Chelated ruthenium catalysts for Z -selective olefin metathesis

Article abstract of DOI:10.1021/ja202818v

We report the development of ruthenium-based metathesis catalysts with chelating N-heterocyclic carbene (NHC) ligands that catalyze highly Z-selective olefin metathesis. A very simple and convenient procedure for the synthesis of such catalysts has been developed. Intramolecular C-H bond activation of the NHC ligand, promoted by anion ligand substitution, forms the appropriate chelate for stereocontrolled olefin metathesis.

Full text of DOI:10.1021/ja202818v

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Chelated Ruthenium Catalysts for Z-Selective Olefin Metathesis
Koji Endo and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
b
Scheme 1. Synthesis of 1b
ABSTRACT: We report the development of ruthenium-
based metathesis catalysts with chelating N-heterocyclic
carbene (NHC) ligands that catalyze highly Z-selective
olefin metathesis. A very simple and convenient procedure
for the synthesis of such catalysts has been developed.
Intramolecular CÀH bond activation of the NHC ligand,
promoted by anion ligand substitution, forms the appro-
priate chelate for stereocontrolled olefin metathesis.
Scheme 2. Synthesis of 4a
ased on the continued development of well-defined catalysts,
olefin metathesis has emerged as a valuable synthetic method
B
for the formation of carbonÀcarbon double bonds.1 Among
the frontiers of catalyst development has been the quest for
Z-selective olefin metathesis catalysts which would enable access
to complex natural products2 and stereoregular unique polymers.3
Specifically, the use of Z-selective catalysts in olefin cross-metath-
esis (CM) represents a promising and useful methodology in
organic chemistry. However, due to the thermodynamic nature
of metathesis,4 most catalysts give a higher proportion of the
thermodynamically favored E olefin isomer. This fundamental
aspect of olefin metathesis has limited its applications in some
areas of chemistry.
times.9 Subsequent intramolecular CÀH bond activation at the
methyl position of the mesityl group resulted in the formation of
4a with concomitant release of pivalic acid (Scheme 2). Such
intramolecular CÀH bond activations assisted by carboxylate
ligands have been reported in other organometallic complexes.10
Based on these previous reports, a plausible mechanism for the
CÀH bond activation in 3a contains a six- (A) or four-membered
(B) transition state (Figure 1). It should be noted that no other
anionic ligands, including sulfonate,5b phosphonate,5b and
trifluoroacetate,11 promote such intramolecular CÀH bond acti-
vation in 1a. An X-ray crystal structure of 4a clearly indicates
CÀH bond activation at the NHC ligand and subsequent forma-
tion of the six-membered chelated complex (Figure 2).
When 1b was reacted with excess silver pivalate, CÀH bond
activation occurred at the adamantyl group, which resulted in the
formation of the five-membered chelate complex [(C10H14)-
H2IMes]RuCl2[dCH-o-(OiPr)C6H4] (4b, Scheme 3). No di-
carboxylate complex was detected as the CÀH bond activation of
1b was too fast, but observation of pivalic acid suggests the same
reaction mechanism of intramolecular CÀH bond activation.
The CÀH bond activation at the adamantyl group and the fast
reaction may be attributed to the ligand geometry in 1b. An X-ray
Recently, some ruthenium-based catalysts which showed
enhanced Z-selectivity have been reported; however, their
selectivity is still not satisfactory for precisely stereocontrolled
syntheses.5 On the other hand, recently developed molybde-
num- and tungsten-based catalysts have shown outstanding
Z-selectivity in CM2 and metathesis homocoupling6 of terminal
olefins. In particular, a bulky aryloxide-substituted Mo catalyst
afforded the cross-coupled product of enol ether and allylben-
zene with 98% of the Z isomer. As has been demonstrated in the
past, Ru- and Mo-based systems show significant differences in
selectivities and utility.7
For general use, metathesis catalysts should be not only
tolerant toward various functional groups and impurities in
reaction media but also readily synthesized from common re-
agents by simple reaction steps. Here, we report chelated Ru
catalysts that catalyze highly Z-selective olefin metathesis, and
their facile synthetic preparation.
We chose [H2IMes2]RuCl2[dCH-o-(OiPr)C6H4] (1a, H2I =
imidazolidinylidene, Mes = mesityl) and the bulkier [H2IM-
esAdm]RuCl2[dCH-o-(OiPr)C6H4] (1b, Adm = 1-adamantyl)
as precursors. 1b was readily synthesized from commercially
available 2 in excellent yield (Scheme 1).8
Reaction of 1a with excess silver pivalate resulted in the
formation of 3a, which was observed only at early reaction
Received: March 28, 2011
Published: May 12, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja202818v J. Am. Chem. Soc. 2011, 133, 8525–8527
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Figure 1. Plausible transition states of the intramolecular CÀH bond
activation in 3a.
Figure 3. X-ray crystal structure and selected bond lengths of 4b.
Displacement ellipsoids are drawn at 50% probability. For clarity, hydro-
gen atoms have been omitted.
Figure 2. X-ray crystal structure and selected bond lengths of 4a.
Displacement ellipsoids are drawn at 50% probability. For clarity,
hydrogen atoms have been omitted.
Scheme 3. Synthesis of 4b
Figure 4. Plot of conversion versus time for the RCM of 5. Reaction
conditions were as follows. 1a:15 1.0 mol % catalyst, 0.1 M substrate,
30 °C, CD2Cl2. 4a: 1.0 mol % catalyst, 0.1 M substrate, 30 °C, C6D6.
4b: 5.0 mol % catalyst, 0.1 M substrate, 70 °C, C6D6.
crystal structure of 1b showed that the adamantyl group is
proximal to a vacant coordination site where the CÀH bond
activation occurs, and the distance between Ru and C12, where a
new bond forms after CÀH bond activation, is relatively short
(2.80 Å).12 The molecular structure of 4b determined by X-ray
crystallography revealed an adamantyl-contained chelate, which
confirmed intramolecular CÀH bond activation at the adamantyl
group (Figure 3).
Complexes having a chelating NHC ligand derived from
intramolecular CÀH bond activation have been reported.13
Most of them were formed during catalyst decomposition, and
none have been reported to have catalytic activity. We investi-
gated at first whether 4a,b were still “active” as olefin metathesis
catalysts, testing them in standard ring-closing metathesis
(RCM) of diethyldiallyl malonate (5)14 and ring-opening me-
tathesis polymerization (ROMP) of norbornene (7). As shown
in Figure 4, 4a,b were metathesis active in the RCM reaction
although activities were lower than with 1a. Because of catalyst
decomposition, conversion by 4a was limited to ∼14%. On the
other hand, 4b was able to achieve almost full conversion at
higher temperature (70 °C). In the ROMP of 7 affording
polynorbornene (8), 4a,b both showed high activities at the
presented conditions.16
0.12 (90% Z isomer) with 4b is among the lowest reported for
Ru-based olefin metathesis catalysts. In addition, the homo-
coupled product 12 afforded by 4b also showed significantly
low E/Z ratio (E/Z = 0.06, 95% Z isomer). The conversion to
11 was improved under THF reflux conditions, maintaining
excellent Z-selectivity (entry 3). This was probably a result of
more efficient removal of ethylene which was generated during
the course of reaction. Unexpectedly, addition of water led to
higher conversions and selectivity for the Z olefin products
(entry 4). This result implies not only that water can be used
to optimize reaction conditions but also that 4b is tolerant
toward water in organic solvent. Thus, dry solvent is not
necessary for 4b in olefin metathesis reactions. This feature
enables easy use of the catalyst in common organic and polymer
syntheses without strict reaction conditions. However, 4b de-
composed immediately when exposed to oxygen in solution,
meaning that degassing of solvent is required to achieve high
conversion. Notably, the reaction was reproducible on a syn-
thetic scale (mmol scale, entry 5).
In summary, we have demonstrated the utility of chelated
ruthenium catalysts for Z-selective olefin cross-metathesis reac-
tions. The Z-selectivity achieved by 4b is the best among
reported Ru-based catalysts and comparable to those achieved
with Mo- and W-based catalysts. Notably, this is the first time
that Z-selectivity in the cross-metathesis of two different olefins
has been demonstrated using a Ru-based catalyst. The Ru cata-
lyst is readily synthesized from common reagents via simple
reaction steps and is stable in the presence of water, which should
Encouraged by the results of the standard metathesis assays,
we then tested the CM of allylbenzene (9) and cis-1,4-diacetoxy-
2-butene (10).14 Selected data of the CM are summarized in
Table 1. Surprisingly, 4a,b gave a much lower E/Z ratio of the
cross-coupled product 11 (entries 1 and 2) compared to their
parent nonchelated catalysts (entries 6 and 7). The E/Z ratio of
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Journal of the American Chemical Society
COMMUNICATION
Table 1. CM of 9 and 10a
11
12
entry
1
cat.
cat. load., mol %b
2.5
solvent
C6H6
temp, °C
time, min
conv,c %
E/Zd
conv,c %
E/Zd
4a
23
10
60
57.5
57.4
32.5
36.4
59.5
64.4
61.6g
69.7
66.3
0.15
0.23
1.44
1.44
0.13
0.12
0.19
0.14
0.14
10.5
10.7
3.10
2.90
3.3
3.0
1.21
0.69
0.07
0.06
0.04
0.03
0.03
5.22
6.86
2
4b
5.0
C6H6
70
30
24.8
26.0
31.6
28.6
NAh
5.9
120
240
240
240
1
3
4
5f
6
4b
4b
4b
1a
5.0
5.0
5.0
2.5
THF
reflux
reflux
reflux
23
THF/H2Oe
THF/H2Oe
C6H6
30
10.2
NAi
NAi
7
1b
2.5
C6H6
23
1
NAi
NAi
30
a Unless otherwise stated, all reactions were carried out using 0.20 mmol of 9, 0.40 mmol of 10, and 0.10 mmol of tridecane (internal standard for
GC analysis) in 1.0 mL of solvent. b Catalyst loading based on 9. c Conversion of 9 to the product determined by GC analysis. d Molar ratio of E and
Z isomers of the product determined by GC analysis. e THF/H2O = 1:1 (by volume). f Reaction was carried out using 1.0 mmol of 9, 2.0 mmol of 10, and
0.050 mmol of catalyst in 5.0 mL of solvent. g Isolated yield. h 12 was obtained with impurities. i GC signal of 12 was too small to quantify.
promote its application in precisely stereocontrolled organic and
polymer syntheses.
Gerber, L. C. H.; Debelouchina, G. T.; Schrock, R. R. Macromolecules
2010, 43, 7515–7522. (c) Torker, S.; M€uller, A.; Chen, P. Angew. Chem.
Int. Ed. 2010, 49, 3762–3766. (d) Flook, M. M.; Ng, V. W. L.; Schrock,
R. R. J. Am. Chem. Soc. 2011, 132, 1784–1786.
’ ASSOCIATED CONTENT
(4) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH: Wein-
heim, 2003; Vols. 1À3.
S
Supporting Information. Experimental procedures and
b
(5) (a) Rosen, E. L.; Sung, D. H.; Chen, Z.; Lynch, V. M.; Bielawski,
C. W. Organometallics 2010, 29, 250–256. (b) Teo, P.; Grubbs, R. H.
Organometallics 2010, 29, 6045–6050.
X-ray data. This material is available free of charge via the Internet at
http://pubs.acs.org.
(6) (a) Jiang, A. J.;Zhao, Y.;Schrock, R. R.;Hoveyda, A. H.J. Am. Chem.
Soc. 2009, 131, 16630–16631. (b) Marinescu, S. C.; Schrock, R. R.; M€uller,
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’ AUTHOR INFORMATION
Corresponding Author
rhg@caltech.edu
(8) Jafarpour, L.; Hillier, A. C.; Nolan, S. P. Organometallics 2002,
21, 442–444.
1
’ ACKNOWLEDGMENT
(9) 3a was detected by H NMR and FAB-MS. See details in the
Supporting Information.
We thank Mr. B. K. Keitz, Mr. M. B. Herbert, and Dr. P. Teo for
helpful discussions and suggestions for this work, Materia, Inc. for
the generous donation of catalysts, and Dr. M. W. Day and Mr.
L. M. Henling for X-ray crystallography. The Bruker KAPPA
APEXII X-ray diffractometer was purchased via an NSF CRIF:MU
award to the California Institute of Technology, CHE-0639094.
This work was financially supported by National Institutes of
Health (NIH 5R01GM031332-27) and Mitsui Chemicals, Inc.
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(12) For X-ray crystal structure and selected bond lengths of 1b, see
the Supporting Information.
(13) (a) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.;
Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem.
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(15) The RCM data were referred to ref 14.
(16) For reaction conditions and results, see the Supporting
Information.
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