Ruthenium-arene catalysts bearing N-heterocyclic carbene ligands for olefin cyclopropanation and metathesis

Article abstract of DOI:10.1007/s00706-015-1492-x

Ruthenium-arene complexes bearing N-heterocyclic carbene (NHC) ligands with the generic formula [RuCl₂(p-cymene)(NHC)] are efficient catalyst precursors for the cyclopropanation of activated olefins with ethyl diazoacetate, and the cis/trans di

Full text of DOI:10.1007/s00706-015-1492-x

Monatsh Chem (2015) 146:1099–1105
DOI 10.1007/s00706-015-1492-x
ORIGINAL PAPER
Ruthenium–arene catalysts bearing N-heterocyclic carbene
ligands for olefin cyclopropanation and metathesis
1,2
Anna M. Maj1,3 Albert Demonceau1 Lionel Delaude1
•
•
•
¨
´
Michael Meret
Received: 27 March 2015 / Accepted: 3 May 2015 / Published online: 20 May 2015
Ó Springer-Verlag Wien 2015
Abstract Ruthenium–arene complexes bearing N-hete-
rocyclic carbene (NHC) ligands with the generic formula
[RuCl2(p-cymene)(NHC)] are efficient catalyst precursors
for the cyclopropanation of activated olefins with ethyl
diazoacetate, and the cis/trans diastereoselectivity of the
reaction markedly depends on the steric bulk of the NHC.
The procedure was successfully applied to styrene, a-
methylstyrene, and various other styrenic derivatives
bearing electron-withdrawing or donating substituents on
their aromatic rings. The reaction of unactivated internal or
terminal alkenes was more sluggish, and the use of nor-
bornene as a substrate afforded only olefin metathesis.
Further investigation of the ring-opening metathesis poly-
merization of this strained cycloolefin in the presence of
trimethylsilyldiazomethane led to high molecular weight
polynorbornene whose microstructure was not significantly
affected by the choice of the NHC ancillary ligand.
Graphical abstract
Keywords Arene complexes Á Cross-metathesis Á
para-Cymene Á Homogeneous catalysis Á
Ring-opening metathesis polymerization Á
Structure–activity relationships
Introduction
Since the first imidazol-2-ylidene derivative was isolated
and characterized by Arduengo in 1991 [1, 2], N-hetero-
cyclic carbenes (NHCs) have become ubiquitous ligands in
organometallic chemistry and in homogeneous catalysis
[3–6]. Over the past 25 years, these stable divalent carbon
species have already afforded countless transition metal
catalysts that have found important practical applications in
olefin chemistry for hydrogenation, hydrosilylation, hy-
droformylation, hydroamination, or Heck reactions, to
name just a few [7–9]. In particular, thanks to the pi-
oneering work of Herrmann [10], Nolan [11], and Grubbs
[12], the replacement of a phosphine with an NHC ligand
onto ruthenium–alkylidene scaffolds led to a rich portfolio
of well-defined catalysts for olefin metathesis [13–16].
In parallel with the development of Grubbs-type
16-electron ruthenium–alkylidene complexes, we found
that coordinatively saturated ruthenium–arene species with
the generic formula [RuCl2(g6-arene)(NHC)] were versa-
tile and efficient catalyst precursors for the ring-opening
Dedicated to the lifework of Professor Franz Stelzer and his
achievements in olefin metathesis and polymer chemistry.
& Lionel Delaude
l.delaude@ulg.ac.be
1
Laboratory of Organometallic Chemistry and Homogeneous
´ `
Catalysis, Institut de Chimie (B6a), Universite de Liege,
`
Sart-Tilman, 4000 Liege, Belgium
2
3
Present Address: Max Planck Institute of Molecular Plant
Physiology, Am Mu¨hlenberg 1, 14476 Potsdam, Germany
´
Present Address: Ecole Nationale Superieure de Chimie de
Lille, Avenue Mendeleıev, BP 90108, 59652
¨
Villeneuve d’Ascq, France
123

´
M. Meret et al.
1100
metathesis polymerization (ROMP) of cyclic olefins when
activated by visible light [17]. A 1:2:4 molar association of
the [RuCl2(p-cymene)]2 dimer (1), an imidazol(in)ium salt,
and a base also provided efficient catalytic systems for
various types of metathesis reactions [18, 19]. Such a
combination required only stable and commercially avail-
able reagents to generate active species in situ, thereby
affording very simple and straightforward experimental
procedures [20].
substitution [22]. Thus, ethyl diazoacetate was slowly
added to a solution containing styrene, and a ruthenium
complex warmed at 60 °C under an inert atmosphere. The
conversion of diazoester was monitored with a gas burette
to measure the volume of nitrogen released. After 24 h, the
reaction mixture was analyzed by gas chromatography
(GC). Products formed included the desired ethyl
2-phenylcyclopropanecarboxylate (10) as a mixture of cis
and trans diastereomers (Scheme 1). In most cases, these
adducts were accompanied by significant amounts of ho-
mologation products, namely ethyl 4-phenylbut-3-enoate
(11) and ethyl 3-phenylbut-2-enoate (12). Formally, these
compounds result from a carbene insertion into a vinylic
C-H bond of styrene, but the exact mechanism of this re-
action, for which the intervention of ruthenacyclobutane
intermediates was postulated, remains uncertain [23]. We
also searched systematically for the peaks of cis- and trans-
stilbene (13) in the chromatograms, as they were indicative
of styrene metathesis. Other possible side-products, which
were identified by GC–MS analyses in previous studies,
include ethyl acrylate, ethyl cinnamate, phenylcyclo-
propane, and 1,2-diphenylcyclopropane [24]. Since they
were formed only in trace amounts in the present work,
their contribution was neglected. Last but not least, the
formal carbene dimers, diethyl maleate, and diethyl fu-
marate (cis- and trans-14, respectively) accounted for the
mass balance.
Early work from our Laboratory had shown that some
ruthenium-based catalysts could be engineered and fine-
tuned to mediate either olefin metathesis or cyclopropa-
nation reactions [21]. Indeed, it was observed that highly
active ruthenium promoters for metathesis were inactive
for cyclopropanation and vice versa. With this in mind, we
anticipated that the least active or inactive [RuCl2(g6-are-
ne)(NHC)] initiators for olefin metathesis should be
efficient catalyst precursors for olefin cyclopropanation.
We therefore launched a detailed study to probe this hy-
pothesis. Herein, we report on the dual catalytic activity of
preformed [RuCl2(p-cymene)(NHC)] complexes toward
the cyclopropanation and the metathesis of various olefinic
substrates.
Results and discussion
To begin our study, we investigated the catalytic activity of
eight representative [RuCl2(p-cymene)(NHC)] complexes
bearing imidazol-2-ylidene (2–8) or imidazolin-2-ylidene
(9) ligands (Fig. 1) in the cyclopropanation of styrene. This
terminal activated olefin is the most common substrate
employed to assess new catalytic systems because it is
much more reactive than other alkenes with electron-
withdrawing substituents or with a high degree of alkyl
A preliminary test was carried out in the presence of the
[RuCl2(p-cymene)]2 dimer (1). When ethyl diazoacetate
was slowly added to a solution of styrene and of this simple
catalyst precursor at 60 °C, ethyl 2-phenylcyclo-
propanecarboxylate (10) was obtained in a modest 58 %
yield, and the cis/trans ratio was 0.56 (Table 1). The in-
troduction of an NHC ancillary ligand onto the metal center
had a beneficial influence on the reaction course as the
cyclopropanation yield reached 78–85 % with a cis/trans
ratio ranging from 0.5 to 0.9. Complex 2 bearing the least
sterically demanding NHC ligand under examination led to
the fastest conversion of the diazoester into active species
(Fig. 2) and afforded the highest cis selectivity. The re-
placement of its N-methyl substituents with bulky and rigid
mesityl groups in complex 8, on the other hand, led to the
highest proportion of trans-isomers. All the ruthenium
compounds of our screening promoted the insertion of the
reactive carbene into the various Csp2-H bonds of styrene
in addition to its C=C double bond, thereby affording
products 11 and 12 in 6–16 % yields. GC analysis also
evidenced the formation of carbene dimers 14 in 4–14 %
yields when the [RuCl2(p-cymene)(NHC)] complexes 2–9
served as catalyst precursors. In the presence of dimer 1,
which was less efficient at promoting cyclopropanation, the
formation of ethyl maleate and fumarate accounted for
Fig. 1 [RuCl2(p-cymene)(NHC)] complexes used in this study
123

Ruthenium–arene catalysts bearing N-heterocyclic carbene…
1101
Scheme 1
Table 1 Reaction of styrene with ethyl diazoacetate catalyzed by complexes 1–9 in chlorobenzene at 60 °C
Catalyst
Cyclopropanation yield/%a
Cis/trans ratiob
Homologation yield/%a
Metathesis yield/%c
1d
2
3
4
5
6
7
8
9
58
83
85
83
79
84
80
84
78
0.56
0.88
0.74
0.63
0.57
0.63
0.60
0.52
0.67
7
10
6
\1
\1
\1
\1
\1
\1
3
8
10
9
16
10
8
\1
\1
Experimental conditions: catalyst (0.005 mmol), 1 cm3 chlorobenzene, 2 cm3 styrene, ethyl diazoacetate (1 mmol) diluted in 1 cm3 styrene and
added in 4 h, 24-h total reaction time at 60 °C
a
Determined by GC based on ethyl diazoacetate
b
Determined by GC
c
Determined by GC based on styrene
d
0.0025 mmol of dimeric catalyst was used
accounted for less than 1 % of the olefin consumption.
Only with [RuCl2(p-cymene)(IMes)] (7) (IMes is 1,3-
dimesitylimidazol-2-ylidene) was stilbene (mostly its
trans-isomer) formed in small but detectable amount. This
observation is in line with previous results from our group,
which had highlighted the superior catalytic activity of
complex 7 in the visible light-induced ROMP of cy-
clooctene
promoted
by
[RuCl2(p-cymene)(NHC)]
compounds 2–9 [17].
Next, we examined the reaction of ethyl diazoacetate
with cyclooctene, a non-activated cycloolefin with respect
to cyclopropanation. Only complexes 2–6 bearing NHC
ligands with alkyl substituents on their nitrogen atoms
served as catalyst precursors in these experiments (cf.
Fig. 1). We reasoned that the use of [RuCl2(p-cyme-
ne)(NHC)] complexes 7–9 bearing the IMes ligand or its
variants would favor olefin metathesis instead of
Fig. 2 Rate of nitrogen evolution from ethyl diazoacetate in styrene
at 60 °C in the presence of complexes 2 (filled square), 3 (filled
circle), and 6 (filled triangle)
35 % of the diazoester conversion. Conversely, styrene
metathesis was essentially repressed under the ex-
perimental conditions adopted. Indeed, in most cases, it
123

´
M. Meret et al.
1102
Scheme 2
Table 2 Reaction of cyclooctene with ethyl diazoacetate catalyzed by complexes 2–6 at 60 °C
Catalyst
Cyclopropanation yield/%a
Endo/exo ratiob
Homologation yield/%a
Metathesis yield/%c
rdcis
2
3
4
5
6
21
18
15
17
17
0.47
0.37
0.37
0.53
0.50
\1
\1
\1
\1
\1
7
6
6
3
4
0.50
0.54
0.39
0.62
0.58
Experimental conditions: catalyst (0.005 mmol), 1 cm3 cyclooctene, ethyl diazoacetate (1 mmol) diluted in 1 cm3 cyclooctene and added in 4 h,
24-h total reaction time at 60 °C
a
Determined by GC based on ethyl diazoacetate
b
Determined by GC
c
Determined by GC based on cyclooctene
d
1
Fraction of cis double bonds within the polyoctenamer determined by H and 13C NMR
cyclopropanation. The experimental procedure set up pre-
viously for styrene was adapted by increasing the alkene
concentration to compensate for its low reactivity. Thus,
the ruthenium complexes and the diazoester were diluted
with neat cyclooctene, and no chlorobenzene was added.
The amount of catalyst (0.5 mol % relative to the diazo-
ester), the reaction time (24 h), and the temperature
(60 °C) were kept unchanged. Under these conditions,
ethyl bicyclo[6.1.0]nonane-9-carboxylate (15) was ob-
tained as a mixture of endo- and exo-diastereomers
(Scheme 2). Yields were, however, far from quantitative
(Table 2), and the formation of ethyl fumarate and maleate
(14) prevailed (up to 60 % based on ethyl diazoacetate).
The cyclopropanation diastereoselectivities were also
unexceptional with endo/exo ratios in the 0.4–0.5 range.
Furthermore, the decomposition rate of the diazo com-
pound was rather slow and remained independent of the
substitution pattern on the NHC ligand (Fig. 3). Beside
formal carbene dimers (14) and cyclopropanes (15), ethyl
2-(cyclooct-2-enyl)acetate (16) resulting from the insertion
of the carbene into an allylic C-H bond of cyclooctene was
Fig. 3 Rate of nitrogen evolution from ethyl diazoacetate in
cyclooctene at 60 °C in the presence of complexes 2 (open square),
3 (open circle), and 6 (open triangle)
also formed in low yield (\1 %). Competitive ROMP of
the cycloolefin constituted a more important side-reaction
and afforded polyoctenamer (17) in small but nonetheless
significant yields (3–7 %).
To further expand the scope of the olefin cyclopropa-
nation with ethyl diazoacetate catalyzed by [RuCl2(p-
123

Ruthenium–arene catalysts bearing N-heterocyclic carbene…
1103
Table 3 Reaction of various olefins with ethyl diazoacetate catalyzed by complex 2 at 60 °C
Olefin
Cyclopropanation yield/%a
Cis/trans ratiob
Homologation yield/%a
Metathesis yield/%c
Styrene
83
84
86
87
83
93
44
39
15
21
0
0.88
0.54
0.62
0.54
0.64
0.97
0.93
0.77
0.55d
0.47d
–
10
6
\1
\1
\1
\1
\1
0
4-Cl-styrene
4-Me-styrene
4-t-Bu-styrene
4-MeO-styrene
a-Me-styrene
1-Octene
5
3
10
\1
6
\1
\1
0
1-Decene
4
Cyclohexene
Cyclooctene
Norbornenee
\1
\1
0
7
42
Experimental conditions: complex 2 (0.005 mmol), 1 cm3 olefin, ethyl diazoacetate (1 mmol) diluted in 1 cm3 olefin and added in 4 h, 24-h total
reaction time at 60 °C
a
Determined by GC based on ethyl diazoacetate
b
Determined by GC
c
Determined by GC based on the olefin
d
Endo/exo ratio determined by GC
Experimental conditions: complex 2 (0.005 mmol), 1 g norbornene, ethyl diazoacetate (1 mmol) diluted in 1 cm3 cyclohexane and added in
e
4 h, 24-h total reaction time at 60 °C
Scheme 3
cymene)(NHC)] complexes, we probed other substrates
than just styrene and cyclooctene. For this purpose, we
elected catalyst precursor 2, which afforded the fastest
conversion of ethyl diazoacetate into dinitrogen (cf. Fig. 2)
and led to the highest cis-diastereoselectivity in the cy-
clopropanation of styrene (cf. Table 1). Among the various
styrene derivatives investigated (Table 3), a-methylstyrene
afforded the highest yield of the analog of 10 (93 %). It
also gave the highest proportion of cis- vs. trans-isomer
(49 %). Para-substitution of the aromatic ring of styrene
with electron-withdrawing (chloro) or donating groups
(methyl, tert-butyl, methoxy) did not affect the cyclo-
propanation yield, which remained close to 85 % in all
cases, but influenced the cis/trans ratio, which fluctuated
between 0.5 and 0.6, down from 0.9 with styrene. Two
terminal linear alkenes (1-octene and 1-decene) were also
cyclopropanated, albeit in modest yields (ca. 40 %). In-
ternal olefins such as cyclohexene and cyclooctene were
much less reactive, and cyclopropanation yields culminated
at roughly 15–20 %. Except with the latter substrate, which
afforded 7 % of polyoctenamer (17), only traces of
metathesis products were detected in most of the reaction
mixtures. In the case of a-methylstyrene and cyclohexene,
they were completely absent. This is most likely due to the
steric constraints encountered in the formation of a tetra-
substituted olefin from a-methylstyrene or to the adverse
thermodynamics for the ring-opening of cyclohexene [25].
Unlike the other olefinic substrates investigated so far,
norbornene was not cyclopropanated by ethyl diazoacetate
in the presence of complex 2 (Table 3). Instead, only
ROMP occurred, and the reaction mixture gelled after the
addition of a few drops of diazoester. After 24 h, the solid
mass was dissolved again in chloroform, and polynor-
bornene was precipitated in methanol. Size-exclusion
chromatography indicated the formation of a high mole-
cular weight polymer (Mn = 140,000) with a broad
dispersity (Mw/Mn = 4.75). This result prompted us to
further investigate the reaction of norbornene employing
ruthenium initiators 2–6 under experimental conditions
more appropriate for the ROMP of this strained cycloolefin
[26, 27]. Thus, trimethylsilyldiazomethane (TMSD) was
elected as a carbene precursor, and the reactions were run
in chlorobenzene using higher dilution conditions than for
cyclopropanation in order to prevent instant jellification
(Scheme 3). Using this revised catalytic system, moderate
to high yields of polynorbornene (up to 81 %) were ob-
tained after 5 h at 60 °C (Table 4). The highest conversion
was reached with complex 5, which afforded a very high
molecular weight polymer. Using NMR spectroscopy, we
were able to determine the proportion of cis and trans
123

´
M. Meret et al.
1104
Table 4 Ring-opening metathesis polymerization of norbornene catalyzed by complexes 2–6 activated by trimethylsilyldiazomethane in
chlorobenzene at 60 °C
Catalyst
Polymer yield/%
103 Man
Mw/Man
rbcis
rcisrctrans
2
3
4
5
6
60
57
46
81
46
88
105
820
600
280
6.8
3.0
2.0
2.3
4.0
0.46
0.41
0.44
0.48
0.44
1.25
1.55
1.55
1.25
1.75
Experimental conditions: catalyst (0.015 mmol), 0.5 g norbornene, 10 cm3 chlorobenzene, trimethylsilyldiazomethane (0.1 mmol) diluted in
1 cm3 chlorobenzene, 5 h at 60 °C
a
Determined by size-exclusion chromatography in THF using monodisperse polystyrene standards
1
b
c
Fraction of cis double bonds within the polymer determined by H and 13C NMR
Blockiness parameter of the polymer determined by 13C NMR
double bonds within the unsaturated chains and to ascertain
their block or random distribution by computing the rcis
and rcisrtrans parameters, respectively [28, 29]. Whichever
ligand was used, polynorbornenes that contained roughly
two trans double bonds for one cis were obtained. The cis/
trans distributions were not completely random, as indi-
cated by values of the rcisrtrans parameter greater than 1.
However, the block arrangement of the polymers was
rather limited and remained unaffected by the exact nature
of the NHC ancillary ligand.
deoxygenated prior to use. The [RuCl2(p-cymene)]2 dimer
(1) was purchased from Strem. The [RuCl2(p–cyme-
ne)(NHC)] complexes 2–9 were prepared according to
literature procedures [17, 30–33]. All the other chemicals
were purchased from Aldrich and used as received. 1H and
13C NMR spectra were recorded at 298 K on a Bruker
DRX 400 spectrometer operating at 400.13 and
100.62 MHz, respectively. Gas chromatography was car-
ried out on a Varian 3900 instrument equipped with a flame
ionization detector and a WCOT fused silica column (sta-
tionary phase: CP-Sil 5CB, column length: 15 m, inside
diameter: 0.25 mm, outside diameter: 0.39 mm, film
thickness: 0.25 lm). Size-exclusion chromatography was
performed in THF at 45 °C on a SFD S5200 autosampler
liquid chromatograph equipped with a SFD 2000 refractive
index detector and a battery of 4 PL gel columns fitted in
series (particle size: 5 lm; pore sizes: 105, 104, 103, and
In conclusion, we have shown that ruthenium–arene
complexes bearing N-heterocyclic carbene ligands were
efficient catalyst precursors for the cyclopropanation of
activated olefins with ethyl diazoacetate. Compared to the
[RuCl2(p-cymene)]2 dimer (1), the [RuCl2(p-cymene)-
(NHC)] complexes 2–9 afforded higher yields of ethyl
2-phenylcyclopropanecarboxylate (10) in the cyclopropa-
nation of styrene. The steric bulk of the NHC ligand also
markedly influenced the cis/trans selectivity of the reac-
tion. The procedure was successfully extended to a-
methylstyrene and to various other styrenic derivatives
bearing electron-withdrawing or donating substituents on
their aromatic rings. The reaction of unactivated internal or
terminal alkenes was more sluggish, and the use of nor-
bornene as a substrate afforded only olefin metathesis.
Further investigation of the ROMP of this strained cy-
cloolefin in the presence of trimethylsilyldiazomethane
showed that high molecular weight polynorbornene was
formed and that the choice of the NHC ancillary ligand had
little impact on its microstructure.
2
3
˚
10 A; flow rate: 1 cm /min). The molecular weights (not
corrected) are reported vs. monodisperse polystyrene
standards used to calibrate the instrument.
Typical procedure for the reaction of styrene
with ethyl diazoacetate
A 10 cm3 two-neck flask equipped with a magnetic stirring
bar and capped with a three-way stopcock was charged
with a ruthenium complex (0.005 mmol). The reactor was
purged of air by applying three vacuum/nitrogen cycles
before 1 cm3 dry chlorobenzene and 2 cm3 styrene were
added. The solution was stirred for a few minutes in an oil
bath thermostated at 60 °C. Ethyl diazoacetate (0.1141 g,
1 mmol) was diluted up to 1 cm3 with styrene in a 1 cm3
syringe. This diazoester solution was slowly added to the
reaction mixture with a syringe pump over a 4-h period.
Stirring was maintained for an additional 20 h at 60 °C.
The rate of nitrogen evolution was monitored by a water
column connected to the reaction flask via the three-way
Experimental
All the syntheses were carried out under a dry inert at-
mosphere using standard Schlenk techniques. Solvents and
olefins were distilled from appropriate drying agents and
123

Ruthenium–arene catalysts bearing N-heterocyclic carbene…
1105
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mixture was analyzed by gas chromatography, and its
composition was established by comparison with authentic
samples.
´
´
9. Dıez-Gonzalez S, Marion N, Nolan SP (2009) Chem Rev
Typical procedure for the ROMP of norbornene
109:3612
10. Weskamp T, Kohl FJ, Hieringer W, Gleich D, Herrmann WA
(1999) Angew Chem Int Ed 38:2416
A ruthenium complex (0.015 mmol) was placed in a
25 cm3 round-bottom flask equipped with a magnetic
stirring bar and capped with a three-way stopcock. The
reactor was purged of air by applying three vacuum/argon
cycles before 5 cm3 dry chlorobenzene and a solution of
0.5 g norbornene (5.3 mmol) in 5 cm3 chlorobenzene were
added with syringes. The reaction mixture was stirred for a
few minutes in an oil bath thermostated at 60 °C before
trimethylsilyldiazomethane (0.1 M in chlorobenzene,
0.10 mmol) was added via a syringe. It was kept at 60 °C
for 5 h, cooled to room temperature, and diluted in chlo-
roform. The resulting viscous solution was slowly poured
in 600 cm3 methanol under vigorous stirring. The pre-
cipitated polymer was washed with methanol and dried
overnight under high vacuum. It was analyzed by NMR
spectroscopy and by size-exclusion chromatography.
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Acknowledgments The financial support of the ‘‘Fonds de la
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