de France, CNRS UMR 8181, Unité de
Catalyse et de Chimie du Solide - UCCS, UArtois, Faculté des
Sciences Jean Perrin, SP18, 62307 Lens Cedex, France, 2Université
de Technologie de Compiègne, Transformation Intégrée de la Matière
Renouvelable, EA 4297 UTC/ESCOM Centre de recherches de
Royallieu, BP 20529, F-60205 Compiègne Cedex, France and
3Université Lille Nord de France, UCEIV, ULCO, 145, Avenue
Maurice Schumann, MREI 1, F-59140 Dunkerque, France
doi:10.3762/bjoc.8.167
Received: 05 June 2012
Accepted: 08 August 2012
Published: 06 September 2012
This article is part of the Thematic Series "Superstructures with
cyclodextrins: Chemistry and applications".
Email:
Guest Editor: H. Ritter
Frédéric Hapiot* - frederic.hapiot@univ-artois.fr; Eric Monflier* -
eric.monflier@univ-artois.fr
© 2012 Bricout et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
amphiphilic phosphanes; biphasic system; cyclodextrins; micelles
Abstract
In this study, we showed that the addition of randomly modified β-cyclodextrin (RAME-β-CD) in aqueous medium could have a
beneficial impact on the catalytic performances of phosphane-based aggregates in the Pd-catalyzed cleavage of allyl carbonates
(Tsuji–Trost reaction). The RAME-β-CD/phosphane supramolecular interactions helped explain the catalytic results. The presence
of RAME-β-CD in the aqueous compartment improved the phosphane-based aggregate dynamics. The exchanges between the
hydrophobic substrate-containing aggregate core and the catalyst-containing aqueous phase were then greatly favored, resulting in
an increase in the catalytic performances.
Introduction
Facing the need of developing a greener chemistry, chemists tional media appeared to be a promising alternative to toxic
have recently focused their investigations on clean transforma- solvents. The use of supercritical fluids, ionic liquids and water
tion processes to convert organic molecules into products. especially appeared to be an effective solution to limit the envi-
Traditional solvents have especially been incriminated due to ronmental impact of chemical reactions [1-3]. Although these
the pollution problems they raise. Thus, in the context of media all display eco-friendly characteristics such as non-flam-
increased awareness about environmental safety, nonconven- mability and chemical stability, water has an additional advan-
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Beilstein J. Org. Chem. 2012, 8, 1479–1484.
tage of being cheap, nontoxic and available in large amounts. complexes. The stoichiometry and association constant Kass of
As an example, the concept of aqueous-phase organometallic these complexes were measured by isothermal titration calori-
catalysis implemented by E. Kuntz in the 70s clearly illustrates metry (ITC), at a suitable RAME-β-CD/phosphane concentra-
the potential of water as a reaction medium [4,5]. However, tion ratio ensuring the absence of micelles during the whole
while hydrophilic substrates can be readily converted in water, experiment. Under these conditions, the values reflect the inter-
the conversion of hydrophobic substrates constitutes a real chal- action between a single CD with a single phosphane only. A 1/1
lenge not completely solved so far. Recently, we showed that an stoichiometry was found whatever the phosphane, according to
association of cyclodextrins (CDs) and amphiphilic phosphanes the fit between theoretical and experimental heats. The Kass
could significantly improve the catalytic performances in an values were very dependent upon the phosphane structure. The
aqueous rhodium-catalyzed hydroformylation of higher olefins Kass values measured for 1 and 2 (300 and 590 M−1, respective-
[6]. Herein we clearly demonstrate that the beneficial effect of ly) were three orders of magnitude lower than those measured
CDs on a catalytic micellar system can be generalized to for 3 and 4 (326 000 and 210 000 M−1, respectively) indicative
another reaction and other amphiphilic phosphanes. To this end, of the deleterious role played by the diazo function on the inclu-
we undertook a study using the randomly methylated sion phenomenon. In fact, two distinct parameters could be im-
β-cyclodextrin (RAME-β-CD) as an additive in an aqueous plicated to explain the low Kass values observed for 1 and 2.
Pd-catalyzed cleavage reaction of allyl carbonates (Tsuji–Trost First, the hydrophilic diazo function had less affinity for the
reaction) and four amphiphilic phosphanes as aggregate- hydrophobic CD cavity than the biphenyl moiety. Additionally,
building blocks. The RAME-β-CD/phosphane interaction and the phosphane nonlinear geometry resulting from the zig-zag
its consequence on the catalytic results are discussed.
diazo structure also disfavored the recognition process between
the phosphane and the CD cavity.
Results and Discussion
To expand the scope of the CD/amphiphilic phosphane combi- The existence of phosphane CD supramolecular complexes
nation in aqueous-phase organometallic catalysis, four was confirmed by using a 2D NMR T-ROESY sequence sensi-
amphiphilic phosphanes 1–4 (Figure 1) were considered [6-8]. tive to dipolar contacts between the CD host and the phosphane
guest. Correlations were detected on T-ROESY spectra of stoi-
These phosphanes have been chosen for the information they chiometric mixtures of RAME-β-CD and phosphanes 1–4.
could give under catalytic conditions by considering their struc- However, a clear assignment of the CD and phosphane protons
tures and their amphiphilic properties. In this study, each phos- could not be properly made due to a severe broadening of the
phane has been proven to supramolecularly interact with β-CDs. NMR signals (ESI). Accordingly, the native β-CD was chosen
For example, addition of a stoichiometric amount of RAME-β- as host by default. The CD protons could then be assigned as
CD on aqueous solutions of water-soluble phosphanes 1–4 led illustrated in Figure 2 for the 4 β-CD supramolecular com-
to the formation of phosphane RAME-β-CD supramolecular plex.
Figure 1: Water-soluble phosphanes 1–4.
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Beilstein J. Org. Chem. 2012, 8, 1479–1484.
Figure 2: 2D T-ROESY NMR spectrum of a stoichiometric mixture of β-CD and 4 (3 mM each) in D2O at 20 °C.
Cross peaks were detected between the inner CD protons H-3/ the surface tension (Figure 3). This phenomenon was inter-
H-5 and the phosphane aromatic protons. A correlation was also preted in terms of CD/amphiphilic phosphane interaction
detected between some of the cyclohexyl protons and H-3 leading, for high CD concentrations, to an aggregate-destruc-
suggesting a deep phosphane inclusion into the CD cavity. For
the other phosphanes, mixtures of supramolecular complexes
could be detected because of the additional inclusion of the
phosphorus-bound phenyl groups into the CD cavities (ESI). It
should be noted that the signal broadening observed for phos-
phane protons suggested the presence of aggregates in the
medium. Thus, though a 1/1 CD/phosphane mixture was used,
phosphane-based aggregates and native β-CD/phosphane
complexes coexisted in water in this concentration range [9]. In
fact, the mixture of CD/phosphane complexes and aggregates
was a consequence of the competitive equilibria of CD/phos-
phane complexation and phosphane self-assembly to form
micelles.
The surface activity of the four studied phosphanes has already
been the subject of investigations [7,8]. Phosphanes 1–4 proved
to be very surface active as expected from their amphiphilic
structure (ESI). However, we show in this study that the add-
Figure 3: Effect of increasing concentrations of β-CD (solid lines) and
RAME-β-CD (dotted lines) on the surface tension of 1 mM aqueous
solutions of phosphanes 1–4 (20 °C).
ition of increasing amounts of native β-CD or RAME-β-CD in
phosphane-containing solutions produced a significant rise of
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Beilstein J. Org. Chem. 2012, 8, 1479–1484.
Figure 4: Equilibria in a phosphane-based micelle/RAME-β-CD mix-
ture.
Figure 5: Turnover frequency (TOF) as a function of the RAME-β-CD/
phosphane ratio in the Pd-catalyzed cleavage of allyl undecyl
carbonate. Reaction conditions: Pd(OAc)2 = 2.23 μmol, phosphane =
20.1 μmol, RAME-β-CD = 0–241 μmol, H2O = 2 g, allyl undecyl
carbonate = 223 μmol, diethylamine = 446 μmol, dodecane = 110
μmol, heptane = 2 g, 1250 rpm, room temperature. The TOF was
defined as the number of moles of allyl undecyl carbonate converted
per mole of palladium and per hour in the early stage of the reaction
(20–40% of conversion).
turing process. Indeed, in the post-micellar region, increasing
the CD amount resulted in a displacement of the equilibria
towards the CD/phosphane complex. The aggregates were then
partially or totally destroyed depending on the CD concentra-
tion. Mixtures of modified and unaltered phosphane-based
aggregates probably coexisted in water (Figure 4). The above
results corroborate previous studies on CD/amphiphilic phos-
phane interactions [9-13].
Nonlinear curves were obtained indicative of different
processes depending on the CD concentration. The highest
The catalytic performances of the CD/amphiphilic phosphane TOFs were measured for a RAME-β-CD/phosphane ratio of
combination have been evaluated in the aqueous Pd-catalyzed 1:1, while a huge drop in TOF was observed for a RAME-β-
cleavage reaction of allyl undecyl carbonate (Scheme 1).
CD/phosphane ratio of 2:1. Conversely, high RAME-β-CD/
phosphane ratios yielded an increase in TOF. The results could
The reactions were performed at room temperature under be interpreted as follows. First, for a RAME-β-CD/phosphane
nitrogen by using palladium acetate as a catalyst precursor, ratio of around 1:1, the micelle structure was slightly altered
phosphanes 1–4 as hydrosoluble ligands, and diethylamine as because of the interaction of RAME-β-CD with the micelle
an allyl scavenger. RAME-β-CD was chosen as an additive constituents, especially the inclusion process (Figure 4).
because of its ability to adsorb at the aqueous/organic interface
[14-17]. The RAME-β-CD/phosphane ratio was varied to eval- As the micelle dynamics was improved, the exchanges between
uate the impact of the CD concentration on the catalytic the hydrophobic substrate-containing micelle core and the cata-
micellar system. The catalytic results have been translated in lyst-containing aqueous solution were then greatly favored, thus
terms of turnover frequency (TOF) to RAME-β-CD/phosphane leading to an increase in TOF. In that case, the substrate
ratio, thus highlighting the role of RAME-β-CD on the catalytic concentration within the core of the phosphane-based aggre-
performances of the phosphane-based aggregates (Figure 5).
gates remained high. Note that the beneficial effect of RAME-
Scheme 1: Tsuji–Trost reaction mediated by a phosphane-based micelle/RAME-β-CD combination.
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