Intriguing substituent effect in modified Hoveyda-Grubbs metathesis catalysts incorporating a chelating iodo-benzylidene ligand

Article abstract of DOI:10.1039/c2dt32422a

A series of modified Hoveyda-Grubbs catalysts incorporating a chelating iodo-benzylidene ligand were prepared and characterized. The presence of electron-withdrawing ring substituents in the para position to the iodide was found to decrease the catalytic activity, revealing that dissociation of the Ru...I-Ar bond is not the rate-determining step.

Full text of DOI:10.1039/c2dt32422a

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Intriguing substituent effect in modified
Hoveyda–Grubbs metathesis catalysts incorporating
a chelating iodo-benzylidene ligand†
Cite this: Dalton Trans., 2013, 42, 355
Received 11th October 2012,
Accepted 5th November 2012
DOI: 10.1039/c2dt32422a
Michał Barbasiewicz,* Krzysztof Błocki, Maura Malińska and Robert Pawłowski
www.rsc.org/dalton
A series of modified Hoveyda–Grubbs catalysts incorporating a In the current literature the relationship is well-documented,
chelating iodo-benzylidene ligand were prepared and charac- electron acceptors present in the benzylidene ring (e.g. 1b)7
terized. The presence of electron-withdrawing ring substituents in usually improve the catalytic performance, while donors
the para position to the iodide was found to decrease the catalytic display the opposite effect.8,9 Recently we reported a new class
activity, revealing that dissociation of the Ru⋯I–Ar bond is not the of halogen-chelated Hoveyda–Grubbs type catalysts.10 The
rate-determining step.
complexes displayed good application profiles and excellent
stability and were observed solely as cis-Cl2 isomers. Despite
the complexes geometry, usual for latent systems coordinated
with heavier-atom donors (e.g. 2),11 they displayed high per-
formance in model metathesis reactions, comparable to that
of parent catalyst 1a.
Over the last two decades olefin metathesis reactions have
experienced a tremendous growth of interest as a tool in
organic synthesis, and have been the subject of thorough
mechanistic studies.1 Following a wealth of experimental data
reported to date, several concepts regarding structure–activity
correlations have been formulated. For the Hoveyda–Grubbs
catalyst with the chelating iPrO group (1a) it is known that the
rate-determining step of the (pre)catalyst initiation consists of
the formation of 14-electron species and coordination of the
olefinic substrate entering the catalytic cycle.2 Recently three
alternative mechanisms of the process were analysed by DFT
calculations.3,4 Considering the dissociative, interchange and
associative character of the process, several authors have ident-
ified the initiation step of the Hoveyda complex (1a) as inter-
change, with Ru⋯O interaction being partially broken and the
incoming olefin weakly associated with the ruthenium center
in the transition state.3 In turn experimental studies by Plenio
revealed that actually both former mechanisms play an impor-
tant role. For simple olefinic substrates the interchange mech-
anism operates, while for more congested ones, and when
subjected to catalysts with the sterically demanding iPrO
group, the dissociative mechanism predominates.5 Since in
both of the processes the rate-determining step consists of dis-
sociation of the chelate ring, electronic factors influencing its
stability (e.g. related to π-electron delocalization,6 effect of
substituents,7–9 etc.) control the rate of the initiation process.
With the aim of further improvement of the catalytic activity
we tested substitution of the benzylidene ring of complex 3c as
a potential mode of activation. In our report we present
studies of substituted iodine-chelated metathesis catalysts
(3a–e).12
First we synthesized four ruthenium complexes bearing
donor (NMe2, 3a; OMe, 3b) and acceptor (Br, 3d; NO2, 3e) sub-
stituents using the procedure described in our previous
report.10 The obtained catalysts were characterized, and tested
in the benchmark RCM reaction of diethyl diallylmalonate in
1
CD2Cl2 with 1 mol% of catalyst at 25 °C followed by H NMR
(Fig. 1).13
Unexpectedly, the iodocomplexes 3a–e displayed an
unusual structure–activity relationship, as compared with the
effect in Hoveyda-type complexes (1a–b).7–9 Bromo (3d) and
nitro (3e) substituted catalysts were actually less active than 3c,
whereas the presence of donating methoxy (3b) and dimethyl-
amino (3a) substituents also deactivated the catalytic system,
Faculty of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland.
E-mail: barbasiewicz@chem.uw.edu.pl; http://www.aromaticity.pl;
Fax: +48 22 822 5996; Tel: +48 22 822 0211
†Electronic supplementary information (ESI) available. CCDC 902057. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt32422a
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Fig. 3 Ligand exchange experiments with complexes 3c and 3e. (a) 3 (1
equiv.) and 4 (equiv.) were dissolved in CD2Cl2, and kept at r.t. under argon
(33 h).
Fig. 1 Reaction profiles of the ring-closing metathesis determined by 1H NMR.
Activity data of complex 3c were taken from the literature.10
To examine the idea nitrosubstituted complex 3e was charac-
terized by X-ray crystallography (Fig. 2).
The structure of complex 3e displayed a geometry similar to
that reported10 for unsubstituted complex 3c. In both these
structures aromatic rings were slipped and only the benzyli-
dene carbon atom (C23) might efficiently overlap with the ipso
carbon atom of the mesityl ring, suggesting attractive π–π*
interaction. In fact, the distance between the carbon atoms in
3e (3.08 Å, Fig. 2) was reduced, as compared with the same in
3c (3.13 Å).10 The closer arrangement of the ligands in 3e was
in accord with polarization of the RuvCH moiety induced by
the electron-withdrawing nitro group, strengthening inter-
action of the fragment with the electron-rich mesityl ring. To
support the idea we performed ligand exchange experiments
to determine relative affinity of chelating ligands (4) to the
ruthenium metal centre (Fig. 3).
Fig. 2 ORTEP18 drawing of the complex 3e molecule represented by thermal
ellipsoids drawn at the 50% probability level. The solvent molecule (CH2Cl2) was
omitted for clarity.
In two experiments, when complexes 3c and 3e were mixed
with equimolar amounts of the styrene ligand (4e and 4c,
respectively) thermodynamic equilibrium was reached with
considerable preference for the formation of unsubstituted
complex 3c (81 : 19).7c Results of the experiments revealed that
but the effect was rather small. A similar order of activity was the presence of a nitro group on the benzylidene ring in the
observed also for more demanding diethyl allylmethallylmalo- iodine-chelated complexes weakens coordination to the ruthe-
nate (see ESI† for details). To explain the surprising results we nium center and destabilizes the chelate. Thus, the observed
considered two hypotheses: (1) electron acceptors display a sta- substituent effect more probably derives from different
bilizing effect on the cis-Cl2 ruthenium chelates, in contrast to initiation mechanisms of the (pre)catalysts, rather than any
properties of ether-chelated trans-Cl2 Hoveyda complexes (1),7,8 type of stabilizing interaction. Considering the situation in
or alternatively, (2) the observed activity is a result of the which a less stable chelate initiates more slowly than a more
altered initiation mechanism, in which the ruthenium– stable one, we assumed that key Ru⋯I–Ar interaction is prob-
halogen bond is not actually broken in the rate-determining ably not broken in the rate-determining step of the initiation
step. The first variant is likely provided that primary stabiliz- process. Alternative mechanisms, being in accord with the
ation of the ruthenium chelates does not arise from simple observed activities, plausibly involve (2a) rate-limiting isomeri-
Ru⋯I–Ar interaction, for which electron withdrawing groups zation of the complex into the more active trans-Cl2 isomer
present in the position para to the halogen atom reliably (preceded or followed by low-energy barrier Ru⋯I dissociation)
display a weakening effect, and destabilize the chelate. We or (2b) rate-limiting olefin association with the formation of
assumed that the cis-Cl2 geometry displayed by complexes 310 the hexacoordinated structure (associative mechanism, Fig. 4).
may be key to solving the problem. Parallel orientation of the Since activity of cis-Cl2 complexes in olefin metathesis may be
aromatic arm of the NHC ligand and the benzylidene ring due to the presence of small equilibrium amounts of the trans-
suggests possible π-stacking interaction,14 or alternatively π–π* Cl2 species, the investigated iodocomplexes 3 can initiate via
interaction between the electron-rich ipso carbon atom of the rate-limiting ligand isomerization (2a). Interconversion
mesityl ring and the benzylidene RuvCH bond.15–17 between cis and trans isomers of related ruthenium species
356 | Dalton Trans., 2013, 42, 355–358
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series, although the nitrosubstituted ligand (4e) was demon-
strated to bond weaker to the ruthenium center in ligand
exchange experiments. On the basis of the preliminary results
we hypothesize that in the class of iodochelated (pre)catalysts,
the rate-determining step does not involve Ru⋯I–Ar bond
breaking, implying that kinetics and thermodynamics follow
reverse trends.
Acknowledgements
The work was financed by the Polish Ministry of Science and
Higher Education (Grant No. N N204 152436). The authors
thank the Umicore AG for donation of the M31 complex. M. B.
thanks Prof. Karol Grela for support and the opportunity to
perform an independent research program in the field.
Notes and references
Fig. 4 Proposed mechanisms of (pre)catalysts initiation: (2a) rate-limiting iso-
merization, (2b) associative mechanism.
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isomerization, which does not involve opening of the chelate
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ility of the isomers and on the energy barriers connecting
them showed a lower preference for the trans-Cl2 isomers for
either the methoxy or the nitro group.19 Another feasible expla-
nation for the observed behaviour of 3 consists of initiation by
the associative mechanism, with rate-limiting olefin coordi-
nation giving the hexacoordinated ruthenium complex (2b).20
Although the mechanism was demonstrated to be less pre-
ferred in the case of the Hoveyda catalyst,3,4 for the cis-Cl2
structure of complexes 3 it seems to be more plausible. With
only little initial reorganization of one chloride ligand (Cl1,
Fig. 2) the geometry may lead to two different vacant sites for
olefin coordination. Approach of the substrate trans to the
NHC ligand may form two conformers with the bottom-
bonded olefin, oriented cis to the benzylidene RuvCH bond
(3-a1 and 3-a2). The orientation is essential for further trans-
formation into the metallacycle,21 and is characterized by
small steric hindrance to olefin approach. In turn association
of the substrate in the position trans to the RuvCH group (3-
a3,4, side-bonded) is less feasible,22 and requires substantial
system reorganization to enter the metathesis reaction.4
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In contrast to properties of the Hoveyda–Grubbs metathesis
catalyst, for which introduction of the nitro group is a well-
established mode of activation,7 benzylidene ring substitution
in iodine-chelated ruthenium complexes (3a–e) displays a
different effect. Surprisingly, the NO2-substituted complex 3e
appeared to be the least active catalyst in the investigated
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10 M. Barbasiewicz, M. Michalak and K. Grela, Chem.–Eur. J., 17 R. Credendino, L. Falivene and L. Cavallo, J. Am. Chem.
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20 Alternatively, the results can be explained with the inter-
change mechanism of initiation with associative character
(Ia), as postulated for Hoveyda (1a) and nitro-Hoveyda (1b)
complexes.2 However, in contrast to the associative mech-
anism (2b, Fig. 4), the process assumes partial dissociation
of the Ru⋯X–Ar bond in the transition state, which pro-
ceeds more easily for acceptor substituted complexes. Also
activity data of ether-chelated complexes suggest7 that in
this mode of initiation the presence of a nitro group (1b vs.
1a) improves the catalytic activity, in variance with results
obtained for iodochelates 3.
12 We synthesized also some bromochelated complexes, but
the (pre)catalysts displayed very similar activity within the
investigated series. See ESI† for details.
13 For a procedure of NMR activity measurements, see:
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tallics, 2012, 31, 3636.
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H. Plenio, Chem.–Eur. J., 2008, 14, 5465.
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22 Moreover, conformer 3-a4 may be destabilized by steric
repulsion between the substrate and the aromatic ring of
the NHC ligand.
358 | Dalton Trans., 2013, 42, 355–358
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