Mechanosynthesis of Noels-type NHC-Ruthenium Complexes and Applications in Ring-Opening Metathesis Polymerization

Article abstract of DOI:10.1021/acs.organomet.0c00013

The use of ball mills enabled the efficient mechanosynthesis of a variety of N-aryl,N-alkyl imidazolium salts and of corresponding NHC silver(I) complexes. Transmetalation with ruthenium via mechanochemistry allowed the rapid access (from 1.5 min to 1 h)

Full text of DOI:10.1021/acs.organomet.0c00013

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Mechanosynthesis of Noels-type NHC−Ruthenium Complexes and
Applications in Ring-Opening Metathesis Polymerization
́
́
Franco̧ is Quintin, Julien Pinaud, Frederic Lamaty,* and Xavier Bantreil*
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ABSTRACT: The use of ball mills enabled the efficient mechanosyn-
thesis of a variety of N-aryl,N-alkyl imidazolium salts and of
corresponding NHC silver(I) complexes. Transmetalation with
ruthenium via mechanochemistry allowed the rapid access (from 1.5
min to 1 h) to complexes having a similar structure to Noels-type
precatalysts. Evaluation of the complexes in the ring-opening metathesis
polymerization of norbornene in different solvent, including nontoxic
ones, showed a high catalytic activity for one of them, comparable to that
of the Noels catalyst.
diisopropylphenyl) imidazole was examined in a ball mill
ince its discovery, olefin metathesis has become an
Simportant chemical transformation in the academic as (Scheme 1). In a vibratory ball mill (vbm), milling N-
well as in the industrial world.1 In particular, ring-opening
arylimidazoles at 25 Hz with benzyl bromide in a PTFE jar
with a 1 cm diameter stainless-steel ball allowed us to obtain
full conversion into corresponding imidazolium salts 2a and
2b, respectively, in less than 1.5 h. No purification was
required, and a simple extraction and filtration over Celite
furnished the pure compounds in 97 and 93% yield. Similarly,
compounds 2c,d, which are bidentate ligand precursors, were
obtained in 91% yield after milling of 1a,b with bromome-
thylpyridine hydrobromide and sodium bicarbonate for 1.5 h.
Double nucleophilic substitution was also performed using 2,6-
di(bromomethyl)pyridine as alkylating agent. Milling fre-
quency and time were increased to 30 Hz and 3.5 h,
respectively, to finally isolate 2e,f in 94 and 91% yield.
Notably, the mechanochemical approach enabled the use of
smoother conditions and shorter reaction times than those
with solution conditions.9 As a representative example,
synthesis of compound 2e requires 2 days when performed
in refluxing dioxane9a instead of 3.5 h under solvent-free
milling conditions.
metathesis polymerization (ROMP) finds applications in the
synthesis of useful polymer materials such as poly(norbornene)
or poly(dicyclopentadiene).2 In regards to this reaction, the
ruthenium−arene complex [RuCl2(p-cymene)(IMes)] (IMes
= 1,3-bis(mesityl)imidazol-2-ylidene), also known as Noels’
catalyst, was found to be highly active.3 Although benzylidene
Grubbs-like complexes are still under intensive study in order
to develop a universal catalyst, Noels’ type complexes have less
been investigated. Thus, we describe herein the mechanosyn-
thesis of analogs of Noels-like complexes through alkylation of
N-arylimidazoles, silver metalation, and transmetalation in a
ball mill, as well as their evaluation in ROMP of norbornene
(NB). Mechanochemistry, including ball mills and reactive
extrusion, was recognized in 2019 by IUPAC as an innovation
that will change the world.4 This technology allows the
avoidance of solvent in the reaction mixture, thus reducing the
environmental impact of the reaction, and generally permits an
increase of the reaction speed compared to solution-based
methods.5 Additionally, it permits the synthesis of compounds
inaccessible by other means.6 Of note, ball-milling was recently
used for the synthesis of second- and third-generation Grubbs-
and Hoveyda-like complexes through a solvent-free carbene
addition reaction.7 We thus desired to take advantage of our
expertise in the preparation of metal complexes via ball-
milling8 for the synthesis of Noels-type olefin metathesis
catalysts.5b
Imidazolium salts 2a−f were then metalated using silver(I)
oxide under solvent-free mechanochemical conditions. Of
note, as milling jars are not transparent, there is no need to
light protection usually necessary for the synthesis of light-
Received: January 9, 2020
On the basis of our experience on the formation of N,N-
dialkylimidazolium salts,8e alkylation of N-mesityl- and N-(2,6-
https://dx.doi.org/10.1021/acs.organomet.0c00013
© XXXX American Chemical Society
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Scheme 1. Alkylation of N-Arylimidazoles and Silver Metalation
Scheme 2. Mechanosynthesis of Ruthenium Complexes
sensitive silver complexes. Contrary to what was observed in
the metalation of N,N-dialkylimidazolium salts,8e the reaction
mixture was found to be not homogeneous in the milling jar.
To solve this issue, addition of water as liquid grinding
assistant (0.3 μL/mg of reactants) revealed the paramount
conditions.10 Under these conditions, metalation proceeded in
1.5−3 h, and complexes 3a−f could be isolated in yields up to
93% (Scheme 1).
Silver complexes 3a and 3c were then milled with 0.5 equiv
of [Ru(p-cymene)Cl2]2 at 30 Hz for 1 h in a stainless-steel jar
(Scheme 2). Corresponding complexes 4a and 4c were both
isolated in 83% yield. No coordination of the pyridine was
observed in the case of 4c. To increase the scope of ruthenium
complexes and compare them in ROMP, complexes 6 and 7,
featuring methyl and isopropyl groups in place of the mesityl in
the NHC ligand, respectively, were also mechanosynthesized.
After 1 h of milling at 30 Hz, complexes 6 and 7 were isolated
in 94 and 92% yield, respectively. For the pyridine-containing
compounds, addition of a stoichiometric quantity of KPF6 in
the milling media enabled the chelation. However, such
reaction was found sensitive to the milling conditions. Reaction
of 3c with [Ru(p-cymene)Cl2]2 at 30 Hz for 1 h in the
presence of KPF6 yielded a 90:10 mixture of 5a/2c·PF6, which
could not be improved by a prolonged milling. To ensure full
conversion and no side product formation, the milling
frequency was reduced to 25 Hz, and the material of the jar
was changed from stainless steel to PTFE. A softer material
such as PTFE will absorb more energy from the impacts of the
ball than a harder one. 5a could be obtained as a pure
compound after only 15 min of grinding at 25 Hz. Similarly,
complexes 8 and 9, featuring a methyl and isopropyl group
instead of the mesitylene, respectively, could be formed in less
than 3 min at 20 Hz. As the transmetalation occurred
efficiently, a simple recovery and filtration over Celite of the
reaction mixture using 1,3-dioxolane as solvant, a green
alternative to dichloromethane, furnished the pure compounds.
As a comparison, in solution, 3 h of reaction in dichloro-
methane were required to obtain 5a, 8, and 9.11 Notably, the
reaction of silver complexes 3b and 3d, featuring a sterically
hindered 2,6-diisopropylphenyl group, and 3e,f, bearing
B
https://dx.doi.org/10.1021/acs.organomet.0c00013
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Communication
tridentate ligands, with the ruthenium dimer resulted in the
formation of the desired complex but with unidentified and
inseparable side products.
The obtained ruthenium complexes were then evaluated in
the ROMP of NB, using a ratio precatalyst/NB = 1/500
(Table 1). To compare the activity of the different precatalysts,
Table 1. Comparison of Mechanosynthesized Catalysts’
a
Activity in the ROMP of Norbornene
Figure 1. Effect of solvent in the ROMP of norbornene with 4a.
Reaction conditions: norbornene (5.32 mmol), 4a (0.01 mmol),
solvent (2.5 mL), N2, and 35 °C.
entry
Ru cat
t (min)
yield (%)
1
2
3
4
5
6
7
8
9
2
30
96
99
8
3
2
2
0
0
4a
20
40
2
2
2
2
2
2
4c
6
7
5a
8
9
mechanotransmetalation allowed generating Noels’ like
ruthenium coordination complexes in short reaction times,
from 1.5 min to 1 h, and in excellent yields. These complexes
were evaluated in the ROMP of NB in a series of solvents, and
complex 4a exhibited interesting high kinetic activity,
comparable to that of in situ generated Noels catalyst. Further
study on the tuning of the NHC ligand in such complexes is
ongoing in our laboratory.
a
Reaction conditions: norbornene (5.32 mmol), catalyst (0.01
mmol), CH2Cl2 (2.5 mL), N2, and 35 °C.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00013.
■
polymerization reactions were stopped after 2 min of reaction,
by quenching with the addition of ethylvinyl ether, to
determine the yield of isolated polymer. If complex 4a gave
an interesting 30% yield in polynorbornene, then 4c, featuring
a pyridine attached to the NHC, furnished only 8% yield,
probably because of an unproductive coordination/decoordi-
nation of the pyridine during the catalytic cycle (Table 1,
entries 1 and 4). Replacing the mesityl with a methyl or
isopropyl group revealed detrimental as the isolated yield
dropped to 3 and 2%, respectively (Table 1, entries 5 and 6).
Such behavior is in good agreement with the observations by
Noels et al. regarding the ROMP of cyclooctene.12 Complexes
5a, 8, and 9, featuring a pyridine coordinated to the ruthenium
center, were almost inactive in the ROMP of NB, due to a too
strong chelation of the ruthenium (Table 1, entries 7−9).
Attempts to activate these precatalysts by heating or adding
HCl did not allow for any improvement. Thus, complex 4a was
found to be the most active precatalyst for the ROMP of NB.
When the reaction time was increased to 20 and 40 min,
gratifyingly, yields up to 99% were obtained (Table 1, entries 2
and 3). In order to find an ecofriendly solvent for ROMP, 4a
was then evaluated in 1,3-dioxolane and DMC (dimethylcar-
bonate) in addition to dichloromethane (Figure 1). Even if the
reaction was found to be faster in dichloromethane, the kinetic
profiles were remarkable in 1,3-dioxolane and DMC since
almost quantitative yields were obtained in 80 min. In
particular, ROMP proceeded more rapidly in 1,3-dioxolane
than DMC. Of note, ROMP of NB with 4a was compared to
previously reported results using in situ generated Noels
catalyst.13 Interestingly, the kinetics were found to be similar,
proving that well-defined complex 4a with an N-benzyl,N-
mesityl NHC ligand possesses an excellent activity in ROMP.
See the Supporting Information for details.
sı
Synthetic procedures, description of compounds, opti-
mization of the ROMP, copies of the H and 13C NMR
1
spectra of the compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
́
́
́
Frederic Lamaty − IBMM, Universite Montpellier, CNRS,
ENSCM, Montpellier CEDEX 5 34095, France; orcid.org/
0000-0003-2213-9276; Email: frederic.lamaty@
umontpellier.fr
́
Xavier Bantreil − IBMM, Universite Montpellier, CNRS,
ENSCM, Montpellier CEDEX 5 34095, France; orcid.org/
0000-0002-2676-6851; Email: xavier.bantreil@
umontpellier.fr
Authors
Franco
ENSCM, Montpellier CEDEX 5 34095, France
́
is Quintin − IBMM, Universite Montpellier, CNRS,
̧
́
Julien Pinaud − ICGM, Universite Montpellier, CNRS,
ENSCM, Montpellier 34095, France; orcid.org/0000-
0003-4446-3771
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00013
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
The Universite de Montpellier, Centre Nationale de la
In conclusion, mechanochemistry enabled the efficient
synthesis of N-aryl,N-alkyl imidazolium salts and of corre-
sponding heteroleptic NHC-silver(I) complexes. In addition,
Recherche Scientifique (CNRS) and Agence Nationale de la
Recherche (grant no. ANR-16-CE07-0009-01) are acknowl-
edged for funding.
C
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