Spicing up olefin cross metathesis with the renewables estragole and methyl sorbate

Article abstract of DOI:10.1016/j.apcata.2021.118173

Diene moieties conjugated to a carbonyl group are ubiquitous in nature and are present in compounds with relevant biological properties. Herein we investigate the cross metathesis (CM) of the renewable cross partners estragole and methyl sorbate (MeSo) to produce methyl 6-(4-methoxyphenyl)hexa-2,4-dienoate. By the judicious choice of the ruthenium-based metathesis catalysts, as well as the reaction conditions, it was possible to obtain good conversion and selectivity for the desired product in catalyst loadings as low as 50 ppm (0.005 mol%), with a minimal amount of solvent.

Full text of DOI:10.1016/j.apcata.2021.118173

Applied Catalysis A, General 620 (2021) 118173
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Spicing up olefin cross metathesis with the renewables estragole and
methyl sorbate
Leonildo A. Ferreira*, Josiane T. Silva, Raissa G. Alves, Kelley C.B. Oliveira,
Eduardo N. dos Santos*
ˆ
Department of Chemistry, Federal University of Minas Gerais, Av. Antonio Carlos, 6627, Belo Horizonte, 31270-901, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Diene moieties conjugated to a carbonyl group are ubiquitous in nature and are present in compounds with
relevant biological properties. Herein we investigate the cross metathesis (CM) of the renewable cross partners
estragole and methyl sorbate (MeSo) to produce methyl 6-(4-methoxyphenyl)hexa-2,4-dienoate. By the judicious
choice of the ruthenium-based metathesis catalysts, as well as the reaction conditions, it was possible to obtain
good conversion and selectivity for the desired product in catalyst loadings as low as 50 ppm (0.005 mol%), with
a minimal amount of solvent.
Ruthenium catalyst
Olefin metathesis
Sustainable chemistry
1. Introduction
atherosclerotic, antimicrobial, insecticidal, anti-depressant, neuro and
cardiovascular protective properties, to cite a few [37,38]. These are
Olefin metathesis is currently a well-developed methodology for the
examples of molecules which synthesis would benefit from a cross
metathesis approach, using naturally occurring allylbenzenes (e.g.,
estragole and isosafrole) and a conjugated carbonylated-diene molecule,
such as methyl sorbate. Reported herein is, therefore, a detailed inves-
tigation on the cross metathesis of renewable estragole with methyl
sorbate, catalysed by ruthenium-based, second-generation metathesis
catalysts (Fig. 2), in the preparation of a dienoate ester, structurally
analogous to piperovatine [39].
–
elaboration of a diversified number of molecules containing C C double
bonds, ranging from simple commodity structures to more complex and
elaborate pharmaceutical ingredients [1,2]. In particular, olefin
metathesis has been employed as a powerful strategy to access func-
tionalized olefins with high atom-economy, mild reaction conditions,
and enabling the use of substrates containing electron-rich as well as
–
several electron-deficient C C double bonds. Illustrative examples of
the latter class of substrates include cross metathesis (CM) reactions of
fatty acids compounds with acrylic acid [3,4] or maleic acid [5–8] de-
rivatives, providing access to monomer and detergent ingredients, as
well as the CM of essential oils components in the preparation of high
value-added fine chemicals [9–12]. Much less effort has, however, been
devoted for the preparation of conjugate dienoates (and other conju-
gated carbonyl dienes) using olefin cross metathesis [13–19].
Molecules containing a dienyl moiety conjugated to a carbonyl group
(e.g., 2,4-dienoates and 2,4-dienamides) are frequently encountered in
nature (Fig. 1), many possessing interesting and diverse biological
properties [20–23].Moreover, molecules containing conjugated dienes
to a carbonyl group find extensive use as synthetic intermediates
[24–36].
2. Experimental section
2.1. General procedures
All manipulations of air/moisture sensitive materials were per-
formed in a MBraun dry box filled with purified argon. All solvents were
purchased in high purity grades. Anhydrous Sure-seal toluene was pu-
rified using a MBraun solvent purification system and stored in a dry box
under argon atmosphere. Catalysts were obtained from Sigma-Aldrich
(G2 – CAS 246047ꢀ 72-3, HG2 – CAS 301224ꢀ 40-8, G2-iPr – CAS
373640ꢀ 75-6, and HG2-iPr – CAS 635679ꢀ 24-2), Umicore (Ind2 – CAS
536724ꢀ 67-1, HG2-tolyl – CAS 927429ꢀ 61-6 and HG2-MesR – CAS
1025728ꢀ 57-7) or Evonik (RF-2 – CAS 1190427ꢀ 49-6). Estragole (98 %
purity) and undecane (99 % purity) were purchased from Sigma-
Aldrich, distilled in a Kugelhohr apparatus, collected under an argon
Piperovatine, piperine and piperlonguminine, active ingredients
found in plants of the genus Piper, exhibit a wide range of biological
activity, including anticancer, anti-inflammatory, anti-angiogenic, anti-
* Corresponding authors.
E-mail addresses: leonildo.laf@gmail.com (L.A. Ferreira), nicolau@ufmg.br (E.N. dos Santos).
https://doi.org/10.1016/j.apcata.2021.118173
Received 17 February 2021; Received in revised form 5 April 2021; Accepted 21 April 2021
Available online 22 April 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

L.A. Ferreira et al.
Applied Catalysis A, General 620 (2021) 118173
Fig. 1. Selected examples of naturally occurring molecules containing a dienic moiety conjugated to a carbonyl group.
Fig. 2. Ruthenium-based olefin metathesis catalysts used for the CM of estragole with MeSo.
atmosphere, and stored in a dry box over 4 Å molecular sieves. 2,4-Hex-
adienoic acid was purchased from Sigma-Aldrich and used as received.
The synthesis of methyl 2,4-hexadienoate (methyl sorbate) is described
in the Supporting Information. NMR spectra were recorded on a Bruker
Avance DRX-400 spectrometer. Chemical shifts are given in parts per
removal of volatiles by sweeping with a constant argon flow. For a
picture of the reactor, refer to the SI file (Fig. S2). Each glass vial was
loaded with estragole (125
(220ꢀ 1100
dard for GC analysis, 200
200 L aliquot was taken (t =0 h) and then an appropriate amount of the
catalyst was added as a freshly prepared stock toluene solution of pre-
L, totalizing 5 mL of
μL, 0.82 mmol, 1 equiv.), MeSo
μ
L, 1.68–8.4 mmol, 1–10 equiv.), undecane (internal stan-
μ
L, 0.95 mmol) and the reaction solvent. A
million and are referenced against the solvent residual signal (CDCl3
=
μ
1H NMR: 7.26 ppm; 13C NMR: 77.16 ppm). GC analyses were run on a
Shimadzu 2010Plus apparatus equipped with an SH-Rtx-Wax column
(30 m, 0.25 mmID, 0.25 df) and an FID detector. Mass spectra were
recorded in a GC2010/QP2010-GC/MS apparatus with an electron
impact detector at 70 eV. The temperature program for GC analysis is
specified in the Supporting Information.
cisely known concentration (typically 100ꢀ 1000
μ
solvent in each vial). The reactor was tight-closed and taken out of the
dry box and put in a pre-heated aluminium block. A mixture of water/
ethylene glycol at 5 ◦C was circulated into the condenser tips and a
steady argon flow was applied to sweep off the volatiles. Upon
completion, the reaction was exposed to air to quench any remaining
catalytically active specie. 200 μL of each reaction vial were diluted with
2.2. Protocols for the CM reactions
1.0 mL of untreated acetonitrile for CG analysis. Products 1 and 4 were
purified by column chromatography (SiO2, hexane/ethyl acetate 94:6).
Products 2 and 3 were identified by mass spectrometry (SI), and in the
case of 2, comparison with previous reported data [9]. Product 5
(anethole), was identified by co-injection of a pure, commercial, sample.
Kinetic monitoring – In a dry box a three-neck glass round-bottom
CM reactions were prepared inside the dry box in a reactor with
seven compartmentalized wells (HEL7), each one containing 20 mL glass
vials. A PTFE-coated magnetic stirring bar and a tailor-made PTFE
gasket (to prevent cross-contamination) were added in each vial. The
reactor lid contains seven internal condenser tips that partially enters
each of the glass vials. An internal hole in the lid allows the continuous
flask was loaded with estragole (250 μL, 1.64 mmol, 1 equiv.), MeSo
2

L.A. Ferreira et al.
Applied Catalysis A, General 620 (2021) 118173
Scheme 1. Cross metathesis of estragole with methyl sorbate (MeSo).
Table 1
Table 2
G2 loading optimization in the CM of estragole with MeSo.a.
Cross metathesis of estragole with different amounts of MeSo.
Selectivity (%)c,e
Selectivity (%)c,e
Entry
G2 (ppm)
Conv. (%)b,c
TONd
Equiv. of
MeSo
Conv.
Entry
TONd
(%)b,c
1 (E,E/E,Z)
3
4
5
1 (E,E/E,
Z)
3
4
5
1
2f
3
4
5
6
7
1000
1000
500
200
100
70
90
87
86
79
69
63
51
900
60 (23/1)
57 (25/1)
58 (22/1)
57 (18/1)
55 (15/1)
56 (14/1)
53 (13/1)
19
19
19
20
19
19
20
9
10
10
9
870
11
12
15
17
17
20
1
2
3
4
5
2
45
51
60
64
69
9000
49 (14/1)
53 (13/1)
53 (12/1)
53 (11/1)
55 (11/1)
18
20
23
25
25
27
20
13
11
9
4
1720
3950
6900
9000
10,200
4
10,200
12,000
12,800
13,800
6
5
6
8
6
8
9
6
10
10
50
6
aEstragole (125
μ
L; 0.82 mmol, 1 equiv.), MeSo (220ꢀ 1100
μL; 1.68–8.2 mmol,
a
Estragole (125
μ
L; 0.82 mmol, 1 equiv.), MeSo (440
μ
L; 3.28 mmol, 4
2–10 equiv.), estragole:MeSo molar ratio = 1:2ꢀ 1:10, undecane (200
μ
L), G2
(0.041
μ
mol; 50 ppm), toluene (5.0 mL), 60 ◦C, 4 h.
equiv.), estragole:MeSo molar ratio
= 1:4, undecane (200 μL), G2
(0.82ꢀ 0.041
μ
mol; 1000ꢀ 50 ppm), toluene (5.0 mL), 60 ◦C, 4 h.
b
Conversions are based on estragole.
b
c
d
e
Conversions are based on estragole.
Values are within ± 2 % deviation for duplicate reactions.
TON = (mol of estragole converted)/(mol of catalyst).
The selectivity of 2 remained within 1–3 % for all the reactions.
c
d
e
Values are within ± 2 % deviation for duplicate reactions.
TON = (mol of estragole converted)/(mol of catalyst).
The selectivity of 2 remained within 1–3 % for all the reactions. f HG2 was
used instead of G2.
variation in the catalyst loadings from 1000 to 50 ppm (1.0 to 0.005 mol
% versus estragole) afforded an augmented turnover number from 900 to
10,200, respectively, although resulting in a gradual decrease in the
conversion and affecting the selectivity for all the products. Enoate CM
product 2 was formed only in minor amounts (less than 3 % selectivity)
in the range of reaction conditions employed, diverging from reported
data for the CM of MeSo or Weinreb dienamides with a variety of CM-
partners [14]. Such a difference is likely due to the much lower cata-
lyst loadings used in the current study (1.0ꢀ 0.005 mol % versus 5,0–10,
0 mol % in the previous work). Dienoate 1 was formed as the major
product, however, CM product 3, SM product 4 and anethole (5, origi-
nated from estragole isomerization) were also observed. Among the four
possible geometric isomers of 1, only the 2E,4E (hereafter named E,E)
and the 2E,4Z (hereafter named E,Z) were formed. The isomers 2Z,4E
and 2Z,4Z were not observed by GC analysis or in the NMR spectra of
isolated dienoate 1 (Figs. S3-S8 in SI). The E,E/E,Z ratios diminished as
the catalyst loading decreased. At higher amounts of G2, a 23/1 E,E/E,Z
ratio was observed, corresponding to a diastereoselectivity towards 1-E,
E of 96 % (Table 1 - entry 1), while at lower loading, the E,E/E,Z
selectivity experienced a small decrease, affording 93 % of 1-E,E (E,E/E,
Z equal to 13:1, Table 1 - entry 6). As the catalyst loading decreased, the
overall selectivity towards 1 diminished from 60 % (Table 1 - entry 1) to
53 % (Table 1 - entry 6), while the selectivity towards 4 experienced a
near two-fold increase (from 9 to 20 %). At higher catalyst loadings the
primary E,Z-product can be recycled to the thermodynamically more
stable E,E-product, as well as the self-metathesis product 4 can be
recycled to 1 or 3, enhancing the amount of these products at the end of
the reaction. Nevertheless, the excellent productivities observed at
lower catalyst loadings (up to 10,200) stimulated the investigation of
other reaction parameters in attempts to improve the yield and selec-
tivity of 1 under these conditions.
(1320
μ
L; 9.84 mmol, 6 equiv.), estragole:MeSo molar ratio = 1:6,
undecane (400
μ
L) and toluene (0.2 mL). A 10 L aliquot was pipetted
μ
for GC analysis and the reactor was fitted to a reflux condenser under Ar.
After temperature stabilization (50 ◦C), 800
(164
μL of a 205
μ
mol Lꢀ 1 G2
μ
mol, 100 ppm) toluene solution was added via syringe to the
reactor. Aliquots (10 μL) were taken after several time intervals, diluted
with untreated acetonitrile, and analysed by GC-FID.
3. Results and discussion
The cross metathesis (CM) of estragole with methyl sorbate (MeSo)
may result in the formation of different CM products, depending upon
–
which MeSo C C double bond reacts with the propagating alkylidenic
specie to form
a CM product. That is, either the α,β- or the
γ,δ-unsaturations (Scheme 1) can coordinate to the ruthenium centre
and lead to different CM products. The γ,δ-C = C is less hindered and less
electron-deficient, than the α,β-C = C and is expected to react preferably
with the propagating species leading the formation of 1 in detriment of
2. The coupling of estragole in the γ,δ-position of MeSo can occur in two
fashions, one with the ester moiety leading to product 1 and another
with propylidene moiety leading to the CH3-homologation of estragole
(product 3 in Scheme 1). Both ways lead to two stereoisomers (E and Z).
Estragole self-metathesis is also an important concurrent pathway and
–
results in 4. Moreover, C C double bond isomerization of estragole can
also take place, leading to 5.
Initial experiments exploring the CM of estragole with MeSo were
conducted in a 7-well compartmentalized reactor, employing the
second-generation Grubbs metathesis catalyst (G2) in toluene, an
estragole to MeSo molar ratio of 1:4, at 60 ◦C for 4 h. Volatiles formed in
the reactions were swept-off with a continuous flow of argon. The
It is well known the cross metathesis partners molar ratio influences
3

L.A. Ferreira et al.
Applied Catalysis A, General 620 (2021) 118173
Table 3
Table 4
Solvent effects in the CM of estragole with MeSo.a.
Catalyst screening in the CM of estragole with MeSo.a.
Selectivity (%)c,e
Selectivity (%)c,e
Entry
Solvent
Conv. (%)b,c
TONd
Entry
Cat.
Conv. (%)b,c
TONd
1 (E,E/E,Z)
3
4
5
1 (E,E/E,Z)
3
4
5
1
toluene
p-cymene
dcef
60
63
63
49
10
64
73
12,000
12,600
12,600
9800
53 (12/1)
54 (10/1)
60 (14/1)
50 (11/1)
53 (10/1)
58 (12/1)
62 (13/1)
23
23
18
26
28
20
19
13
13
10
14
5
8
1
2
3
4
5
6
7
8
9f
G2
73
67
48
63
48
47
40
53
89
14,600
13,400
9600
62 (13/1)
63 (16/1)
33 (10/1)
53 (11/1)
52 (9/1)
19
19
38
25
26
37
17
19
16
12
10
20
14
9
5
2
8
HG2
7
3
9
G2-iPr
Ind2
8
4
Me-THFg
8
12,600
9600
7
5
MIBKh
2000
11
8
RF2
10
8
6i
toluene
toluene
12,800
14,600
12
12
HG2-iPr
HG2-tolyl
HG2-MesR
G2
9400
34 (10/1)
70 (14/1)
64 (14/1)
65 (14/1)
21
7
7i,j
5
8000
6
10,600
8900
9
7
a
Estragole (125
μ
L; 0.82 mmol, 1 equiv.), MeSo (660
μ
L; 4.92 mmol, 6
10
7
equiv.), estragol:MeSo molar ratio = 1:6, undecane (200
μ
L), G2 (0.041 mol;
μ
a
50 ppm), solvent (5.0 mL), 60 ◦C, 4 h.
Estragole (125
μ
L; 0.82 mmol, 1 equiv.), MeSo (660
μ
L; 4.92 mmol, 6
b
c
equiv.), estragol:MeSo molar ratio = 1:6, undecane (200
μ
L), Cat. (0.041 mol;
μ
Conversions are based on estragole.
50 ppm), toluene (0.5 mL), 50 ◦C, 4 h.
Values are within ± 2 % deviation for duplicate reactions.
TON = (mol of estragole converted)/(mol of catalyst).
The selectivity of 2 remained within 1–2 % for all the reactions.
1,2-Dichloroethane.
b
d
e
f
Conversions are based on estragole.
c
Values are within ± 2% deviation for duplicate reactions.
TON = (mol of estragole converted)/(mol of catalyst).
The selectivity of 2 remained within 1–2% for all reactions.
100 ppm of G2.
d
e
f
g
h
i
2-Methyltetrahydrofuran.
Methyl isobutyl ketone.
50 ◦C.
j
0.5 mL of solvent.
conditions using a series of ruthenium ylidene catalysts (Fig. 2). Rather
surprisingly, Hoveyda-Grubbs catalyst HG2 underperformed G2,
exhibiting smaller estragole conversion and yield for 1 (Table 4 - entry 1
vs. 2). Such behaviour appears to contradict a general trend in
metathesis chemistry for acrylates and other more challenging electron-
deficient olefins, in which phosphine-free catalysts, such as HG2, out-
performs G2 under optimized, low-catalyst loading conditions. The test
was repeated three times, including a different batch of HG2, and the
better performance of G2 over HG2 showed to be consistent. A key point
to account for the better performance of HGII-type over GII-type cata-
lyst, e.g., in acrylate cross metathesis, is the substrate-induced decom-
position of the ruthenacyclobutane intermediate by an enolate formed
upon nucleophilic attack of dissociated phosphine (PCy3) on the acrylate
(Michael addition) [40,41]. Nevertheless, many examples of G2 exhib-
iting equal or superior performances as compared to HG2 are found in
literature, especially when more reactive substrates are employed [42].
In the specific case of CM of estragole with MeSo, the better performance
of G2 may reflect the lower electronic deficiency, and thus the higher
reactivity, of the MeSo γ,δ-unsaturation, as compared to acrylates and
on the outcome of CM reactions. Albeit optimal results for several
electron-deficient CM partners such as acrylates and acrylonitrile is
accomplished in a 1:4 M ratio, other CM partners give better results in
either lower or higher ratios, and thus this parameter must be investi-
gated. In Table 2 the CM of estragole and MeSo was investigated in
molar ratios ranging from 1:2 to 1:10, and a increase in estragole con-
version was observed, reaching a maximum conversion of 69 % when
using a ten-fold excess of the CM partner (for other molar ratios, see also
Table S1). Not surprisingly the proportion of the self-metathesis product
was reduced with the increase in MeSo. Nevertheless, the selectivity
towards formation of target 1 was almost unaffected, especially in
ranges greater than 4-fold equivalents of MeSo with respect to estragole.
Conversely, the amount of the other CM product 3 also increased with
the increase of MeSo. Less obvious is the reason for the considerable
increase in the catalyst stability: the TON increases almost 55 % from
entry 1 to entry 5 in Table 2. These observations can be reconciled if one
considers that the proportion of propagating Ru-species formed by the
initial reaction of the catalyst with MeSo will increase with the increase
in the concentration of this reactant. A direct consequence would be
increasing the cross products in detriment to the self-metathesis product
4. An indirect consequence would be reducing the rate of the catalyst
deactivation by reducing, at least at the early stages of the reaction, the
formation of the highly reactive Ru-methylidene species originated from
the interaction of the catalyst with the terminal double bond of
estragole.
–
acrylonitrile C C double-bonds.
The study was extended to other pre-catalysts (entries 3–8). Gener-
ally speaking, catalysts with bulkier N-heterocyclic carbene (NHC) li-
gands resulted in lower conversions and selectivity for 1 (c.f. entry 1 vs.
3; entry 2 vs. 6). Steric constrictions disfavour the coordination of the
trans-disubstituted MeSo γ,δ-unsaturation with respect to the terminal
–
C
C double bond of estragole, thus favouring the homocoupling.
Similar behaviour was observed previously for the CM of estragole with
methyl acrylate [9]. Conversely, the catalyst containing a NHC with
lower steric hindrance (i.e. HG2-tolyl) favoured a higher selectivity for
1, albeit the conversion was lower probably due to faster catalyst
decomposition via bimetallic pathways (c.f. entry 2 vs. 7). Regarding the
ylidene moiety in the pre-catalysts, it is more difficult to provide gen-
eralizations since both steric and electronic effects seem to play an
important role. Under the reaction conditions employed, catalysts that
usually present equivalent or superior outputs presented a significant
lower performance (c.f. entry 1 vs. 4 and 5; entry 2 vs. 8) for the CM of
estragole with MeSo. This exemplifies the importance of the catalyst
screening in challenging transformations.
Despite better results being obtained with a ten-fold excess of the
CM-partner, further optimizations were carried out using a six-fold
excess of MeSo because this value allows good conversion and TON
with a not so large excess of CM partner employed. A screening of
representative solvents was then performed. p-Cymene and 1,2-dichlo-
roethane (dce) resulted in comparable performances to toluene, with a
slight better selectivity for 1 observed in dce (Table 2 - entry 3). More
polar solvents, Me-THF and the green solvent methyl isobutyl ketone
(MIBK) resulted in poorer conversions. The screening of temperature
(Table S2) and amount of toluene (Table S3) is presented in SI, and
selected conditions were included in Table 3, entry 7, in which estragole
conversion reached 73 % with selectivity for 1 of 62 %. Both from
environmental and economic perspective, the better solvent is no sol-
vent. Thus, it is remarkable that this reaction shows a better perfor-
mance when the amount of solvent is reduced to the minimum necessary
to dissolve the catalyst (c.f. Table 3, entry 7).
Upon the increasing the G2 loading to 100 ppm (Entry 9), the con-
version of estragole reached 89 % and a combined 83 % selectivity for
the CM products with a turnover number of 8900 was achieved. Such
results compare favourably with those obtained when using 1000 ppm
of G2 in the initial experiments (Table 1, entry 1). That is, after all the
optimizations, a near ten-fold increase in the catalyst productivity was
Additional investigation was performed under the optimized
4

L.A. Ferreira et al.
Applied Catalysis A, General 620 (2021) 118173
Fig. 3. Kinetic monitoring plot of estragole
conversion (filled blue circles) and yield of CM
products 1 (filled red diamonds) (a), and inset
showing the yields (GC) for product 3 (unfilled
green circles), 4 (brown squares) and 5 (light
blue triangles) (b). Lines were added with the
only purpose to aid visualization. Conditions:
estragole (250
(1320 L; 9.84 mmol,
(400 L), G2 (0.164 μmol; 100 ppm), toluene
μ
L; 1.64 mmol, 1 equiv.), MeSo
μ
6
equiv.), undecane
μ
(1.0 mL), 50 ◦C (For interpretation of the ref-
erences to colour in this figure legend, the
reader is referred to the web version of this
article).
attained with a smaller amount of the catalyst, with near identical
estragole conversion and a slight improvement in the yield of 1. Note-
worthy is the chemoselectivity achieved for 1 in detriment of 2 under
such low catalyst loadings, as opposed to reported literature values for
similar products obtained from electron-rich olefins with ethyl sorbate
(dienoate:enoate ratio = 8/1) and catalyst loadings as high as 10 mol%
of G2 [15]. This represents a considerable improvement as not only the
catalyst productivity was largely improved, but also it has been
demonstrated that the use of electronic or steric ‘protective strategies’
when performing metathesis reactions with dienic substrates similar to
methyl sorbate may be unnecessary. The presence of the ester func-
tionality suffices for the chemodifferentiation of the olefinic units by
second-generation metathesis catalysts. Under the optimized condition
(Table 4, entry 9), a 1:2 ratio of 39:1 (97 %) was obtained.
Oliveira: Investigation. Eduardo N. dos Santos: Supervision, Writing -
review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors thank Conselho Nacional de Pesquisa Científica e Tec-
´
´
nologica - CNPq (INCT-Catalise) for financial support. Dedicated to
Professor Pierre Dixneuf.
To extract a more comprehensive set of information regarding the
CM of estragole, with methyl sorbate, the progress of the reaction was
monitored over a 2 h period (Fig. 3). The reaction proceeded fast over
the first 15 min and remained virtually unchanged after the allotted
time, which is a clear indicative of catalyst decomposition/deactivation.
Moreover, evidence of consumption of neither the homocoupled product
4 nor CM product 3 leading to the formation of target CM 1 can be
observed in the time-dependent plot, suggesting that the formation of
these by-products, unless prevented, is difficult to be later remediated.
Anethole (5), an estragole isomerization product, could also be observed
from the beginning of the CM reaction, and its formation ceased as the
metathesis reaction stopped (i.e., after 15 min). This observation seems
to rule out ruthenium hydrides or ruthenium nanoparticles as main
isomerization species for this particular transformation, once such spe-
cies would, a priori, keep this side reaction going on until complete
estragole conversion. The isomerization mechanism proposed by Nolan
& Prunet, in which the propagating {Ru = CHR} species is considered to
promote C = C isomerization, appears fitting better the observed profile
for the conversion of estragole into anethole [43,44].
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118173.
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