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Cl
MgBr
R
37a
(0.2 - 0.45 mol%)
+
R
R'
TH F , R T - 6 0°C
1-16h
R'
OMe
F
O
OMe
O
91%
56%
81%
>99%
OMe
S
OMe
N
97 - 99%
89 - 96%
69%
35%
Scheme 4.26 Reactivity of [Pd(SIPr)(m-Cl)Cl]
2
(37a) in Kumada cross-coupling.
NN
NN
NN
Cl
Pd
Cl
Cl
Pd
Cl
Cl
Pd
Cl
N
N
N
Cl
15
38
39
Figure 4.18 Three [Pd(NHC)(PEPPSI)] complexes investigated in a structure-activity
relationship study of the Kumada reaction.
tetra-ortho-substituted biaryls was also reported for the first time using a
Pd-NHC precatalyst. A slight increase in the catalyst loading was required
(0.45 mol%) but the highly sterically hindered products were obtained in
moderate yields (two examples, 35-69%).
Organ and co-workers also studied Kumada coupling using a range of
[Pd(NHC)(PEPPSI)] complexes.
92
In a structure-activity relationship study, it
was shown that the IPr ligand was crucial for the reaction to occur. More-
over, the effect of the substitution of the pyridine throwaway ligand was also
studied (Figure 4.18). Both 3-chloropyridine and pyridine showed good ac-
tivity, with a slight advantage for the pyridine moiety (Tables 4.10 and 4.11).
Wu and co-workers developed carbene adducts of cyclopalladated ferro-
cenylimine for the Kumada reaction.
93
The IPr derivative was found to be the
most active in comparison with other carbenes (IMes or an abnormal car-
bene) and some phosphines (PPh
3
and PCy
3
). A wide range of biaryls were
eciently prepared under mild conditions (r.t.-60 1C) using complex 40
(0.5 mol%, Scheme 4.27).
Another interesting contribution is that of Jin et al.
94
They employed four
different [Pd(NHC)(Cp)Cl] complexes in the reaction. In this case, they found
the best ligand to be SIMes (41, Figure 4.19). Using 1 mol% catalyst loading
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