of catalytically active units. Our hypothesis takes advantage
of the self-supporting strategy, and it could also be conceptu-
ally possible to generate self-supported catalysts or catalyst
precursors with which the catalysis can be conducted
homogeneously; however, the catalysts could be recovered
heterogeneously. In principle, the coupling of an active
catalyst, bearing two active sites with a ligand having two
ligating moieties, would provide the self-supported oligo-
meric (or polymeric) catalyst precursor.
generation of self-supported oligomeric Grubbs/Hoveyda-
type Ru-carbene complexes for ring-closing metathesis.
To generate the self-supported oligomeric Grubbs/Ho-
veyda-type Ru-carbene complex, as shown in Scheme 1,
Scheme 1a
The prerequisite of this strategy would be that the
oligomeric catalyst precursors should be dissociated under
the catalysis condition to generate a homogeneous active
catalyst, and after catalysis, it should be reassociated to
regenerate the oligomeric catalyst precursor again. In this
respect, the structural feature and release and return metath-
esis mechanism of the Grubbs-Hoveyda Ru complex 1 may
provide an excellent opportunity for the generation of this
type of self-supported oligomeric catalyst precursor (Figure
1).3 In the presence of olefinic substrates, the catalytically
a Reagents and conditions: (a) NaH, THF, R,R-dibromo-p-
xylene, rt, 19 h, 78%; (b) HC(OEt)3, NH4BF4, toluene, 110 °C, 6
h, 57%; (c) KHMDS, toluene, rt, 15 min, then (Cy3P)2Cl2Ru-
(dCHPh), 3 h, 48%; (d) NaH, R,R-dibromo-p-xylene, THF, rt, 17
h, 98%.
dimeric dihydroimidazolium tetrafluoroborate 5 and isopro-
poxystyrene 8 having a 1,4-bismethylenephenyl linker were
synthesized starting from 36 and 7,7 respectively. Reaction
of the hydroxy bismesityl 3 with R,R-dibromo-p-xylene
followed by ring formation using triethylorthoformate8
provided the desired dimeric dihydroimidazolium tetrafluo-
roborate 5 in two steps in 45% yield. Reaction of the dimeric
N-heterocyclic carbene generated in situ from 5 with Grubbs
first-generation Ru complex (Cy2P)2Cl2Ru(dCHPh) in tolu-
ene afforded the novel dimeric Grubbs second-generation
Ru-carbene complex9 6 in 48% yield.10
Figure 1. Schematic representation of the self-supported oligomeric
Grubbs/Hoveyda-type Ru complexes.
First, the coupling efficieny of the dimeric Ru-carbene
complex 6 with isopropoxybenzylidene was examined in the
presence of CuCl in methylene chloride at 40 °C affording
active Ru-methylidene species can be dissociated from the
oligomeric matrix of 2, which would be reassociated after
catalysis to form the corresponding catalyst precursor.4
Although numerous immobilization methods to recover
metathesis Grubbs/Hoveyda-type Ru catalysts have been
reported,5 no work has been done to demonstrate the validity
of this strategy to generate self-supported metathesis Ru
complexes. Herein, we report the preliminary results of the
(6) Mayer, M.; Buchmeiser, M. R.; Wurst, K. AdV. Synth. Catal. 2002,
344, 712.
(7) Yao, Q. Angew. Chem., Int. Ed. 2000, 39, 3896.
(8) Hong, S. H.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 3505.
(9) Only few examples of dimeric Ru complexes have been reported to
date. See: (a) Varray, S.; Lazaro, R.; Martinez, J.; Lamaty, F. Organome-
tallics 2003, 22, 2426. (b) Yao, Q.; Motta, A. R. Tetrahedron Lett. 2004,
45, 2447. (c) Mayershofer, M. G.; Nuyken, O.; Buchmeiser, M. R.
Marcromolecules 2006, 39, 3484.
(3) (a) Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda,
A. H. J. Am. Chem. Soc. 1999, 121, 791. (b) Garber, S. B.; Kingbury, J. S.;
Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 8168.
(4) (a) Grubbs, R. H. Handbook of Metathesis; Wiley-VCH: Weinheim,
2003; Vols. 1-3. (b) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001,
34, 18.
(5) For selected immobilization of metathesis Ru catalysts, see: (a)
Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem.
Soc. 2000, 122, 8168. (b) Yao, Q. Angew. Chem., Int. Ed. 2000, 39, 3896.
(c) Jafarpour, L.; Nolan, S. P. Org. Lett. 2000, 2, 4075. (d) Connon, S. J.;
Dunne, A. M.; Blechert, S. Angew. Chem., Int. Ed. 2002, 41, 3835. (e)
Yao, Q.; Zhang, Y. Angew. Chem., Int. Ed. 2003, 42, 3395. (f) Clavier, H.;
Audic, N.; Mauduit, M.; Guillemin, J. C. J. Am. Chem. Soc. 2003, 125,
9248. (g) Yao, Q.; Zhang, Y. J. Am. Chem. Soc. 2004, 126, 74. (h)
Buchmeiser, M. R. New J. Chem. 2004, 28, 549. (i) Matsugi, M.; Curran,
D. P. J. Org. Chem. 2005, 70, 1636. (j) Lee, B. S.; Namgoong, S. K.; Lee,
S.-g. Tetrahedron Lett. 2005, 46, 4501.
(10) In a nitrogen-filled drybox, a solution of imidazolium salt 5 (40.0
mg, 0.042 mmol) and potassium bis(trimethylsilyl)amide (KHMDS, 18.46
mg, 0.092 mmol) in toluene (2 mL) was stirred for 15 min, and then a
solution of Grubbs catalyst first generation (76.16 mg, 0.092 mmol) in
toluene (1 mL) was added. The reaction mixture was transferred to a Schlenk
flask. The flask was capped and removed from the drybox and stirred at
room temperature for 3 h. After evaporation of toluene under a vacuum,
the residue was purified by column chromatography on silica gel (n-hexane/
EtOAc ) 4:1) to give the product 6 (38 mg, 0.02 mmol, 48.4%) as a dark
1
brownish solid. H NMR (CDCl3): δ 19.13 (s, 2H), 8.98 (br s, 2H), 7.35
(m, J ) 8.6 Hz, 2H), 7.15-6.99 (m, 14H), 6.94-6.56 (m, 2H), 5.96 (br s,
2H), 4.40-4.32 (m, 6H), 4.13-3.83 (m, 4H), 3.78-3.21 (m, 4H), 2.76-
2.39 (m, 9H), 2.19-2.15 (s, 9H), 2.04-1.89 (m, 12H), 1.51-1.27 (m, 36H),
1.00-0.91 (m, 36H). 31P NMR (CDCl3) δ 51.3 ppm. Anal. Calcd for
C102H140Cl4N4O2P2Ru2: C, 65.86; H, 7.59; N, 3.01. Found: C, 65.82; H,
7.64; N, 3.52.
3846
Org. Lett., Vol. 9, No. 19, 2007