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NN
NN
n
NN
R
N
N
Cl
Pd
Cl
Cl
Pd
Cl
N
N
N
PdCl
2
Mes
Pd
Mes
Ph
N
N
R
N
N
X
X
n = 2, 3, 4
551
X=I,NCS,CF
3
COO
552
R=Me,t Bu, B n
553
Figure 4.26
Structures of the precatalysts 51, 52 and 53.
Ph
Ph
Ph
Ph
Ph
N
Ph
N
N
O
N
N
N
N
N
N
O
O
N
N
Pd
Pd
Pd
N
N
Ph
Ph
NH
HN
Ph
N
N
O
N
O
O
Ph
Ph
O
O
O
Ph
54a
cis
-
54b
54b
trans
-
54b
Figure 4.27
Structures of some amide-based Pd-NHC complexes.
in the Heck reaction between 4-bromoacetophenone and n-butyl acrylate
(Table 4.20). It was shown that an increase in ring size correlates with an
increase in reactivity. This confirms that increased basicity and steric hin-
drance have a positive influence on the Heck coupling.
Shao and co-workers described the use of [Pd(IPr)(Im)Cl
2
](16a) in the
Heck reaction (Scheme 4.34).
120
The catalyst was effective in the coupling of
various aryl chlorides with styrenes. Both activated and deactivated chlorides
were successfully used and yielded the expected alkenes in good yields using
TBAB as the ionic liquid. Interestingly, the reaction could be conducted
under air.
More recently, Cazin and co-workers determined the reactivity of a range
of [Pd(NHC)(m-Cl)Cl]
2
complexes in Heck coupling (37, Figure 4.17).
121
SIPr
outperformed the other NHC ligands, permitting the coupling of various
activated and deactivated aryl bromides at catalyst loadings as low as 0.002
mol%, with TONs up to 49 500 (Table 4.21).
4.3.2.4 Heck Coupling in Water
To avoid the use of toxic solvents, reactions in water are always desirable.
In this section, some Pd-NHC systems are presented that allow Heck
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