Selective double bond isomerization of allyl phenyl ethers catalyzed by ruthenium metal complexes

Article abstract of DOI:10.1016/j.molcata.2005.10.005

The RuCl₂(PPh₃)₃ and RuCl ₃(AsPh₃)₂·CH₃OH complex catalysts were studied for the double bond isomerization of methyl chavicol and eugenol in the polar aprotic (DMSO, acetonitrile), polar protic (ethanol, methanol, n-propanol, iso-propanol, n-butanol, iso-butanol, n-hexanol) and non-polar (benzene, toluene, n-hexane, cyclohexane, tetrahydrofuran) solvents. The highest conversion of methyl chavicol (99.7%) with 95.4% selectivity of trans-anethole was observed in ethanol using RuCl₂(PPh ₃)₃ catalyst. However, 94.2% conversion of methyl chavicol with 98.6% selectivity of trans-isomer was observed using methanol as a solvent in RuCl₃(AsPh₃)₂·CH₃OH. The kinetics of isomerization of methyl chavicol and eugenol using RuCl ₂(PPh₃)₃ and RuCl₃(AsPh ₃)₂·CH₃OH catalysts in ethanol or methanol involving the effect of substrate concentrations, catalyst amount, solvents and temperature on the initial rate of reaction was studied in details. The initial rates of reaction were observed to increase on increasing the initial concentrations of methyl chavicol, eugenol and catalysts. However, the initial rate of reaction decreased on increasing the solvent concentration. Activation energies for the isomerization of methyl chavicol were calculated using RuCl₂(PPh₃)₃ and RuCl ₃(AsPh₃)₂·CH₃OH catalysts and were found to be 4.3 and 6.0 kJ/mol, respectively. Activation energy for the isomerization of eugenol was found to be 6.9 kJ/mol using RuCl ₂(PPh₃)₃ complex catalyst. The RuCl ₂(PPh₃)₃ complex was recycled five times for the isomerization of methyl chavicol and eugenol.

Full text of DOI:10.1016/j.molcata.2005.10.005

Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
Selective double bond isomerization of allyl phenyl ethers
catalyzed by ruthenium metal complexes
Sumeet K. Sharma, Vivek K. Srivastava, Raksh V. Jasra∗
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), G.B. Marg, Bhavnagar 364002, India
Received 16 July 2005; received in revised form 19 September 2005; accepted 5 October 2005
Available online 9 November 2005
Abstract
The RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH complex catalysts were studied for the double bond isomerization of methyl chavicol and
eugenol in the polar aprotic (DMSO, acetonitrile), polar protic (ethanol, methanol, n-propanol, iso-propanol, n-butanol, iso-butanol, n-hexanol)
and non-polar (benzene, toluene, n-hexane, cyclohexane, tetrahydrofuran) solvents. The highest conversion of methyl chavicol (99.7%) with 95.4%
selectivity of trans-anethole was observed in ethanol using RuCl2(PPh3)3 catalyst. However, 94.2% conversion of methyl chavicol with 98.6%
selectivity of trans-isomer was observed using methanol as a solvent in RuCl3(AsPh3)2·CH3OH. The kinetics of isomerization of methyl chavicol
and eugenol using RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH catalysts in ethanol or methanol involving the effect of substrate concentrations,
catalyst amount, solvents and temperature on the initial rate of reaction was studied in details. The initial rates of reaction were observed to increase
on increasing the initial concentrations of methyl chavicol, eugenol and catalysts. However, the initial rate of reaction decreased on increasing the
solvent concentration. Activation energies for the isomerization of methyl chavicol were calculated using RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH
catalysts and were found to be 4.3 and 6.0 kJ/mol, respectively. Activation energy for the isomerization of eugenol was found to be 6.9 kJ/mol
using RuCl2(PPh3)3 complex catalyst. The RuCl2(PPh3)3 complex was recycled five times for the isomerization of methyl chavicol and eugenol.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Methyl chavicol; trans-Anethole; Eugenol; Isomerization; Kinetics; Ruthenium
1. Introduction
Generally, most of the trans-anethole is extracted from
the natural sources like anise oil (80–90%), star anise oil
(>90%), anise seeds and fennel oil (80%). The demand of
trans-anethole in the world market increased rapidly during last
few years due to its growing applications in various products.
The production of trans-anethole from the natural sources was
not sufficient enough to fulfill the increased demand, there-
fore, other alternative routes starting from anisole [4] and
methyl chavicol [5] have been developed. The synthesis of
trans-anethole from anisole is not favorable from industrial
viewpoint due to multi-step involvement. Therefore, the sin-
gle step preparation of anethole (mixture of trans- and cis-
isomers) from methyl chavicol via double bond isomerization
has drawn a special attention from both industrial and aca-
demic perspectives [6–12]. Presently, trans-anethole is com-
mercially produced by double bond isomerization of methyl
chavicol using liquid bases like KOH and/or NaOH in sto-
ichiometric amounts at 473 K reaction temperature. Under
these conditions 56% conversion is achieved in 12 h reaction
time with a trans:cis (anethole) ratio of 82:18 in the product
mixture.
The allyl phenyl ethers have unique importance in the per-
fumery, fragranceandfoodindustriesandamongtheallylphenyl
ethers, the trans-anethole and isoeugenol have high industrial
demand as intermediates for the synthesis of various perfumery
chemicals [1–3]. Besides, trans-anethole also finds application
in alcoholic beverage industry, food industry and in the formu-
lation of oral sanitation products. It is one of the precursors
for the synthesis of anisic aldehyde, synthetic intermediate for
fragrance and flavor industries. The total annual global pro-
duction of trans-anethole is approximately 0.75 million metric
tonnes. trans-Isoeugenol which is also known as 2-methoxy-
4-(1-propenyl) phenol, is widely used in manufacture of per-
fumeries, stabilizers and antioxidants for plastic and rubber
industries. It is also used in antiseptic and analgesic medicines,
as well as for the production of vanillin.
∗
Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2567562.
E-mail address: rvjasra@csmcri.org (R.V. Jasra).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.10.005

S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
201
Additionally, the anethole obtained from certain natural
2.2. Synthesis and characterization of metal complexes
sources and synthesized from methyl chavicol is always accom-
panied with the trans- and cis-isomers (mostly cis-isomer
>15%). Thermodynamically, trans-isomer is more stable as
compared to cis and commercially trans-isomer is an important
valuable product for various applications. According to food
regulatory instructions, more than 1% cis-isomer cannot be tol-
erated due to its toxicity and sharp, unpleasant taste. On the other
hand, present commercial process using liquid base like KOH
or NaOH produce trans to cis ratio of anethole is 82:18. There-
fore, isomerization process is required in which either cis-isomer
is converted into trans-isomer in the same reaction conditions,
or to develop a suitable catalyst that can restrain the formation
of cis-isomer to minimum quantitative amounts in order that a
fractional distillation may be effective to separate the remaining
cis-isomers from product mixture [8].
The metal complexes, RuCl2(PPh3)3 and RuCl3(AsPh3)2·
CH3OH were synthesized using reported methods [18].
2.2.1. Synthesis of RuCl2(PPh3)3
RuCl3·3H2O (0.2 g) was dissolved in methanol (50 ml) and
six-fold excess (1.2 g) of PPh3 was added. The solution was
refluxed for 4 h under N2 (or Ar) atmosphere. The resulting red-
dish brown crystals of the complex were washed with methanol
followed by diethyl ether and dried in vacuum (yield 75%).
2.2.2. Synthesis of RuCl3(AsPh3)2·CH3OH
RuCl3·3H2O (0.2 g) was dissolved in methanol (50 ml) and
six-fold excess (1.2 g) of AsPh3 was added. The solution was
refluxedfor2 hunderN2 (orAr)atmosphere. Theresultinggreen
crystals of the complex were washed with methanol followed by
diethyl ether and dried in vacuum (yield 70%).
Interestingly, isoeugenol at the commercial level is also being
produced by the prolonged heating of the eugenol with the
stoichiometric amount of liquid base like KOH in the pres-
ence of alcohol, mostly higher alcohols, at higher temperature
[13–15].
All the metal complexes were characterized by Bruker
Avance DPX 200 MHz FT-NMR (1H, 31P) spectroscopy and
Perkin-Elmer Spectrum GX FT-IR spectroscopy. The 31P NMR
spectrum of RuCl2(PPh3)3 (in CD2Cl2) gave a singlet at
41.42 ppm which confirms the formation of the complex.
The appearance of ν(Ru–P) and ν(Ru–As) bands at 517 and
474 cm−1, respectively, confirmed the formation of correspond-
ing ruthenium complexes. C, H, N elemental analysis was done
using Perkin-Elmer CHNS/O 2400 analyzer. Metal Complexes;
C, H calculated (found): RuCl2(PPh3)3: C 67.7 (67.5); H 4.7
(4.5) and RuCl3(AsPh3)2·CH3OH: C 52.2 (52.1); H 4.0 (4.1).
Thus, the existing commercial processes for the synthesis
of trans-anethole from methyl chavicol and trans-isoeugenol
from eugenol via double isomerization reaction possess demer-
its like use of strong liquid base in large amount, longer reaction
time, lower conversion of reactant, lower selectivity of trans-
isomer, higher reaction temperature, post synthesis work-up in
separation of spent KOH from reactants/products mixture, haz-
ardous post reaction effluent disposal problems and separation
of cis-isomer from the products mixture. However, recently we
have successfully demonstrated the applicability of solid base
catalysts [11] like basic alumina, exchanged zeolites and hydro-
talcites, which overcome some of these drawbacks. In another
study, the applicability of transition metal complexes as cata-
lysts was also shown for the isomerization of methyl chavicol
to trans-anethole [16,17].
In this present study, we are reporting selective dou-
ble bond isomerization of methyl chavicol to trans-anethole
and eugenol to trans-isoeugenol using RuCl2(PPh3)3 and
RuCl3(AsPh3)2·CH3OH as catalysts. The detailed kinetic study
comprises the effect of concentration of reactants, catalysts, sol-
vents and reaction temperature on the rate of reaction. The effect
of various solvents, reusability of the catalyst has also been stud-
ied.
2.3. Isomerization reaction and products analysis
Typically, isomerization reaction was carried out in a 50 ml
oven dried double necked round bottom flask in which pre-
calculated amount of metal complex catalyst, solvent and
tetradecane as an internal standard were taken. One neck of
the flask was fitted with refluxing condenser and another neck
of the flask was blocked with silicon rubber septa. The entire
experimental set-up was kept in an oil bath equipped with tem-
perature controlling unit. After attaining the reflux temperature
(358 K) of the mixture, the reactants were fed into the flask
with the help of a glass syringe via silicon rubber septa. After
the set reaction time, the reaction mixture was cooled to room
temperature and filtered. For the kinetic studies, the samples
(0.01 ml) were taken out during the experiment by glass syringe
at different time intervals. The analysis of these samples were
carried out using gas chromatography (Shimadzu 17A, Japan),
having 5% diphenyl and 95% dimethyl siloxane universal cap-
illary column (60 m length and 0.25 mm diameter) and flame
ionization detector (FID). The initial column temperature was
increased from 373 to 433 K at the rate of 5 K/min using nitro-
gen as a carrier gas. The temperature of injection port and FID
were kept 513 and 573 K, respectively, during product analysis.
The retention times for different compounds were determined
by injecting pure compounds under identical gas chromatogra-
phy conditions. The reaction mixture of isomerization of methyl
chavicol and eugenol was further characterized by FT-NMR.
2. Experimental
2.1. Materials
trans-Anethole (99.8%), methyl chavicol (98%), eugenol
(98%), trans-isoeugenol (99.7%) and tetradecane (98%) were
procured from Lancaster, UK and used without further purifi-
cation. All the solvents (analytical grade) were purchased from
Ranbaxy Fine Chemicals Limited, India. Ruthenium trichloride
(RuCl3·3H2O), triphenylphosphine (PPh3) and triphenylarsine
(AsPh3), were procured from E. Merck, USA.

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S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
tants, catalysts, solventsandthereactiontemperature. Therefore,
the kinetic experiments were carried out by varying these param-
eters. In each case, the changes in concentration of reactant were
determined by gas chromatography at fixed time intervals. The
initial rate of reaction was calculated by plotting the concen-
tration of reactant in the range of initial period wherein the
concentrationofthereactantdidnotchangesignificantly(<10%)
with time, which gave a polynomial fit and the analysis was car-
ried out by differential method. Under the kinetics experimental
conditions, trans- and cis-isomers were only the major products
formed during all the isomerization reaction.
To ensure the reproducibility of isomerization of methyl
chavicol and eugenol using metal complex catalysts in the sol-
vent, repeated experiments were carried out under identical
reaction conditions. The results obtained, including conversions
and selectivities were found to be within less than 1% vari-
ation, confirming the reproducibility of the results. In order to
check the material balance of the reaction, the moles of products
formed and moles of reactant consumed were compared. A typ-
ical concentration profile of reactant consumption and products
formation are shown in Fig. 1a (for methyl chavicol isomeriza-
tion) and Fig. 1b (for eugenol isomerization).
3. Results and discussion
Fig. 1. (a) Concentration profile of methyl chavicol isomerization
and products. Reaction conditions: [methyl chavicol] = 6.75 mmol,
3.1. Solvent effect
[RuCl2(PPh3)3] = 5.2 × 10−3 mmol,
[solvent]
(ethanol) = 65.22 mmol,
The nature of the solvent used has potential influence
in the isomerization reaction of the methyl chavicol and
eugenol, as seen from the conversion and selectivity data
given in Tables 1 and 2. The conversion of methyl chavi-
col was observed 4.7 and 8.2% (Table 1; Runs 1 and 12) in
highly polar aprotic solvents, such as DMSO and acetonitrile,
respectively, using RuCl2(PPh3)3 catalyst. Whereas in case of
RuCl3(AsPh3)2·CH3OH catalyst the conversion of methyl chav-
icol was observed 2 and 6% using DMSO and acetonitrile,
reaction time = 2 h, 358 K. (b) Concentration profile of eugenol iso-
merization and products. Reaction conditions: [eugenol] = 6.75 mmol,
[RuCl2(PPh3)3] = 5.2 × 10−3 mmol, [solvent] (ethanol) = 65.22 mmol, reaction
time = 2 h, at 358 K.
2.4. Kinetic analysis and reproducibility
The parameters, which might have pronounced effect on the
rate of isomerization reaction include, concentration of reac-
Table 1
Effect of solvents on the isomerization of methyl chavicol
Run
Solvents
RuCl2(PPh3)3 catalyst
RuCl3(AsPh3)2·CH3OH catalyst*
Refluxing temperature (K)
Conversion of
Selectivity of
Conversion of
Selectivity of
methyl chavicol (%)
trans-anethole (%)
methyl chavicol (%)
trans-anethole (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
DMSO
Chloroform
Benzene
Toluene
Cyclohexane
n-Butanol
n-Hexane
Ethanol
Methanol
iso-Propanol
iso-Butanol
Acetonitrile
Tetrahydofuran
Dichloromethane
4.7
58.5
63.0
82.8
87.4
98.8
67.7
99.7
86.7
99.6
99.2
8.2
80.9
60.4
82.4
77.4
77.8
94.2
86.7
95.4
94.0
94.3
94.0
84.6
88.7
86.2
2.0
72.5
58.2
90.6
34.3
92.6
20.6
93.8
94.2
92.1
93.0
6
82.0
93.4
92.3
93.0
92.2
96.2
93.7
97.2
98.6
96.5
95.8
90
470
338
357
388
359
395
345
358
358
358
385
358
343
321
69.5
61.7
66.4
52.7
95.4
93.4
Reaction conditions: [methyl chavicol] = 6.75 mmol, [solvent] = 65.22 mmol, [solvent*] = 93.75 mmol, [catalyst] = 5.2 × 10−3 mmol, [catalyst*] = 5.88 × 10−3 mmol,
reaction time = 3 h at refluxing temperature.

S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
203
Table 2
Effect of solvents on the isomerization of eugenol
Run
Solvents
RuCl2(PPh3)3 catalyst
RuCl3(AsPh3)2·CH3OH catalyst*
Refluxing temperature (K)
Conversion of
eugenol (%)
Selectivity of
trans-isoeugenol (%)
Conversion of
eugenol (%)
Selectivity of
trans-isoeugenol (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
DMSO
Benzene
Toluene
Cyclohexane
n-Hexane
5
76
71.6
85
2
7
90
85
88
77
75
82
92
81
85
82
87
82
83
80
68
81
470
357
388
359
345
358
358
358
372
385
395
413
358
343
321
433
74.2
97.5
64.3
54
99.8
93.6
99.8
99.7
99.7
99.5
98.2
5.4
43
15
11
14
2
33
11
21
38
27
2.5
4
74
78.6
95.6
84.2
95.2
94
94.6
92.2
91
91
87
82
84
Ethanol
Methanol
iso-Propanol
n-Propanol
iso-Butanol
n-Butanol
n-Hexanol
Acetonitrile
Tetrahydofuran
Dichloromethane
n-Decane
99.1
39
96.6
48
35
Reaction conditions: [eugenol] = 6.75 mmol, [solvent] = 65.22 mmol, [solvent*] = 93.75 mmol, [catalyst] = 5.2 × 10−3 mmol, [catalyst*] = 5.88 × 10−3 mmol, reac-
tion time = 3 h at refluxing temperature.
respectively. Similar behavior with the highly polar aprotic sol-
vents was observed during the double bond isomerization of
eugenol to isoeugenol. The conversion of the eugenol was found
5% in DMSO as a solvent using RuCl2(PPh3)3 catalyst (Table 2;
Run 1).
toluene as a solvent. The non-polar solvents gave slightly lower
conversion as compared to polar protic solvents, for the isomer-
ization of eugenol using RuCl2(PPh3)3 catalyst. The conversion
of eugenol was observed 99.1% in tetrahydrofuran, 97.5% in
toluene and 96.6% using n-decane as solvent. However, lower
selectivities of trans-isomer were observed in case of non-polar
solvents, as compared to polar protic solvents. For example,
the selectivity of the trans-isomer was found to be 87% in
tetrahydrofuran, 74.2% in toluene and 84% in n-decane. The
selectivities of trans-isomers were also influenced significantly
by the nature of solvents used in the isomerization reactions.
The maximum selectivity of trans-isomers was observed with
alcoholic solvents, while the solvent systems other than alcohols
have given relatively lower selectivity for the trans-isomers and
the magnitude of selectivity were almost similar with all the
solvent systems other than alcohols.
The double bond isomerization catalyzed by transition metal
complexes follows, generally, two types of mechanistic path-
ways [16], one is ␲-allylic mechanism (1,3-hydrogen shift) and
other is the alkyl mechanism (1,2-hydrogen shift). Generally,
metal hydride complexes follow the alkyl mechanism. The key
feature of the ␲-allylic mechanism is the ␤-C-H activation, a step
that includes the three-carbon arrangement in ␲-bonding to the
metal. In this mechanism, the proposed active catalyst species
RuCl2(PPh3)2, is formed by the dissociation of RuCl2(PPh3)3
(step I) [19]. The next step II, is the interaction of ␲-allylic
electron of 1 to form a ␲-complex 1c with the coordinatively
unsaturated metal complex, RuCl2(PPh3)2. The equilibrium step
III gives ␲-allyl metal complex 1d through allyl-H-migration
(oxidative additions) from ␥-carbon atom to metal. The hydride
shift (reductive eliminations) from ␲-allyl metal-hydride com-
plex 1d to ␣-carbon atom gives complex 1e via next equilibrium
step IV. The dissociation of active catalyst species from the
complex 1e results the formation of 2 and 3.
The highest conversion (99.7%) of methyl chavicol
with 95.4% selectivity for trans-anethole was observed in
moderate polar protic solvents such as ethanol followed
by iso-propanol (99.6% conversion, 94.3% selectivity), n-
butanol (98.8%, 94.2%) and iso-butanol (99.2%, 94%) in
RuCl2(PPh3)3 catalyzed isomerization reaction. While, in case
of RuCl3(AsPh3)2·CH3OH catalyzed reaction, the highest con-
version of methyl chavicol (94.2%) with 98.6% selectivity
for trans-anethole was observed in methanol (Table 1; Run
9) followed by other polar protic solvents. Similar behavior
was observed in case of double bond isomerization of eugenol
(Table2)toisoeugenolcatalyzedbyRuCl2(PPh3)3complex. The
highest conversion (99.8%) of eugenol with 95.6% selectiv-
ity for trans-isoeugenol was observed in moderate polar pro-
tic solvents, such as ethanol followed by iso-propanol (99.8%
conversion, 95.2% selectivity), n-propanol (99.7%, 94.0%),
iso-butanol (99.7%, 94.6%) and n-butanol (99.5%, 92.2%)
in RuCl2(PPh3)3 catalyzed isomerization reaction. However,
significantly lower conversion and selectivity of eugenol to
isoeugenol were observed in RuCl3(AsPh3)2·CH3OH catalyzed
reaction.
The non-polar solvents showed better performance as com-
pared to the highly polar aprotic solvents for the double
bond isomerization of methyl chavicol and eugenol. The
highest conversion (87.4%) of methyl chavicol was observed
using cyclohexane as a solvent followed by toluene (82.8%)
in RuCl2(PPh3)3 catalyzed isomerization reaction. Whereas
RuCl3(AsPh3)2·CH3OH catalyst showed 90.6% conversion of
methyl chavicol with 93.0% selectivity of trans-anethole with

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S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
Scheme 1. Possible mechanistic pathway for the isomerization of methyl chavicol and eugenol using RuCl2(PPh3)3 catalyst.
For the alkyl mechanism, 1,2-insertion of coordinated alkene
polar protic solvents, probably oxidative protonation of dissoci-
ated form RuCl2(PPh3)2. Another important factor influencing
the conversion of allyl phenyl ether is the rate of the dissolution
of the catalyst precursor. General observations for ruthenium
complexes, reveals that they dissolve faster in polar aprotic sol-
vents, while in benzene they may even stay partially solidified
till the end of the reaction period if the concentration of the
complex is more. However, it may be noted that the proposed
mechanisms in Schemes 1 and 2 needs to be confirmed by spec-
troscopic evidence of the actual intermediate species.
The lower conversion of substrate was found in highly polar
solvents like DMSO, acetonitrile might be attributed to a com-
petition between the solvent and the substrate for a coordinative
place available on the catalyst. In this type of reactions, usually
active solvents such as acetonitrile, DMSO were not performed
well, due to the reason that, these solvents may react with the
Ru complexes by oxidative addition to C–X bond, most likely
deactivate the catalyst resulting in a decreased conversion [23].
Relatively strong coordination properties of DMSO, acetonitrile
are the most important factors causing such a low conversion.
The conversion of methyl chavicol was increased up to 80%
using the non-polar solvents, such as, toluene, cyclohexane, n-
decane, benzene, chloroform, etc. This may be perhaps due to
the coordination properties of these solvents. THF is usually the
most effective solvent probably due to its coordinating proper-
ties.
to Ru–H bond or ␤-elimination might be the rate-determining
step. While for ␲-allylic mechanism, the slowest step is oxida-
tive addition, which leads to the formation of hydride-␣-allyl
complex. From the macroscopic point of view, factors such as
the rate of homogenization of the pre-catalyst, formation of the
active catalyst species, which is involved in the catalytic cycle
may also strongly influence the overall rate of reaction [20].
These two factors are certainly connected with homogenization,
which is in equilibrium with ligand dissociation and formation
of the active catalyst species. There is an additional set of equi-
librium reactions, like coordination of the catalyst by substrate,
products or decomposition of products, which also influences
the rate-determining step [20].
The higher conversion and selectivity obtained in alcohols
may be attributed to easily transformation of the catalytic
precursor a into the active catalyst species c in the reaction
conditions (Scheme 2). In fact, the ruthenium hydride com-
plex may be formed in a preliminary step through an oxida-
tive addition of the alcohol to the complex, and this new
hydride complex could be the catalytically active species hav-
ing a very high activity towards double bond isomerization
[21,22]. Higher catalytic activity of ruthenium hydride complex
has also been reported during the double bond isomerization
of 1,4-diarylbutenes [22], from the comparative activities of
RuCl2(PPh3)3 and HRuCl(PPh3)3 with the replacement of the
former with HRuCl(PPh3)3 showing 80-fold increase in the
maximum rate of reaction. Effect of addition of 3% methanol
to the reaction mixture of RuCl2(PPh3)3 and 1,4-diarylbutenes
resulted into a 10-fold increase in the maximum rate of reaction
and this increase in reaction rate has been attributed to formation
metal hydride complex [22]. The formation of metal hydride is
associated with the oxidative protonation of RuCl2(PPh3)3 in the
In view of higher conversion and selectivity observed with
ethanol as a solvent, further detailed kinetics has been stud-
ied using ethanol as a solvent for the double bond isomeriza-
tion of methyl chavicol and eugenol using Ru/PPh3 complex
catalyst. However, in the case of the isomerization of methyl
chavicol using Ru/AsPh3 complex catalyst, methanol has been
chosen as a solvent for the further detailed kinetic study as the

S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
205
Scheme 2. Possible reaction mechanism for the isomerization of methyl chavicol and eugenol in protic polar solvents.
higher selectivity of trans-isomer was observed with methanol
solvent.
3.1.1. Effect of solvent concentration
The effect of solvent concentration on the initial rate of reac-
tion using RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH complex
catalysts are shown in Fig. 2a and b, respectively, for the isomer-
ization of methyl chavicol. From these figures, one can observed
that the concentration of solvent plays an active role in inhibiting
the rate of reaction.
At 10.87 mmol solvent concentration, the initial rate of reac-
tion was found to be 21.67 mmol/(l s). Further, by increasing the
solvent concentration 10.87–21.74 mmol, the initial rate of reac-
tion was observed to decrease from 21.67 to 6.8 mmol/(l s). This
trend was continued up to the solvent concentration 65.22 mmol,
which resulted into the least of value of the initial rate of reaction
(1.67 mmol/(l s)). Initially, at the lower solvent concentration,
the initial rate of reaction was found to be maximum. As the
concentration of solvent increases, the initial rate of reaction
decreased sharply. Therefore, further kinetic experiments were
performed at 65.22 mmol ethanol concentration because in this
region the initial rate of reaction was not significantly dependent
on the solvent concentration.
In case of RuCl3(AsPh3)2·CH3OH complex catalyst, the ini-
tial rate of reaction decreases from 17.1 to 5.6 mmol/(l s) on
increasing the solvent concentration 15.62–31.25 mmol. There-
fore, for RuCl3(AsPh3)2·CH3OH catalyzed isomerization reac-
tion, the methanol concentration was taken 31.25 mmol for all
kinetic experiments.
Similar behavior of the solvent concentration on the ini-
tial rate of reaction using RuCl2(PPh3)3 complex catalyst was
observed for the eugenol isomerization under studied experi-
mental reaction conditions (Fig. 2a). The concentration of the
solvent (ethanol) for the isomerization of eugenol to isoeugenol
was taken 65.22 mmol for the kinetic experiments.
Fig. 2. (a) Effect of solvent concentration on the isomerization of methyl
chavicol and eugenol using RuCl2(PPh3)3 complex catalyst. Reaction
conditions:
[reactant] = 6.75 mmol,
[RuCl2(PPh3)3] = 5.2 × 10−3 mmol,
[solvent] (ethanol) = 11.0–109.0 mmol, at 358 K. (b) Effect of sol-
vent concentration on the isomerization of methyl chavicol using
RuCl3(AsPh3)2·CH3OH complex catalyst. Reaction conditions: [methyl
chavicol] = 6.75 mmol,
[solvent] (methanol) = 15.0–125.0 mmol, at 358 K.
[RuCl3(AsPh3)2·CH3OH] = 5.88 × 10−3 mmol,

206
S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
methyl chavicol concentration using RuCl2(PPh3)3 complex
catalyst.
The effect of the initial concentration of methyl chavicol on
the initial rate of reaction using RuCl3(AsPh3)2·CH3OH cat-
alyst was also studied (Fig. 3b). The trend of the dependence
of the initial rate of the reaction upon the concentration of
methyl chavicol was observed to be some what different than
RuCl2(PPh3)3 catalyst system. Firstly, the initial rate of reac-
tion was found to be increased linearly in the methyl chavicol
concentration range 0.75–8.1 mmol and after that it remained
constant with further increase in the concentration of methyl
chavicol.
3.2.1.2. Effect of eugenol concentration. The effect of ini-
tial concentration of eugenol on the rate of reaction using
RuCl2(PPh3)3 complex catalyst is shown in Fig. 3a. The trend
observed was almost similar like the isomerization of methyl
chavicol using RuCl2(PPh3)3 catalyst. The initial rate of reaction
increased from 1.5 to 5.0 mol/l, on increasing the concentration
of eugenol 5–18.3 mmol. The initial rate of reaction was found to
be linearly dependent on the concentration of eugenol in entire
range of experiments.
3.2.1.3. Effect of reactants concentration and time on selectivity
of trans-isomers. The trend of the formation of trans-isomers
was also studied and the results obtained are shown in Table 3.
The selectivity of trans-anethole was observed to increase from
70.4 to 95.4% by increasing the reaction time up to 60 min.
After that, the selectivity of trans-anethole was observed to be
independent on the reaction time even it is double. Initially, the
lower selectivity of trans-isomer was observed due to simul-
taneous formation of the cis-isomer (Scheme 1). As the time
increases, the concentration of the reactant decreased, at the
lower concentration of methyl chavicol (<10%), the conversion
of the cis-isomer to the trans-isomer was started, due to ther-
modynamically instability of cis-isomer. It was interesting to
observe that the selectivity of trans-anethole is independent of
the concentration of methyl chavicol. There is no significant
changes in the selectivity of trans-anethole were observed even
the concentration of the methyl chavicol increased from the 3.4
to 10.2 mmol.
Fig. 3. (a) Effect of reactants concentration on the isomerization of methyl
chavicol and eugenol using RuCl2(PPh3)3 complex catalyst. Reaction
conditions: [reactant] = 1.35–18.27 mmol, [RuCl2(PPh3)3] = 5.2 × 10−3 mmol,
[solvent] (ethanol) = 65.22 mmol, at 358 K. (b) Effect of methyl chav-
icol concentration on the isomerization of methyl chavicol using
RuCl3(AsPh3)2·CH3OH complex catalyst. Reaction conditions: [methyl
chavicol] = 1.35–10.15 mmol, [RuCl3(AsPh3)2·CH3OH] = 5.88 × 10−3 mmol,
[solvent] (methanol) = 31.25 mmol, at 358 K.
3.2. Effect of various parameters on the rate of reaction
3.2.1. Effect of reactants concentration
3.2.1.1. Effect of methyl chavicol concentration. The effect
of initial concentration of methyl chavicol on the initial rate
of reaction using RuCl2(PPh3)3 complex catalyst is shown
in Fig. 3a. The initial rate of reaction showed positive order
dependence on the concentration of methyl chavicol and
found to it to increase linearly by increasing the concen-
tration of methyl chavicol. In the present study, the maxi-
mum rate of reaction was found 3.5 mmol/(l s) at 12.1 mmol
Some relatively interesting results were found for the
double bond isomerization of eugenol to isoeugenol using
Table 3
Effect of reaction time and concentration of reactant on the selectivity of trans-isomer
Time (min)
Selectivity of trans-isomer (%)
Moles of methyl chavicol (mmol)
Moles of eugenol (mmol)
3.4
6.8
10.2
3.1
6.2
12.4
5
10
15
30
60
70.4
83.3
84.5
91.2
95.4
95.4
70.7
82.5
84.1
91.4
95.4
95.4
70.5
83.5
84.4
90.8
95.3
95.4
89.7
88
85
93.3
95.5
95.5
90.1
88
85.6
92.5
95.6
95.6
89.8
87.9
85.1
92.2
95.5
95.6
120
Reaction conditions: [solvent] (ethanol) = 65.22 mmol, [RuCl2(PPh3)3] = 5.2 × 10−3 mmol, reaction time = 2 h at 358 K.

S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
207
concentration of RuCl2(PPh3)3 complex from 8.0 × 10−3 to
31.3 × 10−3 mmol.
RuCl2(PPh3)3 catalyst. At the lower reaction time (5 min), the
higher selectivity of trans-isomer was observed. As the reac-
tion time increases from 5 to 15 min, the selectivity of the
trans-isomer decreased 89.7–80% at the 3.1 mmol of eugenol
concentration, due to enhanced formation of cis-isomer. Ini-
tially, the slow formation of the cis-isoeugenol has favored the
higher selectivity of trans-isomer. As time increases, the for-
mation of the cis-isomer increased, however, in this case also,
when the concentration of reactant decreased, the cis-isomer
was converted to the trans-isomer. Again there was no signifi-
cantchangesintheselectivityoftrans-isoeugenolwereobserved
even the concentration of the eugenol increased from the 3.1 to
12.4 mmol.
The effect of the catalyst concentration on the initial rate
of reaction for isomerization of eugenol using RuCl2(PPh3)3
complex catalyst is shown in Fig. 4a. The observed trend on
the effect of catalyst concentration on the initial rate of reaction
showed that, initially, the rate of reaction linearly dependence
on the catalyst concentration up to 6 × 10−3 mmol, after that
the initial rate of reaction was observed to be increased slowly
with increasing catalyst concentration. The similar type of the
trend was observed when methyl chavicol was used as reactant
as described above.
3.3. Rate constant and activation energy
3.2.2. Effect of catalysts concentration
The effect of the catalyst concentration on the initial
rate of reaction for isomerization of methyl chavicol using
RuCl2(PPh3)3 and RuCl3(AsPh3)2·CH3OH catalyst is shown
in Fig. 4a and b, respectively. The initial rate of reac-
tion linearly increased with increasing the catalyst concentra-
tion up to 8.0 × 10−3 mmol (2.4 mmol/(l s)). However, after
8.0 × 10−3 mmol catalyst concentration the extent of increment
of the initial rate of reaction was less as the initial rate of reac-
tion increased from 2.4 to 2.75 mmol/(l s) by increasing the
Theeffectofthetemperatureontherateofreactionfortheiso-
merization of methyl chavicol and eugenol using RuCl2(PPh3)3
and RuCl3(AsPh3)2·CH3OH complex catalysts are shown in
Fig. 5a and b, respectively. It was observed that the initial rate of
reaction increased by increasing the reaction temperature in case
of isomerization of methyl chavicol using RuCl2(PPh3)3 com-
plex catalyst. The lower conversion of methyl chavicol and lower
selectivity of trans-anethole was found below refluxing tempera-
ture of the reaction mixture. Similar types of trends in conversion
Fig. 4. (a) Effect of catalyst concentration on the isomerization of methyl
chavicol and eugenol using RuCl2(PPh3)3 complex catalyst. Reaction condi-
tions: [reactant] = 6.75 mmol, [RuCl2(PPh3)3] = 1 × 10−3–31.3 × 10−3 mmol,
[solvent] (ethanol) = 65.22 mmol, at 358 K. (b) Effect of catalyst concentration
on the isomerization of methyl chavicol using RuCl3(AsPh3)2·CH3OH
complex catalyst. Reaction conditions: [methyl chavicol] = 6.75 mmol,
Fig. 5. (a) Arrhenius plot for the isomerization of methyl chavicol and eugenol
using RuCl2(PPh3)3 complex catalyst. Reaction conditions: [reactant] =
6.75 mmol,
[RuCl2(PPh3)3] = 5.2 × 10−3 mmol,
[solvent]
(ethanol) =
65.22 mmol, temperature = 348–358 K. (b) Arrhenius plot for the isomerization
of methyl chavicol using RuCl3(AsPh3)2·CH3OH complex catalyst. Reaction
conditions: [methyl chavicol] = 6.75 mmol, [RuCl3(AsPh3)2·CH3OH] =
5.88 × 10−3 mmol, [solvent] (methanol) = 31.25 mmol, temperature = 348–
358 K.
[RuCl3(AsPh3)2·CH3OH] = 1.2 × 10−3–23.5 × 10−3 mmol,
[solvent]
(methanol) = 31.25 mmol, at 358 K.

208
S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
4. Conclusions
The regioselective synthesis of trans-anethole from methyl
chavicol via double bond isomerization of methyl chavicol
using RuCl2(PPh3)3, RuCl3(AsPh3)2·CH3OH complex cata-
lysts with details kinetics studies has been reported for the
first time. The effect of different solvents on the conversion
and selectivity towards trans-anethole were studied for the iso-
merization of methyl chavicol to trans-anethole. The highest
conversion (99.7%) with selectivity (95.4%) for trans-anethole
was observed in the ethanol followed by iso-propanol and
methanol in relatively lesser reaction time using RuCl2(PPh3)3
catalyst. Similar results were observed for eugenol isomeriza-
tion in alcoholic solvents using RuCl2(PPh3)3 catalyst. How-
ever, in case of RuCl3(AsPh3)2·CH3OH catalyzed isomeriza-
tion of methyl chavicol, the highest conversion (94.2%) with
selectivity (98.6%) for trans-anethole was observed in the
methanol followed by iso-propanol and ethanol. Clearly, it
was observed that the rate of reaction increased on increas-
ing methyl chavicol, eugenol and catalyst concentrations. The
rate of reaction was found strongly dependent on the solvent
concentration. The rate of reaction decreased with increas-
ing the solvent concentration. Activation energies for the iso-
merization of methyl chavicol under experimental conditions
were found to be 4.3 kJ/mol with RuCl2(PPh3)3 catalyst and
6.0 kJ/mol with RuCl3(AsPh3)2·CH3OH catalyst. For the iso-
merization of eugenol, the activation energy was found to be
6.9 kJ/mol using RuCl2(PPh3)3 complex catalyst. The catalyst
was recycled five times without significant loss in conver-
sion and selectivity for isomerization of methyl chavicol and
eugenol.
Fig. 6. (a) Reusability of RuCl2(PPh3)3 complex catalyst for the isomeriza-
tion of methyl chavicol. Reaction conditions: [methyl chavicol] = 6.75 mmol,
[RuCl2(PPh3)3] = 5.2 × 10−3 mmol, [solvent] (ethanol) = 65.22 mmol, reaction
time = 2 h, at 358 K. (b) Reusability of RuCl2(PPh3)3 complex catalyst for
the isomerization of eugenol. Reaction conditions: [eugenol] = 6.75 mmol,
[RuCl2(PPh3)3] = 5.2 × 10−3 mmol, [solvent] (ethanol) = 65.22 mmol, reaction
time = 2 h, at 358 K.
Acknowledgements
and selectivity were found in the case of isomerization of methyl
chavicol using RuCl3(AsPh3)2·CH3OH complex and isomer-
ization of eugenol. Activation energies for the isomerization of
methyl chavicol under experimental conditions were found to
be 4.3 0.08 kJ/mol (Fig. 5a) with RuCl2(PPh3)3 catalyst and
6.0 0.12 kJ/mol (Fig. 5b) with RuCl3(AsPh3)2·CH3OH cat-
alyst. For the isomerization of eugenol, the activation energy
was found to be 6.9 0.13 kJ/mol (Fig. 5a) using RuCl2(PPh3)3
complex catalyst, which is less than the reported value for
rhodium(III) chloride system (42.6 kJ/mol) [13] and for KOH
system (148 kJ/mol) [15].
Authors are highly thankful to Dr. P.K. Ghosh, Director,
CSMCRI, Bhavnagar, India, for encouragement of this publi-
cation and CSIR Network Program on Catalysis, CSIR, New
Delhi, India, for the financial supports.
References
[1] International Flavors & Fragrances Inc., New Release, New York, N.Y.,
January 28, 2004.
[2] K. Bauer, D. Garbe, H. Surberg, Ullmann Encyclopedia of Industrial
Chemistry, sixth ed., 2002 (Electronic Release).
From above detailed kinetics studies, it was observed that
the RuCl2(PPh3)3 complex showed the higher conversion and
reasonably excellent selectivity for the isomerization of methyl
chavicol and eugenol. Therefore, one reaction was performed
in larger scale of methyl chavicol ([methyl chavicol] = 3.48 mol,
[RuCl2(PPh3)3] = 2.2 mmol, [ethanol] = 32.61 mol, time = 2 h at
358 K). Theresultwasfoundidenticalasthepreviouslyobserved
for the isomerization of 1 g (6.75 mmol; Run 8; Table 1) methyl
chavicol. The catalyst was recycled up to five times for the iso-
merization of methyl chavicol (Fig. 6a) and eugenol (Fig. 6b).
There were not observed any considerable changes in the con-
version and selectivity of respective isomers up to fifth cycles in
both of the cases.
[3] P. Gandilhon, U.S. Patent 4,138,411 (1979).
[4] G.E. Svadkovskaya, L.A. Kheifits, A.I. Platova, V.Y. Denisenkova, CA
73 (1970) 14450r.
[5] P.A. Chaloner, Handbook of Coordination Catalysis in Organic Chem-
istry, Butterworths, London, 1986, p. 415.
[6] A.P. Wagner, Manuf. Chem. 23 (1952) 56.
[7] M.R. Reddy, M. Periasamy, J. Organomet. Chem. 491 (1995) 263.
[8] M. Martan, P.H. Reichenbacher, U.S. Patent 4,038,325 to UOP.
[9] H.S. Lee, G.Y. Lee, Bull. Korean Chem. Soc. 26 (3) (2005) 461.
[10] W. Wu, J.G. Verkade, ARKIVOC ix (2004) 88.
[11] V.K. Srivastava, H.C. Bajaj, R.V. Jasra, Catal. Commun. 4 (2003)
543.
[12] C.H. Bibb, P. Fla, U.S. Patent 2,052,744.
[13] L. Cerveny, A. Krejeikova, A. Marhoul, V. Ruzicka, React. Kinet. Catal.
Lett. 33 (1987) 471.

S.K. Sharma et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 200–209
209
[14] A. Loupy, L. Thach, Synth. Commun. 23 (1993) 2571.
[15] L.N. Thach, C.R. Strauss, J. Chem. Edu. 38 (2000) 76.
[16] S.K. Sharma, V.K. Srivastava, P.H. Pandya, R.V. Jasra, Catal. Commun.
6 (2005) 205.
[17] S.K. Sharma, V.K. Srivastava, R.V. Jasra, U.S. Patent applied.
[18] T.A. Stephenson, G.J. Wilkinson, Inorg. Nucl. Chem. 28 (1966) 945.
[19] S. Kaur, J.V. Crivello, N. Pascuzzi, J. Polym. Sci., Part A: Polym. Chem.
37 (1999) 199.
[20] S. Krompiec, N. Kuzynik, R. Penczek, J. Rzepa, J.M. Biaton, J. Mol.
Catal. A: Chem. 219 (2004) 29.
[21] M.A. Bennett, T.W. Matheson, G. Wilkinson, F.G.A. Stone, F.W. Abel,
Comprehensive Organometallic Chemistry, vol. 4, Pergamon, New York,
1982, p. 942.
[22] J. Blum, Y. Becker, J.C.S., Perkin Trans. II (1972) 982.
[23] A. Salvini, F. Piacenti, P. Frediani, A. Devescovi, M. Caporali, J.
Organomet. Chem. 625 (2001) 255.