Eugenol isomerization promoted by arene-ruthenium(ii) complexes in aqueous media: Influence of the pH on the catalytic activity

Article abstract of DOI:10.1039/c3ra43030h

The catalytic activity of the arene-ruthenium(ii) complexes [RuCl ₂(η⁶-C₆H₅OCH₂CH ₂OH)(L)] (L = P(OMe)₃ (1a), P(OEt)₃ (1b), P(OⁱPr)₃ (1c), P(OPh)₃ (1d), PPh₃ (1e)) in the isomerization of eugenol into isoeugenol has been evaluated. Best results in terms of activity and selectivity were observed with those catalysts containing an aliphatic P-donor ligand (1a-c). Under optimized conditions, full conversions in extremely short reaction times (5 min), and with high levels of trans-selectivity (up to 98%), could be achieved. Addition of both NaOH or H₂SO₄ to the aqueous media resulted in rate enhancements, suggesting two different activation pathways of the pre-catalysts. We have evidenced that sodium hydroxide promotes the release of the η⁶- coordinated arene ligand, while sulfuric acid favours the Ru-Cl bond dissociation and the formation of aquo-derivatives [RuCl(H₂O) (η⁶-C₆H₅OCH₂CH ₂OH)(L)][Cl]. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43030h

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PAPER
Eugenol isomerization promoted by arene–
ruthenium(II) complexes in aqueous media: influence
of the pH on the catalytic activity†
Cite this: RSC Adv., 2013, 3, 19985
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Beatriz Lastra-Barreira, Alba E. Dıaz-Alvarez, Lucıa Menendez-Rodrıguez
and Pascale Crochet
*
The catalytic activity of the arene–ruthenium(II) complexes [RuCl2(h6-C6H5OCH2CH2OH)(L)] (L ¼ P(OMe)3
(1a), P(OEt)3 (1b), P(OiPr)3 (1c), P(OPh)3 (1d), PPh3 (1e)) in the isomerization of eugenol into isoeugenol
has been evaluated. Best results in terms of activity and selectivity were observed with those catalysts
containing an aliphatic P-donor ligand (1a–c). Under optimized conditions, full conversions in extremely
short reaction times (5 min), and with high levels of trans-selectivity (up to 98%), could be achieved.
Addition of both NaOH or H2SO4 to the aqueous media resulted in rate enhancements, suggesting two
different activation pathways of the pre-catalysts. We have evidenced that sodium hydroxide promotes
the release of the h6-coordinated arene ligand, while sulfuric acid favours the Ru–Cl bond dissociation
and the formation of aquo-derivatives [RuCl(H2O)(h6-C6H5OCH2CH2OH)(L)][Cl].
Received 17th June 2013
Accepted 12th August 2013
DOI: 10.1039/c3ra43030h
www.rsc.org/advances
functionalized allylbenzene, into trans-anethole, combining a
high activity with a complete chemo- and stereo-selectivity.8
Introduction
Metal-promoted C]C bond migrations are fundamental
processes in organic chemistry with widespread academic and
industrial applications.1 In particular, the isomerization of
allylbenzenes offers an attractive synthetic approach to the
corresponding b-methylstyrenes, which are common starting
materials in the avour and fragrance industries,2 and valuable
intermediates for the preparation of pharmaceutical com-
pounds.3 Such a transformation has been traditionally pro-
moted by superstoichiometric quantities of strong bases,4 but
in recent years, the use of transition-metal catalysts allowed an
enhancement in the selectivity and in the rate of this type of
reactions, with a concomitant reduction of their environmental
impact.5
During the last few years, in the context of studies on C]C
bond migration processes involving different functionalized
allylic substrates, our research group has brought to light
highly efficient catalytic systems based on Ru(II) and Ru(IV)
complexes.6–9 Among them, the dimeric bis(allyl)–ruthenium(IV)
derivative [{RuCl(m-Cl)(h3:h3-C10H16)}2] (C10H16 ¼ 2,7-dimethy-
locta-2,6-diene-1,8-diyl) featured the best catalytic perfor-
mances in the isomerization of estragole, a natural occurring
However, this ruthenium(IV) precursor suffers from some limi-
tations since competing side reaction, i.e. dimerization, takes
place when the allylbenzene contains an hydroxyl substituent
on the arene ring.9,10 Therefore alternative catalytic systems
must be developed to selectively isomerize such compounds.
In particular, the efficient, rapid and selective transformation
of eugenol into trans-isoeugenol still remains a challenge
(Scheme 1). Previous studies have demonstrated the ability of
heterogeneous basic catalysts to promote this process, however
long reaction times and high temperatures (ca. 200 ꢀC) are
generally required to obtain only low to moderate yields of
isoeugenol, with quite moderate selectivities toward the trans
isomer.11 The low conversions attained in these cases are
attributed to the competing deprotonation of the OH group and
the methylenic hydrogens of the allyl function.11d,i More effec-
tive transformations were achieved with homogeneous cata-
lysts,5c,d,f,12 nonetheless long reaction times are, in general, still
required. Only a ruthenium-based catalyst reported recently by
Grotjahn and co-workers was able to selectively isomerize
eugenol into trans-isoeugenol in a short reaction time.13
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Laboratorio de Compuestos Organometalicos y Catalisis (Unidad Asociada al CSIC),
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Departamento de Quımica Organica e Inorganica, Instituto Universitario de Quımica
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Organometalica “Enrique Moles”, Universidad de Oviedo, Julian Claverıa 8, 33006
Oviedo, Spain. E-mail: crochetpascale@uniovi.es; Fax: +34 985103446
† Electronic supplementary information (ESI) available: Experimental details,
products characterization and NMR spectra of 1b in presence of H2SO4 and
NaOH. See DOI: 10.1039/c3ra43030h
Scheme 1 Catalytic isomerization of eugenol into isoeugenol.
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Table 2 Isomerization of eugenol catalyzed by complexes [RuCl2(h6-p-cyme-
ne)(L)] (2a–e) in ethanola
In the present work, we have studied the catalytic behaviour
of the water-soluble arene–ruthenium(II) complexes [RuCl2(h6-
C6H5OCH2CH2OH)(L)]7g in the isomerization of eugenol into
isoeugenol. The optimization of the reaction conditions allowed
a clean, rapid and highly stereo-selective process using an
environmentally benign solvent (i.e. water).
Entry
Catalyst [L]
Time
Yieldb (%)
Trans : cisb
1
2
3
4
5
2a [P(OMe)3]
2b [P(OEt)3]
2c [P(OiPr)3]
2d [P(OPh)3]
2e [PPh3]
4 h
6 h
8 h
23 h
15 h
>99
>99
>99
1
98 : 2
98 : 2
96 : 4
—
94 : 6
c
Results and discussion
>99
a
Reactions carried out at 80 ꢀC, using 4 mmol of eugenol, 1 mol% of Ru
and 1 mL of ethanol. b GC determined. c Not determined.
In our previous studies on the isomerization of other allylben-
zene derivatives, we evidenced that the use of an alcoholic
reaction medium is crucial for reaching high activities and
selectivities.7i,8,9 Hence, the catalytic behaviour of arene–ruth-
Isomerization of eugenol catalyzed by complexes [RuCl2(h6-
enium(II) complexes [RuCl2(h6-C6H5OCH2CH2OH)(L)] (L
¼
Table
3
C6H5OCH2CH2OH)(L)] (1a–e) in watera
P(OMe)3 (1a), P(OEt)3 (1b), P(OiPr)3 (1c), P(OPh)3 (1d), PPh3 (1e),
in Fig. 1) in the isomerization of eugenol was rst evaluated
using ethanol as solvent. Experiments were performed at 80 ꢀC
with 4 mmol of substrate, 1 mol% of Ru and 1 mL of EtOH
(Table 1), the yield and selectivity of the process being moni-
tored by GC analyses of aliquots each 5 min during the rst
hour and then the interval was increased progressively.
With all these catalysts, isoeugenol, resulting from the C]C
bond migration (Scheme 1), was the only product generated
with no traces of the corresponding dimer being detected. As a
general trend, derivatives containing an aliphatic P-donor
ligand, i.e. complexes 1a–c, have presented higher activities and
trans-selectivities as compared to those containing aromatic
ligands.14 In particular, compounds [RuCl2(h6-C6H5OCH2-
CH2OH){P(OMe)3}] (1a) and [RuCl2(h6-C6H5OCH2CH2OH)
{P(OEt)3}] (1b) allowed complete conversions into isoeugenol
within 40 min with a selectivity towards the trans-isomer of at
least 97% (entries 1 and 2, Table 1).
Entry
Catalyst [L]
Time
Yieldb (%)
Trans : cisb
1
2
3
4
5
1a [P(OMe)3]
1b [P(OEt)3]
1c [P(OiPr)3]
1d [P(OPh)3]
1e [PPh3]
10 min
15 min
15 min
23 h
>99
>99
>99
91
97 : 3
98 : 2
97 : 3
86 : 14
95 : 5
75 min
>99
a
Reactions carried out at 80 ꢀC, using 4 mmol of eugenol, 1 mol% of Ru
and 1 mL of water. b GC determined.
were also evaluated under the same experimental conditions
(Table 2). These complexes proved to be much less efficient
(nal TOF values from 0.04 hꢁ1 to 25 hꢁ1 for 2a–e vs. 0.8 hꢁ1 to
200 hꢁ1 for 1a–e),15 thus evidencing an inuence of the arene
ligand on the reaction outcome.16
Interestingly, when the catalytic reactions were performed
with the complexes 1a–e in water rather than ethanol, the
reaction rates increased substantially, and the stereo-selectiv-
ities remained very high (Table 3; nal TOF values from 4 hꢁ1 to
600 hꢁ1 in water vs. 0.8 hꢁ1 to 200 hꢁ1 in ethanol). This
phenomenon was also observed with the p-cymene complexes
(see data in the ESI†). As an example, full conversion of eugenol
was attained aer 2 hours using [RuCl2(h6-p-cymene){P(OMe)3}]
(2a) in water (vs. 4 hours in EtOH; trans : cis ratio ¼ 98 : 2 in
each case).
For comparative purposes, the catalytic activity of the
p-cymene analogues [RuCl2(h6-p-cymene)(L)] (2a–e, in Fig. 1)
This activity enhancement in aqueous media contrasts with
the results obtained in the isomerization of estragole to anet-
hole, for which alcohols, and especially methanol, were the
solvents of choice to attain good catalytic efficiencies.7i,9 The
different water-solubility of the substrates may be responsible
of this behaviour, eugenol being more soluble in aqueous
medium than estragole due to the presence of a phenol unit in
its structure.17
On the other hand, we also observed that the pH of the
medium signicantly alters the reaction rates (Table 4). For
example, the isomerization process promoted by 1b in water
was accelerated both under basic (pH ¼ 12.9, entry 6) and acidic
conditions (pH ¼ 4.8, entry 1), slower transformations taking
place at almost neutral pH (pH between 7 and 8.5, entries 3
and 4). Similar trends were also observed for catalysts 1a and
1c–e (see details in the ESI†). The most impressive changes in
Fig. 1 Structure of arene–ruthenium(II) complexes 1a–e and 2a–e.
Table
1
Isomerization of eugenol catalyzed by complexes [RuCl2(h6-
C6H5OCH2CH2OH)(L)] (1a–e) in ethanola
Entry
Catalyst [L]
Time
Yieldb (%)
Trans : cisb
1
2
3
4
5
1a [P(OMe)3]
1b [P(OEt)3]
1c [P(OiPr)3]
1d [P(OPh)3]
1e [PPh3]
30 min
40 min
4 h
23 h
6 h
>99
>99
>99
18
97 : 3
98 : 2
95 : 5
87 : 13
91 : 9
>99
a
Reactions carried out at 80 ꢀC, using 4 mmol of eugenol, 1 mol% of Ru
and 1 mL of ethanol. b GC determined.
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Table 4 Influence of pH on the catalytic activitya
Entry
pH
4.8
5.2
7.2
Time
Yieldb (%)
Trans : cisb
1
2
3
4
5
6
10 min
10 min
15 min
30 min
10 min
5 min
7 h
7 h
3.5 h
1.75 h
1.5 h
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
98 : 2
98 : 2
98 : 2
97 : 3
97 : 3
98 : 2
89 : 11
90 : 10
97 : 3
98 : 2
97 : 3
8.5
Scheme 2 Aquation of complex 1b to generate 10b.
10.2
12.9
4.8
12.9
7.2
7c
8c
9d
10d
11d
phenolic derivatives 1,2-dimethoxy-4-allylbenzene, safrole and
allylbenzene (entries 2–4, Table 5). As an example, complete
transformation of allylbenzene into b-methylstyrene was ach-
ieved in 45 min under neutral conditions, while only 20 min
were required in the presence of NaOH (entry 4).
To better understand these rate enhancements, the stoi-
chiometric reactivity of complex 1b towards H2SO4 and NaOH
was explored. First of all, variable quantities of aqueous H2SO4
was added to deuterated water solutions of [RuCl2(h6-
C6H5OCH2CH2OH){P(OEt)3}] (1b), conducing, in all cases, to a
mixture of the aquo-derivative [RuCl(D2O)(h6-C6H5OCH2-
CH2OH){P(OEt)3}][Cl] (10b) and the initial complex 1b (Scheme
2; see NMR spectra in the ESI†).
The generation of 10b, through Ru–Cl bond cleavage and D2O
coordination, is favoured by the presence of H2SO4 since the
relative proportion of 10b increased progressively with the
medium acidity (ca. 20, 32, 37, 43, 45 and 47% of 10b observed
aer addition of 0, 0.5, 1, 5, 10 and 20 equivalents of H2SO4 per
Ru, respectively).19 The equilibrium shi towards the formation
of the ionic aquo-compound 10b under these conditions is most
probably due to the enhanced ionic strength of the medium.20
Hence, these observations suggest that the role of H2SO4 during
the catalytic events is to favour the chloride ligand dissociation.
In this sense, we note that the cleavage of Ru–Cl bond is usually
proposed as a key step in C]C bond migration processes
promoted by chloride–ruthenium catalyst precursors.21 In
agreement with this, a similar rate enhancement was also
achieved by addition of a chloride abstractor (AgNO3) to the
catalytic reaction.22 Reversely, the efficiency of complex 1b
dramatically dropped in the presence of Clꢁ sources, such as
HCl or NaCl.23
4.8
12.9
a
Reactions carried out at 80 ꢀC, using 4 mmol of eugenol, 1 mol% of 1b,
1 mL of water and appropriate quantity of NaOH or H2SO4 to reach the
b
c
pH indicated. GC determined. Using 1 mol% of 1d as catalyst.
d
Using 1 mol% of 2b as catalyst.
activity were achieved with [RuCl2(h6-C6H5OCH2CH2OH)
{P(OPh)3}] (1d), which required only 7 hours at pH ¼ 4.8 or pH ¼
12.9 to reach full conversions (Table 4, entries 7 and 8, nal
TOF ¼ 14 hꢁ1), while incomplete reaction was observed aer 23
hours under neutral conditions (Table 3, entry 4, TOF ¼ 0.8
hꢁ1). In blank experiments, we have checked that NaOH (at
pH ¼ 12.9) or H2SO4 (at pH ¼ 4.8) are not able to promote the
isomerization in absence of the metallic precursor, even aer
several hours of heating (48 h).
The higher water-solubility of eugenol in basic medium, due
to the generation of the corresponding phenolate, could have
explained the increased activity in presence of NaOH. However,
this could be discarded since the rate enhancement was also
observed with other substrates, regardless of the presence or
not of an OH function on the aromatic ring. Thus, basic
conditions improved the isomerization of 2-allylphenol (entry 1,
Table 5),18 a phenolic compound, as well as those of the non-
Table 5 Isomerization of different allylbenzenes catalyzed by 1b in water with
or without basea
On the other hand, treatment of aqueous solutions of 1b
with different amounts of NaOH (0.25, 0.5, 0.75, 1 or 2 equiva-
lents per Ru) gave rise to complex mixtures of unidentied
organometallic compounds. However, NMR analyses clearly
evidenced that partial release of the arene ligand took place.24 In
accord with this, 1H NMR spectra exhibit different sets of
signals between 5.0 and 7.3 ppm. The resonances at 7.30, 6.98
and 6.95 ppm correspond, respectively, to the ortho, para and
meta hydrogen nuclei of the free phenoxyethanol (see the NMR
spectra in the ESI†), while signals between 5.0 and 6.1 ppm
reect the presence of different complexes with a h6-coordi-
nated C6H5OCH2CH2OH ligand.24 The relative proportion of
free arene increases with the quantity of NaOH added to the
medium. Interestingly, this behaviour is specic to the phe-
noxyethanol derivatives since the treatment of aqueous solution
of [RuCl2(h6-p-cymene){P(OMe)3}] with one equivalent of NaOH
does not provoke the arene displacement.25
Substrate
1
Product
Timeb
Trans : cisc
15 min
50 : 50
60 : 40
10 mind
30 min
98 : 2
98 : 2
10 mind
2
3
10 min
5 mind
98 : 2
98 : 2
45 min
96 : 4
97 : 3
4
20 mind
a
Reactions carried out at 80 ꢀC, using 4 mmol of substrate, 1 mol% of
1b and 1 mL of water. b Time required to achieving a full conversion of
the substrate. c GC determined. d Reaction performed in the presence of
NaOH (pH ¼ 12.9).
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Hence, two different types of activations are possible for the
pre-catalysts [RuCl2(h6-C6H5OCH2CH2OH)(PR3)] (1a–e): (i)
through chloride dissociation under neutral and, above all,
under acid conditions, or (ii) by arene release at basic pH. This
chemical behaviour is in accord with the activities reached for
1a–e as function of the pH medium, highly basic and acidic
conditions favouring the catalytic reactions. Moreover, the
differences in activity observed between the phenoxyethanol
(1a–e) and p-cymene (2a–e) complexes in the presence of a base
are thus attributed to the capacity of the formers to lose the
arene ligand (see Table 4 entry 11 vs. entry 6 and ESI†). However,
the higher efficiencies of 1a–e vs. 2a–e under neutral (entry 9 vs.
entry 3 and ESI†) or acid conditions (entry 10 vs. 1 and ESI†)
remain unexplained. In effect, the p-cymene complexes
[RuCl2(h6-p-cymene)(PR3)] (2a–e) are more likely to form the
corresponding aquo-species [RuCl(H2O)(h6-p-cymene)(PR3)]-
[Cl]26 than their phenoxyethanol counterparts (1a–e) and then,
they are expected to be more active. The lower activities of
p-cymene derivatives are perhaps related to a higher steric
congestion due to the presence of a di-substituted arene.
General procedure for the isomerization of eugenol catalyzed
by 1a–e and 2a–e
Under a nitrogen atmosphere, the ruthenium catalyst precursor
(0.04 mmol, 1 mol%), 1 mL of the indicated solvent and eugenol
(0.616 mL, 4 mmol) were introduced into a teon-cap sealed
ꢀ
tube and the mixture was heated at 80 C. The complexes 1a–c
and 2a–c resulted completely soluble under the experimental
conditions used, whereas 1d,e and 2d,e were only partially
soluble. The yield and selectivity of the catalytic process were
monitored by GC analyses of aliquots each 5 min during the
rst hour and then the interval was increased progressively.
Similar procedures have been employed to perform the isom-
erization of 1,2-dimethoxy-4-allylbenzene, safrole, 2-allylphenol
and allylbenzene (Table 5). In all cases, only the product
resulting from the C]C bond migration was generated. The
identity of the isolated products was assessed by comparison of
their NMR spectroscopic data with those reported in the liter-
ature and their fragmentation in GC/MSD. The C]C bond
1
stereochemistry was conrmed by H NMR.
Acknowledgements
Conclusions
´
Financial support from the “Ministerio de Economıa y Com-
In summary, we have developed new efficient and selective
catalytic systems for the isomerization of eugenol into iso-
eugenol based on the arene-ruthenium(II) complexes [RuCl2(h6-
C6H5OCH2CH2OH)(L)] (1a–e). Under optimized conditions, full
transformation was achieved within 5 min, with a selectivity
towards the trans-isomer of 98%. Although these results do not
surpass those recently reported by Grotjahn and co-workers,13
they represent a clear improvement in respect to the other
previous studies.11,12 On the other hand, we have demonstrated
that either NaOH or H2SO4 allow for a rate enhancement. Two
different pre-catalysts activation pathways seem to take place
depending on the reaction conditions. Thus, in the presence of
NaOH, complexes 1a–e evolve through the decoordination of
the h6-arene, while H2SO4 favours the dissociation of the Ru–Cl
bond. In both cases, open coordination sites on the metal are
generated, thus favouring the coordination of the substrate.
petitividad” of Spain (Projects CTQ2010-14796/BQU and
CSD2007-00006) is acknowledged. B. L.-B. and L. M.-R. thank
FICYT of Asturias (PCTI – Severo Ochoa program) and the
Spanish Government and the European Social Fund (FPI
program), respectively, for the award of a PhD grant.
Notes and references
1 See, for example: (a) G. W. Parshall and S. D. Ittel, in
Homogeneous Catalysis, John Wiley & Sons, New York, 1992;
(b) P. W. N. van Leeuwen, in Homogeneous Catalysis:
Understanding the Art, Kluwer Academic Publishers,
Dordrecht, 2004; (c) Metal-Catalysis in Industrial Organic
Processes, ed. G. P. Chiusoli and P. M. Maitlis, RSC
Publishing, Cambridge, 2008.
2 See, for example: (a) P. R. Ashurst, in Food Flavorings, Aspen
Publishers, Maryland, 3rd edn, 1999; (b) K. Bauer, D. Garbe
and H. Surburg, in Common Fragrance and Flavour
Materials, Wiley-VCH, Weinheim, 4th edn, 2001; (c)
G. Reineccius, in Flavor Chemistry and Technology, CRC
Press, Boca Raton, 2nd edn, 2006; (d) I. A. Khan and
E. A. Abourashed, in Leung's Encyclopedia of Common
Natural Ingredients Used in Food, Drugs and Cosmetic, John
Wiley & Sons, Hoboken, 3rd edn, 2010.
3 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, PCT Int. Appl., 2008003007, 2008; (d)
V. V. Kouznetsov and D. R. Merchan Arenas, Tetrahedron
Lett., 2009, 50, 1546 and references cited therein; (e)
A. Sharma, N. Sharma, A. Shard, R. Kumar,
D. Mohanakrishnan, Saima, A. K. Sinha and D. Sahal, Org.
Biomol. Chem., 2011, 9, 5211; (f) M.-A. Constantin,
J. Conrad, E. Merixsor, K. Koschorreck, V. B. Urlacher and
Experimental section
General methods
Organic solvents were dried by standard methods and distilled
under nitrogen before use. All reagents were obtained from
commercial suppliers and used without further purication, with
the exception of compounds [RuCl2(h6-C6H5OCH2CH2OH)(L)]
(1a–e)7g and [RuCl2(h6-p-cymene)(L)] (2a–e),7g,27 which were
prepared by following the methods reported in the literature. GC
measurements were made on a Hewlett-Packard HP6890 equip-
ment using a Supelco Beta-Dexꢀ 120 column (30 m length,
250 mm diameter). GC/MSD measurements were performed with
an Agilent 6890N equipment coupled to a 5973 mass detector
(70 eV electron impact ionization) using a HP-1MS column. Flash
chromatography was performed using Merck silica gel 60 (230–
400 mesh). 31P{1H}, 1H and 13C{1H} NMR spectra were recorded
on Bruker DPX-300 or Bruker AV-400 instruments.
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7 (a) V. Cadierno, P. Crochet, S. E. Garcıa-Garrido and
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J. Gimeno, Dalton Trans., 2004, 3635; (b) P. Crochet, J. Dıez, 14 The extremely low activity of 1d is probably due to the easy
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M. A. Fernandez-Zumel and J. Gimeno, Adv. Synth. Catal.,
deactivation
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generating stable 5-membered metallacycle. See:
of
the
catalytic
species
through
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´
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V. Cadierno, P. Crochet, S. E. Garcıa-Garrido and
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P. Crochet, J. Francos, S. E. Garcıa-Garrido, J. Gimeno and 16 The electronic properties of compounds 1a–e and 2a–e have
N. Nebra, Green Chem., 2009, 11, 1992; (g) B. Lastra- been previously evaluated. See ref. 7g.
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´
of 178 mg Lꢁ1 (at 20 ꢀC) and 2.46 g Lꢁ1 (at 25 ꢀC). Available
´
´
1681; (h) M. A. Fernandez-Zumel, B. Lastra-Barreira,
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¨
M. Scheele, J. Dıez, P. Crochet and J. Gimeno, Dalton
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´
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(k) R. Garcıa-Alvarez, F. J. Suarez, J. Dıez, P. Crochet, 18 Starting from 2-allylphenol, unusually high proportions of
~
´
´
V. Cadierno, A. Antinolo, R. Fernandez-Galan and
the thermodynamically unfavourable cis-isomer were
obtained under both neutral and basic conditions, at short
reaction time. This selectivity is probably due to the ability
of the nal product to chelate the metallic center by
coordination of the oxygen atom and the C]C bond. Cis-
disposition of the olen moiety conduces to less sterically
congested complex. We note that a prolongated heating of
the reaction mixture provokes the gradual isomerization of
cis-2-propenylphenol to trans-2-propenylphenol, a trans : cis
ratio of 90 : 10 (without base) and 91 : 9 (with base) being
obtained aer 22 h. For similar process, see: T. Sato,
N. Komine, M. Hirano and S. Komiya, Chem. Lett., 1999, 441.
19 Formation of 10b is the only process observed in the presence
of H2SO4. The acidic solutions do not suffer any
decomposition even aer 24 h, at least at room temperature.
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´
´
´
9 A. E. Dıaz-Alvarez, P. Crochet and V. Cadierno, Tetrahedron,
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10 As an example, attempts to isomerize 4-allylphenol with
[{RuCl(m-Cl)(h3:h3-C10H16)}2] gave rise to the expected 4-
(prop-1-enyl)phenol along with 1,3-di(4-hydroxyphenyl)-2-
methyl-1-pentene, resulting of the C–C coupling between
two units of the corresponding b-methylstyrene. See ref. 9.
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20 Similar effect has been already reported for a biologically
active ruthenium(II)–arene complex: A. K. Renfrew,
A. D. Phillips, E. Tapavicza, R. Scopelliti, U. Rothlisberger
and P. J. Dyson, Organometallics, 2009, 28, 5061.
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D. J. Burkey, Organometallics, 2003, 22, 1761; (c)
J. Soleimannejad and C. White, Organometallics, 2005, 24,
´
´
2538; (d) C. Aliende, M. Perez-Manrique, F. A. Jalon,
´
21 See for example: (a) R. C. van der Dri, M. Vailati,
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B. R. Manzano, A. M. Rodrıguez and G. Espino,
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´
´
´
159, 163; (b) A. Varela-Alvarez, J. A. Sordo, E. Piedra,
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´
´
´
F. J. Suarez, G. Ujaque and C. Vicent, Chem.–Eur. J., 2012, 25 In this case, a new species was selectively generated.
18, 7749.
Although all attempts to isolate this one in pure form have
failed, avoiding therefore a fully characterization, the 1H
NMR data from its aqueous solution suggested the
formation of [Ru(OH)2(h6-p-cymene){P(OMe)2(OH)}] (see
details in the ESI†).
22 The catalytic isomerization reaction carried out with 1 mol%
of 1b and AgNO3 (1 equiv. per Ru) led to a complete
conversion of eugenol in 5 min, giving rise to isoeugenol
as a 96 : 4 mixture of the trans- and cis-isomers.
23 Catalytic experiments performed in the presence of HCl and 26 As an example, the NMR analyses of 0.14 M solutions of 1b
NaCl (1 equiv. per Ru, 1 mol% of 1b, 80 ꢀC) allowed,
respectively, only 10% and 8% of isoeugenol aer 1 hour
of heating.
and 2b in D2O show the presence of the corresponding
aquo-complex in a 20% and 56%, respectively.
27 (a) M. A. Bennett and A. K. Smith, J. Chem. Soc., Dalton
Trans., 1974, 233; (b) S. A. Serron and S. P. Nolan,
Organometallics, 1995, 14, 4611; (c) E. Hodson and
S. J. Simpson, Polyhedron, 2004, 23, 2695.
24 Ortho, meta and para hydrogen nuclei of the h6-coordinated
C6H5OCH2CH2OH ligand, commonly appear at 5.1–6.7. See
ref. 7g and: (a) J. Soleimannejad, A. Sisson and C. White,
19990 | RSC Adv., 2013, 3, 19985–19990
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