A thermally robust ruthenium phosphonium alkylidene catalyst-the effect of more bulky N -heterocyclic carbene ligands on catalyst performance in olefin metathesis reactions

Article abstract of DOI:10.1139/cjc-2013-0156

Three new ruthenium phosphonium alkylidene complexes incorporating N-heterocyclic carbene ligands with bulky N-aryl groups (2,6-diethyl, L = 1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene (H₂IDEP) and 2,6-diisopropyl, L = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (H ₂ID-i-PP)) were synthesized and characterized. The H ₂ID-i-PP supported complex was found to exhibit excellent thermal stabilities relative to the parent N-mesityl (N-Mes) complexes as well as the H₂IDEP supported complexes. All three phosphonium alkylidenes were evaluated in comparison to the N-Mes derivative and Grubbs second generation catalyst using standard olefin metathesis reactions and conditions. The complex containing the bulky H₂ID-i-PP ligand was found to have excellent activity and longevity in comparison to the other catalysts. Although initiation rates were slow for this sterically bulky precatalyst, its superior activity led to the best overall efficiency in test reactions.

Full text of DOI:10.1139/cjc-2013-0156

9
35
ARTICLE
A thermally robust ruthenium phosphonium alkylidene catalyst — the
effect of more bulky N-heterocyclic carbene ligands on catalyst
performance in olefin metathesis reactions
Erin M. Leitao, Warren E. Piers, and Masood Parvez
Abstract: Three new ruthenium phosphonium alkylidene complexes incorporating N-heterocyclic carbene ligands with
bulky N-aryl groups (2,6-diethyl, L = 1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene (H IDEP) and 2,6-diisopropyl, L = 1,3-bis(2,
2
6
-diisopropylphenyl)imidazolin-2-ylidene (H ID-i-PP)) were synthesized and characterized. The H ID-i-PP supported complex was
2
2
found to exhibit excellent thermal stabilities relative to the parent N-mesityl (N-Mes) complexes as well as the H IDEP supported
2
complexes. All three phosphonium alkylidenes were evaluated in comparison to the N-Mes derivative and Grubbs second
generation catalyst using standard olefin metathesis reactions and conditions. The complex containing the bulky H ID-i-PP
2
ligand was found to have excellent activity and longevity in comparison to the other catalysts. Although initiation rates were
slow for this sterically bulky precatalyst, its superior activity led to the best overall efficiency in test reactions.
Key words: olefin metathesis, ruthenium, catalysis, ligand design, decomposition.
Résumé : On a synthétisé et caractérisé trois nouveau complexes ruthénium phosphonium alkylidène dans lesquels sont intégrés des
ligands carbènes N-hétérocycliques contenant des groupements N-aryles volumineux (2,6-diéthyle, L = 1,3-bis(2,6-diéthylphényl)
imidazolin-2-ylidene (H IDEP) et 2,6-diisopropyle, L = 1,3-bis(2,6-diisopropylphényl)imidazolin-2-ylidene (H IDiPP)). On a constaté
2
2
que les complexes supportés par H ID-i-PP présentaient une excellente stabilité thermique comparativement aux complexes
2
N-mesityl (N-Mes) parents, ainsi qu'aux complexes supportés par H IDEP. On a évalué les trois phosphonium-alkylidènes com-
2
parativement au dérivé N-Mes et au catalyseur de Grubbs de deuxième génération en utilisant des réactions et des conditions
classiques de métathèse des oléfines. L'activité et la longévité du complexe contenant le volumineux ligand H ID-i-PP se sont
2
avérées excellentes comparativement aux autres catalyseurs. Même si les vitesses d'initiation de ce précatalyseur stériquement
volumineux étaient faibles, son activité supérieure a donné lieu a` la meilleure efficacité globale dans les réactions de test.
[
Traduit par la Rédaction]
Mots-clés : métathèse des oléfines, ruthénium, catalyse, conception de ligands, décomposition.
tion.48 The phosphonium alkylidene catalysts exemplified by V
and 1-R -A in Chart 1 eliminate L from the catalyst altogether,
Introduction
Ruthenium-based olefin metathesis catalysts that combine high
activity (turn over frequency) with high thermal stability (turn
over number) are of interest for the continued development and
application of this important technology. Since these two attri-
butes tend to work against each other, imparting both can be a
challenge, one that is met mainly through catalyst modification
via ligand design. Using the catalyst platforms known as the first
3
4
9
50
resulting in very rapidly initiating and highly active catalysts.
These four-coordinate catalysts are accessed by protonating the
terminal carbide ligand formed in stoichiometric metathesis re-
5
1
actions between I or II with Feist's ester. However, as a con-
sequence of this high activity, they tend to suffer from poor
5
2
longevity, particularly at higher temperatures or low concentra-
tions of substrate. We have demonstrated that the dominant deg-
radation pathway for these compounds involves reversible C–H
addition from the o-methyl group on the aryl substituent of the
NHC ligand across the phosphonium alkylidene double bond.
1
and second generation Grubbs complexes or the Grubbs–Hoveyda
complexes2 (Chart 1: I, II and III, IV, respectively), impressive
strides have been made in improving the longevity of these highly
active catalysts. Further efforts have focused on substituting the
3–14
chloride ligands for other anionic ligands,
of alkylidene ligands have also been employed.1
proach has involved extensive modification of N-heterocyclic car-
and a wide variety
(
Scheme 1). Irreversible elimination of the methyl phosphonium
5–29
Another ap-
salt forms a highly reactive chelating alkylidene that may be
trapped with normally reticent substrates like 1,1-dichloroethene.
1,30–45
bene (NHC) ligands
on the generally more active second
5
2
Based on these observations, we set out to stymie the initial C–H
generation ruthenium olefin metathesis catalysts. These investi-
gations have unearthed a number of systematic variations that
can lead to improvements in activity and thermal stability, but
rarely both. The advances in this field have been summarized in
activation by modification of the ortho substitutents on the N-aryl
3
0,35
groups.
In particular, we increased their steric bulk, incorpo-
rating ethyl and iso-propyl groups as shown in Chart 1. The idea
being that this would increase the barriers required to attain the
transition state for C–H addition across the Ru=C bond.5 Exam-
4
6,47
recent reviews.
3
The activity of catalysts I–IV is to a large degree related to the
facility with which they dissociate the phosphine ligand trans to L
and this dissociated phosphine can lead to catalyst decomposi-
ples of catalyst precursors 2-R
-A and 3-R -A (Chart 1) were readily
3
3
prepared and their performance evaluated against the previously
Received 10 April 2013. Accepted 17 May 2013.
E.M. Leitao, W.E. Piers, and M. Parvez. Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada.
Corresponding author: Warren E. Piers (e-mail: wpiers@ucalgary.ca).
Can. J. Chem. 91: 935–942 (2013) dx.doi.org/10.1139/cjc-2013-0156
Published at www.nrcresearchpress.com/cjc on 23 May 2013.

936
Can. J. Chem. Vol. 91, 2013
Chart 1.
in an 87% yield using a slightly different starting complex, namely
1
2
the bis-pyridine adduct (H IDEP)(py) RuCl =C(H)Ph prepared us-
2
2
2
0,56
5
ing methodology developed in our lab.
Attempts to prepare
the missing compound in the series (i.e., 3-i-Pr -Cl) were not suc-
3
cessful (see the Supplementary material section for details on
these attempts). These zwitterionic chloride salts were used as
catalyst precursors due to their superior stability in the solid state,
and easily lead to the highly active, four-coordinate catalysts n-R3-
ClB(C F ) by treatment in situ with the strong, chloride abstract-
6
5 3
5
7–60
ing Lewis acid B(C F ) .
6
5 3
The greater steric bulk of the NHC ligands in compounds
1
2
-R -Cl and 3-Cy -Cl is manifest in their H NMR spectra, as com-
3
3
pared to that of the parent second generation phosphonium al-
kylidene catalyst precursor 1-Cy -Cl. Sharp, time averaged signals
3
for the N-mesityl (N-Mes) groups in 1-Cy -Cl indicate that rotation
3
about the Ru–NHC carbon bond is rapid on the NMR timescale in
1
this species. For 2-Cy -Cl, the H NMR spectrum exhibits broadened
3
resonances for both the N-aryl protons and the NHC backbone pro-
tons that decoalesce at low temperatures, a behavior consistent with
restricted rotation about the NHC carbon–ruthenium bond that ren-
ders the N-aryl groups diastereotopic in the lower temperature re-
gime. This behavior is not accounted for by exclusive restricted
61
rotation about the N-aryl N–C bond. For the bulkiest system, 3-Cy3-
Cl, this rotation is slow enough at room temperature that sharp
1
signals for two diastereotopic N-aryl groups are observable in the H
NMR spectra under these conditions.
Crystals suitable for X-ray diffraction of both 2-i-Pr -Cl and
3
Scheme 1.
2
-Cy -Cl were grown by slow diffusion of pentane into a dichlo-
3
romethane solution of zwitterions; the molecular structures are
shown in Fig. 1, while selected metrical data appear in Table 1,
along with analogous data for the corresponding mesityl substi-
5
6
62
tuted compounds 1-i-Pr -Cl and 1-Cy -Cl for comparison.
3
3
The geometry about the ruthenium center for these complexes
is best described as distorted square pyramidal with the phos-
phonium alkylidene carbon C(1) occupying the apical position.
Due to the greater steric bulk of the ortho groups in compounds
2
-R -Cl, the Ru(1)–C(1) distances of 1.83 Å are slightly elongated in
3
comparison to the N-Mes analogs 1-R -Cl, but otherwise the met-
3
rical parameters in the two new complexes are very similar to
those in the previously reported N-Mes derivatives. Following the
trend for other complexes containing three anionic chloride li-
gands, the chloride ligand trans to the NHC ligand has an elon-
gated Ru(1)–Cl(3) bond relative to the others by ϳ0.12 Å.5 In all
four structures, the isopropyl and cyclohexyl substituents are ro-
tated away from Cl(3) to minimize steric interactions. The C(2)–
Ru(1)–C(1)–P(1) dihedral angles of 157.5(2)°–159.5(3)° were relatively
consistent for the complexes, except for the bulkiest of the series,
0
2
-Cy -Cl, in which this dihedral angle is about 8° greater at
3
studied compounds II and 1-R -ClB(C F ) in several standard me-
3
6 5 3
165.5(3)°.
tathesis applications.38
Thermal stability
Results and discussion
Previously, we have examined the thermal stability of catalysts
1-R -ClB(C F ) by heating in the presence of 1,1-dichloroethene
Synthesis
3
6 5 3
5
2
The syntheses of the imidazolinium salts of 1,3-bis(2,6-
diethylphenyl)imidazolin-2-ylidene (H IDEP·HCl) and 1,3-bis(2,
(Scheme 1), establishing decomposition rate profiles. For com-
parison, identical conditions were used to examine the thermal
stability of the new compounds 2-i-Pr -ClB(C F ) , 2-Cy -
2
6
-diisopropylphenyl)imidazolin-2-ylidene (H ID-i-PP·HCl) are
2
3
6
5 3
3
54,55
straightforward;
deprotonation with potassium hexamethy-
ClB(C F ) , and 3-i-Pr -ClB(C F ) ; these derivatives were gener-
6 5 3 3 6 5 3
ldisilazane (KHMDS) in tetrahydrofuran (THF), followed by recrys-
tallization from hot hexanes gave the free carbenes. These ligands
were installed onto ruthenium by substitution reactions using I
ated in situ by treating the chloride zwitterions with B(C F ) . The
6 5 3
progress of the decomposition of these compounds was moni-
tored by following their disappearance, as well as the appearance
1
(Chart 1) to afford the second generation type catalysts in moder-
of the [MePR ][A] byproduct (Scheme 1), by H NMR spectroscopy.
3
ate yields (59%–80%) upon workup. The standard methodology for
preparing the phosphonium alkylidenes (Scheme 2), involving
At 323 K, the thermal decomposition profile for 1-i-Pr -
3
5
2
ClB(C F ) exhibits clean first-order behavior, but under the
6
5 3
5
1
conversion to the terminal carbides using Feist's ester followed
by protonation with gaseous HCl in CH Cl , worked well with
same conditions that of 2-i-Pr -ClB(C F ) deviates from an expo-
3 6 5 3
nential decay curve (Fig. 2, top). The approximate half-life for the
2
2
these ligands to afford compounds 2-Cy -Cl (incorporating
thermal degradation of 2-i-Pr -ClB(C F ) was found to be
3
3
6 5 3
H IDEP, 57% yield) and 3-Cy -Cl (incorporating H ID-i-PP, 77% yield).
≈120 min (Table 2), shorter than the N-Mes derivative 1-i-Pr3-
ClB(C F ) whose half-life was ϳ200 min. We attribute this ap-
2
3
2
The triisopropylphosphonium derivative 2-iPr -Cl was prepared
3
6 5 3
Published by NRC Research Press

Leitao et al.
937
Scheme 2.
Fig. 1. Thermal ellipsoid diagrams of 2-i-Pr -Cl (top) and 2-Cy -Cl
number of accessible C–H bonds in the ortho ethyl groups of
3
3
(
bottom) (50% probability ellipsoids, solvent molecules and all hy-
2-i-Pr -ClB(C F ) results in more rapid C–H activation (see
3
6 5 3
drogens, except H1A, are omitted for clarity). Selected metrical data
appear in Table 1.
Scheme 1). Ultimately, the more rapid decomposition of this com-
pound relative to the parent N-Mes derivative discouraged further
exploration of its decomposition processes.
Similar decomposition profiles were obtained for the tricyclo-
hexylphosphonium alkylidene complexes n-Cy -ClB(C F ) at
3
6 5 3
343 K; each of these decompositions can be fitted to first-order
behavior (Fig. 2, bottom). An analysis of the thermal decomposi-
tion mixtures by ESI-MS showed very simple spectra for 1-Cy3-
ClB(C F ) and 3-Cy -ClB(C F ) , where only the phosphonium
6
5 3
3
6 5 3
+
salt byproduct [CH PCy ] at m/z 295.3 and very small amounts of
H IMes·H (m/z 307.2) or of H IDiPP·H (m/z 389.4) were observed.
3
3
+
+
2
2
Again, the mass spectrum observed for the ethyl substituted cat-
alyst 2-Cy -ClB(C F ) was somewhat more complex, exhibiting
3
6 5 3
peaks for these two species in addition to significant amounts of
higher molecular weight (MW) peaks in a region indicative of
dimeric and trimeric ruthenium containing species (m/z 800–
1200). It is clear from this data that there are significant differ-
ences in the thermal stability of each of the precatalysts. The ethyl
substituted derivative was again more rapidly consumed in com-
parison to the parent N-Mes compound, but the catalyst with iso-
propyl groups in the ortho positions of the N-aryl ring was much
more stable towards degradation with a half-life of over 2000 min,
which is two orders of magnitude better than the other two cata-
lysts (Table 2).
The ϳ40-fold increase in stability associated with the precata-
lyst 3-Cy -ClB(C F ) is further evidence that the rotation about
3
6 5 3
the Ru–NHC (and N-aryl N–C) bonds are significantly hindered by
the ortho isopropyl groups. It appears that this higher rigidity
+
discourages C–H activation of the ortho groups; since [CH PCy ] is
3
3
the major product of this degradation, we presume that C–H acti-
vation remains the primary initial step in this catalyst's decompo-
sition path. Thus, while introducing ethyl groups instead of
methyl groups appears to result in more facile catalyst decompo-
sition, the use of isopropyl groups has the opposite effect and
gives a much more thermally stable catalyst platform.
proximately two-fold decrease in stability upon replacement of
ortho methyl groups with ethyl substituents (and the nonexpo-
nential decay curve) to the operation of a second, competing
decomposition pathway during the thermal degradation of 2-i-
Pr -ClB(C F ) . ESI-MS analysis of the two reactions showed that
Olefin metathesis reactions
3
8
Grubbs and co-workers have proposed standardized olefin
metathesis reactions and conditions to be used for comparative
evaluation of catalysts. These transformations include ring clos-
ing metathesis (RCM) and cross metathesis (CM) reactions and a
ring opening metathesis polymerization (ROMP) process. Typical
conditions for the metathesis reactions with the phosphonium
3
6 5 3
+
both samples predominantly contain [MeP-i-Pr3] at m/z 175.3,
which is the byproduct from unimolecular C–H activation of the
ortho substituent on the N-aryl ring. In addition, both decompo-
sition mixtures exhibit a small amount of their respective proton-
+
ated NHC ligand at m/z 307.3 (H IMes·H ) and m/z 333.2
alkylidene complexes entail 0.1–5 mol% catalyst loading, CD Cl2
2
2
+
(
H IDEP·H ). However, in the spectrum for 2-i-Pr -ClB(C F ) , a
as solvent, hexamethyldisiloxane as the internal standard, and tem-
peratures between 263 and 298 K. The temperature was reduced for
most reactions relative to the standard conditions (T = 298 K) re-
2
3
6 5 3
third species with a peak at m/z 187.2 was observed; this mass is
consistent with the vinyltriisopropylphosphonium ion, suggest-
ing that one of the ortho ethyl groups may undergo facile dehy-
drogenation to form an ortho vinyl group capable of undergoing
38
ported by Grubbs and co-workers since the phosphonium al-
kylidene complexes are rapidly initiating and required lower
temperatures for convenient spectroscopic monitoring of these re-
5
2
intramolecular metathesis. It is also possible that the greater
Published by NRC Research Press

938
Can. J. Chem. Vol. 91, 2013
Table 1. Selected metrical parameters for complexes 2-R -Cl and 1-R -Cl.
3
3
Parametera
2-i-Pr -Cl
1-i-Pr -Cl
b
2-Cy -Cl
1-Cy -Clc
3
3
3
3
Ru(1)–C(1)
Ru(1)–C(2)
1.830(2)
2.032(2)
1.820(3)
2.036(3)
1.831(4)
2.032(4)
1.815(6)
2.021(5)
Ru(1)–Cl(1)
Ru(1)–Cl(2)
Ru(1)–Cl(3)
2.3344(6)
2.3299(6)
2.4560(6)
96.99(10)
133.75(18)
119.87(17)
164.75(7)
159.74(2)
128.01(14)
157.82(17)
2.3332(13)
2.3353(12)
2.4619(9)
96.58(13)
131.9(2)
2.3310(10)
2.3353(10)
2.4608(10)
97.39(16)
134.7(3)
2.3590(15)
2.3991(15)
2.4038(14)
97.4(2)
133.0(4)
119.3(4)
155.79(16)
168.57(6)
129.3(3)
C(1)–Ru(1)–C(2)
Ru(1)–C(2)–N(1)
Ru(1)–C(2)–N(2)
C(2)–Ru(1)–Cl(3)
Cl(1)–Ru(1)–Cl(2)
Ru(1)–C(1)–P(1)
C(2)–Ru(1)–C(1)–P(1)
a
120.4(2)
118.2(3)
165.04(9)
158.61(3)
126.96(18)
157.5(2)
163.23(11)
159.64(4)
126.9(2)
165.5 (3)
150.7(4)
Distances given in Å, angles and dihedrals given in °.
Data taken from ref. 56.
Data taken from ref. 62.
b
c
Fig. 2. Exponentially fitted first-order plots for the disappearance of
n-i-Pr -ClB(C F ) at 323 K (top) and n-Cy -ClB(C F ) at 343 K
kylidenes, the catalyst was generated by mixing stock solutions of
3
6
5 3
3
6
5 3
n-R -Cl and B(C F ) , while for II, a measured amount of a stock
3
6 5 3
(
1
bottom) with [Ru] = 33.8 mmol/L at 323 K in (CDCl ) , monitored by
solution was loaded into an NMR tube and placed in a dry ice/ace-
tone bath (at −78 °C). Substrate stock solution was subsequently
added via syringe prior to placing in the temperature equilibrated
NMR spectrometer probe. Spectra were acquired at regular inter-
vals, and integration of the substrate proton signals were typically
used to monitor the conversion to product.
o
2 2
H NMR spectroscopy.
The first standard reaction performed was the RCM of the sub-
strate 4,4-dicarbethyoxy-1,6-heptadiene (Scheme 3a). Under these
conditions, all of the phosphonium alkylidene complexes tested
in this reaction catalyzed the transformation to completion with
no sign of precatalyst decomposition (Fig. 3, top). Notably, the
performance of the Grubbs second generation catalyst (II) was
comparably inferior under these conditions. Although the differ-
ences in the overall efficiencies in the series of phosphonium
alkylidene catalysts for this transformation are minimal, subtle
differences in the sigmoidal curve shapes are notable (see also
Fig. SI-1 in the Supplementary data), particularly that observed for
3
-Cy -ClB(C F ) , which exhibits a significant period of minimal
3 6 5 3
activity at the beginning of the reaction, followed by very rapid
conversion of substrate to product. We have previously shown
that this initiation behavior is related to the rate at which ethene
is generated, since ethene more rapidly reacts with the precata-
lysts to generate the highly active species that results upon loss of
5
0
the phosphonium alkylidene. Using this knowledge, we subse-
quently demonstrated that adding ethene to the reaction results
in a much shorter induction period for the catalyst for 3-Cy3-
ClB(C F ) (Fig. 3, bottom). The metathesis event that liberates the
6
5 3
phosphonium alkylidene as a vinylphosphonium salt requires
that the phosphonium alkylidene ligand rotate about the Ru=C
bond to allow for proper approach of the substrate olefinic func-
tionality. This process becomes more difficult as the alkyl group
on the phosphorus get larger; the comparative behavior of cata-
lysts 1–3 suggest that the bulk of the ortho substituent on the
N-aryl group also has a significant impact in raising the barrier to
this process (Scheme 4). Ethene catalyzes the initiation reaction
because of its much smaller size as compared to monosubstituted
substrates, resulting in lowered steric interactions in this initial
metathesis reaction. However, when both the phosphorus alkyls
and the ortho N-aryl groups are large, as in 3-Cy -ClB(C F ) , even
Table 2. Rate constants and half-lives for the decomposition of n-R -
3
1
ClB(C F ) with [Ru] = 33.8 mmol/L in (CDCl ) determined by H NMR
6
5 3
o
2
spectroscopy.
Temperature
(K)
Rate constant
(k, s )
Half-life
(t1/2, min)
−1
Compound
1
2
1
2
-i-Pr -ClB(C F )
6 5 3
323
323
343
343
343
5.9(2) × 10−5
200
120
60
26
2100
3
-i-Pr -ClB(C F )
3 6 5 3
4
-Cy -ClB(C F )
1.9(2) × 10−
4.5(2) × 10
3
6 5 3
3
6 5 3
−4
-Cy -ClB(C F )
3
6
5 3
the reaction with ethene is slow enough to result in a measurable
sigmoidal reaction profile (Fig. 3, bottom). It is the greater stability
imparted to the active species (vide supra) that leads to greater
overall performance by this catalyst.
6
3
-Cy -ClB(C F )
5.5(2) × 10−
3
6 5 3
6
3
actions (Scheme 3). In this way, a series of five precatalysts were
compared: the benchmark second generation Grubbs complex
A benchmarking cross metathesis reaction to form the prod-
uct (E)-methyl-7-(ethanoyloxy)hepta-2-enoate was performed us-
3
8
(
II), the well-studied phosphonium alkylidene derivative 1-Cy -
ing methyl acrylate and 5-hexenyl acetate (Scheme 3b). To drive
the reaction forward, ethene was removed by purging the atmo-
sphere above the reaction solution with an argon flow. As the
3
4
9,50
ClB(C F ) ,
and the three new catalysts 2-i-Pr -ClB(C F ) ,
6
5 3
3 6 5 3
2
-Cy -ClB(C F ) , and 3-Cy -ClB(C F ) . For the phosphonium al-
3 6 5 3 3 6 5 3
Published by NRC Research Press

Leitao et al.
939
Scheme 3.
Fig. 3. Reaction profiles depicting the catalysis of the RCM
reaction of 4,4-dicarbethoxy-1,6-heptadiene at 273 K with phos-
phonium alkylidene catalysts and II (top) and the RCM reaction
of 4,4-dicarbethoxy-1,6-heptadiene at 273 K with and without
added ethene (bottom).
Fig. 4. Reaction profiles depicting the catalysis of the CM reaction
of methyl acrylate and 5-hexenyl acetate at 298 K.
facile reaction and so both the catalyst loading and the tempera-
ture were drastically decreased relative to the other transforma-
tions (Scheme 3c). Reaction profiles are given in Fig. 5. Under these
conditions, Grubbs complex II and 3-Cy -ClB(C F ) were not ef-
3
6 5 3
fective catalysts for ROMP of cyclooctadiene and are thus not
shown in the Fig. 5; however, they were found to be more reactive
at higher temperatures. In contrast, 2-i-Pr -ClB(C F ) showed ex-
3
6 5 3
cellent activity in this transformation, reaching 100% conversion
within only 20 min, while its cyclohexyl substituted congener
2
-Cy -ClB(C F ) was notably less effective. These observations
3 6 5 3
suggest the right balance between the ability to effectively initiate
and the stability of the propagating species is embodied in 2-iPr3-
ClB(C F ) for this transformation under these conditions.
6
5 3
Conclusions
This study has examined the effect of changing the ortho sub-
stituents on the N-aryl groups of the NHC ligand in fast initiating
phosphonium alkylidene olefin metathesis catalysts. By increas-
ing the steric bulk of these groups from methyl (in the parent
complexes) to ethyl and isopropyl groups, dramatic changes to
the thermal stability of the precatalysts and the stability of the
propagating species in a series of test metathesis reactions were
noted. Furthermore, the efficiency of initiation was significantly
impacted. In terms of the thermal stability of the precatalysts,
Scheme 4.
those supported by ethyl substituted ligand H IDEP underwent a
2
more rapid thermal decomposition (accelerated by ϳ2 fold), while
reaction profiles in Fig. 4 show, each phosphonium alkylidene
catalyst was effective in this reaction, achieving >80% conversion
in under 30 min. In comparison, the Grubbs second generation
catalyst (II), while effective, catalyzed the reaction at about half
the rate. Again, sigmoidal reaction profiles were observed for the
n-R -ClB(C F ) catalysts, reflecting the slow initiation rates and
those incorporating the isopropyl substituted donor H ID-i-PP
were remarkably resistant to thermal decomposition (decelerated
2
by ϳ40 fold) in comparison to their H IMes congeners. Moreover,
2
the bulky NHC stabilized system 3-Cy -ClB(C F ) gave propagat-
3
6 5 3
ing species that were more stable relative to those of the other
catalysts probed, resulting in high turnover numbers in several
standard metathesis reactions (for example, >6200 for the RCM of
4,4-dicarbethyoxy-1,6-heptadiene). On the other hand, the bulkier
NHC substituents made these catalysts slower to initiate due to
the steric interactions triggered in the initial metathesis event. As
3
6 5 3
the catalyst with the fastest optimal conversion rate, but the lon-
gest initiation period was the bulky 3-Cy -ClB(C F ) .
3
6 5 3
Finally, the catalysts were compared in a ring opening metath-
esis polymerization (ROMP) of cis,cis-1,5-cyclooctadiene. This is a
Published by NRC Research Press

940
Can. J. Chem. Vol. 91, 2013
Fig. 5. Reaction profiles depicting the catalysis of ROMP of cis,cis-1,5-
The flask was left at this temperature for 120 min and then al-
lowed to warm to room temperature overnight (18 h) under an
atmosphere of gaseous hydrochloric acid. The volatiles were re-
moved in vacuo. The green residue was slurried several times in
1
cyclooctadiene at 263 K in CD Cl as monitored by H NMR spectroscopy.
2
2
10 mL portions of pentane, sonicated, and then dried in vacuo. The
crude solids were purified by dissolving in dichloromethane and
layering with pentane to yield green crystals (0.092 g, 0.15 mmol,
1
2
2 2 HP
8
1
7%). H NMR (CD Cl , 400.2 MHz, 300 K) ␦: 19.63 (d, J = 51 Hz,
3
3
H, Ru=C(H)P-i-Pr ), 7.44 (t, J = 7.6 Hz, 2H, Ar o-CH), 7.29 (d, J
=
3
HH
HH
7
.6 Hz, 4H, Ar p-CH), 4.08 (br s, 4H, CH CH ), 3.31–2.83 (m, 11H,
2 2
overlapping i-Pr CH and CH CH ), 1.29 (t, 12H, CH CH ), 1.17 (d,
2
3
2
3
3
13
1
JHP = 17 Hz, 18H, i-Pr CH3). C{ H} NMR (CD Cl , 100.6 MHz, 300 K)
2
2
␦
: 270.3 (br s, Ru=C(H)P-i-Pr ), 203.4 (s, Ru-C(N) ), 144.9, 143.1, 141.4,
3 2
1
5
29.0 (very br s, quaternary C), 127.4 (s, Ar p-CH), 126.4 (s, Ar o-CH),
1
2 2 2 3 CP
3.8 (br s, CH CH ), 25.2 (very br s, CH CH ), 24.4 (d, J = 37 Hz, i-Pr
2
31
1
CH), 17.6 (d, J = 3 Hz, i-Pr CH ), 14.5 (very br s, CH CH ). P{ H}
CP
3
2
3
NMR (CD Cl , 162.0 MHz, 300 K) ␦: 42.8 (s). The ethyl groups are very
2
2
fluxional as determined by the broad signals. Anal. calcd. for
C H Cl N PRu: C 55.42, H 7.33, N 3.92; found: C 55.48, H 7.45, N 3.67.
33
52
3 2
a result, the characteristic sigmoidal shape of the metathesis re-
Synthesis of 3-Cy -Cl
In a 100 mL round-bottom flask, equipped with a stir bar, (H ID-
3
action profile was more pronounced for catalysts 3 versus those of
5
0
2
catalysts 1 and 2. Indeed, even the ethene-catalyzed initiation of
these catalysts was significantly slower for the bulkier catalysts 3.
Nonetheless, since the peak efficiency of the propagating species
stabilized by this ligand was superior to those attained by the less
bulky catalysts, the performance of catalysts 3 in the test reac-
tions was excellent.
i-PP)(PCy )(Cl) RuϵC (250 mg, 0.294 mmol) was added. On the vac-
3
2
uum line, dichloromethane (50 mL) was vacuum transferred into
the flask (at −78 °C). The solution was allowed to warm to room
temperature to dissolve the solids and then gaseous hydrochloric
acid was added (at −78 °C). The green solution became red within
9
0 s. The flask was left at this temperature for 20 min and then
allowed to warm to room temperature overnight (18 h) under an
atmosphere of gaseous hydrochloric acid. The volatiles were re-
moved in vacuo. The green residue was slurried several times in
Experimental section
For general experimental methods, see the Supplementary material
section.
10 mL portions of pentane, sonicated, and then dried in vacuo.
Crude solids were purified by dissolving in dichloromethane and
layering with pentane to yield green flakes (0.202 g, 0.227 mmol,
Synthesis of 2-Cy -Cl
In a 25 mL round-bottom flask, equipped with a stir bar,
3
1
2
(
H IDEP)(PCy )(Cl) RuϵC (168 mg, 0.209 mmol) was added. On the
77.3%). H NMR (CD
2
Cl
2
, 400.2 MHz, 300 K) ␦: 19.62 (d, JHP = 52 Hz,
2
3
2
3
vacuum line, 20 mL CH Cl was vacuum transferred into the flask
1H, Ru=C(H)PCy ), 7.49, 7.41 (both t, J = 7.6 Hz, 1H each, Ar o-CH),
2
2
3
HH
3
(at −78 °C). The solution was allowed to warm to room tempera-
7.32, 7.26 (both d, J = 7.6 Hz, 2H each, Ar p-CH), 4.16–3.95 (m,
HH
ture to dissolve the solids and then gaseous hydrochloric acid was
added (at −78 °C). The green solution became red within 30 s. The
flask was left at this temperature for 20 min and then allowed to
warm to room temperature overnight (18 h) under an atmosphere
of gaseous hydrochloric acid. The volatiles were removed in
vacuo. The green residue was slurried several times in 10 mL por-
tions of pentane, sonicated, and then dried in vacuo. The crude
solids were purified by dissolving in dichloromethane and layer-
ing with pentane to yield green crystals (0.100 g, 0.120 mmol,
4H, CH CH ), 3.73, 3.58 (m, 4H i-Pr CH), 3.54 (m, 3H, Cy CH), 1.68–
2
2
0.89 (m, 54H, overlapping Cy CH and i-Pr CH signals). 1 C{ H}
3
1
2
3
NMR (CD Cl , 100.6 MHz, 298 K) ␦: 268.4 (br s, Ru=C(H)PCy ), 204.9
2
2
3
(
s, Ru-C(N) ), 148.6, 148.3, 137.0, 136.3 (all s, quaternary C), 129.6,
2
1
29.3 (s, Ar p-CH), 124.8, 124.0 (s, Ar o-CH), 55.2, 54.0 (both s,
1
CH CH ), 34.8 (d, J = 35 Hz, Cy CH), 28.8, 27.1, 26.9, 26.0 (m, Cy
2
2
CP
2
CH ), 28.0, 27.7, 26.6, 25.7 (m, i-Pr CH), 23.7 (d, J = 7 Hz, i-Pr CH3).
2
1
CP
31
P{ H} NMR (CD Cl , 162.0 MHz, 300 K) ␦: 33.5 (s). Notice that the
2
2
aromatic rings and NHC backbone protons are inequivalent sug-
gesting that the i-Pr groups are too bulky to allow fast rotation
about the Ru–NHC bond. Anal. calcd. for C46H Cl N PRu·CH Cl :
1
2
5
7.3%). H NMR (CD Cl , 400.2 MHz, 300 K) ␦: 19.68 (d, J = 50 Hz,
2 2 HP
3
HH
3
1H, Ru=C(H)PCy ), 7.43 (t, J = 7.6 Hz, 2H, Ar o-CH), 7.25 (d, J
=
3
HH
72
3
2
2
2
7
.6 Hz, 4H, Ar p-CH), 4.04 (br s, 4H, CH CH ), 3.14–2.85 (m, 11H,
2 2
C 57.81, H 7.64, N 2.87; found: C 57.80, H 7.45, N 2.80.
overlapping Cy CH and CH CH ), 1.70–1.28 (m, 42H, overlapping
2
3
3
1
1
and Cy CH and CH CH ). C{ H} NMR (CD Cl , 100.6 MHz, 300 K)
2
2
3
2
2
Generation of 2-Cy -ClB(C F )
6 5 3
3
␦
: 270.4 (br s, Ru=C(H)PCy ), 203.7 (s, Ru-C(N) ), 143.5 (very br s,
3 2
In an NMR tube, (H IDEP)(Cl) Ru=C(H)PCy (25 mg, 0.030 mmol)
2
3
3
quaternary C), 129.3 (s, Ar p-CH), 126.3 (s, Ar o-CH), 53.8 (br s,
and B(C F ) (17 mg, 33 ␮mol) were combined in CD Cl (0.5 mL).
6
5 3
2
2
1
CH CH ), 34.1 (d, J = 40 Hz, Cy CH), 27.4, 27.3, 26.6, 25.7 (m, all Cy
2
2
CP
The NMR tube was immediately placed in a dry ice/acetone bath
−78 °C) and inserted into a precooled NMR probe (243 K) to ac-
quire spectra without the compound decomposing at all over-
31
1
CH ), 26.8 (very br s, CH CH ), 14.4 (very br s, CH CH ). P{ H} NMR
2
2
3
2
3
(
(
CD Cl , 162.0 MHz, 297 K) ␦: 32.1 (s). The ethyl groups are very
2 2
fluxional as determined by the broad signals. Anal. calcd. for
C42H64Cl N PRu: C 60.39, H 7.72, N 3.35; found: C 60.13, H 7.51,
1
2
night. H NMR (CD Cl , 399.3 MHz, 243 K) ␦: 17.52 (d, J = 36 Hz,
2
2
HP
3
2
1
H, Ru=C(H)PCy ), 7.52 (br s, 2H, Ar o-CH), 7.34 (br s, 4H, Ar p-CH),
3
N 3.27.
4
.23 (s, 4H, CH CH ), 3.21–2.71 (m, 8H, CH CH ), 2.67 (m, 3H, Cy
2 2 2 3
3
CH), 1.74–0.94 (m, J = 7.6 Hz, 42H, overlapping and Cy CH and
Synthesis of 2-i-Pr -Cl
HH
2
3
1
3
1
1
In a 50 mL round-bottom flask, equipped with a stir bar,
CH
10 Hz, Ru=C(H)PCy
118.3 (very br s, quaternary C), 130.6 (s, Ar o-CH), 127.0 (s, Ar p-CH),
2
CH
3
). C{ H} NMR (CD
2
Cl
2
, 100.6 MHz, 243 K) ␦: 262.0 (d, JCP
=
(
H IDEP)(P-i-Pr )(Cl) RuϵC (100 mg, 0.169 mmol) was added. On
3 2
), 188.2 (s, Ru-C(N) ), 149.3, 146.8, 138.2, 135.7,
2
3
2
the vacuum line, 25 mL CH Cl was vacuum transferred into the
2
2
1
flask (at −78 °C). The solution was allowed to warm to room tem-
perature to dissolve the solids and then gaseous hydrochloric acid
was added (at −78 °C). The green solution became red within 30 s.
54.0 (br s, CH
2 2
CH ), 29.6 (d, JCP = 39 Hz, Cy CH), 27.6, 26.0, 25.8, 24.7
(m, all Cy CH ) 23.6 (very br s, CH CH ), 15.3, 13.5 (very br s,
2
2
3
3
1
1
CH CH ). P{ H} NMR (CD Cl , 162.0 MHz, 243 K) ␦: 54.7 (s).
2
3
2
2
Published by NRC Research Press

Leitao et al.
941
Generation of 2-i-Pr -ClB(C F )
the structures 2-Cy -Cl and 2-i-Pr3-Cl, respectively). These data
3
6
5 3
3
In an NMR tube, (H IDEP)(Cl) Ru=C(H)P-i-Pr (25 mg, 39 ␮mol) and
can be obtained, free of charge, via http://www.ccdc.cam.ac.uk/
products/csd/request (Or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1E2, UK; fax: +44 1223
33603; or e-mail: deposit@ccdc.cam.ac.uk).
2
3
3
B(C F ) (22 mg, 43 ␮mol) were combined in CD Cl (0.5 mL). The
6
5 3
2
2
NMR tube was immediately placed in a dry ice/acetone bath (−78 °C)
and inserted into a precooled NMR probe (243 K) to acquire spectra
1
without the compound decomposing at all overnight. H NMR
2
2 2 HP
Acknowledgement
(
CD Cl , 399.3 MHz, 243 K) ␦: 17.46 (d, J = 36 Hz, 1H, Ru=C(H)P-i-Pr3),
7.49 (br s, 2H, Ar o-CH), 7.35 (br s, 4H, Ar p-CH), 4.29 (s, 4H, CH CH ),
Funding was provided by the Natural Sciences and Engineering
Research Council of Canada (NSERC) in the form of a Discovery
Grant to W.E.P. and a CGS-D scholarship for E.M.L. W.E.P. also
thanks the Canada Council of the Arts for a Killam Research Fel-
lowship (2012-14). Materia Inc. of Pasadena is thanked for provid-
ing Grubbs first and second generation catalysts.
2
2
3
.21–3.18 (br s, 11H, overlapping i-Pr CH and CH CH ), 1.26 (br s, 12H,
2
3
3
13
1
CH CH ), 0.93 (br d, J = 16 Hz, 18H, i-Pr CH3). C{ H} NMR (CD Cl ,
2
3
HP
2
2
100.4 MHz, 243 K) ␦: 260.3 (br s, Ru=C(H)P-i-Pr ), 187.3 (s, Ru-C(N) ),
3 2
149.1, 146.9, 141.6, 138.8, 138.0, 135.5 (br s, quaternary C), 130.8 (s, Ar
p-CH), 127.1 (s, Ar o-CH), 53.8 (br s, CH CH ), 24.7 (very br s, CH CH ),
2
2
2
3
1
2
0.5 (d, J = 39 Hz, i-Pr CH), 17.5 (br s, i-Pr CH ), 14.7 (very br s,
CP 3
1
31
References
CH CH ). P{ H} NMR (CD Cl , 162.0 MHz, 243 K) ␦: 62.4 (s).
2
3
2
2
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2
3
3
and B(C F ) (16 mg, 31 ␮mol) were combined in CD Cl (0.5 mL).
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5 3
2
2
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(
(
1
2
night. H NMR (CD Cl , 399.3 MHz, 243 K) ␦: 17.37 (d, J = 38 Hz,
2
2
HP
3
1
H, Ru=C(H)PCy ), 7.43, 7.41 (both pt, J
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= 7.6 Hz, 1H each, Ar
3
HH
3
HH
(
4
0
H, CH CH ), 3.40, 3.25 (m, 2H, i-Pr CH), 2.21 (m, 3H, Cy CH), 1.72–
2 2
1
3
1
.95 (m, 54H, overlapping Cy CH and i-Pr CH signals). C{ H}
2
3
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NMR (CD Cl , 100.4 MHz, 243 K) ␦: 257.1 (d, JCP = 10 Hz,
2
2
Ru=C(H)PCy ), 188.7 (s, Ru-C(N) ), 149.2, 148.0, 147.9, 146.7, 142.3,
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2
(
9) Blacquiere, J. M.; McDonald, R.; Fogg, D. E. Angew. Chem. Int. Ed. 2010, 49,
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39.8, 138.1, 135.6, 135.4, 134.2 (all br s, quaternary C), 131.1, 130.5 (s,
3
807. doi:10.1002/anie.200906635.
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Ar p-CH), 125.7, 124.6 (s, Ar o-CH), 55.0 (s, CH CH ), 29.8 (d, J =
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2
CP
3
(
8 Hz, Cy CH), 29.1, 28.6, 27.8, 26.3 (m, Cy CH ), 26.0, 25.9, 25.8, 24.5
2
m, i-Pr CH), 23.8, 23.3 (br s, i-Pr CH3). 3 P{ H} NMR (CD Cl ,
1
1
2 2
2
000,39,3451.doi:10.1002/1521-3773(20001002)39:19<3451::AID-ANIE3451>3.
161.6 MHz, 243 K) ␦: 56.1 (s).
0.CO;2-U.
(
12) Sanford, M. S.; Love, J. A.; Grubbs, R. H. Organometallics 2001, 20, 5314. doi:
Thermal decomposition of catalysts n-R -ClB(C F )
6 5 3
10.1021/om010599r.
3
In an NMR tube, L(Cl) Ru=C(H)PR3 (32 ␮mol) and B(C F )
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3
6 5 3
(14) Van Veldhuizen, J. J.; Garber, S. B.; Kingsbury, J. S.; Hoveyda, A. H. J. Am.
(
(
25 mg, 54 ␮mol) were combined along with 1,1-dichloroethene
0.50 mL), (CDCl ) (0.4 mL), and an internal standard (0.004 mmol
Chem. Soc. 2002, 124, 4954. doi:10.1021/ja020259c.
2
2
(15) Bieniek, M.; Bujok, R.; Cabaj, M.; Lugan, N.; Lavigne, G.; Arlt, D.; Grela, K.
hexamethyldisiloxane). The tube was placed in an acetone/dry ice
bath (−78 °C) for transport to the NMR spectrometer and then
placed in the prewarmed NMR probe (323 or 343 K). Proton cycling
experiments were used to acquire spectra at various intervals
during the decomposition and integration of the methyl doublet
of doublets (on the P-i-Pr3 group) in the starting material was
monitored against the decomposition product for complexes n-i-
J. Am. Chem. Soc. 2006, 128, 13652. doi:10.1021/ja063186w.
(
16) Bieniek, M.; Michrowska, A.; Usanov, D. L.; Grela, K. Chem. Eur. J. 2008, 14,
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8
(
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2009, 47, 4209. doi:10.1002/pola.23476.
(
(
Pr -ClB(C F ) and the disappearance of the alkylidene proton at
3
6 5 3
ϳ17.5 ppm was monitored for complexes n-Cy -ClB(C F ) . All of
(20) Dorta, R.; Kelly, R. A.; Nolan, S. P. Adv. Synth. Catal. 2004, 346, 917. doi:10.
002/adsc.200404047.
(21) Furstner, A.; Thiel, O. R.; Lehmann, C. W. Organometallics 2002, 21, 331.
3
6 5 3
1
the complexes showed at least the presence of a protonated NHC
+
ligand and MePR3 by ESI-MS.
doi:10.1021/om0108503.
(
22) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc.
X-ray crystallography
2
000, 122, 8168. doi:10.1021/ja001179g.
Single crystals of 2-i-Pr -Cl and 2-Cy -Cl were grown by dif-
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3
3
doi:10.1002/1521-3773(20021104)41:21<4038::AID-ANIE4038>3.0.CO;2-0.
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fusion of pentane into a dichloromethane solution of zwitteri-
ons. The crystals were coated with Paratone-N oil and measured
on a Bruker D8/APEX II CCD diffractometer, using graphite-
monochromated Mo K␣ radiation (␭ = 0.71073 Å). Further data
collection and structure refinement details are given in Ta-
ble S1 in the Supplementary data.
(
(
(
(
om060620u.
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Supplementary material
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1
Supplementary material (details on general methods, synthe-
ses, and characterization of ligands and catalyst precursors and
the procedures employed for the metathesis reactions and crys-
(
1
(30) Berlin, J. M.; Grubbs, R. H.; Schrodi, Y.; Stewart, I. C. Organometallic Ruthenium
Complexes and Related Methods for the Preparation of Tetra-Substituted and Other
Hindered Olefins; WO/2007/075427, 2007.
tal and refinement data for the structures 2-Cy -Cl and 2-i-Pr3-Cl)
3
is available with the article through the journal Web site at http://
nrcresearchpress.com/doi/suppl/10.1139/cjc-2013-0156. CCDC 933204
and 933205 contain the supplementary crystallographic data for
(
31) Fürstner, A.; Ackermann, L.; Gabor, B.; Goddard, R.; Lehmann, C. W.;
Mynott, R.; Stelzer, F.; Thiel, O. R. Chem. Eur. J. 2001, 7, 3236. doi:10.1002/
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Published by NRC Research Press

942
Can. J. Chem. Vol. 91, 2013
(
(
(
(
(
(
(
32) Weigl, K.; Kohler, K.; Dechert, S.; Meyer, F. Organometallics 2005, 24, 4049.
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