Ruthenium-catalyzed estragole isomerization: High trans-selective formation of anethole

Article abstract of DOI:10.1039/c003901b

Complexes [RuCl₂(η⁶-C₆H ₅OCH₂CH₂OH)(L)] (L = P(OMe)₃ (1a), P(OEt)₃ (1b), P(OⁱPr)₃ (1c), P(OPh)₃ (1d), PPh₃ (1e)) have shown to be efficient catalysts for the isomerization of estragole into anethole, the best activities being obtained in polar solvents (water, methanol, ethanol). Interestingly, a complete selectivity toward trans-anethole could be reached under smooth conditions (80 °C) and in very short times (5-15 min). The catalytic experiments have been performed both under conventional and microwave heating, reaction rates being significantly enhanced under the latter conditions. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c003901b

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/greenchem | Green Chemistry
Ruthenium-catalyzed estragole isomerization: high trans-selective
formation of anethole†‡
Beatriz Lastra-Barreira and Pascale Crochet*
Received 8th March 2010, Accepted 13th May 2010
First published as an Advance Article on the web 11th June 2010
DOI: 10.1039/c003901b
6
i
Complexes [RuCl
P(OPh) (1d), PPh
anethole, the best activities being obtained in polar solvents (water, methanol, ethanol).
2
(h -C
3
6
H
5
OCH
2
CH
2
OH)(L)] (L = P(OMe)
3
(1a), P(OEt)
3
(1b), P(O Pr)
3
(1c),
3
(1e)) have shown to be efficient catalysts for the isomerization of estragole into
Interestingly, a complete selectivity toward trans-anethole could be reached under smooth
◦
conditions (80 C) and in very short times (5–15 min). The catalytic experiments have been
performed both under conventional and microwave heating, reaction rates being significantly
enhanced under the latter conditions.
Introduction
anethole with a trans : cis ratio of 82 : 18, therefore making
necessary an additional separation step of the isomers.
1
Trans-anethole (Fig. 1), also known as trans-methylchavicol
or trans-isoestragole, presents important applications in food
and beverage industries and in the formulation of oral hy-
Alternatively, isomerization of estragole into anethole can be
10,11
promoted by metallic precursors;
nevertheless, most of them
2
12
also conduce to a moderate selectivity in the trans isomer.
3
giene products. Furthermore, it is a valuable intermediate for
Recently, we have reported that arene-ruthenium(II) complexes
4
the synthesis of pharmaceutical compounds and perfumery
6
[
RuCl
2
(h -C
6
H
5
OCH
2
CH OH)(L)] (1a–e, Fig. 2) efficiently iso-
2
5
chemicals. Trans-anethole is a naturally-occurring product
merize allylic alcohols into saturated carbonyl compounds,
which has been traditionally extracted from anise or fennel oils,
albeit with variable proportions of its cis-isomer as an impurity.
However, the increasing industrial demand has made it necessary
to develop alternative synthetic routes. From an academic point
of view, several methodologies have been reported to access to
through an initial C=C metal-catalyzed migration followed by
13
a spontaneous tautomerization of the resulting enol. Then,
their capacity to promote C=C bond migration prompted us to
evaluate their efficiency and selectivity in the transformation of
14
estragole into anethole.
6
anethole through different coupling processes, nevertheless they
are not financially viable. Actually, industrial process is based
on the isomerization of estragole (Fig. 1) promoted by excess
7
of KOH or NaOH. A number of drawbacks are associated to
◦
this procedure: (i) the high temperature required (>200 C), (ii)
the poor conversion into anethole (ca. 60%) and (iii) the huge
quantity of basic wastes generated. Another important aspect
of this process is its stereoselectivity. Effectively, only trans-
anethole is interesting for industry since the cis-isomer presents
◦
Fig. 2 Structure of complexes 1a–e and their solubility in water at 20 C
(S20 C).
◦
Interestingly, these complexes are readily accessible in two-
steps from commercially available starting materials and present
a high solubility and stability in water, therefore allowing
8
a higher toxicity and unpleasant odour and taste. Accordingly,
food regulatory instructions limit the proportion of cis-anethole
9
13
to a maximum of 1%. The current commercial process generates
their use in environmental benign aqueous media. Herein, we
describe their application in the highly trans-selective formation
of anethole from estragole. In this study, the efficiency of
both conventional and microwave heating conditions has been
explored.
Fig. 1 Structure of estragole and anethole.
Results and discussion
Catalytic isomerization of estragole into anethole under
conventional heating
Departamento de Qu ´ı mica Org a´ nica e Inorg a´ nica, Instituto de Qu ´ı mica
Organomet a´ lica “Enrique Moles”, Facultad de Qu ´ı mica, Universidad de
Oviedo, E-33071, Oviedo, Spain. E-mail: crochetpascale@uniovi.es;
Fax: +33-985 10 34 46; Tel: +33-985 10 50 76
6
Firstly, the catalytic activity of complexes [RuCl (h -
2
C
6
H
5
OCH
2
CH
2
OH)(L)] (L = P(OMe)
3
(1a), P(OEt) (1b),
(1d), PPh (1e)) in the isomerization of
3
†
Dedicated to Dr Bernard Demerseman on the occasion of his
retirement.
Electronic supplementary information (ESI) available: Experimental
i
P(O Pr)
3
(1c), P(OPh)
3
3
estragole has been checked in an aqueous medium. Experiments
‡
◦
details. See DOI: 10.1039/c003901b
were performed at 80 C using 4 mmol of substrate, 1 mol% of
This journal is © The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1311–1314 | 1311

View Article Online
a
Table 1 Isomerization of estragole into anethole catalyzed by
Table 2 Influence of the ruthenium loading.
6
a
[
RuCl
2
(h -C
6
H
5
OCH
2
CH
2
OH)(L)] (1a–e) in water.
b
b
Entry
mol% of catalyst
Time/h
Yield (%)
trans : cis
b
b
Entry
Catalyst [L]
Time/h
Yield (%)
trans : cis
c
1
2
3
4
5
6
0.05
0.05
0.2
0.2
1
0.5
22
0.5
2
0.5
0.5
1
76
88
>99
>99
>99
—
1
2
3
4
5
6
7
1a [P(OMe)
3
]
2
>99
>99
>99
>99
>99
>99
98
92 : 8
95 : 5
90 : 10
96 : 4
93 : 7
91 : 9
91 : 9
81 : 19
83 : 17
83 : 17
86 : 14
90 : 10
90 : 10
1b [P(OEt)
3
]
]
]
1.5
0.5
23
4.5
21
0.5
0.5
i
1c [P(O Pr)
1d [P(OPh)
3
3
1e [PPh
3
]
2
c
i
1c [P(O Pr)
3
3
3
]
]
]
a
◦
d
i
Reactions carried out at 80 C, using 4 mmol of estragole, the indicated
b c
1c [P(O Pr)
e
i
loading of 1c and 1 mL of water. GC determined. Not determined.
8
1c [P(O Pr)
84
a
◦
Unless otherwise indicated, reactions were carried out at 80 C, using
b
4
mmol of estragole, 1 mol% of Ru and 1 mL of water. GC determined.
c
◦
d
e
At 35 C. With NaOH (1 equiv. per Ru). With H
per Ru).
2 4
SO (1 equiv.
a
Table 3 Influence of the solvent.
b
b
Entry
Solvent
Time/h
Yield (%)
trans : cis
Ru and 1 mL of water (Table 1 and Scheme 1), the reaction being
monitored by GC analyses of aliquots.
1
2
3
4
5
6
7
8
9
hexane
toluene
1,4-dioxane
THF
tert-butanol
isopropanol
ethanol
methanol
acetonitrile
water
20
17
2
1.5
1.25
0.75
0.5
0.25
18
>99
97
92 : 8
96 : 4
92 : 8
96 : 4
95 : 5
95 : 5
99 : 1
>99% trans
93 : 7
92 : 8
>99
>99
>99
>99
>99
>99
15
c
Scheme 1 Catalytic isomerization of estragole into anethole.
1
0
2
>99
For all the catalysts, complete conversion into anethole was
reached, nevertheless the rate of the process was strongly
dependent of the ligand L coordinated on the metal center. Thus,
complexes containing an aliphatic phosphite (1a–c), i.e. the most
soluble in water (see Fig. 2), gave rise to quantitative yields in
anethole within 2 h (entries 1–3 in Table 1). The best results
a
◦
Reactions carried out at 80 C, using 4 mmol of estragole, 1 mol% of
b
c
1
a and 1 mL of the indicated solvent. GC determined. cis isomer not
detected.
6
i
were obtained with [RuCl
2
(h -C
6
H
5
OCH
2
CH
2
OH){P(O Pr)
3
}]
different solvents (see Table 3). As a general trend, the rate of
the process increased with the polarity of the solvent. Thus,
in hexane or toluene, long reaction times, superior to 17 h,
were necessary to reach significant conversions (entries 1–2,
Table 3). In contrast, in polar solvents, and especially in alcohols,
the isomerization proceeded rapidly. Remarkably, complete
conversion in only 15 min was observed when methanol was
used as solvent (entry 8). Finally, although acetonitrile was one
of the most polar solvent used, the conversion obtained in this
medium was particularly low (15% after 18 h, entry 9). This is
probably due to its capacity to coordinate on the active species,
competing then with the substrate.
As far as selectivity is concerned, methanol and ethanol are
revealed to be the solvents of choice. Effectively, under these
conditions the proportion of trans-anethole obtained was equal
or superior to 99% (entries 7 and 8 in Table 3). In particular,
when the reaction was carried out in methanol no cis isomer was
detected neither by GC analyses nor by H and C{ H} NMR
spectroscopy of the isolated product (see ESI‡), evidencing the
complete stereoselectivity of the process.
(
1c) which quantitatively converted estragole into anethole in
only 30 min (entry 3).
In contrast, the presence of an aromatic ligand in the
coordination sphere of ruthenium resulted in lower catalytic
activities. In particular, the triphenylphosphite derivative (1d)
needed 23 h to transform estragole into anethole (entry 4,
16
15
Table 1). In all the cases, good selectivity in the trans isomer was
observed (90–96%), although the proportion never exceeded the
desired value of 99%. In an attempt to improve the selectivity, the
◦
process has been carried out at lower temperature (35 C) with
the most active complex 1c (entry 6, Table 1). Unfortunately,
the trans-selectivity remained almost unchanged (91% vs. 90%),
while the reaction time dramatically increased (21 h vs. 0.5 h). We
next explored the influence of the pH medium onto the catalytic
performances. Thus, reactions were performed in the presence
of one equivalent per ruthenium of NaOH or H
2
SO . The results
4
1
13
1
obtained under basic conditions were similar to those observed
in neutral water (entry 7 vs. entry 3, Table 1). In contrast, the
addition of an acid was detrimental both to the yield and the
selectivity (entry 8).
We also investigated how the process is affected by changing
the catalyst loading. We observed that raising the quantity of
ruthenium from 1 to 2 mol% did not modify the trans : cis ratio
The catalytic studies in ethanol and methanol have been
extended to the other aliphatic catalysts 1b and 1c (Table 4).
In all the cases, complete conversion was reached in short
reaction times (≤1 h) and excellent trans-selectivities were
observed (≥97%).
17
(
entry 6 vs. entry 5, in Table 2). On the other hand, lower metal
loadings (0.2 or 0.05 mol%) conduced to lower selectivities in
Interestingly, the isomerization of estragole can be easily
scaled-up, affording multiple grams of isolated trans-anethole
in a total stereoselective manner (Scheme 2, see details in the
Experimental section).
the desired trans-isomer (entries 1–4 vs. 5).
6
The isomerization of estragole promoted by [RuCl
OCH CH
OH){P(OMe)
2
(h -
C
6
H
5
2
2
3
}] (1a) has also been tested in
1
312 | Green Chem., 2010, 12, 1311–1314
This journal is © The Royal Society of Chemistry 2010

View Article Online
6
Table 4 Isomerization of estragole catalyzed by [RuCl
2
(h -
was observed under conventional heating, even better catalytic
a
C
6
H
5
OCH
2
CH
2
OH)(L)] (1a–e) in ethanol and methanol.
performances could be achieved in methanol. In particular, it
is noteworthy that catalyst 1a afforded a complete and fully-
selective transformation of estragole into trans-anethole in only
5 min (entry 6).
b
b
Entry
Catalyst [L]
Time/h
Yield (%)
trans : cis
c
1
2
3
4
5
6
1a [P(OMe)
3
]
]
0.5
0.5
1
0.25
0.25
0.42
>99
>99
>99
>99
>99
>99
99 : 1
99 : 1
97 : 3
c
3
1b [P(OEt) ]
c
i
1c [P(O Pr)
1a [P(OMe)
1b [P(OEt)
3
]
d
e
3
>99% trans
Conclusions
d
3
]
99 : 1
98 : 2
d
i
6
1c [P(O Pr)
3
]
In summary, the arene-ruthenium(II) complexes [RuCl
OCH CH (1a), P(OEt)
OH)(L)] (L = P(OMe)
P(O Pr) (1c), P(OPh) (1d), PPh (1e)), and especially the
2
(h -
(1b),
C
6
H
5
2
2
3
3
a
◦
Reactions carried out at 80 C, using 4 mmol of estragole, 1 mol% of
Ru and 1 mL of solvent. GC determined. Ethanol used as solvent.
i
b
c
3
3
3
d
e
Methanol used as solvent. cis isomer not detected.
aliphatic ones, have shown to be effective catalysts to convert
estragole into anethole. The best activity and selectivity toward
trans-anethole were obtained in polar solvents (i.e. water,
ethanol and methanol). In particular, in ethanol and methanol,
it is possible to generate anethole with a contain in trans-
isomer ≥99%, fulfilling therefore the criteria imposed by the Joint
9
FAO/WHO Expert Committee on Food Additives (JECFA).
Very short reaction times are required under conventional heat-
ing (ca. 15 min) and even shorter under microwave-irradiations
Scheme 2 Stereoselective isomerization of estragole in preparative
scale.
(
ca. 5 min). Thus, this process represents a rapid and highly
selective access to trans-anethole compatible with renewable
21
solvents such as water, EtOH or MeOH.
Catalytic isomerization of estragole into anethole under
microwave heating
Experimental
During the last decade, microwave-assisted organic synthesis
has emerged as an important issue in green chemistry,
18,19
Solvents were dried by standard methods and distilled under
since the use of this heating technique significantly shortens
reaction times, therefore minimizing the energy consumption
during the process. Moreover, in many cases it also minimizes
side reactions and improves the yield, selectivity and repro-
ducibility of the process. Therefore, we explored the activity of
catalysts 1a–e in the isomerization of estragole under microwave
heating (Table 5). The first experiments were carried out in
water keeping the ruthenium loading (1 mol%), the substrate
nitrogen before use. All reagents were obtained from com-
6
mercial suppliers with the exception of compounds [RuCl
OCH CH (1a), P(OEt)
OH)(L)] (L = P(OMe)
(1c), P(OPh) (1d), PPh (1e)), which were prepared
2
(h -
(1b),
C
6
H
5
2
2
3
13
3
i
P(O Pr)
3
3
3
following the method reported in the literature. NMR spectra
were recorded on a Bruker DPX300 instrument at 300 MHz
1
31
13
( H), 121.5 MHz ( P) or 75.4 MHz ( C) using SiMe
4
or 85%
H
3
PO as standards. GC measurements were made on a Hewlett-
4
◦
TM
concentration (4 M) and the temperature (80 C) as before (see
Packard HP6890 equipment (Supelco Beta-Dex 120 column;
details in the Experimental section). Under these conditions,
reaction rates attained with all the catalysts were significantly
higher than those obtained under conventional heating. The
most impressive enhancement was observed with complex 1e
which led to 97% yield of anethole after only 15 min (vs
30 m length; 250 mm diameter).
20
Typical procedure for catalytic isomerization of estragole into
anethole under conventional heating
Under a nitrogen atmosphere, the ruthenium catalyst precursor
2
1% yield under conventional heating). The best results in
(
(
0.04 mmol, 1 mol%), 1 mL of the indicated solvent and estragole
0.614 mL, 4 mmol) were introduced into a teflon-cap sealed
terms of yield and selectivity were reached with catalysts 1b
and 1c which gave rise to complete conversion within 15 min
and a trans-selectivity of 94-95% (entries 2–3, Table 5). As
◦
tube. Then, the mixture was heated at 80 C and, the yield and
selectivity of the reaction were monitored by GC analyses of
aliquots.
a
Table 5 Isomerization of estragole under microwave heating.
b
b
Entry
Catalyst
Solvent
Time
Yield (%)
trans : cis
Typical procedure for catalytic isomerization of estragole into
anethole under microwave heating
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1a
1b
1c
water
water
water
water
15 min
15 min
15 min
15 min
15 min
5 min
80
90 : 10
95 : 5
94 : 6
80 : 20
86 : 14
>99
>99
15
97
99
98
80
A pressure-resistant septum-sealed glass microwave reactor was
charged under nitrogen atmosphere with estragole (0.614 mL,
4 mmol), the corresponding ruthenium catalyst precursor
water
c
MeOH
MeOH
MeOH
>99% trans
(
0.04 mmol, 1 mol%), a magnetic stirring bar and 1 mL of
5 min
5 min
99 : 1
98 : 2
distilled water or methanol. The vial was then placed inside
the cavity of a CEM DiscoverꢀR S-Class microwave synthesizer
and power was held at 100 W until the desired temperature was
a
Reactions carried out under MW heating at the indicated temperature,
using 4 mmol of estragole, 1 mol% of Ru and 1 mL of solvent. GC
b
◦
reached (80 C). Microwave power was automatically regulated
c
determined. cis isomer not detected.
for the remainder of the experiment to maintain the temperature
This journal is © The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1311–1314 | 1313

View Article Online
(
monitored by a built-in infrared sensor). The internal pressure
2006, 245, 200; (f) G. R. A. Adair, K. K. Kapoor, A. L. B. Scolan and
J. M. J. Willaims, Tetrahedron Lett., 2006, 47, 8943; (g) G. Erdogan
and D. B. Grotjahn, J. Am. Chem. Soc., 2009, 131, 10354; (h) A.
Scarso, M. Colladon, P. Sgarbossa, C. Santo, R. A. Michelin and G.
Strukul, Organometallics, 2010, 29, 1487.
1 For heterogeneous catalytic systems see, for example: (a) V. K.
Srivastava, H. C. Bajaj and R. V. Jasra, Catal. Commun., 2003, 4,
during the reaction ranged between 15 and 25 psi. After the
time indicated (5 or 15 min), the vial was cooled and yield and
selectivity were determined by GC analyses.
1
Isomerization of estragole into anethole under preparative
conditions (conventional heating)
5
2
43; (b) D. Kishore and S. Kannan, J. Mol. Catal. A: Chem., 2006,
44, 83; (c) C. M. Jinesh, C. A. Antonyraj and S. Kannan, Catal.
Today, 2009, 141, 176; (d) S. K. Sharma, P. A. Parikh and R. V.
Jasra, J. Mol. Catal. A: Chem., 2010, 317, 27.
12 For selective synthesis of trans-anethole see references 10a, e and h.
13 B. Lastra-Barreira, J. D ´ı ez and P. Crochet, Green Chem., 2009, 11,
1681.
4 For other arene-ruthenium complexes able to promote C=C migra-
tion see: (a) H. Brunner, T. Zwack and M. Zabel, Organometallics,
2003, 22, 1741; (b) V. Cadierno, P. Crochet, S. E. Garc ´ı a-Garrido and
J. Gimeno, Dalton Trans., 2004, 3635; (c) Y. Takai, R. Kitaura, E.
Nakatani, T. Onishi and H. Kurosawa, Organometallics, 2005, 24,
Under a nitrogen atmosphere, the precursor 1a (0.081 g,
0
.2 mmol, 1 mol%), 5 mL of distilled MeOH and 3.1 mL of
estragole (20 mmol) were introduced into a teflon-cap sealed
◦
tube and heated at 80 C for 20 min. After this time, GC
1
analysis revealed a complete conversion and a trans-selectivity
of 99%. After removal of the solvent under vacuum, flash
chromatography of the residue using ethyl acetate as eluent
afforded 2.6 g (17.5 mmol, 88%) of analytically pure trans-
anethole.
4
729; (d) P. Crochet, J. D ´ı ez, M. A. Fern a´ ndez-Z u´ mel and J. Gimeno,
´
Adv. Synth. Catal., 2006, 348, 93; (e) A. E. D ´ı az-Alvarez, P. Crochet,
M. Zablocka, C. Duhayon, V. Cadierno, J. Gimeno and J. P. Majoral,
Adv. Synth. Catal., 2006, 348, 1671; (f) P. Crochet, M. A. Fern a´ ndez-
Z u´ mel, J. Gimeno and M. Scheele, Organometallics, 2006, 25, 4846;
Acknowledgements
(
g) V. Cadierno, P. Crochet and J. Gimeno, Synlett, 2008, 1105; (h) M.
This work was supported by Spanish “Ministerio de Educaci o´ n
y Ciencia” (Projects CTQ2006-084885/BQU and CSD2007-
Fekete and F. Jo o´ , Catal. Commun., 2006, 7, 783; (i) V. Cadierno, J.
Francos, J. Gimeno and N. Nebra, Chem. Commun., 2007, 2536; (j) V.
Cadierno, P. Crochet, J. Francos, S. E. Garc ´ı a-Garrido, J. Gimeno
and N. Nebra, Green Chem., 2009, 11, 1992.
5 The low catalytic activity of derivatives 1d and 1e is probably due
their possible deactivation through orthometalation of the aromatic
0
0006 (Consolider Ingenio 2010 program)) and FICYT of
Asturias (Project IB08-036). B.L.-B. thanks FICYT of Asturias
for the award of a Ph.D grant (PCTI-Severo Ochoa program).
1
1
3 3
rings of PPh or P(OPh) (see reference 13).
6 The higher catalytic activities obtained in alcoholic medium are
probably due to the easier ruthenium-chloride bond dissociation
Notes and references
31
1
which constitutes the first step of the process. Accordingly, P{ H}
NMR spectra of 1a–c, recorded at room temperature in methanol,
ethanol or isopropanol, show two signals corresponding to deriva-
1
The IUPAC name of anethole is 1-methoxy-4-(1-propen-1-
yl)benzene.
6
6
2
(a) A. Y. Leung, Encyclopedia of Common Natural Ingredients Used
tives [RuCl
OCH
2
(h -C
6
H
5
OCH
2
CH
2
OH){P(OR)
3
}] and [RuCl(S)(h -
in Food, Drug and Cosmetics, John Wiley & Sons, New York, 1980;
C
6
H
5
2
CH
2
OH){P(OR)
3
}][Cl] (S = solvent molecule). The rel-
6
(
b) P. R. Ashurst, Food Flavorings, Springer, 3rd Edn, 1999.
ative proportion of [RuCl(S)(h -C
which results of the Ru–Cl bond cleavage, increases with the solvent
6
H
5
OCH
2
CH
2
OH){P(OR) }][Cl],
3
3
4
See, for example: G. H. Eirew, U.S. Pat. Appl., US 2009035229, 2009.
See, for example: (a) K.-I. Fujita, T. Fujita and I. Kubo, Phytother.
Res., 2007, 21, 47; (b) E. Enan, PCT Int. Appl., 2008003007, 2008;
polarity (i.e. in the order isopropanol < ethanol < methanol).
17 Catalysts 1d and 1e have also been tested under these conditions
and improved performances are observed, nevertheless the activity
and the selectivity are still unsatisfactory (reaction times ≥2 h, trans-
selectivity ≤91%).
(
c) V. V. Kouznetsov and D. R. Merchan Arenas, Tetrahedron Lett.,
009, 50, 1546 and references cited therein.
2
5
6
A. J. Chalk, in Flavors and Fragances: A world Perspective, ed.
B. M. Lawrence, B. D. Mookherjee and B. J. Willis, Elsevier Science,
Amsterdam, 1988.
See, for example: (a) S. Cabiddu, A. Maccioni and M. Secci,
Ann. Chim., 1962, 52, 1261; (b) A. D. Buss, R. Mason and S.
Warren, Tetrahedron Lett., 1983, 24, 5293; (c) G. Cahiez and H.
Avedissian, Tetrahedron Lett., 1998, 39, 6159; (d) J. A. Miller and
J. W. Dankwardt, Tetrahedron Lett., 2003, 44, 1907; (e) J. C. Roberts
and J. A. Pincock, J. Org. Chem., 2006, 71, 1480.
18 (a) C. O. Kappe, Angew. Chem., Int. Ed., 2004, 43, 6250; (b) B. A.
Roberts and C. R. Strauss, Acc. Chem. Res., 2005, 38, 653; (c) D.
Dallinger and C. O. Kappe, Chem. Rev., 2007, 107, 2563; (d) V.
Polshettiwar and R. S. Varma, Chem. Soc. Rev., 2008, 37, 1546; (e) V.
Polshettiwar and R. S. Varma, Acc. Chem. Res., 2008, 41, 629; (f) S.
Caddick and R. Fitzmaurice, Tetrahedron, 2009, 65, 3325.
19 (a) Microwave-Assisted Organic Synthesis, ed. P. Lidstr o¨ m and
J. P. Tierney, Black Publishing: Oxford, 2005; (b) Microwave in
Organic Synthesis, 2nd ed., ed. A. Loupy, Wiley-VCH: Weinheim,
2006; (c) Microwave methods in Organic Synthesis, ed. M. Larhed and
K. Olofsson, Springer: Berlin, 2006; (d) Aqueous Microwave Assisted
Chemistry, ed. V. Polshettiwar and R. S. Varma, RSC Publishing:
Cambridge, 2010.
7
8
(a) A. P. Wagner, Manuf. Chemist, 1952, 23, 56; (b) H. Pines,
and W. M. Stalick, Base-Catalyzed Reactions of Hydrocarbons and
Related Compounds, Academic Press Inc., New York, 1977.
(a) F. Caujolle and D. Meynier, Acad. Sci., 1958, 246, 1465; (b) J. M.
Taylor, P. M. Jenner and W. I. Jones, Toxicol. Appl. Pharmacol., 1964,
6
2
, 378; (c) J. R. Boissier, P. Simon and B. Le Bourhis, Therapie, 1967,
2, 309.
20 After 15 min under conventional heating the following yields and
selectivity were obtained: 28%, trans : cis = 87 : 13 (1a); 66%, trans :
cis = 95 : 5 (1b); 70%, trans : cis = 91 : 9 (1c); 1%, trans : cis ratio not
determined (1d); 21%, trans : cis = 79 : 21 (1e).
9
See specifications given by the Joint FAO/WHO Expert
Committee on Food Additives (JECFA) available online at:
http://www.fao.org/ag/agn/jecfa-flav.
0 For homogeneous catalytic systems see, for example: (a) I. R.
Baxendale, A.-L. Lee and S. V. Ley, Synlett, 2002, 516; (b) M.
Arisawa, Y. Terada, M. Nakagawa and A. Nishida, Angew. Chem.,
Int. Ed., 2002, 41, 4732; (c) S. K. Sharma, V. K. Srivastava, P. H.
Pandya and R. V. Jasra, Catal. Commun., 2005, 6, 205; (d) H. S.
Lee and G. Y. Lee, Bull. Korean Chem. Soc., 2005, 26, 461; (e) S. K.
Sharma, V. K. Srivastava and R. V. Jasra, J. Mol. Catal. A: Chem.,
21 Note that although ethanol and methanol are not typical “green”
solvents due to their flammability and toxicity (in the case of MeOH),
they represent a better alternative than other organic solvents since
they can be produced from biomass. See, for example: (a) K. Alfonsi,
J. Colberg, P. J. Dunn, T. Fevig, S. Jennings, T. A. Johson, H. P.
Kleine, C. Knight, M. A. Nagy, D. A. Perry and M. Stefaniak, Green
Chem., 2008, 10, 31; (b) F. M. Kerton, in Alternative Solvents for
Green Chemistry, RSC Publishing: Cambridge, 2009.
1
1
314 | Green Chem., 2010, 12, 1311–1314
This journal is © The Royal Society of Chemistry 2010