Comparative investigation of Hoveyda-Grubbs catalysts bearing modified N-heterocyclic carbene ligands

Article abstract of DOI:10.1002/adsc.200700042

This paper reports the structural modification of Hoveyda-Grubbs complexes through the introduction of either an N-alkyl-N′-mesityl heterocyclic carbene, an N-alkyl-N′-(2,6-diisopropylphenyl) heterocyclic carbene, or an N-alkyl-N′-alkyl heterocyclic carbe

Full text of DOI:10.1002/adsc.200700042

FULL PAPERS
DOI: 10.1002/adsc.200700042
Comparative Investigation of Hoveyda–Grubbs Catalysts bearing
Modified N-Heterocyclic Carbene Ligands
a,
b
a
a
Nele Ledoux, * Anthony Linden, Bart Allaert, Hans Vander Mierde,
a,
and Francis Verpoort *
Department of Inorganic and Physical Chemistry, Laboratory of Organometallic Chemistry and Catalysis, Ghent
University, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
a
Phone: (+32)-9-264-4440; fax: (+32)-9-264-4983; e-mail: nele.ledoux@gmail.com or francis.verpoort@UGent.be
Institute of Organic Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
b
Received: January 23, 2007
Abstract: This paper reports the structural modifica- metathesis (CM) reactions. A pronounced influence
tion of Hoveyda–Grubbs complexes through the in- on both catalyst activity and selectivity was found to
troduction of either an N-alkyl-N’-mesityl heterocy- be exerted by the NHC amino substituents, which
clic carbene, an N-alkyl-N’-(2,6-diisopropylphenyl) emphasizes that a rigorously selected steric environ-
heterocyclic carbene, or an N-alkyl-N’-alkyl hetero- ment is critical in olefin metathesis catalyst design.
cyclic carbene. The effect of the modified N-hetero-
cyclic carbene (NHC) ligand was investigated in rep-
resentative ring-opening metathesis polymerization Keywords: catalyst design; crystal structures; meta-
(
ROMP), ring-closing metathesis (RCM) and cross thesis; N-heterocyclic carbenes; ruthenium
Introduction
signed to a release/return mechanism. The isopropox-
ystyrene, which decoordinates duringmetathesis, can
st
[1]
Next to the Grubbs 1 generation catalyst 1a and react again with a ruthenium intermediate to regener-
nd
[2]
[6]
[7]
[8]
the Grubbs 2 generation catalyst 1b , the Hoveyda ate the original catalyst. Steric and electronic ef-
[3]
[4]
catalyst 2a and the Hoveyda–Grubbs catalyst 2b
fects in the isopropoxystyrene ligand sphere were
represent an important class of olefin metathesis ini- thoroughly investigated, and were both found to exert
tiators (Figure 1). Their remarkable stability and reac- a stronginfluence on catalyst activity.
tivity towards electron-deficient substrates create an
In this contribution, we describe the preparation,
[
5]
interestingapplication profile. These aryl-ether che- characterization and catalytic behavior of Hoveyda–
late complexes offer the additional advantage of pos- Grubbsꢀ complexes featuringmodified N-heterocyclic
sible recovery after reaction, which should be as- carbene (NHC) ligands. The introduction of an ali-
Figure 1.
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Comparative Investigation of Hoveyda–Grubbs Catalysts
FULL PAPERS
phatic amino side group into the NHC framework synthetic strategy which uses the bis
A
H
T
E
U
G
profoundly alters the catalytic activity of the resulting 4a and b as startingmaterials. Reaction with an
[9]
complexes. An intriguing example was given by Ble- excess of 2-isopropoxystyrene at elevated temperature
chert et al. who described the N-methyl-N’-mesityl de- allows for the decoordination of one NHC ligand with
rived catalyst 3a which does not improve the catalytic formation of the desired complexes in good yield
activity but induces higher selectivity in diastereose- (Scheme 1, route B). In addition, a synthetic approach
lective ring-closing metathesis (RCM) and significant- inspired by a procedure described by Blechert et al.
[
10]
[4b]
ly alters E/Z ratios in cross metathesis (CM). Our proved successful.
Treatment of Hoveyda catalyst
recent research on Grubbsꢀ catalysts bearing N-alkyl- 2a with the appropriate NHC chloride salt and
N’-(2,6-diisopropylphenyl) heterocyclic carbenes dem- LiHMDS (lithium hexamethyldisilazane) as a base af-
onstrated that replacement of the NHC mesityl ring forded the intermediates 5a–c, which were stirred in
with a 2,6-diisopropylphenyl moiety has a dramatic chloroform to liberate their phosphine ligand
effect. Facile bis-coordination of the NHC ligands was (Scheme 1, route C). Here, it is notheworthy that no
observed, which was assigned to an enhanced phos- bis
phine dissociation rate of the mono(NHC) com- course of the reaction. This strategy, which circum-
These results stimulated us to explore the vents the need for Grubbsꢀ precursors 1c and d, was
effects of a similar NHC modification in Hoveyda– also applied to synthesize complexes 8a and b bearing
A
H
R
U
G
AHCTREUNG
[11]
plexes.
Grubbsꢀ catalysts.
symmetrical aliphatic NHC ligands.
Next to these two types of non-symmetrical NHCs,
Single crystals suitable for X-ray crystal-structure
also symmetrical ligands bearing two aliphatic amino analysis were obtained for 3b and c, 6a and b, and 8a.
side groups were successfully coordinated to the Hov- The resulting structures shown in Figure 2, Figure 3,
eyda precursor 2a. We recently reported on the low Figure 4, Figure 5, and Figure 6 confirm the formation
stability of Grubbsꢀ benzylidene complexes 1c and d, of a single isomer. Selected bond lengths and angles
which prevented their isolation. The lack of stability are provided in Table 1. All complexes display a typi-
was attributed to steric effects resultingin a weak-
cal distorted square pyramidal coordination with the
ened NHC to metal bond. Likely, the sterically less Cl atoms trans to one another and the apical position
[
9a]
demanding ge ometry of Hoveyda–Grubbsꢀ complexes
explains a herein described more fruitful outcome.
occupied by the Ru=C bond. Compared with the stan-
dard Hoveyda–Grubbsꢀ catalyst 2b, all complexes
The NHC ligands described in this paper all incor- bearingan aliphatic NHC amino side gr oup are char-
porate a saturated backbone. NHCꢀs with unsaturated acterized by a slightly decreased RuÀCNN bond
backbones have a longer history; ruthenium carbene length. This indicates a stronger s-donation of the
complexes with NHCꢀs bearing N-cyclohexyl groups NHC ligand caused by the aliphatic amino groups.
nd
were even amongst the first 2 generation catalysts to While for complexes 3a–c, the RuÀO bond remains
[
12]
be described in the literature. In addition, Grubbs- within the same range as for 2b, a slightly longer
type metathesis catalysts with “unsaturated” NHCꢀs bond length is observed for complexes 6a and 6b.
bearingan N-alkyl as well as an N-aryl group have This is probably due to steric requirements demanded
thoroughly been reported in the literature by Fürstner by the presence of a bulkier diisopropylphenyl group.
[13]
[14]
et al.
and Grubbs et al.
It was shown that sub-
Remarkably, complexes 3a–c and 6a and b all have
stantial structural variations can be accommodated at their aromatic amino side group oriented towards
the NHC ligand and eventually lead to designer cata- the benzylidene unit. This formation of only one
lysts with tailor-made properties. As “saturated” isomer is well precedented in the literature for
NHCꢀs generally afford more active metathesis initia- Grubbs-type complexes, and was assigned to a p–p
[
15]
tors than “unsaturated” ones,
we were hopingto
interaction between the two nearly coplanar aromat-
[
10,11,13a,c]
successfully contribute to the continuous search for ic groups.
In the here described Hoveyda–
more efficient catalysts.
Grubbs-type complexes the two aromatic groups are
arranged almost perpendicularly and the observed
molecular feature can thus not be ascribed to an in-
tramolecular p–p stackingas stated formerly for
Grubbsꢀ complexes.
Results and Discussion
In contrast to complexes 2b and c and 3a–c, which
In complexes 3a–c and 6a and b, the a-benzylidene
were prepared through an established route involving proton is located directly underneath the N-aryl
nd
their Grubbsꢀ 2 generation analogue and CuCl as a group of the NHC, however, in complex 8a the N-
[
4a,7,8]
phosphine scavenger (Scheme 1, route A),
com- alkyl group is distorted away from the benzylidene
plexes 6a and b required an alternative protocol. unit. This different arrangement reduces the NHC–
Since the N-(alkyl)-N’-(2,6-diisopropylphenyl) hetero- benzylidene steric interactions and explains the small-
cyclic carbenes induce preferential bis-coordination in er N CÀRu=C angle found for complex 8a.
2
their reaction with 1a, we disclose an unconventional
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Scheme 1.
To explore the catalytic potential of the new com- ROMP activity than the ones substituted with N-
plexes, they were compared with the benchmark cata- alkyl-N’-(2,6-diisopropylphenyl) carbenes (6a–c). Fur-
lysts 2a–c in a few model olefin metathesis reactions. thermore, complexes 6b and 6c fail to reach the reac-
(
Scheme 2) Figure 7 and Figure 8 illustrate how the tivity of their phosphine precursor 2a.
catalysts perform in the ROMP of the low strain The activity trends observed in the RCM of diethyl
cis,cis-cycloocta-1,5-diene (COD) under standard con- diallylmalonate (Figure 9) are somewhat different
ditions. Usinga COD/catalyst ratio of 300, the Hovey-
da–Grubbsꢀ complexes bearingsymmetrical NHC li-
from those observed in the ROMP of COD. Original-
ly, we anticipated that changing the electronic nature
gands (2b, c, and 8a, b) reach full conversion within of the NHC through the introduction of aliphatic
the first measurement. When a COD/catalyst ratio of groups might positively affect the catalytic activity of
3
000 is applied, complexes 8a, b show an increased ac- the correspondingcomplexes. However, we came to
tivity relative to the classic Hoveyda–Grubbsꢀ com- conclude that the influence of the steric bulk plays a
plexes 2b, c. much more compellingrole in determiningthe RCM
The Hoveyda–Grubbsꢀ complexes coordinated with activity. As the steric bulk of the NHC ligand increas-
an N-alkyl-N’-mesityl carbene (3a–c) display a higher es, a decrease in catalyst activity is found. In all three
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Figure 2. The molecular structure of 3b, showing50% prob-
ability ellipsoids. Hydrogen atoms have been omitted for
clarity. Crystals were grown from toluene.
Figure 4. The molecular structure of 6a, showing50% prob-
ability ellipsoids. Crystals were grown from benzene.
Figure 3. The molecular structure of 3c, showing50% prob-
ability ellipsoids. Crystals were grown from CH Cl /acetone.
2
2
series of catalysts we observe a distinct negative steric
bulk-catalytic activity relationship: 3a>3b>3c, 6a>
6
b>6c, 8a>8b.
It is also noteworthy that the highly ROMP-active
Figure 5. The molecular structure of 6b, showing50% prob-
complexes 8a and b display rather modest RCM activ-
ity. This substrate specificity likely results from a
ability ellipsoids. Crystals were grown from CH Cl /MeOH.
2
2
large steric bulk around the ruthenium center, which
hampers coordination of the bulky RCM substrate. A
more demandingsteric environment stems from the
three-dimensional bulk of the aliphatic amino side lenging substrate acrylonitrile (Table 2).
The applicability in CM was examined for the chal-
[
5,8a–b,17]
The
groups compared with the only two-dimensional bulk catalytic activity was measured usingtwo catalyst
of the flat aromatic side groups. This extra bulkiness loadings (2.5 and 5 mol%) and compared with the re-
is expected to cause a greater shielding of the metal sults obtained for the conventional complexes 2a–c.
center and explains the lower RCM activity of all new Phosphine complex 2a demonstrates very poor activi-
complexes compared with the benchmark catalysts ty; the NHC-bearingcomplexes show better results.
2
a–c. These data are in agreement with earlier find- Our modified complexes 3a–c, 6a–c, and 8a, b display
ings, which indicated that future NHC ligand design lower activity than the classic Hoveyda–Grubbsꢀ com-
should focus on a rigorously selected steric environ- plexes 2b and c, and induce different E/Z selectivities.
[
16]
ment, rather than on a tuningof electronic effects.
The complexes bearingsymmetrical NHC li ga nds ( 2b,
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Table 1. Selected bond lengths [Š] and angles [8] with standard uncertainties in parentheses.
[4]
[10]
2
b
3a
3b
3c
6a
6b
8a
Bond Lengths
Ru=C
1.828(5)
1.981(5)
2.261(3)
1.351(6)
1.350(6)
1.821(3)
1.978(3)
2.270(2)
1.341(4)
1.345(4)
1.836(2)
1.964(2)
2.266(2)
1.344(3)
1.353(3)
1.832(3)
1.973(3)
2.260(2)
1.360(4)
1.349(4)
1.839(5)
1.968(6)
2.281(4)
1.353(8)
1.456(9)
1.834(6)
1.980(6)
2.297(4)
1.327(8)
1.343(8)
1.824(3)
1.972(3)
2.274(2)
1.346(4)
1.348(4)
RuÀCNN
RuÀO
N(1)ÀC(1)
N(2)ÀC(2)
Bond Angles
ClÀRuÀCl
156.5(5)
101.5(14)
176.2(14)
153.53(4)
102.68(13)
178.16(10)
153.19(2)
102.9(10)
178.04(8)
157.41(3)
102.52(11)
174.21(11)
151.47(6)
101.5(2)
178.5(2)
152.08(6)
101.20(18)
177.8(2)
154.36(3)
96.68(13)
175.66(10)
N CÀRu=C
2
N CÀRuÀO
2
Conclusions
In summary, a comparison between the classical Hov-
eyda–Grubbsꢀ complexes 2b, c and complexes 3a, b
and 6a, b demonstrates that the introduction of one
aliphatic group into the NHC framework does not im-
prove the catalytic activity in any of the tested meta-
thesis reactions. The introduction of two aliphatic
amino side groups (complexes 8a and b) enhances the
reactivity in the ROMP reaction while the increase of
steric interactions lowers the RCM and CM activity.
The lower activity of the N-alkyl-N’-(2,6-diisopropyl-
phenyl) heterocyclic carbene complexes 6a and b
compared with the N-alkyl-N’-mesityl heterocyclic
carbene complexes 3a and b, may analogously be at-
tributed to a more demandingsteric environment.
These results confirm that the NHCꢀs amino side
groups play a pivotal role in determining the reactivi-
ty and selectivity of the correspondingcatalysts.
While small differences in donor capacities might
cause a significantly different catalytic behavior, it is
plausible that subtle steric differences exert a more
determininginfluence on the activity of the catalysts.
Scheme 2.
Experimental Section
All reactions and manipulations involvingor ag nometallic
compounds were conducted in oven-dried glassware under
an argon atmosphere using standard Schlenk techniques.
Solvents were dried with appropriate dryinga ge nts and dis-
tilled prior to use. COD and allylbenzene were dried over
CaH . Acrylonitrile, stabilized with 35–45 ppm hydroqui-
2
1
none monomethyl ether, was distilled prior to use. H and
C NMR measurements were performed with a Varian
Unity-300 spectrometer. Complexes 2b,
Figure 6. The molecular structure of 8a, showing50% prob-
13
ability ellipsoids. Crystals were grown from CH Cl /hexane.
[4a]
[18]
[10]
2
2
2c
and 3a
were prepared accordingto literature procedures.
A
T
E
N
c and 8a, b) show higher Z selectivity. For all but one
3c) of the complexes coordinated with an asymmetri-
cal NHC, a remarkable E/Z selectivity reversal is ob- CuCl (0.014 g, 0.14 mmol) were weighed into a dry Schlenk
served. flask. 2-Isopropoxystyrene (0.023 g, 0.14 mmol) in CH Cl2
(
A
H
R
U
G
A
H
R
U
G
2
2
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Comparative Investigation of Hoveyda–Grubbs Catalysts
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1
Figure 7. MonitoringROMP of COD via H NMR spectroscopy (208C), COD/catalyst=300, catalyst concentration=
4.52 mM, solvent=CDCl3
1
Figure 8. MonitoringROMP of COD via H NMR spectros-
copy (208C), COD/catalyst=3000, catalyst concentration=
Figure 9. MonitoringRCM of diethyl diallylmalonate via
H NMR spectroscopy (208C), diethyl diallylmalonate/cata-
0.452 mM, solvent=CDCl3
1
lyst=200, catalyst concentration=4.52 mM, solvent=
CD Cl .
(
4
10 mL) was added and the resultingsolution was stirred at
08C for 1 h. The reaction mixture was filtered and concen-
2
2
trated under vacuum. The resultingcrude product was puri-
fied by column chromatography using hexane/CH Cl (1/1)
3
H, p-CH ), 2.23 (s, 6H, o-CH ), 1.99 (m, 2H), 1.81 (s, 6H,
3 3
1
3
2
2
CH ), 1.56 (m, 6H), 1.26 (m, 2H); C NMR (CDCl ): d=
3
3
as an eluent. After concentration of the solvent, the desired
293.3 (Ru=CH), 206.8 (CyNCNAr), 152.6 (Ar-C), 144.5 (Ar-
C), 138.8 (Ar-C), 138.6 (Ar-C), 138.0 (Ar-C), 129.7 (Ar-C),
122.9 (Ar-C), 122.7 (Ar-C), 113.1 (Ar-C), 74.9 [OCH-
complex precipitated as a bright green solid which was fil-
1
tered and vacuum dried; yield: 80%. H NMR (CDCl ): d=
3
16.32 (s, 1H, Ru=CH), 7.51 (t, 1H, Ar-H), 7.05 (s, 2H, Ar-
A
H
R
U
G
H), 6.92 (m, 3H, Ar-H), 5.17 [sept, 1H, OCH(CH ) ], 5.06
A
H
R
U
G
Cy), 26.1 (C-3 Cy), 25.8 (C-4 Cy), 22.3 [CH
A
H
R
U
G
3
2
[
m, 1H, N-CH(Cy)], 3.94 (app. s. 4H, NCH CH N), 2.45 (s,
CH ), 18.4 (o-CH ); anal. calcd. (%) for C H N Cl ORu
2 2
3
3
28 38
2
2
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Table 2. CM of allylbenzene and acrylonitrile. 408C, 3 h, sol-
vent=CH Cl . Conversion and E/Z ratios determined by
6.85 (m, 2H, Ar-H), 5.17 [sept, 1H, OCH(CH ) ], 4.02–3.98
A
H
R
U
G
3 2
(m, 7H, CH N and NCH CH N), 3.14 [m, 2H, CH
A
C
H
T
R
E
U
N
G
(CH ) ],
2
2
3
2
2
3 2
1
H NMR (ArCH R protons allylbenzene: d=3.36, Z-
1.80 [d, 6H, OCH
A
H
R
U
G
A
H
R
U
2
3
2
1
3
isomer: d=3.73, E-isomer: d=3.51).
6H, CH(CH ) ]; C NMR (CDCl ): d=290.3 (Ru=CH),
A
H
R
U
G
3
2
3
2
1
5
10.2 (MeNCNAr), 152.9, 148.8, 143.5, 137.5, 129.8, 129.6,
25.1, 122.5, 122.3, 113.1, 75.4 [OCH(CH ) ], 55.3 (NCH),
1.7 (NCH), 38.7 (NCH), 28.1 [CH
(CH ) ], 22.4 [CH(CH ) ]; anal. calcd. (%)
Catalyst
Loading[mol%]
Conversion [%]
E/Z ratio
AHCTREUNG
3
2
A
H
R
U
G
2
2
2
3
3
3
3
3
3
6
6
6
6
6
6
8
8
8
8
a
b
c
a
a
b
b
c
5
<2
91
93
20
34
33
39
43
44
15
31
12
26
21
31
5
-
A
H
R
U
G
A
H
R
U
G
ACHTREUNG
3
2
3
2
3 2
2.5
2.5
2.5
5
2.5
5
2.5
5
2.5
5
2.5
5
2.5
5
2.5
5
2.5
5
0.7/1
0.5/1
1.9/1
1.8/1
1.5/1
1/1
0.6/1
0.6/1
2.5/1
2.8/1
3.2/1
2.9/1
2.2/1
2.4/1
0.8/1
0.6/1
0.5/1
0.4/1
for C H N Cl ORu (564.57): C 55.32, H 6.43, N 4.96;
found: C 55.32, H 6.43, N 4.94.
2
6
36
2
2
A
H
E
G
Analogously, bis(NHC) complex 4b (0.207 g, 0.23 mmol)
and 2-isopropoxystyrene (0.187 g, 1.15 mmol, 5 equivs.) were
weighed into a dry Schlenk flask and dissolved in toluene
A
H
R
U
G
c
a
a
b
b
c
(
10 mL). The solution was stirred at 808C during2 h. The
reaction mixture was concentrated under vacuum. The dark
green residue was purified by column chromatography using
hexane/CH Cl (1/1) as an eluent. The desired complex was
obtained as a bright green solid, which was washed with
hexane; yield: 84%. H NMR (CDCl ): d=16.30 (s, 1H,
2
2
c
1
3
a
a
b
b
Ru=CH), 7.58 (t, 1H, Ar-H), 7.48 (m, 1H, Ar-H), 7.37 (d,
26
7
30
2
[
H, Ar-H), 6.92 (d, 1H, Ar-H), 6.86 (m, 2H, Ar-H), 5.13
m, 2H, OCH(CH ) and N-CH], 3.92 (app. s. 4H,
NCH CH N), 3.11 [m, 2H, CH
ACHTREUNG
3
2
A
H
R
U
G
2
2
(
m, 2H), 1.81 [d, 6H, OCH
H), 1.20 [d, 6H, CH(CH ) ], 0.87 [d, 6H, CH(CH ) ];
3 2
A
H
R
U
4
A
H
R
U
G
A
C
H
T
R
E
U
N
G
3
2
1
3
(
4
590.6): C 56.94, H 6.49, N 4.74; found: C 56.68 H 6.65 N
.79.
C NMR (CDCl ): d=290.5 (Ru=CH), 207.3 (CyNCNAr),
3
1
1
52.9, 149.0, 143.9, 137.7, 129.6, 129.4, 125.0, 122.6, 122.4,
13.2, 75.0 [OCH(CH ) ], 61.4 (NCH), 54.9 (NCH), 43.5
A
H
R
U
G
3 2
(
NCH), 31.2, 29.9, 28.1, 26.2, 25.9, 25.8, 24.0, 22.5; anal.
(
H IiPCampMes)Cl Ru=CH-o-O-i-PrC H (3c)
2 2 6 4
calcd. (%) for C H N Cl ORu (632.69): C 58.85, H 7.01, N
4.43; found: C 58.62, H 7.00, N 4.40.
3
1
44
2
2
Analogously (H IiPCampMes)(PCy )Cl Ru=CHPh (0.122 g,
0
A
H
R
U
G
2
3 2
.14 mmol), CuCl (0.014 g, 0.14 mmol) and 2-isopropoxystyr-
ene (0.023 g, 0.14 mmol) afforded complex 3c, which was pu-
Complexes 6a–c, Route C: General Procedure
rified by column chromatography using hexane/CH Cl (1/1)
2
2
1
as an eluent; yield: 84%. H NMR (CDCl ): d=16.39 (s,
Complex 2a (1 equiv.) and 1.4 equivs. of the appropriate
NHC chloride salt were weighed into a dry Schlenk flask,
and toluene was added. The resultingsuspension was treat-
ed with LiHMDS (lithium hexamethyldisilazane, 1.0M solu-
tion in toluene, 1.4 equivs.) and stirred at room temperature
for 1 h. The mixture was then filtered to remove residual
salts and concentrated under vacuum. The dark green resi-
due was analyzed as a mixture of the phosphine-bearing
complex 5a/b/c and the desired complex 6a/b/c. To achieve
full formation of the desired complex, the crude product
was dissolved in chloroform and stirred during1 h. The solu-
tion was then evaporated and the remainder was subjected
to column chromatography (hexane/CH Cl 1/1) to obtain
3
1
H, Ru=CH), 7.52 (t, 1H, Ar-H), 7.06 (s, 1H, Ar-H), 7.05
(
s, 1H, Ar-H), 6.91 (m, 3H, Ar-H), 5.52 (m, 1H, N-C H),
a
5
.12 [sept, 1H, OCH
A
H
R
U
G
NCH CH N), 3.16 (m, 1H), 2.50 (m, 2H), 2.45 (s, 3H), 2.23
2
2
(
3
d, 6H), 2.10 (s, 1H), 1.98 (m, 2H), 1.74 (m, 6H), 1.54 (m,
H), 1.29 (s, 3H), 1.22 (s, 3H), 0.94 (d, 1H); C NMR
1
3
(
CDCl ): d=294.5 (Ru=CH), 209.3 (NCN), 152.3, 144.9,
3
1
7
4
1
38.8, 138.6, 138.5, 138.3, 129.8, 129.7, 123.0, 122.7, 113.2,
4.9 [OCH(CH ) ], 59.9 (NCH), 51.7 (NCH), 48.5 (NCH),
ACHTREUNG
3
2
3.6, 42.0, 41.1, 38.8, 34.4, 34.1, 28.0, 23.8, 22.3, 21.8, 21.4,
8.4; anal. calcd. (%) for C H N Cl ORu (644.7): C 59.62,
3
2
44
2
2
H 6.88, N 4.35; found: C 58.13, H 6.65 N 4.27.
2
2
pure complexes 6a (yield: 76%), 6b (yield: 72%), or 6c
yield: 73%).
H IiPCampPr)Cl Ru=CH-o-O-i-PrC H (6c): H NMR
(
A
C
H
T
R
E
U
N
G
(H IMePr)Cl Ru=CH-o-O-i-PrC H (6a), Route B
2 2 6 4
1
(
2
2
6
4
Bis
A
C
H
T
R
E
U
N
G
(NHC) complex 4a (0.152 g, 0.20 mmol) and 2-isopropoxy-
(CDCl ): d=16.36 (s, 1H, Ru=CH), 7.59 (t, 1H, Ar-H), 7.48
3
styrene (0.170 g, 1.05 mmol, 5.25 equivs.) were weighed into
a dry Schlenk flask and dissolved in toluene (5 mL). The so-
lution was heated at 808C for 2 h. After evaporation of the
solvent, the crude product was purified by column chroma-
tography (hexane/CH Cl =2/3). After evaporation of the
(m, 1H, Ar-H), 7.38 (d, 2H, Ar-H), 6.91 (d, 1H, Ar-H), 6.85
(m, 2H, Ar-H), 5.57 (m, 1H, N-C H), 5.11 [sept, 1H, OCH-
a
A
T
E
N
(m, 2H), 2.11 (s, 1H), 1.98 (m, 2H), 1.77 (d, 6H), 1.57 (m,
3H), 1.30 (s, 3H), 1.22 (m, 9H), 1.05 (m, 1H), 0.89 (d, 6H);
2
2
1
3
chromatography solvent, the product was obtained as a light
C NMR (CDCl ): d=291.2 (Ru=CH), 209.8 (NCN), 152.7,
3
1
green solid, and washed with hexane; yield: 75%. H NMR
148.9, 148.6, 144.2, 138.4, 129.6, 125.0, 122.5, 113.3, 75.1
[OCH(CH ) ], 60.0 (NCH), 54.9 (NCH), 48.6 (NCH), 43.3,
42.0, 41.1, 38.8, 34.5, 34.1, 28.1, 26.0, 25.8, 24.0, 23.8, 22.5,
(
CDCl ): d=16.22 (s, 1H, Ru=CH), 7.60 (t, 1H, Ar-H), 7.48
ACHTREUNG
3
3 2
(m, 1H, Ar-H), 7.38 (m, 2H, Ar-H), 6.94 (d, 1H, Ar-H),
1698
asc.wiley-vch.de
ꢁ 2007 Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 1692 – 1700

Comparative Investigation of Hoveyda–Grubbs Catalysts
FULL PAPERS
2
1.9; anal. calcd. (%) for C H N Cl ORu (686.78): C 61.21,
CH Cl . Acrylonitrile (0.14 mL, 2.13 mmol, 43 equivs.) and
3
5
50
2
2
2
2
H 7.34, N 4.08; found: C 61.28 H 7.39 N 4.05.
allylbenzene (0.26 mL, 1.96 mmol, 40 equivs.) were added
and the resultingreaction mixture was stirred at 40 8C
during3 h. The reaction mixture was then analyzed by
A
C
H
T
R
E
U
N
G
(H ICy)Cl Ru=CH-o-O-i-PrC H (8a)
2 2 6 4
1
H NMR spectroscopy.
A
C
H
T
R
E
U
N
G
[H ICy][BF ] (0.201 g, 0.624 mmol, 1.4 equivs.), complex 2a
A
C
H
T
R
E
U
N
G
2
4
(
0.268 g, 0.446 mmol, 1 equiv.) and toluene (5 mL) were
placed in a Schlenk flask. LiHMDS (0.624 mL, 0.624 mmol,
.4 equivs.) was added and the resultingsuspension was
Crystal Structures
1
CCDC-634495–CCDC-634499 contain the supplementary
crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data request/cif.
stirred at room temperature for 1 h. The mixture was then
filtered and the filtrate was concentrated under vacuum to
afford a brownish residue, which was analyzed as the phos-
phine-bearingcomplex 7a (benzylidene a-proton: d=20.82).
Full formation of complex 8a required stirringin chloro-
form (20 mL) for 8 h. Purification was achieved by column
chromatography with gradient elution (CH Cl /hexane 3/1 Acknowledgements
2
2
to 100% CH Cl ). The desired complex was obtained as an
2
2
1
olive green solid; yield: 58%. H NMR (CDCl ): d=18.27
The authors gratefully acknowledge the FWO-Flanders and
the research Fund of Ghent University for generous financial
support during the preparation ofthis manuscript. We wish
to thank Olivier F. Grenelle and Dr. Marc G. Proot ofChev-
ron Technology, Ghent for elemental analyses.
3
(
s, 1H, Ru=CH), 7.67 (m, 2H, Ar-H), 7.07 (m, 2H, Ar-H),
.24 [sept, 1H, OCH(CH ) ], 4.96 (m, 1H, N-CH), 4.72 (m,
H, N-CH), 3.68 (m, 4H, NCH CH N), 2.20 (broad signal,
5
1
4
8
2
ACHTREUNG
3
2
2
2
H), 1.89 (m, 4H), 1.83 (d, 6H), 1.72 (m, 2H), 1.56 (m,
1
3
H), 1.15 (m, 2H); C NMR (CDCl ): d 287.9 (Ru=CH),
3
03.0 (NCN), 153.3, 145.1, 129.8, 123.1, 122.9, 113.6, 75.2
[
OCH(CH ) ], 60.0 (broad signal, NCH), 57.7 (broad signal,
A
C
H
T
R
E
U
N
G
3 2
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5
25
38
2
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(
CDCl ): d=18.74 (s, 1H, Ru=CH), 7.67 (m, 2H, Ar-H),
3
7
.05 (m, 2H, Ar-H), 5.42 (br s, 2H, NC H), 5.20 [sept, 1H,
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0
H), 2.36 (m, 2H), 2.31 (t, 2H), 1.99 (m, 2H), 1.91 (m, 2H),
.83–1.78 (m, 12H), 1.47 (d, 6H), 1.23 (s, 6H), 1.02 (m, 2H),
13
.94 (d, 2H); C NMR (CDCl ): d 289.9 (Ru=CH), 206.3
3
(
NCN), 153.3, 145.1, 129.8, 122.9, 121.8, 113.8, 75.4 [OCH-
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(CH ) ], 59.7 (broad signal, NCH), 58.1 (broad signal,
3
2
NCH), 48.5 (NCH), 43.3, 42.2, 40.4, 38.9, 34.6, 28.3, 23.5,
2.5, 22.2, 21.9; anal. calcd (%) for C H N Cl ORu
2
3
3
49
2
2
(
4
661.75): C 59.90, H 7.46, N 4.23; found: C 59.01 H 7.27 N
.23.
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