Chemistry Reference
In-Depth Information
Table 4.14 Hiyama coupling using well-defined Pd-NHC complexes.
X
R'
43a
-
f
(2 mol%)
+
R'
Si(OMe)
3
R
R
NaOH
1,4-dioxane / H
2
O
80°C
X=I,Br,Cl
Yield (%)
a
43a
Entry
X
Product
43b
43c
43d
43e
43f
499 499 499 499 499
96
1
Br
MeOC
30
7
14
22
31
7
2
Cl
MeOC
499
80 499 499 499 499
3
Br
61
72
81
89
86
93
OMe
4
Br
15
28
41
45
17
22
5
Br
MeOC
41
29
27
22
32
26
6
I
MeO
a
By GC.
X
R
44
44 (1 mol%)
+
Ph
Si(OMe)
3
R
Cs
2
CO
3,
TBAF
1 ,4-dioxane, 80°C
N
N
N
X = Br , Cl
N
Pd
Cl
MeOC
MeO
Cl
4 4
X = Br : 95%
X = Cl: 91%
X = Br : 93%
X = Cl: 47%
X = B r: 81%
X = Cl: Trace
Scheme 4.31 Reactivity of complex 44 in Hiyama cross-coupling.
case in the presence of TBAF. Unfortunately, unactivated and deactivated
chlorides were unreactive.
After 2010, Yus and co-workers developed hydroxy-functionalized imida-
zolium salts for microwave-assisted Hiyama coupling (45, Table 4.15).
104
Like Ghosh and co-workers' system, the reaction uses aqueous NaOH in-
stead of TBAF as the reaction promoter. Microwave irradiation considerably
accelerated the coupling. Surprisingly, the in situ-generated system proved
slightly more active than the well-defined complex and the authors preferred
to use it to explore the scope of the reaction. A wide range of (hetero)biaryls
were thus prepared e
ciently using 0.1 mol% of Pd(OAc)
2
, 0.2 mol% of the
NHC ligand and NaOH (50% aq.) under microwave irradiation for 1 h. In
some cases, TBAB was also used as an additive to increase the reaction yield.
Search WWH ::
Custom Search