Optically active tripodal dendritic polyoxometalates: Synthesis, characterization and their use in asymmetric sulfide oxidation with hydrogen peroxide

Article abstract of DOI:10.1002/ejic.201001111

A series of structurally well-defined enantiopure tripodal allyl dendritic structures bearing three amine groups have been synthesized. The hydrogenation of the allyl groups in the presence of a Pd/C catalyst gave the corresponding enantiopure n-propyl counterparts. Treatment of these n-propyl amino dendrimers with heteropolyacid H₃PW₁₂O₄₀ and excess H ₂O₂ gave the enantiopure n-propyl {PO ₄[WO(O₂)₂]₄}^3- salts. Characterization of these dendritic POM hybrids in solution by NMR spectroscopy, elemental analysis, UV/Vis spectrophotometry, circular dichroism (CD), vibrational circular dichroism (VCD) and fluorimetry indicates the presence of POM-ligand interactions and confirms their optical and chiroptical properties. The hybrid compounds selectively oxidized sulfides to the corresponding chiral sulfoxides with up to 13% enantiomeric excess (ee), highlighting the transfer of chirality from the dendritic wedges to the inorganic cluster. The properties of the POM anion, especially its solubility and regio-and stereoselectivity, are sensitive to the structure of the cation. The catalyst was recovered by precipitation without any discernible loss in activity, selectivity or enantioselectivity over three catalytic cycles at-50 °C. Interestingly, a dendritic effect was noted in the enantioselectivity as the dendritic-POM hybrids are more selective than their non-dendritic counterparts. The ee resulting from chirality transfer to the anionic POM unit is comparable to that obtained in our previous work with monopodal dendritic polyoxometalates (14%) despite the polyvalency of the highly charged tripodal ligand, which is rationalized by different spectroscopic methods. Copyright

Full text of DOI:10.1002/ejic.201001111

FULL PAPER
DOI: 10.1002/ejic.201001111
Optically Active Tripodal Dendritic Polyoxometalates: Synthesis,
Characterization and Their Use in Asymmetric Sulfide Oxidation with
Hydrogen Peroxide
Claire Jahier,[a] Marie-Flora Coustou,[a] Martine Cantuel,[a] Nathan D. McClenaghan,[a]
Thierry Buffeteau,[a] Dominique Cavagnat,[a] Mauro Carraro,[b] and Sylvain Nlate*[a]
Keywords: Asymmetric catalysis / Chirality / Dendrimers / Polyoxometalates / Oxidation
A series of structurally well-defined enantiopure tripodal al-
lyl dendritic structures bearing three amine groups have
been synthesized. The hydrogenation of the allyl groups in
the presence of a Pd/C catalyst gave the corresponding
the transfer of chirality from the dendritic wedges to the inor-
ganic cluster. The properties of the POM anion, especially its
solubility and regio- and stereoselectivity, are sensitive to the
structure of the cation. The catalyst was recovered by pre-
enantiopure n-propyl counterparts. Treatment of these n-pro- cipitation without any discernible loss in activity, selectivity
pyl amino dendrimers with heteropolyacid H3PW12O40 and
excess H2O2 gave the enantiopure n-propyl {PO4[WO-
(O2)2]4}3– salts. Characterization of these dendritic POM hy-
brids in solution by NMR spectroscopy, elemental analysis,
UV/Vis spectrophotometry, circular dichroism (CD), vi-
brational circular dichroism (VCD) and fluorimetry indicates
the presence of POM–ligand interactions and confirms their
optical and chiroptical properties. The hybrid compounds se-
lectively oxidized sulfides to the corresponding chiral sulfox-
ides with up to 13% enantiomeric excess (ee), highlighting
or enantioselectivity over three catalytic cycles at –50 °C.
Interestingly, a dendritic effect was noted in the enantio-
selectivity as the dendritic-POM hybrids are more selective
than their non-dendritic counterparts. The ee resulting from
chirality transfer to the anionic POM unit is comparable to
that obtained in our previous work with monopodal dendritic
polyoxometalates (14%) despite the polyvalency of the
highly charged tripodal ligand, which is rationalized by dif-
ferent spectroscopic methods.
Introduction
it up, is the key requirement. Thus, one of the most impera-
tive issues for chemists is the search for green chemical
transformations. In particular, waste reduction is a key
requirement in the development of new attractive industrial
processes. In this context, a variety of metallodendritic
compounds used as recoverable catalysts have been re-
ported.[8–10] This area of research was pioneered in industry
by van Leeuven in the early 1990s and in the academic
world by the groups of Brunner, van Koten, Ford, and Du-
Bois. However, in terms of green chemistry these systems
have deficiencies on several accounts, especially the con-
tamination of the reaction mixture by metal leaching, and
hence the development of more environmentally friendly
catalytic systems remains a major challenge for chemists.
In this context, the combination of POMs with dendritic
structures can result in catalysts with improved properties.
The fixation of POM catalysts onto dendrimers is a very
promising route that allows the coupling of molecular con-
trol, mechanistic knowledge and the catalytic efficiency of
the POM unit with indispensable methods of recovery and
recycling.[11] To date, numerous inorganic chiral POMs have
been developed by two main synthetic approaches. The first
involves the use of chiral species (organic moieties or metal
complexes) as chirality transfer agents[12] whereas the sec-
ond strategy is based upon spontaneous resolution upon
The quest for new catalytic and highly enantioselective
processes[1] is currently one of the primary challenges in
chemical synthesis. In this context, chiral polyoxometalates
(POMs) have recently become a topic of interest due to
their potential applications in medicine and stereoselective
catalysis.[2] As a result, enantioselective organic transforma-
tions can also be investigated by using POM compounds.
Among numerous applications of POMs,[3–7] catalysis is by
far the most studied owing to the enormous versatility that
POMs offer in the clean synthesis of fine chemicals, includ-
ing their ability to catalyse environmentally friendly reac-
tions. Many POMs are able to activate molecular oxygen or
hydrogen peroxide as primary oxidants in reactions with
water as the byproduct. In addition, many elements of the
oxidant are incorporated into the final product (atom econ-
omy). On the other hand, for sustainable economic develop-
ment, preventing waste, rather than treating it or cleaning
[a] ISM, UMR CNRS No. 5255, Université Bordeaux I,
351 Cours de la Libération, 33405 Talence Cedex, France
Fax: +33-5-4000-6994
E-mail: s.nlate@ism.u-bordeaux1.fr
[b] Department of Chemical Sciences and ITM-CNR,
University of Padova,
Via Marzolo, 1 35131 Padova, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001111.
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S. Nlate et al.
FULL PAPER
crystallization in the absence of any chiral auxiliary to yield lished details of the preparation of non-dendritic optically
conglomerates.[13] However, the resolution of the enantio- active polyoxotungstophosphonates by covalent function-
meric structure is frequently complicated by racemization alization of a Keggin-type POM with a chiral organophos-
such that only in a few cases has the spontaneous resolution phonate. These compounds oxidized methyl p-tolyl sulfide
of enantiopure conglomerates been successfully achieved. to the corresponding chiral sulfoxide with up to 75% con-
Recently, an elegant and promising approach to enantio- version and a maximum of 8% ee.[16] In addition, our pre-
pure POMs based on the kinetic resolution of α1-substi- liminary results showed that the dendritic POM compound
tuted Dawson polyoxometalates by their chiral recognition was more selective (14 vs. 4% ee) than the non-dendritic
by peptides was reported.[14] However, this approach could counterpart. Bonchio and co-workers also observed the de-
be restricted by the overall modest yield (20%) of the iso- pendence of the enantioselectivity of the reaction on the
lated enantiopure POM. Therefore an alternative strategy nature of the ligand. These promising results clearly indi-
for the large-scale synthesis of enantiopure systems is the cate that the catalytic properties of the POM unit are de-
direct attachment of chiral organic structures onto the pendent upon the nature of the organic cation. In this con-
POM species. The optical and chiroptical properties of a text, we need to understand the parameters that play a de-
few enantiopure POMs based on the electrostatic coupling cisive role in achieving maximum activity and selectivity.
of chiral cations to achiral anionic POM species have been Thus, an understanding of the relationship between the
described.[12] To the best of our knowledge, there is only structure of the dendritic wedge, the structure of the POM
one report describing the synthesis and characterization of cluster and the properties of the resulting dendritic POM
enantiopure dendritic POM frameworks and their applica- hybrids will be of great interest as they will allow the devel-
tion in an enantioselective transformation. Indeed, in a pre- opment of enantiopure dendritic POM-based catalysts that
liminary communication, we reported the first example of are able to catalyse enantioselective transformations with
enantiopure dendritic POM hybrids, prepared by the ionic excellent yields and enantiomeric excesses. Our first ap-
coupling of enantiopure monopodal dendritic cations and proach was to tune the structure of the chiral dendritic
the anionic POM, selectively oxidizing thioanisole to the counter-cation. Thus, in our preliminary work we reported
corresponding chiral sulfoxide with up to 14% enantio- the synthesis and study of enantiopure dendritic POM hy-
meric excess (ee).[15] This proof-of-principle catalytic experi- brid catalysts, built by the electrostatic coupling of enantio-
ment demonstrated and confirmed the transfer of chirop- pure monopodal dendritic cations with an achiral trianionic
tical properties from the organic moieties to a catalytically POM (Scheme 1, top). Now we compare the chiroptical and
active POM unit. Recently, Bonchio and co-workers pub- catalytic properties of this prototype family of hybrids with
Scheme 1. Dendritic POM hybrids M and 8a based on monopodal dendritic amine L and tripodal dendritic amine 6a, respectively.
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Tripodal Dendritic Polyoxometalates
those of a new series of enantiopure dendritic POMs built Synthesis of Enantiopure Tripodal Dendritic Amines 6a,b
by assembling enantiopure tripodal dendritic amines and
trianionic POM moieties (Scheme 1, bottom). In this way,
the triply-charged anionic POM will be encapsulated by a
single dendritic structure bearing three ammonium groups.
Herein we describe the synthesis and characterization of
a novel series of chiral dendritic POM hybrids by assemb-
ling chiral tripodal dendritic amines and acidic POM moie-
ties. Electronic absorption experiments as well as circular
dichroism (CD), vibrational circular dichroism (VCD) and
fluorimetry have been performed to obtain information on
the interactions between the dendritic cation and the an-
ionic POM in homogeneous solution. These compounds
were used as recoverable catalysts in the oxidation of sul-
fides, which yielded sulfoxides with enantiomeric excess.
The effects of the structural modifications induced by the
dendritic wedge on the properties of the POM are emphas-
Enantiopure tripodal n-propyl dendritic amines (R)-(+)-
6a, (S)-(–)-6a and (R)-(–)-6b, (S)-(+)-6b were obtained from
methyl 4-(bromomethyl)benzoate (2) and enantiopure 1-
phenylethylamine (1a) and 1-cyclohexylethylamine (1b;
Scheme 2), respectively. The first step involved the reaction
of 4-(bromomethyl)benzoate (2) with amines 1a and 1b (in
excess), which selectively gave the amino benzoates (R)-(+)-
3a, (S)-(–)-3a and (R)-(–)-3b, (S)-(+)-3b, respectively. These
were allylated with allylmagnesium bromide to give the cor-
responding enantiomerically pure diallyl carbinols (R)-(+)-
4a, (S)-(–)-4a and (R)-(–)-4b, (S)-(+)-4b. The hexaallyl tria-
mino tricarbinol dendrimers (R)-(+)-5a, (S)-(–)-5a and (R)-
(–)-5b, (S)-(+)-5b were then obtained in good yields by the
coupling reaction of diallyl carbinols 4a and 4b with 2,4,6-
tris(bromomethyl)mesitylene (7). The hydrogenation of
hexaallyl 5a and 5b in the presence of Pd/C as catalyst gave
ized.
the corresponding n-propyl compounds (R)-(+)-6a, (S)-(–)-
6a and (R)-(–)-6b, (S)-(+)-6b in excellent yields. Spectro-
scopic studies and elemental analysis data are consistent
Results and Discussion
with the proposed dendritic structures. Interestingly, the
molecular peaks of amino dendrimers 6a and 6b were ob-
served in their MALDI-TOF mass spectra at m/z = 1130.60
(calcd. 1130.69, [M – 2H]+) and 1148.86 (calcd. 1148.83,
[M – 2H]+), respectively.
Synthesis and Characterization of Enantiopure Dendritic
POM Hybrids
Enantiopure dendritic POM-based frameworks were
constructed by ionic assembly of the trianionic
{PO4[WO(O2)2]4}3– with enantiopure tripodal dendritic am-
moniums obtained by functionalization of enantiopure
commercially available 1-phenylethylamine (1a) and 1-
Synthesis of Enantiopure Dendritic POM Hybrids 8a,b
Enantiopure dendritic POM salts were prepared by a
cyclohexylethylamine (1b). To compare the work described procedure involving the peroxide-mediated decomposition
in this manuscript and that reported in ref.[15], we focused of H3PW12O40.[11d,11f–11i,17] Following the procedure de-
on the Venturello species.
scribed in ref.[5], the hexa-n-propyl dendritic POM hybrids
Scheme 2. Synthesis of enantiopure tripodal dendritic amines 6a,b.
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Scheme 3. Synthesis of enantiopure tripodal dendritic POM hybrids 8a and 8b.
(R)-(+)-8a, (S)-(–)-8a and (R)-(–)-8b, (S)-(+)-8b were pre- (Figure 1b), which exhibit strong and weak Cotton effects
pared from the tripodal dendritic amines 6a and 6b and the with maxima at 225 nm for (R)-(–)-6b·3HCl and (S)-(+)-
commercial heteropolyacid H3PW12O40 in the presence of 6b·3HCl. These values are comparable to those obtained
an excess of H2O2 in a biphasic mixture of water and for the oxygen-to-tungsten charge-transfer bands of Keggin
dichloromethane (Scheme 3). The dendritic POM hybrids polyoxoanions[12k,18] as well as those obtained in our pre-
(R)-(+)-8a, (S)-(–)-8a and (R)-(–)-8b, (S)-(+)-8b were iso- liminary communication with the Venturello ion based hy-
lated as light-yellow solids in good yields. In contrast to the brids prepared with monopodal dendritic amines.[15] To
series of hybrids built with monopodal amines (Scheme 1, gain an insight into the interactions between the dendritic
top) previously reported by our group, compounds 8a and cation and the POM anion in solution, fluorescence and
8b are insoluble in chloroform and slightly soluble in absorption experiments as well as electronic absorption and
dichloromethane. Their solution investigations were per- vibrational circular dichroism (VCD) spectroscopy of the
formed either in acetone or in acetonitrile.
enantiopure hybrids (R)-(+)-8a and (S)-(–)-8a were carried
The ammonium chloride salts 6a·3HCl (see the Support- out.
ing Information for detailed synthetic procedures and char-
acterization) obtained from 6a can also be used to prepare
the dendritic POM salts 8a. The NMR, IR and elemental
analysis data reported for 8a,b are in agreement with the
structures proposed for these hybrids. The 31P NMR spec-
tra present only one signal for 8a ([D6]acetone, δ =
3.26 ppm) and 8b ([D6]acetone, δ = 3.32 ppm). In addition,
the 1H and 13C NMR spectra confirm the presence of
enantiopure cations around the POM species. Concerning
the 183W NMR measurements of these dendritic POM hy-
brids, we were unable to obtain spectra with a sufficiently
high signal-to-noise ratio, probably because of the low re-
ceptivity of W in dilute systems. For certain POM systems
we have observed 183W NMR spectra, but this becomes in-
creasingly difficult with dendritic POM frameworks even
after accumulation for 1 week in 10 mm tubes. However,
effort has been devoted to the characterization of these hy-
1
brids by H and 13C NMR and IR spectroscopy and espe-
cially elemental analysis. The structures and purity of these
hybrids are supported by the analytical data presented in
the Exptl. Sect.
The chiroptical properties of the enantiopure dendritic
POM salts 8a and 8b were examined in solution by circular
dichroism (CD) and polarimetry. The CD spectra of the
enantiomers are mirror images of one another, as shown in
Figure 1a for (R)-(–)-8b and (S)-(+)-8b, which confirms
their enantiopurity. These spectra show strong and weak
Cotton effects with maxima at 199, 224 and 302 nm. These
can be compared with the CD spectra of the ammonium
chloride counterparts (R)-(–)-6b·3HCl and (S)-(+)-6b·3HCl 2.8ϫ10–6 m).
Figure 1. CD spectra in CH3CN. A: enantiopure dendritic POMs
(R)-(–)-8b and (S)-(+)-8b (c = 5.0ϫ10–6 m); B: the precursor am-
monium chloride salts (R)-(–)-6b·3HCl and (S)-(+)-6b·3HCl (c =
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Tripodal Dendritic Polyoxometalates
Electronic Absorption and Emission Studies
containing species.[20] Thus, with three such electroattrac-
tors acting on the central cycle, significant perturbations
may be anticipated on complex 8a, provided this is the cycle
that is interacting.[20c] This reduced charge-transfer interac-
tion may be correlated with the catalytic behaviour (see be-
low).
The electronic absorption spectra of POM-Arquad and
the tripodal ammonium salt 6a·3HCl as well as the corre-
sponding dendritic POM 8a are shown in Figure 2. For the
ligand salt 6a·3HCl the intense high-energy band at 220 nm
is ascribed to the peripheral aromatic groups whereas the
band arising from the electronically isolated central aro-
matic lies at 260 nm. A significantly different absorption is
observed for the dendritic POM 8a compared with the par-
ent POM (POM-Arquad) and tripodal ligand (in CH2Cl2
and CH3CN). The absorption is not simply a summation of
the absorption bands of the constituent POM and dendritic
chromophores.
Vibrational Absorption and Circular Dichroism (VCD)
Analyses
Vibrational absorption spectroscopy has been used to
obtain information about the interaction between the den-
dritic structures and the POM unit. The infrared spectra of
the ammonium chloride tripodal ligand 6a·3HCl and the
tripodal dendritic POM hybrid 8a in CH2Cl2 solution are
shown in Figure 3. Most of the bands of the POM complex
are related to the corresponding tripodal ligand although
broad bands from the POM unit are observed at around
1075 and 945 cm–1. These spectral contributions are as-
signed to the stretching vibrations of the P–O and W=O
bonds, respectively. This assignment was confirmed experi-
mentally from the infrared spectrum of the non-dendritic
Figure 2. Electronic absorption spectra of POM-Arquad in
CH2Cl2, the hydrochloride salts of the tripodal dendrimer
(6a·3HCl) and monopodal ligand (L·HCl) as well as corresponding
dendritic POMs 8a and M, in CH3CN (c = 10–5 m).
The absorption spectrum of tripod–POM 8a is altered
upon dilution of an acetonitrile solution, especially below
10–4 m, notably in the region of the charge-transfer band,
or on adding water, which indicates a decreased interaction/
dissociation, consistent with the proposed interaction (Fig-
ure S37). The fluorescence emission and excitation spectra
show that the structured POM-Arquad emission is slightly
blueshifted when elaborated with dendritic wedges (Δω ≈
1000 cm–1) whereas the blue fluorescence of the ammonium
ligand 6a·3HCl (ΦF = 0.004, λmax = 303 nm) appears partly
quenched due to the close proximity of the POM in 8a,
although no sensitized emission arising from the POM part
is detected (Figure S38).
All these results are consistent with a ground-state (and
excited-state) complex comprising the anionic POM and at
least one cationic dendritic wedge, necessitating a close in-
teraction. The weaker charge-transfer interaction in the
POM–tripod species may be due to differing charge densi-
ties within the dendron brick 6a·3HCl compared with the
monopodal ligand L·HCl. Indeed, protonation of the
amino groups can have a dramatic effect on the neighbour-
ing aromatics. The CH2NMe2 skeleton has a Hammett con-
stant σp = 0.01 whereas on protonation the CH2N+HMe2
group has a Hammett constant σp = 0.43 (for comparison,
an aldehyde electroattractor has σp = 0.42).[19] In addition,
the transmission of electric fields across a similarly short
Figure 3. IR spectra. A: (R)-(+)-6a·3HCl and (R)-(+)-8a in the
3800–2000 cm–1 range; B: the 1800–800 cm–1 range under the fol-
lowing experimental conditions: co-addition of 50 scans, 4 cm–1
resolution, concentration 40 mm for (R)-(+)-6a·3HCl and (R)-(+)-
distance has previously been observed for an anthracene- 8a in CH2Cl2 solution, path length 250 μm.
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POM-Arquad compound (see Figure S39 in the Supporting W=O spectral region (960–920 cm–1) has been obtained due
Information) and by calculations using density functional to solvent (CDCl3) interference. To confirm the chirality
theory (DFT) of the spectrum of the POM entity (see Fig- transfer from the ligand to the POM cluster, we recorded
ure S40 in the Supporting Information). The interaction be- the VCD spectrum of the enantiopure tripodal dendritic
tween the tripodal ligand and the POM unit results in spec- POM hybrid in CH2Cl2 solution, which allows spectral in-
tral modifications of several ligand bands.
formation up to 900 cm–1 to be acquired. A negative VCD
Indeed, the broad band associated with the stretching vi- band is observed at around 1050 cm–1 arising from the P–
bration of the NH+ group shifts from 2590 cm–1 for the O bonds whereas a positive–negative–positive pattern is de-
ligand alone to 2900 cm–1 for the complex. A smaller inter- tected in the 960–920 cm–1 spectral range for the W=O
action of the NH+ group with the POM (with respect to the bonds. The intensities of these latter bands are significantly
Cl– anion for the ligand) explains this blueshift. A second greater than those observed for the P–O bonds, which indi-
important spectral modification upon complexation is the cates that the W=O mode seems to be a good probe for
enhancement of the bands associated with the aromatic following the chirality transfer from the ligand to the POM
C=C stretching vibrations above 1500 cm–1 due to charge cluster.
transfer between the POM entity and the aromatic rings.
The POM molecule would thus display structural chiral-
Vibrational circular dichroism (VCD) is a well-estab- ity in a chiral environment, which may be anticipated to be
lished method for obtaining information about the chirality the origin of the ee in the sulfide oxidation. This structural
of a sample and in particular for determining the absolute chirality has been clearly evidenced by this VCD study.
configuration and solution conformation of chiral mole- Moreover, DFT calculations of the optimized geometry of
cules.[21] Compared with electronic circular dichroism the chiral dendritic POM complex show this tendency of
(ECD) spectra, VCD spectra are more detailed because of the POM unit to adopt a chiral structure. Indeed, signifi-
the many bands contained in the mid-IR region. The VCD cant VCD intensities have been calculated for the stretching
spectra of (R)-(+)-8a in CH2Cl2 solution is shown in Fig- vibrations of P–O and W=O bonds (see Figure S41 in the
ure 4. The most important contributions to the VCD spec- Supporting Information).
trum of the tripodal dendritic POM hybrid come from vi-
brational modes related to the ligand. In particular, the
VCD bands at 1467 and 1454 cm–1 are associated with the
Catalytic Oxidation of Sulfides Using Enantiopure Tripodal
Dendritic POM Hybrids 8a and 8b
CH3 and CH2 bending vibrations of the propyl groups.
Likewise, the VCD band located at 1204 cm–1 is assigned to
the chiral C–Ph stretching vibrations.
To evaluate the catalytic efficiency of enantiopure den-
dritic POM hybrids 8a and 8b and demonstrate the chir-
optical properties of the POM unit in an asymmetric reac-
tion, we oxidized methyl phenyl sulfide (10), methyl p-tolyl
sulfide (11) and 2-chloroethyl phenyl sulfide (12) with H2O2
(35%) in a biphasic mixture of water and organic solvents.
Table 1 reports the results obtained with dendritic POM
catalysts 8a, 8b and the non-dendritic counterpart 9 in
terms of conversion, product selectivity (sulfoxide vs. sul-
fone) and enantiomeric excess (ee). The synthesis and char-
acterization of the non-dendritic hybrid 9 have been re-
ported in our previous work.[15] The reactions were per-
formed in a mixture of water and CDCl3/CH3CN (19:1,
v/v) at 35, 0 and –50 °C. As shown in Table 1, the oxidation
of methyl phenyl sulfide (10), methyl p-tolyl sulfide (11) and
2-chloroethyl phenyl sulfide (12) selectively gave the corre-
sponding methyl phenyl sulfoxide (13), methyl p-tolyl sulf-
oxide (14) and 2-chloroethyl phenyl sulfoxide (15), together
with a small amount of sulfones 16, 17 and 18, respectively.
Figure 4. VCD spectrum of (R)-(+)-8a in CH2Cl2 solution.
The investigation of chirality transfer from the ligand to In all cases, the selectivity to sulfoxide and the enantiomeric
the POM unit is not straightforward. Indeed, induced chi- excess increased with decreasing reaction temperature with
rality on achiral molecules gives rise to very low VCD sig- yields ranging from 19 to 100% and the ee values ranging
nals and is generally associated with a modified vibrational from 1 to 13% after 5 to 360 min depending on the nature
circular dichroism of the chiral inducer. This behaviour has of the substrate and the reaction temperature. The reaction
been observed previously for the enantiopure monopodal efficiency, selectivity and enantioselectivity were dependent
dendritic POM hybrid.[15] Thus, a VCD band associated upon the nature of the substrate and the reaction tempera-
with the P–O bonds has been observed at around 1050 cm–1 ture. A comparison between substrates 10, 11 and 12 shows
with a very low intensity (similar to the noise of the VCD that methyl p-tolyl sulfide (11) substituted with an electron-
spectrum). Unfortunately, no additional information in the donating group (CH3) accelerated the oxidation reaction
732
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Tripodal Dendritic Polyoxometalates
Table 1. Oxidation of representative sulfides catalysed by tripodal dendritic enantiopure POM salts 8a,b and the non-dendritic 9 using
H2O2.[a]
Entry
Substrate Catalyst
T [°C]
t [min][b]
Conversion [%][c]
Products (yield, %)
ee [%][d]
1
2
3
4
5
6
7
8
10
8a
35
0
–50
35
–50
35
–50
35
5
30
180
5
30
8
30
5
5
5
120
5
120
360
360
100
100
100
100
100
100
100
100
100
100
72
13 (36)
(91)
(81)
(0)
(81)
(68)
(72)
14 (48)
(66)
(77)
(68)
15 (39)
(70)
(19)
(63)
16 (64)
2
11
13
–
9
4
4
1
7
9
3
1
3
4
(9)
(19)
(100)
(19)
(28)
(32)
17 (52)
(34)
(23)
(4)
8b
9
11
12
8a
9
0
10
11
12
13
14
15
–50
–50
35
0
–50
–50
8b
8a
100
100
19
18 (61)
(30)
(0)
8b
70
(7)
2
[a] Reaction conditions: catalyst (0.4 mol-%), substrate (250 equiv.), 35% H2O2 (800 equiv.), solvent [1 mL, mixture of CHCl3/CH3CN
(19:1, v/v)]. [b] The reactions were monitored by 1H NMR spectroscopy. [c] The conversions were determined from the relative intensities
of the 1H NMR signals of the substrate and the product. [d] The enantiomeric excesses were determined by chiral HPLC using a Chiralcel
ASH column, UV detection (254 nm), eluting with hexane/2-propanol (1:1) at a flow rate of 0.5 mLmin–1.[22]
relative to methyl phenyl sulfide (10) and 2-chloroethyl effect was also observed in the ee as the dendritic POMs 8a
phenyl sulfide (12; Table 1, entries 3, 10 and 14). The oxi- and 8b are more enantioselective than their non-dendritic
dation of methyl phenyl sulfide (10) with the phenylethyla- compounds 9 under similar conditions (13 and 9% vs. 4%
mine-based hexa-n-propyl dendritic POM hybrids 8a was ee; entries 3, 5 and 7). Studies on the amount of catalyst
more enantioselective than the oxidation of 11 and 12 at used in the oxidation of methyl phenyl sulfide (10) with
–50 °C (13 vs. 9 and 4%, entries 3, 10 and 14). The high POM hybrids 8a showed that increasing the catalyst con-
selectivity to sulfoxide implies that sulfide oxidation pro- centration increases the oxidation kinetics without signifi-
ceeds significantly faster than the corresponding sulfoxide cantly affecting the ee (see Figure S42 in the Supporting
oxidation under these reaction conditions. For example, in Information). As a result, the best reaction conditions
contrast to 10 and 11, the oxidation of 12 led to a signifi- found up to now to achieve a high yield of chiral sulfoxide
cant amount of sulfone (byproduct). This indicates that the product with a maximum ee are 0.4 mol-% of catalyst 8a, a
mechanism operating in the system is electrophilic oxygen ratio of 1:3.2 substrate/hydrogen peroxide and 1 mL of
transfer as 11 is a more nucleophilic substrate than 10 and CHCl3/CH3CN (19:1, v/v) at –50 °C.
12. Overall, with all the catalysts tested, the catalytic experi-
ments performed at 35 °C were extremely fast (100% con-
version with reaction times between 5 and 8 min) in con-
trast to the reaction performed at –50 °C. However, the
selectivity to sulfoxide and the enantioselectivity were lower
Recovery and Reutilization of Enantiopure Dendritic POM
Hybrid 8a and the Non-Dendritic POM 9
under these conditions (Table 1). It is interesting to note
Three catalytic cycles were performed at –50 °C to test
that all the oxidation reactions performed did not proceed the stability of the dendritic POM hybrid 8a in comparison
in the absence of the POM catalyst when carried out under with the non-dendritic counterpart 9. Methyl phenyl sulfide
similar conditions. The phenylethyl-based POM hybrid 8a (10) was used as a model substrate for the catalyst recycling
was more enantioselective than its cyclohexylethyl counter- experiments. The catalyst was recovered by precipitation af-
1
part 8b (Table 1, entries 3, 5, 10, 11, 14 and 15). Interest- ter each catalytic cycle and checked by H and 31P NMR
ingly, as shown in Table 1, the enantioselectivity is depend- before a new catalytic experiment was performed. The
ent upon the nature of the organic cation. A slight dendritic enantiopure dendritic POM hybrid 8a selectively oxidized
Eur. J. Inorg. Chem. 2011, 727–738
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S. Nlate et al.
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Table 2. Recovery and reuse of enantiopure dendritic POM catalyst 8a and the non-dendritic POM 9 in the oxidation of methyl phenyl
sulfide (10) with H2O2 in three catalytic cycles.[a]
Entry Catalyst
Solvent
Cycles
T
[°C]
t [min][b]
Conversion
[%][c]
Products [%]
13 16
ee [%][d]
Cat. yield
[%][e]
1
2
3
4
5
6
8a
9
CDCl3/CH3CN[f]
1
2
3
1
2
3
–50
–50
–50
–50
–50
–50
120
120
120
30
30
30
94
93
93
100
100
100
85
86
86
70
70
71
9
7
7
30
30
29
13
13
13
4
4
4
85
83
80
82
84
84
CDCl3/CH3CN[f]
[a] Reaction conditions: catalyst (0.4 mol-%), substrate (250 equiv.), 35% H2O2 (800 equiv.), solvent (1 mL). [b] The reactions were moni-
tored by 1H NMR spectroscopy. [c] The conversions were determined from the relative intensities of the 1H NMR signals of the substrate
and the product. [d] The enantiomeric excesses were determined by chiral HPLC using a Chiralcel ASH column, UV detection (254 nm),
eluting with hexane/2-propanol (1:1) at a flow rate of 0.5 mLmin–1.[22] [e] The isolated yields of POM hybrids 8a and 9. [f] Mixture of
CHCl3/CH3CN (9:1, v/v).
the methyl phenyl sulfide (10) to the corresponding sulfox- enantiopure amines such as amino acids or binaphthyl-
ide 13 with 93–94% conversion after 120 min and with 13% based amines with greater potential for chiral induction
ee for each of the three catalytic cycles, as shown in Table 2. along with optimized concomitant electrostatic and charge-
The isolated yields of the recovered catalyst were in the transfer interactions. The development of catalytic systems
range of 80 to 85% (Table 2, entries 1–3). The reaction ki- for this purpose is in progress.
netics revealed no discernible loss of activity, selectivity or
enantioselectivity over the three catalytic cycles. As shown
in Table 2 (entries 4–6), the recyclability of the non-den- Experimental Section
dritic POM hybrid 9 was also investigated. Equally, no dis-
General Remarks: Reagent-grade tetrahydrofuran (THF) and di-
cernible loss of activity, selectivity or enantioselectivity was
ethyl ether were predried with Na foil and distilled from sodium–
observed in the oxidation of sulfide 10 at –50 °C. However,
benzophenone under argon immediately before use. Acetonitrile
a positive dendritic effect was noted in the selectivity to the
sulfoxide and the enantioselectivity (Table 2, entries 1–3 vs.
4–6) as the dendritic POM hybrid 8a was more selective
and more stereoselective than its non-dendritic counterpart
9. This can be explained by the fact that in contrast to the
non-dendritic cation, the dendritic wedges around the
active POM unit lower the reaction kinetics and probably
keep the chiral structure close to the achiral POM.
(CH3CN) was stirred under argon overnight over phosphorus pent-
oxide, distilled from sodium carbonate and stored under argon.
Methylene chloride (CH2Cl2) was distilled from calcium hydride
1
just before use. All other chemicals were used as received. The H,
13C and 31P NMR spectra were recorded at 25 °C with Bruker AC
250 FT (1H: 250.13; 13C: 62.91 MHz) and AC 200 FT spectrome-
ters (1H: 200.16; 13C: 50.33; 31P: 81.02 MHz) at CESAMO (Bor-
deaux, France). All chemical shifts are referenced to Me4Si (TMS).
Mass spectra were performed at the CESAMO with a QStar Elite
mass spectrometer (Applied Biosystems). The instrument was
equipped with an ESI source and spectra were recorded in the posi-
tive mode. The electrospray needle was maintained at 4500 V and
operated at room temperature. Samples were introduced by injec-
tion through a 10 μL sample loop into a 200 μLmin–1 flow of meth-
anol from the LC pump. Elemental analyses were carried out at
the Vernaison CNRS centre. The infrared spectra were recorded in
KBr pellets with a FT-IR Paragon 1000 Perkin–Elmer spectrometer
unless otherwise indicated. Organic oxidation products were iden-
tified by correlation with authentic samples. CD analyses were per-
formed with a Jasco J-715 instrument using CH3CN solutions in
1 cm quartz cells with a resolution of 2 nm, a step size of 0.2 nm,
a response time of 2 s and scan speed of 20–50 nmmin–1, collecting
6–16 acquisitions. Electronic absorption spectra were recorded with
a Varian Cary 5000 spectrometer. Steady-state emission spectra
were recorded with a Fluorolog 212 (SPEX) or Fluorolog-3 fluo-
rimeter and were corrected for instrumental function. The quantum
yield Φ of a dendron was calculated by using the equation Φ =
Φr(I/Ir)(Ar/A)(η2/ηr2) in which r refers to the reference, I is the inte-
grated emission intensity, A is the absorbance at the excitation
wavelength and η is the refractive index of the solvent. An optically
dilute solution of naphthalene in degassed ethanol (Φr = 0.21)[23]
was used as the reference.
Conclusions
We have assembled enantiopure dendritic polyoxomet-
alate frameworks derived from phenyl- and cyclohexyl-
based chiral tripodal amino dendrimers and acidic POM
moieties. The solution CD, UV/Vis and VCD data of the
phenyl-based dendritic POM hybrid 8a indicate an induc-
tion of optical activity in the POM cluster. Interestingly,
these compounds selectively oxidize sulfides to the corre-
sponding chiral sulfoxides with up to 13% ee. These results
are comparable to those reported in our preliminary com-
munication in which a series of dendritic POM hybrids
bearing monopodal dendritic amino cations oxidized sul-
fides with up to 14% ee.[15] The properties of the POM unit,
especially their selectivity in the oxidation of sulfides to
sulfoxides and their enantioselectivity, are sensitive to the
structure of the dendritic cation as well as the reaction tem-
perature. Despite the increased potential for electrostatic in-
teractions with the triply charged tripods compared with
the monocationic monopodal species, a lower charge-trans-
fer interaction is evidenced. We believe that the ee values
IR and VCD spectra were recorded with a ThermoNicolet Nexus
can be increased by elaboration of dendritic structures from 670 FTIR spectrometer equipped with a VCD optical bench.[24] In
734
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Eur. J. Inorg. Chem. 2011, 727–738

Tripodal Dendritic Polyoxometalates
this optical bench the light beam was focused by a BaF2 lens
(191 mm focal length) onto the sample through an optical filter
(depending upon the studied spectral range) and was equipped with
a BaF2 wire grid polarizer (Specac) and a ZnSe photoelastic modu-
lator (Hinds Instruments, Type II/ZS50). The light was then fo-
cused by a ZnSe lens (38.1 mm focal length) onto a 1ϫ1 mm2 Hg
CdTe (ThermoNicolet, MCTA* E6032) detector. IR and VCD
spectra were recorded at a resolution of 4 cm–1 by co-adding 50
and 72000 scans (24 h acquisition time), respectively. The sample
was held in a fixed pathlength cell (200 μm) with KBr windows. IR
and VCD spectra of the two opposite enantiomers of ligands
6a·3HCl and tripodal dendritic POM hybrid 8a were measured in
CH2Cl2 at a concentration of 40 mm. Baseline corrections of the
VCD spectra were performed by subtracting the two opposite en-
antiomer VCD spectra (recorded under the same experimental con-
centration) and dividing by two. In all experiments, the photoelastic
modulator was adjusted for a maximum efficiency at 1400 cm–1.
Calculations were performed with the standard ThermoNicolet
software using Happ and Genzel apodization, de-Haseth phase
correction and a zero-filling factor of one. Calibration spectra were
recorded using a birefringent plate (CdSe) and a second BaF2 wire
grid polarizer following the experimental procedure previously
published.[25] Finally, the solvent absorption has been subtracted
from the IR spectra presented. The geometry optimizations, vi-
brational frequencies, absorption and VCD intensities were calcu-
lated by using the Gaussian 03 program[26] on the SGI Altix XE
1300 computer of the Pôle Modélisation of the Institut des Sciences
Moléculaires (University Bordeaux I). Calculations of the opti-
mized geometry of the POM unit and of the (R)-(+) monopodal
dendritic POM hybrid were performed by using density functional
theory using the B3PW91 functional and 6-31G* basis set, except
on the tungsten atom for which the SDD basis set was used. Vi-
brational frequencies, IR and VCD intensities were calculated at
the same level of theory utilizing the magnetic field perturbation
method with gauge-invariant atomic orbitals.[27] For comparison
with experiment, the calculated frequencies were scaled by 0.968
and the calculated intensities were converted into Lorentzian bands
with half-widths of 7 cm–1.
Cyclohexylethylamino Benzoates (R)-(–)-3b and (S)-(+)-3b: White
solid; yield 1.35 g, 98%. H NMR (250.13 MHz, CDCl3, TMS): δ
1
= 7.97 (d, 2 H, Ar), 7.39 (d, 2 H, Ar), 3.89 (s, 3 H, O-CH3), 3.81
(dd, 2 H, CH2-N), 2.46 (q, 1 H, CH), 1.71 (m, 5 H, CH2 and CH),
1.26 (m, 6 H, CH2), 0.98 (d, 3 H, CH3) ppm. 13C NMR
(62.91 MHz, CDCl3, TMS): δ = 166.9 (C=O), 146.7 (Cq, Ar), 129.6
(CH, Ar), 128.5 (Cq, Ar), 127.9 (CH, Ar), 57.1 (CH), 52.1 (CH2-
N), 51.1 (O-CH3), 42.9 (CH), 29.8, 28.0, 26.8, 26.6, 26.5 (CH2),
16.7 (CH3) ppm. MS (ESI): calcd. for [M + H]+ 276.34; found
276.20. C17H25NO2 (275.39): calcd. C 74.14, H 9.15, N 5.09; found
C 73.78, H 9.08, N 4.89. (R)-(–)-3b: [α]2D0 = –18.7 (c = 10–2 gmL–1,
CHCl3). CD (c = 1.7ϫ10–5 m, CH3CN): λ (Δε) = 200 (5.1), 208
(–0.7), 227 (–2.9), 245 (4.7 m–1 cm–1) nm. (S)-(+)-3b: [α]2D0 = 18.2 (c
= 10–2 gmL–1, CHCl3). CD (c = 1.7ϫ10–5 m, CH3CN): λ (Δε) =
200 (–5.1), 208 (+0.7), 227 (+2.9), 245 (–4.7 m–1 cm–1) nm.
General Procedure for the Synthesis of the Amino Diallyl Carbinols
4a and 4b: In a Schlenk tube, allyl bromide (2.5 equiv.) was slowly
added to a mixture of magnesium (3 equiv.) in anhydrous diethyl
ether (25 mL) cooled to 0 °C. Then a solution of amino benzoate
3 (1 equiv.) in anhydrous diethyl ether (20 mL) was added to the
reaction mixture at 0 °C. The mixture was stirred for 12 h at room
temperature. Then a solution of 6 m NH4Cl (10 equiv.) was added
to the reaction mixture and the product was extracted with diethyl
ether and dried with sodium sulfate. The solvent was removed un-
der vacuum and the product purified by silica gel column
chromatography, eluting with a petroleum ether/diethyl ether (7:3,
v/v) to give the corresponding amino diallyl dicarbinol.
Phenylethylamino Diallyl Carbinols (R)-(+)-4a and (S)-(–)-4a:
1
Colourless oil; yield 4.78 g, 96%. H NMR (250.13 MHz, CDCl3,
TMS): δ = 7.34 (m, 9 H, Ar), 5.63 (m, 2 H, CH=CH2), 5.12 (m, 4
H, CH=CH2), 3.85 (q, 1 H, CH), 3.63 (dd, 2 H, CH2-N), 2.61 (dd,
4 H, CH2-CH=CH2), 1.40 (d, 3 H, CH3) ppm. 13C NMR
(62.91 MHz, CDCl3, TMS): δ = 145.4 (Cq, Ar), 144.4 (Cq, Ar),
138.8 (Cq, Ar), 133.5 (CH=CH2), 128.5 (CH, Ar), 128.1 (CH, Ar),
127.9 (CH, Ar), 126.9 (CH, Ar), 126.7 (CH, Ar), 125.4 (CH, Ar),
119.1 (CH=CH2), 75.0 (Cq, C-OH), 57.8 (CH), 51.3 (CH2-N), 46.8
(CH2-CH=CH2), 24.5 (CH3) ppm. MS (ESI): calcd. for [M + H]+
322.47; found 322.22. C22H27NO (321.46): calcd. C 82.20, H 8.47,
N 4.36; found C 81.88, H 8.39, N 4.55. (R)-(+)-4a: [α]2D0 = 28.0 (c
= 10–2 gmL–1, CHCl3). CD (c = 2.5ϫ10–5 m, CH3CN): λ (Δε) =
216 (–0.6), 235 (+0.8 m–1 cm–1) nm. (S)-(–)-4a: [α]2D0 = –28.0 (c =
10–2 gmL–1, CHCl3). CD (c = 2.5ϫ10–5 m, CH3CN): λ (Δε) = 216
(0.7), 235 (–0.5 m–1 cm–1) nm.
Synthesis of Enantiopure Tripodal Dendritic Amines 6a and 6b
General Procedure for the Synthesis of the Amino Benzoates 3a and
3b: A mixture of 1 equiv. of the corresponding enantiopure amine 1
(3 equiv.), methyl 4-(bromomethyl)benzoate (2; 1 equiv.) and N,N-
diisopropyl(ethyl)amine (3 equiv.) in CH3CN (15 mL) was stirred
for 48 h at room temperature. After removal of the solvent under
vacuum, the residue was extracted with diethyl ether, washed with
water and dried with sodium sulfate. The solvent was removed un-
der vacuum and the product was purified by silica gel column
chromatography, eluting with petroleum ether/diethyl ether (9:1,
v/v).
Cyclohexylethylamino Diallyl Carbinols (R)-(–)-4b and (S)-(+)-4b:
1
Colourless oil; yield 2.29 g, 95%. H NMR (250.13 MHz, CDCl3,
TMS): δ = 7.30 (m, 4 H, Ar), 5.59 (m, 2 H, CH=CH2), 5.07 (m, 4
H, CH=CH2), 3.73 (dd, 2 H, CH2-N), 2.62 (dd, 4 H, CH2-
CH=CH2), 2.43 (q, 1 H, CH), 1.68 (m, 5 H, CH2 and CH), 1.23
(m, 6 H, CH2), 0.96 (d, 3 H, CH3) ppm. 13C NMR (62.91 MHz,
Phenylethylamino Benzoates (R)-(+)-3a and (S)-(–)-3a: Colourless
CDCl3, TMS):
δ = 144.3 (Cq, Ar), 139.3 (Cq, Ar), 133.6
1
oil; yield 3.51 g, 95%. H NMR (250.13 MHz, CDCl3, TMS): δ =
(CH=CH2), 127.9 (CH, Ar), 125.4 (CH, Ar), 119.1 (CH=CH2),
75.0 (Cq-OH), 57.3 (CH), 51.2 (CH2-N), 46.9 (CH2-CH=CH2),
42.9 (CH), 29.9, 28.0, 26.8, 26.7, 26.5 (CH2), 16.8 (CH3) ppm. MS
(ESI): calcd. for [M + H]+ 328.18; found 328.27. C22H33NO
(327.51): calcd. C 80.68, H 10.16, N 4.89; found C 79.24, H 9.87,
N 4.28. (R)-(–)-4b: [α]2D0 = –20.2 (c = 10–2 gmL–1, CHCl3). (S)-(+)-
4b: [α]2D0 = 20.7 (c = 10–2 gmL–1, CHCl3).
8.00 (d, 2 H, Ar), 7.32 (m, 7 H, Ar), 3.91 (s, 3 H, O-CH3), 3.80 (q,
1 H, CH), 3.68 (dd, 2 H, CH2-N), 1.39 (d, 3 H, CH3) ppm. 13C
NMR (62.91 MHz, CDCl3, TMS): δ = 167.1 (C=O), 146.2 (Cq,
Ar), 145.4 (Cq, Ar), 129.7 (CH, Ar), 128.6 (CH, Ar), 128.0 (CH,
Ar), 127.1 (CH, Ar), 126.7 (CH, Ar), 57.7 (CH), 52.1 (CH2-N),
51.3 (O-CH3), 24.6 (CH3) ppm. MS (ESI): calcd. for [M + H]+
270.35; found 270.16. C17H19NO2 (269.34): calcd. C 75.81, H 7.11,
N 5.20; found C 76.46, H 7.23, N 5.43. (R)-(+)-3a: [α]2D0 = 30.0 (c
= 10–2 gmL–1, CHCl3). CD (c = 3.8ϫ10–5 m, CH3CN): λ (Δε) =
General Procedure for the Synthesis of the Hexaallyl Triamino
Tricarbinols 5a and 5b: 2,4,6-Tris(bromomethyl)mesitylene (7;
1 equiv.) and K2CO3 (4 equiv.) were added to a solution of the cor-
207 (–8.0), 218 (–2.3), 235 (3.3 m–1 cm–1) nm. (S)-(–)-3a: [α]2D0
=
–30.0 (c = 10–2 gmL–1, CHCl3). CD (c = 3.8ϫ10–5 m, CH3CN): λ responding enantiopure amino diallyl carbinol (3.6 equiv.) in
(Δε) = 207 (5.6), 218 (1.9), 235 (–3.6 m–1 cm–1) nm.
CH3CN (10 mL). The mixture was stirred for 16 h at reflux. After
Eur. J. Inorg. Chem. 2011, 727–738
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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735

S. Nlate et al.
FULL PAPER
removal of the solvent under vacuum, the product was extracted
with dichloromethane, washed with water and dried with sodium
sulfate. The solvent was removed under vacuum and the product
was purified by silica gel column chromatography, eluting with pe-
troleum ether/diethyl ether (7:3, v/v).
(CH, Ar), 126.6 (CH, Ar), 124.9 (CH, Ar), 77.1 (Cq, C-OH), 56.4
(CH), 52.6 (CH2-N), 47.4 (CH2-N), 45.3 (CH2), 16.9 (CH2), 16.5
(CH3), 14.7 (CH3), 12.6 (CH3, Ar) ppm. MS (MALDI-TOF):
calcd. for [M – 2H]+ 1130.69; found 1130.60. C78H105N3O3
(1132.71): calcd. C 82.71, H 9.34; found C 81.64, H 9.34. (R)-(+)-
6a: [α]2D0 = 60.0 (c = 10–2 gmL–1, CHCl3). CD (c = 3.6ϫ10–6 m,
CH3CN): λ (Δε) = 194 (34.5), 205 (–8.0), 222 (+18.0 m–1 cm–1) nm.
(S)-(–)-6a: [α]2D0 = –60.0 (c = 10–2 gmL–1, CHCl3). CD (c =
3.6ϫ10–6 m, CH3CN): λ (Δε) = 194 (–36.0), 205 (6.0), 222
(–17.0 m–1 cm–1) nm.
Phenylethyl Hexaallyl Triamino Tricarbinols (R)-(+)-5a and (S)-
(–)-5a: White solid; yield 1.0 g, 91%. 1H NMR (250.13 MHz,
CDCl3, TMS): δ = 7.18 (m, 27 H, Ar), 5.55 (m, 6 H, CH=CH2),
5.07 (m, 12 H, CH=CH2), 3.86 (q, 3 H, CH), 3.58 (m, 6 H, CH2-
N), 3.46 (m, 6 H, CH2-N), 2.55 (dd, 12 H, CH2-CH=CH2), 2.16
(s, 3 H, OH), 2.12 (s, 9 H, CH3 Ar), 1.49 (d, 9 H, CH3) ppm. 13C
NMR (62.91 MHz, CDCl3, TMS): δ = 144.0 (Cq, Ar), 142.6 (Cq,
Ar), 139.1 (Cq, Ar), 138.1 (Cq, Ar), 133.7 (CH=CH2), 133.3 (Cq,
Cyclohexylethyl Hexa-n-propyl Triamino Tricarbinols (R)-(–)-6b and
1
(S)-(+)-6b: White solid; yield 3.2 g, 92%. H NMR (250.13 MHz,
CDCl3, TMS): δ = 7.04 (m, 12 H, Ar), 3.42 (dd, 6 H, CH2-N), 3.01
Ar), 129.0 (CH, Ar), 128.5 (CH, Ar), 127.7 (CH, Ar), 126.6 (CH, (dd, 6 H, CH2-N), 1.97 (m, 6 H, CH and OH), 1.79 (s, 9 H, CH3,
Ar), 124.9 (CH, Ar), 119.0 (CH=CH2), 75.1 (Cq, C-OH), 56.5 Ar), 1.56 (m, 15 H, CH2 and CH), 1.34 (m, 12 H, CH2), 1.12 (m,
(CH), 52.6 (CH2-N), 47.4 (CH2-N), 46.8 (CH2-CH=CH2), 16.5 12 H, CH2), 0.86 (m, 6 H, CH2), 0.72 (m, 6 H, CH2), 0.64 (t, 18
(CH3), 12.6 (CH3, Ar) ppm. MS (MALDI-TOF): calcd. for [M –
2H]+ 1118.60; found 1118.75. C78H93N3O3 (1120.61): calcd. C
83.60, H 8.36; found C 83.46, H 8.38. (R)-(+)-5a: [α]2D0 = 60.0 (c =
10–2 gmL–1, CHCl3). CD (c = 3.5ϫ10–6 m, CH3CN): λ (Δε) = 195
(3), 206 (–0.2), 223 (+1.7 m–1 cm–1) nm. (S)-(–)-5a: [α]2D0 = –60.0 (c
= 10–2 gmL–1, CHCl3). CD (c = 3.5ϫ10–6 m, CH3CN): λ (Δε) =
195 (–2.4), 206 (0.6), 223 (–1.9 m–1 cm–1) nm.
H, CH3) ppm. 13C NMR (62.91 MHz, CDCl3, TMS): δ = 144.5
(Cq, Ar), 138.3 (Cq, Ar), 138.0 (Cq, Ar), 133.1 (Cq, Ar), 129.7 (CH,
Ar), 124.5 (CH, Ar), 77.1 (Cq, C-OH), 57.0 (CH), 53.1 (CH2-N),
48.4 (CH2-N), 45.4 (CH2), 41.3 (CH), 31.2 (CH2), 30.5 (CH2), 26.6
(CH2), 26.4 (CH2), 16.7 (CH2), 15.9 (CH3), 14.4 (CH3), 10.9 (CH3,
Ar) ppm. MS (MALDI-TOF): calcd. for [M – 2H]+ 1148.83; found
1148.86. C78H123N3O3 (1150.85): calcd. C 81.41, H 10.77; found C
80.52, H 10.80. (R)-(–)-6b: [α]2D0 = –54.0 (c = 10–2 gmL–1, CHCl3).
CD (c = 2.8ϫ10–6 m, CH3CN): λ (Δε) = 197 (–16.0), 209 (–4.0),
224 (7.5 m–1 cm–1) nm. (S)-(+)-6b: [α]2D0 = 54.0 (c = 10–2 gmL–1,
CHCl3). CD (c = 2.8ϫ10–6 m, CH3CN): λ (Δε) = 197 (19.0), 209
(4.0), 224 (–9.0 m–1 cm–1) nm.
Cyclohexylethyl Hexaallyl Triamino Tricarbinols (R)-(–)-5b and (S)-
(+)-5b: White solid; yield 1.04 g, 92%. 1H NMR (250.13 MHz,
CDCl3, TMS): δ = 7.03 (m, 12 H, Ar), 5.36 (m, 6 H, CH=CH2),
4.86 (m, 12 H, CH=CH2), 3.41 (dd, 6 H, CH2-N), 3.00 (dd, 6 H,
CH2-N), 2.46 (q, 3 H, CH), 2.38 (dd, 12 H, CH2-CH=CH2), 1.98
(s, 9 H, CH3, Ar), 1.34 (m, 15 H, CH2 and CH), 1.10 (m, 12 H, General Procedure for the Synthesis of the Enantiopure Dendritic
CH2), 0.82 (d, 9 H, CH3), 0.64 (m, 6 H, CH2) ppm. 13C NMR
(62.91 MHz, CDCl3, TMS): δ = 143.9 (Cq, Ar), 138.8 (Cq, Ar),
138.1 (Cq, Ar), 138.1 (Cq, Ar), 133.5 (CH=CH2), 133.1 (Cq, Ar),
129.9 (CH, Ar), 124.7 (CH, Ar), 119.0 (CH=CH2), 75.2 (Cq, C-
POM Salts 8a and 8b: H2O2 (35% in water) was added to an aque-
ous solution of commercial heteropolyacid H3PW12O40. The mix-
ture was stirred at room temperature for 30 min and then a CH2Cl2
solution of the corresponding amino tricarbinol compound was
OH), 57.1 (CH), 53.1 (CH2-N), 48.4 (CH2-N), 46.9 (CH2- added and the mixture was stirred for an additional hour. The or-
CH=CH2), 41.4 (CH), 31.2 (CH2), 30.6 (CH2), 26.6 (CH2), 26.5
(CH2), 26.4 (CH2), 16.0 (CH3), 11.0 (CH3, Ar) ppm. MS (MALDI-
TOF): calcd. for [M – 2H]+ 1136.74; found 1136.93. C78H111N3O3
(1138.76): calcd. C 82.27, H 9.82; found C 83.02, H 9.56. (R)-(–)-
5b: [α]2D0 = –58.0 (c = 10–2 gmL–1, CHCl3). CD (c = 3.5ϫ10–6 m,
CH3CN): λ (Δε) = 197 (–9.3), 208 (–2.0), 225 (+3.3 m–1 cm–1) nm.
(S)-(+)-5b: [α]2D0 = 58.0 (c = 10–2 gmL–1, CHCl3). CD (c =
ganic layer was dried with Na2SO4 and the solvents evaporated
under vacuum to provide the desired dendritic POM material as a
light-yellow solid.
Tris(phenylethyl) Hexa-n-propyl Tricarbinol Dendritic POM Salts
(R)-(+)-8a and (S)-(–)-8a: Yield 1.9 g, 86%. 1H NMR
(250.13 MHz, [D6]acetone, TMS): δ = 7.56 (br., 27 H, Ar), 4.56
(br., 12 H, CH2-N), 3.70 (br., 3 H, CH), 2.93 (br., 9 H, CH3, Ar),
1.75 (br., 12 H, CH2), 1.41 (br., 9 H, CH3), 1.29 (br., 12 H, CH2),
0.79 (br., 18 H, CH3) ppm. 13C NMR (62.91 MHz, CD3CN, TMS):
δ = 145.1 (Cq, Ar), 142.7 (Cq, Ar), 137.7 (Cq, Ar), 137.6 (Cq, Ar),
133.0 (Cq, Ar), 128.2 (CH, Ar), 127.9 (CH, Ar), 127.3 (CH, Ar),
126.1 (CH, Ar), 124.6 (CH, Ar), 75.8 (Cq, C-OH), 66.0 (CH), 51.8
(CH2-N), 46.8 (CH2-N), 45.0 (CH2), 16.2 (CH2), 15.6 (CH3), 14.3
(CH3), 13.6 (CH3, Ar) ppm. 31P NMR (81.02 MHz, [D6]acetone,
3.5ϫ10–6 m, CH3CN):
(–3.8 m–1 cm–1) nm.
λ (Δε) = 197 (11.3), 208 (0.7), 225
General Procedure for the Synthesis of Hexa-n-propyl Triamino
Tricarbinols 6a and 6b: Pd/C catalyst (10%) was added to a THF
solution (20 mL) of the corresponding triamine hexaallyl tricarbi-
nol 5 in a thick-walled tube capped with a Young’s stopcock. The
tube was flushed, pressurized with hydrogen, sealed and stirred at
room temperature for 3 h. The solvent was removed under vacuum
and the residue was extracted with dichloromethane and filtered
through Celite. After evaporation of the solvent, the hexa-n-propyl
triamino tricarbinol compound was purified by silica gel column
chromatography with petroleum ether/diethyl ether (6:4, v/v) as elu-
ent.
85% H PO ): δ = 3.26 (PO ) ppm. IR (KBr): ν = 3489 (br, s), 2957
˜
3
4
4
(s), 2928 (s), 2871 (s), 1701 (m), 1604 (m), 1455 (m), 1108 (m),
1081 (m, P–O), 1062 (m, P–O), 951 (m, W=O), 834 (m, O–O) cm–1.
C78H108N3O27PW4 (2286.05): calcd. C 40.98, H 4.76, W 32.17;
found C 41.85, H 4.80, W 34.70. (R)-(+)-8a: [α]2D0 = 9.0 (c =
10–2 gmL–1, CHCl3). CD (c = 5.0ϫ10–6 m, CH3CN): λ (Δε) = 196
(10.0) 220 (6.0 m–1 cm–1) nm. (S)-(–)-8a: [α]2D0
= –9.0 (c =
Phenylethyl Hexa-n-propyl Triamino Tricarbinols (R)-(+)-6a and
10–2 gmL–1, CHCl3). CD (c = 5.0ϫ10–6 m, CH3CN): λ (Δε) = 196
(–12.9), 220 (–9.6 m–1 cm–1) nm.
1
(S)-(–)-6a: White solid; yield 2.6 g, 93%. H NMR (250.13 MHz,
CDCl3, TMS): δ = 7.21 (m, 27 H, Ar), 3.85 (q, 3 H, CH), 3.62 (dd,
6 H, CH2-N), 3.40 (m, 6 H, CH2-N), 2.12 (s, 9 H, CH3, Ar), 1.75 Tris(cyclohexylethyl) Hexa-n-propyl Tricarbinol Dendritic POM
(m, 12 H, CH2), 1.48 (m, 9 H, CH3), 1.26 (m, 6 H, CH2), 1.04 (m, 6 Salts (R)-(–)-8b and (S)-(+)-8b: Yield 2.3 g, 86%. 1H NMR
H, CH2), 0.84 (t, 18 H, CH3) ppm. 13C NMR (62.91 MHz, CDCl3, (250.13 MHz, CDCl3, TMS): δ = 7.73 (br., 18 H, Ar), 7.31 (br., 18
TMS): δ = 144.8 (Cq, Ar), 142.6 (Cq, Ar), 138.7 (Cq, Ar), 138.1
(Cq, Ar), 133.3 (Cq, Ar), 128.9 (CH, Ar), 128.5 (CH, Ar), 127.7
H, Ar), 4.41 (br., 12 H, CH2-N), 3.00 (br., 3 H, CH), 2.17 (br., 9
H, CH3, Ar), 1.67 (br., 36 H, CH3, Ar and CH2), 1.13 (br., 27 H,
736
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Eur. J. Inorg. Chem. 2011, 727–738

Tripodal Dendritic Polyoxometalates
CH3), 0.89 (br., 18 H, CH2), 0.81 (br., 54 H, CH3) ppm. 13C NMR
(62.91 MHz, CDCl3, TMS): δ = 147.4 (Cq, Ar), 139.6 (Cq, Ar),
139.4 (Cq, Ar), 134.8 (Cq, Ar), 131.3 (CH, Ar), 126.8 (CH, Ar),
77.7 (Cq, C-OH), 58.6 (CH), 54.8 (CH2-N), 50.1 (CH2-N), 47.8
(CH2), 47.4 (CH2), 43.1 (CH), 33.0 (CH2), 32.3 (CH2), 28.2 (CH2),
28.0 (CH2), 18.4 (CH2), 17.5 (CH3), 15.9 (CH3), 12.2 (CH3,
Ar) ppm. 31P NMR (81.02 MHz, [D6]acetone, 85% H3PO4): δ =
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3.32 (PO ) ppm. IR (KBr): ν = 3458 (br, s), 2965 (s), 2876 (s), 1701
˜
4
(m), 1609 (m), 1465 (m), 1089 (m, P–O), 1058 (m, P–O), 964 (m,
W=O), 843 (m, O–O) cm–1. C78H126N3O27PW4 (2304.20): calcd. C
40.66, H 5.51, W 31.92; found C 41.57, H 5.39, W 30.90. (R)-(–)-
8b: [α]2D0 = –12.0 (c = 10–2 gmL–1, CHCl3). CD (c = 5.0ϫ10–6 m,
CH3CN): λ (Δε) = 199 (7.0), 224 (12.9), 302 (–2.7 m–1 cm–1) nm.
(S)-(+)-8b: [α]2D0 = +12.0 (c = 10–2 gmL–1, CHCl3). CD (c =
5.0ϫ10–6 m, CH3CN): λ (Δε) = 199 (–8.4), 224 (–7.7), 302
(3.6 m–1 cm–1) nm.
[3]
[4]
General Procedure for the Catalytic Oxidation Reactions with
Enantiopure Dendritic POM Hybrids and for the Catalyst Recovery
Experiments: The POM catalyst and the substrate (250 equiv.) were
dissolved in solvent (1 mL). An aqueous solution of H2O2 (35% in
water) was added to the reaction mixture at the appropriate tem-
1
perature. The latter was stirred and monitored by H NMR spec-
[5]
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A. Proust, R. Thouvenot, P. Gouzerh, Chem. Commun. 2008,
16, 1837–1852.
troscopy. Upon completion of the reaction, the organic layer was
separated and concentrated under vacuum to about 1 mL. The cat-
alyst was precipitated by addition of Et2O (5 mL). The solid was
filtered, washed with diethyl ether (3ϫ2 mL) and dried under vac-
uum to yield the POM catalyst as a white solid (see Table 2), which
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1
was analysed by H and 31P NMR spectroscopy before its use in a
new catalytic experiment. The diethyl ether solution was evaporated
under vacuum and the oxidized product was purified by silica gel
column chromatography (petroleum ether/diethyl ether, 1:9, v/v).
The enantiomeric excesses were determined by chiral HPLC using
a Chiralcel ASH column and UV detector (254 nm), eluting with
hexane/2-propanol (1:1) at a flow rate of 0.5 mLmin–1. Retention
times [min]: (R)-13: 22.5; (S)-13: 30.9; (R)-14: 21.4; (S)-14: 31.2;
(R)-15: 28.5; (S)-15: 39.1.[21] The catalyst was recovered following
the typical procedure and conditions described above for the first
cycle, the organic solvent and the reactants being adjusted to the
amount of catalyst used.
[7]
[8]
[9]
Supporting Information (see footnote on the first page of this arti-
cle): NMR, IR, CD, fluorescence, and absorption spectra of amines
and dendritic-POM compounds are given. Theoretical VCD spec-
tra of the Venturello species [PW4O24]3- and detailed catalytic ex-
periments data are also presented.
[10]
Acknowledgments
S. N. and C. J. thank the Agence National de la Recherche (grant
ANR-06-BLAN-0215), the University of Bordeaux, the Centre
National de la Recherche Scientifique (CNRS), the European Sci-
ence Foundation (ESF), the European Cooperation in Science and
Technology (COST D40 action), the University of Padova, the
European Research Council under the European Community’s
Seventh Framework Programme (FP7/2007-2013/ERC, grant no.
208702) and the Région Aquitaine for financial support. We thank
Prof. S. Quideau and Dr. D. Deffieux (ISM) for helpful discussions
and HPLC facilities and Ms. Gloria Modugno for CD experiments.
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Received: October 18, 2010
Published Online: January 4, 2011
738
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Eur. J. Inorg. Chem. 2011, 727–738