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at room temperature, they were able to couple various aryl and heteroaryl
chlorides.
4.3.1.4 Stille Coupling
Owing to the toxicity of the stannyl derivatives, the Stille reaction
95
is being
used less and less as a cross-coupling reaction and has slowly been replaced
by more environmentally benign methods. Nevertheless, this reaction always
proved to be very ecient. There are very few reports in the literature of the
use of Pd-NHC systems, most of them being prior to the award of the Nobel
Prize in Chemistry in 2010.
In 1999, Herrmann and co-workers were the first to investigate the use of
well-defined Pd-NHC complexes to promote Stille coupling.
96
The mixed
phosphine-NHC complexes 42 were found to combine the stability of the
bis(carbene) complexes and the reactivity of the bis(phosphine) complexes.
PPh
3
was found to be the best phosphine to promote the coupling of various
bromides with tributylphenyltin, affording the corresponding coupling
products in good yields (Table 4.12).
In a similar study, Herrmann et al. confirmed that PPh
3
was better than
other phosphines at promoting the coupling.
97
The presence of an aromatic
Table 4.12 Pioneering work using Pd-NHC precatalysts for the Stille reaction.
X
SnBu
3
42
(1 mol %)
R
N
N
+
R
Toluene , 110°C
17 - 25 h
I
Pd
I
X=Cl,Br
PR
3
42
Yield (%)
a
Entry
X
Product
PR
3
11
1
Br
-
MeOC
499
2
Br
PPh
3
MeOC
9
3
Br
PCy
3
MeOC
68
4
Br
P(Fur)
3
MeOC
91
5
Br
PPh
3
82
6
Br
PPh
3
MeO
4
7
Cl
PPh
3
MeOC
a
By GC.
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