Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin with oxygen catalyzed by a combination of CoCl2 and N-hydroxyphthalimide

Article abstract of DOI:10.1007/s11164-013-1039-x

Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin (4-hydroxy-3-methoxybenzaldehyde), in methanol, with molecular oxygen at atmospheric pressure as oxidant and a combination of cobaltous chloride and N-hydroxyphthalimide (NHPI) as catalyst, has been investigated. The effect of reaction conditions on conversion and selectivity for vanillin was studied systematically. Selectivity for vanillin could be enhanced by optimizing the molar ratio of 2-methoxy-p-cresol to NHPI, the amount of sodium hydroxide, reaction time, reaction temperature, and the volume of methanol, which determined the concentration of the reactants. Under the optimized conditions the yield of vanillin was 90.1 %. Springer Science+Business Media Dordrecht 2013.

Full text of DOI:10.1007/s11164-013-1039-x

Res Chem Intermed
DOI 10.1007/s11164-013-1039-x
Liquid-phase oxidation of 2-methoxy-p-cresol
to vanillin with oxygen catalyzed by a combination
of CoCl and N-hydroxyphthalimide
2
Yuecheng Zhang
Jiquan Zhao
•
Xiujuan Li
•
Xiaohui Cao
•
Received: 19 December 2012 / Accepted: 8 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin (4-hydroxy-3-
methoxybenzaldehyde), in methanol, with molecular oxygen at atmospheric pres-
sure as oxidant and a combination of cobaltous chloride and N-hydroxyphthalimide
(
NHPI) as catalyst, has been investigated. The effect of reaction conditions on
conversion and selectivity for vanillin was studied systematically. Selectivity for
vanillin could be enhanced by optimizing the molar ratio of 2-methoxy-p-cresol to
NHPI, the amount of sodium hydroxide, reaction time, reaction temperature, and the
volume of methanol, which determined the concentration of the reactants. Under the
optimized conditions the yield of vanillin was 90.1 %.
Keywords 2-Methoxy-p-cresol ꢀ Vanillin ꢀ Oxidation ꢀ Molecular oxygen ꢀ
Cobaltous chloride ꢀ N-hydroxyphthalimide
Abbreviations
NHPI N-hydroxyphthalimide
Introduction
Vanillin (4-hydroxy-3-methoxybenzaldehyde) is an important material extensively
used in foods, cosmetics, tobacco, pharmaceuticals, and many others [1, 2].
Although vanillin as a natural product can be obtained by cultivation of vanilla
plants, most of that used commercially is produced by chemical synthesis, because
of its low price. Vanillin is synthesized worldwide from many starting materials
including o-methoxyphenol [3–5], lignin [6–9], eugenol or isoeugenol [10, 11],
p-cresol [12], and 4-hydroxybenzaldehyde [13]. From an industrial perspective the
Y. Zhang ꢀ X. Li ꢀ X. Cao ꢀ J. Zhao (&)
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130,
People’s Republic of China
e-mail: zhaojq@hebut.edu.cn
123

Y. Zhang et al.
methods above suffer from one or several disadvantages including long reaction
routes and complex production processes, scarce raw materials, low yield,
expensive reagents, and serious pollution; these processes are, therefore, inefficient
and cause serious effluent problems.
2
-Methoxy-p-cresol, a natural product which can be extracted from creosote, is
inexpensive and readily available. Therefore, use of 2-methoxy-p-cresol as a
starting material to produce vanillin by one-step oxidation not only satisfies
increasing demand for natural flavors but also has the advantages of low effluent
disposal, good overall process economics, and high-quality product. Both
environmentally and economically, use of molecular oxygen as oxidant is the
best choice, because of its ready availability and water as the only by-product of
the reaction. Oxidation of 2-methoxy-p-cresol to vanillin with molecular oxygen as
oxidant has been studied [12, 14]. The process reported by Nishizawa et al. [14]
was the first method of oxidation of 2-methoxy-p-cresol to vanillin with molecular
oxygen as oxidant and soluble cobaltous chloride as catalyst; the yield of vanillin
was 60 %. More recently, Chandalia and colleagues [12] used cobaltous chloride
as catalyst to synthesize vanillin with air as an oxidant, but the yield of vanillin
was lower than 60 %. Thereafter, several attempts were made to increase the yield
of vanillin by improving the catalyst. The methodology is usually to introduce a
second transition metal, for example Mn, Ni, Cu, or Fe, as cocatalyst with
cobaltous chloride or oxide, to increase the selectivity for vanillin [15–19]. Some
Schiff-base transition metal complexes have also been used as catalysts for
oxidation of 2-methoxy-p-cresol to vanillin [20]. Although some improvement of
the yield of vanillin has been achieved, high pressure or high temperature was
needed, which resulted in the formation of hyperoxidation by-products in the
reaction.
Ishii et al. [21] have successfully developed a strategy for generation of carbon
radicals from hydrocarbons by use of N-hydroxyphthalimide (NHPI) as catalyst and
oxygen as oxidant, in the presence or absence of a cobalt species, under mild
conditions. This method can successfully convert alkanes into alcohols, ketones, and
carboxylic acids. For example, oxidation of toluene by molecular oxygen at normal
pressure and room temperature in acetic acid is achieved by the use of NHPI and
Co(II) species as combined catalyst. However, achieving high selectivity for
aldehydes is difficult, because of their reactivity [22, 23]. Recently, direct oxidation
of aromatic alcohols and methyl aromatic compounds to aldehydes with molecular
oxygen catalyzed by NHPI in the presence or absence of transition metals has been
reported [24, 25]. The work indicated it was necessary to conduct the reaction in
neutral or alkaline medium to achieve high selectivity for aldehydes. This suggests
the combination of NHPI and cobaltous chloride as catalyst may enable efficient
oxidation of 2-methoxy-p-cresol to vanillin in alkaline medium. Preliminary
experiments confirmed the hypothesis was plausible. Therefore, the effects of
different reaction conditions, for example reaction time, temperature, catalyst
loading, solvent volume, and quantity of base on the conversion of 2-methoxy-p-
cresol and selectivity for vanillin were investigated. Herein, we report the results of
the investigation.
1
23

Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin,
Materials and methods
Materials and apparatus
NHPI was purchased from Acros Organics. 2-Methoxy-p-cresol, cobaltous chloride
(
CoCl ꢀ6H O), sodium hydroxide (NaOH), and HPLC-grade methanol, acetonitrile,
2
2
and phosphoric acid were obtained from Tianjin Fuchen Chemical Reagent Factory,
China. All the chemicals were used as received. HPLC was performed with a
Shimadzu (Kyoto Japan) SPD-10ATvp plus chromatograph equipped with an SPD-
10Avp detector and C18 reversed-phase column (4.6 mm i.d. 9 250 mm). The
mobile phase was 25 % acetonitrile–water containing 0.1 % phosphoric acid.
Catalytic procedure
Oxidation reactions were conducted in a 10-ml two-necked flask equipped with a
condenser and a thermometer. A balloon filled with oxygen was connected to the
top of the condenser. All reactions were conducted at atmospheric pressure and
stirred with an electromagnetic stirrer at a constant rate.
In a typical process, a mixture of NaOH (21.7 mmol) and methanol (3.0 ml) was
introduced into the flask. The mixture was vigorously stirred at room temperature.
When the NaOH had dissolved completely, 2-methoxy-p-cresol (3.6 mmol) was
introduced and the mixture was stirred for half an hour to ensure the substrate had
been converted into its sodium salt. Cobaltous chloride and NHPI were then added
quickly. Chilled water was circulated through the condenser to minimize loss of the
solvent. The mixture was heated to the desired temperature (65 °C), then the oxygen
from the balloon was introduced, and controlled by use of a triple valve. After a
specific time, introduction of oxygen was stopped and the reaction mixture was left
to cool to room temperature. The cooled reaction mixture was analyzed as described
below.
Analysis of the reaction mixture
After reaction the whole reaction mixture was accurately weighed to obtain the
total weight (w ). An accurately weighed amount (approx. 0.1 g) of the reaction
t
mixture (w ) was placed in a 10-ml volumetric flask, then an appropriate amount
0
of concentrated hydrochloric acid was added to neutralize the NaOH and the
sodium salt of the substrate and product. The neutralized mixture was dissolved
in mobile phase to a constant volume. The amounts of 2-methoxy-p-cresol and
vanillin (ws0 and w ) were determined by HPLC with use of an external
v0
standard.
The weight of vanillin in the reaction mixture can be obtained by use of Eq. 1:
wv ¼ ðwt ꢁ wv0Þ= w0ðgÞ
The weight of 2-methoxy-p-cresol left in the reaction mixture can be obtained by
ð1Þ
use of Eq. 2:
123

Y. Zhang et al.
ws ¼ ðwt ꢁ ws0Þ= w0ðgÞ
moles of 2-methoxy-p-cresol reduced
ð2Þ
Conversion(% ) ¼
Selectivity(% ) ¼
ꢁ 100 %
ꢁ 100 %
ð3Þ
moles of 2-methoxy-p-cresol charged
moles of vanillin obtained
ð4Þ
ð5Þ
moles of 2-methoxy-p-cresol reduced
Yield ¼ Conversion ꢁ Selectivity
Results and discussion
A few preliminary experiments showed the reaction proceeded readily when
catalyzed by a combination of cobaltous chloride and NHPI in the presence of
NaOH in methanol under atmospheric oxygen pressure at elevated temperature. The
selectivity for vanillin was lower than 70 %, because of the formation of side
products, for example 3-methoxy-4-hydroxybenzyl alcohol, 3-methoxy-4-hydroxy-
benzyl methyl ether, 3-methoxy-4-hydroxybenzoic acid, and tarry material. To
obtain vanillin with high selectivity the effects of reaction time, amounts of NHPI
and NaOH, reaction temperature, and solvent volume were studied.
Effect of reaction time
First, the effect of reaction time on the reaction was investigated at 50 °C; the
results are shown in Table 1. It can be seen that conversion of 2-methoxy-p-cresol
increased with time, as expected, and was[99.0 % after 6 h. Selectivity for vanillin
also increased with time, initially, reaching a maximum after reaction for 7 h, after
which it decreased gradually with time. GC–MS analysis indicated the main product
was vanillin, with 3-methoxy-4-hydroxybenzyl alcohol as major by-product and
other, minor, by-products including 3-methoxy-4-hydroxybenzoic acid and
3-methoxy-4-hydroxybenzyl methyl ether (Fig. 1). In fact, much work has focused
on oxidation of p-cresol to p-hydroxybenzaldehydes with oxygen similar to that of
Table 1 Effect of reaction time on the oxidation of 2-methoxy-p-cresol
Entry
Time (h)
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
5
6
2
4
6
7
8
9
60.0
85.0
50.0
58.0
66.7
72.2
68.6
64.9
30.0
49.3
66.7
72.2
68.6
64.9
[99.0
[99.0
[99.0
[99.0
Reaction conditions: 2-methoxy-p-cresol 3.623 mmo, molar ratio of sodium hydroxide to 2-methoxy-p-
cresol 5.7:1, cobaltous chloride 0.017 mmol, NHPI 0.072 mmol, reaction temperature 50 °C, methanol
5
ml, atmospheric oxygen pressure
1
23

Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin,
CH OCH3
2
CH OH
3
OCH3
OH
CH3
CH OH
CHO
CO H
2
2
CoCl , NHPI
2
O2
O2
OCH3
+
O2
NaOH
OCH3
OCH3
OCH3
OH
OH
OH
OH
NaOH
disproportionation
Fig. 1 Pathways of formation of vanillin and by-products
2
-methoxy-p-cresol to vanillin [26–28]. On the basis of the results obtained by
GC–MS analysis and reported in literature we can infer the formation pathways of
vanillin and by-products (Fig. 1).
From the pathways it is obvious that 2-methoxy-p-cresol was first oxidized to
3
-methoxy-4-hydroxybenzyl alcohol, a key intermediate in the reaction. This
intermediate was then transformed to vanillin with time, which increased the
selectivity for vanillin. With the reaction time was extended further, some vanillin
was converted into to 3-methoxy-4-hydroxybenzoic acid by over oxidation or
disproportionation (Fig. 1). As a result, selectivity for vanillin decreased.
Effect of NHPI loading
The effect of amount of NHPI on the oxidation reaction was studied at 50 °C
keeping the amount of cobaltous chloride constant at 0.017 mmol. The results are
given in Table 2, from which it can be seen that conversion of 2-methoxy-p-cresol
was always higher than 99.0 % after 7 h; however, selectivity for vanillin changed
with amount of NHPI. Selectivity for vanillin was only 59.5 % if no NHPI was used
Table 2 Effect of NHPI loading on the oxidation of 2-methoxy-p-cresol
Entry
NHPI/substrate (mol %)
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
0
1
2
4
[99.0
[99.0
[99.0
[99.0
59.5
65.0
72.2
65.1
59.5
65.0
72.2
65.1
Reaction conditions: 2-methoxy-p-cresol 3.623 mmol, molar ratio of sodium hydroxide to 2-methoxy-p-
cresol 5.7:1, cobaltous chloride 0.0173 mmol, reaction temperature 50 °C, reaction time7 h, methanol
5
ml, atmospheric oxygen pressure
123

Y. Zhang et al.
and increased to 72.2 % when the amount of NHPI was increased to 2 % of the
amount of 2-methoxy-p-cresol. The selectivity for vanillin then began to decrease
with further increase in amount of NHPI. According to the literature, neat NHPI can
catalyze the oxidation of benzyl alcohols by molecular oxygen, forming the
corresponding aldehydes [29, 30]. As shown in Fig. 1, the reaction producing
vanillin proceeded with 3-methoxy-4-hydroxybenzyl alcohol as intermediate,
hence, low NHPI loading suppressed further oxidation of 3-methoxy-4-hydroxy-
benzyl alcohol to vanillin; high NHPI loading accelerated the transformation
process, which increased selectivity for vanillin for a specific time. However, if the
NHPI loading is too high, the intermediate 3-methoxy-4-hydroxybenzyl alcohol will
be quickly transformed to vanillin, thus increasing the contact time of vanillin with
oxygen and NaOH. As a result some of the vanillin generated is turned into
3
-methoxy-4-hydroxybenzoic acid, as shown in Fig. 1.
Effect of temperature
The effect of temperature on both conversion of 2-methoxy-p-cresol and selectivity
for vanillin was studied in the temperature range 45–70 °C. The results, listed in
Table 3, indicated that conversion was higher than 99.0 % in this temperature range.
Selectivity for vanillin increased from 57.5 to 80.8 % when the temperature was
increased from 45 to 70 °C, remaining almost constant in the range 65–70 °C.
Because the reaction was carried out at atmospheric pressure and the solvent was
methanol, the effects of temperature higher than 70 °C could not be determined.
One reason for the high selectivity for vanillin at high temperature is that high
temperature promotes transformation of the intermediate to vanillin. Another reason
is that NaOH was not completely dissolved at lower temperatures, as has also been
observed by others for similar conditions [28]. Incomplete dissolution of NaOH
prevented complete conversion of 2-methoxy-p-cresol to its sodium salt, which left
some free 2-methoxy-p-cresol to form some other oxidation-coupling products
under these reaction conditions.
Table 3 Effect of temperature on the oxidation of 2-methoxy-p-cresol
Entry
Temperature (°C)
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
5
6
45
50
55
60
65
70
[99.0
[99.0
[99.0
[99.0
[99.0
[99.0
57.5
72.2
73.8
78.0
80.2
80.8
57.5
72.2
73.8
78.0
80.2
80.8
Reaction conditions: 2-methoxy-p-cresol 3.623 mmol, molar ratio of sodium hydroxide to 2-methoxy-p-
cresol 5.7:1, cobaltous chloride 0.0173 mmol, NHPI 0.0725 mmol, reaction time 7 h, methanol 5 ml,
atmospheric oxygen pressure
1
23

Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin,
Effect of amount of sodium hydroxide
The concentration of (NaOH) has significant effect on the liquid-phase oxidation of
methyl aromatic compounds, including alcohols [26, 28]. Hence, it was important to
study the effect of NaOH concentration on 2-methoxy-p-cresol oxidation. The
experiments were conducted by varying the molar ratio of NaOH to 2-methoxy-p-
cresol from 4.5 to 6.5 for a constant amount of 2-methoxy-p-cresol (3.623 mmol) in
5
ml methanol at 65 °C. The results, shown in Table 4, indicate that conversion was
higher than 99.0 % and that selectivity for vanillin increased from 61.1 to 84.0 % in
the NaOH-to-substrate molar ratio range from 4.5:1–6.5:1. It was observed that a
large excess of sodium was necessary to maintain high selectivity for vanillin. Even
when the NaOH to 2-methoxy-p-cresol molar ratio was 5.5, by-product formation
was still more than 25.0 %. Lower selectivity for vanillin under these conditions
was because of oxidative dimerization of 2-methoxy-p-cresol (Fig. 2), similar to
dimerization of p-cresol in its liquid-phase oxidation [28]. It is known that the
phenolic hydroxyl group of p-cresol derivatives interferes with oxidation of the
substrates to their corresponding aldehydes [26, 28], it is necessary to completely
convert p-cresol derivatives into their sodium phenolate salts with a large excess of
NaOH; this reduces oxidative dimerization of 2-methoxy-p-cresol.
Effect of volume of methanol
The volume of methanol affects the concentrations of the reactants in the mixture. It
is, therefore, a crucial condition that must be studied. The effect of volume of
methanol on the reaction was studied in the range 2–6 ml while keeping the
amounts of the other reactants constant. The results, listed in Table 5, indicate that
conversion of 2-methoxy-p-cresol was always higher than 99.0 %. However,
selectivity for vanillin was found to change with methanol volume. Selectivity for
vanillin increased sharply from 77.6 % to a maximum of 90.1 % when the volume
of methanol was increased from 2 to 3 ml; it then decreased slowly as the volume of
methanol was increased further, and reached 76.3 % when the volume of methanol
6 ml. It was found that low methanol volume resulted in a viscous reaction medium
that affected mass transfer of the reactants. If the volume of methanol was too high,
Table 4 Effect of amount of sodium hydroxide on the oxidation of 2-methoxy-p-cresol
Entry
NaOH/substrate (mol:mol)
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
5
6
4.5
5.0
5.5
5.7
6.0
6.5
[99.0
[99.0
[99.0
[99.0
[99.0
[99.0
61.1
74.1
79.7
80.2
83.3
84.0
61.1
74.1
79.7
80.2
83.3
84.0
Reaction conditions: 2-methoxy-p-cresol 3.623 mmol, cobaltous chloride 0.0173 mmol, NHPI
.0725 mmol, reaction temperature 65 °C, reaction time 7 h, methanol,5 ml, atmospheric oxygen
pressure
0
123

Y. Zhang et al.
OH
OH
OH
H CO
H CO
3
3
OCH3
O2
CH3
CH3
CH3
Fig. 2 Oxidative dimerization of 2-methoxy-p-cresol
Table 5 Effect of volume of methanol on the oxidation of 2-methoxy-p-cresol
Entry
Methanol (ml)
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
5
2
3
4
5
6
[99.0
[99.0
[99.0
[99.0
[99.0
77.6
90.1
84.7
83.3
76.3
77.6
90.1
84.7
83.3
76.3
Reaction conditions: 2-methoxy-p-cresol 3.623 mmol, molar ratio of sodium hydroxide to 2-methoxy-p-
cresol 6.0:1, cobaltous chloride 0.0173 mmol, NHPI 0.0725 mmol, reaction temperature 65 °C, reaction
time7 h, atmospheric oxygen pressure
however, this resulted in low reactant (e.g. NaOH) concentration, leading to
dimerization of 2-methoxy-p-cresol (Fig. 2) and nucleophilic attack of CH O– on
3
the benzyl carbon of the intermediate 3-methoxy-4-hydroxybenzyl alcohol, leading
to the formation of 3-methoxy-4-hydroxybenzyl methyl ether, as observed by
others [28, 31, 32].
Conclusions
Liquid-phase oxidation of 2-methoxy-p-cresol with atmospheric oxygen as oxidant,
catalyzed by a combination of cobaltous chloride and NHPI in the presence of
NaOH, has been used for synthesis of vanillin. The effect of reaction conditions on
conversion and selectivity for vanillin was studied systematically. This investigation
showed that selectivity for vanillin was strongly dependent on reaction time,
reaction temperature, molar ratio of NHPI to substrate, amount of NaOH, and
volume of methanol, which determines the concentration of the reactants. Under the
optimized conditions—2-methoxy-p-cresol 3.623 mmol, molar ratio of NaOH to
2
-methoxy-p-cresol 6.0:1, cobaltous chloride 0.0173 mmol, NHPI 0.0725 mmol,
methanol 3 ml, reaction temperature 65 °C, and reaction time 7 h—the yield of
vanillin reached 90.1 %. This process is not only economic but also environmental
friendly.
Acknowledgments The authors are grateful for financial support from the National Natural Science
Foundation of China (no. 21276061).
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Liquid-phase oxidation of 2-methoxy-p-cresol to vanillin,
References
1
2
3
4
. S.R. Rao, G.A. Ravishankar, J. Sci. Food Agric. 80(3), 289–304 (2000)
. H. Priefert, J. Rabenborst, A. Steinb u¨ chel, Appl. Microbiol. Biotechnol. 56(3–4), 296–314 (2001)
. H.O. Mottern, J. Am. Chem. Soc. 56(10), 2107–2108 (1934)
. M. Bolognini, F. Cavani, L.D. Pozzo, L. Maselli, F. Zaccarelli, B. Bonelli, M. Armandi, E. Garrone,
Appl. Catal. A 272(1–2), 115–124 (2004)
5. H. Wynberg, Chem. Rev. 60(2), 169–184 (1960)
6
. E.A. Borges da Silva, M. Zabkova, J.D. Ara u´ jo, C.A. Cateto, M.F. Barreiro, M.N. Belgacem,
A.E. Rodrigues, Chem. Eng. Res. Des. 87(9), 1276–1292 (2009)
7
8
9
. J.D.P. Ara u´ jo, C.A. Grande, A.E. Rodrigues, Chem. Eng. Res. Des. 88(8), 1024–1032 (2010)
. I.A. Pearl, J. Am. Chem. Soc. 64(6), 1429–1431 (1942)
. G. Wu, M. Heitz, E. Chornet, Ind. Eng. Chem. Res. 33(6), 718–723 (1994)
1
0. I.B. Adilina, T. Hara, N. Ichikuni, S. Shimazu, J. Mol. Catal. A 361–362, 72–79 (2012)
1. V. Augugliaro, G. Camera-Roda, V. Loddo, G. Palmisano, L. Palmisano, F. Parrino, M.A. Puma,
Appl. Catal. B 111–112, 555–561 (2012)
1
1
1
1
1
1
1
1
1
2. S.N. Sharma, S.B. Chandalia, J. Chem. Technol. Biol. 49(2), 141–153 (1990)
3. D.F. Taber, S. Patel, T.M. Hambleton, E.E. Winkel, J. Chem. Educ. 84, 7–1158 (2007)
4. K. Nishizawa, K. Hamada, T. Aratani, EP0 012 939 (1979)
5. W. Jia, H. Wang, A. Qiu, C. Zhu, CN 1167750A (1997)
6. A. Qiu, C. Zhu, J Lai, CN1264695A (2000)
7. R. Chen, R. Chen, CN102295543A (2011)
8. Z. Yu, S. Yan, H. Liu, CN102030625A (2011)
9. J. Wang, A. Zhao, G. Lu, G. Guo, Y. Guo, W. Zhan, Z. Zhang, W. Han, Y. Wang, X. Liu, X. Gong.
CN102091637A (2011)
0. J. Hu, J. Mao, H. Li, Z. Chen, CN102381950A (2012)
2
2
2
2
1. Y. Ishii, S. Sakaguchi, Catal. Today 117(1–3), 105–113 (2006)
2. Y. Aoki, S. Sakaguchi, Y. Ishii, Tetrahedron 61(22), 5219–5222 (2005)
3. F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti, C. Punta, R. Paganelli, G.F. Pedulli,
F. Fontana, Org. Proc. Res. Dev. 8(2), 163–168 (2004)
2
4. P. Lahtinen, J.U. Ahmad, E. Lankinen, P. Pihko, M. Leskel a¨ , T. Repo, J. Mol. Catal. A 275(1–2),
228–232 (2007)
2
2
2
2
2
3
3
3
5. W. Yu, C. Zhou, D. Tong, T. Xu, J. Mol. Catal. A 365, 194–202 (2012)
6. Y. She, W. Wang, G. Li, Chin. J. Chem. Eng 20(2), 262–266 (2012)
7. V.S. Kshirsagar, J.M. Nadgeri, P.R. Tayade, C.V. Rode, Appl. Catal. A 339(1), 28–35 (2008)
8. C.V. Rode, M.V. Sonar, J.M. Nadgeri, R.V. Chaudhari, Org. Pro. Res. Dev. 8(6), 873–878 (2004)
9. R.A. Sheldon, I.W.C.E. Arends, Adv. Synth. Catal. 346(9–10), 1051–1071 (2004)
0. Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 343(5), 393–427 (2001)
1. K.T. Li, P.Y. Liu, Appl. Catal. A 272(1–2), 167–174 (2004)
2. M. Shimizu, Y. Watanabe, H. Orita, T. Hayakawa, K. Takehira, Tetrahedron Lett. 32(18), 2053–2056
(1991)
123