L-proline-derived dendritic tetrakis(diperoxotungsto)phosphate: Synthesis and enantioselective oxidation catalysis

Article abstract of DOI:10.1002/ejic.201101145

As a part of our ongoing research towards the synthesis of chiral dendritic polyoxometalate (DENDRI-POM) hybrids that are able to catalyze oxidation of organic substrates with high selectivity and enantioselectivity, we have prepared an enantiopure DENDRI-POM salt by coupling a well-defined L-proline-derived tetrapropylammonium dendron with the tetrakis(diperoxotungsto) phosphate [PO₄{WO(O₂)₂}₄] ^3-. This DENDRI-POM hybrid was characterized by infrared, NMR spectroscopy, polarimetry, and elemental analysis. The data obtained are consistent with the structure in which the trianionic POM is surrounded by three L-proline-based dendrons. This DENDRI-POM oxidized sulfides with moderate activity and very low enantioselectivities (up to 4 %), whereas alkenes were efficiently oxidized with up to 37 % enantiomeric excess (ee), and good yields. Although these ee values are still not satisfactory for practical asymmetric synthesis, 37 % ee represents the best enantioselectivity reported to date in the oxidation of organic substrate with chiral POM hybrids. Studies of temperature, solvent, and catalyst loading effects on the outcome of the reaction indicate that the ee is sensitive to the nature of the solvent. Moreover, the catalyst was recovered at low temperature and reused while retaining its activity and enantioselectivity. These results contrast and complement our reported results in which DENDRI-POMs based on phenylethylamine-derived ligands efficiently oxidize sulfides with up to 14 % ee, whereas alkenes were not oxidized with this family of chiral POM compounds. Thus, the influence of the nature of the chiral countercation on the POM properties is again observed. L-Proline-derived dendritic tetrakis(diperoxotungsto)phosphate was prepared byassembling enantiopure L-proline-derived ligands with the anionic polyoxometalate (POM) unit. This DENDRI-POM catalyzes the oxidation of alkenes to the corresponding epoxides with 37 % ee. This ee value is the best enantioselectivity reported to date in the oxidation of organic substrates with chiral POM hybrids. Copyright

Full text of DOI:10.1002/ejic.201101145

FULL PAPER
DOI: 10.1002/ejic.201101145
L
-Proline-Derived Dendritic Tetrakis(diperoxotungsto)phosphate: Synthesis and
Enantioselective Oxidation Catalysis
Claire Jahier[a] and Sylvain Nlate*[a]
Keywords: Asymmetric catalysis / Oxidation / Chirality / Dendrimers / Polyoxometalates
As a part of our ongoing research towards the synthesis of
chiral dendritic polyoxometalate (DENDRI-POM) hybrids
that are able to catalyze oxidation of organic substrates with
high selectivity and enantioselectivity, we have prepared an
enantiopure DENDRI-POM salt by coupling a well-defined
ee values are still not satisfactory for practical asymmetric
synthesis, 37% ee represents the best enantioselectivity re-
ported to date in the oxidation of organic substrate with chi-
ral POM hybrids. Studies of temperature, solvent, and cata-
lyst loading effects on the outcome of the reaction indicate
that the ee is sensitive to the nature of the solvent. Moreover,
the catalyst was recovered at low temperature and reused
while retaining its activity and enantioselectivity. These re-
sults contrast and complement our reported results in which
DENDRI-POMs based on phenylethylamine-derived ligands
efficiently oxidize sulfides with up to 14% ee, whereas al-
kenes were not oxidized with this family of chiral POM com-
pounds. Thus, the influence of the nature of the chiral
countercation on the POM properties is again observed.
L-proline-derived tetrapropylammonium dendron with the
tetrakis(diperoxotungsto)phosphate [PO4{WO(O2)2}4]3–. This
DENDRI-POM hybrid was characterized by infrared, NMR
spectroscopy, polarimetry, and elemental analysis. The data
obtained are consistent with the structure in which the tri-
anionic POM is surrounded by three L-proline-based den-
drons. This DENDRI-POM oxidized sulfides with moderate
activity and very low enantioselectivities (up to 4%),
whereas alkenes were efficiently oxidized with up to 37%
enantiomeric excess (ee), and good yields. Although these
ing potential, and overall their catalytic efficiency are
closely related to the structure of the dendritic wedge. Re-
cently, we have reported a series of chiral dendritic polyoxo-
metalates (DENDRI-POMs) constructed by electrostatic
interactions between enantiopure dendritic ammonium ions
and an achiral polyanion that efficiently catalyze the asym-
metric oxidation of sulfides with up to 14% enantiomeric
excess (ee), thereby highlighting the transfer of chirality
from the dendritic wedges to the inorganic cluster.[10]
Shortly after this report, the group of Bonchio published a
report on nondendritic, optically active polyoxotungsto-
phosphonates prepared by covalent functionalization of a
Keggin-type POM with a chiral organophosphonate, which
oxidized methyl p-tolyl sulfide to the corresponding chiral
sulfoxide with up to 75% conversion and a maximum of
8% ee.[11] With these promising results, our attention was
turned to improve the catalytic efficiency of these hybrids,
in terms of reactivity and stereoselectivity, by tuning the
structure of the chiral dendritic countercation. Thus, we
have studied the catalytic properties of enantiopure DEN-
DRI-POM hybrids constructed by electrostatic coupling of
an achiral trianionic POM with enantiopure dendritic cat-
ions that bear one and three ammonium groups, respec-
tively.[10] We discovered a substantial improvement in the
stability, activity, and selectivity of the oxidation of sulfides
with catalysts built with the monocationic ligand versus the
triammonium ligand, but the asymmetric induction was not
Introduction
Polyoxometalates (POMs)[1] are an important class of in-
organic compounds with highly interesting properties that
render them attractive candidates for potential applications
in a variety of fields such as catalysis, biology, magnetism,
optics, and medicine.[2–6] In this context, the construction
of enantiomerically pure inorganic–organic materials have
received much more attention due to their potential use in
the area of medicine and asymmetric catalysis.[7] Among
numerous application of POMs, catalysis is by far the most
studied on account of the enormous versatility that POMs
offer in the clean synthesis of fine chemicals, including their
ability to catalyze environmentally friendly reactions. Al-
though a wide variety of chiral catalysts (including transi-
tion-metal-based complexes and organocatalysts) have been
developed for the asymmetric oxidation of organic sub-
strates with high activity and enantioselectivity,[8] few of
them are based on POM clusters.[7,9] Among those reported
so far, dendritic structures are rare,[10] although the proper-
ties of POMs such as their stability, solubility, their recycl-
[a] IECB-CBMN, UMR CNRS No. 5248,
Université Bordeaux 1,
2 Rue Robert Escarpit, 33607 Pessac Cedex, France
Fax: +33-5-4000-2215
E-mail: s.nlate@iecb.u-bordeaux.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101145.
Eur. J. Inorg. Chem. 2012, 833–840
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
833

C. Jahier, S. Nlate
FULL PAPER
significantly affected by this modification of the chiral ing tetraallyl–pyrrolidine compound 5. Attempts to prepare
ethylamine-based dendritic structure. Being interested in the the triaallyl derivative of 2 or the hexaallyl derivative of 5
development of highly enantioselective DENDRI-POM by allylation of the corresponding carbinol with allyltri-
catalysts, we herein report the synthesis and characteriza- methylsilane in the presence of BF3 failed. The hydrogena-
tion of an enantiopure POM salt by coupling the pyrrolidi- tion of 5 in the presence of Pd/C as catalyst gave the corre-
nylmethanol-based dendritic cation to the Venturello poly- sponding tetrapropyl compound 6 in 93% yield.
anion [PO4{WO(O2)2}4]3–. We focused on this trianionic
All these compounds were characterized by NMR spec-
POM to compare the work described in this manuscript troscopy, mass spectrometry, and elemental analysis. The
with that previously reported in our group.[10] This POM data obtained are consistent with the proposed pyrrolidine-
hybrid was used as catalyst in the asymmetric oxidation of based structure. The enantiopure DENDRI-POM salt 7
aromatic alkenes with a better ee.
(Scheme 2) was prepared from the commercially available
Keggin heteropolyacid H3PW12O40 and the tetrapropyl–
pyrrolidine 6, in the presence of an excess amount of H2O2,
following the procedure that involves peroxide-mediated de-
composition of H3PW12O40. According to reported studies,
H3PW12O40 is decomposed in the presence of H2O2 to
form the dinuclear peroxotungstate [{WO(O2)2(H2O)}2O]2–
Results and Discussion
Preparation and Characterization of
DENDRI-POM 7
L-Proline-Derived
The l-proline-derived DENDRI-POM 7 was prepared and the trianionic peroxophosphotungstate [PO4{WO-
by electrostatic assembly of the l-proline-based dendron 6 (O2)2}4]3–.[10,13] The latter reacts selectively with 6 in a bi-
with the Venturello polyanion [PO4{WO(O2)2}4]3– by treat- phasic mixture of water and dichloromethane to give the
ment of 6 with the commercially available heteropolyacid DENDRI-POM 7, whereas the dinuclear peroxotungstate
H3PW12O40 and an excess amount of hydrogen peroxide in [{WO(O2)2(H2O)}2O]2– remained in the aqueous phase of
a biphasic medium of water and dichloromethane. To com- the reaction mixture and was isolated as a potassium salt
pare the work described in this manuscript with that re- in the presence of potassium chloride. DENDRI-POM 7
ported in the literature,[10] we focused on the Venturello po- was obtained as a light yellow solid in 90% yield. Spectro-
lyanion. l-2-(Hydroxydipropylmethyl)-1-{[4-(hydroxydipro- scopic studies, and infrared and elemental analysis data re-
pylmethyl)phenyl}pyrrolidine (6) was obtained from the l- ported for 7 are consistent with the proposed structure in
proline methyl ester 1, as summarized in Scheme 1.
which the polyanion [PO4{WO(O2)2}4]3– is surrounded
The l-proline methyl ester 1 was first synthesized accord- by three tetrapropyl–pyrrolidine–ammonium dendrons
ing to the published method.[12] The allylation of 1 with (Scheme 2). Only one signal was observed in the 31P NMR
allylmagnesium bromide gives the enantiopure l-2-(diall- spectrum for 7 (δ = 2.85 ppm, CDCl3).
ylhydroxymethyl)pyrrolidine 2 in 97% yield. The coupling
of 2 with methyl 4-(bromomethyl)benzoate (3) affords the and demonstrate the chiroptical properties of the POM unit
corresponding (diallylhydroxymethyl)pyrrolidinyl-substi- in an asymmetric reaction, we have performed the oxidation
To evaluate the catalytic efficiency of DENDRI-POM 7
tuted methyl benzoate 4 in 94% yield. The latter was all- of methyl phenyl sulfide and selected alkenes in the presence
ylated with allylmagnesium bromide to give the correspond- of H2O2.
Scheme 1. Synthesis of l-proline-derived dendron 6.
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 833–840

l-Proline-Derived Dendritic Tetrakis(diperoxotungsto)phosphate
building blocks for the synthesis of fine chemicals and phar-
maceuticals.[15] Thus, asymmetric epoxidation of alkenes is
an interesting alternative to the kinetic resolution of such
compounds.[16] To compare the catalytic efficiency of hybrid
7 to those of DENDRI-POMs prepared from ethylamine-
based dendritic ligands described in the literature,[10] we
have evaluated the performance of POM hybrid 7 in the
asymmetric epoxidation of trans-stilbene (11), α-trans-
methylstilbene (12), styrene (13), α-methylstyrene (14), and
methyl cinnamate (15) with aqueous hydrogen peroxide
(35%) under similar conditions. As shown in Table 1, the
oxidation of aromatic alkenes 11, 12, 13, and 14 with hybrid
7 gave the corresponding trans-stilbene oxide (16), α-trans-
methylstilbene oxide (17), phenyloxirane (18), and 2-
methyl-2-phenyloxirane (19) if the reaction was performed
at 50 °C.
Low substrate conversion was observed at room temp. in
the case of 11, whereas 12 led quantitatively to the corre-
sponding oxide 17 even at room temperature. An enantio-
meric excess (ee) of 37 and 11% was respectively obtained
in the oxidation of 11 and 12 (Table 1, entries 3–6). Interest-
ingly, the enantioselectivity was not dependent upon the re-
action temperature, contrary to the oxidation of sulfides.
Compounds 13 and 14 were inert at room temp. However,
significant substrate conversion was obtained by heating the
reaction mixture to 50 °C, unfortunately with almost no
enantioselectivity (ee values up to 2% were obtained; en-
tries 8 and 10). Compound 15 was totally inert under the
experimental conditions used. Thus, epoxide 20 was not ob-
tained, even by increasing respectively the reaction tempera-
ture above 50 °C, the reaction time and the catalyst concen-
tration (entries 11 and 12). It is clearly shown in Table 1
that the enantioselectivity depends on the nature of the sub-
Scheme 2. Synthesis of DENDRI-POM salt 7.
Catalytic Oxidation of Methyl Phenyl Sulfide and Selected
Alkenes with
H2O2
L-Proline-Derived DENDRI-POM 7 and
To evaluate the catalytic performance of enantiopure strate. Whereas the oxidation of 11 gave the corresponding
DENDRI-POM 7, we performed the asymmetric oxidation epoxide 16 with 37% ee and 44% yield at room temp. in
of methyl phenyl sulfide 8 and selected aromatic alkenes by 48 h, 12 was quantitatively oxidized to epoxide 17 in 24 h,
using aqueous hydrogen peroxide (35%) in a biphasic mix- but with low ee (11%). The same trend was observed in the
ture of water and organic solvents. The results summarized oxidation of 13 and 14, as the latter was slightly more reac-
in Table 1 show that hybrid 7 catalyzed the oxidation of tive than 13, but with very low ee (entries 7–10). In ad-
methyl phenyl sulfide 8 to the corresponding methyl phenyl dition, 15, which bears a withdrawing substituent on the
sulfoxide 9 and a small amount of sulfone 10, with 100% alkene group, is not oxidized under these conditions (entries
conversion and only 2% enantioselectivity (ee), after 2 h at 11 and 12). These results indicate that the reaction kinetics
room temperature (Table 1, entry 1). Whereas the reactivity and enantioselectivity in the oxidation of alkenes with hy-
of 7 toward methyl phenyl sulfide 8 decreased with a de- brid 7 and H2O2 are controlled by the electronic structure
crease in the reaction temperature to –50 °C, no significant and the bulkiness of the alkene. These results are in agree-
change in the ee value was observed (2 versus 4%, entries ment with those reported by Beller’s group in the asymmet-
1 and 2). These results contrast with our recently reported ric epoxidation of aromatic alkenes with iron-based cata-
work with a series of DENDRI-POM salts prepared from lysts and H2O2.[18] Beller has demonstrated that the
ethylamine-based dendrons.[10] These DENDRI-POM cata- enantioselectivity in alkene epoxidation is controlled by ste-
lysts efficiently oxidized sulfides to the corresponding sulf- ric and electronic factors. The mechanism that operates in
oxides with generally 100% conversion and up to 14% ee our system is in agreement with an electrophilic oxygen
after times that range from 5 to 360 min, even at low tem- transfer, as previously observed for oxidation reactions with
perature (–50 °C). In the work described in this manuscript, DENDRI-POM salts of the Venturello ion,[10] as 12 is a
the proline-based DENDRI-POM 7 oxidized methyl phenyl more nucleophilic and more reactive substrate than 11, 13,
sulfide 8 with only 73% conversion and 4% ee after 10 h at 14, and 15.
–50 °C. These results showed once again the influence of
To evaluate the influence of the polarity of solvents on
the structure of the cation on the properties of the POM the reaction kinetics and the enantioselectivity, we exam-
anion. Furthermore, optically active oxiranes represent key ined the oxidation of 11 by POM hybrid 7 in chloroform,
Eur. J. Inorg. Chem. 2012, 833–840
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 1. Catalytic oxidation of methyl phenyl sulfide 8[a] and representative aromatic alkenes[b] by proline-derived DENDRI-POM 7,
using H2O2.
Entry
Substrate
T [°C]
t [h][c]
Conv. [%][d]
Products
ee [%][e]
1
2
3
4
5
6
7
8
8
r.t.
–50
r.t.
50
r.t.
50
r.t.
50
r.t.
50
2
100
73
44
100
97
100
0
76
0
89
0
9 (83); 10 (17)
9 (70); 10 (3)
16
2[f]
10
48
48
24
24
72
72
72
48
72
72
4[f]
11
12
13
14
15
37 (S,S)
37 (S,S)
11 (S,S)
17
18
19
20
11
–
1 (R)
–
2 (R)
–
9
10
11
12
r.t.
50
0
–
[a] Reaction conditions: catalyst (0.4 mol-%), substrate (250 equiv.), 35% H2O2 (800 equiv.), CDCl3 (1 mL). [b] Reaction conditions: cata-
lyst (1 mol-%), substrate (250 equiv.), 35% H2O2 (800 equiv.), solvent CDCl3 (1 mL). [c] Reactions were monitored by 1H NMR spec-
1
troscopy. [d] Conversion determined from the relative intensities of the H NMR spectroscopic signals of the substrate and the product.
[e] Enantiomeric excess determined by chiral HPLC using a Chiralcel ASH column, UV detection (254 nm), eluting with hexane/2-
propanol (9.5:0.5) at a flow rate of 0.5 mLmin–1.[14] [f] Enantiomeric excess 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.[17]
dichloromethane, toluene, acetonitrile, acetone, and tetra- to be accelerated to some extend compared with that in
hydrofuran in the presence of H2O2 at 50 °C. For better CH2Cl2 and toluene, and the ee was increased from 19 and
comparison, the reaction time (48 h) and the catalyst con- 28%, respectively, in CH2Cl2 and toluene, to 37% in CDCl3
centration (1 mol-%) were kept constant for all experiments. (Table 2, entries 1–3).
As detailed in Table 2, it is noteworthy that the nature of
We also investigated the influence of the catalyst concen-
the solvent has a dramatic effect on the reaction kinetics tration on the reaction kinetics and ee values. Thus, 11 was
and the ee values. In CH3CN, acetone, and THF, for exam- oxidized with hybrid 7 in chloroform in the presence of
ple, no oxidation of 11 to the corresponding epoxide 16 H2O2 at 50 °C. The results, which are summarized in
occurred (entries 4–6). However, in CDCl3, CH2Cl2, and Table 3, showed that increasing the catalyst concentration
toluene, 11 was oxidized to epoxide 16 with conversions increased the reaction kinetics without affecting the ee. At
that ranged from 43 to 100% and ee values that ranged a low catalyst concentration (0.4 mol-%), no substrate con-
from 19 to 37% (Table 2, entries 1–3).
version was observed after 96 h at 30 °C, whereas only 14%
The reaction seemed to be favorable in less polar and conversion was obtained after 48 h at 50 °C (Table 3, entries
noncoordinating solvents. It becomes apparent that out of 1 and 5). By using 2 mol-% of catalyst, the conversion was
all solvents evaluated in the oxidation of sulfides and al- complete after 48 h at 50 °C (entry 7). In all cases, the ee
kenes with POM hybrid 7, the reaction in CDCl3 was found value of epoxide 16 (37%) was not affected by the catalyst
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l-Proline-Derived Dendritic Tetrakis(diperoxotungsto)phosphate
Table 2. Effects of solvents on yield and ee values of (S,S)-16.[a]
(5·HCl) were found to be inert in oxidation reactions under
similar conditions. Thus the presence of the polyoxomet-
alate unit is essential to the catalytic activity.
Catalyst Recovery and Reutilization
Entry
Solvent
Conv. [%][b]
ee [%][c]
A significant advantage of dendritic catalysts is that they
can be easily separated from substrates and product
through precipitation due to the solubility properties of
these macromolecules. Therefore, four catalytic cycles were
performed at room temperature and at 50 °C to test the
stability of the l-proline-based DENDRI-POM 7 in the
oxidation of 11 with 1 mol-% of catalyst and H2O2. The
catalyst was recovered by precipitation after each catalytic
cycle and checked by 1H and 31P NMR spectroscopy before
a new catalytic experiment was performed. At room tem-
perature, trans-stilbene 11 was oxidized without any dis-
cernible loss in activity and enantioselectivity over at least
four cycles, whereas at 50 °C, the conversion (34 to 15%)
and the isolated yield (62 to 21%) of the recycled catalyst
decreased significantly after the first cycle. The 31P NMR
spectrum of the recovered catalyst showed the appearance
of new peaks at δ = –0.2 and –16.1 ppm, thus indicating a
structural change of the polyanion. This structural change
is probably due to the decomposition of the POM unit at
50 °C. It was noticed, however, that the decomposition of
the catalyst did not affect the ee value of 37%.
1
2
3
4
5
6
CDCl3
CH2Cl2
toluene
CH3CN
acetone
THF
100
71
43
0
0
0
37 (S,S)
19 (S,S)
28 (S,S)
–
–
–
[a] Reaction conditions: catalyst (1 mol-%), substrate (250 equiv.),
35% H2O2 (800 equiv.), solvent (1 mL). [b] Conversion determined
from the relative intensities of the H NMR spectroscopic signals
of the substrate and the product. [c] Enantiomeric excess deter-
mined by chiral HPLC using a Chiralcel ASH column, UV detec-
tion (254 nm), eluting with hexane/2-propanol (9.5:0.5) at a flow
rate of 0.5 mLmin–1.[14]
1
concentration. Increasing the catalyst loading from 2 to
5 mol-% did not increase significantly the reaction kinetics
(entries 3 and 4), which is an interesting feature in terms of
the development of environmentally friendly processes.
Table 3. Effects of catalyst concentration on the reaction efficiency
and ee values of (S,S)-16.[a]
Conclusion
In the quest for efficient enantioselective POM-based cat-
alysts, we have designed and synthesized an l-proline-de-
rived enantiopure ligand and assembled it with the Ven-
turello trianion [PO4{WO(O2)2}4]3– by electrostatic interac-
tion. This DENDRI-POM oxidized methyl phenyl sulfide
with low activity and very low ee (up to 4%), whereas aro-
matic alkenes were efficiently oxidized to the corresponding
oxirane with up to 37% ee. These results are not compar-
able with those previously reported in our group, in which
DENDRI-POM salts based on the Venturello anion and
enantiopure phenylethylamine-derived dendrons oxidized
phenyl methyl sulfide with better activity and ee (up to
14%), whereas alkenes were not oxidized with this family
of POM catalysts.[10] Although the enantioselectivity (37%)
of the reaction is still not satisfactory for practical asym-
Entry Cat. [%] T [°C]
t [h][b]
Conv. [%][c]
ee [%][d]
1
2
3
4
5
6
7
0.4
1
2
5
0.4
1
30
96
48
48
48
48
48
48
0
8
44
53
14
53
100
–
37
37
37
37
37
37
50
2
[a] Reaction conditions: catalyst, substrate (250 equiv.), 35% H2O2
(800 equiv.), solvent (1 mL). [b] Reactions were monitored by 1H
NMR spectroscopy. [c] Conversion determined from the relative in-
tensities of the 1H NMR spectroscopic signals of the substrate and
the product. [d] Enantiomeric excess determined by chiral HPLC
using a Chiralcel ASH column, UV detection (254 nm), eluting
with hexane/2-propanol (9.5:0.5) at a flow rate of 0.5 mLmin–1.[14]
By varying the oxidant/substrate ratio, no significant metric synthesis, to the best of our knowledge, this ee value
change was observed in either the reaction kinetics and the represents the best enantioselectivity obtained up to now
enantioselectivity. It thus appears that the best reaction with POM-based frameworks in enantioselective transfor-
conditions identified up to now to achieve a high yield and mations. To improve this ee value by varying the reaction
a maximum ee in the oxidation reactions with POM-based parameters, we found that, in contrast to the reaction tem-
catalysts are the oxidation of 11 to the corresponding 16, perature, the nature of the solvent could dramatically influ-
with 2 mol-% of l-proline-based dendritic hybrid 7, a ratio ence the ee of the reaction. These new results complement
of 1:3.2 substrate/hydrogen peroxide, and 1 mL of CDCl3 our previous work and clearly confirm that some key pa-
at 50 °C. It is interesting to note that all the reactions per- rameters in catalysis with POMs, such as activity, selectivity,
formed did not proceed in the absence of POM catalyst and enantioselectivity, can be improved by modifying the
when carried out under similar conditions. In addition, the local environment of polyanions. Further studies are being
l-proline-derived ligand 5 and its hydrochloride salt carried out in our laboratory.
Eur. J. Inorg. Chem. 2012, 833–840
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. Jahier, S. Nlate
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H, CH2), 1.72–1.69 (m, 2 H, CH2) ppm. 13C NMR (50.33 MHz,
Experimental Section
1
CDCl3, TMS): δ = 166.9 (C=O), 145.9 (Cq, Ar), 134.1 (d, JC,H
=
General: Reagent-grade THF and diethyl ether were predried with
Na foil and distilled using sodium–benzophenone under argon im-
mediately before use. Acetonitrile (CH3CN) was stirred under ar-
gon overnight over phosphorus pentoxide, distilled from sodium
carbonate, and stored under argon. Methylene chloride (CH2Cl2)
was distilled from calcium hydride just before use. All other chemi-
cals were used as received. The 1H, 13C, 31P NMR spectra were
recorded at 25 °C with a Bruker AC 250 FT spectrometer (1H:
250.13 MHz; 13C: 62.91 MHz) and a Bruker AC 200 FT spectrome-
ter (1H: 200.16 MHz; 13C: 50.33 MHz; 31P: 81.02 MHz) at CES-
AMO (Bordeaux, France). All chemical shifts are referenced to
Me4Si (TMS). Mass spectra were performed by the CESAMO with
a QStar Elite mass spectrometer (Applied Biosystems). The instru-
ment is required to have an ESI source, and spectra were recorded
in the positive mode. The electrospray needle was maintained at
4500 V and operated at room temperature. Samples were intro-
23.06 Hz, CH=CH2), 129.6 (CH, Ar), 128.6 (Cq, Ar), 127.8 (CH,
Ar), 117.9 (CH=CH2), 75.8 (Cq–OH), 70.2 (CH–N), 62.3 (CH2-
N), 54.1 (CH2–N), 51.9 (O–CH3), 41.0 (CH2–CH=CH2), 27.2
(CH2), 24.8 (CH2) ppm. MS (ESI): m/z = 330.21 [M + H]+; found
330.45. C20H27NO3: calcd. C 72.92, H 8.26; found: C 72.47, H 8.02.
[α]2D0 = –136.5 (c = 10–2 gmL–1, CHCl3).
Tetraallyl–Pyrrolidine Compound 5: In a Schlenk tube, allyl brom-
ide (0.40 mL, 0.56 g, 4.64 mmol) was slowly added to a mixture of
magnesium (0.135 g, 5.57 mmol) in anhydrous diethyl ether
(10 mL) cooled to 0 °C. Then a solution of methyl benzoate deriva-
tive 4 (0.61 g, 1.86 mmol) in anhydrous diethyl ether (5 mL) was
added to the reaction mixture at 0 °C. The mixture was stirred for
12 h at 30 °C. A solution of 6 m NH4Cl (15 mL) was then added
to the reaction mixture. The product was extracted with diethyl
ether (3ϫ15 mL), and dried with Na2SO4. The solvent was re-
moved under vacuum, and the product was purified by silica gel
column chromatography and eluted with petroleum ether/diethyl
ether (9:1 v/v). Compound 5 was isolated as colorless oil in 87%
duced by injection through
a 10 μL sample loop into a
200 μLmin–1 flow of methanol from the LC pump. Elemental
analyses were performed at the Vernaison CNRS center at Lyon-
Villeurbanne (France). The infrared spectra were recorded in KBr
pellets with a FTIR Paragon 1000 Perkin–Elmer spectrometer un-
less otherwise indicated. Organic oxidation products were identified
by correlation to authentic samples.
1
(617 mg) yield. H NMR: δ = 7.36–7.29 (m, 4 H, Har), 5.99–5.81
(m, 2 H, CH=CH2), 5.70–5.57 (m, 2 H, CH=CH2), 5.13–5.01 (m,
2
8 H, CH=CH2), 3.82 (dd, JH,H = 11.31 Hz, 2 H, CH2–N), 2.88–
2.69 (m, 2 H, CH and CH2–N), 2.71–2.51 (dm, 4 H, CH2–
CH=CH2), 2.49–2.33 (m, 1 H, CH2–N), 2.34–2.18 (dm, 4 H, CH2–
CH=CH2), 1.89–1.84 (m, 2 H, CH2), 1.71–1.67 (m, 2 H, CH2) ppm.
13C NMR (50.33 MHz, CDCl3, TMS): δ = 144.4 (Cq, Ar), 138.6
(Cq, Ar), 134.2 (CH=CH2), 133.4 (CH=CH2), 127.9 (CH, Ar),
125.2 (CH, Ar), 119.0 (CH=CH2), 117.7 (CH=CH2), 75.6 (Cq–
OH), 74.9 (Cq–OH), 70.1 (CH–N), 62.4 (CH2–N), 55.0 (CH2–N),
46.7 (CH2–CH=CH2), 41.0 (CH2–CH=CH2), 27.2 (CH2), 24.8
(CH2) ppm. MS (ESI): m/z = 382.27 [M + H]+; found 382.57.
L-2-(Diallylhydroxymethyl)pyrrolidine (2): In a Schlenk tube, allyl
bromide (4.2 mL, 5.90 g, 48.61 mmol) was slowly added to a mix-
ture of magnesium (1.40 g, 58.33 mmol) in anhydrous diethyl ether
(25 mL) cooled to 0 °C. Then a solution of l-proline methyl ester[12]
(1; 1.61 g, 9.72 mmol) in anhydrous diethyl ether (10 mL) was
added to the reaction mixture at 0 °C. The mixture was stirred for
1 h at room temperature. Then a solution of 6 m NH4Cl (25 mL)
was added to the reaction mixture. The product was extracted with
diethyl ether (3ϫ30 mL) and dried with sodium sulfate. The sol-
vent was removed under vacuum and the product was purified by
silica gel column chromatography and eluted with petroleum ether/
diethyl ether (9:1 v/v) to give compound 2 as colorless oil in 97%
yield (1.71 g). 1H NMR (200.16 MHz, CDCl3, TMS): δ = 5.90–
5.82 (m, 2 H, CH=CH2), 5.12–5.06 (m, 4 H, CH=CH2), 3.16–3.09
(m, 1 H, CH), 2.99–2.90 (m, 2 H, CH2–N), 2.67 (br., 2 H, OH and
NH), 2.40–2.07 (m, 4 H, CH2–CH=CH2), 1.79–1.69 (m, 4 H, CH2)
ppm. 13C NMR (50.33 MHz, CDCl3, TMS): δ = 134.0 (CH=CH2),
117.6 (CH=CH2), 73.4 (Cq–OH), 63.7 (CH–N), 46.4 (CH2–N),
42.8 (CH2), 40.3 (CH2), 25.6 (CH2–CH=CH2) ppm. MS (ESI): m/z
= 182.10 [M + H]+; found 182.29. C11H19NO: calcd. C 72.88, H
10.56; found: C 72.67, H 10.44. [α]2D0 = –114.0 (c = 10–2 gmL–1,
CHCl3).
C25H35NO2: calcd. C 78.70, H 9.25; found C 78.54, H 8.89. [α]2D0
–140.0 (c = 10–2 gmL–1, CHCl3).
=
Tetrapropyl–Pyrrolidine Compound 6: Pd/C catalyst (10%) was
added to a solution of 5 (0.52 g, 1.36 mmol) in THF (20 mL) in a
thick-walled tube capped with a Young stopcock. 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 filtered through Celite.
The solvent was removed under vacuum and the product was puri-
fied by chromatography on a silica gel column and eluted with
petroleum ether/diethyl ether (4:6 v/v). Compound 6 was obtained
as colorless oil in 93% (493 mg) yield. 1H NMR (200.16 MHz,
3
CDCl3, TMS): δ = 7.33 (s, 4 H, Har), 3.78 (dd, JH,H = 11.31 Hz,
2 H, CH2–N), 2.90–2.79 (m, 3 H, CH and CH2–N), 2.51–2.49 (m,
1 H, CH2–N), 1.86–1.72 (m, 6 H, CH2), 1.70–1.65 (m, 2 H, CH2),
1.55–1.52 (m, 2 H, CH2), 1.37–1.28 (m, 8 H, CH2), 1.12–1.10 (m,
Methyl L-4-[2-(Diallylhydroxymethyl)pyrrolidin-1-yl]benzoate (4): A
mixture of l-proline diallyl carbinol 2 (0.95 g, 5.24 mmol), methyl
4-(bromomethyl)benzoate 3 (1.60 g, 6.81 mmol), and N,N-diisopro-
pylethylamine (0.88 g, 6.81 mmol) in CH3CN was stirred for 24 h
at 30 °C. 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 under vacuum and the
product purified by silica gel column chromatography and eluting
with petroleum ether/diethyl ether (7:3 v/v). Compound 4 was ob-
3
3
2 H, CH2), 0.91 (t, JH,H = 7.90 Hz, 6 H, CH3), 0.85 (t, JH,H
=
5.98 Hz, 6 H, CH3) ppm. 13C NMR (50.33 MHz, CDCl3, TMS): δ
= 145.1 (Cq, Ar), 138.3 (Cq, Ar), 127.8 (CH, Ar), 125.1 (CH, Ar),
76.9 (Cq–OH), 75.6 (Cq–OH), 70.2 (CH–N), 62.8 (CH2–N), 54.9
(CH2–N), 45.2 (CH2), 38.5 (CH2–CH–CH2), 27.2 (CH2), 24.9
(CH2), 17.0 (d, CH2), 16.7 (CH2), 14.9 (d, CH3), 14.4 (CH3) ppm.
MS (ESI): m/z = 390.34 [M + H]+; found 390.63. C25H35NO2:
calcd. C 77.07, H 11.12; found C 76.89, H 11.19. [α]2D0 = –135.5 (c
= 10–2 gmL–1, CHCl3).
tained as
a
colorless oil in 94% (1.62 g) yield. 1H NMR
3
(200.16 MHz, CDCl3, TMS): δ = 7.98 (d, JH,H = 6.88 Hz, 2 H,
Har), 7.41 (d, JH,H = 6.75 Hz, 2 H, Har), 5.91–5.84 (m, 2 H,
CH=CH2), 5.13–5.03 (m, 4 H, CH=CH2), 3.88 (dd, JH,H
3
Enantiopure Dendritic POM Hybrid 7: H2O2 (4.6 mL, 35% in
water) was added to a solution of commercial heteropolyacid
H3PW12O40 (0.098 mmol) in water. The mixture was stirred at
2
=
12.08 Hz, 2 H, CH2–N), 3.89 (s, 3 H, O–CH3), 2.91–2.82 (m, 2 H,
CH and CH2–N), 2.67 (br., 2 H, OH and NH), 2.36–2.32 (dm, 4 room temperature for 30 min.
A solution of 6 (100 mg,
H, CH2–CH=CH2), 2.19–2.17 (m, 1 H, CH2–N), 1.88–1.81 (m, 2 0.256 mmol) in CH2Cl2 (5 mL) was added, and the mixture was
838
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 833–840

l-Proline-Derived Dendritic Tetrakis(diperoxotungsto)phosphate
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M. Carraro, G. Scorrano, U. Kortz, Adv. Synth. Catal. 2005,
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stirred for 30 min. The CH2Cl2 layer was washed with water and
dried with sodium sulfate. The product was obtained by removing
the solvent under vacuum to give a yellow solid in 90% (204 mg)
[3]
1
yield. H NMR (200.16 MHz, CDCl3, TMS): δ = 7.32 (br., 12 H,
Har), 4.32 (br., 6 H, CH2–N), 3.64 (br., 6 H, CH2–N), 3.28 (br., 3
H, CH–N), 2.32 (br., 6 H, CH2), 2.11 (br., 12 H, CH2), 1.82 (br.,
18 H, CH2), 1.31 (br., 24 H, CH2), 0.91 (br., 6 H, CH2), 0.88 (br.,
36 H, CH3) ppm. 13C NMR (50.33 MHz, CDCl3,TMS): δ = 148.9
(Cq, Ar), 131.3 (CH, Ar), 126.7 (Cq, Ar), 126.2 (CH, Ar), 76.9
(Cq–OH), 74.2 (Cq–OH), 74.0 (CH–N), 60.3 (CH2–N), 54.8 (CH2–
N), 45.0 (CH2), 37.7 (d, CH2–CH=CH2), 26.3 (CH2), 23.5 (CH2),
16.6 (CH2), 16.4 (d, CH2), 14.4 (d, CH3), 14.2 (CH3) ppm. 31P
NMR (121.49 MHz, CDCl3): δ = 2.85 (PO4) ppm. FTIR (KBr
[4]
[5]
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plates): ν = 2965.7 (s), 1461.8 (m), 1114.7 (m, P–O), 1083.6 (m, P–
˜
O), 954.9 (s, W=O), 839.5 (m, O–O). C75H132N3O30PW4: calcd. C
38.79, H 5.73, P 1.33, W 31.67; found C 39.43, H 6.03, P 1.29, W
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[6]
[7]
General Procedure for the Catalytic Oxidation Reactions with Den-
dritic POM 7 and for the Catalyst Recovery Experiment: 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 temperature. The latter
1
was stirred and monitored by H NMR spectroscopy. Upon com-
pletion of the reaction, the organic layer was separated and concen-
trated under vacuum to about 0.2 mL. The catalyst was precipi-
tated by addition of Et2O (5 mL). The solid was filtered, washed
with diethyl ether (3ϫ1 mL), and dried under vacuum to yield the
1
POM catalyst as a white solid, which was analyzed 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 chromatography on a silica gel
column [petroleum ether/diethyl ether (1:9 v/v)]. The enantiomeric
excesses were determined by chiral HPLC using a Chiralcel ASH
column, UV detector (254 nm), and eluting with hexane-2-propa-
nol (1:1) in the case of methyl phenyl sulfide (8) and (9.5:0.5) in
the case of alkenes, at a flow rate of 0.5 mLmin–1. The catalyst was
recovered following the typical procedure and conditions described
above for the first cycle. The organic solvent and the reactants were
adjusted to the amount of the catalyst used.
[8]
[9]
Supporting Information (see footnote on the first page of this arti-
cle): NMR and IR spectra of compounds 4, 5, 6, and 7.
Acknowledgments
S. N. and C. J. thank the Agence National de la Recherche (ANR)
(grant number ANR-06-BLAN-0215), the University of Bordeaux,
the Centre National de la Recherche Scientifique (CNRS), and the
European Cooperation in Science and Technology (COST) (D40
action). We thank Ms Murielle Berlande (ISM) for HPLC experi-
ments.
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Eur. J. Inorg. Chem. 2012, 833–840
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Received: October 18, 2011
Published Online: January 9, 2012
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Eur. J. Inorg. Chem. 2012, 833–840