Enantiopure dendritic polyoxometalates: chirality transfer from dendritic wedges to a pom cluster for asymmetric sulfide oxidation

Article abstract of DOI:10.1002/chem.200901512

The synthesis and characterization of four enantiopure polyoxometalate- cored dendrimers, which contain n-propyl and epoxy groups, based on electrostatic interactions between enantiopure dendritic quaternary ammonium ions and an a chiral trianionic POM has been reported. The optical and chiroptical properties of the n-propyl-terminated POM-cored dendrimers have been demonstrated in solution by UV/Vis spectroscopy, circular dichroism (CD) and vibrational circular dichroism (VCD) spectroscopy, and fluorimetry. The solution CD, UV/Vis and VCD data indicate a significant induced optical activity in the POM cluster. The appropriate elaboration of optimized dendritic structures enabled the use of a range of POM species, and represented a promising approach to efficient, recoverable, and highly enantioselective dendritic POM future catalysts.

Full text of DOI:10.1002/chem.200901512

COMMUNICATION
DOI: 10.1002/chem.200901512
Enantiopure Dendritic Polyoxometalates: Chirality Transfer from Dendritic
Wedges to a POM Cluster for Asymmetric Sulfide Oxidation
Claire Jahier,[a] Martine Cantuel,[a] Nathan D. McClenaghan,[a] Thierry Buffeteau,[a]
Dominique Cavagnat,[a] Francine Agbossou,[b] Mauro Carraro,[c] Marcella Bonchio,[c] and
Sylvain Nlate*[a]
Dedicated to Professor Pierre H. Dixneuf in recognition of his contribution to the field of homogeneous catalysis
The quest for new catalytic and highly enantioselective
processes is currently one of the most intensively studied
areas in chemical synthesis.[1] In this context, chiral polyoxo-
metalates (POMs) have become a topic of recent interest
owing to their potential application in medicine and asym-
metric catalysis.[2] Among numerous applications of POMs,
catalysis is by far the most studied, owing to the enormous
versatility that POMs offer in the clean synthesis of fine
chemicals. However, few chiral POM compounds for asym-
metric catalysis are known.[3] Of those reported, none are
based on dendritic structures, although dendritic counter
cations are known to play a critical role in determining the
properties of anionic POMs.[4] Two main approaches have
been developed for the synthesis of chiral POM-based
frameworks. The first involves the use of a chiral species (or-
ganic moieties or metal complexes) as a chirality transfer
agent.[5] The second strategy is based on spontaneous resolu-
tion upon crystallization in the absence of any chiral auxili-
ary, which yields conglomerates.[6] For example, enantiopure
hafnium-substituted POMs, which are obtained by spontane-
ous resolution upon crystallization in the absence of any
chiral source have been reported.[6a] Recently, two enantio-
merically pure 3D chiral POM-based architectures were de-
veloped from achiral moieties without any chiral auxilia-
AHCTUNGTRENNUNG
ry.[6b–c] However, despite their potential applications in catal-
ysis and separation, enantiopure POM frameworks are still
under represented. The search for suitable enantiomerically
pure POM-based hybrid materials remains a challenging
issue in synthetic chemistry and materials science. To date,
chiral dendritic POM systems, as well as the study of chirop-
tical activity of enantiopure POM-based frameworks in
asymmetric catalysis have, to the best of our knowledge,
never been reported.
Herein, we report the synthesis and characterization of
four enantiopure polyoxometalate-cored dendrimers, which
contain n-propyl [(R)-(+)-6 and (S)-(À)-6] and epoxy [(R)-
(+)-7 and (S)-(À)-7] groups, based on electrostatic interac-
tions between enantiopure dendritic quaternary ammonium
ions and an achiral trianionic POM. To the best of our
knowledge, these compounds represent the first examples of
optically active dendritic POM systems. Importantly, the
POM cluster displays chiroptical effects, indicating chirality
transfer from the enantiopure dendritic ammonium ions.
The optical and chiroptical properties of the n-propyl-ter-
minated POM-cored dendrimers (R)-(+)-6 and (S)-(À)-6
have been demonstrated in solution by UV/Vis spectrosco-
py, circular dichroism (CD) and vibrational circular dichro-
ism (VCD) spectroscopy, as well as fluorimetry. In addition,
the use of 6 as a catalyst in the asymmetric oxidation of thi-
oanisole with aqueous H2O2 provides the corresponding op-
tically active sulfoxide with up to 14% enantiomeric excess
(ee) as a result of a chirality transfer process from the den-
dritic wedge to the catalytically active POM unit. Despite
the modest enantioselectivity, this proof-of-principle catalyt-
ic experiment demonstrates and confirms the transfer of chi-
roptical properties from organic moieties to a catalytically
active POM unit. Interestingly, the catalyst was recovered
and reused in three successive cycles without discernable
[a] C. Jahier, Dr. M. Cantuel, Dr. N. D. McClenaghan, Dr. T. Buffeteau,
Dr. D. Cavagnat, Dr. S. Nlate
ISM, UMR CNRS No. 5255, Universitꢀ Bordeaux I
351 Cours de la Libꢀration, 33405 Talence Cedex (France)
Fax : (+33)5-4000-6994
E-mail: s.nlate@ism.u-bordeaux1.fr
[b] Dr. F. Agbossou
UCCS UMR CNRS N8 8181, Universitꢀ de Lille
59652 Villeneuve d’Asq Cedex (France)
[c] Dr. M. Carraro, Prof. M. Bonchio
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/chem.200901512.
Chem. Eur. J. 2009, 15, 8703 – 8708
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8703

loss in activity, selectivity, or enantioselectivity. Catalyst
preparation involves the electrostatic coupling of the Ven-
turello peroxophosphotungtate [PO4{WO(O2)2}4]3À, known
for its efficiency in oxidation reactions,[4d–g,7] with designed
enantiopure dendritic cations obtained from enantiomeri-
cally pure 1-phenylethylamine [(R)-(+)-1 and (S)-(À)-1].
Although enantiopure architectures based on Keggin-type
[BW12O40]5À POMs have been recently reported,[5k] the tri-
anionic [PO4{WO(O2)2}4]3À moiety has never been used as a
building unit to make chiral frameworks. Enantiopure den-
dritic POM-based frameworks (R)-(+)-6 and (S)-(À)-6, as
well as (R)-(+)-7 and (S)-(À)-7 were built up from ionic as-
semblies of the tri-anionic [PO4{WO(O2)2}4]3À with the cor-
responding enantiopure dendritic cations. The synthesis of
the enantiopure dendritic cations involved the functionaliza-
tion of commercially available
prepare compounds 6 and 7. A water-soluble dinuclear per-
oxotungstate decomposition product [{WO(O2)2A
(H2O)}2O]2À
CTHUNGTRENNUNG
was easily removed because it remained in the aqueous
phase of the reaction mixture.[4d–g,7d,8] The NMR and IR
spectroscopy and the elemental analysis data reported for
the dendritic POM hybrids were in agreement with the
structures shown in Scheme 1. Only one signal was obtained
in the 31P NMR spectrum for 6 (CDCl3, d=3.36 ppm) and 7
([D6]DMSO, d=À10.51 ppm). The chiroptical properties of
6 have been examined in solution by CD spectroscopy and
polarimetry. The CD spectra of (R)-(+)-6 and (S)-(À)-6 are
mirror images of one another (Figure 1a), confirming their
enantiopurity, and show strong and weak Cotton effects
with maxima at 222 and 298 nm, respectively. These can be
compared with the CD spectra of the ammonium chloride
1-phenylethylamine (R)-(+)-1
and (S)-(À)-1 to tetraallyl dicar-
binol (R)-(+)-4 and (S)-(À)-4
and tetra-n-propyl dicarbinol
(R)-(+)-5 and (S)-(À)-5, respec-
tively (Scheme 1).
The treatment of
1 with
methyl 4-(bromomethyl)ben-
zoate (2) gave the correspond-
ing amino dibenzoate (R)-(+)-3
and (S)-(À)-3, which was ally-
lated by allylmagnesium bro-
mide to give the tetraallyl dicar-
binol (R)-(+)-4 and (S)-(À)-4.
All attempts to prepare the cor-
responding hexaallyl amino de-
rivatives by allylation of 4 by
using allyltrimethylsilane in the
presence of BF3 failed. The hy-
drogenation of (R)-(+)-4 and
(S)-(À)-4 in the presence of Pd/
C catalyst gave the n-propyl
counterparts (R)-(+)-5 and (S)-
(À)-5. The treatment of amines
4 and 5 with an aqueous solu-
tion of the commercial hetero-
polyacid H3PW12O40 and an
excess of H2O2 provided the
enantiomerically pure n-propyl
dendritic salts (R)-(+)-6 and
(S)-(À)-6, as well as the epoxy
dendritic salts (R)-(+)-7, (S)-
(À)-7 of [PO4{WO(O2)2}4]3À in a
biphasic mixture of water and
dichloromethane. The ammoni-
um chloride compounds 4·HCl
and 5·HCl (see the Supporting
Information for synthetic proce-
dures and characterization de-
tails) obtained from 4 and 5, re-
Scheme 1. Synthesis of enantiopure dendritic POMs (R)-(+)-6, (S)-(À)-6, (R)-(+)-7, and (S)-(À)-7. i) N,N-di-
AHCTUNGTERGiNNUN sopropylethylamine, CH3CN, 308C, 24 h; ii) allylmagnesium bromide, Et2O, 0–308C, 12 h; iii) Pd/C (10%),
spectively, can also be used to THF, RT, 3 h; iv) H3PW12O40, H2O2, H2O, CH2Cl2, RT.
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Enantiopure Dendritic Polyoxometalates
COMMUNICATION
dritic POM hybrids, owing to their high solubility in organic
solvents. The reaction conditions were optimized with re-
spect to the solvent and temperature used (see page S27 in
the Supporting Information) to achieve a high yield of the
chiral sulfoxide product and to maximize enantioselectivity.
We found that thioanisole was quantitatively oxidized by
using (R)-(+)-6 and (S)-(À)-6 at À508C after 30 min to
yield 93% of sulfoxide with 14% ee, together with a small
amount of sulfone (7%). The reaction did not proceed in
the absence of the dendritic POM catalyst when carried out
under similar conditions. Interestingly, a dendritic effect was
noted in the enantioselectivity. Indeed, compared with the
enantiopure nondendritic compounds 11 (Scheme 1), the
dendritic compounds 6 were more selective (14 versus
4% ee). The catalyst was easily recovered by precipitation
with diethyl ether, and was checked by 31P NMR spectrosco-
py before a new catalytic cycle was conducted. Three cata-
lytic cycles were performed without any discernable loss in
activity, selectivity, and enantioselectivity. Despite the
modest ee, to the best of our knowledge this is the first time
that chirality transfer from organic moieties to the POM
unit in an organic–POM hybrid framework has been demon-
strated and confirmed (by the ee values) through a catalytic
reaction. In addition, an important consideration with
regard to catalytic activity and chirality transfer concerns
POM–wedge interactions in solution because these com-
plexes may be anticipated to be dynamic in nature. To gain
an insight into interactions in a homogeneous solution, fluo-
rescence and absorption experiments, as well as electronic
absorption and VCD spectroscopy of (R)-(+)-6 and (S)-(À)-
6 were carried out. The absorption spectra (in CH2Cl2 and
CH3CN) were not simply a summation of the absorp-
tion bands associated with constituent POM and dendron
chromophores (see Figure 2). Indeed, an additional ab-
sorption band between 280 and 350 nm was observed
with respect to free dendron (Figure 2b) and nondendritic
Figure 1. CD spectra in CH3CN of a) enantiopure dendritic POMs (R)-
(+)-6 and (S)-(À)-6 (c=2.3ꢂ10À5 m) and their precursors and b) tetra-n-
propyl dicarbinol ammonium chloride (R)-(+)-5·HCl and (S)-(À)-5·HCl
(c=1.0ꢂ10À5 m). Ellipt. =ellipticity.
counterparts (R)-(+)-5·HCl and (S)-(À)-5·HCl that exhibit
strong and weak Cotton effects with maxima at 200 and
228 nm, respectively (Figure 1b). These values are compara-
ble to those obtained for oxygen-to-tungsten charge-transfer
bands of Keggin polyoxoanions.[5k,9]
POM–Arquad ([{nC18H37(75%)+nC16H33(25%)}2N(CH3)2]+
-
3
[PO4{WO(O2)2}4]3À, in which Arquad=[{nC18H
nC16H33A(25%)}2N
(CH3)2], Figure 2c).[7b]
37ACHTUNGTRENNUNG(75%)+
C
ACHTUNGTRENNUNG
To demonstrate the chiroptical properties of the POM
unit in an asymmetric reaction, we performed the oxidation
of thioanisole to the corresponding sulfoxide with H2O2
(35%) in a biphasic mixture of water and CDCl3 by using
the enantiopure compound 6 (0.4%, Scheme 2).
Instead of epoxy compounds 7, the n-propyl derivatives 6
have been used for all of the experiments performed to in-
vestigate the chiroptical properties of this new series of den-
Figure 2. Electronic absorption spectra of a) dendritic POM salt (S)-(À)-
6, b) tetra-n-propyl dicarbinol amine (S)-(À)-5·HCl in CH3CN, and
c) POM-Arquad in CH2Cl2. Inset: luminescence spectrum of a) (S)-(À)-6
and b) (S)(À)-5·HCl in CH3CN.
Scheme 2. Enantiopure dendritic POM-catalyzed asymmetric oxidation
of phenylmethylsulfide 8 to phenylmethylsulfoxide 9.
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S. Nlate et al.
The latter absorption band is consistent with significant
ground-state interaction with at least one chiral auxiliary,
even at micromolar concentrations. This interaction appears
to be unaltered upon adding aliquots of water, including
under simulated catalytic conditions (4.10À3 m catalyst+800
equiv H2O2 (35%)). However, when even higher concentra-
tions of water were used, ionic complex formation appeared
to be increasingly disfavored. Fluorescence emission and ex-
citation spectra showed that structured POM–Arquad emis-
sion was slightly blueshifted when it was with dendritic
wedges (Dw=ca. 1000 cmÀ1), whereas the blue ammonium
ligand 5·HCl fluorescence (fF =0.03, lmax =320 nm) ap-
peared to be quenched owing to the close proximity of the
POM. All of these results are consistent with a ground-state
(and excited-state) complex comprising the anionic POM
and at least one cationic dendritic wedge, necessitating close
interaction (see pages S23 and S24 in the Supporting Infor-
mation). Futhermore, VCD spectroscopy is a well-estab-
lished method to obtain information about the chirality of a
sample and, in particular, for determination of the absolute
configuration and solution conformation of chiral mole-
cules.[10] Compared with those of electronic circular dichro-
ism (ECD) spectra, VCD spectra are more detailed because
of the many bands contained in the mid-infrared (mid-IR)
region. The IR spectra of 5 and 6 in CDCl3 solution are re-
ported in the Supporting Information (see page S25). Most
of the bands of complex 6 are related to ligand 5, whereas
broad bands of the central POM are observed at 1060 and
967 cmÀ1. These spectral contributions are assigned to
Figure 3. Experimental VCD spectra of (R)-(+)-6 and (S)-(À)-6 in the
1750–950 cmÀ1 region under the following experimental conditions: coad-
dition of 36000 scans (12 h acquisition time), resolution (4 cmÀ1), concen-
tration (0.6m) in CDCl3 solution, path length (250 mm).
À
stretching vibrations of P O and W=O bonds, respectively.
This assignment has been confirmed by calculations at the
density functional theory (DFT) level of optimized geome-
try, vibrational frequencies, and IR intensities of the POM
entity (see page S26 in the Supporting Information). VCD
spectra of (R)-(+)-6 and (S)-(À)-6 are shown in Figure 3.
The two VCD spectra are perfect mirror images, indicat-
ing identical optical purity of the two enantiomers. The most
important contributions in the VCD spectrum come from
the vibrational modes related to the ligand. In particular,
the VCD bands at 1468 and 1452 cmÀ1 are associated with
the CH2 and CH3 bending vibrations of the propyl groups.
Two other VCD bands with significant intensities are locat-
ed at 1383 and 1206 cmÀ1. They are assigned to the chiral
Figure 4. Comparison of VCD spectra of (R)-(+)-5 and (R)-(+)-6 in the
1750–950 cmÀ1 region.
on the POM entity, a detailed examination of the VCD
spectra in the 1100–1000 cmÀ1 region reveals a negative–pos-
itive bisignate shape at about 1050 cmÀ1 from higher to
lower frequency for the R-(+)-6 complex. Even if the inten-
sity of these VCD bands (at 1054 and 1043 cmÀ1) are very
low and close to the noise of the VCD spectra, we attribute
these contributions to chirality transfer from the ligand to
the POM cluster. Indeed, the POM molecule would thereby
display a structural chirality in a chiral environment, which
may be anticipated to be the origin of the ee in sulfide oxi-
dation. This structural chirality has been experimentally ob-
served. Moreover, DFT calculations on the optimized geom-
etry of the isolated molecule show this tendency of the
À
À
C NH bending and C Ph stretching vibrations, respectively.
The investigation of chirality transfer from the ligand to the
POM unit is not straightforward. Indeed, induced chirality
on achiral molecules gives rise to very low VCD signals and
is generally associated with a modified VCD of the chiral in-
ducer. This behavior is illustrated in Figure 4 in which the
comparison of VCD spectra of R-(+)-5 and R-(+)-6 is
shown in the 1750–950 cmÀ1 region.
The VCD bands related to the CH2 and CH3 bending vi-
brations of the propyl groups exhibit opposite signs, whereas
the signs of the VCD bands in the 1420–1100 cmÀ1 region
are not modified. This result suggests that the conformations
of the propyl groups are changed in the complex with re-
spect to the ligand alone. Concerning the induced chirality
POM molecule to adopt a chiral structure of the PACHTUNGTRENNUNG(WO6)
fragments. Indeed, significant VCD intensities have been
À
calculated for the stretching vibrations of P O bonds, even
if the molecule was not in a chiral environment (see Fig-
ure S37 in the Supporting Information). The initial catalytic
results described herein, as well as the UV/Vis absorption,
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Enantiopure Dendritic Polyoxometalates
COMMUNICATION
C108H156 N3PW4O30: C 47.29, H 5.73, W 26.81; found: C 47.18, H 5.99, W
27.48; (R)-(+)-6: [a]D20 =67.3 (c=10À2 gmLÀ1, CHCl3); CD (c=2.3.10À5 m,
CH3CN): 222 (De=5.3) and 297 nm (De=À0.6); (S)-(À)-6: [a]2D0 =À67.6
(c=10À2 gmLÀ1, CHCl3); CD (c=2.3.10À5 m, CH3CN): 219 (De=À5.6)
and 297 nm (De=+0.2).
the CD and VCD reported data, clearly demonstrate and
confirm a chirality extention from the enantiopure dendritic
wedges to the POM cluster and their close interaction in so-
lution.
Compound 7: Yield 70%; 1H NMR (250.13 MHz, DMSO, TMS): d=
In conclusion, we have built enantiopure dendritic POM
frameworks by assembling chiral dendritic amines and
acidic POM moieties. Compounds 6 and 7 represent the first
examples of enantiopure dendritic POM hybrids. The solu-
tion CD, UV/Vis and VCD data of 6 indicate a significant
induced optical activity in the POM cluster. In addition,
compound 6 selectively oxidizes thioanisole to the corre-
sponding chiral sulfoxide with 14% ee. Despite the modest
ee, the present reaction demonstrates chirality transfer to
the POM unit in an asymmetric transformation. We believe
that the appropriate elaboration of optimized dendritic
structures will enable the use of a range of POM species,
and represents a promising approach to efficient, recovera-
ble, and highly enantioselective dendritic POM future cata-
lysts. Further work in this area is in progress.
À
7.66–7.29 (m, 39H; Ar), 4.49 (q, 3H; CH), 4.43 (dd, 12H; CH2 N), 3.71–
À
À
4.15 (br, 18H; CH2 O, CH O, and CH2), 1.86 ppm (d, 24H; CH2);
13C NMR (62.91 MHz, DMSO, TMS): d=145.0 (Cq, Ar), 137.7 (Cq, Ar),
134.6 (CH, Ar), 127.6 (CH, Ar), 125.3 (CH, Ar), 117.0 (CH, Ar), 114.4
À
À
(Cq, Ar), 79.1 (CHO), 74.6 (Cq OH), 64.8 (CH2 O), 56.6 (CH), 52.6
À
À
(CH2 N), 46.4 (CH2 CHO), 41.1 (CH), 30.2 (CH2), 15.1 ppm (CH3);
31P NMR (81.02 MHz, DMSO, 858 H3PO4): d=À10.51 ppm (PO4); IR
(KBr pellet): n=3413 (brs), 2955 (s), 2932 (s), 2859 (s), 1722 (m), 1679
À
À
(w), 1632 (m), 1087 (s, P O), 968 (s, W=O), 845 (s, O O), 574 (m),
548 cmÀ1 (w); elemental analysis calcd (%) for C108H132 N3PW4O42: C
44.57, H 4.57, W 25.27; found: C 45.36, H 4.76, W 25.98. (R)-(+)-7:
[a]2D0 =64.3 (c=10À2 gmLÀ1
,
DMSO); (S)-(À)-7: [a]2D0 =À64.0 (c=
10À2 gmLÀ1, DMSO).
General procedure for catalytic reactions and catalyst recycling: The
POM catalyst 6 (4.10À3 mmol) and thioanisole 8 (250 equiv) were dis-
solved in CDCl3 (1 mL). An aqueous solution of H2O2 (35%, 800 equiv)
was added to the reaction mixture at the appropriate temperature. The
latter was stirred and monitored by 1H NMR spectroscopy. Upon reac-
tion completion, the catalyst was precipitated by addition of Et2O
(5 mL). The solid was filtered, washed with diethyl ether (3ꢂ2 mL), and
dried under vacuum to yield the POM catalyst as a white solid, which
was analyzed by 1H and 31P NMR spectroscopy before its use in a new
catalytic experiment. The diethyl ether solution was evaporated under
vacuum and the sulfoxide was purified by chromatography on a silica-gel
column (petroleum ether/diethyl ether 1:9; v/v). The enantiomeric excess-
es were determined by chiral HPLC by using a Chiralcel ASH column,
UV detector (254 nm), eluting with hexane/isopropanol (1:1) at a flow
rate of 0.5 mLminÀ1. Retention time: (R)-9 22.5, (S)-9 30.9 min.[11]
Experimental Section
General procedure for the synthesis of enantiopure dendritic POM 6 and
7: For the synthesis of the amino dibenzoate compounds, a mixture of
the corresponding enantiomerically pure 1-phenylethylamine, methyl 4-
(bromomethyl)benzoate, and N,N-diisopropylethylamine in CH3CN was
stirred for 24 h at 308C. After removal of the solvent under vacuum, the
residue was extracted with diethyl ether, washed with water, and dried
over sodium sulfate. The solvent was removed under vacuum and the
product was purified by chromatography on a silica-gel column, eluting
with a petroleum ether/diethyl ether (9:1; v/v) mixture. The enantiopure
amino tetraallyl dicarbinol was prepared by adding a diethyl ether solu-
tion of amino dibenzoate to a diethyl ether solution of freshly prepared
allylmagnesium bromide. The mixture was stirred at 308C for 12 h, then
a water solution of NH4Cl 6m was added, and the organic layer was
washed with water and dried over Na2SO4. After removal of the solvent
under vacuum, the product was purified by chromatography on a silica-
gel column, eluting with a petroleum ether/diethyl ether (7:3; v/v) mix-
ture. The corresponding amino tetra-n-propyl dicarbinol was prepared by
adding Pd/C catalyst (10%) to a solution of an amino tetraallyl com-
pound in THF in a thick-walled tube capped with a Youngꢃs stop-cock.
The tube was flushed, pressurized with hydrogen, sealed, and stirred at
room temperature for 3 h. The solvent was removed under vacuum, the
residue was extracted with diethyl ether, and the solution was filtered
through celite and evaporated. Then, the enantiopure dendritic polyoxo-
metalate salts were prepared by adding H2O2 (35% in water) to a solu-
tion of commercially available heteropolyacid H3PW12O40 in water. The
mixture was stirred at room temperature for 30 min, then a CH2Cl2 solu-
tion of the tetra-branched amino dicarbinol compound was added, and
the mixture was stirred for an additional hour. For 6, the CH2Cl2 layer
was dried over Na2SO4 and evaporated under vacuum, providing the de-
sired dendritic POM material as a light-yellow solid, whereas the CH2Cl2
insoluble 7 was collected as a precipitate.
Acknowledgements
Financial Support from the ANR-06-BLAN-0215 grant (C.J., S.N., and
F.A.), the University of Bordeaux 1, the CNRS, the ESF COST D40
action, the Region Aquitaine, and the University of Padova, is gratefully
acknowledged. We thank Prof. S. Quideau and Dr. D. Deffieux (ISM) for
helpful discussions and HPLC facilities, and Ms. Gloria Modugno for CD
experiments.
Keywords: asymmetric oxidation
·
chirality transfer
·
dendrimers
dichroism
·
polyoxometalates
·
vibrational circular
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À
7.52–7.03 (br; Ar), 4.38–3.70 (br; CH and CH2 N), 2.21 (s; OH), 2.02 (d;
CH3), 1.67 (br; CH2), 1.17 (br; CH2), 0.97 (br; CH2), 0.74 ppm (br; CH3);
13C NMR (62.91 MHz, CDCl3, TMS): d=145.2 (Cq, Ar), 143.3 (Cq, Ar),
138.6ACHTUNGTRENNUNG(Cq, Ar), 128.5 (CH, Ar), 128.3 (CH, Ar), 128.2 (CH, Ar), 126.9
À
À
(Cq, Ar), 125.3 (CH, Ar), 77.3 (Cq OH), 56.2 (CH), 53.5 (CH2 N), 45.5
(CH2), 17.0 (CH2), 14.7 (CH3), 13.6 ppm (CH3); 31P NMR (81.02 MHz,
CDCl3, 858 H3PO4): d=3.26 ppm (PO4); IR (KBr pellet): n=3479 (brs),
À
2957 (s), 2932 (s), 2871 (s), 1724 (m), 1109 (m), 1053 (m, P O), 947 (m,
À1
À
W=O), 852 (m, O O), 577 cm (w); elemental analysis calcd (%) for
Chem. Eur. J. 2009, 15, 8703 – 8708
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: June 4, 2009
Published online: July 16, 2009
8708
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Chem. Eur. J. 2009, 15, 8703 – 8708