Is the Hoveyda-Grubbs complex a vinylogous fischer-type carbene? Aromaticity-controlled activity of ruthenium metathesis catalysts

Article abstract of DOI:10.1002/chem.200800704

Three naphthalene-based analogues (4a-c) of the Hoveyda-Grubbs metathesis catalyst exhibited immense differences in reactivity. Systematic structural and spectroscopic studies revealed that the ruthenafurane ring present in all 2-isopropoxyarylidene chelates possesses some aromatic character, which inhibits catalyst activity. This aromatic stabilization within the chelate ring may be controlled by variation of the polycyclic core topology as was demonstrated for tetraline and phenanthrene derivatives (4d,e). General conclusions about a new mode of ligand-structure tuning in catalytic systems are presented.

Full text of DOI:10.1002/chem.200800704

DOI: 10.1002/chem.200800704
Is the Hoveyda–Grubbs Complex a Vinylogous Fischer-Type Carbene?
Aromaticity-Controlled Activity of Ruthenium Metathesis Catalysts
Michał Barbasiewicz,*[a, c] Anna Szadkowska,[a] Anna Makal,[b] Katarzyna Jarzembska,[b]
[a]
Krzysztof Wozniak,[b] and KarolGreal *
´
Abstract: Three naphthalene-based an-
alogues (4a–c) of the Hoveyda–Grubbs
metathesis catalyst exhibited immense
differences in reactivity. Systematic
structural and spectroscopic studies re-
vealed that the ruthenafurane ring
present in all 2-isopropoxyarylidene
chelates possesses some aromatic char-
acter, which inhibits catalyst activity.
This aromatic stabilization within the
chelate ring may be controlled by var-
iation of the polycyclic core topology
as was demonstrated for tetraline and
phenanthrene derivatives (4d,e). Gen-
eral conclusions about a new mode of
ligand-structure tuning in catalytic sys-
tems are presented.
Keywords: aromaticity · catalysis ·
ligand design · metallacycles · meta-
thesis
Introduction
With the recognition of the major impact of olefin metathe-
sis in modern organic synthesis,[1] a surge of productive re-
search efforts in ruthenium-complex design has been wit-
nessed in recent years, which is driven by the challenge to
enhance the industrial potential of Grubbs catalysts (1 and 2
Scheme 1).[2] Since the initial observation by Hoveyda[3a] and
Blechert[3b] of improved catalytic properties of 2-alkoxyben-
[a] Dr. M. Barbasiewicz, A. Szadkowska, Prof. K. Grela
Institute of Organic Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, 01224 Warsaw (Poland)
Fax : (+48)22 632 66 81
Scheme 1. Selected metathesis catalysts.
Homepage: http://zinc.icho.edu.pl/
E-mail: michal.barbasiewicz@chemie.uni-erlangen.de
grela@icho.edu.pl
zylidene complex (3a), continuous efforts were spent
toward development of new catalytic systems balanced be-
tween stability and activity.[4] In this field we demonstrated
that reducing the electron-density on the chelating oxygen
atom by adding acceptor groups at the aromatic benzylidene
ring (e.g. NO2 in 3b)[4a,b] and in the alkyl ether chain[4c] fa-
cilitated the initiation step, thus improving the rate of the
metathesis reaction. The second strategy was based on the
out-of-plane distortion of the alkoxy substituent, which
´
[b] A. Makal, K. Jarzembska, Prof. K. Wozniak
Department of Chemistry
Warsaw University, Pasteura 1
02093 Warsaw (Poland)
[c] Dr. M. Barbasiewicz
Current address: Institut für Organische Chemie
Friedrich-Alexander Universität Erlangen-Nürnberg
Henkestraße 42, 91054 Erlangen (Germany)
Fax : (+49)9131 85 26865
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800704. It contains details of
complete ligand and catalyst syntheses, their characterization data
and spectral reproductions, numerical data of kinetic experiments and
details of X-ray structures (for compounds 3b, 4a, 4b, 4d, and 4e).
À
weakens Ru O coordination and is induced by steric con-
gestion with a phenyl group (3c)[4d,e] or by incorporation of
the chelating oxygen atom into the folded six-membered
cyclic structure 3d.[4f] On the other side the broad applica-
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FULL PAPER
tion profile of the metathesis reaction demands not only the
most efficient catalytic systems, for example, in polymer
chemistry slowly initiating so-called “latent catalysts” are
sought after. Accordingly substitution of an alkoxystyrene
ligand into strongly chelating nitrogen[5] and sulfur[6] conge-
ners led to complexes of diminished activity, which allow
longer handling of a monomer/catalyst mixture before the
polymerization process.
In spite of these elucidations most new useful complexes
were discovered by trial and error, as many opposing effects
superimpose in the final catalytic output. Thus further for-
mulation of the theoretical background to systematize re-
sults and promote further developments was required. In
this report we present a new structure-reactivity concept,
based on evaluation of the aromatic character of the chelate
ring, which may serve as a tool to probe subtle electronic ef-
fects in catalytic systems and shed light on the aromaticity
phenomena.
Results and Discussion
In one of our investigations of the structure-activity relation-
ships of metathesis catalysts we synthesized a simple deriva-
tive of the parent Hoveyda catalyst, the stable naphthalene-
based complex (4a, Scheme 2 and Figure 1a).
Figure 1. X-ray crystallographic structures[7] of complexes 4a (a) and 4b
(b) represented by thermal ellipsoids drawn at the 50% probability level.
Especially unusual was the magnitude of this ligand
effect. Simple modifications of the Hoveyda complex lead
usually to changes in the catalytic activity, but none of them
could so dramatically influence this system, especially
toward deprivation of catalytic activity (usually steric con-
gestions result in diminished stability and improved catalytic
output). For a deeper understanding of these differences,
structural crystallographic studies of the presented com-
plexes were carried out. Casual analysis of structures of the
synthesized compounds confirmed general structural similar-
ities with complex 3a in ligand orientation and the charac-
Scheme 2. New complexes 4a–c synthesized by a ligand exchange reac-
tion with 2.
Unexpectedly, the structural modification led to complete
loss of activity toward common substrates in the initial
screening of model RCM and CM reactions. This result
seemed to be surprising as the close structural similarity
with the parent Hoveyda complex 3a could not justify this
behavior. First we assumed that some specific detail of the
structure must inhibit the ligand dissociation step. The inter-
action of the flagpole hydrogen atom of the second aromatic
ring (position 8 at naphthalene core) with the carbene
moiety or the NHC structure were considered among
others.
To solve this riddle we synthesized the remaining isomers
of the naphthalene-based catalysts for a systematic compari-
son of the effect of the aromatic ring annelated on the
parent complex 3a. Surprisingly, we observed in a prelimina-
ry screening that only one complex (4b, Figure 1b) exhibit-
ed catalytic activity comparable to the parent Hoveyda cata-
lyst, whereas two others (4a and 4c) were completely inef-
fective toward a set of model RCM substrates.
À
teristic Ru O bond length varied to a minor extent, in the
range of 0.05 Š. This suggested that none of the known con-
cepts of the structure-activity relationship of the Grubbs–
Hoveyda type complexes could unambiguously explain the
experimental results. Consideration of simple electron-densi-
ty effects acting on the stability of the chelate predicts simi-
lar properties for catalysts 4a and 4b because both of them
are based on a 2-naphthol core. On the other hand, the
proximity of the flagpole hydrogen atom of the second-clos-
est aromatic ring to the Ru=CH group and the mesitylene
ring of the NHC ligand exerts an effect present in complex
4a only.
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M. Barbasiewicz, K. Grela et al.
Benzylidene ruthenium complexes belong to the class of
the carbene complexes of moderate electron density. Reac-
tion with ethyl vinyl ether may transform them into elec-
tron-rich Fischer-type carbene complexes of substantially di-
minished activity (the property used sometimes for quench-
ing metathesis reactions).[8] In fact the parent Hoveyda–
Grubbs catalyst, on the basis of the so-called vinylology con-
cept,[9] can be classified as a Fischer-type complex, in which
the isopropoxy group functions as a mesomeric donor for a
through-bond p-conjugation with the carbene moiety. One
of the arguments toward this interpretation is the planar
configuration of the chelating oxygen atom, which ensures
sp2 hybridization and conjugation with the aromatic ring
(Scheme 3, left).
not be accidental and pointed our attention towards subtle
effects arising from the topology of electron distribution at
the naphthalene core.
The electronic properties of polyaromatic hydrocarbons
(PAHs) are the matter of continuous theoretical and syn-
thetic studies.[14] In this area a simple graph-topology driven
concept (the Clar rule)[15,16c] allows a qualitative evaluation
of their stability. The more aromatic (Clar) sextets that can
be written for an investigated structure, the greater the pre-
dicted stability. On the basis of this theory, angular PAHs
(phenanthrene, chrysene, picene, etc.) differ from linear
ones (anthracene, naphthacene, pentacene, etc.) in a way
that is related to the number of aromatic sextets per total
number of rings.[15,17] Not only simple aromatic benzene
rings can be regarded as building blocks for these systems.
The resonance-assisted hydrogen-bonding concept, intro-
duced by Gilli,[18] extended this methodology to other cyclic
structures, such as, benzoannelated b-ketoenolates, in which
hydrogen bonding between hydroxy and carbonyl groups
constitutes a novel pseudoaromatic ring within the polycyc-
lic structure. Recently, such a conjugation effect for systems
of various topologies was investigated theoretically.[16] Ac-
cordingly, both hydrogen and lithium bonds in chelate rings
fill the criteria for resonance stabilization, the latter even
more efficiently, probably due to better orbital overlap.[16c]
These studies also confirmed the substantial differences be-
tween linear and bent polyarenes and qualitatively support-
ed our presumptions.
In fact, the conjugation effect in the investigated ruthenium
chelates may be generalized as a cyclic ring current with
some aromatic character. Namely, the electronic properties of
naphthalene-based catalysts 4a–c may resemble structures of
higher (tricyclic) all-carbon analogues, phenanthrene and an-
thracene (considering extension of the structure by a chelate
ring) and inherit the differences of their aromatic properties.
In this respect the concept of metalloaromaticity[19] intro-
duced by Calvin and Wilson in 1945[20] is richly evoked to
explain the properties of organometallic compounds in
terms of their stability, reactivity and structure. Numerous
studies were conducted in this field for example, synthesis of
classes of iridacycles,[19b,c] ferrabenzenes,[21] ruthenaben-
zenes,[22] molybdafuranes,[23] as well as simple acac[24] and dii-
mine complexes.[19d] At this moment we realized that aro-
matic stabilization of the chelate may be responsible for the
unexpected lack of reactivity of complexes 4a and 4c, which
operate at the initiation step (stabilizes the chelate for the
dissociation[20,25] or retards the reaction with the olefin in an
associative[26] mechanism) and may justify the pronounced
“on/off” effect in the catalytic activity as amplifying small
energetic differences by the kinetic pattern of the precata-
lyst activation process.
Scheme 3. Electronic character of the ruthenium complex 3a.[10]
The structure of naphthalene exhibits substantial bond al-
ternation,[11] which is generally attributed to differences in
bond orders and manifestly lower aromatic stabilization
(calculated per one ring) as compared with the ideal hexag-
onal aromatic prototype, benzene. Partial double-bond char-
À
À
acter of C1 C2 and single-bond character of C2 C3 (num-
bered for the naphthalene core) led us to assume that only
À
structures built on the C1 C2 side of naphthalene (4a, 4c)
are able to establish efficient conjugation within the chelate
ring. On the other side, 4b resembles the parent Hoveyda
complex 3a, in which the aromatic stabilization (ring cur-
rent) of the substituted benzene reduces this conjugation.[12]
We supposed that except for some minor random effects dis-
played in these structures, the main structural difference be-
tween naphthalene isomers is the propensity to conduct
electronic p density, which affects the conjugation between
À
OR and Ru=CH groups. This conjugation of the substitu-
ents increases the electron density at the ruthenium center
and reduces the rate of the reaction with olefinic substrates.
In subsequent studies we compared the spectral data (1H
and 13C NMR spectra) of compounds 4a–c as a probe of
their electronic properties. Taking the NMR data of 3a as a
point of reference we observed that both inactive catalysts
(4a, 4c) exhibited a substantial deshielding effect (ca.
+1.6 ppm) at the alkylidene hydrogen atom and a slight
shielding effect for the alkylidene carbon nucleus (ca.
À6.5 ppm). In fact p-donor heteroatoms of Fischer-type car-
benes cause a shielding effect on the alkylidene proton[13]
compared with unsubstituted complexes and cannot justify
the opposite trend observed for complexes 4a and 4c. The
magnitude of the differences in the NMR chemical shifts for
the proton located at the conjugated chelate ring seemed to
To prove this concept we evaluated the general criteria of
aromaticity present in the literature:[27,28] structural (bond
equalization) and magnetic (diamagnetic ring current inves-
tigated by NMR), and compared them with the energetic
one (based on resonance stabilization), which was attributed
to the activity pattern. The results are presented in Table 1.
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Aromaticity-Controlled Activity of Metathesis Catalysts
FULL PAPER
Table 1. Selected parameters of complexes 3a,b and 4a–e applied for
evaluation of the aromatic character of a chelate ring.
Complex
Activity
d(1H)
[ppm][b]
HOMA[c]
Clar prediction[d]
at 08C[a]
ACHTREUNG
3a
active
16.56
18.15
16.75
18.19
16.39
18.36
16.47
0.983
0.877
nonconjugated
conjugated
nonconjugated
conjugated
nonconjugated
partially conjugated
nonconjugated
4a[e]
4b
inactive[f]
active
0.952
4c[e]
4d
inactive[f]
active
n. d.[g]
0.985
4e[e]
3b
inactive[h]
active
0.910
0.968/0.983[i]
[a] Precise data are presented in Figure 3. [b] Signal of benzylidene
proton measured in CDCl3. [c] Calculated for the homocarbon aromatic
ring adjacent to the chelate according to the ref. [29]. [d] The character
of the chelate is based on analogy with polyaromatic hydrocarbons (see
the text below). [e] System with strongly conjugated chelate. [f] Active at
1108C. [g] We failed to obtain good quality crystals suitable for X-ray
measurements. [h] Active at 808C. [i] Two molecules present in the crys-
tal structure.
In spectroscopic studies, the strong deshielding effect
present in complexes 4a and 4c was perfectly consistent
with the diamagnetic ring current observed in all aromatic
systems, giving reliable proof for the formulated concept.
Next, the harmonic oscillator model of aromaticity
(HOMA)[29] was applied for the analysis of X-ray structures.
This evaluation technique is based on parametrization of the
degree of bond-length alternation giving numerical values
close to 1 (for highly aromatic symmetrical structures with
Scheme 4. Structural analogies[32] of complexes 4a–c. Metallacycles (left)
mimic electron distribution of polycyclic hydrocarbons—phenanthrene
and anthracene (right). According to the Clar rule substantial aromatic
character of the chelate is predicted for complexes 4a and 4c. For 4b
three possible structures contribute and the symmetrical one is slightly
favored.[15, 17a]
À
equal C C distances) and 0 (for hypothetical cyclic polyenes
with alternating single and double bonds). Our analysis was
limited to evaluation of local aromaticity of adjacent all-
carbon rings as no parametrization was accessible for Ru=
[30]
À
CH and O Ru bonds. The obtained results were consis-
tent with expectations based on simple analysis of aromatic
ring distribution based on Clar rule considerations[15,17a] with
chelate ring-expanding polycyclic character of the ligand
core (middle ring of reduced aromaticity for structure 4a
and highly aromatic for complex 4b, see Scheme 4). Howev-
er, these changes were minor, probably due to the generally
low magnitude of aromatic stabilization of a ruthenacycle,
as compared to other aromatic systems.[31]
For the purpose of a decisive proof of a topology-based
reactivity profile of complexes 4a–c we decided to apply the
very subtle effects that govern the electronic structure of
polyaromatic systems in order to restore the catalytic activi-
ty of inactive complex 4a. A partially saturated analogue of
4a, based on the tetraline core (4d), and phenanthrene de-
rivative 4e were synthesized. We expected that both modifi-
cations would reduce the conjugation of the metallocycle
and restore the catalytic activity of the complexes
(Scheme 5).
Scheme 5. Structural changes were applied to restore catalytic activity
similar to that of complex 4a. A saturated ring was introduced to give
tetraline derivative 4d (top) and the extended polyaromatic system of
phenanthrene derivative 4e was employed (bottom, see text for details).
modification far from the coordination sphere of the ruthe-
nium ion (Scheme 5 and Figure 2), as presumably meaning-
less in the sense of current mechanistic concepts of reactivi-
ty.
By analogy between the investigated naphthalene-based
catalysts with the corresponding polyaromatics of similar
topological structure (anthracene and phenanthrene,
Scheme 4) the ligand of 4a was expanded by the next aro-
matic ring, leading to the metalla-analogue of chrysene 4e.
Once again, simple considerations suggested that this modi-
fication should, at least partially, restore the catalytic activi-
ty[16c] due to possible inversion of the aromatic sextet distri-
Saturation of the distal aromatic ring gave complex 4d,
which was designed to have similar electronic properties to
the parent Hoveyda complex 3a, with a single aromatic
ring. At the same time the change of geometry was only
minor, leaving possible steric hindrance between the alkyli-
dene group and the mesitylene ring of the NHC ligand rela-
tively intact. In the second investigated structure we chose a
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M. Barbasiewicz, K. Grela et al.
~
&
^
*
Figure 3. Activity profiles of complexes 3b ( ), 4b ( ), 3a ( ), 4d ( ) in
Figure 2. X-ray crystallographic structure[7] of complexes 4d (a) and 4e
(b) represented by thermal ellipsoids drawn at the 50% probability level.
The isopropoxy group in 4e exhibits disorder in the crystal lattice.
&
^
RCM reactions of N,N-diallyltosylamine at 08C (top) and 3a ( ), 4e ( ),
~
and 4a and 4c ( ) at 808C (bottom).
bution within the polyaromatic core according to the Clar
rule.[15]
their application as latent systems and to evaluate the
strength of the ligand effect, which controls precatalyst acti-
vation and release of active catalytic species. In refluxed tol-
uene (1108C) both complexes showed similar activity, lead-
ing to the RCM product of diethyl diallylmalonate in ap-
proximately 45% conversion (ca. 60% for 2.5 mol% of cat-
alyst, Figure 4). After the fast initiation step, reaction prog-
ress was stopped within 4–6 h, which can be attributed to
catalyst decomposition at elevated temperature.
The results of systematic activity studies performed on a
model RCM reaction of N,N-diallyltosylamine with 1 mol%
of the catalyst are presented in Figure 3.
Initial screening at 08C (Figure 3, top) demonstrated simi-
lar catalytic performance for complexes 3a and 4b as inter-
mediate between highly-active nitro catalyst 3b and tetra-
line derivative 4d of slightly diminished output. Under these
conditions complexes 4a, 4c, and 4e were completely inert
and the product of an RCM reaction was not detected in re-
action mixtures. For more precise differentiation, analogous
reactions were performed at 808C in dichloroethane
(Figure 3, bottom). Under these conditions the activity of
complexes 4a and 4c was still negligible (<5% conversion),
whereas the “restored activity” complex 4e reacted much
more efficiently, consistent with our expectations.
~
Figure 4. Activity profiles of complexes 4a ( ) and 4c (&) at 2.5 mol%
Finally, we set out to initiate inert complexes 4a and 4c
under more vigorous conditions to demonstrate the scope of
&
^
catalyst loading, and 4a ( ) and 4c ( ) at 1 mol% catalyst loading in
RCM reactions of diethyl diallylmalonate at 1108C.
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Aromaticity-Controlled Activity of Metathesis Catalysts
FULL PAPER
solid was filtered off and the filtrate was concentrated under vacuum.
The product was purified by column chromatography. Elution with cyclo-
hexane, then cyclohexane/ethyl acetate (1:1) removed 4a as a light green
band. The solvent was evaporated and the product was dissolved in a
small amount of CH2Cl2, then methanol was added until green crystals
precipitated. The precipitate was filtered off, washed with methanol and
dried in vacuo to afford complex 4a (0.030 g, 22%) as a light green solid.
According to the presented concept and the expected in-
ductive[4a–c] and steric[4d–f] modes of catalyst activation, a
third novel mode, aromatic conjugation of a chelate, can be
varied in a continuous way, as the aromatic stabilization
may differ for aromatic systems of diverse topologies.
Kinetic studies of complexes 4a–e
At 08C: Two sets of RCM reactions were used as a test to compare the
activity of the catalysts: cyclization of N,N-diallyltosylamine and N,N-
di(3-buten-1-yl)tosylamine in CH2Cl2 at 08C. Typically 1 mol% of the
catalyst (0.004 mmol) was added to a solution of substrate (0.350 mmol)
and an internal standard in CH2Cl2 (17.5 mL) at 08C. The reaction was
run at 08C under argon and samples were taken after 5, 10, 20, 30, 45,
and 60 min and 2, 4, and 6 h, and were analyzed by GC.
Conclusion
The presented results demonstrate that simple qualitative
analysis of conjugated cyclic structures with Clarꢁs rule may
help explain and serve as a tool for fine-tuning the ligand ef-
fects in the design of novel catalytic systems. As many con-
jugated ligands, such as, ketoenolates,[22] Schiff bases,[10,33]
salens,[34] and diiminopyridines[35] have numerous practical
applications, the precise mechanism of their action is still a
matter of discussion. In the presented model, bond order al-
ternation in the naphthalene moiety enables the manipula-
tion of conjugation within the chelate ring,[20,25] whereas
other structural parameters remain almost intact. In the
design of Hoveyda–Grubbs type metathesis catalysts this
idea reveals that the isopropoxy group functions not only as
a simple chelating arm, but is an inherent part of the conju-
gated ring influenced by the p-electrons of the ligand. De-
pending on the topology of the ligand core this effect may
be controlled in a broad range from minor to substantial, in
which the aromatic character of the chelate inhibits the ini-
tiation step and decreases the catalytic activity of the car-
bene complex. Application of metathesis technology justifies
development of systems of any type, the pharmaceutical and
fine-chemical industries ideally require highly active cata-
lysts in ppm amounts and at low temperatures,[1e] whereas
dormant catalysts with retarded initiation are useful in
metathesis polymerization processes.[5] As both types of
metathesis catalysts find important practical applications,
the concept formulated herein can be of practical impor-
tance since it delivers a better understanding of the mode of
action of the complexes[36] and can lead to new ruthenium
chelates of tailored activity.
At 808C: Two sets of RCM reactions were used as a test to compare the
activity of catalysts: cyclization of N,N-diallyltosylamine in C2H4Cl2 and
in toluene at 808C. Typically 1 mol% of the catalyst (0.004 mmol) was
added to a solution of substrate (0.350 mmol) and an internal standard in
17.5 mL of solvent at 808C. The reaction was run at 808C under argon
and samples were taken after 1, 2, 3, 4, 5, and 6 h, and were analyzed by
GC.
At 1108C: Typically 1 mol% of the catalyst (0.004 mmol, 2.4 mg) was
added to the solution of substrate (diethyl diallylmalonate) (0.350 mmol)
and an internal standard in toluene (17.5 mL) at room temperature. The
reaction was run at 1108C under an argon atmosphere for 6 h and sam-
ples were taken for GC after: 15 and 30 min and 1, 2, 3, 4, 5, and 6 h.
The same procedure was applied for 2.5 mol% of the catalysts
(0.180 mmol substrate scale).
Acknowledgements
´
The authors thank Professor J. Lewinski (Faculty of Chemistry, Warsaw
University of Technology) for his commitment to science and the educa-
tion of young co-workers, which was always stimulating and grounded a
fundamental knowledge helpful in formulation of this concept. We thank
Dr S. Harutyunyan for some early experiments (preparation of 4a) made
during her sabbatical stay in 2002. X-ray single crystal measurements
were accomplished at the Structural Research Lab. of the Chemistry De-
partment, Warsaw University, Poland. SRL has been established with fi-
nancial support from the European Regional Development Fund in the
Sectoral Operational Programme “Improvement of the Competitiveness
of Enterprises, years 2004–2006” project no: WKP 1/1.4.3/1/2004/72/72/
165/2005/U. Support from the Foundation for Polish Science for K. G.
(professorship Mistrz), A. M. (program START) and program NOVUM
is greatly acknowledged. Part of this work was supported by grant N
N204 0302 33 from the Ministry of Science and Higher Education
(K. W.).
ExperimentalSection
Generalmethods : For a detailed description of the experimental meth-
ods and instruments used, see Supporting Information. CCDC-698596
(3b), CCDC-698342 (4a), CCDC-698343 (4b), CCDC-698344 (4d), and
CCDC-698345 (4e) 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/data_re-
quest/cif.
[1] For selected reviews on olefin metathesis, see: a) T. M. Trnka, R. H.
Grubbs, Acc. Chem. Res. 2001, 34, 18–29; b) R. H. Grubbs, Hand-
book of Metathesis, Wiley, Weinheim, (Germany), 2003; c) S . J.
Connon, S. Blechert, Angew. Chem. 2003, 115, 1944–1968; Angew.
Chem. Int. Ed. 2003, 42, 1900–1923; d) D. Astruc, New J. Chem.
2005, 29, 42–56; e) H. Clavier, K. Grela, A. Kirschning, M. Mauduit,
S. P. Nolan, Angew. Chem. 2007, 119, 6906–6922; Angew. Chem. Int.
Ed. 2007, 46, 6786–6801.
[2] For an industrial perspective, see: A. M. Thayer, Chem. Eng. News
2007, 85, 37–47.
[3] a) S. B. Garber, J. S. Kingsbury, B. L. Gray, A. H. Hoveyda, J. Am.
Chem. Soc. 2000, 122, 8168–8179; b) S. Gessler, S. Randl, S. Ble-
chert, Tetrahedron Lett. 2000, 41, 9973–9976.
Representative procedure for the synthesis of catalysts 4a–e: (Unopti-
mized for specific cases with polycyclic ligands). 2-Isopropoxy-1-vinyl-
naphthalene (0.051 g; 0.24 mmol), CuCl (0.024 g; 0.24 mmol), and
CH2Cl2 (15 mL) were placed in a Schlenk flask. Afterwards, Grubbs
second-generation carbene complex (0.170 g; 0.20 mmol) was added and
the resulting solution was stirred under argon at 408C for 30 min. From
this point on, all manipulations were carried out in air with reagent-grade
solvents. The reaction mixture was concentrated under vacuum and the
resulting material was dissolved in ethyl acetate (ca. 10 mL), a white
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114, 4210–4212; Angew. Chem. Int. Ed. 2002, 41, 4038–4040; b) A.
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9336
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Chem. Eur. J. 2008, 14, 9330 – 9337

Aromaticity-Controlled Activity of Metathesis Catalysts
FULL PAPER
À
ical for reaction with ethyl vinyl ether (NHC)(Cl)2
OMe.
A
fective conjugation within the chelate ring, compare: a) T. M. Kry-
´
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the contribution of a quinonoid-type structure, which precluded ef-
Received: April 4, 2008
Published online: September 3, 2008
Chem. Eur. J. 2008, 14, 9330 – 9337
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9337