A self-generating, highly active, and recyclable olefin-metathesis catalyst

Article abstract of DOI:10.1002/1521-3773(20021018)41:20<3835::AID-ANIE3835>3.0.CO;2-4

Sequential ring-opening-metathesis polymerization and cross-metathesis reactions allow a one-pot synthesis of the illustrated ruthenium catalyst. This conceptually novel polymer-bound catalyst shows excellent metathesis activity and recyclability.

Full text of DOI:10.1002/1521-3773(20021018)41:20<3835::AID-ANIE3835>3.0.CO;2-4

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aspire to preparing ruthenium alkylidenes with reactivity
profiles that correspond to those of homogeneous catalysts,
combined with a facile recoverymethodologythat involves a
simple change in the polarityof the solvent to allow easy
separation of the catalyst from the substrate. The only
previous report that involves this method detailed a variant
of 4 attached to polyethylene glycol (PEG) chain which could
be precipitated with diethyl ether.[3f] The levels of residual
ruthenium in the products is a critical issue in all heteroge-
neous systems. We report herein the design and preparation of
a polymer-bound, homogeneous, recyclable, and highly effi-
cient ruthenium catalyst. The polymer support also seems
capable of scavenging ruthenium after the reaction, thus
allowing the formation of metathesis products that contain
extremelylow ruthenium levels.
A Self-Generating, Highly Active, and
Recyclable Olefin-Metathesis Catalyst**
Stephen J. Connon, Aideen M. Dunne, and
Siegfried Blechert*
As the world becomes more cost-conscious and environ-
mentallyaware, the pursuit of new transition-metal catalysts
that are both active and recyclable has become increasingly
important to synthetic organic chemists. Recently, the advent
of efficient homogeneous ruthenium alkylidene precatalysts
1 6[1] have propelled the olefin-metathesis reaction to the
It is known that ring-opening-metathesis polymerization
(ROMP) products of substituted oxanorbornene derivatives
are often soluble in organic solvents with the notable
exceptions of methanol, diethyl ether, or diethyl ether/hexane
mixtures. This led us to postulate that a ruthenium alkylidene
incorporated into these products could serve as a recyclable
and easilyassembled catalsyt without the need for the
purchase of and attachment to often expensive solid-phase
resins. It was thus conceivable that 10 (Scheme 1) could serve
forefront of contemporaryorganic chemistryas a mild
and efficient method for C C-bond formation.[2] Given
ꢀ
this new-found significance, it is not surprising that the
development of reusable catalysts has been the goal of
a number of research groups.[3,4]
Various recycling strategies have been reported:
a) Recoveryof the homogeneous catalyst bychroma-
[1c,d]
tographyof the crude reaction mixture
b) Filtration of the reaction mixture to separate the
heterogeneous catalyst bound to a polymeric solid
e,g j]
support[3a
c) Selective precipitation of the homogeneous catalyst
from solution after the reaction.[3f]
Chromatographic recoveryfurnishes pure catalsyt
for reuse, but is time-consuming and generates consid-
erable amounts of silica and solvent waste. Heteroge-
neous catalysts are easily recovered with minimal
waste, however with one reported exception[3i] these catalysts
are much less active than their homogeneous counterparts. By
employing a selective catalyst-precipitation strategy, one can
Scheme 1. Synthesis of catalyst 11.
as a precursor for such a catalyst through consecutive (one-
pot) ROMP and cross-metathesis (CM) attachment of the
ruthenium moietyto the polymer. Compound 10 contains an
isopropoxy styrene moiety necessary for catalyst formation
and an oxanorbornene ring destined to become the polymer
support after ROMP. Furthermore, the second ether linkage is
positioned ortho to the chelating isopropoxygroup. This is an
important consideration: bycorrelation with catalyst 6 and
from preliminarystudies conducted in our laboratories, we
were aware that the presence of alkoxygroups in this ortho
position leads to catalysts that display strongly enhanced
activityrelative to 5.[1f]
[*] Prof. Dr. S. Blechert, Dr. S. J. Connon, Dr. A. M. Dunne
Institut f¸r Chemie, Technische Universit‰t Berlin
Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax : (þ 49)30-3142-3619
E-mail: blechert@chem.tu-berlin.de
[**] S.J.C thanks the Alexander von Humboldt foundation for a postdoc-
toral fellowship. We would also like to thank the Fonds der
Chemischen Industrie for financial support. We thank S. Gessler for
helpful discussions.
Aldehyde 9 was readilyprepared in good yield (76%) by
the action of the potassium salt of 7 on bromide 8 in N,N-
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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dimethylformamide (DMF).[5] Subsequent Wittig olefination
gave styrene derivative 10 in 79% yield. Treatment of 10 with
2 (1 mol%) in CH2Cl2 at 208C in the presence of CuCl as a
phosphane scavanger gave polymer-bound catalyst 11
(Scheme 1).
Table 1. Activityof 13 in various metathesis reactions.
Substrate[a] Product[b]
Conversion [%][c]
> 98
Catalyst 11 was tested in simple ring-closing-metathesis
(RCM) reactions and gave encouraging results. Excellent
conversions were obtainable for up to four consecutive cycles;
however, long reaction times (7 12 h) were required for
quantitative conversion. We speculated that the sluggish
activitywas in part due to the difficultyin forming the
catalytically active 14-electron species[6] in the presence of
99 equivalents of ligand and thus decided to lower the ligand/
catalyst ratio by a factor of 10. To avoid a situation in which
two catalyst moieties could exist in close proximity to one
another, an oxanorbornene benzoate co-monomer 12 was
added to act as a spacer. This was prepared from the same
batch of alcohol used to prepare bromide 8, and so had the
same endo/exo isomer ratio (ca. 1:1) as styrene 10. For ease
and simplicityof catalyst preparation, no attempt was made to
separate these endo/exo mixtures, which arise from the Diels
Alder reaction used to prepare 8.[5] In our experience, the
rates of polymerization of these isomers do not differ greatly
in anycase. Thus, reaction of 2, 10, and 12 (1:10:30) at room
temperature in CH2Cl2 gave quantitative polymerization of
both oxanorbornene compounds 10 and 12 after 10 min to
give a red solution, indicative of the presence of 2. Addition of
CuCl gave a green solution after heating at reflux for 1 h.
Workup gave polymer-bound 13 in 93% yield (Scheme 2).
> 98
> 98
> 98
> 98
> 98
> 98
> 98[d]
[a] Conditions: 13 (1 mol%), CH2Cl2 (0.05m), 208C. [b] Reaction times
(min) in parentheses. [c] Determined by 1H NMR spectroscopic analysis.
[d] 13 (0.05 mol%), allyltrimethylsilane (1.0 equiv).
Scheme 2. Preparation of catalyst 13.
eight-membered rings 22 27 with quantitative conversion.
The formation of trisubstituted olefin 23 and the compatibility
of 13 with potentially chelating hydroxy- and amide-function-
alized substrates 16 and 17 further underlines the synthetic
utilityof this catalyst. In all previous literature reports 2.5
5% of the polymer-bound catalyst and higher temperatures
(usually45 8C) were required to promote efficient meta-
thesis.[3] In the atom-economic and efficient ROM-CM
reaction only0.05 mol% of catalyst was necessaryto give
quantitative conversion of 21.
Having established the reactivityof 13 under mild con-
ditions, we then turned to the critical question of recyclability.
It was found that after metathesis, 13 could be conveniently
precipitated and filtered off bythe addition of diethyl ether or
hexane. In the RCM of N-tosyldiallylamine (30), 13 can be
recycled easily under the conditions outlined previously in
Table 1, with quantitative conversion for each of seven
consecutive cycles (Table 2).
Polymer 13 is soluble in most common organic solvents but
not in diethyl ether and hexane. To our delight, the catalyst
loading can be determined (typically in the range 0.08
0.1 mmolgꢀ1) byrelative integration of the broad alkylidene
signal at d ¼ 16.7 ppm and the signal for the aromatic ortho
proton (d ¼ 8.0 ppm) of the benzoate moietyin the 1H NMR
spectrum (a 1:60 ratio of alkylidene/benzoate indicates
quantitative ROMP and CM). Thus, expensive, time-consum-
ing, and often inconvenient ruthenium analysis is not re-
quired. This loading indicates a high level of incorporation
(i.e. efficient CM) of the ruthenium moietyonto the styrene-
substituted polymer backbone, which was reproducible be-
tween polymer batches. ROMP-based catalyst 13 was found
to have impressive activityin RCM, ROM-CM, and tandem
metathesis reactions (Table 1).
With only1 mol% (measurable by 1H NMR spectroscopy)
of 13 at 208C it was possible to generate five-, six-, seven-, and
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Table 2. Recyclability of 13 in the RCM of 30.
solution-phase activitywith high and convenient recyclability
without requiring the purchase of often expensive solid-
support resins. The catalyst is prepared from inexpensive
starting materials in a straightforward manner; the generation
of the solid-support and attachment of the ruthenium moiety
is possible in one convenient step through sequential ROMP
and CM. The products from metathesis reactions involving
these catalysts also contain unparalleled low levels of residual
ruthenium. The conceptuallynovel catalyst 13 represents a
balance between the high activityof suitablysubstituted
analogues of 6, and the recyclability of 5; possible through
ligand design and stoichiometry.
Studies on the exploitation of these flexible systems in new
metathesis applications are underway. One example is the
incorporation of a modified fluorescence marker into the
catalyst to identify trace quantities of polymer in the reaction
products. Investigation of the product bymass spectrometryis
another option. The results of these and other investigations
will be reported in due course.
Cycle
t [h]
Conversion [%][a]
1
2
3
4
5
6
7
8
1
1
1
1
1
2
4
72
> 98
> 98
> 98
> 98
> 98
> 98
> 98
75
1
[a] Determined by H NMR spectroscopic analysis.
[7]
Total-reflection X-rayfluorescence (TXRF) analysis
of
the products after catalyst separation indicated that a
maximum of 0.004% ruthenium was present in each of the
first four cycles. This value is approximately an order of
magnitude lower than that of contemporaryliterature systems
for the removal of ruthenium after the reaction[3,8] and even
two orders of magnitude lower than that of the best previously
reported polymer-bound catalysts.[3] It seems likely, therefore,
that ruthenium remains attached to the polymer even after
catalyst deactivation. Importantly, a ruthenium-saturated
analogue of 13 (i.e. all styrene moieties loaded) was consid-
erablyless recyclable under these conditions, thus indicating
that the recyclability of 13 is largelydue to the presence of an
excess of Lewis basic ligand, which captures the catalytically
active species after the reaction. When the small amount of
ruthenium used (1 mol% over seven cycles) is taken into
account, 13 compares veryfavorablyin terms of recyclability
with literature systems.[3] In addition to being more recyclable,
13 displayed a much enhanced metathesis activity relative to 5
in the RCM of 30,[9] and was comparable in terms of reaction
rate to benchmark catalyst 2. For example, in the RCM of 30
at high dilution (0.01m, CH2Cl2, 208C, in air) the time taken to
reach 80% conversion to 31 was 65 min in the presence of 13,
whereas to 119 min were required when using non-polymer-
bound catalyst 5.[9]
Given the mode of formation of 13, it seemed likelythat a
high degree of self-generation of the polymer support would
be possible. As expected, when just 0.5 mol% of 13 was added
to a solution of 10 and 12 (1:3) in CH2Cl2, polymerization was
complete after 10 min to afford 13 (off-white color) and no
signal for the alkylidene was observed in the 1H NMR
spectrum. This ruthenium-deficient version of 13 could then
be loaded with commerciallyavailable 2 to a preselected level
under standard conditions (Scheme 3). It is envisaged that this
development will allow greater control of the catalyst-loading
process.
Received: June 12, 2002 [Z19469]
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recyclable ROMP-based catalyst,[10] which combines good
[4] Recently a recyclable molybdenum-based asymmetric catalyst was
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Scheme 3. Self generation of the polymer support.
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[5] Concise procedures for the preparation of 7, 8, 9, 10, 12, and 13 from
inexpensive commerciallyavailable starting materials, together with a
general metathesis procedure using 13 are available in the Supporting
Information.
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6543 6554.
[7] We thank Prof. B. O. Kolbesen, Institut f¸r Anorganische Chemie,
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