Chemistry Reference
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Pd(OAc)
2
(3 mol%)
IPr.HCl (3 mol% )
X
R'
+
R'
Si(OMe)
3
R
R
TBAF
1,4-dioxane / THF
60 - 80°C
X=Br,Cl
MeOC
MeO
X=Br:100%
X=Cl:100%
X=Br:93%
X=Cl:29%
X=Cl:19%
MeOC
N
NC
X=Br:81%
X=Cl:81%
X=Br:100%
X=Cl:100%
X=Cl:100%
Scheme 4.30 Reactivity of the in situ-generated system in Hiyama cross-coupling.
N
NN
NN
R
R'
R
R
Cl
Pd
Cl
Br
Pd
Br
N
N
43a
:R=2,6-
i
Pr
2
C
6
H
3
43b
: R = 2,6-Et
2
C
6
H
3
43c
: R = 2,4,6-Me
3
C
6
H
2
43d
:R=2,6-Me
2
C
6
H
3
43e
:R=
i
Pr , R' = CH
2
Ph
43f
:R=CH
2
CONH
t
Bu, R' = CH
2
Ph
Figure 4.20
Structure of the well-defined complexes used in Hiyama coupling.
It was not until 2009 that a second report of Hiyama coupling using a Pd-
NHC system appeared, when Ghosh and co-workers reported the use of well-
defined PEPPSI-type precatalysts in the reaction (43, Figure 4.20).
102
Various
improvements to the original protocol were outlined: the use of a fluoride
source was not necessary to promote the reaction and the coupling occurred
in air and in a mixed aqueous medium (dioxane-water). Because of the
similar electronegativities of Si and C, the silane is a poor nucleophile. This
explains why Hiyama coupling generally requires fluoride anions in order to
proceed. Nevertheless, in this report, NaOH promoted the reaction. Using
this system, aryl bromides could be eciently coupled to phenylsiloxane;
however, 4-chloroacetophenone was found to be unreactive and vinyl-
trimethoxysilane reacted in a sluggish manner (Table 4.14).
In the same year, Chen and co-workers reported the use of complex 44 in
the cross-coupling (Scheme 4.31).
103
The reaction between 4-chloro-
acetophenone and phenyltrimethoxysilane was found to be successful in this
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