chapter 2 palladium catalysts in suzuki cross- coupling reaction

commonly used ligand type in the literature since NHCs have high electron-donor
characteristics, promoting oxidative addition step. Even deactivated aryl chlorides can
be used effectively in the presence of NHCs (Miura 2004).
The application of NHC ligands, 13 (Figure 2.10), to the Suzuki–Miyaura
coupling was first reported in 1998 by Hermann (Hermann 1998) (Table 2.2, entry 9).
Palladium adducts of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 14
(IMes) (Figure 2.10), have been reported to be active catalysts for Suzuki cross-
couplings of a wide variety of aryl chlorides with arylboronic acids (Zhang et al. 1999)
(Table 2.2, entry 10).
2.2.2.4. Base Effect
Unlike other cross-coupling reactions involving transmetallation, the Suzuki
reaction requires the use of a base (Leadbeater 2005, Miyaura 2002).
Although the mechanism of oxidative addition and reductive elimination are
reasonably well understood, less is known about the transmetallation step because the
mechanism highly depends on reaction conditions, especially base used for the coupling
(Miyaura and Suzuki 1995). It has been suggested that there are mainly two role of the
base during the transmetallation;
1. to form a more electron-rich intermediate with the boronic acid which is more
reactive than the original boronic acid towards attack of Pd(II) complexes.
2. to form an alkoxy palladate from arylpalladium halide (Genet and Saignac
1999) (Figure 2.11).
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