Unsymmetrical Ru-NHC catalysts: A key for the selective tandem Ring Opening-Ring Closing alkene Metathesis (RO-RCM) of cyclooctene

Article abstract of DOI:10.1039/c1dt11643f

Dissymmetry for selectivity: NHC ligand with two different pendant group allows the selective formation of cyclic oligomers in place of polymers opening new strategy to generate macrocycles. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11643f

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Unsymmetrical Ru-NHC catalysts: a key for the selective tandem Ring
Opening–Ring Closing alkene Metathesis (RO–RCM) of cyclooctene†
Santosh Kavitake,a Manoja K. Samantaray,a Richard Dehn,b Stephan Deuerlein,b Michael Limbach,b,c
Jo¨rg A. Schachner,b Erwan Jeanneau,d Christophe Cope´ret*a,e and Chloe´ Thieuleux*a
Received 31st August 2011, Accepted 21st September 2011
DOI: 10.1039/c1dt11643f
Dissymmetry for selectivity: NHC ligand with two differ-
ent pendant group allows the selective formation of cyclic
oligomers in place of polymers opening new strategy to
generate macrocycles.
Scheme 1 Tandem RO–RCM of cis-cyclooctene.
While Ring Opening Metathesis Polymerisation (ROMP) is typ-
ically thermodynamically highly favored in most cases and can
thus be easily performed with a wide range of substrate and
catalysts including the most simple systems based on WCl6/R3Al,
RuCl3/BuOH and Re2O7/Al2O3, combining Ring Opening (RO)
and Ring Closing Metathesis (RCM) to generate selectively cyclic
oligomers as intermediates for fine-chemical applications remain a
challenge.1 Unlike RCM2 and ROMP,3 the tandem Ring Opening-
Ring Closing Metathesis reaction (RO–RCM)4 is often limited
by thermodynamic constrains, requiring very low concentration
and very specific temperature to allow conversion or to avoid
polymer formation.5 Additionally, in spite of these optimal
conditions, running the reaction at high conversions often leads
to high oligomers formation and low selectivity in cyclic dimers
(Scheme 1).
Recently, the groups of Chen6 and Buchmeiser7 have developed
unsymmetrical catalysts for sequence-selective copolymerization
of norbornene and cyclooctene by Ring Opening Metathesis
Polymerization (ROMP). This success could be attributed to the
design of dual configuration catalysts, where norbonene, the most
reactive but bulkiest substrate, selectively reacts with the active site
with the more opened configuration (substrate control), whereas
the active site with most crowded configuration selectively reacts
with the less reactive but less hindered substrate (catalyst control).
While catalyst architecture with dual-configuration allows dis-
crimination between two substrates in the same catalytic reaction
(ROMP), can it favour one reaction over another with the same
substrate, for instance ROMP vs. Ring Closing Metathesis (RCM)
thus allowing a selective tandem Ring Opening–Ring Closing
Metathesis (RO–RCM)?
Here, we show the dramatic influence of the nature of the
NHC ligands on the selective formation of cyclic oligomers in
the RO–RCM of cyclooctene8 using unsymmetrical Ru-NHC9
homogeneous catalysts based on NHC unit possessing one mesityl
substituent and one alkyl substituent (propyl or benzyl) in place
of two mesityl ligands as found in the most common Grubbs-
II like catalysts (G-II10 and N-II,11 Scheme 2a). RO–RCM of
cyclooctene was investigated with unsymmetrical catalysts (RuPr
and RuBn, Scheme 2b) and their performances were compared
to those of the classical symmetric catalysts with saturated or
unsaturated NHC ligand (G-II and N-II). The catalytic tests were
carried out using the following standard conditions: 0.01%mol of
Ru (cyclooctene:Ru = 10 000), ~20 mM cyclooctene solution in
toluene, 25 ◦C for 50–60 h using eicosane (0.41 mol equiv.) as
the internal standard (Figure S1†) to monitor the conversion of
aUniversite´ de Lyon, C2P2, UMR 5265 (CNRS -CPE Lyon Universite´
Lyon 1), 43 Bd. du 11 Novembre 1918, 69616, Villeurbanne Cedex, France.
E-mail: thieuleux@cpe.fr
bBASF SE, Basic Chemicals Research, Carl-Bosch-Strasse 38, 67056,
Ludwigshafen, Germany
cCaRLa – Catalysis Research Laboratory, Im Neuenheimer Feld 584, 69120,
Heidelberg, Germany
dCentre de Diffractome´trie Henri Longchambon - Universite´ Lyon 1, 43, Bd
du 11 Novembre 1918, F-69616, Villeurbanne, France
eETH Zu¨rich, Department of Chemistry, Wolfgang-Pauli-Str., 10, CH-8093,
Zurich, Switzerland. E-mail: ccoperet@inorg.chem.ethz.ch
† Electronic supplementary information (ESI) available: Synthesis proce-
dures, characterizations, catalytic reactivity test, X-ray crystal structure
analysis. See DOI: 10.1039/c1dt11643f
Scheme 2 a) Homogeneous catalysts (G-II and N-II) with symmetrical
NHC and b) unsymmetrical NHC, RuPr and RuBn.
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Fig. 1 a) Conversion (%) vs. time (h) for all catalysts [G-II ( ), N-II ( ), RuPr ( ), RuBn ( )]. b) Mass balance vs. conversion (%) for all the catalysts [G-II
( ), N-II ( ), RuPr ( ), RuBn ( )]. c) Selectivity of cyclic oligomers vs. conversion, at constant mass balance for RuPr [dimer ( ), trimer ( ), tetramer ( ),
pentamer ( )]. d) Selectivity of cyclic oligomers vs. conversion, at constant mass balance for RuBn [dimer ( ), trimer ( ), tetramer ( ), pentamer ( )].
Table 1 Mass balance of cyclic oligomers and their selectivity at 30%
conversion of cyclooctene
cyclooctene into cyclic oligomers (dimers up to pentamers) by gas
chromatography.
While all systems led to full conversion of cyclooctene within
24 h without any initiation period (Fig. 1a), the catalysts display
different initial rates and are ranked as follows G-II (TOF of 9720
h-1) > N-II (TOF of 4180 h-1) > RuPr (TOF of 2340 h-1) ꢀ RuBn
(TOF of 505 h-1). More importantly, they display very different
mass balance and product selectivities. While unsymmetrical
catalysts (RuPr and RuBn) led to a high mass balance reaching
a constant value of 80% after 10% conversion, G-II and N-II
exhibit a rather low mass balance (30–40%) up to 90% conversion
(Fig. 1b), before sharply increasing up to 80% mass balance. This
is consistent with the formation of polymers (or large oligomers)
for the latter, which are then converted to smaller oligomers via
backbiting at higher conversion, while smaller cyclic oligomers
(up to pentamers) are preferentially and directly obtained with
the former. In terms of selectivity, the major products are the
dimer and trimer along with the minor tetramer and pentamer
cyclic oligomers (Fig. 1, S2 and Table 1). In particular the dimer
and trimer are formed in a 2 : 1 ratio with selectivities of ca. 50%
and 25%, respectively for unsymmetrical catalysts. In contrast,
symmetrical ones lead at best to less than 35% and 5% selectivities
in dimer and trimer respectively. Overall, there are two families
of catalysts: the symmetric ones favoring polymer formation
(ROMP) vs. unsymmetrical ones having high productivity in cyclic
oligomers via RO–RCM.
Cyclic oligomers selectivity (%)
Catalysts Dimer Trimer Tetramer Pentamer Mass balance (%)
G-II
N-II
RuPr
RuBn
25
32
48
50
3
5
24
24
1
2
11
10
1
1
5
4
30
40
88
88
Additionally, for these unsymmetrical RO–RCM catalysts,
analysis of the evolution of product selectivities vs. conversion
at constant mass balance provides insight on the nature of the
product formation: the dimer is a primary product (high slightly
decreasing selectivity with time). Other cyclic oligomers, whose
selectivity increases vs. conversion, (Figure S2d and S2e†) are
obtained by secondary processes (metathesis involving products,
e.g. dimer), but in the specific case of the trimer, it is also probably
formed as a primary product considering its high selectivity at low
conversion.
Dramatic selectivity-switch due to the presence of unsym-
metrical ligand is reminiscent of what was reported regarding
the sequence-selective copolymerization of norbornene and cy-
clooctene using unsymmetrical phosphine6 or NHC ligands.7
This sequence selective ROMP, providing alternated cy-
clooctene/norbonene copolymer, originates from the catalyst
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architecture having dual-site configuration, which discriminates
between two substrates in the same catalytic reaction (ROMP),
because of the swing of the alkylidene ligand between two non-
equivalent “positions” (two different configuration associated to
two different steric environments) for each metathesis cycle.
Here, in the RO–RCM of a single substrate, cyclooctene,
the alternating site reactivity also explains the observed high
selectivity towards dimer and trimer and their 2 : 1 ratio, by
alternatively favouring one reaction over another, Ring Opening
(ROM) vs. Ring Closing (RCM) Metathesis (propagation vs.
backbiting). In the proposed mechanism (Scheme 3), the Ru-
NHC complexes enter the catalytic cycle by de-coordinating
the phosphine ligand to generate a reactive 14-electron complex
having the alkylidene unit on the most open configuration (AMOST).
Further reaction of AMOST with cyclooctene would yield BLESS via
ROM with a swing of the propagating alkylidene ligand from
the most open to the less open configuration. At this stage,
3. When backbiting occurs on the closest alkene (BB1), it yields
back CMOST and cyclooctene (non-productive).
From BMOST, with the alkylidene in the open configuration,
further reaction with cyclooctene leads to CLESS, for which
backbiting is favored, releasing dimer and regenerating AMOST
.
Such proposed mechanism explains the preferred formation of
dimer and trimer products including the observed 2 : 1 ratio, if
one considers that all productive backbiting reactions on remote
alkenes from DLESS are equally favored (BB2 ª BB3).
In conclusion, we have shown that Ru-alkene metathesis cata-
lysts bearing unsymmetrical NHC-units provide selectively cyclic
dimer and trimer products over polymers from cyclooctene. This is
in sharp contrast with symmetrical homologues, which lead mainly
to polymers. This unexpected selectivity is due to a controlled
tandem ROM-RCM reaction, where one configuration favors
intramolecular backbiting and the other one coordination of an
additional substrate (intermolecular), thus leading to RO–RCM.
This opens new perspectives in forming selectively macrocycles
from other cyclic alkenes via metathesis, and we are currently
further investigating the scope of this reaction.
B
LESS could regenerate AMOST through backbiting (non-productive)
or react with cyclooctene to yield CMOST via ROM with the
propagating chain on the most open configuration. With this CMOST
configuration, backbiting is disfavored, while further reaction with
cyclooctene is favored giving DLESS. Now the propagating chain
is on the less open configuration thus favoring backbiting over
tandem coordination of cyclooctene and ROM. Here backbiting
can occur on the different alkene moieties along the growing
chain:
The project was funded by BASF SE and ANR-08-BLAN-
0151-01. We thank Laurent Veyre for technical advice.
Notes and references
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Scheme 3 Proposed mechanism for the tandem Ring Opening–Ring
Closing Metathesis of cyclooctene, selective formation of dimer and trimer.
(Mesityl represents the small group while Propyl or Benzyl represent the
bulky group, BBn = backbiting, n = position of alkene from the carbene
carbon).
1. When backbiting occurs on the alkene at the terminal position
(BB3), trimer is formed and AMOST is regenerated,
2. When backbiting occurs on the alkene at the inter-
mediate position (BB2), dimer is formed and the intermedi-
ate BMOST is formed with the alkylidene on the most open
site,
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