Carbonyl-inserted organo-hybrids of a Dawson-type phosphovanadotungstate: Scope and chemoselective oxidation catalysis

Article abstract of DOI:10.1021/ol202430e

The Dawson-type polyoxometalate (POM) [P₂V₃W ₁₅O₆₂]^9- is a prototype for inclusion of carbonyls of amides, ureas, carbamates, and thiocarbamates into polyoxometallic structures. The carbonyl-inserted POMs catalyze the oxidation of sulfides. Chemoselectivity depends primarily on the proton content of the POM, but it is also influenced by the organic substituent.

Full text of DOI:10.1021/ol202430e

ORGANIC
LETTERS
2011
Vol. 13, No. 22
5990–5993
Carbonyl-Inserted Organo-Hybrids of a
Dawson-Type Phosphovanadotungstate:
Scope and Chemoselective Oxidation
Catalysis
†
Julie Oble, Benoıt Riflade, Amandine Noel, Max Malacria, Serge Thorimbert,*
†
†
†
,†
€
ˆ
Bernold Hasenknopf,* and Emmanuel Lacote*
,†
,†,‡
^
ꢀ
UPMC Univ Paris 06, Institut Parisien de Chimie Moleculaire (UMR CNRS 7201),
C. 229, 4 place Jussieu, 75005 Paris, France, and Institut de Chimie des Substances
Naturelles - CNRS, Av. de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
serge.thorimbert@upmc.fr; bernold.hasenknopf@upmc.fr; emmanuel.lacote@icsn.
cnrs-gif.fr
Received September 7, 2011
ABSTRACT
The Dawson-type polyoxometalate (POM) [P2V3W15O62]9ꢀ is a prototype for inclusion of carbonyls of amides, ureas, carbamates, and
thiocarbamates into polyoxometallic structures. The carbonyl-inserted POMs catalyze the oxidation of sulfides. Chemoselectivity depends
primarily on the proton content of the POM, but it is also influenced by the organic substituent.
Because of their diversity, polyoxometalates (POMs)
form a rich family of molecular transition metal clus-
ters, which can be adapted for applications in chemistry
and biology.1 Grafting of organics to POMs to generate
organicꢀinorganic hybrids is a prime tool to modulate
or expand their properties.2
The [P2V3W15O62]9ꢀ phosphovanadotungstate 1 is an
attractive platform. The three capping vanadium atoms
make it an oxidant, which can be used catalytically.3
Hybrids can be generated upon coupling with tris-hydro-
xymethylmethane derivatives RC(CH2OH)3.4
We recently reported that exchanging one sp3 oxygen in
that triol family with a suitably positioned sp2 one from
amide carbonyls led to incorporation of the latter into
†
ꢀ
UPMC Univ Paris 06, Institut Parisien de Chimie Moleculaire (UMR
CNRS 7201).
‡ Institut de Chimie des Substances Naturelles - CNRS.
ꢀ
(3) (a) Keita, B.; Contant, R.; Mialane, P.; Secheresse, F.; de Oliveira,
(1) (a) Long, D.-L.; Tsunashima, R.; Cronin, L. Angew. Chem., Int.
Ed. 2010, 49, 1736–1758. (b) Hill, C. L. J. Mol. Catal. A: Chem. 2007,
262, 2–6. (c) Coronado, E.; Gimenez-Saiz, C.; Gomez-Garcia, C. J.
Coord. Chem. Rev. 2005, 249, 1776–1796. (d) Hasenknopf, B. Front.
P.; Nadjo, L. Electrochem. Commun. 2006, 8, 767–772. (b) Lopez, X.; Bo,
C.; Poblet, J. M. J. Am. Chem. Soc. 2002, 124, 12574–12582. (c) Weiner,
H.; Finke, R. G. J. Am. Chem. Soc. 1999, 121, 9831–9842.
(4) (a) Hou, Y.; Hill, C. L. J. Am. Chem. Soc. 1993, 115, 11823–11830.
For other examples of derivatization of [P2W15V3O62]
9ꢀ, see: (b) Zeng,
^
Biosci. 2005, 10, 275–287. (e) Hasenknopf, B.; Micoine, K.; Lacote, E.;
Thorimbert, S.; Malacria, M.; Thouvenot, R. Eur. J. Inorg. Chem. 2008,
5001–5013.
(2) (a) Proust, A.; Thouvenot, R.; Gouzerh, P. Chem. Commun. 2008,
1837–1852. (b) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P.
Chem. Rev. 2010, 110, 6009–6048.
H.; Newkome, G. R.; Hill, C. L. Angew. Chem., Int. Ed. 2000, 39, 1771–
1774. (c) Pradeep, C. P.; Long, D.-L.; Newton, G. N.; Song, Y. F.;
Cronin, L. Angew. Chem., Int. Ed. 2008, 47, 4388–4391. (d) Han, Y.;
Xiao, Y.; Zhang, Z.; Liu, B.; Zheng, P.; He, S.; Wang, W. Macromole-
cules 2009, 42, 6543–6548.
r
10.1021/ol202430e
Published on Web 10/26/2011
2011 American Chemical Society

[P2V3W15O62]9ꢀ.5 This makes it complementary to the
seminal triol-capping of the same structure4 because it
allows π-conjugation between the organic and inorganic
parts.
Table 1. Insertion of Ureas, Carbamates, and Thiocarbamates
into (TBA)5H4-1
Herein we report that (i) the carbonyl of trivalent
functions (ureas, carbamates, thiocarbamates) can also
be inserted into 1 and (ii) the new organo-hybrids not
only retain the oxidative properties of [P2V3W15O62]9ꢀ
but also can lead to improved chemoselectivity in the
oxidation of sulfides.
The diol-carbonyl ligands 2aꢀq used in this work were
prepared from commercially available amino-diols which
were functionalized at nitrogen with isocyanates (ureas),
chloroformates (carbamates), or isothiocyanates (thiocarb-
amates, see Supporting Information (SI) for details).
All ligands were subjected to the conditions devised
for the coupling of diolamide derivatives to TBA5H4-1,
i.e. 20 mol % of p-toluenesulfonic acid (PTSA) in
dimethylacetamide (DMAc) under microwave heating
at 80 °C. The new POMs 3aꢀq were obtained in high
yields in all but two cases in the carbamate series (Table 1,
entries 12 and 16). Ligand 2l led too rapidly to an
unreactive oxazolidinone under the reaction conditions,
and ligand 2p gave a complex mixture of POMs. Thus,
other sp2 oxygens than amides can be included into poly-
oxometalates.6
To check whether an acidic proton on the nitrogen was
required for the insertion to happen, we carried out the
reaction first onto tertiary amide 4a. The previous condi-
tions (20 mol % PTSA) afforded a mixture of 5a and
another unidentified POM (Table 2, entry 1). Gratifyingly,
5a was obtained in high yield when excess PTSA was used
(1.5 equiv, 82%, entry 2). These modified conditions were
applied to the preparation of the tertiary carbamate (5b,
85%, entry 3) and urea (5c, 87%, entry 4) derivatives. All
reactions worked smoothly.
prod.,
entrya
diol
X
R1
R2
yield (%)
1
2a
2b
2c
2d
2e
2f
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
O
Et
Et
Et
Et
Et
Et
Et
Et
Et
H
3a, 86
3b, 89
3c, 84
3d, 90
3e, 88
3f, 96
3g, 70
3h, 72
3i, 82
3j, 87
3k, 92
ꢀ
2
pNO2-C6H4
pCF3-C6H4
Ph
3
4
5
pTol
6
pMeO-C6H4
pPh-C6H4
2,5-Me2-C6H4
Et
7
2g
2h
2i
8
9
10
11
12
13
14
15
16
17
2j
Ph
H
2k
2l
Bn
Et
Et
Et
Et
Et
H
O
mCF3C6H4
Ph
2m
2n
2o
2p
2q
O
3m, 80
3n, 90
3o, 87
ꢀ
O
pTol
O
pMeO-C6H4
Ph
O
S
Et
Et
3q, 78
a Cations of 1 are TBA5H4 (TBA = tetra-n-butylammonium; PTSA =
p-toluene sulfonic acid; DMAc = dimethylacetamide).
The new products were characterized by NMR, IR, and
ESI-MS, which gave concurring proof of the carbonyl
insertion into the inorganic framework. The POMs were
obtained mostly as TBA salts with varying amounts of
dimethylammonium cations (DMA) coming from the
decomposition of dimethylacetamide. Those amounts
were quantified by NMR and elemental analyses (SI).
Table 2. Insertion of Tertiary Amides and Carbamates
^
(5) (a) Li, J.; Huth, I.; Chamoreau, L.-M.; Hasenknopf, B.; Lacote,
E.; Thorimbert, S.; Malacria, M. Angew. Chem., Int. Ed. 2009, 48, 2035–
2038. This communication is part of a research program devoted to the
finding of new or modified properties of known POMs via the design
of organic hybrids thereof. See: (b) Micoine, K.; Hasenknopf, B.;
^
equiv
PTSA
prod.,
Thorimbert, S.; Lacote, E.; Malacria, M. Angew. Chem., Int. Ed. 2009,
48, 3466–3468. (c) Boglio, C.; Micoine, K.; Derat, E.; Thouvenot, R.;
entrya
diol
X, R
yield (%)
^
Hasenknopf, B.; Thorimbert, S.; Lacote, E.; Malacria, M. J. Am. Chem.
Soc. 2008, 130, 4553–4561. (d) Micoine, K.; Hasenknopf, B.; Thorim-
1
2
3
4
4a
4a
4b
4c
ꢀ, Ph
0.2
1.5
1.5
1.5
ꢀ
^
bert, S.; Lacote, E.; Malacria, M. Org. Lett. 2007, 9, 3981–3984.
ꢀ, Ph
5a, 82
5b, 85
5c, 87
^
(e) Thorimbert, S.; Hasenknopf, B.; Lacote, E. Isr. J. Chem. 2011, 51,
275–280.
O, pMeOC6H4
NH, Ph
(6) For examples of CdO insertion in POMs, see: (a) Mak, T. C. W.;
Li, P.; Zheng, C.; Huang, K. J. Chem. Soc., Chem. Commun. 1986, 1597–
1598. (b) Chen, Q.; Liu, S. C.; Zubieta, J. Angew. Chem., Int. Ed. Engl.
1988, 27, 1724–1725. (c) Lee, C. W.; So, H.; Lee, K. R. Bull. Korean
Chem. Soc. 1988, 9, 362–364. (d) Lemonnier, J. F.; Floquet, S.; Kachmar,
A.; Rohmer, M. M.; Benard, M.; Marrot, J.; Terazzi, E.; Piguet, C.; Cadot,
E. Dalton Trans. 2007, 3043–3054. (e) Kortz, U.; Savelieff, M. G.; Ghali,
F. Y. A.; Khalil, L. M.; Maalouf, S. A.; Sinno, D. I. Angew. Chem., Int. Ed.
2002, 41, 4070–4073. See also refs 2b and 5c.
a Cations of 1 are TBA5H4 (TBA = tetra-n-butylammonium;
PTSA = p-toluene sulfonic acid; DMAc = dimethylacetamide).
Because of the presence of the three vanadiums, vana-
dotungstate 1 is used for oxidations.3 However, there is to
Org. Lett., Vol. 13, No. 22, 2011
5991

date only one example of use of a catalytically active triol-
hybrid reported.4a
retain some activity after the replacement of one sp3 oxo of
the POM framework with an sp2 one.
The redox properties of representative examples of urea
and carbamate hybrids were investigated by cyclovoltam-
metry. Two reductions were observed between 0 and
ꢀ1 V vs Fcþ/Fc (Fc = ferrocene) in acetonitrile at a fast
scan rate. Whereas the first reduction is pseudoreversible,
the second wave is large and likely corresponds to the
superposition of two redox processes. Indeed, at a slow
scan rate and on a freshly polished electrode, the second
process splits into two waves. Detailed analysis of the
second process is hampered by the strong adsorption of
the compounds on the electrode surface.
The electrochemical potentials fall into the same range as
those for triol- and diol-amide substituted [P2V3W15O62]9ꢀ
(Table 3).4a,5a The new hybrid compounds should therefore
be able to promote the same redox reactions as the pre-
viously reported derivatives. One notes variations of the
potential with different substituents in the para position of
the aromatic ring. However, in contrast to what was
observed with diol-amide ligands, there is no direct correla-
tion between the electron-donating or -accepting properties
(Hammett parameters) and the value of the reduction
potential. This indicates that, in ureas and carbamates, there
is no through-bond interaction between the R groups and
the polyoxometallic framework.
Four POMs (one urea (3i), two carbamates (3k, 3o), and
one amide5a (3r, R1 = p-Me-C6H4)) derivatives were
selected across the board for that purpose. In a typical
experiment tetrahydrothiophene was treated with tBuOOH
in the presence of POM 3k (0.017 mol %) in acetonitrile at
rt. This delivered 88% of the corresponding sulfoxide 6
(Table 4, entry 1; the yields were measured by NMR using
trimethoxybenzene as internal standard). No sulfone was
observed in any of those reactions.
Control experiments showed that no reaction took place
without a POM using either TBHP or H2O2. Also, tetra-
butylammonium (as TBABr or TBAOH, 1 equiv) alone
did not catalyze the oxidation. This unambiguously de-
monstrates that the POM is a catalyst which needs to be
reactivated by a stoichiometric reoxidant. The reaction
was not limited to 3k but worked well also with 3o (77%,
entry 3), 3i (entry 4), and 3r (entry 5). Gratifyingly, the
catalyst could be recycled upon precipitation in diethyl
ether and reused (90%, entry 6).
Most interestingly, the reaction also worked in the pre-
sence of H2O2 (30% in water) as an oxidant (entry 2). This is
important from a green chemistry point of view and shows
that the hybrids are not much sensitive to water. For H2O2
also the POM acted as a catalyst. Our hybrids compare in
fact very well to the purely inorganic 1 (entry 7).
Inthe hypothesisthatthe vanadium sites of the POM are
involved in the oxidation, one can anticipate that organic
ligands on the vanadiums might modify the reactivity of
the system and allow fine-tuning of the catalyst. We thus
decided to approach this issue from the chemoselectivity
angle.
Table 3. Redox Potentials for the First Two Reduction Pro-
cesses of Representative Urea and Carbamate-Inserted
Vanadotungstatesa
1st process
E1 (mV vs Fc)
2nd process
POM
ΔEp (mV)
E2 (mV vs Fc)
3b
3d
3e
3f
ꢀ273
ꢀ353
ꢀ246
ꢀ323
ꢀ332
ꢀ343
ꢀ307
84
ꢀ620
ꢀ659
ꢀ621
ꢀ630
ꢀ538
ꢀ611
ꢀ600
168
108
162
132
239
102
Table 4. Use of Carbonyl-Inserted POMs As Catalysts for the
Oxidation of Sulfides
3m
3n
3o
a Redox potentials were measured by cyclovoltammetry at rt in
MeCN (0.1 M TBAPF6) on glassy carbon electrode at 300 mV sꢀ1
.
3
Values are quoted vs ferrocene as internal standard. For the quasi-
reversible first process the half-wave potentials and peak differences are
given. Broad waves were observed for the second process, and only the
reduction peak potential is reported.
entry
POM
cations
oxidant
yield (%)
1
2
3
4
5
6
7
3k
3k
3o
3i
(TBA)4.8(DMA)0.2
(TBA)4.7(DMA)0.3
(TBA)4.6(DMA)0.4
(TBA)4.6(DMA)0.4
(TBA)4.6(DMA)0.4
(TBA)4.6(DMA)0.4
(TBA)5.3H3.7
tBuOOH
H2O2
88
96
77
97
85
90a
75
tBuOOH
tBuOOH
tBuOOH
tBuOOH
tBuOOH
The known POM-catalyzed oxidation of sulfides7 to
sulfoxides is a good benchmark for catalytic oxidations
with the carbonyl-inserted organo-POMs, provided they
3r
3r
1
a Reaction carried out after recycling of 3r by precipitation.
(7) Selected recent examples: (a) Carraro, M.; Nsouli, N.; Oelrich, H.;
Sartorel, A.; Soraru, A.; Mal, S. S.; Scorrano, G.; Walder, L.; Kortz, U.;
Bonchio, M. Chem.;Eur. J. 2011, 17, 8371–8378. (b) Jahier, C.;
Coustou, M.-F.; Cantuel, M.; McClenaghan, N. D.; Buffeteau, T.;
Cavagnat, D.; Carraro, M.; Nlate, S. Eur. J. Inorg. Chem. 2011, 727–
738. (c) Khenkin, A. M.; Leitus, G.; Neumann, R. J. Am. Chem. Soc.
2010, 132, 11446–11448. (d) Mizuno, N.; Uchida, S.; Kamata, K.;
Ishimoto, R.; Nojima, S.; Yonehara, K.; Sumida, Y. Angew. Chem.,
Int. Ed. 2010, 49, 9972–9976. (e) Nisar, A.; Lu, Y.; Zhuang, J.; Wang, X.
Angew. Chem., Int. Ed. 2011, 50, 3187–3192. See also ref 4a.
Two different sulfides (tetrahydrothiophene and phenyl
methyl sulfide, 1 equiv each) were reacted with 1 equiv of tert-
butyl hydroperoxide in acetonitrile at rt. After consumption
of the co-oxidant (50% complexion with regard to 6 þ 7), the
6:7 ratio was measured by NMR.
5992
Org. Lett., Vol. 13, No. 22, 2011

When (TBA)5.3H3.7-1 was used, almost no selectivity
was observed (6:7 = 60:40, Table 5, entry 1). The selectiv-
ity was improved to 70:30 after cation metathesis to all
TBAs (entry 2). This set the standard to which some of the
hybrids were compared. To get a broad picture of all types
of carbonyl-inserted POMs, we added two amides to the
selection (3sꢀt; Table 5,5a). For easy comparison of the
diversely substituted POMs, and unless otherwise noted,
t-BuOOH was selected asstoichiometric oxidant because it
works in a purely organic medium (no water).
Table 5. Chemoselective Oxidations
In the urea-inserted series, alkyl (entries 3ꢀ4), electron-
poor (entries 5ꢀ7), electron-rich (entries 10ꢀ11), and neu-
tral (entries 8ꢀ9) aromatic substituents all led to noticeable
improvement in the selectivities. Those depended on the
cation distribution. With around two protons, the selectiv-
ities were close to 70:30 (entries 3 and 8). With 0.3ꢀ0.4
dimethylammoniums (DMAs), the selectivities improved to
approximately 85:15 (entries 5, 9ꢀ10), with the exception of
3c, which was more selective (93:7, entry 7). Gratifyingly,
the all-TBA POMs were highly selective, independently of
the nature of the urea substituent (entries 4, 6, 11).
A similar behavior was observed with carbamates (entries
12ꢀ16). The DMA salts led to chemoselectivities around 80:20
(entries 12, 13, 15), while the TBA salts of 3m and 3o worked
with increased selectivities (up to 96:4, entries 14 and 16).
Amides behaved like the urea and carbamate hybrids
(entries 17ꢀ21). While the DMA-containing salts were still
less chemoselective than the TBA ones (compare entries
17 and 20 to 19 and 21), some differences appeared.
p-Nitroaryl substituted 3s was substantially less selective
than p-methoxyaryl 3t, even, or especially, their TBA salts
(compare entries 19 and 21).
ent.
R (POM)
cationsa
6b
7b
1
ꢀ (1)
ꢀ (1)
TBA5.3H3.7
TBA9
60
70
67
96
82
93
93
72
87
83
95
86
78
96
72
96
79
72
88
90
95
40
30
33
4
2
3
EtNH (3a)
TBA2.85H2.15
TBA5
4
EtNH (3a)
5
pNO2C6H4NH (3b)
pNO2C6H4NH (3b)
pCF3C6H4NH (3c)
PhNH (3d)
TBA4.7DMA0.3
TBA5
18
7
6
7
TBA4.65DMA0.35
TBA3.45H1.55
TBA4.7DMA0.3
TBA4.7DMA0.3
TBA5
7
8
28
13
17
5
9
PhNH (3d)
10
11
12
13
14
15
16
17
18c
19
20
21
pMeOC6H4NH (3f)
pMeOC6H4NH (3f)
BnO (3k)
TBA4.7DMA0.3
TBA4.6DMA0.4
TBA5
14
22
4
PhO (3m)
PhO (3m)
pMeOC6H4O (3o)
pMeOC6H4O (3o)
pNO2C6H4 (3s)
pNO2C6H4 (3s)
pNO2C6H4 (3s)
pMeOC6H4 (3t)
pMeOC6H4 (3t)
TBA4.6DMA0.4
TBA5
28
4
TBA4.7DMA0.3
TBA4.7DMA0.3
TBA5
21
28
12
10
5
Switching to H2O2 as a stoichiometric oxidant led to a
slightly diminished, but similar, selectivity for the one
example we selected to carry out (entry 18).
TBA4.65DMA0.35
TBA5
Overall the hybrid catalysts have two main features.
• The chemoselectivity is primarily correlated to the
proton content in the cations of the salts. The more
protons, the less selective the system. When all protons
are removed, then the catalysts are highly selective,
much more than the purely inorganic polyanion.
• The chemoselectivity can in some cases be potentially
tuned by a simple change of the electronic properties
of the organic substituents, albeit with a less wide
amplitude than with the protons. This is true for the
amides, which agrees with our previously published
observations.5a With ureas and carbamates, the pre-
sence of the additional heteroelement attached to the
carbonyl probably buffers the electronic influence of
the organic substituents, which explains the lack of
tunability in those cases.
a Measured by NMR and elemental analysis. DMA = dimethyl
ammonium. b NMR ratios. c H2O2 was used as the oxidant.
accessible in that way. We also showed that carbonyl
insertion did not block the catalytic activity of the
vanadium core of the POMs. On the contrary, one can
fine-tune the chemoselectivity of the sulfide oxidation
upon appropriate choice of cations and organic substitu-
ents. Grafting of organics to POMs thus proves again to be
a good way to access new or enhanced properties. Further
work on these organo-POMs will focus on asymmetric
oxidations.
Acknowledgment. We thank CNRS, UPMC, and ANR
(Grants JC05-41806 and 08-PCVI-0005 to E.L., S.T., and B.
H.) for financial support of this work. FR2769 is acknowl-
edged for technical assistance.
To conclude, we have expanded the nature of the
carbonyl oxygens that can be inserted into phosphova-
nadotungstate [P2V3W15O62]9ꢀ. Secondary and tertiary
amides, ureas, carbamates, and thiocarbamates are
suitable functional groups. This generalization of our
method significantly widens the diversity of organo-POMs
Supporting Information Available. Characterization of
all new compounds and cyclic voltammograms. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 22, 2011
5993