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D. Lachkar, E. Lacote / C. R. Chimie xxx (2015) 1e4
2
counterions 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 when weaker interactions (such as H-bonding) are
considered.
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
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 squaramide
slightly from
d
¼ ꢀ12.2 and ꢀ10.0 ppm to ꢀ13.6 and
ꢀ10.7 ppm in 1 and e13.9 and ꢀ11.0 ppm in 2. The 1H NMR
signal of the amide proton at
d
¼ 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 because we are in an apolar medium) [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 than when 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. 1. Catalysts used in this work.
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Please cite this article in press as: D. Lachkar, E. Lacote, Bifunctional organocatalysis with squaramide-containing Dawson
Lachkar, David
