Micellar promoted alkenes isomerization in water mediated by a cationic half-sandwich Ru(II) complex

Article abstract of DOI:10.1016/j.ica.2016.06.004

Micellar media in water provide a simple and efficient environment to favor the double bond isomerization of terminal alkenes catalyzed by the cationic half-sandwich complex 1 at 95 °C. The micellar medium favors both catalyst dissolution in water by means of ion-pairing with the preferred anionic surfactants as well as substrate dissolution thus favoring its conversion into products.

Full text of DOI:10.1016/j.ica.2016.06.004

Inorganica Chimica Acta xxx (2016) xxx–xxx
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Research paper
Micellar promoted alkenes isomerization in water mediated by a
cationic half-sandwich Ru(II) complex
⇑
Laura Sperni, Alessandro Scarso, Giorgio Strukul
Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari di Venezia, via Torino 155, 30172 Mestre Venezia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Micellar media in water provide a simple and efficient environment to favor the double bond isomeriza-
tion of terminal alkenes catalyzed by the cationic half-sandwich complex 1 at 95 °C. The micellar medium
favors both catalyst dissolution in water by means of ion-pairing with the preferred anionic surfactants as
well as substrate dissolution thus favoring its conversion into products.
Received 15 April 2016
Received in revised form 30 May 2016
Accepted 3 June 2016
Available online xxxx
Ó 2016 Elsevier B.V. All rights reserved.
GS dedicates this work to the late Professor
Rino A. Michelin, companion of many
chemical adventures and friend of a
lifetime.
Keywords:
Allylbenzenes
Isomerization
Ruthenium
Micellar media
Surfactants
1
. Introduction
Recently the stringent constraints required by the environmen-
impact for the environment. The latter aspect is clearly demon-
strated by the fact that its E-factor value is considered equal to zero
[4]. In all recent solvent selection guides [6] developed by different
chemical and pharmaceutical companies, water is always consid-
ered as highly desirable [7]. Recent studies emphasized also that
reactions in water can benefit from extra features like enhanced
selectivity (chemo-, regio-, stereo- and enantio-) if compared to
the use of organic media for the same process [8]. Because of its
many advantages [9], water is witnessing a sort of renaissance as
an alternative reaction medium [10]. The peculiarity of water is
often related to the hydrophobic effect that is responsible for most
of the increased selectivities performed by reactants both when
dissolved in water and under ‘‘on water” conditions [11]. On the
other hand, the same properties cause also some challenging draw-
backs, in primis the generally low solubility of organic substrates
and of most metal complex catalysts that in some cases are also
inactivated/decomposed by this medium. In order to overcome
such limitations, traditional catalysts are modified making use of
water soluble ligands bearing polar and/or charged tags [12] to
increase their solubility.
tal legislation in many countries are imposing to industry new
paradigms and methods pushing towards drastic modifications in
the way in which fine chemicals and pharmaceuticals are pro-
duced. We are currently living in a new era where industrial syn-
thesis must comply with the implementation of sustainability
criteria [1] based on the almost 20 year old twelve principles of
green chemistry [2]. Among others, the use of catalytic rather than
stoichiometric methods and the avoidance of organic solvents or
their replacement with less hazardous media are often the first
steps to convert an old synthetic method into a more sustainable
one. In an industrial chemical production the choice of the solvent
is not trivial [3] because about half of the total amount of waste is
constituted by the solvent [4,5] and, quite commonly, the simplest
way to dispose it is eventually incineration to recover heat. Chlori-
nated, aromatic and highly polar solvents like DMF and DMSO are
usually considered for use only if no alternatives are possible.
Water, as the most abundant liquid on earth, is the only one that,
together with low cost and high safety, provides almost negligible
A simpler and more straightforward approach consists in the
use of surfactants whose role is to cope with reagents and cata-
lysts. The addition of surfactants to water leads to the spontaneous
formation (self-assembly) of micelles as supramolecular aggregates
⇑
Corresponding author.
E-mail address: strukul@unive.it (G. Strukul).
http://dx.doi.org/10.1016/j.ica.2016.06.004
020-1693/Ó 2016 Elsevier B.V. All rights reserved.
0
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L. Sperni et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
that, thanks to their apolar core and polar surface, are able to
dissolve apolar substrates and catalysts in their core but also ionic
catalysts close to the surface. Micellar catalysis [8e,13,14] has been
an alternative approach to traditional catalysis in organic media for
years, but because of the recent, more stringent environmental
concerns, it is becoming a real alternative gaining pace also in
drug productions [15]. Several are in fact the examples of reactions
that benefit from the use of micellar media vs. organic media on
grounds such as reaction rate, selectivity at all levels, ease of
product isolation and recyclability [16].
Micellar catalysis is particularly suited for reactions where soft
Lewis acidic metal catalysts interact with soft Lewis basic sub-
strates, in particular reactions involving late transition metals
and unsaturated substrates are usually unaffected by the presence
of water or by the nature of the surfactant. In this field our research
group has obtained very good results using micellar catalysis in
water in e.g. alkyne hydration [17] or alkene hydroformylation
2. Experimental
2.1. General
1H NMR were recorded at 298 K, on a Bruker AVANCE 300 spec-
trometer operating at 300.15 MHz. d values in ppm are relative to
SiMe . GC analyses were performed on HP Series II 5890 instru-
4
ment equipped with a 30 m HP5 capillary column, using He as
gas carrier and FID. GC–MS analyses were performed on a GC Trace
GC 2000 instrument equipped with a 30 m HP5-MS capillary col-
umn using He gas carrier and coupled with a MS Thermo Finnigan
Trace MS quadrupole with Full Scan method.
Solvents and reactants were used as received; otherwise they
were purified according to literature methods [28]. TLC analysis
were performed on TLC PolygramÒ Sil G/UV254 of 0.25 mm thick-
ness and flash-chromatography separations were performed on sil-
ica gel Merck 60, 230–400 mesh [29].
[
18] and epoxidation [14b], or sulfoxidation [14c], mediated by
cationic Pt(II) catalysts using economic, traditional, commercially
available anionic surfactants like e.g. sodium dodecyl sulfate,
observing in some cases possible catalyst recycling.
Half-sandwich complexes of ruthenium bearing aryl or
cyclopentadienyl ligands turned out to be interesting catalysts
for a series of reactions. Examples span from the allylic substitu-
tion reaction [19], to the Diels-Alder cycloaddition reaction with
high selectivity towards the exo product [20], to the [2 + 2 + 2]
cyclotrimerization of alkynes leading to aromatic compounds
5
2
.2. Synthesis of [Ru(
g
-C
5
Me
5
)(CH
3
CN){P(OEt)
3 2 4
} ]BPh 1
5-C
[Ru(g
5
Me
5
)(CH
3
CN){P(OEt)
3
}
2
]BPh 1. An excess of P(OEt)
4
3
(
6.6 mmol, 1.1 mL) was added to a solution of the chloro-com-
5
pound RuCl(
5 5 3 2
g -C Me )(PPh ) 4 (220 mg, 0.3 mmol) in 10 mL
toluene, and the solution was stirred for 2 h. The solvent and the
excess phosphite were removed under reduced pressure to give
an oil, which was dissolved in ethanol (8 mL). An excess of acetoni-
trile (6 mmol, 0.32 mL) and an excess of NaBPh
05 mg) in ethanol (2 mL) were added and the reaction mixture
4
(0.6 mmol,
[
21] and to the cyclopropanation of alkenes with diazoacetate car-
2
bene precursors with good selectivity for the formation of the cis
isomer [22], just to name a few. As long as intramolecular reactions
are concerned, the Ru-catalyzed isomerization of alkenes has been
known for decades [23] including some recent examples based on
monocationic Ru(II) species bearing 1-phenyl-indenyl and chloride
anionic ligands [24].
Recently we successfully investigated the nitrile hydration reac-
tion to amides in micellar media catalyzed by neutral half-sand-
wich Ru(II) complexes bearing cymene, phosphite and chlorine
ligands [25]. Spurred by these positive results related to a good sol-
ubilization of both catalyst and substrate in water, we became
interested with the double bond migration in terminal alkenes to
give the corresponding internal isomers. The reaction has some
interesting industrial applications like e.g. the estragole, eugenol,
and safrole isomerization into the corresponding internal alkenes
stirred for 24 h. The pale-yellow solid which separated was filtered
and crystallized from dichloromethane and ethanol; yield P65%.
1
H NMR (CD
2
Cl
2
, 20 °C) d: 7.65–6.87 (m, 20H, Ph), 3.95 (m, 12H,
CN), 1.31 (s, 15H, CH Me ), 1.29 (t, 18H,
phos); P{ H} NMR (CD Cl , 20 °C) d: 145.3 (s); Anal. Calcd
68BNO Ru (928.89): C, 62.07; H, 7.38; N, 1.51; Found:
C, 61.89; H, 7.44; N, 1.43%.
CH
CH
2
), 1.69 (t, 3H, CH
3
3
C
5
5
31
1
3
2
2
for C48
H
6 2
P
2.3. Catalytic procedure
In a 3 mL vial were introduced the surfactant followed by water
(
1 mL). The mixture was stirred until complete dissolution and for-
mation of a clear solution. To this, catalyst 1 (0.005 mmol) was
added and the mixture was stirred for 10 min, followed by allyl-
benzene (50 eq. with respect to 1). The mixture was heated at
[
26] used as fragrances, and can be carried out with both homoge-
9
5 °C under stirring at 750 rpm. After 18 h the aqueous reaction
neous and heterogeneous catalysts [27].
mixture was extracted two times with ethyl acetate and the
Herein we report an alternative supramolecular approach to
this reaction utilizing readily available and cheap surfactants to
promote the terminal alkene isomerization reaction in the pres-
ence of 1 (Scheme 1). To the best of our knowledge the use of
micellar environments has never been investigated for this
reaction.
organic phase was analyzed by GC and GC–MS. The structure of
1
the isomerized products were confirmed by GC–MS and H NMR.
3. Results and discussion
3.1. Synthesis and characterization of 1
With the aim of developing a simple small cationic half-sand-
wich complex reminiscent of those already known to operate in
the alkene isomerization reaction but focusing on an easy solubi-
lization in water in the presence of micellar aggregates, we
designed complex 1 as a good synthetic target bearing the pen-
tamethylcyclopentadienyl ligand, two phosphite neutral ligands
and one neutral labile acetonitrile ligand that could be easily dis-
placed by the incoming alkene substrate but not by water
molecules.
BPh -
+
4
Ru
N
(
EtO)
3
P
P(OEt)
3
1
R
R
The synthesis of complex 1 was achieved in overall two syn-
2
H
O, surfactant
3
2
thetic steps starting from RuCl
3
Á3(H
)(PPh
the method reported in the literature from RuCl
2
O) in a one pot reaction. Ini-
4 was prepared following
Á3(H O) by reac-
5
tially, complex RuCl(
g
-C
5
Me
5
3 2
)
Scheme 1. Alkene isomerization reaction mediated by the half-sandwich Ru(II)
complex 1 in water under micellar conditions.
3
2
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L. Sperni et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
3
⁄
tion with pentamethylcyclopentadienyl ligand (Cp ) leading to
5
5 5 4
[(g -C Me )RuCl] that was further reacted with triphenylphos-
phine [30]. The obtained intermediate species 4 was not isolated
but directly reacted with an excess of triethylphosphite (Scheme 2)
in order to lead to the displacement of the two molecules of triph-
enylphosphine. By addition of sodium tetraphenylborate in the
presence of an excess of acetonitrile the halogen ligand was
removed and substituted with the neutral acetonitrile, making
the overall complex 1 cationic in the presence of the weakly
coordinating tetraphenylborate counteranion (Scheme 2). The last
⁄
step enabled to completely substitute all ligands except the Cp
without requiring isolation of the intermediate species.
The synthesized complex, dissolved in chloroform-d, showed in
1
the
H NMR spectrum aromatic resonances typical of the
tetraphenylborate anion, the presence of the methyl substituents
⁄
of the Cp at 1.31 ppm, the ethyl moieties of the phosphite ligands
at 3.95 and 1.31 ppm and the presence of the coordinated acetoni-
trile as confirmed by the existence of a singlet resonance at
1
.69 ppm (Fig. 1B). As long as the 31P NMR spectrum is concerned,
a singlet resonance was observed at 145.3 ppm indicating the sym-
metry of the molecule (Fig. 1D). Some resonances were observed
also when changing the medium from an organic chlorinated sol-
vent to D
typical anionic surfactant able to solubilize cationic complexes
Fig. 1A and C). In fact, as clearly observed in the 1H spectrum
2
O in the presence of sodium dodecyl sulfate (SDS) as a
(
reported in Fig. 1A, all the aromatic resonances of the anion of
the complex are clearly present in the spectrum in deuterated
water, while the resonances of the cationic part of the complex
are covered by the resonances of the surfactant. On the other hand,
3
1
the P NMR spectrum clearly shows the singlet at 144.7 ppm con-
firming the presence of the transition metal complex in the micelle
(Fig. 1C). No NMR signals are observed in pure water as the catalyst
is not soluble.
Attempts were made to investigate the positioning of the cata-
lyst with respect to the micellar aggregates using 2D NMR NOESY
and DOSY experiments. No useful information was obtained due to
the overlap of the resonances of the cationic part of the metal cat-
alyst and those of the surfactant. However, the cationic nature of
the complex and the anionic nature of the surfactant seem to sug-
gest that the catalyst is likely located close to the external surface
of the micelle, as has already been observed with other cationic
phosphine containing noble metal complexes [14c].
Fig. 1. A) 1H NMR spectrum of the Ru(II) complex 1, 4 mM in D
2
O in the presence of
1
SDS 50 mM; B) H NMR spectrum of the Ru(II) complex 1, 4 mM in chloroform-d; C)
3
1
P NMR spectrum of the Ru(II) complex 1, 4 mM in D
2
O in the presence of SDS
31
5
0 mM; D) P NMR spectrum of the Ru(II) complex 1, 4 mM in chloroform-d.
different media ranging from organic solvents to water with the
addition of a wide range of possible surfactants [31] (Table 1) seek-
ing for the best medium to dissolve all species and favor the iso-
merization reaction.
3.2. Catalytic studies
The isomerization to a-methylstyrene was initially investigated
in organic media observing that while toluene was not a good sol-
The isomerization of allyl benzene derivatives such as estragole,
eugenol, and safrole to the corresponding internal alkenes used as
fragrances are all important industrial processes, seeking for both
high productivity and high selectivity for the E isomer. In our
investigation, we initially carried out as test reaction the isomer-
ization of allylbenzene in the presence of 2 mol% of catalyst 1 in
Table 1
Allylbenzene isomerization mediated by Ru(II) catalyst 1 in different media.
#
Reaction medium
Yield
a
(
%)
1
2
3
4
5
Toluene
1,2-dichloroethane (DCE)
<2
49
<2
<2
5
H
H
H
2
2
2
O
P(OEt)
3
O/cetyl trimenthylammonium bromide (CTAB)
O/N-dodecyl-N,N-dimethyl-3-amonium-1-propan
Ru
Ru
Ph
3
P
Cl
3
(EtO) P
Cl
P(OEt)3
3
PPh
sulfonate (DDAPS)
4
6
7
8
9
H
H
H
H
H
2
2
2
2
2
O/Triton X–100
O/TPGS
O/TPGS-750-M
O/dioctyl solfosuccinate sodium salt (DOSS)
O/sodium dodecylbenzenesulfonate (SDBS)
12
7
3
7
CH
3
CN
BPh -
+
4
10
<2
20
21
Ru
(EtO)
3
P
N
1
2
11 H O/sodium dodecylsulfate (SDS)
NaBPh
4
P(OEt)
3
12
2
H O/sodium hexadecyl sulfonate (SHS)
Scheme 2. One pot direct synthesis of the final catalytically active complex 1 from
complex 4 by substitution of the phosphine ligand with an excess of triethylphos-
phite and subsequent in situ replacement of the chloride ligand with acetonitrile.
Experimental conditions: [allylbenzene] = 200 mM; [1] =4 mM (2 mol%); [surfac-
tant] = 50 mg, for TPGS-750-M 2 w/w%; water 1 mL, 95 °C, 18 h.
a
Determined by extraction with ethyl acetate and GC analysis.
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4
L. Sperni et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
vent leading to negligible yield for the internal alkene product, in
chlorinated solvent the reaction was much more favored observing
Table 3
Scope of the terminal alkene isomerization reaction mediated by Ru(II) catalyst 1.
#
1
Substrate
Product
Yield (%)a
38
E:Za
4
9% yield (E:Z 98:2) after 18 h at 95 °C (entries 1 and 2, Table 1).
The reaction in pure water did not proceed at all due to lack of
99:1
solubility of both substrate and catalyst (entry 3, Table 1). A series
of surfactants were then investigated, observing that neither catio-
nic tensides like cetyl trimethylammonium bromide (CTAB), nor
zwitterionic N-dodecyl-N,N-dimethyl-3-amonium-1-propan sul-
fonate (DDAPS, entry 5, Table 1) favored the reaction with yields
in all cases lower than 10%. Among neutral surfactants, only Tri-
ton-X100 (entry 6, Table 1) led to a minimal product formation
O
O
O
O
2
36
98:2
O
O
O
O
3
4
35
22
98:2
98:2
O
O
O
O
while D-
and DL-
TPGS-750-M) turned out to be inefficient in solubilizing the Ru
a-tocopherol polyethylene glycol 1000 succinate (TPGS)
a
-tocopherol methoxypolyethylene glycol succinate
(
OH
OH
complex in water (entries 7 and 8, Table 1). As long as anionic sur-
factants are concerned, dioctyl sulfosuccinate (DOSS, entry 9,
Table 1) and sodium dodecylbenzenesulfonate (SDBS, entry 10,
Table 1) did not allow sufficient solubilization of the catalyst and
consequently alkene isomerization. Conversely, two anionic sur-
factants, in particular sodium dodecyl sulfate (SDS, entry 11,
Table 1) and sodium hexadecyl sulfonate (SHS, entry 12, Table 1)
led both to catalyst solubilization and formation of the isomerized
product in 20 and 21% yield, respectively (E:Z 99:1 in both cases).
Even in the latter case yields observed are not very high but com-
parable (even if lower) to DCE as solvent.
Even if defining the structure of the micellar medium at 95 °C
may be uncertain, this result is undoubtedly due to a solubilization
effect of catalyst 1 in the reaction medium, as demonstrated by
Fig. 1 as opposed to pure water where the complex is completely
insoluble. This point is further supported by the effect of surfactant
concentration reported in Table 2. We recall that a similar behavior
by micellar media at high temperature was observed in the hydra-
tion of nitriles catalyzed by Ru complexes [25] and in the hydro-
formylation of terminal alkenes catalyzed by Pt complexes [18].
Transition metal catalyzed isomerization reactions generally
proceed through two possible pathways (i) metal-hydride addition
elimination to and from the coordinated substrate and (ii) olefin
oxidative addition to produce hydridoallyl metal intermediates.
Given the aprotic nature of the solvation sphere surrounding the
catalyst (toluene, DCE, the hydrophobic part of the micelles) in
the present case in-situ hydride formation seems quite unlikely.
On this basis it seems reasonable to suggest that the isomerization
reaction is likely to proceed via the second possible pathway with
prior displacement of acetonitrile by the incoming allylbenzene
substrate, as has been recently found by Nolan and coworkers for
similar cyclopentadienyl Ru(II) complexes [32].
Experimental conditions: [substrate] = 200 mM; [1] =4 mM (2 mol%); [SDS] = 60 mM;
water 1 mL, 95 °C, 18 h.
a
Determined by extraction with ethyl acetate and GC analysis.
In order to optimize the catalytic systems, the reactions with
the two best anionic surfactants were repeated in the presence of
different amounts of SDS and SHS as reported in Table 2. From
the data reported, it is evident that both for SDS and SHS there is
a typical profile of product yield vs. surfactant concentration char-
acterized by an increase of activity for concentrations above the c.
m.c. (that is lower than 10 mM for both surfactants) up to a max-
imum value followed by a decrease. This typical behavior can be
attributed to an initial increase of activity due to better solubiliza-
tion of both catalyst and substrate within the micelles where they
can interact forming the corresponding isomerization product, fol-
lowed by a decrease in catalytic activity in the presence of higher
amounts of surfactant due to a dilution effect. It is worth noting
that, upon optimization of the amount of surfactant employed, it
was possible to obtain catalytic activities comparable to the use
of a chlorinated solvent such as DCE when using SDS at a concen-
tration of 60 mM or using SHS at a concentration of 100 mM.
To extend the catalytic protocol and investigate the scope of the
reaction, we tested catalyst 1 in the micellar medium using a series
of terminal alkenes as reported in Table 3. Simple terminal alkenes
like 1-octene and 1-dodecene did not react under the optimized
experimental conditions. Conversely, when investigating allylben-
zene derivatives endowed with methoxy substituents (Table 3)
moderate yields were observed but the selectivity for the E isomer
was in all cases almost complete as with allylbenzene. The sub-
strates reported in Table 3 are all examples of important starting
materials for the fragrance industry [33] like estragole (entry 1),
3
(
,4-dimethoxyallylbenzene (entry 2), safrole (entry 3) and eugenol
entry 4).
Attempts to recycle the Ru catalyst were carried out as in Table 2
Table 2
Allylbenzene isomerization mediated by Ru(II) catalyst 1: effect of the concentrations
of SDS and SHS.
entry 2 on allylbenzene with SDS extracting the product of the
reaction with ethyl acetate and adding fresh substrate to the aque-
ous micellar phase. Unfortunately the second run of the reaction
led to the formation of only 12% of isomerized internal alkene.
#
Medium
Surfactant concentration (mM)
Yield (%)a
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
2
O/SDS
2
O/SDS
2
O/SDS
2
O/SDS
2
O/SDS
2
O/SDS
2
O/SHS
2
O/SHS
2
O/SHS
2
O/SHS
2
O/SHS
2
O/SHS
25
60
15
42
34
20
15
15
8
19
43
20
21
18
1
31
H and P NMR experiments carried out at the end of the first
125
250
370
500
25
reaction clearly demonstrated the partial decomposition of the
original complex during the reaction. On the other hand, the reac-
tion on allylbenzene with SDS could be scaled up testing the cat-
alytic system on 4 mmol substrate, observing yield of 39% and E:
Z ratio of 98:2, thus demonstrating the reliability of the catalytic
method developed.
50
100
200
330
450
1
1
1
0
1
2
4
. Conclusion
Experimental conditions: [allylbenzene] = 200 mM; [1] =4 mM (2 mol%); water
1
mL, 95 °C, 18 h.
a
In conclusion, herein we reported a simple and efficient alkene
Determined by extraction with ethyl acetate and GC analysis. In all cases E:Z
9
9:1.
isomerization method based on 2 mol% of catalyst 1 operating in
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L. Sperni et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
5
water in the presence of anionic surfactants that showed compara-
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[
[
ble activity with respect to the use of traditional chlorinated
organic solvents. The key feature of the catalytic system is the
micellar medium obtained by simple addition of commercially
available SDS or SHS that favor the close contact between the apo-
lar alkene substrate with the cationic catalyst that is likely to be
positioned close to the surface of the anionic micelles. The opti-
mized catalytic system was applied to a series of allylbenzenes that
are precursors to fragrances showing excellent E-isomer selectivity
and overall good performance with respect to the use of organic
solvents. The latter was retained even when the reaction was
scaled up to substrate gram size.
[
(
(
b) K. Holmberg, Eur. J. Org. Chem. (2007) 731;
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[
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