Carborane as a tunable tag for Ru catalysts: Generating an anion-appended recyclable and robust catalyst suitable for the noncovalent binding concept

Article abstract of DOI:10.1021/ol7018218

Ruthenium carbene complexes 9 with a closo-1,2-C₂B ₁₀H₁₁ tag and 10 with an ionic [nldo-7,6-C ₂B₉Hu]- tag were synthesized. Both 9 and 10 are highly reactive catalysts for olefin metathesis reactions.

Full text of DOI:10.1021/ol7018218

ORGANIC
LETTERS
2007
Vol. 9, No. 21
4263-4266
Carborane as a Tunable Tag for Ru
Catalysts: Generating an
Anion-Appended Recyclable and Robust
Catalyst Suitable for the Noncovalent
Binding Concept
Guiyan Liu, Jianzheng Zhang, Bin Wu, and Jianhui Wang*
Department of Chemistry, College of Science, Tianjin UniVersity, Tianjin 300072, PRC
wjh@tju.edu.cn
Received July 29, 2007
ABSTRACT
-
Ruthenium carbene complexes 9 with a closo-1,2-C2B10H11 tag and 10 with an ionic [nido-7,8-C2B9H11
]
tag were synthesized. Both 9 and 10
are highly reactive catalysts for olefin metathesis reactions. Importantly, 10 is a robust and recyclable anion-appended catalyst that was
suitable for noncovalent binding with many cationic resins. At least ten recycles were achieved for RCM of the selected substrate using 10
as the catalyst in ionic liquids.
Immobilization of homogeneous catalysts on support materi-
als is very attractive, as it could offer more practical
advantages for such types of catalysts.1 Compared to the
traditional covalent-binding immobilizations, the recently
developed noncovalent method via electrostatic interactions
is simpler, is more convenient for recycling and reuse of
the support materials, and is quite apt for industrial continu-
ous-flow processes.2 However, it requires cation- or anion-
appended homogeneous catalysts or catalysts with precursors
for appending anions or cations.
Since the discovery of well-defined modern ruthenium
catalysts 1-4,3 olefin metathesis has gained a position of
interesting significance.4 Various strategies have been em-
ployed to reuse and recycle these catalysts.5 One such
(3) (a) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2039-2041. (b) Schwab, P.; Grubbs, R.
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R. H. Org. Lett. 1999, 1, 953-956. (f) Huang, J.; Stevens, E. D.; Nolan, S.
P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674-2678. (g) Scholl,
M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999,
40, 2247-2250.
(1) Blaser, H.-U.; Pugin, B. Supported Reagents and Catalysts in
Chemistry; RSC: Cambridge, 1998; Vol. 216, p 101.
(2) (a) Horn, J.; Michalek, F.; Tzschucke, C. C.; Bannwarth, W. Top.
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Warrington, B. H.; Watts, P.; Wong, S. Y. F.; Zhang, X. Tetrahedron 2002,
58, 4735-4757. (e) Kirschning, A.; Solodenko, W.; Mennecke, K. Chem.-
Eur. J. 2006, 12, 5972-5990.
(4) (a) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH:
Weinheim, Germany, 2003; Vols. 1-3. (b) Schrock, R. R.; Hoveyda, A.
H. Angew. Chem., Int. Ed. 2003, 42, 4592-4633. (c) Trnka, T. M.; Grubbs,
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10.1021/ol7018218 CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/20/2007

strategy is to introduce a cation-appended ligand on the
ruthenium metal center, which is being intensively used for
immobilization of transition metal catalysts.6 For example,
Ru carbene complexes can be made soluble in the aqueous
phase using cationic phosphine ligands.7 Introduction of a
cationic imidazolium to Hoveyda’s boomerang ligand to
make the catalyst suitable for use in ionic liquids (ILs)8 and
binding the ammonium-substituted Hoveyda-Grubbs cata-
lyst to commercial resins9 are the other methods adopted for
reuse and recycle. However, reports on anion-appended Ru
carbene complexes are rare. In view of the fact that many
polymers are cationic resins, there is a strong desire for
synthesizing Ru carbene complexes incorporating anion-
appended ligands for anchoring the complexes to such
cationic resins through noncovalent binding. But, this
remained a challenging issue, partially due to the high
sensitivity of the ruthenium carbene to many different anions
via ligand exchange protocol.10
With this objective in mind, we have designed and
synthesized a carborane-tagged isopropoxy styrene ligand,
which on binding to the Ru carbene complex would render
it with some different and useful properties, and more
importantly, it may potentially offer a robust and recyclable
catalyst incorporating anion-appended ligands suitable for
noncovalent binding to cationic support materials. Herein,
we report the synthesis and the activity of carborane-tagged
ruthenium catalyst 9 and the carbollide-tagged ruthenium
catalyst 10 in the ring-closing metathesis reaction (Scheme
1).
Scheme 1. Synthesis of Ruthenium Catalysts 9 and 10
Carborane of formula o-C2B10H12, a commercially avail-
able derivative of boron hydrides, has been intensively used
for pharmaceutical purposes,11 in material chemistry,12 and
in metallocarborane chemistry.13 Under strong basic condi-
tions or in the presence of n-Bu4N+F-, a formal {BH}-
vertex can be removed selectively from the neutral o-C2B10H12
to form [nido-7,8-C2B9H12]- (named the carbollide anion).14
It is thought worthwhile to make use of this stable anion for
the noncovalent immobilization of the ruthenium carbene
catalyst to the cationic supports.
(5) Reviews: (a) Kingsbury, J. S.; Hoveyda, A. H. In Polymeric Materials
in Organic Synthesis and Catalysis; Buchmeiser, M. R., Ed.; Wiley-VCH:
Weinheim, 2003; p 467. (b) Buchmeiser, M. R. New J. Chem. 2004, 28,
549-557. (c) Clavier, H.; Grela, K.; Kirschning, A.; Mauduit, M.; Nolan,
S. P. Angew. Chem., Int. Ed. Engl. 2007, DOI: 10.1002/anie 200605099.
(6) Miao, W.; Chan, T. H. Acc. Chem. Res. 2006, 39, 897-908.
(7) (a) Lynn, D. M.; Mohr, B.; Grubbs, R. H.; Henling, L. M.; Day, M.
W. J. Am. Chem. Soc. 2000, 122, 6601-6609. (b) Mohr, B.; Lynn, D. M.;
Grubbs, R. H. Organometallics 1996, 15, 4317-4325. (c) Lynn, D. M.;
Mohr, B.; Grubbs, R. H. J. Am. Chem. Soc. 1998, 120, 1627-1628. (d)
Kirkland, T. A.; Lynn, D. M.; Grubbs, R. H. J. Org. Chem. 1998, 63, 9904-
9909.
(8) (a) Yao, Q.; Sheets, M. J. Organomet. Chem. 2005, 690, 3577-
3584. (b) Yao, Q.; Zhang, Y. Angew. Chem., Int. Ed. 2003, 42, 3395-
3398. (c) Audic, N.; Clavier, H.; Mauduit, M.; Guillemin, J.-C. J. Am. Chem.
Soc. 2003, 125 (31), 9248-9249. (d) Clavier, H.; Audic, N.; Guillemin,
J.-C.; Mauduit, M. J. Organomet. Chem. 2005, 690, 3585-3599.
(9) Michrowska, A.; Mennecke, K.; Kunz, U.; Kirschning, A.; Grela,
K. J. Am. Chem. Soc. 2006, 128 (40), 13261-13267.
(10) (a) Tanaka, K.; Bohm, V. P. W.; Chadwick, D.; Roeper, M.;
Braddock, D. C. Organometallics 2006, 25, 5696-5698. (b) Monfette, S.;
Fogg, D. E. Organometallics 2006, 25, 1940-1944. (c) Vygodskii, Y. S.;
Shaplov, A. S.; Lozinskaya, E. I.; Filippov, O. A.; Shubina, E. S.; Bandari,
R.; Buchmeiser, M. R. Macromolecules 2006, 39, 7821-7830.
(11) For an excellent review, see: Hawthorne, M. F. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 950-984.
(12) Wei, X.; Carroll, P. J.; Sneddon, L. G. Organometallics 2006, 25,
609-621.
(13) (a) Xie, Z. Coord. Chem. ReV. 2006, 250, 259-272. (b) Xie, Z.
Acc. Chem. Res. 2003, 36, 1-9. (c) Saxena, A. K.; Hosmane, N. S. Chem.
ReV. 1993, 93, 1081-1124.
As illustrated in Scheme 1, commenced from the com-
mercially available 2,4-dihydroxybenzaldehyde 5, the syn-
thesis of closo-1,2-carborane-tagged ligand 8 was prepared
in a straightforward manner in good yield. Treatment of 8
with 2 in the presence of CuCl in CH2Cl2 at 40 °C, as
described by Hoveyda and co-workers,4c resulted in the
exchange of the styrene group to give neutral closo-1,2-
carborane-tagged ruthenium complex 9 in good yield (78%).
Reaction of 8 with tetrabutylammonium fluoride (TBAF) in
mixture solvents of THF and water opens the carborane cage
to give an intermediate, followed by ligand exchange with
2 which afforded the anionic [nido-7,8-C2B9H11]--tagged
ruthenium complex 10 in 64% yield as greenish crystalline
solid.
Interestingly, 9 and 10 have very different solubility due
to the different properties of the carborane tag, which could
be used for different tasks. Compound 9 exhibits very good
solubility in nonpolar solvents, such as benzene, toluene, and
hexanes, while 10 is very soluble in polar solvents, such as
alcoholic solvents, acetone, and acetyl acetate.
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Org. Lett., Vol. 9, No. 21, 2007

Both the closo-1,2-carborane-tagged Ru carbene 9 and the
anionic [nido-7,8-C2B9H11]--tagged 10 are highly active
catalysts for ring-closing metathesis (RCM) reactions. Diethyl
diallymalonate 11 was chosen as the test diene to investigate
the catalytic activity of 9 and 10 in its RCM. The relative
conversion rates for RCM by 9 and 10 under different
conditions are illustrated in Figure 1. At 0 °C, 9 exhibits
Table 1. Application of Catalyst 9 and 10 on Different
Substratesa
Figure 1. Relative conversion rates of 9 and 10 using 1 mol % of
catalyst in CH2Cl2: (a) at 0 °C; (b) at 30 °C.
better performance than 10, showing that the closo-1,2-
carborane motif has less effect on the Ru metal center than
[nido-7,8-C2B9H11]-. This could be due to the competitive
coordination to the nearby Ru center of the anionic [nido-
7,8-C2B9H11]- tag with the vinyl group of 11. Nevertheless,
at 30 °C the RCM activity of 9 and 10 was enhanced and
high conversions were achieved (>98%) after 2 h, which is
comparable with those of 4 under similar reaction condi-
tions.15
Although the Ru carbene complex bearing an anionic
phosphine ligand has been reported,7a to the best of our
knowledge, 10 is the first anion-appended Ru carbene
catalyst that possesses high actiVity to RCM reaction. Further
study showed that ring closure of N-protected substrates or
oxygen-containing substrates leading to either a five-, six-,
or seven-membered structure with a di- or trisubstituted
double bond was obtained in high conversions (>95%) by
either 9 or 10. Even the cross metathesis of steric bulk-
substituted vinyl phenol 25 was also achieved in good
conversion for both 9 and 10 (see Table 1).
a
Reactions were conducted at 30 °C using 0.5 mol % of catalyst in
CH2Cl2.
evaluated for their performance toward the RCM of 13 in
1-butyl-3-methylimidazolium hexafluorophosphate (BMI·
PF6) ionic liquids (ILs). As shown in Table 2, with low
catalyst loading (2.5%), the anionic [nido-7,8-C2B9H11]--
tagged catalyst 10 can be recycled and reused at least 10
times with only a slight decrease in activity with respect to
the activity for the first run, which is comparable to those
of the cation-appended ruthenium carbene catalysts. In
contrast, a dramatic decrease in the conversion of 13 was
found after the first run when 9 was used to promote RCM
in ILs. The result clearly indicates that 10 was binding into
the ILs via electrostatic interactions though the anionic [nido-
7,8-C2B9H11]- tag and thus solves the leaching problem that
has happened for 9 during the recycle and reuse of the
catalyst.
Moreover, the anionic [nido-7,8-C2B9H11]--tagged 10 is
a robust and recyclable RCM catalyst. Both 9 and 10 were
(14) (a) Wiesboeck, R. A.; Hawthorne, M. F. J. Am. Chem. Soc. 1964,
86, 1642-1643. (b) Garrett, P. M.; Tebbe, F. N.; Hawthorne, M. F. J. Am.
Chem. Soc. 1964, 86, 5016-5017. (c) Hawthorne, M. F.; Young, D. C.;
Garrett, P. M.; Owen, D. A.; Schwerin, S. G.; Tebbe, F. N.; Wegner, P. A.
J. Am. Chem. Soc. 1968, 90, 862-868. (d) Fox, M. A.; Gill, W. R.;
Herbertson, P. L.; MacBride, J. A. H.; Wade, K.; Colquhoun, H. M.
Polyhedron 1996, 15, 565-571. (e) Tomita, H.; Luu, H.; Onak, T. Inorg.
Chem. 1991, 30, 812-815.
In summary, we have developed closo-1,2-carborane-
tagged Ru carbene 9 and robust and recyclable [nido-7,8-
(15) Ritter, T.; Hejl, A.; Wenzel, A. G.; Funk, T. W.; Grubbs, R. H.
Organometallics 2006, 25, 5740-5745.
Org. Lett., Vol. 9, No. 21, 2007
4265

ment of immobilization of catalyst via electrostatic interac-
tions for different tasks in organic synthesis, the anionic
[nido-7,8-C2B9H11]- tagging strategy described here could
be applied to design other transition-metal-based catalysts.
Studies on the application of 10 with other cationic support
materials are currently underway in this lab.
Table 2. Comparative Recycling and Reuse of 9 and 10 in
BMI‚PF6
cycle (% convn.)
Acknowledgment. This work was supported by Tianjin
catalyst
1
2
3
4
5
6
7
8
9
10
University.
9
10
>98
30
-
-
-
-
-
-
-
-
>98a >98a >98a >98a >97a >97a >97a >97a >97b >95c
Supporting Information Available: Experimental details
and analytical data for this work described (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
a
b
c
Reaction time for each run: 1 h; 2 h; 4 h.
C2B9H11]--tagged 10 which is suitable for the noncovalent
binding concept. Given the growing interest in the develop-
OL7018218
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Org. Lett., Vol. 9, No. 21, 2007