Synthesis and reactivity of ruthenium phosphite indenylidene complexes

Article abstract of DOI:10.1021/om300703p

The synthesis of the four olefin metathesis precatalysts Caz-1a-d, featuring the NHC ligand N,N′-bis(2,4,6-trimethylphenyl)imidazolin-2- ylidene (SIMes) and four different phosphites (P(OⁱPr)₃, P(OPh)₃, P(OEt)₃,

Full text of DOI:10.1021/om300703p

Article
pubs.acs.org/Organometallics
Synthesis and Reactivity of Ruthenium Phosphite Indenylidene
Complexes
Xavier Bantreil,† Albert Poater,‡ Cesar A. Urbina-Blanco,† Yannick D. Bidal,† Laura Falivene,§
́
Rebecca A. M. Randall,† Luigi Cavallo,§ Alexandra M. Z. Slawin,† and Catherine S. J. Cazin*,†
†EaStCHEM School of Chemistry, University of St Andrews, St Andrews KY16 9ST, U.K.
‡
́
́
Institut de Quımica Computacional, Departament de Quımica, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Spain
§Dipartimento di Chimica e Biologia, Universita
̀
di Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy.
S
* Supporting Information
ABSTRACT: The synthesis of the four olefin metathesis
precatalysts Caz-1a−d, featuring the NHC ligand N,N′-
bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) and
four different phosphites (P(OiPr)3, P(OPh)3, P(OEt)3, and
P(OMe)3), is reported. The complexes are readily synthesized
from commercially available [RuCl2(3-phenylinden-1-ylidene)-
(pyridine)(SIMes)] (Ind-III) in yields of up to 88%. These
complexes adopt an unusual cis configuration between the
phosphite and the NHC ligands. NMR experiments and
computational studies confirm that the cis complexes are
thermodynamically favored in comparison to their trans
counterparts. In addition, the isomerization from trans to cis occurs via a mononuclear and non-dissociative mechanism.
Among the four precatalysts, cis-Caz-1a, featuring a P(OiPr)3 ligand, displays the highest activity in ring-closing metathesis and
cross-metathesis transformations. Experiments at low catalyst loadings demonstrated the potential of this catalyst, allowing better
conversions than with commonly used commercially available precatalysts.
INTRODUCTION
■
Olefin metathesis is now considered among the most powerful
tools in organic and organometallic chemistry.1 Its importance
was recognized by the 2005 Nobel Prize in chemistry to
Grubbs, Schrock, and Chauvin.2 The efficiency of metathesis
catalysts is also highlighted by the ever-increasing number of
applications spanning from the synthesis of biologically active
compounds to that of novel polymers.3 Dating back to 1992,
when Grubbs reported the first-generation ruthenium catalyst
[RuCl2(PCy3)2(CHPh)],4 numerous studies have targeted
the development of longer-living and more active precatalysts.5
The replacement of a labile phosphine ligand by a strongly σ-
donating N-heterocyclic carbene (NHC) afforded the corre-
sponding [RuCl2(NHC)PCy3(CHPh)] complexes, which
displayed increased reactivity and stability (G-II, Figure 1).1f,g,6
The introduction of 1-isopropoxy-2-vinylbenzene to form o-
isopropoxybenzylidene instead of the benzylidene moiety
afforded chelated catalysts,7 the best known being the Hoveyda
catalyst8 (Hov-II, Figure 1). Further modifications of the
chelating ligand allowed for the formation of precatalysts
recognized as highly efficient systems.9 More recently, replacing
the benzylidene by a 3-phenylinden-1-ylidene has given rise to a
new family of complexes.10 These precatalysts have proven
highly stable and efficient in various metathesis transformations.
Studies focusing on the variation of the NHC on these
architectures have been reported.11 However, G-II and Ind-II
Figure 1. Previously reported ruthenium complexes relevant to the
present study.
find their limitations in catalysis involving challenging
substrates, where the former does not tolerate harsh conditions
and the latter requires 2−5 mol % of catalyst to perform
reactions effectively.
Several studies have been reported on the modulation of
ruthenium complexes by replacement of the tricyclohexylphos-
Received: July 25, 2012
Published: October 16, 2012
© 2012 American Chemical Society
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phine ligand by other phosphines,12 NHCs,13 Schiff bases,14
and pyridine,15 in the benzylidene and indenylidene families.
Unfortunately, the expensive PCy3 is generally accepted as the
best throwaway ligand to perform exchange reactions and
generate novel catalysts motifs. In this context, we recently
reported the synthesis of a phosphite-containing ruthenium
complex (cis-Caz-1a, Figure 1).16a This study was based on the
known synergistic effect between strongly π acidic phosphites
and strongly σ donating NHCs on metals.17 Such a beneficial
effect has already been demonstrated, affording extremely long-
lived and highly active precatalysts in Ni- and Pd-catalyzed
coupling reactions.18
downfield shift of the phosphorus peak from trans-Caz-1a-d to
the corresponding cis-Caz-1a-d. The electronic parameters of
the phosphites appear to have an effect on the reaction; an
excess of the less electron donating phosphite P(OPh)3 was
required in order to convert all of the Ind-III into the new
complex.
As previously demonstrated with P(OiPr)3,16a trans- and cis-
Caz-1a−d were found to be kinetic and thermodynamic
products, respectively; heating the reaction mixture at 40 °C in
CH2Cl2 allowed for full conversion to the cis isomers (Table 1).
a
Table 1. Synthesis of cis-Caz-1a−d Complexes
cis-Caz-1a16a was readily synthesized by reacting commer-
cially available Ind-III19 with P(OiPr)3 (Scheme 1). During the
Scheme 1. Synthesis of Phosphite-Containing Ruthenium
Indenylidene Complex Caz-1a
b
entry P(OR)3 (equiv) θ (°)
complex
time (h) yield (%)
1
2
3
4
P(OMe)3 (1)
P(OEt)3 (1)
P(OiPr)3 (1)
P(OPh)3 (4)
107
109
128
130
cis-Caz-1b
cis-Caz-1c
cis-Caz-1a
cis-Caz-1d
3
5
57
88
84
76
15
15
a
Reaction conditions: Ind-III (1 equiv), phosphite (1−4 equiv),
CH2Cl2, 40 °C. Tolman cone angle.23
b
exchange reaction, when the reaction was performed at room
temperature, two isomers, trans- and cis-dichloro, were
observed. The trans isomer was found to be the kinetic
product of the reaction, and heating to 40 °C afforded full
conversion to the unusual cis isomer, the thermodynamic
product. cis-Caz-1a represents a rare example of a Ru
metathesis complex adopting a cis configuration while solely
bearing monodentate ligands. Indeed, to date, monodentate
phosphine containing cis-dichloro ruthenium complexes had
rarely been observed20 or were formed with the assistance of a
hydroxy-substituted NHC.21 Furthermore, this represented, to
the best of our knowledge, the first example of an indenylidene-
type complex displaying such a configuration. However, the cis-
dichloro geometry has been recently found in a limited number
of Hoveyda-type catalysts, with metal centers effectively bearing
a bidentate ligand.22 In addition to having a fascinating
structure, cis-Caz-1a showed interesting properties in ring-
closing metathesis (RCM), especially at low catalyst loadings.
Indeed, cis-Caz-1a was found to be latent16a and required
thermal activation; complete conversions in RCM could be
achieved with catalyst loadings as low as 0.02 mol % with
heating in refluxing toluene. Knowing that a synergy between
the phosphite and the NHC on the ruthenium could lead to
even more promising precatalysts, we proceeded to investigate
the potential of phosphites other than P(OiPr)3. These findings
are reported in this contribution.
1
Reactions were monitored by H and 31P{1H} NMR until full
conversion to the cis isomers was achieved. In all cases, the
initial formation of trans-Caz-1a−d was observed, followed by
equilibration to the thermodynamically favored cis isomers.
The trans/cis isomerization is strongly dependent on the
phosphite cone angle. Bulky phosphites such as P(OiPr)3 and
P(OPh)3 required longer reaction times than the smaller
phosphites P(OMe)3 and P(OEt)3 (Table 1).23 However,
complexes cis-Caz-1a−d could be isolated in good yields,
ranging from 57% for cis-Caz-1b to 88% for cis-Caz-1c. The
lower yield for cis-Caz-1b was due to isolation issues, as several
MeOH washes were necessary to obtain the analytically pure
complex.
In order to fully characterize cis-Caz-1a−d, single crystals
suitable for X-ray diffraction studies were grown by slow
evaporation of solutions of complexes in CH2Cl2/n-dodecane
or THF/n-dodecane or by recrystallization in nitromethane.
Graphical representations of cis-Caz-1a−d are provided in
Figure 2 and selected bond lengths and angles in Table 2.
Crystallographic data of Ind-III are included in the table for
comparison.19a The distances of the carbene to the metal center
Ru−C(1) range from 2.063(6) to 2.080(4) Å, which are slightly
longer than Ru−C(1) in Ind-III but are in the same range as
that of the previously reported [RuCl2(Ind)(PAr3)(SIMes)],
(Ind = 3-phenylinden-1-ylidene).12c Interestingly, the Ru−C(1)
bond was found to be slightly longer in cis-Caz-1d, which
features P(OPh)3, than in the other phosphites. The C(1)−
Ru(1)−P(1) angle between the phosphite and the NHC ranges
from 97.88° (for P(OMe)3) to 100.06° (for P(OiPr)3), giving
support that steric repulsion between NHC and P(OMe)3 is
less important than with P(OiPr)3.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Complexes cis-Caz-
1a−d. Initial syntheses of complexes Caz-1a−d were
performed by reacting Ind-III with the phosphites P(OiPr)3,
P(OMe)3, P(OEt)3, and P(OPh)3 in CH2Cl2 at room
temperature. In each case, the 31P{1H} NMR spectrum of the
crude product showed the formation of two complexes (signals
between 110 and 135 ppm, whereas free phosphites appear
around 128−145 ppm). Each spectrum showed a significant
Mechanistic Investigation. Previous kinetic experiments
permitted to obtain the thermodynamic parameters of the
trans/cis isomerization, ΔH⧧ = 22.6 kcal mol−1 and ΔS⧧ = −4.2
cal mol−1 K−1. These data indicated that the isomerization
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Figure 2. Graphical representations of cis-Caz-1a−d. Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes [RuCl2(Ind)(L)(SIMes)] (Ind = 3-Phenylinden-1-
ylidene, L = P(OR)3, Pyridine)
L
P(OiPr)3
P(OPh)3
P(OEt)3
P(OMe)3
pyridine
Ru(1)−C(24)
Ru(1)−C(1)
Ru(1)−P(1)
Ru(1)−Cl(1)
1.881(8)
2.067(7)
2.249(2)
2.3974(19)
2.4036(18)
98.7(3)
1.866(4)
1.871(6)
2.063(6)
2.2273(17)
2.3899(15)
2.3972(15)
98.2(2)
1.882(4)
2.065(4)
2.2423(10)
2.3774(11)
2.4287(9)
96.64(15)
88.60(11)
97.88(10)
7.1(4)
1.911(19)
2.051(9)
2.080(4)
a
2.2179(12)
2.3797(11)
2.3868(11)
98.74(17)
88.98(13)
99.17(11)
2.128(8)
2.385(4)
2.385(4)
98.3(8)
Ru(1)−Cl(2)
C(24)−Ru(1)−C(1)
C(24)−Ru(1)−P(1)
C(1)−Ru(1)−P(1)
N(2)−C(3)−C(4)−N(5)
b
90.5(2)
90.20(18)
98.58(16)
4.6(6)
95.3(7)
c
100.06(19)
6.7(7)
166.4(7)
2.5(4)
0.0
a
b
c
The distance Ru−NPy is given. The angle C(24)−Ru−NPy is given. The angle C(1)−Ru-NPy is given.
a
Table 3. Comparison of the X-ray and DFT Ru−X Bond Distances in Selected Complexes
1a
1b
1c
1d
X-ray
DFT
X-ray
DFT
X-ray
DFT
X-ray
DFT
Ru−P
Ru−NHC
Ru−Ind
Ru−Cl
Ru−Cl
2.249
2.067
1.881
2.404
2.397
2.276
2.057
1.897
2.433
2.398
2.242
2.065
1.882
2.429
2.253
2.056
1.898
2.428
2.397
2.227
2.063
1.871
2.397
2.390
2.259
2.054
1.897
2.433
2.398
2.218
2.080
1.866
2.387
2.380
2.247
2.066
1.899
2.420
2.396
b
c
2.377
c
a
b
All distances in Å. Ru−Cl bond trans to the P(OR)3. Ru−Cl bond trans to the NHC.
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Table 4. DFT Calculated Bond Dissociation Energy of the Ligand L in the Cis Isomer (BDE) and Energy Difference between
a
the BDE of the Cis and Trans Isomers, ΔEc/t, of a Series of Ligands L in [RuCl2(Ind)(L)(SIMes)] Complexes
ΔEc/t
BDE
b
b
b
L
gas
7.6
toluene
CH3NO2
toluene
CH3NO2
toluene
toluene
P(OMe)3
P(OEt)3
P(OiPr)3
P(OCy)3
P(OPh)3
PMe3
7.3
6.7
8.0
7.7
6.4
7.9
7.2
8.9
21.8
21.0
15.3
15.2
12.1
20.5
14.7
−2.9
−3.9
3.5
40.3
42.7
44.0
46.7
38.1
38.1
36.0
23.8
29.3
32.6
21.4
6.8
4.0
3.5
4.2
7.2
7.7
4.6
4.2
4.8
7.9
8.3
4.7
4.6
5.3
7.1
7.8
−3.5
−6.1
−16.4
−15.7
−5.9
−6.1
−1.8
−4.4
−14.7
−14.5
−6.6
−4.7
1.8
−0.1
−2.6
−13.8
−7.9
−3.4
3.7
PEt3
PiPr3
−1.3
−11.9
−11.9
−3.9
−1.9
0.8
−10.8
−7.9
−0.7
−1.3
PCy3
PPh3
Py
−4.5
b
9.2
a
All values in kcal/mol. Positive ΔEc/t values imply that the cis isomer is favored. M06 calculated values.
followed a mononuclear and nondissociative mechanism,16a
which is further supported by the fact that the reaction rate
remained unaltered in the presence of an excess of free
phosphite. DFT calculations were undertaken to gain insight
into the unusual preference of the Caz-1a−d systems to form
the cis isomer. These results are summarized in Tables 3 and 4.
Before discussing the relative stability of the cis and trans
isomers, we briefly focus on the ability of the selected
computational recipe to reproduce correctly the coordination
distance of the various ligands around the Ru center (see Table
3). The data reported in Table 3 indicate that the combination
of the BP86 functional together with the SDD effective core
potential for Ru and the SVP basis set for main-group atoms
reproduces the experimental distances with an absolute mean
error of only 0.019 Å, which can be considered as a more than
satisfactory agreement, and it is consistent with previous
work.24 More specifically, the DFT geometries nicely reproduce
the experimental evidence that the Ru−Cl bond trans to the
P(OR)3 ligand is slightly longer than the Ru−Cl bond trans to
the NHC ligand (on average, by 0.018 Å in the experimental
structures versus 0.031 Å in the DFT structures).
Moving to the relative energy of the cis and trans isomers,
the cis isomer is calculated to be more stable than the trans
isomer for all the P(OR)3-based systems considered, in
agreement with the experimental findings, independent of the
solvent considered (see the positive ΔEc/t of Table 4). This is
in contrast to the pyridine- and the PR3-based complexes
where, with the exception of PMe3 in CH3NO2, the trans
isomer is preferred instead (see the negative ΔEc/t of Table 4).
This difference is not sensitive to the nature of the R group for
P(OR)3, with ΔEc/t spanning the rather small window of
roughly 3.5 kcal/mol in the gas phase, while the bulkiness of R
group has a major role in the case of PR3. The ΔEc/t values
show a fairly good linear correlation with the Tolman cone
angle for P(OR)3 and PR3, respectively (Figure 3).
Figure 3. Plot of the gas-phase DFT calculated ΔEc/t versus the
Tolman cone angle.23
toluene: i.e. polar solvents favor the cis isomer due to its higher
dipole moment. The relative stabilities of the cis and trans
isomers scarcely depend on the specific functional used
(compare the ΔEc/t values in toluene calculated with the
BP86 and M06 functionals in Table 4), although the M06
functional is slightly biased toward the cis isomer. This is
reasonable, considering that the M06 functional was fitted to
reproduce a large data set which includes also dispersion
stabilized systems and the cis isomer presents the bulkier NHC
and P(OR)3 ligands in a relative cis orientation, which
maximizes dispersion interaction between them.
Focusing on the absolute BDE, we next compared P(OMe)3
and PMe3, since steric effects should be minimal. The values in
toluene reported in Table 4 indicate that the BDE of P(OMe)3
in the trans isomer (14.6 kcal/mol) is 7.7 kcal/mol smaller than
the BDE of PMe3 (22.3 kcal/mol). This difference can be easily
rationalized, considering the lower basicity of phosphites
relative to that of phosphines. Conversely, in the cis isomer
the BDE of P(OMe)3 (21.8 kcal/mol) is 1.3 kcal/mol greater
than the BDE of PMe3 (20.5 kcal/mol), indicating that in the
cis isomer P(OMe)3 binds more strongly than PMe3 to the Ru
center, despite its reduced basicity. Structural analysis indicates
that the Ru−P(OMe)3 bond length in the cis isomer, 2.26 Å, is
0.07 Å shorter than in the trans isomer, 2.33 Å, whereas the
Ru−PMe3 bond length in the cis isomer, 2.34 Å, is only 0.05 Å
shorter than in the trans isomer, 2.39 Å. Furthermore, the
average P−O bond in the cis isomer, 1.66 Å, is 0.01 Å longer
than in the trans isomer, 1.65 Å, which is a classical sign of
Figure 3 also indicates that the different behavior between
P(OR)3 and PR3 is entirely electronic in nature, since the trend
observed for P(OR)3 is clearly shifted at higher ΔEc/t relative to
the trend observed of PR3. Furthermore, it indicates that
smaller ligands can be accommodated slightly better than larger
ligands in the rather hindered position cis to the SIMes ligand.
Finally, the solvent effect was evaluated, and in agreement with
previous results,25 for both P(OR)3 and PR3, the cis isomer is
stabilized more than the trans isomer in a high-polarity solvent
such as nitromethane than in low-polarity solvents such as
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a
Table 5. Comparative Evaluation of Complexes cis-Caz-1a−d
a
b
1
Reaction conditions: substrate (0.25 mmol), precatalyst (0.5 to 5 mol %), toluene (0.1 M), 80 °C. Conversions were determined by H NMR
based on substrates 2, 4 and 6 average of 2 reactions. TBDMS = tert-butyldimethylsilyl.
Figure 4. Comparative catalytic studies at low catalyst loading. Reaction conditions: 9 (0.5 mmol), precatalyst (0.5 mol %), toluene (0.1 M, 5 mL)
80 °C.33
back-donation from the metal into the empty π* orbitals
corresponding to P−O bonds.26,27 The strength of the Ru−
P(OMe)3 bond in the various isomers was evaluated with the
Mayer bond order (MBO),28 which is a valuable tool in the
analysis of the bonding in main-group compounds and has also
been used to characterize transition-metal systems.29,30 The
Ru−P(OMe)3 MBO in the cis isomer, 0.98, is 0.21 greater than
in the trans isomer, 0.77, whereas the Ru−PMe3 MBO in the
cis isomer, 0.78, is only 0.12 greater than in the trans isomer,
0.66, supporting the idea of a back-donation from the metal to
the phosphites.
As a final remark, the absolute BDE strongly depends on the
specific functional used. A simple comparison of the BDE in
toluene calculated with the BP86 and M06 functionals indicates
that the M06 functional, as expected, shows much stronger
binding in all cases. However, the M06 functional predicts that
the bulky and more basic PCy3, with a BDE of 37.2 kcal/mol,
binds to the trans isomer similarly to the small PMe3, with a
BDE of 38.2 kcal/mol, which is somewhat surprising
considering the steric encumbrance of PCy3. In this regard,
the BP86 functional predicts that PMe3, with a BDE of 22.3
kcal/mol, binds much better than PCy3, with a BDE of 10.6
kcal/mol. A recent benchmark study has indicated that both
functionals in combination with the solvation model adopted
here are probably underestimating and overestimating,
respectively, the relative BDE of small and large phosphines.31
Precatalyst Activity Comparison. The new complexes
were evaluated in diene and enyne ring-closing metathesis
(RCM) and cross-metathesis (CM) of benchmark substrates
(Table 5). Preliminary experiments showed that cis-Caz-1a
displayed a latent character.16a Thus, reactions were performed
at 80 °C in toluene. A general trend was found between
reactivity and phosphite substitution. Triisopropyl phosphite
and triphenyl phosphite containing precatalysts cis-Caz-1a,d
exhibited comparable efficiency, the former being slightly more
active. Indeed, in the presence of 1 mol % of precatalyst, after
30 min, complete conversion of 2 was achieved with cis-Caz-1a,
while traces of 2 could still be detected with cis-Caz-1d (Table
5, entries 1 and 6). Even clearer evidence of cis-Caz-1a being
faster than cis-Caz-1d was found when reactions with substrates
4 and 6 were carried out (Table 5, entries 8 and 13 versus 12
and 19). Finally, cis-Caz-1b,c, featuring trimethyl and triethyl
phosphite, respectively, were similarly active but less so than cis-
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Caz-1a,d, requiring longer reaction times to reach full
conversions.
coordination to the ruthenium could promote simultaneous
dissociation of the labile ligand, we investigated if the smallest
olefin, ethylene, could coordinate to Caz-1a. However,
calculations indicate that, in the presence of a Ru−indenylidene
bond, only isomer A of Figure 5 is stable, even if it is very high
in energy. The other isomers tested, B and C, are not stable,
and ethylene detached from the ruthenium during geometry
optimization.
The activity of cis-Caz-1a−d was also compared to those of
commercially available precatalysts in the RCM of hindered
N,N-dimethallyltosylamine 9 at relatively low catalyst loading
(Figure 4).32 The commercially available Ind-II, Ind-III, G-II,
and Hov-II were not able to achieve more than 60%
conversions in the benchmark transformation. Due to their
increased stability, Ind-II and Hov-II were the most efficient of
the “traditional” precatalysts. G-II initiated as fast as the other
precatalysts but did not permit more than 17% conversion;
Ind-III gave no conversion. In comparison, phosphite-
containing precatalysts cis-Caz-1b,c (bearing P(OMe)3 and
P(OEt)3, respectively) displayed a very slow rate of conversion
in addition to an activation period of approximately 1.75 h. The
contrast is striking with cis-Caz-1a,d, featuring P(OiPr)3 and
P(OPh)3, respectively; these precatalysts were able to reach full
conversions in less than 7 h. In these cases, an induction period
was also observed, confirming the latent character of such
complexes. Catalyst efficiency can thus be classified as follows:
cis-Caz-1a (P(OiPr)3) > cis-Caz-1d (P(OPh)3) ≫ cis-Caz-1b
(P(OMe)3) ≈ cis-Caz-1c (P(OEt)3).
The influence of the concentration was investigated in the
RCM of 9 with precatalyst cis-Caz-1a.34 The reaction was
conducted at three different concentrations: 0.05, 0.1, and 1 M.
The rate of the reaction appeared similar whatever the
concentration, indicating that concentrated reactions could be
carried out without affecting the reaction outcome.
The reactivity trend observed above could be put in direct
relation with the affinity of the different phosphites to the metal
center (Table 4). Indeed, whether in the cis or trans complexes,
the binding energies of the phosphites to the ruthenium follow
the opposite trend: P(OMe)3 > P(OiPr)3. In addition,
regardless of the phosphite used, the binding is stronger in
the cis than in the trans configuration, suggesting that, during
the mechanism, decoordination of the phosphite in a trans
position should be more facile.35
In the case of the Ru−methylidene bond that should be
formed when P(OiPr)3 coordinates back to the ruthenium
center after some productive metathesis events, the energies of
all isomers A−C were located. Isomer C, which derives from a
trans coordination of ethylene to the methylidene moiety, is the
most stable isomer. The difference in behavior in comparison to
trans-Caz-1a can be attributed to the different bulkiness of the
indenylidene and methylidene groups, which is evidenced in
the rather larger Cl−Ru−C(indenylidene) angles in trans-Caz-
1a relative to those in the methylidene analogue (Figure 5).
The bulky indenylidene moiety pushes the Cl atoms toward the
vacant coordination position around the Ru atom, preventing
olefin coordination as the sixth ligand. This suggests that the
mechanism of activation of Caz-1a, with olefins bulkier than
ethylene, could proceed via dissociation of P(OiPr)3. In
contrast, in the case of the Ru−methylidene complex an
associative mechanism of activation cannot be excluded.
Reaction Scope. In order to explore the tolerance of cis-
Caz-1a to different functionalities, a wide range of substrates in
various metathesis transformations was studied. The reactions
were performed in toluene at 80 °C in the presence of 1−5 mol
% of cis-Caz-1a, the higher catalyst loading being only necessary
for the formation of 14 and 22 featuring a tetrasubstituted
double bond (Table 6). The RCM of unhindered malonate
derivatives was achieved in short reaction times (less than 1 h)
and in good yields. Indeed, di- and trisubstituted cyclopentenes
12 and 3 were obtained in quantitative yields. Nonetheless,
highly constrained substrate 13 could not be fully cyclized, even
after 24 h at 80 °C, and was isolated in 70% yield (Table 6,
entry 3). Six- and seven-membered rings 16 and 18 were
obtained in excellent yields; interestingly, no increase in the
reaction time was required in comparison to five-membered-
ring formation. Of note, a concentration of 0.05 M was
necessary to obtain 18 without parasitic polymerization.
The RCM of nitrile-containing substrates 19 and 21 (Table
6, entries 6 and 7) was also studied. Nonhindered cyclopentene
20 was isolated in good yield; however, cis-Caz-1a was unable
to achieve complete conversion of 21, and only a 55% yield of
22 was obtained. The cyclization of tosylamine-based olefins
was found to be very efficient regardless of steric hindrance and
ring size (Table 6, entries 8−12); a small increase in the
reaction time was needed for larger ring sizes, and a catalyst
loading of only 2 mol % was necessary to achieve the
cyclization of 9 and 29 to obtain tetrasubstituted five- and six-
membered rings in good yields.
To obtain more insight into the activation/initiation steps,
additional DFT calculations were performed (Figure 5).
Considering that recent results indicate that in the case of
Hoveyda−Grubbs catalysts the activation step could have an
associative-displacement character,36 which suggests that olefin
Amide- and ether-based substrates were also efficiently
cyclized, with yields spanning from 80% to 99%. Increasing the
ring size from six to seven members proved routine, as products
34, 36, 38, and 40 were obtained in excellent yields in less than
1 h (Table 6, entries 14−17).
Enyne ring-closing metathesis is a powerful tool enabling the
synthesis of dienes that can undergo further Diels−Alder
reaction and thus readily furnish bicyclic compounds.
Substrates 4 and 41 were fully converted after 30 min;
however, 5 was isolated in only 75% yield (Table 7, entries 1
Figure 5. (top) Energies (kcal/mol) of various isomers of Caz-1a with
an ethylene coordinated molecule. Reported energies are in toluene
with cis-Caz-1a + C2H4 taken as reference at 0 kcal/mol (top). M06
values are reported in parentheses. (bottom) Schematic representation
of relevant angles in Ru−indenylidene and Ru−methylidene isomers
of trans-Caz-1a.
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a
Table 6. Ring-Closing Metathesis Behavior of cis-Caz-1a
Table 7. Enyne Ring-Closing Metathesis Behavior of cis-Caz-
1a
a
a
Reaction conditions: substrate (0.25 mmol), cis-Caz-1a (1 mol %),
b
toluene (0.1 M), 80 °C. Average of two runs. Conversions were
determined by H NMR based on enyne. Isolated yields are given in
parentheses. 5 mol % of precatalyst was used. Ts = tosylate.
1
c
partners allowed the isolation of the desired products, in good
yields compared to previously reported results;11c thus proving
once more that cis-Caz-1a displays good tolerance toward a
variety of functional groups. However, allyltosylamine 51 was
found to be incompatible with the catalytic system, as no
conversion to 52 was observed.
Ester-containing substrates 55 and 57 bearing varied chain
lengths were also coupled with methyl acrylate 7 in good yields
(Table 8, entries 5 and 6). Both products were isolated as the E
isomers, identified by 1H NMR. Reaction of eugenol 59
(essential oil of clove) with acrolein 49 was found to be
efficient and did not require protection of its phenolic moiety
(Table 8, entry 7). Finally, p-chlorostyrene 61 reacted well with
methyl acrylate 7 and gave 62 in 81% yield with an E/Z ratio of
20:1 (Table 8, entry 8). It is worth noting that no formation of
self-metathesis compounds was observed during the screening
of substrates.
Evaluation at Low Catalyst Loading. Low catalyst
loading experiments in RCM, enyne metathesis, and CM
were performed to study the limitations of cis-Caz-1a. Easily
cyclized substrates in RCM, i.e. affording di- or trisubstituted
cyclic olefins, were accessed with excellent conversions with as
low as 0.02 mol % catalyst (Table 9, entries 1−8); increasing
the ring size to form six- or seven-membered rings did not
require higher loading than 0.05 mol %, while increasing the
steric hindrance to form trisubstituted olefins required slightly
higher catalyst loadings. Tetrasubstituted olefins 10 and 30
were formed using as little as 0.1 mol % of cis-Caz-1a (Table 9,
entries 10 and 11), whereas only 75% conversion of malonate
derivative 13 was achieved using 0.5 mol % of precatalyst
(Table 9, entry 9). To the best of our knowledge, these results
represent the lowest catalyst loadings affording complete
conversion of substrates 10 and 30. While more active catalysts
for the ring-closing metathesis of less hindered olefins have
been reported,15b,32 only a few examples of catalysts as robust
as cis-Caz-1a have been described in the literature,13d,e,32a
making it an excellent compromise between reaction condition
harshness and tolerance to steric hindrance. For this reason, it
represents a superior choice to more popular commercially
available catalysts.38
a
Reaction conditions: substrate (0.25 mmol), cis-Caz-1a (1 mol %),
b
toluene (0.1 M), 80 °C. Average of two runs. Conversions were
determined by H NMR based on diene. Isolated yields are given in
parentheses. 5 mol % of precatalyst was used. 0.05 M concentration
was used. 2 mol % of precatalyst was used. Ts = tosylate.
1
c
d
e
and 2). A longer reaction time was necessary to convert
hindered compound 43. Once again, a relatively low isolated
yield of 71% was obtained; such behavior could result from
parallel polymerization reactions that can easily occur at
elevated temperature. While substrate 45 remained unchanged
after 24 h, more hindered enyne 47 was efficiently cyclized in 3
h (Table 7, entries 4 and 5).37 In conclusion, precatalyst cis-
Caz-1a allowed the formation of dienes from enynes in short
reaction times with acceptable yields.
Finally, the ability of precatalyst cis-Caz-1a to promote
intermolecular cross-metathesis (CM) was investigated (Table
8). CM reactions are more difficult than their RCM
counterparts, as side-formation of self-metathesis products
may occur. Several substrates were reacted in the presence of 2
mol % of cis-Caz-1a and 2 equiv of alkene partners in toluene at
80 °C. Silylated compound 6 was efficiently coupled with
various olefins (Table 8, entries 1−4). The use of methyl
acrylate 7, acrolein 49, and diallylic acetate 53 as alkene
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a
Table 8. Cross-Metathesis Behavior of cis-Caz-1a
a
b
Reaction conditions: substrate (0.25 mmol), alkene partner (0.5 mmol), cis-Caz-1a (2 mol %), toluene (0.1 M), 80 °C. Average of two runs;
c
1
isolated yields. E/Z ratios were determined by H NMR. Only 1 equiv of alkene partner was used.
ppm) at 298 K. Elemental analyses were performed by the University
In CM it was found that a loading of 0.1−0.2 mol % was
required to achieve good conversions. Compounds 56 and 58
could be isolated in comparable yields than with 2 mol % of
catalyst (Table 9, entries 14 and 15 and Table 8, entries 5 and
6). However, in order to limit self-metathesis of the substrate
under the slightly harsher reaction conditions, 5 equiv of the
less electron rich alkene partner was required. In addition, for
59 and 61, a lower concentration of 0.1 M was required. The
use of acrolein 49 seemed detrimental to the metathesis
reaction, as only 12% of CM product 60 was isolated.
of St Andrews Analytical Service. Calorimetric studies were performed
on a CALVET C80 solution calorimeter. The complexes cis-Caz-1a
and trans-Caz-1a were synthesized according to literature procedur-
es.16a
Synthesis of Complexes cis-Caz-1b−d: General Procedure.
Under an inert atmosphere, phosphite (1−4 equiv) was added to a
solution of Ind-III in dichloromethane (0.05 M). The mixture was
stirred for the appropriate time at 40 °C, cooled to room temperature,
and concentrated in vacuo (1 mL). Pentane (10 mL) was added, and
the precipitate was collected by filtration and washed with pentane (3
× 5 mL).
Dichloro[N,N′-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene](3-
phenylinden-1-ylidene)(trimethyl phosphite)ruthenium (cis-Caz-
1b). According to the general procedure, 1 equiv of trimethyl
phosphite was used and the reaction mixture was stirred for 3 h at 40
°C. Additional washes of the solid with cold MeOH (3 × 5 mL)
afforded the product as a brown solid (366 mg, 57%). 1H NMR
(CD2Cl2, 400 MHz): δ (ppm) 8.84 (dd, 1H, 3JHH = 7.3 Hz, 4JHH = 0.8
Hz, indenylidene CH7), 7.66−7.88 (m, 2H, indenylidene CH9), 7.53−
7.41 (m, 3H, indenylidene CH10 and CH11), 7.34 (ddd, 1H, 3JHH = 7.3
CONCLUSION
■
The family of mixed NHC−phosphite complexes has been
extended, and the synthesis and characterization of the three
new catalysts cis-Caz-1b−d, bearing phosphites featuring
various electronic and steric properties, has been reported. All
complexes present an unusual cis configuration between the
NHC and the phosphite ligand. Mechanistic and computational
studies confirmed that the cis isomer is more stable than the
trans form for all the P(OR)3-based systems considered and
indicated that the different behavior between P(OR)3 and PR3
is entirely electronic in nature. Additionally, experimental and
computational studies support a mononuclear nondissociative
mechanism for the isomerization from trans-Caz-1 to cis-Caz-1.
Catalytic studies demonstrated that precatalysts cis-Caz-1a−d
are able to promote ring-closing, cross, and enyne metatheses
in excellent yields even at low catalyst loading. Indeed, in RCM,
catalyst loadings as low as 0.02 mol % for “easily cyclized
substrates” and 0.1 mol % for more challenging ones, leading to
tetrasubstituted olefins, were enough to reach full conversions,
demonstrating the robust character of the cis-Caz-1 family of
catalysts.
4
3
Hz, JHH = 0.8 Hz, indenylidene H5), 7.27 (ddd, 1H, JHH = 7.3 Hz,
3
4JHH = 0.8 Hz, indenylidene H6), 7.06 (d, JHH = 7.3 Hz, 1H,
indenylidene H4), 7.03 (s, 1H, mesityl CH), 7.02 (s, 1H, mesityl CH),
6.38 (s, 1H, indenylidene H2), 6.28 (s, 1H, mesityl CH), 6.23 (s, 1H,
mesityl CH), 4.03−3.63 (m, 4H, carbene H4′ and H5′), 3.23 (d, 9H,
3JPH = 10.7 Hz, POCH3), 2.68 (s, 3H, mesityl CH3), 2.62 (s, 3H,
mesityl CH3), 2.37 (s, 3H, mesityl CH3), 2.35 (s, 3H, mesityl CH3),
1.99 (s, 3H, mesityl CH3), 1.71 (s, 3H, mesityl CH3). 13C{1H} NMR
2
(CD2Cl2, 100.6 MHz): δ (ppm) = 292.7 (d, JCP = 23.9 Hz,
2
indenylidene C1), 208.5 (d, JCPcis = 14.6 Hz, carbene C2′), 144.0 (s,
CIV), 141.2 (d, 3JCP = 2.5 Hz, indenylidene C7a), 140.4 (d, 3JCP = 13.5
Hz, indenylidene C2H), 139.2 (s, CIV), 139.1 (s, CIV), 138.8 (s, CIV),
138.3 (s, CIV), 137.0 (s, CIV), 136.5 (s, CIV), 136.4 (s, CIV), 135.3 (s,
CIV), 135.1 (s, CIV), 130.6 (s, mesityl CH), 130.1 (s, indenylidene C7),
130.0 (s, mesityl CH), 129.7 (s, mesityl CH), 129.5 (s indenylidene
C6), 129.3 (indenylidene C5), 129.3 (s, indenylidene C10), 129.0 (s,
indenylidene C11), 127.3 (s, indenylidene C9), 117.3 (s, indenylidene
C4H), 53.4 (br s, OCH3), 52.6 (s, carbene C5′H2), 51.9 (s, carbene
C4′H2), 21.5 (s, mesityl CH3), 21.1 (s, mesityl CH3), 20.9 (s, mesityl
CH3), 19.4 (s, mesityl CH3), 19.1 (s, mesityl CH3), 18.3 (s, mesityl
CH3). 31P{1H} NMR (CD2Cl2, 162 MHz): δ (ppm) 132.2. Anal.
Calcd for C39H45Cl2N2O3PRu: C, 59.09; H, 5.72; N, 3.53. Found: C,
59.38; H, 6.07; N, 3.37.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were performed under an
inert atmosphere of argon or nitrogen using standard Schlenk line and
glovebox techniques. Solvents were dispensed from a solvent
purification system. All other reagents were used without further
1
purification. H, 13C{1H}, and 31P{1H} 1D and 2D nuclear magnetic
resonance (NMR) spectra were recorded on a Bruker AVANCE 400
Ultrashield spectrometer using the residual solvent peak as reference
(CHCl3, δH 7.26 ppm, δC 77.16 ppm; CH2Cl2, δH 5.32 ppm, δC 53.80
Dichloro[N,N′-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene](3-
phenylinden-1-ylidene)(triethyl phosphite)ruthenium (cis-Caz-1c).
According to the general procedure, 1 equiv of triethyl phosphite was
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a
Table 9. Experiments at Low Catalyst Loading with cis-Caz-1a
a
Reaction conditions for RCM (entries 1−11): substrate (0.25 mmol), cis-Caz-1a, toluene (0.5 M), reflux, or neat, 120 °C for substrates 2, 11, 13.
Reaction conditions for enyne-RCM (entries 12 and 13): substrate (0.25 mmol), cis-Caz-1a, toluene (0.1 M), reflux. Reaction conditions for CM
b
(entries 14−17): substrate (0.25 mmol), alkene partner (1.25 mmol), cis-Caz-1a, toluene (0.5 M), reflux. Average of two runs. Conversions were
c
d
1
determined by H NMR based on substrates. Isolated yields are given in parentheses. A concentration of 0.05 M was used. Isolated yields are
e
given. E/Z ratios are given in parentheses. A concentration of 0.1 M was necessary to avoid self-metathesis of the substrate.
used and the reaction mixture was stirred for 5 h at 40 °C. The
129.5 (s, indenylidene C6H), 129.3 (s, indenylidene C5H), 129.2 (s,
indenylidene C10H), 128.9 (s, indenylidene C11H), 127.3 (s,
indenylidene C9H), 117.3 (s, indenylidene C4H), 62.5 (br s,
CH2CH3), 52.5 (s, carbene C5′H2), 51.9 (s, carbene C4′H2), 21.5 (s,
mesityl CH3), 21.1 (s, mesityl CH3), 20.9 (s, mesityl CH3), 19.4 (s,
mesityl CH3), 19.4 (s, mesityl CH3), 19.2 (s, mesityl CH3), 16.4 (s,
CH2CH3), 16.3 (s, CH2CH3). 31P-{1H} NMR (CD2Cl2, 162 MHz): δ
(ppm) 127.5. Anal. Calcd for C42H51Cl2N2O3PRu: C, 60.43; H, 6.16;
N, 3.36. Found: C, 60.57; H, 6.57; N, 3.60.
1
product was obtained as a brown solid (617 mg, 53%). H NMR
(CD2Cl2, 400 MHz): δ (ppm) 8.85 (dd, 1H, 3JHH = 7.3 Hz, 4JHH = 0.8
Hz, indenylidene H7), 7.65 (m, 2H, indenylidene H9), 7.52−7.42 (m,
3
4
3H, indenylidene H11 and H10), 7.34 (ddd, 1H, JHH = 7.3 Hz, JHH
=
0.8, indenylidene H6), 7.28 (ddd, 1H, JHH = 7.3 Hz, JHH = 0.8 Hz,
3
4
indenylidene H5), 7.08 (d, 1H, JHH = 7.3 Hz, indenylidene H4), 7.03
3
(s, 1H, mesityl CH), 7.01 (s, 1H, mesityl CH), 6.56 (s, 1H,
indenylidene H2), 6.31 (s, 1H, mesityl CH), 6.20 (s, 1H, mesityl CH),
'
'
Dichloro[N,N′-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene](3-
phenylinden-1-ylidene)(triphenylphosphite)ruthenium (cis-Caz-1d).
According to the general procedure, 4 equiv of triphenyl phosphite was
used and the reaction mixture was stirred for 15 h at 40 °C. The
4.01−2.99 (m, 10H, carbene H4, H5 and CH2CH3), 3.55−3.26 (br s,
3H, CH2CH3), 2.68 (s, 3H, mesityl CH3), 2.67 (s, 3H, mesityl CH3),
2.42 (s, 3H, mesityl CH3), 2.34 (s, 3H, mesityl CH3), 1.98 (s, 3H,
1
mesityl CH3), 1.71 (s, 3H, mesityl CH3), 1.05 (br s, 9H, CH2CH3).
product was obtained as a brown solid (500 mg, 76%). H NMR
2
13C{1H} NMR (CD2Cl2, 100.6 MHz): δ (ppm) = 292.5 (d, JC−P
=
3
(CD2Cl2, 400 MHz): δ (ppm) 8.72 (d, 1H, JHH = 7.3 Hz,
23.9 Hz, indenylidene C1), 208.7 (d, JCPcis = 14.6 Hz, carbene C2′),
2
indenylidene H7), 7.52−7.43 (m, 1H, indenylidene H9), 7.42−7.28
(m, 5H, indenylidene H6, H10, and H11), 7.23 (ddd, 1H, 3JHH = 7.3 Hz,
4JHH = 0.9 Hz, indenylidene H5), 7.14−6.59 (br m, 18H, P(OPh)3 CH,
mesityl CH and indenylidene H4), 6.32 (s, 1H, indenylidene H2), 6.24
(br s, 2H, mesityl CH), 4.09−3.71 (m, 4H, carbene H4′ and H5′), 2.81
(s, 3H, mesityl CH3), 2.70 (s, 3H, mesityl CH3), 2.46 (s, 3H, mesityl
143.5 (s, CIV), 141.2 (d, JCP = 3.1 Hz, indenylidene C7a), 140.4 (d,
3
3JCP = 14.6 Hz, indenylidene C2H), 139.1 (s, CIV), 138.8 (s, CIV),
138.3 (s, CIV), 138.2 (s, CIV), 137.0 (s, CIV), 136.6 (s, CIV), 136.5 (s,
CIV), 135.5 (s, CIV), 135.2 (s, CIV), 130.4 (s, mesityl CH), 130.1 (s,
indenylidene C7H), 130.0 (s, mesityl CH), 129.8 (s, mesityl CH),
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Organometallics
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CH3), 2.12 (s, 3H, mesityl CH3), 2.04 (s, 3H, mesityl CH3), 1.63 (s,
3H, mesityl CH3). 13C{1H} NMR (CD2Cl2, 100.6 MHz): δ (ppm)
293.2 (d, 2JCP = 24.7 Hz, indenylidene C1), 207.2 (d, 2JCPcis = 13.5 Hz,
(RYC-2009-05226 to A.P.) and the Catalan Government (BE-
2011 to A.P.) for funding.
3
carbene C2′), 151.6 (d, JCP = 13.2 Hz, P(OPh)3 CIV), 143.9 (s, CIV),
REFERENCES
3
3
141.3 (d, JCP = 2.0 Hz, indenylidene C7a), 140.3 (d, JCP = 14.6 Hz,
indenylidene C2H), 139.7 (s, CIV), 139.1 (s, CIV), 138.5 (s, CIV), 138.4
(s, CIV), 136.8 (s, CIV), 136.6 (s, CIV), 136.3 (s, CIV), 135.7 (s, CIV),
134.3 (s, CIV), 130.8 (s, mesityl CH), 130.7 (s, mesityl CH), 130.4 (s,
indenylidene H7), 130.3 (s, mesityl CH), 130.1 (s, mesityl CH), 129.5
(s indenylidene C5H), 129.3 (s, indenylidene C6H), 129.3 (s, P(OPh)3
CmH), 128.9 (s, indenylidene C10H), 128.8 (s, indenylidene C9H),
128.0 (s, indenylidene C11H), 125.1 (s, P(OPh)3 CpH), 122.0 (s,
P(OPh)3 CoH), 117.6 (s, indenylidene C4H), 52.9 (s, carbene C5′H2),
52.1 (s, carbene C4′H2), 21.6 (s, mesityl CH3), 20.9 (s, mesityl CH3),
20.8 (s, mesityl CH3), 19.6 (s, mesityl CH3), 19.6 (s, mesityl CH3),
19.2 (s, mesityl CH3). 31P{1H} NMR (CD2Cl2, 162 MHz): δ (ppm)
113.2. Anal. Calcd. for C54H51Cl2N2O3PRu (978.94): C, 66.25; H,
5.25; N, 2.86. Found: C, 66.39; H, 5.15; N, 2.99.
■
(1) For reviews on metathesis, see: (a) Furstner, A. Angew. Chem., Int.
̈
Ed. 2000, 39, 3013. (b) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res.
2001, 34, 18. (c) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-
VCH: Weinheim, Germany, 2003. (d) Astruc, D. New J. Chem. 2005,
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Chem. Rev. 2010, 110, 4865. (I) Luan, X.; Dorta, R.; Leitgeb, A.;
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Transition Metal Catalysis and Organocatalysis; Cazin, C. S. J., Ed.;
Springer: London, 2011; Vol. 32, p 63.
(2) For the Nobel lecture see: (a) Chauvin, Y. Adv. Synth. Catal.
2007, 349, 27. (b) Grubbs, R. H. Adv. Synth. Catal. 2007, 349, 34.
(c) Schrock, R. R. Adv. Synth. Catal. 2007, 349, 41.
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J. M.; Hanson, P. R. Chem. Rev. 2004, 104, 2239. (c) Nicolaou, K. C.;
Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490. (d) Van
de Weghe, P.; Bisseret, P.; Blanchard, N.; Eustache, J. J. Organomet.
Chem. 2006, 691, 5078. (e) Donohoe, T. J.; Orr, A. J.; Bingham, M.
Calorimetric Measurements. Two separate solutions containing
the complex and the ligand were prepared: stock solution A,
containing 30 mg (0.048 mmol) of [Ru(μ-Cl)Cl(η6-cymene)]2 in
CH2Cl2 (0.75 mL), and stock solution B, containing the ligand
P(OR)3 (0.293 mmol, 6 equiv) in CH2Cl2 (2 mL). The first container
of the cell was charged with stock solution B (0.75 mL) and the
second container with stock solution A (0.3 mL). The cell was placed
in the calorimeter and the temperature stabilized at 30 °C. The
solutions were then mixed, and a thermogram was recorded. The
reported enthalpy of reaction is based on an average of three
consistent measurements. −ΔHrxn (kcal/mol): P(OPh)3, 34.1 0.4;
́
Angew. Chem., Int. Ed. 2006, 45, 2664. (f) Gradillas, A.; Perez-Castells,
J. Angew. Chem., Int. Ed. 2006, 45, 6086. (g) Hoveyda, A. H.;
Zhugralin, A. R. Nature 2007, 450, 243. (h) Kotha, S.; Lahiri, K. Synlett
2007, 2767. (i) Compain, P. Adv. Synth. Catal. 2007, 349, 1829.
(j) Hoveyda, A.; Malcolmson, S.; Meek, S.; Zhugralin, A. Angew.
Chem., Int. Ed. 2010, 49, 34.
P(OMe)3, 42.8
0.6; P(OEt)3, 42.9
0.5; P(OiPr)3, 43.2
0.4.
Relative bond dissociation energies (BDE) (kcal/mol): P(OPh)3,
17.05; P(OMe)3, 21.40; P(OEt)3, 21.45; P(OiPr)3, 21.60.34
Computational Details. All calculations were performed with the
Gaussian09 package (Revision A.1)39 at the BP86 GGA level40 using
the SDD ECP on Ru41 and the split-valence plus one polarization
function SVP basis set on all main-group atoms during geometry
optimizations.42 The reported energies have been obtained via single
point calculations on the BP86 geometries with triple-ζ valence plus
polarization function TZVP basis set for main group atoms using the
M06 functional.43 Solvent effects, toluene and nitromethane, were
included with the PCM model.44
(4) (a) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H.; Ziller, J. W. J.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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H. J. Am. Chem. Soc. 1999, 121, 791.
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Chem. Soc. 2000, 122, 8168.
Text, tables, figures, and CIF files giving detailed experimental
procedures for catalysis, for DFT calculations and calorimetric
studies, UV−vis spectra of cis-Caz-1a and trans-Caz-1a, NMR
spectra of complexes and metathesis products, and crystallo-
graphic data for cis-Caz-1b−d. This material is available free of
charge via the Internet at http://pubs.acs.org. The CIF files of
crystal structures for cis-Caz-1b-d have also been deposited
with the CCDC (nos. 862342−862344, respectively). These
data can be obtained free of charge from http://www.ccdc.cam.
ac.uk.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cc111@st-andrews.ac.uk.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Royal Society (University
Research Fellowship to C.S.J.C.), the EC (CP-FP 211468-2
EUMET and CIG09-GA-2011-293900), the Spanish MINECO
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(34) See the Supporting Information for details.
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was inadvertently published on the web on October
16, 2012, before all author corrections were applied.
Corrections were made to Scheme 1, Table 1, and experimental
section and the corrected version reposted on October 22,
2012.
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