Ammonium-tagged ruthenium-based catalysts for olefin metathesis in aqueous media under ultrasound and microwave irradiation

Article abstract of DOI:10.3762/bjoc.15.16

The influence of microwave and ultrasonic irradiation on the performance of ammonium-tagged Ru-based catalysts in olefin metathesis transformations in aqueous media was studied. Differences in the catalytic activity in correlation with the nature of the present counter ion and the size of the N-heterocyclic carbene (NHC) ligand were revealed. The presented methodology allows for preparation of a variety of polar and non-polar metathesis products under environmentally friendly conditions.

Full text of DOI:10.3762/bjoc.15.16

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Ammonium-tagged ruthenium-based catalysts
for olefin metathesis in aqueous media under
ultrasound and microwave irradiation
Łukasz Gułajski1, Andrzej Tracz1, Katarzyna Urbaniak1, Stefan J. Czarnocki1,
Michał Bieniek1 and Tomasz K. Olszewski*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 160–166.
1Apeiron Synthesis SA, Duńska 9, 54-427 Wrocław, Poland and
2Wrocław University of Science and Technology, Faculty of
Chemistry, Wybrzeże Wyspiańskiego 29, 50-370 Wrocław, Poland
doi:10.3762/bjoc.15.16
Received: 23 August 2018
Accepted: 22 December 2018
Published: 17 January 2019
Email:
Tomasz K. Olszewski* - tomasz.olszewski@pwr.edu.pl
This article is part of the thematic issue "Progress in metathesis
chemistry III".
* Corresponding author
Keywords:
Associate Editor: M. Rueping
catalysis; green chemistry; microwave; N-heterocyclic carbene; olefin
metathesis; ruthenium; ultrasound
© 2019 Gułajski et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The influence of microwave and ultrasonic irradiation on the performance of ammonium-tagged Ru-based catalysts in olefin me-
tathesis transformations in aqueous media was studied. Differences in the catalytic activity in correlation with the nature of the
present counter ion and the size of the N-heterocyclic carbene (NHC) ligand were revealed. The presented methodology allows for
preparation of a variety of polar and non-polar metathesis products under environmentally friendly conditions.
Introduction
Olefin metathesis is well established as a powerful transformat- which there is currently an increasing demand, especially in
ion used for effective and elegant creation of new carbon–car- industrial practice, as a replacement for those with major regu-
bon double bonds [1,2]. The development of commercially latory issues such as chlorinated (dichloromethane, 1,2-
available, stable and effective catalysts for that reaction [3-6] dichloroethane) or aromatic solvents (toluene, benzene) [13-16].
made possible for its wide application not only in academia but In that aspect olefin metathesis in aqueous media appears to be
also in industry [7-12]. However, there is still a large interest in an interesting alternative, especially in the case of preparation
improving the catalytic activity of the existing Ru-based me- of biologically important molecules [17-20] as well as of highly
tathesis catalysts as there is no universal catalyst for all the me- polar compounds. Thus far, several strategies were applied to
tathesis transformations. This is especially true for olefin me- facilitate olefin metathesis in water including the development
tathesis reactions carried out with the use of green solvents, for of specially designed water-soluble catalysts [21-28], addition
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Beilstein J. Org. Chem. 2019, 15, 160–166.
of organic solvents [29-31], or use of additives such as for ex- reduce the energetic cost of the processes, the replacement of
ample calixarenes or cyclodextrins [32,33], chloride salts [34], mechanical mixing and/or heating of the reacting species with
vitamin E-based amphiphiles [35], dodecyltrimethylammonium microwave (μW) [46-48] and ultrasonic irradiation (US) [49-
bromide (DTAB) [36], polymerised cyclooctadiene (COD) and 55] appears as a promising approach. Both methods were shown
cyclooctene (COE) [37], sodium dodecyl sulphate (SDS) [38] in the past to be responsible for shortening the reaction time,
or DL-α-tocopherol methoxypolyethylene glycol succinate solu- increasing the reaction yield or even favour the formation
tion (TPGS-750-M) [39], to improve the solubility of reacting of the desired product when compared to traditional protocols
species and/or performance of the catalyst. Recent progress in [56,57]. In the case of olefin metathesis, examples of applica-
the flourishing field of micellar catalysis and the use of surfac- tion of those techniques are well documented for organic sol-
tants that self-aggregate in water into micelles in which the vents [58-66], surprisingly, examples describing reactions in
hydrophobic core provides an environment for effecting homo- aqueous media are scarce and thus merit further investigation
geneous reactions between organic molecules has been [67,68].
reviewed by Scarso et al. [40] and very recently by Lipshutz
and co-workers [41]. Worth mentioning are also reports of In line with our ongoing research on synthesis of catalysts for
heterogenous and recyclable catalysis able to mediate metathe- olefin metathesis and to expand the utility of ammonium-tagged
sis in aqueous media [42-45]. Although the aforementioned ex- ruthenium-based catalysts [69-76], herein we present the use of
amples show a significant progress in the olefin metathesis in such catalysts for olefin metathesis in aqueous media promoted
aqueous media, some limitations such as complex structure of by microwave and ultrasound irradiation.
the tailored catalysts and thus difficulties associated with their
synthesis, or the need to use additives or co-solvents to improve Results and Discussion
the solubility of reacting species, still remain. Therefore, further The structures of the catalysts 1–5 used in this work are
development of catalytic systems would provide a complimen- depicted in Figure 1.
tary extension to the scope of this interesting transformation.
Catalyst 1b was prepared by alkylation of the non-ionic tertiary
Furthermore, in the continuous search for new sustainable amine-containing analogue with methyl iodide [71]. Com-
protocols for chemical reactions to induce new reactivates or plexes 1c,d were prepared from their commercially available
Figure 1: Structures of the Ru-based catalysts used in this study.
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Beilstein J. Org. Chem. 2019, 15, 160–166.
corresponding chloride salt 1a [72] by exchange of the Cl− reaction performed under ultrasound irradiation proved to be
counter-ion to PF6− or BF4− [76]. The exchange was performed ca. 10% more productive with both catalysts compared to the
in water, and after addition of NH4PF6 or NH4BF4 the formed classical conditions. On the other hand, microwave irradiation
catalysts were collected by filtration. Catalyst 2c was prepared turned out to be less effective leading to a drop in the reaction
from the new complex 2a using a similar procedure (see Sup- yield for 1a (48%) and a slightly increased yield in the case of
porting Information File 1 for details). The complexes 3a [75] 4b (55%). In the homometathesis reaction of allyl alcohol 8
and 5 are commercially available and catalyst 4b was obtained (Table 1, entries 3 and 4) both catalysts (5 mol %) produced the
according to a literature procedure from commercially avail- desired product again with quite similar yields under classical
able Grubbs 2nd generation catalyst through ligand exchange conditions. However, the use of microwave or ultrasound irradi-
[68]. In general, the solubility of the catalysts containing Cl− as ation promoted the undesired isomerisation of the C=C bond,
counter ion in water is good (e.g., 50 mg mL−1 for 1a or 3a) thus lowering the yields of the desired product 9 (Table 1,
whereas for those with I− as counter-ion is much lower (e.g., entries 3 and 4). This result is in agreement with the known fact
4.0 mg mL−1 for 1b). In turn catalysts bearing PF6− or BF4− as that in protic solvents ruthenium hydrides are formed leading to
counter ions are not soluble in water [71].
isomerisation byproducts [66]. Finally, we were pleased to see
that the use of ultrasound or microwave irradiation were benefi-
We have started our study with the comparison of the catalytic cial for the CM of alcohol 8 with methyl acrylate (10, Table 1,
activity of complex 4b, having the ionic tag attached to the entries 5 and 6) resulting not only in increased conversion but
benzylidene ligand, with that of catalyst 1a, bearing an ionic tag also reducing the amount of the unwanted product of self-me-
placed on the N-heterocyclic carbene (NHC) fragment. As tathesis of 8.
model reactions we have selected the ring-closing metathesis
(RCM) of the water-soluble substrate 6, the homometathesis of In general, the results obtained with catalysts 4b and 1a were
alcohol 8, and more challenging, the cross metathesis (CM) be- comparable. However, we expected that 1a should be much
tween alcohol 8 and the electron-deficient cross partner methyl more effective because it remains tagged after the initiation
acrylate (10, Table 1).
step. This unexpected catalytic activity might be due to the fact
that catalysts 4b and 1a have different counter ions and there-
All reactions were run at 36 °C in D2O promoted either by fore we decided to examine if there is an influence of counter
microwave (µW) or ultrasound (US) irradiation, and for com- ions on the catalytic activity. To achieve this we used ana-
parison purposes also with standard magnetic stirring. In the logues of 1a bearing different counter ions (1b–d) and also
case of the RCM (Table 1, entries 1 and 2) both tested catalysts included catalysts having differently sized NHC ligands (2a,c).
(1 mol %) under classical conditions exhibited similar activities For testing the catalysts performances, we selected the RCM of
with 4b being slightly less active (52 vs 48%, respectively). The the water-soluble substrate 12 (Table 2).
Table 1: Effect of microwave (μW) and ultrasound (US) irradiation on RCM, homometathesis and CM in water mediated by complexes 1a and 4b.
Entry
Substrate
Product
Ru complex
Classical conditionsa
USa
µWa
1b
2b
1a
4b
52
48
63
59
48
55
7
6
8
3c
4c
1a
4b
81 (78)
77 (88)
73 (60)
38 (66)
64 (68)
75 (84)
9
5c
6c
1a
4b
69 (74)
35 (45)
71 (79)
71 (80)
81 (88)
80 (86)
11
aConversion and selectivity (in parentheses, referring to the formation of an aldehyde, having a signal at 9.60 ppm, resulting from double bond migra-
tion) have been determined based on 1H NMR. bReaction conditions: D2O, catalyst (1 mol % Ru), c 0.1 M, 36 °C, 2 h. cReaction conditions: D2O,
catalyst (5 mol % Ru), c 0.1 M, 36 °C, 2 h.
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Beilstein J. Org. Chem. 2019, 15, 160–166.
Table 2: Effect of the counter ion and substituents size of the NHC ligand in catalysts 1b–d, and 2a,c on their efficiency in the RCM of substrate 12 in
water under μW and US irradiation.a
Entry
Substrate
Product
Ru
Classical conditionsb
USb
µWb
1
2
3
4
5
6
7
1a
1b
1c
1d
2a
2c
5
33
13
53
35
54
33
3
41
53
49
46
12
32
1
58
77
81
72
51
61
1
13
12
aReaction conditions: D2O, Ru catalyst (0.25 mol %), c 0.2 M, 36 °C, 2 h. bConversions determined based on NMR.
Under the reaction conditions the classical catalyst 5 increasing size of the NHC ligand. This result suggests that a
(0.25 mol %) was not soluble resulting in poor yields and justi- fast propagation ensured by a smaller carbene ligand rather than
fying the use of modified catalysts. For the ammonium NHC- robustness ascribed to larger catalysts is a prerequisite for the
tagged catalysts (0.25 mol %), the use of microwave irradiation efficient metathesis in homogeneous aqueous conditions.
was more productive than ultrasound treatment. This effect was Except in the case of catalyst 3a, exhibiting the lowest activity
most pronounced in the case of catalysts with low solubility in under classical conditions, we noted a positive effect of US in-
water such as 1b or the insoluble catalysts 1d and 2c. Addition- creasing the reaction yield from 22 to 61%.
ally, under classical conditions, we observed a higher activity of
catalyst 2a with Cl− as counter ion and a larger NHC ligand Finally, we have tested the influence of μW and US irradiation
when compared to 1a. In turn, the use of US and μW had an on the RCM of lipophilic substrates 14, 16 and 18 in water
inverse effect on that reaction furnishing lower yields of the (Table 4).
product in the case of 2a when compared with 1a. In case of
catalysts bearing a large hexafluorophosphate counter ion Regardless of the conditions and substrate used, the lowest
(PF6−; 1c and 2c) an increase of the NHC’s size had a negative yields were observed for the water-soluble complexes 1a and
effect on the catalyst performance (Table 2, entries 3 and 6, re- 2a. This is most probably due to the reduced stability of those
spectively).
catalysts in aqueous medium and additionally to their limited
contact with the substrates, being in a different phase (organic
Examining further the influence of the steric hindrance of the layer), and in a process that resembles more to a “heterogen-
NHC ligand we tested complexes 1a, 2a and 3a (1 mol %) all eous reaction”. In turn, reactions with the use of catalysts 1b,
with Cl− as counter ion in the RCM of polar substrate 6 1c, 1d and 2c with much lower solubility in water gave signifi-
(Table 3).
cantly better results. With only two exceptions the use of μW or
US irradiation provided poorer results when compared to the
In order to maintain homogeneity of the reaction mixture addi- classical conditions. Only catalysts 1c and 2c bearing PF6− as
tion of isopropanol (iPrOH) was necessary. Under the applied counter ion performed better with both, μW and US irradiation.
conditions the activity of the tested complexes decreased with However, this effect was observed only for the simplest sub-
Table 3: Effect of the size of the NHC ligand in the catalysts 1a, 2a, and 3a on their catalytic efficiency in the RCM of substrate 6 under μW and US ir-
radiation.a
Entry
Substrate
Product
Ru
Classical conditionsb
USb
µWb
1
2
3
1a
2a
3a
61
43
22
48
38
61
51
36
39
7
6
aReaction conditions: D2O/iPrOH 2:1 (v/v), Ru catalyst (1 mol %), 36 °C, 2 h. bConversions determined based on NMR.
163