Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 395

O

Cl

Pd(OA c) 2 (1.5 mol%)
Bu 4 NB r-Bu 4 NAc
12 0°C, 3 h

+

MeO

O

90%

MeO

Scheme 9.55 Example of arylation by an electron-rich aryl chloride in a ''phosphine-
free'' system in an ionic liquid medium.

CO 2 Bu

Br CO 2 Bu

Pd(OAc) 2
Bu 4 NBr-Bu 4 NAc
100°C, 0.5 h

Cl Cl

CO 2 Bu

Ph

120°C, 3 h

Ph

Scheme 9.56 Example of sequential alkenylation in an ionic liquid.

Cl Ph

(10) (0.04 mol%)
Ca(OH) 2, NMP
160°C, 6 h

MeO MeO 75%

NO N
Pd

P d

N O N

2ClO 4 -

(10)

Scheme 9.57 Stable Pd chelate with macrocyclic ligand (10) as precatalyst in the

Mizoroki-Heck reaction of electron-rich chloroarenes.

with styrene to give the respective stilbene in high yield without the need to
use the standard stabilizing agent Bu 4 NBr (Scheme 9.57). A very low loading
of precatalyst (0.0375 mol% Pd) is used. Given that apparently active Pd
species are slowly and partially released from the precatalyst, it is evident
that Pd species can be fairly reactive in the oxidative addition to an unac-
tivated C-Cl bond without special stable accelerating ancillary ligands.
Another recent example of fitting the precatalyst to the reaction of unac-
tivated aryl chlorides and bromides was reported by Pons and co-workers,
who used macromolecular chelators (11-13) (Scheme 9.58). 220 Chloro- and