A Ruthenium Catalyst for Olefin Metathesis Featuring an Anti-Bredt N-Heterocyclic Carbene Ligand

Article abstract of DOI:10.1002/adsc.201501140

A ruthenium complex bearing an "anti-Bredt" N-heterocyclic carbene was synthesized, characterized and evaluated as a catalyst for olefin metathesis. Good conversions were observed at room temperature for the formation of di- A nd tri-substituted olefins b

Full text of DOI:10.1002/adsc.201501140

UPDATES
DOI: 10.1002/adsc.201501140
A Ruthenium Catalyst for Olefin Metathesis Featuring an Anti-
Bredt N-Heterocyclic Carbene Ligand
a,b
c
c,
a,
David Martin, Vanessa M. Marx, Robert H. Grubbs, * and Guy Bertrand *
a
UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of
California, San Diego, La Jolla, CA 92093-0343, USA
E-mail: gbertrand@ucsd.edu
Present address: UMR CNRS 5250, UniversitØ Grenoble-Alpes “DØpartement de Chimie MolØculaire”, B.P. 53, 38041
b
Grenoble Cedex 9, France
c
Department of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
E-mail: rhg@caltech.edu
Received: December 14, 2015; Revised: January 13, 2016; Published online: February 9, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501140.
[
8]
Abstract: A ruthenium complex bearing an “anti-
Bredt” N-heterocyclic carbene was synthesized,
characterized and evaluated as a catalyst for olefin
metathesis. Good conversions were observed at
room temperature for the formation of di- and tri-
substituted olefins by ring-closing metathesis. It also
allowed for the ring-opening metathesis polymeri-
zation of cyclooctadiene, as well as for the cross-
metathesis of cis-1,4-diacetoxy-2-butene with allyl-
benzene, with enhanced Z/E kinetic selectivity over
classical NHC-based catalysts.
kyl)(amino)carbenes 4 (CAACs) and of di(amido)-
[9]
carbenes 5 (DACs) have demonstrated interesting
activities and selectivities for several ring-closing
metathesis and cross-metathesis reactions.
stance, DAC-ruthenium complex 5 catalyzes the ring-
closing metathesis (RCM) of tetra-substituted olefins,
a challenging transformation that is accomplished by
[10–12]
For in-
[11,13]
only a limited range of metathesis catalysts.
How-
ever, note that this was found to be also the case for
catalyst 6, which bears a non-electrophilic NHC with
a similar steric environment to 5. The beneficial effect
of p-accepting properties of the ligand is more obvi-
Keywords: olefin metathesis; ruthenium; stable car-
benes
N-Heterocyclic carbene ligands have become key
components of the [L X Ru=CHR] olefin metathesis
2
2
[
1]
catalysts. The replacement of one phosphine in cata-
lyst 1 by an imidazolidinylidene (catalyst 2) results in
significant enhancements (Scheme 1), which have
been attributed to the superior electronic donation of
the carbene ligand and the ensuing increased affinity
[
2,3]
of the ruthenium center for p-acidic olefins.
Note
that an extra gain in stability can be achieved by re-
placement of the remaining phosphine with a chelating
[
4]
ether (catalyst 3).
For some time, the carbene ligand on ruthenium
had been limited to imidazolylidenes, imidazolidinyli-
[5,6]
denes and closely related cyclic di(amino)carbenes.
These so-called NHCs are poor p-acceptors and cover
[
7]
a narrow range of s-donation. More recently, sever-
al examples of p-accepting stable cyclic carbenes have
been developed, thus providing new opportunities for
tuning the ruthenium center. Complexes of cyclic (al- Scheme 1.
Adv. Synth. Catal. 2016, 358, 965 – 969
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
965

David Martin et al.
UPDATES
ous in the CAAC series. Indeed, complex 4a has the
highest reported TON value (340,000) for ethenolysis
[12]
reactions to date. It vastly surpasses the more tradi-
tional catalysts 1–3 (TONs of only 2,000–5,000). Note
that sterics also play a role, as 4b is more active than
4
c or 4d by several orders of magnitude. More gener-
ally, both subtle changes in electronic and steric prop-
erties of the ligands can account for dramatic changes
in activity, stability or selectivity. Therefore there is
no single best ruthenium catalyst for all metathesis
transformations, and new improvements in the field
[13]
require a continuous effort to design new catalysts.
We recently reported “anti-Bredt” N-heterocyclic
carbenes featuring one nitrogen atom in a strained
[
14]
bridgehead position. These ligands are significantly
more p-accepting than NHCs, while retaining their
strong s-donation. They also feature a unique low
steric hindrance on the side of the bridgehead nitro-
gen. Interestingly, they were found to be superior to
CAAC and cyclic di(amino)carbenes as ligands for
the gold-catalyzed intermolecular hydroamination of
Figure 1. Solid state structure of 7 with ellipsoids drawn at
5
0% probability level. Hydrogen atoms have been omitted
for clarity. Selected bond lengths (Š) and angles (8): Ru1À
C1 1.926(6), Ru1ÀO1 2.324(4), Ru1ÀC26 1.813(7), Ru1ÀCl1
2
1
.3374(16), Ru1ÀCl2 2.3278(17), N1ÀC1 1.452(8), N2ÀC1
[
15]
alkynes by hydrazine.
In addition, they allow for
.330(8), N1ÀC1ÀN2 114.6(5), C1ÀRu1ÀC26 105.4(3), Cl1À
the challenging gold(I)-catalyzed hydroarylation of
À
Ru1 Cl2 154.27(7).
[
16]
N,N-di(alkyl)anilines. Herein we report the synthe-
sis of the “anti-Bredt” NHC ruthenium complex 7
and its evaluation as a catalyst, using a set of standar- which is due to the superior p-accepting capabilities
[
13]
dized olefin metathesis reactions.
Complex 7 was synthesized by deprotonation of the
of the carbene.
The N-aryl substituent of the stable carbene is lo-
anti-Bredt imidazolium salt 8 in THF at À788C, gen- cated above the benzylidene ligand. Interestingly, the
erating the free carbene, followed by the addition in same orientation was found in CAAC-complexes 4.
situ of ruthenium complex 9 (Scheme 2). The con- This was interpreted as the result of negative steric in-
sumption of 9 and the formation of free triphenyl- teractions, which would prevent the alkyl groups of
3
1
phosphine were monitored by P NMR spectroscopy. the quaternary carbon of CAAC to approach the ben-
[10]
After completion of the reaction and work-up, com- zylidene proton. This explanation is clearly irrele-
plex 7 was isolated as green crystals in 89% yield and vant in the case of the anti-Bredt NHC ligand, which
fully characterized, including a single crystal X-ray features a non-bulky distorted amino group in place
[
17]
diffraction study.
of the quaternary carbon of CAACs. The preferred
In the solid state, the ruthenium center of 7 has conformation of 7 in the solid state could be governed
a distorted square-pyramidal environment, the apical by unfavorable steric interactions between the chlor-
position being occupied by the benzylidene ligand ides and the N-2,6-di(isopropyl)phenyl groups. This is
(
(
Figure 1). The Ru1ÀC1 (192.6 pm) and Ru1ÀO1 suggested by the examination of the hypothetical
232.4 pm) bond lengths are comparable to those in structure resulting from the simple 1808 rotation of
[
10]
CAAC-complexes 4. In contrast, the tetrahydropyr- the ligand around the CÀRu bond of 7, which brings
imidin-2-ylidene complex 6 features a significantly the isopropyl groups in very close proximity to the
[
11]
longer Ru1ÀC1 bond length (204.8 pm). This is con- chlorine atoms (see the Supporting Information). The
sistent with an increased double bond character of the shortest carbon-chloride distance is 278 pm, the Van
metal-carbene bond of the anti-Bredt NHC complex, der Walls radii of these atoms being 170 pm and
1
75 pm, respectively. However, note that we found no
evidence that the solid-state conformation is main-
tained in solution.
Complex 7 promotes the classical ring-closing meta-
thesis (RCM) of diethyl dialkylmalonate 10a. After
3
0 min at room temperature, 90% of the starting ma-
terial was converted into the corresponding cyclopen-
tene 11a with 1% catalyst loading. Complete conver-
sion was reached within 2 h (Table 1, entries 1 and 2).
In contrast, catalysts bearing bulky CAAC 4c and 4d,
Scheme 2. Synthesis of complex 7.
9
66
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2016, 358, 965 – 969

UPDATES
A Ruthenium Catalyst for Olefin Metathesis
Table 1. Ring-closing metathesis of malonate derivatives.
Table 2. Cross-metathesis of cis-1,4-diacetoxy-2-butene with
allylbenzene.
Entry
Catalyst
Time [h]
Conversion (% E)
Entry
Catalyst
Substrate
Temp.
Time
Conv.
1
2
3
4
5
7
7
2
3
1
3
0.5
0.5
1
52% (70)
98% (70)
79% (91)
72% (91)
60% (66)
1
2
7
7
2
3
4b
4c
4d
5
6
7
7
2
3
4b
7
7
2
3
4b
5
10a
10a
10a
10a
10a
10a
10a
10a
10a
10b
10b
10b
10b
10b
10c
10c
10c
10c
10c
10c
10c
258C
258C
308C
308C
308C
608C
608C
608C
608C
308C
308C
308C
308C
308C
608C
1008C
1008C
1008C
608C
1008C
1008C
30 min
2 h
30 min
20 min
15 min
3 h
90%
99%
97%
98%
95%
97%
95%
85%
74%
90%
90%
95%
95%
95%
0%
[
[
[
a]
a]
b]
[
[
[
[
[
[
[
a]
a]
a]
a]
a]
b]
b]
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
4b
[
[
a]
[13]
Ref.
Ref.
b]
[10b]
10 h
20 min
30 min
1 h
0
1
2
3
4
5
6
7
8
9
0
1
(
1
entries 20 and 21). Note that even after a night at
1
12 h
008C complex 7 could still be detected by H NMR.
[
[
[
a,c]
a,c]
a]
45 min
45 min
1 h
12 h
12 h
1 h
1 h
>48 h
20 h
24 h
This suggested that the catalyst could survive the ex-
perimental conditions and was just inert toward the
substrate.
Catalyst 7 was also tested in cross-metathesis reac-
tions. With 2.5% catalyst loading, the reaction be-
tween cis-1,4-diacetoxy-2-butene and allylbenzene
reached 98% conversion in 3 h. Relative to catalysts 2
and 3, 7 is slightly less active, but exhibits an en-
hanced Z/E ratio at higher conversion (Table 2, en-
tries 1–3). Note that similar kinetic selectivity was re-
ported with CAAC-based catalyst 4b (entry 4).
The catalyst was significantly less active in cross-
metathesis of hex-5-en-1-yl acetate with methyl acry-
late, a very challenging electron-poor olefin. Indeed
0%
[
[
[
[
[
a,c]
a,c]
a]
26%
64%
0%
29%
19%
b]
b]
6
[
[
[
a]
[10a]
Ref.
Ref.
Ref.
b]
c]
[11]
[13]
6-membered ring NHC 5, and DAC 6 require higher only 23% conversion was obtained after 20 h under
temperature to reach decent conversions (entries 6– standard conditions, whereas the reaction reaches
). Complex 7 compares well with benchmark cata- completion within 2–8 h with catalysts 2 and 3
lysts 2 and 3 and CAAC-complex 4b, which benefits (Table 3).
from a smaller N-aryl substituent than in 4c and 4d Finally, we confirmed that 7 could promote ring-
entries 3–5). This remains the case for the synthesis opening metathesis polymerization (ROMP).
9
[1g,18]
(
For
of the more challenging trisubstituted olefin 11b (en- this reaction the stability of the catalyst has usually
tries 10–14). However, whereas 90% of the starting a marginal role, the catalytic activity having the major
[13]
material 10b was transformed within 30 min in pres- contribution to the overall catalytic performance.
ence of 1% of 7, the conversion did not improve after According to NMR monitoring, the polymerization of
one night, likely due to decomposition of the catalyst.
Complex 7 did not catalyze the formation of the Table 3. Cross-metathesis of methyl acrylate with hex-5-en-
1
-yl acetate.
tetra-substituted olefin 11c from malonate (entries 15
and 16). CAAC ligands (catalysts 4a–c), which are
comparable to anti-Bredt NHCs in terms of electronic
properties, suffer from the same limitation, whereas 2
and 3 afford up to 64% conversion (entries 17–19).
Robustness of the catalyst is known to play a key role
in the very challenging RCM of 10c, which typically
occurs at elevated temperatures. This is well illus-
trated by 5 and 6, which are moderately active in
RCM (entries 8 and 9), but have a good thermal sta-
bility and still allow for the formation of 11c at 1008C
Entry
Catalyst
Time [h]
Conversion
[
13]
1
2
3
7
2
3
20
2
8
23%
98%
96%
[
[
a]
a]
[
a]
[13]
Ref.
Adv. Synth. Catal. 2016, 358, 965 – 969
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
967

David Martin et al.
UPDATES
Table 4. Ring-opening metathesis polymerization of cyclooc-
ta-1,5-diene.
dry and oxygen-free solvents. Toluene, THF and pentane
were distilled over sodium under argon atmosphere prior
use. Dichloromethane was dried via elution through a sol-
vent column drying system. CD Cl and C D were distilled
2
2
6
6
from CaH . Allylbenzene, tridecane, and cis-1,4-diacetoxy-2-
2
butene were distilled from anhydrous potassium carbonate
prior to use. KHMDS, ethyl acrylate, hex-5-en-1-yl acetate,
and anthracene were purchased from Aldrich, stored under
Ar, and otherwise used as received. 1,5-Cyclooctadiene was
distilled under Ar immediately prior to the polymerization
Entry
Catalyst
Time [min]
Conversion
1
7
1
2
3
60
60
6
98%
32%
99%
99%
[
[
[
a]
a]
a]
2
3
4
1
13
reaction. H and C NMR spectra were recorded on Bruker
Avance 300 spectrometer. Chemical shifts are given relative
6
to SiMe and referenced to the residual solvent signal. Melt-
4
[
a]
[13]
Ref.
ing points were measured with an Electrothermal MEL-
TEMP apparatus. All metathesis reactions were conducted
[
13]
according to the protocol described in ref.
cyclooctadiene occurred in 1 hour in the presence of
.1% of 7. This is in line with the results obtained in
0
{
3-(2,6-Diisopropylphenyl)-1,3-diaza[3.3.1]non-2-
the previous test reactions. Indeed this fair activity is
moderate when compared to benchmark catalysts 2
and 3, which afford 99% polymerization within 6 min
under similar conditions (Table 4).
ylidene]dichloro(2-isopropoxyphenylmethylene)-
ruthenium (7)
[14]
Under an argon atmosphere, amidinium triflate
8
In conclusion, the anti-Bredt NHC ruthenium com- (140 mg, 0.32 mmol) was dissolved in THF (6 mL) and
slowly added to a stirred solution of KHMDS (70 mg,
plex 7 was synthesized, characterized and evaluated
0
.35 mmol) in THF (3 mL) at À788C. After 5 min, Cl Ru[=
as a catalyst for olefin metathesis reactions. It was
found to be highly active in RCM, with good and fast
conversions being observed at room temperature for
the formation of di- and tri-substituted olefins. How-
ever, it was inactive for the formation of tetra-substi-
tuted olefins. It allowed for ROMP, as well as for the
2
[
19]
CHC H -o-(i-Pr)]PPh 9 (158 mg, 0.27 mmol)
in THF
6
4
3
(10 mL) was added. The resulting mixture was warmed up
to room temperature and stirred overnight. The volatiles
were removed under vacuum. The residues were washed
two times with pentane and the product was extracted three
times with toluene. The combined toluene fractions were
cross-metathesis of cis-1,4-diacetoxy-2-butene with al- concentrated under vacuum and pentane was slowly layered,
lylbenzene with a 70% E selectivity at 98% conver- affording compound 7 as green crystals; yield: 145 mg
+
sion. It is a poor catalyst for the more challenging (89%); mp 130–1338C. MS: m/z=605.1638 [M+H] , calcu-
1
lated for C H N ORuCl : 605.1636; H NMR (CDCl3,
3
3
cross-metathesis of hex-5-en-1-yl acetate with methyl
acrylate.
The fact that these results are reminiscent of the ac-
tivity of catalyst 4b is in line with the similarity of
anti-Bredt NHCs and CAACs in terms of electronic
properties. Previous screening of CAAC-based cata-
lysts demonstrated the importance of tuning the steric
29 41
2
2
00 MHz): d=16.2 (s, 1H), 7.6–7.2 (m, 4H), 6.9–6.8 (m,
H), 5.30 (m, 1H), 5.09 (pseudo sept, J=6 Hz, 1H), 3.8–3.6
m, 3H), 3.51 (d, J=12 Hz, 1H), 3.36 (d, J=12 Hz, 1H),
.18 (sept, J=6 Hz, 2H), 2.62 (m, 1H), 2.2–1.5 (m, 4H),
.79 (d, J=6 Hz, 3H), 1.77 (d, J=6 Hz, 3H), 1.30 (d, J=
Hz, 3H), 1.24 (d, J=6 Hz, 3H), 0.94 (d, J=6 Hz, 3H),
(
3
1
6
0
1
3
.88 (d, J=6 Hz, 3H); C NMR (CDCl , 75 MHz): d=295.8
environment of the ruthenium center, especially by (Ru=CH), 253.3 (RuCN ), 152.5 (Caro), 146.7 (Caro), 146.5
3
2
varying the N-substituent of the carbene ligand. How- (Caro), 144.1 (Caro), 142.0 (Caro), 130.2 (CHaro), 129.6 (CHaro),
ever, in the case of anti-Bredt NHCs, only the free 125.5 (CHaro), 125.3 (CHaro), 123.1 (CHaro), 121.9 (CHaro),
1
13.0 (CHaro), 74.6 (OCH), 62.2 (CH ), 52.3 (CH ), 50.5
carbene bearing a 2,3-di(isopropyl)phenyl amino
group is available. To date, all attempts to deproto-
nate precursors of these carbenes featuring smaller N-
substituents have led to side-reactions under kinetic
control, such as the formation of azomethine ylides.
Therefore, the synthetic challenge of increasing the
structural diversity of anti-Bredt NHC ruthenium
complexes remains to be addressed.
2
2
(
(
(
CH ), 30.9 (CH), 29.7 (CH ), 29.2 (CH ), 28.2 (CH ), 27.7
CH ), 25.9 (CH ), 25.7 (CH ), 24.1 (CH ), 23.9 (CH), 22.2
CH ).
2 2 2 3
3
3
3
3
3
Acknowledgements
This work was financially supported by the DOE (DE-
FG02-13ER16370), the NIH (5R01M031332), the NSF
(
CHE-1212767) and NSERC (fellowship to VMM). Thanks
Experimental Section
are due to B. Donnadieu for the X-ray diffraction study, Dr.
David VanderVelde (Caltech NMR facility) for assistance
with NMR experiments, and Dr. Scott Virgil (Caltech Center
for Catalysis and Chemical Synthesis) for assistance with GC
experiments.
General Considerations
Experiments were performed under an inert atmosphere of
dry argon, using standard Schlenk and dry-box techniques,
9
68
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2016, 358, 965 – 969

UPDATES
A Ruthenium Catalyst for Olefin Metathesis
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