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Pd pre-catalyst
R'
R
N
L
n
Pd(0)
R''
R
X
Reductive
elimination
Oxidative
addtion
R
R
R'
L
n
Pd(II)
N
Pd(II)L
n
R
X
R''
R'
H
N
Pd(II)L
n
R''
X
+ base
HNR'R''
Base
or
R
Base.HX
L
n
Pd(II)
OR
+
HNR 'R' '
Amine
coordination
Scheme 4.59 Mechanism of the amination reaction.
new ligands with various steric and electronic properties, in order to pro-
mote cross-coupling reactions, has been repeatedly emphasized. In the
palladium-catalysed amination reaction, recent efforts have been devoted
to the design of new bulky ligands.
9e
For example, in 2011, Organ and co-
workers reported the use of [Pd(IPent)(PEPPSI)] for amination using sec-
ondary alkyl amines.
169
The reactivity of the latter complex was directly
compared with that of the [Pd(IPr)(PEPPSI)] congener. The IPent ligand
had a tremendous effect on the catalysis, allowing the coupling of various
aryl chlorides and amines, which was hardly possible with the IPr-based
system (Scheme 4.60). The conditions described by Organ and co-workers
utilized Cs
2
CO
3
as the base, affording great functional group tolerance.
However, high catalyst (4 mol%) and base (3 equiv.) loadings were usually
required to achieve excellent results. In the same report, rate and compu-
tational calculations were also investigated to understand better the differ-
ences between the two catalytic systems. The rate studies highlighted the
fact that [Pd(IPr)(PEPPSI)] is strongly dependent on the electronic nature
of the chloride, whereas [Pd(IPent)(PEPPSI)] is less sensitive. The computa-
tional studies showed that amine coordination/deprotonation was the rate-
limiting step. At this stage, the IPent complex was found to bind to the
amine much more strongly than the IPr analogue. The combination of the
two factors resulted in a much more active catalyst
for amination
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