Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 390

Ph

Cl Ph

PdCl 2 (2.5 mol% )
L(40mol%)
H 2 O , 120°C, 2 h

83%

Ph Ph

Ph

N N

N

PhHN P P P P

NHPh

N N

Ph Ph

Scheme 9.52 Example of a phosphazane ligand in the ''base-free'' Mizoroki-Heck
reaction of electron-rich chloroarenes.

agents. Moreover, even the addition of base was not needed because the
ligand was observed to serve as such. Such a dual function makes it neces-
sary to use a huge excess of such a ligand (2.5 mol% Pd per 40 mol% ligand)
comparable to the substrate loading, assuming that such compounds can
take several protons (Scheme 9.52). Such systems were used with a wide
range of aryl iodides and bromides at 100 1C and chlorides at 120 1C. Hence
it can be safely concluded that this unusual ligand indeed shows un-
ambiguous evidence of ligand acceleration combined with excellent per-
formance in neat aqueous media. The catalyst was even found to be
recyclable up to 10 times. 195

9.2.3 Systems with Ill-Defined Catalytic Species
Mizoroki-Heck reactions involving Pd species without definite stable ancil-
lary ligands, as already introduced, fall into two distinct categories. Those
using iodoarenes and selected reactive electron-deficient bromoarenes can
take place in the presence of any palladium source. On the other hand, the
reactions of less reactive bromo- and chloroarenes require careful opti-
mization of simple catalytic systems for a particular application and a proper
choice of palladium precatalyst.
There were two seminal works that set the milestones in this chemistry,
both involving precatalysts containing the well-known tris-o-tolylphosphine
ligand, but delivering activity far beyond the expectations based on the
knowledge gained previously on systems with this and other phosphine
ligands. Spencer was the first to discover that even less reactive substrates
such as reactive electron-deficient aryl chlorides or bromides can be made to
react in a system that utilizes a simple palladium precatalyst, if conditions
such as base, solvent, temperature and palladium loading are thoroughly
optimized. 45,46,196,197 The idea was not immediately appreciated, however,
because such catalytic systems were unstable and inconvenient for practical
use. The reactions in such systems often end up swiftly in the formation of
palladium black and sudden termination, hence many researchers con-
sidered them to be poorly reproducible and give them up for more reliable
ligand-accelerated systems.