Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: Synthesis, structure, and catalytic activity

Article abstract of DOI:10.1002/chem.200800470

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy₃)₂Cl₂Ru=CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ringopening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-l,4-diac_etoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-l,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60%. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail.

Full text of DOI:10.1002/chem.200800470

FULL PAPER
DOI: 10.1002/chem.200800470
Ruthenium-Based Olefin Metathesis Catalysts Coordinated with
Unsymmetrical N-Heterocyclic Carbene Ligands: Synthesis, Structure, and
Catalytic Activity
Georgios C. Vougioukalakis and Robert H. Grubbs*[a]
Abstract: A series of ruthenium-based
olefin metathesis catalysts coordinated
with unsymmetrical N-heterocyclic car-
bene (NHC) ligands has been prepared
and fully characterized. These com-
plexes are readily accessible in one or
two steps from commercially available
[(PCy3)2Cl2Ru=CHPh]. All of the com-
plexes reported herein promote the
ring-closing of diethyldiallyl and dieth-
ylallylmethallyl malonate, the ring-
opening metathesis polymerization of
1,5-cyclooctadiene, and the cross meta-
thesis of allyl benzene with cis-1,4-diac-
etoxy-2-butene, in some cases surpass-
ing in efficiency the existing second-
generation catalysts. Especially in the
cross metathesis of allyl benzene with
cis-1,4-diacetoxy-2-butene, all new cat-
alysts demonstrate similar or higher ac-
tivity than the second-generation
ruthenium catalysts and, most impor-
tantly, afford improved E/Z ratios of
the desired cross-product at conversion
above 60%. The influence of the un-
symmetrical NHC ligands on the initia-
tion rate and the activation parameters
for the irreversible reaction of these
ruthenium complexes with butyl vinyl
ether were also studied. Finally, the
synthesis
of
the
related
chlorodicarbonyl
AHCTREUNG
complexes allowed for the study of the
electronic properties of the new un-
symmetrical NHC ligands that are dis-
cussed in detail.
Keywords: carbene ligands · homo-
geneous catalysis · olefin metathe-
sis · rhodium · ruthenium
Introduction
The development of ruthenium-based metathesis catalysts
with high air and moisture stability and functional-group tol-
erance has led to the use of olefin metathesis, one of the
[1]
À
most powerful carbon carbon bond-forming reactions, in a
variety of applications in organic synthesis and polymer
chemistry.[2] The synthesis of the first well-defined rutheni-
um olefin metathesis initiator (1) was published in 1992.[3]
However, this early catalyst was only effective in the ring-
opening metathesis polymerization (ROMP) of highly
strained olefins. Although the basic structure of the current-
ly used ruthenium-based catalysts still resembles that of the
original complex, comprised of a ruthenium alkylidene, two
halides, and two neutral ligands, contemporary catalysts (2–
5) are much more robust and functional-group tolerant. For
example, the first-generation catalyst 2 has much better
functional group compatibility than all of the early-transi-
tion-metal olefin metathesis initiators.[4] Replacement of one
of the tricyclohexylphosphine ligands with the bulky N-het-
erocyclic carbene (NHC) ligand H2IMes produced rutheni-
um complex 3, which displays improved catalytic activity,
while maintaining the high functional group tolerance and
[a] Dr. G. C. Vougioukalakis, Prof. Dr. R. H. Grubbs
Arnold and Mabel Beckman Laboratory of Chemical Synthesis
Division of Chemistry and Chemical Engineering
California Institute of Technology
1200 East California Blvd, Pasadena, CA 91125 (USA)
Fax : (+1)626-564-9297
E-mail: rhg@caltech.edu
Supporting information for this article is available on the WWW
1
under http://dx.doi.org/10.1002/chem.200800470. It contains H, 13C,
19F, and 31P NMR spectra of catalysts 8–13, plots with additional cata-
lytic evaluation data for 6–13, initiation kinetics plots and tables for
catalysts 6–13, and IR spectra for Rh complexes 29–33.
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thermal stability of 2.[5] Furthermore, substitution of the
phosphine ligands for bidentate carbenes (complexes 4 and
5) led to ruthenium-based metathesis catalysts with even
higher thermal stability.[6] More recent studies have fostered
the development of ruthenium-based catalysts that initiate
asymmetric olefin metathesis reactions,[7] and have led to
metathesis applications in aqueous and protic solvent sys-
tems,[8] and the challenging formation of tetrasubstituted
[9]
À
carbon carbon double bonds.
Unsymmetrically substituted five-membered NHCs,
namely having two different exocyclic substituents adjacent
to the carbenic center, have also been successfully utilized
as ligands in ruthenium metathesis catalysts.[7b–e,10–17] Other
unsymmetrical carbenic frameworks that have been recently
incorporated in ruthenium-based catalysts include a series of
cyclic
N
N
Unsymmetrical NHC frameworks, especially those with an
aliphatic amino side group, were initially used in order to in-
crease the electron-donating ability of the NHCs and, there-
fore, enhance the catalytic activity of the corresponding
complexes.[11,13,16] Additionally, it has been found that the
utilization of unsymmetrical NHCs alters the selectivity in
diastereoselective ring-closing metathesis (RCM) reactions
and the E/Z selectivity in cross-metathesis (CM) reac-
tions.[12,15,17]
thesis of these rhodium(I) complexes was carried out by
transmetalation from the corresponding Ag(I) complexes.
Results and Discussion
Synthesis and characterization of the new ruthenium com-
plexes: The synthesis of carbene precursors 18a–c is
straightforward (Scheme 1), following a modification of pre-
viously reported procedures.[15,20] The appropriate anilines
Building upon our preliminary studies on the develop-
ment of unsymmetrical catalysts 6 and 7, bearing an NHC
that has one mesityl and one 2,6-difluorophenyl group,[15] we
now report the synthesis and catalytic evaluation of unsym-
metrical catalysts 8–13. These catalysts were fully character-
ized, and their catalytic performance was evaluated in
RCM, ROMP, and CM reactions. Some complexes were
found to surpass the commercially available second-genera-
tion catalysts in efficiency. Moreover, in the CM of allyl
benzene with cis-1,4-diacetoxy-2-butene, catalysts 8–13 pro-
vide improved E/Z selectivity of the desired cross-product
at conversions higher than 60%. The initiation rates and the
activation parameters in the irreversible reaction of catalysts
8–13 with butyl vinyl ether were also studied. The rate-limit-
ing step for the initiation of all phosphine-containing cata-
lysts was found to be phosphine dissociation. The kinetics in
the initiation of the phosphine-free catalysts are more com-
plicated; however, the related experiments suggest that
oxygen dissociation is not rate-determining. The electron-
donating ability of the new unsymmetrical NHCs was finally
studied in the rhodium(I) complexes of the general formula
[Rh(CO)2Cl
A
Scheme 1. Synthesis of 4,5-dihydroimidazolium chlorides 18a–c.
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Homogeneous Catalysis
FULL PAPER
were initially treated with oxalyl chloride, ethylchlorooxo-
acetate, or phenylchlorooxoacetate (14a–c) to afford the
corresponding condensation products 15a–c. Upon addition
of the second aniline, oxalamides 16a–c were isolated.
These were reduced with BH3·THF and then treated with
HCl to furnish dihydrochloride salts 17a–c that, upon treat-
ment with triethyl orthoformate, cyclize to imidazolinium
chlorides 18a–c. Complexes 8, 10, and 12 were isolated in
46–80% yield by generation of the free carbene from 18a–c,
in situ with potassium hexamethyldisilazane (KHMDS), and
then reacted with ruthenium source 2 in benzene. Treatment
of 8, 10, and 12 with o-isopropoxy-b-methylstyrene afforded
the phosphine-free analogues 9, 11, and 13 in 57–85% yield.
Compounds 8–13 are air stable in the solid state and can be
purified by silica gel chromatography.
Complexes 8–13 were completely characterized by NMR
spectroscopy and HRMS (see the Supporting Information).
Even at temperatures as low as À708C, the NHCs of com-
plexes 9, 11, and 13 rotate fast on the NMR timescale and,
therefore, only one absorption is observed in the benzyli-
dene region of the 1H NMR spectra (around 16 ppm) for
these phosphine-free complexes. In contrast, phosphine-con-
taining complexes 8, 10, and 12, in solution, are a mixture of
two rotational isomers (ꢀ1:4 for complexes 6, 8, and 12,
and ꢀ2:3 for complex 10), with the major rotamer being
the one with its mesityl ring located above the benzylidene
group.[21] The higher minor/major rotational isomer ratio in
10, relative to that in complexes 6 and 8, may be due to a
more efficient slipped p–p stacking interaction between the
2,3,4,5,6-pentafluorophenyl and the benzylidene group in 10,
compared to the p–p interactions between the 2,6-difluoro-
phenyl or the 2,4,6-trifluorophenyl and the benzylidene
groups in 6 and 8.[22] Single crystals of good quality for X-
ray analysis from complexes 8, 9, and 13 were also ob-
tained.[23] As depicted in Figures 1–3, all complexes exhibit a
distorted square-pyramidal geometry with the Ru=C benzyl-
Figure 2. Crystal structure of catalyst 9. Ellipsoids are drawn at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
À
À
lengths [Š] and angles [8]: Ru(1) C(19) 1.8307(18), Ru(1) C(1)
À
À
À
1.9722(17), Ru(1) O(1) 2.2502(12), Ru(1) Cl(1) 2.3303(5), Ru(1) Cl(2)
2.3420(5), C(19)-Ru(1)-C(1) 101.80(7), C(19)-Ru(1)-O(1) 79.79(6), C(1)-
Ru(1)-O(1) 176.00(6), C(19)-Ru(1)-Cl(1) 103.00(6), C(1)-Ru(1)-Cl(1)
88.83(5), O(1)-Ru(1)-Cl(1) 87.23(4), C(19)-Ru(1)-Cl(2) 97.99(6), C(1)-
Ru(1)-Cl(2) 97.05(5), O(1)-Ru(1)-Cl(2) 86.31(4), Cl(1)-Ru(1)-Cl(2)
156.547(17).
Figure 3. Crystal structure of catalyst 13. Ellipsoids are drawn at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
À
À
lengths [Š] and angles [8]: Ru(1) C(22) 1.8340(9), Ru(1) C(1) 1.9725(9),
À
À
À
Ru(1) O(1) 2.2451(7), Ru(1) Cl(2) 2.3284(3), Ru(1) Cl(1) 2.3393(2),
C(22)-Ru(1)-C(1) 101.58(4), C(22)-Ru(1)-O(1) 79.87(3), C(1)-Ru(1)-
O(1) 177.42(3), C(22)-Ru(1)-Cl(2) 104.07(3), C(1)-Ru(1)-Cl(2) 89.08(3),
O(1)-Ru(1)-Cl(2) 88.49(2), C(22)-Ru(1)-Cl(1) 96.45(3), C(1)-Ru(1)-Cl(1)
96.34(3), O(1)-Ru(1)-Cl(1) 85.59(2), Cl(2)-Ru(1)-Cl(1) 157.287(9).
idene bond occupying the apical position and the Cl atoms
trans to one another. This geometry, as well as the bond
lengths and angles in 8, 9, and 13, are quite similar to those
of parent complexes 3 and 5. Moreover, in the solid state,
the mesityl group in complexes 8, 9, and 11[23] is located
above the benzylidene moiety, as has been also observed in
complexes 6 and 7.[15]
Figure 1. Crystal structure of catalyst 8. Ellipsoids are drawn at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
À
À
lengths [Š] and angles [8]: Ru(1) C(19) 1.826(2), Ru(1) C(1) 2.0602(18),
À
À
À
Ru(1) Cl(1) 2.3878(5), Ru(1) Cl(2) 2.3927(4), Ru(1) P(1) 2.4403(5),
C(19)-Ru(1)-C(1) 97.61(8), C(19)-Ru(1)-Cl(1) 90.17(6), C(1)-Ru(1)-Cl(1)
93.25(5), C(19)-Ru(1)-Cl(2) 106.08(6), C(1)-Ru(1)-Cl(2) 83.33(5), Cl(1)-
Ru(1)-Cl(2) 163.679(17), C(19)-Ru(1)-P(1) 96.29(6), C(1)-Ru(1)-P(1)
165.65(5), Cl(1)-Ru(1)-P(1) 90.246(16), Cl(2)-Ru(1)-P(1) 89.429(16).
Ring-closing metathesis (RCM) activity of the new catalysts:
RCM is the most frequently utilized olefin metathesis reac-
tion in organic synthesis.[1b,24] We initially chose to study the
catalytic activity of the new complexes in the RCM of di-
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R. H. Grubbs and G. C. Vougioukalakis
ethyldiallyl malonate (19) to disubstituted cycloalkene 20 by
1H NMR spectroscopy (Figure 4). Interestingly, the plots of
cycloalkene 20 conversion versus time reveal that both 6
induction period and an increased decomposition rate, as il-
lustrated from the curvature in the corresponding logarith-
mic plot (Supporting Information). Finally, as clearly illus-
trated in Figure 4, complexes 12 and 13 are very poor olefin
metathesis catalysts. Phosphine-containing catalyst 12 exhib-
its high initial activity, but this activity decreases during the
course of the reaction, due to its high decomposition rate,
leveling off at 59% conversion. In sharp contrast, phos-
phine-free complex 13 has a very long induction period
(11 min to reach 5% conversion), leading the reaction to
>97% conversion in 180 min.
The RCM of diethylallylmethallyl malonate (21, Figure 5)
leads to the formation of a trisubstituted five-membered
Figure 5. RCM of diene 21 with catalysts 3, 5, and 6–11, at 308C.
ring (22). Due to steric effects, this reaction is more chal-
lenging than the RCM of diethyldiallyl malonate (19). Over-
all, the reactivity trends for this more demanding RCM re-
action were found to be similar to those observed for the
RCM of 19. Thus, catalysts 6 and 8 show similar activity,
being once again the most efficient catalysts in this study
and outperforming second-generation catalysts 3 and 5.
Here, the reduced stability of catalyst 10 becomes more evi-
dent. After an initial period of high activity, the reaction
rate slows drastically and eventually complex 10 does not
catalyze this reaction to completion, reaching the final 90%
conversion after 100 min. Complexes 12 and 13 are the most
inefficient catalysts for this transformation in the present
study. Under the standard reaction conditions (308C, 1%
catalyst loading, 0.1m substrate, CD2Cl2), 12 levels off at
30% conversion after 8 min, whereas it takes 13 h for com-
plex 13 to lead the reaction to 95% conversion (Supporting
Information).
Figure 4. RCM of diene 19 with catalysts 3, 5, and 6–13, at 308C.
and 8 effect the cyclization of 19 more efficiently than the
second-generation phosphine-containing catalyst 3, with cat-
alyst 8 being the most efficient of all (>97% conversion in
9min). Complex 10 is also more reactive than 3, at least at
the beginning of the RCM reaction (evidently due to its
high initiation rate), but eventually slows down due to a de-
crease in its catalytic activity. This decrease in catalytic ac-
tivity (decomposition) is also observed in the curvature in
the ln[19] plot versus time for 10 (Supporting Information).
Nevertheless, complex 10 eventually displays >97% conver-
sion in 40 min. On the other hand, phosphine-free catalysts
7, 9, and 11 are less efficient than parent complex 3. In fact,
increasing the number of fluorine atoms on the N-aryl sub-
stituent, in going from complex 7 to 9 and eventually to 11,
results in lower activity, with catalyst 11 being the least effi-
cient of all in this series (>97% conversion in 100 min).
This low efficiency of catalyst 11 arises from its prolonged
The formation of tetrasubstituted double bonds through
RCM is even more challenging and typically requires high
catalyst loadings and elevated reaction temperatures due to
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Homogeneous Catalysis
FULL PAPER
the increased steric bulk of the substrates.[9] In the model
RCM reaction of diethyldimethallyl malonate catalysts 6
and 8 are the most efficient, affording 30% and 21% of the
ring-closed product respectively, after four days at 308C
(Supporting Information). Complexes 11 and 13 do not ring-
close diethyldimethallyl malonate. Catalysts 3 and 5 catalyze
the same reaction in 17% and 6% respectively.
poor E/Z cross-product selectivity.[26] It is also important to
note that in CM, the E/Z selectivity at high conversion is
governed by thermodynamic factors; that is, secondary
metathesis promotes isomerization of the product to the
thermodynamically favored E isomer. The development of
catalysts that could efficiently control E/Z selectivity in CM
reactions still represents a major challenge.[15,19,24,26] In our
present study, we chose to evaluate catalysts 8–13 in the CM
of allyl benzene (25) with cis-1,4-diacetoxy-2-butene (26).
As shown in Figure 7, catalysts 6–11 demonstrate activity
similar to or higher than the second-generation catalysts 3
and 5. While catalysts 3 and 5 lead the reaction to 79% and
72% conversion, respectively, catalysts 9–11 are more effi-
cient reaching a ꢀ88% yield of the desired cross-product
(27). As illustrated in the plots of the E/Z ratio of cross-
product versus conversion to cross-product (Figure 7), cata-
lysts 6–11 are also more Z selective than catalysts 3 and 5,
at conversions above 60%. For example, catalyst 3 affords
an E/Z ratio of ꢀ10 at 79% conversion, whereas at the
same conversion catalysts 6–11 give an E/Z ratio of about
5.5.
Ring-opening metathesis polymerization (ROMP) activity
of the new catalysts: The ROMP of cyclic strained olefins is
one of the earliest commercial application of olefin metathe-
sis.[1,25] As such, we studied the catalytic activity of the new
unsymmetrical catalysts in the ROMP of 1,5-cyclooctadiene
1
(23) by H NMR spectroscopy. The conversion to the prod-
uct polyalkenamer (24) over time with catalysts 3 and 5–11
is represented in Figure 6. Interestingly, the reactivity trends
Figure 6. ROMP of 23 with catalysts 3, 5, and 6–11, at 308C.
observed for the fluorinated phosphine-free catalysts (7, 9,
and 11) in RCM invert in the ROMP of 1,5-cyclooctadiene.
Thus, catalyst 11 is the most reactive fluorinated phosphine-
free catalyst, leading the reaction to >99% conversion in
60 min. In this reaction, complexes 3, 5, and 8 are the most
efficient catalysts, showing very similar behavior with re-
spect to reactivity. Moreover, since the stability of a catalyst
does not have a very important impact in the ROMP of
23,[24] complex 12 is also a competent catalyst in this reaction
(>99% conversion in 13 min, Supporting Information).
Cross-metathesis (CM) activity of the new catalysts: When
compared to RCM and ROMP, CM is certainly an under-
utilized olefin metathesis transformation. The basic reason
lies in the fact that CM is more challenging, as it lacks the
entropic driving force of RCM and the ring-strain release of
ROMP. Additionally, CM reactions often lead to relatively
low statistical yields of the desired cross-product, as well as
Figure 7. CM of 25 with 26 using catalysts 3, 5, and 6–11.
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R. H. Grubbs and G. C. Vougioukalakis
Finally, complexes 12 and 13 are rather poor CM catalysts,
showing lower reactivity than all other catalysts in this
study. Additionally, the E/Z profile of catalysts 12 and 13 is
very similar to that of catalysts 3 and 5 (Supporting Infor-
mation).
The rate constants for all phosphine-containing catalysts
were found to be independent of olefin concentration rela-
tive to [Ru], suggesting rate-determining phosphine dissoci-
ation. The large and positive values for DS° for all phos-
phine-containing catalysts in Table 1 are also indicative of a
dissociative mechanism.[28] Complex 12 has the lowest free
energy of activation, being the fastest initiating catalyst in
the present study. Unfortunately, as mentioned above, the
lack of stability makes this complex a very poor olefin meta-
thesis catalyst. As also illustrated in Table 1, catalysts 6 and
8 have essentially the same free energy of activation, initiat-
ing about 4 times faster than the parent complex 3.
The rate constants in the reactions of the phosphine-free
catalysts with butyl vinyl ether, in sharp contrast to the cor-
responding reactions of the phosphine-containing catalysts,
show a first-order dependence on olefin concentration rela-
tive to [Ru] (Supporting Information). Moreover, the DS°
values for the phosphine-free complexes in Table 1 are both
negative and large, also indicative of an associative mecha-
nism. The values for DH° in the two cases of phosphine-
containing and phosphine-free catalysts are quite different
as well.[30] Catalyst 13 has the highest free energy of activa-
tion in Table 1, being the poorest initiating catalyst in the
present study. As discussed in the previous sections, this was
also observed in the RCM, ROMP, and CM reactivity of
catalyst 13.
Kinetic studies on the initiation of the new catalysts: We
next determined the activation parameters for the initiation
of catalysts 7–13 by utilizing the rapid, quantitative, and ir-
reversible reaction of these complexes with butyl vinyl
ether. This reaction affords a Fischer carbene, generally con-
sidered to be an inactive metathesis species.[27] The initiation
experiments were carried out in [D8]toluene (catalysts 8 and
10–13) or [D6]benzene (catalysts 7 and 9), according to liter-
ature procedures.[28] The reactions between complexes 7–13
and butyl vinyl ether were carried out at four or five differ-
ent temperatures, at 0.15m olefin concentration; the activa-
tion parameters were then extracted from the resulting
Eyring plots (Supporting Information). The obtained activa-
tion parameters are summarized in Table 1 and a represen-
tative Eyring plot is shown in Figure 8. To probe the de-
pendence of the initiation rate on the olefin concentration,
the initiation rates were also measured at 0.45 and 1.35m
olefin concentration at a selected temperature (Supporting
Information).
Studies on the electronic properties of the new unsymmetri-
cal NHCs: Electron-rich and bulky NHC ligands display ex-
cellent s-donating ability similar to electron-rich phosphines.
One of the most straightforward methods to study the elec-
tronic properties of an NHC ligand is through the measure-
ment of the carbonyl stretching frequencies in the corre-
Table 1. Activation parameters for the initiation of catalysts 3, 5, and 6–
13.
Catalyst
DH°
DS°
DG° (303 K)
[kcalmolÀ1
]
[calmolÀ1 KÀ1
]
[kcalmolÀ1
]
3[a]
6[b]
8
10
5[c]
7
27Æ2
27Æ2
+13Æ6
23.0Æ0.4
+19Æ921.78
+16Æ5
Æ0.08
21.67Æ0.03
Æ0.07
sponding [Rh(CO)2X
(NHC)] complexes.[31] Since the CO
A
26Æ2
26Æ3
+15Æ922.05
À19Æ3
stretching frequencies (wavenumbers) in these complexes
are inversely proportional to the p-back-donation from the
metal center, the more basic the NHC the more important
the p-back-donation from the metal and, therefore, the
lower the stretching frequency of the CO ligands.
15.2Æ0.8
12Æ2
20.69Æ0.02
21.94Æ0.06
22.09Æ0.08
21.62Æ0.04
17.9Æ0.2
À32Æ5
9
14Æ2
À26Æ7
11
12
13
16Æ1
À20Æ3
38Æ2
+66Æ8
17.6Æ0.7
À22Æ2
24.07Æ0.07
Chlorodicarbonyl(carbene)rhodium(I) complexes 29–33
T
[a] Reference[28]. [b] Reference [15]. [c] Reference [29].
were easily prepared by transmetalation from the corre-
sponding AgI complexes. For the preparation of 29, 30, and
32 we used [{Rh(CO)2Cl}2] as rhodium source, whereas in
the case of 31 and 33 we used [{Rh(cod)Cl}2] (cod=cyclo-
G
octadiene). Complex 32, bearing a symmetrical NHC with
o-fluorinated aryl groups, was synthesized for comparison
reasons.[32] FT-IR spectroscopy of complexes 29–33 was car-
ried out in CH2Cl2. The measured CO stretching frequencies
I and II (Table 2) indicate that the parent ligand (H2IMes)
has the highest s-donating ability in this series of ligands. In
contrast, the ligand that induces the lowest electron density
at the metal center is the tetrafluorinated symmetrical NHC
in 32, with the unsymmetrical N-pentafluorophenyl NHC in
31 being the second least basic ligand. Finally, the NHCs in
Rh complexes 29, 30, and 33 have identical s-donor and p-
acceptor properties. Overall, there is a shift of the CO
stretching frequencies in Rh complexes 28–33 as a function
Figure 8. Eyring plot for the reaction of catalyst 9 with butyl vinyl ether.
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Homogeneous Catalysis
FULL PAPER
tiation of the phosphine-containing catalysts by the well-es-
tablished dissociative mechanism. In the case of the phos-
phine-free catalysts the initiation mechanism is most proba-
bly associative. Finally, the synthesis of the related
[Rh(CO)2Cl(NHC)] complexes allowed the study of the
A
electronic properties of the new unsymmetrical NHC li-
gands. It was found that although there is a shift of the CO
stretching frequencies in these Rh complexes as a function
of the number of fluorine atoms on the N-aryl substituents
of the NHC, there is no correlation between the electronic
properties of the NHCs and the efficiency of the corre-
sponding ruthenium catalysts for the present series of cata-
lysts.
Experimental Section
Materials and general procedures: Unless otherwise indicated, all com-
pounds were purchased from Aldrich or Fisher. Catalyst 2 was obtained
from Materia. Silica gel used for the purification of organometallic com-
plexes was obtained from TSI Scientific, Cambridge, MA (60 Š, pH 6.5–
7.0). Oxomesitylacetyl chloride was prepared by a method reported by
Mol and co-workers.[11] All reactions involving metal complexes were
conducted in oven-dried glassware under an argon or nitrogen atmos-
phere with anhydrous solvents, by using standard Schlenk and glovebox
techniques. Anhydrous solvents were obtained by elution through a sol-
vent column drying system.[34] The screening of the catalysts, in ring-clos-
ing metathesis, cross metathesis, and ring-opening metathesis polymeri-
zation reactions was conducted according to literature procedures.[24]
NMR spectra were measured on Varian Inova 500 and Varian Mercury
300 spectrometers. NMR chemical shifts are reported in ppm downfield
from Me4Si, by using the residual solvent peak as internal standard for
1H and 13C, and H3PO4 (d=0.0 ppm) for 31P. Gas chromatography data
was obtained using an Agilent 6850 FID gas chromatograph equipped
with a DB-Wax Polyethylene Glycol capillary column (J&W Scientific).
IR spectra were recorded on a Perkin-Elmer Paragon 1000 spectropho-
tometer. X-ray crystallographic structures were obtained by Larry M.
Henling and Dr. Michael W. Day of the California Institute of Technolo-
gy Beckman Institute X-ray Crystallography Laboratory. CCDC-660407
(8), CCDC-660408 (9) and CCDC-627196 (13) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Table 2. Carbonyl stretching frequencies for rhodium complexes 28–33.
Compound
nCO I [cmÀ1
]
nCO II [cmÀ1
]
28[a]
29
30
31
32
2081
2081
2081
2083
2085
2081
1996
2000
2000
2005
2005
2000
33
[a] Reference [31a].
of the number of fluorine atoms on the N-aryl sybstituents
of the NHC. However, with the exception of the phosphine-
free catalysts in RCM reactions, there is no evident correla-
tion between the electronic properties of the NHCs and the
efficiency of the corresponding ruthenium catalysts for the
present series of catalysts.[33]
N-(2,4,6-Trifluorophenyl)oxanilic acid ethyl ester (15a): 2,4,6-Trifluoroa-
niline (2.94 g, 20.0 mmol, 1.0 equiv) and dry triethylamine (2.79 mL,
20.0 mmol, 1.0 equiv) were dissolved in dry THF (40 mL) under nitrogen.
This solution was cooled to 08C, and ethyl chlorooxoacetate (2.22 mL,
20.0 mmol, 1.0 equiv) was added dropwise. Precipitation of a white solid
(triethylammonium chloride) occurred immediately upon addition. The
suspension was allowed to stir for 16 h, warming to room temperature.
The solid was filtered off, washed with diethyl ether (50 mL), and the
combined organic layer was washed with an aqueous saturated NH4Cl so-
lution until pH 6. This organic layer was then washed with brine (80 mL)
and dried over MgSO4. The solvent was removed under reduced pressure,
leaving a yellow solid that was washed with hexanes (3”6 mL) to afford
15a as a white crystalline solid (3.31 g, 13.4 mmol, 67% yield). 1H NMR
(300 MHz, CDCl3, 258C): d=8.31 (brs, 1H), 6.81–6.76 (m, 2H), 4.43 (q,
Conclusion
A series of ruthenium-based metathesis catalysts bearing un-
symmetrical NHC ligands has been prepared and completely
characterized. All of these complexes are competent cata-
lysts for ring-closing metathesis, cross metathesis, and ring-
opening metathesis polymerization, in some cases surpassing
in efficiency the existing second-generation catalysts. In the
cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-
butene, all new catalysts afford improved E/Z ratios of the
desired cross-product at conversions above 60%. The influ-
ence of the unsymmetrical NHC ligands on the initiation
rate and the activation parameters in the reaction of the cor-
responding ruthenium complexes with butyl vinyl ether was
also studied. The results from these experiments suggest ini-
3J
A
N
(75 MHz, CDCl3, 258C; due to extensive fluorine coupling, coupling con-
stants are not given and resonances are reported as peaks): d=163.47–
163.08 (m), 160.02–159.69 (m), 156.62–156.32 (m), 154.90, 101.50–100.78
(m), 64.11, 14.16 ppm; 19F{1H} NMR (282 MHz, CDCl3, 258C): d=
À107.80, À113.93 ppm; HRMS (FAB+): m/z calcd for C10H9NO3F3 [M+]:
248.0535; found: 248.0536.
Chem. Eur. J. 2008, 14, 7545 – 7556
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7551

R. H. Grubbs and G. C. Vougioukalakis
N-(2,4,6-Trifluorophenyl)-N’-mesityloxalamide (16a): Compound 15a
(2.47 g, 10.0 mmol, 1.0 equiv) was suspended in 2,4,6-trimethylaniline
(2.53 mL, 18.0 mmol, 1.8 equiv) in a dry Schlenk tube under nitrogen.
The tube was sealed and the suspension was stirred at 1808C for 16 h.
Upon being cooled to room temperature, the reaction mixture solidified.
The orange-brown solid was washed with diethyl ether (4”5 mL) and
hexanes (2”5 mL), leaving 16a as a white solid (1.41 g, 4.2 mmol, 42%
yield). 1H NMR (300 MHz, CDCl3, 258C): d=9.07 (brs, 1H), 8.81 (brs,
1H), 6.90 (s, 2H), 6.79–6.74 (m, 2H), 2.29 (s, 3H), 2.18 ppm (s, 6H);
13C{1H} NMR (75 MHz, CDCl3, 258C; due to extensive fluorine coupling,
coupling constants are not given and resonances are reported as peaks):
d=159.91–159.62 (m), 158.74, 157.41, 156.55–156.25 (m), 137.94, 134.93,
129.58, 129.26, 101.44–100.71 (m), 21.17, 18.52 ppm; 19F{1H} NMR
(282 MHz, CDCl3, 258C): d=À108.18, À113.65 ppm; HRMS (FAB+):
m/z calcd for C17H16N2O2F3 [M+]: 337.1164; found: 337.1164.
to extensive fluorine coupling and the existence of two rotational iso-
mers, coupling constants are not given and resonances are reported as
peaks): d=297.57 (major), 293.43 (minor), 223.44, 222.80, 164.00–163.84
(m), 162.94–162.76 (m), 160.89–160.69 (m), 155.69–155.42 (m), 152.31–
152.21 (m), 152.02–151.86 (m), 151.21, 142.89–142.75 (m), 138.87, 138.24,
136.90–136.77 (m), 136.59, 135.09, 131.02–130.50 (m), 130.00, 129.18,
128.91, 128.53, 128.02–127.83 (m), 101.87–101.44 (m), 101.03–100.60 (m),
53.10–53.05 (m), 52.76, 52.22, 52.20, 32.15, 32.02, 31.96, 31.83, 29.26,
28.00, 27.92, 27.84, 26.39, 26.36, 21.07, 20.86, 19.68, 18.28 ppm; 31P{1H}
NMR (121 MHz, CD2Cl2, 258C) d=31.94 (s, minor), 27.33 ppm (s,
major); 19F{1H} NMR (282 MHz, CD2Cl2, 258C): d=À105.01 (d, major,
J
FF =18 Hz), À106.15 (brs, major), À108.25 (d, minor,
JFF =18 Hz),
À114.02 ppm (brs, minor); HRMS (FAB+): m/z calcd for
C43H56N2F3Cl2PRu [M+]: 860.2554; found: 860.2536.
[RuCl2(1-(2,4,6-trifluorophenyl)-3-mesityl-4,5-dihydroimidazol-2-yli-
À
dene)(=CH-o-iPrO Ph)] (9): In a glovebox, a vial was charged with
complex 8 (172 mg, 200 mmol, 1.0 equiv), toluene (4 mL), and o-isopro-
poxy-b-methylstyrene (175 mg, 4.0 mmol, 20.0 equiv). The dark red solu-
tion was stirred for 10 min and then left inside the capped vial without
stirring at room temperature. After 48 h the desired complex had precipi-
tated as dark green crystals. The supernatant brown liquid was decanted
off; the crystals were washed with pentanes (3”10 mL) and dried in
vacuo to afford complex 9 (109mg, 171 mmol, 85% yield). Crystals suita-
ble for X-ray crystallography were grown at room temperature by slow
diffusion of hexanes into a solution of 9 in benzene. 1H NMR (500 MHz,
CD2Cl2, 258C): d=16.10 (s, 1H), 7.56–7.53 (m, 2H), 7.12 (s, 2H), 6.94–
N-(2,4,6-Trifluorophenyl)-N’-mesityl-1,2-ethanediamine dihydrochloride
(17a): In a dry, high pressure tube containing 16a (1.68 g, 5.0 mmol,
1.0 equiv), BH3·THF (1m in THF) (25 mL, 25.0 mmol, 5.0 equiv) was
added under nitrogen. The tube was sealed and the solution was stirred
at 708C for 16 h. Once the reaction mixture had cooled to room tempera-
ture, the clear yellowish solution was slowly added to methanol (50 mL)
at 08C. Concentrated aqueous HCl solution (1.8 mL) was also slowly
added at 08C. When all bubbling ceased, the solvent was removed under
reduced pressure. The resulting solid was dissolved in methanol and the
solvent was again removed under reduced pressure. This was repeated
twice more to remove the remaining boron as B(OMe)3. The remaining
A
6.89(m, 4H), 5.00 (septet, 3J
(s, 3H), 2.30 (s, 6H), 1.36 ppm (d, 3J
ACHTREUNG
white solid was finally washed with diethyl ether (2”5 mL) to provide
the desired product as a white powder (1.53 g , 4.0 mmol, 80% yield).
1H NMR (300 MHz, [D6]DMSO, 258C): d=7.11–7.05 (m, 2H), 6.96 (s,
2H), 3.59–3.33 (m, 4H), 2.48 (s, 6H), 2.37 ppm (s, 3H); 13C{1H} NMR
(75 MHz, [D6]DMSO, 258C; due to extensive fluorine coupling, coupling
constants are not given and resonances are reported as peaks): d=
152.51–152.34 (m), 138.26, 131.82, 130.31, 124.95–124.60 (m), 116.97–
116.64 (m), 111.86–111.43 (m), 50.57, 41.32, 20.16, 17.57 ppm; 19F{1H}
NMR (300 MHz, [D6]DMSO, 258C): d=À123.19, À125.29ppm; HRMS
(FAB+): m/z calcd for C17H20N2F3 [M+]: 309.1579; found: 309.1587.
AHCTREUNG
(125 MHz, CD2Cl2, 258C; due to extensive fluorine coupling, coupling
constants are not given and resonances are reported as peaks): d=
292.19, 215.59, 164.56–164.32 (m), 163.31–163.14 (m), 163.31–163.14 (m),
162.43–162.32 (m), 161.27–161.10 (m), 152.45, 144.53, 139.46, 137.93,
137.50, 129.91, 129.85, 122.61, 122.51, 113.11, 101.45–101.04 (m), 75.50,
53.15, 51.84, 21.29, 21.16, 17.96 ppm; 19F{1H} NMR (282 MHz, CD2Cl2,
258C): d=À103.97, À106.17 ppm; HRMS (FAB+): m/z calcd for
C28H29N2OF3Cl2Ru [M+]: 638.0653; found: 638.0658.
N-(2,3,4,5,6-Pentafluorophenyl)-N’-mesityloxalamide (16b): Compound
15b (1.35 g, 6.0 mmol, 1.0 equiv) was added to a solution of 2,3,4,5,6-pen-
tafluoroaniline (2.42 g, 13.2 mmol, 2.2 equiv) in dry THF (60 mL) under
nitrogen. After stirring the reaction mixture at room temperature for 2 h,
the solvent was evaporated under reduced pressure. The remaining solid
was washed with hexanes (3”10 mL) and dried under high vacuum to
afford the desired compound as a white solid (2.12 g, 5.7 mmol, 95%
yield). 1H NMR (300 MHz, CDCl3, 258C): d=9.14 (brs, 1H), 8.68 (brs,
1H), 6.92 (s, 2H), 2.30 (s, 3H), 2.20 ppm (s, 6H); 13C{1H} NMR (75 MHz,
CDCl3, 258C; due to extensive fluorine coupling, coupling constants are
not given and resonances are reported as peaks): d=158.68, 157.12,
144.83–144.44 (m), 142.61–142.14 (m), 141.48–141.19 (m), 139.85–139.41
(m), 139.23–138.75 (m), 138.10, 136.55–136.06 (m), 134.80, 129.30, 129.12,
111.07–110.52 (m), 21,08, 18.34 ppm; 19F{1H} NMR (282 MHz, CDCl3,
1-(2,4,6-Trifluorophenyl)-3-mesityl-4,5-dihydroimidazolium
chloride
(18a): A suspension of 17a (1.53 g, 4.0 mmol, 1.0 equiv) in triethylortho-
formate (13.3 mL, 80.0 mmol, 20.0 equiv) was heated at 1358C for 30 min
under nitrogen. Upon cooling to room temperature, the solids were fil-
tered of and washed with diethyl ether (3”10 mL) to provide the desired
product as
a white solid (1.28 g ,
3.6 mmol, 90% yield). 1H NMR
(300 MHz, [D6]DMSO, 258C): d=9.59 (s, 1H), 7.64–7.58 (m, 2H), 7.07
(s, 2H), 4.58–4.64 (m, 4H), 2.30 (s, 6H), 2.26 ppm (s, 3H); 13C{1H} NMR
(75 MHz, [D6]DMSO, 258C; due to extensive fluorine coupling, coupling
constants are not given and resonances are reported as peaks): d=
162.15–161.19(m), 140.54, 135.78, 131.32, 130.17, 124.79–124.79(m),
111.93–111.39 (m), 103.15–102.37 (m), 52.20, 52.06, 21.24, 17.78 ppm;
19F{1H} NMR (282 MHz, [D6]DMSO, 258C): d=À105.45, À116.29ppm;
HRMS (FAB+): m/z calcd for C18H18N2F3 [M+]: 319.1422; found:
319.1421.
258C): d=À143.95 (d,
J
C
À161.85 ppm (t,
A
A
C17H13N2O2F5 [M+]: 372.0897; found: 372.0893.
À
N
N
N-(2,3,4,5,6-Pentafluorophenyl)-N’-mesityl-1,2-ethanediamine
dihydro-
and KHMDS (337 mg, 1.0 mmol, 2.0 equiv) were stirred in benzene
(10 mL) at room temperature for 30 min. Catalyst 2 (411 mg, 500 mmol,
1.0 equiv) was added as a solid in one portion, and the reaction flask was
taken out of the glove box and heated under a nitrogen atmosphere at
808C for 30 min. The solution was concentrated to 2 mL in vacuo and
poured onto a column packed with TSI Scientific silica gel. The complex
was eluted with hexanes/diethyl ether (2:1) as a red band. This was con-
centrated in vacuo, transferred in a glove box, dissolved in the minimum
amount of benzene and lyophilized to afford the desired complex as a
violet solid (345 mg, 401 mmol, 80% yield). Crystals suitable for X-ray
crystallography were grown at room temperature by slow diffusion of
hexanes into a solution of 8 in benzene. 1H NMR (500 MHz, CD2Cl2,
258C): d=19.48 (s, 1H-minor), 19.12 (s, 1H-major), 7.47–6.88 (m, 9H-
major, 9H-minor), 4.10–3.90 (m, 4H-major, 4H-minor), 2.62–1.00 ppm
(m, 42H-major, 42H-minor); 13C{1H} NMR (125 MHz, CD2Cl2, 258C; due
chloride (17b): This was synthesized analogously to 17a starting with 16b
(white powder, 60% isolated yield). 1H NMR (300 MHz, [D6]DMSO,
258C): d=6.95 (s, 2H), 3.76 (t, 3J
N
N
6 Hz, 2H), 2.44 (s, 6H), 2.20 ppm (s, 3H); 13C{1H} NMR (75 MHz,
[D6]DMSO, 258C; due to extensive fluorine coupling, coupling constants
are not given and resonances are reported as peaks): d=139.92–139.63
(m), 139.50–139.18 (m), 138.99, 136.89–136.01 (m), 134.25–133.88 (m),
132.60, 131.39, 130.99, 124.46–124.12 (m), 50.94, 41.81, 20.81, 18.39 ppm;
19F{1H} NMR (282 MHz, [D6]DMSO, 258C): d=À159.54 to À159.65 (m),
À165.66 to À165.81 (m), À174.82 to À174.92 ppm (m); HRMS (FAB+):
m/z calcd for C17H18N2F5 [M+]: 345.1390; found: 345.1396.
1-(2,3,4,5,6-Pentafluorophenyl)-3-mesityl-4,5-dihydroimidazolium
chlo-
ride (18b): A suspension of 17b (834 mg, 2.0 mmol, 1.0 equiv) in triethyl-
orthoformate (6.6 mL, 40.0 mmol, 20.0 equiv) was heated at 1358C for
7552
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 7545 – 7556

Homogeneous Catalysis
FULL PAPER
5 min under argon. After cooling to room temperature, the solids were
filtered of and washed with diethyl ether (4”2 mL) to provide the de-
sired product as a white solid (622 mg, 1.7 mmol, 87% yield). 1H NMR
(300 MHz, CDCl3, 258C): d=10.47 (s, 1H), 6.89(s, 2H), 4.68 (t, 3J-
J
C
G
Phenyl chlorooxoacetate (14c): Phenol (15.64 g, 166.2 mmol, 1.0 equiv)
was dissolved in dry CH2Cl2 (150 mL) and oxalyl chloride (14.5 mL,
166.2 mmol, 1.0 equiv) was slowly added through a syringe. This solution
was cooled to 08C and dry triethylamine (23.2 mL, 166.2 mmol,
1.0 equiv) was added dropwise over a period of 30 min. Precipitation of a
white solid (triethylammonium chloride) was observed. The final orange
suspension was allowed to stir for 45 min as the reaction mixture warmed
to room temperature. The solid was filtered off, and the solvent of the fil-
trate was removed under reduced pressure to afford a tan solid. This was
suspended in hexanes (200 mL) and filtrated again to remove the remain-
ing triethylammonium chloride. The solvent was removed under reduced
pressure to afford 14c as an off-white solid (16.95 g, 92.1 mmol, 55%
yield). 1H NMR (300 MHz, CDCl3, 258C): d=7.55–7.50 (m, 2H), 7.44–
7.40 (m, 1H), 7.34–7.32 ppm (m, 2H); 13C{1H} NMR (75 MHz, CDCl3,
258C): d=161.38, 154.26, 150.39, 130.27, 127.66, 120.94 ppm; HRMS
(EI+): m/z calcd for C8H5ClO3 [M+]: 183.9927; found: 183.9928.
A
N
13C{1H} NMR (75 MHz, CDCl3, 258C; due to extensive fluorine coupling,
coupling constants are not given and resonances are reported as peaks):
d=162.10, 145.21–144.94 (m), 143.99–143.48 (m), 141.87–141.69 (m),
141.16, 139.96–139.65 (m), 136.66–136.27 (m), 134.78, 130.25, 130.08,
111.98–111.54 (m), 52.59, 52.03, 21.25, 17.92 ppm; 19F{1H} NMR
(282 MHz, CDCl3, 258C): d=À145.77 to À145.84 (m), À151.62 to
À151.75 (m), À160.23 to À160.36 ppm (m); HRMS (FAB+)m/z calcd for
C18H16N2F5 [M+]: 355.1234; found: 355.1245.
A
À
ylidene}
N
A
2.0 equiv), KHMDS (100 mg, 500 mmol, 2.0 equiv), and complex
2
(206 mg, 250 mmol, 1.0 equiv) were simultaneously dissolved in benzene
(5 mL). The reaction flask was taken out of the glove box and heated
under a nitrogen atmosphere at 808C for 20 min. The solution was con-
centrated to 2 mL in vacuo and poured onto a column packed with TSI
Scientific silica gel. The complex was eluted with hexanes/diethyl ether
(2:1) as a red band. This was concentrated in vacuo, transferred in a
glove box, dissolved in the minimum amount of benzene and lyophilized
to afford the desired complex as a violet solid (170 mg, 190 mmol, 76%
yield). 1H NMR (300 MHz, CD2Cl2, 258C): d=19.66 (s, 1H-minor), 19.07
(s, 1H-major), 7.55–6.99 (m, 7H-major, 7H-minor), 4.16–3.81 (m, 4H-
major, 4H-minor), 2.64–1.01 ppm (m, 42H-major, 42H-minor); 13C{1H}
NMR (125 MHz, CD2Cl2, 258C; due to extensive fluorine coupling and
the existence of two rotational isomers, coupling constants are not given
and resonances are reported as peaks): d=297.98 (major), 292.73
N-(2,6-Difluorophenyl)oxanilic acid phenyl ester (15c): 2,6-difluoroani-
line (5.05 mL, 50.0 mmol, 1.0 equiv) and dry triethylamine (13.9mL,
100.0 mmol, 2.0 equiv) were dissolved in dry THF (250 mL) under nitro-
gen. This solution was cooled to 08C, and 14c (11.07 g, 60.0 mmol,
1.2 equiv) was added in one portion. Precipitation of a white solid (tri-
ethylammonium chloride) occurred immediately upon addition. The sus-
pension was allowed to stir for 16 h as the reaction mixture warmed to
room temperature. The solid was filtered off and washed with diethyl
ether (200 mL). The combined organic layer was initially washed with an
aqueous saturated NH4Cl solution, until pH 6, then with brine (300 mL),
and dried over Na2SO4. The solvent was removed under reduced pressure
leaving a red viscous liquid that was vigorously stirred with hexanes until
a white-pink solid precipitated. This was washed with hexanes (3”6 mL)
to afford 15c as a white solid (9.32 g, 33.6 mmol, 67% yield). 1H NMR
(300 MHz, CDCl3, 258C): d=8.48 (brs, 1H), 7.48–7.41 (m, 2H), 7.35–
(minor), 223.71 (d, 2J
N
N
164.82, 151.35, 150.71, 147.48–147.31 (m), 146.29–145.92 (m), 145.53–
145.28 (m), 144.20–143.92 (m), 143.25–143.00 (m), 139.05–138.66 (m),
138.24–138.00 (m), 137.23–136.85 (m), 136.48–136.25 (m), 135.93, 134.34,
131.60–131.22 (m), 130.67–130.20 (m), 129.75, 129.54, 128.993, 128.51,
127.83–127.44 (m), 126.41–126.14 (m), 125.41–125.10 (m), 117.56–117.31
(m), 116.00–115.76 (m), 52.63, 52.49, 52.16, 31.87, 31.74, 31.69, 31.56,
29.00, 27.63, 27.55, 27.51, 27.43, 26.08, 25.98, 20.74, 20.54, 19.31,
17.92 ppm; 31P{1H} NMR (121 MHz, CD2Cl2, 258C) d=32.47 (s, minor),
27.04 ppm (s, major); 19F{1H} NMR (282 MHz, CD2Cl2, 258C): d=
7.17 (m, 4H), 7.04 ppm (t, 3J(H,H)=8 Hz, 2H); 13C{1H} NMR (75 MHz,
CDCl3, 258C; due to extensive fluorine coupling, coupling constants are
not given and resonances are reported as peaks): d=159.51, 159.45,
158.79, 156.16, 156.10, 154.45, 150.40, 146.84, 129.93, 129.23, 129.10,
128.98, 127.09, 121.19, 112.41–112.29 (m), 112.14–112.06 ppm (m); 19F{1H}
NMR (282 MHz, CDCl3, 258C): d=À117.01; HRMS (FAB+): m/z calcd
for C14H10NO3F2 [M+]: 278.0629; found: 278.0637.
À136.09, À144.05, À144.35, À152.96 (t, J
(19F,19F)=24 Hz), À161.35, À161.42, À161.79(brs),
À163.45 (brs), À166.07 ppm (t, J
(19F,19F)=24 Hz); HRMS (FAB+): m/z
calcd for C43H54N2F5Cl2PRu [M+]: 896.2366; found: 896.2363.
[RuCl2{1-(2,3,4,5,6-pentafluorophenyl)-3-mesityl-4,5-dihydroimidazol-2-
ACHTREUNG
ACHTREUNG
N-(2,6-Difluorophenyl)-N’-(2,6-diisopropylphenyl)oxalamide (16c): Com-
pound 15c (5.54 g, 20.0 mmol, 1.0 equiv) was suspended in 2,6-diisopro-
pylaniline (12.68 mL, 70.0 mmol, 3.5 equiv) in a dry Schlenk tube under
nitrogen. The tube was sealed and the suspension was stirred at 1808C
for 16 h. The resulting brown solid was washed with hexanes (5”15 mL)
leaving 16c as a white solid (5.05 g, 14.0 mmol, 70% yield). 1H NMR
(300 MHz, CDCl3, 258C): d=8.94 (brs, 1H), 8.75 (brs, 1H), 7.39–7.22
AHCTREUNG
ACHTREUNG
À
ylidene}(=CH-o-iPrO Ph)] (11): Inside a glovebox, a vial was charged
with complex 10 (120 mg, 134 mmol, 1.0 equiv), toluene (2 mL), and o-iso-
propoxy-b-methylstyrene (472 mg, 2.7 mmol, 20.0 equiv). The deep red
solution was stirred for 20 min and then left inside the capped vial with-
out stirring at room temperature. After 48 h the vial was removed from
the glove box and placed in an oil bath at 608C for 30 min. Upon cooled
to room temperature, the reaction mixture was poured onto a column
packed with TSI Scientific silica gel. The complex was eluted with hex-
anes/diethyl ether (2:1) as a green band. This was concentrated in vacuo,
transferred in a glove box, dissolved in the minimum amount of benzene
and lyophilized. The obtained solid was finally washed with pentanes (4”
1 mL) to afford the desired complex as a green powder (52 mg, 77 mmol,
57% yield). Crystals suitable for X-ray crystallography were grown at
room temperature by slow diffusion of hexanes into a solution of 11 in
benzene. 1H NMR (300 MHz, CD2Cl2, 258C): d=16.00 (s, 1H), 7.58–7.53
(m, 4H), 7.04 (t, 3J
2H), 1.22 (d, 3J(H,H)=7 Hz, 12H); 13C{1H} NMR (75 MHz, CDCl3,
258C; due to extensive fluorine coupling, coupling constants are not
given and resonances are reported as peaks): d=158.56, 158.41, 156.02,
155.81, 153.22, 146.06, 129.50, 129.25, 128.72, 123.99, 112.41–112.33 (m),
112.18–112.10 (m), 29.16, 23.89 ppm; 19F{1H} NMR (282 MHz, CDCl3,
258C): d=À116.95 ppm; HRMS (FAB+): m/z calcd for C20H23N2O2F2
[M+]: 361.1728; found: 361.1731.
N-(2,6-Difluorophenyl)-N’-(2,6-diisopropylphenyl)-1,2-ethanediamine di-
hydrochloride (17c): This compound was synthesized analogously to 17a
starting with 16c (white powder, 67% isolated yield). 1H NMR
(300 MHz, [D6]DMSO, 258C): d=7.41–7.20 (m, 3H), 6.99–6.94 (m, 2H),
6.73–6.63 (m, 1H), 3.76–3.66 (m, 2H), 3.28–3.19(m, 4H), 1.14 ppm (d, 3J-
(m, 1H), 7.12 (s, 2H), 6.97–6.90 (m, 3H), 5.04 (septet, 3J
1H), 4.28–4.19(m, 4H), 2.46 (s, 3H), 2.29(s, 6H), 1.40 ppm (d,
(H,H)=6 Hz, 6H); 13C{1H} NMR (125 MHz, CD2Cl2, 258C; due to ex-
ACHTREUNG
A
ACHTREUNG
extensive fluorine coupling, coupling constants are not given and reso-
nances are reported as peaks): d=155.00, 154.62, 151.33, 151.08, 150.70,
143.35, 142.10, 130.43, 130.21, 126.33, 112.72–112.32 (m), 112.14–111.92
(m), 27.89, 25.18 ppm; 19F{1H} NMR (282 MHz, [D6]DMSO, 258C): d=
À129.06 ppm; HRMS (FAB+): m/z calcd for C17H21N2F2 [M+]: 291.1673;
found: 291.1672.
tensive fluorine coupling, coupling constants are not given and resonan-
ces are reported as peaks): d=271.96, 216.34, 152.21, 147.67–147.59 (m),
145.62–145.57 (m), 143.97, 143.42–143.16 (m), 141.37–141.11 (m), 139.36,
139.21–138.84 (m), 137.41, 136.86, 129.92, 129.60, 122.34, 115.85–115.59
(m), 112.84, 75.27, 51.58, 20.95, 20.83, 17.61 ppm; 19F{1H} NMR
(282 MHz, CD2Cl2, 258C): d=À134.98 (d, J
A
Chem. Eur. J. 2008, 14, 7545 – 7556
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7553

R. H. Grubbs and G. C. Vougioukalakis
1-(2,6-Difluorophenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazolium
2 mL) and dried under high vacuum to afford the desired complex as a
powder. (65–75% isolated yield).
chloride (18c): This compound was synthesized analogously to 18a start-
ing with 17c (white solid, 98% yield). 1H NMR (300 MHz, [D6]DMSO,
General method for the synthesis of chlorodicarbonyl(carbene) rho-
A
3
258C): d=9.55 (s, 1H), 7.66–7.41 (m, 6H), 4.72 (t, J
A
dium(I) complexes 31 and 33: Inside a glovebox, the corresponding 4,5-
dihydro-imidazolium chloride (500 mmol, 1.0 equiv), along with silver(I)
oxide (250 mmol, 0.5 equiv), and 4 Š molecular sieves (170 mg) were sus-
pended in CH2Cl2 (4 mL) in the dark. The reaction mixture was stirred at
room temperature for 1.5 h, passed through a short celite pad, and then
4.43 (t, 3J
3J(H,H)=7 Hz, 6H), 1.20 ppm (d, 3J
A
(H,H)=7 Hz, 2H), 1.29(d,
A
ACHTREUNG
(75 MHz, [D6]DMSO, 258C; due to extensive fluorine coupling, coupling
constants are not given and resonances are reported as peaks): d=
161.44, 158.82, 155.47, 146.68, 142.10, 131.88, 130.54, 125.60, 114.35–
113.60 (m), 54.79, 52.21, 28.63, 25.43, 24.29 ppm; 19F{1H} NMR
(282 MHz, [D6]DMSO, 258C): d=À119.89 ppm; HRMS (FAB+): m/z
calcd for C21H25N2F2 [M+]: 343.1986; found: 343.1987.
added to a solution of [{Rh(cod)Cl}2] (123 mg, 250 mmol, 0.5 equiv) in
G
CH2Cl2 (2 mL).The new reaction mixture was stirred for an additional
1.5 h in the dark and then passed through a short celite pad. This solution
was taken out of the glove box, and poured onto a column packed with
TSI Scientific silica gel. The column was initially flushed with CH2Cl2.
A
À
The desired [Rh
A
G
dazol-2-ylidene}
U
G
CH2Cl2 (2:100) as a yellow band. This was concentrated in vacuo, trans-
ferred in a glove box, dissolved in the minimum amount of benzene and
lyophilized. The obtained solid was finally washed with pentanes (2”
1 mL) to afford a yellow powder. This complex was then dissolved in a
dry and deoxygenated 1:1 THF/toluene mixture (3 mL). CO was bubbled
through the reaction mixture for 1 h at room temperature. The solvent
was removed in vacuo, and the resulting solid was washed with pentanes
(2”2 mL) and dried under high vacuum to afford the desired cis-
772 mmol, 1.3 equiv) and KHMDS (140 mg, 772 mmol, 1.3 equiv) were
stirred in benzene (30 mL) at room temperature for 80 min. Complex 2
(443 mg, 551 mmol, 1.0 equiv) was added as a solid in one portion, and
the reaction flask was taken out of the glove box and stirred under a ni-
trogen atmosphere at room temperature for 4 h. The solvent was com-
pletely removed in vacuo. The residual solid was dissolved in a mixture
of hexanes/diethyl ether (3 mL, 5:1) and poured onto a column packed
with TSI Scientific silica gel. The desired complex was eluted with hex-
anes/diethyl ether (5:1) as a brown band. This was concentrated in vacuo,
transferred in a glove box, dissolved in the minimum amount of benzene,
and lyophilized to afford complex 12 as a brown solid (212 mg, 240 mmol,
44% yield). 1H NMR (500 MHz, CD2Cl2, 258C): d=19.41 (s, 1H-minor),
19.13 (s, 1H-major), 7.52–6.69 (m, 11H-major, 11H-minor), 4.16–3.89 (m,
4H-major, 4H-minor), 2.65–0.90 ppm (m, 47H-major, 47H-minor);
13C{1H} NMR (125 MHz, CD2Cl2, 258C; due to extensive fluorine cou-
pling and the existence of two rotational isomers, coupling constants are
not given and resonances are reported as peaks): d=297.11 (major),
294.19 (minor), 223.08, 222.45, 192.43, 162.40, 160.36, 152.06, 151.25,
138.78, 138.15, 136.89, 136.72, 135.32, 134.59, 131.36, 131.29, 131.21,
130.12, 129.98, 129.78, 129.16, 128.67, 128.42, 127.92, 126.66, 112.79,
112.63, 111.90, 111.74, 53.17, 52.16, 35.70, 35.22, 34.00, 33.87. 32.03, 31.90,
31.77, 29.19, 27.96, 27.88, 27.20, 27.11, 26.55, 26.41, 21.06, 20.86, 19.70,
18.32 ppm; 31P{1H} NMR (121 MHz, CD2Cl2, 258C) d=31.59(s, minor),
27.59ppm (s, major); 19F{1H} NMR (282 MHz, CD2Cl2, 258C): d=
À110.26 (s, major), À117.22 ppm (s, minor); HRMS (FAB+): m/z calcd
for C46H63N2F2Cl2PRu [M+]: 884.3118; found: 884.3126.
[Rh(CO)2Cl
A
yield, two steps).
Characterization of chlorodicarbonyl{1-(2,6-difluorophenyl)-3-mesityl-
4,5-dihydroimidazol-2-ylidene}rhodium(I) (29): 1H NMR (300 MHz,
CD2Cl2, 258C): d=7.48–7.38 (m, 1H), 7.18–7.01 (m, 4H), 4.22–4.00 (m,
4H), 2.39(s, 6H), 2.35 ppm (s, 3H); 13C{1H} NMR (125 MHz, CD2Cl2,
258C; due to extensive fluorine coupling, coupling constants are not
given and resonances are reported as peaks): d=207.65 (d,
1J(103Rh,13C)=42 Hz), 185.42 (d, 1J(103Rh,13C)=54 Hz), 182.35 (d,
1J(103Rh,13C)=74 Hz), 160.82–160.45 (m), 158.81–158.45 (m), 138.88,
136.01, 136.86, 134.56, 129.94, 129.86, 129.79, 129.43, 129.33, 129.19,
112.33–111.67(m), 51.86, 51.72, 20.72, 18.17, 17.94 ppm; 19F{1H} NMR
(282 MHz, CD2Cl2, 258C): d=À116.42 ppm; HRMS (FAB+): m/z calcd
for C20H18ClF2O2N2Rh [M+]: 494.0080; found: 494.0084.
Characterization of chlorodicarbonyl{1-(2,4,6-trifluorophenyl)-3-mesityl-
4,5-dihydroimidazol-2-ylidene}rhodium(I) (30): 1H NMR (300 MHz,
CD2Cl2, 258C): d=7.01 (s, 2H), 6.91–6.85 (m, 2H), 4.20–4.00 (m, 4H),
2.38 (s, 6H), 2.35 ppm (s, 3H); 13C{1H} NMR (125 MHz, CD2Cl2, 258C;
due to extensive fluorine coupling, coupling constants are not given and
resonances are reported as peaks): d=208.01 (d, 1J(103Rh,13C)=41 Hz),
[RuCl2{1-(2,6-difluorophenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimi-
À
dazol-2-ylidene}(=CH-o-iPrO Ph)] (13): This complex was synthesized
and purified analogously to complex 9 starting with 12 (green powder,
85% yield). Crystals suitable for X-ray crystallography were grown at
room temperature by slow diffusion of hexanes into a solution of 13 in
benzene. 1H NMR (500 MHz, CD2Cl2, 258C): d=16.10 (s, 1H), 7.64 (t, 3J-
1
1
185.33 (d, J(103Rh,13C)=53 Hz), 182.30 (d, J(103Rh,13C)=74 Hz), 163.14–
162.90 (m), 161.15–160.54 (m), 158.92–158.67 (m), 138.96, 135.73, 134.42,
109.65, 101.11–100.71 (m), 51.94, 51.67, 20.70, 17.90 ppm; 19F{1H} NMR
(282 MHz, CD2Cl2, 258C): d=À106.66, À113.07 ppm (brs); HRMS
(FAB+): m/z calcd for C20H17ClF3O2N2Rh [M+]: 511.9986; found:
511.9980.
G
ACHTREUNG
U
ACHTREUNG
AHCTREUNG
Characterization of chlorodicarbonyl{1-(2,3,4,5,6-pentafluorophenyl)-3-
1H NMR
A
N
ACHTREUNG
mesityl-4,5-dihydroimidazol-2-ylidene}rhodium(I)
(31):
7 Hz, 6H); 13C{1H} NMR (125 MHz, CD2Cl2, 258C; due to extensive fluo-
rine coupling, coupling constants are not given and resonances are re-
ported as peaks): d=289.19, 215.68, 162.85, 160.82, 152.74, 148.48, 144.00,
137.47, 131.42, 131.33, 131.26, 130.02, 129.65, 125.16, 122.50, 122.23,
113.12, 112.55, 112.53, 112.40, 112.37, 75.50, 56.28, 51.66, 28.16, 25.70,
23.79, 21.49 ppm; 19F{1H} NMR (282 MHz, CD2Cl2, 258C): d=
À107.90 ppm; HRMS (FAB+): m/z calcd for C31H36N2OF2Cl2Ru [M+]:
662.1217; found: 662.1195.
(300 MHz, CD2Cl2, 258C): d=7.02 (s, 2H), 4.24–4.04 (m, 4H), 2.37 (s,
6H), 2.35 ppm (s, 3H); 13C{1H} NMR (125 MHz, CD2Cl2, 258C; due to
extensive fluorine coupling, coupling constants are not given and reso-
nances are reported as peaks): d=208.50 (d, 1J(103Rh,13C)=41 Hz),
185.07 (d, J(103Rh,13C)=54 Hz), 182.04 (d, J(103Rh,13C)=74 Hz), 146.00–
145.75 (m), 143.94–143.78 (m), 142.78–142.35 (m), 140.63–140.39 (m),
139.25, 138.96–138.67 (m), 136.95–136.69 (m), 135.60–135.38 (m), 134.15,
129.45, 129.26, 116.44–115.93 (m), 52.08, 51.60, 20.72, 17.88 ppm; 19F{1H}
NMR (282 MHz, CD2Cl2, 258C): d=À142.99 (brs), À154.39to À154.54
(m), À162.63 to À162.76 ppm (m); HRMS (FAB+): m/z calcd for
C19H15ClF5ON2Rh [M+ÀCO]: 519.9848; found: 519.9856.
1
1
General method for the synthesis of chlorodicarbonyl(carbene) rho-
A
dium(I) complexes 29, 30, and 32: In a glovebox, the corresponding 4,5-
dihydro-imidazolium chloride (500 mmol, 1.0 equiv), along with silver(I)
oxide (250 mmol, 0.5 equiv), and 4 Š molecular sieves (170 mg) were sus-
pended in CH2Cl2 (4 mL) in the dark. The reaction mixture was stirred at
room temperature for 1.5 h, passed through a short celite pad, and then
added to a solution of [{Rh(CO)2Cl}2] (97 mg, 250 mmol, 0.5 equiv) in
CH2Cl2 (2 mL). This reaction mixture was stirred for an additional 1.5 h
in the dark and then passed through a short celite pad. The solvent was
removed in vacuo, and the resulting solid was washed with pentanes (5”
Characterization of chlorodicarbonyl{1,3-bis(2,6-difluorophenyl)-4,5-dihy-
A
droimidazol-2-ylidene}rhodium(I) (32): 1H NMR (300 MHz, CD2Cl2,
258C): d=7.55–7.43 (m, 2H), 7.19–7.09 (m, 4H), 4.24–4.19 ppm (m, 4H);
13C{1H} NMR (125 MHz, CD2Cl2, 258C; due to extensive fluorine cou-
pling, coupling constants are not given and resonances are reported as
peaks): d=210.15 (d, 1J(103Rh,13C)=40 Hz), 185.42 (d, 1J(103Rh,13C)=
54 Hz), 181.75 (d, 1J(103Rh,13C)=74 Hz), 160.49, 160.18, 158.47, 158.18,
7554
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 7545 – 7556

Homogeneous Catalysis
FULL PAPER
130.60–130.34 (m), 112.38–112.11 (m), 52.17, 51.77 ppm; 19F{1H} NMR
(282 MHz, CD2Cl2, 258C): d=À116.66 ppm; HRMS (FAB+): m/z calcd
for C17H10ClF4O2N2Rh [M+]: 487.9422; found: 487.9421.
Hoveyda, J. Am. Chem. Soc. 2002, 124, 4954–4955; c) J. J. Van Veld-
huizen, D. G. Gillingham, S. B. Garber, O. Kataoka, A. H. Hoveyda,
J. Am. Chem. Soc. 2003, 125, 12502–12508; d) D. G. Gillingham, O.
Kataoka, S. B. Garber, A. H. Hoveyda, J. Am. Chem. Soc. 2004, 126,
12288–12290; e) J. J. Van Veldhuizen, J. E. Campbell, R. E. Giudici,
A. H. Hoveyda, J. Am. Chem. Soc. 2005, 127, 6877–6882; f) T. W.
Funk, J. M. Berlin, R. H. Grubbs, J. Am. Chem. Soc. 2006, 128,
1840–1846; g) J. M. Berlin, J. M. Goldberg, R. H. Grubbs, Angew.
Chem. 2006, 118, 7753–7757; Angew. Chem. Int. Ed. 2006, 45, 7591–
7595; h) R. E. Giudici, A. H. Hoveyda, J. Am. Chem. Soc. 2007, 129,
3824–3825.
Characterization of chlorodicarbonyl{1-(2,6-difluorophenyl)-3-(2,6-diiso-
propylphenyl)-4,5-dihydroimidazol-2-ylidene}rhodium(I) (33): 1H NMR
(300 MHz, CD2Cl2, 258C): d=7.49–7.39 (m, 2H), 7.32–7.29 (m, 2H),
7.13–7.07 (m, 2H), 4.22–4.07 (m, 4H), 3.27 (septet, 3J
ACHTREUNG
1.39(d, 3J
A
ACHTREUNG
NMR (125 MHz, CD2Cl2, 258C; due to extensive fluorine coupling, cou-
pling constants are not given and resonances are reported as peaks): d=
208.64 (d, 1J(103Rh,13C)=42 Hz), 185.35 (d, 1J(103Rh,13C)=54 Hz), 182.35
(d, 1J(103Rh,13C)=75 Hz), 160.39, 158.38, 147.12, 134.14, 130.03–129.87
(m), 129.68, 124.47, 112.12, 111.96, 54.75, 51.50, 28.24, 26.37, 23.37 ppm;
19F{1H} NMR (282 MHz, CD2Cl2, 258C): d=À116.40 ppm; HRMS
(FAB+): m/z calcd for C22H24ClF2ON2Rh [M+ÀCO]: 508.0600; found:
508.0590.
[8] a) D. M. Lynn, B. Mohr, R. H. Grubbs, L. M. Henling, M. W. Day, J.
Am. Chem. Soc. 2000, 122, 6601–6609; b) D. M. Lynn, R. H.
Grubbs, J. Am. Chem. Soc. 2001, 123, 3187–3193; c) T. Rçlle, R. H.
Grubbs, Chem. Commun. 2002, 1070–1071; d) J. P. Gallivan, J. P.
Jordan, R. H. Grubbs, Tetrahedron Lett. 2005, 46, 2577–2580;
e) S. H. Hong, R. H. Grubbs, J. Am. Chem. Soc. 2006, 128, 3508–
3509; f) M. T. Mwangi, M. B. Runge, M. B. Bowden, J. Am. Chem.
Soc. 2006, 128, 14434–14435; g) J. B. Binder, I. A. Guzei, R. T.
Raines, Adv. Synth. Catal. 2007, 349, 395–404.
Acknowledgements
[9] a) J. M. Berlin, K. Campbell, T. Ritter, T. W. Funk, A. Chlenov,
R. H. Grubbs, Org. Lett. 2007, 9, 1339–1342; b) I. C. Stewart, T.
Ung, A. A. Pletnev, J. M. Berlin, R. H. Grubbs, Y. Schrodi, Org.
Lett. 2007, 9, 1589–1592.
[10] A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W. Lehmann,
R. Mynott, F. Stelzer, O. R. Thiel, Chem. Eur. J. 2001, 7, 3236–3253.
[11] M. B. Dinger, P. Nieczypor, J. C. Mol, Organometallics, 2003, 22,
5291–5296.
This research was supported in part by the National Institutes of Health
and by a Marie Curie International Fellowship to G.C.V. within the 6th
European Community Framework Program. The authors thank Larry M.
Henling and Dr. Michael W. Day for the X-ray crystallographic analyses
and Dr. Mona Shahgholi for performing the mass spectrometric analyses.
Materia Inc. is acknowledged for providing generous gifts of catalyst 2.
[12] K. Vehlow, S. Maechling, S. Blechert, Organometallics 2006, 25, 25–
28.
[13] N. Ledoux, B. Allaert, S. Pattyn, H. V. Mierde, C. Vercaemst, F. Ver-
poort, Chem. Eur. J. 2006, 12, 4654–4661.
[14] N. Ledoux, B. Allaert, A. Linden, P. Van Der Voort, F. Verpoort,
Organometallics 2007, 26, 1052–1056.
[15] G. C. Vougioukalakis, R. H. Grubbs, Organometallics 2007, 26,
2469–2472.
[16] N. Ledoux, A. Linden, B. Allaert, H. V. Mierde, F. Verpoort, Adv.
Synth. Catal. 2007, 349, 1692–1700.
[17] P. A. Fournier, S. K. Collins, Organometallics 2007, 26, 2945–2949.
[18] D. R. Anderson, V. Lavallo, D. J. OꢁLeary, G. Bertrand, R. H.
Grubbs, Angew. Chem. 2007, 119, 7400–7403; Angew. Chem. Int.
Ed. 2007, 46, 7262–7265.
[19] G. C. Vougioukalakis, R. H. Grubbs, J. Am. Chem. Soc. 2008, 130,
2234–2245.
[20] A. W. Waltman, R. H. Grubbs, Organometallics, 2004, 23, 3105–
3107.
[1] a) Olefin Metathesis and Metathesis Polymerization (Eds.: K. J. Ivin,
J. C. Mol), Academic Press, San Diego, 1997; b) Handbook of Meta-
thesis (Ed.: R. H. Grubbs), Wiley-VCH, Weinheim, 2003.
[2] Representative review articles on the applications of olefin metathe-
sis: a) R. H. Grubbs, S. J. Miller, G. C. Fu, Acc. Chem. Res. 1995, 28,
446–452; b) M. Schuster, S. Blechert, Angew. Chem. 1997, 109,
2124–2144; Angew. Chem. Int. Ed. Engl. 1997, 36, 2036–2056;
c) R. H. Grubbs, S. Chang, Tetrahedron 1998, 54, 4413–4450;
d) S. K. Armstrong, J. Chem. Soc. Perkin Trans. 1 1998, 371–388;
e) K. J. Ivin, J. Mol. Catal. A 1998, 133, 1–16; f) M. R. Buchmeiser,
Chem. Rev. 2000, 100, 1565–1604; g) A. Fürstner, Angew. Chem.
2000, 112, 3140–3172; Angew. Chem. Int. Ed. 2000, 39, 3012–3043;
h) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18–29;
i) S. J. Connon, S. Blechert, Angew. Chem. 2003, 115, 1944–1968;
Angew. Chem. Int. Ed. 2003, 42, 1900–1923; j) R. R. Schrock, A. H.
Hoveyda, Angew. Chem. 2003, 115, 4740–4782; Angew. Chem. Int.
Ed. 2003, 42, 4592–4633; k) R. H. Grubbs, Tetrahedron 2004, 60,
7117–7140; l) T. J. Donohoe, A. J. Orr, M. Bingham, Angew. Chem.
2006, 118, 2730–2736; Angew. Chem. Int. Ed. 2006, 45, 2664–2670.
[3] S. T. Nguyen, L. K. Johnson, R. H. Grubbs, J. W. Ziller, J. Am.
Chem. Soc. 1992, 114, 3974–3975.
[4] a) P. Schwab, M. B. France, J. W. Ziller, R. H. Grubbs, Angew.
Chem. 1995, 107, 2179–2181; Angew. Chem. Int. Ed. Engl. 1995, 34,
2039–2041; b) P. Schwab, R. H. Grubbs, J. W. Ziller, J. Am. Chem.
Soc. 1996, 118, 100–110.
[5] M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953–
956.
[6] a) J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus, Jr., A. H. Hov-
eyda, J. Am. Chem. Soc. 1999, 121, 791–799; b) S. B. Garber, J. S.
Kingsbury, B. L. Gray, A. H. Hoveyda, J. Am. Chem. Soc. 2000, 122,
8168–8179; c) H. Wakamatsu, S. Blechert, Angew. Chem. 2002, 114,
2509–2511; Angew. Chem. Int. Ed. 2002, 41, 2403–2405; d) K.
Grela, S. Harutyunyan, A. Michrowska, Angew. Chem. 2002, 114,
4210–4212; Angew. Chem. Int. Ed. 2002, 41, 4038–4040; e) A. Mi-
chrowska, R. Bujok, S. Harutyunyan, V. Sashuk, G. Dolgonos, K.
Grela, J. Am. Chem. Soc. 2004, 126, 9318–9325; f) A. Hejl, M. W.
Day, R. H. Grubbs, Organometallics 2006, 25, 6149–6154.
[21] Comparison of the 1H NMR chemical shifts in the two rotational
isomers of 6, 8 and 10 with that in parent complex 3 suggests that
the major rotational isomer in these unsymmetrical complexes con-
tains a mesityl ring located above the benzylidene moiety. This was
confirmed for complexes 6 and 8 by acquiring low-temperature
1H NMR spectra (À708C, CD2Cl2) of the crystals studied by X-ray
crystallography, which showed only the rotational isomer that has its
mesityl ring located directly above the benzylidene group. These
spectra showed a ꢀ5:95 ratio for the two benzylidene protons, as
compared to the room temperature ratio of ꢀ2:8. Unfortunately, it
was not possible to obtain any diagnostic information regarding this
issue from NOE experiments in solution.
[22] For two review articles on p–p stacking interactions, see: a) J. H.
Williams, Acc. Chem. Res. 1993, 26, 593–598; b) C. Janiak, J. Chem.
Soc. Dalton Trans. 2000, 3885–3896; for two recent publications on
intramolecular p–p stacking interactions see: c) D. Mandal, B. D.
Gupta, Organometallics, 2006, 25, 3305–3307; d) D. Mandal, B. D.
Gupta, Organometallics, 2007, 26, 658–670.
[23] Low-quality crystals of complex 11 were also obtained. X-ray analy-
sis on different samples of these single crystals showed the same
connectivity and orientation of the ligands as in the crystal struc-
tures of the complexes 8, 9, and 13.
[7] a) T. J. Seiders, D. W. Ward, R. H. Grubbs, Org. Lett. 2001, 3, 3225–
3228; b) J. J. Van Veldhuizen, S. B. Garber, J. S. Kingsbury, A. H.
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R. H. Grubbs and G. C. Vougioukalakis
[24] For a comparative catalytic evaluation study of the most commonly-
used ruthenium-based olefin metathesis catalysts, see: T. Ritter, A.
Hejl, A. G. Wenzel, T. W. Funk, R. H. Grubbs, Organometallics
2006, 25, 5740–5745.
[25] C. Slugovc, Macromol. Rapid Commun. 2004, 25, 1283–1297.
[26] A. K. Chatterjee, T. L. Choi, D. P. Sanders, R. H. Grubbs, J. Am.
Chem. Soc. 2003, 125, 11360–11370.
nism of olefin metathesis catalysts with chelating alkylidenes are
currently underway and will be published in due course.
[31] a) K. Denk, P. Sirsch, W. A. Herrmann, J. Organomet. Chem. 2002,
649, 219–224; b) M. Mayr, K. Wurst, K. H. Ongania, M. R. Buchme-
iser, Chem. Eur. J. 2004, 10, 1256–1266; c) W. A. Herrmann, J.
Schütz, G. D. Frey, E. Herdtweck, Organometallics 2006, 25, 2437–
2448.
[32] For two ruthenium-based metathesis catalysts coordinated with this
NHC, see: T. Ritter, M. W. Day, R. H. Grubbs, J. Am. Chem. Soc.
2006, 128, 11768–11769.
[33] For a cyclic voltametry study on the p-face donor properties of
NHCs in some related ruthenium-based metathesis catalysts, see:
a) M. Süßner, H. Plenio, Chem. Commun. 2005, 5417–5419; see
also: b) S. Leuthꢂußer, D. Schwarz, H. Plenio, Chem. Eur. J. 2007,
13, 7195–7203.
[27] J. Louie, R. H. Grubbs, Organometallics 2002, 21, 2153–2164.
[28] a) M. S. Sanford, M. Ulman, R. H. Grubbs, J. Am. Chem. Soc. 2001,
123, 749–750; b) M. S. Sanford, J. A. Love, R. H. Grubbs, J. Am.
Chem. Soc. 2001, 123, 6543–6554; according to these studies, the
higher activity of the second- versus the first-generation ruthenium-
based metathesis catalysts originates from the increased affinity of
the NHC-substituted ruthenium center for p-acidic olefins relative
to s-donating phosphine, and not from the increased labilization of
the phosphine due to the large trans-effect of the NHC ligands, as
originally believed.
[34] A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen, F. J.
Timmers, Organometallics 1996, 15, 1518–1520.
[29] A. H. Hejl, Ph.D. thesis, California Institute of Technology (USA),
2007.
[30] The kinetics in the initiation of the phosphine-free catalysts are
much more complicated than for the phosphine-containing ones.
Detailed studies designed to fully understand the initiation mecha-
Received: March 13, 2008
Published online: July 21, 2008
Please note: Minor changes have been made to this Manuscript since its
publication in Chemistry—A European Journal Early View. The Editor.
7556
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Chem. Eur. J. 2008, 14, 7545 – 7556