LETTER
O’Keefe, S. J.; Zaller, D. M.; Schmatz, D. M.; Doherty, J. B.
C–S Coupling with Heterogeneous NHC-Ni Catalyst
2521
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(18) Synthesis of Silica-Coated Magnetic Particles: A two-
necked round-bottomed flask (RBF) containing NH4OH
(250 mL) and deionized (DI) H2O (250 mL) was vigorously
stirred with an overhead stirrer at 500 rpm. A freshly
prepared aq solution of iron chloride (100 mL) containing
FeCl2·4H2O (2.57 g, 12.93 mmol) and FeCl3·6H2O (6.18 g,
22.6 mmol) was dropwise mixed with NH4OH solution for
30 min. The resultant mixture was stirred for additional 1 h
at r.t., then the formed MNPs were separated using a magnet
(4000 gauss). The MNPs were washed with DI H2O (20 ×)
and MeOH (5 ×) by repeating magnet separation and
decantation. The MNPs (1.35 g) dried in vacuo were treated
with 1% 3-aminopropyltriethoxysilane in CHCl3 (80 mL) at
60 °C for 4 h. The aminated MNPs were washed with CHCl3
(10 ×) and MeOH (5 ×) by repeating magnet separation and
decantation. The MNPs (1 g) dried in vacuo (elemental
analysis, N%: 0.382%, 0.27 mmol/g) were treated with 2%
TEOS (tetraethyl orthosilicate) in EtOH and shaken at 30 °C
for 12 h. The particles were washed with EtOH (6 ×)
repeating magnet separation and decantation.
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(20) Preparation of Catalyst 5 [MNP-Si-NHC(Pyr)-Ni] and
Catalyst 7 [MNP-Si-NHC(Bz)-Ni]: The aforementioned
magnetic nanoparticles (MNPs) were redispersed in toluene
and the resulting solution was heated to reflux. Then, 1-(3-
triethoxysilylpropyl)-2-imidazoline was added. After 24 h,
the solution was cooled to r.t. MNPs were concentrated
magnetically by using an external permanent magnet and
washed with toluene and EtOH. Imidazolinyl-functionalized
MNPs were redispersed and reacted with 2-picolyl chloride
or benzyl chloride at 80 °C in CHCl3 for 12 h, affording 4
and 6. After washing, the loadings of imidazoles on MNP-Si
were determined by their nitrogen contents by elementary
analysis (4, 0.47 mmol/g; 6, 0.67 mmol/g). The NHC ligand
functionalized MNPs were then redissolved in a mixture
of Ni(acac)2 at 60 °C in DMSO under basic conditions
(Scheme 2). After 12 h, the mixture was cooled to r.t., and
then catalyst 5 [MNP-Si-NHC(Pyr)-Ni] or catalyst 7 [MNP-
Si-NHC(Bz)-Ni] were magnetically separated by using the
external magnet. The prepared catalysts were washed with
CHCl3, EtOH, and H2O subsequently.
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(21) General Procedure for C–S Cross-Coupling Reaction: To
a mixture of 10 mol% of catalyst in DMF (1 mL) and Cs2CO3
(2 mmol) were added an aryl halide (1 mmol) and a thiol
Synlett 2010, No. 16, 2518–2522 © Thieme Stuttgart · New York