Cationic Silica-Supported N-Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis

Article abstract of DOI:10.1002/anie.201510678

Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ-donating N-heterocyclic carbene ligands with weak σ-donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well-defined silica-supported catalysts, [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)⁺][B(Ar^F)₄^-] [IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, B(Ar^F)₄=B(3,5-(CF₃)₂C₆H₃)₄] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.

Full text of DOI:10.1002/anie.201510678

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International Edition: DOI: 10.1002/anie.201510678
German Edition: DOI: 10.1002/ange.201510678
Supported Catalysts
Cationic Silica-Supported N-Heterocyclic Carbene Tungsten Oxo
Alkylidene Sites: Highly Active and Stable Catalysts for Olefin
Metathesis
Margherita Pucino, Victor Mougel, Roman Schowner, Alexey Fedorov, Michael R. Buchmeiser,*
and Christophe CopØret*
Abstract: Designing supported alkene metathesis catalysts
with high activity and stability is still a challenge, despite
significant advances in the last years. Described herein is the
combination of strong s-donating N-heterocyclic carbene
ligands with weak s-donating surface silanolates and cationic
tungsten sites leading to highly active and stable alkene
metathesis catalysts. These well-defined silica-supported cata-
surface siloxy and either one alkyl or amide ligand, respec-
tively. This conclusion has inspired the synthesis of catalysts
with enhanced dissymmetry at the metal site.[1c,11] One of the
most recent and compelling examples is the use of poor
s-donor surface silanolates[12] with strong s-donating thiolate
ligands in silica-supported tungsten oxo alkylidene catalys-
ts,[4e,15] which display substantially enhanced activity towards
terminal olefins.
The recent report of highly active and stable tungsten oxo
alkylidenes supported by an NHC[6c] as a strong s-donor
ligand thus opens new avenues to significantly improve the
activity of silica-supported metathesis catalysts by combining
cationic intermediates and a strong s-donating NHC ligand
with a weak s-donating surface silanolate. Herein we report
the synthesis and characterization of silica-supported neutral
and cationic tungsten oxo alkylidene complexes bearing an
NHC ligand and show the unprecedented activity and
ꢀ
=
=
lysts,
ꢀ
[( SiO)W( O)( CHCMe2Ph)(IMes)(OTf)]
and
+
F
À
=
=
[( SiO)W( O)( CHCMe2Ph)(IMes) ][B(Ar )4 ] [IMes =
1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, B(ArF)4 =
B(3,5-(CF3)2C6H3)4] catalyze alkene metathesis, and the cat-
ionic species display unprecedented activity for a broad range
of substrates, especially for terminal olefins with turnover
numbers above 1.2 million for propene.
I
n recent years, tremendous advances in the design of alkene
and alkyne metathesis catalysts have been made for both
homogeneous and heterogeneous systems.[1–3] In particular,
the development of highly active tungsten oxo alkylidene
molecular catalysts[4] and the adaption of N-heterocyclic
carbene (NHC) ligands, widely used in ruthenium-based
alkene metathesis catalysts,[5] to molybdenum and tungsten
systems[6] have significantly widened the range of applications
of d0-metal-based olefin metathesis catalysts.
stability of the supported cationic olefin metathesis catalyst.
[13]
=
=
Grafting W( O)( CHCMe2Ph)(IMes)(OTf)(OtBuF6)
=
=
(1) and the related cationic species [W( O)( CHCMe2Ph)-
(IMes)(OtBuF6)+] [B(ArF)4 ] (2) onto SiO2–700 afforded the
À
corresponding materials 1@SiO2 and 2@SiO2, respectively
(Scheme 1). Quantification by solution NMR spectroscopy of
tBuF6OH released upon grafting indicates that about 47 and
51% of the surface silanols (0.26 mmolgÀ1) reacted with 1 and
2, respectively. This data is in agreement with the tungsten
loading of 2.51% (1@SiO2, 0.13 mmolgÀ1) and 2.67%
(2@SiO2, 0.14 mmolgÀ1) as determined by elemental analysis
and the presence of residual silanols interacting with nearby
aromatic residues according to infrared (IR) spectroscopy (a
broad band at 3647 cmÀ1; Figure S25 in the Supporting
Information).[9c]
For heterogeneous catalysts, major advances have been
possible through surface organometallic chemistry,[3,7] which
enables the generation of well-defined active sites at the
surface of oxide supports.[8] This approach identified the
electronic dissymmetry at the metal center as a key factor for
greatly improved activity of silica-supported Schrock alkyli-
dene complexes.[9] DFT calculations[10] have revealed that this
increase in activity is due to the easier coordination of the
olefin substrate to the metal-alkylidene sites and the destabi-
lization of metallacyclobutane intermediates because of the
presence of both weak and strong s-donor ligands, that is, one
1
The H magic angle spinning (MAS) NMR spectrum of
1@SiO2 showed resonances at d = 11.2, 7.1, and 2.1 ppm,
which were assigned to the alkylidene, the aromatic, and
methyl proton signals, respectively (Figure S19). A similar
1H MAS spectrum was observed for 2@SiO2 (Figure S21),
with peaks at d = 10.4, 7.4 and 1.9 ppm, accounting for the
alkylidene, aromatic, and methyl moieties, respectively. As
generally observed for supported tungsten alkylidene com-
plexes, the carbene signals were not detected at natural
abundance in the 13C cross-polarization (CP) MAS spectra,
but all the other signals corresponding to the ligands were
observed (Figures S20–S22). The 11B NMR spectrum of
2@SiO2 shows only one signal at d = À6.3 ppm and it is
[*] M. Pucino, Dr. V. Mougel, Dr. A. Fedorov, Prof. Dr. C. CopØret
Department of Chemistry and Applied Biosciences, ETH Zürich
Vladimir-Prelog-Weg 2, 8093 Zurich (Switzerland)
E-mail: ccoperet@ethz.ch
R. Schowner, Prof. Dr. M. R. Buchmeiser
Institute of Polymer Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
E-mail: michael.buchmeiser@ipoc.uni-stuttgart.de
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201510678.
À
consistent with the presence of isolated B(ArF)4 anions as
observed for the molecular precursor 2 (Figure S23).
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and 2 showed high activity [TOF = 170 minÀ1 (TOF = turn-
over frequency), with fast equilibrium conversion, ca. 3 min].
The catalyst 2@SiO2 can however be recycled more than three
times without loss of catalytic activity. In addition, quantifi-
cation of the number of active sites indicated that more than
95% of the tungsten centers in 2@SiO2 are active (see the
Supporting Information). More importantly, 2@SiO2 dis-
played an unprecedented activity for the metathesis of 1-
nonene, a terminal olefin. TOFs as high as 2460 minÀ1 could
be measured at a 50 ppm loading, about two orders of
magnitude higher than the best well-defined silica-supported
catalyst known to date under the same reaction conditions. In
contrast, the molecular catalyst 2 showed faster deactivation
in the metathesis of 1-nonene at a 0.02% catalyst loading,
with the conversion reaching only 80%. This difference
presumably arises from the low stability of the methylidene
species generated from 2 and illustrates the advantage of site
isolation of the corresponding supported catalysts, which
prevents deactivation through dimerization.[9b] This excep-
tionally high stability allowed a decrease in the catalyst
loading to 10 ppm in the 1-nonene metathesis with 2@SiO2,
thus reaching a TON of about 61100, at the expense of the
TOF, which fell to 830 minÀ1. This loss of activity (TOF) is
attributed to poisoning of active species with adventitious
impurities not removed by the purification protocol.
Scheme 1. Grafting 1 and 2 onto SiO2–700. B(ArF)4 =B(3,5-(CF3)2C6H3)4,
Mes=mesityl, Tf =trifluoromethanesulfonyl.
As the alkylidene signals of supported species are difficult
to detect, because of their large chemical shift anisotropy and
low site density,[14] we prepared 13C-enriched compounds both
at the alkylidene and NHC carbene positions, 1* and 2*,
respectively. The 13C-enriched silica-supported complexes
1*@SiO2 and 2*@SiO2 were synthesized and characterized
1
by 13C CPMAS and two-dimensional (2D) H-13C HETCOR
NMR spectroscopy (see the Supporting Information). Alky-
lidene signals were observed at d = 300 and 296 ppm for
1*@SiO2 and 2*@SiO2, respectively, and the NHC carbene
was observed at d = 186 ppm for 1*@SiO2 and d = 181 ppm
for 2*@SiO2. The spectroscopic characterization of 1@SiO2
and 2@SiO2, combined with elemental (C, H, B and F; see the
Supporting Information) analyses, are in agreement with the
The activity of 2@SiO2 in self-metathesis, ring-closing
metathesis, and cross-metathesis with functionalized sub-
strates was also investigated (Table 2). 2@SiO2 displayed high
activity for functionalized substrates, which is generally
challenging for supported catalysts. With ethyl oleate, fast
equilibrium conversion is observed at a 0.1 mol% loading.
Decreasing the catalyst content to 10 ppm allows a maximum
TON of about 12000 with a TOF3min of 400 minÀ1. 2@SiO2 is
also active for the ethenolysis of ethyl oleate where TONs of
up to 930 were achieved. Ring-closing metathesis of diallyl
ether proceeded with a TON of 780, while the molecular
precursor 2 was inactive,[6c] and likely results from the
increased stability provided by grafting.
The heterogeneous nature of 2@SiO2 allowed us to
investigate its activity in gas-phase reactions under flow
conditions. Remarkably, the catalyst 2@SiO2 achieved more
than 1200000 turnovers in the self-metathesis of propene
within 6 days, with a metathesis selectivity greater than 99%,
and with only about 40% deactivation over that period.
In summary, grafting neutral and cationic tungsten oxo
alkylidene NHC complexes onto SiO2–700 afforded the well-
ꢀ
=
=
proposed structures [( SiO)W( O)( CHCMe2Ph)(IMes)
+
F
À
ꢀ
=
=
(OTf)] and [( SiO)W( O)( CHCMe2Ph)(IMes) ][B(Ar )4 ]
as the main species in 1@SiO2 and 2@SiO2, respectively.
To harness the potential of 1@SiO2 and 2@SiO2 in alkene
metathesis, we first tested them with the benchmark sub-
strates cis-4-nonene and 1-nonene (Table 1).[4c,d,5a,b, 12,15] Both
materials are active in self-metathesis of cis-4-nonene and 1-
nonene at 308C, but they significantly differ in activity.
1@SiO2 showed moderate activity with cis-4-nonene, while
2@SiO2 displays high activity, similar to some of the best
silica-supported tungsten alkylidene catalysts.[4c,d,5a,b, 12] For
comparison, 1 was inactive under similar reaction conditions,
Table 1: Catalytic activity of 1@SiO2 and 2@SiO2 in self-metathesis of
cis-4-nonene and 1-nonene in toluene at 308C.
ꢀ
=
=
defined surface species [( SiO)W( O)( CHCMe2Ph)-
+
ꢀ
=
=
(IMes)(OTf)] and [( SiO)W( O)( CHCMe2Ph)(IMes) ]-
[a]
Substrate
Catalyst mol% TOF3min
Time to equilibrium[b]
cis-4-nonene 1@SiO2 0.1
cis-4-nonene 2@SiO2 0.1
cis-4-nonene 2@SiO2 0.02
10 (3%)
90 (27%)
120 (7%)
5 h (490)
5 min (510)
1 h (2510)
Table 2: Catalytic activity of 2@SiO2 with functionalized substrates in
toluene.
1-nonene
1-nonene
1-nonene
1-nonene
1@SiO2 0.02
2@SiO2 0.02
2@SiO2 0.002 2460 (15%)
2@SiO2 0.001 830 (2%)
4 (<1%) 26% in 24 h (1300)
Substrate
T [8C]
mol%
TON[a]
420 (25%)
5 h (4530)
ethyl oleate
diallyl ether
diallyl diphenylsilane
ethyl oleate ethenolysis
70
30
30
60
0.001
0.1
0.1
11874 (1 h)
780 (24 h)
880 (24 h)
930[b] (1 h)
74% in 24 h (37090)
60% in 24 h (61070)
[a] TOF at 3 min, given in minÀ1 and the corresponding conversion is
given within parentheses. [b] TON given within parentheses. Also given
is the percent conversion for cases in which equilibrium was not
achieved.
0.1
[a] Time is given within parentheses. [b] 82% selectivity for ethenolysis,
18% of homodimerization products.
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À
[B(ArF)4 ]. Both complexes are active in alkene metathesis,
[5] a) T. Weskamp, F. J. Kohl, W. Hieringer, D. Gleich, W. A.
Herrmann, Angew. Chem. Int. Ed. 1999, 38, 2416 – 2419; Angew.
Chem. 1999, 111, 2573 – 2576; b) M. Scholl, T. M. Trnka, J. P.
Morgan, R. H. Grubbs, Tetrahedron Lett. 1999, 40, 2247 – 2250;
c) J. Huang, E. D. Stevens, S. P. Nolan, J. L. Petersen, J. Am.
Chem. Soc. 1999, 121, 2674 – 2678; d) G. C. Vougioukalakis,
R. H. Grubbs, Chem. Rev. 2010, 110, 1746 – 1787; e) V. M. Marx,
A. H. Sullivan, M. Melaimi, S. C. Virgil, B. K. Keitz, D. S.
Weinberger, G. Bertrand, R. H. Grubbs, Angew. Chem. Int. Ed.
2015, 54, 1919 – 1923; Angew. Chem. 2015, 127, 1939 – 1943.
[6] a) M. R. Buchmeiser, S. Sen, J. Unold, W. Frey, Angew. Chem.
Int. Ed. 2014, 53, 9384 – 9388; Angew. Chem. 2014, 126, 9538 –
9542; b) S. Sen, R. Schowner, D. A. Imbrich, W. Frey, M.
Hunger, M. R. Buchmeiser, Chem. Eur. J. 2015, 21, 13778 –
13787; c) R. Schowner, W. Frey, M. R. Buchmeiser, J. Am.
Chem. Soc. 2015, 137, 6188 – 6191.
but the cationic species displayed unprecedented activity and
stability in the metathesis of olefins, coupled with improved
functional-group tolerance by comparison with other silica-
supported complexes. Most remarkable are the high activity
and stability with terminal olefins, thus allowing over
a 1 million TON in propene self-metathesis with 2@SiO2 to
be achieved. These results may open a new prospective for the
use of well-defined catalysts in chemical processes. The results
provide further evidence that enhancing the dissymmetry at
the metal center by using a strong s-donating ligand such as
an NHC in silica-supported d0-metal alkylidene complexes
leads to an increased activity in olefin metathesis and that site
isolation provided by the support leads to exceptionally high
stability.
[7] a) C. CopØret, M. Chabanas, R. P. Saint-Arroman, J. M. Basset,
Angew. Chem. Int. Ed. 2003, 42, 156 – 181; Angew. Chem. 2003,
115, 164 – 191; b) S. L. Wegener, T. J. Marks, P. C. Stair, Acc.
Chem. Res. 2012, 45, 206 – 214; c) M. M. Stalzer, M. Delferro,
T. J. Marks, Catal. Lett. 2015, 145, 3 – 14.
Acknowledgments
[8] a) C. CopØret, New J. Chem. 2004, 28, 1 – 10; b) C. CopØret,
Dalton Trans. 2007, 5498 – 5504.
We are grateful to Dr. R. Verel for his assistance with NMR
measurements, Dr. Wolfgang Frey for single-crystal X-ray
analysis, and Dr. P. Zhizhko for performing reproducibility
tests. A.F. thanks the Holcim Stiftung for a Habilitation
Fellowship. The authors are grateful to the Scientific Equip-
ment Program of ETH Zürich and the Swiss National Science
Foundation (RꢀEquip grant 206021_150709/1) for financial
support.
[9] a) M. Chabanas, A. Baudouin, C. CopØret, J. M. Basset, J. Am.
Chem. Soc. 2001, 123, 2062 – 2063; b) F. Blanc, C. CopØret, J.
Thivolle-Cazat, J. M. Basset, A. Lesage, L. Emsley, A. Sinha,
R. R. Schrock, Angew. Chem. Int. Ed. 2006, 45, 1216 – 1220;
Angew. Chem. 2006, 118, 1238 – 1242; c) B. Rhers, A. Salameh,
A. Baudouin, E. A. Quadrelli, M. Taoufik, C. CopØret, F.
Lefebvre, J. M. Basset, X. Solans-Monfort, O. Eisenstein,
W. W. Lukens, L. P. H. Lopez, A. Sinha, R. R. Schrock, Organo-
metallics 2006, 25, 3554 – 3557; d) F. Blanc, J. Thivolle-Cazat,
J. M. Basset, C. CopØret, A. S. Hock, Z. J. Tonzetich, A. Sinha,
R. R. Schrock, J. Am. Chem. Soc. 2007, 129, 5779 – 5779.
[10] a) A. Poater, X. Solans-Monfort, E. Clot, C. CopØret, O.
Eisenstein, J. Am. Chem. Soc. 2007, 129, 8207 – 8216; b) X.
Solans-Monfort, C. CopØret, O. Eisenstein, J. Am. Chem. Soc.
2010, 132, 7750 – 7757; c) X. Solans-Monfort, C. CopØret, O.
Eisenstein, Organometallics 2012, 31, 6812 – 6822; d) X. Solans-
Monfort, C. CopØret, O. Eisenstein, Organometallics 2015, 34,
1668 – 1680; e) V. Mougel, C. B. Santiago, P. A. Zhizhko, E. N.
Bess, J. Varga, G. Frater, M. S. Sigman, C. CopØret, J. Am. Chem.
Soc. 2015, 137, 6699 – 6704.
[11] a) S. J. Malcolmson, S. J. Meek, E. S. Sattely, R. R. Schrock,
A. H. Hoveyda, Nature 2008, 456, 933 – 937; b) I. Ibrahem, M.
Yu, R. R. Schrock, A. H. Hoveyda, J. Am. Chem. Soc. 2009, 131,
3844 – 3845; c) A. H. Hoveyda, S. J. Malcolmson, S. J. Meek,
A. R. Zhugralin, Angew. Chem. Int. Ed. 2010, 49, 34 – 44; Angew.
Chem. 2010, 122, 38 – 49; d) E. M. Townsend, J. Hyvl, W. P.
Forrest, R. R. Schrock, P. Müller, A. H. Hoveyda, Organo-
metallics 2014, 33, 5334 – 5341.
[12] V. Mougel, C. CopØret, Chem. Sci. 2014, 5, 2475 – 2481.
[13] During the course of this study we could determine the crystal
structure of 1. Details are given in the Supporting Information
(see Figure S54).
Keywords: metathesis · N-heterocyclic carbene ·
structure elucidation · supported catalysts · tungsten
How to cite: Angew. Chem. Int. Ed. 2016, 55, 4300–4302
Angew. Chem. 2016, 128, 4372–4374
[1] a) R. R. Schrock, Angew. Chem. Int. Ed. 2006, 45, 3748 – 3759;
Angew. Chem. 2006, 118, 3832 – 3844; b) R. H. Grubbs, Angew.
Chem. Int. Ed. 2006, 45, 3760 – 3765; Angew. Chem. 2006, 118,
3845 – 3850; c) S. J. Meek, R. V. OꢀBrien, J. Llaveria, R. R.
Schrock, A. H. Hoveyda, Nature 2011, 471, 461 – 466; d) B. K.
Keitz, K. Endo, P. R. Patel, M. B. Herbert, R. H. Grubbs, J. Am.
Chem. Soc. 2012, 134, 693 – 699; e) C. Wang, F. Haeffner, R. R.
Schrock, A. H. Hoveyda, Angew. Chem. Int. Ed. 2013, 52, 1939 –
1943; Angew. Chem. 2013, 125, 1993 – 1997; f) R. K. M. Khan, S.
Torker, A. H. Hoveyda, J. Am. Chem. Soc. 2013, 135, 10258 –
10261.
[2] a) B. Haberlag, M. Freytag, C. G. Daniliuc, P. G. Jones, M.
Tamm, Angew. Chem. Int. Ed. 2012, 51, 13019 – 13022; Angew.
Chem. 2012, 124, 13195 – 13199; b) A. Fürstner, Angew. Chem.
Int. Ed. 2013, 52, 2794 – 2819; Angew. Chem. 2013, 125, 2860 –
2887.
[3] C. CopØret, A. Comas-Vives, M. P. Conley, D. Estes, F. Nunez-
Zarur, A. Fedorov, V. Mougel, H. Nagae, P. A. Zhizhko, Chem.
Rev. 2016, 116, 323 – 421.
[14] a) F. Blanc, C. CopØret, A. Lesage, L. Emsley, Chem. Soc. Rev.
2008, 37, 518 – 526; b) F. Blanc, J. M. Basset, C. CopØret, A.
Sinha, Z. J. Tonzetich, R. R. Schrock, X. Solans-Monfort, E.
Clot, O. Eisenstein, A. Lesage, L. Emsley, J. Am. Chem. Soc.
2008, 130, 5886 – 5900.
[4] a) D. V. Peryshkov, R. R. Schrock, M. K. Takase, P. Muller,
A. H. Hoveyda, J. Am. Chem. Soc. 2011, 133, 20754 – 20757;
b) D. V. Peryshkov, R. R. Schrock, Organometallics 2012, 31,
7278 – 7286; c) M. P. Conley, V. Mougel, D. V. Peryshkov, W. P.
Forrest, D. Gajan, A. Lesage, L. Emsley, C. CopØret, R. R.
Schrock, J. Am. Chem. Soc. 2013, 135, 19068 – 19070; d) M. P.
Conley, W. P. Forrest, V. Mougel, C. CopØret, R. R. Schrock,
Angew. Chem. Int. Ed. 2014, 53, 14221 – 14224; Angew. Chem.
2014, 126, 14445 – 14448; e) V. Mougel, M. Pucino, C. CopØret,
Organometallics 2015, 34, 551 – 554.
[15] F. Allouche, V. Mougel, C. CopØret, Asian J. Org. Chem. 2015, 4,
528 – 532.
Received: November 18, 2015
Published online: March 1, 2016
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