Stable ruthenium indenylidene complexes with a sterically reduced NHC ligand

Article abstract of DOI:10.1039/c2cc37514a

Stable and active Ru olefin metathesis catalyst 7 bearing a sterically reduced NHC ligand was synthesized. Upon action of pyridine, 7 forms complex 24, the first example of an Ru-indenylidene catalyst bearing a labile pyridine ligand and a sterically less demanding NHC ligand.

Full text of DOI:10.1039/c2cc37514a

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Stable ruthenium indenylidene complexes with
a sterically reduced NHC ligand†
Cite this: DOI: 10.1039/c2cc37514a
Christian Torborg, Grzegorz Szczepaniak, Adam Zieli n´ ski, Maura Mali n´ ska,
Krzysztof Wo ´z niak and Karol Grela*
Received 15th October 2012,
Accepted 6th December 2012
DOI: 10.1039/c2cc37514a
www.rsc.org/chemcomm
Stable and active Ru olefin metathesis catalyst 7 bearing a sterically
reduced NHC ligand was synthesized. Upon action of pyridine, 7 forms
complex 24, the first example of an Ru–indenylidene catalyst bearing a
labile pyridine ligand and a sterically less demanding NHC ligand.
The importance of olefin metathesis for modern synthetic
1
chemistry is illustrated by numerous applications in organic
Scheme 1 Synthesis of 7.
2
and polymer chemistry. Since the introduction of the ‘first
3
,4
generation’ Ru-based catalysts, the basic structure has been
modified leading to numerous different catalyst families,
including the most prominent mixed N-heterocyclic carbene
the robustness of the aforementioned catalysts, especially, of
the 1,3-bis(2-methylphenyl)-4,5-dihydroimidazol-2-ylidene (oTol)
substituted ones, like 2.
(
NHC)–phosphine metathesis catalysts, described as ‘second
5
generation’ (Fig. 1). The replacement of one labile phosphine
ligand with a non-labile NHC led to an increased longevity of
the active species and therefore better catalyst performance.
Recently, the size reduction of the substituents of the N-bound
aryl rings was found to be beneficial for the formation of tetra-
substituted alkenes, a transformation which has been considered
as challenging for some time.
example 2, often lack sufficient stability, as the facilitated rotation
around the N–C bond opens pathways to decomposition.
Grubbs et al. and Grisi et al. showed that the stability of such
complexes can be greatly enhanced by backbone substitution of
the NHC, leading to highly efficient catalysts like 3. Despite these
advances, we sought for a more straightforward way to increase
As indenylidene bearing complexes such as 1 and 4 are known
17–18
to be more stable than their benzylidene counterparts,
we
envisioned that an indenylidene catalyst 7 containing oTol would
show sufficient stability as well as activity (Scheme 1).
Addition of a commercially available M1 catalyst (5) to an excess
of a carbene formed in situ from the corresponding chloride salt 6
and potassium hexamethyldisilazide (KHMDS) in toluene provided
6
7
8–12
However, those complexes, for
7
in 55% yield as a red solid. Crystals suitable for X-ray diffraction
13–14
aryl
analysis were grown from a dichloromethane–pentane mixture. In
15
16
contrast to 4, as well as to previously reported similar Hoveyda type
16b
complexes, only one conformer, in which the o-tolyl substituents
of the NHC are in the syn-conformation, was found in the crystal
(Fig. 2). Notably, the ORTEP diagram of 7 further displays the
3-phenyl group of the indenylidene moiety being on the other side
as the methyl groups of the o-tolyl substituents. As a result, the
metal centre is considerably less shielded on one side of the
complex than on the other. Hence, the structure remarkably differs
19
from the corresponding SIPr-substituted complex 4, in which the
aryl rings are almost perpendicular to the NHC ring. Other struc-
tural parameters resemble those observed in other second genera-
tion indenylidene complexes.
Fig. 1 Selected second generation catalysts.
This catalyst was surprisingly stable in a solid form as well as in
solution. Only slight degradation of 7 was observed in a toluene-d
8
1
solution at room temperature. In fact, according to the H-NMR
University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland.
E-mail: klgrela@gmail.com
spectrum, 20% of the intact catalyst was still present even after 13
days. In contrast, benzylidene complex 2 was already fully decom-
posed after that time under identical conditions (see the ESI†).
†
Electronic supplementary information (ESI) available: Experimental proce-
dures, NMR and X-ray data. See DOI: 10.1039/c2cc37514a
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Table 1 Comparison of 2 and 7 in RCM of tetra-substituted olefins
Yield [%]
Entry Substrate
Product
Loading (mol%)
2
7
a
1
5
45
75
b
2
0.5
0.5
69
55
98
55
Fig. 2 Molecular structure of 7. Ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are omitted for clarity.
c
3
The catalytic performance of 7 was investigated in the RCM of
6 6
diethyl dimethallylmalonate 8 at 40 1C in C D . For comparison,
commercially available 1, 2 and 10 were applied under identical
conditions (Fig. 3). As expected, mesityl-substituted 1 performed
a
6 6
Conditions: 0.06 mmol 8, C D (0.1 M), 40 1C, 1 h; conversions
b
calculated by NMR. Conditions: 0.5 mmol 11, toluene (0.1 M),
6
c
0 1C, 1 h; shown are isolated yields. Conditions: 0.5 mmol 13,
poorly with this very challenging substrate; both 2 and 7 initiated toluene (0.1 M), 60 1C, 3.5 h; shown are isolated yields.
rapidly at the chosen temperature, with 7 providing a higher
conversion under these conditions. The oTol containing Hoveyda
1
5
catalyst 10, one of the most active catalysts for this reaction,
showed a longer initiation period than 2 and 7.
Table 2 Model RCM, CM and enyne metathesis reactions for 7 and 24
Isolated
Catalyst (mol%) yield [%]
Notably, for the RCM of other sterically demanding olefins, Entry Substrate
rivals the most efficient catalysts reported so far (see Table 1).
Product
7
Good results obtained with this novel catalyst prompted us
to further test 7 in a set of RCM, cross metathesis (CM) and
enyne metathesis reactions (Table 2). RCM of 15 proceeded in
satisfying yield at room temperature and at low catalyst loading
7
(0.5)
a
1
2
74
61
b
(Table 2, entry 1). The substrate 17 underwent enyne metathesis
15
16
24
(0.5)
in the presence of 2 mol% of 7 in DCM at rt (Table 2, entry 3).
In addition, the new catalyst showed high activity and excellent
selectivity in the CM of 19 with methyl acrylate (Table 2,
entry 4).
7
(2)
c
3
99
91
7
(2)
d
4
a
b
Conditions: 0.5 mmol 15, toluene (0.1 M), rt, 18 h. Conditions:
c
0
.5 mmol 15, DCM (0.1 M), rt, 18 h. Conditions: 0.5 mmol 17, DCM
d
(
0.1 M), rt, 3 h. Conditions: 0.5 mmol 19, toluene (0.1 M), 60 1C, 1 h.
Pyridine-substituted catalysts (‘third generation’, Scheme 2)
are superior to phosphine containing ones in respect of faster
initiation, as the pyridine ligand is only weakly coordinated to
the metal centre and thus more easily released, forming the
2
0
reactive 14-electron species. Unfortunately, those catalysts
also tend to decompose more rapidly in reaction mixtures,
and therefore are eventually outperformed by their phosphine
1
0
and NHCewg-containing analogues. In contrast, indenylidene-
2
1
type third generation catalysts 22, 23 are rather stable.
Considering the stabilizing effect of the indenylidene moiety on
Fig. 3 RCM of diene 8 at 40 1C in C
6
D
6
. Lines are intended as visual aids.
third generation catalysts as well as on 7, we became interested in
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In summary, complex 7, bearing the oTol ligand combines
the high stability of indenylidene-type complexes with the high
catalytic activity of catalysts containing sterically less demanding
NHC ligands. As a result, 7 has been successfully applied in a
number of RCM, CM and ene–yne metathesis reactions. In
addition, the excellent stability of 7 allowed for preparation of
the first example of the 3rd generation indenylidene catalyst
bearing a sterically reduced NHC ligand.
The authors wish to thank the Foundation for Polish Science
‘
‘Team Programme’’ co-financed by the EU European Regional
Development Fund-Operational Program Innovative Economy
007/2013. M. Barbasiewicz, S. Czarnocki, M. Wilczek and O.
2
Ablialimov are gratefully acknowledged for technical assis-
tance. K. Zukowska is kindly acknowledged for proofreading
Scheme 2 Selected third generation catalysts and the synthesis of 24.
:
the manuscript.
Notes and references
1
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Fig. 4 Molecular structure of 24. Ellipsoids are drawn at the 50% probability
level. Hydrogen atoms and crystallization solvent molecules are omitted for
clarity.
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To the best of our knowledge, a third generation catalyst containing
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has not been reported yet in the scientific literature.
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6a
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C–H activation. Conversely, dissolving 7 in a minimal amount of
pyridine, followed by recrystallization from a pentane–dichloro-
methane mixture gave complex 24 in 76% yield (Scheme 2).
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of 24 are in the syn conformation. The Ru–C18 distance
1
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1
1
1
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2
1
to the corresponding SIPr complex 23, while the Ru–C1
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(
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showed activity in RCM of diene 15 (Table 2, entry 2). Mindful
of the remarkable performances of 3rd generation catalysts in
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2
1
20 H. Clavier, J. L. Petersen and S. P. Nolan, J. Org. Chem., 2006,
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ring-opening polymerization (ROMP) reactions, we continue
the study and plan to report the activity of 24 in ROMP in
due time.
6
2
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun.