[(NHC)(NHCewg)RuCl2(CHPh)] complexes with modified NHCewg ligands for efficient ring-closing metathesis leading to tetrasubstituted olefins

Article abstract of DOI:10.1002/chem.200903275

Imidazolium salts (NHC_ewg-HCl) with electronically variable substituents in the 4,5-position (H,H or C1,C1 or H,NO₂ or CN ₅CN) and sterically variable substituents in the 1,3-position (Me,Me or Et,Et or iPr,iPr or Me,iPr) were synthesized and converted into the respective [AgI(NHC)_ewg] complexes. The reactions of [(NHC)RuCl ₂(CHPh)(Py)₂] with the [AgI(NHQw₈)] complexes provide the respective [(NHC)(NHC_ewg)RuCl₂(CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring-closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2-0.5 mol% at 80°C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC_ewg group with l,3-Me,iPr and 4,5-Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL-NHC)-(NHC_cwg)RuCl₂(CHPh)] and [(FLNHC_ewg)(NHC)RuCl₂(CHPh)], with a dansyl fluorophore (FL)-tagged electron-rich NHC ligand (FL-NHC) and an electron-deficient NHC ligand (FLNHC_ewg) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore-tagged ligand. In this manner, it was shown for ring-opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC_ewg ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.

Full text of DOI:10.1002/chem.200903275

FULL PAPER
DOI: 10.1002/chem.200903275
ACHTUNGTRENNUNG[(NHC)ACHTUNGTRENNUNG(NHCewg)RuCl2CAHTNUGRTEN(NUGN CHPh)] Complexes with Modified NHCewg Ligands
for Efficient Ring-Closing Metathesis Leading to Tetrasubstituted Olefins**
Volodymyr Sashuk, Lars H. Peeck, and Herbert Plenio*[a]
Abstract:
Imidazolium
salts
substrate conversion, catalyst loadings
of 0.2–0.5 mol% at 808C in toluene are
sufficient. The complex with the best
catalytic activity in such RCM reac-
tions and the fastest initiation rate has
an NHCewg group with 1,3-Me,iPr and
4,5-Cl,Cl substituents and can be syn-
thesized in 95% isolated yield from the
ruthenium precursor. To learn which
one of the two NHC ligands acts as the
leaving group in olefin metathesis reac-
tions two complexes, [(FL-NHC)-
N
(CHPh)] and [(FL-
2A(CHPh)], with a
(NHCewg·HCl) with electronically vari-
able substituents in the 4,5-position
(H,H or Cl,Cl or H,NO2 or CN,CN)
and sterically variable substituents in
the 1,3-position (Me,Me or Et,Et or
iPr,iPr or Me,iPr) were synthesized and
converted into the respective [AgI-
)G
CHTUNGTRENNUNG
ACHTUNGTRENNUNG
CHTUNGTRENNUNG
G
ligand.
In
this
A
ACHTUNGTRENNUNG
The catalytic activity of such complexes
in ring-closing metathesis (RCM) reac-
tions leading to tetrasubstituted olefins
was studied. To obtain quantitative
Keywords: fluorescence · N-hetero-
cyclic carbenes · olefin metathesis ·
ring-closing metathesis · ruthenium
Introduction
Ruthenium complexes bearing two identical N-heterocyclic
carbenes (NHC) were first reported by Herrmann et al.[1]
and later by Grubbs et al.[2] and found to display modest ac-
tivities in olefin metathesis reactions.[3] In hindsight, this is
not surprising as NHC ligands are now renowned for their
ability to bind strongly to metal centers.[4] In the related
Grubbs 2nd-generation complexes, the initiation reaction
proceeds by dissociative substitution of PCy3 with an olefin;
the analogous pathway is likely for complexes with two
NHC ligands.[5] Consequently, the presence of a second
slow down the initiation reaction.[3a,6] As a consequence, this
class of complexes was considered to be less attractive for
olefin metathesis and thus rarely studied.[6–7] However, Ver-
poort et al. showed that the second NHC ligand in
NHC ligand in [(NHC)2RuCl2ACTHNUTRGNEUGN(CHPh)] can be expected to
[(NHC)2RuCl2ACHTUNTGRNEUNG(CHPh)] complexes appears to be more labile
[a] V. Sashuk, L. H. Peeck, H. Plenio
than expected and can be replaced by isopropoxystyrene to
produce a Grubbs–Hoveyda-type complex.[8] Recently Sij-
besma et al. also demonstrated for closely related complexes
that a polymer-tagged NHC dissociates from ruthenium
under the influence of ultrasound irradiation.[9]
Organometallic Chemistry and Homogeneous Catalysis
Petersenstr. 18, Technische Universitꢀt Darmstadt
64287 Darmstadt (Germany)
E-mail: plenio@tu-darmstadt.de
[**] NHC=N-heterocyclic carbene
We recently reported that attaching strongly electron-
withdrawing groups to NHCs provides ligands (termed
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903275.
Chem. Eur. J. 2010, 16, 3983 – 3993
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3983

NHCewg) with significantly reduced donor abilities roughly
comparable to PCy3.[10] This led us to believe that NHCewg li-
gands could act as more facile leaving groups, just like PCy3
in 2nd-generation Grubbs complexes. On testing such com-
plexes in RCM reactions, it was discovered that [(NHC)-
ACHTUNGTRENNUNG(NHCewg)RuCl2ACHTUNGTRENNUNG(CHPh)] 1 renders catalysts with unprece-
dented activities for RCM (ring-closing metathesis) reac-
tions leading to tetrasubstituted olefins.[11] The synthesis of
such olefin-metathesis products still poses significant prob-
lems with Grubbs-type complexes and until recently
2.5–5 mol% of catalyst and elevated temperatures were re-
quired for such reactions.[12] More recently, some improve-
ments were reported and attributed to a reduced bulk of the
NHC ligand,[13] to the use of NHC ligands with restricted-
conformational freedom,[14] and by applying low polarity,
perfluorinated solvents.[15] In 2009, work from Grubbs fo-
cused on the optimization of catalytic conditions for a single
substrate (diethyl dimethallylmalonate) and reported im-
pressive catalytic efficiencies at 0.2 mol% loading for this
substrate.[16]
Scheme 1. Synthesis of [AgI
5 h, RT; b) R4-I (neat), 48 h, 858C; c) Ag2O, CH2Cl2, 508C.
(NHC)] complexes. a) R3-I, DMSO, KOH,
ACHTUNGTRENNUNG
Following our initial report, we have now facilitated and
generalized synthetic access to [(NHC)
ACHTNUGERTN(NUGN NHCewg)RuCl2-
ACHTUNGTRENNUNG
pounds enables us to modify the steric and the electronic
nature of the NHCewg groups, leading to further improved
performance in RCM reactions of sterically hindered sub-
strates and to perform mechanistic studies aimed at eluci-
dating the basic mechanism of olefin metathesis reactions
with such complexes.
Results and Discussions
Synthesis of [(NHC)
understand which properties of the NHCewg ligand influence
the performance of [(NHC)(NHCewg)RuCl2A(CHPh)] com-
ACHTUGTNRENNU(G NHCewg)RuCl2ACHUTNGTREN(NUGN CHPh)] complexes: To
A
CHTUNGTRENNUNG
Scheme 2. Synthesis of new [(NHC)
ACHTUTGNRNEUNG(NHC)ewgRuCl2HCATUNGTREN(NUGN CHPh)] complexes
5a–g and the cyclic voltammetry-derived RuII/III redox potentials E1/2
.
plexes in various RCM reactions, we systematically varied
the nature of the R1, R2, R3, and R4 substituents in imidazo-
linium salts to allow the fine-tuning of the steric and elec-
tronic properties of the respective NHCewg ligand. The re-
spective imidazoles 1 (R1,R2 =H,H; Cl,Cl; H,NO2, and
CN,CN) are commercially available. Imidazole 1 and the re-
sulting 1-alkylimidazoles 2 are reacted with the respective
alkyl iodides to produce the imidazolium salts 3 in excellent
yields. The reaction of the dialkylimidazolium salts with
Ag2O (following a synthesis by Bielawski et al. for R3,R4 =
Me,Me)[17] results in the facile formation of the respective
(see Table 1–3). With the exception of the 4,5-H,H-substitut-
ed complex 5a, all complexes are air- and moisture-stable
and can be purified by simple chromatography. Alternative-
ly, it is also possible to synthesize complexes 5 directly from
[(NHC)RuCl2ACHTNUTRGNEUNG(CHPh)(py)2] and the respective imidazolium
salt by utilizing Cs2CO3 as the base. However, the yields of
5 are significantly lower when using this direct route.
[AgI
G
Electrochemistry of [(NHC)ACTHNGUTRENN(UG NHCewg)RuCl2ACHTUNGERTN(NUNG CHPh)] com-
NHC-transfer reagents.[18]
plexes 5: To obtain information on the donation of the re-
spective NHC ligands, the RuII/III redox potentials of com-
plexes 5 in the electronic and steric series were determined
(listed in Scheme 2).[19] The cyclic voltammetry derived
Finally, the respective [(NHC)ACTHNUTRGNEN(UG NHCewg)RuCl2ACHTUTGNREN(NUGN CHPh)]
complexes 5 (for numbering see Scheme 1 or Table 1) were
obtained from the reaction of [(NHC)RuCl2A(CHPh)(py)2]
with [AgI(NHCewg)] 4 (Scheme 2). The conversions for this
redox potentials range from E1/2 =+0.482 V for 5a to E1/2 =
final step are nearly quantitative, whereas the isolated yields
of the complexes 5 range between 70–95%. Complex 5g is
available in an isolated yield of 95%. This is important as
this complex later turns out to be the most active precatalyst
+0.711 V for 5d and display a reversible electrochemistry
for all complexes. Within the electronic series, the increase
in the redox potentials is correlated with the presence of
more electron-withdrawing groups at the NHCewg ligand,
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Efficient Ring-Closing Metathesis Leading to Tetrasubstituted Olefins
FULL PAPER
which provides evidence for a decreased donor ability of the
respective NHCewg ligand.[10a,20] This is qualitatively in
accord with observations by Bielawski et al. who studied in
observed with complex 1 (48%) recently reported by us.[11]
Somewhat unexpectedly, the performance of complexes 5a–
d in the RCM of diethyl dimethallylmalonate is comparable,
despite significant differences in the electron donation of
the respective NHCewg ligands.
We next decided to expand the Cl,Cl-substituted NHCewg
complexes derived from 5b to probe the influence of steric
bulk in the NHCewg on the catalytic performance for the
same RCM reaction (Table 1, entry 1).[21] In the series of
complexes 5b, 5e, 5 f, and 5g, the methyl groups in 5b are
successively replaced by the more bulky ethyl and isopropyl
units. The effect of steric modifications on the performance
of the respective ruthenium complexes is more pronounced
than the electronic effect. The RCM yields when using the
detail the electron donation in [(NHC)RhClACHTNUTRGNE(NUG cod)] (cod=
1,5-cyclooctadiene) and [(NHC)Rh(CO)2Cl] by using the
same NHC ligands as in the electronic series described here.
The steric properties of the NHCewg ligands (steric series,
Scheme 2) were modified (with constant R3,R4 =Cl, Cl) in a
systematic manner by introducing various N-alkyl substitu-
ents with variable bulk, ranging from R1,R2 =Me to R1,R2 =
iPr. The minor changes in the redox potentials of complexes
5 demonstrate that the variable bulk of the N,N-dialkyl
groups exerts only a minor effect on the donating properties
of NHCewg in the respective ruthenium complexes. The
subtle changes in the redox potentials nicely reflect the in-
creased donor capacity of secondary versus primary alkyl
groups and the usefulness of redox potentials for the evalua-
tion of NHC donation.[10a] Based on the present data, we
conclude that the steric and electronic properties of NHCewg
ligands can be manipulated almost independently from each
other.
[(NHC)ACTHNUGTRENN(UG NHCewg)RuCl2ACHUTNGTREN(NGUN CHPh)] complexes 5b, 5e, 5 f, and
5g range from 56–84% (Table 1, entry 1 and Figure 1). All
of the newly synthesized complexes are characterized by su-
perior performance relative to complex 1 and even the least
efficient one of the new complexes gives better yields than
complex 1 for this test reaction.[11] The best results in terms
of final yield and rate of the RCM reaction are obtained for
complex 5g with an unsymmetrical NHCewg ligand (R1 =Me,
R2 =iPr). This complex is characterized by a combination of
high activity and relatively fast initiation. Whereas some
ruthenium complexes reported here require up to 24 h at
808C to reach full conversion of the substrates, RCM reac-
tions that utilize 5g are normally finished within 6–8 h at
808C. At this point, we also tried to further improve the per-
formance of complexes 5. This was based on the assumption
that the main decomposition pathway of the RCM catalysts
RCM reactions utilizing [(NHC)ACTHNUTRGENN(UG NHCewg)RuCl2ACHTUNTGREN(NUGN CHPh)]:
With a broad range of available complexes, we first evaluat-
ed the influence of variable NHCewg donor strength on the
performance of the [(NHC)ACTHNUTRGENNUG(NHCewg)RuCl2CAHTUNGTERN(NUGN CHPh)] com-
plexes 5a–d in the difficult ring-closing metathesis reaction
of diethyl dimethallylmalonate at 808C (Table 1, entry 1).
With a 0.5 mol% loading, the desired product is formed in
74–81% yield, which is significantly better than the yields
Table 1. RCM reactions leading to tetrasubstituted, five-membered cyclic olefins.[a]
Yield [%]
Entry
Product
Catalyst
loading
AHCTUNGTRENGN[UN mol%]
1
Me,Me
H,H
5a
Me,Me
Cl,Cl
5b
Me,Me
H,NO2
5c
Me,Me
CN,CN
5d
Et,Et
Cl,Cl
5e
iPr,iPr
Cl,Cl
5 f
Me,iPr
Cl,Cl
5g
Me,iPr
H,NO2
5h
0.5
0.5
0.2
48
74
30
78
37
81
41
74
56
58
84
75
97[b]
50[b]
36
34, 77[b]
1
2
0.5
0.2
98
96
98
94
88
98
99
65
36[b]
3
4
5
0.05
93
99
97
99
99
99
94
99
98
0.2
0.1
99
68
0.5
91
99
99
99
[a] General procedure for metathesis screen: 0.2 mmol olefinic substrate in toluene (10 mL, 0.02m), T=808C, 20 h, Ru complexes 5a–h were added as a
stock solution (3 mmolLÀ1 in toluene). Substrate conversion determined by GC. [b] T=1008C.
Chem. Eur. J. 2010, 16, 3983 – 3993
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3985

H. Plenio et al.
(1,2-dichlorobenzene) at 808C under a dynamic vacuum of
100 mbar. However, this led to no improvement as with a
0.2 mol% loading of 5d product M1 was formed in 45%
yield under standard conditions, but in only 25% yield with
applied dynamic vacuum. It appears that the complete and
fast removal of ethene is unfavorable. This came as a sur-
prise as the presence of ethene in closed-vessel reactions is
not favorable for RCM reactions.[24]
To demonstrate the broad applicability of these com-
plexes for the synthesis of tetrasubstituted olefins, we tested
a number of additional RCM transformations (Table 1, en-
tries 2–5, Table 2, entries 1–6, and Table 3, entries 1–3). In
almost all of the test reactions, complex 5g with N,N’-Me,iPr
substituents shows the best catalytic performance. This is
why we decided to also realize the N,N’-Me,iPr pattern with
(H,NO2) substituents in the backbone of complex 5h. But
the performance of this complex cannot rival that of com-
plex 5g (Table 1–3). However, complex 5b (5-(Me,Me),
(Cl,Cl) is nearly as good as 5g. In general, the differences in
the series of complexes are less pronounced as anticipated
and this is one reason why we refrained from screening all
Figure 1. Time–conversion curves for the RCM of diethyl dimethallylmal-
onate by using the complexes 5a–g.
under the present conditions occurs via the unstable
{(NHC)RuCl2ACHTUNGTRENNUNG
(CH2)} intermediate.[22] This species obviously
forms more easily in the presence of the reaction product
ethene. Analogous to our previous ADMET (acyclic diene
metathesis) experiments,[23] we carried out the RCM reac-
tion of diethyl dimethallylmalonate in a high-boiling solvent
Table 2. RCM reactions leading to tetrasubstituted, six-membered cyclic olefins.[a]
Yield [%]
Entry
Product
Catalyst
loading
AHCTUNGTRENGN[UN mol%]
1
Me,Me
H,H
5a
Me,Me
Cl,Cl
5b
Me,Me
H,NO2
5c
Me,Me
CN,CN
5d
Et,Et
Cl,Cl
5e
iPr,iPr
Cl,Cl
5 f
Me,iPr
Cl,Cl
5g
Me,iPr
H,NO2
5h
0.5
0.2
0.2
99
99
59
99
79
99
55
99
68
86
78[b]
79[b]
1
2
3
0.5
99
0.5
0.2
0.2
99
64
56
87
69
61
72
98
99
90
65
75[b]
0.5
0.2
0.2
98
99
81
99
95
99
81
99
94
63
87
53
95
69
93
49[b]
69[b]
4
5
0.5
0.2
82
95
32
84
43
78
99
33
55[b]
6
0.5
84
86
85
79
94
[a] General procedure for metathesis screen: 0.2 mmol olefinic substrate in toluene (10 mL, 0.02m), T=808C, 20 h, Ru complexes 5a–h were added as a
stock solution (3 mmolLÀ1 in toluene). Substrate conversion determined by GC. [b] T=1008C.
3986
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Efficient Ring-Closing Metathesis Leading to Tetrasubstituted Olefins
Table 3. RCM reactions leading to tetrasubstituted, seven-membered cyclic olefins.[a]
FULL PAPER
Yield [%]
Me,Me
CN,CN
5d
Entry
Product
Catalyst
loading
AHCTUNGTRENGN[UN mol%]
1
Me,Me
H,H
5a
Me,Me
Cl,Cl
5b
Me,Me
H,NO2
5c
Et,Et
Cl,Cl
5e
iPr,iPr
Cl,Cl
5 f
Me,iPr
Cl,Cl
5g
Me,iPr
H,NO2
5h
0.5
0.2
0.2
60
95
57
98
57
92
53
89
79
95
59
33
67
40[b]
1
2
3
0.2
0.5
63
46
85
59
76
70
63
84
46
[a] General procedure for metathesis screen: 0.2 mmol olefinic substrate in toluene (10 mL, 0.02m), T=808C, 20 h, Ru complexes 5a–h were added as a
stock solution (3 mmolLÀ1 in toluene). Substrate conversion determined by GC. [b] T=1008C.
possible combinations of complexes 5a–h and the various
substrates.
M1. At such temperatures complex 5d with the least elec-
tron-donating NHCewg ligand shows the best substrate con-
versions (Table 1, entry 1). With a 0.5 mol% loading of 5d
virtually quantitative conversion was observed and with
0.2 mol% loading 77% (Table 1, entry 1). This excellent
performance is comparable to that of the best complexes re-
cently reported in a careful optimization study by the
Grubbs group for the same substrate.[16] Nonetheless, it ap-
pears that the least electron-donating NHCewg ligand with
R3,R4 =CN is a less efficient leaving group, as complex 5d
requires significantly higher temperatures for reaction. For
other RCM reactions higher reaction temperatures are not
helpful and the peak performance is normally observed at
808C. For entry 2 (Table 1) and entries 1, 4, and 5 (Table 2)
complex 5d is less efficient than 5g at the same temperature
and when comparing to the 808C performance of 5g. We
therefore studied one RCM reaction (Table 3, entry 1) at a
range of different temperatures, employing a 0.2 mol%
loading of complex 5g under standard conditions. The fol-
lowing RCM results were observed: T=1008C, 33% conver-
sion; 908C, 41%; 808C, 59%; and 708C, 30%. After lower-
ing the reaction temperature to 608C, almost no conversion
was observed.
The new complex 5g requires a two-to-three times lower
catalyst loading than complex 1 in RCM reactions, which in
turn was already significantly more active than complexes
reported by others.[12c,e,13–15] As an example, the formation of
M9 by RCM was reported to require 5 mol% for quantita-
tive conversion,[25] whereas with complex 5g as little as
0.5 mol% was sufficient. For a more detailed comparison of
different ruthenium complexes (1, Grubbs 1st and 2nd-gen-
eration, Grubbs–Hoveyda) in RCM reactions leading to tet-
rasubstituted olefins, the reader is referred to our previous
publication.[11] Here we are only going to discuss the work
of others published in 2009 and the substrate conversions
not studied previously.
The synthesis of M10 (Table 2, entry 5) was studied by
Grela et al. and found to require 5 mol% of a Grubbs–
Hoveyda-type catalyst for quantitative product formation,[25]
whereas with 5g as little as 0.5 mol% produces a 99% yield.
The formation of M11 (Table 2, entry 6) requires 1 mol% in
fluorinated solvents for 99% product formation,[15a] whereas
0.5 mol% of complex 5g in toluene solvent was already suf-
ficient for nearly quantitative conversion (94%). We also
tested a few difficult RCM reactions leading to di- or trisub-
stituted cyclic olefins. Numerous groups studied the RCM
reactions of Ph2Si(CH2CH=CH2)2 (Table 1, entry 4) and typ-
ically 5 mol% of ruthenium complex were needed for quan-
titative conversion.[26] In our experiments with complex 5g,
as little as 0.2 mol% were sufficient and even with
0.1 mol% catalyst loading a respectable 68% conversion is
obtained.
This clearly shows that [(NHC)ACTHNUTRGENN(UG NHCewg)RuCl2ACHTUNGTRENNUNG(CHPh)]
complexes are not rapidly initiating species. However, the
slow initiation reaction appears coupled with a high stability
of the precatalyst, since complexes 5 still show significant
activity after being heated to 808C for 20 h. This property
may very well be responsible for the high activity of com-
plexes 5 in the conversion of otherwise difficult substrates:
the high stability of the precatalysts appears to lead to the
slow generation of active species at elevated temperatures,
at a rate that appears to be required for the formation of
An increase of the reaction temperature from 80 to 1008C
changes the ranking of complexes 5a–g for the synthesis of
Chem. Eur. J. 2010, 16, 3983 – 3993
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3987

H. Plenio et al.
tetrasubstituted olefins. This explanation is compatible with
the fact that following initiation and loss of one NHC
ligand, [(NHC)ACHTUNGTRENNUNG(NHCewg)RuCl2ACHTUNTGREN(NUNG CHPh)] probably forms the
same active species as Grubbs II or Grubbs–Hoveyda-type
complexes.
tron-withdrawing substituents are better leaving groups than
NHCewg with weakly electron-withdrawing groups.[11] How-
ever, the detailed catalytic study undertaken now sheds
some doubts on this simple mechanistic picture. We there-
fore decided to study in more detail the leaving group quali-
ty of different NHCewg ligands, which also concerns the ini-
tiation of the RCM reactions. Complexes 5a–d were dis-
solved in [D5]pyridine and the rate for the substitution of
the NHCewg ligands by pyridine determined by 1H NMR
spectroscopy (by the respective integrals of the benzylidene
CHPh signals). Pyridine replaces the NHCewg group and
Two sulfoxides (Table 1, entry 5 and Table 3, entry 3) and
the respective thioethers and sulfoxides were tested in RCM
reactions. Both the thioether and the sulfoxide could not be
converted to the respective cyclic products by using the cat-
alysts reported here, but we are not aware that this transfor-
mation is possible with other RCM catalysts. This failure is
most likely attributed to the coordination of the sulfur lone
pair to ruthenium resulting in catalyst inhibition. On the
other hand, the reaction leading to the cyclic sulfone M5
(Table 1, entry 5) was successful; with a 0.5 mol% loading a
99% conversion was observed. The synthesis of this sulfone
had previously been reported by using 5 mol% Grubbs II
complex,[27] whereas M14 (Table 3, entry 3) was not obtained
previously by RCM reactions.
[(NHC)RuCl2ACHTNUTRGNEUNG(CHPh)(py)2] is formed, as evidenced by the
1
disappearance of the d=19.75 ppm H NMR spectroscopic
shift of the benzylidene proton (CHPh). Instead a signal at
d=19.95 ppm grows in, which corresponds to the benzyli-
dene proton in [(NHC)RuCl2ACHTNUGRTENUNG(CHPh)(py)2]. Based on the
NMR spectroscopic experiments, the kobs for the NHCewg
substitution by pyridine for the electronic series were calcu-
lated (Table 4). There is no apparent correlation of these
Table 4. kobs for NHCewg–pyridine exchange.
Crystal structure analysis of 5d: We have determined the
crystal structure of 5d (Figure 2), but the structural features
are unremarkable relative to Grubbs 2nd-generation com-
A
N
(CHPh)]
kobs [sÀ1
]
5 f: R1,R2 =iPr,iPr; R3,R4 =Cl,Cl
5g: R1,R2 =Me,iPr; R3,R4 =Cl,Cl
5b: R1,R2 =Me,Me; R3,R4 =Cl, Cl
5e: R1,R2 =Et,Et; R3,R4 =Cl,Cl
electronic series
378.0Æ45.4
113.4Æ8.3
106.2Æ2.7
67.0Æ6.1
5c: R,Rꢂ=Me; R3,R4 =H,NO2
5d: R1, R2 =Me,Me; R3,R4 =CN,CN
5b: R1, R2 =Me,Me; R3,R4 =Cl,Cl
547.2Æ10.4
255.6Æ5.0[a]
106.2Æ2.7[a]
[a] Errors from curve fitting, the real error should be higher as the con-
versions for the reaction monitored are not quantitative.
rates with the rate of the initiation reaction for the RCM re-
actions. It is therefore likely that the NHC substitution by
pyridine is not a good model reaction for the initiation reac-
tion. Unfortunately, the rates obtained from the pyridine-ex-
change experiments are also not correlated with the electron
donation or the sterics of the respective NHCewg ligand.
However, as pointed out by Bielawski et al., the bonding of
the NHC ligands used here is more complicated.[17] Apart
from being good s-donors (to a variable extent) the degree
of p-back bonding is significant especially for the cyano-sub-
stituted NHC ligands,[17,28] which should influence the leav-
ing-group properties of the respective NHCewg ligand.[29]
Figure 2. Crystal structure analysis of 5d. Important bond lengths and
À
À
À
angles: Ru C
240.94(18) pm; (NHC)C-Ru-CAHCTNUGTRENNNUG
G
ACHTUGNTRENUN(GN NHCewg): 207.8, Ru Cl: 238.6(2),
plexes.[3b] Both NHC ligands are tilted away from the ben-
zylidene to utilize the empty space trans to Ru=CHPh. The
À
two Ru C
(NHC) bond lengths in 5d are almost identical.
In the case of [(NHC)
idine reaction produces a second (major) product (d=
18.09 ppm) in addition to [(NHC)RuCl2A(CHPh)(py)2] (ca.
60:40 ratio). To test whether this complex results from the
exchange of the NHC ligand in 5a instead of the expected
NHCewg ligand, we synthesized the symmetrical bis-NHC
complex by Herrmann et al.[1] However, the reaction of this
complex in [D5]pyridine is characterized by decomposition
and no resonance at d=18.09 ppm was observed.
ACHTUTGNRNEUNG(NHC)ewgRuCl2HCATUNGTREN(NUGN CHPh)] 5a the pyr-
This is consistent with the observation that variable electron
donation of NHC ligands exerts only a subtle influence on
structural parameters.[10a] Based on the evaluation of the
steric shielding of the ruthenium atom by the five ligands,
primarily by the two NHC units, it also appears unlikely
that the substitution of NHCewg by pyridine or in the initia-
tion reaction by olefins occur by an associative mechanism.
CTHUNGTRENNUNG
Leaving-group quality of NHC ligands in [(NHC)-
A
E
Fluorophore-tagged
NHC
ligands
and
[(NHC)-
A
CHTUNGTRENNUNG
3988
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Chem. Eur. J. 2010, 16, 3983 – 3993

Efficient Ring-Closing Metathesis Leading to Tetrasubstituted Olefins
FULL PAPER
NHC–pyridine substitution reactions put our simple mecha-
nistic hypothesis of NHCewg ligands as efficient leaving
groups during olefin metathesis reactions in jeopardy. As it
is doubtful whether the pyridine experiments serve as good
models for the initiation step in olefin metathesis, we decid-
ed to study olefin metathesis reactions by using different
dansyl group. For both complexes 11 and 15 the fluores-
cence intensity during the olefin metathesis reaction was
probed. Since ruthenium in Grubbs-type complexes is
known to quench the fluorescence of the dansyl dye,[30c] the
dissociation of the fluorophore tag was expected to lead to a
drastic increase in the fluorescence intensity.
[(NHC)
G
N
Accordingly, solutions of 11 and 15 display only weak
fluorescence, whereas the free NHC ligands 9 and 12 are
characterized by strong fluorescence emission. It is impor-
tant to realize that the high sensitivity of fluorescence detec-
tion allows the observation of ligand dissociation events
under real catalytic conditions; that is, in the low-concentra-
tion regime.
We first tested the ruthenium complexes 11 and 15 in
pure CH2Cl2 at room temperature, to learn more about the
time-dependent fluorescence evolution resulting from ad-
ventitious decomposition of the ruthenium complexes in the
absence of substrates (blind experiment). With 11 a signifi-
cant increase in fluorescence was observed, which reflects
the lower stability and decomposition of less-stable 11
(Figure 3, trace a). The fluorescence of a solution of 15 in-
creases only slowly, indicative of the high stability of 15
(Figure 4, trace a). Next we probed the substitution of NHC
ligands with pyridine. For complex 15, virtually the same
fluorescence–time curve as in the blind experiment is ob-
tion in this respect was which NHC is liberated during an
olefin metathesis reaction and which NHC remains ligated
to ruthenium. We and others recently discovered that tag-
ging of certain ligands in metal complexes with fluorescent
dyes can provide valuable mechanistic information due to
changes in the fluorescence intensity in the course of cata-
lytic reactions.[30] Transition metals are able to quench the
fluorescence of fluorescent dyes, depending on the distance
between the fluorophore and the metal.[31] This is why
ligand dissociation reactions can give rise to changes in the
fluorescence properties. Consequently a new fluorophore-
tagged FL-NHCewg 9·HCl was synthesized (Scheme 3),
whereas FL-NHC 12·HCl was available from a previous
study.[30a] The respective complexes [(FL-NHCewg)-
R
ACHTUNGTRENNUNG
cedures (Schemes 3 and 4). In those complexes, either the
NHC or the NHCewg ligand is tagged with a fluorescent
Scheme 3. Synthesis of the dansyl-tagged NHC ligand and [(NHC)(FL-
NHCewg)RuCl2ACHTUNGTRENNUNG(CHPh)] complex. a) NaH, BrCAHTUNGTRNEN(UGN CH2)3NHBoc, THF, 608C,
12 h; b) MeI, 408C, 24 h; c) 4m HCl, dioxane, RT, 30 min; d) dansyl-
chloride, iPr2NEt, MeOH, 2 h, RT; e) Ag2O, CH2Cl2, 408C, 60 min;
Figure 3. Fluorescence-time curve of 11 (1.2ꢃ10À6 m): a) in CH2Cl2, b) in
pyridine, and c) during the ROMP reaction in CH2Cl2.
f) [(NHC)RuCl2ACHTUNGTRENNUNG(CHPh)(py)2], 608C, toluene, 60 min. RFL =dansylamide.
Scheme 4. Synthesis of the dansyl-tagged NHC ligand and [(NHC)-
(NHCewg)RuCl2A(CHPh)] complex. a) KOtBu, THF, 508C, 2 h; b) pyridine,
RT, 15 min; c) [AgI(NHC)], toluene, 608C. RFL =dansylamide.
G
N
Figure 4. Fluorescence-time curve of 15 (1.2ꢃ10À6 m): a) in CH2Cl2, b) in
pyridine, and c) during the ROMP reaction in CH2Cl2.
G
Chem. Eur. J. 2010, 16, 3983 – 3993
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3989

H. Plenio et al.
served, showing that the FL-NHC ligand remains bonded to
ruthenium (Figure 4, trace b). In complex 11, with the
dansyl-tag attached to the NHCewg ligand, the fluorescence
intensity increases much faster than the background reaction
(Figure 4, trace b). This indicates the release of the fluoro-
phore-tagged NHCewg. We next tested the application of flu-
orophore tags in RCM. However, the high temperatures
(808C) needed for such reactions led to a significant fluores-
cence signal in the blind experiment. This is why we chose
the ring-opening metathesis ploymerization (ROMP) reac-
tion of norbornene instead, as it can be carried out at room
temperature. Under the specified conditions (0.0025 mol%
of 11 or 15, T=308C), the polymerization of norbornene is
complete within less than 100 min. The monomer/ruthenium
ratio of 40.000:1 also provides evidence for the excellent
and more strongly donating NHC ligands; we have now
demonstrated that it is also worth taking a different view, by
studying applications for NHCewg ligands with a weaker
donor capacity.
Experimental Section
General: All chemicals were purchased as reagent grade from commer-
cial suppliers and were used without further purification unless otherwise
noted. Dansyl chloride was prepared from dansyl acid according to the
literature procedure.[32] Solvents were dried by passing over Al2O3 and/or
by storing over molecular sieves unless otherwise noted. 1,2-Dichloroben-
zene and pyridine were degassed by the freeze-pump-thaw cycle tech-
nique. Flash column and preparative TLC were performed by using silica
gel 60 (0.063–0.20 mesh ASTM). TLC was performed by using silica gel
60 F254 (0.2 mm) on alumina plates. NMR spectra were recorded on
Bruker DRX500 and Bruker DRX300. The chemical shifts (d) are given
in ppm relative to TMS, coupling constants J are in Hz. MS spectra were
recorded on a Finnigan MAT95 spectrometer. GC experiments were run
on a Clarus 500 GC with autosampler and FID detector. Column: Varian
ROMP activities of the [(NHC)ACTHNUTRGENN(UG NHCewg)RuCl2ACHTUTGNREN(NUGN CHPh)]
complexes. In the present experiments, the precatalyst con-
centration was 1.2ꢃ10À6 m. During the ROMP reaction uti-
lizing complex 15, the fluorescence again remains almost
constant. This provides firm evidence that the FL-NHC
ligand remains ligated to the ruthenium during the olefin
metathesis reaction. Quite in contrast, the fluorescence in-
tensity of 11 increases rapidly and reaches saturation at
around the time the ROMP reaction is finished. This clearly
demonstrates that olefin metathesis with [(FL-NHCewg)-
CP-Sil
8 CB (l=15 m, diameter=0.25 mm, dF=1.0 mm), N2 (flow:
17 cmsÀ1; split 1:50); injector-temperature: 2008C, detector temperature:
2708C. Temperature program: isotherm 608C for 5 min, heating to 3008C
with 258CminÀ1, isotherm for 5 min. The identity of all GC product
peaks was established by GCMS on a Finnigan MAT GCMS. The spec-
troscopic data (1H NMR) of the isolated products are identical to those
reported in the literature. Cyclic voltammetry: EG&G 263A-2 potentio-
stat. Cyclic voltammograms were recorded in dry CH2Cl2 under an argon
atmosphere at ambient temperature. A three-electrode configuration was
employed. The working electrode was a Pt disk (diameter: 1 mm) sealed
in soft glass with a Pt wire as counter electrode. The pseudo reference
electrode was an Ag wire. Potentials were calibrated internally against
the formal potential of octamethylferrocene (À10 mV (CH2Cl2) vs. Ag/
AgCl). NBu4PF6 (0.1 molLÀ1) was used as supporting electrolyte. UV/Vis
spectra were recorded on a Zeiss Specord S10 spectrometer. Fluores-
cence spectra were recorded on a J&M FL3095 spectrometer; fluores-
ceine was used as a reference standard.
ACHTUNGTRENNUNG(NHC)RuCl2ACHTUNGTRENNUNG(CHPh)] complexes goes along with the disso-
ciation of the NHCewg ligand, whereas the other NHC ligand
remains coordinated to ruthenium.
Conclusion
We have demonstrated the facile synthesis of nine optimized
[(NHC)
(NHCewg)RuCl
N
General procedure for the synthesis of 1-alkylimidazoles 1: Solid KOH
(0.84 mg, 15 mmol, 1.5 equiv) was added to a solution of imidazole
(10 mmol, 1 equiv) in DMSO (20 mL). The mixture was stirred at RT for
5 h. Next the appropriate alkyl iodide (11 mmol, 1.1 equiv) was added
and the solution was stirred at 508C for 2 days. After this time, the reac-
tion mixture was diluted with water (500 mL) and extracted with CHCl3
(2ꢃ20 mL). The solvent was removed from the combined organic frac-
tions affording the product.
lent yields. The catalytically most efficient complex 5g is
available in 95% yield from easily available starting materi-
als. The new complexes show unprecedented activities in
RCM reactions leading to a variety of tetrasubstituted ole-
fins. Specifically, complex 5g generates the respective RCM
products in excellent yields at 0.2–0.5 mol% loading. Fur-
thermore, we now have a toolbox of catalytically active
complexes at hand, which we are currently testing in various
other olefin metathesis reactions. The modification of the
leaving group quality of the NHCewg ligand allows the easy
fine-tuning of the catalytic activities for different olefin
metathesis reactions—despite the fact that following the ini-
tiation reaction, the same catalytically active species as in
olefin metathesis reactions for Grubbs 2nd-generation or
Grubbs–Hoveyda complexes appears to be formed. Evi-
dence for this was obtained through the use of fluorophore-
tagged NHC ligands. This approach represents a new tool
for homogeneous catalysis, which can provide valuable
mechanistic evidence in the low-concentration regime. In
the present case, it was shown that NHCewg ligands act as ef-
ficient leaving groups in olefin metathesis reactions.
1-Isopropyl-4,5-dichloroimidazole: Orange-brown oil; yield: 84%;
1H NMR (300 MHz, CDCl3): d=7.42 (s, 1H), 4.34 (septet, J=6.6 Hz,
1H), 1.47 ppm (d, J=6.6 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=132.3,
125.9, 112.5, 49.0, 22.5 ppm.
1-Ethyl-4,5-dichloroimidazole: Red oil; yield: 94%; 1H NMR (300 MHz,
CDCl3): d=7.40 (s, 1H), 3.97 (q, J=7.3 Hz, 2H), 1.44 ppm (t, J=7.3 Hz,
3H); 13C NMR (75 MHz, CDCl3): d=133.6, 126.0, 113.0, 41.4, 15.5 ppm.
General procedure for the synthesis of imidazolium salts 2: A mixture of
1-alkylimidazole (2 mmol, 1 equiv) and the respective alkyl iodide (5–
7 equiv) was heated at 858C for 2 days in a closed vessel. The obtained
suspension was filtered and washed with ether affording the desired
product.
1,3-Diisopropyl-4,5-dichloroimidazolium iodide: Grey precipitate; yield:
1
52%; H NMR (300 MHz, [D6]DMSO): d=9.49 (s, 1H), 4.68 (septet, J=
6.6 Hz, 1H), 1.54 ppm (d, J=6.6 Hz, 6H); 13C NMR (75 MHz,
[D6]DMSO): d=133.0, 118.1, 53.1, 21.4 ppm.
1-Isopropyl-3-methyl-4,5-dichloroimidazolium iodide: Off-white precipi-
tate; yield: 97%; 1H NMR (300 MHz, CDCl3): d=9.59 (s, 1H), 4.66 (s,
J=6.6 Hz, 1H), 3.82 (s, 3H), 1.50 ppm (d, J=6.6 Hz, 6H); 13C NMR
(75 MHz, CDCl3): d=135.1, 119.3, 117.5, 52.6, 35.0, 21.4 ppm.
Finally, it is important to note that up until now the re-
search into new NHC ligands was focused on creating more
3990
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Chem. Eur. J. 2010, 16, 3983 – 3993

Efficient Ring-Closing Metathesis Leading to Tetrasubstituted Olefins
FULL PAPER
1-Isopropyl-3-methylimidazolium iodide: Orange precipitate; yield: 75%;
1H NMR (300 MHz, CDCl3): d=9.73 (s, 1H), 7.49 (s, 2H), 4.68 (septet,
J=6.6 Hz, 1H), 3.96 (s, 3H), 1.47 ppm (d, J=6.6 Hz, 6H); 13C NMR
(75 MHz, CDCl3): d=134.8, 123.4, 120.1, 52.9, 36.6, 22.7 ppm.
3.00–1.40 (brs, 12H), 2.26 (s, 3H), 2.35 (s, 3H), 2.25 ppm (s, 3H);
13C NMR (125 MHz, CDCl3): d=303.2 (m), 222.2, 187.7, 151.0, 140.0,
139.9, 138.8, 137.7, 137.2, 134.6, 129.7, 129.4, 129.1, 127.7, 116.7, 116.5,
51.6, 51.1, 35.0, 34.7, 20.9, 20.0, 18.2 ppm (brs); HRMS (EI): m/z: calcd
for C33H36N4Cl4Ru:· 732.0847 [M]+; found: 732.0877.
1-Isopropyl-3-methyl-4-nitroimidazolium iodide: Yellow precipitate;
yield: 70%; 1H NMR (300 MHz, [D6]DMSO): d=9.66 (s, 1H), 9.27 (s,
1H), 4.74 (septet, J=6.6 Hz, 1H), 4.07 (s, 3H), 1.52 ppm (d, J=6.6 Hz,
6H); 13C NMR (75 MHz, [D6]DMSO): d=138.2, 123.3, 54.4, 37.3,
Complex 5c: Two rotamers 1:1.7; reaction time: 15 min; chromatogra-
phy: (cyclohexane/EtOAc 3:1). Evaporation of the eluent and washing
1
with pentane affords the product as a brown precipitate (75%). H NMR
À
22.0 ppm, one peak (C NO2) is not observed.
(500 MHz, CDCl3): d=19.19 (s, 1H), 19.15 (s, 1H), 7.90–7.60 (m, 5H),
7.75 (s, 1H), 7.47 (t, J=7.5 Hz, 2H), 7.12 (t, J=7.5 Hz, 4H), 7.05 (s, 2H),
7.03 (s, 2H), 6.72 (brs, 4H), 4.20–3.80 (m, 8H), 3.52 (s, 3H), 3.29 (s, 3H),
3.00–1.40 (brs, 12H), 2.93 (s, 3H), 2.69 (s, 3H), 2.64 (brs, 12H), 2.37 (s,
3H), 2.34 (s, 3H), 2.26 (s, 3H), 2.24 ppm (s, 3H); 13C NMR (125 MHz,
CDCl3): d=304.3 (m), 221.1, 220.9, 197.7, 196.5, 151.0, 150.9, 140.1, 140.0,
139.3, 139.2, 138.9, 137.9, 137.9, 137.1, 137.0, 134.6, 134.4, 130.0, 129.9,
129.5, 129.4, 129.4, 127.9, 127.9, 126.0, 125.4, 51.7, 51.1, 38.5, 38.0, 37.1,
36.9, 20.9, 20.0, 18.2 ppm (brs); HRMS (EI): m/z: calcd for
C33H39N5Cl2O2Ru: 709.1561 [M]+; found: 709.1516.
1,3-Diethyl-4,5-dichloroimidazolium iodide: Off-white powder; yield:
79%; 1H NMR (300 MHz, CDCl3): d=9.55 (s, 1H), 4.22 (q, J=7.3 Hz,
4H), 1.44 ppm (t, J=7.3 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=135.4,
118.3, 44.0, 13.8 ppm.
General procedure for the synthesis of [AgIACTHNUTRGNE(NUG NHC)] complexes 4: A
Schlenk tube containing Ag2O (0.4 mmol, 1 equiv) and the appropriate
imidazolium salt (0.8 mmol, 2 equiv) was filled with CH2Cl2 (3 mL) under
an argon atmosphere and stirred at 508C until the silver oxide dissolved.
Finally the reaction mixture was added dropwise to diethyl ether or pen-
tane to precipitate the complex.
Complex 5d: Reaction time: 30 min; chromatography: (cyclohexane/
EtOAc 4:1). The crude product was dissolved in a minimal amount of
CH2Cl2 and was added to pentane (40 mL)—crystals started growing in a
few minutes. Green crystals were collected by decantation of the mother
liquor (116 mg, 89%). 1H NMR (500 MHz, CDCl3): d=19.14 (s, 1H),
7.65 (brs, 2H), 7.51 (t, J=7.5 Hz, 1H), 7.15 (t, J=7.5 Hz, 2H), 7.05 (s,
2H), 6.69 (brs, 2H), 4.15–3.92 (m, 4H), 3.41 (s, 3H), 2.79 (s, 3H), 2.62
(brs, 6H), 2.45–1.80 (brs, 6H), 2.36 (s, 3H), 2.26 ppm (s, 3H); 13C NMR
(125 MHz, CDCl3): d=307.6 (m), 219.9, 200.5, 150.9, 140.2, 139.2, 138.1,
137.2, 136.8, 134.3, 130.3, 130.0, 129.6, 129.5, 128.2, 115.5, 115.2, 106.8,
106.7, 51.7, 51.1, 37.1, 36.9, 21.0, 20.1, 18.1 ppm; HRMS (EI): m/z: calcd
for C35H38N6Cl2Ru: 714.1566 [M]+; found: 714.1571.
1,3-Diisopropyl-4,5-dichloroimidazolinium-silver(I) iodide: White precipi-
tate; yield: 72%; 1H NMR (300 MHz, CDCl3): d=5.00 (septet, J=
6.6 Hz, 1H), 1.63 ppm (d, J=6.6 Hz, 6H); 13C NMR (75 MHz, CDCl3):
d=182.9, 116.3, 55.3, 22.5 ppm.
1-Isopropyl-3-methyl-4,5-dichloroimidazolinium-silver(I) iodide: Brown
solid (69%); 1H NMR (300 MHz, CDCl3): d=4.92 (s, J=6.8 Hz, 1H),
3.87 (s, 3H), 1.57 ppm (d, J=6.8 Hz, 6H); 13C NMR (75 MHz, CDCl3):
d=181.8, 117.4, 115.4, 53.8, 38.1, 22.4 ppm.
1-Isopropyl-3-methyl-4-nitroimidazolinium-silver(I) iodide: Yellow solid;
yield: 80%; 1H NMR (500 MHz, [D6]DMSO): d=8.99 (s, 1H), 4.92
(septet, J=6.6 Hz, 1H), 4.13 (s, 3H), 1.50 ppm (d, J=6.6 Hz, 6H);
13C NMR (125 MHz, [D6]DMSO): d=187.1, 139.7, 123.1, 55.2, 39.6,
22.9 ppm.
Complex 5e: Reaction time: 30 min; chromatography: (cyclohexane/
EtOAc 2:1). Evaporation of the eluent and washing with pentane affords
the product as a green precipitate (83%). 1H NMR (500 MHz, CDCl3):
d=19.31 (s, 1H), 7.95 (brs, 2H), 7.43 (t, J=7.4 Hz, 1H), 7.12 (t, J=
7.4 Hz, 2H), 7.04 (s, 2H), 6.91 (brs, 1H), 6.06 (s, 1H), 4.89–0.67 (m, 3H),
4.29–3.62 (m, 4H), 3.60–2.94 (m, 4H), 2.94–2.82 (m, 9H), 2.65 (s, 3H),
2.18–1.64 (m, 3H), 2.11 (s, 3H), 0.38 ppm (t, J=7.3 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d=297.9 (m), 222.4, 187.6, 151.3, 139.8, 138.7, 137.8,
137.2, 135.1, 130.4, 129.8, 129.5, 129.3, 129.0, 128.7, 127.9, 116.9, 116.1,
51.6, 51.4, 44.5, 43.6, 21.1, 21.0, 20.1, 18.4 (brs), 16.3, 14.5 ppm; HRMS
(EI): m/z: calcd for C35H42N4Cl4Ru: 760.1207 [M]+·; found: 760.1224.
1,3-Diethyl-4,5-dichloroimidazolinium-silver(I) iodide: Off-white solid;
yield: 98%; 1H NMR (300 MHz, CDCl3): d=4.29 (q, J=7.3 Hz, 4H),
1.41 ppm (t, J=7.3 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=182.7,
115.6, 45.2, 15.4 ppm.
1-Isopropyl-3-methylimidazolinium-silver(I) iodide: Off-white glue; yield:
80%; 1H NMR (300 MHz, CDCl3): d=7.53 (s, 1H), 7.40 (s, 1H), 4.90–
4.70 (m, 1H), 3.82 (s, 3H), 1.43 ppm (d, J=6.6 Hz, 6H); 13C NMR
(75 MHz, CDCl3): d=180.3, 122.8, 118.3, 52.9, 38.3, 23.4 ppm.
General procedure for the synthesis of [(NHC)
5: A dry Schlenk flask containing [(NHC)RuCl
0.18 mmol) and [AgI(NHC)] (0.13 mmol) was evacuated and backfilled
with argon three times. Toluene (5 mL) was added by syringe and the re-
action mixture was stirred at 658C until the reaction was finished (ca. 10–
20 min, TLC). Finally the volatiles were evaporated on a rotavap and the
residue purified by column chromatography.
G
2ACHTUNGTRNE(NUNG CHPh)]
Complex 5 f: Reaction time: 30 min; chromatography: (cyclohexane/
EtOAc 4:1). Evaporation of the eluent and washing with pentane affords
the product as an olive precipitate (90%). 1H NMR (500 MHz, CDCl3):
d=19.40 (s, 1H), 8.97 (brs, 1H), 7.43 (t, J=7.5 Hz, 1H), 7.11 (brs, 2H),
7.06 (s, 1H), 7.04 (s, 1H), 6.96 (s, 1H), 6.80 (brs, 1H), 6.06 (s, 1H), 4.47
(septet, J=7.0 Hz, 1H), 4.10–3.70 (m, 4H), 3.67 (septet, J=7.0 Hz, 1H),
2.82 (s, 3H), 2.65 (s, 3H), 2.51 (s, 3H), 2.37 (s, 3H), 2.14 (s, 3H), 1.79 (s,
3H), 1.45 (d, J=7.0 Hz, 3H), 1.14 (d, J=7.0 Hz, 3H), 0.95 (d, J=7.0 Hz,
3H), 0.33 ppm (d, J=7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=
299.7 (m), 221.7, 189.1, 151.4, 139.1, 138.4, 137.8, 137.5, 137.4, 137.1,
135.6, 131.6 (brs), 130.2, 129.7, 129.7, 129.0, 128.9, 126.9 (brs), 116.4,
115.6, 56.5, 53.7, 51.7, 51.5, 22.0, 21.7, 21.4, 21.0, 21.0, 19.9, 19.8, 19.1,
18.7, 17.9 ppm; HRMS (EI): m/z: calcd for C37H46N4Cl4Ru: ·788.1546
[M]+; found: 788.1503.
CTHUNGTRENNUNG
ACHTUNGTRENNUNG
Complex 5a: Reaction time: 30 min; chromatography: (cyclohexane/
EtOAc 2:1). It is strongly recommended to use a short column and de-
gassed eluent to minimize the decomposition of this complex. The crude
product was dissolved in a minimal amount of CH2Cl2 and added to pen-
tane (40 mL). The solution was cooled to À208C and was next placed in
an ultrasonic bath causing precipitation of a green solid. The product was
collected by decantation of the mother liquor (60%). 1H NMR
(500 MHz, CDCl3): d=19.21 (s, 1H), 7.70 (brs, 2H), 7.41 (t, J=7.5 Hz,
1H), 7.08 (t, J=7.5 Hz, 2H), 7.02 (s, 2H), 7.00–6.00 (brs, 2H), 6.62 (s,
1H), 6.51 (s, 1H), 4.26–3.72 (m, 4H), 3.20 (s, 3H), 3.00–1.40 (brs, 12H),
2.64 (s, 3H), 2.33 (s, 3H), 2.24 ppm (s, 3H); 13C NMR (125 MHz, CDCl3):
d=299.5 (m), 223.8, 186.1, 151.1, 140.1, 140.0, 138.6, 137.6, 137.4, 137.2,
134.9, 129.7, 129.4, 128.5, 127.7, 122.4, 121.9, 51.7, 51.2, 37.2, 36.6, 21.0,
20.9, 20.1, 18.3 ppm (brs); HRMS (EI): m/z: calcd for C33H40N4Cl2Ru:
·664.1628 [M]+; found: 664.1665.
Complex 5g: Two rotamers 1:1.3; reaction time: 30 min; chromatogra-
phy: (cyclohexane/EtOAc 2:1). Evaporation of the eluent and washing
with pentane affords the product as a green precipitate (95%). 1H NMR
(500 MHz, CDCl3): d=19.34 (s, 1H), 19.24 (s, 1H), 7.73 (brs, 1H), 7.48–
7.40 (m, overlapped triplets, 2H), 7.16–7.08 (m, overlapped triplets, 4H),
7.05 (s, 2H), 7.00 (s, 2H), 4.40 (sep, J=6.9 Hz, 1H), 4.20–3.75 (m, 8H),
3.70 (sep, J=7.0 Hz, 1H), 3.21–0.67 (m, 18H), 3.13 (s, 3H), 2.82 (s, 3H),
2.66 (s, 3H), 2.53 (s, 3H), 2.50 (s, 3H), 2.37 (s, 3H), 2.35 (s, 3H), 2.22 (s,
3H), 2.17ACTHNUTRGNE(NUG s, 3H), 1.78 (s, 3H), 0.97 (d, J=6.1 Hz, 3H), 0.44 ppm (d, J=
Complex 5b: Reaction time: 30 min; chromatography: (cyclohexane/
EtOAc 4:1). Evaporation of the eluent and washing with pentane affords
the product as a green precipitate (90%). 1H NMR (500 MHz, CDCl3):
d=19.19 (s, 1H), 7.70 (brs, 2H), 7.45 (t, J=7.5 Hz, 1H), 7.11 (t, J=
7.5 Hz, 2H), 7.04 (s, 2H), 6.68 (brs, 2H), 4.20–3.80 (m, 4H), 3.17 (s, 3H),
6.1 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=303.5 (m), 299.7 (m),
222.4, 222.0, 188.7, 188.6, 151.4, 151.3, 140.4, 140.3, 140.1, 139.3, 139.0,
138.4, 137.9, 137.6, 137.4, 137.3, 137.2, 135.5, 135.1, 130.1, 130.0, 129.9,
129.7, 129.6, 129.2, 129.1, 128.0, 118.6, 118.3, 114.8, 114.2, 56.6, 54.2, 51.8,
Chem. Eur. J. 2010, 16, 3983 – 3993
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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H. Plenio et al.
51.8, 51.7, 51.3, 35.6, 34.8, 22.1, 21.9, 21.2, 20.2, 20.1, 20.0, 19.7, 18.8, 18.4,
18.0 ppm; HRMS (EI): m/z: calcd for C35H42N4Cl4Ru: ·760.1207 [M]+;
found: 760.1241.
ditions); attempts to separate the product by column chromatography
failed and mainly led to decomposition. It was decided to use impure
product for the synthesis of the corresponding silver complex 10. This
complex was prepared according to a standard procedure and used with-
out purification for the synthesis of the corresponding ruthenium com-
plex, which explains the low product yield.
Complex 5h: Two rotamers 1:2.3; reaction time: 15 min; chromatogra-
phy: (cyclohexane/EtOAc 4:1). Evaporation of the eluent and washing
1
with pentane affords the product as a brown precipitate (88%); H NMR
(500 MHz, CDCl3): d=19.25 (s, 1H), 19.24 (s, 1H), 9.00–6.00 (m, 5H),
7.80 (s, 1H), 7.63 (s, 2H), 7.48–7.43 (m (overlapping triplets), 2H), 7.15–
7.09 (m (overlapping triplets), 4H), 7.06 (brs, 4H), 6.99 (brs, 1H), 6.16
(brs, 1H), 4.33 (septet, J=7.0 Hz, 1H), 4.20–3.80 (m, 8H), 3.51 (septet,
J=7.0 Hz, 1H), 3.50 (s, 3H), 2.85 (s, 3H), 2.81 (brs, 6H), 2.70 (brs,
12H), 2.50 (brs, 6H), 2.38 (s, 3H), 2.37 (s, 3H), 2.25 (s, 3H), 2.18 (s, 3H),
1.78 (brs, 3H), 1.23 (brs, 3H), 0.88 (brs, 3H), 0.33 ppm (brs, 3H);
13C NMR (125 MHz, CDCl3): d=301.5 (m), 221.3, 220.8, 197.8, 196.2,
151.2, 151.0, 140.8, 140.1, 139.8, 139.4, 139.2, 138.4, 138.0, 137.9, 137.3,
137.1, 135.2, 134.6, 130.5, 130.4, 130.0, 129.8, 129.6, 129.4, 129.0, 127.9,
121.6, 121.0, 54.4, 52.1, 51.7, 51.6, 51.5, 51.2, 37.0, 36.9, 24.8, 21.7, 21.1,
21.0, 20.0, 19.9, 18.8, 18.6, 18.2, 17.8 ppm; HRMS (EI): m/z: calcd for
C35H43N5O2Cl2Ru: 737.1948 [M]+; found: 737.1829.
[(FL-NHCewg
Schlenk flask containing [(NHC)RuCl
and [Ag(FL-NHC)] (10) was evacuated and backfilled with argon three
)
(NHC)RuCl
R
A dry
AHCTUNGTRENNUNG
times. Toluene (4 mL) was added by a syringe and the reaction mixture
was stirred at 658C for 1 h. Then, the solvent was evaporated with a rota-
vap and the residue purified by chromatography (silica, cyclohexane/
EtOAc 2:1) under an argon atmosphere (the solution of the complex is
sensitive to air). Evaporation of the solvent gave the pure product as a
1
green solid (43 mg, 49%). H NMR (500 MHz, CDCl3): d=19.32 (s, 1H),
19.13 (s, 1H), 8.54 (d, J=8.5 Hz, 1H), 8.49 (d, J=8.5 Hz, 1H), 8.33 (d,
J=8.5 Hz, 1H), 8.23 (d, J=8.5 Hz, 1H), 8.13 (d, J=8.5 Hz, 1H), 8.08 (d,
J=8.5 Hz, 1H), 7.93 (brs, 2H), 7.60–6.40 (m, 22H), 5.39 (t, J=5.5 Hz,
1H), 5.30 (t, J=5.5 Hz, 1H), 4.15–3.80 (m, 8H), 3.20–1.10 ppm (m, 66H).
13C NMR (125 MHz, CDCl3): d=300.2, 300.1, 221.5, 220.4, 188.6, 186.9,
151.7, 151.6, 151.1, 150.8, 140.0, 139.5, 138.9, 138.5, 138.0, 137.9, 137.3,
136.9, 136.8, 135.1, 134.9, 134.8, 130.2, 129.9, 129.8, 129.8, 129.7, 129.5,
129.5, 129.4, 129.0, 128.6, 128.6, 128.3, 128.1, 128.0, 127.7, 127.5, 126.4,
122.9, 119.6, 119.5, 117.5, 115.8, 115.2, 51.7, 51.2, 51.1, 48.2, 45.4, 45.3,
40.6, 39.2, 35.0, 34.1, 31.2, 30.0, 29.6, 29.2, 27.0, 26.9, 25.7, 20.9, 20.8, 19.9,
19.8, 18.1 ppm; MS (ESI): m/z: calcd for C47H54Cl3N6O2SRu: 975.2 [M+
Synthesis of a NHCewg fluorophore-tagged [(FL-NHCewg)ACTHUNGTERN(NUG NHC)RuCl2-
ACHTUNGTRENNUNG
1-(3-aminopropyl)-4,5-dichloroimidazole (tert-butoxycarbonyl (Boc)-pro-
tected) 6: 3-Bromopropylamine was protected with a Boc group accord-
ing to a literature procedure.[33] The corresponding imidazole was synthe-
sized according to a modified literature procedure.[34] 4,5-Dichloroimida-
zole (0.93 g, 6.8 mmol, 1 equiv) in THF (10 mL) was added to a suspen-
sion of NaH (0.27 g, 6.8 mmol, 1 equiv) in dry THF (50 mL) dropwise at
08C and left warming to ambient temperature. Then, 3-bromopropyla-
mine (Boc-protected, 1.62 g, 6.8 mmol, 1 equiv) in THF (10 mL) was
added and stirred at 608C overnight. NaBr was filtered off and the resi-
due was purified by column chromatography (CHCl3/MeOH/Et3N
95:4:1). Evaporation of the solvent affords the product as a yellowish vis-
·
ÀCl]; found: 975.3.
Synthesis of NHC fluorophore-tagged [(FL-NHC)RuCl2ACTHNUTRGNNEG(U CHPh)ACHTUNGTRENNUNG(PCy3)]
(13): A heat-gun dried Schlenk flask containing Grubbs I generation
complex (300 mg, 0.36 mmol) and the dansyl-tagged NHC salt[30a] 12
(500 mg, 0.50 mmol) was evacuated and back-filled with argon three
times. THF (6 mL) was added by syringe and the mixture was heated to
458C. Next, potassium tert-amylate solution (0.8 mL, 1.7m in toluene)
was added to the suspension and stirred at the same temperature for 2 h.
The solvent was removed in vacuo and the residue purified by column
chromatography (cyclohexane/EtOAc 1:1) to give 13 as a red-brown film
(180 mg, 34%). 1H NMR (300 MHz, CDCl3): d=19.10, 8.56 (m, 2H),
8.47 (t, J=6.5 Hz, 2H), 8.18 (m, 2H), 7.56–7.49 (m, 4H), 7.28–7.18 (m,
3H), 7.09–6.90 (m, 5H), 6.90 (m, 1H), 6.35 (brs, 2H), 4.04–3.75 (m, 4H),
3.38 (s, 2H), 3.19 (m, 8H), 2.95 (s, 2H), 2.89 (s, 6H), 2.88 (s, 6H), 2.65–
2.03 (m, 23H), 1.50–0.70 ppm (m 30H); 13C NMR (125 MHz, CDCl3):
very broad resonances from rotamers.
1
cous oil (1.28 g, 64%). H NMR (300 MHz, CDCl3): d=7.51 (s, 1H), 4.65
(brs, 1H), 3.98 (t, J=7.2 Hz, 2H), 3.16 (brs, 2H), 1.96 (quintet, J=
7.2 Hz, 2H), 1.44 ppm (s, 9H); 13C NMR (75 MHz, CDCl3): d=156.1,
126.1, 79.8, 134.4, 43.8, 37.4, 30.8, 28.3 ppm.
Synthesis of imidazolium salt 7: Methyl iodide (3 mL) was added to the
imidazole (1.3 g, 4.3 mmol, 1 equiv) and the formed solution was stirred
at 408C for 1 day in a closed vessel. Addition of diethyl ether precipitat-
ed the off-white product, which was filtered off (1.56 g, 83%). 1H NMR
(300 MHz, CDCl3): d=10.60 (s, 1H), 5.48 (brs, 1H), 4.39 (t, J=6.6 Hz,
2H), 4.03 (s, 3H), 3.35–3.15 (m, 2H), 2.24 (quintet, J=7.2 Hz, 2H),
1.39 ppm (s, 9H); 13C NMR (75 MHz, CDCl3): d=156.2, 137.1, 119.9,
119.0, 79.4, 47.0, 37.0, 35.8, 28.5, 28.3 ppm.
Synthesis of fluorophore-tagged [(FL-NHC)
Pyridine (0.5 mL) was added to [(FL-NHC)RuCl
A
(CHPh)] (15):
(PCy3)] (13)
C
ACHTUNGTRENNUNG
(170 mg) in a Schlenk tube. The solution turned from green to brown
rapidly within a few minutes. Cold pentane was added after 5 min to pre-
Deprotection of 7: HCl (4m) in dioxane (5 mL) was added to a solution
of the imidazolium salt 7 (436 mg, 1 mmol) in CH2Cl2 (4 mL) and stirring
was continued at RT for 15 min. Addition of diethyl ether precipitated
the off-white product (8, 269 mg, 96%), which was filtered off. 1H NMR
(300 MHz, [D6]DMSO): d=9.83 (s, 1H), 8.41 (brs, 3H), 4.41 (t, J=
6.6 Hz, 2H), 3.83 (s, 3H), 2.89 (m, 2H), 2.16 ppm (t, J=6.6 Hz, 2H);
13C NMR (75 MHz, [D6]DMSO): d=136.7, 119.4, 118.0, 45.5, 35.2, 35.0,
26.0 ppm.
cipitate green solid. The supernatant was decanted and [(FL-
a
NHC)RuCl CHTUNTGERN(NUNG CHPh)(py)2] (14) dried in vacuo (80 mg, 49%). This com-
2A
plex was used directly for the reaction with 1,3-dimethylimidazolinium-
AgI according to the general procedure. Purification by column chroma-
tography (cyclohexane/EtOAc 1:1) gave
(NHC)RuCl2A
(CHPh)]. Yield: (20 mg, 25%). 1H NMR (500 MHz,
CDCl3): d=19.09 (s, 1H), 8.56 (m, 2H), 8.47(m, 2H), 8.19 (m, 2H), 7.59
a green solid [(FL-NHC)-
A
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
Introduction of the dansyl fluorophore 9: The double salt 8 (233 mg,
0.83 mmol, 1 equiv) was dissolved in N,N’-diisopropylethylamine (0.4 mL,
2.49 mmol, 3 equiv) and a suspension of dansylchloride (5 mL, 224 mg,
0.83 mmol, 1 equiv) in MeOH (5 mL) was added. After the mixture had
been stirred for 2 h at RT, the solvent was removed on a rotavap and
water (100 mL) was added to the residue and then extracted with CH2Cl2
(2ꢃ10 mL). The organic phase was washed with water and dried over
(m, 2H), 7.56–7.52 (m, 4H), 7.39 (m, 1H), 7.18 (t, J=6.5 Hz, 2H), 7.05
(s, 2H), 7.01 (t, J=6.5 Hz, 2H), 6.72 (brs, 2H), 4.03 (m, 2H), 3.89 (m,
2H), 3.41 (s, 2H), 3.33 (s, 2H), 3.23 (brs, 8H), 3.08 (s, 3H), 2.88 (m,
12H), 2.70–2.40 (m, 17H), 2.20 ppm (brs, 6H); 13C NMR (125 MHz,
CDCl3): d=303.5, 222.1, 187.4, 151.7, 151.0, 140.2, 138.7, 138.5, 137.4,
137.3, 136.4, 132.7, 132.6, 130.6, 130.6, 130.5, 130.1, 129.7, 129.7, 129.3,
128.0, 128.0, 127.9, 123.2, 119.9, 119.8, 116.8, 116.5, 115.2, 115.2, 62.3,
62.2, 52.6, 52.6, 51.7, 51.1, 45.7, 45.4, 35.1, 34.9, 20.1 ppm; MS (ESI): m/z:
calcd for C65H77Cl3N10O4S2Ru: 1334.4 [M+·ÀCl]; found: 1333.7.
MgSO4. Precipitation with ether gave
a light-yellow solid (210 mg).
1H NMR (500 MHz, CDCl3): d=10.06 (s, 1H), 8.45 (d, J=8.5 Hz, 1H),
8.34 (d, J=8.5 Hz, 1H), 8.11 (d, J=7.0 Hz, 1H), 7.53 (t, J=6.0 Hz, 1H;
NH), 7.39 (t, J=8.0 Hz, 2H), 7.08 (d, J=8.0 Hz, 1H), 4.37 (t, J=6.6 Hz,
2H), 3.83 (s, 3H), 3.02 (m, 2H), 2.83 (s, 6H), 2.12 ppm (t, J=6.6 Hz,
2H). 13C NMR (125 MHz, CDCl3): d=151.6, 141.6, 138.2, 130.0, 129.6,
128.8, 128.6, 125.5, 123.1, 121.5, 119.6, 119.5, 115.3, 47.1, 45.4, 39.8, 35.5,
27.8 ppm. The crude product contains ca. 30% of an unidentified impuri-
ty with the dansyl moiety (which also forms under different reaction con-
NHC–pyridine substitution experiments: The appropriate complex
5
(2.5·10À6 mol) was weighed into an NMR tube under argon. The tube was
filled with dried and degassed [D5]pyridine (99.5% deuteration) under
an atmosphere of argon. The exchange of NHC ligands against pyridine
at 313 K was observed by NMR spectroscopy (Bruker ARX 300 MHz)
through the changes in the benzylidene proton. To keep track of the con-
3992
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 3983 – 3993

Efficient Ring-Closing Metathesis Leading to Tetrasubstituted Olefins
FULL PAPER
version of the [(NHC)
sulting in [(NHC)RuCl2A
zylidene resonances and the signal of [D4H1]pyridine were compared at
the beginning and the end of the substitution reaction. For 5b: 80, 5c:
50, 5d: 60, 5e: 95, 5 f: 75, and 5g: 90% of the signal intensity are ob-
served at the end of the reaction.
G
E
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CHTUNGTRENNUNG
Fluorescence experiments: A cuvette under an argon atmosphere was
loaded with 1.5·10À7 (for the blind experiment and pyridine exchange) or
3.75·10À9 mol (norbornene polymerization) of complex 11 or 15 (from a
stock solution in CH2Cl2) and diluted with a solvent (3 mL, CH2Cl2 or
pyridine). The cuvette was placed in the holder, which allows the solution
to be stirred and thermostatted (308C). The dansyl-tagged complex was
added to a norbornene solution in CH2Cl2 (10 mg, 30 mL from CH2Cl2
stock solution). Fluorescence intensities were recorded at 518 nm by
using excitation at 350 nm.
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Acknowledgement
This work was supported by the DFG. We wish to thank Dipl.-Ing. F.
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Received: November 30, 2009
Published online: February 22, 2010
Chem. Eur. J. 2010, 16, 3983 – 3993
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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