Oxidation of aromatic aldehydes and ketones by H2O 2/CH3ReO3 in ionic liquids: A catalytic efficient reaction to achieve dihydric phenols

Article abstract of DOI:10.1016/j.tet.2004.12.025

A convenient and efficient application of hydrogen peroxide/ methyltrioxorhenium in ionic liquids [bmim]BF₄ and [bmim]PF ₆ for the oxidation of hydroxylated and methoxylated benzaldehydes and acetophenones to the corresponding phenols is described. Good yields of products were obtained in short reaction times.

Full text of DOI:10.1016/j.tet.2004.12.025

Tetrahedron 61 (2005) 1821–1825
Oxidation of aromatic aldehydes and ketones by
H2O2/CH3ReO3 in ionic liquids: a catalytic efficient reaction to
achieve dihydric phenols
a
a
b
Roberta Bernini,a, Antonietta Coratti, Gianfranco Provenzano, Giancarlo Fabrizi
*
and Daniela Tofanic
a
`
Dipartimento A.B.A.C, Universita degli Studi della Tuscia, Via S. Camillo De Lellis, 01100 Viterbo, Italy
b
`
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Universita degli Studi di Roma “La Sapienza”,
P.le A. Moro 5, 00185 Roma, Italy
c
`
Dipartimento di Ingegneria Meccanica e Industriale, Universita “Roma Tre”, Via della Vasca Navale 79, 00146 Roma, Italy
Received 5 August 2004; revised 17 November 2004; accepted 9 December 2004
Available online 25 December 2004
Abstract—A convenient and efficient application of hydrogen peroxide/methyltrioxorhenium in ionic liquids [bmim]BF4 and [bmim]PF6 for
the oxidation of hydroxylated and methoxylated benzaldehydes and acetophenones to the corresponding phenols is described. Good yields of
products were obtained in short reaction times.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
peroxide/selenium compounds,11 sodium perborate and
sodium percarbonate,12 m-chloroperbenzoic acid13 were
other useful reagents to convert aromatic aldehydes to the
corresponding phenols, but these procedures required long
reactions time, in some cases up to 30 h. Recently, Sn-Beta
zeolites were used to activate hydrogen peroxide14 provid-
ing an interesting example of good reactivity of aromatic
aldehydes in short reaction times.
The dihydric phenols such as catechol, hydroquinone and
their methyl ethers, are high value chemicals. They are
widely used as agrochemicals, antioxidants, pharmaceuti-
cals, flavoring agents, polymerization inhibitors. For
example, catechol is the basic chemical for the synthesis
of the 4-tert-butylcatechol, an industrial antioxidant for
foods and cosmetics1 and for the synthesis of the
pharmaceutical adrenalone;2 guaiacol is used as good
starting material for the industrial synthesis of vanillin, a
prime flavor molecule;3 hydroquinone is largely employed
in photographic processes.4
In recent years, methyltrioxorhenium (CH3ReO3, MTO)15
has been shown to possess interesting catalytic properties in
oxidation reactions with hydrogen peroxide as oxygen atom
donor.16 The reactive intermediate is a bis-peroxo metal
[CH3ReO(O2)2] complexes dpRe (Fig. 1) whose structure
was established by X-ray analysis.17 Recently, an efficient
oxidation of hydroxylated and methoxylated benzaldeydes
with hydrogen peroxide/methyltrioxorhenium in conven-
tional solvents was described: under these conditions
between 16 and 24 h were necessary to achieve complete
oxidation.18
Classical synthesis of this kind of phenols can be performed
by catalytic oxidation of aromatic hydrocarbons,5 hydroxyl-
ation of phenols,6 oxidative decarboxylation of aryl
carboxylic acids,7 hydrolysis of halobenzenes.8 Alterna-
tively, the oxidation of benzaldeydes and acetophenones
with alkaline hydrogen peroxide was an easy synthetic
pathway (Dakin reaction).9 However, under these con-
ditions, reactions were too slow to be commercially
attractive. Hydrogen peroxide/boric acid,10 hydrogen
Keywords: Oxidation; Hydrogen peroxide/methyltrioxorhenium; Ionic
liquids; Benzaldehydes; Acetophenones; Dihydric phenols.
* Corresponding author. Tel.: C39 0761357230; fax: C39 0761357230;
e-mail: berninir@unitus.it
Figure 1. Catalytic system H2O2/CH3ReO3.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.12.025

1822
R. Bernini et al. / Tetrahedron 61 (2005) 1821–1825
During recent years, the ionic liquids [bmim]BF4 and
[bmim]PF6, where [bmim]C is the 1-butyl-3-methyl-
imidazolium cation, were used as substitutes for molecular
solvents in catalytic reactions (Fig. 2).19 These kind of
liquids have several interesting and benign properties: they
show low volatility, chemical, physical and thermal stability
and can furthermore be recycled and reused.20 Moreover,
catalytic oxidations proceed faster than in conventional
solvents.21 For example, hydrogen peroxide/boric acid in
[bmim]PF6 converted aromatic aldehydes into phenols very
quickly (1–2 h)22 and the m-chloroperbenzoic acid
promoted Baeyer–Villiger oxidation of simple ketones and
benzaldehydes was achieved in 2–4 h.23
conversion of substituted aromatic aldehydes and ketones
in [bmim]BF4 and [bmim]PF6 to the corresponding phenols.
Good yields of products in short reaction times and under
controlled conditions were obtained.
2. Results and discussion
As a model substrate, for the initial investigations, we chose
4-methoxybenzaldeyde 1 (Scheme 1). Our results are
summarized in Table 1. Both in [bmim]BF4 and [bmim]PF6,
the oxidation to 4-methoxyphenol 2 are faster at 50 8C and
better in terms of yield and selectivity (entries 3, 4), than at
25 8C (entries 1, 2); small amount of hydroquinone 3 and
4-methoxybenzoic acid 4 were also isolated. Our results
also showed that the nature of the anion of the ionic liquid
was not important for the reactivity of the substrate and the
reaction times. When the experiments were carried out
under identical conditions but omitting the catalyst, we
found that 4-methoxyphenol 2 was formed only in traces
(!5%). Experimentally, all the reactions were carried out at
50 8C for 4 h, the products were then selectively and
quantitatively extracted from the ionic liquid solution by
several extractions with small portions of diethyl ether. The
second run was performed by adding fresh substrate and
hydrogen peroxide to the ionic liquid solution: the catalytic
system was still stable and efficient for five successive
recycling experiments as reported in Table 2. However as
general trend, we found a small increase in the yield of the
4-methoxybenzoic acid 4, most probably as a consequence
of added water.
Figure 2. Ionic liquids [bmim]BF4 and [bmim]PF6.
By continuing our studies into oxidations using the catalytic
system hydrogen peroxide/methyltrioxorhenium in ionic
liquids,24 we report now the results on the oxidative
Good results were obtained with activated benzaldehydes
such as 5, 7, 11 and 13. The corresponding phenols 3, 8, 9,
14 were obtained in good yields (Scheme 2, Table 3, entries
1–8). Nevertheless, benzaldehydes with meta-substituted
electron-donating groups 16, 19 and unsubstitued benzal-
dehyde 22, failed to oxidize to the Dakin products: in these
Scheme 1. Oxidative conversion of 4-methoxybenzaldehyde 1.
a
Table 1. Oxidation of 4-methoxybenzaldeyde 1 with hydrogen peroxide/methyltrioxorhenium in [bmim]BF4 and [bmim]PF6
Entry
Ionic liquid
Conv. (%)
Yield (%) of 2
Yield (%) of 3
Yield (%) of 4
1b
2b
3c
4c
[bmim]BF4
[bmim]PF6
[bmim]BF4
[bmim]PF6
20
18
O98
O98
95
98
77
75
—
—
10
10
—
—
13
15
a Conversions and yields were determined after chromatographic purification of reaction mixtures.
b H2O2 (35% water solution): 8 equiv; CH3ReO3 (2%), 25 8C, 24 h.
c H2O2 (35% water solution): 4 equiv; CH3ReO3 (2%), 50 8C, 4 h.
Table 2. Oxidation of 4-methoxybenzaldeyde 1 with H2O2/CH3ReO3 in [bmim]BF4 and in [bmim]PF6 as a recyclable systema
Run no.b
Conv. (%)c
Yield (%) of
2c
Yield (%) of
3c
Yield (%) of
4c
Conv. (%)d
Yield (%) of
2d
Yield (%) of
3d
Yield (%) of
4d
1
2
3
4
5
6
O98
95
90
88
84
77
76
77
60
64
68
10
13
11
16
14
11
13
11
12
24
22
22
O98
95
88
85
80
75
75
73
64
60
56
10
10
8
11
12
14
15
15
19
25
28
30
66
68
a Yields and conversions were calculated after chromatographic purification of the reaction mixture.
b After each run, the successive one was performed adding only fresh substrate and oxidant to the ionic liquid solution under the same experimental conditions.
c Data in [bmim]BF4.
d Data in [bmim]PF6.

R. Bernini et al. / Tetrahedron 61 (2005) 1821–1825
1823
Scheme 2. Oxidation of benzaldehydes with hydrogen peroxide/methyltrioxorhenium in [bmim]BF4 and [bmim]PF6.
Table 3. Experimental data of oxidations depicted in Scheme 3a
Entry
Substrate
Phenols
Benzoic acid Conditionsb
Conv. (%)
Yield (%) of Yield (%) of
phenols
benzoic acid
1
2
3
4
5
6
7
8
5
5
7
3
3
8 (9)
8 (9)
9
6
6
[bmim]BF4, H2O2 (2 equiv), 50 8C, 2 h
O98
95
O98
O98
96
90
90
90
O98
O98
95
95
95
95
95
5
5
15
15
16
13
20
22
O98
O98
95
98
98
[bmim]PF6, H2O2 (2 equiv), 50 8C, 2 h
[bmim]BF4, H2O2 (4 equiv), 50 8C, 4 h
[bmim]PF6, H2O2 (4 equiv), 50 8C, 4 h
[bmim]BF4, H2O2 (4 equiv), 50 8C, 6 h
[bmim]PF6, H2O2 (4 equiv), 50 8C, 6 h
[bmim]BF4, H2O2 (4 equiv), 50 8C, 4 h
[bmim]PF6, H2O2 (4 equiv), 50 8C, 4 h
[bmim]BF4, H2O2 (4 equiv), 50 8C, 4 h
[bmim]PF6, H2O2 (4 equiv), 50 8C, 2 h
[bmim]BF4, H2O2 (10 equiv), 50 8C, 24 h
[bmim]PF6, H2O2 (10 equiv), 50 8C, 24 h
[bmim]BF4, H2O2 (10 equiv), 50 8C, 24 h
[bmim]PF6, H2O2 (10 equiv), 50 8C, 24 h
10
10
12
12
15
15
18
18
21
21
24
24
75 (10)c
75 (10)c
84
7
11
11
13
13
16
16
19
19
22
22
9
87
80
78
—
—
—
—
—
14
14
17
17
20
20
23
23
9
10
11
12
13
14
95
—
98
a Yields and conversions were calculated after chromatographic purification of the reaction mixtures.
b CH3ReO3 (2%).
c In parentheses were reported the yields of phenol 9.
cases the main products were the corresponding benzoic
acids 18, 21, 24 (Scheme 2, Table 3, entries 9–14).
alkaline work-up. Benzoic acids 4, 6, 10 and 12 were
isolated in low yields.
Besides aromatic aldehydes, we tested the efficiency of the
hydrogen peroxide/methyltrioxorhenium catalytic system in
the conversion of the aromatic ketones 25, 27, 29 and 31 to
the corresponding phenols (Scheme 3). Our results showed
lower reactivity at 50 8C for the acetophenones used: we
found that by raising the temperature up to 80 8C, phenols 2,
3, 8 and 9 can be obtained after 3–5 h (Table 4) in good to
high yields. The esters 26, 28, 30, 32, obtained as secondary
products, can be easily converted into phenols by simple
3. Conclusions
C-2 and C-4 hydroxylated or methoxylated benzaldehydes
and acetophenones were converted to the corresponding
phenols in good yields and short reaction times, with the
hydrogen peroxide/methyltrioxorhenium catalytic system,
used in ionic liquids. After extraction of the products, the
ionic liquid solution still showed catalytic activity. Work is
Scheme 3. Oxidative conversion of substituted acetophenones with hydrogen peroxide/methyltrioxorhenium in [bmim]BF4 and [bmim]PF6.

1824
R. Bernini et al. / Tetrahedron 61 (2005) 1821–1825
a
Table 4. Oxidation of substituted acetophenones with hydrogen peroxide/methyltrioxorhenium in [bmim]BF4 and [bmim]PF6
Entry
Substrate
Ester
Phenols
Conditionsb
Conv. (%)
Yield (%)
of phenols
Yield (%)
of the ester of the ben-
zoic acid
Yield (%)
1
2
3
4
5
6
7
8
25
25
27
27
29
29
31
31
26
26
28
28
30
30
32
32
2 (3)
2 (3)
[bmim]BF4, H2O2 (8 equiv), 80 8C, 5 h
[bmim]PF6, H2O2 (3 equiv), 80 8C, 3 h
[bmim]BF4, H2O2 (8 equiv), 80 8C, 5 h
[bmim]PF6, H2O2 (3 equiv), 80 8C, 3 h
[bmim]BF4, H2O2 (8 equiv), 80 8C, 5 h
[bmim]PF6, H2O2 (3 equiv), 80 8C, 3 h
[bmim]BF4, H2O2 (8 equiv), 80 8C, 5 h
[bmim]PF6, H2O2 (6 equiv), 80 8C, 5 h
90
96
98
91
87
95
95
90
76 (10)c
11
3
3
19
—
—
15
29
—
—
56 (22)c
3
3
8
8
9
9
90
O98
55
71
47
10
—
30
—
53
55
45
a Yields and conversions were calculated after chromatographic purification of the reaction mixtures.
b H2O2 (35% water solution); CH3ReO3 (2%).
c In parentheses were reported the yield of phenol 3.
in progress to test the chemoselectivity of these conditions
in more complex molecules, that possess other oxidizable
groups.
OCH3), 6.76 (4H, d, JZ1.6 Hz, Ar-H); dC (CDCl3,
200 MHz): 55.9, 115.0, 116.2, 149.6, 153.4; m/z (EI) 124
(MC, 80.1%).
4.2.2. Hydroquinone (3). White solid, mp 169–171 8C,
(lit.,10 170–171 8C). dH (DMSO-d6, 200 MHz): 5.69 (2H, s,
Ar-H), 7.73 (2H, s, Ar-H); dC (DMSO-d6, 200 MHz): 115.8,
149.8; m/z (EI) 110 (MC, 100%).
4. Experimental
All commercial products were of the highest grade available
and were used without further purifications. Hydrogen
peroxide was 35% aqueous solution (Aldrich). Synthesis of
[bmim]BF4 and of [bmim]PF6 were carried out according to
Ref. 25. Thin layer chromatography was carried out using
Merck silica gel 60F-254 plates with UV indicator. Reaction
products were purified by flash chromatography on columns
packed with silica gel, 230–400 mesh. NMR spectra were
recorded on a Bruker AC 200 spectrometer and are reported
in d values. Mass spectra were recorded on a VG 70/250S
spectrometer with an electron beam of 70 eV. Gas
chromatography-mass spectroscopy of the reaction products
were performed using a SPB column (25 m!0.30 mm and
0.25 mm film thickness) and an isothermal temperature
profile of 60 8C for the first 2 min, followed by a 10 8C/min
temperature gradient to 280 8C for 15 min. The injector
temperature was 280 8C. Chromatography grade helium was
used as the carrier gas. Melting points were determined with
a Bu¨chi apparatus and are uncorrected.
4.2.3. 4-Methoxybenzoic acid (4). White solid, mp 178–
180 8C, (lit.,26 179–181 8C). dH (CDCl3/CD3OD, 200 MHz):
3.79 (3H, s, OCH3), 6.85 (2H, d, JZ14.5 Hz, Ar-H), 7.94
(2H, d, JZ14.5 Hz, Ar-H); dC (CDCl3/CD3OD, 200 MHz):
55.2, 114.6, 131.8, 163.3, 168.7; m/z (EI) 152 (MC, 85.3%).
4.2.4. 2-Methoxyphenol (8). Liquid dH (CDCl3, 200 MHz):
3.86 (3H, s, OCH3), 6.85–7.02 (4H, m, Ar-H); dC (CDCl3,
200 MHz): 55.8, 110.8, 114.6, 120.2, 121.4, 145.7, 146.6;
m/z (EI) 124 (MC, 68.8%).
4.2.5. Catechol (9). White solid, mp 102–104 8C, (lit.,10
103–104 8C). dH (CDCl3/CD3OD, 200 MHz): 6.45–6.51
(2H, m, Ar-H), 6.55–6.64 (2H, m, Ar-H); dC (CDCl3/
CD3OD 200 MHz): 114.9, 119.7, 144.2; m/z (EI) 110 (MC,
100%).
4.2.6. 2-Methoxybenzoic acid (10). White solid, mp 183–
184 8C, (lit.,27 184–185 8C). dH (CDCl3, 200 MHz): 3.98
(3H, s, OCH3), 6.73–7.11 (2H, m, Ar-H), 7.49 (1H, t, JZ
8.5 Hz, Ar-H), 8.02 (1H, d, JZ7.8 Hz, Ar-H); dC (CDCl3,
200 MHz): 56.6, 111.8, 117.5, 121.8, 133.4, 135.1, 158.3,
166.3; m/z (EI) 152 (MC, 35.3%).
4.1. General procedure for the oxidation of benzaldeydes
and acetophenones with H2O2/CH3ReO3 in ionic liquids
The substrate (1.0 mmol) was dissolved in [bmim]BF4 or
[bmim]PF6 (1 ml). Then, hydrogen peroxide (35% aqueous
solution, 2–8 equiv) and CH3ReO3 (0.02 mmol) were
added. Reactions were monitored by thin layer chromato-
graphy and by chromatography-mass spectroscopy. After
extraction with diethyl ether and evaporation of the organic
solvent, the products were purified by flash chromatography
using dichloromethane as eluent. Their identity was
confirmed by 1H, 13C NMR and GC-MS analyses,
comparing the experimental data with those of authentic
compounds.
4.2.7. 2-Hydroxybenzoic acid (12). White solid, mp 159–
161 8C, (lit.,27 158–160 8C). dH (CDCl3/DMSO-d6,
200 MHz): 6.80–6.95 (2H, m, Ar-H), 7.45–7.55 (1H, m,
Ar-H), 7.82 (1H, d, JZ9.0 Hz, Ar-H); dC (CDCl3/DMSO-
d6, 200 MHz): 112.7, 116.9, 118.7, 130.2, 135.2, 161.5,
172.1; m/z (EI) 138 (MC, 40.5%).
4.2.8. 3,4-Dimethoxyphenol (14). White solid, mp 78–
80 8C (lit.,10 79 8C). dH (CDCl3, 200 MHz) 3.75 (3H, s,
OCH3), 3.77 (3H, s, OCH3); 6.32 (1H, dd, JZ2.7, 8.5 Hz,
Ar-H), 6.44 (1H, d, JZ2.7 Hz, Ar-H), 6.68 (1H, d, JZ
8.5 Hz, Ar-H); dC (CDCl3, 200 MHz): 55.7, 56.5, 100.6,
105.8, 112.5, 142.9, 149.8, 150.2; m/z (EI) 154 (MC,
63.2%).
4.2. Hydrolysis of esters 27, 29, 31 and 30
Alkaline hydrolysis were carried out according to Ref. 11.
4.2.1. 4-Methoxyphenol (2). White solid, mp 55–57 8C,
(lit.,10 56–57 8C). dH (CDCl3, 200 MHz): 3.75 (3H, s,

R. Bernini et al. / Tetrahedron 61 (2005) 1821–1825
1825
7. Bujis, W.; Frijns, L. H. B.; Offermans, M. R. J.; EP Patent
434140, 1991.
4.2.9. 3,4-Dimethoxybenzoic acid (15). White solid, mp
179–182 8C, (lit.,27 180–182 8C). dH (CDCl3, 200 MHz):
3.91 (3H, s, OCH3); 3.92 (3H, s, OCH3); 6.88 (1H, d, JZ
8.4 Hz, Ar-H), 7.56 (1H, s, Ar-H), 7.74 (1H, dd, JZ1.6,
8.4 Hz, Ar-H); dC (CDCl3, 200 MHz): 55.9, 56.0, 110.2,
112.3, 124.5, 148.6, 153.6, 165.4, 171.5; m/z (EI) 182 (MC,
100%).
8. Furuya, M.; Nakajima, H.; JP Patent 050070391, 1993.
Furuya, M.; Nakajima, H.; JP Patent 03206060, 1991.
9. (a) Dakin, H. D. Am. Chem. J. 1909, 42, 477–499. (b) Dakin,
H. D. Org. Synth. 1933, 149–154. (c) Hocking, M. B. Can.
J. Chem. 1973, 51, 2384–2392. (d) Hocking, M. B.; Bhandari,
K.; Shell, B.; Smyth, T. A. J. Org. Chem. 1982, 47,
4208–4215.
4.2.10. 3-Methoxybenzoic acid (18). White solid, mp 104–
105 8C (lit.,10 105–107 8C). dH (CDCl3, 200 MHz): 3.83
(3H, s, OCH3); 7.14–7.17 (1H, dd, JZ8.3, 0.9 Hz, Ar-H),
7.37 (1H, t, JZ8.0 Hz, Ar-H), 7.71 (1H, dd, JZ7.8, 1.2 Hz,
Ar-H); dC (CDCl3, 200 MHz): 55.3, 114.3, 120.3, 122.6,
129.4, 130.5, 159.5, 172.1; m/z (EI) 152 (MC, 100%).
10. Roy, A.; Reddy, K. R.; Mohanta, P. K.; Ila, H.; Junjappa, H.
Synth. Commun. 1999, 29, 3781–3791.
11. Syper, L. Synthesis 1989, 167–172.
12. (a) Kabalka, G. W.; Reddy, N. K.; Narayana, C. Tetrahedron
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Tetrahedron 1995, 51, 6145–6166.
4.2.11. 3-Hydroxybenzoic acid (21). White solid, mp 202–
204 8C (lit.,27 202–203 8C). dH (CDCl3/CD3OD, 200 MHz):
6.93 (1H, dd, JZ8.1, 2.5 Hz, Ar-H), 7.37–7.46 (1H, t, JZ
7.8 Hz, Ar-H), 7.37–7.46 (2H, m, Ar-H); dC (CDCl3,
200 MHz): 116.1, 120.2, 121.4, 129.2, 131.2, 156.6,
169.2; m/z (EI) 138 (MC, 100%).
13. (a) Godfrey, I. M.; Sargent, M. V.; Elix, J. A. J. Chem. Soc.,
Perkin Trans. 1 1974, 1353. (b) Hannan, R. L.; Barber, R. B.;
Rapoport, H. J. Org. Chem. 1979, 44, 2152. (c) Franck, R. W.;
Gupta, R. M. J. Org. Chem. 1985, 50, 1984–1985.
14. Corma, A.; Fornes, V.; Iborra, S.; Mifsud, M.; Renz, M.
J. Catal. 2004, 221, 67–76.
15. Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 1638–1641.
4.2.12. Benzoic acid (24). White solid, mp 121–123 8C
(lit.,10 123–124 8C). dH (CDCl3, 200 MHz): 7.45–7.50 (2H,
m, Ar-H), 7.59–7.64 (1H, m, Ar-H), 8.13 (2H, d, JZ7.5 Hz,
Ar-H); dC (CDCl3, 200 MHz): 128.4, 130.2, 133.8, 172.5;
m/z (EI) 122 (MC, 64.7%).
16. Kuhn, F. E.; Groarke, M. In Herrmann, W. A., Cornils, B.,
Eds.; Applied Homogeneous Catalysis with Organometallic
Compounds; Wiley-VCH: New York, 2002; Vol. 3,
pp 1304–1318.
17. Herrmann, W. A.; Fischer, R. W.; Scherer, W.; Rauch, M. U.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1157–1160.
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Acknowledgements
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We are sincerely grateful to Italian ‘Consorzio Inter-
universitario Nazionale “La Chimie per L’Ambiente”
INCA for its financial support.
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