Ruthenium(iv) catalysts for the selective estragole to trans-anethole isomerization in environmentally friendly media

Article abstract of DOI:10.1039/c0gc00417k

Several ruthenium(iv) complexes have been tested as potential catalysts for the isomerization of estragole into anethole using water and glycerol as alternative green reaction media. Best results in terms of activity and E-selectivity were obtained with the dimeric species [{RuCl(μ-Cl) (η³:η³-C₁₀H₁₆)} ₂] (C₁₀H₁₆ = 2,7-dimethylocta-2,6-diene-1,8- diyl) and the mononuclear derivative [RuCl₂(η³: η³-C₁₀H₁₆){P(OMe)₃}]. In particular, using a ruthenium loading of 1 mol%, almost quantitative and stereoselective formation of trans-anethole (trans/cis ratios = 99:1) could be reached at 80°C in short times (5-30 min) employing water-MeOH (EtOH) or glycerol-MeOH (EtOH) mixtures (1:1 v/v) as solvent. Recyclability issues have also been addressed.

Full text of DOI:10.1039/c0gc00417k

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PAPER
Ruthenium(IV) catalysts for the selective estragole to trans-anethole
isomerization in environmentally friendly media†
Beatriz Lastra-Barreira, Javier Francos, Pascale Crochet* and Victorio Cadierno*
Received 9th August 2010, Accepted 23rd November 2010
DOI: 10.1039/c0gc00417k
Several ruthenium(IV) complexes have been tested as potential catalysts for the isomerization of
estragole into anethole using water and glycerol as alternative green reaction media. Best results in
3
3
terms of activity and E-selectivity were obtained with the dimeric species [{RuCl(m-Cl)(h :h -
C10H16)}2] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) and the mononuclear derivative
3
3
[RuCl2(h :h -C10H16){P(OMe)3}]. In particular, using a ruthenium loading of 1 mol%, almost
quantitative and stereoselective formation of trans-anethole (trans/cis ratios = 99 : 1) could be
reached at 80 ◦C in short times (5–30 min) employing water–MeOH (EtOH) or glycerol–MeOH
(EtOH) mixtures (1 : 1 v/v) as solvent. Recyclability issues have also been addressed.
and alcoholic beverages,2,6a in the formulation of oral hygiene
Introduction
products8 or as an advanced intermediate for the preparation of
pharmaceutical compounds9 and perfumery chemicals,2,6a has
made extraction from natural sources not sufficient to supply
the market, hence the need to produce it synthetically.10
The catalytic isomerization of olefins is a well established
process in organic chemistry with widespread academic and
industrial applications.1 Transformation of allyl-benzenes into
the corresponding 1-propenyl derivatives is a clear example of
the synthetic utility of this textbook reaction since the latter
are common starting materials in the flavour and fragrance
industries.2 In this sense, the selective isomerization of estragole
1 (1-allyl-4-methoxybenzene), readily accessible by distillation
of crude sulfate turpentine,3 into trans-anethole 2 (1-methoxy-
4-((E)-1-propenyl)benzene) represents nowadays the main route
used for the large-scale production of this chemical (Scheme 1),4,5
a naturally-occurring compound which has been traditionally
extracted from anise or fennel oils.6,7 The increasing demand of
2, widely employed by industry to enhance the flavour of foods
Currently, the industrial isomerization of estragole to anet-
hole is performed with excess of KOH,10,11 a procedure that
presents sever◦al major drawbacks: (i) a high temperature is
required (200 C), (ii) yields are only moderate (ca. 60%), (iii)
a large quantity of basic wastes are generated, which leads
to inconvenient pollution effects, and (iv) the process is not
stereoselective, i.e. a mixture of trans and cis isomers is formed in
82 : 18 ratio, making necessary an energy-consuming fractional
distillation step to obtain trans-anethole 2 in pure form.12
This stereoselectivity issue deserves to be highlighted: only the
trans isomer is marketed since the cis one is characterized
by its toxic nature and unpleasant organoleptic properties.13
Accordingly, the food regulatory instructions given by the Joint
FAO/WHO Expert Committee of Food Additives (JECFA)
limit the cis-anethole content to a maximum of 1% for human
use.14 To overcome all these limitations, several protocols using
heterogeneous15 and homogeneous16 metal-based catalysts have
been devised. However, only a few generate anethole enriched
above 97% in its trans isomer.16a,e,h Consequently, the search
for efficient and selective catalytic systems able to promote the
estragole 1 to trans-anethole 2 isomerization still remains a
challenge for synthetic chemists.
Scheme 1 The estragole 1 to trans-anethole 2 isomerization.
Departamento de Qu´ımica Orga´nica e Inorga´nica. Instituto Universitario
de Qu´ımica Organometa´lica “Enrique Moles” (Unidad Asociada al
CSIC). Universidad de Oviedo, Julia´n Claver´ıa 8, 33006, Oviedo, Spain.
E-mail: crochetpascale@uniovi.es (PC), vcm@uniovi.es (VC);
Fax: (+34)985103446; Tel: (+34)985102985
On the other hand, chemical transformations are currently
experiencing a deep change to meet sustainability criteria
imposed by the Green Chemistry principles.17 One of them
is to circumvent the use of hazardous solvents, as they are
responsible for a large part of the waste generated by the
chemical processes. The wanted characteristics for a green
† Electronic supplementary information (ESI) available: Copies of the
1
1H and 13C{ H} NMR and GC spectra of trans-anethole generated at
35 ◦C using 5f as catalyst in water (Scheme 4). Copy of the GC spectrum
of the cis/trans-anethole mixture generated at 80 ◦C using 5f as catalyst
in water (entry 11 in Table 2). See DOI: 10.1039/c0gc00417k
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Table 1 Estragole to anethole isomerization in environmentally
friendly media using the bis(allyl)-ruthenium(IV) complexes 3 and 4a
solvent include: no toxicity, no flammability, high availability,
obtaining from renewable sources and biodegradability.18 Water
has been for long time the first solvent of choice regarding the
aforementioned considerations.18 Indeed, it is now well-accepted
that water is a reliable alternative to the organic, petroleum-
based, solvents commonly used by the chemical community.19
In recent years, with the increase in biodiesel production world-
wide, the availability of glycerol has tremendously increased
and finding new applications for this low-cost raw material
has become an urgent necessity.20 As water, glycerol meets the
requirements needed to be considered as a green solvent. In
fact, its use as an alternative medium for organic reactions has
emerged as a promising new field of research that is waiting to
be explored in depth.21,22
Entry Cat. % Ru
Solvent Temp. Time Yieldb E/Zb
1
2
3
3
3
4
4
3
3
3
3
3
3
3
3
3
1 mol (%)
1 mol (%)
1 mol (%)
1 mol (%)
1 mol (%)
1 mol (%)
H2O
Glycerol 80 ◦
H2O
80 ◦
80 ◦
C
C
C
C
C
C
C
C
C
C
C
C
C
30 min >99% 97 : 3
45 min >99% 97 : 3
3 h
6 h
>99% 94 : 6
>99% 95 : 5
4
Glycerol 80 ◦
5c
6d
7
H2O
H2O
80 ◦
80 ◦
80 ◦
80 ◦
10 min >99% 97 : 3
15 min >99% 97 : 3
0.5 mol (%) H2O
2 h
>99% 96 : 4
8
9
10
11
12
13e
2 mol (%)
1 mol (%)
1 mol (%)
1 mol (%)
1 mol (%)
1 mol (%)
H2O
H2O
H2O
H2O
30 min >99% 97 : 3
15 min >99% 97 : 3
5 min >99% 95 : 5
Despite this growing interest to develop environmentally
benign and safe processes, estragole to trans-anethole iso-
merizations using green reaction media have been neglected.
Only in the context of a broader study by our own group,
some experiments in water have been described employing
100 ◦
120 ◦
35 ◦
21 h
24 h
>99% 99 : 1
94% 98 : 2
Glycerol 35 ◦
H2O
80 ◦
15 min >99% 96 : 4
6
elaborated (h -arene)-ruthenium(II) complexes as catalysts.23
a Reactions performed under N2 atmosphere using 2 mmol of substrate
and 0.5 cm3 of water or glycerol. b Determined by GC. c Reaction
performed in the presence of 1 mol% of NaOH. d Reaction performed
in the presence of 1 mol% of H2SO4. e Reaction performed under MW
heating at the indicated temperature (initial MW power 100 W).
However, as shown in Scheme 2, the results obtained in this
medium were not completely satisfactory in terms of trans/cis
stereoselectivity. Only the use of methanol as solvent allowed to
generate anethole with a trans-selectivity ≥99% with these Ru(II)
catalysts.
Results and discussion
Initial exploratory experiments were performed at 80 ◦C with
2 mmol of estragole, a ruthenium loading of 1 mol% and 0.5 cm3
of the appropriate solvent (water or glycerol), monitoring the
course of the reactions by GC analyses of aliquots. Under these
conditions, we found that, regardless of the solvent employed,
both Ru(IV) complexes are able to generate anethole in almost
quantitative yield (≥99% by GC) after 0.5–6 h of heating (entries
1–4 in Table 1). However, the reactions proceeded significantly
faster in water than in glycerol (entry 1 vs. 2 and 3 vs. 4). From
these initial studies the dimeric species [{RuCl(m-Cl)(h :h -
C10H16)}2] (3) emerged as the catalyst of choice due to its
high efficiency (quantitative conversions in less than 1 h) and
remarkable selectivity towards the desired trans-anethole isomer
(E/Z ratios = 97 : 3) (entries 1–2).
3
3
Scheme 2 Estragole to anethole isomerization in methanol and water
using hydrophilic (h -arene)-ruthenium(II) complexes.
6
With the aim of finding more readily accessible catalysts for
this key transformation, we turned our attention to the commer-
cially available bis(allyl)-ruthenium(IV) derivatives [{RuCl(m-
Several attempts to improve the selectivity of the process were
then performed using 3 under aqueous conditions. In this sense,
we firstly explored the influence of the pH medium. To this
end, reactions were performed in the presence of one equivalent
of NaOH or H2SO4 per ruthenium (entries 5 and 6). Faster
transformations were in both cases observed but, in terms of
selectivity, the results obtained were identical to those reached
in neutral water. No influence of the catalyst loading on the
selectivity was observed (entries 7–8). Finally, the effect of the
temperature was explored (entries 9–11) and, to our delight,
we found that an almost complete selectivity towards trans-
anethole can be reached (E/Z ratios = 99 : 1) performing the
catalytic reaction at 35 ◦C (entry 11). However, an extremely
long reaction time (21 h) was needed to totally consume the
starting estragole. Similarly, improvement of the selectivity was
3
3
Cl)(h :h -C10H16)}2] (3; C10H16 = 2,7-dimethylocta-2,6-diene-1,8-
3
2
3
diyl) and [RuCl2(h :h :h -C12H18)] (4; C12H18 = dodeca-2,6,10-
triene-1,12-diyl) (see Fig. 1),24 whose outstanding ability to pro-
mote C C migrations in water has been largely demonstrated.25
Thus, in this work an evaluation of their suitability for the selec-
tive estragole to trans-anethole isomerization in environmentally
friendly media (water and glycerol) is presented.
◦
also noticed at 35 C when glycerol was used as solvent (entry
12), albeit an incomplete reaction was in this case observed
Fig. 1 Structure of the bis(allyl)-ruthenium(IV) complexes 3 and 4.
308 | Green Chem., 2011, 13, 307–313
after 24 h. By contrast, at higher temperatures (100–120 ◦C),
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Table 2 Estragole to anethole isomerization in environmentally
friendly media using the mononuclear bis(allyl)-ruthenium(IV) com-
plexes 5a–na
the reactions were significantly faster but less selective (entries
9–10). This temperature effect on the stereoselectivity is quite
surprising since, as previously observed by others,15b,d an increase
in temperature would lead to a greater selectivity towards
the thermodynamically more stable trans-isomer.26 Degradation
of the catalytically active species with temperature could be
responsible of these unexpected results. In accordance with the
negative effect of temperature on the stereoselectivity, it is not
surprising that a poor result was also obtained under MW
heating, the only positive effect of microwave irradiations being
the improvement of the reaction rate (entry 13).27
Entry
Catalyst
Solvent
Time
Yieldb
E/Zb
1
2
3
4
5
6
7
8
5a
5a
5b
5b
5c
5c
5d
5d
5e
5e
5f
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
Glycerol
H2O
6 h
10 h
3 h
>99%
>99%
>99%
>99%
>99%
91%
97%
96%
95%
92%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
>99%
99%
87 : 13
91 : 9
94 : 6
95 : 5
94 : 6
93 : 7
95 : 5
94 : 6
93 : 7
93 : 7
95 : 5
95 : 5
95 : 5
95 : 5
92 : 8
92 : 8
95 : 5
93 : 7
93 : 7
94 : 6
93 : 7
94 : 6
88 : 12
92 : 8
94 : 6
95 : 5
93 : 7
93 : 7
>99% transd
98 : 2
99 : 1
Seeking to find a more competitive catalyst, a series of
6 h
3
3
mononuclear derivatives [RuCl2(h :h -C10H16)(L)] (5a–n) were
synthesized by cleavage of the chloride bridges of [{RuCl(m-
21 h
24 h
21 h
24 h
24 h
24 h
30 min
1 h
1 h
2 h
1 h
2 h
9 h
9 h
1 h
6 h
3 h
6 h
1 h
3 h
3
3
Cl)(h :h -C10H16)}2] (3) with phosphines (5a–e), phosphites (5f–
i), isocyanides (5j–k), carbon monoxide (5l) or nitriles (5m–n)
(see Scheme 3; details are given in the Experimental).28 We
reasoned that the introduction of these ligands, presenting quite
different electronic and steric properties, on the coordination
sphere of ruthenium should exert some influence on the outcome
of the isomerization process in terms of activity as well as
trans/cis selectivity. The ability of these mononuclear species to
promote the estragole to anethole isomerization was evaluated
in both water and glycerol. Table 2 provides a summary of the
results obtained performing the catalytic reactions at 80 ◦C with
a metal loading of 1 mol%.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29c
30c
31c,e
5f
5g
5g
5h
5h
5i
5i
5j
5j
5k
5k
5l
5l
5m
5m
5n
5n
5f
3 h
9 h
3 h
3 h
6 h
24 h
9 h
5f
5f
>99%
a Reactions performed at 80 ◦C under N2 atmosphere using 2 mmol
of substrate and 0.5 cm3 of water or glycerol. b Determined by GC.
c Reaction performed at 35 ◦C. d Cis isomer not detected. e Catalyst
generated in situ from [{RuCl(m-Cl)(h :h -C10H16)}2] (3) and P(OMe)3.
Scheme 3 Synthesis of the mononuclear bis(allyl)-ruthenium(IV) com-
plexes 5a–n.
3
3
As shown in the table, despite all the complexes synthesized
were found to be active catalysts in the isomerization of
estragole, none of them proved to be more active or selective
complexes 5b–e, that also dissolve completely in water and
glycerol due to the presence of the hydrophilic phosphine ligands
TPPMS, PTA, PTA-Bn and DAPTA,28 were comparatively
much less effective (entries 3–10). The electronic nature of the
ligands seems to play a key role on the catalytic activity of these
mononuclear species, the best results being in general observed
when p-accepting ligands, such as phosphites, isocyanides and
carbon monoxide, are coordinated to ruthenium.
In complete accordance with the results obtained with dimer
3, the selectivity of 5f towards trans-anethole could be improved
by performing the catalytic reactions at low temperature (35 ◦C;
entries 29–30 vs. 11–12). In particular, using water as solvent
(entry 29) a quantitative conversion of estragole into anethole
could be reached after 6 h in a total stereoselective manner
(cis isomer not detected by GC). Remarkably, using these
optimal reaction conditions, the isomerization process could be
easily scaled-up without detrimental effect on the stereoselec-
tivity. Thus, as shown in Scheme 4, starting from 20.2 mmol
of estragole, 2.89 grams of analytically pure trans-anethole
were isolated (96% yield) after extraction with diethyl ether
3
3
than dimer 3. In fact, only [RuCl2(h :h -C10H16){P(OMe)3}] (5f)
showed an effectiveness comparable to that of 3, being able
to convert quantitatively 1 into 2 after only 30 min (water) or
1 h (glycerol) of heating, albeit with a slightly lower trans/cis
selectivity (95 : 5; entries 11–12). In the rest of the cases, longer
reaction times were needed to attain similar conversions and,
in general, higher amounts of the undesirable cis isomer were
formed. At this point it should be mentioned that, as previously
observed with complexes 3 and 4, a two-phase system (water
or glycerol/organic products) is in all cases formed, with the
ruthenium catalyst remaining mainly in the aqueous or glycerolic
phase (in some cases a suspension distributed between the
3
3
two phases was observed).29 Regarding complex [RuCl2(h :h -
C10H16){P(OMe)3}] (5f) both phases were completely homoge-
nous because of the unexpectedly high solubility of this complex
in water and glycerol (8.3 and 2.5 mg mL-1, respectively, at
20 ◦C). However, solubility grounds are not the only factor
responsible for the high catalytic activity shown by 5f since
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higher than that observed under the same conditions in water
or glycerol (entries 3–4 vs. 1–2 and 11–12 vs. 9–10), reducing
the content of the undesirable cis isomer in the final anethole
to a maximum of 1%. Improvement of the activity was also
in most cases observed. Interestingly, this high stereoselectivity
was maintained when H2O–MeOH (EtOH) or glycerol–MeOH
(EtOH) mixtures (1 : 1 v/v) were used as solvent, the reactions
proceeding even faster when compared to those performed using
a single solvent alone (quantitative conversions after 5–30 min;
entries 5–8 and 13–16). In particular, quantitative conversion of
estragole into anethole (trans/cis ratio = 99 : 1) could be reached
after only 5 min of heating performing the catalytic reactions
with [RuCl2(h :h -C10H16){P(OMe)3}] (5f) in H2O–MeOH or
H2O–EtOH mixtures (entries 13–14; 10 min if glycerol-based
mixtures are employed: see entries 15–16). The beneficial effect
of methanol and ethanol can be attributed to their ability
to generate the catalytically active ruthenium-hydride species,
via a b-hydride elimination process on the corresponding Ru-
alcoholate intermediates.
Scheme 4 Stereoselective isomerization of estragole in preparative
scale.
(3 ¥ 10 cm3) and subsequent filtration of the combined ethereal
1
1
solutions over silica-gel (copies of the H and 13C{ H} NMR
and GC spectra obtained are included in the ESI†). It is also
important to note that, the in situ generated catalyst (just by
3
3
3
3
mixing the dimeric precursor [{RuCl(m-Cl)(h :h -C10H16)}2] (3)
and two equivalents of trimethyl phosphite) presents a catalytic
performance similar to that of the isolated complex 5f (entry
31 in Table 2). This convenient one-pot procedure, based on the
use of two commercially available air-stable reagents without the
need of any purification, is an additional proof of the potential
utility of 5f for a practical application.
Remarkably, the use of all these mixtures of solvents led
again to biphasic reaction media (solvents/products). As in
the precedent cases, the ruthenium catalysts remained dissolved
mainly in the solvents phase, leading to almost completely
homogenous solutions even in the case of the poorly soluble
6
In our previous work using (h -arene)-ruthenium(II) com-
plexes (Scheme 2),23 the best results were obtained using
methanol and ethanol as solvents (quantitative conversions after
15–30 min of heating at 80 ◦C with trans-selectivities ≥99%). This
3
3
fact prompted us to study the behaviour of [{RuCl(m-Cl)(h :h -
3
3
dimer [{RuCl(m-Cl)(h :h -C10H16)}2] (3) (an illustrative example
3
3
C10H16)}2] (3) and [RuCl2(h :h -C10H16){P(OMe)3}] (5f) in these
media. We note that, although methanol and ethanol are not
commonly considered as “green” solvents, they represent a
good alternative to the widely used petroleum-based ones since
they can be produced from biomass. In fact, their use is highly
recommended, especially in the pharmaceutical industry.18c,30
As shown in Table 3, performing the catalytic reactions at
80 ◦C with a ruthenium loading of 1 mol%, the selectivity of
the isomerization process in MeOH and EtOH was in all cases
is given in Fig. 2).
Table 3 Estragole to anethole isomerization using complexes 3 and 5f
in different solventsa
Fig. 2 The biphasic system resulting from the reaction described in
entry 7 of Table 3.
Entry Cat. Solvent
Time
Yieldb
E/Zb
1
2
3
4
5
6
7
8
3
H2O
Glycerol
MeOH
EtOH
30 min >99% 97 : 3
45 min >99% 97 : 3
30 min >99% 99 : 1
30 min >99% 99 : 1
10 min >99% 99 : 1
10 min >99% 99 : 1
30 min >99% 99 : 1
30 min >99% 99 : 1
30 min >99% 95 : 5
The appearance of a biphase, along with the remarkable
activity and trans-selectivity shown by complexes 3 and 5f
in these mixtures of solvents, prompted us to study their
recyclability. To this end, mixtures H2O–MeOH and glycerol–
MeOH were used, separating the final anethole with the aid
of a Pasteur pipette and washing the aqueous or glycerolic
phase with n-heptane (3 ¥ 2 cm3) prior to a new addition of
estragole. Gratifyingly, as shown in Table 4, the aqueous or
glycerolic phase containing the catalysts could be re-used up to
five consecutive runs. However, an important decrease of the
activity, accompanied by a slight loss of the trans-selectivity,
was observed after each cycle. Best results were obtained using
the mononuclear complex 5f in glycerol–MeOH, which after
five consecutive runs was still able to generate anethole in 96%
yield and high stereoselectivity (trans/cis ratio = 95 : 5) after
24 h of heating (entry 4). Partial decomposition of the catalysts
3
3
3
3
H2O–MeOHc
H2O–EtOHc
Glycerol–MeOHc
Glycerol–EtOHc
H2O
3
3
3
9
5f
5f
5f
5f
5f
5f
5f
5f
10
11
12
13
14
15
16
Glycerol
MeOH
EtOH
1 h
>99% 95 : 5
10 min >99% >99% transd
10 min >99% 99 : 1
H2O–MeOHc
H2O–EtOHc
Glycerol–MeOHc
Glycerol–EtOHc
5 min
5 min
>99% 99 : 1
>99% 99 : 1
10 min >99% 99 : 1
10 min >99% 99 : 1
a Reactions performed at 80 ◦C under N2 atmosphere using 2 mmol of
substrate and 0.5 cm3 of the indicated solvent. b Determined by GC.
c 1 : 1 v/v ratio. d Cis isomer not detected.
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Table 4 Estragole to anethole isomerization using complexes 3 and 5f:
Experimental
Catalyst recyclinga
General methods
The manipulations were performed under inert N2 atmosphere
using vacuum-line and standard Schlenk or sealed-tube
techniques. All reagents were obtained from commercial
3
3
suppliers with the exception of compounds [{RuCl(m-Cl)(h :h -
Entry Cat. Solvent
Cycle Time
Yieldb E/Zb
10 min >99% 99 : 1
3
2
3
3
3
C10H16)}2] (3),32 [RuCl2(h :h :h -C12H18)] (4),33 [RuCl2(h :h -
1
2
3
4
3
H2O–MeOHc
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
5
C10H16)(PPh3)] (5a),34 [RuCl2(h :h -C10H16)(TPPMS)] (5b),35
3
3
6 h
17 h
24 h
>99% 96 : 4
>99% 93 : 7
>99% 92 : 8
[RuCl2(h :h -C10H16)(PTA)] (5c),35 [RuCl2(h :h -C10H16)(PTA-
3
3
3
3
3
3
3
3
Bn)] (5d),35 [RuCl2(h :h -C10H16)(DAPTA)] (5e),35 [RuCl2(h :h -
C10H16){P(OMe)3}] (5f),36 [RuCl2(h :h -C10H16){P(OEt)3}]
3
3
3
Glycerol–MeOHc
H2O–MeOHc
30 min >99% 99 : 1
6 h
17 h
24 h
5 min
15 min >99% 98 : 2
9 h
24 h
10 min >99% 99 : 1
30 min >99% 99 : 1
1 h
6 h
24 h
3
3
3
3
(5g),37 [RuCl2(h :h -C10H16){P(OPh)3}] (5i),38 [RuCl2(h :h -
>99% 97 : 3
>99% 97 : 3
C10H16)(CO)] (5l),34 [RuCl2(h :h -C10H16)(NCMe)] (5m)39
3
3
99%
97 : 3
and [RuCl2(h :h -C10H16)(NCPh)] (5n),37 which were prepared
following the methods reported in the literature. Infrared spectra
were recorded on a Perkin–Elmer 1720-XFT spectrometer.
Elemental analyses were performed with a Perkin–Elmer 2400
microanalyzer. GC measurements were made on a Hewlett–
Packard HP6890 equipment using a Supelco Beta-DexTM 120
column (30 m length; 250 mm diameter). NMR spectra were
recorded on a Bruker DPX-300 instrument at 300 MHz (1H),
75.4 MHz (13C) or 121.5 (31P) using SiMe4 or 85% H3PO4 as
standards. DEPT experiments have been carried out for all the
compounds reported. The numbering for protons and carbons
of the 2,7-dimethylocta-2,6-diene-1,8-diyl skeleton is as follows:
3
3
5f
5f
>99% 99 : 1
>99% 94 : 6
96% 93 : 7
Glycerol–MeOHc
>99% 98 : 2
>99% 97 : 3
96%
95 : 5
a Reactions performed at 80 ◦C under N2 atmosphere using 2 mmol of
substrate and 0.5 cm3 of the indicated solvent. b Determined by GC.
c 1 : 1 v/v ratio.
seems to be responsible for their lower activity and selectivity
after each catalytic cycle. In this sense, analysis by 31P{ H}
1
NMR spectroscopy of the crude anethole generated using 5f
showed the presence of trace amounts of trimethylphosphate
(dP 0.3 ppm),31 resulting from the oxidation of free P(OMe)3
ligand. The ruthenium content in crude anethole was also inves-
tigated by means of inductively coupled plasma-atomic emission
spectroscopy (ICP-AES) analysis. The weight percentage of
ruthenium in samples generated from complexes 3 and 5f in
glycerol/methanol mixtures was 280 and 320 ppm, respectively,
confirming that Ru-leaching also takes place during recycling.
Preparation of complexes [RuCl2(g3:g3-C10H16)(L)] (L =
P(OiPr)3 (5h), CNBn (5j), CNCy (5k))
3
3
A purple solution of dimer [{RuCl(m-Cl)(h :h -C10H16)}2] (3)
(0.308 g, 0.5 mmol) in acetone (20 cm3) was treated, at
room temperature, with the corresponding two-electron donor
ligand L (1 mmol) for 1 h. The resulting yellow solution was then
evaporated to dryness, and the resulting solid residue washed
with diethyl ether (3 ¥ 5 cm3) and dried in vacuo.
Conclusions
In summary, we have developed new ruthenium-based catalytic
systems able to promote the isomerization of estragole into
anethole. To the best of our knowledge, these systems constituted
3
3
by the bis(allyl)-ruthenium(IV) complexes [{RuCl(m-Cl)(h :h -
3
3
C10H16)}2] and [RuCl2(h :h -C10H16){P(OMe)3}], along with
6
the ruthenium(II) examples [RuCl2(h -arene)(L)] previously re-
[RuCl2(g3:g3-C10H16){P(OiPr)3}] (5h). Yellow solid; Yield:
92% (0.475 g); IR (KBr): n 491 (w), 858 (m), 701 (w), 743 (m),
755 (m), 859 (w), 882 (m), 976 (s), 1006 (s), 1105 (s), 1138 (w),
1176 (w), 1370 (m), 1382 (m), 1452 (w), 2860 (w), 2926 (w), 2974
ported by us,23 are the most efficient catalysts reported to
date for this transformation. The best results in terms of
stereoselectivity have been observed when using methanol as
solvent. In this medium, anethole is generated with a trans-
selectivity ≥99%, thus fulfilling the strict criteria imposed by the
FAO/WHO committee. More importantly, such a remarkable
trans-selectivity is maintained when the catalytic reactions are
performed in water–methanol and glycerol–methanol mixtures.
In addition, the use of these environmentally friendly media
resulted also in an improvement of the reaction rates compared
to those observed using one of the three solvents alone. Overall,
the results presented herein represent the first green synthetic
approach to the industrially relevant additive trans-anethole.
1
1
(w) cm-1; 31P{ H} NMR (CDCl3): d 117.4 (s) ppm; H NMR
(CDCl3): d 1.35 and 1.39 (d, 3JHH = 6.0 Hz, 9H each, CHMe2),
2.18 (s, 6H, Me), 2.65 (br, 2H, H4 and H6), 3.39 (br, 2H, H2
and H10), 3.51 (br, 2H, H5 and H7), 4.54 (d, 3JHP = 8.5 Hz, 2H,
H1 and H9), 4.80 (m, 3H, CHMe2), 5.13 (br, 2H, H3 and H8)
ppm; 13C{ H} NMR (CDCl3): d 20.6 (s, Me), 23.8 (s, CHMe2),
1
2
36.7 (s, C4 and C5), 62.7 (d, JCP = 7.9 Hz, C1 and C8), 71.1
2
2
(d, JCP = 10.7 Hz, CHMe2), 109.0 (d, JCP = 15.2 Hz, C3 and
C6), 124.0 (s, C2 and C7) ppm; Elemental analysis calcd (%) for
C19H37O3Cl2PRu: C 44.19, H 7.22; found: C 44.04, H 7.31.
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[RuCl2(g3:g3-C10H16)(CNBn)] (5j). Yellow solid; Yield: 73%
(0.310 g); IR (KBr): n 493 (w), 601 (w), 693 (m), 735 (s), 784
(w), 854 (m), 962 (m), 1025 (m), 1308 (w), 1354 (m), 1456 (s),
2215 (vs), 2855 (w), 2912 (w), 3006 (w) cm-1; 1H NMR (C6D6):
d 2.28 (br, 2H, H4 and H6), 2.31 (s, 6H, Me), 3.23 (br, 2H, H2
and H10), 4.03 (br, 2H, H5 and H7), 4.10 (s, 2H, NCH2), 4.73 (br,
2H, H1 and H9), 5.39 (br, 2H, H3 and H8), 7.07–7.21 (m, 5H, Ph)
of Ph.D. grants. Authors would like to thank an anonymous
referee for their helpful comments and suggestions.
Notes and references
1 See, for example: (a) Metal-Catalysis in Industrial Organic Processes,
ed. G. P. Chiusoli and P. M. Maitlis, RSC Publishing, Cambridge,
2008; (b) P. W. N. M. van Leeuwen, in Homogeneous Catalysis:
Understanding the Art, Kluwer Academic Publishers, Dordrecht,
2004; (c) G. W. Parshall and S. D. Ittel, in Homogeneous Catalysis,
John Wiley & Sons, New York, 2nd edn, 1992.
2 See, for example: (a) G. Reineccius, in Flavor Chemistry and Technol-
ogy, CRC Press, Boca Raton, 2nd edn, 2006; (b) K. Bauer, D. Garbe
and H. Surburg, in Common Fragrance and Flavour Materials, Wiley-
VCH, Weinheim, 4th edn, 2001; (c) P. R. Ashurst, in Food Flavorings,
Aspen Publishers, Maryland, 3rd edn, 1999; (d) A. J. Chalk, in
Flavors and Fragrances: A World Perspective, ed. B. M. Lawrence,
B. D. Mookherjee and B. J. Willis, Elsevier Science Publishers,
Amsterdam, 1988.
3 (a) T. Derfer, in The Chemistry of Turpentine in “Naval Stores”,
ed. D. Zinkel and J. Russell, Pulp Chemicals Association,
New York, 1992; (b) See also the following report: “Test Plan
for Estragole” submitted by the Flavor and Fragrance High
Production Volume Consortia (FFHPVC) to the United States
Environmental Protection Agency (EPA) in 2002. Available on-line at
http://www.epa.gov/hpv/pubs/summaries/estragole/c14022tc.htm
(accessed July 26, 2010).
4 Other routes for the large-scale synthesis of anethole, involving the
treatment of anisole with propionic acid derivatives or propionalde-
hyde, are known: (a) G. E. Svadkowskaya, L. A. Kheifits, A. I. Platova
and K. Denisenlova, USSR Patent, SU 261380, 1970; (b) K. Bauer
and R. Molleken, Ger. Offen., DE 2418974, 1978.
1
ppm; 13C{ H} NMR (C6D6): d 21.2 (s, Me), 37.9 (s, C4 and C5),
48.1 (s, NCH2), 63.7 (s, C1 and C8), 109.1 (s, C3 and C6), 125.4
(s, C2 and C7), 126.3, 128.3 and 128.9 (s, CH of Ph), 130.0 (s, C
of Ph), 132.8 (s, Ru-CN) ppm; Elemental analysis calcd (%) for
C18H23Cl2NRu: C 50.83, H 5.45, N 3.29; found: C 50.90, H 5.36,
N 3.20.
[RuCl2(g3:g3-C10H16)(CNCy)] (5k). Yellow solid; Yield: 64%
(0.267 g); IR (KBr): n 531 (w), 656 (w), 670 (s), 787 (w), 856
(m), 960 (w9, 1025 (m), 1127 (w), 1269 (w), 1321 (w), 1456 (m),
2196 (vs), 2851 (w), 2934 (m), 2994 (w) cm-1; 1H NMR (C6D6):
d 0.90–1.65 (m, 10H, CH2), 2.29 (br, 2H, H4 and H6), 2.35 (s,
6H, Me), 3.25 (m, 3H, NCH, H2 and H10), 4.05 (br, 2H, H5 and
H7), 4.76 (br, 2H, H1 and H9), 5.40 (br, 2H, H3 and H8) ppm;
1
13C{ H} NMR (C6D6): d 21.3 (s, Me), 22.4, 25.1 and 32.3 (s,
CH2 of Cy), 38.01 (s, C4 and C5), 55.1 (s, CH of Cy), 63.7 (s,
C1 and C8), 108.7 (s, C3 and C6), 125.1 (s, C2 and C7), 131.9 (s,
Ru-CN) ppm; Elemental analysis calcd (%) for C17H27Cl2NRu:
C 48.92, H 6.52, N 3.36; found: C 49.04, H 6.60, N 3.44.
5 Anethole formation can also be achieved through different C–C
coupling and reduction reactions. However, these methods are not
economically viable alternatives for its large-scale production. See,
for example: (a) J. C. Roberts and J. A. Pincock, J. Org. Chem., 2006,
71, 1480; (b) J. A. Miller and J. W. Dankwardt, Tetrahedron Lett.,
2003, 44, 1907; (c) G. Cahiez and H. Avedissian, Tetrahedron Lett.,
1998, 39, 6159; (d) A. D. Buss, R. Mason and S. Warren, Tetrahedron
Lett., 1983, 24, 5293; (e) S. Cabiddu, A. Maccioni and M. Secci, Ann.
Chim., 1962, 52, 1261.
6 See, for example: (a) I. A. Khan and E. A. Abourashed, in Leung’s
Encyclopedia of Common Natural Ingredients Used in Food, Drugs
and Cosmetics, John Wiley & Sons, Hoboken, 3rd edn, 2010;
(b) B. Sima´ndi, A. Dea´k, E. Ro´nyai, G. Yanxiang, T. Verres, E.
Lemberkovics, M. Then, A. Sass-Kiss and Z. Va´mos-Falusi, J. Agric.
Food Chem., 1999, 47, 1635; (c) D. Q. Tuan and S. G. Ilangantileke,
J. Food Eng., 1997, 31, 47.
7 Anethole is also a principal constituent of other essential oils
derived from medicinal plants with anti-oxidant, anti-inflammatory,
gastroprotective, psycholeptic and anaesthetic properties: P. M. G.
Soares, R. F. Lima, A. de Freitas Pires, E. P. Souza, A. M. S. Assreuy
and D. N. Criddle, Life Sci., 2007, 81, 1085 and references cited
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8 See, for example: (a) G. H. Eirew, U.S. Pat,. Appl., US 2009035229,
2009; (b) U. Glenert, Eur. J. Pharmacol., Mol. Pharmacol. Sect., 1992,
226, 43; (c) J. B. Epstein and M. M. Schubert, Oral Surg., Oral Med.,
Oral Pathol., 1987, 64, 179.
9 See, for example: (a) D. Mansuy, A. Sassi, P. M. Dansette and M. Plat,
Biochem. Biophys. Res. Commun., 1986, 135, 1015; (b) K.-I. Fujita,
T. Fujita and I. Kubo, Phytother. Res., 2007, 21, 47; (c) E. Enan,
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10 Some data on anethole production and consumption can be found in
the report: “Test Plan for Anethole” submitted by FFHPVC to EPA
(http://www.epa.gov/hpv/pubs/summaries/anethole/c14069tc.
htm; accessed July 26, 2010).
11 (a) A. P. Wagner, Manuf. Chemist, 1952, 23, 56; (b) H. Pines and W. M.
Salik, in Base-Catalyzed Reactions of Hydrocarbons and Related
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12 Crystallization at low temperature can also be used to purify trans-
anethole: C. B. Davies, U.S. Pat. Appl., US 4902850, 1990.
General procedure for the catalytic reactions
Under nitrogen atmosphere, the ruthenium catalyst precursor
(0.01 mmol of 3 or 0.02 mmol of 4 and 5a–n; 1 mol% of
Ru), 0.5 cm3 of the indicated solvent and estragole (0.307 cm3,
2 mmol) were introduced into a teflon-capped sealed tube. Then,
the mixture was heated at 80 ◦C or 35 ◦C in an oil-bath for the
indicated time. The course of the reaction was monitored by
taking regularly samples of ca. 20 mL which after extraction
with CH2Cl2 (3 cm3) were analyzed by GC [Supelco Beta-DexTM
120 (30 m le◦ngth; 250 mm diameter) column; helium 4 mL min-1,
◦
◦
160 C, 10 C min-1 to 210 C: 1.68 min (estragole), 1.91 min
(cis-anethole) and 2.10 min (trans-anethole)].
Catalyst recycling
After completion of the reaction, the mixture was allowed to
reach the room temperature and two phases appeared. Most of
anethole was separated with the aid of a Pasteur pipette and the
remaining extracted from the aqueous or glycerolic phase with
n-heptane (3 ¥ 2 cm3). To the aqueous or glycerolic phase a new
load of estragole (0.307 cm3, 2 mmol) was then added and the
mixture heated at 80 ◦C in an oil-bath for the indicated time.
Acknowledgements
This work was supported by the Spanish MICINN (Projects
CTQ2006-08485/BQU, CSD2007-00006 and CTQ2010-
14796/BQU) and the Gobierno del Principado de Asturias
(FICYT Project IB08-036). B.L.-B. thanks FICYT of Asturias
(PCTI-Severo Ochoa program) and J.F. thanks MEC of Spain
and the European Social Fund (FPU program) for the award
312 | Green Chem., 2011, 13, 307–313
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27 The combined use of MW-irradiation, as a nonclassical low-energy-
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28 Abbreviations
benzenesulfonate
used:
TPPMS
salt;
=
3-diphenylphosphino-
1,3,5-triaza-7-
= 1-benzyl-3,5-diaza-
sodium
PTA
=
phosphatricyclo[3.3.1.13,7]decane; PTA-Bn
1-azonia-7-phosphatricyclo[3.3.1.13,7]decane chloride; DAPTA
3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane.
=
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©