Assessing the ligand properties of 1,3-dimesitylbenzimidazol-2-ylidene in ruthenium-catalyzed olefin metathesis

Article abstract of DOI:10.1039/c2dt31520c

The deprotonation of 1,3-dimesitylbenzimidazolium tetrafluoroborate with a strong base afforded 1,3-dimesitylbenzimidazol-2-ylidene (BMes), which was further reacted in situ with rhodium or ruthenium complexes to afford three new organometallic products.

Full text of DOI:10.1039/c2dt31520c

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Assessing the ligand properties of 1,3-dimesitylbenzimidazol-2-ylidene
in ruthenium-catalyzed olefin metathesis†
Yannick Borguet,a Guillermo Zaragoza,b Albert Demonceaua and Lionel Delaude*a
Received 11th July 2012, Accepted 3rd September 2012
DOI: 10.1039/c2dt31520c
The deprotonation of 1,3-dimesitylbenzimidazolium tetrafluoroborate with a strong base afforded 1,3-
dimesitylbenzimidazol-2-ylidene (BMes), which was further reacted in situ with rhodium or ruthenium
complexes to afford three new organometallic products. The compounds [RhCl(COD)(BMes)] (COD is
1,5-cyclooctadiene) and cis-[RhCl(CO)2(BMes)] were used to probe the steric and electronic parameters
of BMes. Comparison of the percentage of buried volume (%VBur) and of the Tolman electronic
parameter (TEP) of BMes with those determined previously for 1,3-dimesitylimidazol-2-ylidene (IMes)
and 1,3-dimesitylimidazolin-2-ylidene (SIMes) revealed that the three N-heterocyclic carbenes (NHCs)
had very similar profiles. Nonetheless, changes in the hydrocarbon backbone subtly affected the
stereoelectronic properties of these ligands. Accordingly, the corresponding [RuCl2(PCy3)(NHC)-
(vCHPh)] complexes displayed different catalytic behaviors in the ring-closing metathesis (RCM) of
α,ω-dienes. In the benchmark cyclization of diethyl 2,2-diallylmalonate, the new [RuCl2(PCy3)(BMes)-
(vCHPh)] compound (1d) performed slightly better than the Grubbs second-generation catalyst (1a),
which was in turn significantly more active than the related [RuCl2(PCy3)(IMes)(vCHPh)] initiator (1b).
For the formation of a model trisubstituted cycloolefin, complex 1d ranked in-between catalyst precursors
1a and 1b, whereas in the RCM of tetrasubstituted cycloalkenes it lost its catalytic efficiency much more
rapidly.
metathesis, whose most prominent representatives include the
Introduction
16-electron benzylidene complexes of type 17 and the related
indenylidene compounds of type 2 (Fig. 1).8
Since the first imidazol-2-ylidene derivative was isolated and
characterized by Arduengo in 1991,1 stable N-heterocyclic car-
benes (NHCs) have become efficient tools in organic synthesis
and catalysis.2 Over the past two decades, these divalent carbon
species have emerged as powerful nucleophilic organocatalysts
for asymmetric synthesis.3 They have also afforded an impres-
sive range of transition-metal complexes that have found count-
less applications in homogeneous catalysis.4 In particular, NHC
ligands have largely contributed to the advent of highly efficient
ruthenium catalysts for olefin metathesis and related reactions.5
Indeed, mixed complexes bearing both a phosphine and an NHC
ligand were quickly identified as superior metathesis initiators
compared to their bis(phosphine) or bis(NHC) analogues.6
These observations sparked the development of a broad family
of so-called second-generation ruthenium initiators for olefin
Extensive catalytic screening,9–13 supported by mechanistic
investigations14 and theoretical studies,15,16 allowed us to better
understand the role of the NHC ligand in complexes of type 1 or
2. It was soon recognized that the presence of large N-aryl sub-
stituents on the imidazole ring system, such as the mesityl
(2,4,6-trimethylphenyl) or 2,6-diisopropylphenyl groups, usually
provided the right balance of steric protection and electronic
aLaboratory of Organometallic Chemistry and Homogeneous Catalysis,
Institut de Chimie (B6a), Université de Liège, Sart-Tilman par 4000
Liège, Belgium. E-mail: l.delaude@ulg.ac.be; Fax: +32 4 366 3497
bUnidade de Difracción de Raios X, Universidade de Santiago de
Compostela, Edificio CACTUS, Campus Vida, 15782 Santiago de
Compostela, Spain
†Electronic supplementary information (ESI) available: Detailed crystal-
lographic analysis and cif files for compounds 1d and 9. CCDC 808482
and 891838. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt31520c
Fig. 1 Second-generation ruthenium–benzylidene (1) and indenylidene
(2) catalysts for olefin metathesis (PCy3 = tricyclohexylphosphine,
Mes = 2,4,6-trimethylphenyl).
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Scheme 2 Synthesis of [(BMes)Cr(CO)5] (6). Reaction conditions: (a)
MesNH2, Pd(OAc)2, BINAP, NaOBut, PhCH3, reflux, 12 h; (b) [Cr-
(CO)6], BuLi, 12-crown-4, THF, −78 °C, 15 min then 0 °C, 30 min; (c)
Me3SiCl, −78 °C, 15 min, then 0 °C, 30 min, and RT, 3 h.
unstable and decomposed rapidly into metathetically inactive
species in air. More detailed investigations led to the isolation of
degradation products 3 and 4, whose molecular structures were
determined by X-ray diffraction analysis.33 The mechanism pos-
tulated to rationalize their formation is illustrated in Scheme 1. It
involves the activation of ortho C–H bonds on both phenyl sub-
stituents of the NHC ligand with the assistance of the dissociated
phosphine present in solution and is supported by DFT calcu-
lations performed independently by the groups of Suresh34 and
Cavallo.35
Scheme 1 Possible mechanism for the decomposition of complex 1c.
We reasoned that protecting the ortho-positions of the
N-phenyl substituents in complex 1c with alkyl groups would
alleviate its tendency to decomposition and should result in a
new, promising second-generation catalyst for olefin metathesis.
Therefore, we launched a research program to investigate the
ligand properties of benzimidazol-2-ylidene derivatives bearing
bulky aromatic substituents on their nitrogen atoms. Several
attempts to synthesize suitable precursors for these compounds
had met with failure,36 and we are only aware of a single report
that describes the synthesis of pentacarbonylchromium(0)
complex 6 bearing the 1,3-dimesitylbenzimidazol-2-ylidene
ligand (nicknamed BMes).37 In this case, the NHC was
assembled directly on the metal upon reaction of the lithium 1,2-
phenylenediamide derived from 5 with [Cr(CO)6] in the presence
of 12-crown-4, followed by treatment with chlorotrimethylsilane
(Scheme 2). Given the limited scope of this ingenious strategy
and the poor yield attained, we searched for a more satisfactory
entry to the chemistry of the BMes ligand. Eventually, we
managed to prepare 1,3-dimesitylbenzimidazolium tetrafluoro-
borate (8) via the intermediacy of dihydrophenazine 7 formed
upon oxidation of N,N′-dimesityl-1,2-benzenediamine (5) with
sodium periodate on wet silica gel, followed by electrocyclic
ring closure (Scheme 3).38
donation to many NHC-based catalytic systems, while avoiding
ortho-metalation side-reactions.17,18 Thus, complexes 1a19 and
2a20 featuring the 1,3-dimesitylimidazolin-2-ylidene ligand
(nicknamed SIMes) have become standard catalysts for olefin
metathesis and are now commercially available.
Tinkering with their hydrocarbon backbone offers additional
options to fine-tune the stereoelectronic properties of NHC
ligands. Hence, modifications of the heterocycle core have
attracted a great deal of attention from the catalysis commu-
nity.21,22 It should be pointed out, however, that introducing vari-
ations on the C4 or C5 positions of the imidazole ring system is
often more challenging from a synthetic point of view than alter-
ing the nature of the N1 or N3 substituents.23 In parallel with the
development of the “saturated” imidazolin-2-ylidene ligand
SIMes, early investigations focused on the use of the corres-
ponding “unsaturated” imidazol-2-ylidene compound nicknamed
IMes (Fig. 1). These studies led to complexes 1b24 and 2b25 that
were usually found to be less active than compounds 1a and
2a,9,12,13,26,27 although the exact reasons for this discrepancy are
still unclear.16,28,29
Several fused polycyclic NHCs were successfully designed
for asymmetric organocatalysis,30 but they were seldom used as
ligands in ruthenium-promoted olefin metathesis reactions.18,19,31
To the best of our knowledge, only one example of a ruthenium–
benzylidene catalyst sporting a benzimidazolylidene ligand has
been reported so far. Thus, in 2007 Grubbs et al. disclosed the
synthesis of complex 1c featuring the 1,3-diphenylbenzimidazol-
2-ylidene ligand (Scheme 1).32 This compound was highly
In this contribution, we first assess the steric and electronic
properties of the 1,3-dimesitylbenzimidazol-2-ylidene ligand
(BMes) using rhodium complexes. Then, we report on the syn-
thesis of the second-generation ruthenium–benzylidene complex
[RuCl2(PCy3)(BMes)(vCHPh)] (1d) and we probe its catalytic
activity in ring-closing metathesis (RCM) using benchmark
reactions.
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Table 1 Steric and electronic parameters of mesityl-based NHC
ligands coordinated to rhodium
c/cm–1
TEPd/cm–1
a
NHC
%VBur
δRh–C b/ppm
ν
ˉCO
IMes
BMes
SIMes
31.7e
30.0
183.6f
196.4
212.8f
2038.5f
2040.0
2040.5f
2051.0
2052.2
2052.6
32.7g
a Computed using the web-based application SambVca46 from the XRD
structures of the corresponding [RhCl(COD)(NHC)] complexes. b 13C
NMR chemical shift of the carbene carbon in [RhCl(COD)(NHC)]
complexes. c Average wavenumber for the stretching vibration of CO
ligands in [RhCl(CO)2(NHC)] complexes. d Tolman electronic parameter
calculated using eqn (1). e Based on the data from ref. 47. f Taken from
ref. 40. g Based on the data from ref. 48.
Scheme 3 Synthesis of 1,3-dimesitylbenzimidazolium tetrafluoro-
borate (8). Reaction conditions: (a) MesNH2, Pd(OAc)2, P(But)3,
NaOBut, PhCH3, 110 °C, overnight; (b) NaIO4, SiO2, H2O, CH2Cl2, RT,
t
24 h; (c) BuCO2CH2Cl, AgOTf, KOAc, CH2Cl2, 50 °C, 48 h; (d)
HBF4, H2O, RT, 1 h.
Fig. 2 ORTEP representation of [RhCl(COD)(BMes)] (9) with thermal
ellipsoids drawn at the 50% probability level. Hydrogen and disordered
atoms were omitted for clarity. Selected bond lengths (Å) and angles (°):
Rh1–C1 2.029(3), Rh1–Cl1 2.3556(9), Rh1–C22 2.096(3), Rh1–C23
2.118(3), Rh1–C26 2.180(4), Rh1–C27 2.197(3), C1–Rh1–Cl1 87.52
(8), N1–C1–N2 104.1(2), C1–N1–C4–C5 93.1(3), C1–N2–C13–C14
−84.3(4).
Scheme 4 Synthesis of [RhCl(COD)(BMes)] (9) and cis-[RhCl-
(CO)2(BMes)] (10).
Results and discussion
exactly average between those recorded for [RhCl(COD)(IMes)]
and [RhCl(COD)(SIMes)] (Table 1).‡ This trend is not unprece-
dented in the literature. For example, the chemical shifts of the
carbenoid centers in chromium complexes with the general
formula [(NHC)Cr(CO)5] increase in the following order: imida-
zol-2-ylidenes (186–200 ppm)42,43 < benzimidazol-2-ylidenes
(200–210 ppm)37,43 < imidazolin-2-ylidenes (217–225 ppm).44
This evolution was explained by a parallel decrease in the
π-charge density on C2 when aromatic conjugation in the
6π-electron imidazole ring is shared with the benzannulated
moiety or disrupted by saturation of the C4vC5 double bond.45
Bright yellow crystals of [RhCl(COD)(BMes)] (9) were grown
by evaporating a dichloromethane solution and subjected to
X-ray diffraction analysis (Fig. 2).† As expected, a square-planar
arrangement of the ligands around the metal center was
observed, with the NCN plane of the carbene approximately per-
pendicular to the coordination plane of rhodium. The cycloocta-
diene ring presented a little disorder, with two different
Evaluation of the steric and electronic properties of BMes
Over the past few years, several distinct methods were developed
to quantify the steric and electronic properties of NHC
ligands.39–41 Among them, the study of [RhCl(COD)(NHC)]
(COD is 1,5-cyclooctadiene) and cis-[RhCl(CO)2(NHC)] deriva-
tives to probe steric parameters and electronic factors, respect-
ively, is perhaps the most popular one, owing to the
straightforward preparation, high stability, and low toxicity of the
rhodium complexes involved. Hence, we first synthesized
[RhCl(COD)(BMes)] (9) by deprotonating benzimidazolium salt
8 with potassium tert-butoxide to generate the BMes carbene in
situ, followed by cleavage of the [RhCl(COD)]2 dimer
(Scheme 4). The desired product was isolated in 91% yield after
purification by column chromatography.
13C NMR analysis of complex 9 in CD2Cl2 revealed the pres-
ence of a highly deshielded doublet, whose chemical shift
(196.4 ppm) and multiplicity (1JRh–C = 52.4 Hz) left little doubt
about the successful coordination of a carbene ligand to the
metal center. It is noteworthy that this chemical shift is almost
‡(183.6 + 212.8)/2 = 198.2 ppm.
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displacement of the COD ligand and afforded the cis-dicarbonyl
complex 10 in 63% yield (Scheme 4). The IR spectrum of this
compound was recorded in CH2Cl2 to allow a direct comparison
of the ν values with previous measurements carried out in the
ˉCO
same way on [RhCl(CO)2(IMes)] and [RhCl(CO)2(SIMes)] by
Wolf and Plenio (Table 1).40 The Tolman electronic parameter
(TEP) was then computed using the linear regression proposed
by Dröge and Glorius to correlate data obtained for Rh and Ni
complexes (eqn (1)).41 For the series of N,N′-dimesityl substi-
tuted NHCs under scrutiny, the electron-donating character
increased in the order SIMes < BMes < IMes. This sequence is
in good agreement with the evolution of 13C NMR chemical
shifts discussed above for the C2 carbenic center and further sup-
ports the idea that benzimidazole-derived NHCs have intermedi-
ate electronic properties compared to imidazole and imidazoline
derivatives.
Fig. 3 Graphical representation of the sphere used to calculate the
%VBur parameter for the mesityl-based NHC ligands IMes, BMes and
SIMes.
TEP=cmꢀ1 ¼ 0:8001νˉCORh;average=cmꢀ1 þ 420
ð1Þ
It is important to stress that the steric and electronic para-
meters under investigation differed only marginally when
switching from IMes to BMes or SIMes (Δ%VBur = 2.7%, Δν
ˉCO
orientations in a ca. 65/35 ratio for the C–C single bonds linking
the CvC double bond trans to the halogen. Altogether, the
various bond lengths and angles were in line with those reported
previously for other complexes of the same family,49 including
[RhCl(COD)(IMes)]47 and [RhCl(COD)(SIMes)].48
= 2.0 cm−1). Electrochemical measurements could help further
refine this analysis as they are often more sensitive than TEP
determinations for probing the donor properties of NHC
ligands,40,53 especially in ruthenium catalysts for olefin meta-
thesis.54 From the data gathered here, it is nonetheless obvious that
changes in the hydrocarbon backbone of an NHC ligand have
only a limited influence on its steric and electronic properties, as
long as the core heterocycle remains a five-membered ring with
two nitrogen atoms. Likewise, introduction of methyl or halogen
substituents on the C4 and C5 positions of IMes had only a
minor impact on the %VBur and TEP parameters.22 Expanding
the heterocycle to a 6, 7 or 8-membered ring is a better option to
alter more substantially the %VBur parameter.55 Modulation of
the N1 and N3 substituents provides another suitable approach to
adjust the steric properties of NHC ligands, due to their proxi-
mity to the metal center. Unless they bear strong electron-donat-
ing or withdrawing functional groups,56 exocyclic alkyl or aryl
groups do not, however, have a strong electronic impact, because
of the lack of conjugation with the donor C2 atom. Arguably, the
most convenient approach developed so far to broadly tune the
donor properties of imidazole-based NHC ligands involves the
In order to quantify the steric demand of NHC ligands,
Cavallo and Nolan have defined a parameter called the “percen-
tage of buried volume” (%VBur), which gives a measure of the
space occupied by a ligand in the first coordination sphere of a
metal center (Fig. 3).50,51 The bulkier an NHC ligand is, the
greater its %VBur. Several refinements were brought to the set of
atom radii and sphere radius used for the calculations and the
resulting algorithm can now be run via a user-friendly interface
on the internet.46 We have used this front-end to compute the
%VBur parameter of BMes based on the crystal structure of
[RhCl(COD)(BMes)] (9). The default processing parameters
were kept unchanged (sphere radius: 3.5 Å; distance from the
center of the sphere: 2.10 Å; mesh spacing: 0.05 Å; Bondi radii
scaled by 1.17).
Because only a small area around the nitrogen atoms is mainly
responsible for the value of %VBur (cf. Fig. 3), rather similar
behaviors were expected for BMes and the other mesityl-based
NHCs, IMes and SIMes. We were surprised, however, to find out
that the “aromatic” backbone led to a slightly lower value of
%VBur than its “unsaturated” or “saturated” counterparts
(Table 1).52 Indeed, given that the N–C–N angle of the carbene
ligand in complex 9 (104.1(2)°) lies in-between those recorded
for IMes (103.50(14)°)47 and SIMes (107.29(12)°)48 in the
respective [RhCl(COD)(NHC)] complexes, one would have
expected an intermediate %VBur value for BMes compared to its
“unsaturated” and “saturated” analogues. It is noteworthy that
our experimental results fit nicely with earlier theoretical calcu-
lations based on DFT optimized geometries, which had already
predicted a slightly lower %VBur parameter for BMes (31.2)
compared to IMes (31.6) and SIMes (32.7).46 Crystal structures
obtained for ruthenium catalysts were therefore examined to
further probe the tendency observed with rhodium complexes
(vide infra).
derivatization of an enolate group as
a
reactive C4–C5
backbone.57
Synthesis of [RuCl2(PCy3)(BMes)(vCHPh)] (1d)
Deprotonation of benzimidazolium salt 8 with potassium bis(tri-
methylsilyl)amide followed by ligand exchange with the Grubbs
first-generation catalyst [RuCl2(PCy3)2(vCHPh)] led to the
mixed complex 1d featuring the BMes ligand (Scheme 5). This
product was isolated in 82% yield after removal of the accompa-
nying free phosphine by column chromatography under an inert
atmosphere. It was fully characterized by 1H, 13C, and 31P NMR
spectroscopy and its crystal structure was determined by X-ray
diffraction analysis (Fig. 4).† The 1H and 13C NMR spectra were
recorded at 223 K to allow the observation of the signals due to
the meta-hydrogen atoms on the mesityl substituents, which
Dissolving the [RhCl(COD)(BMes)] complex 9 in dichloro-
methane and bubbling carbon monoxide for 15 min led to the
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Scheme 6 Ruthenium-catalyzed RCM of diethyl 2,2-diallylmalonate
(DEDAM, 11a) and diethyl 2-allyl-2-(2-methylallyl)malonate (11b).
Scheme 5 Synthesis of [RuCl2(PCy3)(BMes)(vCHPh)] (1d).
Fig. 5 Time course of the RCM of diethyl 2,2-diallylmalonate (11a)
using ruthenium catalysts 1a, 1b, and 1d (1 mol%) in CD2Cl2 at 30 °C.
Fig. 4 ORTEP representation of [RuCl2(PCy3)(BMes)(vCHPh)] (1d)
with thermal ellipsoids drawn at the 50% probability level. Hydrogen
atoms were omitted for clarity. Selected bond lengths (Å) and angles (°):
Ru1–C1 2.060(6), Ru1–C26 1.835(6), Ru1–Cl1 2.3781(16), Ru1–Cl2
2.3920(16), Ru1–P1 2.4252(16), C1–Ru1–P1 163.98(17), Cl1–Ru1–Cl2
168.35(6), C1–Ru1–C26 97.8(2), Cl1–Ru1–C26 102.6(2), N1–C1–N2
103.7(5), C2–N1–C8 120.6(5), C7–N2–C17 121.3(4), C1–N1–C8–C9
82.3(7), C1–N2–C17–C22 103.3(7).
Catalytic tests
The catalytic efficiency of complex 1d was evaluated in the ring-
closing metathesis (RCM) of four model α,ω-dienes and com-
pared to the activities of second-generation catalysts 1a and 1b
using standard experimental procedures defined by Grubbs and
co-workers.26 We first investigated the RCM of diethyl 2,2-di-
allylmalonate (DEDAM, 11a) in CD2Cl2 at 30 °C (Scheme 6).
Reactions were carried out using 1 mol% of ruthenium initiator
and monitored by 1H NMR spectroscopy. Under these con-
ditions, an almost quantitative conversion of the substrate into
cyclopentene diester 12a took place in ca. 40 min with
[RuCl2(PCy3)(BMes)(vCHPh)] (1d) (Fig. 5). This new com-
pound even displayed a slightly higher activity than the Grubbs
second-generation catalyst 1a at the beginning of the reaction.
As reported previously, replacement of the SIMes ligand with
IMes led to a slower reaction.26 Thus, with catalyst precursor 1b
the conversion after 40 min was 82% and it took about 90 min
to reach completion.
Our second benchmark was the RCM of diethyl 2-allyl-2-(2-
methylallyl)malonate (11b) to form the trisubstituted cycloolefin
12b (Scheme 6). In this reaction, complex 1d displayed catalytic
activity intermediate between those recorded by Grubbs et al.
with catalyst precursors 1a and 1b (Fig. 6).26 Indeed, with the
“saturated” ancillary ligand the reaction was complete in 75 min,
whereas conversions of 93 and 83% were obtained within the
same period of time with the “aromatic” and “unsaturated”
NHCs, respectively.
completely disappeared in the baseline at room temperature. In
the solid state, complex 1d exhibited a distorted square pyrami-
dal geometry with the benzylidene moiety occupying the apical
position. No significant deviations were observed between the
spectroscopic data acquired for this new compound and those
reported previously for the analogous second-generation ruthe-
nium–benzylidene catalysts 1a58 and 1b.24
We were particularly interested in extracting the %VBur
parameter of BMes from the crystal structure of 1d in order to
determine whether the steric pressure exerted by the benzimida-
zolylidene ligand was indeed slightly inferior to those displayed
by IMes and SIMes, as observed in the [RhCl(COD)(NHC)]
complexes (vide supra). In the [RuCl2(PCy3)(NHC)(vCHPh)]
series, however, the %VBur computed for BMes (30.3) was inter-
mediate between those obtained from the molecular structures of
complex 1b (30.1) and the Grubbs second-generation catalyst 1a
(31.3). These fluctuations confirm the structural flexibility of
mesityl-based NHCs and their ability to fit with the crowding
around the metal center.29,51 They also show that changes
between “saturated”, “aromatic”, and “unsaturated” backbones
have only a subtle, unpredictable influence on the buried
volume.
Last but not least, we have also compared the metathetical
activities of complexes 1a, 1b, and 1d in the RCM of diethyl
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precursor 1d, whereas the second-generation ruthenium–alkyli-
dene 1b remained active over much longer periods.10 Hence, the
presence of mesityl substituents on the nitrogen atoms of BMes
did not prevent the rapid decomposition of compound 1d when
heated at 80 °C.
Conclusion and perspectives
Upon deprotonation with a strong base, benzimidazolium salt 8
afforded the carbene ligand BMes, which was further reacted in
situ with rhodium or ruthenium complexes to afford a small,
albeit representative, selection of new organometallic products.
The rhodium compounds, [RhCl(COD)(BMes)] (9) and cis-
[RhCl(CO)2(BMes)] (10), were used to probe the steric and elec-
tronic features of 1,3-dimesitylbenzimidazol-2-ylidene. Compari-
son of the %VBur and TEP parameters of BMes with those
determined previously for IMes and SIMes revealed that the
three mesityl-based NHCs had very similar profiles. Nonethe-
less, changes between “saturated”, “aromatic”, and “unsaturated”
backbones in the central heterocyclic part of these ligands subtly
affected their stereoelectronic properties.
Fig. 6 Time course of the RCM of diethyl 2-allyl-2-(2-methallyl)malo-
nate (11b) using ruthenium catalysts 1a, 1b, and 1d (1 mol%) in CD2Cl2
at 30 °C.
Accordingly, the corresponding [RuCl2(PCy3)(NHC)-
(vCHPh)] complexes displayed different catalytic behaviors in
the ring-closing metathesis of α,ω-dienes. In the benchmark
cyclization of diethyl 2,2-diallylmalonate (11a), the new
[RuCl2(PCy3)(BMes)(vCHPh)] compound (1d) performed
slightly better than the Grubbs second-generation catalyst (1a),
which was in turn significantly more active than the related
[RuCl2(PCy3)(IMes)(vCHPh)] initiator (1b). For the formation
of trisubstituted cycloolefin 12b, complex 1d ranked in-between
catalyst precursors 1a and 1b, whereas in the RCM of tetrasub-
stituted cycloalkenes 12c and 14 it lost its catalytic efficiency
much more rapidly.
Scheme 7 Ruthenium-catalyzed RCM of diethyl 2,2-bis(2-methyl-
allyl)malonate (11c) and N,N-bis(2-methylallyl)tosylamide (13).
Table 2 RCM of α,ω-dienes catalyzed by complexes 1a, 1b, or 1d
(5 mol%) in toluene-d8 at 80 °C
Substrate
Catalyst
Time/h
Yield/%
Ref.
Altogether, this study underlines how difficult it is to predict
the activity of closely related metathesis initiators based on struc-
tural information. It also confirms that an optimization of the cat-
alytic system is required for each individual substrate in order to
reach the highest possible efficiency.13 In this respect, the suc-
cessful introduction of a bulky 1,3-diarylbenzimidazol-2-ylidene
ligand instead of the more common imidazol-2-ylidene or imida-
zolin-2-ylidene derivatives offers additional options to fine-
tuning the steric and electronic properties of ruthenium–alkyli-
dene catalyst precursors, and we are currently pursuing further
investigations in this direction.
11c
11c
11c
13
13
13
1a
1b
1d
1aa
1b
1d
5
5
5
7
24
24
57
47
19
90
95
41
12
12
This work
59
10
This work
a 3.5 mol%.
2,2-bis(2-methylallyl)malonate (11c) and N,N-bis(2-methylallyl)-
tosylamide (13) (Scheme 7). Because the formation of tetrasub-
stituted double bonds is one of the most difficult reactions to
achieve via olefin metathesis, these experiments were performed
in toluene-d8 at 80 °C using 5 mol% of ruthenium initiator.
Under these conditions, none of the catalysts under examination
afforded a quantitative yield of diethyl 3,4-dimethylcyclopent-3-
ene-1,1-dicarboxylate (12c) after 5 h (Table 2). Nevertheless,
complexes 1a and 1b bearing, respectively, the SIMes and IMes
ligands significantly outperformed catalyst 1d in this transfor-
mation. This tendency was confirmed in the more facile cycliza-
tion of tosylamide 14. Once again, much higher yields were
obtained with catalyst precursors 1a and 1b than with complex
Experimental
General information
All the reactions were carried out using standard Schlenk tech-
niques under a dry argon atmosphere. Solvents were distilled
from appropriate drying agents and deoxygenated prior to use.
1,3-Dimesitylbenzimidazolium tetrafluoroborate (8)38 and the
substrates for RCM reactions12,60 were prepared according to the
literature. Petroleum ether refers to the hydrocarbon fraction of
bp 40–60 °C and was purchased from Labotec. Chromatography
was performed on silica gel 60 (60 Å nominal pore diameter,
0.063–0.200 mm particle size) supplied by Biosolve. All the
other chemicals were purchased from Aldrich and used as
1
1d. Moreover, monitoring the time course of the reaction by H
NMR spectroscopy revealed that conversion of the two sub-
strates stopped increasing already after 10 min using catalyst
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

View Article Online
1
received. Unless otherwise specified, H, 13C{1H}, and 31P{1H}
was stirred for 30 min at room temperature. After a rapid decan-
tation, the supernatant carbene solution was cannulated under
NMR spectra were recorded at 298 K with a Bruker DRX 400 or
a Bruker Avance 250 spectrometer. Chemical shifts are listed in
parts per million downfield from TMS and are referenced from
the solvent peaks or TMS. Infrared spectra were recorded with a
Perkin-Elmer Spectrum One FT-IR spectrometer. Elemental ana-
lyses were carried out in the Laboratory of Pharmaceutical
Chemistry at the University of Liège.
argon into
a
second flask containing
a
solution of
[RuCl2(vCHPh)(PCy3)2] (100 mg, 0.12 mmol, 1 equiv.) in
toluene (7 mL). The reaction mixture was stirred overnight at
room temperature. The solvent was evaporated in vacuo and the
residue was purified by column chromatography under argon.
Tricyclohexylphosphine was first eluted with n-pentane. The
desired product was then eluted as a dark orange band with
n-pentane–diethyl ether (9 : 1 v/v) as eluents. The solvents were
evaporated and the solid dried under high vacuum to yield a
dark-orange powder (89 mg, 82%).
Synthesis of [RhCl(COD)(BMes)] (9)
In a 50 mL Schlenk flask, [RhCl(COD)]2 (50.7 mg, 0.1 mmol)
and potassium tert-butoxide (25.3 mg, 0.23 mmol, 2.3 equiv.)
were stirred for 45 min at room temperature in THF (25 mL).
Next, 1,3-dimesitylbenzimidazolium tetrafluoroborate (8)
(100 mg, 0.23 mmol, 2.3 equiv.) was added and the reaction
mixture was stirred overnight at room temperature. It was
brought back to air and filtered on a 0.45 μm membrane. The
filtrate was evaporated in vacuo. The residue was dissolved in
dichloromethane–pentane and purified by column chromato-
graphy on silica gel using petroleum ether–ethyl acetate (3 : 1
v/v) as eluents to afford the title compound (Rf = 0.39) as a
yellow powder (113 mg, 91%).
1H NMR (250 MHz, CD2Cl2, 223 K): δ = 19.01 (1 H, s,
RuvCHPh), 8.87 (1 H, d, J = 8.0 Hz, CHPh), 7.39 (1 H, dd, J =
7.2 Hz, CHPh), 7.14 (1 H, s, CHMes), 7.10 (4 H, m, 2 CHPh + 2
CHBenzimid), 7.05 (1 H, s, CHMes), 6.97 (1 H, d, J = 7.4 Hz,
CHPh), 6.87 (1 H, s, CHMes), 6.70 (1 H, d, J = 7.0 Hz, CHBenzimid),
6.57 (1 H, d, J = 8.0 Hz, CHBenzimid), 5.79 (1 H, s, CHMes), 2.42
(3 H, s, CH3), 2.34 (3 H, s, CH3), 2.21 (3 H, s, CH3), 2.07 (3 H,
s, CH3), 1.96 (3 H, s, CH3), 1.54 (3 H, s, CH3), 1.6–0.6 (33 H,
m, Cy). 13C{1H} NMR (62.5 MHz, CD2Cl2, 223 K): δ = 294.8
(RuvCHPh), 203.6 (JP–C = 82.2 Hz, C2NHC), 150.3, 139.2,
138.63, 138.55, 137.5, 136.6, 135.8, 135.30, 135.25, 135.21,
135.18, 132.7, 131.4, 131.1, 129.6, 129.1, 128.9, 128.4, 128.20,
128.15, 127.8, 126.9, 122.5, 110.0, 109.0, 30.2, 29.9, 29.5, 28.8,
28.0, 27.4, 27.3, 25.5, 20.7 (CH3), 20.6 (CH3), 18.76 (CH3),
18.71 (CH3), 17.8 (CH3), 17.2 (CH3). 31P{1H} NMR (101 MHz,
CD2Cl2, 298 K): δ = 31.9. Calc. for C50H65Cl2N2PRu: C, 67.0;
H, 7.3; N, 3.1%. Found: C, 67.2; H, 7.9, N, 3.3%.
1H NMR (400 MHz, CD2Cl2): δ = 7.16 (2 H, dd, J = 3.1 and
6.1 Hz), 7.15 (2 H, s), 7.12 (2 H, s), 6.84 (2 H, dd, J = 3.1 and
6.0 Hz), 4.63–4.41 (2 H, m, CH COD), 3.54–3.42 (2 H, m, CH
COD), 2.44 (6 H, s), 2.26 (6 H, s), 1.98 (6 H, s), 1.91–1.80 (4
H, m, CH2 COD), 1.68–1.53 (4 H, m, CH2 COD). 13C{1H}
NMR (101 MHz, CD2Cl2): δ = 196.4 (d, JRh–C = 52.4 Hz),
139.2, 138.3, 135.43, 135.40, 135.39, 133.4, 129.8, 128.7,
123.2, 110.5, 97.8 (CH COD), 97.7 (CH COD), 68.4 (CH
COD), 68.3 (CH COD), 32.8 (CH2 COD), 28.3 (CH2 COD),
21.0 (CH3 Mes), 19.4 (CH3 Mes), 18.0 (CH3 Mes). Calc. for
C33H38ClN2Rh: C, 66.0; H, 6.4; N, 4.7%. Found: C, 66.3; H,
6.6, N, 4.6%.
X-Ray diffraction studies
Crystal data were collected on a Bruker APPEX II (for 1d) or a
Bruker Smart-CCD-1000 diffractometer (for 9) using graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å) from a fine-
focus sealed tube source at 100 K. Computing data and reduction
was made with the APPEX II software.61 The structure of 1d
was solved using DIRDIF200862 and the structure of 9 with
SIR2004.63 They were refined by full-matrix, least-squares based
on F2 by SHELXL.64 An empirical absorption correction was
applied using SADABS.65 All non-hydrogen atoms were aniso-
tropically refined and the hydrogen atom positions were calcu-
lated and refined using a riding model.
Synthesis of cis-[RhCl(CO)2(BMes)] (10)
Carbon monoxide was bubbled in a solution of [RhCl(COD)-
(BMes)] (9) (50 mg, 0.08 mmol) and CH2Cl2 (10 mL) for
15 min at room temperature. The solvent was evaporated and the
residue was washed with a minimal amount of n-pentane at
0 °C, leaving a light yellow solid (29 mg, 63%).
IR (NaCl, CH Cl ): ν = 2082 (trans) and 1998 (cis) cm−1
.
ˉCO
2
2
Crystal data for [RhCl(COD)(BMes)] (9). Bright yellow crys-
tals were obtained by slow evaporation of a concentrated solution
in dichloromethane under an inert atmosphere. C33H38ClN2Rh,
M = 601.01, crystal dimensions: 0.33 × 0.14 × 0.10 mm, mono-
clinic, a = 13.077(3), b = 11.325(3), c = 19.269(4) Å, β =
99.374(3)°, V = 2815.5(11) Å3, T = 100 K, space group P21/n, Z
= 4, 23 108 measured reflections, 5540 independent reflections,
Rint = 0.05, final R1 values [I > 2σ(I)] = 0.034, final wR(F2) =
0.082.
1H NMR (400 MHz, CD2Cl2): δ = 7.32 (2 H, dd, J = 3.1 and
6.1 Hz), 7.12 (4 H, s), 7.01 (2 H, dd, J = 3.1 and 6.1 Hz), 2.43
(6 H, s), 2.10 (12 H, s). 13C{1H} NMR (101 MHz, CD2Cl2): δ =
187.6 (d, JRh–C = 44.5 Hz), 185.2 (d, JRh–C = 53.8 Hz), 182.9
(d, JRh–C = 73.9 Hz), 139.9, 136.3, 134.8, 132.4, 129.6, 124.5,
111.5, 21.1, 18.2.
Synthesis of [RuCl2(vCHPh)(PCy3)(BMes)] (1d)
1,3-Dimesitylbenzimidazolium tetrafluoroborate (8) (81 mg,
0.18 mmol, 1.5 equiv.) was weighed in a Schlenk tube and
placed under an argon atmosphere. Toluene (3 mL) and a 0.5 M
solution of potassium bis(trimethylsilyl)amide in toluene
(0.43 mL, 0.22 mmol, 1.8 equiv.) were added and the mixture
Crystal data for [RuCl2(vCHPh)(PCy3)(BMes)] (1d). Dark
orange crystals were obtained by slow evaporation of a
n-pentane solution under an inert atmosphere, C50H65Cl2N2PRu,
M = 896.98, crystal dimensions: 0.21 × 0.06 × 0.05 mm, mono-
clinic, a = 9.7323(5), b = 37.4221(19), c = 12.4794(5) Å, β =
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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90.151(2)°, V = 4545.0(4) Å3, T = 100 K, space group P21/n,
4, 46 648 measured reflections, 6516 independent
Organocatalysis, Catalysis by Metal Complexes, ed. C. S. J. Cazin,
Springer, Dordrecht, 2011, vol. 32.
Z
=
3 For selected, recent reviews, see: (a) V. César, S. Bellemin-Laponnaz and
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reflections, Rint = 0.143, final R1 values [I > 2σ(I)] = 0.057, final
wR(F2) = 0.112.
RCM of α,ω-dienes 11a and 11b
A 2 mL volumetric flask capped with a septum was charged
with a ruthenium complex (3.2 μmol) under argon. Dried and
degassed dichloromethane-d2 (2 mL) was added with a dried
syringe under argon. An NMR tube capped with a septum was
charged with this stock solution (0.50 mL, 0.8 μmol of catalyst)
and dichloromethane-d2 (0.3 mL) under argon. The sample was
thermostated at 30 °C in the NMR probe before the substrate
(11a: 19.3 μL or 11b: 20.5 μL, 0.08 mmol, 0.1 M) was added
with a dried microsyringe under argon. Experimental data points
were collected using the Bruker automation software. The con-
version of diethyl 2,2-diallylmalonate (11a) was computed from
the integrals of the allyl methylene protons in the starting
material (δ = 2.53, d) and the product (δ = 2.90, s). The conver-
sion of diethyl 2-allyl-2-(2-methylallyl)malonate (11b) was
determined similarly from the signals of the allyl methylene
protons in the starting material (δ = 2.59, s and 2.56, m) and the
product (δ = 2.79, m and 2.85, m).
7 T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18–29.
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RCM of α,ω-dienes 11c and 13
9 C. W. Bielawski and R. H. Grubbs, Angew. Chem., Int. Ed., 2000, 39,
2903–2906.
An NMR tube capped with a septum was charged with a ruthe-
nium complex (4 μmol, 5 mol%) under argon. Dried and
degassed toluene-d8 (0.8 mL) was then introduced under argon.
The sample was thermostated at 80 °C in the NMR probe before
the substrate (11c: 21.6 μL or 13: 22.3 μL, 0.08 mmol, 0.1 M)
was added with a dried microsyringe under argon. Experimental
data points were collected using the Bruker automation software.
The conversion of diethyl 2,2-bis(2-methylallyl)malonate (11c)
and 4-methyl-N,N-bis(2-methylallyl)benzenesulfonamide (13)
was computed from the integrals of the allyl methylene protons
in the starting material (δ = 2.65, s or 3.68, s, respectively) and
the product (δ = 2.81, s or 3.88, s, respectively).
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Acknowledgements
The financial support of the “Fonds de la Recherche Scien-
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