Poly(fluoroalkyl acrylate)-Bound Ruthenium Carbene Complex: A Fluorous and Recyclable Catalyst for Ring-Closing Olefin Metathesis

Article abstract of DOI:10.1021/ja037394p

The synthesis of a fluorous olefin metathesis catalyst derived from the Grubbs second-generation ruthenium carbene complex is described. The air stable fluorous polymer-bound ruthenium carbene complex 1 shows high reactivity in effecting the ring-closing

Full text of DOI:10.1021/ja037394p

Published on Web 12/16/2003
Poly(fluoroalkyl acrylate)-Bound Ruthenium Carbene Complex: A Fluorous
and Recyclable Catalyst for Ring-Closing Olefin Metathesis
Qingwei Yao* and Yiliang Zhang
Department of Chemistry and Biochemistry, The Michael Faraday Laboratories,
Northern Illinois UniVersity, DeKalb, Illinois 60115
Received July 18, 2003; E-mail: qyao@niu.edu
Since reported in 1994 by Horva´th,1 fluorous biphasic chemistry
Scheme 1. Synthesis of Fluorous Ru-Catalyst 1
has evolved into a rich and fruitful research area due to its potential
as a new paradigm of green chemical processes.2 The development
of various fluorous reagents and catalysts enables fluorous technol-
ogy to be increasingly applied to the isolation of reaction products
and the recovery of recyclable catalysts. We now wish to report
examples of olefin metathesis using a fluorous ruthenium catalyst.
The fluorous catalyst 1 effects the ring-closing metathesis (RCM)
of a broad spectrum of diene and enyne substrates in minimally
fluorous solVent systems (PhCF3/CH2Cl2, 1:9-1:49 v/v) with high
reactivity. The catalyst can be readily recycled by fluorous
extraction and repeatedly reused.
Table 1. Recycling and Reuse of Fluorous Ru-Catalyst 1 in the
Ring-Closing Metathesis of Test Dienes 9-11
The advent of several well-defined ruthenium catalysts, most
notably the Grubbs-type Ru alkylidenes 23a and 3,3b has fueled the
widespread application of olefin metathesis.4 A major recent
advance in this area is the discovery by Hoveyda et al. of the robust
and recyclable Ru catalysts 4 and 5.5 We6 and others7 have
established that various polymeric and solid supports can be used
to immobilize these catalysts. The development of fluorous olefin
metathesis catalysts8 is a highly desirable goal since this would
lead to a further expansion of the scope of this important reaction.
a
b
Bath temperature. Cycles 1-15: with 0.5 mmol 9 and 1 mol % 1
c
for 1 h; cycles 16-20: with 1 mmol 9 and 0.5 mol % 1 for 2 h. With 0.5
mmol 10 and 1 mol % 1 for 1 h. Determined by 1H NMR spectroscopy.
d
e
With 0.5 mmol 11 and 2 mol % 1 for 1.5 h.
RCM quantitatively in the presence of only 0.5-1 mol % of 110 in
the solvent system PhCF3/CH2Cl2 (1:19 v/v).11 Importantly, 1 can
be cleanly recovered by FC-72 extraction and repeatedly reused.12
The catalyst remained highly active even after 20 cycles in the case
of 9. Diene 11 also cyclized cleanly to give the trisubstituted cyclic
olefin 14 with 2 mol % of 1. The catalyst was recycled and reused
for six runs with only a very slight drop in its activity.13
Having established the recyclability and reusability of 1, we next
examined its performance in the RCM of a variety of di-, tri-, and
tetrasubstituted diene and enyne substrates leading to the formation
of various carbocyclic and heterocyclic olefins (Table 2). The
benchmark diene 15 cyclized with excellent conversion in the
presence of 1 mol % of 1 for up to seven cycles, further
corroborating the high level of recyclability of 1. With one single
batch of 1 (2 mol %), the trisubstituted dienes 17-21 underwent
RCM to form the respective products, all in excellent conversion.
Similarly, another batch of 1 was used to catalyze the reactions of
substrates 27-29 in a consecutive manner with good to excellent
yields. Finally, 1 proved to be reactive toward the RCM of
tetrasubstituted diene 30, albeit a higher loading of 1 and a longer
reaction time were required to achieve a high conversion. With
the more challenging diene 31,15 the reaction went only to half
completion even after an extended reaction time. Remarkably,
Incorporation of perfluoroalkyl groups2 into the ligands would
conceivably render a Ru catalyst fluorous. Given the capricious
nature of the Ru catalysts, however, even a cautious alteration of
the ligand structure may result in an unpredictable change in
activity. We therefore elected to use a suitably fluorinated polymer9
as a fluorous tag2g to the catalyst. To this end, fluorous polymer 8
bearing a bidentate isopropoxystyrene ligand5 was designed and
synthesized as shown in Scheme 1. The commercially available
fluoroacrylate 6 was copolymerized with acryloyl chloride in the
presence of AIBN under heating in PhCF3. The resulting acyl
chloride copolymer was then coupled to the known ligand 76 under
standard conditions to give 8 as a white powder. Polymer 8 is fully
soluble in fluorous solvents such as PhCF3 and FC-72 but insoluble
in common organic solvents such as CH2Cl2 and EtOAc. 1H NMR
spectroscopy (500 MHz) revealed a clean incorporation of the
styrene ligand in about 1:10 ratio relative to the fluoroalkyl groups,
corresponding to a loading of 0.19 mmol/g of the ligand. Treatment
of 8 with catalyst 3 in the presence of CuCl5b resulted in the
exchange of the styrene groups in >90% conversion to deliver the
air stable greenish powder 1.
The activity of catalyst 1 was evaluated using three test dienes,
as shown in Table 1. The disubstituted dienes 9 and 10 underwent
9
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J. AM. CHEM. SOC. 2004, 126, 74-75
10.1021/ja037394p CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Activity and Recyclability of Fluorous Ru-Catalyst 1a
Acknowledgment. This work was supported by grants from
the NIH (GM63522) and the Petroleum Research Fund, adminis-
tered by the American Chemical Society (36466-G1). We thank
Ms. Adalie Rodriguez Motta for sharing some preliminary results
with the RCM of diene 30.
Supporting Information Available: Experimental details and full
characterization of all new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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(10) The loading of 1 was estimated based on full conversion of the styrene
ligand of 8. The estimated value represents an upper limit of the actual
amount of the catalyst, which should be slightly lower due to the presence
of a small amount of free ligand that remained unreacted (less than 10%
as revealed by 500 MHz 1H NMR spectroscopy).
a
Unless otherwise noted, all reactions were performed with 0.50 mmol
substrate and 2 mol % 1 in PhCF3/CH2Cl2 (1:19 v/v, 10 mL) at 50 °C.b
Determined by 500 MHz H NMR. Yield of isolated pure product.
mol % 1 for all cycles (1-7). In PhCF3/CH2Cl2 (1:49 v/v, 25 mL). 30
(0.25 mmol) and 5 mol % 1 in PhCF3/CH2Cl2 (1:9 v/v, 5 mL). 30 (0.25
mmol) and 5 mol % 1 in PhCF3 (5 mL). 31 (0.10 mmol) and 5 mol %
1 in PhCF3 (2 mL). 23 (0.25 mmol) and 2 mol % 1 in PhCF3/CH2Cl2
(1:9 v/v, 5 mL).
1
c
d
1
e
f
g
h
i
(11) In the PhCF3/CH2Cl2 (1:9 to 1:49 v/v) mixed solvent system the reaction
was monophasic, whereas the biphasic system FC-72/CH2Cl2 (1:1 v/v)
gave less satisfactory results (see the Supporting Information for details).
(12) The recycling of 1 can simply be carried out in the air by fluorous
extraction of the crude reaction mixture dissolved in EtOAc with FC-72.
The FC-72 solvent can be conveniently recycled and reused provided an
efficient cooling trap is attached to the evaporator.
(13) For further recycling and reuse of 1, see the Supporting Information.
(14) Only the less volatile PhCF3 was used as the solvent when an extended
reaction time was required.
however, the recovered catalyst still remained highly active, further
attesting to the stability and recyclability of 1.
In conclusion, we have shown that catalyst 1 can be readily
assembled from an easily accessible fluorous polymer. Compared
with catalyst 3 and other immobilized Ru catalysts, 1 has the
combined advantage of high reactivity and a high level of
recyclability and reusability. Given the growing interest in fluorous
chemistry and the widespread application of olefin metathesis, 1
should prove to be of unique utility in both areas.
(15) Yao, Q. Org. Lett. 2002, 4, 428.
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