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NN
NN
Pd
Pd
Cl
Cl
Ph
Ph
[Pd(SIPr)(cin)Cl]
7
[Pd(IPr)(cin)Cl]
6
Ph
Ph
Ph
Ph
p-Tol
p-Tol
p-Tol
p-Tol
NN
NN
Ph
Ph
p-Tol
Ph
p-Tol
Ph
p-Tol
p
-To l
Pd
Pd
Cl
Cl
Ph
Ph
[Pd(IPr*
Tol
)(cin)Cl]
778
[Pd(IPr*)(cin)Cl]
9
Figure 4.35 Precatalysts compared in the direct arylation of benzothiophene.
Table 4.24 C omparison of precatalysts under optimized
conditions for the direct arylation of benzothiophene.
[Pd(NHC)(cin)Cl]
(0.025 mol%)
Br
+
K
2
CO
3,
DMA
PivOH (30 mo l%)
140°C, 16 h
S
S
Conversion (%)
a
Precatalyst
[Pd(SIPr)(cin)Cl] (7)
80
[Pd(IPr)(cin)Cl] (6)
75
[Pd(IPr*)(cin)Cl] (9)
57
[Pd(IPr*
Tol
)(cin)Cl] (78)
58
a
By GC.
was used to study the scope of the reaction and similar results were obtained
to those using 76 (Scheme 4.54).
In 2012, Nolan's group also investigated the use of well-defined Pd-NHC
precatalysts in the arylation of heterocycles.
155
The reactivities of four dif-
ferent [Pd(NHC)(cin)Cl] complexes were investigated and the effect of the
bulk of the NHC ligand was studied (6, 7, 9, 78, Figure 4.35). Bulky ligands
(IPr* and IPr*
Tol
) were not found to be advantageous and SIPr was the most
ecient NHC to promote the reaction (Table 4.24). s-Donation did not play a
crucial role, as IPr and SIPr gave similar results. Benzothiophene, thiophene
and imidazopyridine were successfully and e
ciently arylated at low catalyst
loadings (0.1-0.01 mol%) using deactivated, activated and sterically hin-
dered aryl bromides (Scheme 4.55).
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