from the drawback that many of their catalytic active sites
are in the interior of the support. Therefore, they are, in
general, less accessible for the substrates so that it is often
necessary to use a high loading of these palladium-based
catalysts in a typical reaction. On the other hand, ionic liquids
(ILs) as environmentally acceptable solvents for organic
reactions have also attracted attention in recent years because
they have a very low vapor pressure and can be used to
replace organic solvents. Among them, imidazolium-based
ionic liquids have been extensively utilized as reaction media
for a number of palladium-catalyzed coupling reactions that
in fact act as in situ imidazole carbene ligands with transition
7
metals. However, ionic liquids are very expensive, and
therefore, for many practical applications, it is more desirable
to minimize the amount of ILs in reaction processes on the
basis of economic criteria. Very recently, Mehnert and co-
workers reported a new, interesting method for the nonco-
+
Valent immobilization of a homogeneous phosphine-Rh
4
complex/[bmim][BF ] solution onto the surface of silica
8
modified with a monolayer of ionic liquid. The catalyst was
then successfully used for hydrogenation and hydrofor-
8
mylation of olefins. However, it has been shown that the
metal complex can leach from the surface because the
4
adsorbed [bmim][BF ], which served as the reaction phase,
a
is partially dissolved in the organic phase and therefore
restricted long-period catalyst recovery. A possible strategy
to circumvent the aforementioned drawbacks could be based
on the simultaneous covalent immobilization of the metal
complex/ionic liquid matrix onto high-surface inorganic
solids such as silica. Along the line of this hypothesis, herein
we wish to disclose a new concept for in situ generation of
Pd-NHC complexes in an imidazolium-type ionic liquid
matrix (which is prefunctionalized with a trimethoxysilyl-
propyl group) and then simultaneous grafting of the whole
system on the surface of silica. The preparation procedure
for this new concept is shown in Scheme 1.
(a) (MeO)
0 °C Ar, 8 h. (c) 100 °C, 4 h. (d) SiO
3
Si(CH
2
)
3
Cl, toluene, reflux, 24 h. (b) Pd(OAc)
2
, 50-
6
2
, CHCl , reflux, Ar, 24 h.
3
The ionic liquid matrix N-3-(3-trimethoxysilylpropyl)-3-
methyl imidazolium chloride (1) was first synthesized by
the reaction of N-methylimidazole with the corresponding
(3-chloropropyl)trimethoxysilane in refluxing toluene. Then,
2
a sub-stoichiometric amount of Pd(OAc) was allowed to
react under an inert atmosphere with an excess of 1 at 50-
60 °C for 8 h and then at 100 °C for 4 h to afford a clear,
pale green-yellow solution (Figure 1, Supporting Informa-
tion). In situ C NMR analysis of this solution showed two
peaks at 160 and 161 ppm concerning two regioisomers of
9
1
3
2
, which clearly supports the formation of a NHC-Pd
(
4) For more recent leading references on heterogeneous palladium-
5,7d
catalyzed Heck reaction, see: (a) Crudden, C. M.; Sateesh, M.; Lewis, R.
J. Am. Chem. Soc. 2005, 127, 10045. (b) Liang, L.; Zhang, L. X.; Shi, J.
L.; Yan, J. N. Appl. Catal. A 2005, 283, 85. (c) Okumura, K.; Nota, K.;
Yoshida, K.; Niwa, M. J. Catal. 2005, 231, 245. (d) Shimizu, K. I.; Koizumi,
S.; Hatamachi, T.; Yoshida, H.; Komai, S.; Kodama, T.; Kitayama, Y. J.
Catal. 2004, 228, 141. (e) Pr o¨ kl, S. S.; Kleist, W.; Gruber, M. A.; K o¨ hler,
K. Angew. Chem., Int. Ed. 2004, 43, 1881. (f) Mandal, S.; Roy, D.;
Chaudhari, R. V.; Sastry, M. Chem. Mater. 2004, 16, 3714. (g) Horniakova,
J.; Raja, T.; Kubota, Y.; Sugi, Y. J. Mol. Catal. A: Chem. 2004, 217, 73.
h) Srivastava, R.; Venkatathri, N.; Ratnasamy, P. Tetrahedron Lett. 2003,
4, 3649. Choudary, B. M.; Mahdi, S.; Chowdari, N. S. J. Am. Chem. Soc.
002, 124, 14127. (i) K o¨ hler, K.; Heidenreich, R. G.; Krauter, J. G. E.;
Piestsch, J. Chem.-Eur. J. 2002, 8, 622. (j) Mori, K.; Yamaguchi, K.; Hara,
T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2002, 124,
complex. The resulting solution was then diluted with dry
CHCl and further reacted with SiO under reflux for 24 h
to form the corresponding coValently anchored NHC-Pd/
IL matrix system 3. FT-IR spectroscopy of 3 showed a new
broad absorption band at 1537 cm (CdC stretching) along
with bands at 1956, 2982 (for aliphatic C-H stretching),
and 3171 cm (unsaturated C-H stretching), respectively.
This clearly indicates the successful covalent attachment of
3
2
9
-1
-1
(
4
2
the above-mentioned matrix onto the silica surface (Figure
2
, Supporting Information). TGA/DTG analysis of 3 shows
1
1572.
5) Schwartz, J.; B o¨ hm, V. P. W.; Gardiner, M. G.; Grosche, M.;
Hermann, W. A.; Hieringer, W.; Raudaschl-Sieter, G. Chem.-Eur. J. 2000,
, 1773.
6) (a) Byun, J. W.; Lee, Y. S. Tetrahedron Lett. 2004, 45, 1837. (b)
Kim, J. H.; Kim, J. W.; Shokouhimehr, M.; Lee, Y. S. J. Org. Chem. 2005,
a weight loss due to the desorption of water below 100 °C
followed by a second small weight loss centered at 150 °C
which is owing to the loss of inner water molecules as well
as the loss of MeOH upon further condensation of unreacted
methoxy groups. This is finally followed by a set of weight
losses centered at 280 °C corresponding to the elimination
of the surface-bound organic groups (Figure 3, Supporting
Information). This indicates that catalyst 3 is thermally stable
up to 280 °C.
(
6
(
7
0, 6714.
(7) (a) Vallin, K. S.; Emilsson, P.; Larhed, M.; Hallberg, A. J. Org. Chem.
2
002, 67, 6243. (b) Mathew, C. J.; Smith, P. J.; Welton, T.; White, A. J.
P.; Williams, D. J. Organometallics 2001, 20, 3848. (c) Jain, N.; Kumar,
A.; Chauhan, S. M. S. Tetrahedron 2005, 61, 1015. (d) Xu, L.; Chen, W.;
Xiao, J. Organometallics 2000, 19, 1123.
(
8) (a) Mehnert, C. P.; Mozeleski, E. J.; Cook, R. A. Chem. Commun.
002, 3010. (b) Mehnert, C. P.; Cook, R. A.; Dispenziere, N. C.; Afework,
M. J. Am. Chem. Soc. 2002, 124, 12932.
2
(9) See Supporting Information for details.
1238
Org. Lett., Vol. 8, No. 6, 2006