Glycol-modified molybdate catalysts for efficient singlet oxygen generation from hydrogen peroxide

Article abstract of DOI:10.1039/b704238h

Pretreatment of molybdate-exchanged layered double hydroxides in polyalcohols such as ethylene glycol affords heterogeneous catalysts showing largely improved oxidant efficiency compared to the unmodified materials. The Royal Society of Chemistry.

Full text of DOI:10.1039/b704238h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Glycol-modified molybdate catalysts for efficient singlet oxygen
generation from hydrogen peroxide
Joos Wahlen,a Dirk De Vos,a Walther Jary,b Paul Alstersc and Pierre Jacobs*a
Received (in Cambridge, UK) 20th March 2007, Accepted 27th April 2007
First published as an Advance Article on the web 17th May 2007
DOI: 10.1039/b704238h
1
presence of citronellol, O2 generated from H2O2 (eqn (1)) can
either react with this trap (eqn (2)) or can be physically quenched
by the solvent, the solid catalyst, or citronellol itself (eqn (3)). The
reaction was carried out at 25 uC in N,N-dimethylformamide
(DMF) as the solvent. H2O2 (50 wt.%) was added in small
portions (Mo/H2O2 5 1/50) in order to favour the formation of
the labile oxotriperoxo-Mo species.1 The citronellol/Mo molar
ratio was 100/1.
Pretreatment of molybdate-exchanged layered double hydro-
xides in polyalcohols such as ethylene glycol affords hetero-
geneous catalysts showing largely improved oxidant efficiency
compared to the unmodified materials.
The confinement of molybdate (MoO422) anions to the surface of
III
layered double hydroxides (LDHs, [MII
M ] ?
x/n
x(OH)2][Xn2
12x
zH2O) is by now a recognized route to heterogeneous catalysis for
the generation of singlet molecular oxygen (1O2) from hydrogen
peroxide.1 Although these Mo-LDH catalysts show improved
catalytic activity compared to soluble molybdate, immobilization
leads to a significant drop of the H2O2 efficiency, presumably
because a relatively large amount of the formed 1O2 is quenched by
collision with the support matrix. Indeed, the surface of an LDH is
covered with numerous hydroxyl groups, which are known to be
efficient 1O2 quenchers.2 As a consequence, the oxidant efficiency
of Mo-LDHs in the peroxidation of b-citronellol, a typical olefinic
compound, is at most half of that observed for homogeneous
MoO422. Therefore, overstoichiometric amounts of H2O2 are
required to reach complete substrate conversion. Herein we
report that the pretreatment of Mo-LDHs in polyalcohols such
as ethylene glycol significantly improves the oxidant efficiency of
the catalysts compared to the parent Mo-LDHs.
MoꢀLDHꢀEG
2H2O2 DCDCCA 2H2Oz1O2
(1)
ð2Þ
solvent
citronellol
solid
1O2 DCDCCA 3O2
(3)
In Fig. 1, the yield of citronellol hydroperoxides is plotted
against the number of equivalents (equiv.) of H2O2 added with
respect to citronellol. Theoretically, if the above-mentioned
physical quenching processes are not operating, 2 equiv. of H2O2
are sufficient for complete conversion of citronellol (100% H2O2
efficiency). Fig. 1 clearly shows that the H2O2 utilization of the
Mo-LDHs pretreated in glycolic solvents is more efficient than
that of the parent Mo-LDH. The efficiency increases from 20% for
the unmodified Mo-LDH to about 60% for Mo-LDH-EG.
First, a (NO32)-[Mg,Al]LDH support with a Mg/Al molar ratio
of 2/1 was prepared by co-precipitation of Mg(II) and Al(III)
nitrate salts in the presence of aqueous NaOH. Next, partial ion
exchange of nitrate for molybdate yielded (MoO422,NO32)-
22
[Mg,Al]LDH containing 0.2 mmol MoO4
per gram.1 This
material was used for further treatment with various glycols.3–6
The general procedure involved heating air-dry Mo-LDH (1 g) in
ethylene glycol (50 mL) for 12 h at 80 uC.3 The product (Mo-
LDH-EG) was centrifuged, thoroughly washed with acetone, and
air-dried at room temperature. A similar treatment of the Mo-
LDH was carried out in different glycols, simple alcohols and
other polar organic solvents.
The H2O2 efficiency of the various catalysts was assessed by
chemical trapping of the formed 1O2 with b-citronellol. This olefin
1
contains two types of allylic hydrogen atoms and reacts with O2
to yield an equimolar mixture of allylic hydroperoxides.
Quantification of the formed allylic hydroperoxides by GC
analysis allows the determination of the H2O2 efficiency. In the
aCentre for Surface Chemistry and Catalysis, K.U. Leuven, Kasteelpark
Arenberg 23 Box 2461, 3001 Leuven, Belgium.
Fig. 1 Peroxidation of citronellol with unmodified Mo-LDH ($) and
Mo-LDHs treated in 1,2-propanediol (#), ethylene glycol (&), glycerol
(m), and 1,3-propanediol (%). Reaction conditions: 0.5 g of Mo-LDH-X
(0.1 mmol Mo), 10 mmol citronellol, H2O2 added in 5 mmol portions,
10 mL of DMF, 25 uC. GC analysis after centrifugation and reduction
with (CH3)3P. Hydroperoxide selectivity .95% in all cases.
E-mail: pierre.jacobs@biw.kuleuven.be; Fax: +32 16 32 1998;
Tel: +32 16 32 1610
bDSM Fine Chemicals Austria, P.O. Box 296, 4021 Linz, Austria
cDSM Pharmaceutical Products – Advanced Synthesis, Catalysis and
Development, P.O. Box 18, 6160 MD Geleen, The Netherlands
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 2333–2335 | 2333

View Article Online
catalyst. Apparently, the presence of a glycolic moiety in the
modifying agent is of primary importance for the preparation of a
heterogeneous catalyst showing high oxidant efficiency.
Having identified Mo-LDH-EG as a highly efficient catalyst,
the recycling and reuse of this material were examined. At the end
of the reaction, the solid Mo-LDH-EG was recovered by simple
centrifugation, washed with acetone, and the catalyst along with
replenished reagents and solvent was reused for nine cycles with
minimal loss of activity (Fig. 2). The oxidant efficiency remained
equally high during all runs. This recycle experiment clearly shows
that the efficiency-enhancing effect of the EG treatment is stable in
time. Moreover, elemental analysis (ICP-MS) showed no leaching
of molybdate from the LDH support.
Fig. 2 Hydroperoxide yield and reaction time ($) for the peroxidation
of citronellol catalyzed by Mo-LDH-EG. (Yield axis starts at 90%.)
Reaction conditions: 1 g of Mo-LDH-EG (0.2 mmol Mo), 20 mmol
citronellol, 80 mmol H2O2 added in 10 mmol portions, 10 mL of DMF,
25 uC. Hydroperoxide selectivity .99% in all cases.
Characterization of the Mo-LDH-EG catalyst by X-ray
diffraction, infrared spectroscopy and solid-state NMR indicated
that ethylene glycol is interacting with aluminium located at the
outer surface and at the edges of the LDH crystals; intercalation
does not occur to any significant extent. At this time we presume
that the EG pretreatment induces a transformation of the outer
LDH surface, e.g., by relocating the molybdate ions, or by
changing the sorption properties of the surface. Both factors likely
Complete conversion of citronellol is obtained using 4 equiv. of
H2O2, whereas the unmodified Mo-LDH requires at least 8 equiv.
of H2O2. Treatment in 1,2-propanediol yields a slightly more
efficient catalyst than Mo-LDH-EG. On the other hand, 1,3-
propanediol and glycerol provide less efficient catalysts compared
to Mo-LDH-EG, but the efficiency is still significantly higher than
that observed for Mo-LDH. In contrast, treatment of Mo-LDHs
in 2-methoxyethanol, 2,2,2-trifluoroethanol, simple alcohols such
as tert-butanol or ethanol, or other polar organic solvents such as
DMF under similar conditions did not result in an improvement of
the oxidant efficiency compared to the unmodified Mo-LDH
1
favour the access of citronellol to the O2-producing centres, or
1
alternatively, the easy escape of O2 from the LDH surface.
In order to explore the synthetic scope of the Mo-LDH-EG
catalyst, the peroxidation of some typical olefinic compounds was
attempted (Table 1).{ Dienes such as a-terpinene (Entry 1) react
1
with O2 via a [4 + 2]-cycloaddition and yield the corresponding
endoperoxides as the product. On the other hand, alkyl-substituted
a
Table 1 Peroxidation of olefinic compounds with singlet oxygen generated from Mo-LDH-EG and H2O2
H2O2
(equiv.) t (h) Products
Distribution Conversion Selectivity
(%)
Entry Substrate
1
(%)
(%)
3.3
3
3
—
98
86
2
3
2.5
3.5
—
99
97
99
99
4
53 : 47
4
5
4.5
42 : 15 : 43 98
99
5
4
4
8
3.5
5
5
49 : 51
52 : 48
47 : 53
96
87
94
99
98
99
6b
7c
8
9
4
5
3.5
5
42 : 58
42 : 58
92
99
99
99
10
11c
8
16
13
15
59 : 41d
61 : 39d
88
89
98
91
a
Reaction conditions: 5 mmol olefin and 0.25 g of Mo-LDH-EG (Mg/Al 5 2, 0.2 mmol Mo/g) were stirred at 25 uC in 5 mL of DMF. H2O2
(50 wt.%) was added in 2.5 mmol portions. GC analysis after centrifugation and reduction with excess (CH3)3P (except for Entry 1). 5 mL of
b
c
methanol. Unmodified Mo-LDH. Pairs of enantiomers.
d
2334 | Chem. Commun., 2007, 2333–2335
This journal is ß The Royal Society of Chemistry 2007

View Article Online
alkenes react with 1O2 via the ene reaction, and mixtures of regio-
isomeric allylic hydroperoxides with a unique product distribution
are obtained.7 This is illustrated by the oxidation of simple acyclic
and cyclic alkyl-substituted alkenes such as 2,3-dimethyl-2-butene
(Entry 2), 2-methyl-2-heptene (Entry 3) and 1-methyl-1-cyclo-
hexene (Entry 4). These substrates typically require 3 to 5 equiv. of
H2O2 to reach complete conversion. Reduction of the allylic
hydroperoxides yields the corresponding allylic alcohols. Although
product mixtures are obtained in most cases, many of these
compounds are difficult to obtain by other synthetic procedures.
This work was supported by the European Commission
(SUSTOX project, G1RD-CT-2000-00347) and the Belgian
Government (IAP project on Supramolecular Chemistry and
Catalysis). JW acknowledges FWO Vlaanderen for a postdoctoral
fellowship.
Notes and references
{ Products were identified by GC-MS, 1H and 13C NMR and by
comparison with authentic samples prepared by photochemical oxidation
in the presence of tetraphenylporphine (chloroform) or rose bengal
(methanol) as photosensitizer. CAUTION: H2O2 and alkyl hydroperoxide
solutions are strongly oxidizing and should be handled with appropriate
precautions.
1
The regioselectivity of the reaction of O2 generated from Mo-
LDH-EG and H2O2 is similar to known solution photochemistry.
For example, 1-methyl-1-cyclohexene gave a hydroperoxide
mixture showing the same product distribution pattern as that
observed for photooxidations.8 Oxyfunctionalized alkenes such as
the monoterpene citronellol (Entry 5) cleanly yield the allylic
hydroperoxide products, no oxidation of the primary alcohol
functionality being observed. Photooxidation of citronellol is the
first step in the preparation of rose oxide, a well-known perfumery
ingredient used in rose and geranium perfumes.9 Reaction in
methanol as the solvent is somewhat slower and slightly less
efficient than in DMF (Entry 6). Using the unmodified Mo-LDH
catalyst instead of Mo-LDH-EG, twice as much H2O2 is required
to reach high conversions (Entry 7). For comparison, the
1 (a) F. van Laar, D. De Vos, D. Vanoppen, B. Sels, P. A. Jacobs,
A. Del Geurzo, F. Pierard and A. Kirsch-De Mesmaeker, Chem.
Commun., 1998, 267; (b) B. F. Sels, D. E. De Vos, P. J. Grobet,
F. Pierard, A. Kirsch-De Mesmaeker and P. A. Jacobs, J. Phys. Chem.
B, 1999, 103, 11114; (c) B. F. Sels, D. E. De Vos, P. J. Grobet and
P. A. Jacobs, Chem.–Eur. J., 2001, 7, 2547; (d) F. M. P. R. van Laar,
D. E. De Vos, F. Pierard, A. Kirsch-De Mesmaeker, L. Fiermans and
P. A. Jacobs, J. Catal., 2001, 197, 139; (e) D. E. De Vos, J. Wahlen,
B. F. Sels and P. A. Jacobs, Synlett, 2002, 367; (f) J. Wahlen, D. E.
De Vos, B. F. Sels, V. Nardello, J.-M. Aubry, P. L. Alsters and
P. A. Jacobs, Appl. Catal., A, 2005, 293, 120.
2 For studies on the quenching of 1O2 by silica gel, see: (a) K.-K. Iu and
J. K. Thomas, J. Am. Chem. Soc., 1990, 112, 3319; (b) E. L. Clennan
and M. F. Chen, J. Org. Chem., 1995, 60, 6004.
3 J. L. Guimara˜es, R. Marangoni, L. P. Ramos and F. Wypych, J. Colloid
Interface Sci., 2000, 227, 445.
peroxidation of citronellol in methanol using homogeneous
22
MoO4
requires 4 equiv. of H2O2 for complete conversion,
whereas in DMF, 6 equiv. are required to reach 80% conversion.10
Other heterogeneous catalysts such as La(OH)3 (12 equiv.)11 or
La-zeolites (8 equiv.)12 show far less efficient utilization of H2O2.
For linalool (Entry 8), a monoterpene containing an allylic alcohol
functionality, peroxidation occurs at the isolated, electron-rich 6,7-
double bond. Epoxidation of the less electron-rich, allylic double
bond was not observed. Using 5 equiv. of H2O2, full conversion of
linalool is obtained after 5 h (Entry 9). The derived allylic alcohols
are intermediates for 3,7-dimethyl-1,5,7-octatrien-3-ol, which can
be used as a perfume or flavouring component.13 Next, Mo-LDH-
EG was used for the peroxidation of allylic alcohols to the
corresponding hydroperoxy homoallylic alcohols.14 Mesitylol (4-
methyl-3-penten-2-ol, Entry 10) was selected as a typical allylic
alcohol showing relatively low reactivity towards 1O2. The photo-
oxygenation of mesitylol is the first step in the synthesis of 1,2,4-
trioxanes. Some of these compounds show high antimalarial
activity against Plasmodium falciparum.15 Use of 8 equiv. of H2O2
resulted in 90% conversion. Very high selectivity towards the
hydroperoxy homoallylic alcohols was observed. Notably, almost
no epoxidation of the double bond and no oxidation of the
secondary alcohol group were observed. Using the unmodified
Mo-LDH catalyst, twice as much H2O2 is required to reach similar
conversions in the peroxidation of mesitylol (Entry 11). Moreover,
the hydroperoxide selectivity is rather low due to competitive
epoxidation of the double bond. The stereoselectivities of the
peroxidation catalyzed by Mo-LDH-EG in aqueous DMF are in
accordance with known photochemical oxidations in polar organic
solvents.14 Thus, relatively low diastereoselectivities are observed
due to competitive hydrogen bonding of the allylic alcohol group
with DMF and water, rather than with the attacking 1O2.
4 T. Stanimirova and T. Hibino, Appl. Clay Sci., 2006, 31, 65.
5 For reports on the thermal treatment of aluminium hydroxide in glycols,
see: (a) M. Inoue, Y. Kondo and T. Inui, Chem. Lett., 1986, 1421; (b)
M. Inoue, Y. Kondo and T. Inui, Inorg. Chem., 1988, 27, 215.
6 The use of glycerol as a swelling agent for the preparation of LDH
intercalates with organic or inorganic anions is well known. For
example, see: (a) E. D. Dimotakis and T. J. Pinnavaia, Inorg. Chem.,
1990, 29, 2393; (b) H. C. B. Hansen and R. M. Taylor, Clay Miner.,
1991, 26, 311.
7 (a) M. Prein and W. Adam, Angew. Chem., Int. Ed. Engl., 1996, 35, 477;
(b) E. L. Clennan, Tetrahedron, 2000, 56, 9151; (c) M. C. DeRosa and
R. J. Crutchley, Coord. Chem. Rev., 2002, 233, 351; (d) J. Wahlen, D. E.
De Vos, P. A. Jacobs and P. L. Alsters, Adv. Synth. Catal., 2004, 346,
152; (e) E. L. Clennan and A. Pace, Tetrahedron, 2005, 61, 6665.
8 Y. Araki, D. C. Dobrowolski, T. E. Goyne, D. C. Hanson, Z. Q. Jiang,
K. J. Lee and C. S. Foote, J. Am. Chem. Soc., 1984, 106, 4570.
9 (a) P. Esser, B. Pohlmann and H.-D. Scharf, Angew. Chem., Int. Ed.
Engl., 1994, 33, 2009; (b) K. Bauer, D. Garbe and H. Surburg, Common
Fragrance and Flavor Materials – Preparation, Properties and Uses,
Wiley-VCH, Weinheim, 1997, p. 139; (c) P. Kraft, J. A. Bajgrowicz,
C. Denis and G. Fra´ter, Angew. Chem., Int. Ed., 2000, 39, 2981.
10 V. Nardello, S. Bogaert, P. L. Alsters and J.-M. Aubry, Tetrahedron
Lett., 2002, 43, 8731.
11 V. Nardello, J. Barbillat, J. Marko, P. T. Witte, P. L. Alsters and
J.-M. Aubry, Chem.–Eur. J., 2003, 9, 435.
12 (a) J. Wahlen, D. De Vos, S. De Hertogh, V. Nardello, J.-M. Aubry,
P. Alsters and P. Jacobs, Chem. Commun., 2005, 927; (b) J. Wahlen,
S. De Hertogh, D. E. De Vos, V. Nardello, S. Bogaert, J.-M. Aubry,
P. L. Alsters and P. A. Jacobs, J. Catal., 2005, 233, 422.
13 G. Ohloff and W. Giersch, CH 596121, 1978.
14 (a) W. Adam and B. Nestler, J. Am. Chem. Soc., 1992, 114, 6549; (b)
W. Adam and B. Nestler, J. Am. Chem. Soc., 1993, 115, 5041; (c)
W. Adam, H.-G. Bru¨nker, A. S. Kumar, E.-M. Peters, K. Peters,
U. Schneider and H. G. von Schnering, J. Am. Chem. Soc., 1996, 118,
1899.
15 (a) A. G. Griesbeck, T. T. El-Idreesy, M. Fiege and R. Brun, Org. Lett.,
2002, 4, 4193; (b) G. Bez, B. Kalita, P. Sarmah, N. C. Barua and
D. K. Dutta, Curr. Org. Chem., 2003, 7, 1231; (c) A. G. Griesbeck,
T. T. El-Idreesy, L.-O. Ho¨inck, J. Lex and R. Brun, Bioorg. Med. Chem.
Lett., 2005, 15, 595.
In conclusion, pretreatment of Mo-LDHs in glycols yields
heterogeneous catalysts showing superior H2O2 efficiency com-
pared to the unmodified materials.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 2333–2335 | 2335