Heptane-soluble ring-closing metathesis catalysts

Article abstract of DOI:10.1021/ol071210k

Terminally vinyl-functionalized polyisobutylene (PIB) oligomers can be easily transformed into end-functionalized PIB-bound Ru metathesis catalysts. The nonpolar catalysts so prepared can be used as solutions in heptane and recycled by a gravity-based extraction after addition of a heptane-immiscible polar solvent. This paper describes the synthesis and the recycling of a PIB-supported second-generation Hoveyda-Grubbs catalyst for Ru-catalyzed ring-closing metathesis and ring-opening metathesis polymerizations.

Full text of DOI:10.1021/ol071210k

ORGANIC
LETTERS
2007
Vol. 9, No. 17
3259-3261
Heptane-Soluble Ring-Closing
Metathesis Catalysts
Chayanant Hongfa,† Jianhua Tian,† Hassan S. Bazzi,*,‡ and David E. Bergbreiter*,†
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012,
and Department of Chemistry, Texas A&M UniVersity at Qatar, P.O. Box 23874,
Doha, Qatar
bergbreiter@tamu.edu; bazzi@ tamu.edu
Received May 22, 2007
ABSTRACT
Terminally vinyl-functionalized polyisobutylene (PIB) oligomers can be easily transformed into end-functionalized PIB-bound Ru metathesis
catalysts. The nonpolar catalysts so prepared can be used as solutions in heptane and recycled by a gravity-based extraction after addition
of a heptane-immiscible polar solvent. This paper describes the synthesis and the recycling of a PIB-supported second-generation Hoveyda
−
Grubbs catalyst for Ru-catalyzed ring-closing metathesis and ring-opening metathesis polymerizations.
Ruthenium-catalyzed ring-closing metathesis, cross metath-
esis, and ring-opening metathesis polymerization have
developed into broadly useful methodology.1 These homo-
geneously catalyzed reactions are equally useful in complex
syntheses and in the formation of materials. However, as is
true for other homogeneously catalyzed reactions, strategies
to recover and reuse catalysts are of interest. For these Ru
catalysts, these strategies include using soluble polymers,
insoluble polymers, and boomerang insoluble polymer-
supported catalysts.2 Water and scCO2 soluble ligands that
enable catalyst use or recovery in environmentally benign
solvents have been developed.3 Ionic liquids have also been
used to recycle catalysts.4 Here we describe an alternative
approach using a heptane-soluble polymer to prepare a
second-generation Hoveyda-Grubbs catalyst that is recover-
able and reusable in hydrocarbon solvents by liquid/liquid
or liquid/solid separations after catalysis.
Two strategies are possible with a heptane-soluble catalyst.
First, a catalyst selectively soluble in a heptane-rich phase
of a liquid/liquid biphasic mixture can be recovered in that
phase after a monophasic reaction. This can be done by using
thermomorphic or latent biphasic conditions or by product
extraction with a heptane-immiscible solvent.5 A second
approach is to simply carry out the reaction in heptane and
to rely on the fact that many polar organic products are not
† Texas A&M University.
‡ Texas A&M University at Qatar.
(1) (a) Chauvin, Y. Angew. Chem., Int. Ed. 2006, 45, 3740-3747. (b)
Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760-3765. (c) Trnka, T.
M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29.
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Langemann, K.; Liebl, M.; Six, C.; Leitner, W. J. Am. Chem. Soc. 2001,
123, 9000-9006. (b) Hong, S. H.; Grubbs, R. H. J. Am. Chem. Soc. 2006,
128, 3508-3509.
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(5) (a) Bergbreiter, D. E.; Liu, Y. S.; Osburn, P. L. J. Am. Chem. Soc.
1998, 120, 4250-4251. (b) Bergbreiter, D. E.; Osburn, P. L.; Smith, T.;
Li, C.; Frels, J. D. J. Am. Chem. Soc. 2003, 125, 6254-6260.
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J.; Dunne, A. M.; Blechert, S. Angew. Chem., Int. Ed. 2002, 41, 3835-
3838. (c) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J.
Am. Chem. Soc. 2000, 122, 8168-8179. (d) Jafarpour, L.; Nolan, S. P.
Org. Lett. 2000, 2, 4075-4078. (e) Kingsbury, J. S.; Hoveyda, A. H. J.
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Chem. Soc. 2004, 126, 74-75.
10.1021/ol071210k CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/20/2007

especially soluble in heptane. If such a product were a solid,
filtration or decantation can then separate the catalyst and
product. Both approaches are described below.
The general strategy for synthesis of a heptane-soluble
metathesis catalyst relied on the use of commercially
available vinyl-terminated polyisobutylene. While this mate-
rial can be >90% dCH2 terminated polyisobutylene 1, some
samples contain as much as 20% of the internal double bond
(e.g., polyisobutylene 2). A Friedel-Crafts alkylation of
either structurally isomeric polyisobutylene oligomer, how-
ever, yields the same 4-(polyisobutyl)phenol product 3.6
Formylation of this oligomeric phenol by using paraform-
aldehyde produces the 2-hydroxy-5-(polyisobutyl)benzalde-
hyde 4. This product can in turn be converted into the
oligomeric styrene derivative 5 by using a Wittig olefination.
Exchange of the methylene group with the benzylidene group
of the commercially available second-generation Grubbs
catalyst 6 produced the desired hepane-soluble catalyst 7
(Scheme 1). The last step of this synthesis also used flash
Figure 1. 1H NMR spectrum of a polyisobutylene-bound ruthenium
metathesis catalyst 7 showing changes in the i-Pr heptet in inset a
and b for 5 and 7, respectively.
CH of the isopropyloxy shifts from δ 4.51 in 5 to δ 4.87 in
7 (cf. Figure 1, insets a and b).
Other than having a polyisobutylene-containing ligand that
makes it heptane soluble, the catalyst 7 was unexceptional
in its reactivity, converting a variety of R,ω-dienes into cyclic
olefins at room temperature (Table 1). The catalyst could
be recycled multiple times. The recycling of 7 was ac-
complished several ways. The most standard approach used
Scheme 1. Synthesis of a PIB-Supported Second-Generation
Hoveyda-Grubbs Catalyst
Table 1. RCM Reactions with Catalyst 7a
column chromatography to separate the PIB-bound Ru
complex 7 from starting materials. Chromatography is not
usually used in the synthesis of polymer-bound catalysts but
is feasible for 7 just as it is feasible for PEG-bound or
dendrimer-bound catalysts.2c,3b As is the case for other
terminally functionalized polymer-bound ligands/catalysts,
all of the intermediates in these syntheses could be character-
1
ized by H NMR spectroscopy.6-8 Figure 1 illustrates the
sort of 1H NMR spectra that can be obtained for species like
catalyst 7. This spectrum establishes that catalyst 7 like its
low molecular weight analogue involves the isopropyloxy
group in the Ru coordination sphere as the heptet due to the
product
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
9
66%
72%
84%
67%
62%
75%
81%
84%
76%
70%
75%
94%
93%
84%
80%
94%
98%
99%
84%
88%
99%
98%
99%
93%
96%
11
13
15
17
(6) Li, J.; Sung, S.; Tian, J.; Bergbreiter, D. E. Tetrahedron 2005, 61,
12081-12092.
(7) Dickerson, T. J.; Reed, N. N.; Janda, K. D. Chem. ReV. 2002, 102,
3325-3344.
a Yields in cycles 1-5 are of products isolated by solvent removal and
increase cycle to cycle because of saturation of the heptane phase by
products.
(8) Bergbreiter, D. E.; Blanton, J. R.; Chandran, R.; Hein, M. D.; Huang,
K. J.; Treadwell, D. R.; Walker, S. A. J. Polym. Sci., Part A: Polym. Chem.
1989, 27, 4205-4226.
3260
Org. Lett., Vol. 9, No. 17, 2007

for dienes like 8 and 16 was to carry out the reaction in
heptane as a solvent and to then extract the product with
acetonitrile. In such cases, the less dense heptane phase
contained the catalyst 7 and recycling simply involved
addition of fresh substrate.
in the catalytic cycle always has a hydrocarbon soluble ligand
may lower Ru leaching.
The reactivity of catalyst 7 was also tested in ring opening-
metathesis polymerization (ROMP) reactions.10 In this case,
the use of 7 is expected to generate an amphiphilic block
copolymer since the ligand’s PIB chain will be the end group
of the product polymer chain. The ROMP reactions of 18
and 19 with catalyst 7 were studied in THF at room
temperature (Scheme 2), using a monomer:initiator ratio of
While catalyst 7 was in our experience equivalent in
reactivity to the commercially available 6, its heptane
solubility and the generally poor solvating ability of heptane
toward many polar organic products makes another recycling
scheme possible. For example, using the substrates 10, 12,
and 14 in Table 1, we were able to carry out a reaction
through at least five cycles by simply adding a heptane-
soluble starting material to the heptane solution of the
catalyst. After 1 h, the heterocyclic product precipitates from
heptane leaving a solution of the catalyst that can be
separated by forced siphon and reused. The apparent
increases in yields from cycles 1-5 in Table 1 reflect
saturation of the heptane phase by products.9 If a catalyst
were used through multiple cycles this would pose little
problem unless the product or byproducts were heptane
soluble.9b
Scheme 2. ROMP Reactions with Catalyst 7
30:1. 1H NMR showed quantitative conversion in both
reactions after 20 min. The reactions were then quenched
with ethyl vinyl ether, and the resulting polymers were
isolated. Catalyst 7 is thus useful in RCM or ROMP
chemistry.
In summary, the studies here show that polyisobutylene
oligomers are an excellent heptane selectively soluble
polymer support for the second-generation Hoveyda-Grubbs
catalyst. The derivatization and chemistry of these oligomers
can be easily followed by conventional spectroscopy. The
activity of the PIB-supported catalyst is analogous to that
of other soluble polymer or nonsupported catalysts.
The efficiency of separation of catalyst and product was
evaluated by ICP-MS analysis, using a sample of product 9
and 11 from the first and third reaction cycles of a cyclization
like that of Table 1. Combustion of product 9 and digestion
of any residue in concentrated sulfuric acid showed the
presence of ca. 20 ppm Ru in the product phase. This
translates into recovery of ca. 97% of the Ru for a second
cycle. The loss of Ru presumably reflects the fact that the
present scheme requires the Ru-methylidene intermediate
formed from 7 to be recaptured by 5 formed in situ for
complete Ru recovery. Inefficiencies in this process and loss
of Ru to another phase are most notable in the higher level
of Ru contamination in experiment where 11 precipitated
from heptane. In this case, ca. 96% of the Ru catalyst was
still recovered, but the solid product contained ca. 1000 ppm
of Ru. This high concentration of Ru is due to the small
volume of the product phase and partitioning of Ru species
onto the small volume of the polar solid that is formed. This
suggests that designing other systems where the Ru species
Acknowledgment. Support of this project from the
National Science Foundation (D.E.B., CHE-0446107), the
Robert A. Welch Foundation (D.E.B., A-639), and the Qatar
Foundation (H.S.B.) is gratefully acknowledged.
Supporting Information Available: Experimental pro-
cedures and characterization data for the synthesis of PIB-
bound ligands and catalyst 7 and procecures for the ring-
closing reactions and ring-opening polymerization reactions.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(9) (a) Bergbreiter, D. E.; Li, J. Chem. Commun. 2004, 1, 42-43. (b)
Bergbreiter, D. E.; Osburn, P. L.; Frels, J. D. J. Am. Chem. Soc. 2001, 123,
11105-11106.
(10) (a) Buchmeiser, M R. Chem. ReV. 2000, 100, 1565-1604. (b)
Grubbs, R. H.; Tumas, W. Science 1989, 243, 907-915.
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