support.4 Such PE oligomers exhibit thermomorphic so-
lubility and are soluble in hot nonpolar solvents but
completely insoluble at room temperature in any solvent.
Transition metal complexes bound to such oligomers can
catalyze homogeneous reactions at elevated temperatures
but can be isolated as solids on cooling. Recently, DuPont
reported the preparation of PEolig-bound porphyrin and
phthalocyanine metal complexes that can be used as catalysts
for radical polymerizations,5 and we recently described
similar PEolig-supported salen/metal catalysts.6 Here we
describe how PEolig-bound NHC ligands can be used in
ring-closing metathesis.
typically complete in <5 min. Recovery of 5 was effected
by cooling the reaction mixture to room temperature to
precipitate 5. Centrifugation was used to separate this solid
PEolig-bound Ru catalyst from the toluene solution of the
product. Added toluene facilitated separation of the liquid
product containing phase from the catalyst. This separa-
tion was followed by a second washing step with more
toluene to remove any residual product from the solid.
Removal of the toluene from the combined liquid phase
under reduced pressure yielded product that was pure by
1H and 13C NMR spectroscopy. Recycling experiments
were then carried out. The process of recycling small
amounts of a PEolig catalyst and handling of the small
amount of precipitated powder was aided by adding
additional PE that had not been functionalized with a
catalyst or ligand.4b,d,5 Using this method, complex 5 could
be recovered and reused up to four times in good yields.
In our initial studies, a thermomorphic PEolig-bound
NHC-Ru complex 5 was prepared using commercially
available PEolig-OH7 and 4 (Scheme 1). In this chemistry,
Scheme 1. Preparation of PEolig-Supported NHC-Ru Complex 5
Table 1. Recyclability Data for Catalyst 5a
conv (%)
substrateb
1
2
3
4
5
6
98
94
99
96
93
93
95
83
80
92
50
71
63
À
8
10
À
PEolig-OH was mesylated to form the PEolig-mesylate 1
(PEolig-OMs4c) which was then allowed to react with
N-mesityl imidazole 2 to give a PEolig-imidazolium mesy-
late salt whose counterion was exchanged with LiCl to
form 3. Then the PEolig-NHC-Ru complex 5 was prepared
by reaction of 3 with KHMDS and 4. After cooling, 5
was isolated by filtration as a brownish/green powder.
1H NMR spectroscopy of 5 showed the characteristic ben-
zylidene proton of an Ru benzylidene at 16.6 ppm.
The Ru complex 5 was then examined as a catalyst for
RCM using three different substrates (Table 1). These
experiments were conducted at 65 °C in degassed toluene.
Since one of our goals was to test if Ru leaching was a
problem, these reactions were carried out with 5 mol % of
the Ru catalyst. At this catalyst loading, the reactions were
a RCM reactions were carried out with substrate (0.41 mmol) and 5
(5 mol %) in toluene (3 mL) under N2 at 65 °C for 5 min unless otherwise
noted. b Yields based on 1H NMR conversions. Recycling of 5 consisted
of cooling the reaction mixture, adding 10 mL of toluene, followed by
centrifugation and decantation of the toluene containing products.This
process was repeated to remove any residual product.
Inspection of the isolated product 9 provided visual
evidence that using complex 5 gives rise to a product with
lower Ru contamination than is the case when 9 is formed
using other Ru catalysts for RCM (Figure 1). Comparison
of the color of 9 formed using a low-molecular weight
HoveydaÀGrubbs second generation catalyst, using a
PIB-bound Ru catalyst where Ru recovery is dependent on
a “boomerang” process that employed a liquid/liquid
separation, and using 5 shows that 5 is visually unconta-
minated by Ru. A more quantitative analysis of the
efficiency of 5 at removing Ru from the products was
evaluated by inductively coupled mass spectrometry (ICP-
MS) of the amount of Ru in 9 formed using 5. Analysis of
the product formed from reaction of 8 contained <0.65%
(77 ppm) of charged Ru, a value similar to the ca. 0.5% Ru
leaching seen in prior work from our group that used
similar Ru catalyst loadings and a liquid/liquid separation
of a PIB-bound NHC ligated Hoveyda-Grubbs catalyst.3
Furthermore, the results here for reactions that used 5 mol %
(4) (a) Bergbreiter, D. E. J. Polym. Sci., Part A: Polym. Chem. 2001,
39, 2351. (b) Bergbreiter, D. E.; Chen, Z.; Hu, H.-P. Macromolecules
1984, 17, 2111. (c) 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. (d) Bergbreiter, D. E.;
Chandran, R. J. J. Am. Chem. Soc. 1985, 107, 4792. (e) Bergbreiter,
D. E.; Osburn, P. L.; Wilson, A.; Sink, E. M. J. Am. Chem. Soc. 2000,
122, 9058.
(5) Older, C. M.; Kristjandsdottir, S.; Ritter, J. C.; Tam, W.; Grady,
M. C. Chem. Ind. 2009, 123, 319.
(6) Bergbreiter, D. E.; Hobbs, C.; Hongfa, C. J. Org. Chem. 2011, 76,
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nicaldatasheets (accessed April 2011).
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