Efficient synthesis of styrene derivatives through ethenolysis of renewable propenylbenzenes

Article abstract of DOI:10.1016/j.mcat.2021.111631

Functionalized styrenes were obtained by the ethenolysis of renewable 1-propenylbenzenes in a very efficient synthetic pathway. Some of the products are valuable food & flavor ingredients (4-vinylguaiacol) or locust pheromone (4-vinylanisole). The catalysts employed were ruthenium-alkylidene complexes bearing a N-heterocyclic carbene as a ligand, which bulkiness proved to be important for the catalysis output. The judicious choice the reaction conditions was critical to enable near quantitative yields under mild conditions in short reaction times. More strikingly, the catalyst load could be reduced to 0.01 mol%, keeping good conversion and selectivity.

Full text of DOI:10.1016/j.mcat.2021.111631

Molecular Catalysis 509 (2021) 111631
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Efficient synthesis of styrene derivatives through ethenolysis of
☆
renewable propenylbenzenes
1
*
Jesus A. Avenda n˜ o Villarreal, Artur V. Granato , F a´ bio G. Delolo, Eduardo N. dos Santos
Departamento de Química, Universidade Federal de Minas Gerais, 31270-901, Belo Horizonte, MG, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Functionalized styrenes were obtained by the ethenolysis of renewable 1-propenylbenzenes in a very efficient
synthetic pathway. Some of the products are valuable food & flavor ingredients (4-vinylguaiacol) or locust
pheromone (4-vinylanisole). The catalysts employed were ruthenium-alkylidene complexes bearing a N-het-
erocyclic carbene as a ligand, which bulkiness proved to be important for the catalysis output. The judicious
choice the reaction conditions was critical to enable near quantitative yields under mild conditions in short
reaction times. More strikingly, the catalyst load could be reduced to 0.01 mol%, keeping good conversion and
selectivity.
Metathesis
4
4
-vinylguaiacol
-vinylanisole
Ruthenium catalyst
1
. Introduction
The use of renewable platforms to produce chemicals of high added
methodologies have drawbacks associated with the large amount of
rejects, including salt formation due to the need of stoichiometric
addition of a base or organometallic reagents [17]. In this scenario, the
ethenolysis of renewables rises as a promising alternative to obtain
styrene derivatives generating less problematic side products, and in
smaller quantities [10,18].
value is a field of emerging scientific and industrial interest [1]. In this
scenario, the carbon-carbon bond formation catalysed by
transition-metal complexes plays a key role, enabling sustainable routes
for useful compounds [2]. Olefin metathesis is a chemical reaction,
Some propenylbenzenes are suitable starting materials used to pre-
pare a variety of compounds, including fine chemicals [19-23]. The
ethenolysis of 1-propenylbenzenes such as anethole, isoeugenol and
isosafrole gives direct access to functionalized styrenes (Scheme 1).
Also, 2-propenylbenzenes such as estragole, eugenol, safrole and
methyl-eugenol can be obtained from natural sources even in larger
which formally involves the exchange in substituents between C C
–
double bonds. It is catalysed by transition metal compounds and is
largely used in the chemical industry for alkene transformation [3].
Since the development of robust, well-defined ruthenium catalysts such
as those presented in Chart 1, olefin metathesis has become a more
versatile tool in organic synthesis [4]. When metathesis involves the
quantities as compared to 1-propenylbenzenes [24]. Combining C C
–
reaction of a compound containing an internal C
ethylene, it is named ethenolysis. The synthesis of
saturated methyl esters from unsaturated fatty acids methyl esters
FAME) is an example of the emerging industrial uses of ethenolysis [5],
and its application to other renewable feedstock is also well established
6-9].
Styrene derivatives are valuable building blocks for biologically
–
C double bond with
double bond isomerization and metathesis is a way of increasing the
synthetic potential of metathesis [20,25,26], and a very elegant method
to convert 2-propenylbenzenes into functionalized styrenes through the
isomerization/metathesis approach was reported by Gooβen and
co-workers [10]. In this method, one isomerization catalyst, namely [Pd
α
-olefins and -un-
ω
(
[
4 9 3 2
(C H ) PBr] , and a Ru-ylidene metathesis catalyst are loaded in the
same reaction vessel, and operate in an orthogonal manner. The 2-pro-
penyl double bond migrates to the 1-propenyl position and then ethe-
nolysis takes place. Despite the elegance, this methodology has some
drawbacks in terms of practical applications: the use of an expensive
catalyst to perform the isomerization, a high catalyst loading (3.0 mol%)
active molecules, fine chemicals, and polymers [10]. There are several
routes to obtain functionalized styrenes, both classic and catalytic.
Among the latter, the metal catalyzed cross-coupling methodologies
occupy
a prominent position [11-16]. Nevertheless, even these
☆
Dedicated to Professor Christian Bruneau.
Corresponding author.
*
E-mail address: nicolau@ufmg.br (E.N. dos Santos).
Present address: UNIBH 30575–180, Belo Horizonte, MG, Brazil
1
https://doi.org/10.1016/j.mcat.2021.111631
Received 23 March 2021; Received in revised form 2 May 2021; Accepted 5 May 2021
Available online 25 May 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

J.A. Avenda n˜ o Villarreal et al.
Molecular Catalysis 509 (2021) 111631
◦
of an also expensive metathesis catalyst, and a long reaction time (16 h).
In an alternative approach, the ethenolysis of 1-propenylbenzenes was
carried with an ethylene surrogate (1,7-octadiene, 10 equivalents) at
for one hour and then distilled (50 C; 2 mmHg) using a Kugelrohr
distillation apparatus and collected under argon. Isoeugenol (99%) and
the methyl isoeugenol (99%) were purified in the same way, but distilled
◦
room temperature for 2 h, employing 0.5 mol% of a PPh
3
-analogous of
at higher temperature (90 C; 2 mmHg). After purification, all substrates
GII to reach NMR yields of up to 92% [27].
presented as colorless liquids. The n-undecane (>99%) was degassed by
bubbling argon (1 min of bubbling for each mL). Ruthenium metathesis
catalysts GI, GII, HII (Sigma-Aldrich), M73Pr and M73Mes (Umicore)
were used as received, as were ethylene (99.9%, Praxair).
Considering the plethora of catalysts capable of promoting the
isomerization of 2-propenylbenzene into 1-propenylbenzenes [24],
including the industrially employed potassium hydroxide [28], it seems
that the critical part of this synthetic route is the stability and robustness
of the ethenolysis catalyst.
2
.2. Representative procedure for ethenolysis
In recent years, ruthenium metathesis catalysts containing a cyclic
(
alkyl)(amino)carbene (CAAC) as stabilizing ligands have emerged as
Ethenolysis was carried out in a stainless steel reactor with a pressure
highly efficient promoters for ethenolysis and other metathesis appli-
cations [29-32]. Turnovers as high as 340,000 have been reported in
some instances [33]. Nevertheless, this high performance seems to be
closely related to a high degree of purification of the reactants, espe-
cially ethylene [6], which can hamper the economic viability of indus-
trial processes. Furthermore, the intellectual property situation of the
catalysts has to be taken into account for industrial implementation.
Herein we decided to focus our efforts in developing reaction con-
ditions that allow high turnover numbers for commercial ruthenium
catalysts employing reactants purified by inexpensive methods.
gage, an inlet needle valve, a ball valve and a dip tube with needle valve.
A PTFE-coated magnetic stirring bar was put inside the reactor, which
was closed and maintained under vacuum at room temperature for 1 h.
In a glovebox, a Schlenk tube was charged with anethole (1), (0.367 mg,
2
.5 mmol), undecane (0.195 mg, 1.25 mmol) and toluene (7 mL) in a
glovebox. A solution of ruthenium catalyst (M73Pr) was prepared in
ꢀ 1
toluene (10 mg.mL ), and 0.103 mL (containing 1.25
μ
mol of M73Pr,
0
.05 mol%) of this solution was transferred to a Schlenk containing
toluene (3 mL). Both Schlenk tubes were sealed with J. Young PTFE
stopcock and taken out of the glovebox. The reactor was charged with
the substrate solution through the ball valve using Schlenk techniques,
2
. Experimental
◦
heated to 60 C and purged 3 times with ethylene. The catalyst solution
was added in the reactor with a syringe through the ball valve under a
slight flow of ethylene. The ball valve was closed and the ethylene
pressure was raised to 5 bar. The reaction was monitored by collecting
samples (0.2 mL) through the dip tube without depressurization. The
samples were exposed to atmosphere, diluted in untreated toluene (1.0
mL) and immediately analyzed by gas chromatography (GC).
2
.1. General procedures
All manipulations of air or water sensitive compounds were carried
out under dry argon using a glovebox or standard Schlenk line tech-
1
niques. NMR spectra were recorded on a Bruker NanoBay ( H, 400 MHz;
1
3
C 100 MHz) spectrometer and referenced to residual protic solvent
signal.
Anhydrous THF (>99.9%, Sigma-Aldrich) and anhydrous toluene
>99.8%, Sigma-Aldrich) were used as received. Trans-anethole (99%)
2
.3. Product analysis
(
◦
GC quantifications were performed on a Shimadzu GC2010 Plus in-
was heated (80 C) with Magnesol® (5% m/m) under vigorous stirring
strument equipped with an auto-sampler, fitted with a nonpolar Rtx®
Chart 1. Examples of commercial ruthenium metathesis catalysts.
2

J.A. Avenda n˜ o Villarreal et al.
Molecular Catalysis 509 (2021) 111631
column (30 m length, 250
μ
m internal diameter) and a flame ionization
1-propenylbenzene to optimize the reaction conditions. In the ethenol-
◦
detector (FID). GC analysis conditions: injection port, 310 C; split ratio
ysis of 1 (Scheme 2), two main products were observed: 4-vinylanisole
◦
◦
◦
′
5
0:1; FID, 320 C; oven temperature 50 C for 5 min, heating at 15 C/
(2) and 4,4 -dimethoxystyrene (3). The isomeric estragole was
◦
min to 280 C, kept for 5 min. Carrier gas: H
2
(UHP grade), make-up gas
observed in minor amounts and the formation of co-products propene
and 2-butene could be inferred, although they were not quantified due
to analytical limitations. In Table 1, a catalyst screening under various
reaction conditions is presented. The initial experiments were per-
formed with a catalyst loading of 1 mol% with respect to anethole in
2
N (UHP grade). Conversions and yields in catalytic runs were deter-
mined having undecane as internal standard. The GC/MS analyses were
performed using a Shimadzu GC2010/QP2010-GC/MS instrument
employing an electron impact detector at 70 eV. The GC/MS conditions
were identical to that of the GC/FID setup, except for the carrier gas
◦
THF, 60 C and 10 bar of ethylene.
(
helium). The products were purified by passing the reaction solution
The catalysts bearing tricyclohexylphosphine as ligand (GI, GII) gave
low conversion for ethenolysis of 1 (Table 1, Entries 1,2). GI is a useful
catalyst for, e.g., methyl oleate ethenolysis, but presents a very low
activity for anethole under the reaction conditions employed. Probably,
trough a small column containing 1 g of basic alumina (activated, basic,
Aluminum oxide, Brockmann I, Sigma-Aldrich), then, the solvent was
1
13
evaporated and the product analyzed by H NMRand C NMR.
the trans configuration of its propenylic C
omatic ring conjugation makes this substrate less reactive than methyl
oleate, which has a cis, non-conjugate C C double bond. GII presents a
–
C double bond and the ar-
2
.4. Analytical data
-vinyl anisole (2): Mass spectrometry (m/z/relative intensity): 134/
–
significantly better activity, which is expected as the catalyst containing
the N-heterocyclic carbene (NHC) ligand binds the olefin more effi-
ciently. Nevertheless, even with GII the conversion was not yet satis-
factory as only 30% was achieved after 16 h (entry 2). Conversely, HII
catalyst gave a much better result, reaching 94% yield (entry 3) under
the same reaction conditions. Although both GII and HII generate the
4
+
1
1
00 [M] , 119/60, 91/56, 65/20. H NMR (400 MHz, CDCl
3
): δ 7.37 (d,
= 8.8 Hz, Ar-H), 6.69
3
3
3
2
H,
J
H
–
H
= 8.8 Hz, 2 Ar-H), 6.89 (d, 2H,
J
H H
–
3
(
dd, 1H,
CH ), 5.15 (d, 1H,
C NMR (100 MHz, CDCl
CH=), 130.67 (aromatic carbon), 127.58 (2 aromatic carbons), 114.12
J
H
–
H
= 17.6 Hz, -CH=), 5.63 (d, 1H,
J
H
–
H trans = 18 Hz,
), 3.83 (s, 3H, Ar–OCH ).
): δ 159.6 (aromatic carbon), 136.44 (Ar-
3
=
2
J
H
–
H cis = 10.8 Hz, =CH
2
3
1
3
3
3
same catalytic species, the released PCy in GII can be detrimental for
). The 1H NMR
3
the catalyst activity and stability. Not only this ligand can rebind the
catalytically active species, but also it can attack Ru-methylidene in-
termediates leading to catalyst decomposition paths, as demonstrated by
Fogg and co-workers [36,37]. The analogous M73 is slightly less active,
showing that the leaving alkylidene can also influence on the system
performance. Perhaps more surprising is that M73Pr gives a better yield
for 2 than HII. Steric features of the stabilizing ligand seem to be critical
for a good performance of the catalyst in challenging metathesis re-
(
=CH
2
), 111.75 (2 aromatic carbons), 55.47 (Ar–OCH
1
3
and C NMR signals assignments were confirmed by comparison to the
literature values [34].
2
‑methoxy-4-vinylphenol (5): Mass spectrometry (m/z/relative in-
+
1
tensity): 150/100 [M] , 135/85, 107/40, 91/10, 77/36. H NMR
spectrum (400 MHz, CDCl ): δ 7.28 (m, 1H, 1 Ar-H), 6.98 (m, 2H, Ar-H),
= 14.1 Hz, -CH=), 5.94 (s, 1H, Ar-OH), 5.67 (d, 1H,
H trans = 17.6 Hz, =CH H cis = 10.8 Hz, =CH ),
.91 (s, 3H, Ar–OCH ). C NMR (100 MHz, CDCl ): δ 146.7, 145.7
3
.71 (dd, 1H, 3
6
3
J
H H
–
), 5.20 (d, 1H, 3
J
H
–
2
J
H
–
2
1
3
actions [38,39]. SIPr is a bulkier NHC ligand than H IMes and the
2
3
3
3
former may disfavor the coordination of anethole in an intermediate
(
aromatic carbons), 136.74 (Ar-CH=), 120.11 (aromatic carbon),
1
containing a bezylidene moiety, which is necessary to make the stilbene
derivative 3. Thus, the path of the cross metathesis involving the coor-
dination of the less encumbered ethylene is favored with this bulkier
version of the catalyst [40].
1
14.12 (=CH
2
), 111.75 (2 aromatic carbons), 55.87 (Ar–OCH
3
). The H
1
3
NMR and C NMR signals assignments were confirmed by comparison
to the literature values [10].
1
,2-dimethoxy-4-vinylbenzene (7): Mass spectrometry (m/z/relative
+
1
Considering that THF is nowadays a non-recommended solvent in
recent solvent guides [41], we decided to employ the more acceptable
and now standard toluene as solvent for olefin metathesis in the ethe-
nolysis of 1-propenylbenzenes. We were pleased to find that the
replacement was even advantageous as full conversion and near quan-
titative yield of 2 was achieved using the catalyst M73Pr (Table 1, entry
intensity): 164/100 [M] , 149/45, 121/20, 103/25, 91/37, 77/25.
NMR spectrum (400 MHz, CDCl ): δ 7.42 (m, 2H, 2 Ar-H), 7.18 (m, 1H,
= 14.2 Hz, -CH=), 5.89 (d, 1H,
H
3
Ar-H), 6.91 (dd, 1H, 3
3
J
H
–
H
J
H H trans
–
), 4.15 (s, 3H,
3
=
17.4 Hz, =CH
), 4.12 (s, 3H, Ar–OCH
aromatic carbon), 136.44 (Ar-CH=), 130.67 (aromatic carbon), 127.58
), 111.75 (2 aromatic carbons),
2
), 5.42 (d, 1H,
J
H
–
H cis = 10.8 Hz, =CH
2
). 1 C NMR (100 MHz, CDCl
3
Ar–OCH
3
3
3
): δ 159.6
(
6
). In order to verify whether or not 16 h were really necessary, we made
(
2 aromatic carbons), 114.12 (=CH
2
) The 1H NMR and C NMR signals assignments were
13
the reaction in a shorter reaction time (2 h) and no decrease in yield was
observed (Table 1, entry 7). Once obtained more satisfactory reaction
conditions, we addressed the issue of catalyst loading. It was possible to
reduce 20 times the catalyst loading (from 1 to 0.05 mol%; c.f. entry 7
and 8) without significant loss in yield. Further reduction to 0.01 mol%
still leaded to 87% yield (entry 9), resulting in a turnover number of
5
5.47 (Ar–OCH
3
confirmed by comparison to the literature values [35].
. Results and discussion
3
3
.1. Ethenolysis of anethole (1)
8
700, which is considered appropriate for fine chemicals applications.
Reducing ethylene pressure from 10 to 5 bar did not compromise the
yield (c.f. entry 8 and 10). This pressure reduction has important im-
plications from the technical point of view, as equipment to operate up
to 7 bar are considerably cheaper than for higher pressures. More
For the ethenolysis of 1-propenylbenzenes, we employed THF as
◦
solvent, 60 C, ethylene (10 bar) as initial conditions [10] for the
screening of the ruthenium-based catalysts presented in Chart 1 (GI, GII,
HII, M73, M73Pr). Anethole (1) was used as
a representative
Scheme 1. Ethenolysis of 1-propenylbenzenes to obtain functionalized styrenes.
3

J.A. Avenda n˜ o Villarreal et al.
Molecular Catalysis 509 (2021) 111631
Scheme 2. Ethenolysis of anethole (1).
Table 1
a
Ethenolysis of anethole (1) .
Entry
Ru catalyst
[Ru] mol%
Solvent
Conversion [%]
Yield 2 [%]
1
2
3
4
5
6
GI
1
THF
5
3
GII
1
THF
30
99
89
99
99
99
97
88
97
81
26
94
88
97
99
99
96
87
96
80
HII
1
THF
M73
1
THF
M73Pr
M73Pr
M73Pr
M73Pr
M73Pr
M73Pr
M73Pr
1
THF
1
toluene
toluene
toluene
toluene
toluene
toluene
b
7
1
b
8
b
0.05
0.01
0.05
0.05
9
b, c
1
1
0
1
b, c, d
a
Reaction conditions: anethole (1) (2.5 mmol; 0.367 mg); solvent (10 mL);
◦
ethylene (10 bar), 60 C, 16 h. Yields determined by GC using undecane as the
internal standard.
b
2
h.
c
ethylene (5 bar).
d
untreated anethole.
importantly, we used ethylene of standard purity (99.9%), without any
additional treatment. Finally, we checked the relevance of the
pre-treatment of anethole with magnesium silicate. Entry 11 was carried
out with untreated anethole and the results were significantly worse
than those with treated anethole (c.f. entry 8 and 11). The presence of
impurities such as bases or peroxides may be detrimental to the catalyst
activity even in ppm level and the pre-treatment with magnesium sili-
cate followed by distillation proved to be important. It is also worth-
while mentioning that, while the presence of cis-anethole is forbidden
for the use in flavors, mixtures of cis- and trans-anethole (Z and E iso-
mers, respectively) obtained industrially [28] could be used in this
methodology. Actually, the presence of cis-anethole is even desirable
because it is more reactive in metathesis than the trans isomer.
Fig. 1. Ethenolysis of anethole (1) kinetic curve. Reaction conditions: anethole
(
2.5 mmol); Ru catalyst (M73Pr - 0.05 mol%); toluene (10 mL); ethylene (5
◦
bar), 60 C. Yields determined by GC using undecane as the internal standard.
certain liquors [42]. More strikingly, 2 was recently reported as an
aggregating pheromone of migratory locusts (Locusta migratoria), a
plague responsible for enormous damages in agriculture [43]. Product 5
is also a valuable commercial flavor, usually produced by biological
routes [44].
4. Conclusions
In Fig. 1 the kinetic profile of the reaction carried under the same
conditions as in entry 10, Table 1 is presented. It is rather surprising that
after 15 min a conversion of 93% was already obtained and the turnover
In this work we developed practical reaction conditions for the
ethenolysis of renewable 1-propenylbenzenes. We were able to reduce
the ethylene pressure by a factor of 2, the reaction time by a factor
greater than 8 and, moreover, the catalyst loading by a factor of up to
300 as compared to previous results [10], while keeping good yields.
The choice of the pre-catalyst showed to be critical for the success of the
process. Competent catalysts for other ethenolysis processes failed in the
ethenolysis of 1-propenylbenzenes. Noteworthy is that the pre-catalyst
containing the more sterically encumbered NHC ligand (SIPr) reduced
the self-metathesis of the propenylbenzenes. Thus, this study provided
important insights about the structural features relevant to the catalyst
stability and selectivity in the field of ethenolysis.
ꢀ
1
frequency considering the first 10 min of reaction is c.a. 10,000 h
.2. Ethenolysis of 1-propenylbenzenes
Under optimized conditions, the ethenolysis of other 1-propenylben-
.
3
zenes (i.e. isoeugenol and methylisoeugenol) was carried out and the
results are presented in Table 2. The self-metathesis product (homo-
dimer) was observed only in minor amount and the vinyl products (5, 7)
were also obtained in high yields (Table 2, entry 2 and 3). It was ex-
pected that ethenolysis of isoeugenol would present an additional
challenge, as isoeugenol was reported to form the undesired dehy-
drodiisoeugenol under certain metathesis conditions [10]. However,
this product was not observed under the conditions employed in this
work.
Considering that 4-vinylanisole (2) is an aggregating pheromone for
migratory locusts, and has a potential to help in the control of this
agriculturally relevant plague and 4-vinylguaiacol (5) is an established
valuable aroma, the methodology described herein seems a promising
alternative to produce these valuable chemicals directly from renewable
Product 2 is a naturally occurring aroma compound used, e.g., in
4

J.A. Avenda n˜ o Villarreal et al.
Molecular Catalysis 509 (2021) 111631
Table 2
a
Ethenolysis of 1-propenylbenzenes .
Entry
1
Substrate
Product
Conversion (%)
Yield (%)
96
97
2
3
98
97
96
97
a
◦
Reaction conditions: 1-propenylbenzene (2.5 mmol); catalyst M73Pr (1.25
determined by GC using undecane as the internal standard.
μ
mol; 1.03 mg; 0.05 mol%); toluene (10 mL); ethylene (5.0 bar), 60 C, 2 h. Yields
feedstock in a scale compatible with the fine chemicals segment.
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