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C. R. Chimie 19 (2016) 113e116Contents lists avaiComptes Rendus Chimie
www.sciencedirect.comPreliminary communication/CommunicationBifunctional organocatalysis with squaramide-containing
Dawson organo-polyoxotungstates
Organocatalyse bifonctionnelle par des organo-polyoxotungstates
contenant une fonction squaramide
David Lachkar a, Emmanuel Lacôte a, b, *
a ICSN-CNRS, Bâtiment 27, 1, avenue de la Terrasse, 91198 Gif-sur-Yvette cedex, France
b Universite de Lyon, Institut de chimie de Lyon, UMR 5265 CNRS‐Universite Lyon -1‐ESCPE Lyon, 43, bd du 11-Novembre-1918, 69616
Villeurbanne, Francea r t i c l e i n f o
Article history:
Received 20 December 2014
Accepted 11 May 2015
Available online 22 December 2015
Keywords:
Polyoxometalates
Homogeneous catalysis
Hydrogen bonds
Organocatalysis
Organiceinorganic hybrids* Corresponding author. Universite de Lyon, Instit
UMR 5265 CNRS‐Universite Lyon-1‐ESCPE Lyon, 43,
1918, 69616 Villeurbanne, France.
E-mail address: emmanuel.lacote@univ-lyon1.fr
http://dx.doi.org/10.1016/j.crci.2015.05.006
1631-0748/© 2015 Academie des sciences. Publish
creativecommons.org/licenses/by-nc-nd/4.0/).a b s t r a c t
Two squaramide-containing organo-polyoxometalates (POMs) have been prepared, which
efficiently catalyze the addition of dicarbonyl derivatives onto nitroolefins. The POM sur-
face enhances the reactivity of the squaramide, and the hybrid acts as a bifunctional
catalyst, where the highly charged POM surface in the organic medium is a base, which
deprotonates the nucleophile and the squaramide activates the nitroalkenes via H-bonds.
This opens new perspectives to POM-based catalysis.
© 2015 Academie des sciences. Published by Elsevier Masson SAS. This is an open access
article under the CCBY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).r é s u m é
Deux organo-polyoxometallates (POMs) contenant une fonction squaramide ont ete
prepares. Ils catalysent l'addition de composes dicarbonyles sur des nitro-olefines. La
surface du POM augmente la reactivite du squaramide. L'hybride agit ainsi comme un
catalyseur bifonctionnel, pour lequel un oxo de la surface du POM agit en tant que base en
milieu organique et deprotone le nucleophile. Le squaramide active la nitro-olefine au
moyen de liaisons hydrogene. Ce travail ouvre de nouvelles perspectives en catalyse par les
POMs.
© 2015 Academie des sciences. Published by Elsevier Masson SAS. This is an open access
article under the CCBY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Polyoxometalates (POMs) are nanosized anionic clusters
of highly oxidized early transition metals and oxygen,
whose composition can be finely tuned. They have there-
fore been used as active agents in scores of catalytic re-
actions as oxidants, [1] Brønsted [2] and Lewis [3] acids,ut de chimie de Lyon,
bd du 11-Novembre-
(E. Lacôte).
ed by Elsevier Masson SAS.bases, [4] etc. The recent introduction of methods for the
easy preparation of organo-POMs [5] has led to more
complex catalytic systems [6]. The introduction of a flexible
covalent linker between the POM cluster and a catalytically
active unit can lead to synergistic effects of the catalyst
with the POM surface, and new or improved reactivity
[6a,f].
The use of organo-POMs is particularly crucial for POM-
based organocatalysis since POMs are inorganic structures,
which cannot be used in organocatalysis except (rarely) asThis is an open access article under the CC BY-NC-ND license (http://
D. Lachkar, E. Lacôte / C. R. Chimie 19 (2016) 113e116114counterions of protic organocatalysts [2]. The POM in these
systems remains mostly passive, limiting the dehydration
of the products, [2a] possibly by buffering the reaction
medium [6f]. In addition, the charge in the POM is diffused
on the surface, and mainly concentrated inside the poly-
oxotungstic cage [7]. This might limit the scope of the
method whenweaker interactions (such as H-bonding) are
considered.
Using covalent hybrids should bring new possibilities.
We have recently illustrated this potential by tethering a
substituted DMAP catalyst to a Dawson hybrid poly-
oxotungstate, which not only improved the reactivity, but
also changed the outcome of the reaction relative to purely
organic systems [6a]. We deduced that the cationic charge
that developed when the pyridinium intermediate was
formed led to folding toward the POM surface by way of an
electrostatic interaction with the polyanionic cluster. A
handful of other examples of synergistic behavior of POMs
with a Lewis acidic corner, [4bec] organometallic species,
[6f] or catalytic counterions [8] have been reported.
Because this reactivity enhancement is specific to organo-
POMs we wish to extend its scope.
As a follow-up of our assessment of POMs in organo-
catalysis, we wondered whether hydrogen bonds could be
harnessed to change reactivities. Since H-bond-based
organocatalysis is another very important ensemble of
organic reactions [9a] and H-bonds are increasingly seen as
crucial features that direct organo-POM reactivity, [10] we
decided to examine whether the surface of organo-POMs
could also work in synergy with H-bonding organo-
catalysts. We selected squaramides for this purpose [11].
The POM platform chosen was the a2 organotin-
substituted Dawson polyoxotungstate of general formula
[P2W17O61(SnR)]7e (Fig. 1). We selected an amide to tether
the squaramides, [10a] which was further separated from
the POM via a semi-flexible 1,3-xylylene diamine linker.
The NeH bond of the amide is likely to establish an H-bond
first to an oxo of the POM surface, directing the squaramideFig. 1. Catalysts used in this work.also toward the surface. At the same time, the xylylene
spacer was introduced to prevent the association of being
too tight and block any reaction.
Following our coupling method, an excess of squar-
amides (2 equiv.) was added to a solution of the mixed
anhydride TBA6[a2-P2W17O61SnCH2CH2C(]O)] in acetoni-
trile in the presence of Et3N [12]. Organo-POMs 1 and 2
were precipitated after 4 h at rt. The 31P signals moved
slightly from d¼12.2 and 10.0 ppm to 13.6 and
10.7 ppm in 1 and e13.9 and11.0 ppm in 2. The 1H NMR
signal of the amide proton at d¼ 7.99 ppm in 1 (resp.
7.96 ppm in 2) confirms the covalent linkage between the
Dawson structure and squaramides. In addition, benzyl CH2
and aromatic signals of the squaramide ligand were found
on both hybrid structures. ESI-MS analysis of 1 and 2
showed the presence of molecular ions for both POMs
stripped of 3e6 counterions, which confirmed their
structures.
With the two tentative catalysts in hand, we examined
the addition of diketones onto nitroalkenes. In the first
experiment (Table 1, entry 1), pentanedione 5 was reacted
with nitroalkene 6 with 0.5 mol % of 1 in toluene at rt. No
reaction took place. A quick screening of solvents showed
that no reaction took place in ethyl acetate or DCE (Table 1,
entries 2e3), but that both acetonitrile and dichloro-
methane were suitable for the reaction (53% and 54% yield
respectively, entries 4e5). We selected dichloromethane
for the rest of the work. The yield increased upon heating
and with a slightly increased catalyst loading (to 1.8 mol %,
99% yield, entries 6e7). These optimized conditions also
worked with POM 2, albeit with a slightly lower yield (93%,
entry 8).
We next carried out a series of control experiments to
assess the role of all the components. First, no reaction took
place without catalyst (entry 9), or in the presence of either
the lacunary Dawson [P2W17O61]10e (which has basic oxido
sites [4a,13]), or the benzylamide Dawson hybrid TBA7[P2-
W17O61Sn(CH2CH2C(])ONHBn] (entry 13).
On the other hand, both squaramides 3 and 4 which
mimic the organic part of the hybrid led to the product with
average yields (45% and 34%, entries 11e12). Finally a bi-
nary 1:1 system consisting of the benzylamide hybrid and
squaramide 3 led to only 12% of 7.
Taken together, these observations show that the
squaramide part is required for reactivity, and that the POM
alone does not catalyze the addition. We suggest that the
squaramide complexes with the nitroalkene via H-bond
activation (Fig. 2), [11] and that the POM surface helps the
deprotonation of the dicarbonyl partner (which is only
possible becausewe are in an apolarmedium) [4a]. The two
activations are needed: the hybrid is a bifunctional catalyst.
Gratifyingly, when the squaramide and the POM are not
tethered, the reaction is less productive thanwhen they are
attached (compare entries 7 and 11).
We next examined the scope of the reaction under the
optimized conditions (1.8 mol % of catalyst 1 or 2 in
refluxing dichloromethane for 18 h). Aryl nitroolefin sub-
strates with either electron-withdrawing or electron-
donating groups (Table 2, entries 1e5) reacted equally
well. On the other hand, if the substitution of one of the
methyl substituents on the dicarbonyl derivative was
Fig. 2. Tentative model for the reactivity.
Table 2
Scope of the POM-squaramide organocatalysis.
Entry R1 R2 Ar
1 Me Me o-(NO2)-C6H4
2 Me Me Ph
3 Me Me p-Me-C6H4
4 Me Me p-Cl-C6H4
5 Me Me p-F-C6H4
6 Et Et Ph
7 R1 ¼ R2 ¼ p-MeO-C6H5 Ph
8 t-Bu t-Bu Ph
9 Ph Me Ph
10 MeO Me Ph
11 MeO MeO Ph
12 R1, R2¼ (CH2)3 Ph
Table 1
Organo-POM-based H-bond organocatalysis: optimization of the conditions.
Entry Solvent T Catalyst (mol %) Yield (%)a
1 Tol. rt 1 (0.5) eb
2 AcOEt rt 1 (0.5) eb
3 DCE rt 1 (0.5) eb
4 MeCN rt 1 (0.5) 53c
5 DCM rt 1 (0.5) 54
6 DCM D 1 (1.0) 70
7 DCM D 1 (1.8) 99
8 DCM D 2 (1.8) 93
9 DCM D e eb
10 DCM D K10[P2W17O61] eb
11 DCM D 3 (1.8) 45
12 DCM D 4 (1.8) 34
13 DCM D TBA7[P2W17O61Sn(CH2CH2C(¼)ONHBn] (1.8) eb
14 DCM D TBA7[P2W17O61Sn(CH2CH2C(¼)ONHBn] (1.8)/3 (1.8) 12
a Unless otherwise specified the reactions took 13 h.
b No reaction took place.
c Reaction was stopped after 8 h.
D. Lachkar, E. Lacôte / C. R. Chimie 19 (2016) 113e116 115possible (entry 9), switching from pentanedione to hepta-
nedione led to a dramatic loss in yield (entry 6), while
further increase of the steric requirement of the dione was
even more detrimental (entries 7e8). Cyclic diketones did
not react (entry 12). Finally, a b-ketoester with a steric
requirement similar to that of pentanedione worked well
(entry 10), but a malonate did not (entry 11).
The reaction is sensitive to steric hindrance (compare
entries 1, 6, and 8) on the dicarbonyl partner, as well as its
acidity (compare entry 1 with the more acidic diketone
with entry 11 with the less acidic dimethyl malonate), but
not at all to modifications on the nitroalkene. This is
consistent with the model presented in Fig. 2, in which the
key fact is the deprotonation by the POM surface.Prod. Yd. with 1 (%) Yd. with 2 (%)
7 99 93
8 99 90
9 96 94
10 96 94
11 99 88
12 35 39
13 19 15
e S.M. S.M.
14 99 98
15 97 83
e S.M. S.M.
e S.M. S.M.
D. Lachkar, E. Lacôte / C. R. Chimie 19 (2016) 113e116116Malonates do not work well, as only the more acidic aryl
derivatives give some products (entries 7 and 11). It is not
clear to us why cyclohexane diketone did not lead to the
expected adduct despite its more acidic protons (entry 12).
Finally, we examined how the catalyst could be recycled.
Diethyl ether was added to the reaction mixture presented
in Table 2, entry 1. This led to the precipitation of 1. After
filtration, 1 could be re-used in a new reaction, without
significant loss of reactivity. The isolated yield dropped a
little (95% vs. 99%). This could be achieved four times in a
row, however with a regular (albeit low) decrease in yield
(99%, 95%, 91%, 85%, and 83%).
To conclude, we have introduced two squaramide-
containing organo-polyoxometalates which efficiently
catalyzed the addition of dicarbonyl derivatives onto
nitroolefins, and can be recycled. Seen from the POM point
of view, this is a new addition to the family of organic re-
actions opened to POM catalysis and to the growing num-
ber of POM-based bifunctional catalytic systems in which
the POM surface acts in synergy with the substituents
embedded [4b,c] or grafted [6a,f] onto the POM,making the
hybrid better than the mere sum of its components. Seen
from the organic point of view, this is another evidence that
specifically designed organic molecules can generate new
applications in POM chemistry via supramolecular in-
teractions. This interaction between organic molecules and
POMs in organic solvents has been long overlooked. That is
no longer the case. Its further study should lead to other
significant advances in POM (organic) chemistry.
Acknowledgments
We thank the “Centre national de la recherche scienti-
fique” (ICSN) for funding, and N. Elie, K. Hammad and J.-F.
Gallard (ICSN) for NMR and MS analyses.
Appendix A. Supplementary data
Supplementary data related to this article can be found
at http://dx.doi.org/10.1016/j.crci.2015.05.006.
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Bifunctional organocatalysis with squaramide-containing Dawson organo-polyoxotungstates

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