Highly efficient C-H insertion reactions of hydrogen peroxide catalyzed by homogeneous and heterogeneous methyltrioxorhenium systems in ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2005.02.056

A convenient and efficient C-H insertion reaction of environment friendly H₂O₂ into representative hydrocarbon derivatives by homogeneous methyltrioxorhenium (MTO), heterogeneous poly(4-vinylpyridine)/ methyltrioxorhenium (PVP/MTO) and microencapsulated polystyrene/ methyltrioxorhenium (PS/MTO) systems in ionic liquids, is described. In some cases a higher activity was observed if compared with the same reaction in molecular solvents. The heterogeneous catalysts are stable systems under the reaction conditions and can be recycled for more transformations.

Full text of DOI:10.1016/j.tetlet.2005.02.056

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2427–2432
Highly efficient C–H insertion reactions of hydrogen
peroxide catalyzed by homogeneous and heterogeneous
methyltrioxorhenium systems in ionic liquids
Gianluca Bianchini,a Marcello Crucianelli,a,* Francesco De Angelis,a
Veronica Nerib and Raffaele Saladinob,*
a
`
Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita dell’Aquila, via Vetoio, I-67010 Coppito (AQ), Italy
b
`
INFM, Dipartimento A.B.A.C., Universita della Tuscia, via S.Camillo De Lellis, I-01100 Viterbo, Italy
Received 27 January 2005; revised 8 February 2005; accepted 9 February 2005
Abstract—A convenient and efficient C–H insertion reaction of environment friendly H2O2 into representative hydrocarbon deriva-
tives by homogeneous methyltrioxorhenium (MTO), heterogeneous poly(4-vinylpyridine)/methyltrioxorhenium (PVP/MTO) and
microencapsulated polystyrene/methyltrioxorhenium (PS/MTO) systems in ionic liquids, is described. In some cases a higher activity
was observed if compared with the same reaction in molecular solvents. The heterogeneous catalysts are stable systems under the
reaction conditions and can be recycled for more transformations.
ꢀ 2005 Elsevier Ltd. All rights reserved.
In recent years the need for designing ÔcleanÕ and ÔgreenÕ
industrial processes has created a very significant
amount of interest in the use of non-volatile ionic liquids
in chemical synthesis and catalysis.1 Ionic liquids pos-
sess, inter alia, the following desirable properties: they
are liquids below 100 ꢁC and with a relative low viscos-
ity, allowing kinetic control; they are solvents for a wide
range of inorganic, organic and polymeric materials;2
they can exhibit Brønsted and Lewis acidity as well as
superacidity, enabling many catalytic processes. Finally,
some ionic liquids are commercially available since they
can be prepared readily and economically. As such,
ionic liquids are thought to offer a range of significant
improvements compared to conventional molecular sol-
vents especially in view of the fact that there are several
ionic liquids that can be synthesized, making it possible
to design an ionic liquid, which is specific with the reac-
tion requirements (Ôdesigner solventÕ).
dazolium hexafluorophosphate [BMIM][PF6],3 1-ethyl-
3-methylimidazolium tetrafluoroborate [EMIM][BF4]4
and 1-ethyl-3-methylimidazolium bis-triflic amide
[EMIM][Tf2N]5 salts, which are oxygen and water stable
compounds. Although some attention has been focused
on catalytic oxidations in ionic liquids,6 to date there are
no examples of C–H insertion reactions of environment
friendly hydrogen peroxide (H2O2) into hydrocarbon
derivatives. The oxidation of hydrocarbons is an impor-
tant and challenging area in industrial and commodity
chemistry.7 Most of the existing processes use toxic
and often stoichiometric oxidants and molecular sol-
vents eventually producing wastes.
In the last decade methyltrioxorhenium (MTO) has been
used in several organic transformations.8 Furthermore,
MTO showed excellent conversions and selectivities
for the epoxidation of olefins in ionic liquids. Under
these experimental conditions, the bis-peroxo [Me-
ReO(O2)2]g2-rhenium complex was more reactive than
the monoperoxo [MeRe(O)2O2] complex, while the
opposite is true for the reaction conducted in conven-
tional molecular solvents.9,10
Advances in oxidation chemistry that utilize ionic liq-
uids have been obtained with 1-n-butyl-3-methylimi-
Keywords: C–H insertion reaction; Heterogeneous catalysis; Hydrogen
peroxide; Methyltrioxorhenium; Ionic liquid.
As erroneously reported in a recent communication,11a
MTO cannot be retained into ionic liquids during
liquid/liquid extraction of the reaction mixture, due to
its known high solubility in a wide range of molecular
*
Corresponding authors. Tel.: +39 862 433780; fax: +39 862 433753
(M.C.); tel.: +39 761 357284; fax: +39 761 357242 (R.S.); e-mail
addresses: cruciane@univaq.it; saladino@unitus.it
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.056

2428
G. Bianchini et al. / Tetrahedron Letters 46 (2005) 2427–2432
solvents.11b Thus, novel synthetic strategies should be
designed to obtain MTO catalytic systems that are really
immobilized in ionic liquids. Recently, we described the
first example of oxidation of hydrocarbons with H2O2
and heterogeneous MTO systems in ethanol and acetic
acid.12 These catalysts showed to be efficient systems
also for the activation of H2O2 in the oxidation of aro-
matic derivatives, alkenes and terpenes.13 Heteroge-
neous catalysis allows the easy separation of products
making possible the recycling and reuse of the catalyst
with environmental and economical advantages.
than 5% conversion of substrates took place under
otherwise identical conditions. The oxidation results
are summarized in Tables 1–6 and Schemes 1–5.
The oxidation of triphenylmethane 1 with MTO and
heterogeneous MTO catalysts I–V in [BMIM][PF6]
and CHCl3 (25% v/v) at 45 ꢁC, yielded benzophenone
6 as the only recovered product (Scheme 1). In accor-
dance with the data previously reported,12 it is reason-
able to suggest that triphenyl carbinol (not shown)
was a reactive intermediate in the oxidation. To the best
of our knowledge, this is the first example of oxidative
dearylation of tertiary aromatic hydrocarbon to ketone
in ionic liquid. Catalysts I–V showed both conversion
values of substrate (45–53%) and yield of product (80–
91%) comparable with MTO (Table 1, entries 2–6 vs
1). In the family of poly(4-vinylpyridine) catalysts, pyr-
idine N-oxide derivatives II and IV were less efficient
than parent pyridine derivatives I and III (e.g., Table
1, entries 2–4 vs 3–5), compound III being the best cata-
lyst. The behaviour of microencapsulated V was similar
to poly(4-vinylpyridine) catalysts affording 6 in 45%
conversion and 88% yield (Table 1, entry 6). It is inter-
esting to note that catalysts I–V showed a higher reactiv-
ity in the oxidation of 1 in [BMIM][PF6]–CHCl3 than in
ethanol.12 Comparable results in molecular solvents
were obtained only in acetic acid, which is known to effi-
ciently increase the reactivity of MTO.12,17
Herein we describe the unprecedented catalyzed C–H
insertion reactions of environment friendly H2O2 into
representative hydrocarbon derivatives, by means of
heterogeneous MTO systems in ionic liquids. Reactions
were performed using catalysts based on the heterogeni-
zation of MTO on easily available, nontoxic and low
expensive poly(4-vinylpyridine) and poly(4-vinylpyr-
idine N-oxide) 2% [PVP-2/MTO (I) and PVPN-2/MTO
(II), respectively], and 25% [PVP-25/MTO (III) and
PVPN-25/MTO (IV), respectively], cross-linked with
divinylbenzene.14 Microencapsulated catalyst PS-2/
MTO (V) based on the physical entrapment of MTO
on polystyrene 2% cross-linked with divinylbenzene
was also used.15 The structures of catalysts are reported
in Figure 1.
Briefly, triphenylmethane 1, benzhydrol 2, 1-phenyl-
ethanol 3, cis-1,2-dimethylcyclohexane 4 and adaman-
tane 5 (1 mmol) dissolved in [BMIM][PF6] or [EMIM]-
[Tf2N] (1 mL; in the case of 1 a low amount of CHCl3
was used in order to increase the solubility of the sub-
strate) were added portionwise with supported catalysts
I–V (100 mg, loading factor 1) and H2O2 (4–6 equiv) at
25–60 ꢁC. At the end of the reaction, the mixture was
extracted with diethyl ether. After separation, the ethe-
real layer was added with a catalytic amount of MnO2,
filtrated and evaporated. The ionic liquid layer contain-
ing catalysts I–V may be used in successive experiments
without further purification (see forward). Reaction
products were characterized by GC–MS analyses and
by comparison with authentic samples.16 Oxidations
with MTO under similar experimental conditions were
performed as references. In the absence of catalyst, less
To evaluate the generality of this procedure, benzhydrol
2 and 1-phenyl-ethanol 3 were oxidized under similar
experimental conditions. The oxidation of 2 with H2O2
and catalysts I–V in [BMIM][PF6] at 45 ꢁC gave 6 in
76–83% yield and 73–94% conversion of substrate
(Scheme 2, Table 2, entries 2–6). Catalysts I–V showed
both conversion values of substrate and yields of prod-
uct comparable with MTO. Again, pyridine N-oxide
derivatives were less efficient than parent pyridine deriva-
tives (e.g., Table 2, entries 2–4 vs 3–5), compounds I and
III being the best catalysts. Microencapsulated catalyst
V showed a similar behaviour (Table 2, entry 6). The
equilibrium constant for the interaction between pyr-
idine N-oxide and MTO is of the same order of
magnitude than that measured for pyridine.18 On the
other hand, the pyridine N-oxide/MTO adduct shows
MTO
MTO
MTO
N
MTO
N
O
N
O
N
MTO
n
n
n=2%, PVP-2/MTO (I),
n=25%, PVP-25/MTO (III)
n=2%, PVPN-2/MTO (II),
n=25%, PVPN-25/MTO (IV)
PS-2/MTO (V)
Figure 1.

G. Bianchini et al. / Tetrahedron Letters 46 (2005) 2427–2432
Table 1. Oxidation of 1 with MTO and polymer supported MTO catalysts
2429
Entry
Catalysta
Solventb
Time (h)
T (ꢁC)
Conv. (%)
Yield (%)
1
2
3
4
5
6
MTO
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
65
48
48
48
48
48
45
45
45
45
45
45
57
48
51
51
53
49
93
89
81
91
80
88
PVP-2/MTO (I)
PVPN-2/MTO (II)
PVP-25/MTO (III)
PVPN-25/MTO (IV)
PS-2/MTO (V)
a PVP-2/MTO: poly(4-vinylpyridine) 2% cross-linked. PVP-25/MTO: poly(4-vinylpyridine) 25% cross-linked. PVPN-2/MTO: poly(4-vinylpyridine
N-oxide) 2% cross-linked. PVPN-25/MTO: poly(4-vinylpyridine N-oxide) 25% cross-linked. PS-2/MTO: polystyrene 2% cross-linked.
b Ionic liquid mixed with 25% v/v of CHCl3.
Table 2. Oxidation of 2 with MTO and polymer supported MTO catalysts
Entry
Catalysta
Solvent
Time (h)
T (ꢁC)
Conv. (%)
Yield (%)
1
2
3
4
5
6
MTO
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
24
24
24
24
24
24
45
45
45
45
45
45
98
78
73
91
81
94
88
83
79
82
76
78
PVP-2/MTO (I)
PVPN-2/MTO (II)
PVP-25/MTO (III)
PVPN-25/MTO (IV)
PS-2/MTO (V)
a PVP-2/MTO: poly(4-vinylpyridine) 2% cross-linked. PVP-25/MTO: poly(4-vinylpyridine) 25% cross-linked. PVPN-2/MTO: poly(4-vinylpyridine
N-oxide) 2% cross-linked. PVPN-25/MTO: poly(4-vinylpyridine N-oxide) 25% cross-linked. PS-2/MTO: polystyrene 2% cross-linked.
Table 3. Oxidation of 3 with MTO and polymer supported MTO catalysts
Entry
Catalysta
Solvent
Time (h)
T (ꢁC)
Conv. (%)
Yield (%)
1
MTO
EtOH
EtOH
24
24
24
24
24
24
24
44
48
48
48
48
48
60
60
60
60
60
60
60
rt
35
26
20
22
22
18
23
98
80
84
75
80
88
12
10
8
2
PVP-2/MTO (I)
PVPN-2/MTO (II)
PVP-25/MTO (III)
PVPN-25/MTO (IV)
PS-2/MTO (V)
PVP-2/MTO (I)
MTO
3
EtOH
EtOH
4
10
8
5
EtOH
EtOH
AcOH
6
8
7
12
91
84
82
86
80
85
8
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
9
PVP-2/MTO (I)
PVPN-2/MTO (II)
PVP-25/MTO (III)
PVPN-25/MTO (IV)
PS-2/MTO (V)
rt
10
11
12
13
rt
rt
rt
rt
a PVP-2/MTO: poly(4-vinylpyridine) 2% cross-linked. PVP-25/MTO: poly(4-vinylpyridine) 25% cross-linked. PVPN-2/MTO: poly(4-vinylpyridine
N-oxide) 2% cross-linked. PVPN-25/MTO: poly(4-vinylpyridine N-oxide) 25% cross-linked. PS-2/MTO: polystyrene 2% cross-linked.
Table 4. Stability of catalysts III and V in the oxidation of 1 and 2 in [BMIM][PF6]
Entry
Reagent
Product
Catalyst
Conversion (%)
Run no. 1a
Run no. 2
Run no. 3
Run no. 4
1
2
3
4
1
6
III
V
51 (91)b
49 (88)
91 (82)
94 (78)
35 (80)
47 (88)
32 (76)
85 (78)
33 (78)
45 (86)
30 (75)
83 (77)
—
—
1
6
26
2
III
6
—
82 (76)
V
a After the first reaction, following runs were performed adding only fresh substrate and oxidant to the ionic liquid, and working under the same
experimental conditions.
b Yields of benzophenone 6 are given in parentheses.
the highest value of the energy for the transition state in
the epoxidation of olefins19 and oxygen-donor adducts
of MTO usually show lower reactivity than MTO.20
catalyst used (Scheme 3, Table 3, entries 9–13). Note-
worthy a higher reactivity was observed if compared
to that showed in both ethanol and acetic acid at
60 ꢁC (less than 5% conversion of substrates took
place at room temperature). Under these latter
conditions compound 7 was recovered in 8–12% yield
and 18–26% conversion of substrate (see Table 3,
entries 2–7).21
The oxidation of 3 with H2O2 and catalysts I–V in
[BMIM][PF6], at room temperature, afforded acetophe-
none 7 as the only recovered product in 80–86% yield
and 75–88% conversion of substrate depending on the

2430
G. Bianchini et al. / Tetrahedron Letters 46 (2005) 2427–2432
Table 5. Oxidation of 4 with MTO and polymer supported MTO catalysts
Entry
Catalysta
Solvent
Time (h)
T (ꢁC)
Conv. (%)
Yield (%)
1
2
3
4
5
6
MTO
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
[BMIM][PF6]
72
72
72
72
72
72
45
45
45
45
45
45
87
81
78
68
71
80
88
79
75
78
76
81
PVP-2/MTO (I)
PVPN-2/MTO (II)
PVP-25/MTO (III)
PVPN-25/MTO (IV)
PS-2/MTO (V)
a PVP-2/MTO: poly(4-vinylpyridine) 2% cross-linked. PVP-25/MTO: poly(4-vinylpyridine) 25% cross-linked. PVPN-2/MTO: poly(4-vinylpyridine
N-oxide) 2% cross-linked. PVPN-25/MTO: poly(4-vinylpyridine N-oxide) 25% cross-linked. PS-2/MTO: polystyrene 2% cross-linked.
Table 6. Oxidation of 5 with MTO and polymer supported MTO catalysts
Entry
Catalysta
Solvent
Time (h)
T (ꢁC)
Conv. (%)
Yield (%)
1
2
3
4
5
6
MTO
[EMIM][Tf2N]
[EMIM][Tf2N]
[EMIM][Tf2N]
[EMIM][Tf2N]
[EMIM][Tf2N]
[EMIM][Tf2N]
48
48
48
48
48
48
60
60
60
60
60
60
43
63
78
69
79
72
68
79
68
75
69
72
PVP-2/MTO (I)
PVPN-2/MTO (II)
PVP-25/MTO (III)
PVPN-25/MTO (IV)
PS-2/MTO (V)
a PVP-2/MTO: poly(4-vinylpyridine) 2% cross-linked. PVP-25/MTO: poly(4-vinylpyridine) 25% cross-linked. PVPN-2/MTO: poly(4-vinylpyridine
N-oxide) 2% cross-linked. PVPN-25/MTO: poly(4-vinylpyridine N-oxide) 25% cross-linked. PS-2/MTO: polystyrene 2% cross-linked.
H
O
catalyst
catalyst
H2O2, solvent
Ph
Ph
45 ˚C
H2O2, solvent
60 ˚C
H
Ph
Ph
OH
Ph
6
1
9
5
Scheme 1.
Scheme 5.
OH
formations. It is noteworthy that the recycled catalyst V
showed comparable selectivity in the oxidation even after
four runs, affording acetophenone 6 as the only recov-
ered product in high yield (see Table 4, entries 2 and 4).
However, in the case of the recycled catalyst III, a de-
crease of the reactivity was observed during the first recy-
cling step (run no. 2), after which the value of conversion
of substrates 1 and 2 become constant in the range of 30–
35% (see Table 4, entries 1 and 3). Probably, a partial
leaching of the catalyst was responsible for this
behaviour.
O
catalyst
H2O2, solvent
45 ˚C
Ph
Ph
Ph
Ph
6
2
Scheme 2.
OH
O
catalyst
H2O2, solvent
Ph
CH3
Ph
CH3
7
The efficiency of oxygen-atom insertion by MTO and
heterogeneous MTO catalysts in ionic liquids was further
demonstrated through their ability to oxidize saturated
hydrocarbons, such as cis-1,2-dimethylcyclohexane 4
and adamantane 5. The reaction of 4 with both MTO
and heterogeneous MTO catalysts I–V performed in
[BMIM][PF6] at 45 ꢁC, gave the corresponding cis-1,2-
dimethylcyclohexan-1-ol 8 both in high yield and conver-
sion of substrate (Scheme 4, Table 5), showing a selective
oxidation at the tertiary C–H bond. As shown in Table 5,
compounds I and V were the best heterogeneous catalyst
systems affording 8 in 79% and 81% yield, and 81% and
80% conversion, respectively (Table 5, entries 2 and 6). A
similar selectivity in the oxidation of 4 was previously ob-
served with dimethyldioxirane,22 a stoichiometric oxi-
dant characterized by a ÔbutterflyÕ transition state, in
the oxygen-atom insertion reaction, similar to that
hypothesized for MTO.8
3
Scheme 3.
CH3
CH3
catalyst
H
OH
CH3
CH3
H2O2, solvent
45 ˚C
4
8
H
H
Scheme 4.
To evaluate the stability of heterogeneous MTO catalysts
in ionic liquids the oxidation of 1 and 2 with catalysts III
and V in [BMIM][PF6], was repeated in successive trans-

G. Bianchini et al. / Tetrahedron Letters 46 (2005) 2427–2432
2431
M. J.; McCormac, P. B.; Seddon, K. R. Green Chem. 1999,
1, 23.
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ˆ
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daram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168–1178.
In the oxidation of adamantane 5 the ionic liquid
[EMIM][Tf2N] was used as reaction solvent in order to
increase the solubility of substrate. When the reaction
was performed at 60 ꢁC in the presence of both MTO
and heterogeneous MTO catalyst systems I–V, 1-adam-
antanol 9 was selectively obtained in acceptable yield
and conversion of substrate (Scheme 5, Table 6). Note-
worthy, in these latter cases catalysts I, III and V were
found more reactive and selective than MTO (Table 6,
entry 1 vs entries 2, 4 and 6), even if usually a catalytic
species loses part of its catalyst efficiency after the hetero-
genization process. An enhancement of the reactivity of
heterogeneous MTO catalysts with respect to parent
MTO was previously observed in the oxidation of phe-
nol and anisole derivatives to quinones with hydrogen
peroxide.13a Catalysts II and IV showed a reactivity sim-
ilar to MTO (Table 6, entries 3 and 5). Again, poly(4-
vinylpyridine) catalysts I and III were more selective
than their corresponding N-oxide derivatives II and IV
(Table 6, entries 2 and 4 vs entries 3 and 5), catalyst I
being the best reactive system.
6. For some recent examples of catalytic oxidation in ionic
liquids: (a) Kuhn, F. E.; Zhao, J.; Abrantes, M.; Sun, W.;
¨
Afonso, C. A. M.; Branco, L. C.; Gonc¸alves, I. S.;
Pillinger, M.; Roma˜o, C. C. Tetrahedron Lett. 2005, 46,
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47–52; (b) Valente, A. A.; Petrovski, Z; Branco, L. C.;
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C.; Nunes, C. D.; Gonc¸alves, I. S. J. Mol. Catal. A: Chem.
2004, 218, 5–11; (c) Li, Z.; Xia, C.-G. J. Mol. Catal. A:
Chem. 2004, 214, 95–101; (d) Tong, K.-H.; Wong, K.-Y.;
Chan, T. H. Org. Lett. 2003, 5, 3423–3425; (e) Peng, J.;
Shi, F.; Gu, Y.; Deng, Y. Green Chem. 2003, 5, 224–226;
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In conclusion, various hydrocarbons can be efficiently
oxidized to corresponding ketones or alcohols by
MTO and heterogeneous MTO catalyst systems I–V in
ionic liquids using H2O2 as environment friendly oxi-
dant. The efficiency and selectivity of catalysts I–V were
sensitive to the nature of the polymeric support, poly(4-
vinylpyridine) and polystyrene affording the best perform-
ances. In several cases the activity of catalysts I–V in io-
nic liquids was greater than that observed in molecular
solvents. Moreover, in the oxidation of adamantane cata-
lysts I, III and V were more reactive and selective than
parent MTO. While MTO was not entirely retained in
ionic liquids during extraction of the reaction mixtures
with diethyl ether, catalyst V was easily recycled and
used for successive transformations with similar selecti-
vity and reactivity. Future work is therefore in progress,
in order to exploit further on this efficient and environ-
ment respectful procedure.
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¨
Scherbaum, A.; Herrmann, W. A. J. Organomet. Chem.
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Tetrahedron Lett. 2003, 44, 8991–8994; (b) In our hands,
an extraction yield major than 85% of MTO from
[EMIM][BF4] was measured using diethyl ether as mobile
phase, with a concomitant dramatic decrease of the
catalytic activity.
12. Bianchini, G.; Crucianelli, M.; De Angelis, F.; Neri, V.;
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Acknowledgements
MIUR (PRIN COFIN 2003) ÔLa catalisi dei metalli di
transizione nello sviluppo di strategie sintetiche innova-
tive di eterocicliÕ is acknowledged for financial support.
References and notes
15. Catalysts I–V were prepared as reported in Ref. 14. In
summary, MTO (256 mg, 1.0 mmol) was added to a
suspension of the appropriate resin (1.0 g) in ethanol
(4 mL). The mixture was stirred during 1 h by a magnetic
stirring. The solvent was removed by filtration, and the
catalyst was washed with ethyl acetate and finally dried
under high vacuum.
16. The reaction mixtures were analyzed by a Hewlett Packard
6890 Series gas chromatograph equipped with a FID,
using a 30 m · 0.32 mm · 0.25 lm film thickness (cross-
linked 5% phenylmethylsiloxane) column and nitrogen as
carrier gas. The identification of the peaks by GC–MS has
been performed by means of a Varian 2000 GC–MS
instrument, using the same column. Yields and conver-
sions of the reactions have been quantified using n-octane
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