Several decompostion pathways involve bimetallic species
due to the propensity for ruthenium to dimerize via thermo-
dynamically stable chloride (4 and 6) or carbide bridges (5)
(Figure 2).4b-e One approach to prevent these undesirable
cannot realize all the benefits of solid-phase catalysis, such as
desirable continuous flow processes.
Scheme 1. Synthesis of Ruthenium Complexes 15 and 16
Figure 2.
Various dimeric ruthenium decomposition products.4
bimolecular decomposition pathways is to immobilize the
catalyst onto a solid support that inhibits intermolecular
catalyst-catalyst interactions via site isolation.6 Furthermore,
supported catalysts have the added advantages of generating
products free of ruthenium contamination and possess the ability
to be recovered and subsequently recycled. A number of reports
have been published employing various strategies to obtain
solid-supported olefin metathesis catalysts.7 These consist of
anchoring the catalytic moiety, via a number of positions within
the catalyst framework, to a variety of solid supports, such as
organic polymers or inorganic oxides. Of the various strategies,
immobilization through a chelating benzylidene has been the
most widely employed. These catalysts operate via a release/
return phenomenon8 with all the catalytic activity arising from
a homogeneous species, which is susceptible to the same
bimetallic decomposition pathways. Likewise, such systems
(4) (a) Ulman, M.; Grubbs, R. H. J. Org. Chem. 1999, 64, 7202–7207.
(b) Amoroso, D.; Yap, G. P. A.; Fogg, D. E. Organometallics 2002, 21,
3335–3343. (c) Amoroso, D.; Snelgrove, J. L.; Conrad, J. C.; Drouin, S. D.;
Yap, G. P. A.; Fogg, D. E. AdV. Synth. Catal. 2002, 344, 757–763. (d)
Hong, S. H.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2004, 126,
7414–7415. (e) Hong, S. H.; Wenzel, A. G.; Salguero, T. T.; Day, M. W.;
Grubbs, R. H. J. Am. Chem. Soc. 2007, 129, 7961–7968. (f) Hong, S. H.;
Chlenow, A.; Day, M. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 46,
5148–5151. (g) van Rensburg, W. J.; Steynberg, P. J.; Meyer, W. H.; Kirk,
M. M.; Forman, G. S. J. Am. Chem. Soc. 2004, 126, 14332–14333.
(5) (a) Berlin, J. M.; Campbell, K.; Ritter, T.; Funk, T. W.; Chlenov,
A.; Grubbs, R. H. Org. Lett. 2007, 9, 1339–1342. (b) Stewart, I. C.; Ung,
T.; Pletnev, A. A.; Berlin, J. M.; Grubbs, R. H.; Schrodi, Y. Org. Lett.
2007, 9, 1589–1592. (c) Anderson, D. R.; Lavallo, V.; O’Leary, D. J.;
Bertrand, G.; Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 46, 7262–7265.
(d) Vougioukalakis, G. C.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130,
2234–2245. (e) Chung, C. K.; Grubbs, R. H. Org. Lett. 2008, 10, 2693–
2696.
Other strategies involve immobilization via alternative X-type
ligands that replace the ancillary chlorides, such as fluorinated
carboxylates,9 or via functionalized NHC ligands.10 The latter
is a very attractive approach as the NHC forms a strong bond
to the ruthenium center and is the most substitutionally inert
ligand within the catalyst coordination sphere.11 Herein, we
report the syntheses of two triethoxysilyl functionalized NHC
ligands (10 and 13) and their use in the generation of ruthenium
complexes 15 and 16.12 The subsequent grafting of these
complexes onto silica is described and the utility of the silica-
supported analogues as heterogeneous catalysts is evaluated.
The synthesis of 10 began with the allylation of bisimine 7
followed by reduction to furnish diamine 8. This was treated
with HCl and triethyl orthoformate to generate the imidazolium
(6) (a) Collman, J. P.; Belmont, J. A.; Brauman, J. J. J. Am. Chem. Soc.
1983, 105, 7288–7294. (b) Drago, R. S.; Pribich, D. C. Inorg. Chem. 1985,
24, 1983–1985. (c) Tollner, K.; Popovitv-Biro, R.; Lahav, M.; Milstein, D.
Science 1997, 278, 2100–2102. (d) Annis, D. A.; Jacobsen, E. N. J. Am.
Chem. Soc. 1999, 121, 4147–4154.
(7) For recent review articles, see: (a) Buchmeiser, M. R. New. J. Chem.
2004, 28, 549–557. (b) Coperet, C.; Basset, J.-M. AdV. Synth. Catal. 2007,
349, 78–92. (c) Clavier, H.; Grela, K.; Kirschning, A.; Mauduit, M.; Nolan,
S. P. Angew. Chem., Int. Ed. 2007, 46, 6786–6801.
(8) (a) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda,
A. H. J. Am. Chem. Soc. 1999, 121, 791–799. (b) Hoveyda, A. H.;
Gillingham, D. G.; van Veldhuizen, J. J.; Kataoka, O.; Garber, S. B.;
Kingsbury, J. S.; Harrity, J. P. A. Org. Biomol. Chem. 2004, 2, 8–23.
1262
Org. Lett., Vol. 11, No. 6, 2009