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O
[Pd(IPr)(allyl)Cl] (84)
(10 mol%)
N
I
+
N
R
R
K
3
PO
4,
toluene, 90°C
CO balloon
NN
O
OTBS
O
OMe
O
O
N
N
N
N
Pd
BocN
Cl
81%
79%
85%
82%
[Pd(IPr)(allyl)Cl]
84
O
O
O
S
N
N
N
87%
57%
35%
Scheme 4.67 Carbonylative amination using [Pd(IPr)(allyl)Cl].
investigated and also the [Pd(IPr)(allyl)Cl] complex (84). The latter (10 mol%)
was found to be particularly active to promote the reaction in toluene at
90 1C in the presence of K
3
PO
4
as the base and using a simple CO balloon as
the carbonyl source. This permitted the synthesis of an interesting library of
acylpyrrole compounds in good yields. Notably, the reaction also worked
with activated aryl bromides but failed with other aryl bromide congeners.
4.3.6.1.4 Alternative Bases. The use of alternative and milder bases to in-
crease functional group tolerance in the amination reaction is very import-
ant, because the commonly used metal alkoxide bases sometimes lead to
undesired side reactions. Whereas KOtBu does not tolerate 4-chloro-
benzoate, potassium 2,2,5,7,8-pentamethylchroman-6-oxide (98,pK
a
E
11.4)
was found to be tolerated by the substrate.
186
Moreover, the reactivity of the
base in amination was discovered to be between those of KOtBu and Cs
2
CO
3
using [Pd(IPent)(PEPPSI)] (4) as the precatalyst. Any attempt to modify the
structure of the base resulted in lower reactivity than that of the chroman
oxide derivative. A small range of compounds was prepared to show the ad-
vantage of the new base over KOtBu (Scheme 4.68).
4.3.6.2 C-S Bond Formation
The formation of C-S bonds using Pd-NHC precatalysts has hardly been
reported in the literature. This is notably because sulfur species deactivate
the palladium catalysts.
The first report was published in 2010 by Liu and co-workers, who pre-
pared three complexes to investigate the reaction: the monocarbene dimer
complex 99a, the biscarbene complex 99b and the mixed phosphine-NHC
complex 99c (Figure 4.42).
187
Reaction conditions were optimized using
4-bromoacetophenone and 4-methylbenzenethiol as substrates, highlighting
the eciency of 99c in the reaction. KOtBu and toluene were selected as the
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