Chemistry Reference
In-Depth Information
Table 4.7 Recyclability of 29.
29
(0.5 mol%)
KO
t
Bu
i
PrOH, 80°C
MeOC
Cl
+
B(OH)
2
MeOC
Cycles
1
2
3
4
5
6
7
8
9
10
Ti e (h)
4
4
4
4
4
4
5
5
6
7
Yield (%)
91
93
92
92
93
90
87
89
82
83
proved to be both active and recyclable for the Suzuki reaction,
68
and also
magnetic IPr-functionalized silica nanoparticles (30, Figure 4.16).
69
After
coupling, the latter material could be isolated by applying an external
magnetic field and therefore could be reused without loss of activity. An-
other interesting contribution by Alam and Sarkar highlights the use of
the nanostructured mesoporous solid supported catalyst MCM41 (31,
Figure 4.16).
70
This heterogeneous catalyst was successfully used in the
Suzuki coupling of aryl bromides and iodides and could be reused after
simple filtration from the reaction mixture.
4.3.1.1.8 Suzuki Coupling Using Pseudohalides. Halides are most often
the coupling partner of choice for Suzuki coupling. However, some recent
examples involving pseudohalides can be highlighted. For example, in
2010 Luo and co-workers reported the reaction with aryltriazenes using a
polymer-supported Pd-NHC catalyst (32, Table 4.8).
71
The coupling pro-
ceeded smoothly and the biaryls were obtained in good yields at room
temperature. Interestingly, iodide and bromide atoms were not affected by
the reaction conditions and hence could be tolerated by the system. Of
note, the catalyst is also recyclable.
Arylsulfonates were also found to be suitable coupling partners for the
first time using a Pd-NHC precatalyst.
72
Both tosylates and mesitylates af-
forded the expected biaryls using 1 mol% of [Pd(IPr)(Im)Cl
2
] 16a,
Figure 4.10), K
3
PO
4
H
2
O as the base in morpholine as the solvent. However,
tosylates were better coupling partners (Scheme 4.12).
4.3.1.2 Negishi Coupling
Despite being one of the first cross-couplings to have been discovered,
73
the
Negishi reaction appears to be far less developed than the Suzuki-Miyaura
reaction. This is nevertheless a very powerful tool, notably because of its high
tolerance to functional groups. The coupling involves the reaction between a
halide or pseudohalide and an organozinc derivative. In this section, we
focus on the main advances in the field using Pd-NHC systems, before and
after the award of the 2010 Nobel Prize in Chemistry.
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