New ruthenium(II) indenylidene complexes bearing unsymmetrical N-heterocyclic carbenes

Article abstract of DOI:10.1021/om300783g

New robust and air-stable ruthenium(II) indenylidene second-generation precatalysts with unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized. These complexes were studied using ring-closing metathesis (RCM) of diethyl diallylmalonate (5; DEDAM) as a model substrate. Two new complexes containing OMe and NMe₂ groups in the NHC ligand, namely [1-(2,4,6-trimethylphenyl)-3-(2-methoxybenzyl)-2-imidazolidinylidene] dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (4a) and [1-(2,4,6-trimethylphenyl)-3-(2-N,N-dimethylaminobenzyl)-2- imidazolidinylidene]dichloro(3-phenyl-1H-inden-1-ylidene) (tricyclohexylphosphine)ruthenium(II) (4b), show increased activity in comparison to the parent [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene] dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (Umicore M2). Interestingly, the complex [1-(2,4,6-trimethylphenyl)-3-(2- thiomethylbenzyl)-2-imidazolidinylidene]dichloro(3-phenyl-1H-inden-1-ylidene) (tricyclohexylphosphine)ruthenium(II) (4c), bearing a SMe substituent, showed very low activity in the model RCM reaction with diethyl diallylmalonate (5). Application of the active complexes for metathesis reactions with a broad spectrum of olefins in commercial-grade solvents in air was examined.

Full text of DOI:10.1021/om300783g

Communication
pubs.acs.org/Organometallics
New Ruthenium(II) Indenylidene Complexes Bearing Unsymmetrical
N-Heterocyclic Carbenes
Osman Ablialimov, Mariusz Kędziorek, Christian Torborg, Maura Malinska, Krzysztof Wozniak,
́
́
and Karol Grela*
̇
Department of Chemistry, Warsaw University, Zwirki i Wigury Street 101, 02-089 Warsaw, Poland
S
* Supporting Information
ABSTRACT: New robust and air-stable ruthenium(II) indenylidene second-
generation precatalysts with unsymmetrical N-heterocyclic carbene (NHC)
ligands were synthesized. These complexes were studied using ring-closing
metathesis (RCM) of diethyl diallylmalonate (5; DEDAM) as a model substrate.
Two new complexes containing OMe and NMe2 groups in the NHC ligand,
namely [1-(2,4,6-trimethylphenyl)-3-(2-methoxybenzyl)-2-imidazolidinylidene]-
dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II)
(4a) and [1-(2,4,6-trimethylphenyl)-3-(2-N,N-dimethylaminobenzyl)-2-imidazolidinylidene]dichloro(3-phenyl-1H-inden-1-
ylidene)(tricyclohexylphosphine)ruthenium(II) (4b), show increased activity in comparison to the parent [1,3-bis(2,4,6-
trimethylphenyl)-2-imidazolidinylidene]dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (Umi-
core M2). Interestingly, the complex [1-(2,4,6-trimethylphenyl)-3-(2-thiomethylbenzyl)-2-imidazolidinylidene]dichloro(3-
phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (4c), bearing a SMe substituent, showed very low activity
in the model RCM reaction with diethyl diallylmalonate (5). Application of the active complexes for metathesis reactions with a
broad spectrum of olefins in commercial-grade solvents in air was examined.
which were further cyclized with triethyl orthoformate to the
lefin metathesis is a versatile tool in organic chemistry for
Othe formation of carbon−carbon double bonds. In this corresponding NHC salts.14 Synthesis of 3c was accomplished
1
through the reaction of the S-methylated thiosalicylic acid 1c
with N-(2,4,6-trimethylphenyl)-1,2-diaminoethane using 1,1′-
carbonyldiimidazole (CDI). With these NHC salts in hand, we
prepared ruthenium complexes 4a−c. Following the conven-
tional route by the reaction of in situ generated carbene with
commercially available dichloro(3-phenyl-1H-inden-1-ylidene)-
bis(tricyclohexylphosphine)ruthenium(II) (M1), these com-
plexes were obtained in moderate yields. Purification by silica
gel chromatography and further recrystallization from a DCM/
n-pentane mixture afforded carmine, air-stable solids (Scheme
1).
respect it was widely acknowledged by a Nobel Prize to R. R.
Schrock, Y. Chauvin, and R. H. Grubbs.2−4
Olefin metathesis still attracts a great deal of attention from
organic chemists not only in academia5 but also in industry.6
The state of the art of olefin metathesis catalysts open up new
vistas for more efficient methods in tedious natural product
synthesis.7 Nowadays, there are a great number of precatalysts
known with various NHC ligands designed for olefin
metathesis, and many of them are commercially available.8 It
is well-known that modifications on the NHC’s moiety can
induce profound changes in the reactivity pattern of the
resulting precatalysts.9 Moreover, the unsymmetrical NHC
ligands allow diversification of the steric bulkiness in the vicinity
of the ruthenium and therefore change the catalytic properties
of the resulting complexes.10−12 The range of the ruthenium
metathesis precatalysts developed up to this point is broad, but
there is still need for more complexes which are air-stable and
remain robust in solvents of commercial-grade purity. There-
fore, we turned our attention to ruthenium indenylidene
precatalysts, since they are robust and efficient in the olefin
metathesis reactions.13 We expected that formal replacement of
one of the traditional mesityl groups in the NHC ligand by a
flexible benzyl group bearing a donor substituent might allow
additional coordination of the latter to the metal.
Catalytic activities of prepared precatalysts were investigated
in RCM reactions with diethyl diallylmalonate (5) (Scheme 2,
Figure 1). As a reference, commercially available Umicore M2
precatalyst was used.
In this reaction precatalysts 4a,b both show high metathetic
activity and significantly outperform commercial precatalyst
M2. Encouraged by these promising results, we decided to
investigate the performance of the new precatalysts on a wider
range of substrates (Table 1). In general, the reactions were
monitored by GC until full consumption of the substrate. Ring-
closing ene-yne metathesis (RCEYM) was investigated with the
standard test substrate 7. In this reaction precatalyst 4a was
slightly slower than its analogue 4b; however, both showed a
shorter reaction time for the conversion to the final product 8
The synthesis of the new imidazolinium salts 3a,b was
achieved by the condensation of N-(2,4,6-trimethylphenyl)-1,2-
diaminoethane and the respective easily accessible aldehydes.
The resulting diimines were reduced in situ to diamines 2a,b,
Received: August 13, 2012
© XXXX American Chemical Society
A
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Organometallics
Communication
of the new complexes 4a,b, as full conversion was achieved just
in 3 h, in comparison to 8 h for the M2 precatalyst. In principle,
the new precatalysts with unsymmetrical NHC ligands 2a,b
outperform the M2 precatalyst in the majority of tested
reactions.
Scheme 1. Synthetic Pathways for the Synthesis of the New
Indenylidene Precatalysts
a
Blechert et al. showed that diastereoselective ring-rearrange-
ment metathesis (DRRM) of protected cyclopentene 16 with
Grubbs first-generation and Hoveyda−Grubbs first-generation
precatalysts does not proceed in a diastereoselective manner,
while their second-generation analogues gave poor diaster-
eoselectivities (trans:cis d.r. = 2:1) (Scheme 3).15 We expected
that precatalysts bearing unsymmetrical NHC moieties would
increase the diastereoselectivity. Therefore, we applied
precatalysts 4a,b, as well as commercially available precatalyst
M2, to this particular reaction. Interestingly, M2 precatalyst
gave the same result (trans:cis d.r. = 2:1) as Grubbs and
Hoveyda−Grubbs second-generation precatalysts. In contrast,
application of the new precatalysts 4a,b resulted in much higher
diastereoselectivity for the substrate 16 (trans:cis d.r. = 4.7:1 for
4a and trans:cis d.r. = 4.5:1 for 4b; see Table 2). Precatalyst 4a
showed, to the best of our knowledge, the highest
diastereoselectivity for the substrate 16 ever reported.15
In comparison with the symmetrical ligand 1,3-dimesityl-4,5-
dihydroimidazol-2-ylidene (SIMes), the newly designed unsym-
metrical NHC ligands certainly change the reactivity of
indenylidene precatalysts 4a,b. In order to find out whether
the benzyl group interacts through the heteroatom with the
ruthenium core, a single crystal of precatalyst 4a was grown and
analyzed by X-ray diffraction (Figure 2).
a
Legend: (a) N-(2,4,6-trimethylphenyl)-1,2-diaminoethane, HCOOH
cat., MeOH, room temperature, 24 h then NaBH4, MeOH, room
temperature, 12 h; (b) 1,1'-carbonyldiimidazole (CDI), MeCN, 0 °C
to room temperature, 3 h then N-(2,4,6-trimethylphenyl)-1,2-
diaminoethane, 0 °C to room temperature, reflux 2 h next LiAlH4,
THF, 0 °C to reflux, 4 h; (c) (OEt)3CH, HCl, 100 °C, 4 h; (d)
C5H11OK, M1, toluene, room temperature to 65 °C, 0.5 h.
Single crystals of 4a grown from a DCM/n-pentane mixture
crystallize in the orthorhombic P212121 space group symmetry
(for the experimental details see the Supporting Information).
There is one molecule of precatalyst in the asymmetric unit
with the five-coordinated ruthenium atom. The 4a molecules
pack into compact layers in the crystal lattice, with the methoxy
groups directed between the layers (see Figure S2 in the
Supporting Information). The bond length between the
ruthenium atom Ru(1) and the carbon atom C(1) is
2.059(4) Å, whereas the Ru(1)−P(1) bond is equal to
2.472(1) Å. The trigonal-bipyramidal geometry around the
Ru(1) atom is distorted, with the C(1)−Ru(1)−P(1) angle
equal to 160.0(1)° and the Cl(1)−Ru(1)−Cl(2) angle equal to
159.37(4)°. This led to the reduction of the space between the
cyclohexyl ring of the tricyclohexylphosphine ligand and the
phenyl ring of the NHC ligand. The other important structural
parameters are the following: the Ru(1)−C(22) bond is
1.863(4) Å, whereas the bond lengths to the chloride atoms are
2.409(1) and 2.401(1) Å for Cl(1) and Cl(2), respectively. A
high degree of freedom for the benzyl group and steric
repulsion from the tricyclohexylphosphine ligand and chlorine
atom are probably responsible for the lack of interaction
between the oxygen atom and the metal center, at least in the
solid state.
Scheme 2. RCM of Diethyl Diallylmalonate (5)
However, during the catalytic cycle after dissociation of the
tricyclohexylphosphine it is feasible that such coordination
takes place and influences the stability of the 14-electron
catalyst.16
Figure 1. RCM conversion of diethyl diallylmalonate (5). Legend: (a)
60 °C in C6D5CD3.
Interestingly, the sulfur-containing complex 4c exhibits very
low activity in comparison to 4a,b in olefin metathesis of
DEDAM (5), showing just 7% conversion after 1.5 h at 30 °C
in DCM. Even after the reaction time was extended to 24 h,
precatalyst 4c showed practically no change in the conversion.
However, in the same reaction this precatalyst resuscitated in
in comparison to the M2 precatalyst. In general, amide-based
substrate 9 required higher temperature for the full conversion
to the product 10 in a reasonable time. Both new precatalysts
4a,b in cross-metathesis of 11 and 12 showed E selectivity
comparable to that of the M2 precatalyst. Cyclization to the
seven-membered ring in product 15 demonstrated high activity
B
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Organometallics
Communication
Table 1. Preparative RCM, CM, and Ene-Yne Reactions
a
b
c
0.1 M in CH2Cl2 (commercial grade HPLC). Reactions were performed in toluene (commercial grade HPLC) in air at 50 °C. Isolated yields after
d
1
flash chromatography. E:Z ratio was determined by H NMR spectroscopy.
Scheme 3. DRRM of Cyclopentene (16)
Table 2. Diastereoselective Ring-Rearrangement Metathesis
with Different Precatalysts
a
a
precatalyst
conversion (%)
trans:cis
b
Gru I, Hov I
95
95
1:1
Figure 2. ORTEP representation of complex 4a. Hydrogen atoms
have been omitted for clarity. Thermal ellipsoids are shown at the 50%
probability level.
b
Gru II, Hov II
2:1
M2
4a
>95
>95
2:1
4.7:1
Scheme 4. Possible Interaction between NHC and Ru
Center of 4c in Solution
4b
>95
4.5:1
a
b
Determined by 1H NMR spectroscopy Results reported by Blechert
et al.15
toluene at 60 °C, showing 40% conversion after 25 min (Figure
1). Recently, Lemcoff studied in detail Ru−SII chelation in a
number of ruthenium benzylidene complexes containing
thioalkyl ligands. In all cases these S-chelated complexes
exhibited strongly decreased activity in metathesis (latent
precatalysts).17 Also in the case of a thio-analogue of trans-
dichloro Hoveyda precatalyst obtained by us, very low catalytic
potency was observed.18 Therefore, we suppose that, at least in
solution, there must be some interaction between the Lewis
basic group in the ortho position in the benzylic arm of the
NHC ligand and the ruthenium center (Scheme 4) that
influences the catalyst reactivity profile. Unfortunately, despite
many attempts, we were unable to grow single crystals of good
quality.
precatalysts with those NHC ligands were prepared, and their
catalytic activities have been comparatively studied for
formation of a disubstituted double bond in the RCM reaction
with diethyl diallylmalonate (5). Two complexes, containing
OMe (4a) and NMe2 (4b) substituents in the NHC ligands,
were found to outperform commercially available catalyst M2.
This trend was retained also in other RCM and RCEYM
reactions performed in air in commercial grade solvents. Complex
In conclusion, three new unsymmetrical NHCs have been
synthesized in a convenient fashion. Three indenylidene
C
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Organometallics
Communication
their Application. In Catalysis of Organic Reactions: Twenty-second
Conference; Prunier, M. L., Ed.; CRC Press: Boca Raton, FL, 2008;
Chemical Industries Series Vol. 123, p 568.
4c, containing the SMe substituent, showed in contrast
practically no reactivity in the benchmark reaction of 5. To
better understand the structure−reactivity relationship asso-
ciated with unsymmetrical benzyl-substituted NHC ligands, the
synthesis of other precatalysts and their studies will be reported
in due course.
ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*Tel: +48-22-8220211 ext. 420. Fax: +48-22-8220211. E-mail:
klgrela@gmail.com.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We wish to thank the Foundation for Polish Science (TEAM
Programme cofinanced by the EU European Regional
Development Fund, Operational Program Innovative Economy
2007-2013. O.A. thanks Dr. M. Barbasiewicz, Dr. P. Horeglad,
́
and Dr. W. Kosnik for fruitful discussions. O.A. is also grateful
to Dr. R. Loska for the initial work.
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