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R
8
(0.2 - 2 mol% )
R
R'
R'
X
+
B(OH)
2
KO
t
Bu, toluene
RT - 65°C
X=Cl,Br
O
O
N
N
O
X=Br:96%
X=Br:95%
X=Br:88%
O
O
X=Cl:86%
X=Cl:78%
X = Cl: 72%
Scheme 4.4 Preparation of tetra-ortho-substituted biaryls using [Pd(anti-(2,7)-
SICyoctNap)(cin)Cl] (8).
Table 4.2 Comparison of ligand bulk in [Pd(NHC)(cin)Cl].
Entry
NHC in [Pd(NHC)(cin)Cl]
%V
bur
1
IPr
36.7
2
SIPr
37.0
3
anti-(2,7)-SICyoctNap
42.0
4
IPr*
44.6
Inspired by the work of Dorta and co-workers and noting that bulky lig-
ands were key to the success of the reaction, Nolan's group next decided to
apply [Pd(IPr*)(cin)Cl] (9) to this challenging transformation.
24
The IPr*
ligand was found to be bigger than the previously reported anti-(2,7)-
SICyoctNap (Table 4.2) and the reactivity was found to be slightly superior.
Notably, 1 mol% of the complex was sucient to promote the coupling at
room temperature. The combination between KOH and DME was found to
be the best system in this case, allowing the preparation of a wide range of
biaryls at room temperature or using gentle heating (65 1C) (Scheme 4.5). It
should be noted that the use of KOH is preferable to KOtBu in this case as
KOH is both cheaper and milder than KOtBu. Steric maps of the
[Pd(IPr*)(cin)Cl] complex exhibited a similar twist of the ligand around the
metal centre.
In 2012, Tu et al. investigated the reactivity of robust acenaphthoimida-
zolylidene PEPPSI-based palladium complexes in the preparation of tetra-
ortho-substituted biaryls.
37
Complex 10 (Figure 4.7) proved highly ecient in
the reaction in comparison with well-established PEPPSI systems reported in
the literature. They attributed this higher activity to a stronger s-donation
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