Enantiopure 24-armed dendritic polyoxometalates: Synthesis and evaluation as recoverable catalysts for asymmetric sulfide oxidation

Article abstract of DOI:10.1016/j.ica.2016.05.019

Enantiopure 24-armed dendritic polyoxometalate (DENDRI-POM) hybrids were prepared by ionic coupling of enantiopure 8-armed n-propyl dendritic ammoniums with a catalytically active peroxophosphotungstate trianion {PO₄[WO(O₂)₂]₄}^3-. The catalytic properties of these DENDRI-POM hybrids were evaluated in the oxidation of thioanisole as a model reaction and compared to those of the 12-armed n-propyl analogous previously reported in our group. Up to 10% enantiomeric excess (ee) was obtained, indicating a negative effect on the reaction rate and the enantioselectivity, whereas the selectivity to the chiral sulfoxide versus sulfone was improved. This study AIDS understanding of how the structure and size of the dendritic wedge around the POM can influence its catalytic properties, especially regarding enantioselectivity. Four catalytic cycles were performed without any obvious change in the activity, selectivity and enantioselectivity.

Full text of DOI:10.1016/j.ica.2016.05.019

Inorganica Chimica Acta 450 (2016) 81–86
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Enantiopure 24-armed dendritic polyoxometalates: Synthesis and
evaluation as recoverable catalysts for asymmetric sulfide oxidation
a
b
b
a,
⇑
Claire Jahier , Rachid Touzani , Sghir El Kadiri , Sylvain Nlate
a
UMR 5248 CBMN, CNRS-Université de Bordeaux-ENITAB, Institut Européen de Chimie et Biologie, 2 rue Robert Escarpit, F-33607 Pessac, France
Université Mohammed 1er, Faculté des Sciences, Laboratoire de Chimie Appliquée et Environnement, Oujda, Morocco
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Enantiopure 24-armed dendritic polyoxometalate (DENDRI-POM) hybrids were prepared by ionic cou-
Received 30 March 2016
Accepted 9 May 2016
Available online 17 May 2016
pling of enantiopure 8-armed n-propyl dendritic ammoniums with a catalytically active peroxophospho-
3À
4
tungstate trianion {PO [WO(O
2
2
) ]
4
}
. The catalytic properties of these DENDRI-POM hybrids were
evaluated in the oxidation of thioanisole as a model reaction and compared to those of the 12-armed
n-propyl analogous previously reported in our group. Up to 10% enantiomeric excess (ee) was obtained,
indicating a negative effect on the reaction rate and the enantioselectivity, whereas the selectivity to the
chiral sulfoxide versus sulfone was improved. This study aids understanding of how the structure and size
of the dendritic wedge around the POM can influence its catalytic properties, especially regarding enan-
tioselectivity. Four catalytic cycles were performed without any obvious change in the activity, selectivity
and enantioselectivity.
Keywords:
Dendrimer
Chirality
Polyoxometalate
Catalysis
Asymmetric oxidation
Ó 2016 Elsevier B.V. All rights reserved.
1
. Introduction
The design of active, selective and recoverable chiral
electrostatic interactions, that results in hybrids in which POM
units are encapsulated within the chiral organic structure [15].
Among POM-based hybrids reported so far, those built from den-
dritic structures are rare, although it has been shown that the
properties of DENDRI-POM hybrids can be tuned by varying the
dendritic structure around the POM species. For several years we
have been working on the design, synthesis and characterization
of chiral DENDRI-POM hybrids, and the investigation of their chiral
properties, especially the effects of chiral dendritic structures on
POM properties. These chiral DENDRI-POMs are prepared by elec-
trostatic coupling of enantiopure dendritic cations with achiral
POM anions. They were shown to catalyze oxidation reactions with
enantioselectivity, demonstrating chirality transfer from the den-
dritic cations to the active POM unit [16–18]. At the same time,
the group of Bonchio published non-dendritic optically active poly-
oxotungstophosphonates based on the covalent coupling of the
polyoxometalate (POM)-based catalysts has become a topic of
recent interest due to their promising applications in stereoselec-
tive catalysis [1,2]. Organic transformations catalyzed with POMs
have been largely investigated [3], owing to the enormous versatil-
ity that POMs offer in the clean synthesis of fine chemicals, includ-
ing their ability to catalyze environmentally-friendly reactions.
Although a wide variety of chiral catalysts have been developed
for the asymmetric oxidation of organic substrates with high activ-
ity and enantioselectivity [4,5], only a few of them are based on
POM clusters [6–10]. Due to their promising applications in enan-
tioselective transformations, the search for efficient strategies to
prepare chiral POMs is one of the most pressing aspects in POM
chemistry research, although the preparation of chiral POMs in
solution is generally unsuccessful because of rapid racemisation
8
À
lacunary [
c
-SiW10
O
36
]
with a chiral organophosphonate species
[
11]. Thus, a useful and easy approach is the grafting of a chiral
that catalyzes the oxidation of sulfides to the corresponding chiral
sulfoxide with up to 75% conversion and a maximum of 8% ee [19].
Previously, we showed that enantiopure 12-armed n-propyl DEN-
DRI-POM hybrids oxidized sulfides with up to 14% ee if the
dendritic cations around the POM catalyst are built from
1-phenylethylamine, as well as aromatic alkenes with up to 37%
ee when the dendritic cations are built from L-proline [18]. These
results highlight the influence of the dendritic cation on the prop-
erties of the POM anion. Thus, in order to prepare more efficient
organic structure to the POM unit through covalent [6,11–14] or
ionic [15] bonding. Whereas the covalent bonding approach can
be restricted due to the lacking of grafting function on POM
structures [11], the most general alternative method is the cou-
pling of achiral anionic POMs with chiral organic cations through
⇑
Corresponding author. Fax: +33 5 4000 2215.
E-mail address: s.nlate@iecb.u-bordeaux.fr (S. Nlate).
http://dx.doi.org/10.1016/j.ica.2016.05.019
020-1693/Ó 2016 Elsevier B.V. All rights reserved.
0

8
2
C. Jahier et al. / Inorganica Chimica Acta 450 (2016) 81–86
and selective chiral DENDRI-POM catalysts, investigation of the
structure/properties relationship between the anionic POM and
the dendritic cation are required. Our first approach was to tune
the size of the dendritic structure around the POM and to investi-
gate the effect of the increased bulkiness on the POM properties.
The organization of dendritic structures around the POM is
assumed to confer new specific properties to the whole DENDRI-
POM hybrid. For example, the encapsulation of POMs within large
chiral dendritic structures is expected to create an efficient chiral
environment in the vicinity of the POM, and thus improve the
transfer of chiroptical properties from dendritic structures to the
catalytically active POM. Herein, to investigate the effect of the
increased size of dendritic cations on the properties of the POM
anion, we report the synthesis and characterization of enantiopure
13C NMR (62.91 MHz, CDCl
, Ar), 133.2 (CH@CH ), 119.3 (CH@CH
(CH, Ar), 75.6 (C -OH), 46.6 (CH ). Anal. Calc. for C20
(314.42): C, 76.40; H, 8.33. Found: C, 76.31; H, 8.31%.
3
, TMS): dppm = 156.1 (C
q
, Ar), 147.2
(C
q
2
2
), 114.4 (CH, Ar), 111.1
q
2
26 3
H O
2
.2.1.2. 1-Chloropropyltetraallyldicarbinol compound 3. A mixture of
tetraallyldicarbinolphenol (3.31 g, 10.52 mmol), 1.25 mL of
-bromo-3-chloropropane (1.99 g, 12.62 mmol) and CO
2.18 g, 15.78 mmol) in 10 mL of DMF, was stirred for 24 h at
5 °C. The reaction mixture was then extracted with diethyl ether
2
1
K
2
3
(
3
(
3 Â 30 mL) and the resulting solution was washed with water and
dried over sodium sulfate. The solvent was removed under vacuum
and the product purified by chromatography on a silica gel column
eluting with a petroleum ether/diethyl ether mixture (5:5; v/v) to
2
4-armed n-propyl DENDRI-POMs and compare their chiroptical
provide compound 3 as colorless oil. Yield: 3.70 g, 90%.
and catalytic properties with those of the 12-armed n-propyl DEN-
DRI-POM analogous [16], in the oxidation of thioanisole.
1
H NMR (200.16 MHz, CDCl
3
, TMS): dppm = 6.93 (s, 1H, Ar), 6.89
), 5.10 (m, 8H, CH@CH ), 4.12 (t,
ACl), 2.58 (dd, 8H, CH ), 2.24 (m,
), 2.22 (s, 2H, OH). C NMR (50.33 MHz, CDCl , TMS):
, Ar), 147.2 (C , Ar), 133.3 (CH@CH ), 119.2
), 115.0 (CH, Ar), 109.9 (CH, Ar), 75.2 (C , CAOH), 64.1
AO), 46.8 (CH ), 41.5 (CH ACl), 32.3 (CH ). Anal. Calc. for C23
31ClO (390.94): C, 70.66; H, 7.99. Found: C, 70.12; H, 7.43%.
(
2
2
d
(
(
H
s, 2H, Ar), 5.57 (m, 4H, CH@CH
2
2
H, CH
H, CH
2
AO), 3.75 (t, 2H, CH
2
2
1
3
2
3
2
. Experimental
ppm = 158.7 (C
CH@CH
CH
q
q
2
2
q
2.1. General remarks
2
2
2
2
-
3
Reagent-grade tetrahydrofuran (THF) and diethyl ether were
predried over Na foil and distilled from sodium-benzophenone
under argon immediately prior to use. Acetonitrile (CH CN) was
stirred under argon overnight over phosphorus pentoxide, distilled
from sodium carbonate, and stored under argon. Methylene chlo-
2
1
5
.2.1.3. 1-Iodopropyl tetraallyl dicarbinol compound 4. A mixture of
-chloropropyltetraallyldicarbinol compound (2.18 g,
.58 mmol) and NaI (1.67 g, 11.15 mmol) in 15 mL of 2-butanone
3
3
was stirred for 6 h at 80 °C. After removal of the solvent under vac-
uum, the residue was extracted with CH Cl
(3 Â 20 mL). The
resulting solution was washed with a saturated aqueous solution
of Na , dried over sodium sulfate, and then filtered. The sol-
vent was removed under vacuum to provide the iodo compound
2 2
ride (CH Cl ) was distilled from calcium hydride just before use.
1
13
31
2
2
All other chemicals were used as received. The H, C, P NMR
spectra were recorded at 25 °C with a Brüker AC 250 FT spectrom-
2 2 3
S O
1
13
eter ( H: 250.13, C: 62.91 MHz) and a Brüker AC 200 FT spec-
1
13
31
trometer ( H: 200.16, C: 50.33, P: 81.02 MHz) at the CESAMO
1
4
as yellow oil. Yield: 2.61 g, 97%. H NMR (200.16 MHz, CDCl
3
,
(
Bordeaux, France). All chemical shifts are referenced to Me
4
Si
TMS): dppm = 6.93 (s, 1H, Ar), 6.85 (s, 2H, Ar), 5.57 (m, 4H, CH@CH
2
),
(
TMS). Mass spectra were performed by the CESAMO on a QStar
5
.10 (m, 8H, CH@CH
2
), 4.04 (t, 2H, CH
), 2.21 (m, 2H, CH ), 2.16 (s, 2H, OH).
, TMS): dppm = 158.6 (Cq, Ar), 147.2
), 115.0 (CH, Ar), 109.9
-O), 46.8 (CH ), 33.1 (CH ),
INa: [M+Na] = 505.39.
I (482.39): C, 57.27; H,
2 2
AO), 3.37 (t, 2H, CH
AI),
Elite mass spectrometer (Applied Biosystems). The instrument is
required with an ESI source and spectra were recorded in the pos-
itive mode. The electrospray needle was maintained at 4500 V and
operated at room temperature. Samples were introduced by injec-
tion through a 10 mL sample loop into a 200 mL/min flow of
methanol from the LC pump. Elemental analyses were carried
out at the Vernaison CNRS center.
2
.59 (dd, 8H, CH
2
2
1
3
C NMR (50.33 MHz, CDCl
3
(
Cq, Ar), 133.3 (CH@CH
CH, Ar), 75.2 (C , C-OH), 67.1 (CH
.52 (CH -I). MS (ESI) Calc. for C23
Found: 505.12. Anal. Calc. for C23
.48. Found: C, 57.60; H, 6.42%.
2 2
), 119.2 (CH@CH
(
2
q
2
2
2
+
2
H
H
31
O
3
31
O
3
6
2
.2. Preparation of 24-armed enantiopure DENDRI-POMs (S)-(À)-8a
2.2.1.4. Enantiopure phenylethyloctaallyltetracarbinol amines (S)-(-)-
and (R)-(+)-8b
6a and (R)-(+)-6b. To a mixture of 1-phenylethylamine (S)-(À)-5a
3
or (R)-(+)-5b respectively (0.57 g, 4.70 mmol) in 15 mL of CH CN
2
(
2
.2.1. Synthesis of enantiopure octa-n-propyltetracarbinol amines
S)-(À)-7a and (R)-(+)-7b
.2.1.1. Tetraallyldicarbinolphenol 2. In a Schlenk tube, allyl bro-
were added 1-iodopropyl tetraallyl dicarbinol phenyl dendron 4
(6.81 g, 14.11 mmol) and 2.46 mL of N,N-diisopropylethylamine
(1.82 g, 14.11 mmol). The mixture was stirred at 50 °C for 48 h at
room temperature. After removal of the solvent under vacuum,
the product was extracted with diethyl ether (3 Â 20 mL), washed
with water and dried over sodium sulfate. The solvent was
removed under vacuum and the product was purified by chro-
matography on silica-gel column eluting with petroleum ether/
mide (11.22 g, 92.75 mmol) was slowly added to a mixture of mag-
nesium (2.67 g, 111.30 mmol) in 25 mL of anhydrous diethyl ether,
cooled at 0 °C. Then, a solution of dimethyl-5-hydroxyisophtalate 1
(
3.88 g, 18.55 mmol) in 15 mL of anhydrous diethyl ether was
added by canula into the reaction mixture at 0 °C. The mixture
was stirred for 15 min at room temperature. Then, a solution of
diethyl ether (2:8; v/v) to provide (S)-(À)-6a and (R)-(+)-6b as a
1
6
4
M NH Cl (35 mL) was added to the reaction mixture, and the pro-
yellow oil. Yield: 2.73 g, 70%. H NMR (200.16 MHz, CDCl
3
, TMS):
duct was extracted with diethyl ether (3 Â 30 mL), then dried over
sodium sulfate. The solvent was removed under vacuum and the
product purified by chromatography on a silica gel column, eluting
with a petroleum ether/diethyl ether mixture (6:4; v/v) to give the
corresponding amino phenol tetraallyl dicarbinol dendron 2 as a
d
ppm = 7.26 (m, 5H, Ar), 6.91 (s, 2H, Ar), 6.80 (s, 4H, Ar), 5.58 (m,
8H, CH@CH ), 5.09 (m, 16H, CH@CH ), 3.93 (m, 5H, CH AO and
CH), 2.58 (m, 20H, CH ACH@CH and CH AN), 2.23 (s, 4H, OH),
1.90 (m, 4H, CH ), 1.35 (d, 3H, CH ). C NMR (50.33 MHz, CDCl
TMS): dppm = 159.0 (C , Ar), 147.1 (C , Ar), 144.2 (C , Ar), 133.4
), 127.9 (CH, Ar), 127.7 (CH, Ar), 126.5 (CH, Ar), 119.1
2
2
2
2
2
2
1
3
2
3
3
,
q
q
q
colorless oil. Yield: 5.40 g, 93%. 1H NMR (250.13 MHz, CDCl
(CH@CH
(CH@CH
(CH
(CH
3
,
2
TMS): dppm = 8.53 (d, 2H, Ar), 7.74 (s, 1H, Ar), 5.59 (m, 4H,
2
), 114.7 (CH, Ar), 109.9 (CH, Ar), 75.2 (C
), 46.5 (CH AN), 27.8 (CH
: [M+H] = 830.54. Found:
q
, CAOH), 65.9
CH@CH
2
), 5.08 (m, 8H, CH@CH
2
), 2.58 (dd, 8H, CH
2
), 2.35 (s, 2H,
2
AO), 59.0 (CH), 46.8 (CH
). MS (ESI) Calc. For C54H71NO
6
2
2
2
), 16.0
+
OH).
3

C. Jahier et al. / Inorganica Chimica Acta 450 (2016) 81–86
83
8
30.51. Anal. Calc. for C54
H
71NO
6
(830.14): C, 78.12; H, 8.62. Found:
completion of the reaction, the organic layer was separated and
concentrated under vacuum to about 0.3 mL. The catalyst was pre-
cipitated by addition of Et O (5 mL). The solid was filtered, washed
2
20
À2
À1
C, 77.52; H, 8.36%. S-(À)-6a: [
a
]
D
= -70.1 (c = 10 g mL , CHCl
3
).
2
0
À2
À1
R-(+)-6b: [
a]
D
= +70.2 (c = 10 g mL , CHCl
3
).
with diethyl ether (3 Â 1 mL) and dried under vacuum to yield the
2
.2.1.5. Enantiopure phenylethylocta-n-propyltetracarbinol amines
POM catalyst as a slightly-yellow solid. The recovered catalyst was
1
31
(
S)-(À)-7a and (R)-(+)-7b. Pd/C catalyst (10%) was added to a
analyzed by H and P 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 Chiracel ASH column and UV detector (254 nm), eluting
THF (20 mL) of solution of the corresponding phenylethyloctaallyl-
tetracarbinol amines (S)-(À)-6a and (R)-(+)-6b in a thick-walled
tube capped with a Young’s stopcock. The tube was flushed, pres-
surized with hydrogen, sealed and stirred at room temperature for
4
h. The solvent was removed under vacuum and the residue was
À1
extracted with diethyl ether and filtered through Celite. After evap-
oration of diethyl ether the enantiopure phenylethylocta-n-propy-
ltetracarbinol amines (S)-(À)-7a and (R)-(+)-7b was obtained as a
with hexane/2-propanol (1:1, v/v) at a flow rate of 0.5 mL min
[20]. Retention times [min]: (R)-10: 22.5; (S)-10: 30.9. The catalyst
was recovered following the typical procedure and conditions
described above for the first cycle, the organic solvent and the reac-
tants being adjusted to the amount of catalyst used.
colorless solid. Yield: 1.55 g, 95%. 1H NMR (200.16 MHz, CDCl
,
3
TMS): dppm = 7.20 (m, 5H, Ar), 6.80 (s, 1H, Ar), 6.74 (s, 1H, Ar),
6
4
3
.69 (s, 4H, Ar), 3.98 (m, 1H, CH), 3.86 (m, 4H, CH
H, CH AN), 1.83 (m, 4H, CH ), 1.35 1.67 (m, 16H, CH
H, CH ), 1.20 (m, 8H, CH ), 0.97 (m, 8H, CH ), 0.75 (t, 24H, CH
C NMR (50.33 MHz, CDCl , TMS): dppm = 158.8 (C , Ar), 147.6
, Ar), 144.1 (C , Ar), 127.9 (CH, Ar), 127.7 (CH, Ar), 126.5 (CH,
Ar), 114.5 (CH, Ar), 109.3 (CH, Ar), 98.3 CH, Ar), 77.3 (C , CAOH),
2
AO), 2.75 (m,
), 1.27 (d,
).
2
2
2
3
. Results and discussion
3
2
2
3
1
3
3
q
3.1. Synthesis and characterization of 24-armed DENDRI-POM hybrids
(
C
q
q
(
S)-(À)-8a and (R)-(+)-8b
q
6
2
7.3 (CH
7.8 (CH
2
AO), 59.0 (CH), 46.5 (CH
2
), 45.2 (CH
2
AN), 33.2 (CH
2
),
:
To build the 24-armed DENDRI-POM hybrids (S)-(À)-8a and (R)-
2
), 16.7 (CH ), 14.4 (CH ). MS (ESI) Calc. For C54H87NO
3
3
6
+
+
(+)-8b enantiomers, we designed and prepared octa-n-propylte-
tracarbinol amines (S)-(À)-7a and (R)-(+)-7b and coupled them
[
M+H] = 846.66. Found 846.64 [M] . Anal. Calc. for C54H87NO
6
(
7
846.27): C, 76.64; H, 10.35. Found: C, 76.26; H, 10.39%. S-(À)-
3À
2
0
À2
À1
20
with the catalytically active tri-anion {PO
4
[WO(O
2
)
2
]
4
}
by elec-
a: [
a]
D
= À63.5 (c = 10 g mL , CHCl
3 D
). R-(+)-7b: [a] = +63.5
À2
À1
trostatic interaction, as summarized in Scheme 1.
(
c = 10 g mL , CHCl
3
).
2
.2.2. Preparation of enantiopure 24-armed n-propyl dodecacarbinol
Synthesis of enantiopure 24-armed n-propyl DENDRI-POM hybrids
(S)-(À)-8a and (R)-(+)-8b
dendritic POMs (S)-(À)-8a and (R)-(+)-8b
(23.0 mL, 757.6 mmol, 35% in water) was added to an
aqueous solution of commercial heteropolyacid
1.310 g, 0.454 mmol) in water. The mixture was stirred at room
H O
2 2
The first step in the synthesis of dendritic amines (S)-(À)-7a and
3 40
H PW12O
(R)-(+)-7b involves the allylation of dimethyl-5-hydroxyisophta-
late 1 with allylmagnesium bromide to give the tetraal-
(
temperature for 30 min. A solution of the corresponding pheny-
lyldicarbinolphenol 2. The reaction of 1-bromo-3-chloropropane
with the tetraallylphenol 2 gives the corresponding 1-chloropropy-
ltetraallyldicarbinol compound 3. The latter reacts with NaI in
butanone to give the iodo derivative 4. The treatment of
1-phenylethylamine (S)-(À)-5a and (R)-(+)-5b with 1-iodopropyl-
tetraallyl 4 gave the enantiopure octaallyl amines (S)-(À)-6a and
(R)-(+)-6b in good yields. The hydrogenation of (S)-(À)-6a and
(R)-(+)-6b in the presence of Pd/C catalyst gave the octa-n-propyl-
tetracarbinol counterparts (S)-(À)-7a and (R)-(+)-7b. The reaction
of dendritic amines (S)-(À)-7a and (R)-(+)-7b with an aqueous
lethyl octa-n-propyl tetracarbinol amines (S)-(À)-7a or (R)-(+)-7b
(
2 2
1.0 g, 1.181 mmol) in CH Cl (5 mL) was added and the mixture
was stirred for 30 min. The organic layer was washed with water
and dried over sodium sulfate. The product was obtained by
removing the solvent under vacuum to provide DENDRI-POM (S)-
(
À)-8a or (R)-(+)-8b as a light-yellow solid. Yield: 1.560 g, 93%.
1
H NMR (200.16 MHz, CDCl
3
, TMS): dppm = 7.70–7.32 (br, 15H,
AO and CH), 2.34 (br,
AN, OH and NH), 1.71 (br, 57H, CH and CH ), 1.25 (br,
), 1.02 (m, 24H, CH ), 0.81 (br, 72H, CH ). C NMR
, TMS): dppm = 157.8 (C , Ar), 147.9 (C , Ar),
, Ar), 129.9 (CH, Ar), 129.5 (CH, Ar), 128.9 (CH, Ar),
15.2 (CH, Ar), 109.5 (CH, Ar), 109.3 (CH, Ar), 76.6 (C , C-OH),
-N), 25.8 (CH ),
Ar), 6.81 (br, 18H, Ar), 4.07 (br, 15H, CH
2
3
(
1
1
6
1
d
2
2
7H, CH
6H, CH
2
2
3
1
3
2
2
3
solution of the commercially-available heteropolyacid H
and H in a biphasic medium of water and dichloromethane
yielded enantiopure DENDRI-POM hybrids (S)-(À)-8a and (R)-(+)-
3 40
PW12O
50.33 MHz, CDCl
3
q
q
2 2
O
34.4 (C
q
3À
q
8b, that have a {PO
4
[WO(O
2
2
) ]
4
}
trianion at the core.
4.8 (CH
7.7 (CH
, 85°H
2
-O), 63.9 (CH), 48.5 (CH
2
1
), 45.2 (CH
2
2
The dendritic POM hybrids (S)-(À)-8a and (R)-(+)-8b were
3
2
3
), 16.8 (CH ), 14.3 (CH
3
). P NMR (81.02 MHz, acetone-
easily isolated from the organic phase as a light yellow solids in
À1
6
3
PO ): dppm = 3.01 (PO
4
4
). FT-IR (KBr, cm ): = 3423 (br, s),
excellent yields, whereas
tungstate decomposition product [{WO(O
a
water-soluble dinuclear peroxo-
O]2 was
À
965 (s), 2938 (s), 2876 (s), 1599 (m), 1440 (m), 1167 (m), 1060
2
)
(H
2 2
O)}
2
(
m, PAO), 915 (m, W = O), 879 (m, O-O), 739 (w), 708 (w). Anal.
Calc. for C162 (3692.16): C, 52.70; H, 7.21; W,
9.91. Found: C, 53.44; H, 7.98; W, 19.35%. (S)-(À)-8a:
removed from the aqueous phase due to its solubility in water.
H
264
N
3
O
42PW
4
For comparison with the work reported for 12-armed n-propyl
3
À
1
DENDRI-POMs, we focused on the {PO
4
[WO(O
2
)
2
]
4
}
trianion
2
0
À
À1
20
[
a
]
D
= À93.4 (c = 10 g mL , CHCl
3
). (R)-(+)-8b : [
a]
D
= +93.4
and n-propyl-based dendritic amines for the preparation of the
24-armed DENDRI-POM analogous described in this manuscript.
Moreover, the hydrophobic environment created by n-propyl
groups around the hydrophilic POM can facilitate the capture of
less polar substrates such as sulfides and release the more polar
oxidized product in the reaction medium. This feature minimizes
the over-oxidation reaction of sulfoxide, giving rise to high selec-
tivity with respect to the chiral product. All the experimental data
reported in the Section 2 for dendritic amines and DENDRI-POM
hybrids (S)-(À)-8a and (R)-(+)-8b are in agreement with the struc-
ture proposed for these compounds (Scheme 1). Interestingly, the
À2
À1
(
c = 10 g mL , CHCl
3
).
2
.3. General procedure for the catalytic oxidation reactions with
enantiopure dendritic POMs (S)-(À)-8a and (R)-(+)-8b and for
catalyst recovery experiments
The POM catalyst and the substrate (250 equiv.) were dissolved
3 2 2
in 1 mL of CDCl . An aqueous solution of H O (35% in water) was
added to the reaction mixture at the appropriate temperature. The
latter was stirred and monitored by 1H NMR spectroscopy. Upon

8
4
C. Jahier et al. / Inorganica Chimica Acta 450 (2016) 81–86
MeO
O
O
BrMg
OH
Cl
Br
OH
NaI
2-Butanone
80°C, 6h
OH
HO
HO
Cl
I
O
O
Et O, 0°C,
K CO ,
2
2
3
r.t.,15 min
DMF 35°C,
24h
HO
MeO
HO
HO
2
3
4
1
(iPr)
2
NEt
NH
2
CH CN,
3
(
S)-(-)-5a
50°C, 48h
HO
OH
HO
O
OH
(
R)-(+)-5b
O
O
HN
+
NH
O O
HO
+
O
O
3-
HO
O
O
HO
HO
W
O
P
O
O
O
O
W
OH
O
O
O
O O
O
W O
HO
HO
O
O
O
O
O
2
H , Cat.
HO
HO
O
O
OH
W
3 40
H PW12O
Pd/C
N
O
N
O
O
O
THF, r.t., 4h
H O /H O
2
2
2
+
H
N
O
(S)-(-)
-6a
HO
O
(S)-(-)-7a
R)-(+)-7b
HO
OH
HO
(R)-(+)-6b
(
OH
HO
(
S)-(-)-8a
(
R)-+)-8b
Scheme 1. Synthesis of enantiopure 24-armed n-propyl DENDRI-POM hybrids (S)-(À)-8a and (R)-(+)-8b.
molecular peaks of dendritic amines (S)-(À)-6a and (R)-(+)-6b and
octa-n-propyl dendrons (S)-(À)-7a and (R)-(+)-7b were observed in
yields ranging from 7% to 87% and up to 10% ee after 30 to 480 min
reaction (Table 1, entries 3–19). Increasing the catalyst loading
increased the reaction rate and the enantioselectivity, the best
reaction conditions being when thioanisole 9 is oxidized with
0.8 mol% of 24-n-propyl DENDRI-POM hybrids (S)-(À)-8a and (R)-
+
their mass spectra at m/z = 830.51 (Calc. 830.54 [M+H] ) and
+
1
8
46.66 (Calc. 846.29 [M+H] ) respectively. Compared to the H
NMR spectra of dendritic amines (S)-(À)-7a and (R)-(+)-7b (see
the Supporting information (SI), Fig. S9), the signals are signifi-
cantly broadened and shifted upfield in the case of DENDRI-POM
hybrids (S)-(À)-8a and (R)-(+)-8b (Fig. S11, SI), indicating an elec-
trostatic interaction between dendritic cations and the POM trian-
2 2
(+)-8b in the presence of 1 equiv of H O , at 0 °C (Table 1, entries
18,19 and 21).
Compare to enantiopures 12-armed n-propyl DENDRI-POMs,
the oxidation of thioanisole with 24-armed n-propyl DENDRI-
POMs 8a-b indicated a significant decrease in the reaction kinetics
and the enantioselectivity. Whereas 12-armed POM hybrid oxidize
sulfide 9 to sulfoxide 10 (80%) and sulfone 11 (15%), with 11% ee
after 15 min, at 0 °C, the 24-armed analogous oxidizes thioanisole
to sulfoxide 9 with 49% yield and 5% ee after 30 min, without any
trace of sulfone 11, in similar conditions (Table 1, entry 3). The
reaction kinetics and the enantioselectivity are slowed down in
the oxidation of sulfide 9 with the 24-armed n-propyl DENDRI-
POM hybrids, whereas the selectivity to the chiral sulfoxide is
improved. Thus, the increase of the bulkiness around the anionic
POM catalyst with larger n-propyl-based dendritic ammoniums
decrease the reaction kinetics and the enantioselectivity, whereas
the selectivity to the chiral sulfoxide is improved. This result indi-
cates that the increase of the size of the dendritic structure around
the POM catalyst promotes the selectivity to the chiral sulfoxide,
but diminishes the reaction rate and the chiral environment
around the POM, probably due to the decreased interaction
between dendritic cations and the anionic POM. The negative effect
on the reaction rate may arise from the fact that the increased bulk
around the POM unit caused by the 24-peripheral-n-propyl groups
prevents the easy access of substrate molecules to the catalytic
POM center and thus slows down the reaction rate. It is known that
more favorable interaction between substrate and catalyst gives
higher enantioselectivity [21]. Despite the increased bulk around
the POM catalyst that slows down the reaction kinetics, the
hydrophobic environment around the POM tends to capture sulfide
molecules and compensate the decrease of the reaction rate. Thus,
3
1
ion. The P NMR spectra of DENDRI-POM (S)-(-)-8a and (R)-(+)-8b
showed a single sharp peak at d = 3.01 ppm and the IR spectra
3
À
showed characteristic bands of the POM {PO
1
4
[WO(O
2
)
]
2 4
}
at
À1
060 (PAO), 915 (W = O) and 879 cm (OAO), which suggest that
the Venturello species is retained intact in the dendritic hybrid
Figs. S13 and S14, SI).
(
3.2. Catalytic oxidation of methyl phenyl sulfide with DENDRI-POMs
(
S)-(À)-8a and (R)-(+)-8b using H
2 2
O
To evaluate the catalytic efficiency of enantiopure DENDRI-POM
hybrids (S)-(À)-8a and (R)-(+)-8b in an asymmetric reaction, thioa-
nisole 9 was used as a model substrate and oxidized with H
35%) in a biphasic mixture of water and CDCl . The reaction kinet-
ics were monitored over time by plotting the ratio between the
intensity of the disappearing 1H NMR signal of CH
S group of the
substrate 9 at 2.48 ppm and the growing peaks of CH SO and CH
SO groups of the corresponding sulfoxide 10 and sulfone 11 at
.73 and 3.05 ppm respectively. The enantiomeric excesses (ee)
2 2
O
(
3
3
3
3
-
2
2
were monitored by chiral HPLC. The reaction conditions were opti-
mized with respect to the temperature, the catalyst and the oxi-
dant to achieve a high yield of the chiral sulfoxide product with
a maximum of ee values. As shown in Table 1, DENDRI-POM
hybrids 8a-b selectively oxidize thioanisole 9 to the corresponding
chiral sulfoxide 10, together with a small amount of sulfone 11. In
all cases, the selectivity to sulfoxide product as well as the enan-
tioselectivity increased by lowering the reaction temperature, with

C. Jahier et al. / Inorganica Chimica Acta 450 (2016) 81–86
85
Table 1
Catalytic oxidation of thioanisole 9 by 24-armed DENDRI-POM hybrids (S)-(À)-8a and (R)-(+)-8b using H
2 2
O
.a
Entry
Catalysts (mol-%)
2
H O
2
(equiv.)
Temperature (°C)
Time (min)c
Conversion (%)d
Products
ee
10
11
(%)e
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
(S)-(À)-8a
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
2
2
1
1
1
1
1
3.2
1
35
35
0
0
0
À10
À10
À10
À10
À50
À50
À50
0
0
0
0
0
0
0
35
0
30
60
30
60
120
30
71
90
49
90
90
26
48
80
91
4
64
75
49
87
83
26
48
77
85
4
7
15
0
3
7
0
0
3
6
0
0
0
2
14
0
4
7
0
3(S)
3(S)
5(S)
5(S)
5(S)
7(S)
7(S)
7(S)
7(S)
7(S)
8(S)
8(S)
4(S)
4(S)
2(S)
2(S)
2(S)
10(S)
10(S)
2(S)
9(S)
60
120
180
30
120
480
60
120
60
120
240
30
1
1
1
1
1
1
1
1
1
1
2
2
7
7
43
67
100
56
79
90
59
93
70
60
43
65
86
56
75
83
59
87
64
60
b
(S)-(À)-8a
60
30
30
6
7
0
(R)-(+)-8b
(R)-(+)-8b
b
a
b
c
Reaction conditions: catalyst (0.4 mol%), substrate (250 equiv.), CDCl
Reaction conditions: catalyst (0.8 mol%).
3
(1 mL).
1
Reactions were monitored by H NMR.
d
e
Conversion determined from the relative intensities of the 1H NMR signals of the substrate and the product.
Enantiomeric excess determined by chiral HPLC using a Chiralcel ASH column, UV detection (254 nm), eluting with hexane/isopropanol (9.5:0.5) at a flow rate of
À1
0
.5 mL min [20].
by lengthening the reaction time, higher conversion rates can be
reached, even at lower temperatures. Interestingly, this hydropho-
bic environment created by the n-propyl groups prevents the
over-oxidation by the release of the sulfoxide product in the polar
reaction medium. Compared to 12-armed n-propyl DENDRI-POMs,
the decrease of the enantioselectivity in the oxidation of thioani-
sole 9 with 24-armed n-propyl DENDRI-POMs may be explained
by the fact that the increased hydrophobicity around the POM
probably weakens the interaction between the POM and the chiral
ammoniums, and thus lessens the chirality transfer between the
two components. In the case of 12-armed n-propyl DENDRI-POMs,
an additional interaction between aromatic groups of chiral
organic cations and the aromatic ring of the substrate (thioanisole)
tend to keep the substrate closer to the POM unit and the chiral
environment, which increases the reaction rate and the enantiose-
lectivity. In order to check the efficiency and stability of the
reported with the 12-armed n-propyl DENDRI-POM analogous,
the increased bulkiness around the POM unit with large size den-
dritic cations has a negative effect on the reaction rate and the
enantioselectivity, whereas the selectivity to chiral sulfoxide is
improved. It appears that the catalyst efficiency is strongly influ-
enced by the microenvironment around the POM unit. Increasing
the bulk around the POM and the hydrophobicity between the
POM units weakens the chiral environment around the POM that
gives low ee values. Four catalytic cycles were performed without
any obvious change in activity, selectivity and enantioselectivity.
The construction of DENDRI-POMs based on a covalent coupling
of dendritic structures with greater potential for chiral induction
and POM units should be crucial in promoting the enantioselectiv-
ity. Further work in this area is in progress.
Acknowledgements
2
4-armed DENDRI-POM in catalytic conditions, we performed four
catalytic cycles in the oxidation of thioanisole 9 with 8a, at 0 °C.
The results obtained are comparable to those reported in Table 1
Financial support from the University of Bordeaux (France), the
CNRS (France), the ANR-06-BLAN-0215 (France), the CNRST
(
entry 4) without any obvious change in activity, selectivity and
(
Morocco) is gratefully acknowledged.
enantioselectivity.
Appendix A. Supplementary material
4
. Conclusions
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2016.05.019.
Enantiopure 24-armed n-propyl DENDRI-POM hybrids were
prepared by ionic bonding of enantiopure 8-armed n-propyl
phenylethylamine-derived ammoniums with the Venturello trian-
References
3
À
ion {PO
4 2 2 4
[WO(O ) ] } . These DENDRI-POMs oxidized thioanisole
to the sulfoxide with up to 10% ee. Compared to the results
[1] Dimitris E. Katsoulis, Chem. Rev. 98 (1998) 359.

8
6
C. Jahier et al. / Inorganica Chimica Acta 450 (2016) 81–86
[
2] M.T. Pope, A. Müller (Eds.), Polyoxometalate Chemistry: From Topology via
[11] B. Hasenknopf, K. Micoine, E. Lacôte, S. Thorimbert, M. Malacria, R. Thouvenot,
Eur. J. Inorg. Chem. (2008) 5001.
[12] K. Micoine, B. Hasenknopf, S. Thorimbert, E. Lacôte, M. Malacria, Angew. Chem.
Int. Ed. 48 (2009) 3466.
[13] X. Fang, T.M. Anderson, C.L. Hill, Angew. Chem. Int. Ed. 44 (2005) 3540.
[14] M. Lu, J. Kang, D. Wang, Z. Peng, Inorg. Chem. 44 (2005) 7711.
[15] S. Nlate, C. Jahier, Eur. J. Inorg. Chem. (2013) 1606.
[16] C. Jahier, M. Cantuel, N.D. McClenaghan, T. Buffeteau, D. Cavagnat, F. Agbossou,
M. Carraro, M. Bonchio, S. Nlate, Chem. Eur. J. 15 (2009) 8703.
[17] M.-F. Jahier, M. Coustou, N.D. Cantuel, T. McClenaghan, D. Buffeteau, M.
Cavagnat, S. Carraro, Eur. J. Inorg. Chem. 727–738 (2011).
[18] C. Jahier, S. Nlate, Eur. J. Inorg. Chem. (2012) 833.
Self-Assembly to Applications, Kluwer Academic, Dordrecht, 2001.
3] S.-S. Wang, G.-Y. Yang, Chem. Rev. 115 (2015) 4893.
4] A. Berkessel, H. Gröger, Asymmetric Organocatalysis, Wiley-VCH, Weinheim,
Germany, 2005.
5] T. Katsuki, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, 2nd ed., Wiley-
VCH, New York, 2000.
[
[
[
[
[
[
[
6] R. Ishimoto, K. Kamata, K. Suzuki, K. Yamaguchi, N. Mizuno, Dalton Trans. 44
(
2015) 10947.
7] Q. Han, C. He, M. Zhao, B. Qi, J. Niu, C. Duan, J. Am. Chem. Soc. 135 (2013)
0186.
8] Y. Wang, H. Li, W. Qi, Y. Yang, Y. Yan, B. Li, L. Wu, J. Mater. Chem. 22 (2012)
181.
1
9
[19] M. Carraro, G. Modugno, A. Sartorel, G. Scorrano, M. Bonchio, Eur. J. Inorg.
Chem. (2009) 5164.
[20] A. Ozanne-Beaudenon, S. Quideau, Tetrahedron Lett. 47 (2006) 5869.
[21] F.D. Furia, G. Licini, G. Modena, J. Org. Chem. 61 (1996) 5175.
9] W. Adam, P.L. Alsters, R. Neumann, C.R. Saha-Möller, D. Seebach, R. Zhang, Org.
Lett. 5 (2003) 725.
[
10] W. Adam, P.L. Alsters, R. Neumann, C.R. Saha-Möller, D. Seebach, A.K. Beck, R.
Zhang, J. Org. Chem. 68 (2003) 8222.