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Pd pre-catalyst
Base.HX
L
n
Pd(0)
R
X
Base
Oxidative
addtion
X
X
L
n
Pd(II)
H
L
n
Pd(II)
R
R'
R'
β
-hydride
elimination
R
L
X
R'
X
R'
Pd(II)L
n
Pd(II)L
n-1
R
Migratory
insertion
H
H
R
L
Scheme 4.33 General mechanism of the Heck reaction.
described earlier in Section 4.3.1. Indeed, the precatalyst is first activated to
its active Pd(0) form, then oxidative addition of the halide or pseudohalide
takes place. As has already been discussed, chlorides are both the most
dicult but also the most desirable substrates to activate. The following step
is fundamentally different, as it involves the coordination of the alkene to
the metal centre instead of a transmetallation. Next, a migratory insertion
allows the formation of the desired C-C bond. Finally, the product is re-
leased after b-hydride elimination and the Pd(0) active species is regenerated
by the action of a base.
4.3.2.1 The Beginning
The use of Pd-NHC systems in the Heck reaction was pioneered by
Herrmann in 1995
19a
and 1998.
19b
Using complexes 47a and 47b and an
in situ-generated system (47c), the coupling of deactivated bromides, acti-
vated bromides and activated chlorides with n-butyl acrylate was successfully
achieved at high temperatures (Figure 4.22, Table 4.17). The stability of the
Pd-NHC bond was found to be essential to the success of the reaction at
such high temperatures. The use of the in situ-generated system even per-
mitted TONs up to 250 000. No induction period was observed in com-
parison with the well-defined system. Notably, for the coupling of aryl
chlorides, [N(nBu)
4
]Br was used as an additive, acting as a reducing agent.
After this discovery, the Heck reaction became a proof of concept reaction
for assessing the reactivity of new catalytic systems, like the Suzuki-Miyaura
reaction. Many reports exist on the reactivity of Pd-NHC complexes in the
Heck reaction and very interesting reviews already cover most of the work.
9c,d
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