A new family of halogen-chelated hoveyda-grubbs-type metathesis catalysts

Article abstract of DOI:10.1002/chem.201202817

Coordination, not insertion: New ruthenium benzylidenes with a chelating halogen atom were easily prepared and showed excellent stability and activity as metathesis catalysts (see figure). Structure-activity studies reveal that strength of the ruthenium-halogen interaction can be tuned across a wide range to set up a family of latent to active catalysts. Copyright

Full text of DOI:10.1002/chem.201202817

COMMUNICATION
DOI: 10.1002/chem.201202817
A New Family of Halogen-Chelated Hoveyda–Grubbs-Type Metathesis
Catalysts
Michał Barbasiewicz,* Michał Michalak, and Karol Grela*[a]
Halocarbons are among the most common organic com-
pounds and are applied as solvents, pesticides, and refriger-
ants. They also offer opportunities for further functionaliza-
tion due to the unique properties of the carbon–halogen
bond. These transformations can be carried out by both stoi-
chiometric and catalytic processes, the latter represented by
the enormous variety of reactions that are catalysed by tran-
sition metals.[1] Apart from the catalytic processes based on
Figure 1. Selected metathesis catalysts with coordinating atoms of oxygen
À
oxidative insertion of the metal center to the C X bond, or-
(1a and b), sulfur (2), and nitrogen (3).[10,14] Mes=2,4,6-trimethylphenyl.
ganic halides can be applied in coordination chemistry be-
cause of their ability to act as acceptors and donors of elec-
tron density, the latter being a rather rare occurrence. Al-
though electron-deficient iodoarenes can form stable com-
plexes with nucleophilic pyridine derivatives to give various
architectures through halogen bonding,[2,3] manifestation of
the s-donor properties of organic halides is far less
common. Few examples, such as the formation of hydrogen
bonds with halogens,[4] the anchimeric assistance of epoxide
opening in the synthesis of chlorosulpholipid,[5] and studies
in which metal–halogen coordination was observed[6] or
postulated as an intermediate interaction,[7] have been de-
scribed in the literature.
The application of catalyzed olefin metathesis for the
transformation of complex substrates is recognized in cur-
rent organic chemistry. Pioneered by early ruthenium[8] and
molybdenum alkylidenes,[9] catalysts have evolved into more
sophisticated structures that contain N-heterocyclic carbene
(NHC) motifs and the chelating benzylidene ligand (see
1a).[10] In further development, the inspiring Hoveyda–
Grubbs complex was replicated in numerous sulfur,[11] nitro-
gen,[12] phosphorus, and selenium[13] derivatives,[14] giving
access to a palette of new structures and rich structure–ac-
tivity correlation data (Figure 1).
improves catalyst performance.[15] In turn, electronic effects
are based on the control of Ru···O coordination; for exam-
ple, the nitro group that is present on the benzylidene ring
(1b) facilitates the opening of the chelate, which increases
catalyst activity.[16] Stability of the chelate bond can also be
increased by the electronic effect that is prompted by de-
signed naphthalene ligands. The origin of this stabilization
was investigated in detail and led to the conclusion that the
initiation rate of the complexes is governed by p-electron
delocalization of the chelate ring.[17] However, some aspects
of the (pre)catalyst initiation remain under debate,[18] and
numerous synthetic studies are being targeted towards this
essential process by the preparation of new complexes with
modified chelating ligands.
In our project, we focused on the synthesis of ruthenium
complexes that are chelated with s-donors of limited coordi-
nation ability and are intrinsically unable to support chelate
formation. We expected that the relatively weak interaction
could be easily strengthened with p-electronic effects,[17a,b]
providing stable complexes of peculiar characteristics. As
weak donors, we considered covalently bonded halogen
atoms, which are a rare class of ligands in coordination
chemistry. This type of interaction has been postulated in
only a few benzylidene–ruthenium complexes described in
the literature, and only for structures with the donor atom
occupying the sixth coordination site and, usually, weakly
bonded to the metal center.[19–21] To the best of our knowl-
edge, stable pentacoordinated ruthenium complexes in
which one of the ligands is a covalently bonded halogen
atom remain unknown. Starting from this point, we investi-
gated the reaction between the commercially available com-
plex 4 and 2-halostyrene derivatives under ligand-exchange
conditions. Surprisingly, we observed that in the reaction
with 2-iodopropenylbenzene, complex 5a was formed in
good yield (72%; Scheme 1).[22]
Among examples of substituted 1a-type complexes, some
general factors that affect the activity of the catalysts have
been described. Blechert and Wakamatsu demonstrated a
steric effect in which substituents at the ortho position to
the chelating OiPr group caused a structural distortion that
[a] Dr. M. Barbasiewicz, Dr. M. Michalak, Prof. K. Grela
Faculty of Chemistry, Warsaw University
Pasteura 1, 02-093 Warsaw (Poland)
Fax : (+48)22-822-5996
E-mail: klgrela@chem.uw.edu.pl
barbasiewicz@chem.uw.edu.pl
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202817.
Chem. Eur. J. 2012, 18, 14237 – 14241
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Scheme 1. Synthesis of complex 5a.
The iodo complex 5a was easily synthesized by simple
heating of the substrates in toluene at 808C for 30 min, cool-
ing to room temperature, and filtration. The procedure was
very convenient and appeared to be superior to chromato-
graphic purification, in which partial decomposition of the
product was observed. The isolated 5a was stable in air in
the solid state and in solution (CD2Cl2), and was character-
ized by using NMR and IR spectroscopy, MS, and elemental
and X-ray analysis (Figure 2).
Figure 3. Complexes 5a–e and the p-electronic effects applied for stabili-
zation. [a] See reference [22].
In the next step, we explored the synthesis of bromo de-
rivatives. Although complex 5c did not precipitate from a
ligand-exchange reaction between 4 and 2-bromopropenyl-
benzene in toluene, we confirmed its formation with
1H NMR spectroscopy.[22] To enhance the stability of the
bromine chelation, two different electronic effects were ap-
plied. An electron-donating dimethylamino group, intro-
duced at the para position of the benzylidene ring, stabilized
the complex 5e. In another structure, the p-electron delocal-
ization effect[17] predicted for the complex with a bromo-
naphthalene derivative resulted in the formation of 5d. In
both cases, the stable complexes 5d and 5e precipitated
from the reaction mixtures were isolated in good yields
(60% and 54%, respectively) and were characterized with
spectral and X-ray measurements (see the Supporting Infor-
mation for details).
Figure 2. X-ray structure of 5a. Thermal ellipsoids are shown at the 50%
À
À
probability level. Selected bond lengths: Ru(1) C(1) 2.012(4); Ru(1)
1
In the H NMR spectra of the prepared complexes, benz-
À
À
Cl(3) 2.3562(10); Ru(1) Cl(4) 2.3711(10); Ru(1)···I(1) 2.6458(6); Ru(1)
À
À
À
C(22) 1.836(3); C(22) C(23) 1.465(5); C(23) C(24) 1.394(5); C(24) I(1)
2.095(3) ꢁ.
AHCTUNGTREGyNNNU lidene-proton (Ru=CH) resonances for the naphthalene-
based catalysts 5b and 5d (d=18.26 and 17.97 ppm, respec-
tively) were deshielded in comparison with their parent
(benzene-based) structures 5a and 5e (d=18.06 and
17.75 ppm, respectively). However, the shift attributed to
the induced current of the chelate rings was rather small
(Dd=0.2 ppm) in comparison with differences in naphtha-
lene-based Hoveyda–Grubbs complexes (Dd=1.6 ppm).[17]
Next, the synthesized complexes were tested in a model
ring-closing metathesis (RCM) reaction of diethyl diallyl-
malonate (0.2m) with 5 (1 mol%) in non-degassed CD2Cl2
The structure of the complex 5a revealed a cis geometry
of the chloride ligands, which is usual for heavier coordinat-
ing atoms[13] and some nitrogen-based systems.[12] A slipped
parallel orientation of the benzylidene and mesityl groups of
the NHC backbone suggested a stabilizing stacking interac-
tion.[23] Striking bonding properties of the covalently-bonded
À
iodine atom were displayed in C I···Ru coordination
1
(2.6458(6) ꢁ), constituting an almost right angle with the
carbon atom of the ligand (89.46(10)8). Other parameters,
such as bond lengths and valence angles of the benzylidene
unit, NHC moiety, and chloride ligands, do not diverge from
related structures that have been described in the literature.
We synthesized the naphthalene-based derivative 5b
(87%), in which p-electron delocalization was expected to
strengthen the stability of the chelate ring (Figure 3), as
compared with 5a.[17] The complex displayed the same cis-
Cl2 geometry, with only minor structural changes in the
at 258C and analyzed with H NMR spectroscopy. The re-
sults are presented in Figure 4. Surprisingly, all halogen-co-
ordinated complexes appeared to be quite active in RCM
reactions, and were comparable with the commercial Hovey-
da (Hov II) and Grubbs second generation (Gr II) catalysts.
As depicted in Figure 4, the bromoderivatives 5d and 5e
displayed similar activity that was balanced between
(Hov II) and (Gr II) activity, whereas the iododerivatives 5a
and 5b were less active than Hov II and Gr II.[24] Although
metathesis technology demands both highly active and
latent catalysts, the latter should display excellent stability
when prolonged reaction times and increased temperatures
À
bond length Ru···I (2.6294(6) ꢁ) and valence angle C I···Ru
(90.07(16)8).
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Halogen-Chelated Hoveyda–Grubbs-Type Metathesis Catalysts
COMMUNICATION
We then attempted the protonation of complex 5e to
transform the electron-donating amino group into an ammo-
nium salt with acceptor properties. It is known that the
treatment of a NEt2-substituted Hoveyda–Grubbs catalyst
with a Brønsted acid improves its performance from latent
to active by weakening the iPrO···Ru coordination.[25] Ex-
pectedly, 5e·H+ as a salt of camphor-10-sulfonic acid ap-
peared to initiate more rapidly than 5e, but deactivated
within about one hour after 93% of substrate conversion in
a model RCM reaction of diethyl diallylmalonate (1 mol%;
CD2Cl2, 258C). The result suggests that the studied complex
5c is close to the borderline of stability[22] and that the pro-
tonated form of 5e is an example of an in situ over-boosted
catalyst suffering from a limited performance. Finally, we
applied the complexes 5a, 5b, and 5d for routine, model
metathesis reactions of both straightforward and more diffi-
cult substrates. In most cases, the new family of catalysts ex-
hibited excellent activity (Scheme 2).
Figure 4. Activity profiles of complexes 5a, 5b, 5d, 5e, 1a (Hov II), and
Gr II (1 mol%) in RCM reactions of diethyl diallylmalonate in non-de-
gassed CD2Cl2 at 258C. Gr II=(SiMes)ACHTNUTRGNEUGN(PCy3)Cl2Ru=CHPh; SiMes=1,3-
bis(2,4,6-trimethylphenyl)-2-imidazolin-2-ylidene, Cy=cyclohexyl.
are required to transform less reactive substrates. To deter-
mine the stability of the catalysts at higher temperatures,
the most latent complex 5b was tested in a model RCM re-
action at elevated temperatures under air. Surprisingly, at
even a slightly elevated temperature (408C), the system acti-
vated more rapidly, and at 558C, it proceeded rapidly with-
out signs of catalyst deactivation (Figure 5). The behavior
Scheme 2. Selected examples of olefin metathesis reactions catalysed by
complexes 5a–d. [a] See reference [16b].
Although only a limited number of observations are avail-
able, some conclusions about the new family of catalysts fea-
Figure 5. Activity profiles of complex 5b (1 mol%) in RCM reactions of
diethyl diallylmalonate in non-degassed CD2Cl2 at 25, 40, and 558C
(sealed tube).
À
turing unique Ru···X Ar coordination can be drawn. Halo-
carbons belong to a class of ligands of very low basicity[26]
and quantitative data of the relative ligating abilities of hal-
oarenes available in the literature is limited.[6c] However, the
donor strength of covalently bonded halogen atoms is ex-
pected to follow the order of increasing atomic numbers
(F<Cl<Br<I), with heavier elements being better s-
donors, as observed for 15 and 16 group coordinating atoms
displayed by 5b is a desirable feature[11b] and demonstrates
that the halogen-chelated complexes are resistant toward
oxidative addition, contrary to NHC–ruthenium complexes
derived from 2,6-dichloroaniline, which easily decompose
[20]
À
along an Ar Cl activation pathway.
Chem. Eur. J. 2012, 18, 14237 – 14241
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M. Barbasiewicz, K. Grela and M. Michalak
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ACHTUNGTRENNUNG
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À
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relationships requires further studies. We believe that these
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À
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was stirred at 808C for 30 min under argon[28] and a green precipitate was
formed. The reaction mixture was then cooled to RT and filtered through
a Schott filter, and the crude product was washed with toluene and dried
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CCDC-901488 (5a), CCDC-901489 (5b), CCDC-901490 (5d), and
CCDC-901491 (5e) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk_request/cif.
´
A. Makal, K. Wozniak, R. Kadyrov, K. Grela, Organometallics 2009,
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Acknowledgements
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[14] E. Tzur, A. Szadkowska, A. Ben-Asuly, A. Makal, I. Goldberg, K.
The project was financed by the Polish Ministry of Science and Higher
Education (grant no. N N204 152436). K.G. and M.M. thank the Seventh
Framework Program (grant no. CPFP 211468–2 EUMET) for support.
The authors thank the Umicore AG for a donation of the M31 complex.
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Keywords: catalysis · coordination modes · delocalization ·
halogens · metathesis · ruthenium
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[22] A characteristic 1H NMR resonance of the benzylidene proton of
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mopropenylbenzene (0.009 g), and CD2Cl2 (1 mL) were combined in
an NMR tube and kept at 408C for 30 min under air.
Received: August 6, 2012
Published online: October 2, 2012
Chem. Eur. J. 2012, 18, 14237 – 14241
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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