Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 400

(reagent concentrations, palladium loading, time, temperature), they
showed that the performance of the catalytic system, including palladacyclic
precursors based on different scaffolds, is practically insensitive to the na-
ture of the precatalyst, and also base and solvent taken from a series com-
monly used for such reactions. The structural type of palladacycles taken for
the study (hybrid monomeric complexes with a highly labile tosylate ligand
to facilitate ligand exchange) permitted effective preactivation under rela-
tively mild conditions. This study was performed not with a reactive aryl
halide and standard olefin, but with a more challenging case using an
electron-rich aryl bromide and ethylene, which even better reflects the
relevance of the results for adequate positioning of catalytic systems based
on such precatalysts.
Among the most unusual SRPCs is palladium chloride deposited on wool,
reported by Lei and co-workers. 229b However exotic and unusual this ma-
terial might seem, its reported performance is very high. The precatalyst was
able to service the reactions of unactivated bromoarenes not only with
standard olefins, but also with some disubstituted alkenes (methacrylates,
1,1-diphenylethylene, a-methylstyrene) in high yields under remarkably mild
conditions at 80 1C in neat water in the presence of a very small amount of
PEG-400 (0.2% solution, which is much weaker than the usually used con-
centrations of such phase-transfer agents) (Scheme 9.66). Under such con-
ditions, the reaction systems are heterogeneous and probably the only way
for such a system to work is via absorption of organic reagents on the surface
of the wool.
Among other systems that have been reported since 2008 is 2,2 0 -diamino-
6,6 0 -dimethyldiphenyl, used as a ligand in a typical system of Pd(OAc) 2
(1 mol%), L, K 2 CO 3 , DMF, 140 1C, exhibiting modest activity for Mizoroki-
Heck reactions of aryl iodides and bromides with standard olefins and just a
single example of an unactivated aryl bromide. 230 Similar activity was reported
for the Pd complex of the tetraphosphine N,N,N 0 ,N 0 -tetra(diphenylpho-
sphinomethyl)-1,2-ethylenediamine. 231 N-C-N pincers, derivatives of iso-
xazole, such as 22 (Scheme 9.67) and isothiazole showed typical performances

PdCl 2 on wool
( 5.5 mol%)
NaOAc, PEG-400
80°C, 24 h

Br CO 2 Me

+

CO 2 Me

94%

Scheme 9.66 Palladium on wool as an effective precatalyst.

O O

N Pd

Br
(22)

N

Scheme 9.67 New precatalyst (22) for the Mizoroki-Heck reaction of reactive aryl
bromides and iodides with standard olefins.